有害废物的生物修复-综述(english)

Bioremediation of Hazardous Wastes—A Review Satinder K.Brar1;M.Verma2;R.Y.Surampalli3;K.Misra4;R.D.Tyagi5;N.Meunier6;and J.F.Blais7Abstract:Intensive industrialization generates hazardous wastes comprising organics,inorganics,heavy metals,and munitions that need to be tackled in a safe monly employed physicochemical technologies have paved the way to ecofriendly bioremediation processes.Bioremediation uses natural as well as recombinant microorganisms to break down toxic and hazardous substances by aerobic and anaerobic means.They can be applied on site͑in situ͒or off site͑ex situ͒,mediated by mixed microbial consortia and/or pure microbial strains and.plants͑phytoremediation͒or even natural attenuation.They include several processes—bioventing,biosparging, biostimulation,bioaugmentation,bioleaching,fungal bioremediation,and biosorption.Bioremediation also encompasses ex situ engi-neered methods like bioreactors and enzyme catalyzed breakdown.The success of bioremediation is governed by three important factors—availability of microbes,accessibility of contaminants,and a conducive environment.This review discusses various bioreme-diation technologies,listing the advantages as well as disadvantages andfield application,if any.DOI:10.1061/͑ASCE͒1090-025X͑2006͒10:2͑59͒CE Database subject headings:Abatement and removal;Biodegradation;Explosives;Heavy metals;Inorganic chemicals;Organic chemicals;Hazardous wastes.IntroductionThe environment contaminated with toxic chemicals,and by-products are posing a gigantic task of treatment and/or man-agement.These toxic contaminants,primarily of anthropogenic origin,are broadly classified as metal,nonmetal,metalloid, inorganic,and organic compounds͑Cluis2004͒.Organic con-taminants comprise aliphatic,alicyclic,aromatic,and polycyclic aromatic hydrocarbons,which include halogenated and non-halogenated compounds,pesticides,and explosives.Inorganic pollutants may be metals such as Ag,Al,As,Be,Cd,Cr,Cu,Hg, Fe,Ni,Pb,Sb,Se,Zn,and radioactive elements and their deriva-tives͑Meagher2000;Allen2002͒.The principal concern associ-ated with contaminants is toxicity and health risk to humans.It is therefore essential to contain or mitigate these organic and inorganic contaminants so as to prevent them from contaminating surface and groundwater by dissolution or dispersion͑Gomez and Bosecker1999;McLaughlin et al.2000;Evans and Furlong 2003͒.Currently,various processes—physical,chemical,and biologi-cal either singly or in combination—are available to treat and manage these wastes.However,some uneconomical physico-chemical methods are slowly losing ground due to inherent prob-lems of secondary contamination and nonsustainable control of contaminants͑Vidali2001͒.Furthermore,stricter regulatory standards imposed by various countries on decontamination of contaminated sites have created an interest in bioremediation approach͑Hattan et al.2003͒.In particular the Resource Conservation and Recovery Act͑RCRA͒of1976promulgated with amendments in1986by the U.S.EPA ͑EPA1986͒regulate the generation,transportation,treatment, storage,and disposal of hazardous waste.The RCRA advocates adoption of ecofriendly remediation options like bioremediation or natural attenuation.Bioremediation involves enhanced degradation of toxic com-pounds by transforming them into innocuous substances,specifi-cally,carbon dioxide and water.The process can be carried out either on site͑in situ bioremediation͒by taking advantage of in-digenous microorganisms or introduction of bacterial or fungal strains and in bioreactors͑ex situ bioremediation͒to achieve com-plete detoxification of hazardous compounds͑Gibson and Sayler 1992;Rosenberg1993͒.Microorganisms are ubiquitous and possess incredible meta-bolic systems to degrade and utilize various toxic compounds as a source of energy for their growth via aerobic respiration,anaero-bic respiration,fermentation,and cometabolism.They possess characteristic degradative enzymes for biodegradation of respec-tive contaminants through aerobic and/or anaerobic/anoxic ually,aerobic