Black carbon (BC) has drawn great interest in the environmental field due to the strong capability to sequestrate organic contaminants as well as the potential to mediate their redox reactions. However, the underlying mechanisms for the redox mediator effects of BC, particularly the role played by the surface O-containing groups, are still not well understood. Due to the heavy use, nitroaromatic compounds (NACs) are widely present in surface and subsurface environments, while their transformation reactions are often initiated by the reduction of the nitro group(s). As sulfide is a common reducing agent existing in natural anaerobic environments, we herein investigated the sulfide-induced reduction of nitrobenzene, a model NAC, in anoxic aqueous suspensions of varying-sized biochar fractions, including dissolved BC (size &lt; 0.45 μm), colloidal BC (0.45 μm &lt; size &lt; 1 μm), and rest BC (size &gt; 1 μm). The organic carbon (OC)-normalized pseudo-first-order rate constant (kOC) followed an order of dissolved BC &gt; colloidal BC &gt; bulk BC &gt; rest BC. Formation of polysulfides from sulfide oxidation in the presence of different BC fractions was demonstrated by the UV-vis absorbance spectra of the supernatant separated from BC suspensions. Significantly higher nitrobenzene reduction was observed for the supernatant than for the particulate BC remnant, implying an indispensible role of polysulfides in the reaction. It was proposed that the surface O-containing groups (e.g., quinones) catalyze the oxidation of sulfide to polysulfides, which are strong nucleophilic reagents to reduce nitrobenzene. This hypothesis was verified by the observed mediation effects of BC subjected to different oxidation-reduction surface treatments, as well as electron paramagnetic resonance (EPR) measurements of BC after sequential reactions with sulfide. The results demonstrate that the surface oxygen-containing groups of black carbon can oxidize sulfide to polysulfides and therefore effectively facilitate the reduction of nitroaromatic compounds.

Aqueous-phase reduction of nitroaromatic compounds (NACs) by sulfide mediated by carbonaceous materials

255th National Meeting (2018)

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