Influence of ultraviolet wavelengths on kinetics and selectivity for N-gases during TiO2 photocatalytic reduction of nitrate

For drinking water applications, photocatalytic reduction processes beneficially transform aqueous nitrate to innocuous nitrogen gases (N-gases) but can produce nitrite and ammonia as undesirable aqueous by-products. We hypothesize that by-product selectivity is a function of light source and photon fluence dose, such that discrete wavelengths can increase yield of desirable N-gases. Experiments performed under different wavelength irradiation (ultraviolet- [UV] A, B, C) reduced nitrate in water to differing extents based on pH over the range of 1-8 or the presence of soluble organic electron donors. At an equivalent photon fluence dose, the most rapid nitrate loss in acidic solutions occurred using a combination of three UV-light emitting diodes (285 nm, 300 mu, 365 nm) closely followed by a polychromatic medium pressure UV lamp. A polychromatic xenon light source was least effective in reducing nitrate. Nitrite is an important intermediate during photocatalytic reduction of nitrate. Nitrite absorbs 330-380 nm light with high quantum efficiency. Thus, polychromatic or monochromatic light sources with strong UV-A emission more rapidly convert nitrite to by-products than UV-C monochromatic light sources. Nitrous acid (HONO) has a higher molar absorptivity (epsilon) and quantum efficiency than nitrite ion (PKa = 3.39) around 350-370 nm. Selectivity towards N-gases is bifurcated at the nitrite intermediate and is strongly influenced by direct photolysis instead of photocatalytic reduction. Thus, the selectivity of by-products can be controlled by delivering light in the 350-370 nm wavelength range, where it enables photocatalytic processes to rapidly initiate NO3- reduction and delivers photons for direct photolysis of HONO.