Schwertmannite Photocatalytic Reduction Of Cr (Ⅵ) By Small Molecular Weight Organic Acids

In recent years, chromium (Cr) contamination in soils and natural waters originating from various industrial processes such as mining, leather tanning, electroplating is a significantly environmental concern. In natural environment, chromium presents mainly in two oxidation states:Cr (Ⅵ) and Cr (Ⅲ). Cr (Ⅵ) is highly soluble, mobile and toxic to humans, animals and plants. Conversely, Cr (III) has relatively low toxicity and mobility, and is one of the micronutrient elements in humans. Thus, the approaches that convert Cr (VI) into Cr (Ⅲ) in contaminated soils and waters have received considerable attention. Schwertmannite is an Fe (Ⅲ)-oxyhydroxysulfate that primarily forms in iron-and sulfate-rich acid mine drainage (AMD). This paper selected Cr (Ⅵ) as a target and mainly studies the kinetics and mechanism of schwertmannite photocatalytic reduction of Cr (Ⅵ) by small molecular weight organic acids (SOAs) under an illumination of mimic solar light.In chapter Ⅰ:A new type oxyhydroxysulfate of iron schwertmannite was obtained through oxidation of ferrous sulfate by Acidithiobacillus ferrooxidans cells (i.e. A. ferrooxidans LX5). The target product was analyzed and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. The adsorption of Cr (Ⅵ) by schwertmannite in0.01M phosphate medium has also been studied. The results demonstrated that the ocherous precipitates formed through oxidation of ferrous sulfate by A. ferrooxidans LX5for72h were identified as pure schwertmannite and were spherical particles of uniform size with a diameter of approximately2.5μm, having no obvious characteristic pin-cushion morphology. The adsorption of Cr (Ⅵ) by schwertmannite was completely inhibited by0.01M phosphate at pH3.0-5.0and25℃.In chapter Ⅱ:A series of bath experiments were carried out to investigate the photoreduction of Cr(Ⅵ)by oxalic acid catalyzed by schwertmannite. The results indicated that schwertmannite or oxalic acid alone was unable to effectively transform Cr (Ⅵ) to Cr (Ⅲ) even under an irradiation of mimic solar light. The photoreduction of Cr (Ⅵ) was significantly enhanced in the coexistence of schwertmannite and oxalic acid because Fe (Ⅲ) on schwertmannite was extracted by oxalic acid and then Fe (Ⅲ)-oxalic acid complexes with high photochemical activity formed. The photocatalytic reduction of Cr (VI) was strongly dependent on pH, schwertmanite loading and the initial concentrations of oxalic acid. Lower pH and higher concentration of oxalic acid benefit the photoreduction of Cr (Ⅵ). Under the experimental conditions, Cr (Ⅵ) was almost completely removed within50min at pH3.0, with a removal rate of99.2%; the optimum dosage of schwertmannite was0.6g/L and the optimum molar ratio of oxalic acid/Cr (Ⅵ) is3:1. The schwertmannite-catalytic photoreduction of Cr (Ⅵ) in the presence of excess oxalic acid obeyed to zero-order kinetic with respect to Cr (Ⅵ). The surface morphology of schwertmanite basically had no change after the photoreduction of Cr (Ⅵ) and schwertmannite could be resued. Our research has further comfirmed that schwertmannite has good photo-catalytic ability and can be used to reality.In chapter Ⅲ:In this study, the effects of schwertmannite on the photoreduction of Cr (Ⅵ)by SOAs(citric acid, tartaric acid)were investigated at pH3.0-5.0and25℃in batch experiments.The results indicated that an addition of schwertmannite significantly enhanced the photoreduction of Cr (Ⅵ) by SOAs under lighting condition; the catalysis of schwertmannite under sunlight is superior to that under mimic solar light. Of the tested three SOAs, the efficiency for photocatalytic reduction of Cr (Ⅵ) was in the order of oxalic acid>citric acid>tartaric acid.The effects of various factors such as pH, schwertmanite loading and the initial concentrations of SOAs and substrate on the potoreduction of Cr (Ⅵ) and its mechanism were also discussed. It was found that the photocatalytic reduction of Cr (Ⅵ) was accelerated with decreasing pH or increasing SOAs concentration; the optimum molar ratio of citric acid/Cr (Ⅵ) and tartaric acid/Cr (Ⅵ) is respectively3:1and5:1; and the reaction obeyed to zero-order kinetic model in a wide range of pH. A possible mechanism for photoreduction of Cr (Ⅵ) by SOAs assisted with schwertmannite was proposed. Partial Fe (Ⅲ) on the structure of schwertmannite was extracted by SOAs, and then Fe (Ⅲ)-SOA complexes with high photochemical activity formed. Further, Fe (Ⅱ) together with organic acid radicals, CO2·-and02·-were generated through a metal-ligand-charge-transfer (MLCT) pathway, leading to a rapid reduction of Cr (Ⅵ). It was indicated that schwertmannite as a potentially excellent photo-catalyst would play an important role in treatment of Cr (Ⅵ)-contaminated water combined with organics and sunlight.