Ozonation of phosphonate antiscalants used for reverse osmosis desalination: parameter effects on the extent of oxidation

The recovery of usable product water in brackish water reverse osmosis (RO) desalination is limited by the potential for membrane scaling. This limited recovery results in high costs of disposal of the concentrate (waste) stream. Concentrate treatment to improve recovery is vital, and treatment that includes antiscalant oxidation prior to precipitation and solid/liquid separation might allow increased precipitation through antiscalant deactivation. Ozone and hydrogen peroxide were used to oxidize phosphonate-type antiscalants under varying conditions of water composition, pH, ozone dose, and antiscalant type; orthophosphate (P) is an oxidation product and was used to measure the extent of oxidation. Antiscalant oxidation increases with ozone dose (1-10mg/L O₃) and in the presence of calcium, from 1mg/L P to 7mg/L P for an antiscalant concentration of 27mg/L as P. The addition of hydrogen peroxide causes only minor increases in antiscalant oxidation. The extent of oxidation varies with pH as a function of metal-ligand speciation, with the doubly-protonated metal-ligand species dominating the reactivity of the antiscalant. In the presence of calcium, the primary oxidation pathway for phosphonate antiscalants is through direct reaction with ozone (70-80% of measured P). In the absence of calcium, ozone and hydroxyl radicals appear to contribute relatively equally to oxidation. These results suggest phosphonate antiscalants can be oxidized with relatively low ozone doses, thereby deactivating the chelating behavior of the antiscalant and allowing increased precipitation of scaling salts such as calcium and magnesium.