Remediation of soil contaminated with heavy metals-A Review

Remediation of soil contaminated with heavy metals-A Review

Abstract - Soil contamination is a major problem arising worldwide as result of speedy urbanization and industrialization.Theunorganizeddischargeofdomesticand industrialeffluentcontaminates soilandwaterwithvariety of toxic chemicals. The presence of toxic metals in soil and waterarehazardousforecosystem.Chemicallycontaminated soils are not suitable for agriculture and construction purposes due to their poor physical and chemical properties. Such soils need remediation before use for any application. Various methods are available for decontamination of such soils for soil improvement. However, methods for removal of toxic heavy metals need more research inputs. The Chemical remediation of soil contaminated with hazardous heavy metals by washing with chelating agents was reported by many researchers. This review presents the different methodology adopted by researchers for remediation of soil contaminatedwith heavy metals.

Key Words: Soil Contamination, Soil Remediation, Toxic HeavyMetals,SoilWashing,Chelation.

1. INTRODUCTION

Soil and water get polluted with heavy metals due to industrialprocessesandimproperdisposalofgarbage[1,2].

Arsenic, cadmium, chromium, copper, lead, nickel and mercury are generally present in polluted soil and water. Theseheavymetalsaretoxictohumanhealthandecosystem [3,4].Heavymetalmaygetsweptawayfromcontaminated areas to new areas by wind and rain. Prevention of contamination of soil with heavy metal is very essential becausecleaningofcontaminatedsoilsisverydifficultand expensive.

Methodsforremediationofcontaminatedsoilforreducing theharmfuleffectsofheavymetalincludeexcavation,insitu remediation, phytoremediation and chemical extraction [5,6].

2. METHODOLOGIES

Differentmethodologiesofsoilremediationarediscussed herebasedonliteraturesurvey.Excavationandremovalof thecontaminatedsoilisarapidexsitu remediationmethod bywhichcompleteremovalofthecontaminantsispossible [10]. Disadvantage of excavation method is that the contaminantsaresimplymovedtoadifferentplace,where theywillcontaminateanothersiteaswellasduringremoval and transport of contaminated soil there is risk of

contaminated soil falling to other places. Excavation is a costlyprocessandcostincreasefurtheriflargeareaistobe excavated.

Somein-situremediationmethodsarebeingusedwhich donotremoveheavymetalbutimmobilizetheminsoil[7]. OnincreasingpHofsoilto6.5orhigher,solubilityofcationic metals get decreased which makes them less available to plants thus blocking their entry in food chain but on increasingpH,solubilityof anionicmetalsisincreasedwhich makes them more available to plants. Drained soil have improvedsoilaerationwhich allowmetalstogetoxidized and makethemlesssolubleinwaterthuslessavailableto plantsbutchromiumismoreavailabletoplants inoxidized forms. Availability of chromium to plants is reduced on gettingittreatedwithorganicmatter.

Availability of cationic metals to plants get reduced on applicationofphosphatetocontaminatedsoilbecauseitreact with cationic heavy metal to form metal-phosphate compounds.Duetoextremelylowsolubilitypotentialvalues of metal-phosphate compounds, these compounds do not enterthefoodchainbygettingabsorbedintoplantsaswellas thesecompoundsdoesnotgetleachedtothegroundwater. Phosphate-basedmetalstabilizationisaveryrapidandeasy toapplyprocess.Phosphatereagentcanbeappliedinawet ordryformandcanbeusedtostabilizemetalsinsituorex situwithin24to48hoursofapplication.Radionuclidessuch asthorium,uranium,radiumandcesiumwerealsoconverted to safe and stable metal-phosphate compounds. Waste streams toxicity levels (leachability) of metal phosphate compoundsofHg,Se,Zn,Pb,Ba,CrandCd arereportedtobe below Universal TreatmentStandard (UTS) and Resource conservation&recoveryact(RCRA)regulatedlevelssothese metal phosphate compounds can be left on the site rather thantransportedoffthesitetoahazardouslandfill.Costof in- situ fixation of heavy metal is about half the cost of excavation and disposal of soil contaminated with heavy metal becauseitsavesontransportationanddisposalcost [8,9].

EPA(EnvironmentalprotectionagencyofUSA)approved phosphate-based chemical stabilization process is commercially vailable non-hazardous method that permanently immobilizeawiderangeofheavymetals.The treated soils contain extremely stable metal-phosphate compounds which are stable to long-term pH related deterioration.EPA’sToxicCharacteristicLeachingProcedure (TCLP) test and Multiple Extraction Procedure (MEP) test

measurelong-termstabilityofmetal-phosphatecompounds because it simulates the long term leaching effect of environmental exposure of stabilized metals. Ten MEP extractionssimulate1,000yearsofmetalsstabilityandeach TCLPextractionsimulates100yearsofstability.MEP,TCLP, Synthetic Precipitate Leaching Procedure (SPLP), redox potential, bioavailability and Germany’s DIN ( Deutsches Institute fur Normung) leachate test established the effectivenessofthismetalsstabilizationprocess.

