Use of carbon-based composites to enhance performance of TiO for the simultaneous removal of nitrates and organics from aqueous environments.

The simultaneous photocatalytic removal of nitrate from aqueous environment in presence of organic hole scavenger using TiO has long been explored. However, the use of unmodified TiO in such reaction resulted in non-performance or release of significant amount of undesirable reaction products in the process, a problem that triggered surface modification of TiO for enhanced photocatalytic performance. Previous studies focused on decreasing rate of charge carrier recombination and absorption of light in the visible region. Yet, increasing active sites and adsorption capacity by combining TiO with a high surface area adsorbent such as activated carbon (AC) remains unexploited. This study reports the potential of such modification in simultaneous removal of nitrates and oxalic acid in aqueous environment. The adsorptive behaviour of nitrate and oxalic acid on TiO and TiO/AC composites were studied. The Langmuir adsorption coefficient for nitrate was four times greater than that of oxalic acid. However, the amount of oxalic acid adsorbed was about 10 times greater than the amount of nitrate taken up. Despite this advantage, the materials did not appear to produce more active photocatalysts for the simultaneous degradation of nitrate and oxalic acid. The photocatalytic activity of TiO and its carbon-based composites was improved by combination with CuO particles. Consequently, 2.5 CuO/TiO exhibited the maximum photocatalytic performance with 57.6 and 99.8% removal of nitrate and oxalic acid, respectively, while selectivity stood at 45.7, 12.4 and 41.9% for NH, NO and N, respectively. For the carbon based, 2.5 CuO/TiO-20AC showed removal of 12.7% nitrate and 80.3% oxalic acid and achieved 21.6, 0 and 78.4% selectivity for NH, NO and N, respectively. Using the optimal AC loading (20 wt%) resulted in significant decrease in the selectivity for NH with no formation of NO, which unveils that selectivity for N and low/no selectivity for undesirable products can be manipulated by controlling the rate of consumption of oxalic acid. In contract, no nitrate reduction was observed with CuO promoted TiO-T and its TiO-(T)-20AC, which may be connected to amorphous nature of TiO-T and perhaps served as charge carrier trapping sites that impeded activity.