bioremediation has higher degradation efficiency than anaerobic processes and is widely employed de-pending on the chemical nature of the contaminant͑Ahlet and Peters2001͒.Both aerobic and anaerobic processes can also be1Ph.D.Student,INRS-ETE,Universitédu Québec,490,Rue de la Couronne,Québec,Canada G1K9A9.2Ph.D.Student,INRS-ETE,Universitédu Québec,490,Rue de la Couronne,Québec,Canada G1K9A9.3Engineer Director,U.S.EPA,P.O.Box-17-2141,Kansas City, KS66117.4Scientist“E”,Naval Materials Research Laboratory͑NMRL͒, Defense Research and Development Organization͑DRDO͒,Ambernath 421506,India.5Professor,INRS-ETE,Universitédu Québec,490,Rue de la Couronne,Québec,Canada G1K9A9.E-mail:tyagi@ete.inrs.ca 6Post-Doctoral Fellow,École de technologie supérieure,1100,Rue Notre-Dame Ouest,Montréal,Québec,Canada,H3C1K3.7Professor,INRS-ETE,Universitédu Québec,490,Rue de la Couronne,Québec,Canada G1K9A9.Note.Discussion open until September1,2006.Separate discussions must be submitted for individual papers.To extend the closing date by one month,a written request must befiled with the ASCE Managing Editor.The manuscript for this paper was submitted for review and pos-sible publication on July6,2005;approved on July6,2005.This paper is part of the Practice Periodical of Hazardous,Toxic,and Radioactive Waste Management,V ol.10,No.2,April1,2006.©ASCE,ISSN1090-025X/2006/2-59–72/$25.00.sequentially utilized to reduce the toxicity or complexity of the contaminant.Rate of biodegradation can be further improved by use of contaminant adapted/acclimatized microorganisms and/or genetically improved bacteria at contaminated sites,referred to as“bioaugmentation”͑Quan et al.2004͒.Biodegradation can also be enhanced by adding essential nutrients to stimulate the growth of indigenous microorganisms,called“biostimulation”͑Trindade et al.2005͒.Bioremediation is a function of various factors—existence of a microbial population capable of degrading con-taminants,contaminant bioavailability to microbes and environ-mental factors such as temperature,pH,nutrients͑organic, inorganic,and their availability͒,electron acceptor͑s͒,redox po-tential,water activity,osmotic pressure,and concentration of contaminants͑Thakur2004͒.This review discusses various methods such as microbial/ bioleaching,fungal bioremediation,biosparging,bioventing,bio-stimulation,bioaugmentation,biologicalfixation,enzyme-catalyzed treatment,biosorption,and natural attenuation involved in bioremediation practices.Various methods entailed are based on either aerobic or anaerobic biotransformation processes.Bio-logical reactors,a direct simulation of bioremediation,have also been dealt with in a short manner.A brief summary of different bioremediation strategies for typical hazardous wastes is illus-trated in Fig.1.MetalsMetals form a large proportion of the pollution load of different contaminants present in the environment.Risks associated with pollution of soil by heavy metals for several years are an unavoid-able phenomenon͑Allen2002͒.In US,metals pollution ranges from45to70%of total pollutants in contaminated sites͑EPA 1997͒.In effect,Pb,Cd,Zn,Ni,Cr and As are on the list of ten common contaminants most often traced at the sites indexed by the Superfund program and Department of Defense͑EPA1997; Mulligan et al.2001͒.In order to advocate environmental poli-cies,most of the industrialized countries endowed themselves or are in the process of doing so,to establish laws and norms aiming at generic criteria on metal contamination in soils͑U.K.Department of the Environment1987;U.K.Department forEnvironment,Food,and Rural Affairs2002;EPA1992;Nedwedand Clifford1997;NATO/CCMS1998;Pronk2000;Abollinoet al.2002͒.However,there are many disparities in these reg-ulatory norms between countries depending on specific toxicityrelevance.In the1980s,metal remediation methods implied removalof contaminated sites and landfilling in cells for safety and/orconfinement of contaminants using physical barriers made upof steel,cement,or other impervious materials͑EPA1997;Papassiopi et al.1999;Mulligan et al.2001͒.Many of these tech-nologies no longerfind use due to their cost and their adverseeffects on the environment.This meant exploration of variousother methods,especially physicochemical methods like electro-acoustics;physicochemical separation͑gravimetric,flotation,magnetic separation͒based on hydrometallurgy principles;leach-ing with inorganic͑H2SO4,HCl,HNO3,etc.