Aquatic transformation of phosphite under natural sunlight and simulated irradiation

The phototransformation of phosphite (HPO₃^2-, H₂PO₃^-, +3) from Lake Taihu water (THW) under natural sunlight was evaluated. No direct phosphite photoreaction was observed under sunlight. Suspended solids were shown to play important roles in the indirect photoreaction of phosphite in lake water. The phototransformation of phosphite followed pseudo-first-order reaction kinetics and the kinetics constants (k, d^-1) decreased as: 0.0324 (original THW), 0.0236 (sterilized THW), 0.0109 (filtered THW) and 0.0102 (sterilized filtered THW). Original THW with 1 mmol L^-1 NO₃^- added was used to simulate the phosphite removal in lakes with serious N pollution. The results showed that the phototransformation was accelerated (with k increased to 0.0386-0.0463 d^-1), and sterilization or filtration shown little effect to the transformation, as the half-lives of phosphite drew closer. Under simulated irradiation in NO₃^- system, increasing NO₃^- concentration or decreasing pH value promoted phototransformation. The addition of Fe³⁺ or Fe²⁺ accelerated photooxidation, while the addition of Mn²⁺ or Cd²⁺ inhibited phototransformation. Br^-, NO₂^- and HCO₃^- in environmental concentrations decreased phototransformation, and HCO₃^- showed the strongest inhibition. Suwannee River humic acid or Suwannee River fulvic acid strongly inhibited the photooxidation process, and the inhibiting effects varied with their structure. Phosphite photooxidation was strongly inhibited by adding isopropanol or sodium azide as reactive oxygen species (ROS) quenchers. Electron spin resonance analysis indicated that OH was a main oxidant produced in this system. The increased amount of phosphate coincided with the decreased amount of phosphite, which indicated that the transformation product of phosphite was phosphate. Phosphite is a considerable component of the P redox cycle in Lake Taihu.