TiO ₂ / Fe ^{0 Photocatalytic Reduction Of Nitrate In Water}

With the development of industry and agriculture and the improvement of people’s living standard, the nitrate pollution has turn into a serious environmental problem. Nitrate is an important part of total nitrogen. In addition to the nitrate from external sources, the nitrite and ammonium nitrogen in the water can also be transformed to nitrate. However, due to the high solubility and the stable properties, the nitrate in the water is difficult to be eliminated through general physical or chemical methods. In recent years, the main approaches adopted to eliminate the nitrate include physical chemistry method, chemical reduction method, biological denitrification method and photocatalytic reduction method. Among these approaches, the chemical reduction and photocatalytic reduction methods, represented by the zero-valent iron reduction method and the titanium dioxide photocatalytic method, have bright prospect due to their merits of good effectiveness and steady operation. In this study, the sponge iron is chosen as the carrier and a new photocatalyst TiO2/Fe0is prepared by slow hydrolysis of steam. The performance the prepared photocatalyst for nitrate reduction is investigated, aiming at opening up a new creative path for controlling the nitrate pollution in the water.In this study, the nitrate in the water was transformed by sponge iron. The effects of particle size, solid-to-liquid ratio and coexisting ions on the transformation were investigated, and the transformed products and total iron from dissolution were analyzed contrastively. By dynamic tests, the effect of the hydraulic retention time and the nitrate concentration of influent were also investigated. The experimental results showed that, the pH value has an important effect on the transformation of nitrate by sponge iron, and specifically, a lower pH value was more beneficial. Due to the existed Cl-and SO42-will compete with NO3-for the binding sites in the surface of sponge iron, the transformation of nitrate might be inhibited to some extent. The hydraulic retention time has a great influence on the transformation, and under the experimental conditions, the optimum hydraulic retention time is7.7min with average NO3--N concentration of16.6mg-L-1in effluent. Products analysis demonstrates that the main products of sponge iron reacting with nitrates is NH4+-N. In this research, we invented a novel catalyst-TiO2/Fe0, which was developed by coating TiO2onto the surface of Fe0. The SEM and XRD analysis showed the crystal form of catalyst prepared had been the typical characteristics of anatase type and loading TiO2has a little impact on partical size of catalyst and there is little Fe0transforming Fe3O4in the roasting process of catalyst. This paper studied the catalytic performance of catalyst prepared, mainly investigating the impact of the preparation method, calcination temperature, capacity of TiO2, lamp type and dissolved oxygen on the nitrate conversion.The result indicated that treatment efficiency of the catalyst which was developed by using slow steam hydrolysis method was the best and the optimal parameter of nitrate conversion was that, the calcination temperature was450℃, capacity of TiO2was7.57%, dominant wavelength365nm UV lamp as the light source and nitrogen as the shielding gas.The performance of catalyst to nitrate reduction was evaluated via nitrate conversion rate and selectivity for nitrogen. Using formic acid as the hole scavenger, the photocatalytic activities and the selectivity for N2were investigated under the UV irradiation for photocatalytic reduction of nitrate solution. The result indicated that formic acid as the hole scavenger had great effect on the nitrate conversion. The effect of the formic acid concentration, the influent nitrate concentration, hydraulic retention time on nitrate conversion were further studied. The result demonstrated that, when the formic acid concentration was0.027mol.L-1, influent nitrate was50mg.L-1and hydraulic retention time was6.7min, the catalyst for nitrate reduction is the best photocatalytic activity. After12h UV irradiation, the conversion rate of nitrate reached88.2%and nitrogen selectivity was60.9%.From the mechanism of nitrate reduction, TiO2/Fe0catalyst and formic acid combined action to drive the nitrate reduction. Formic acid as the hole scavenger not only adjusted the pH of the solution but also promoted the reduction of nitrate due to the generated CO2radical which has great reducing capacity. TiO2/Fe0mainly palyed the role on the electron donor to reduction the nitrate in the water.