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Unravelling the Recent Developments in the Production Technology and Efficient Applications of Biochar for Agro-Ecosystems
Unravelling the Recent Developments in the Production Technology and Efficient Applications of...
Kumari, Khushbu;Kumar, Raushan;Bordoloi, Nirmali;Minkina, Tatiana;Keswani, Chetan;Bauddh, Kuldeep
agriculture Review Unravelling the Recent Developments in the Production Technology and Efﬁcient Applications of Biochar for Agro-Ecosystems 1 , † 1 , † 1 2 2 , Khushbu Kumari , Raushan Kumar , Nirmali Bordoloi , Tatiana Minkina , Chetan Keswani * 1 , and Kuldeep Bauddh * Department of Environmental Sciences, Central University of Jharkhand, Ranchi 835205, India Academy of Biology and Biotechnology, Southern Federal University, Rostov-on-Don 44090, Russia * Correspondence: email@example.com (C.K.); firstname.lastname@example.org (K.B.) † These authors contributed equally to this work. Abstract: Considerable interest is being shown in using biochar production from waste biomass with a variety of disciplines to address the most pressing environmental challenges. Biochar produced by the thermal decomposition of biomass under oxygen-limited conditions is gaining popularity as a low-cost amendment for agro-ecosystems. The efﬁciency of biochar formation is affected by temperature, heating rate, feedstock type, particle size and reactor conditions. Properties such as pH, surface area and ash content of produced biochar increases with increasing temperatures. Biochar produced at lower heating rates may have high porosity and be beneﬁcial for morphological changes in the soil. Biochar can help to enhance soil health and fertility as well as improve agricultural yield. As a result, biochar can assist in increasing food security by promoting sustainable agricultural systems and preserving an eco-friendly environment. Biochar is also widely being used as a sorbent for organic and inorganic pollutants, owing to its large surface area, allowing it to be immobilized from soil with ease. The functional groups and charges present on the surface of biochar play an important role in pollutants removal. This review focuses on the mechanisms of biochar production using different waste materials as a feed stock, factors that inﬂuence biochar quality as well as Citation: Kumari, K.; Kumar, R.; application of biochar in agricultural soil and their reclamation as well. This article also discusses Bordoloi, N.; Minkina, T.; knowledge gaps and future perspectives in the ﬁeld of biochar-based toxic-pollution remediation. Keswani, C.; Bauddh, K. Unravelling the Recent Developments in the Keywords: agro-ecosystem; biochar; food security; pollutants; reclamation Production Technology and Efﬁcient Applications of Biochar for Agro-Ecosystems. Agriculture 2023, 13, 512. https://doi.org/10.3390/ agriculture13030512 1. Introduction Waste utilization is one of the utmost important aspects of environmental management. Academic Editor: Nguyen V. Hue Converting waste to biochar is a practical solution for addressing sustainable production Received: 17 January 2023 systems and maintaining an eco-friendly environment. Biochar is a solid by-product of the Revised: 16 February 2023 thermal conversion of biomass or feedstock materials in oxygen-less environments [1,2]. Accepted: 18 February 2023 Biochar can also be deﬁned as low-cost carbonaceous (65–90%) material made by pyrolyz- Published: 21 February 2023 ing waste biomass in the absence of oxygen [3,4]. It can be produced from a variety of feedstock materials, including agricultural waste, animal manure, paper products and so on. The biochar production rate is primarily dependent on heating values and temperatures (~350 1000 C) [5,6]. It has been well identiﬁed that feedstocks and thermochemical Copyright: © 2023 by the authors. decomposition methods and their temperature and durations can all affect the physicochem- Licensee MDPI, Basel, Switzerland. ical properties of biochar . As a result, the properties of biochar derived from various This article is an open access article feedstock materials differ as it relates to high surface area, maximum cation exchange distributed under the terms and capacity, high porosity, stability and functional groups, which makes biochar suitable for a conditions of the Creative Commons Attribution (CC BY) license (https:// variety of applications [8,9]. Thus, fast and easy preparation, eco-friendliness, reusability creativecommons.org/licenses/by/ and cost-effectiveness are some advantages of biochar [10,11]. 4.0/). Agriculture 2023, 13, 512. https://doi.org/10.3390/agriculture13030512 https://www.mdpi.com/journal/agriculture Agriculture 2023, 13, 512 2 of 26 Recently, biochar application combined with previously adapted practices, such as organic farming and chemical fertilizer-based agriculture, is being promoted for managing agricultural sustainability [12–15]. An investigation of biochar ’s function in composting and the use of biochar–compost mixtures in agricultural systems were identiﬁed to be the newest study areas . According to studies, using biochar in conjunction with other farm management techniques (including tillage, nutrient, residue and irrigation management) may be a better plan of action in the event of climate change, especially in places with restricted water supplies [12,17]. Several studies are available to prove the potential of biochar in achieving environmental and agronomical beneﬁts. These may help in climate change mitigation, enhanced crop productivity, increased soil microbial biomass, etc. The properties of biochar play an important role in determining soil organic matter, moisture availability, fertilizer use efﬁciency, nutrient uptake and leaching, etc. Improving soil mois- ture and nutrient availability are crucial components of biochar to increase plant nutrition and decrease nutrient leaching, particularly of nitrogen (N), phosphorus (P) and potassium (K). Biochar can improve crop yields and soil quality when combined with other organic additives such as manures . Refs. [19,20] observed that mixing biochar with mineral fertilizer has a considerable impact on nutrient use efﬁciencies and maize productivity. Ref.  studied the effects of biochar on enhancing nitrogen uptake efﬁciency amended in Chinese paddy ﬁelds, which increases the yields of rice. Similarly, ref.  observed that peanut hull biochar increased the concentration of nutrients (N, P, K, magnesium (mg) and calcium (Ca)) and pH. Several studies have been conducted to investigate the feasibility of using biochar made from plant residues, animal manure, pharmaceuticals and biosolids to adsorb pesticides and toxic metals [23–27]. They have also mentioned that the ability of biochar to adsorb pollutants is dependent on the feedstock materials and their physiochem- ical properties. Biochar is a promising renewable resource for the economy with several environmental beneﬁts [16,28,29]. As a result, biochar has been produced using variety of feedstocks and other materials and three distinct technologies: pre-treatment, thermal technologies and post-treatment technologies [30–32], as mentioned in Figure 1. Ref.  mentioned that different methods such as pyrolysis, hydrothermal carbonization and gasi- ﬁcation can be used to produce biochar. They also have stated that pyrolysis methods are appropriate for producing biochar from feedstock materials. However, research on various components of biochar has primarily taken place in industrialized nations including the United States, Australia, South America and Europe, as well as a few emerging nations such as China . Therefore, there is a lot of room to investigate how biochar might be used to manage agriculture in a number of developing and resource-poor nations such as rainfed areas with low water availability, such as sub-Saharan Africa and South and Southeast Asia . Furthermore, there are relatively few studies on the use of biochar in dry, tropical Indian agro-ecosystems with cropping systems based on previously modiﬁed organic and chemical fertilizer applications. These agro-ecosystems have a signiﬁcant capacity to store carbon; hence, investigating the function of biochar is crucial in the context of climate change scenarios . The existing state of knowledge is mostly based on small-scale laboratory and green- house conditions. The properties of biochar may vary depending on the feedstock used and the method of production. The availability of feedstock, economic beneﬁts, energy requirements and environmental problems (if any) associated with its large-scale produc- tion and use are still being explored. Preliminary evidence suggests that biochar can play an important role in addressing the problems caused by climate change and threats to agro-ecosystem sustainability. This review discusses the principles and concepts of producing biochar and the factors that inﬂuence biochar ’s quality and its applications. The aim of this study is to summarize biochar ’s properties produced from various feedstocks materials and also their applications in agro-ecosystems. This review is aimed at assisting researchers throughout the world in the selection of appropriate biochar produced at different conditions to promote agriculture and environmental sustainability without reducing crop output. Agriculture 2023, 13, x FOR PEER REVIEW 3 of 29 summarize biochar’s properties produced from various feedstocks materials and also their applications in agro-ecosystems. This review is aimed at assisting researchers throughout the world in the selection of appropriate biochar produced at different condi- Agriculture 2023, 13, 512 3 of 26 tions to promote agriculture and environmental sustainability without reducing crop out- put. Figure 1. Representative workﬂow of the technologies/methodologies used for the production of biochar. Figure 1. Representative workflow of the technologies/methodologies used for the production of biochar. 