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

Highlights: • Low-dose S-NZVI achieves the complete removal of low-concentration EDTA-Cd. • S-NZVI has extremely high salinity resistance which can efficiently remove 2000 μg/L of EDTA-Cd under 5% salinity conditions. • The working pH range of S-NZVI is relatively wide which can almost completely remove EDTA-Cd in the pH range of 2–7. • S-NZVI shows excellent removal performance in complex water quality where various substances coexist. Achieving Cd removal standards is a difficult task due to the strict Cd discharge standards for industrial wastewater. 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 wastewater. EDTA-Cd (2000 μg/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 μg/L, which meets the China Environmental 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 (Na₂EDTA, imidazole, 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.