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Biosurfactant-facilitated remediation of metal-contaminated soils.

Biosurfactant-facilitated remediation of metal-contaminated soils. Biosurfactant-facilitated Remediation of Metal-contaminated Soils Raina M. Miller Department of Soil and Water Science, University of Arizona, Tucson, Arizona whole cells or microbial are used Bioremediation of metal-contaminated wastestreams has been successfully demonstrated. Normally, exopolymers to concentrate and/or precipitate metals in the wastestream to aid in metal removal. Analogous remediation of metal-contaminated soils is more complex because microbial cells or large exopolymers do not move freely through the soil. The use of microbially produced surfactants (biosurfac- is an alternative with potential for remediation of metal-contaminated soils. The distinct advantage of biosurfactants over whole cells or tants) A a wide exopolymers is their small size, generally biosurfactant molecular weights are less than 1500. second advantage is that biosurfactants have variety of chemical structures that may show different metal selectivities and thus, metal removal efficiencies. A review of the literature shows that complexation capacities of several bacterial exopolymers was similar to the complexation capacity of a rhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 9027. - Environ Health Perspect 1 03(Suppl 1):59-62 (1995) Key words: metals, biosurfactant, surfactant, bioremediation, remediation, soil Introduction wall, translocation of the metal into the (8). The smallest pores can act as a filter with volatilization of the metal as a result of and Remediation of soil contaminated cell, for metal-containing microorganisms 2+ potentially toxic metal cations such as Pb a biotransformation reaction, and the for- large colloidal-metal complexes and pre- mation of metal precipitates by reaction vent transport of the metal through the Zn, Cr +, Cd2 , and Hg2+ has tradition- involved the excavation and transport with extracellular polymers or microbially soil. Movement of metals can also be ally waste anions as or metals into of contaminated soil to hazardous produced such sulphide phos- retarded by the diffusion of sites for Due to the phate (3). immobile zones created by small soil pores. landfilling. great in zones expense of traditional remediation, and In situ bioremediation of metal-conta- The presence of metals immobile recent U.S. Environmental Protection minated soils presents a more complex sep- can lead to extensive tailing, prolonging the metals from Agency (U.S. EPA) regulations that require aration problem due to the presence of soil. flushing required to remove pretreatment prior to landfilling (1), alter- The soil surface area as well as the mineral the soil. are of the soil nate cost-effective remedial techniques and organic matter composition needed. This has led to increasing interest will determine the amount of metal sorbed. Discussion in the application of microorganisms and Metal sorption occurs through one of three Biosurfactants microbial to in situ remediation of mechanisms: cation-exchange, metal-ligand products or has metal-contaminated surface and subsurface complexation to soil, metal complexa- One biologic technique that potential soils. tion with soil matter Sorption for removal of metals from soil is the use of organic (7). lim- Technologies using microorganisms by any of these mechanisms effectively microbially produced surfactants (biosur- and microbial to remove metals its the availability of metals for removal by Biosurfactants have the potential products factants). is the the have been successfully applied to waste- flushing. Another complicating factor to impact the major factors that cause streams such as industrial of a soil for a metal. In many removal of heavy metals from soil to be so sewage sludge, selectivity with a effluents, and mine water. Approaches used instances soils are contaminated difficult, namely, sorption, rate-limited in these microbial-metal mixture of and the relative affinity mass transfer, and resistance to aqueous- systems exploit metals, in mix- interactions to concentrate and separate of the soil for any given metal this phase transport. Biosurfactants are pro- from the wastestream. These inter- ture varies. is both a function of duced and metals Selectivity by plants, animals, many con- actions, which are described in several ionic radius, for instance the sorption of different microorganisms (9). When Z , include metal bind- Hg2+> Cd2+> and of electron to remediation of cont- recent reviews (2-6), sidering approaches ing to the cell surface or within the cell Cu2+> Ni2+ > Co2+> aminated