2. Methodology 2. Methodology An exhaustive literature search was conducted using Scopus with the search key- An exhaustive literature search was conducted using Scopus with the search key- words for this review being “biochar and agro-ecosystem, agricultural productivity, crop words for this review being “biochar and agro-ecosystem, agricultural productivity, crop production, crop yield, economics of biochar, biochar production, etc.” Science Direct and production, crop yield, economics of biochar, biochar production, etc.” Science Direct and Google Scholar searches were also carried out and studies were deemed to be eligible for Google Scholar searches were also carried out and studies were deemed to be eligible inclusion in the review study. The literature cited in this review was published from 2003 to for 2022,inclusio and the papers m n in the ajorly review targeted we study re from . The journal literatur articles, be ook cited chapte in rs, b this ooks, review was published from conference papers and technical reports published between 2003 and 2022. Relevant data 2003 to 2022, and the papers majorly targeted were from journal articles, book chapters, from each study were extracted on the biochar production process, feedstock used, books, conference papers and technical reports published between 2003 and 2022. Relevant data from each study were extracted on the biochar production process, feedstock used, biochar preparation methodology, factors impacting the process and yield of biochar and biochar physiochemical characteristics. Both pot and ﬁeld experiments were documented in the review to increase the number of investigations. Further, the journals of interest were Science of the Total Environment, Journal of Environmental Quality, Journal of Environ- mental Management, Journal of Agriculture, Ecosystem and Environment, Environmental Pollution, Agronomy, Biochar, Plant Physiology and Biochemistry, Bioresource Technology, Chemosphere, etc., which were searched up to 2022 in order to delve deeper into the relevant literature. We selected and cited 209 articles from the total number of studies found, out of which the total number of articles (including research papers and review papers) was 198 followed by 5 book chapters, 4 books and 2 conference papers. Around 70% of the documents cited were from the years 2016 to 2022. 3. Biochar Production Process The making of biochar is simple and suitable for many regions around the world; however, large-scale development requires optimization and economic evaluation. Further- more, the process of producing biochar ranges from traditional kilns and earth mounds to engineered systems that depend on ﬂat beds or ﬂuidized reactors for pyrolysis, gasiﬁcation or other methodologies . Agriculture 2023, 13, 512 4 of 26 3.1. Feedstocks There are many potential feedstock materials used in the production of biochar, in- cluding wood and agricultural wastes, leaves, rice husks and straw, paper sludge, food waste, manure, bagasse, etc. [37–41] Feedstocks for biochar production are plentiful and inexpensive, primarily derived from agricultural biomass and solid waste [42,43]. Further- more, agricultural waste feedstock materials (rice, wheat and maize straws), forest residues, etc. are used to make biochar, as mentioned in Figure 2. Biochar produced from invasive terrestrial plant species and different aquatic weeds can aid in invasion management while also protecting the environment . Similarly, crop waste and wood are both energy-rich feedstocks that lend themselves well to pyrolysis [45,46]. Some wastes such as manure or sewage sludge may be appropriate for biochar production, if they can be produced at very high temperatures . Various types of animal waste (chicken or poultry manure, sheep manure, duck manure) have proven to be valuable soil amendments after being pyrolyzed, because of their higher nutrients content . In addition, the application of biochar improved waste management and provided valuable nutrients (such as N, P, K) to the soil as well as metal sorption on contaminated land . Knowing how initial feedstock characteristics inﬂuence biochar properties is critical in terms of feedstock. Feed- stocks have been found to have a signiﬁcant impact in the production of biochars with distinctly varied chemical characteristics . Wood-based biochars contain more C and less plant-available nutrients, whereas manure-based biochars exhibit the opposite trend, and grass-based biochars often fall midway between woody and manure biochars . The total C content in biochars is frequently increased since most feedstocks include signiﬁcant C concentrations; however, feedstock selection has a considerable impact on biochar C content. Wood-based biochars included more C than biochars manufactured from other feedstocks, owing to a lack of other elements (e.g., N, S, P, K, Ca, and P), resulting in a lesser Agriculture 2023, 13, x FOR PEER REVIEW 5 of 29 C-dilution effect in wood-based biochars . Figure 2. Feedstocks utilization and their applications. Figure 2. Feedstocks utilization and their applications. 3.2. Methodology In relation to temperature, residence time and heating rate, there are several methods for producing biochar [51,52]. There are some thermochemical processes such as pyroly- sis, gasification, hydrothermal carbonization and torrefaction that are currently being used to produce biochar and other bio-based products, as shown in Table 1. Table 1. Methodologies for producing biochar. Temperature (°C) Methods Residence Time References 300–700 Slow pyrolysis hour-days [24,51,53–56] 500–1000 Fast pyrolysis <2 s Agriculture 2023, 13, 512 5 of 26 3.2. Methodology In relation to temperature, residence time and heating rate, there are several methods for producing biochar [51,52]. There are some thermochemical processes such as pyrolysis, gasiﬁcation, hydrothermal carbonization and torrefaction that are currently being used to produce biochar and other bio-based products, as shown in Table 1. Table 1. Methodologies for producing biochar. Temperature ( C) Methods Residence Time References 300–700 Slow pyrolysis hour-days 500–1000 Fast pyrolysis <2 s ~750–900 Gasiﬁcation 10–20 s [24,51,53–56] 180–300 Hydrothermal carbonization 1–16 h ~290 Torrefaction ~10–60 min These methods affect the chemical composition and physical state of the feedstocks used. Furthermore, during the valorization process, feedstocks (cellulose, hemicellulose, lignin, and pectin) are depolymerized and fragmented, from which small amounts of condensable bio-oil and non-condensable gases are produced [57,58]. 3.2.1. Pyrolysis Pyrolysis is a thermal decomposition of biomass material in the absence of oxygen at a speciﬁc temperature, pressure and resident time that is required for complete com- bustion [59,60]. However, the ﬁnal products may be produced in signiﬁcant quantities. Pyrolysis can be categorized into slow and fast pyrolysis based on temperature, pressure and residence time [61,62]. Fast pyrolysis is a direct thermochemical process that can emulsify feedstocks or biomass into biochar with liquid bio-oil, which has a greater energy potential [62,63]. Fast pyrolysis occurs under three conditions: ﬁrst, when biomass is pyrolyzed at temperature >100 C/min, the particles of the obtained biochar are distinct in size; second, particles and pyrolysis fumes are released in 0.5–2 s with less time at high temperatures; and third, temperatures in a moderate, fast pyrolysis treatment completed at 400–600 C . In view of this, fast pyrolysis advancement is obligatory to keep the fume residence time in the reaction chamber to a minimum in order to achieve high bio-oil quality . Similarly, slow pyrolysis is carried out at higher temperatures (350–550 C) in the absence of O . As a result, the biomass containing cellulose, hemicellulose and lignin produces 30% more biochar than fast pyrolysis (12%) or gasiﬁcation (10%) . The mechanism is observed by signiﬁcantly reducing the degree of polymerization, which consists of two reaction processes: (a) Slow pyrolysis, which includes cellulose decom- position at a higher residence time and temperature rate and (b) fast pyrolysis, which is done at a higher heating rate by rapid volatilization with formed levoglucosan . Moreover, the hydroxymethyl furfural produced by the dehydration of levoglucosan can decompose to produce both liquid and gaseous products