Bromate reduction in water by catalytic hydrogenation using metal–organic frameworks and sodium borohydride

Abstract

Catalytic hydrogenation of bromate is a promising approach to remove bromate from water considering that this approach not only removes bromate but also converts it to bromide. However, the reduction of metal catalysts and bromate reduction using a continuous H₂ purge inevitably leads to waste of H₂ gas. Thus, using solid H₂-releasing compounds (e.g., NaBH₄) to generate H₂ in situ seems to be more advantageous because solid H₂-releasing compounds are relatively safe and easy to handle. Recently, Metal–Organic Frameworks (MOFs) have attracted enormous attention owing to their superior physical and chemical characteristics in CO₂ capture and H₂ storage. Some non-noble-metal MOFs also have been extensively used as catalysts, including hydrolysis of NaBH₄ for H₂ production. Nevertheless, few studies have studied using MOFs as heterogeneous catalysts to produce hydrogen in situ and then reduce bromate. In fact, metallic catalysts coupled with NaBH₄ have not been evaluated for bromate reduction. Thus, this study aims to investigate whether MOFs/NaBH₄ can be an effective system to reduce bromate. The iron-bearing MOF, MIL-88A, and the cobalt-bearing MOF, ZIF-67, were selected owing to their non-noble metallic contents. While NaBH₄ alone was found to reduce bromate, the combination of MOFs/NaBH₄ considerably accelerated the hydrolysis of NaBH₄, thereby reducing bromate to bromide. The effect of NaBH₄ dosage was found to be the most influential on the removal efficiency and kinetics, while the effect of MOF loading was insignificant. Elevated temperatures and relatively acidic conditions were also more favorable for the bromate reduction. In the presence of other anions, MOFs/NaBH₄ still successfully reduced bromate to bromide. MOFs/NaBH₄ can be also re-used for the bromate reduction and ZIF-67/NaBH₄ even exhibited almost 100% recyclability after 5 cycles. The above-mentioned results indicate that MOFs can successfully facilitate the hydrolysis of NaBH₄ and subsequently reduce bromate to bromide in water.