Microbial and peat effects on the oxidation of alpha-(4-pyridyl-1-oxide)-N-tert-butyl-nitrone by hydroxyl radicals in soil

Abstract

The Fenton reaction involves the reaction between H₂O₂ and ferrous iron that yields a hydroxyl radical (·OH). H₂O₂ has been used to generate ·OH in soil and aquifer material to oxidize undesirable contaminants. However, non-target chemical species, both natural and anthropogenic also react with ·OH. That is, non-target chemical species can "scavenge" ·OH and limit the "oxidation capacity" of the treatment system. A laboratory study was conducted to determine whether soil microbiota or soil organic matter in the form of peat would scavenge ·OH and compete with a probe compound (alpha-(4-pyridyl-1-oxide)-N-tert-butyl-nitrone (4-POBN)) for ·OH. 4-POBN was used because of the numerous advantages regarding its fate in soil and ease of analysis, however, it would be rare to find this probe as a contaminant in ground water or soil. A soil slurry composed of silica sand (SiO₂), goethite (FeOOH), 4-POBN and either a suspension of catalase-positive bacterium (Shewanella putrefaciens) or Pahokee peat was amended with H₂O₂. An analytical technique was used to estimate the steady-state ·OH concentration, the rate of ·OH production, the natural scavenging conditional rate constant, and the rate of ·OH scavenging. The oxidation efficiency was calculated which provided an empirical measure of treatment. Parameter values were compared to evaluate the roles of microbes and peat as determinants of ·OH availability under the conditions of the experiment. Microbes did not significantly effect the rate of ·OH production. The steady-state ·OH concentration was estimated to be approximately the same in the microbial-amended and unamended reactors, and the treatment efficiency was slightly greater in microbial-amended reactors. These results indicate that microbes had a minimal scavenging role. Pahokee peat increased the rate of ·OH production which was partially attributed to the iron contained in the peat. The conditional natural scavenging reaction rate constant was lower in the peat-amended reactors, and the steady-state ·OH concentration was greater in the peat-amended reactors. The rate of ·OH scavenging and the total ·OH scavenged were greater in the peat-amended reactors. Although the conditional natural scavenging rate constant was lower in peat-amended reactors, the rate of scavenging increased. This was attributed to the increased concentration of ·OH. The degradation rate of 2-chlorophenol, a common ground water contaminant, was successfully predicted using 4-POBN transformation data obtained under similar experimental conditions.