such as syngas and bio-oil . Ref.  conducted an experiment to produce biochar from lignocellulosic feedstocks using a slow pyrolysis method and compared its sustainability impact to that of direct biomass combustion. Similarly, ref.  found that biochar obtained through pyrolysis produces higher quality bio-oil than alternative treatments. Therefore, the ﬁndings observed that the effect of pyrolysis is only dependent on the energy supplied during pre-treatment processes. Additionally, ref.  observed that the mechanism of hemicellulose decomposi- tion is correlated with cellulose, because oligosaccharides are formed after hemicellulose depolymerization. This can happen when reaction occurs between intramolecular rear- rangement, decarboxylation, depolymerization and aromatization. In accordance with this, the building blocks of lignin are linked with the -O-4 bond that breaks in the lignin de- composition mechanism and produces free radicals. Such free radicals absorb protons from other species, resulting in the decomposition of organic compounds and move towards the other molecules, showing chain propagation in the produced biochar. The advantage Agriculture 2023, 13, 512 6 of 26 of pyrolysis is that it is a zero-waste process and the disadvantage is that it is not being suitable for biomass with a high moisture content. Several more products can be produced Agriculture 2023, 13, x FOR PEER REVIEW 7 of 29 through the pyrolysis method , as mentioned in Figure 3. Figure 3. Products produced through the pyrolysis method. Figure 3. Products produced through the pyrolysis method. 3.2.2. Hydrothermal Carbonization 3.2.2. Hydrothermal Carbonization Hydrothermal carbonization is one of the cost-effective methods to produce hydro- char. In this, hydrochar can be produced at low temperature (180–250 °C) [71,72]. In the Hydrothermal carbonization is one of the cost-effective methods to produce hydrochar. hydrothermal process, the hydrochar is produced by dried feedstocks, which is different In this, hydrochar can be produced at low temperature (180–250 C) [71,72]. In the hy- from pyrolysis . The process involved in the hydrothermal method is as follows: (a) Feedstocks can be mixed with water and placed in a closed reactor to gradually provide drothermal process, the hydrochar is produced by dried feedstocks, which is different temperature stability and (b) their various temperatures result in a variety of products from pyrolysis . The process involved in the hydrothermal method is as follows: such as biochar produced at a temperature below 250 °C, bio-oil in between 250–400 °C, which is known as hydrothermal liquefaction, and gaseous substances such as syngas (a) Feedstocks can be mixed with water and placed in a closed reactor to gradually provide (CO, CO2, H2 and CH4) at temperatures above 400 °C, which is known as hydrothermal temperature stability and (b) their various temperatures result in a variety of products gasification . During the hydrothermal carbonization process, the intermediate prod- such as biochar produced at a temperature below 250 C, bio-oil in between 250–400 C, ucts 5-hydroxymethylfurfural and its derivatives are formed as a result of the reactions involved such as fragmentation, dehydration and isomerization . Additionally, the which is known as hydrothermal liquefaction, and gaseous substances such as syngas hydrochar is produced through the process of polymerization, condensation and intra- (CO, CO , H and CH ) at temperatures above 400 C, which is known as hydrother- 2 2 4 molecular dehydration . The feedstock with a high lignin content creates more com- mal gasiﬁcation . During the hydrothermal carbonization process, the intermediate plicated mechanisms due to its high molecular weight and complex nature. The decom- position of lignin commences in the reaction between dealkylation and hydrolysis, which products 5-hydroxymethylfurfural and its derivatives are formed as a result of the reac- produces phenolic product such as catechols, phenols, syringols, etc. [76,77] Therefore, tions involved such as fragmentation, dehydration and isomerization . Additionally, the produced hydrochar is an intermediate of repolymerization and crosslinking [62,78]. However, the lignin content in biomass is not dissolved in the liquid phase and is con- the hydrochar is produced through the process of polymerization, condensation and in- verted into hydrochar. The advantages of hydrothermal carbonization include effective tramolecular dehydration . The feedstock with a high lignin content creates more utilization of biomass as a clean and convenient solid fuels, and a disadvantage is that it complicated mechanisms due to its high molecular weight and complex nature. The decom- is difficult to collect the products and higher requirements of equipment. position of lignin commences in the reaction between dealkylation and hydrolysis, which 3.2.3. Gasification produces phenolic product such as catechols, phenols, syringols, etc. [76,77] Therefore, Gasification is the breakdown of carbonaceous materials into gaseous products such the produced hydrochar is an intermediate of repolymerization and crosslinking [62,78]. as syngas, including CO, CO2, H2, CH4 and some hydrocarbons in the reactor that contain oxygen and steam at high temperatures . Syngas production is solely reliant on the However, the lignin content in biomass is not dissolved in the liquid phase and is converted feedstock reaction stability at high temperatures . However, as the temperature rises, into hydrochar. The advantages of hydrothermal carbonization include effective utilization CO and H2 levels increase while CO2 and CH4 levels decrease. The main product of the of biomass as a clean and convenient solid fuels, and a disadvantage is that it is difﬁcult to gasification process is syngas and char is the by-product with the lowest yield . The processes involved in gasification are as follows: (a) Feedstock or biomass drying or com- collect the products and higher requirements of equipment. plete moisture destruction, evaporation without energy recovery and the moisture con- tent of biomass influences its drying process and (b) gasification involves a combustion or 3.2.3. Gasiﬁcation Gasiﬁcation is the breakdown of carbonaceous materials into gaseous products such as syngas, including CO, CO , H , CH and some hydrocarbons in the reactor that contain 2 2 4 oxygen and steam at high temperatures . Syngas production is solely reliant on the feedstock reaction stability at high temperatures . However, as the temperature rises, CO and H levels increase while CO and CH levels decrease. The main product of 2 2 4 the gasiﬁcation process is syngas and char is the by-product with the lowest yield . The processes involved in gasiﬁcation are as follows: (a) Feedstock or biomass drying or complete moisture destruction, evaporation without energy recovery and the moisture content of biomass inﬂuences its drying process and (b) gasiﬁcation involves a combustion or oxidation reaction with several feedstock materials that have reactive and combustible properties in a gasiﬁer to produce CO, CO and water . The advantage of gasiﬁcation 2 Agriculture 2023, 13, 512 7 of 26 is that it has high efﬁciency energy recovery; the disadvantage lies with its complex technology and high investment and operating costs. 3.2.4. Torrefaction Torrefaction is a new and advanced method for making biochar [83,84]. The biomass Agriculture 2023, 13, x FOR PEER REVIEW 8 of 29 is gradually heated to a high temperature of 300 C in an oxygen-deﬁcient environment and it produces a solid, uniform biochar with less moisture and greater energy content oxidation reaction with several feedstock materials that have reactive and combustible than raw biomass . During the torrefaction process, the moisture content and some properties in a gasifier to produce CO, CO2 and water . The advantage of gasification volatile organic compounds are evaporated from the biomass in the reaction chamber . is that it has high efficiency energy recovery; the disadvantage lies with its complex tech- nology and high investment and operating costs. The processes involved in torrefaction are described in Figure 4. There are three main techniques, which are as follows: (a) The biomass is dried in steam at temperatures no 3.2.4. Torrefaction higher than 260 C and a residence time of 10 min, which is referred to as steam torrefaction, Torrefaction is a new and advanced method for making biochar [83,84]. The biomass is gradually heated to a high temperature of 300 °C in an oxygen-deficient environment (b) wet torrefaction, also known as hydrothermal carbonization, occurs when biomass is and it produces a solid, uniform biochar with less moisture and greater energy content mixed with water at a temperature of 180–260 C and a residence time of 40 min and than raw biomass . During the torrefaction process, the moisture content and some (c) in oxidative volatile o torr rganefaction, ic compoundin s are which evaporabiomass ted from the b that iomas can s in the r beetr aceated tion cham with ber . oxidizing agents The processes involved in torrefaction are described in Figure 4. There are three