Accelerated Reduction Of Azo And Nitro Compounds By Redox-active Lignite

Lignite is generally regarded as the lowest rank of coal due to its relatively poor heat content.However,it is widely found around the world and its recoverable reserves are very abundant.Currently,lignite is mainly used as fuel and sorbents of pollutants.Lignite contains large amounts of humic-like substances,iron and other metal minerals,which are generally believed to be redox active and capable of promoting the electron transfer.Therefore,in this study,different lignite samples were assayed for their redox activities,the impacts of which on sulfide-or microbe-mediated reduction of azo/nitro compounds were also explored.Lignite samples originated from Inner Mongolia,Shanxi,Yunnan,Xinjiang and an anthracite sample from Shanxi were selected to study their influences on chemical reduction of azo dyes by sulfide.Mediated electrochemical reduction and oxidation analysis indicated that the lignite samples possessed a high electron accepting capacity but negligible electron donating capacity.And the promotion effects of lignite samples on chemical reduction seemed to be determined by their electron accepting capacities.It was found that the lignite-mediated decolorization performance increased with the increase of sulfide concentration(0-3.0 mM),lignite dosage(0-300 mg/L)and salinity(0-6% NaCl).Over 80% decolorization could be maintained in eight successive rounds of operation,demonstrating the persistent acceleration effects of lignite.Measurement and comparison of reaction rate of individual steps not only further confirmed the redox mediator activity of lignite,but also identified the first step,i.e.,the reduction of lignite by sulfide to be the rate-limiting step of lignite-mediated azo dye decolorization.Additionally,redox transformation was observed with lignite's oxygenated moieties and iron components,which were believed to contribute to lignite's redox mediator activity.The effects of these five coal samples on reduction of nitrobenzene by Shewanella oneidensis MR-1 were also studied.It was found that they could all promote the bioreduction of nitrobenzene and the stimulating effects of lignite samples correlated with their contents of oxygen-containing groups.Nitrobenzene concentration(10-200 mg/L),lignite dosage(10-200 mg/L),and cell concentration(0.04-0.28 g/L)in the systems could affect the reduction rate of nitrobenzene.The oxygen-containing group contents of lignite were increased by HNO3(2 M)treatment.And the promotion effects of HNO3-treated lignite on nitrobenzene bioreduction were higher than that of original lignite.However,improvement by HNO3 treatment leveled off at higher HNO3 concentrations(5-10 M),which might cause the significant dissolution and loss of iron and other mediator substances in the lignite.The humic acid and HCl extractable-iron in lignite could participate in electron transfer during nitrobenzene bioreduction.Cells contacted directly with the cells tofacilitate the transfer of biogenic electrons and the reduction of nitrobenzene.Based on the findings of this study,redox-active lignite could be applied to develop efficient mediator materials for the reductive treatment of wastewater containing oxidative pollutants.