Measurement of Hydroxyl Radical Activity in a Soil Slurry Using the Spin Trap α-(4-Pyridyl-1-oxide)- N-tert-butylnitrone

The spin trap compound α-(4-pyridyl-1-oxide) N - tert -butylnitrone (4-POBN) served as a probe to estimate the activity of Fenton-derived hydroxyl radicals ( • OH) in a batch suspension comprised of silica sand and crushed goethite ore. The rate of probe disappearance was used to analyze system kinetics and oxidation efficiency under heterogeneous, Fenton-driven conditions. The • OH−(4-POBN) adduct produced by the reaction of 4-POBN with • OH was observed via electron paramagnetic resonance to confirm the mechanism of 4-POBN transformation. The disappearance of 4-POBN with • OH in the sand/goethite slurry amended with H 2 O 2 (150 mM) was first-order in 4-POBN. Under the conditions of this experiments, the effective steady-state • OH concentration ([ • OH] ss ) was estimated at 1.2 × 10 - 15 and 6.5 × 10 - 16 M, when 4-POBN was initially present at 700 and 1440 μM, respectively. Observed kinetics were in qualitative agreement with a conceptual model in which H 2 O 2 is consumed in both Fenton and non-Fenton reactions and 4-POBN competes with other slurry constituents for • OH. Reaction of H 2 O 2 with • OH was an appreciable sink for • OH, but the reaction did not contribute significantly to H 2 O 2 depletion. Nonproductive reactions involving H 2 O 2 (those that did not contribute to • OH production) represented a significant source of overall reaction inefficiency, as defined.