Catalytic oxidation of organic pollutants in wastewater via a Fenton-like process under the catalysis of HNO3-modified coal fly ash

The performance of acid-modified coal fly ash (CFA) was tested for use as a catalyst of a Fenton-like process in p-nitrophenol (p-NP) wastewater treatment. The results show that HNO3-modified coal fly ash (HFA) has a better catalytic capacity (96.6% p-NP removal rate) than those (<92% p-NP removal rate) of CFA modified by other acids (HCl, H2SO4, and H3PO4). The removal rate of the p-NP by the adsorption of HFA is less than 2.96%, which can be thought of as negligible compared to that removed by the catalytic oxidation process. Under the optimum experimental conditions (reaction time = 60 min, C-H2O2 = 170 mg L-1, C-HFA = 10.0 g L-1, pH = 2.0, mixing speed = 150 rpm, and temperature = 25 degrees C), a 98% p-NP removal rate is observed. HFA has an acute response to the change of temperature and higher temperature is welcomed (9.3% and 98% of the p-NP was removed at 25 and 50 degrees C, respectively, at 5 min). HFA can be reused 9 times with >91% of the p-NP removal rate, revealing an outstanding catalytic stability and reusability. The catalytic mechanism of HFA was discussed by comparing the physicochemical features of HFA with raw CFA and by proposing the reactions which occur in the Fenton-like process.