Mechanism and kinetics of halogenated compound removal by metallic iron: Transport in solution, diffusion and reduction within corrosion films

A detailed kinetic model comprised of mass transport (k_tra), pore diffusion (k_dif), adsorption and reduction reaction (k_rea), was developed to quantitatively evaluate the effect of corrosion films on the removal rate (k_obs) of halogenated compounds by metallic iron. Different corrosion conditions were controlled by adjusting the iron aging time (0 or 1 yr) and dissolved oxygen concentration (0–7.09 mg/L DO). The k_obs values for bromate, mono-, di- and tri-chloroacetic acids (BrO₃⁻, MCAA, DCAA and TCAA) were 0.41–7.06, 0–0.16, 0.01–0.53, 0.10–0.73 h⁻¹, with k_tra values at 13.32, 12.12, 11.04 and 10.20 h⁻¹, k_dif values at 0.42–5.82, 0.36–5.04, 0.30–4.50, 0.30–3.90 h⁻¹, and k_rea values at 14.94–421.18, 0–0.19, 0.01–1.30, 0.10–3.98 h⁻¹, respectively. The variation of k_obs value with reaction conditions depended on the reactant species, while those of k_tra, k_dif and k_rea values were irrelevant to the species. The effects of corrosion films on k_dif and k_rea values were responsible for the variation of k_obs value for halogenated compounds. For a mass-transfer-limited halogenated compound such as BrO₃⁻, an often-neglected k_dif value primarily determined its k_obs value when pore diffusion was the rate-limiting step of its removal. In addition, the value of k_dif might influence product composition during a consecutive dechlorination, such as for TCAA and DCAA. For a reaction-controlled compound such as MCAA, an increased k_rea value was achieved under low oxic conditions, which was favorable to improve its k_obs value. The proposed model has a potential in predicting the removal rate of halogenated compounds by metallic iron under various conditions.