Mineral‐catalyzed Fenton‐like oxidation of sorbed chlorobenzenes

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Abstract
The oxidation of 1,3,5‐trichlorobenzene, 1,2,3,4‐tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene sorbed on hematite (α‐Fe2O3), a naturally occurring soil mineral, by catalyzed hydrogen peroxide was investigated using the hematite as the sole source of the iron catalyst. Partitioning of the chlorobenzenes onto the hematite was documented and essentially each of the chlorobenzenes was initially found in the sorbed state. Subsequent desorption measurements using gas‐purge methodology showed that the rate of chlorobenzene desorption decreased as a function of chlorine substitution. The first‐order rate constants for the desorption of 1,3,5‐trichlorobenzene, 1,2,3,4‐tetrachlorobenzene, and pentachlorobenzene were 0.091, 0.051, and 0.029 hr−1, respectively. Hexachlorobenzene desorption was undetectable over 144 hr. Hematite‐chlorobenzene slurries were treated with H2O2 concentrations ranging from 0.1 to 5% at pH 3. The degradation of 1,3,5‐trichlorobenzene, 1,2,3,4‐tetrachlorobenzene, and pentachlorobenzene using H2O2 concentrations ≤1% proceeded at rates less than their corresponding rates of desorption, suggesting that desorption controlled the rates of oxidation. When H2O2 concentrations ≥2% were used, the degradation rates of the three lower chlorobenzenes exceeded the rates of desorption suggesting that oxidation was occurring, at least in part, in the sorbed state. Hexachlorobenzene was not degraded by H2O2 concentrations ≤5%. Hydroxyl radical generation rates were not significantly different in all of the hematite‐catalyzed systems, indicating that surface catalysis mechanisms were saturated with respect to H2O2 (i.e., were characterized by 0‐order kinetics), even at the lowest H2O2 concentrations. The data show that sorption significantly affects rates of mineral‐catalyzed H2O2 oxidations, and that different mechanisms (e.g., surface‐catalyzed oxidation) may be occurring at H2O2 concentrations ≥2%.

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