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Front. Environ. Sci. Eng. 2020, 14(5): 84 https://doi.org/10.1007/s11783-020-1263-8 REVIEW ARTICLE Remediation of soil and groundwater contaminated with organic chemicals using stabilized nanoparticles: Lessons from the past two decades 1,2* 3* 4* 4 5 5 Zhengqing Cai , Xiao Zhao , Jun Duan , Dongye Zhao (✉) , Zhi Dang , Zhang Lin 1 National Engineering Lab for High-concentration Refractory Organic Wastewater, East China University of Science and Technology, Shanghai 200237, China 2 Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China 3 College of Water Resources & Civil Engineering, China Agricultural University, Beijing 100083, China 4 Environmental Engineering Program, Department of Civil Engineering, Auburn University, Auburn, AL 36849, USA 5 School of Environment and Energy, South China University of Technology, Guangzhou 510006, China HIGH LIGHTS GRAPHIC A BSTRA C T � Overviewed evolution and environmental appli- cations of stabilized nanoparticles. � Reviewed theories on particle stabilization for enhanced reactivity/deliverability. � Examined various in situ remediation technolo- gies based on stabilized nanoparticles. � Summarized knowledge on transport of stabi- lized nanoparticles in porous media. � Identiﬁed key knowledge gaps and future research needs on stabilized nanoparticles. AR TICL E I N F O ABSTRA CT Article history: Due to improved soil deliverability and high reactivity, stabilized nanoparticles have been studied for Received 16 March 2020 nearly two decades for in situ remediation of soil and groundwater contaminated with organic pollutants. While large amounts of bench- and ﬁeld-scale experimental data have demonstrated the potential of the Revised 30 April 2020 innovative technology, extensive research results have also unveiled various merits and constraints Accepted 4 May 2020 associated different soil characteristics, types of nanoparticles and particle stabilization techniques. Available online 15 June 2020 Overall, this work aims to critically overview the fundamental principles on particle stabilization, and the evolution and some recent developments of stabilized nanoparticles for degradation of organic contaminants in soil and groundwater. The speciﬁc objectives are to: 1) overview fundamental Keywords: mechanisms in nanoparticle stabilization; 2) summarize key applications of stabilized nanoparticles for Stabilized nanoparticle in situ remediation of soil and groundwater contaminated by legacy and emerging organic chemicals; In-situ remediation 3) update the latest knowledge on the transport and fate of stabilized nanoparticles; 4) examine the merits and constraints of stabilized nanoparticles in environmental remediation applications; and 5) identify the Organic contaminant knowledge gaps and future research needs pertaining to stabilized nanoparticles for remediation of Soil remediation contaminated soil and groundwater. Per instructions of this invited special issue, this review is focused on Groundwater contributions from our group (one of the pioneers in the subject ﬁeld), which, however, is supplemented Fate and transport by important relevant works by others. The knowledge gained is expected to further advance the science and technology in the environmental applications of stabilized nanoparticles. © The Author(s) 2020. This article is published with open access at link.springer.com and journal.hep. com.cn ✉ Corresponding author 1 Introduction E-mail: email@example.com Groundwater is a vital drinking water source in many parts These authors contributed equally to this work. of the world. For example, groundwater accounts for 18% Special Issue—Accounts of Aquatic Chemistry and Technology Research of China’s annual total water consumption (610 billion m ) (Responsible Editors: Jinyong Liu, Haoran Wei & Yin Wang) 2 Front. Environ. Sci. Eng. 2020, 14(5): 84 (O’Carroll et al., 2013; MWR, 2015), and makes up one remove or degrade organic contaminants in soil and third of potable water supplies in England (WHO, 2006). groundwater, including pump and treat (P&T), permeable Yet, with the rapid urbanization and industrialization over reactive barriers (PRBs), soil replacement, soil washing, the past decades, large volumes of soil and groundwater electrokinetic removal/degradation, phytoextraction, and have been contaminated by various legacy and emerging thermal treatment (Gong et al., 2018). For instance, since organic chemicals. For instance, in China, the widespread the 1980s, P&T has been widely applied to contaminated environmental pollution has caused extensive groundwater sites; but this ex-situ technique suffers from poor contamination, and ca.80% of the extractable shallow efﬁciency, contaminant redound, tailing and back diffu- groundwater was found polluted (MEE, 2016). sion, long remediation time, and high energy consumption Of the various priority contaminants, chlorinated (O’Connor et al., 2018). Likewise, since its invention in solvents such as trichloroethene (TCE), tetrachloroethene the 1990s, PRBs have been widely employed in the (PCE), and carbon tetrachloride (TeCA) have been the subsurface to intercept and transform contaminants in most widely studied legacy pollutants in soil and ground- groundwater (Obiri-Nyarko et al., 2014). In 1991, water. For instance, TCE was detected in over 1000 of the O’Hannesin and Gillham (1992) applied granular zero 1699 Superfund Sites in the US, and according to a US valent iron (ZVI) in a PRB for in situ removing TCE and Geological Survey report, PCE, TCE and TeCA were PCE in groundwater. Other than ZVI, many other materials detected in 8.9%, 5.1% and 4.7%, respectively, of the have been tested for in situ remediation of groundwater >5000 wells studied throughout the conterminous United contaminated with halogenated organics, phenolic com- States (Moran et al., 2007). Due to continued uses in many pounds, pharmaceuticals, and nitroaromatics (Guan et al., industrial sectors, thousands of sites have been found 2015). For instance, zeolite (Vignola et al., 2011), calcite contaminated with chlorinated solvents throughout Asia, (Turner et al., 2008), pyrite (Wang et al., 2020), combined Europe and other industrialized areas in the world over the calcium peroxide and straw/biochar (Liu et al. 2019) have past decades, and the concentration of the chlorinated been used or tested as PRB ﬁlling materials. In addition, solvents in groundwater was found to reach the mg/L level biochar (Tang et al., 2013), mackinawite (FeS) (Jeong and (Azzellino et al., 2019; Squillace and Moran, 2007). Hayes, 2007; Duan et al., 2019b), FeS-modiﬁed ZVI In addition, numerous other legacy and emerging (Duan et al., 2019a; Kim et al., 2011; 2013), and MnO - organic contaminants have been detected in contaminated activated persulfate (Mazloomi et al., 2016) have been soil and groundwater, such as pesticides, dioxins, poly- tested to remediate soil and groundwater contaminated chlorinated biphenyls (PCBs), polycyclic aromatic hydro- with organic pollutants. However, PRBs are held back not carbons (PAHs), pharmaceuticals and personal care only by the high installation cost, but also the reactive products (PPCPs), ﬂame retardants, and plasticizers. For lifetime of the active materials, and may not be suitable for examples, PAHs were found in >600 of the 1408 National many soil geological and hydraulic conditions; and the Priorities List sites designated by the US Environmental bulk materials are limited to only amending surface soil for Protection Agency (EPA) (Duan et al., 2015). A recent the poor deliverability in soil. study in Ireland revealed that mecoprop, phenoxyacetic Over the last two decades or so, the development of acid, and 2,4-dichlorophenol were detected in 36%,39%, innovative nanomaterials, in particular, stabilized nano- and 26% of the 730 groundwater samples collected in 6 particles, has brought about some major changes in agricultural sites (McManus et al., 2017). According to a groundwater and soil remediation. Stabilized nanoparticles national reconnaissance of pharmaceuticals and other offer some unprecedented advantages over traditional bulk organic contaminants in the US groundwater, which materials, including much larger speciﬁc surface area, analyzed groundwater samples from a network of 47 higher activity, and soil deliverability. Especially, because sites across 18 states, organic contaminants were detected of the improved soil transportability, stabilized nanoparti- in 81% of the samples, with the most frequently detected cles can be directly delivered into the source zone in compounds including N,N-diethyltoluamide (35%), contaminated soil or deep aquifers to destroy the bisphenol A (30%), tri(2-chloroethyl) phosphate (30%), contaminants in situ (Gong et al., 2018). Figure 1 shows sulfamethoxazole (23%), and 4-octylphenol monoethox- a conceptualized schematic of in situ dechlorination of ylate (19%) (Barnes et al., 2008). More recently, per- and TCE and polychlorinated biphenyls (PCBs) as well as polyﬂuoroalkyl substances (PFAS) have been found at reduction of nitrobenzene by injection of stabilized ZVI more than 400 military sites in the US. nanoparticles into the contaminated source area. Compared These organic chemicals are often recalcitrant to natural to conventional remediation technologies, the in situ degradation and can be retained in soil and groundwater remediation by stabilized nanoparticles offers some key for decades or even hundreds of years, causing long-term advantages, including: 1) it is less destructive and threat to the environmental and human health. more cost effective, 2) it can proactively attack pollutants To mitigate the associated environmental risks, various in source zones, and thus cuts down the remediation remediation methods have been studied or applied to timeframe (Karn et al., 2009; Zhao et al., 2016), and 3) it Zhengqing Cai et al. Remediation of soil and groundwater by stabilized nanoparticles 3 can reach contaminant plumes in deep aquifers or in areas (O’Carroll et al., 2013; Wu and Zeng, 2018). While where conventional technologies cannot be applied. these studies have demonstrated tremendous potential of The concept of direct injection of nanoscale ZVI (nZVI) the stabilized nanoparticles, they also revealed some for in situ dechlorination in the subsurface was ﬁrst challenges in the engineering applications, including: proposed in 1997 by Wang and Zhang (1997). After 4 1) highly stable and well dispersed nanoparticles are years, Elliott and Zhang (2001) conducted the ﬁrst ﬁeld test required to facilitate delivery of the nanoparticles in the for in situ dechlorination by delivering non-stabilized subsurface and to avoid plugging the porous media; 2) the bimetallic (Fe/Pd) particles into a contaminated subsur- mobility of the injected nanoparticles need to be further face. However, this study and many follow-on studies improved to achieve homogeneous distribution of the revealed that the nanoparticles are prone to rapid nanoparticles in the target zones with a signiﬁcant radius of aggregation, hindering the transport and deliverability of active zone; 3) the long-term impacts on the local the nanoparticles (Elliott and Zhang, 2001). biogeochemical conditions remain poorly understood; To facilitate particle delivery, various particle stabiliza- and 4) the long-term fate and transport of the delivered tion methods have been investigated in the past 20 years. nanoparticles need to be investigated. Typically, some macromolecules are coated on the surface Overall, this work aims to critically overview the of nanoparticles either during the nanoparticle formation evolution and some recent development of stabilized (pre-aggregation stabilization) or after the particles (post- nanoparticles for degradation of organic contaminants in aggregation stabilization) are formed, and the resulting soil and groundwater. The speciﬁc objectives are to: electrostatic or/and steric repulsion forces keep the 1) overview fundamental mechanisms in nanoparticle nanoparticles from aggregation (Zhao et al., 2016). The stabilization; 2) summarize key applications of stabilized earliest work on the particle stabilization for environmental nanoparticles for in situ remediation of soil and ground- applications was for stabilizing ZVI nanoparticles, where water contaminated by legacy and emerging organic Zhao and coworkers at Auburn University ﬁrst invented a chemicals; 3) update the latest knowledge on the method for preparing highly stable ZVI nanoparticles by environmental impacts, fate and transport of stabilized adding a low cost stabilizer (starch or carboxymethyl nanoparticles; 4) address the merits and constraints of cellulose (CMC)) during the particle synthesis (He and stabilized nanoparticles in environmental remediation Zhao, 2005,2007). The resulting nanoparticles have been applications; and 5) examine the knowledge gaps and considered the most deliverable ZVI nanoparticles so far, future research direction of stabilized nanoparticles for and have been tested or applied in several ﬁeld scale tests remediation of contaminated soil and groundwater. or practices for in situ degradation of chlorinated solvents Per instructions of this invited special issue, this review in soil and groundwater (see Section 3.1). Following the is focused on contributions from our group, which, similar particle stabilization mechanisms, our group and however, is supplemented by important relevant works others have also developed several other stabilized by others. The knowledge gained is expected to further nanoparticles, such as Fe O , FeS, Fe-Mn binary oxides, advance the science and technology in the environmental 3 4 and Fe (PO ) . applications of stabilized nanoparticles. 3 4 2 Stabilized nanoparticles offer some unique features over conventional granular or powder materials, including: 1) stabilized nanoparticles remain dispersible in water and 2 Stabilized nanoparticles soil, maximizing soil deliverability and the speciﬁc surface area and reactivity of the nanoparticles, 2) the particle size, 2.1 Basic chemistry for synthesis of engineered transportability and reactivity may be manipulated by nanoparticles using stabilizers of different physical-chemical properties (e.g., molecular weight, degree of substitution, functional Generally, there are two strategies to fabricate nano-sized groups, and hydrophobicity), especially when the pre- particles (Wiesner and Bottero, 2007; Zhao et al., 2016), aggregation stabilization technique is applied (Zhao et al., including 1) top-down, namely to break down the large 2016), and 3) stabilized nanoparticles may be directly pieces of materials into nano-sized particles through delivered into the source zones to remediate contaminated physical methods such as ball-milling and grinding; and soil/groundwater in situ. 2) bottom-up, namely to build up the nano-sized materials Numerous bench-scale studies have been reported over from atomic or molecular entities. The bottom-up approach the last decade or so to demonstrate the effectiveness of has been more commonly used to prepare nanoparticles with better controlled properties, and thus will be the focus stabilized nanoparticles for potential in situ degradation of in this review. organic contaminants in soil and groundwater (He et al., Typically, iron-based nanoparticles, such as ZVI, 2007; Wei et al., 2010; Swindle et al., 2014). In addition, magnetite and FeS, are synthesized through redox increasingly more ﬁeld-scale studies have been also carried reactions and/or precipitation processes in aqueous solu- out, which have unveiled the pros and cons of the 2+ 3+ tion starting with Fe and/or Fe . The formation of nanoparticle-based in situ remediation technologies 4 Front. Environ. Sci. Eng. 2020, 14(5): 84 Fig. 1 Schematic description of in situ remediation of TCE/PCBs and nitrobenzene by directly delivering stabilized nZVI into contaminated source zone. nanoscale clusters typically undergoes 4 steps (Wiesner widely used, though different precursors and reducing and Bottero, 2007): 1) formation of zero-charged pre- agents have also been employed (Zhao et al., 2016). cursors, usually with redox reactions, hydroxylation and Equation (1) illustrates the reductive formation of complexation involved; 2) nucleation, namely the zero- elemental Fe from Fe(II) or Fe(III). charged precursors assemble and condense through olation 2þ 3þ 0 Fe ðor Fe Þþ reducing agent ↕ ↓Fe (1) or oxolation; 3) growth of the nuclei to saturation or solubility limit for the precursors; and 4) aging stage, at The nucleation of the resulting Fe gives the clustered which the minimum activation energy is reached and ZVI particles or aggregates. Typically, inert or anoxic thermodynamically stable clusters/nanoparticles are conditions are desired during the synthesis to assure formed. To produce nanoparticles with desired size, efﬁcient reduction and avoid oxidation of the ZVI crystalline structure, and morphology, all of these 4 steps particles. Sodium/potassium borohydride is a strong should be well-controlled. reducing agent but comes with a relatively high cost. For environmental applications (e.g., water treatment Consequently, some cheaper and “greener” reducing and soil remediation), several key criteria must be taken agents were also tested such as tea-based polyphenolic into account in the material synthesis, including: 1) the compounds (Hoag et al., 2009) or sorghum bran extracts nanoparticles must be non-toxic, 2) the synthesis should (Njagi et al., 2011), but the resulting ZVI nanoparticles avoid uses of toxic and expensive organic solvents, i.e., exhibited different morphologies and size distributions and aqueous solution based synthesis is preferred, and 3) the weakened reactivity due to the slower nucleation and overall process should be simple, low-cost and environ- particle growth rates. mentally benign. FeS nanoparticles have drawn extensive interests in In line with these criteria, iron-based nanomaterials have environmental remediation recently for the moderate been the most preferred nanomaterials for environmental reducing power, longer-lasting reactive life than ZVI, uses, such as ZVI, iron sulﬁde (FeS), magnetite (Fe O ), 3 4 and strong sorption capacity toward many heavy metals iron phosphate (Fe (PO ) ), binary metal oxides (Fe-Mn 3 4 2 (Gong et al., 2016a). Conventionally, FeS nanoparticles oxides), and sulﬁdated ZVI (S-nZVI). can be synthesized through mixing stoichiometric amounts ZVI has been the most studied iron-based nanoparticles 2– of Fe(II) and S under inert or anoxic conditions as shown (Liu et al., 2015). Nano-sized ZVI materials (typically in Eq. (2) (Gong et al., 2016a), aggregates of nanoscale primary particles) can be synthe- 2þ 2– sized through high energy ball-milling (a top-down Fe þ S ↕ ↓FeS (2) method), or reductive precipitation (bottom-up), or gas- phase reduction of nanoscale iron oxide (Zhao et al., 2016). Magnetite (Fe O ) nanoparticles have been shown to 3 4 Wang and Zhang (1997) pioneered the use of ZVI for offer high adsorption capacities toward many important treating chlorinated solvents in water, where clustered ZVI contaminants such as arsenic and chromium due to the larger speciﬁc surface area (Liang et al., 2012; Liang and particles were prepared by reducing Fe(III) using NaBH . Zhao, 2014). Generally, magnetite particles are prepared Since then, the borohydride reduction method has been Zhengqing Cai et al. Remediation of soil and groundwater by stabilized nanoparticles 5 þ 2– per the classical precipitation approach (Anderson, 1956). H S↕ ↓2H þ S (11) Typically, a base solution (NaOH or NH ) is introduced In the two-step method, Fe is ﬁrst formed according to dropwise into the solution containing Fe(III) and Fe(II) at a Eq. (1), then a sulfur source is introduced to react with molar ratio of 2:1, thereby transforming Fe(III) and Fe(II) 2+ remaining Fe to form FeS via Eq. (2) but in the presence into FeOOH and Fe(OH) , respectively, as shown in Eqs. 0 0 of Fe , resulting in a core-shell structure FeS-on-Fe (3) and (4). During the follow-on aging stage, FeOOH particles (Duan et al., 2019a). reacts with Fe(OH) to form the magnetite particles Eq. (5). 