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), pore diffusion (k), adsorption and reduction reaction (k), was developed to quantitatively evaluate the effect of corrosion films on the removal rate (k) 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 values for bromate, mono-, di- and tri-chloroacetic acids (BrO, MCAA, DCAA and TCAA) were 0.41-7.06, 0-0.16, 0.01-0.53, 0.10-0.73 h, with k values at 13.32, 12.12, 11.04 and 10.20 h, k values at 0.42-5.82, 0.36-5.04, 0.30-4.50, 0.30-3.90 h, and k values at 14.94-421.18, 0-0.19, 0.01-1.30, 0.10-3.98 h, respectively. The variation of k value with reaction conditions depended on the reactant species, while those of k, k and k values were irrelevant to the species. The effects of corrosion films on k and k values were responsible for the variation of k value for halogenated compounds. For a mass-transfer-limited halogenated compound such as BrO, an often-neglected k value primarily determined its k value when pore diffusion was the rate-limiting step of its removal. In addition, the value of k 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 value was achieved under low oxic conditions, which was favorable to improve its k value. The proposed model has a potential in predicting the removal rate of halogenated compounds by metallic iron under various conditions.