͒or organic ͑CH3COOH,etc͒acids;electrodeposition and electrocoagulation; membrane separation;solvent extraction;ion exchange;adsorp-tion and electrochemical methods͑Rumeau1989;Hinchee et al.1990;Rajeshwar and Ibanez1997;Duyvesteyn1998;Blais et al.1999,2001,2003;Cimino et al.2000;Juang and Wang2000a,b;Mulligan et al.2001;Wendt and Kreysa2001;Mercier et al.2002a,b;Fiset et al.2002;Arévalo et al.2002;Meunier et al.2002,2003,2004a,b͒.Despite the existence of a wide range of physi-cochemical methods,they are losing popularity owing to the costsand risks of secondary pollution associated with them.Thus,useof microorganisms like bacteria,fungi,and algae to treat environ-ments charged with heavy metals is an increasingly interestingavenue but with limitedfield application at present͑Kapoor andViraraghavan1995,1997͒.At this juncture,it would be appropri-ate to enumerate the role of various existing biological methods inmetal detoxification which contribute to bioremediation strategy. Microbial Leaching/BioleachingCertain microorganisms have immense metabolic capacity toallow passage of metals into solution catalyzed by microbial andmainly bacterial activity.This is called bioleaching,and requiresthe presence of microorganisms able to proliferate inextreme Fig.1.Summary of different bioremediation strategies for typical hazardous wastesecosystems͑strongly acidic pH,highly oxidizing conditions,high concentration of metal ions in solution͒and ready to draw their energy from oxidation of mineral sulfides͑Lundgren and Silver 1980͒.In fact,extraction of metals by biological solubilization has been extensively exploited and used for several years in bio-hydrometallurgy͑Torma1986͒.Direct oxidation of metal sulfides has been utilized for several metals:Cd,Ni,Zn,Co,Pb,Cu,Fe,Ga,Mn,and Sb͑Torma1986; Rossi1990͒.The indirect mode of metal sulfides involves oxida-tion by Fe2+ions that produce SO and metals in ionic form.The Thiobacillus then oxidizes SO to H2SO4and Acidithiobacillus ferrooxidans oxidizes Fe͑II͒to Fe͑III͒.The cycle regenerates with Fe͑III͒as summed up in the equation2Fe2++12O2+2H+→H2O+2Fe3+͑1͒Metal sulfides͑M S͒are oxidized by ferric ions asMS+2Fe3+→S0+M2++2Fe2+͑2͒Subsequently,elemental sulfur is reoxidized to sulfuric acid by Acidithiobacillus ferrooxidans or other species of Thiobacillus or sulfur-oxidizing bacteriaS0+1.5O2+H2O→H2SO4͑3͒The pH of the medium decreases and the redox potential in-creases contributing to solubilization of metallic oxides͑M O͒and carbonates͑M CO3͒MO+H2SO4→MSO4+H2O͑4͒MCO3+H2SO4→MSO4+H2O+CO2͑5͒Several bacteria,viz.,Thiobacillus ferrooxidans and Thioba-cillus thiooxidans,now called Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans,were regarded as prime players in the oxidation of metal sulfides͑Kelly and Wood2000͒.In the last decade,other bacteria like Leptospirillum ferrooxidans have also been found to oxidize ferrous ion and solubilize pyrites but with lower efficacy than T.ferrooxidans.The bacterium Thiobacillus cuprinus,isolated for thefirst time in Germany,oxidizes several metal sulfides with a notable preference for Cu sulfides͑Huber et al.1986;Huber and Stetter1990͒.The euryhaline species Thio-bacillus prosperus,with the ability to grow in the presence of6% NaCl,is able to oxidize reduced forms of sulfur,ferrous ion,and metal sulfides͑Huber and Stetter1990͒.Another species,Thioba-cillus plumbophilus,isolated from a uranium mine,possessed a unique metabolism,growing only on galena͑PbS͒,hydrogen sul-fide͑H2S͒,and molecular hydrogen as energy sources,and unable to grow by using ferrous ion or other reduced sulfur compounds ͑Drobner et al.1992͒.Metal sulfides as energy source have also been utilized by several species of moderate or