Photo-induced Phosphate Release from Organic Phosphorus Decomposition Driven by Fe(Ⅲ)-oxalate Complex in Lake Water

Abstract

The phosphate released from organic phosphorus photo-decomposition has a significantly influence on the phosphorus levels in the water column in lakes. In order to reveal the effect of organic phosphorus photo-decomposition on phosphate level in lake water, the phosphate released from organic phosphorus photo-decomposition driven by Fe(Ⅲ)-oxalate complex under UV-Vis and sunlight irradiation was investigated in natural lake water using glyphosate as the model organic phosphorus. The effects of pH and initial concentration of Fe(Ⅲ), oxalate and glyphosate on the phosphate released from glyphosate photolysis were studied. The results showed that phosphate could be released from glyphosate degradation by Fe(Ⅲ)-oxalate complex under UV-Vis and sunlight irradiation. The concentration of phosphate reached 0.25 mg·L-1 and 0.18 mg·L-1 under UV-Vis and sunlight irradiation for 60 and 720 min, respectively. The amount of phosphate released increased with the increase of the initial concentration of Fe(Ⅲ), as well as the increasing oxalate and glyphosate concentration in lake water. However, the increase of pH could significantly inhibit this process in the reaction system. The concentration of phosphorus decreased with the addition of isopropanol, which indicated that the hydroxyl radical (·OH) was one of the main active oxygen species of Fe(Ⅲ)-oxalate complex. The rates of·OH production for Fe(Ⅲ)-oxalate/UV-Vis and Fe(Ⅲ)-oxalate/sunlight systems were 0.52×10-2 μmol·(L·min)-1 and 0.03×10-2 μmol·(L·min)-1, respectively. The steady-state concentrations of hydroxyl radical (·OH) for the Fe(Ⅲ)-oxalate/UV-Vis conditions were 4.74×10-16 mol·L-1 and 0.27×10-16 mol·L-1 for the Fe(Ⅲ)-oxalate/sunlight system.