Use of Salicylic Acid as a Model Compound to Investigate Hydroxyl Radical Reaction in an Ozonation–Membrane Filtration Hybrid Process

In this study, experiments were conducted using salicylic acid (SA) as a hydroxyl radical probe to determine the importance of hydroxyl reactions in ozone–membrane filtration processes. Four processes were investigated: ozonation alone, membrane filtration alone, ozonation–ceramic membrane filtration, and ozonation–iron oxide-coated ceramic membrane filtration. Experiments were conducted at two different pH values: pH ca. 2.5 and pH ca. 7.0. The results show that at pH values <3.0, SA was not removed by either the reaction with molecular ozone, filtration, or sorption. The reaction rate increased significantly at pH values >7.0, suggesting that hydroxyl radical reactions control the reaction mechanism at higher pH. The results also show that the salicylate ion sorbed to the membrane, whereas SA acid did not. Greater than 95% removal of SA was achieved with the iron oxide-coated membranes compared to 80% percentage with the uncoated membrane (ozone dosage of 0.25 mg/min, treatment time: 240 min). The presence of 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-dihydroxybenzoic acid (2,5-DHBA) in the permeate were confirmed using GC/MS. 2,3-DHBA was found to be the predominate byproduct. DBHA is known to form as a result of the reaction of the hydroxyl radical with SA. A reaction mechanism is suggested to explain the enhanced SA removal by the hybrid process.