Mechanisms involved in the superior performance of the continuous Fe2+ dosing scheme over the single Fe2+ dosing scheme was thoroughly investigated. The kinetics and stoichiometry of the phenol removal/persulfate consumption strongly depended on the volumetric or molar Fe2+ feeding rate, Fe2+ concentration in the feed solution, and Fe2+ feeding mode (continuous or single dose). The process performance was determined by the molar Fe2+ feeding rate rather than the volumetric Fe2+ feeding rate or the Fe2+ concentration in the feed solution. The phenol degradation rate increased as the molar Fe2+ feeding rate increased to 2.77 mmol/min but decreased as the Fe2+ feeding rate increased further. The sulfate radical was predominant radical species formed in continuous Fe2+ dosing mode. The hydroxyl and sulfate radicals were both important in single Fe2+ dose mode. The presence of hydroxyl radicals in single Fe2+ dosing mode decreased the amount of phenol oxidation that occurred, probably because the hydroxyl radicals were readily scavenged by soil organic matter. Continuous Fe2+ dosing facilitated phenol mineralization, which was indicated by total organic carbon measurements and toxicity tests performed using Hyalella azteca.
Keywords: Continuous Fe(II) dosing; Persulfate; Sediment; Soil organic matter; Sulfate radical.
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