FePO4 activated sulfite autoxidation for simultaneous pollutant degradation and phosphorus release

FePO4 (FP) as a precursor of lithium iron phosphate batteries has been mass-produced in recent years and may face the problem of overproduction. Also, the phosphorus removal process in wastewater treatment plants produces large amounts of sludge containing FP. It is a huge waste of both iron and phosphorus resources if such FP cannot be utilized. This work explores a new strategy for utilizing FP to activate sulfite (S(IV)) autoxidation for simultaneous pollutant degradation and phosphorus release in water. Acid orange 7 (AO7) and several pharmaceutical and personal care products (PPCPs) were chosen as the target pollutants for the FP-S(IV) system and the influence of pH, FP and S(IV) dosage were investigated. Typically, up to 89.5% AO7 (10 mg L-1) and above 60% PPCPs (10 mu M) were oxidized within 90 min by adding 1 g L-1 FP and 2 mM S(IV) into the water at the initial pH 4.0. Results of quenching experiments and electron spin resonance analysis confirmed SO5 center dot- as the dominant reactive species for AO7 oxidation. Meanwhile, two stages of phosphorus release from FP were observed along with the AO7 oxidation and their linear correlation over the pH range 3.0-8.0 was proven. The first stage (0-5 min) was thought to be dependent on the initial pH and was obviously faster than the second stage (10-45 min), whose rate increased linearly with S(IV) dosage. Phosphorus released high up to 24.3 mu M at 90 min with 4 mM S(IV). Moreover, the mechanisms of FP activated sulfite autoxidation as well as phosphorus release were proposed, and the AO7 degradation pathway was investigated. This study provides a novel possibility for FP waste utilization and potential recovery of phosphorus resources.