Role of Humic Acid and Quinone Model Compounds in Bromate Reduction by Zerovalent Iron

Abstract

Experiments were conducted to examine the role of humic acid and quinone model compounds in bromate reduction by Fe(0). The reactivity of Fe(0) toward bromate declined by a factor of 1.3-2.0 in the presence of humic acid. Evidence was obtained that the quick complexation of humic acid with iron species and its adsorption passivated the iron surface and decreased the rate of bromate reduction by Fe(0). On the other hand, in the long run, the reduced functional groups present in humic acid were observed to regenerate Fe(II) and reduce bromate abiotically. Compared with the case of humic acid only, the simultaneous presence of Fe(II) and humic acid significantly increased the bromate removal rate. Fe(III)/Fe(II) acted as a catalyst in the oxidation of humic acid by bromate. Anthraquinone-2,6-disulfonate (AQDS) and lawsone did not cause any significant effect on the bromate reduction rate by Fe(0). However, the redox reactivity of lawsone in the presence of Fe(III) was evident, while AQDS did not show any under the tested conditions. The difference was attributable to the presence/ absence of reducing functional groups in the model compounds. The electron spin resonance further demonstrated that the redox functional groups in humic acid are most likely quinone-phenol moieties. Although the bromate reduction rate by regenerated Fe(II) is a few times slower than that by Fe(0), the reactive Fe(II) can be, alternatively, reductively formed to maintain iron surface activation and bromate reduction to prolong the lifetime of the zerovalent iron.