Availabilityofanionicmetalssuchasarsenictoplantsget increasedonapplicationofphosphatetothesoilsoanionic metals can not be fixed by phosphate-based chemical stabilizationprocess.Asisclearfromthediscussion,suitable immobilization technique can be applied only if nature of heavymetalpresentinsoilisknown.

Phytoremediation of contaminated soil for removal of heavymetalsorconvertingheavymetalstoharmlessformis donebyusingvegetationandmicrobiota.Phytoremediation atacontaminatedsitecanbepossibleonlyifplantusedfor phytoremediationcansurviveinthatclimateandistolerant topresenceofheavymetalsinsoil.Costofphytoremediation is less than the cost of excavation or in situ fixation but processofphytoremediationtakelongertimetoaccomplish thanothertreatmentmethods[12].Phytoremediationisvery efficientwhenheavymetalsinlowtomediumconcentrations arespreadoverawidearea.Phytoremediation canbeused inconjunctionwithotherremedialmethods.

Chemicalextractionorsoilwashingis ex-situtechnique inwhich anaqueous solution of extractingagentisaddedto soilcontaminatedwith heavymetals [11].Soilwashingis very cost-effective and ensures rapid cleanup of a contaminated site by completely removing heavy metals [10,13].

Washing of soil with water can remove only small amountofheavymetalsbecausetheyoccurinsorbedstate and are sparingly soluble in water [14,15]. For effective removalof

heavy metals, some chemicals are to be added to the washingwaterwhichcan transferheavymetalsfromthesoil into solution [16].Chemical used for soil washing mustbe capableofbreakingstrongbondsbetweenthesoilandmetals and dissolving the metals. Surfactants, cyclodextrins, chelatingagentsand acidsare being used for soil washing [17-23]. Suitability of chemical used for effective soil washingisdependentontypeof soiland heavymetal.The identificationandquantificationof heavymetal inthesoil beforeandaftersoilwashing isnecessarytodesignefficient soil washing procedure [24]. Strong acids (nitric, hydrochloric) used as extracting agents for soil washing dissolve clay mineral and degrade the soil crystalline structure therefore organic acids (oxalic, citric, formic, acetic, succinic, maleic, lactic, fumaric acids) and chelating agents (ethylene-diaminetetraacetic acid (EDTA),

diethylenetriaminepentaaceticacid(DTPA), nitrilotriacetic acid(NTA),citricacid,ferricchloride)are beingpreferredas alternativestomineral acid[25-31].

Whensoilcontaminatedwithheavymetaliswashedwith aqueoussolutionofchelatingreagent,it bindwithmetalto form complex. Complex of chelating reagent and heavy metal isalsocalledmetalchelatewhichis soluble inwater therefore metal get removedfromthecontaminated soil [32-37].EDTAiswidelyusedforwashing ofcontaminated soil becauseofitsabilitiestoformwatersolublecomplexes withalmostallheavymetal [38-47].

Ferricchloride(FeCl3)andEDTAwereusedbyBilginetal. [48] toextract Cu,CdandZnfromthecontaminatedsoil. After1hofsoilwashing, 90.57%ofCd, 73.22%ofCuand 99.59%of Zn wasremovedfromcontaminatedsoil using 0.01MEDTA aswashingsolutionwithliquid/soilratioof20. UdovicandLestanreportedthat73%ofPb,23%ofZnand 74 % Cd was removed from contaminated soil by using EDTA aswashingsolution[49].

The best removal efficiencies for Cd, Cu and Zn were obtainedwhensoilwaswashedwith0.01MFeCl3aswashing solutionwithliquid/soilratioof 20 on rotaryshakerat 200±5rpmfor 2h. Onincreasingwashingtimefrom0.5to2 hpercentremovalofCdincreasedfrom83.14to 98.18.On increasingwashingtimeto2h percent removalofCuandZn alsoincreasedto97.48%and98%. Limetal.[50]reported that95%ofPb,89% ofCdand90%ofNi wasremovedfrom contaminatedsoil.

Soil washing with EDTA and FeCl3 solutions is environmentfriendlyanddueto highliquid/soilratioof20, removal efficiency for removal of Cd, Cu and Zn from contaminatedsoilsgotimproved.RemovalefficiencyofEDTA for removal of Cd, Cu and Zn from contaminated soil was lowerthanthatofFeCl3 butEDTAprovidedshorterwashing timethanFeCl3 forthe removalprocess.

ExtractionofCdfromcontaminatedsoilbysoilwashing with aqueous solution of Calcium chloride and FeCl3 is reportedtobe veryefficient,cost-effectiveandenvironment friendly [51-53].Makino et al.,reportedthat soilwashing withFeCl3 wasmoreeffectivethanwith calciumchlorideor hydrochloricacid[52].

