Removal And Remediation Of Arsenic In Arsenic-containing Sediments By Microbial Reduction

Arsenic is a toxic metalloid element widely found in nature,mainly in the form of realgar,orpiment,arsenopyrite and other sulfide or associated with copper,lead,zinc and other mines.These arsenic-containing minerals were found in Hunan,Guizhou,Xinjiang,Sichuan,Inner Mongolia and other provinces.As a result of human activities such as the exploitation,smelting,processing and discharge of arsenic-containing mineral resources,arsenic is highly enriched in soil and groundwater,especially arsenic pollution in industrial tailings,which seriously worsens the quality and safety of soil in nearby farmland.The accumulation of high arsenic concentrations in the soil will be partially absorbed by crops growing on it,and the entry of crops into the human food chain will harm human health,leading to cardiovascular disease,lung cancer,kidney cancer and other chronic diseases and even cancer.Arsenic absorption efficiency of rice plants is higher than that of other grain crops.Arsenic pollution in paddy fields has become an important pollution problem affecting human health.In this study,the flooding condition of paddy soil was simulated,and the direct action of two kinds of reducing microorganisms and the synthesis of the mineral ferrous sulfide were used to effectively remove arsenic.Therefore,in this paper,firstly,the effects of sulfate reducing bacteria（SRB）and iron reducing bacteria（Shiwanella,IRB）on soil samples of arsenic-containing ferrihydrite were used to evaluate the enhancement of the migration ability of arsenic from arsenic-containing ferrihydrite to solution.Then,the biological nano ferrous sulfide（FeS）was synthesized by SRB and iron reducing bacteria（JF-5）.The production and stability of FeS were studied,and the migration and adsorption capacity of FeS were discussed.The main results are:（1）The action of SRB and IRB released more arsenic fixed in arsenic-containing ferrihydrite into the solution,and the total amount of arsenic released was much higher than that of pure water,about 500 times higher.Different microbial action sequences could change the species and total amount of arsenic released,and SRB followed by IRB treatment（the IRB-SRB）had the highest total arsenic release,the single-cycle release was 2.28 mg·g^-1.In the microbial injection column experiment,the total arsenic release was increased to 10.87 mg·g^-1（SRB and IRB acting simultaneously,the SRB+IRB）,and the total arsenic release effect under the other treatments was IRB-SRB(7.54 mg·g^-1),SRB-IRB(IRB followed by SRB,6.48 mg·g^-1)and control group(pure water,0.01 mg·g^-1).The reduction dissolution of arsenic-containing ferrihydrite is the main cause of arsenic release,and the FeS formed during the experiment could affect the release of arsenic.Attenuated total reflectance-Fourier transform infrared spectroscopy（ATR-FTIR）confirmed that the vibrational bands of arsenate(AsO₄^3-,872cm^-1)and arsenite(AsO₃^3-,790 cm^-1)in arsenic-containing ferrihydrite were weakened or invisible with the injection of microbial.X-ray photoelectron spectroscopy（XPS）results further showed that,compared with the original arsenic-containing ferrihydrite,the total arsenic of the samples was reduced to 20%-58%after biological reduction,and the SRB+IRB treatment had the best effect（58.07%）.It showed that the experimental treatment of microbial enhanced arsenic release is effective.（2）The mixture of SRB and JF-5 at different ratios produced different amounts of biological FeS.When the molar Fe:S was 0.2,the maximum amount of FeS was 2400mg·L^-1.After the formation of biological nano FeS,adding 0.1wt%sodium carboxymethyl cellulose（CMC）could slow down the aggregation and sedimentation of FeS,indicating that microbial synthesis of nano FeS is feasible and the yield is considerable,and CMC can effectively strengthen the stability of FeS system and improve the performance of FeS.The convection-diffusion model can describe the migration behavior of biological nano FeS after stabilization（CMC-FeS）particles well.The correlation linearity of CMC-FeS particles at different flow rates was 0.35-0.85,while the correlation of pure water-FeS particles was low（R²=0.20）,CMC-FeS had better migration performance than pure water-FeS.At the flow rate of 180 mL·h^-1,the migration behavior was close to the convection-diffusion model（0.85）,and the permeability in the column was the best,with an average permeability coefficient of243.97 cm·h^-1.The adsorption capacity of CMC-FeS for As（Ⅴ）was strong,the maximum adsorption capacity of As（Ⅴ）by CMC-FeS was 14.04 mg·g^-1.Increasing the concentration of FeS slowed down the migration and penetration,deposited more in the column porous media,and the adsorption effect of As（Ⅴ）became worse.In conclusion,this study achieved significant arsenic release effect with microorganisms and methods.Bionano FeS are produced in large quantities,and CMC-FeS have good stability and migration performance.