3þ – Fe þ 3OH ↕ ↓FeðOHÞ ↕ ↓FeOOH þ H O 2.2 Principles of nanoparticle stabilization 3 2 (3) In the nanoscale, surface forces far exceed gravity. As such, surface interactions dominate the physical stability of 2þ – Fe þ 2OH ↕ ↓FeðOHÞ (4) the nanoparticles. Moreover, nanoparticles possess very high surface energy and thus are thermodynamically 2FeOOH þ FeðOHÞ ↕ ↓Fe O þ 2H O (5) 2 3 4 2 unstable, i.e., they tend to agglomerate into larger particles and/or react with the media. Agglomeration of nanopar- Iron or calcium phosphate compounds have been found ticles usually occurs in three manners (Zhao et al., 2016), 2+ effective for sequestrating heavy metals such as Pb and including 1) Ostwald ripening, i.e., smaller or ‘immature’ 2+ Cu through the formation of metal phosphate precipitates particles may dissolve and become feeding materials for and surface complexation (Liu and Zhao, 2007, 2013). Liu larger particles, leading to an increase of the mean particle and Zhao (2007) synthesized iron phosphate nanoparticles size; 2) arrested precipitation (precipitation facilitated by through a straightforward stoichiometric precipitation formation of nucleation centers); and 3) attractive interac- approach per Eq. (6) (Liu and Zhao, 2007), tions between particles (e.g., van der Walls and magnetic 2þ 3– forces). 3Fe þ 2PO þ 8H O↕ ↓Fe ðPO Þ ⋅8H O (6) 4 2 3 4 2 2 Depending on the extent, aggregation can alter the Binary metal oxides are commonly found in lithosphere physico-chemical properties of the particles and affect the and pedosphere, and show high afﬁnity to metalloid anions environmental uses of nanoparticles. For instance, aggre- (As or Se oxyanions). Fe-Mn binary oxides have been the gated nanoparticles may partially or completely lose the most studied binary particles for treating arsenic and other characteristics of nanoscale particles such as high speciﬁc metal/metalloids in water. Typically, Fe-Mn binary oxides surface area, high reactivity, high-surface-to-volume ratio, particles are prepared by reacting Fe(II) with KMnO to and size-dependent physico-chemical properties. In addi- form a mixed phase of Fe O and MnO , following the 2 3 2 tion, aggregated nanoparticles are much less transportable stoichiometry of Eqs. (7) and (8) (An and Zhao, 2012), in soil or sediment (He et al., 2007). Consequently, particle stabilization is often required to resist aggregation and to 2þ – – 3Fe þ MnO þ 4OH þ 3H O 4 2 obtain a stable dispersion for intended uses. According to the classic Derjaguin-Landau-Verwey- ↕ ↓3FeðOHÞ þ MnO þ H (7) 3 2 Overbeek (DLVO) theory, the net interaction energy between particles is the sum of repulsive energy and 2FeðOHÞ ↕ ↓Fe O þ 3H O (8) attraction energy (Phenrat et al., 2008). Typical attractive 3 2 3 2 forces include van der Waals and magnetic attraction, S-nZVI has been recently synthesized through one-step whereas repulsive forces include electrostatic double layer or two-step synthesis method to enhance the reactivity and repulsion, osmotic repulsion and elastic-steric repulsion. selectivity of pristine ZVI (Kim et al., 2011; Gong et al., Coating nanoparticles with a proper stabilizers and at an 2017; Duan et al., 2019a). In the one-step method, a appropriate concentration can increase the energy barrier mixture of boronhidride and dithionite solution is dropwise between two approaching nanoparticles by enhancing the 3+ added to the Fe solution. The dithionite decomposes repulsive forces. through Eqs. (9)–(11) to produce sulﬁde (Kim et al., 2011), A stabilizer can function in two ways to increase the and Fe and FeS were simultaneously formed via Eqs. (1) dispersion stability: 1) surface modiﬁcation, i.e., charged and (2) in one pot, stabilizer molecules are attached to particle surfaces, inducing electrostatic repulsion between like-charged surfaces; and 2) network or steric stabilization, i.e., 2– – 2– 2S O þ H O ↕ ↓2HSO þ S O (9) 2 4 2 3 2 3 stabilizer molecules (usually long-chained macromole- cules) are attached on the surface to form a network to 2– 2– þ S O þ S O þ 2H O þ H 2 4 2 3 2 induce steric or osmotic separation of the nanoparticles. Accordingly, three particle stabilization mechanisms are ↕ ↓3HSO þ H S (10) often cited, namely, 1) electrostatic stabilization (charged 3 2 6 Front. Environ. Sci. Eng. 2020, 14(5): 84 stabilizers are sorbed on the surface to create/enhance the than pre-agglomeration approach in terms of both particle electrostatic double layer repulsion due to Coulombic size and reactivity. For instance, Cho and Choi (2010) forces); 2) steric stabilization (osmotic repulsion occurs found that pre-agglomeration stabilized ZVI nanoparticles when the layers of macromolecules on approaching were more reactive than bare ZVI particles when tested for particles overlap); and 3) electrosteric stabilization (the dechlorination of chlorinated solvents, whereas Phenrat combination of electrostatic and steric repulsions. In some et al. (2009) reported that the post-agglomeration stabiliza- cases, network stabilization may also refer to particle tion did not enhance the reactivity of bare nZVI. In separation due to the formation of a dense viscous gel addition, for reactive nanoparticles like ZVI, the sonication matrix between two particles due to hydrogen bonding and process could also induce elevated corrosion of the polymer entanglements, which may occur at high doses of particles, resulting in signiﬁcant reactivity loss (Tratnyek large viscous macromolecules (Comba and Sethi, 2009). et al., 2011). However, it is probably more accurate to refer to this type of particles as networked or bridged nanoparticles because 2.3 Effect of stabilizer on particle reactivity for target they often appear as large ﬂocs that settle by gravity and contaminants cannot form a stable suspension. Stabilizers may be introduced into a dispersion before or In a typical aqueous suspension, stabilizer molecules will after the aggregates are formed, which are termed as pre- distribute between the aqueous phase and the nanoparticle agglomeration and post-agglomeration stabilization, surface. As shown in Fig. 3, stabilizers can inﬂuence the respectively (Fig. 2). For pre-agglomeration stabilization, interactions between nanoparticles and a target contami- stabilizers are added before or during the nucleation and nant in a number of ways. First, adsorption of stabilizers on growth and aging of nanoparticles, and thus, pre- the particle surface can alter the surface properties, such as agglomeration stabilization often results in smaller and the surface charge/potential and accessibility. As a result, more uniform nanoparticles. For example, highly stable the modiﬁed surface becomes more selective toward stabilized ZVI nanoparticles were prepared in the presence different contaminants. For example, coating negatively CMC as a stabilizer (Fig. 2). In contrast, in post- charged CMC on Fe O or Fe-Mn binary nanoparticles 3 4 agglomeration stabilization, the stabilizers are applied turned the surface much more negative than that for starch- after the aggregates of nanoparticles are formed, where the modiﬁed counterparts. Consequently, starch-modiﬁed formed aggregates are broken into ﬁner particles via nanoparticles showed more favorable adsorption for external energy (e.g., sonication) in the presence of a arsenate (An and Zhao, 2012; Liang et al., 2012; Liang stabilizer. In this case, the size of the resulting particles will and Zhao, 2014); conversely, CMC-stabilized FeS nano- be dependent on efﬁciency of the aggregate breakage, and particles were more favorable for taking up cationic heavy 2+ thus, post-agglomeration stabilization is often less efﬁcient metals such as Hg (Gong et al., 2014). Second, stabilizer Fig. 2 Conceptualized illustration of nanoparticle aggregation and stabilization. Zhengqing Cai et al. Remediation of soil and groundwater by stabilized nanoparticles 7 Fig. 3 Effects of stabilizers on interactions between nanoparticles and target contaminants. molecules may compete with the target contaminants for 2) synthetic or natural macromolecules or polyelectrolytes; the adsorption/reaction sites on the nanoparticle surface, or 3) viscosity modiﬁers; 4) oil emulsiﬁers; and 5) micro- adsorption of stabilizers may block some of the reactive scale solid supports or coatings. sites or render the sites less accessible. This is particularly Surfactants are widely used surface modiﬁers to improve the case when small-molecule stabilizers are used. For the dispersion stability and the mobility of nanoparticles. instance, previous studies on stabilized Pd, Fe-Pd and FeS Both anionic and cationic surfactants can improve the nanoparticles indicated that glucose-modiﬁed Pd nanopar- electrostatic repulsion between nanoparticles resulting in ticles were less reactive as a catalyst than CMC-stabilized enhanced particle stability. Surfactant molecules can exist Pd because the adsorbed glucose layer was much denser in the aqueous phase as monomers, aggregates and than the CMC layer (He and Zhao, 2008; Gong et al., micelles. Typically, micelles function better to disperse 2014). Moreover, the presence of too much stabilizer on colloids/nanoparticles. However, the formation of micelles the particle surface can inhibit the contaminant mass requires a dosage higher than the critical micelle transfer and reactivity (Gong et al., 2014). Third, some concentration, which may impede its practicality for ﬁeld organic stabilizers (especially those with quinone and applications. In addition, the toxicity of surfactants and the phenol moieties) may serve as a catalyst or electron shuttle possible solubilization effect on problem contaminants to facilitate redox reactions between the nanoparticles should also be considered. (e.g., ZVI and FeS) and the contaminants (Tratnyek et al., Natural bio-polymers including starch, guar gum, 2011). Lastly, while most stabilizer molecules are expected xanthan