main such as gas is used in the combustion process to generate heat energy. Torrefaction is an techniques, which are as follows: (a) The biomass is dried in steam at temperatures no incomplete pyrolysis process that requires a temperature of 200–300 C, a residence time higher than 260 °C and a residence time of 10 min, which is referred to as steam torrefac- tion, (b) wet torrefaction, also known as hydrothermal carbonization, occurs when bio- of more than 30 min, a heating rate of 50 C/min and the absence of oxygen . Several mass is mixed with water at a temperature of 180–260 °C and a residence time of 40 min researchers explained that the torrefaction process is divided into four stages: heating, and (c) in oxidative torrefaction, in which biomass that can be treated with oxidizing drying (including pre-drying and post-drying), torrefaction and cooling [53,88]. In heating, agents such as gas is used in the combustion process to generate heat energy. Torrefaction is an incomplete pyrolysis process that requires a temperature of 200–300 °C, a residence the biomass is heated until it is completely dry (like moisture and biomass evaporation) time of more than 30 min, a heating rate of 50 °C/min and the absence of oxygen . at a given temperature. In drying, the biomass is dried (complete drying of moisture and Several researchers explained that the torrefaction process is divided into four stages: heating, drying (including pre-drying and post-drying), torrefaction and cooling [53,88]. biomass evaporation) at a temperature of 100 C and 200 C, referred to as pre-drying and In heating, the biomass is heated until it is completely dry (like moisture and biomass post-drying, respectively. As a result, by using the appropriate temperature, the volume of evaporation) at a given temperature. In drying, the biomass is dried (complete drying of biomass is reduced. Torrefaction is the ﬁnal process of producing biochar at a temperature moisture and biomass evaporation) at a temperature of 100 °C and 200 °C, referred to as pre-drying and post-drying, respectively. As a result, by using the appropriate tempera- of 200 C and maintaining stability at that temperature throughout the manufacturing ture, the volume of biomass is reduced. Torrefaction is the final process of producing bi- process. In cooling, the produced biochar is allowed to cool at room temperature. The ochar at a temperature of 200 °C and maintaining stability at that temperature throughout advantages the of matorr nufact efaction uring proces include s. In coolinthe g, thepr production oduced biochof ar is all syngas owed to and cool a biofuels t room and biochar temperature. The advantages of torrefaction include the production of syngas and biofu- production, and the disadvantage is that it requires extensive gas cleaning. els and biochar production, and the disadvantage is that it requires extensive gas cleaning. Figure 4. Physiochemical properties of biochar. Figure 4. Physiochemical properties of biochar. 3.2.5. Flash Carbonization 3.2.5. Flash Carbonization Flash carbonization is a process whereby biomass can burn at a high pressure (1 to 2 MPa) and temperature (300–600 °C) for a residence time of 30 min; as a result, about 40% Flash carbonization is a process whereby biomass can burn at a high pressure (1 to 2 MPa) and temperature (300–600 C) for a residence time of 30 min; as a result, about 40% solid carbon materials are released . The methodology of ﬂash carbonization seems to be very limited in the literature and is not widely used. 4. Factors Affecting the Process and Yield of Biochar Feedstocks such as organic or natural materials are often used to produce biochar, but there are several factors that affect the process and yield of biochar, such as temperature, preparation methods and particle size [62,89]. Agriculture 2023, 13, 512 8 of 26 4.1. Temperature Temperature is the only dependent factor inﬂuencing the production of biochar and its properties. Properties such as pH, surface area and the ash content of the produced biochar increase with increasing temperature but the yield of biochar decreases [90,91]. Furthermore, biochar produced at low pyrolysis temperatures has a cellulose crystal struc- ture, which limits the material’s diverse organic character . In addition, when organic feedstocks are used to make biochar at higher temperatures (400–700 C), a large amount of carbon in a poly-condensed aromatic structure is released, as is an ion exchange functional group due to decarboxylation and dehydration . The surface area and adsorption isotherm of biochar produced at high temperatures (300–600 C) are directly affected . Maximum biochar yield and C-O and C-H bond can be affected by low temperatures (250–400 C) . Ref.  made biochar using thermogravimetric, thermochemical and infrared spectral methods at both low and high temperatures. They observed that CO and water were released at low temperatures, whereas CO, H and CH were released at high 2 4 temperatures. As a result, at high temperatures, biochar contains more volatile matter and releases aldehydes and other oxygen-containing aliphatic compounds, while at low tem- peratures, carbohydrate decomposition and aromatic compounds such as toluene, phenol, benzene and others are released [95,96]. Although it was observed that changes in pyroly- sis temperatures affect the sorbent capacity of organic compounds such as nitrobenzene, dinitrobenzene, benzene, naphthalene and catechol , Ref.  found that producing 2 1 biochar at low (450 C) and high (600 C) temperatures results in low (0.7–13.6 m g ) and 2 1 high (243.7–401.0 m g ) surface areas, respectively. Various biochar studies have found a signiﬁcant relationship between biochar surface area and pyrolysis temperature [24,98]. 4.2. Preparation Methods Heating rates The pyrolysis temperature (low or high) and heating rates inﬂuence the preparation of biochar. Heating rates may also have an impact on the yields of bio-oil, syngas and the surface structures of produced biochar . However, high heating rates and optimal feed- stock temperatures improve bio-oil quality, whereas low heating rates and low feedstock temperatures favor biochar production . Therefore, various feedstocks materials can be pyrolyzed or valorized at different temperatures with different heating rates. Biochar made from pine sawdust, at lower heating rates (20 Cs ), may have high porosity and be beneﬁcial for soil amendment [92,101]. However, when biochar is produced at a high heat- ing rate (500 Cs ), the cell structure of the produced pine sawdust biochar is demolished via devolatilization. Feedstocks The type of feedstock has a signiﬁcant impact on the properties and chemical com- positions of biochar . Hassan et al.  also described oxygen-rich but low-hydrogen feedstock materials, including sugar, which are non-graphitizing and form a strong net- work of cross-links that immobilize the structure and unite the crystalline phase into a solid mass. During pyrolysis, sugars turn into liquids and the resultant biochar does not have its original physical structure nor does it have high porosity . Furthermore, feed- stocks with high carbon and nitrogen concentrations are used to produce biochar through pyrolysis . 4.3. Particle Size Biochar particle size is an important property that is primarily inﬂuenced by the type of feedstocks used, as well as temperature and heating rates [91,92]. These factors also have an impact on the rates of heat conduction in reaction mechanisms within the particles. Due to uniform heat transfer, fast pyrolysis produces smaller particle sizes of biochar and yields the greatest amount of bio-oil and gas-product yields, whereas, large particles (1.8 mm) have high temperature gradients, resulting in lower core temperatures and low particle Agriculture 2023, 13, 512 9 of 26 surfaces [102,103]. As a result, the obtained biochar has the highest yields while bio-oil and produced gas yields decrease. Ref.  investigated biochar produced from paulownia wood particles of different sizes and they found that the maximum biochar yield obtained was 28% with particle size ranges from 0.224 to 0.425 mm, at a temperature of 500 C with a heating rate of 223.15 C min under an oxygen-less atmosphere. 5. Properties of Biochar The properties of biochar are an important indicator to determine its application for various purposes. Biochar ’s physiochemical characteristics such as density, pore volume, pore size, surface area, water holding capacity (WHC), pH, elemental composition, en- ergy content, ﬁxed carbon, volatile matter and cation exchange capacity are of particular interest because they are intimately linked to its functionality and applications. Different physiochemical properties of biochar are summarized in Figure 4. 5.1. Physical Properties Biochar physical properties are likely to be important in understanding how biomass feedstocks and pyrolysis conditions relate to biochar ’s environmental impacts. This section focuses on the physical properties of biochar. Table 2 shows biochar generated from various feedstocks and their physiochemical characteristics as established by various researchers. 