Compound-Specific Isotope Analysis Reveals Sulfidation-Induced Mechanistic Shifts during Decabromodiphenyl Ether Debromination by Nanoscale Zero-Valent Iron

Sulfidated nanoscale zero-valent iron (S-nZVI) is widely used for reductive dehalogenation, yet how sulfidation alters the reaction mechanism of highly halogenated contaminants during multistep transformation remains poorly understood. Here, BDE-209 debromination by S-nZVI across a range of S/Fe molar ratios was investigated by integrating reaction kinetics, product analysis, and compound-specific isotope analysis (CSIA). Under the CSIA experimental conditions, k_obs exhibited a volcano-type response to sulfidation, reaching 0.25 h^-1 at S/Fe = 0.48, whereas ε_C shifted from -1.71‰ for pristine nZVI to -3.16‰ at S/Fe = 0.12 and remained more negative thereafter. Notably, increasing S/Fe from 0 to 0.12 changed k_obs only marginally (0.05 to 0.07 h^-1) but nearly doubled the magnitude of ε_C, indicating a sulfidation-induced mechanistic shift in isotope-sensitive C-Br bond cleavage before substantial rate enhancement occurred. Product analysis further showed that the distributions of first-step debromination intermediates were affected by their differential subsequent transformation, limiting their use as direct indicators of parent-compound bond cleavage. Together with the shift in cumulative positional debromination toward a greater para contribution, the isotope response was consistent with an increased relative contribution of surface-adsorbed atomic hydrogen mediated reduction after sulfidation. Collectively, these findings demonstrate that sulfidation alters parent-compound reductive transformation behavior and thereby influences multistep debromination and product distributions, highlighting the importance of considering contaminant transformation behavior alongside removal efficiency when evaluating reductive remediation processes.