Abstract
Iron (hydr)oxides are ubiquitous earth materials that have high adsorption capacities for toxic elements and degradation ability towards organic contaminants. Many studies have investigated the reactivity of synthetic magnetite, while little is known about natural magnetite. Here, we first report the reactivity of natural magnetites with a variety of elemental impurities for catalyzing the decomposition of H2O2 to produce hydroxyl free radicals (•OH) and the consequent degradation of p-nitrophenol (p-NP). We observed that these natural magnetites show higher catalytic performance than that of the synthetic pure magnetite. The catalytic ability of natural magnetite with high phase purity depends on the surface site density while that for the magnetites with exsolutions relies on the mineralogical nature of the exsolved phases. The pleonaste exsolution can promote the generation of •OH and the consequent degradation of p-NP; the ilmenite exsolution has little effect on the decomposition of H2O2, but can increase the adsorption of p-NP on magnetite. Our results imply that natural magnetite is an efficient catalyst for the degradation of organic contaminants in nature.
Highlights
Euhedral activity of magnetite in the degradation of organic pollutants by hydrogen peroxide (H2O2), while that of Ni has an inhibitory effect[12]
Various micro-intergrowth textures have been observed such as sandwich-type and trellis-type ilmenite lamella in the magnetite matrix[16], which are remarkably different from those synthetic ones, and may exert great effects on the reactivity of magnetite towards organic contaminants
Natural magnetite can potentially serve as a heterogeneous catalyst for the degradation of various organic compounds through the heterogeneous catalytic reaction since H2O2 is usually present in rain water and soil[17]
Summary
Activity of magnetite in the degradation of organic pollutants by hydrogen peroxide (H2O2), while that of Ni has an inhibitory effect[12]. Natural magnetite can potentially serve as a heterogeneous catalyst for the degradation of various organic compounds through the heterogeneous catalytic reaction since H2O2 is usually present in rain water and soil[17] In this respect, the reactivity of magnetite may be enhanced by either promoting the generation of hydroxyl radicals from H2O2, or increasing the adsorption capacity for the organic contaminants. The chemical composition and texture of the samples were characterized using X-ray diffraction (XRD), electron probe microanalysis (EPMA), inductively coupled plasma atomic emission spectroscopy (ICP-AES), scanning electron microscope (SEM) with backscattering electron (BSE) imaging, X-ray absorption fine structure (XAFS) spectroscopy as well as BET specific surface area and surface site density measurements The capacity of these magnetite samples in generating hydroxyl radicals (OH), and their capacity in catalyzing the degradation of p-nitrophenol (p-NP) were assessed by batch experiments. We chose p-NP as the model organic contaminant, because p-NP is one type of hazardous waste that is mainly produced during chemical processes, including petrochemical manufacturing, oil refining, rubber, wood preservation operations, pulp and paper mills as well as in the production of pesticides, paints and plastics. p-NP is highly persistent, bioaccumulative and toxic, and listed as priority pollutant by US Environmental Protection Agency[19,20]
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