Kinetics and corrosion products of aqueous nitrate reduction by iron powder without reaction conditions control

Although considerable research has been conducted on nitrate reduction by zero-valent iron powder (Fe⁰), these studies were mostly operated under anaerobic conditions with invariable pH that was unsuitable for practical application. Without reaction conditions (dissolved oxygen or reaction pH) control, this work aimed at subjecting the kinetics of denitrification by microscale Fe⁰ (160-200 mesh) to analysis the factors affecting the denitrification of nitrate and the composition of iron reductive products coating upon the iron surface. Results of the kinetics study have indicated that a higher initial concentration of nitrate would yield a greater reaction rate constant. The reduction rate of nitrate increased with increasing Fe⁰ dosage. The reaction can be described as a pseudo-first order reaction with respect to nitrate concentration or Fe⁰ dosage. Experimental results also suggested that nitrate reduction by microscale Fe⁰ without reaction condition control primarily was an acid-driven surface-mediated process, and the reaction order was 0.65 with respect to hydrogen ion concentration. The analyses of X-ray diffractometry and X-ray photoelectron spectroscopy indicated that a black coating, consisted of Fe₂O₃, Fe₃O₄ and FeO(OH), was formed on the surface of iron grains as an iron corrosion product when the system initial pH was lower than 5. The proportion of FeO(OH) increased as reaction time went on, whereas the proportion of Fe₃O₄ decreased.