Identification and manipulation of active centers on perovskites to enhance catalysis of peroxymonosulfate for degradation of emerging pollutants in water.

Perovskites (the general formula of ABO3) with versatile substrates can serve as desirable catalysts to initiate advanced oxidation processes (AOPs) for environmental remediation. However, the knowledge regarding the active centers remains piecemeal and unclear, such as how the redox metal centers of B site, inert metals of A site, oxygen vacancies, and direct oxidation of catalysts govern the chemical degradation of aqueous pollutants. This study aimed to identify principal alternations in physicochemical and electrical properties of ABO3-based perovskites modified with partial/overall substitution at A/B sites and synthesized at different conditions. In order to probe varied catalytic activity of these catalysts, ofloxacin (OFX) was used as a model micro-pollutant. Results showed that the OFX degradation by activation of peroxymonosulfate (PMS) with LaFeO3 perovskite was favored by the Sr substitution at A site, Cu substitution at B site, and increasing calcination temperature. Evolution of 1O2, •OH and SO4•- have proven for efficient OFX oxidation, as evidenced by results from in-situ electron paramagnetic resonance (EPR) analyses and quenching tests. Specifically, the introduction of Sr at A site can facilitate PMS self-decomposition to produce more 1O2 due to the increased abundance of surface oxygen vacancies. In contrast, the Cu substitution at B site improved the surface oxygen vacancies, as well as the electrical conductivity, which can further accelerate •OH and SO4•- generation for the OFX degradation. This study provides deeper insights into the underlying mechanisms governing the catalytic activity of perovskites. These findings build a basis for better decontamination of hazardous environmental organic pollutants.

Characterization and reactivity of biogenic manganese oxides for ciprofloxacin oxidation.

Biogenic manganese oxides (BioMnOx) were synthesized by the oxidation of Mn(II) with Mn-oxidizing bacteria Pseudomonas sp. G7 under different initial pH values and Mn(II) dosages, and were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and UV-Vis absorption spectroscopy. The crystal structure and Mn oxidation states of BioMnOx depended on the initial pH and Mn(II) dosages of the medium. The superoxide radical (O(·-)2) was observed in Mn-containing (III/IV) BioMnOx suspensions by electron spin resonance measurements. BioMnOx(0.4)-7, with mixed valence of Mn(II/III/IV) and the strongest O(·-)2 signals, was prepared in the initial pH 7 and Mn(II) dosage of 0.4 mmol/L condition, and exhibited the highest activity for ciprofloxacin degradation and no Mn(II) release. During the degradation of ciprofloxacin, the oxidation of the Mn(II) formed came from biotic and abiotic reactions in BioMnOx suspensions on the basis of the Mn(II) release and O(·-)2 formation from different BioMnOx. The degradation process of ciprofloxacin was shown to involve the cleavage of the hexatomic ring having a secondary amine and carbon-carbon double bond connected to a carboxyl group, producing several compounds containing amine groups as well as small organic acids.

Mechanism of the anodic oxidation of 4-chloro-3-methyl phenol in aqueous solution using Ti/SnO2-Sb/PbO2 electrodes.

Electrochemical oxidation of 4-chloro-3-methyl phenol (CMP) was examined using Ti/SnO(2)-Sb/PbO(2) anodes. The physicochemical properties of the electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. The degradation was studied by monitoring the total organic carbon (TOC) removal of CMP, and variation of the concentration of intermediates by high-performance liquid chromatography (HPLC), ion chromatography (IC) and gas chromatography/mass spectrometry (GC/MS). The mineralization of CMP is confirmed to be controlled by mass transfer or by both chemical reaction and mass transfer. Hydroxyl radicals (OH) and active chlorine on the electrode surface had a dominant role in the electro-oxidation process. The chloride element in CMP was immediately driven away from parent substance by OH attack, and then accelerated the ring cleavage of methyl-p-benzoquinone, which was formed during the anodic oxidation of CMP. Ultimately, the chlorine of CMP was mainly transformed to hypochlorite and chloride ion in aqueous solution. Additionally, formic acid and acetic acid were relatively stable products that were not electro-oxidized efficiently in our experiments. The degradation pathway of CMP is proposed on the basis of these results.

Preparation of a titanium dioxide photocatalyst codoped with cerium and iodine and its performance in the degradation of oxalic acid.

A novel class of visible light-activated photocatalysts was prepared by codoping TiO(2) with cerium and iodine (Ce-I-TiO(2)). The particles were characterized using the Brunauer-Emmett-Teller method, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and UV-Visible light absorption. Particles of Ce-I-TiO(2) had greater photoabsorption in the 400-800 nm wavelength range than iodine-doped TiO(2) (I-TiO(2)). The effects on the photocatalytic degradation of oxalic acid under visible light or UV-Visible light irradiation were investigated. The photocatalytic activity of Ce-I-TiO(2) calcined at 673 K was significantly higher than that of Ce-I-TiO(2) calcined at 773 K and I-TiO(2) calcined at 673 K in aqueous oxalic acid solution under visible light or UV-Visible light irradiation. Under visible light irradiation, oxalic acid was first adsorbed on the surface of the catalysts rather than reacted with free radicals in the bulk solution, and then oxidized by (·)OH(ads) to CO(2), which was verified by studying the effects of nitrogen purging and scavengers, as well as by gas chromatography/mass spectrometry.