Solar photocatalytic degradation of propyl paraben in Al-doped TiO2 suspensions

The photocatalytic degradation of propyl paraben (PP) over Al-doped TiO2 suspensions under simulated solar radiation was investigated. The catalysts were prepared by a co-precipitation method with the dopant content varying from 0.04 to 2 wt%. The catalyst with 0.04 wt% dopant was more effective than the rest, presumably due to a more efficient separation of the photogenerated charges associated with a greater crystallite size. Subsequent experiments with this catalyst showed that the rate of PP degradation (i) generally increased with increasing catalyst concentration in the range 125-1000 mg/L, and decreasing PP concentration (in the range 210-1687 mu g/L), and (ii) was impeded at alkaline conditions (pH range 3-9). Moreover, degradation in pure water was always faster than in actual (i.e. bottled water, treated wastewater, surface water) or synthetic (i.e. pure water spiked with humic acid, t-butanol, chloride and bicarbonate ions) water matrices, pinpointing the competitive behavior of non-target water constituents. Conversely, the addition of persulfate had a beneficial effect on the rate of degradation since it acted both as electron acceptor and a source of extra sulfate radicals.