Immobilization Of Cadmium In Sediment Via Zero-valent Iron Mediated By Hydrogen Autotrophic Microbes

The problem of heavy metal pollution in sediments has become increasingly prominent.Thus,it is particularly important to explore an efficient and safe remediation technology for contaminated sediments.Among these,in-situ immobilization is highly favored.Zero-valent iron（Fe⁰）is a highly efficient iron-based material for the in-situ immobilization of heavy metals through reduction,adsorption,and co-precipitation,etc.,however,it is difficult to corrode in anoxic sediments.In this study,the synergistic system mediated by nitrate or sulfate was explored for the remediation of Cd-contaminated sediment via the introduction of hydrogen autotrophic microbes,utilizing their hydrogen-consuming metabolism to promote Fe⁰ corrosion.The results showed that under the mediation of nitrate,the combination of hydrogenotrophic denitrification and hydrogen evolution corrosion of Fe⁰ promoted continuous Fe⁰ corrosion,while inducing the production of large amounts of reactive secondary minerals such as magnetite,lepidocrocite and green rust on the iron surface.After 7d of remediation,the concentration of Cd（II）in the overlying water decreased by 83.1%,which was 2.9 times higher than that of the Fe⁰ system.In addition,Cd（II）in sediment was transformed from unstable acid-soluble and reducible state to more stable oxidizable and residue state,with enhanced acid leaching resistance and greatly reduced biological mobility and availability.Increasing the nitrate concentration and Fe⁰ dosage within a certain range were also beneficial for the enhanced Cd（II）immobilization in this system,whereas the optimum dosage needed to be adapted to the microbial activity.In the sulfate-mediated biotic Fe⁰ system,hydrogenotrophic desulfurization with sulfate as electron acceptor facilitated Fe⁰ electron transfer,thereby maintaining Fe⁰ corrosion.Besides,active minerals such as magnetite,lepidocrocite and green rust were produced on the iron surface under biomineralization.In addition,the sulfate reduction metabolism also produced Cd S to be deposited on the mineral surface.Finally,through the interaction mechanism of adsorption,complexation and precipitation,the system successfully achieved82.7%Cd（II）immobilization efficiency in the overlying water,and the percentage of residual Cd（II）in the sediment was increased by 171.4%compared with the original sediment,which greatly reduced the risk of secondary pollution of the sediment.Comparing the mediating effects of two oxygen anion salts on the remediation of Cd（II）-contaminated sediment,sulfate-mediated enhancement was slightly better than nitrate-mediated.The same mechanism involved in both was the use of microbial hydrogen consumption activity as a transfer medium,with H₂ as an electron donor and nitrate and sulfate as electron acceptors to promote anaerobic corrosion of Fe⁰,which later induced the production of strongly adsorptive iron oxides.Different from nitrate,sulfate also stimulated the formation of cadmium sulfide from the reduction product by sulfate-reducing bacteria（SRB）in mixed anaerobic microbes with Cd（II）,which had a lower solubility product constant,further optimizing the immobilization.In summary,nitrate or sulfate-mediated biotic Fe⁰had the potential to be a stabilizing technology for the treatment of Cd（II）in sediment by breaking down reaction limitations and improving the remediation of Cd（II）sediment with the help of multiple elemental transformation processes involving microbes,such as iron,nitrogen and sulfate.