Effect of pH, charge separation and oxygen concentration in photoelectrocatalytic systems: active chlorine production and chlorate formation

In this study, photoanodic chlorine generation and chlorate formation were investigated using a nanoporous TiO2 photo-electrode under an illumination intensity of 7.4 mW/cm2 UV and biased at +1.0 V vs. SCE. Chlorine formation increased over time finally reaching a steady-state concentration of 42 mg/l in 45 min. On the other hand, chlorate formation was observed after only 5 min irradiation and increased over time. The effect of pH, oxygen concentration and charge separation on chlorine generation and chlorate formation was also investigated. Chlorine production was practically constant over a pH range of 4.0–8.0, but decreased slightly at higher pH values. Chlorate production could be detected at pH values ≥7.0. A significant effect of oxygen concentration on chlorate formation was observed. Chlorate was significantly reduced under nitrogen gas. Chlorine production in the separated photoanode compartment was about 10 fold higher than that in a combined photoanode–cathode system.

Chlorine generation and chlorate formation was studied using a PEC treatment system under different conditions and the following conclusions may be drawn: 1. A PEC experiment was employed at initial pH of 8.0. The pH in the photoanode compartment decreased over time yielding a value of 3.2 after 45 min of reaction time. A constant concentration of 42 mg/l of active chlorine was achieved after 45 min in the photoanode chamber while chlorate concentration was found to be 0.141 mg/l after 45 min but slightly decreasing over time. A complementary PEC experiment was performed under a controlled pH of 8.0. Chlorine production was almost the same while chlorate formation was found to be 1.3 mg/l and increased over time probably due to the high ClO– levels at a constant pH of 8.0. 2. Chlorine generation was also investigated under different initial pH values. A constant active chlorine production was achieved (around 40 mg/l) in the pH range of 3.2–8.0. This production decreased sharply at initial pH > 8.0. Chlorate formation was observed at initial pH values <7.0 and increased with increasing pH up to 9.0. Subsequently chlorate concentrations decreased due to reducing levels of chlorine production at these higher pH values. 3. The effect of oxygen on chlorine generation was not significant. On the other hand, its effect was more significant with respect to chlorate production with chlorates decreasing about 52% by removing oxygen and 75% under nitrogen. 4. Two different PEC (separated and combined photoanode-cathode) configurations were also used to investigate charge separation on the chlorine generation and chlorate formation. Chlorine generation (4.2 mg/l) in the combined photoanode– cathode system was 10 fold lower than that (39 mg/l) in the separated photoanode cell. On the other hand, chlorate concentrations of 0.428 mg/l, observed in the combined system at initial pH = 8.0 was around 3 times higher than that measured in the separated photoanode side under same initial concentration.