5.1.1. Surface Area Biochar has a large surface area that provides high performance sorption capacity for soil nutrients and organic and inorganic contaminants . Biochar produced at higher temperatures is more likely to be chemically and biologically resistant, which is an essential quality to consider when presenting it as a potential option in favor of carbon sequestration in the environment or soil . Moreover, biochar loses its surface usefulness when pyrolyzed at high temperatures due to increased aromatic condensation processes . As a result, biochar produced at lower and moderate temperatures (400–600 C) can have a broad range of surface functions . Due to the very heterogeneous nature of biochar, which is dependent on both unstable and stable C compositions, the surface chemistry of biochar varies signiﬁcantly . The surface chemistry of biochar, which typically exhibits a variety of hydrophobic and hydrophilic functions in both basic and acidic environments, has a large impact on surface behaviors . The heterogeneous character of biochar surfaces is attributed to the intricacy of surface chemistry, which impacts its interaction with a wide spectrum of inorganic and organic molecules found in the environment . Fur- thermore, soil nutrients such as N, P, K and sulphur are incorporated into the stable C part of biochar materials, resulting in surface heterogeneity due to changes in electronegativity between aromatic carbon and heteroelements . 5.1.2. Density and Porosity 3 3 Biochar has a lower bulk density (0.6 g cm ) than soil (1.25 g cm ) . Due to the mixing or diluting impact, biochar application is likely to diminish the density of the bulk soil. When the density difference between the materials is considerable, the intensity of this effect can be extremely large. In the long run, biochar may reduce bulk density by reacting with soil particles and improving aggregation and porosity . The latter necessitates the continuous monitoring of changes in bulk density over long periods of time. Most biochar research is short-term (less than four years), which may not completely reﬂect the long-term impacts of biochar. Biochar reduces bulk density in general; however, the level of these changes varies subject to the soil condition and biochar application rate. Increasing pyrolysis temperatures lead to higher porosities of the ﬁnal product. Ref.  found that grass chars have much larger porosities than woody biochars, with porosities exceeding 80% for treatment temperatures of 350 to 700 C and lower temperature dependence over this range than wood. Agriculture 2023, 13, 512 10 of 26 5.1.3. Pore Volume and Pore Size Distribution Simply knowing the biochar surface area is not enough for its applicability. Some gases, for example, may be difﬁcult to reach through a huge surface area made up of numerous extremely small pores, limiting the biochar ’s ability to absorb the relevant gas despite the large surface area. Similarly, plenty of nanometer-sized pores have no relevance on plant-available water because plants are unable to defeat the capillary pressures that keep water in these small pores . Biochar pores are divided into macropores (pore diameters of 1000–0.05 m), mesopores (0.05–0.002 m) and micropores (0.002–0.0001 m) . Micropores make up a substantial part of the pore constitution of biochars, accounting for more than 80% of the total pore volume [115,116]. In untreated agricultural residues (straw and stalk), the number of micropores was determined to be less than 10% of the overall pore volume . The pore properties of biochar determine how it adsorbs pollutants and interacts with soil to change the soil’s physical processes . Many biochar-induced ecological roles are also determined by the pore structure of the biochar. Biochar ’s pore spaces create suitable environments that are ideal for microbial and fungus communities [119,120]. 5.1.4. Hydrophobicity and Water Holding Capacity The hydrologic characteristics of biochars are affected by two main processes that occur during pyrolysis: the decrease in functional groups reduces the material’s afﬁnity for water, and the rise in porosity alters the amount of water that may be adsorbed. WHC is de- termined by the porosity of the biochar ’s bulk volume, while hydrophobicity is determined by surface functional groups. Because these qualities can have opposing or overlapping impacts, they are not always easy to tell apart. The term hydrophilic refers to a surface that attracts water, while hydrophobic refers to one that repels it. Escalating the pyrolysis temperature is expected to increase the hydrophobic character of the biochar by eliminating more polar surface functional groups and raising the aromaticity of the biochar . This is reﬂected in the decline in the O/C-ratio . Porosity and interconnectivity of the pores are important factors in a material’s WHC or its capability to contain and retain water . As a result, biochars made at high temperatures should store more water in their porous structures . Despite the fact that biochar made at low temperatures has a porous structure, it may be difﬁcult to access due to smaller pore sizes, less interconnectivity and the presence of tar components plugging the pores . 5.2. Chemical Properties 5.2.1. Elemental Composition One of the fundamental principles of biochar synthesis is to change the chemical composition of raw biomass, speciﬁcally to increase its carbon content. Biochar is mainly composed of carbon content with trace amounts of N, P, K, Ca, Na, Mg, Al and Cu . Due to high-temperature pyrolysis, most of the H, O, N and S will be lost during the synthesis process, leaving only aromatic C in the biochar . Biochar ’s elemental content, on the other hand, is affected by the source material, pyrolysis substrates, time and method . Biochar formed from biosolids is often richer in N, P, K, Ca, Na, Mg, Al and Cu than biochar derived from plant material. Ref.  discovered that four leguminous straw biochars had more nutrients (Ca, Mg and K) than ﬁve non-leguminous plant biochars. The elemental content of biochar is affected by the pyrolysis temperature, with higher pyrolysis temperatures resulting in lower total N in the biochar . 5.2.2. Energy Content The energy content of biochar increases with increasing temperature and is deter- mined by the carbon concentration in biomass. Heat treatment at 700 C increases the 1 1 energy content of raw biomass from 15 to 20 MJkg to 30–35 MJkg for biochar . This amount of energy is comparable to raw anthracite . At temperatures between 250 and 350 C, the most substantial rise in energy content occurs. The heating value is Agriculture 2023, 13, 512 11 of 26 1 1 raised from less than 20 MJkg to values of 25–30 MJkg within this 100 C range. The change in energy content after 400 C is negligible. Lengthening the residence duration has a favorable inﬂuence on the heating value, causing it to rise even more. However, as compared to the temperature, the effect of residence duration is minor. In the torrefaction range, increasing the residence period from one to two (or even three) hours adds only a few MJkg to the heating value. Maintaining the initial residence time while increasing the temperature by 20–50 K can have the same impact . 5.2.3. Fixed Carbon and Volatile Matter Fixed carbon is the carbon content that remains in the solid structure after the volatile components have been removed. It is determined by 100% reduced moisture content, ash content and volatile matter. In order to replace fossil carbon carriers, some biochar applications, particularly metallurgical ones, require very high ﬁxed carbon levels of more than 90 or even 95% . The carbon content of raw biomass is ﬁxed at 10–30% and does not change signiﬁcantly before entering the torrefaction range . The quantity of ﬁxed carbon increases to roughly 50–60% between 250 and 350 C. Regardless of the fact that this limited temperature range has the highest effect on ﬁxed carbon content, levels greater than 90% require temperatures of 700 C. It should also be emphasized that these are ash-free ﬁgures and that the ﬁnal product used contains a large quantity of ash. The devolatilization process increases the ﬁxed carbon content while decreasing the volatile matter. 5.2.4. pH-Value An increase in biochar pH can have the direct effect of an increase in alkalinity. The pH value of biochar is crucial for agricultural uses such as soil amendment. With pH ranging from 5 to 7.5, raw biomass is often slightly acidic or moderately basic . The most prevalent functional groups that are removed during pyrolysis are carboxyl, hydroxyl and formyl groups. Furthermore, during the process, the relative content of the ash, which is likewise basic in nature, is raised. An increasing degree of carbonization causes an increase in pH . For temperatures exceeding 500 C, the maximum pH values feasible are in the 10 to 12 range. Table 2. Biochar produced from various feedstocks and their physiochemical characteristics. Physical properties Chemical Properties References Temp. SA D PV Ash Feedstock Method pH C/N O/C H/C 2 3 1 ( C) (m /g) (nm) (cm g ) (%) Hydrothermal Sewage sludge 300 57.66 NA NA 6.5 8 0.19 0.12 NA  liquefaction Hydrothermal Bamboo 200 9.32 14.75 0.136 5.18 133 0.67 0.11 0.95  carbonization Hydrothermal Bamboo 240 7.63 11.36 0.067 5.31 98.5 0.34 0.08 1.08  carbonization Hydrothermal 280 5.18 11.3 0.021 5.32 92.1 0.26 0.07 0.71 Bamboo  carbonization Pepper stalk Pyrolysis 600 71.3 3.2 0.006 10.8 32.9 0.09 0.32 10.6  Pine sawdust Pyrolysis 650 130 NA 0.0138 9.6 260 NA 0.05 NA  Biogas Residue Pyrolysis 400 4 10.1 0.009 10.6 23.5 0.23 0.68 28.7  Biogas Residue Pyrolysis 600 3.3 18.1 0.013 11.9 24.9 0.23 0.32 31.6  Biogas Residue Pyrolysis 800 7.1 11 0.016 11.6 28.5 0.24 0.24 31.1  Rice husk Pyrolysis 500–600 NA NA NA 9.5 77.4 0.26 0.61 27.6  NA: Not Available; SA: Surface Area; D: Density; PV: Pore Volume. 