Solar photocatalysis of a recalcitrant coloured effluent from a wastewater treatment plant

Abstract

A photocatalytic study of a coloured effluent from a wastewater treatment plant was carried out in a pilot plant using compound parabolic collectors (CPC) in order to find out the best conditions for colour removal, since the discharge limit for this parameter is not achieved after conventional wastewater treatment. The interaction between ultraviolet natural radiation and TiO₂ strongly enhanced the colour degradation rate. Different TiO₂ concentrations were tested and the optimum concentration achieved was 200 mg L⁻¹. The use of peroxydisulfate (S₂O₈²⁻) as an additional electron scavenger gave a noticeable effect on colour and dissolved organic carbon (DOC) removal due to the formation of additional powerful oxidant species (OH˙ and SO₄˙⁻). However, hydrogen peroxide (H₂O₂) as additional oxidant was more efficient on colour degradation than S₂O₈²⁻. The amount of energy necessary for the same colour removal (C/C₀ < 0.1) was 6, 14 and 80 kJ_UV L⁻¹, respectively, for the following systems using sunlight: 200 mg L⁻¹ TiO₂ + 5 mM H₂O₂, 200 mg L⁻¹ TiO₂ + 2 mM S₂O₈²⁻ and 200 mg L⁻¹ TiO₂. The first-order kinetic constants (0.487, 0.207 and 0.053 L kJ⁻¹) and initial degradation rates (32.1, 10.0 and 2.2 mg kJ⁻¹) showed the same behaviour.