sites, there are several apparent configuration, e.g., Fe Mn + the of to the use of biosurfactants advantages +> Thus, (7). difficulty removing specific metals from soil may rather than synthetic ones; they are vary. be biodegradable, they may cost-effective, This was at the Joint United States- paper presented in them in Movement of metals soils during soil and it may be possible to produce Mexico Conference on and Fate, Transport, Interactions of Metals held 14-16 1993 in is also limited the natural het- situ at contaminated sites. April flushing by Arizona. Tucson, In erogeneities that occur in soil texture, general, surfactants are amphoteric This research was the National supported by and matter content. These molecules of a tail and structure, organic consisting nonpolar Institute of Environmental Health Sciences Superfund and the U.S. Environmental heterogeneities result in the development a polar/ionic head. In aqueous solution, Project IP42ES4940 no. Protection Agency (grant R818620). of networks of soil Soil surfactants reduce surface tension accu- complex pores. by to Dr. R.M. Address correspondence Miller, pores vary greatly in size ranging from less mulating at interfaces and facilitating the of Soil and Water of Department Science, University 2 the size of a bac- formation of emulsions between of AZ 85721. 621- than approximately liquids Arizona, Tucson, Telephone (602) pm, 7231. Fax 621-1647. (602) terial cell, to as large as 0.2 mm in diameter different At low concentration, polarities. Environmental Health Perspectives R.M. MILLER contact between the bio- surfactants are present as individual mole- there are species level differences in the may allow direct of the structure of biosurfactants. For surfactant and the sorbed metal. The cules. However, as the concentration chemical surfactant is increased, a concentration is instance, the rhamnolipids produced by potential for biosurfactant-mediated des- no further change in interfa- various Pseudomonas sp. differ both in the orption of metals is indicated by a study by reached where (30). In this cial takes place. The amount of number of rhamnose molecules (1 to 2) Blakeburn and Scamehorn properties concentra- length of the lipid moiety. study, a positively charged surfactant surfactant needed to reach this and the is called the critical micelle concentra- Biosurfactant molecular weights range from (cetylpyridinium chloride) was used to tion surfactant to although regenerate activated carbon beds saturated tion (CMC). At the CMC, approximately 500 1500 mw, solute (4- molecules aggregate to form structures some exceptions exist, e.g., Pseudomonas with a negatively charged organic or micelles. The on hexadecane have been tert-butylphenol). The regeneration process such as bilayers, vesicles, strains growing of the organic solute and size of aggregate formed depends reported to produce protein-containing sur- involved desorption type and on the solu- with molecular by a surfactant solution, followed by on the surfactant structure face-active substances solution. Micelles are the smallest weights of up to 14,300 (11). removal of the surfactant-organic tion pH (10). less than Analagously, the removal of cationic metals basic structure formed, generally Biosurfactants in Remediation nm in diameter. A micelle is composed from soil would employ anionic biosurfac- interest has focused principally on tants to desorb cationic metals for subse- of a monolayer of surfactant molecules To date, where the heads are oriented toward the use of surfactants to remove organic quent removal by flushing. polar from soil. Studies of organic A study by Beveridge and Pickering the surrounding aqueous solution and the contaminants are oriented toward the contaminants have shown that both bio- (31) examined the effect of a range of syn- nonpolar tails and synthetic (19-29) sur- thetic anionic, and neutral surfac- hydrophobic center of the micelle. Vesicle logical (16-18) cationic, in size and range from factants can enhance either the chemical tants on the sorption of metals by clays. structures are next nm in diameter. removal or the biodegradative removal of The cationic surfactants used were found 10 nm to more than 500 are of surfactant bilayers, organic contaminants from soil. While to reduce the sorption of Cu, Pb, Cd, and Vesicles composed the for use Zn by montmorillonite, probably through which are similar in structure to biological these studies indicate potential In the polar of surfactants to facilitate the removal of competition by the cationic surfactant for membranes. aqueous solution, the literature contains the clay surface (cation surfactant heads of a bilayer face the out- metals