Study On The Soil Heavy Metal Remediation By Sodium Alginate Coated Sulfur-modified Nanoscale Zero-valent Iron

Soil leaching technology has become one of the hot technologies researched in the field of soil heavy metal pollution remediation in recent years due to its advantages of high efficiency,short cycle time,and flexible operation.However,the simultaneous recovery of multiple heavy metals in the leaching process has been less studied,and the difficulty of leaching wastewater treatment has limited its large-scale application.In this study,sulfur-modified nanoscale zero-valent iron(S-nZVI)was prepared by liquid-phase reduction method and covered with sodium alginate(SA)to form gel beads(SAS-nZVI),and the adsorption capacity of SAS-nZVI on Pb(Ⅱ)in water and its adsorption mechanism were analyzed.The combination(SAS-nZVI-CA)of SAS-nZVI and citric acid(CA)was used for leaching and remedying of Pb-Cd composite contaminated soil to investigate the effect of leaching on the removal and recovery of heavy metals under different leaching conditions,and to evaluate the effect of leaching on the soil in terms of changes in the basic physicochemical properties and heavy metal morphology before and after leaching.The main conclusions were obtained as follows.(1)XRD,SEM,EDS,and FTIR analyses showed that S-nZVI was successfully loaded into SA,and the modified nZVI showed good dispersion with improved agglomeration and passivation.The removal progress of Pb(Ⅱ)from water by SAS-nZVI was more consistent with the Langmuir isotherm model and the pseudo-second-order kinetic model.The removal mechanism of Pb(Ⅱ)removal by SAS-nZVI in an aqueous solution may include adsorption,reduction,surface complexation,and co-precipitation processes.In addition,the removal efficiency of SAS-nZVI for Pb(Ⅱ)in an aqueous solution could still reach 82.75% after five cycles of recycling.(2)When the CA concentration was 0.1 mol/L and the liquid-solid ratio was 20 at room temperature,the removal rates of Pb and Cd from soil were increased by 14.9% and18.36%,respectively,after the addition of 2.0 g/L of SAS-nZVI.When SAS-nZVI was added at 3.0 g/L,the recoveries of Pb and Cd from the leaching solution(Rr/w)and soil(Rr/s)reached 84.94%,59.82%,and 64.77%,57.69%.The leaching desorption process of SAS-nZVI-CA for Pb and Cd was well fitted with the dual constant equation,indicating that the desorption kinetic process of Pb and Cd desorption was heterogeneous diffusion process and the elution process was controlled by diffusion factors.(3)The addition of SAS-nZVI could,to a certain extent,attenuate the adverse effects of single citric acid leaching on soil fertility,reduce soil nutrient loss and appropriately replenish soil organic matter,compared with single CA leaching.Moreover,SAS-nZVICA could effectively reduce the unstable state content of Pb and Cd in the soil,thus reducing the mobility and toxicity of Pb and Cd in the soil.