Use of carbon-based composites to enhance performance of TiO2 for the simultaneous removal of nitrates and organics from aqueous environments

摘要

The simultaneous photocatalytic removal of nitrate from aqueous environment in presence of organic hole scavenger using TiO2 has long been explored. However, the use of unmodified TiO2 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 TiO2 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 TiO2 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 TiO2 and TiO2/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 TiO2 and its carbon-based composites was improved by combination with Cu2O particles. Consequently, 2.5 Cu2O/TiO2 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 NH4+, NO2- and N-2, respectively. For the carbon based, 2.5 Cu2O/TiO2-20AC showed removal of 12.7% nitrate and 80.3% oxalic acid and achieved 21.6, 0 and 78.4% selectivity for NH4+, NO2- and N-2, respectively. Using the optimal AC loading (20wt%) resulted in significant decrease in the selectivity for NH4+ with no formation of NO2-, which unveils that selectivity for N-2 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 Cu2O promoted TiO2-T and its TiO2-(T)-20AC, which may be connected to amorphous nature of TiO2-T and perhaps served as charge carrier trapping sites that impeded activity.