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

- A kinetic model including kobs, ktra, kdif and krea was proposed to elucidate the role of corrosion films.
- The trends of kobs, ktra, kdif and krea values varied with the reactant species, operation time and O2 content were explored.
- The effects of corrosion films on kdif and krea values were responsible for the variation of kobs value.
- Various product compositions were probably attributed to mass diffusion.

A detailed kinetic model comprised of mass transport (ktra), pore diffusion (kdif), adsorption and reduction reaction (krea), was developed to quantitatively evaluate the effect of corrosion films on the removal rate (kobs) 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 kobs 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 ktra values at 13.32, 12.12, 11.04 and 10.20 h−1, kdif values at 0.42-5.82, 0.36-5.04, 0.30-4.50, 0.30-3.90 h−1, and krea values at 14.94-421.18, 0-0.19, 0.01-1.30, 0.10-3.98 h−1, respectively. The variation of kobs value with reaction conditions depended on the reactant species, while those of ktra, kdif and krea values were irrelevant to the species. The effects of corrosion films on kdif and krea values were responsible for the variation of kobs value for halogenated compounds. For a mass-transfer-limited halogenated compound such as BrO3−, an often-neglected kdif value primarily determined its kobs value when pore diffusion was the rate-limiting step of its removal. In addition, the value of kdif might influence product composition during a consecutive dechlorination, such as for TCAA and DCAA. For a reaction-controlled compound such as MCAA, an increased krea value was achieved under low oxic conditions, which was favorable to improve its kobs value. The proposed model has a potential in predicting the removal rate of halogenated compounds by metallic iron under various conditions.

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