Doped Cu0 and sulfidation induced transition from R-O• to •OH in peracetic acid activation by sulfidated nano zero-valent iron-copper

Peracetic acid (PAA) has emerged as a new effective oxidant for various contaminants degradation through advanced oxidation process (AOP). In this study, sulfidated nano zero-valent iron-copper (S-nZVIC) with low Cu doping and sulfidation was synthesized for PAA activation, resulting in more efficient degradation of sulfamethoxazole (SMX, 20 μM) and other contaminants using a low dose of catalyst (0.05 g/L) and oxidant (100 μM). The characterization results suggested that S-nZVIC presented a more uniform size and distribution with fewer metal oxides, as the agglomeration and oxidation were inhibited. More significantly, doped Cu0 and sulfidation significantly enhanced the generation and contribution of •OH but decreased that of R-O• in S-nZVIC/PAA/SMX system compared with that of nZVIC and S-nZVI, accounting for the relatively high degradation efficiency of 97.7% in S-nZVIC/PAA/SMX system compared with 85.7% and 78.9% in nZVIC/PAA/SMX and S-nZVI/PAA/SMX system, respectively. The mechanisms underlying these changes were that (i) doped Cu° could promote the regeneration of Fe(Ⅱ) for strengthened PAA activation through mediating Fe(Ⅱ)/Fe(Ⅲ) cycle by Cu(Ⅰ)/Cu(Ⅱ) cycle; (ii) S species might consume part of R-O•, resulting in a decreased contribution of R-O• in SMX degradation; (iii) sulfidation increased the electrical conductivity, thus facilitating the electron transfer from S-nZVIC to PAA. Consequently, the dominant reactive oxygen species transited from R-O• to •OH to degrade SMX more efficiently. The degradation pathways, intermediate products and toxicity were further analyzed through density functional theory (DFT) calculations, liquid chromatography-mass spectrometry (LC-MS) and T.E.S.T software analysis, which proved the environmental friendliness of this process. In addition, S-nZVIC exhibited high stability, recyclability and degradation efficiency over a wide pH range (3.0∼9.0). This work provides a new insight into the rational design and modification of nano zero-valent metals for efficient wastewater treatment through adjusting the dominant reactive oxygen species (ROS) into the more active free radicals.