Remediation of Cr(VI)-contaminated soil by double-modified nanoscale zero-valent iron: performance and mechanism

This study utilized biochar (BC) and carboxymethyl cellulose (CMC) modified nanoscale zero-valent iron (nZVI) to synthesize a novel composite material. The composite material was applied to the remediation of Cr(VI)-contaminated soil to evaluate its remediation performance and investigate the remediation mechanism.

The BC supported CMC stabilized nZVI composite (CMC-nZVI@BC) was prepared by the liquid-phase reduction method. The composites were analyzed and tested by SEM, XRD and FTIR characterization and Cr(VI) removal experiments in solution. The immobilization efficiency of the composites on Cr(VI) in soil was evaluated by testing the soil Cr(VI) content. The toxicity characteristic leaching procedure (TCLP) and simplified bioaccessibility extraction test (SBET) were used to assess the leachability and in vitro bioaccessibility of Cr in soil to humans. The changes in Cr speciation were evaluated by the BCR sequential extraction method. The changes in soil physicochemical properties and remediation mechanism of CMC-nZVI@BC were also investigated.

The results of characterization and batch experiments in solution showed that the nZVI in CMC-nZVI@BC was in the form of long chains, which were encapsulated by CMC and loaded on the BC surface. The agglomeration and surface passivation of the nanoparticles were significantly improved, and the modified nZVI exhibited better stability and reactivity. Soil remediation experiments showed that after adding 3 g/kg CMC-nZVI@BC to 100 g of contaminated soil, the immobilization efficiency of Cr(VI) in the soil reached 97.96% after 21 days of incubation, which was 1.88 times greater than that of the single nZVI system. This process significantly promoted the conversion of unstable acid soluble Cr to other stable forms. The TCLP and SBET leaching Cr concentrations decreased by 97.47% and 98.40%, respectively, indicating that CMC-nZVI@BC greatly reduced the leaching toxicity and bioavailability of Cr. In addition, the soil pH, CEC, SOM, and available iron content increased to different degrees after the application of CMC-nZVI@BC. The mechanisms of Cr(VI) stabilization in soil primarily included extraction, adsorption, reduction, and coprecipitation.

In this study, CMC-nZVI@BC composites were successfully synthesized. Soil remediation experiments proved that CMC-nZVI@BC is an ideal material to remediate Cr(VI)-contaminated soil and provides a reference for future research.