UV/chlorine oxidation of the phosphonate antiscalant 1-Hydroxyethane-1, 1-diphosphonic acid (HEDP) used for reverse osmosis processes: Organic phosphorus removal and scale inhibition properties changes

Abstract

Reverse osmosis (RO) technology plays an increasingly important role in municipal wastewater reclamation. However, the antiscalants used in RO systems showed adverse effects to the ecosystem: impending the removal of hardness from RO concentrates; inducing phosphorus pollution in receiving water; enhancing the trace metal migration in the environment. In this study, UV/chlorine advanced oxidation process was used to oxidize a typical phosphoric antiscalant (1-Hydroxyethane-1, 1-diphosphonic Acid, HEDP). UV/chlorine showed significant synergetic effects on HEDP degradation compared to UV irradiation or chlorination alone. Compared to UV/H2O2 oxidation, UV/chlorine process is more efficient for HEDP transformation with chlorine dosages ranging from 0.1 to 0.4 mmoL/L. Chorine dosage showed dual effects on HEDP oxidation by UV/chlorine: the increasing trend of transformation efficiency of HEDP got slower with increasing chlorine dosage. The transformation efficiency of HEDP by UV/chlorine oxidation decreased from 39% to 14% with pH increasing from 4.5 to 9.0, likely due to the higher quantum yields and lower radical quenching rates of HOCl than those of OCl−. The transformation efficiency of HEDP decreased 14% and 42% with 30 mM of chloride and bicarbonate, respectively. The presence of nitrate promoted the oxidation of HEDP by UV/chlorine: the transformation efficiency increased 5% and 83% with the presence of 5 mM and 30 mM nitrate, respectively. Based on the static scale inhibition tests, UV/chlorine oxidation is effective at removing the scale inhibition ability of HEDP. During UV/chlorine process, the maximum scale inhibition ratio decreased from 66% to 34% as the removal of phosphonate ligand from HEDP increased to 80%.