N-nitrosodimethylamine removal by a novel silver/sulfur-coated nanoscale zero-valent iron/activated carbon composite: Adsorption kinetics, mechanisms, and degradation pathways

Abstract

The removal of disinfection by-products from water is of critical importance to achieve the water purification. N-nitrosodimethylamine (NDMA), an emerging nitrogenous disinfection by-product produced by the chlorine-ammonia disinfection, remains significantly hazardous even at low concentrations. Herein, an activated carbon (AC)-supported nanoscale zero-valent iron (nZVI) coated with sulfur (S) and silver (Ag) composite (Ag@S-nZVI/AC) was synthesized, characterized via several techniques and evaluated for NDMA removal performance. The results demonstrated that crystalline Ag and mackinawite (FeS) coated amorphous nZVI spheres were successfully anchored on AC. The removal of NDMA at ultra-lower concentrations by the Ag@S-nZVI/AC composite was better fitted by pseudo-second-order (PSO) kinetic and Freundlich isotherm models. NDMA was chemically adsorbed in multiple layers on heterogeneous surface of the Ag@S-nZVI/AC composite via spontaneous endothermic process. The adsorption constant (KF) of NDMA removal by Ag@S-nZVI/AC at T=298 K was 9.89 times greater than that of NDMA removal by pristine AC. The van der Waals forces, hydrogen bonds and surface-mediated electron transfer were the main mechanisms responsible for the highly effective remediation of ultra-lower concentrations of NDMA from drinking water. The decomposition of NDMA was significantly enhanced by Ag@S-nZVI/AC, resulting in the generation of more dimethylamine (DMA), ammonium and the nitrogen-containing compounds. The decomposition process involved the oxidation of FeS, S and Fe0 surface as well as the hydrogenation of Ag. The post-leaching concentrations of Fe, Ag and S in treated drinking water were lower than the standard permissible limits.