Studies Of SRB On Regeneration Treatment Of Chelators After Soil Washing Heavy Metals Contaminated Soil And Mechanisms

In recent years,the situation of heavy metal contaminated soil is a severe problem in China.As an effective technology for rapid and permanent remediation of soil contaminated with heavy metal,soil washing has increasingly received attention.However,the hazards of residual washing agents to soil and the heavy metal containing waste water producing secondary pollution problems remain unresolved.In this research,sulfate reducing bacteria（SRB）was used to treat the soil washing waste,and the removal effect of chelated metals in the single and composite washing wastewater as well as the heavy metal removal mechanism were studied,in order to achieve the recycling and harmless disposal of the polymetallic waste leachate.In addition,the use of recycled leachate to re-leach heavy metal contaminated soil provides a scientific basis for solving the disposal and discharge of soil leachate,as well as reducing the economic and environmental costs of soil leachate remediation.The results are shown as follows:（1）The experimental results indicated that SRB could significantly reduce the chelated Cu content in the eluent within seven days.When the content of Cu is fixed at 50 mg·L^-1,the removal efficiency of SRB on Cu²⁺,Cu-EDTA,Cu-GLDA,Cu-CA,and Cu-MC can reach more than 60%.The highest molar ratios of Cu to EDTA,GLDA,CA and MC that SRB can withstand are 1:10,1:25,1:100 and 1:25,respectively.Compared with single chelator EDTA,GLDA and CA,the mixed chelators（MC）has a milder toxicity to the bacteria.Therefore,MC can not only maintain preferably leaching efficiency,but also be more conducive to the treatment and recycling of leaching waste solution after leaching.（2）The mechanism experiment showed that the removal of complex heavy metals by SRB mainly included three ways:bacterial redox,bacterial extracellular polymeric substances and dead bacterial adsorption,among which the redox of SRB was the main pathway.The extracellular polymer extracted from SRB has a certain adsorption effect on chelated Cu,and the adsorption capacity of Cu²⁺,Cu-EDTA,Cu-GLDA,Cu-CA and Cu-MC are 210.96,204.75,193.64,190.95 and 200.63 mg·g^-1,respectively.The kinetic equilibrium experiment showed that the adsorption process was more consistent with the quasi-second order kinetic equation.SRB had a small adsorption effect on chelated Cu,and the removal efficiency was 3.92-6.65%.Bio-scanning electron microscopy（SEM）and X-Ray diffraction（XRD）analysis revealed the main precipitated product was Cu S,SRB mainly through redox to remove chelated Cu.（3）The full-factor experiments suggested that SRB can effectively remove high concentrations of chelated Cd,Pb,Cu and Zn.The maximum removal efficiency of chelated Cu was 73.87%,followed by chelated Cd at 46.51%,and the maximum removal efficiency of chelated Pb and Zn were 17.14%and5.80%.The removal efficiency of each heavy metal in the chelated polymetallic system is the chelated Cu>chelated Cd>chelated Pb>chelated Zn,and the removal of each chelated heavy metal had a synergistic effect.（4）The actual soil leaching experiments revealed that,compared with EDTA,MC reduced the dosage of EDTA by 80%,and at the same time showed better removal efficiency for the chelated Cd,Pb,Cu and Zn.The removal efficiency of chelated Cd,Pb,Cu and Zn in EDTA and MC leaching waste treated by SRB reached the best at 11 days,and the best removal efficiency of heavy metals were 89.63%,27.11%,98.08%,8.46%and 72.62%,22.63%,80.12%,1.15%,respectively.All have better removal effect.In addition,SRB treated two kinds of leacheate can be recycled.After regeneration,MC still showed a high removal efficiency of Cd,Pb and Cu in the soil,which could reach 34.96%,45.17%and 45.05%of the original MC,respectively.