Effect of initial solution pH on 4-nitrophenol oxidation through homogeneous/heterogeneous photo-Fenton process using goethite/ H2O2 system

The photodegradation of 4-nitrophenol (4-NP) through homogeneous/heterogeneous photo-Fenton process was examined in the presence of goethite (alpha-FeOOH)/H2O2/UV system in solution at different pH values. The degradation of 4-NP was achieved with excellent performance. Conversion reached 100%, 92% and 83% in the first 120 min of irradiation for acidic, point of zero charge (pzc) and weakly alkaline pH, respectively. Meanwhile, 69% of degradation was reached under strong alkaline pH conditions and this resulted in the weakest removal percentage under comparable experimental conditions. The rate of photocatalytic degradation followed pseudo-first order kinetics with kinetic constants of 6.30 x 10(-2) min(-1), 2.37 x 10(-2) min(-1), 1.73 x 10(-2) min(-1) and 0.70 x 10(-2) min(-1) at pH = 3.0, 5.5, 9.0 and 11.0, respectively. The iron(II) species dissolved in the solution enhanced the photocatalytic degradation and the hydroxyl radicals (OH) production. The involvement of free radicals was evidenced by using scavengers and the results revealed that OH is the main active species. The stability of goethite to repeated uses was investigated too and, after five runs, the removal efficiency of 4-NP was still complete, with values attaining 100%.