Efficient Treatment of Phosphonate-Laden Wastewater by CaO2/UV: Partial Oxidation, Complexation, and Coprecipitation

Unlike the case for inorganic phosphate (Pi), removing organic phosphorus (Po) is more challenging. Conventional methods for removing Po involve a two-step route, where Po is first converted to Pi by an advanced oxidation process, followed by subsequent chemical precipitation/coagulation. Here, we propose a one-step CaO2/UV method for simultaneous conversion and coprecipitation of phosphonates, a representative type of organic phosphorus compound. We discovered that the total P removal (82.0%) is much higher than the conversion efficiency of Po to Pi (26.7%) under a dose of 1.5 mM CaO2 and 4 h UV irradiation for nitrilotris-methylene phosphonic acid (NTMP) treatment, providing new directions for treating Po compounds without completely breaking down the molecular structure. We further found that a CaO2/Po molar ratio of 1.5 is ideal for inducing the partial breakdown of the NTMP structure, Ca-phosphate formation, and coprecipitation of NTMP and its degradation intermediates. Beyond that, we noted that the presence of Ca2+ enhances the reactivity of NTMP with hydroxyl radicals, as supported by the improved conversion efficiency of Po to Pi, i.e., 37.4% vs 9.1% under pHi 11. We also demonstrated the preliminary feasibility of the CaO2/UV system in treating the reverse osmosis concentrate. Our study offers a robust one-step approach and sheds new insights on the synergistic role of partial oxidation, complexation, and coprecipitation in dealing with phosphonate-laden wastewater.