gum have been used as neutral stabilizers for to be adsorbed on the surface, some remain dissolved in the engineered nanoparticles (Zhao et al., 2016), where solution. These free molecules can complex with the target particle stabilization is achieved through the steric or 2+ contaminants (e.g., CMC-Hg ), resulting in increased network repulsion mechanism. In contrast, engineered solubility/mobility of the target chemicals. functional polymers or macromolecules, such as CMC, The overall effects of stabilizers are the sum of all the poly acrylic acid (PAA), polyaspartate, and poly styrene interactions. Therefore, the selection of the most suitable sulfonate, are all negatively charged, which offer better stabilizer should consider a number of factors, including particle stabilization through concurrent electrostatic and type of the nanoparticles, properties of the target steric repulsion mechanisms. Overall, these bio-polymers contaminants, stabilizer molecular size, charge, and or synthetic polymers (especially, polysugars) are not only functional groups, environmental friendliness, and cost. effective stabilizers for many environmentally relevant When used for soil or groundwater remediation, environ- nanoparticles (e.g., ZVI, iron oxides, and Pd), but low-cost mental factors including soil properties, particle deliver- and environmental friendly. There have been many of these ability, and fate and transport of the stabilizers and macromolecules available on the market, with different nanoparticles, should be taken into account as well. molecular weights (few hundreds to million Daltons), degrees of substitution (DS), and viscosity. The abundant 2.4 Common types of stabilizers options provide a convenient means to manipulate the nanoparticle growth and size by use of a suitable stabilizer For environmental remediation uses, stabilizers can be or a combination of two or more different stabilizers. divided into ﬁve groups (Zhao et al., 2016): 1) surfactants; Some high molecular-weight macromolecules may 8 Front. Environ. Sci. Eng. 2020, 14(5): 84 stabilize nanoparticles by increasing the suspension solvents using stabilized ZVI nanoparticles has been one viscosity and network effect. For instance, xanthan gum of the most studied subjects over the last two decades or so. (Comba and Sethi, 2009) and guar gum (Sakulchaicharoen Early studies showed that bare ZVI particles appear as et al., 2010) have been used as viscosity modiﬁers to micron to millimeter scale aggregates, which are hardly inhibit aggregation of ZVI nanoparticles. mobile or deliverable in soil (Schrick et al., 2004). Since Oil emulsiﬁers can modify the hydrophobicity of the invention of the starch- and CMC-stabilized ZVI nanoparticles, which are often desirable for remediation nanoparticles (He and Zhao, 2005; He et al., 2007), a great of dense non-aqueous phase liquids (DNAPLs). For deal of effort has been devoted to developing various instance, vegetable oil (along with some surfactants) was stabilized or surface modiﬁed nanoparticles that can be introduced in ZVI suspension to facilitate particle delivery directly delivered into the contaminated soil. As shown in and inhibit the particle corrosion in groundwater (Quinn Fig. 3, CMC-stabilized Fe-Pd nanoparticles were transpor- et al., 2005). table through a loamy sand column within 30 s under Solid supports or protective solid coatings may also keep gravity, while bare Fe-Pd nanoparticles were completely nanoparticles from aggregating. For instance, SiO or C- blocked on top of the sand column (He et al., 2007). In based materials (biochar, carbon nanoparticles, and carbon addition, the CMC coating also mitigates adverse effects of microspheres) have been used to support ZVI nanoparti- the nanoparticels on biota. For instance, Lee et al. (2008) cles (Zheng et al., 2008; Sunkara et al., 2010; Wei et al., found that bare nZVI may invade and deactivate E.coli 2019), where nanoparticles are embedded on the surface or cells (Fig. 3), while Dong et al. (2016) reported that the in the porous structure of the supports. However, such presence of CMC coating reduced the cytotoxicity of ZVI supported nanoparticles are not directly deliverable in soil, nanoparticles due to surface electrostatic repulsive forces and as such, they are more suitable for water treatment or between the CMC-coated particles and the negatively uses in PRBs in groundwater remediation. charges cells. In fact, the presence of the polysaccharide Overall, our knowledge on particle stabilization has stabilizers may induce some fortuitous positive effects. For come a long way. The use of stabilizers, especially in the example, in a pilot-scale study, He et al. (2010) reported pre-agglomeration stabilization process, can facilitate that polysaccharide stabilizers (like CMC) could serve as a formation of well stabilized aqueous suspensions of carbon source to stimulate the local bacteria activity and desired size and reactivity. Depending on the type of induce biodegradation of chlorinated solvents after nanoparticles and their uses, different stabilizers may be delivery into the subsurface. used. To this end, there is a need for engineered stabilizers of controlled molecular weight (MW) and structure, 3.1 Reductive degradation of organic pollutants functionality, and viscosity to optimize the performances of the resulting nanoparticles. Chlorinated solvents are the most widespread organic contaminants and have been listed as the priority contaminants in soil and groundwater (Stroo et al., 2003; 3 Stabilized nanoparticles for degradation Zimmermann et al., 2020). Typical chlorinated solvents of organic pollutants in soil and water include PCE, TCE, 1,1,2-trichloroethane (TCA), chloro- form (CF) and other chlorinated aliphatic hydrocarbons For environmental cleanup, the most promising uses of (CAHs). ZVI nanoparticles (usually with Pd or another stabilized nanoparticles are for in situ remediation of novel or transition metal as the catalyst) have been contaminated soil due to the improved soil deliverability of extensively studied for dechlorination of these chlorinated the nanoparticles. In situ degradation of chlorinated hydrocarbons. Equations (12)–(13) illustrate the redox Fig. 4 Mechanisms of reductive dechlorination of TCE by ZVI-based bimetallic nanomaterials (a) or sulﬁdated ZVI (b) (He et al., 2018). Zhengqing Cai et al. Remediation of soil and groundwater by stabilized nanoparticles 9 reactions in a typical dechlorination process, and Fig. 4 were rapidly degraded with the highest degradation rate depicts the reaction mechanisms. occurred in the ﬁrst week of the injections. The concentrations of the chlorinated solvents rebounded to 0 2þ – Fe ↕ ↓Fe þ 2e the pre-injection levels after ~2 weeks, indicating exhaus- tion of the ZVI's reactivity. However, the injection of E ¼ – 0:44 V at pH 7 (12) CMC-Fe/Pd initiated a biological dechlorination that started after four weeks of the ﬁrst injection and lasted – þ – throughout the monitoring period. After ~600 days, the RCl þ 2e þ H ↕ ↓RH þ Cl combined concentration of TCE, PCE and their biode- gradation byproducts in the two monitoring wells remained E ¼ 0:5– 1:5 V at pH 7 (13) 40% and 61% lower than the pre-injection level. This was the ﬁrst ﬁeld evidence suggesting that CMC-Fe/Pd Typically, the dechlorination occurs through an initial facilitated a rapid abiotic dechlorination in the early stage adsorption followed by reductive breakage of the carbon- and then initiated a long-lasting biotic dechlorination halogen bonds. Usually, a small fraction (~1%)ofa process with CMC and H as additional sources of carbon secondary metal such as Pd, Ni or Cu is incorporated on and electrons. ZVI to catalyze the dechlorination rate. As depicted in One of the critical drawbacks of stabilized ZVI Fig. 4(a), the introduction of a metal catalyst can facilitate nanoparticles has been the relatively short reactive lifetime electron transfer and lead to production of more reactive (hours to days) due to competitive side reactions such as atomic hydrogen ( H). In particular, H adsorbed on the corrosion by water or dissolved oxygen (DO). As such, metal surface was found the predominant contributor to stabilized ZVI nanoparticles should be prepared on site and TCE dechlorination (He et al., 2018). In addition, particle used right before an attempt injection. To extend the stabilization can also speed up the reaction rate. Earlier, He reactive life and improve the reaction selectivity, S-nZVI and Zhao (2005) found that starch stabilized Fe-Pd have been prepared in recent years (He et al., 2018; Duan bimetallic nanoparticles showed 37 times faster dechlor- et al., 2019a). As shown in Fig. 4(b), the sulﬁdation may ination rate for TCE than bare Fe-Pd nanoparticles, and facilitate the electron transfer while suppressing the side later, the authors found that CMC-stabilized Fe-Pd corrosion reactions (He et al., 2018). Compared to pristine nanoparticles offered two times faster dechlorination rate Fe-Pd, S-nZVI showed a 190 folds faster TCE degradation than starch-stabilized Fe-Pd nanoparticles due to increased rate and 36 folds greater electron efﬁciency (He et al., speciﬁc surface area and the catalytic activity (He and 2018). He et al. (2018) also claimed that TCE dechlorina- Zhao, 2008). Zhang et al. (2011) reported the ﬁrst tion is more favorable at the FeS sites in S-nZVI, while systematic study of degradation of soil-sorbed TCE by other side reactions (e.g., corrosion) occur predominantly CMC-stabilized Fe-Pd nanoparticles and found that the on the Fe O sites; speciﬁcally, the FeS sites contributed x y x TCE sorbed by soil with higher soil organic matter (SOM) ~72% to the TCE degradation based on the electron was more recalcitrant to the reductive dechlorination. The utilization efﬁciency while Fe O contributed only ~28%. x y possible reasons include: 1) SOM may lessen the stabilizer Moreover, S-nZVI degrades TCE mainly through electron effect, 2) adsorption of SOM may block the reaction sites transferring on the FeS