Continuous Fe(II)-dosing scheme for persulfate activation: Performance enhancement mechanisms in a slurry phase...

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.