5.2.5. Cation Exchange Capacity The cation exchange capacity (CEC) of a material is the quantity of exchangeable 2+ 2+ + + 4+ cations it can hold (e.g., Ca , Mg , K , Na , NH ) . It is used to characterize the fertility of soils as a result of negative surface charges attracting cations (because almost all nutrients used by plants and microbes are taken up in their ionic form) . Biochar has a high CEC due to its porous nature. Biochar is a form of ﬁxed carbon that will provide microbial communities with a long-term habitat . Biochars formed at relatively low production temperatures, where the surface area has greatly expanded compared to Agriculture 2023, 13, 512 12 of 26 the feedstock but sufﬁcient functional groups remain in the structure to give negative charges, have the highest CEC. A study conducted by  determined the average CEC of biochars made from various feedstocks (oak, pine, grass) at various pH levels (1.5–7.5). The result shows that CEC (51.9 cmolckg ) was the highest for biochar made at the lowest temperature (250 C) as compared to biochar made at higher temperatures. They also 1 1 observed that CEC decreases from 32.7 cmolckg at 300 C to 5 cmolckg at >800 C of the produced biochar. The application of biochar for soil supplement boosts the overall cation exchange capacity of the soil when produced under the right conditions . 6. Application of Biochar in Agro-ecosystems Declining soil fertility caused by infelicitous application of synthetic fertilizers is also a big worry for agricultural systems, especially in arid and semi-arid regions of the world . Increasing the number of chemical fertilizers used during the green revolu- tion initially increased agricultural yield, but it also caused a rapid deterioration in soil fertility and quality, which later disrupted the long-term viability of soil systems [12,35]. Thus, it is necessary to create new strategies that emphasize sustainability in terms of productivity, resource use, soil quality and accessibility for farmers . Crop productivity could be increased by strengthening soil quality by increasing soil organic matter (SOM) through the application of biochar . Earlier research supported the use of biochar to enhance soil quality and crop yield in various parts of the world [35,138]. The application of biochar has been viewed as a viable tool to address the complex issues of soil quality deterioration, waste management and boosting crop productivity [15,17,139]. In addition, researchers reported improvements in soil C sequestration and soil hydro-physical prop- erties such as water holding capacity due to the modiﬁcation of some of the soil physical properties such as porosity, texture, structure and aggregate stability under biochar-applied soils [17,137,138]. 6.1. Effect of Biochar on Soil Nutrients Cycle Biochar contributes to an improvement in the N cycle and/or additional N supply and increased soil N retention, and its use efﬁciency has been linked to increased plant productivity [140,141]. When biochar is added to soil, it not only improves soil fertility, but also provides micro and macro nutrients as required. According to a meta-analysis by , biochar may have a greater tendency to improve soil fertility through managing the N cycle than to supply nitrogen. The soil N cycle may be affected by biochar via a number of processes, including N adsorption or desorption by biochar, which can lower or raise the amount of inorganic nitrogen in the soil and alter the amount of soil miner- alizable substrates, which in turn inﬂuences the microbial processes of N mineralization or immobilization (i.e., labile organic compounds) and shifts the equilibrium between the nitriﬁcation and denitriﬁcation processes by changing the characteristics of the soil (i.e., pH and aeration) . However, the surface features of biochar (such as surface area, acidic functional groups, and CEC) as well as the species and quantity of NH N and NO - N 4+ 3 in soils greatly inﬂuence the extent and dominating processes of N cycling as altered by biochar . Ref.  documented that P-solubilizing bacteria (Pseudomonas and Bacillus) were shown to be more prevalent in biochar-amended soil, which indicated that the ﬁxed P forms in soil minerals, SOM or biochars might be solubilized or changed into accessible P. 6.2. Effect of Biochar on Soil Biological Processes Plant growth is directly impacted by changes in the soil biota . For soil microor- ganisms, biochar ’s porous structure makes a suitable shelter that guards against predation or desiccation . According to several reports, biochar promotes the growth of my- corrhizal fungus. The aggregation and structural stability of soil are both inﬂuenced by mycorrhizal fungi, which also help plants absorb nutrients and water. However, biochar has the potential to attract bacteria, making them less susceptible to leaching. Accord- ing to , adding biochar to paddy soil boosted bacterial abundance by 161%, with Agriculture 2023, 13, 512 13 of 26 Gram-positive bacteria being more affected by biochar than Gram-negative bacteria. The application of biochar also alters the N -ﬁxing bacteria that produce ammonia (NH ) from 2 3 atmospheric N . The reported impact of biochar on biological nitrogen ﬁxation has been attributed to a variety of different mechanisms . It has been observed that raising biochar application rates increases biological nitrogen ﬁxation [150,151]. 6.3. Effect of Biochar on Crop Growth and Productivity There has been a sharp rise in interest in biochar and its impact on crop productivity. Applications of biochar to soils have been found to increase plant growth and yield in several recently published sources [152–154]. Ref.  examined the association between biochar inputs and productivity (yield and above-ground biomass) for many crops and reported that biochar on average enhanced agricultural production by 10%. Further, authors reported that the liming impact (raised pH) and the better water holding capacity of the soil are the two key factors of biochar that account for increased productivity. Several studies have found that adding biochar to soil reduces bulk density while increasing WHC. The increase in WHC after biochar addition is ascribed to the biochar ’s large surface area and porosity, which contribute to improved water usage efﬁciency and thus plant productivity . The increase in WHC caused by biochar additions could be more noticeable in sandy soils, where the limited surface area of their particles and the presence of macro-pores limit their capacity to hold water. Ref.  proposed that biochar addition could improve the WHC of desert soils, resulting in greater plant growth. Ref.  articulated that the application of biochar increased the crop yields of soybean, maize, wheat and rice crops by 16%, 17%, 19% and 22%, respectively, over the control treatment without a biochar application. Similarly, ref.  stated that crop yield increased by 11% on average when the crop was cultivated with a biochar amendment. All of these studies are highly encouraging but there is a worry that applying fresh or pure biochar to soil, because of its high carbon content, may eventually cause the soil’s nitrogen to become immobilized, which would negatively affect plant growth and reduce crop output [156–158]. The crop response, however, varies from adverse to favorable depending on the properties of the biochar, how it is applied and the pre-existing soil conditions [13,15]. Furthermore, owing to the differences in soil buffering ability, different biochar application rates were recommended for different texture soils . They found that a low application rate of Thai traditional kiln biochar derived from Eucalyptus camaldulensis (1%) was suitable for coarse-textured soil with low buffering capacity. However, a larger biochar rate (2%) was proposed for ﬁne-textured soil, which had a higher buffering capacity than coarse-textured soil. These ﬁndings suggested that the function of biochar was strongly related to pyrolysis temperatures, soil and plant types and application rates. Therefore, it is crucial to carefully examine the impacts of biochar on a speciﬁc soil system before using it widely. The effects of biochar on various crops’ growth and productivity have been shown in Table 3. 