from soil, negative sites on the tails are sandwiched very little actual information concerning exchange). In contrast, cationic surfactants side while nonpolar effect on sorption of metals by between the heads. Thus, the environment surfactant removal of metal contaminants. had little a vesicle is The goal of the use of surfactants for illite or kaolinite, which was attributed to a both inside and outside hydro- is of ion due to philic (aqueous) while the environment both organics and metals similar; smaller influence exchange of the non- increase the apparent water solubility of the surface properties of these clays. within the bilayer, composed to facilitate the the anionic surfactants tested surfactant tails, is hydrophobic. the contaminant of interest Surprisingly, polar can also exist as flexible sheets or removal by biodegradation or flushing. seemed to increase the sorption of the metals Bilayers it should be noted that there are in this test system. The authors suggest planar bilayers which are the largest of the However, A sheet some differences between metal-conta- that this may have been due to formation basic surfactant structures. bilayer key soils which is essentially unlimited in size. If the solu- minated and organic-contaminated of metal-surfactant species, precipi- is the that must be considered. The most obvious tated from solution or sorbed to the clay tion on both sides of the bilayer surfaces. the and behavior of the difference is that unlike organics, metals same, properties will cannot be In some cases met- Although it is well-known that micro- two bilayer surfaces be identical. biodegraded. but transformation metals from solu- However, if the bilayer is at an interface, als may be transformed bial cells can complex the often increases metal there is little information in the e.g., air-water or liquid-liquid, bilayer only toxicity (e.g., tion, -- A the use of biosurfac- asymmetric properties. Hg++ CH -Hg'). second difference literature concerning may develop and tants to complex metals. Other microbial Typically, CMCs of biosurfactants range to be considered between organic is that of the as bacterial and algal from 1 to 200 mg/I (11). metal contaminants organics products such most concern are neutral molecules, while have been shown to Biosurfactants are produced by many exopolysaccharides as cationic a of metals. bacterial The chemical metals are most often found bind variety Emulsan, pro- different genera. varies since contaminant duced Acinetobacter RAG- I was found structure of biosurfactants widely, species. Thus, sorption by but all biosurfactants described thus far in on the chemical properties of both to bind to 240 uranium (UO22+)/mg depends up pg the choice of emulsan a Pseudomonas the literature are anionic or nonionic. the soil and the contaminant, (32). Similarly, to ura- can be classified into several surfactant used for contaminant complexa- exopolysaccharide bound up 96 pg Biosurfactants tion will be exopolymer (33). A study of cad- broad glycolipids, lipopeptides, important. nium/mg groups: an Arthrobacter and The addition of a biosurfactant may mium complexation by lipopolysaccharides, phospholipids, The of heavy metals from showed that cadmium acids/neutral lipids (11-13). promote desorption exopolysaccharide fatty first is was best-studied of biosurfac- solid phases in two ways. The binding (3.3 pg/mg exopolymer) largest and group which include the of the free form of less than that of uranium tants are the glycolipids, through complexation significantly in This A bound the metal residing solution. (34). Klebsiella exopolysaccharide sophorose-, rhamnose-, trehalose-, sucrose-, the of the amounts of cadmium 1 and fructose-lipids. Both biosurfactant decreases solution-phase activity comparable (1 as well as and are affected by metal and, therefore, promotes desorption pg/mg exopolymer) copper (22 yield composition Le Chatelier's The exopolymer) (35). A study of sev- conditions including carbon according to principle. pg/mg growth culture medium nutrients second is that under conditions of reduced eral marine Pseudomonas sp. exopolysac- source, (e.g., biosurfactants will accu- charides showed of nitrogen, phosphate, iron), temperature, interfacial tension, complexation copper, When In mulate at the solid-solution interface. This iron, lead, nickel, and zinc (36). pH, and agitation (14,15). addition, Environmental Health Perspectives 60 BIOSURFACTANT-FACILITATED REMEDIATION OFMETALS studied separately, the affinity of the surfactant used in this study was a facilitate the movement of metal-rhamno- Pseudomonas exopolysaccharides for the monorhamnolipid produced by Pseudo- lipid vesicles in