Low dose of sulfur-modified zero-valent iron for decontamination of trace Cd(II)-complexes in high-salinity wastewater

Achieving Cd removal standards is a difficult task due to the strict Cd discharge standards for industrial wastewa-ter. Moreover, the low concentration of Cd remaining in industrial wastewater after pretreatment often exists in a complex state, and the wastewater has a high salinity. Hereupon, we propose to use a small amount of sulfur-modified zero-valent iron (S-NZVI) to remove residual low-concentration Cd complexes in high-salinity waste -water. EDTA-Cd (2000 mu g/L) was completely removed when the dose of S-NZVI was only 0.05 g/L. Moreover, the removal process was almost unaffected by salinity. Even when the salinity was 5%, the adsorption capacity still reached 39.5 mg/g, and the concentration of residual Cd was less than 50 mu g/L, which meets the China Environ-mental Protection Administration emission standards (less than 0.1 mg/L). In addition, S-NZVI can almost completely remove EDTA-Cd in the pH range of 2-7. It shows good removal performance for the other four Cd carboxyl complexes (DTPA, citrate, glycine, and tartrate). Furthermore, S-NZVI also shows good performance in the case of high concentrations of coexisting ions (CaCl2, MgCl2, Na2SO4, NaNO3) and organics (Na(2)EDTA, im-idazole, thiourea, acetone). However, the performance of S-NZVI is certainly inhibited by the presence of complexing substances or reducing substances. The mechanism EDTA-Cd removal by S-NZVI is that S-NZVI leaches Fe3+ into the solution, and the Fe3+ completes the replacement of EDTA-Cd. The LMCT produced by EDTA-Fe under natural light promotes the replacement process, and finally, the released Cd2+ is captured by S-NZVI and removed as CdS and Fe-O-Cd. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The use of sulfur-modified zero-valent iron (S-NZVI) has been proven effective in removing residual low-concentration Cd complexes in high-salinity wastewater, even under extremely high salinity conditions, meeting the emission standards set by the China Environmental Protection Administration.