sites, whereas the reactive atomic for ZVI nanoparticles, 3) SOM may compete with TCE for hydrogen mechanism played only a minor role (He et al. electron donors, and 4) SOM may suppress the catalytic 2018). Fan et al. (2017) pointed out that sulﬁdation of effect of Pd (Zhang et al., 2011). Moreover, this study nZVI may offer the following advantages: 1) it can demonstrated that the addition of some surfactants can generate more FeS phases thereby enhancing the enhance TCE desorption and degradation effectiveness by dechlorination process, 2) sulﬁdation can suppress the CMC-Fe/Pd, and the overall effect depends on the formation of iron oxides on the particle surface resulting in physiochemical properties of surfactants and soil char- less undesired reactions, and 3) it may immobilize metals acteristics (Zhang et al., 2011). by forming sparingly soluble metal sulﬁdes. He et al. (2010) conducted a pilot-scale ﬁeld study on in Cai et al. (2018b) studied CMC-stabilized ZVI nano- situ degradation of PCE, TCE and PCBs by delivering particles for reductive removal of nitrobenzene (NB) in CMC-Fe/Pd into the contaminated subsurface. Two water and a ﬁeld soil (Cai et al., 2018b). The materials injections were administered, and the concentrations of displayed 3.7 times higher reactivity toward NB degrada- the contaminants were followed for ~600 days. In the ﬁrst tion than bare ZVI based on the pseudo-ﬁrst order reaction –1 injection, ~150 gallons of CMC-Fe/Pd (0.2 g/L) were rate constants (0.643 vs. 0.175 min ). The study also gravity-fed into the 50-ft (15.2 m) deep unconﬁned aquifer. revealed that the degradation reaction proceeded as NB ! After one month, another batch of ~150 gallons were nitrosobenzene ! phenylhydroxylamine ! aniline, delivered but at a higher concentration (1.0 g/L). Analyses where aniline is easily biodegradable (Zhao et al., 2019). of PCE and TCE in the monitoring wells (located 1.5 and Moreover, the stabilized nanoparticles at 0.6 g/L were able 3.0 m from the injection well) indicated that PCE and TCE to nearly completely degrade soil-sorbed NB (0.01 mmol/ 10 Front. Environ. Sci. Eng. 2020, 14(5): 84 g). By comparing the NB desorption and degradation rates, tions of SOM (>30 mg/L) decreased the rate constant for the availability of electrons was found to be the rate- nearly 88% (Zhang et al., 2013). These studies conﬁrmed limiting step in the degradation of soil-sorbed NB. that CMC or other similar polysaccharides may serve as While stabilized nanoparticles have shown to be a effective stabilizers for preparing stable noble metal promising remediation technology, there are still several catalysts. To take advantages of the high catalytic activity technical issues that need to be addressed. First, although of the stabilized nanoparticles, and to facilitate treating stabilized nanoparticles were initially contemplated to be contaminants in water in standard reactors (e.g., batch or used for in situ remediation of contaminated soil, most ﬁxed-bed column), the nanoparticles can be deposited on studies so far have been focused on testing the particles’ low-cost supporting materials such as activated alumina reactivity in the aqueous phase. As such, there exists a data and/or activated carbons. High temperature calcination gap on the reactivity and transport behaviors of stabilized may not be needed although moderate thermal treatment nanoparticles when used for treating soil-sorbed organic (~300 °C) can consolidate the particle loading and burn off pollutants. Second, while CMC-stabilized nanoparticles the stabilizer after the loading (Zhang et al., 2013). appeared to be most transportable in soil, controlled deliverability of stabilized nanoparticles in the desired 3.3 Oxidative degradation of organic chemicals using source zone remains a challenge, in most cases, the stabilized nanoparticles technology is limited by the limited transport distance. PPCPs have been widely detected in groundwater, surface Third, more information is needed on the performances of water, and soil owing to their widespread consumption and stabilized nanoparticles under actual ﬁeld conditions, and poor removal efﬁciency by conventional water treatment more pilot- and/or ﬁeld scale data are yet to be collected to processes (Cai et al., 2018a; Hu et al., 2019; Wang et al., identify the most suitable stabilizers as well as the physical, 2019). While reductive degradation is often more effective geological, biogeochemical and hydrodynamic conditions. for halogenated organics, oxidation is the common Fourth, the long-term reactivity, fate and transport of the degradation path for many PPCPs. As such, stabilized delivered nanoparticles and the stabilizers need to be oxidizing nanoparticles have been prepared and tested to investigated. Fifth, the impacts of delivered nanoparticles degrade PPCPs in groundwater and soil (Chen et al., 2012; on the soil physico-chemical properties, the local biogeo- Han et al., 2015; Han et al., 2017b). chemical conditions, and the stability of other co-existing MnO has been a known oxidant and can oxidize contaminants need to be investigated. 2 pharmaceuticals (Du et al., 2018). For example, estradiol can be oxidized by MnO to form estrone Eq. (14) and 2- 3.2 Stabilized nanoparticles as a catalyst 2 hydroxyestradiol Eq. (15) (Jiang et al., 2009): Stabilizers have been widely used in fabricating more C H O þ MnO þ 2H ↕ ↓ 18 24 2 2 reactive catalytic nanoparticles for water or soil treatment. Elemental Pd is a powerful catalyst and nanosized Pd 2þ C H O þ Mn þ 2H O (14) particles (with an average size of 2.4 nm) were synthesized 18 22 2 2 through a facile NaBH reduction method with CMC as the stabilizer (Liu et al., 2008). The catalytic activities of C H O þ MnO þ 2H ↕ ↓ 18 24 2 2 CMC-stabilized Pd nanoparticles were examined through TCE hydrodechlorination reactions, and the observed 2þ C H O þ Mn þ H O (15) 18 24 3 2 pseudo ﬁrst-order reaction rate constant was increased from 224 to 828 L/min/g and the mean particle size of Pd However, the reaction rate with non-stabilized MnO decreased from 4.7 to 2.5 nm when CMC content increased particles is rather slow due to the low speciﬁc surface area from 0.001 to 0.050 wt.% (Liu et al., 2008). The work also and limited reactive sites. To enhance the reactivity and demonstrated the size-effect and the more active roles of facilitate soil deliverability, CMC-stabilized MnO nano- corner and edge atoms of the Pd nanoparticles (Liu et al., particles were prepared and tested for degradation of 2008). To facilitate water treatment uses of the stabilized aqueous and soil-sorbed 17β-estradiol (Han et al., 2015; Pd nanoparticles, Bacik et al. (2012) loaded CMC-Pd onto Han et al., 2017b). The CMC stabilization technique a commercial porous Al O support through an incipient resulted in discrete and rather uniform-sized MnO 2 3 2 wetness impregnation technique. The CMC-Pd nanoparti- nanoparticles, with a mean particle size of 36.8410.17 –3 cles were well-dispersed on the support and composite nm at a CMC-to-MnO molar ratio of 1.3910 . More- materials offered >7 times greater activity when used for over, CMC-stabilized MnO nanoparticles displayed much TCE hydrodechlorination compared to commercial alu- greater speciﬁc surface area, improved reactivity and mina supported Pd particles (Zhang et al., 2013). Low improved soil deliverability. For example, when tested for concentrations of SOM (< 10 mg/L) exhibited negligible oxidative degradation of estradiol in water, the apparent effect on TCE hydrodechlorination, while high concentra- pseudo ﬁrst-order rate constant (k ) at pH 7 was increased a Zhengqing Cai et al. Remediation of soil and groundwater by stabilized nanoparticles 11 –1 –1 from 0.067 h for non-stabilized MnO to 0.071 h for technology must consider the soil properties especially CMC-stabilized MnO ,and the24hremovalwas the SOM content and adsorption/desorption behaviors of increased by 9% (Han et al., 2015). The advantages of the contaminants. stabilized MnO nanoparticles became more evident when It should be noted that MnO is a relatively weak 2 2 the nanoparticles were used to degrade soil-sorbed oxidant, so it may not completely mineralize PPCPs, estradiol. After 96 h of reactions, 83% of estradiol in a rather, it may transform the chemicals into less toxic –4 soil slurry system was degraded using 210 mol/L of byproducts. Thus, the MnO oxidation may be combined CMC-stabilized MnO nanoparticles, while only 70% of with other processes such as advanced oxidation processes estradiol was degraded by the same dosage of non- (AOP), if complete mineralization is desired (Du et al., stabilized MnO particles. The improved reactivity was 2018). attributed to the protection of the CMC coating that Stabilized ZVI nanoparticles may be used to induce complexes with inhibitive soil components (such as DOM, Fenton-like reactions under oxic conditions to oxidize 2+ 2+ Ca ,Mn , and their complexes). Moreover, CMC- organic contaminants in water via reactive oxygen species stabilized MnO displayed much improved soil transport- (ROS) (Joo and Zhao, 2008). Compared to the classical ability or deliverability. At a low injection pressure of 2.14 Fenton reactions, the nanoparticle-induced Fenton process psi, the breakthrough of CMC-stabilized MnO through a can proceed at relatively higher pH (>6). For instance, Joo sandy loam soil bed occurred at ~3 pore volumes (PVs), and Zhao (2008) prepared and tested CMC-stabilized Fe/ and full breakthrough was reached at ~7 PVs with the C/C Pd bimetallic nanoparticles for degradation of lindane and plateau maintained at ~0.90 (i.e. 90% of the inﬂuent level). atrazine (Joo and Zhao, 2008). Batch kinetic tests showed Stabilized MnO nanoparticles were found evenly dis- that under oxic conditions, the nanoparticles facilitated tributed along the column bed (Han et al., 2015). The soil Fenton-like reactions, which led to oxidation of 65% of deliverability enabled the nanoparticles to be used for in lindane within 10 min (initial concentration = 1 mg/L, ZVI situ oxidative