a complex system like soil. metals generally followed the order: lead >> monas aeruginosa ATCC 9027. This work While the use of microorganisms and copper = iron > zinc > nickel. Interestingly, showed that of the + in a mM microbial products, e.g., biosurfactants, in 92% Cd 0.5 complexation capacity was species specific solution of Cd(NO3)2 was complexed by a bioremediation of metal-contaminated soils in this study with up to one order of mag- 5-mM solution of rhamnolipid, a complex- shows promise, the development of reme- nitude difference in metal complexation ation of 22 rhamnolipid. This value dial technologies will require further study pg/mg Pseudomonas In is comparable to the cadmium complexa- in several areas. For instance, soils contain between different sp. sum- mary, metal complexation with exopoly- tion capacities reported for Arthrobacter numerous cations that may compete with mers seems to exhibit both metal selectivity (34) and Klebsiella (35) exopolymers (3.3 metal contaminants for biosurfactant com- and metal complexation capacities that are and 11 pg/mg exopolymer, respectively). plexation sites. Therefore, the selectivity of in species specific. Cadmium complexation by rhamnolipid biosurfactants for metals both solution Exopolysaccharides differ from biosur- was stable from pH 6.0 to 7.0. Cryo-elec- and in soil systems must be investigated. tron of the factants in that they are large (c.f., molecu- microscopy rhamnolipid struc- There is also relatively little information lar weight of emulsan is 9.8 x 105), and tures formed in the presence of cadmium about biosurfactant structure and structure in have minimal surface activity, although shows vesicles ranging size from 10 to sizes, or the effect of biosurfactant-metal they exhibit strong affinities for oil-water 300 nm in diameter, with a size distribu- interactions on these structures. Clearly, in interfaces (37). Biosurfactants may offer an tion as follows: 71% of the vesicles were biosurfactant structure size and charge will over in the the 10 to 50 nm range, 26% of the vesicles affect movement of biosurfactant-metal advantage exopolysaccharides in remediation of soils because of their com- the 51 to 250 range, and 3% of the vesi- complexes through the soil. In addition, paratively small size. The potential for use cles were > nm in diameter The structure size and charge will affect the 250 (39). in of biosurfactants removal of metals from small size of these metal-rhamnolipid vesi- access of biosurfactants to soil pores and soils is indicated by a study of biosurfactant cles and the absence of metal precipitates in the interaction of biosurfactant therefore, complexation of cadmium (38). The bio- the metal-rhamnolipid mixtures should with sorbed metals. REFERENCES 1. Peters RW, Shem L. Use of chelating agents for remediation of Gray NCC, eds). New York:Marcel Dekker, 1987;89-120. heavy metal contaminated soil. In: Environmental Remediation. 16. Falatko DM, Novak JT. Effects of biologically produced sur- Washington:American Chemical Society, 1992;70-84. factants on the mobility and biodegradation of petroleum 2. Brierley CL. Bioremediation of metal-contaminated surface hydrocarbons. Water Environment Res 64:163-169 (1992). and groundwaters. Geomicrobiology J 8:201-223 (1990). 17. Harvey Elashvili I, Valdes JJ, Kamely D, Chakrabarty AM. S, 3. Hughes MN, Poole RK. Metals and Microorganisms. New Enhanced removal of Exxon Valdez spilled oil from Alaskan York:Chapman and Hall, 1989;303-358. gravel by a microbial surfactant. Biotechnol 8:228-230 (1990). 4. Lester JN, ed. Heavy Metals in Wastewater and Sludge 18. Jain DK, Lee Trevors JT. Effect of addition of Pseudomonas H, Treatment Processes, Vol II: Treatment and Disposal. Boca aeruginosa UG2 inocula or biosurfactants on biodegradation Raton, FL:CRC Press, 1987;15-40. selected hydrocarbons in soil. J Ind Microbiol 10:87-93 (1992). 5. Macaskie LE. The application of biotechnology to the treat- 19. Aronstein BN, Alexander M. Surfactants at low concentrations ment of wastes produced from the nuclear fuel cycle: biodegra- stimulate of sorbed in samples of biodegradation hydrocarbons dation and bioaccumulation as a means of treating radionuclide- aquifer sands and soil slurries. Environ Toxicol Chem streams. Crit Rev 11:41-112 11:1227-1233 (1992). containing Biotechnol (1991). 6. Volesky ed. Biosorption of Heavy Metals. Boca Raton, 20. Edwards DA, Laha Liu Luthy RG. Solubilization and B, S, Z, FL:CRC Press, 1990;8-43. biodegradation of hydrophobic organic compounds in soil- 7. Sposito G. The Chemistry of Soils. New York:Oxford aqueous systems with nonionic surfactants. In: Transport and Remediation of Subsurface Contaminants (Sabatini DA, Knox University Press, 1989:148-169. 