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Semiconductor photocatalytic reduction is a relatively new technique for the removal of dissolved metal ions in wastewater. In this paper, physical adsorption and photocatalytic reduction of eight environmentally signi"cant metal ions in TiO suspensions of Degussa P25 and Hombikat UV100 were investigated. Critical scrutiny of the potential versus pC and pH diagrams reveals that any particular oxidation or reduction reaction can be promoted thermodynamically over other reactions by adding di!erent reactants. In order to elucidate better understanding of the kinetics of the photocatalytic process, operating parameters such as dissolved oxygen, presence of ferric and ferrous ions, organic reductants and their concentrations were examined in detail. It was observed that the presence of dissolved oxygen inhibits while the presence of organic reductants promotes photocatalytic reduction.

Arsenic contamination has been found in the groundwater of several countries. Photocatalysis can rapidly oxidize arsenite (As(III)) to less labile and less toxic arsenate (As(V)), which then can be removed by adsorption onto photocatalyst surfaces. This study investigates the photocatalytic oxidation of As(III) to As(V) as a function of As(III) concentration, pH, catalyst loading, light intensity, dissolved oxygen concentration, type of TiO2 surfaces, and ferric ions to understand the kinetics and the mechanism of As(III) oxidation in the UV/TiO2 system. Photocatalytic oxidation of As(III) to As(V) takes place in minutes and follows zero-order kinetics. Benzoic acid (BA) was used as a hydroxyl radical (.OH) scavenger to provide evidence for the .OH as the main oxidant for oxidation of As(III). The .OH radical was independently generated by nitrate photolysis, and kinetics of As(III) oxidation by the .OH radical was determined. Formation of salicylic acid (SA) from the oxidation of BA by .OH also demonstrates the involvement of .OH in the mechanism of As(III) oxidation. The effect of Fe(III) on As(III) oxidation at different pH values with and without TiO2 under UV light was examined. The results suggest that .OH is the dominant oxidant for As(III) oxidation. Two commercially available TiO2 suspensions, Degussa P25 and Hombikat UV100, were tested for the removal of arsenic through oxidation of As(III) to As(V) followed by adsorption of As(V) onto TiO2 surfaces. Results showed that complete removal of arsenic below the World Health Organization drinking water limit of 10 microg/L could be achieved.

An existing side-fired steam reformer is simulated using a rigorous model with proven reaction kinetics, incorporating aspects of heat transfer in the furnace and diffusion in the catalyst pellet. Thereafter, “optimal” conditions, which could lead to an improvement in its performance, are obtained. An adaptation of the nondominated sorting genetic algorithm is employed to perform a multiobjective optimization. For a fixed production rate of hydrogen from the unit, the simultaneous minimization of the methane feed rate and the maximization of the flow rate of carbon monoxide in the syngas are chosen as the two objective functions, keeping in mind the processing requirements, heat integration, and economics. For the design configuration considered in this study, sets of Pareto-optimal operating conditions are obtained. The results are expected to enable the engineer to gain useful insights into the process and guide him/her in operating the reformer to minimize processing costs and to maximize profits.

Semiconductor photocatalytic reduction is a relatively new technique for the removal of dissolved toxic metal ions from wastewater. In this paper, adsorption and photocatalytic reduction of Cr(VI) to Cr(III) inaqueous solutions by UV/TiO2 photocatalysis has been investigated. It has been observed that the pH of the solution plays an important role in this reaction. An acidic medium is favorable for Cr(VI) photocatalytic reduction, where 94% of Cr(VI) was photoreduced within 1 h at pH 3 when 2 g/L of TiO2 was used as the slurry. An adsorption study shows that the photocatalytic reduction mainly occurs on the surface of TiO2. The presence of Fe(III) improved the photocatalytic reduction of Cr(VI) because it was observed that an additional reaction between Fe(II) and Cr(VI) takes place in the UV/TiO2 reduction process. A new combination of photocatalytic reduction and metal ion coprecipitation using Fe(OH)3 for complete removal of aqueous Cr [Cr(VI) as well as Cr(III)] was designed, which reduced the chromium concentration from 30 ppm to 17 ppb for a simulated wastewater. Thermodynamic analysis showed that TiO2 cannot photoreduce Cr(III) to Cr(0), but reduction is possible with ZnS. When kinetic experiments were performed, it was observed that more than 86% of Cr(III) could be photoreduced to Cr(0) in 5 h with a ZnS catalyst.

This paper reviews the current knowledge on the occurrence, biodegradation, and photooxidation of nonylphenol (NP), octylphenol (OP), and bisphenol-A (BPA) in aquatic environment. Generally, the concentrations determined were 0.006-32.8, < 0.001-1.44, and 0.0005-4.0 mu g L(-1) for NP, OP, and BPA respectively in river waters worldwide. Anthropogenic activities that can lead to run-off and storm water discharge may contribute to such concentrations in rivers. Pathways for biodegradation of NP and BPA appear to be similar. The influence of ferric ions, oxalate, hydrogen peroxide, and dissolved organic matter (DOM) on the photooxidation of NP and BPA in natural water is presented. Several techniques including nanofiltration, adsorption, sonochemical, photocatalytic, chlorination, ozonation, and ferrate(VI) oxidation for removals of NP, OP, and BPA are also reviewed.

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