Mechanisms for sulfide-induced nitrobenzene reduction mediated by a variety of different carbonaceous materials: Graphitized carbon-facilitated electron transfer versus quinone-facilitated formation of reactive sulfur species.

Abstract

Although it has long been known that carbonaceous materials (CMs) can facilitate the reduction of organic contaminants by sulfide, the underlying mechanisms and controlling factors, particularly the surface property dependence, are not well understood. Here, sulfide-induced nitrobenzene reduction was explored as a model reaction to compare the mediation efficiency of a variety of CMs, including rice straw-derived black carbon (R-BC) and pine wood-derived black carbon (P-BC), a commercial activated carbon (AC), multi-walled carbon nanotube (MCNT), and graphite. Given the same load (250mg L-1 ), the observed pseudo-first-order rate constant (kobs ) of nitrobenzene reduction was ordered as AC>R-BC>MCNT>P-BC>graphite. The surface area-normalized rate constant (kSN ) was ordered as R-BC>graphite>MCNT>AC>P-BC. Neither the kobs nor the kSN followed the order of mediator's electron conductivity (graphite>MCNT>AC>P-BC>R-BC). For the low-graphitized R-BC and P-BC, increasing surface oxygen content by HNO3 oxidation enhanced nitrobenzene reduction, whereas decreasing the content by NaBH4 reduction impeded the reaction. Opposite trends were observed with the high-graphitized AC, MCNT, and graphite. The quinone moieties of low-graphitized CMs were found to facilitate nitrobenzene reduction by serving as one-electron acceptors to generate reactive reducing sulfur species (polysulfides and polysulfide free radicals) from sulfide. In contrast, the surface oxygen groups of high-graphitized CMs suppressed the reaction by lowering the electron conductivity. These results demonstrate that the types of CMs and their surface chemistry properties are key determinants in mediating redox transformation of organic contaminants.