Enhanced reduction of aqueous bromate by catalytic hydrogenation using the Ni-based Metal-organic framework Ni(4,4′-bipy)(1,3,5-BTC) with NaBH4

Abstract

A metal-organic framework catalyst containing immobilized Ni (Ni(4,4′-bipy)(1,3,5-BTC); MOF-Ni) was first synthesized for the enhanced selective reduction of aqueous bromate to environmentally benign bromide, and catalytic reduction was successfully achieved with NaBH4 in a batch reaction system. The reactivity of MOF-Ni for bromate reduction was optimized under different conditions (i.e., catalyst loading, NaBH4 dosage, suspension pH, and coexisting anions). Under the optimized conditions, Ni(4,4′-bipy)(1,3,5-BTC) showed complete bromate removal in 15 min with the fastest bromate reduction kinetics (k = 0.365 min−1) and the highest bromate removal capacity (2.25 mmol·g−1·h−1). The stability and reactivity of Ni(4,4′-bipy)(1,3,5-BTC) for bromate removal were enhanced by decelerating the passivation of MOF-Ni sites with adsorbed active hydrogen species. The material could remove bromate selectively in the presence of coexisting anions with slightly enhanced reactivity. MOF-Ni exhibited 100% removal of bromate throughout 6 continuous cycles of durability testing. Metal leaching from the structure of MOF-Ni was negligible over repeated cycling, and its structure was stable during the continuous reactions, suggesting that durable and stable Ni(4,4′-bipy)(1,3,5-BTC) could be applied to bromate treatment processes in real water and wastewater treatment plants. The experimental results provide a background understanding of the synthesis of a reactive MOF-Ni catalyst for the enhanced reduction of aqueous bromate and the development of novel catalytic water and wastewater treatment processes for the durable and stable selective removal of aqueous bromate.