<i>In-Situ</i> Remediation of Chromium Contaminated Soil by Biochar Co-Modified by CMC and Nano Zero-Valent Iron: Effects of Acid Rain and Freeze-Thaw Conditions

In this study, by simulating acid rain and freeze-thaw conditions, the remediation effect of biochar modified by carboxymethyl cellulose stabilized nano zero-valent (CMC-nZVI@BC) on hexavalent chromium Cr(VI) contaminated soil was explored. Simulated acid rain was prepared according to the actual situation in Shenyang, China. Soil samples underwent 16 freeze-thaw cycles (frozen at −15°C for 12 h, then cultured at 15°C for 12 h). Samples were taken to measure pH, conductivity, cation exchange capacity (CEC), soil organic matter (SOM) and Cr(VI) concentrations. The toxicity characteristic leaching procedure (TCLP) and simplified bioaccessibility extraction test (SBET) were conducted to assess the leachability and in vitro bioaccessibility of Cr. ICP-OES was used to test the Cr(VI) and total Cr ions concentrations. The analysis of soil Cr forms adopted the BCR continuous extraction method. CMC-nZVI@BC showed improved dispersion stability and oxidation resistance with optimal CMC loading (0.25%) and nZVI ratio (4:3). After 14 days, Cr(VI) removal decreased from 97.7% to 92.9%. Under acid rain and freeze-thaw conditions, Cr(VI) concentrations in leachate remained below the 0.05 mg/L standard. TCLP, SBET, and 16S rRNA results confirmed that CMC-nZVI@BC reduced Cr leaching toxicity and bioavailability, while enhancing soil microorganism richness and diversity. <i>In-situ</i> remediation of chromium contaminated soil revealed that CMC-nZVI@BC could remain stable and not cause pollution to groundwater under the conditions of acid rain leaching and freeze-thaw cycles, providing certain references for the treatment of Cr(VI) contaminated sites.