Keetal.reportedthat tartaricacidisanenvironmentallyfriendly extractant for remediation of heavy metal contaminatedsoils.Tartaricacid, inthepHrangeof3.5-4.0, waseffectiveforremovalof50%-60%ofCd,40%-50%ofPb, 40%-50%ofCuand20%-30%ofZnfromthecontaminated soilwithin24h.HighermasstransfercoefficientsforCdand PbthanthoseforCuandZnmadetheremovalofCdandPb moreefficient[54]. Labastidaetalreportedthat44%lead wasremovedfromsoilonremediationwithEDTA. Soilafter remediation was having a final concentration of 363.50 ± 43.50mg/kg whichisbelowUSEPAstandard[55].

Jiangetal.extractedcopperandnickelfromcontaminated soil by washing it with chitosan (biodegradable chelating agent),EDTAandsodiumcitrate[56].Extractionefficiencyof threechelatingagents forremovalofcopperandnickelfrom the contaminated soil decrease in the order : chitosan > EDTA > sodium citrate. Extraction efficiency of any of the chelatingagentswashigherfor nickelthanforcopperwhich is consistentwiththerelativestabilityof chelatecomplexes of these twometals.Efficiencyofmetalextraction bysoil washingisdependentonpH of solutionofchelatingagent. Extraction efficiency of chitocan solution for removal of copperandnickel fromthecontaminatedsoil increased with decreasing its pH and extraction was most efficient when its pH was 3 - 3.5. Availability of heavy metal is increasedinacidiccondition.AtpH3-3.5,43.36%ofcopper and37.07%ofnickelwereextractedfromcontaminatedsoil. Extraction efficiency of heavy metal is increased on increasing concentration of chelating solution. Percentage removal of nickel from contaminated soil increased from 56.22% to 63.34% whenconcentrationofchitosansolution increased from 0.1- 0.4g/l.Theremovalofcopper from contaminated soil increased from 30.99% to 43.49% on increasing concentrationof chitosansolutionfrom0.1to0.3 g/l and percentage ofcopperremovaldidnotincreaseon further increasing the concentration of chitosan solution beyond 0.3g/l. After 2 h of washing of contaminated soil with chelating solution, 38.72% of copper and 51.25% of nickelwereremoved.Extendingthereactiontimefrom2hto 8 h did not affect the extraction efficiency significantly. A reactiontimeof2hwaschosenas theoptimalreactiontime for extractionofcopperandnickel.Extractionefficienciesof copperandnickelincreasedonincreasingliquid/soilratio. Whenchelatingsolution/soilratiowasincreasedfrom5to 20,theremovalofcopperincreasedfrom26.34%to47.22%, while the removal of nickel increased from 41.75% to 66.13%. Liquid/soil ratio was increased by adding same concentration(0.2g/L)ofchelatingsolutiontocontaminated soil which means that chitosan amount was increased on increasingliquid/soilratio.Highliquid/soilratiocanprevent cloggingofthesoilduringwashing[33]butgeneratelarge amount of wastewater, which would increase the cost soil washing. Optimum condition for removal of nickel and copper from contaminated soil were obtained by adding chitosansolutionofconcentrationof0.3g/landliquid/soil ratioof10ml/gto1gofsoilsampleandwereagitatedata speedof120rpmatroomtemperaturefor2hours.

TheremovaloftoxicmetalssuchasPb,Zn,CuandCdby soil washingwithchelatorsisa viableremediationoption. Ethylenediamine tetraacetate (EDTA) is by far the most tested,strongchelatorthateffectivelyremovestoxicmetals fromthesoil.ThereisconcernthatEDTA-toxicmetalchelates could leach from remediated soil and contaminate the groundwater[57].Readilybiodegradablechelatorssuchas N,N-bis (carboxymethyl) -l- glutamate(GLDA), iminodisuccinate(IDS),3-hydroxy-2,2′-iminodisuccinicacid (HIDS), S,S ethylenediamine-disuccinate (EDDS),

methylglycinediacetate(MGDA),andnitrilotriacetate(NTA) havebeenintroducedintosoilwashingandarebeingtested aspossiblesubstitutes[58-60].

3. CONCLUSIONS

Differentapproacheshavebeenproposedintheliterature to assess the sequestering ability of ligands toward metal cations. This review aims to give a methodological contributioninthechoiceofchelatingagentsfor soilwashing remediation based on literature survey. The methods here proposedallowsdeterminingboththesuitabilityoftheligand for scavenging the target metal ions, and the minimal concentrationofligandtobeused,minimizingtheadverse effectsonessentialmetalions.Thisisparticularlyrelevant whenstudyingtherealapplicationofawashingligandtoa specificsoil,knowingthetotalconcentrationsofmetalstobe removed before the experimental determination of its behavior.Theremediationofmetal-contaminatedsoilsusing synthetic chelates for soil washing and for enhancing phytoextraction by plants has become one of a number of wellstudiedclean-uptechniquesinthelasttwodecades.

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