Mechanism and kinetics of halogenated compound removal by metallic iron: Transport in solution, diffusion and...

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.