degradation of the estradiol or likely other dose = 0.5 g/L, Pd = 0.8% of Fe, initial pH = 7.9–8.4, ﬁnal PPCPs sorbed in soil. Up to 88% of water-leachable 17β- pH = 6.2–6.9). While the particle stabilization greatly estradiol was degraded when an estradiol-laden soil was enhanced the anaerobic degradation of lindane, the CMC treated with 22–130 PVs of a CMC-stabilized MnO coating was found to consume nearly 50% of the hydroxyl suspension (MnO = 0.174 g/L) (Han et al., 2017b). radicals generated from the nanoparticles-mediated Fenton The degradation involves a ﬁrst-step adsorption of the process, leading to lowered degradation efﬁciency despite solutes on the particle surface and then the oxidation faster reaction rate. Therefore, more oxidation-resistant reaction. As such, the degradation effectiveness can be stabilizers should be explored for this purpose. inﬂuenced by factors that affect adsorption and reactivity Compared to reductive nanoparticles, much less infor- of MnO , such as particle size, surface area, surface charge, mation is available on stabilized oxidative nanoparticles. and accessibility of the reactive sites. Lower pH was found The degradation pathway of oxidative process is not well to favor the reaction, which is attributed to the proton– understood, and the environmental impacts of the reaction catalyzed reduction of MnO via Eq. (16). In addition, by-products as well as the oxidative nanoparticles need to lower pH is associated with higher reduction potential, be investigated. Recent works have indicated that high 2+ lower surface charge, and less adsorption of Mn on the concentrations of stabilized Fe O and FeS nanoparticles 3 4 particle surface. Some leachable soil components such as under oxic conditions can cause oxidative stress and tissue 2+ Ca and organic matter were found to inhibit the reaction damage toward zebraﬁsh (Zheng et al., 2018a,b). in the early stage, but promoted the reaction in the longer More reactive materials are needed, which can either term. The inhibition was due to the rapid uptake of DOM directly extract electrons from the target contaminants or and cations onto the nanoparticles surface, blocking some facilitate generation of highly reactive oxidizing species to of the reactive sites; however, over the longer run, DOM completely mineralize the target contaminants. While 2+ may serve as a scavenger for Mn generated in the redoc solutions of strong oxidants such as permanganate or reaction process, alleviating the inhibitive effect (Han persulfate have been used to oxidize soil-sorbed organic et al., 2015). contaminants, the solution form of these chemicals bears þ – 2þ with some critical limitations, including: 1) the solution 1=2MnO ðsÞþ 2H þ e ↕ ↓1=2Mn ðaqÞþ H O (16) 2 2 may move along with the groundwater and may spread and The desorption rate of estradiol from soil was found to cause undesired side effects, and 2) due to the limited critically affect the degradation effectiveness. If the contact time with the target contaminants, the reactivity desorption is too fast, it may be ﬂushed away to the may not be well utilized and the effectiveness is severely downstream of the groundwater by the injected nanopar- limited by the desorption rate of the contaminants from the ticle slurry, resulting in limited contact with the nanopar- soil. Instead, once delivered, reactive nanoparticles may ticles; conversely, slow desorption may limit the overall stay attached to the soil matrix and offer prolonged reactive degradation rate. Therefore, the use of the in situ life without affect the down-gradient ﬂow. The particle 12 Front. Environ. Sci. Eng. 2020, 14(5): 84 stabilization technique may also be extended to prepare 4 Transport of stabilized nanoparticles photoactive materials for oxidative treatment of persistent organic chemicals by preparing photoactive semiconduc- As stated above, for in situ remediation of soil and tors in the presence of a stabilizer. For instance, Xu et al. groundwater, it is desirable to deliver the nanoparticles into (2020a) prepared a type of iron oxide/carbon sphere the contaminated source zone, or to create a reactive zone composite material in the presence glucose that serve as by evenly distributing the nanoparticles in the target space. both a carbon source and a stabilizer, and the new material In general, non-stabilized particles are hardly deliverable showed much enhanced photoactivity toward perﬂuorooc- in typical soil or sediment due to the strong soil ﬁltration tanoic acid (PFOA). and/or straining effects. As such, proper particle stabiliza- tion is required to facilitate direct injection of the reactive 3.4 Adsorptive removal of persistent organic chemicals nanoparticles into the source zone. This in situ remediation using stabilized nanoparticles method is particularly advantageous when the contami- nants are located deep in the aquifer or when surface Stabilized nanoparticles may also be used for adsorptive remediation actions are not possible (e.g., when a removal of persistent organic pollutants (POPs) in water or contaminant plume is located under an existing structure). immobilization of POPs in soil or sediment. Gong et al. Alternatively, a permeable reactive zone may be built (2016b) prepared stabilized magnetite nanoparticles around a contaminant plume to contain its spreading by (Fe O ) for removing PFOA from water. Batch kinetic 3 4 directly delivering stabilized nanoparticles without digging experiments revealed that the starch-stabilized nanoparti- out the soil. To this end, understanding the transport cles facilitated fast PFOA uptake with a sorption properties of stabilized nanoparticles that are delivered in equilibrium time of 30 min, and provided 2.4 times higher the soil is critical to set up the injection points and injection adsorption capacity (maximum Langmuir capacity = 62.5 pressure, to assess the effective area, and to evaluate the mg/g) than non-stabilized magnetite aggregates due to the maximum travel distance of the nanoparticles. It is also smaller particle size and larger speciﬁc surface area. noteworthy that the transport behavior, and thus the Fourier transform infrared (FTIR) spectra suggested that suitability of the direct injection method, may be affected the main PFOA removal mechanism was inner-sphere by the physical and biogeochemical properties of the complexation. Moreover, when tested in wheat germina- porous media, such as hydraulic conductivity, mineral tion, the starch-stabilized magnetite nanoparticles were compositions, zeta potential, pH, and NOM (Lefevre et al., able to mitigate the toxic effect of PFOA on the seeding 2016; Han et al., 2017a; Cai et al., 2018b; Ji et al., 2019). growth. The results demonstrated promise of stabilized Fe O nanoparticles as a “green” adsorbent for effective 3 4 4.1 Nanoparticle aggregation and transport theory removal/immobilization of PFOA in soil and groundwater (Gong et al., 2016b). The classical DLVO theory has been widely adopted to Much more work has been done with stabilized interpret the aggregation behavior of nanoparticles (Liu et nanoparticles for in situ immobilization of metals and al., 2016). According to the theory, interactions between metalloids (Liu et al., 2015; Zhao et al., 2016). However, nanoparticles are governed by a superposition of van der much less information is available for organic chemicals. Waals attractive forces and electrostatic double layer As adsorption of organic contaminants often involve forces. Typically, the van der Waals attractive forces carbonaceous materials, stabilized carbonaceous materials between nanoparticles are approximated by assuming may be developed. For instance, Liu et al. (2016) prepared spherical nanoparticles, but the unique shapes and stabilized multi-walled carbon nanotubes (MWNTs) using compositions of nanoparticles induce the inaccuracy. CMC, starch and leonardite humic acid (LHA), and found Moreover, coating nanoparticles with an organic stabilizer the stabilization effectiveness ranked as CMC>starch> causes additional steric repulsion forces, rendering the LHA. For chemicals of both hydrophobic and lipophobic particle interactions occur only in the secondary minimum properties, such as PFOA or perﬂuorooctanesulfonic acid zone. As such, the forces, such as bridging, osmotic, steric, (PFOS), stabilized composite materials consisting of hydrophobic, Lewis acid-base, and magnetic forces, can be carbonaceous materials and metal oxides may be devel- of equivalent magnitude as van der Waals attractive forces oped to induce corporative adsorption mechanisms. For and electrostatic double layer forces. To deal with the instance, Xu et al. (2020 a,b) reported a new class of iron restrictions of the classical model and to take into account oxide/carbon sphere and carbon-bismuth phosphate com- the surface heterogeneities, several extended DLVO posite materials. The composite materials were able to (XDLVO) models have been developed to evaluate the adsorb PFOA through interactions with both the head particle-particle and particle-collector interactions under carboxylic groups and the structural –CF groups instead of more realistic conditions and/or in the presence of an the head only or tail only adsorption modes when organic coating (Phenrat et al., 2007; Hotze et al., 2010). individual metal oxides or activated carbon are used. The transport of nanoparticles in saturated porous media Zhengqing Cai et al. Remediation of soil and groundwater by stabilized nanoparticles 13 is typically interpreted by the ﬁltration theory (Zhang et al., between the nanoparticles. He and Zhao (2007) found 2017). According to the classical ﬁltration theory, that CMCs of higher MW resulted in much smaller ZVI nanoparticles are deposited on porous media following nanoparticles and improved transportability. Saleh et al. two consecutive steps: 1) transport of nanoparticles to the (2008) and Liang et al. (2012) tested transport behaviors of matrix surface by Brownian diffusion, interception, and ZVI particles modiﬁed through the