extremely thermophilic bacteria like Acidianus brierleyi͑Sulfolobus brierleyi)at temperatures from45to75°C͒;Acidianus infernus͑proliferating between65 and96°C͒;and Metallosphaera sedula͑a facultatively thermo-philic chemoautotroph͒which oxidized elemental sulfur as well as ferrous iron or metal sulfides͑Zillig et al.1980;Segerer et al. 1986;Huber et al.1986͒.Interestingly,three bacterial strains distinct from Sulfobacillus thermosulfidooxidans͑standard strain thermotolerans asporogenes͒were also isolated for their ability to oxidize elemental sulfur,ferrous iron,and metal sulfides ͑Kovalenko and Malakhova1983;Golovacheva et al.1987͒. Recently,Dufresne et al.͑1993͒isolated Sulfobacillus(S.di-sulfidooxidans),able to use ferrous iron as energy source and grow on reduced inorganic sulfur compounds͑elemental sulfurand organo-sulfur compounds such as glutathione,cystamine,thi-anthrene,thiourea,cysteine,cystine,and benzothiazole͒.This wide array of microbial strains with distinct characteris-tics possesses an extraordinary ability to cope with metal stressesand clean the environment,in return aiding in maintaining thestatus quo of ecological cycles.Traditionally,thiobacilli or othersulfur-oxidizing bacteria have been employed in metal recoveryby bacterial leaching of ores and mining residues͑Torma1987;Morin and Ollivier1989;Tuovinen et al.1991͒with recent ex-tension of these studies into metal removal from waste streams ͑Blais et al.2004͒.Fungi-mediated metal leaching has been carried out by twodistinct types—white rot fungus͑saprotrophs͒and mycorrhizae ͑symbiotic association between plant roots and fungi͒.Typical fungal detoxification processes comprise heterotrophic͑chemoor-ganotrophic͒leaching by Penicillium simplicissimum to leach Znfrom insoluble ZnO-laden industrialfilter dust by induced pro-duction of citric acid͑Ͼ100mM͒͑Schinner and Burgstaller1989;Franz et al.1991,1993͒.Siderophore-mediated metal solu-bilization,where metal is adsorbed to biomass and/or precipi-tated,resulting inflocculation and decrease in bioavailability,isalso an alternate mechanism͑Diels et al.1999;Gadd2004͒.Fungi have also been known to carry out biomethylation of toxicarsenic complexes to volatile dimethyl͓͑CH3͒2HAs͔or tri-methylarsine͓͑CH3͒3As͔and also other redox transformations ͑Tamaki and Frankenberger1992͒;Gharieb et al.1999;Lovley 2000͒.Moreover,reduction of Hg͑II͒to Hg͑0͒by fungi resultsin diffusion of elemental Hg out of cells,which can be utilizedto mobilize Hg from contaminated soil sites͑Silver1998;Hobman et al.2000͒.Ectomycorrhizal͑ECM͒fungi have also been known to in-crease availability of metals in the rhizosphere by solubilizingminerals,including metal-containing rock phosphates,via pro-duction of organic acids or proton extrusion͑Leyval et al.1997͒,typically in acidic soils͑Bradley et al.1981,1982͒.In fact,excesstranslocation of metals within mycorrhizal fungi exerted moretoxicity on fungal biomass,indicated by decline in propagule den-sity and infectivity in metal-polluted soils.This makes them goodbioindicators of soil contamination͑Grodzinskaya et al.1995͒.Moreover,mycorrhizal colonization of plant roots in a contami-nated soil can be a sign of metal detoxification/nonbioavailability.Additionally,fungi could also perform metal/metal-complexsorption to cellular surfaces,and even cationic species can beaccumulated within cells via membrane transport systems ofvarying affinity and specificity͑White et al.1997;Gadd andSayer2000͒.There are numerous studies documented on free-living,pathogenic and plant symbiotic fungi associated with for-mation of calcium oxalate crystals from solubilized calcium ͑Gadd1999͒.Other than calcium,fungi can also produce other metal oxalates and metal-bearing minerals,e.g.,Cd,Co,Cu,Mn, Sr,and Zn͑White et al.1997;Gadd1999;Sayer et al.1999͒. Biodegradation of metallocyanide complexes by mixed fungi cultures of Fusarium solani,Trichoderma polysporum,F.oxy-sporum,Scytalidium thermophilum,and Penicillium miczynski have been successfully investigated by Barclay et al.