Mechanism of Persulfate Activation by Phenols

The activation of persulfate by phenols was investigated to further the understanding of persulfate chemistry for in situ chemical oxidation (ISCO). Phenol (p<i>K</i>_a = 10.0) activated persulfate at pH 12 but not at pH 8, suggesting activation occurred only via the phenoxide form. Evaluation of the phenoxide activation mechanism was complicated by the concurrent activation of persulfate by hydroperoxide anion, which is generated by the base catalyzed hydrolysis of persulfate. Therefore, phenoxide activation was investigated using pentachlorophenoxide at pH 8.3, midway between the p<i>K</i>_a of pentachlorophenol (p<i>K</i>_a = 4.8) and that of hydrogen peroxide (p<i>K</i>_a = 11.8). Of the two possible mechanisms for phenoxide activation of persulfate (reduction or nucleophilic attack) the results were consistent with reduction of persulfate by phenoxide with oxidation of the phenoxide. The concentration of phenoxide required for maximum persulfate activation was low (1 mM). The results of this research document that phenoxides activate persulfate via reduction; phenolic moieties ubiquitous to soil organic matter in the subsurface may have a significant role in the activation of persulfate during its injection into the subsurface for ISCO. Furthermore, the results provide the foundation for activation of persulfate by other organic anions without the toxicity of phenols, such as keto acids

Degradation of Perfluorooctanoic Acid by Reactive Species Generated through Catalyzed H₂O₂ Propagation Reactions

Perfluorinated compounds, which are environmentally persistent and bioaccumulative contaminants, cannot currently be treated in the subsurface by <i>in situ</i> technologies. Catalyzed H₂O₂ propagation (CHP) reactions, which generate hydroxyl radical, hydroperoxide anion, and superoxide anion, were investigated for treating perfluorooctanoic acid (PFOA) as a basis for <i>in situ</i> chemical oxidation remediation of groundwater. Using 1 M H₂O₂ and 0.5 mM iron­(III), PFOA was degraded by 89% within 150 min. Hydroxyl radical does not react with PFOA, but systems producing only superoxide promoted 68% PFOA degradation within 150 min. In systems producing only hydroperoxide, the level of PFOA degradation was 80% over 150 min. The generation of near-stoichiometric equivalents of fluoride during PFOA degradation and the lack of detectable degradation products suggest PFOA may be mineralized by CHP. CHP process conditions can be adjusted during treatability studies to increase the flux of superoxide and hydroperoxide to treat PFOA, providing an easily implemented technology