6.4. Effect of Biochar on Plant Physiology A number of physiological indicators responded to biochar treatments as a result of soil, biochar type and other factors. Biochar soil amendment, for example, reduced the leaf chlorophyll content in highland rice cultivated in nutrient-poor soils . Ref.  conducted a pot study and found that the application of cotton-sticks-derived biochar increased photosynthetic rate, transpiration rate and sub-stomatal CO concentration, as well as the concentrations of chlorophylls, carotenoids, lycopene, anthocyanin, ascorbic acid and protein. Furthermore, ref.  reported that when soil treated with mixed- wood biochar at a level of 3 kg m soil, the photosynthetic rate was enhanced three-fold, stomatal conductance was increased 1.7-fold and a 5% rise in chlorophyll ﬂuorescence was observed. It was reported that the application of biochar in poor sandy soils improved plant growth by improving soil–plant water relations (enhanced relative water content and leaf osmotic potential) and photosynthesis (condensed stomatal resistance and stimulated photosynthesis rate by increasing the electron transport rate of photosystem II) under Agriculture 2023, 13, 512 14 of 26 well-watered and drought conditions . The increased water-holding capacity of biochar-amended soils can be used to predict the overall increase in plant accessible water . Biochar amendment improved plant physiology in wheat and maize cultivated in sandy loam soil, whereas the addition of biochar had a signiﬁcantly positive effect on + + photosynthetic rate, stomatal conductance and xylem K and Na in comparison to the control soil . Furthermore, lettuce (Lactuca sativa L.) plants grown in biochar-treated soil had higher leaf water potential, absorption rates, transpiration rates and water usage efﬁciency . According to , biochar improved soil adsorption of Na and raised plant xylem K content, enhancing potato tuber output. The study also observed that applying biochar had a favourable residual effect on lowering Na+ uptake in the next wheat crop . As a result, biochar has the potential to ameliorate salinity-induced mineral absorption reductions and may be a novel approach for mitigating the impacts of salinization in arable and polluted soils . Table 3. Effects of biochar on crop growth and productivity. Feed Stock Used Biochar Rate Agricultural Crop Soil Type Effects References Application of biochar at the rate of 20 t ha increased rice yield Sandy clay loam Wheat straw Rice  0, 5, 20 and 40 t ha (16.89%) and N use efﬁciency (pH = 5.15) (10.14%) over the control treatment. Application of biochar enhanced Typic hapludalfs Acacia arabica wood Maize 2.5% yield of maize crops over the  500 g m (pH = 7.78) bio-fertilizer control treatment. Results showed that plant dry biomass grown on soil amended with maize-residue biochar in Loamy sand Maize residue 2% v/v Lettuce combination with microbial  (Luvisol) (pH = 6.2) inoculants was signiﬁcantly increased by 5.8–18% compared to the control plants. Combined application of biochar and farmyard manure reduced Alluvial inceptisol the rate of chemical fertilizers Rice husk Wheat  5 t ha with improved soil properties as (pH = 7.5) well as crop productivity in dry, tropical agro-ecosystems. The increase in growth and grain yield recorded in the biochar-treated soil compared to Sandy clay loam Maize Maize residues 20 t ha the unamended plot might be  (pH = 5.12) attributed to the improvement in maize water use and nutrient availability. Biochar treatment had a Sludge biosolids and Sandy clay loam signiﬁcantly higher crop yield 10% v/v Sorghum  garden waste (pH = 5.2) (21%) compared to compost treatment. Biochar application without a fertilizer signiﬁcantly increased Sandy clay loam Peanut shell 1.5% w/w Rapeseed Lettuce  root (153%), shoot (219%) and (pH = 7.61) total biomass (186%) compared to control conditions (only soil). Application of willow wood biochar increased grain yield by Willow wood Maize Ferralsol (pH = 5.6)  10 t ha 13%, compared to control conditions. Barley yields were 30% higher in Acidic eutric nitisol Acacia wood Barley  10 t ha soil amendments with biochar (pH = 4.97) compared to control conditions. Applications of biochar increased Willow wood 10 t ha Peanut Ferralsol (pH = 6.2) pod yield by 24%, compared to  control conditions. Agriculture 2023, 13, 512 15 of 26 7. Application of Biochar for Reclamation of Contaminated Agro-ecosystems Biochar has a wide variety of environmental applications due to its abundance in feedstock, large surface area, microporosity and ion exchange capacity . Biochar is gaining popularity as a soil amendment because it has the capacity to mitigate climate change by sequestering carbon from the atmosphere into the soil . It also improves soil properties and fertility by increasing moisture, nutrient retention and microbial activity, resulting in increased crop productivity . On the other hand, biochar has the ability to decontaminate polluted soils. These various potential beneﬁts have been cost-effective and environmentally friendly for environmental restoration. 7.1. Reclamation of Inorganic and Organic Pollutants In a recent study, the application of biochar has been suggested for the removal of metal contaminated soil and water . Several studies have already been conducted or investigated into the quality of biochar and its ability to eliminate or reduce pollution loads from contaminated agricultural soil (Table 4). According to a recent study, the mechanisms behind the removal of heavy metal with biochar amendments can be linked to electrostatic interactions and precipitation reactions. Because of the decreased zeta potential and increased CEC, there is greater negative charge on the soil surface when biochar is used . In terms of precipitation, the signiﬁcantly elevated soil pH resulting from biochar amendments could contribute to a decrease in heavy metal mobilisation. In different conditions, various oxidates, phosphates and carbonates would form. For example, a novel precipitate was observed on Pb-loaded biochar derived from sludge at an initial pH 5 and was used as a lead–phosphate silicate . Ref.  conducted an experiment on the effects of biochar obtained from rice straw on the mobility and bioavailability of Cu(II), Pb(II) and Cd(II) in Ultisol soil . By increasing the biochar amendment dose, acid extractable Cu(II) and Pb(II) fell by 19.7–100% and 18.8–77.0%, respectively. The reducible Pb(II) for treatments with 3% and 5% biochar was two and three times greater than that of samples without biochar when these heavy metals were supplied at 5 mmolkg . Another study  used microanalyses techniques to investigate the ability of biochar to immobilise and retain As, Cd and Zn from a multi-element contaminated sediment derived soil, ﬁnding that biochar reduced Cd and Zn concentrations by 300- and 45-fold, respectively. Heavy metals in the soil can be immobilised, allowing them to be held in the soil and released at a slower rate, resulting in less environmental damage. The use of biochar to remove organic pollutants from soil is critical, notably for the removal of fungicides, herbicides and pesticides, as well as industrial chemicals such as volatile organic compounds, polycyclic aromatic hydrocarbons (PAHs) and other pol- lutants . The interactions of these pollutants with various properties of biochar often direct the abstraction processes. Organic pollutants are removed primarily through chemisorption (electrophilic contact), physisorption (hydrophilic, electrostatic attraction/repulsion via-electron donor-acceptor, pore diffusion and H bonding), chemical transformation (through a reductive reaction or electrical conductivity) and through biodegradation (by di- verse microorganisms located on the surface and in the micropores of biochar) . Biochar interactions with organic pollutants are inﬂuenced by pH, pyrolysis temperature, feedstock type and pollutant ratios to biochar. Biochar is desirable for the removal of nonpolar organic contaminants due to its greater surface area and microporosity at higher pyrolysis temperatures . As the pyrolysis temperature increases above 500 C, aromaticity, low polarity and acidity of biochar increase, resulting in the loss of O– and H–containing functional groups. When O–bearing functional groups are reduced, hydrophobic interac- tions accelerate; however, biochar produced at temperatures below 500 C may contain more O– and H– bearing functional groups, giving it a strong afﬁnity for polar organic molecules . Table 4 shows that biochar has the ability to reduce and degrade organic contaminants in soil. Agriculture 2023, 13, 512 16 of 26 7.2. Reclamation of Pesticides The effect of biochar on heavy metal and organic pollutant remediation is proposed as a cost-effective and environmentally acceptable solution for managing polluted environ- ments. Pesticides, on the other hand, are intentionally put in soil or other environmental compartments in agriculture to control pests and diseases. From the standpoint of humans and ecosystems, greater sorption and decreased dissipation of pesticides in the presence of biochar may reduce the risk of environmental pollution and human exposure. Furthermore, from an agricultural standpoint, decreased bioavailability and plant uptake may boost crop production and reduce chemical residues in crops. However, because a pesticide aimed at controlling speciﬁc