A heterogeneous NiO catalyst was prepared by a precipitation process using nickel nitrate with oxalic acid and tested for heterogeneous oxidation of benzoic acid (BA) in the presence of peroxymonosulfate (PMS). It was found that the synthetic NiO is highly effective in heterogeneous activation of PMS to produce sulfate radicals ( ) and hydroxyl radicals (· OH), and also presents stable performance in the heterogeneous activation of PMS for BA degradation. Physicochemical properties of the NiO catalyst were characterized by several techniques, such as thermogravimetric analysis, Brunauer-Emmett-Teller, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. It was found that NiO and NiOOH were formed on the synthetic NiO catalyst and were stably distributed on the catalyst surface. Nearly 95% decomposition could be achieved in 30 min at the conditions of 500 ml 20 μM BA solution, 0.25 g catalyst, and [PMS]:[BA] = 30:1. The heterogeneous reactions, the effects of PMS concentration, and catalyst dosage on the BA degradation were investigated. The heterogeneous BA degradation reactions followed first-order kinetics. Additionally, quenching experiments proved that the dominant radical in the solution was · OH. The experiments results also showed that this approach is effective for the degradation of many other pollutants (such as tetracycline hydrochloride, 2, 4-dichlorophenol, Acid orange 7, rhodamine B, and methyl red). PRACTITIONER POINTS: A novel NiO material was fabricated for degradation of benzoic acid. The synthetic NiO catalyst comprised active NiO and NiOOH. The main radical for benzoic acid removal rate was · OH. A plausible mechanism for catalyzed degradation of the benzoic acid was proposed.
Keywords: NiO; heterogeneous activation; hydroxyl radicals; peroxymonosulfate; sulfate radicals.
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