Heterogeneous Ozonation of Model Drinking Water Contaminants Using Cbv-720 Zeolite
Advanced Oxidation Processes (AOPs) are designed to remove aqueous organic contaminants through their reaction with hydroxyl radicals (HO∙). Ozone is classified as an AOP due to its ability to produce hydroxyl radicals, as well as its ability to oxidize a wide range of organic contaminants. Due to the unselective nature of hydroxyl radicals, many organic and inorganic co-solutes can act as scavengers, reducing the efficiency of an AOP. Here we show a probe system for testing the hypothesis that adsorptive ozonation of contaminants may yield a more selective environment that favors neutral contaminant oxidation over oxidation of co-solutes.
Initial experiments examined the hydroxyl radical probe 3-nitro-α α, α,-trifluorotoluene (TFNT) at pH 7.8 during continuous ozone exposure with varying amounts of CBV 720 (SiO2:Al2O3 = 30, unit cell 24.3 and surface area = 780 m2/gram) zeolite with 0.05 wt% resulting in the fastest oxidation rates. Baseline results were obtained by observing the oxidation rate of TFNT in the presence and absence of a 0.05 wt% zeolite suspension. In the interest of higher sample throughput, a dosed ozone reaction system was developed. Inside this system, experiments investigated the behavior of HO∙ and its affinity for TFNT adsorbed to the zeolite. Addition of the radical scavenger bicarbonate ion decreased TFNT oxidation rates in solution, consistent with the effect of a negatively charged surface repelling carbonate from the local environment of the reaction, as well as the ability of carbonate to effectively scavenge HO∙. A second vi HO∙ scavenger, dichloroacetic acid (DCAA), was added which would not adsorb to the zeolite but remained in solution. This allowed an investigation into the behavior of HO∙ in the presence of a scavenger adsorbed to zeolite and a scavenger in solution.
Oxidation rates were not increased by the addition of zeolite during ozonation. The zeolite demonstrated the ability to sequester TFNT on its surface, making it harder for HO∙ to react and decreasing oxidation rates while having little to no effect on DCAA oxidation. Despite the inhibition of TFNT oxidation, the process was not stopped completely and TFNT was still mostly removed