͑1998͒.Hence,processes based on various fungal strains couldprovide efficient management and/or degradation of toxic pollut-ants and/or its derivatives.However,for a practical and sustain-able approach,proper measures should be taken before adaptingfungal bioremediation processes singly or integrated with othertechniques.Metal SpeciationBenign metal speciation in environment by fungi is a widely stud-ied and implemented bioremediation mode͑Lovley and Coates 1997;Eccles1999͒.This mechanism is associated with mobiliza-tion or immobilization processes that control transportation of metal species between soluble and insoluble phases͑White et al. 1997,1998;Sreekrishnan and Tyagi1994;Vachon et al.1994͒. BiosorptionThis method has been employed as an effective and economical alternative to conventional methods of detoxification and recov-ery of toxic or invaluable metals from industrial wastewater ͑Aksu and Açikel1999͒.It entails use of live or dead biomass and/or its derivatives to adsorb metal ions with ligands or func-tional groups located on external microbial surfaces.In this re-spect,several yeasts,algae,fungi,bacteria and some other aquatic species have been isolated and characterized͑Brierley1990; Prasetyo1992;V olesky and Holan1995;Aksu and Açikel1999; Vieira and V olesky2000͒.Various powerful commercial bio-sorbants such as BIO-FIX,AMT-BIOCLAIM,and AlgaSORB have also been already developed͑Brierley1990͒.The biosorbant BIO-FIX is made up of high-density polysul-fone balls with the ability to dissolve in dimethylformamide ͑DMF͒.Biomass of algae,yeasts,and bacteria is killed thermi-cally,dried,and mixed with a polysulfone-DMF solution to form balls.On biosorption,elution of metals contained in the balls was carried out using dilute mineral acids͑Jeffers et al. 1989;Brierley1990;Jeffers and Corwin1993͒.Similarly, AMT-BIOCLAIM comprises Bacillus bacteria,treated in a strong caustic solution,washed with water,and immobilized as porous balls on polyethyleneimine and glutaraldehyde͑Brierley1990; Brierley and Brierley1993͒.The biosorbant AlgaSORB contains alga,Chlorella vulgaris and others,killed and immobilized in a silica gel matrix͑Darnall et al.1989;Brierley1990͒.The most commonly used immobilizing agents or matrices are alginate, polyacrylamine,polysulfone,silica gel,cellulose,and glutaralde-hyde͑Atkinson et al.1998͒.Removal of metal species by biosor-bants was influenced by several parameters—the specific surface area of the biosorbant and physicochemical parameters of the solution like temperature,pH,initial metal ion concentration,and concentration of biomass.Other factors influencing the presence of more than one metal species are the number of metal ions competing for adsorption sites,the respective concentrations of each metal species,interactions between species,and residence time͑Aksu and Açikel1999͒.For reasons of economics,biomass of industrial waste origin, for example,Saccharomyces cerevisiae yeast and Rhizopus arrhizus from citric acid production industries,offers a very in-teresting alternative͑Atkinson et al.1998͒.Filamentous soil fungi such as Aspergillus niger,Mucor rouxxi,Rhizopus arrhizus,and Trichoderma viridae have also been used as commercial biosor-bants of potentially toxic elements͑Kapoor and Viraraghavan 1995;Gonzalez-Chavez et al.2002,2004͒.Moreover,regeneration and reuse properties of the biomass, acquisition,and immobilization cost in a matrix or an easily re-coverable support,are also very important factors.In conclusion, biosorbants seem to offer an interesting commercial prospect for removal of metals from industrial effluents with scope forfield application.PhytoremediationThis technology,which made its debut in the1990s,comprises growing plants on contaminated sites so that polluting compo-nents percolate through the radical system of the plants and ac-cumulate in various parts͑roots,stems,leaves,etc.