The underlying mechanisms for the catalytic effects of black carbon (BC) on abiotic redox transformation of organic contaminants have not been well understood. Herein we investigated the reduction of nitrobenzene by sulfide in aqueous solutions mediated by three different demineralized BC (rice straw carbon, wood charcoal, and diesel soot) as a model reaction using batch technique combined with electron paramagnetic resonance (EPR) spectroscopy. The reduction of nitrobenzene was pronouncedly facilitated by the presence of BC (10 mg/L) when compared with that in the absence of BC. On the basis of either unit surface area or unit mass, the three BC exhibited much stronger catalytic effects than oxygen group-free pure graphite. The catalytic effect of BC was also found to positively correlate with the content of surface O-containing groups of different rice straw carbon samples modified by oxidation with HNO3 or by reduction with NaBH4. The supernatant separated from the aqueous suspension of sulfide-prereacted BC showed strong reducing capability, whereas the supernatant from the suspension of sulfide-prereacted graphite had negligible reducing capability. Addition of tert-butanol (a radical scavenger) markedly suppressed nitrobenzene reduction for both BC suspension and supernatant separated from BC suspension. EPR and/or ultraviolet (UV)-visible analyses demonstrated the simultaneous occurrence of nitrobenzene free radical anion and polysulfide free radicals in these samples. The combined results led us to propose a new mechanism that the surface O-containing groups of BC oxidize sulfide to form soluble polysulfides which are strong one-electron reductants and reduce nitrobenzene to a corresponding free radical anion. This hypothesis was corroborated by the observation that the intensity of the free radical signal of BC increased with increasing concentration and also reaction time of sulfide. Further density function theory (DFT) calculations revealed that the quinoid moieties conjugated with other fused aromatic rings at the surface of BC help to stabilize unpaired electrons donated by sulfide. Our findings indicate a previously unrecognized, universally effective mechanism that does not rely on electron conducting or shuttling for BC-mediated reduction of nitroaromatic compounds by sulfide.

New insights into black carbon-mediated reduction of nitroaromatic compounds by sulfide in aqueous solution

256th National Meeting (2018)

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