8. Donahue Miller Shickluna Soils: An RC, eds). Washington:American Chemical Society, RL, RW, JC. Introduction to Soils and Plant Growth, 5th ed. Englewood 1992; 159-168. 21. Edwards DA, Liu Z, Luthy RG. Enhancing polynuclear aro- Cliffs, NJ:Prentice-Hall Inc., 1983;61. 9. Zajic JE, Panchel CJ. Bio-emulsifiers. CRC Crit Rev Microbiol matic uptake into bulk solution with amphiphilic colloidal 5:39-66 (1976). Wat Sci Tech aggregates. 26:2341-2344 (1992). PR. III: 22. Fountain JC. Field tests of surfactant flooding: mobility control 10. Cantor CR, Schimmel Biophysical Chemistry, part of Francisco: of In: and The Behavior Biological Macromolecules. San dense nonaqueous-phase liquids. Transport WH Freeman of Subsurface Contaminants Knox and Co., 1980;1327-1371. Remediation (Sabatini DA, F. Structure of Chemical 11. Lang S, Wagner and properties biosurfactants. RC, eds). Washington:American Society, 1992; In: 182-191. Biosurfactants and Biotechnology (Kosaric N, Cairns WL, New York:Marcel 23. Green G. The use of surfactants in the bioremediation of Gray NCC, eds). Dekker, 1987;21-45. 12. Fiechter A. Biosurfactants: moving towards industrial applica- contaminated soils. Technical EPA/101/F- petroleum report tion. Trends Biotech Environmental Protection 10:208-217 (1992). 90/013. Washington:U.S. Agency, E. Microbial surfactants. Crit Rev 1990. 13. Rosenberg Biotechnol 3:109-132 (1986). 24. Laha solubilization of Liu Z, S, Luthy RG. Surfactant poly- in 14. Hommel RF, Ratledge, C. Biosynthetic mechanisms of low cyclic aromatic hydrocarbon compounds soil-water suspen- sions. Sci Tech molecular weight surfactants and their precursor molecules. In: Wat 23:475-485 (1991). Biosurfactants: Production, Properties, Applications (Kosaric 25. Liu M, Roy D. Washington of hydrophobic organic from cont- N, ed). New York:Marcel Dekker, 1993;3-63. aminated sand with a surfactant. Min Metallur Process 15. Syldatk C, Wagner F. Production of biosurfactants. In: 9:206-208 (1992). Brooks Biosurfactants and Biotechnology (Kosaric N, Cairns WL, 26. McDermott JB, Unterman R, Brennan MJ, RE, Volume 103, Supplement 1, February 1995 R.M. MILLER Mobley DP, Schwartz CC, Dietrich DK. Two strategies for (1983). PCB soil remediation: biodegradation and surfactant extrac- 33. Marques AM, Bonet Simon-Pujol MD, Fuste MC, R, tion. Environ Progress 8:46-51 (1989). Congregado F. Removal of uranium by an exopolysaccharide 27. Palmer C, Sabatini DA, Harwell JH. Sorption of hydrophobic from Pseudomonas sp. Apple Microbiol Biotech 34:429-431 organic compounds and nonionic surfactants with subsurface (1990). materials. In: Transport and Remediation of Subsurface 34. Scott JA, Palmer SJ. Cadmium biosorption by bacterial Contaminants (Sabatini DA, Knox RC, eds). Washington: exopolysaccharide. Biotech Lett 10:21-24 (1988). American Chemical Society, 1992; 169-181. 35. Bitton Freihofer V. Influence of extracellular polysaccha- G, 28. West CC. Surfactant-enhanced solubilization of tetrachloroeth- rides on the toxicity of copper and cadmium toward Klebsiella ylene and degradation products in pump and treat remediation. aerogenes. Microbial Ecol 4:119-125(1978). In: Transport and Remediation of Subsurface Contaminants 36. Corpe WA. Metal-binding properties of surface materials from (Sabatini DA, Knox RC, eds). Washington:American Chemical marine bacteria. Dev Ind Microbiol 16:249-255 (1975). Society, 1992;149-158. 37. Gutnick DL, Shabtai Y. Exopolysaccharide bioemulsifiers. In: 29. West CC, Harwell JH. Surfactants and subsurface remediation. Biosurfactants and Biotechnology (Kosaric Cairns WL, N, Environ Sci Technol 26:2324-2330 (1992). Gray NCC, eds). New York:Marcel Dekker, 1987;21 1-246. 30. Blakeburn DL, Scamehorn JF. Surfactant-enhanced carbon 38. Tan Champion JT, Artiola JF, Brusseau ML, Miller RM. H, regeneration. In: Surfactant-based Separation Processes. New Complexation of cadmium by a rhamnolipid biosurfactant. York:Marcel Dekker, 1989;205-230. Environ Sci Technol in press (1994). 31. Beveridge A, Pickering WF. The influence of surfactants on the 39. Champion JT, Gilkey JC, Lamparski Miller RM. H, Retterer, of metal ions Water Res Electron microscopy of morphol- adsorption heavy by clays. 17:215-225 rhamnolipid (biosurfactant) (1983). ogy: effects of pH, cadmium, and octadecane. J ColIloid 32. Zosim Gutnick D, Rosenberg E. Uranium binding by emul- Interface Sci (in press). Z, san and emulsanosols. Biotechnol Bioeng 25:1725-1735 62 Environmental Health Perspectives http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Environmental Health Perspectives Pubmed Central

Biosurfactant-facilitated remediation of metal-contaminated soils.

