Enhanced immobilization of cadmium in contaminated paddy soil by biochar-supported sulfidized nanoscale zero-valent iron

PurposeNanoscale zero-valent iron (nZVI) is increasingly used to enhance the immobilization of cadmium (Cd) in paddy soil, but its application has limitations by various inherent disadvantages, such as easy passivation, agglomeration, and poor electron transfer. To overcome the drawback, biochar-supported sulfidized nZVI (S-nZVI/BC) was synthesized and added into Cd-contaminated paddy soil to investigate its immobilization performance and remediation mechanisms for Cd.Materials and methodsThe synthesized materials were characterized by various techniques such as SEM, TEM, XPS, and FTIR. Sequential extraction method was adopted to examine the chemical speciation of Cd. The diethylenetriamine pentaacetic acid (DTPA)-extractable Cd and toxicity characteristic leaching procedure (TCLP)-leachable Cd were measured to evaluate the availability and leaching toxicity of Cd. The changes of soil properties and the reaction mechanism between Cd and S-nZVI/BC were also analyzed.Results and discussionSequential extraction procedure suggested that S-nZVI/BC was effective in transforming mobile Cd to stable speciation with the proportion of residual speciation increased by 32.04% after 49 days of treatment. The DTPA-extractable Cd content decreased from 2.14 to 0.42 mg & BULL;kg(-1), indicating the availability of Cd was significantly reduced. Meanwhile, the immobilization efficiency of TCLP-leachable Cd was increased by 53.70%, showing that S-nZVI/BC reduced the leaching toxicity and environmental risk of Cd. Also, it was found that the addition of S-nZVI/BC increased the soil pH, redox potential (Eh), and organic matter (OM), of which pH was the main factor influencing soil Cd mobility. Furthermore, characterization results of S-nZVI/BC after remediation clarified that the stabilization mechanism of Cd was mainly dominated by the adsorption, complexation, and precipitation of Cd2+ with BC and the iron oxide/hydroxide or FeSx shell of S-nZVI and secondary iron minerals (CdFe2O4).ConclusionOverall, S-nZVI/BC could be an effective material for the remediation of Cd-contaminated soil and alleviate the hazards likely posed to the environment. This work offers a new avenue for practical applications of nZVI-based materials in contamination remediation.