pests or weeds must be accessible to be effective, lower pes- ticide efﬁcacy due to biochar application is undesirable in agricultural soils . Some studies have noticed that the efﬁciency of insecticides is compromised in the presence of biochar . Ref.  found that the fumigant 1,3- dichloropropene had less efﬁcacy in biochar-amended soil. The results showed that the dose of 1,3-dichloropropene that doubled in the soil amended with 1% biochar achieved full activity against nematode survival. Despite the lower efﬁcacy, adequate nematode control was accomplished with 0.5% and 1% biochar at a 1,3- dichloropropene dose on the low end of the recommended rates range. However, if the biochar ’s adsorption strength is too high, appropriate insect or weed control will be impossible to achieve. In this situation, biochar with a higher surface area would be regarded as undesirable, as biochar with a higher surface area has been found to have a higher sorption capacity. Ref.  discovered a strong inﬂuence of biochar application on the efﬁcacy of artrazine in soil in the management of ryegrass weed and suggested that the dose in biochar supplemented soil (1% biochar by weight) may need to be increased by 3–4 times to attain the required weed control. They also mentioned that the impact of biochar on herbicide efﬁcacy was determined by the chemistry of the herbicide molecule and its method of action. However, it was recently revealed that the sorption capacity of biochar decreases with age, which could be crucial for herbicide efﬁcacy control in biochar-added soils . It is critical to strike a balance between biochar ’s potentially beneﬁcial inﬂuence on pesticide clean-up and its detrimental impact on pesticide efﬁcacy. Table 4 depicts biochar ’s ability to remove pesticides from contaminated soil. 7.3. Reclamation of Other Pollutants Biochar amendment can also immobilise other contaminants in the environment, such as radionuclides and nutritional elements . Agricultural waste management has be- come one of the most pressing environmental challenges in recent years since signiﬁcant amounts of organic waste are generated as a result of intensive agricultural activity. Agri- cultural wastes, such as crop straw and animal manure, are high in organic components and other elements that plants require, making them ideal for amending agricultural land with the goal of improving soil properties. These wastes can help recycle nutrients, increase soil organic matter levels and improve soil characteristics . However, dumping agricultural waste into the soil without ﬁrst treating it can cause a slew of issues. For example, manure application carries a high risk of runoff and leaching of manure-derived components such as N and P, which might endanger streams and lakes; uncooked sludge carries a risk of ex- cessive heavy metal levels, which can harm the environment. Converting agricultural trash to biochar is a useful waste management approach, especially since agricultural wastes have little potential to slow down climate change. Composting is one of the most frequently accepted methods for recycling agricultural wastes, as it avoids some of the drawbacks associated with direct land application of raw wastes, such as phytotoxicity . Biochar is used as a bulking agent that can aid this process as a structural and drying supplement as well as a source of carbon and energy for microbes . Agriculture 2023, 13, 512 17 of 26 Table 4. Potential of biochar produced from various feedstocks for the removal of organic and inorganic pollutants from soil. Pollutants Biochar Pyrolysis Temp. ( C) Biochar Doses Removal Efﬁciency References Feedstock (%) Inorganic and organic pollutants 2+ Pb Compost and poultry manure 500 3% 89  2+ Zn Sewage sludge 400 5% 51.2  Dibutyl phthalate Bamboo 650 1% 87.5  2+ Cd Eucalyptus wood 500 2% 80  2+ Pb Poultry litter 500 2% 99.8  2+ Cd Bamboo 750 5% 56  Phenanthrene Conifer 600 0.5% 100  2+ Cu Poultry manure 420 1% 99  Diethyl phthalate Bamboo 820 0.5% 90  Tylosin Hardwood 850 10% 66  Atrazine Dairy manure 450 5% >66  Pesticides Pyrimethanil Rice husk 650 1% 82–84  Simazine Poultry 450 2% 50–56  Chlorpyrifos Plant residues 600 5 g/L 42–47  Atrazine Organic waste 400 4% 47–52  Carbofuran Empty fruit bunches 600 10% 71–80  8. Economic Consideration of Biochar Production Biochar has recently sparked signiﬁcant scientiﬁc attention due to its ability to increase agricultural yields and carbon sequestration potential. However, less emphasis has been dedicated to measuring biochar economic value, even though it is necessary for any massive application. A few studies have conducted detailed cost-beneﬁt assessments (CBA) of biochar as a soil amendment. The cost of biochar from feedstock to soil application is mostly determined by the cost of the initial biomass resources used and the ﬁnal production process. Transportation of biomass materials is regarded as a major factor in the economical use of biochar. However, biochar production may be acceptable if the revenues from the above values outweigh the economic costs of raising, harvesting, shipping and storing the biomass feedstock, as well as those of pyrolysis, transportation and application of biochar. As a result, the net margin of generating biochar might be increased by using less expensive feedstock and a promising processing technology . Ref.  stated that some pre- treatment operations (such as drying and size reduction) are also covered by feedstock costs, while others are covered by using plant capital and operational costs (e.g., reception, storage, feeding). Large-scale production of biochar will result in agronomic and economic beneﬁts. For example, the economic balance is determined by the yield of the crops to which biochar is applied and the proﬁt made from surplus harvest. This implies that crop selection, soil type and biochar application, both in quality and quantity, are critical since they affect the economic balance [208,209]. 9. Conclusions and Future Prospects In the 21st century, global climate change is a vital concern among scientiﬁc researchers. Agro-ecosystems are a major concern in relation to climate change, and the use of biochar has proven itself to reduce the effects of malpractices occurring in agriculture systems. Biochar production reveals a different type of biomass that is used as feedstock and the use of biochar may extend a chance to limit the number of issues related to global climate change. The application of biochar in agricultural ﬁelds helps to increase overall soil quality and fertility, also improving nutrient sorption capacity, which substitutes fertilizer use. The review shows that biochar is extremely effective in increasing crop production and yield. The strategies for the application of biochar as a soil additive can cut down the greenhouse gases emission viz CO , CH and N O in the atmosphere, which results in a reduction of 2 4 2 climate change. The efﬁciency of the biochar majorly depends on the pyrolysis temperature, feedstock and the pyrolysis process. Functional groups such as carboxyl and hydroxyl Agriculture 2023, 13, 512 18 of 26 groups on biochar ’s surface aid in contaminant removal. When creating recoverable biochar for a variety of environmental applications, economic consequences and recyclability should be taken into account. The majority of biochar research is still in the laboratory phase. However, environmental factors tend to be more complex than laboratory conditions, resulting in ambiguity about biochar ’s environmental impact. Therefore, more in situ experiments are needed to determine the true impact of biochar on the environment, such as environmental microorganisms, before it is used on a broad basis. Furthermore, studies are also required to ﬁnd new activation strategies to better understand the adsorption and desorption mechanisms for speciﬁc contaminants. The research of microbial populations and their interactions with biochar in soil is still in the infant stage. Microbe growth and development in the presence of biochar, as well as the impact of biochar characteristics on the microbial community, must be thoroughly investigated. More research into microbial activity during mineralization and soil remediation is needed. Furthermore, the interactions, modiﬁcations and binding mechanisms of biochar with soil must be thoroughly explored. Author Contributions: Writing—original draft preparation by K.K. and R.K.; Conceptualization and editing by N.B.; K.B., T.M. and C.K. All authors have read and agreed to the published version of the manuscript. Funding: CK gratefully acknowledges the ﬁnancial support from the Ministry of Science and Higher Education of the Russian Federation project on the development of the Young Scientist Laboratory (no. LabNOTs-21-01AB) and by the Strategic Academic Leadership Program of the Southern Federal University (“Priority 2030”). 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Multidisciplinary Digital Publishing Institute
Unravelling the Recent Developments in the Production Technology and Efficient Applications of Biochar for Agro-Ecosystems
, Volume 13 (3) –
Feb 21, 2023
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