Environmental Health Perspectives , Volume 103 (Suppl 1) – Feb 1, 1995

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Abstract

Biosurfactant-facilitated Remediation of Metal-contaminated Soils Raina M. Miller Department of Soil and Water Science, University of Arizona, Tucson, Arizona whole cells or microbial are used Bioremediation of metal-contaminated wastestreams has been successfully demonstrated. Normally, exopolymers to concentrate and/or precipitate metals in the wastestream to aid in metal removal. Analogous remediation of metal-contaminated soils is more complex because microbial cells or large exopolymers do not move freely through the soil. The use of microbially produced surfactants (biosurfac- is an alternative with potential for remediation of metal-contaminated soils. The distinct advantage of biosurfactants over whole cells or tants) A a wide exopolymers is their small size, generally biosurfactant molecular weights are less than 1500. second advantage is that biosurfactants have variety of chemical structures that may show different metal selectivities and thus, metal removal efficiencies. A review of the literature shows that complexation capacities of several bacterial exopolymers was similar to the complexation capacity of a rhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 9027. - Environ Health Perspect 1 03(Suppl 1):59-62 (1995) Key words: metals, biosurfactant, surfactant, bioremediation, remediation, soil Introduction wall, translocation of the metal into the (8). The smallest pores can act as a filter with volatilization of the metal as a result of and Remediation of soil contaminated cell, for metal-containing microorganisms 2+ potentially toxic metal cations such as Pb a biotransformation reaction, and the for- large colloidal-metal complexes and pre- mation of metal precipitates by reaction vent transport of the metal through the Zn, Cr +, Cd2 , and Hg2+ has tradition- involved the excavation and transport with extracellular polymers or microbially soil. Movement of metals can also be ally waste anions as or metals into of contaminated soil to hazardous produced such sulphide phos- retarded by the diffusion of sites for Due to the phate (3). immobile zones created by small soil pores. landfilling. great in zones expense of traditional remediation, and In situ bioremediation of metal-conta- The presence of metals immobile recent U.S. Environmental Protection minated soils presents a more complex sep- can lead to extensive tailing, prolonging the metals from Agency (U.S. EPA) regulations that require aration problem due to the presence of soil. flushing required to remove pretreatment prior to landfilling (1), alter- The soil surface area as well as the mineral the soil. are of the soil nate cost-effective remedial techniques and organic matter composition needed. This has led to increasing interest will determine the amount of metal sorbed. Discussion in the application of microorganisms and Metal sorption occurs through one of three Biosurfactants microbial to in situ remediation of mechanisms: cation-exchange, metal-ligand products or has metal-contaminated surface and subsurface complexation to soil, metal complexa- One biologic technique that potential soils. tion with soil matter Sorption for removal of metals from soil is the use of organic (7). lim- Technologies using microorganisms by any of these mechanisms effectively microbially produced surfactants (biosur- and microbial to remove metals its the availability of metals for removal by Biosurfactants have the potential products factants). is the the have been successfully applied to waste- flushing. Another complicating factor to impact the major factors that cause streams such as industrial of a soil for a metal. In many removal of heavy metals from soil to be so sewage sludge, selectivity with a effluents, and mine water. Approaches used instances soils are contaminated difficult, namely, sorption, rate-limited in these microbial-metal mixture of and the relative affinity mass transfer, and resistance to aqueous- systems exploit metals, in mix- interactions to concentrate and separate of the soil for any given metal this phase transport. Biosurfactants are pro- from the wastestream. These inter- ture varies. is both a function of duced and metals Selectivity by plants, animals, many con- actions, which are described in several ionic radius, for instance the sorption of different microorganisms (9). When Z , include metal bind- Hg2+> Cd2+> and of electron to remediation of cont- recent reviews (2-6), sidering approaches ing to the cell surface or within the cell Cu2+> Ni2+ > Co2+> aminated sites, there are several apparent configuration, e.g., Fe Mn + the of to the use of biosurfactants advantages +> Thus, (7). difficulty removing specific metals from soil may rather than synthetic ones; they are vary. be biodegradable, they may cost-effective, This was at the Joint United States- paper presented in them in Movement of metals soils during soil and it may be possible to produce Mexico Conference on and Fate, Transport, Interactions of Metals held 14-16 1993 in is also limited the natural het- situ at contaminated sites. April flushing by Arizona. Tucson, In erogeneities that occur in soil texture, general, surfactants are amphoteric This research was the National supported by and matter content. These molecules of a tail and structure, organic consisting nonpolar Institute of Environmental Health Sciences Superfund and the U.S. Environmental heterogeneities result in the development a polar/ionic head. In aqueous solution, Project IP42ES4940 no. Protection Agency (grant R818620). of networks of soil Soil surfactants reduce surface tension accu- complex pores. by to Dr. R.M. Address correspondence Miller, pores vary greatly in size ranging from less mulating at interfaces and facilitating the of Soil and Water of Department Science, University 2 the size of a bac- formation of emulsions between of AZ 85721. 