͒.Plants have a natural capacity to accumulate essential heavy metals͑Fe,Mn, Zn,Cu,Mg,Mo,and Ni͒from soil or water for their growth and development.Certain plants can also accumulate heavy metals with unknown biological functions such as Cd,Cr,Pb,Ag,As, and Hg.The in situ͑on-site treatment͒phytoremediation has extensive commercialization in Europe and the United States͑Meharg and Cairney2000;Gaur and Adholeya2004͒.This technology is lim-ited as contamination of soil should not exceed a certain depth so that the roots of the plant are in contact with the metal pollutants.Moreover,climatic conditions and the bioavailability of metalsare pertinent factors to be evaluated.Similarly,it often takes alonger time period to decontaminate a site due to the limitedgrowth rate of a selected plant species and confinement to the areacovered by roots.It could also be necessary to proceed throughseveral cycles of culture and harvest to restore a site completely.Lastly,once contaminated,vegetation must be disposed of in anappropriate manner͑Mulligan et al.2001͒.Other InorganicsThe bioremediation of inorganic compounds is well com-prehended by involving mechanisms such as bioassimilation,bio-degradation,biosorption,biomagnification,bioaccumulation,biotransformation,and biovolatilization͑Zumriye2005͒.Bio-chemical pathways and microbial metabolism have also beenused for removal of another class of toxic compounds—cyanides ͑Stephen2004͒.It specifically takes place by hydrolytic,oxida-tive,reductive,and substitution/transfer reactions͑Raybuck1992;Dubey and Holmes1995;Adjei and Ohta1999;Yanese et al.2000;Barclay et al.2002;Kwon et al.2002;Ezzi-Mufaddal andLynch2002͒.Radioactive environmental waste sites left over from nuclearweapons production during the cold war are a gigantic challengeto decontaminate as reported by the U.S.Department of Energy ͑DOE͒͑Macilwain1996;McCullough et al.1999͒.However, bioremediation can play a pivotal role in decontamination of radioactive sites.The bacterium Deinococcus geothermalis ͑Ferreira et al.1997͒is remarkable not only for its extreme re-sistance to ionizing radiation but also for its ability to grow at temperatures as high as55°C͑Ferreira et al.1997͒and in the presence of chronic irradiation͑Daly2000͒.Given the need to develop bioremediating bacteria for treatment of radioactive high-temperature waste environments,D.geothermalis and D.murrayi were tested for their transformability,and many genetic modifi-cations have been made to enhance their efficiency͑Liu et al. 2003;Brim et al.2003͒.Several models and experimental sys-tems have also been studied to enhance bioremediation of radio-nuclides͑McCullough et al.1999;Wang et al.2003;Cazzola et al.2004͒.A bacterium,Geobacter sulfurreducens,possesses ex-traordinary capabilities to transport electrons and”reduce”metal ions͑even radioactive substances͒as part of its energy-generating metabolism͑“Bacterium”2003͒.Several ecoremediation tech-nologies based on biological methods like wild plants,known as hyperaccumulators͑Watanabe1997;Dushenkov et al.1999͒, genetically engineered plants͑Pena and Seguin2001;Singh et al.2003͒,fungi ͑Gray 1998͒,and natural ͑Entry and Watrud 1998;Watanabe 2001͒or genetically modified microrganisms ͑Lasat et al.1998;Lange et al.1998;Pieper and Reineke 2000;Sayler and Ripp 2000͒have been developed for radioactive decontami-nation.Extended studies on genetically engineered strains may hold future prospects for in situ degradation or detoxification of radioactive sites.OrganicsOrganic contaminants like pesticides,organochlorines,polychlo-rinated biphenyls ͑PCBs ͒,polycyclic aromatic hydrocarbons ͑PAHs ͒,synthetic dyes,wood preservatives,munitions waste and synthetic polymers can be either degraded or converted into less toxic forms by bioremediation ͑Pointing 2001;Evans and Furlong 2003͒.This bioremediation could be mediated by bacteria,fungi,a cocktail of microorganisms,or plants or even a combination of all ͑U.S.Geological Survey 1997;Nealson 2003͒.In most cases,xenobiotics are extremely resistant to biodegra-dation by native flora and fauna ͑Fernando et al.1990͒.At this juncture,ligninolytic fungi have impeccable ability to cause deg-radation due to their nonspecific enzyme system ͑Novotny et al.2004͒.These organics have been further elaborated and are