N-doped biochar-catalyzed dechlorination of carbon tetrachloride in sulfide-containing aqueous solutions: Performances, mechanisms and pathways.

Nitrogen-doped biochar (N-BC) has been widely concerned in the field of environmental protection. This study verified the alfalfa-based N-BC pyrolyzed at different temperatures is able to catalyze carbon tetrachloride (CT) dechlorination in sulfide-containing aqueous solutions under normal environmental pH range (6.3 ∼ 8.3) effectively, with Cl-, trichloromethane (CF), CS2 and HCO3- as predominated products. Higher pyrolysis temperature resulted in larger specific surface area, more pores and better catalytic capacity. The N-BC had a good tolerance to water matrix in catalyzing CT dechlorination by sulfide, while the higher pH value or higher dosage of sulfide or N-BC was beneficial to catalytic CT dechlorination. The generation of CS2 was the major CT dechlorination pathway, attributing to the SN2 nucleophilic substitution by newborn C-SS- structure generating from the interaction between pyridine-N and sulfide. Besides, generation of CF via hydrogenolysis process was the minor CT dechlorination pathway, owing to the enhanced electron transfer by pyrrole-N, graphitic-N and quinones on surface of N-BC. It was the first time that N-BC was found to be effective in catalyzing the hydrogenolysis process of CT dechlorination. This study emphasized the importance of N-BC in restoring chlorinated hydrocarbons polluted aquatic environment containing sulfide, such as sediments.