621- than approximately liquids Arizona, Tucson, Telephone (602) pm, 7231. Fax 621-1647. (602) terial cell, to as large as 0.2 mm in diameter different At low concentration, polarities. Environmental Health Perspectives R.M. MILLER contact between the bio- surfactants are present as individual mole- there are species level differences in the may allow direct of the structure of biosurfactants. For surfactant and the sorbed metal. The cules. However, as the concentration chemical surfactant is increased, a concentration is instance, the rhamnolipids produced by potential for biosurfactant-mediated des- no further change in interfa- various Pseudomonas sp. differ both in the orption of metals is indicated by a study by reached where (30). In this cial takes place. The amount of number of rhamnose molecules (1 to 2) Blakeburn and Scamehorn properties concentra- length of the lipid moiety. study, a positively charged surfactant surfactant needed to reach this and the is called the critical micelle concentra- Biosurfactant molecular weights range from (cetylpyridinium chloride) was used to tion surfactant to although regenerate activated carbon beds saturated tion (CMC). At the CMC, approximately 500 1500 mw, solute (4- molecules aggregate to form structures some exceptions exist, e.g., Pseudomonas with a negatively charged organic or micelles. The on hexadecane have been tert-butylphenol). The regeneration process such as bilayers, vesicles, strains growing of the organic solute and size of aggregate formed depends reported to produce protein-containing sur- involved desorption type and on the solu- with molecular by a surfactant solution, followed by on the surfactant structure face-active substances solution. Micelles are the smallest weights of up to 14,300 (11). removal of the surfactant-organic tion pH (10). less than Analagously, the removal of cationic metals basic structure formed, generally Biosurfactants in Remediation nm in diameter. A micelle is composed from soil would employ anionic biosurfac- interest has focused principally on tants to desorb cationic metals for subse- of a monolayer of surfactant molecules To date, where the heads are oriented toward the use of surfactants to remove organic quent removal by flushing. polar from soil. Studies of organic A study by Beveridge and Pickering the surrounding aqueous solution and the contaminants are oriented toward the contaminants have shown that both bio- (31) examined the effect of a range of syn- nonpolar tails and synthetic (19-29) sur- thetic anionic, and neutral surfac- hydrophobic center of the micelle. Vesicle logical (16-18) cationic, in size and range from factants can enhance either the chemical tants on the sorption of metals by clays. structures are next nm in diameter. removal or the biodegradative removal of The cationic surfactants used were found 10 nm to more than 500 are of surfactant bilayers, organic contaminants from soil. While to reduce the sorption of Cu, Pb, Cd, and Vesicles composed the for use Zn by montmorillonite, probably through which are similar in structure to biological these studies indicate potential In the polar of surfactants to facilitate the removal of competition by the cationic surfactant for membranes. aqueous solution, the literature contains the clay surface (cation surfactant heads of a bilayer face the out- metals from soil, negative sites on the tails are sandwiched very little actual information concerning exchange). In contrast, cationic surfactants side while nonpolar effect on sorption of metals by between the heads. Thus, the environment surfactant removal of metal contaminants. had little a vesicle is The goal of the use of surfactants for illite or kaolinite, which was attributed to a both inside and outside hydro- is of ion due to philic (aqueous) while the environment both organics and metals similar; smaller influence exchange of the non- increase the apparent water solubility of the surface properties of these clays. within the bilayer, composed to facilitate the the anionic surfactants tested surfactant tails, is hydrophobic. the contaminant of interest Surprisingly, polar can also exist as flexible sheets or removal by biodegradation or flushing. seemed to increase the sorption of the metals Bilayers it should be noted that there are in this test system. The authors suggest planar bilayers which are the largest of the However, A sheet some differences between metal-conta- that this may have been due to formation basic surfactant structures. bilayer key soils which is essentially unlimited in size. If the solu- minated and organic-contaminated of metal-surfactant species, precipi- is the that must be considered. The most obvious tated from solution or sorbed to the clay tion on both sides of the bilayer surfaces. the and behavior of the difference is that unlike organics, metals same, properties will cannot be In some cases met- Although it is well-known that micro- two bilayer surfaces be identical. biodegraded. but transformation metals from solu- However, if the bilayer is at an interface, als may be transformed bial cells can complex the often increases metal there is little information in the e.g., air-water or liquid-liquid, bilayer only toxicity (e.g., tion, -- A the use of biosurfac- asymmetric properties. Hg++ CH -Hg'). second difference literature concerning may develop and tants to complex metals. Other microbial Typically, CMCs of biosurfactants range to be considered between organic is that of the as bacterial and algal from 1 to 200 mg/I (11). metal contaminants organics products such most concern are neutral molecules, while have been shown to Biosurfactants are produced by many exopolysaccharides as cationic a of metals. bacterial