dis-cussed in subsequent sections.The significance of bioremediation as a major decontamination strategy can also be gauged from Fig.2where the percent use of different commonly practiced remediation technologies are evaluated based on vendor supplied data ͑EPA REACH IT 2004͒.Fungal BioremediationSynthetic dyes/pesticides/PCBs are introduced into the environ-ment by the agricultural,sanitization,textile dyeing,paint,refin-ery,and electrical industries ͑Meharg and Cairney 2000;Novotny et al.2001͒.These contaminants are of great environmental concern because of their abundance and toxic,carcinogenic,and teratogenic effects on animals and humans.Specifically,pesticides are linked to toxic effects and popula-tion declines at higher trophic levels ͑Alloway and Ayres 1993͒.Fortunately,bacteria and several soil fungi ͑e.g.,Fusarium ,Peni-cillium ͒are now known to degrade pesticides with greater effi-cacy ͑Twigg and Socha 2001͒.While most soil microorganisms are ubiquitous and occur in a variety of moist soils,fungal species possess higher efficiency of pesticide degradation even in arid and semiarid soil conditions ͑Baarschers and Heitland 1986;Bumpus et al.1993;Twigg and Socha 2001͒.Additionally,highly recalci-trant pesticides like the chlorinated triazine herbicide 2-chloro-4-ethylamine-6-isopropylamino-1,3,4-triazine ͑atrazine ͒have been transformed by the white-rot fungi ͑WRF ͒P .chrysosporium and Pleurotus pulmonarius ,yielding hydroxylated and N -dealkylated metabolites ͑Masaphy et al.1993;Mougin et al.1994;Beaudette et al.1998;Van Acken et al.1999͒.Moreover,white-rot fungi come equipped with a panoply of enzymes,particularly peroxi-dases,giving them an edge over bacteria,which require preconditioning/acclimatization to grow in any recalcitrant me-dium ͑Barr and Aust 1994͒.On the other hand,even the recom-mended dose of a pesticide in fields might be deleterious to a newly reclaimed calcareous soil with low populations of ectomy-corrhizae ͑Abd-Alla et al.2000͒.Despite this limitation,a role for ectomycorrhizae cannot be ruled out in degradation of several synthetic dyes,pentachlorophenol,endosulfan,and DDT ͑Ryan and Bumpus 1989;Yadav and Reddy 1993͒.The presence of healthy consortia of microorganisms in soil assures that certain pesticides can be used safely in the environment,albeit under the recommended application dose.Despite the safe usage,more studies need to be carried out at the molecular level to ascertain the specific enzymes participating in pesticide degradation ͑Shimazu et al.2001͒.Nonetheless,this has not been an impediment in developing WRF-based soil bioremediation of PCBs ͑Van Acken et al.1999;Pointing 2001͒.Novotny et al.͑2001͒successfully demonstrated in vivo degradation of a broad selection of recalcitrant com-pounds ͑dyes,polyaromatic hydrocarbons,PCBs ͒under a variety of conditions,except that the aforestated enzyme levels must be sufficiently high.Several WRF-like organisms like Phanero-chaete chrysosporium,Bjerkandera adusta,Pleurotus ostreatus ,and Trametes versicolor have the innate ability to degrade low-chlorinated congeners of PCBs with or without stereoselectivity ͑Yadav et al.1995;Beaudette et al.1998;Kubatova et al.2001͒.Mycorrhizal fungi,in particular ectomycorrhizae and ericoids,are efficient remediators of a wide range of PCBs ͑Donnelly et al.1993;Meharg et al.1997;Green et al.1999͒.Ligninolytic fungi are able to aerobically degrade a wide va-riety of recalcitrant organic pollutants,including various types of dyes like azo and triphenylmethane dyes,anthraquinone,phthalo-cyanine,and heterocyclic dyes ͑Ollikka et al.1993;Paszczynski and Crawford 1995;Swamy and Ramsay 1999;Novotny et al.2001;Pointing 2001;Baldrian 2004͒.Other than the organism P .chrysosporium ,Pycnoporus sanguineus,Bjerkandera adusta,Trametes hispida ,and Pleurotus eryngii have also been shown to have accounted for decolorization of dyes ͑Heinfling et al.1998;Rodriguez et al.1999;Kasinath et al.2003͒.EventheFig. 2.Different technologies adopted for decontamination of hazardous wastes ͑data derived from EPA REACH IT 2004͒。