Abiotic transformation of hexabromocyclododecane by sulfidated nanoscale zerovalent iron: Kinetics, mechanism and influencing factors

•S-nZVI showed superior reactivity towards HBCD over FeS, S2− and nZVI.•HBCD was debrominated sequentially by S-nZVI to form less-brominated products.•FeS on S-nZVI solid may participate as both reactant and catalyst.•The optimal solution pH was 5.0 for HBCD transformation by S-nZVI.•The presence of typical inorganic ions slowed HBCD transformation by S-nZVI.

Recent studies showed that sulfidated nanoscale zerovalent iron (S-nZVI) is a better reducing agent than nanoscale zerovalen iron (nZVI) alone for reductive dechlorination of several organic solvents such as trichloroethylene (TCE) due to the catalytic role of iron sulfide (FeS). We measured the rates of transformation of hexabromocyclododecane (HBCD) by S-nZVI and compared them to those by FeS, nZVI, and reduced sulfur species. The results showed that: i) HBCD (20 mg L−1) was almost completely transformed by S-nZVI (0.5 g L−1) within 12 h; ii) the reaction with β-HBCD was much faster than with α- and γ-HBCD, suggesting the diastereoisomeric selectivity for the reaction by S-nZVI; and iii) the reaction with S-nZVI was 1.4-9.3 times faster than with FeS, S2− and nZVI, respectively. The study further showed that the HBCD reaction by S-nZVI was likely endothermic, with the optimal solution pH of 5.0, and could be slowed in the presence of Ca2+, Mg2+, NO3−, HCO3− and Cl−, and by increasing ionic strength, solvent content and initial HBCD concentration, or decreasing the S-nZVI dosage. GC-MS analysis showed that tetrabromocyclododecene and dibromocyclododecadiene were the products. XPS spectra indicated that both Fe(II) and S(-II) on the S-nZVI surface were oxidized during the reaction, suggesting that FeS might act as both catalyst and reactant. The study not only demonstrated the superiority of S-nZVI over other well-known reactive reagents, but also provided insight to the mechanisms of the reaction.

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