Parathion degradation and toxicity reduction in solar photocatalysis and photolysis

The solar photocatalytic degradation of methyl parathion was investigated using a circulating TiO₂/solar light reactor. Under solar photocatalysis condition, parathion was more effectively degraded than solar photolysis and TiO₂-only conditions. With solar photocatalysis, 20 mg/L of parathion was completely degraded within 60 min with a TOC decrease of 63% after 150 min. The main ionic byproducts during photocatalysis recovered from parathion degradation were mainly as NO₃⁻, NO₂⁻ and NH₄⁺, 80% of the sulphur as SO₄²⁻, and 5% of phosphorus as PO₄³⁻. The organic intermediates 4-nitrophenol and methyl paraoxon were also identified, and these were further degraded in solar photocatalytic condition. Two different bioassays (Vibrio fischeri and Daphnia magna) were used to test the acute toxicity of solutions treated by solar photocatalysis and photolysis. The Microtox test using V. fischeri showed that the toxicity expressed as EC₅₀ (%) value increased from 5.5% to >82% in solar photocatalysis, indicating that the treated solution is non-toxic, but only increased from 4.9 to 20.5% after 150 min in solar photolysis. The acute toxicity test using D. magna showed that EC₅₀ (%) increased from 0.05 to 1.08% under solar photocatalysis, but only increased to 0.12% after 150 min with solar photolysis, indicating the solution is still toxic. The pattern of toxicity reduction parallels the decrease in TOC and the parathion concentrations.