Study On Removal Of Hexavalent Chromium In Groundwater Using Nano-FeS

As an important water source for municipal,agricultural and industrial,groundwater plays an important role in safeguarding the living water of urban and rural residents,supporting social and economic development and maintaining the ecological balance.Because of the complicated hydrogeological conditions of groundwater,the treatment and remediation of it are of difficult,high cost and long period,once polluted,the environmental and ecological damage is often difficult to reverse.Chromium is of high vertical migration ability in the vadose zone,once migrates into the aquifer,under the influence of the hydrodynamic condition,migration-dispersion effect may take place,causing widespread pollution of groundwater and seriously affecting the human health and ecological environment.Thus,chromium contamination should never be ignored,and the causation of groundwater contamination has aroused increasing attention.In situ reaction zone technology mainly injects reaction reagent or microbes into underground environment,and the reaction zone is thereby created through the reaction of injected reagent and contaminant,which could effectivelytackling,fixing or degradingcontaminants.The main cost of in situ reaction technology is the construction of injection wells,and expensive infrastructure construction costs are avoided;and the operation cost is relatively low,while the repair range is not limited to the depth of plume,and has less interference on the surrounding environment.In the actual site remediation,the right choice of reaction reagentis the foundation for a successful reaction zone,and optimizing the biogeochemical processes in the underground environment can greatly improve the repairing effect for contaminants.In recent years researchers have proposed nano-FeS as the reductant for the in situ remediation of chromiuM contaminated groundwater,while nanoparticles can be oxidizedeasily,and the mobility in the underground environment is limited,so the modification of nano-FeS for enhancing its stability and mobility is of great importance for further application.This study is based on the application of namo-FeS in remediatingchromium contaminated groundwater,nano-FeS and CMC stabilised FeS(CMC-FeS)was synthesized followinghomogeneous precipitation method,and the stability and reactivity were comparedfor its further applicability as in situ reaction reagent in chromium-contaminated groundwater remediation.Meanwhile the influence of Cr(Ⅵ)removal by Fe(Ⅱ),S(-Ⅱ)and the mixing solution of Fe(Ⅱ)and S(-Ⅱ)was investigated,the product of nano-FeS(CMC-FeS)and hexavalent chromium was also detected for the analysis of the reaction mechanism,thus providing theoretical foundation for the application of FeS in chromium groundwater remediation.Research results obtained are as follows:The appearance of synthesized nano-FeS is filamentous with a length between 20 to 50 nM,and it has good oxidation resistance but it might settle down in a short time,exhibiting poor stability.The shape of synthesized CMC-FeS is nearly spherical with a particle size distribution in the range of 20 to 50 nm,the nanoparticles also has good dispersibility,oxidation resistance and stability,which implies a promising application in the remediation of groundwater cantamination,FeS type,pH,reaction temperature and initial Cr(Ⅵ)concentration influence the reaction of nano-FeS and Cr(Ⅵ).The removal of freshly prepared FeS suspension is 7.88,2.71 and 1.02 times compared with commercially availableFeS powder,dried FeS,and CMC-FeS suspension respectively.pH mainly affects the dissolution of FeS(CMC-FeS)and the presence form of chromium,resulting difference of removal efficiency and the reaction mechanism as a function of pH conditions.The calculated Ea of the reaction was 44.88kJ/mol for nano-FeS(38.42kJ/mol for CMC-FeS),which indicates a chemical controlled process rather than diffusion controlled reactions to some extent.When the dosage of nano-FeS(CMC-FeS)is certain,the reaction rate and removal of Cr(Ⅵ)drops rapidly with the increase of initial Cr(Ⅵ)concentration.In the preparation process,the addition of CMC hindered the contact between Cr(Ⅵ)and Fes,and the filamentous FeS was more easily to contact with Cr(Ⅵ)compared with the spherical CMC-FeS,but the removal of CMC-FeS and FeS suspension was similar within 2h,considering the well oxidation resistance and stability,CMC-FeS may serve as an more preferable in situ injection reagent in chromium contaminated groundwater remediation.The addition of CMC decreased the Ea,making the reaction much easier to happen.In the practical site remediation,the reaction reagent dosage should be determined according to the contaminant concentration of Cr(VI),in order to get considerable remediation effect.At pH 7.0-9.0,the removal of Cr(VI)by the mixing solution of Fe(II)and sulfide was observed to be more effective than the summing of separate ferrous and sulfide ions at pH 7.0～9.0,the promoting effect was presumed by the catalysis of ferric iongenerated during the reduction process.The solubility of FeS(CMC-FeS)is greatly affected by the solution pH.At pH<3,≤ FeS could dissolve Fe2+ and S2-adequately,both as a strong reductant for Cr(Ⅵ),and the products were iron and chromium oxide;When at pH 5-12,Cr(VI)firstly adsorbed on the surface of FeS,and then be reduced.The products were the coprecipitation of iron,chromium and sulfur;Cr(VI)mainly exists in the form of CrO42-at pH≥12,and the pH of the solution is far overweigh the isoelectric point of FeS(-7.5),the electrostatic repulsion between Cro42-and FeS is not conducive for the adsorption of Cr(VI)on FeS surface,causing the decline of Cr(VI)removal.