CaO₂/UV Process for One-Step Phosphorus Removal and Recovery from Hypophosphite: Simultaneous Oxidation and Precipitation

Abstract

Hypophosphite (P(I)) is ubiquitous in some waste streams. It may contribute to the eutrophication of water bodies, yet conventional methods are ineffective in treating P(I)-laden wastewater. Here, we propose a novel CaO2/ultraviolet (UV) system that can simultaneously remove P(I) and recover Ca-phosphate. This system lies in the simultaneous release of OH–, Ca2+, and H2O2, with the last being activated and producing reactive oxygen species (ROS) under UV irradiation. Subsequently, the generated ROS converts P(I) to phosphite and phosphate, forming Ca-phosphate with the released Ca2+ at pH 11.0. Beyond activating H2O2, UV light also promotes the dissolution of CaO2 powders. The system removes 97.0% of 1.0 mM P(I) in 6 h with dosing 2.0 mM CaO2. The presence of 5.0 mM HCO3 – drops P(I) removal from 97.0 to 14.1% by competing with P(V) toward Ca2+. Nitrate and humic acid show similar effects by adsorbing UV light or quenching ROS. However, we may overcome the adverse effects with an enhanced CaO2 dose. For example, with 10 mM CaO2, the CaO2/UV system removed more than 78% of non-ortho P from actual electroless rinse water. These results confirmed that CaO2/UV is an exciting all-in-one system for treating P(I)-laden wastewater and recovering valuable P products.

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

“…Our previous study on hypophosphite removal in the same system highlights the critical role of oxidation and precipitation . We believe that the exact mechanism applies here.…”

“…Our previous study on hypophosphite removal in the same system highlights the critical role of oxidation and precipitation. 44 We believe that the exact mechanism applies here. CaO 2 dissolution in bulk solution brings Ca 2+ , H 2 O 2 , and high pH (Figures 1c and S2).…”

“…In this case, the liquid samples were acidified without filtration. Then, the P i concentration was analyzed by a spectrophotometric method at a wavelength of 700 nm using a blue molybdenum complex. , In some cases, NTMP and its degradation intermediate products were also detected by Q-Exactive Liquid Chromatography-Quadrupole Orbitrap Mass Spectrometry (HPLC-MS, Q-Exactive, Thermo Fisher).…”

“…53 Additionally, the self-decomposition of H 2 O 2 is also recorded at pH > 10. 44 In short, although the initial concentration was set at the same level (1.5 mM), the effective H 2 O 2 dose under different bulk pH values was significantly different. Therefore, the conversion efficiency of P o to P i is lowered with an increased pH in the absence of Ca 2+ .…”

“…XRD characterization fails to identify any crystalline phase in the recovered solids except CaCO 3 (Figure 2c), which results from undissolved parts of commercial CaO 2 . 44 This points out that either the Ca-phosphate solids are in an amorphous phase or the crystalline phase is covered by amorphous or organic substances. Indeed, XPS characterization reveals the presence of P, Ca, C, O, and N (Figures 2b and S3).…”