title: Reduction of nitroaromatics sorbed to black carbon by direct reaction with sorbed sulfides.
abstract: Sorption to black carbons is an important sink for organic contaminants in sediments. Previous research has suggested that black carbons (graphite, activated carbon, and biochar) mediate the degradation of nitrated compounds by sulfides by at least two different pathways: reduction involving electron transfer from sulfides through conductive carbon regions to the target contaminant (nitroglycerin) and degradation by sulfur-based intermediates formed by sulfide oxidation (RDX). In this study, we evaluated the applicability of black carbon-mediated reactions to a wider variety of contaminant structures, including nitrated and halogenated aromatic compounds, halogenated heterocyclic aromatic compounds, and halogenated alkanes. Among these compounds, black carbon-mediated transformation by sulfides over a 3-day time scale was limited to nitroaromatic compounds. The reaction for a series of substituted nitroaromatics proceeded by reduction, as indicated by formation of 3-bromoaniline from 3-bromonitrobenzene, and inverse correlation of log kobs with energy of the lowest unoccupied molecular orbital (ELUMO). The log kobs was correlated with sorbed sulfide concentration, but no reduction of 3-bromonitrobenzene was observed in the presence of graphite and sulfite, thiosulfate, or polysulfides. Whereas nitroglycerin reduction occurred in an electrochemical cell containing sheet graphite electrodes in which the reagents were placed in separate compartments, nitroaromatic reduction only occurred when sulfides were present in the same compartment. The results suggest that black carbon-mediated reduction of sorbed nitroaromatics by sulfides involves electron transfer directly from sorbed sulfides rather than transfer of electrons through conductive carbon regions. The existence of three different reaction pathways suggests a complexity to the sulfide-carbon system compared to the iron-carbon system, where contaminants are reduced by electron transfer through conductive carbon regions.
authors: †Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States.|∥Department of Civil and Environmental Engineering, Villanova University, Villanova, Pennsylvania 19085, United States.
authors: Wenqing
authors: W
authors: Xu
authors: †Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States.|‡Department of Environmental Sciences, Connecticut Agricultural Experiment Station, 123 Huntington Street, New Haven, Connecticut 06504-1106, United States.
authors: Joseph J
authors: JJ
authors: Pignatello
authors: §Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, United States.
authors: William A
authors: WA
authors: Mitch
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D017554:Carbon Compounds, Inorganic; D004579:Electron Transport; D004785:Environmental Pollutants / Q000737:chemistry*; D006108:Graphite / Q000737:chemistry; D015394:Molecular Structure; D009574:Nitro Compounds / Q000737:chemistry*; D009578:Nitrobenzenes / Q000737:chemistry; D005996:Nitroglycerin / Q000737:chemistry; D010084:Oxidation-Reduction; D053260:Soot / Q000737:chemistry*; D013440:Sulfides; D013455:Sulfur / Q000737:chemistry; D013885:Thiosulfates / Q000737:chemistry
chemical_list: D000814:Aniline Compounds; D017554:Carbon Compounds, Inorganic; D004785:Environmental Pollutants; D009574:Nitro Compounds; D009578:Nitrobenzenes; D053260:Soot; D013440:Sulfides; D013885:Thiosulfates; C477543:o-bromonitrobenzene; C092035:carbon sulfide; C075716:3-bromoaniline; D013455:Sulfur; D002244:Carbon; D006108:Graphite; C032915:polysulfide; D005996:Nitroglycerin