Remediation Of Cadmium-contaminated Soil By Sulfidated Nanoscale Zero-valent Iron: The Efficiency And Mechanism

About 20 million hectares of cultivated soil in China are contaminated by cadmium,threatening the ecosystem and human health.Nanoscale zero-valent iron（nZVI）has been successfully applied to remediate metal（loid）s-contaminated soil,such as As,Cr,Pb,owing to its excellent adsorption capacity and reactivity.However,the remediation efficiency of nZVI on cadmium-contaminated soil is not so good,and unstable.Sulfidation can enhance the performance and stability of nZVI and also improve the removal capacity of Cd.However,there is no publication on testing the effectiveness of sulfidated nanoscale zero-valent iron（S-nZVI）for the remediation of Cd-contaminated soil.In this study,we firstly use S-nZVI to treat the Cd-contaminated soil under different conditions and investigate the change of Cd speciation after remediation.Then spectroscopy was used to characterize the magnetic material separated from the soil,so as to explore the Cd immobilization mechanism by S-nZVI in soil and the oxidation of S-nZVI.A combination of sludge biochar（BC）and S-nZVI（BC+S-nZVI）was used as soil amendment,evaluating the effect of BC/S-nZVI mass rations on Cd speciation after remediation.Finally,the main results of this study are shown as below:（1）The effect of different soil conditions on Cd speciation.The content of exchangeable（EX）cadmium was 84-95%in untreated soil.After treated with S-nZVI,the EX fraction decreased to 0.1-1%at the soil moisture of 50%within 30 d.Under the identical conditions,Cd was mainly related with the EX fraction（76-88%）after reaction with nZVI.Among different S-nZVI dosages(1,5,10 g kg^-1),5 g kg^-1of S-nZVI was the best dosage to reduce EX fraction by more than 97.6%,and converted them into relatively stable iron-manganese oxides-bound（OX）cadmium fraction（56-82%）.Initial soil p H（5.3,6.0 and 7.5）had little effect on Cd immobilization by S-nZVI.The available Cd was mostly converted to OX fraction（72-92%）after reaction with S-nZVI.（2）Mechanism of Cd immobilization by S-nZVI.SEM characterization showed that Cd distributed on the whole particles and was closely related to the spatial distribution of the elements of Fe,S,and O.Combining the results of XRD and Raman spectra,Cd O and Cd S formed during the remediation with the forming ofα-Fe OOH andγ-Fe OOH in 30 d.The Cd S was produced by the replacement reaction between Cd and Fe S on the surface of S-nZVI or the reaction between Cd and HS^-,while the existence of Cd O might be related to iron oxides and S-nZVI.Meanwhile,the kinds of iron oxides（α-Fe₂O₃,γ-Fe₂O₃,and Fe₃O₄）were abundant after 30 d.The formation of iron oxide could synergistically adsorb cadmium.（3）The effect of combination of BC and S-nZVI on Cd speciation after remediation.The combination of BC and S-nZVI（BC+S-nZVI）was conducive to the long-term stability of remediation due to the stable properties of BC and the well remediation efficiency of low-dose S-nZVI in the short term.The EX fraction increased significantly after 15 days of application of 20 g kg^-1BC alone or low dosage of S-nZVI(1 g kg^-1).While the content of EX fraction decreased to 9%in BC+S-nZVI treatment（mass ratio of 20:1）,which was lower than that of 20 g kg^-1BC（16%）and 1 g kg^-1S-nZVI（53%）,and the change of soil p H value was little.In summary,the combined use of BC and low dose of S-nZVI improved the long-term stability of remediation,and effectively slowed down the increase of EX fraction of Cd.