Fenton-like oxidation of organic pollutants in the presence of iron (II, III) oxides

Firstly, PCP was chosen as a model pollutant, to investigate the oxidation of PCP on the surface of magnetite used as heterogeneous catalyst. Oxidation experiments were carried out under various experimental conditions at neutral pH and correlated with the adsorption behavior. The surface reactivity of magnetite was evaluated by conducting the kinetic study of both H2O2 decomposition and PCP oxidation experiments. The occurrence of the optimum values of H2O2 and magnetite concentrations for the effective degradation of PCP could be explained by the scavenging reactions with H2O2 or iron oxide surface. All batch experiments indicate that Fenton-like oxidation of PCP was controlled by surface mechanism reaction and the species compete with each other for adsorption on a fixed number of surface active sites. The apparent degradation rate was dominated by the rate of intrinsic chemical reactions on the oxide surface rather than the rate of mass transfer. Raman analysis suggested that the sorbed PCP was removed from magnetite surface at the first stage of oxidation reaction. All X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy and chemical analyses showed that the magnetite catalyst exhibited low iron leaching, good structural stability and no loss of performance in second reaction cycle. Secondly, Rhodamine B (RhB) was chosen as a model compound pollutant. Two types of iron (II, III) oxides were used as heterogeneous catalysts and characterized by XRD, Mossbauer spectroscopy, BET surface area, particle size and chemical analyses. The catalytic efficiency of iron (II, III) oxide to promote Fenton-like reaction was examined at neutral pH. The adsorption to the catalyst changed significantly with the pH value and the sorption isotherm was fitted using the Langmuir model for both solids. Both sorption and FTIR results indicated that surface complexation reaction may take place in the system. The variation of oxidation efficiency against H2O2 dosage and amount of exposed surface area per unit volume was evaluated and correlated with the adsorption behavior in the absence of oxidant. There is also an optimum amount of H2O2 value for the degradation of RhB. The phenomena could also be explained by the scavenging effect of hydroxyl radical by H2O2 or by iron oxide surface (like the oxidation of PCP). Sorption and decolourization rate of RhB as well as H2O2 decomposition rate were found to be depended on the surface characteristics of iron oxide. The kinetic oxidation experiments showed that structural FeII content strongly affect the reactivity towards H2O2 decomposition and therefore RhB decolourization. Finally, the effect of chelating agent on the heterogeneous Fenton reaction rate of pentachlorophenol in the presence of magnetite was investigated. Six kinds of chelating agents including oxalate, EDTA, CMCD, tartarate, citrate and succinate were chosen. The PCP oxidation rate in this system was significantly improved by using chelating agents at neutral pH. The kinetic rate constant was increased by 5.7, 4, 3.2, 2.4, 2.5 and 1.7 times with oxalate, EDTA, CMCD, tartarate, citrate and succinate, respectively. The enhancement factor of heterogeneous oxidation rate was found to be not correlated with that of dissolved iron dissolution amount. In homogeneous Fenton system (dissolved Fe2+ or Fe3+), EDTA-driven reaction showed the highest oxidation rate, while oxalate seems to be more efficiency in heterogeneous Fenton system (Fe3O4). This observation could be explained by the inactivation of iron surface sites which become unavailable for the interactions with H2O2 to initiate Fenton-like reactions. These results demonstrated that the chelating agent-promoted dissolution of magnetite did not play the key role in determining the efficiency of heterogeneous Fenton oxidation.