Efficient degradation of p-nitrophenol by Fe@pomelo peel-derived biochar composites and its mechanism of simultaneous reduction and oxidation process.

In this study, waste pomelo peels (PP) mixed with iron salts was treated successively with hydrothermal and pyrolyzing carbonization processes to obtain Fe(0) containing biochar composites (Fe@PP-Hy-Py) and the catalytic degradation of p-nitrophenol (PNP) using these Fe@PP-Hy-Py composites was studied. The results showed that the hydrothermal pre-treatment of the mixture of iron salts and pomelo peels was favorable for the incorporation of iron precursor within biomass network, which enabled copolymerization during the following pyrolysis. Through the pyrolysis process, the iron precursor was reduced in situ to amorphous Fe(0) dopped into the carbonaceous matrix, which conversely decreased the defect and disorder degree of pseudo-graphitic carbons and catalyzed the formation of environmental persistent free radicals (EPFRs). Degradation tests showed that the composites obtained at 600 °C with the theoretical Fe mass loading of 10% exhibited the greatest PNP degradation efficiency. Over 90% of 10 mg/L PNP was removed in 2 min under both N and air conditions with 1.0 g/L of catalyst level. The degradation kinetics of PNP were all well fitted by the pseudo-first-order kinetics model with k of Fe@PP-Hy-Py being 0.953 min. HPLC-QTOF/MS analysis demonstrated that both oxidation and reduction of PNP occurred as indicated by the detection of 4-aminophenol and ring opening compounds. The Fe(0) on the Fe@PP-Hy-Py was responsible for the reduction of PNP, while oxidation was induced by EPFRs. This study highlights the feasibility of synthesizing active heterogeneous Fe(0)-biochar composites by hydrothermal-pyrolysis route and the associated mechanisms of pollutant degradation.