Catalytic reduction of nitrophenols and dyes by HKUST-1/hydrogel composite

The effective removal of nitrophenols from wastewater is crucial due to their high carcinogenic risk. This research presents the development of a copper-based metal-organic framework (HKUST-1) integrated into a chitosan-graft-poly(acrylic acid) hydrogel. The hydrogel composite was evaluated as a catalyst for reducing nitrophenols and dyes using sodium borohydride (NaBH₄) as a reducing agent. Various conditions were investigated for the reduction of 4-nitrophenol (4-NP) to the less harmful 4-aminophenol (4-AP), including catalyst dosage, NaBH₄ concentration, initial 4-NP concentration, temperature, and pH. The catalyst was able to completely reduce 4-NP within 25 minutes at a dosage of 3 g L^-1 and a NaBH₄ concentration of 300 mM. The reduction rate increased with higher temperatures, with an Arrhenius activation energy of 54.4 kJ mol^-1. Common anions found in surface water, such as Cl^-, NO₃ ^-, SO₄ ^2-, and HCO₃ ^-, had a slight impact on the reduction rate of 4-NP. When tested in real water environments, the reduction rate decreased, but complete conversion was still achieved. Additionally, the composite successfully reduced 100% of 2-nitrophenol, 100% of methyl orange, and 69% of Congo red. Overall, the hydrogel composite has shown significant potential as a catalyst for reducing various organic pollutants with high efficiency and easy separation through filtration.