post-agglomeration gravitational sedimentation, and 2) deposition of the stabilization approach using a high MW (125 kg/mol) poly nanoparticles to the matrix surface (Kretzschmar et al., (methacrylic acid)-b-(methyl methacrylate)-b-(styrene sul- 1999). He et al. (2009) reported the ﬁrst systematic study fonate) triblock copolymer, a low MW polyaspartate on the transport of CMC-stabilized ZVI nanoparticles biopolymer, and the surfactant sodium dodecyl benzene through various porous media, and reported that Brownian sulfonate (MW = 348.5 g/mol). While all the stabilizers diffusion was the predominant mechanism for the ﬁltration rendered the zeta-potential of nZVI more negative, and the of the nanoparticles, whereas gravitational sedimentation stabilizers with larger MW resulted in more negative zeta- also played an important role, which account for 30% of potential and more transportable ZVI nanoparticles the overall single-collector contact efﬁciency for coarse through a sand column. However, caution needs to be glass beads and 6.7% for a sandy soil. exercised that the higher the MW, the more viscous the It should be noted that the classical ﬁltration model does stabilizer solution, which may impede the transport of not distinguish adsorption from other ﬁltration removal nanoparticles in ﬁeld soil. So far, CMC with MW of 90,000 mechanisms, although adsorption can play important roles has been most widely used as a stabilizer for a host of in the overall removal of the nanoparticles (Han et al., nanoparticles and has been shown most effective. 2017b; Zhang et al., 2017). To overcome this drawback, He et al. (2009) investigated the breakthrough behaviors Zhang et al. (2017) developed a modiﬁed transport model of CMC-stabilized ZVI nanoparticles (size = 18.12.5 by incorporating a Langmuir-type adsorption rate law into nm) through four saturated model porous media: sandy the classic convection-dispersion equation. Using experi- soil, clean sand, coarse and ﬁne glass beads, and simulated mentally derived adsorption parameters, the model was the transport performance using both classical ﬁltration able to assess the role of adsorption in the transport of theory and a modiﬁed convection–dispersion equation CMC-stabilized ZVI nanoparticles. Based on the experi- with a ﬁrst-order removal rate law. A constant concentra- mental and modeling data, the ﬁltration removal was found tion plateau (C /C ) was observed at full breakthrough, e 0 to be primary mechanism for particle retention at low ﬂow ranging from 0.69 for the soil to 0.99 for glass beads. The velocities, whereas adsorption becomes more signiﬁcant at particle removal and maximum travel distance (L ) were max elevated ﬂow rates (Zhang et al., 2017). found strongly dependent on the interstitial ﬂow velocity, but only modestly affected by up to 40 mmol/L of calcium. 4.2 Transport of stabilized nanoparticles in porous media He et al. (2009) also proposed a correlation method to estiamte the L based on ﬂow velocity (or injection max Stabilizers can affect the particle size, surface charge and pressure). The simulation results indicate that once interactions between the nanoparticles and the collectors, delivered, 99% of the nanoparticles are expected to stay and thus affect the particle transportability in porous media in the soil matrix within 16 cm at a groundwater ﬂow (He et al., 2007). Liu et al. (2016) studied effects of CMC, velocity of 0.1 m/day, but may travel over 146 m at a ﬂow starch and LHA on the aggregation and stabilization of velocity of 61 m/d. Later, An et al. (2015) studied tranaport MWCNs in aqueous suspensions. The researchers found of CMC- or starch-stabilized Fe-Mn binary oxides that while all three stabilizers inhibited aggregation of the nanoparticles and found that their transport distance can nanoparticles, the stabilization mechanisms differed, be harnessed by manipulating the injection pressure or the namely, the coating of negatively charged CMC enhanced injection ﬂow rate. electrophoretic mobility, the neutral starch slightly curbed Johnson et al. (2013) investigated transport of CMC- electrophoretic mobility, and LHA hardly affected electro- stabilized nZVI in a ﬁeld-scale large 3D model aquifer phoretic mobility of the particles. Moreover, CMC (10 m10 m2.4 m deep), and suggested that the very- stabilizes the nanoparticles through enhanced electrostatic aggressive ﬂow conditions were necessary to achieve repulsion, primary energy barrier and steric hindrance, 2.5 m of nZVI transport using a hydraulically constrained whereas starch and LHA work primarily through steric ﬂow path between injection and extraction wells. The hindrance (Liu et al., 2016). Consequently, CMC demon- authors also indicated that the particle injection altered the strated to be the most effective stabilizer. Among various groundwater ﬂow, likely due to hydrogen bubble forma- reported commercial stabilizers, CMC exhibited 1–2 tion, which diverted the nZVI away from the targeted ﬂow orders of magnitude lower attachment efﬁciency other path. Using a spectrophotometric method, the authors commercial polymers (He et al., 2009). asserted that deployment of unoxidized nZVI for ground- Coating of CMC or other polyelectrolytes of higher MW water remediation would likely be difﬁcult. on nanoparticles induces a higher charge density and steric The ﬁeld study by He et al. (2010) showed that when barriers, resulting in enhanced electrosteric repulsion benchmarked against the bromide tracer, approximately 14 Front. Environ. Sci. Eng. 2020, 14(5): 84 37.4% and 70.0% of the injected Fe were detected in the et al., 2013). For instance, Zhang et al. (2017) investigated ﬁrst monitoring well (1.5 m from the injection well) the effects of aluminum oxide and iron oxide on the following the two injections, conﬁrming the mobility or transport of CMC-ZVI nanoparticles by column break- deliverability of CMC-Fe/Pd under the ﬁeld soil setting. through experiments, and observed that aluminum oxide Moreover, the soil deliverability was further boosted when and iron oxide coatings on quartz sand enhanced particle the injection pressure was elevated. retention, reducing the full breakthrough plateau (C/C ) Bennett et al. (2010) carried out a series of three single from 0.90 for plain sand to 0.76 when either of the metal well push-pull ﬁeld tests to investigate the transportability oxides was coated on the sand. Both experimental and of CMC-nZVI (0.2 or 1.0 g/L) or CMC-Fe/Pd (0.33 g/L) in modeling resulting conﬁrmed that the presence of both a saturated aquifer. Monitoring the Fe concentration in the metal oxides increased the adsorption capacity of the extracted groundwater indicated that the stabilized nano- nanoparticles, with the k (adsorption coefﬁcient) ads particles were transportable in the soil, but the mobility increased by a factor of 1.6‒1.8, and the k (ﬁltration ﬁl dropped with time, possibly due to the soil ﬁltration effect. coefﬁcient) increased by ~2.2 compared to the plain sand. The results also suggested that the advective nanoparticle At lower pore velocities, ﬁltration was the primary transport may be enhanced by circulating the groundwater/ mechanism for particle retention; however, at elevated nanoparticle suspension between two wells and by velocities, adsorption became more signiﬁcant. The maintaining high post-injection groundwater velocities. presence of NOM (40–80 mg-C/L) and ionic strength The deviation between the bench-scale laboratory data (up to 200 mmol/L CaCl ) had negligible effect on the and some of the larger-scale results can be attributed to breakthrough proﬁles of the nanoparticles. While a water- many factors, including: 1) heterogeneity of ﬁeld condi- soluble neutral starch was also able to stabilize the tions (e.g., hydraulic conducted, adsorption and ﬁltration nanoparticles, much larger (mean hydrodynamic diameter characteristics of the soil), 2) particle stabilization condi- = 303 nm) were obtained, leading to a higher particle tions (type and concentration of CMC and the nanoparti- retention than CMC-nZVI. Moreover, the narrower pore cles), and 3) injection pressure. While sufﬁciently high size and larger speciﬁc surface area will result in more injection pressure should be supplied to facilitate particle collisions, which are favorable for nanoparticle retention transport, too high hydraulic pressure may lead to the through the ﬁltration mechanism. He et al. (2009) “caking” effect at the wall of the injection well, leading to reporeted that media with >1.5 times greater speciﬁc clogging the entrance pores. Instead of “pushing” from the surface area provided >10% greater removal efﬁciency for injection well, a “pulling” technique may be exercised by CMC-stabilized nZVI. extracting groundwater from a monitoring well. In all In addition to the material heterogeneity, variation of cases, the bottleneck for the direct injection approach groundwater ﬂow should be taken into account. Under remains to be the insufﬁcient transportability or deliver- elevated external pressure, the injected nanoparticle ability. Therefore, more effective particle stabilization suspension may take a different path from the normal strategies are needed. In addition, it is worth noting that the groundwater ﬂow pattern, and sometimes it may be forced advective delivery (injection) of nanoparticle suspensions to be seeped out from the ground surface (e.g., in the case pushes away the existing aqueous contaminants without of shallow and unconﬁned aquifer). sufﬁcient contact or reaction, and thus, direct injection of In some real-world 3D systems, gravity may affect the nanoparticles is best directed toward the stationary/sorbed particle ﬂow pattern especially for metallic nanoparticles. contaminants or residual non-aqueous phase liquids within Kanel et al. (2008) tested the 2-D transport of PAA- source zones (Bennett et al., 2010). stabilized ZVI nanoparticles through a two-dimensional sand box under saturated, steady-state ﬂow conditions, and 4.3 Factors affecting transport of stabilized nanoparticles