Enhanced reductive removal of bromate using Acid-Washed Zero-Valent iron in the presence of oxalic acid

- Oxalic acid (OA) is used to mediate bromate reduction using acid-washed ZVI.
- Bromate reduction is remarkably enhanced by AZVI in the presence of OA = 250 μM.
- Addition of OA prevents the formation of passivation layer of iron oxides/hydroxides.
- The presence of OA enables AZVI to be recycled efficiency even without regeneration.

While zero-valent iron (ZVI) has been demonstrated to reduce bromate to bromide, ZVI rapidly loses its reductive power because it is oxidized and a passivation layer forms on its surface. In this study, oxalic acid (OA) is introduced to prevent the formation of passivation layer and maintain reductive reactivity of acid-washed ZVI (AZVI). Despite the fact that AZVI is able to remove bromate, bromate reduction by AZVI is remarkably enhanced in the presence of a very small amount of OA (250 μM). OA can coordinate with oxidized iron species (Fe3+) and thus prohibit the formation of passivation layer of iron oxides and hydroxides, allowing AZVI to remain highly reactive for reducing bromate to bromide completely. AZVI + OA also exhibits much lower activation energy than AZVI, indicating that AZVI + OA is much preferred for bromate reduction over AZVI. While bromate reduction by AZVI is considerably hindered under alkaline conditions, the presence of OA lessens the adverse effect and especially retains the reductive reactivity of ZVAI under weakly alkaline conditions. OA is also proven to make AZVI highly reactive even in the presence of co-existing anions without efficiency loss. More importantly, the presence of OA enables AZVI to be reused for bromate reduction quite efficiently even without acid-washing regeneration. These findings reveal that OA-mediated AZVI can enhance bromate reduction, enable AZVI to be highly reactive even under unfavorable conditions, and improve recyclability of AZVI for bromate reduction. These features validate that introduction of OA into bromate reduction by AZVI is advantageous and promising.

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