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-pyridy-M-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 H₂O₂ (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 x 10-¹⁵ and 6.5 x 10^{- 16} M, when 4-POBN was initially present at 700 and 144 μM, respectively. Observed kinetics were in qualitative agreement with a conceptual model in which H₂O₂ is consumed in both Fenton and non-Fenton reactions and 4-POBN competes with other slurry constituents for ·OH. Reaction of H₂O₂ with ·OH was an appreciable sink for ·OH, but the reaction did not contribute significantly to H₂O₂ depletion. Nonproductive reactions involving H₂O₂ (those that did not contribute to ·OH production) represented a significant source of overall reaction inefficiency, as defined.