found that the nanoparticle plume migrated downward as it moved horizontally through the porous media, indicating Accordingtothe ﬁltration theory, soil retention of that the density gradients inﬂuenced on two-dimensional nanoparticles involves transport of nanoparticles to the transport. A variable-density groundwater ﬂow model collector’s surface and then deposition of particles to the SEAWAT was able to simulate the observed density-driven soil matrix. Physical parameters such as surface coating transport patterns. agents, ﬂow velocity, surface properties of soil and Field water matrix and chemistry may alter the physical- nanoparticles, and the accessible surface area can affect chemical properties of the nanoparticles that are observed the mass transfer of the nanoparticles, whereas the solution in the bench scale. For instance, Swindle et al. (2014) and surface chemistry will govern the kinetics of the compared the size-dependent reactivity of magnetite particle deposition (Kretzschmar et al., 1999; He et al., nanoparticles (~6 nm, ~44 nm, and ~90 nm) in a ﬁeld 2009; Zhang et al., 2017). setting to a laboratory analog. Field results indicated that The mineral surface of the porous media may interact an organic coating developed on the particle surfaces, with stabilizers and/or nanoparticles, thereby affecting which inhibited the reactivity and dissolution of the adsorption/ﬁltration and transport of nanoparticles (Liu nanoparticles, with the amount of dissolution decreasing Zhengqing Cai et al. Remediation of soil and groundwater by stabilized nanoparticles 15 as particle size decreased, which reversed the size- investigated through numerous bench- and ﬁeld-scale dependent reactivity trends observed in laboratory inves- studies. tigations. This review overviews the evolution of stabilized NOM may act as a stabilizer or bridging agent affecting nanoparticles with respect to environmental cleanup uses, particle aggregation and transport (Su, 2017). For encompassing the fundamental principles and bench- to instances, the coating of humic acid (HA) was found to ﬁeld-scale experimentations toward an innovative in situ lower the pH of magnetite nanoparticles, promoting the remediation technology using stabilized nanoparticles. The PZC mobility of nanoparticles in negatively charged soil matrix merits and limitations of the remediation are discussed. (Hu et al., 2010). Cuny et al. (2015) reported that This review also revealed some critical research needs. adsorption of HA on iron-based nanoparticles induced a In addition to the technology gaps mentioned in various more negative zeta potential, which did not alter the sections, the following future research needs are identiﬁed: particle size but positively affected the particle mobility. 1) While the particle stabilization technique can greatly However, the details about the impacts of different types improve the soil deliverability of nanoparticles, the and concentrations of HA on the aggregation and transport transport distance remains a bottle neck for effective of nanoparticles are lacking. application of the technology, especially for soil of low Solution pH and ionic strength play a critical role in the permeability. There is a need to further modify the aggregation of nanoparticles by regulating their surface stabilization technique to facilitate the deliverability and potentials, which may be used to manipulate retention or distribution of stabilized nanoparticles into the target transport of nanoparticles. For example, the pH of contaminated source zones. PZC nZVI is generally lower than that of soil matrix, thus more 2) Other surface modiﬁers than organic macromolecules nanoparticles may be retained in a soil matrix by adjusting should be sought to achieve particle stabilization, extended the pH to a level (e.g., 6.4) where the surfaces of the reactive lifetime and reaction selectivity toward the target nanoparticles and soil matrix are positively charged (Kim contaminants. In this regard, recent works showed that et al., 2012). It should be noted, however, that too low pH sulﬁdation of ZVI in combination with CMC stabilization can cause dissolution of metal-based nanoparticles, and showed both enhanced stability and dechlorination promote undesired corrosion of nZVI (Cai et al., 2018b), reactivity; and loading sulfur on ZVI enhances particle while too high pH may induce metal hydroxides hydrophobicity and thus selectivity toward hydrophobic precipitations inhibiting the particle reactivity. compounds. Yet, cautions need to be exercised that the Ionic strength, especially polyvalent cations, is expected addition of the surface modiﬁers may increase the particle to cause double-layer compression and facilitate particle size and impede the transportability when used for in situ aggregation and collector-particle interactions (Saleh et al., remediation of soil. 2008). However, CMC appears to be able to resist such 3) Information on the long-term effectiveness and effects common groundwater conditions. For instance, He reactivity of the injected nanoparticles is lacking. Such et al. (2009) and Zhang et al. (2017) observed that the long-term monitoring data, especially in the ﬁeld-scale, are 2+ presence of Ca at up to 40 mmol/L only moderately critical for assessing the technology effectiveness and affected the transport of CMC-stabilized nZVI. However, optimizing the process design. much greater effect of ionic strength should be expected 4) Although there has been no evidence showing that for non-stabilized nanoparticles. For instance, Tosco et al. stabilized nanoparticles pose signiﬁcant toxic effects on (2012) reporeted that under natural ﬂow conditions, biota under environmentally relevant conditions and synthetic ferrihydrite nanoparticles were able to transport dosages, long-term monitoring data are needed to address over 5-30 m at the normal ionic strength (2-5 mmol/L) in the environmental fate and impacts of the nanoparticles the tested European aquifers, but only traveled a few delivered in the subsurface. Further studies are needed to meters when the ionic strength was elevated to 10 mmol/L. investigate how delivered nanoparticles affect the biogeo- chemical conditions and mobility of other chemicals (especially heavy metals) in the subsurface, in particular 5 Concluding remarks and prospects under ﬁeld conditions; likewise, the effects of local environmental conditions on the fate, transport and Building upon the classical colloid physics and chemistry, transformation of the nanoparticles should be investigated. our understanding of stabilized nanoparticles has come a 5) The effect of the delivered nanoparticles on the long way in the last two decades or so, and the momentum hydraulic conductivity should be further conﬁrmed at the in their environmental remediation remains strong and ﬁeld scale and over extended period of time. diverse, especial in the ﬁeld of in situ remediation of soil 6) Mechanistically sounder transport model that couples and groundwater. 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Occurrence and fate of Zheng T, Zhan J, He J, Day C, Lu Y, Mcpherson G L, Piringer G, John V typical antibiotics in wastewater treatment plants in Harbin, North- T (2008). Reactivity characteristics of nanoscale zerovalent iron- east China. Frontiers of Environmental Science & Engineering, 13: 20 Front. Environ. Sci. Eng. 2020, 14(5): 84 silica composites for trichloroethylene remediation. Environmental contaminants in the subsurface using compound-speciﬁc chlorine Science & Technology, 42(12): 4494–4499 isotope analysis: A review of principles, current challenges and Zimmermann J, Halloran L J S, Hunkeler D (2020). Tracking chlorinated applications. Chemosphere, 244: 125476 Dr. Zhengqing Cai obtained his Ph.D. in Dr. Dongye Zhao is the Engineering Environmental Engineering from Auburn Alumni Chair Professor in the Civil Engi- University in 2016. Following postdoctoral neering Department of Auburn University. research at Fudan University, he joined the He received his Ph.D. in environmental faculty of East China University of Science engineering from Lehigh University in and Technology in 2018. His research 1998. His research focuses on development interests focus on the photochemical degra- of stabilized nanomaterials for soil/ground- dation of organic contaminants and envir- water remediation and photocatalysts for onmental nanotechnologies. destruction of persistent organic pollutants. Dr. Xiao Zhao has been an associate Dr. Zhi Dang is a Professor in the School professor in the College of Water Resources of Environment and Energy at the South & Civil Engineering at China Agricultural China University of Technology. His University since 2017. He received his Ph. research ﬁelds are focused on the release, D. from Auburn University, USA, in 2015, migration, and fate of heavy metals from and then carried out a postdoctoral study at mining areas and the remediation of soil Tsinghua University. He has published over contaminated by heavy metals. 35 peer-reviewed articles focusing on environmental nanotechnologies. Dr. Jun Duan is current a postdoctoral Dr. Zhang Lin is a professor in the School researcher in the college of environmental of Environment and Energy at the South science and engineering at Peking Univer- China University of Technology. She sity. He received his Ph.D. from Auburn obtained her Ph.D. from the Institute of University, USA, in 2019. His research Chemistry of the Chinese Academy of interests focus on the synthesis and applica- Sciences in 1999. Her current research tion of nanomaterials for remediation of interests are focused on the resources contaminated water and soil. He has recovery and energy utilization of the published 11 peer-reviewed articles. solid wastes.
Frontiers of Environmental Science & Engineering – Springer Journals
Published: Oct 1, 2020
Keywords: Stabilized nanoparticle; In-situ remediation; Organic contaminant; Soil remediation; Groundwater; Fate and transport
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