Ozonation of phosphonate antiscalant 1-hydroxyethane-1,1-diphosphonic acid in reverse osmosis concentrate: Kinetics, phosphorus transformation, and anti-precipitation property changes

Abstract

1-hydroxyethane-1,1-diphosphonic acid (HEDP) is an intensively used phosphonate antiscalant in reverse osmosis (RO) process. However, HEDP in RO concentrate can increase the eutrophication risks, chelate with heavy metals and inhibit the precipitation treatment of RO concentrate. Ozonation was found to be effective in removing HEDP and mitigating its risks. The removal efficiencies of HEDP and total organic carbon after 20 min of ozonation were 75.2% and 41.6%, respectively. The rate constant of HEDP with ozone and •OH was 0.58 M−1 s-1and 5.4 × 108 M−1 s−1, respectively. Organic phosphorus products and orthophosphates were formed during the process, accounting for 16.7% and 60% of the total phosphorus, respectively. A kinetic model of phosphorus transformation was established. Simulation results indicated that the apparent rate constants of orthophosphate formation from HEDP and organic phosphorus product were 1.1 × 10−3 s−1 and 1.3 × 10−3 s−1, respectively. pH was an important factor affecting HEDP degradation and phosphorus transformation. As the pH value increased from 3 to 9, the removal efficiency of HEDP increased from 6.8% to 95%, and the ratio of orthophosphate formation to HEDP removal increased by more than 3.4 times. The precipitate morphology analysis and scale inhibition ratio tests showed that ozone effectively reduced the anti-precipitation property of HEDP. The dose–response curves of HEDP samples with different ozonation extents indicated that the ozonation products of HEDP didn’t show anti-precipitation effect. The apparent rate constant of HEDP degradation in RO concentrate was 26% lower than that in UPW. The ratios of ozone consumption to HEDP removal were 7.7–9.4 mg-O3/mg-HEDP during ozonation of HEDP in actual RO concentrate.