The chemical metals are most often found bind variety Emulsan, pro- different genera. varies since contaminant duced Acinetobacter RAG- I was found structure of biosurfactants widely, species. Thus, sorption by but all biosurfactants described thus far in on the chemical properties of both to bind to 240 uranium (UO22+)/mg depends up pg the choice of emulsan a Pseudomonas the literature are anionic or nonionic. the soil and the contaminant, (32). Similarly, to ura- can be classified into several surfactant used for contaminant complexa- exopolysaccharide bound up 96 pg Biosurfactants tion will be exopolymer (33). A study of cad- broad glycolipids, lipopeptides, important. nium/mg groups: an Arthrobacter and The addition of a biosurfactant may mium complexation by lipopolysaccharides, phospholipids, The of heavy metals from showed that cadmium acids/neutral lipids (11-13). promote desorption exopolysaccharide fatty first is was best-studied of biosurfac- solid phases in two ways. The binding (3.3 pg/mg exopolymer) largest and group which include the of the free form of less than that of uranium tants are the glycolipids, through complexation significantly in This A bound the metal residing solution. (34). Klebsiella exopolysaccharide sophorose-, rhamnose-, trehalose-, sucrose-, the of the amounts of cadmium 1 and fructose-lipids. Both biosurfactant decreases solution-phase activity comparable (1 as well as and are affected by metal and, therefore, promotes desorption pg/mg exopolymer) copper (22 yield composition Le Chatelier's The exopolymer) (35). A study of sev- conditions including carbon according to principle. pg/mg growth culture medium nutrients second is that under conditions of reduced eral marine Pseudomonas sp. exopolysac- source, (e.g., biosurfactants will accu- charides showed of nitrogen, phosphate, iron), temperature, interfacial tension, complexation copper, When In mulate at the solid-solution interface. This iron, lead, nickel, and zinc (36). pH, and agitation (14,15). addition, Environmental Health Perspectives 60 BIOSURFACTANT-FACILITATED REMEDIATION OFMETALS studied separately, the affinity of the surfactant used in this study was a facilitate the movement of metal-rhamno- Pseudomonas exopolysaccharides for the monorhamnolipid produced by Pseudo- lipid vesicles in a complex system like soil. metals generally followed the order: lead >> monas aeruginosa ATCC 9027. This work While the use of microorganisms and copper = iron > zinc > nickel. Interestingly, showed that of the + in a mM microbial products, e.g., biosurfactants, in 92% Cd 0.5 complexation capacity was species specific solution of Cd(NO3)2 was complexed by a bioremediation of metal-contaminated soils in this study with up to one order of mag- 5-mM solution of rhamnolipid, a complex- shows promise, the development of reme- nitude difference in metal complexation ation of 22 rhamnolipid. This value dial technologies will require further study pg/mg Pseudomonas In is comparable to the cadmium complexa- in several areas. For instance, soils contain between different sp. sum- mary, metal complexation with exopoly- tion capacities reported for Arthrobacter numerous cations that may compete with mers seems to exhibit both metal selectivity (34) and Klebsiella (35) exopolymers (3.3 metal contaminants for biosurfactant com- and metal complexation capacities that are and 11 pg/mg exopolymer, respectively). plexation sites. Therefore, the selectivity of in species specific. Cadmium complexation by rhamnolipid biosurfactants for metals both solution Exopolysaccharides differ from biosur- was stable from pH 6.0 to 7.0. Cryo-elec- and in soil systems must be investigated. tron of the factants in that they are large (c.f., molecu- microscopy rhamnolipid struc- There is also relatively little information lar weight of emulsan is 9.8 x 105), and tures formed in the presence of cadmium about biosurfactant structure and structure in have minimal surface activity, although shows vesicles ranging size from 10 to sizes, or the effect of biosurfactant-metal they exhibit strong affinities for oil-water 300 nm in diameter, with a size distribu- interactions on these structures. Clearly, in interfaces (37). Biosurfactants may offer an tion as follows: 71% of the vesicles were biosurfactant structure size and charge will over in the the 10 to 50 nm range, 26% of the vesicles affect movement of biosurfactant-metal advantage exopolysaccharides in remediation of soils because of their com- the 51 to 250 range, and 3% of the vesi- complexes through the soil. In addition, paratively small size. The potential for use cles were > nm in diameter The structure size and charge will affect the 250 (39). in of biosurfactants removal of metals from small size of these metal-rhamnolipid vesi- access of biosurfactants to soil pores and soils is indicated by a study of biosurfactant cles and the absence of metal precipitates in the interaction of biosurfactant therefore, complexation of cadmium (38). The bio- the metal-rhamnolipid mixtures should with sorbed metals. REFERENCES 1. Peters RW, Shem L. Use of chelating agents for remediation of Gray NCC, eds). New York:Marcel Dekker, 1987;89-120. heavy metal contaminated soil. In: Environmental Remediation. 16. Falatko DM, Novak JT. Effects of biologically produced sur- Washington:American Chemical Society, 1992;70-84. factants on the mobility and biodegradation of petroleum 2. Brierley CL. Bioremediation of metal-contaminated surface hydrocarbons. Water Environment Res 64:163-169 (1992). and groundwaters. 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Biotechnol Bioeng 25:1725-1735 62 Environmental Health Perspectives

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Environmental Health PerspectivesPubmed Central

Published: Feb 1, 1995

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