Measurement of hydroxyl radical activity in a soil slurry using the spin trap α-(4-pyridyl-1-oxide)-N-tert-butylnitrone

The spin frap compound alpha-(4-pyridyl-1-oxide) N-tert-butylnitrone (4-POBN) served as a probe to estimate the activity of Fenton-derived hydroxyl radicals ((OH)-O-.) 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)-O-.-(4-POBN) adduct produced by the reaction of 4-POBN with (OH)-O-. was observed via electron paramagnetic resonance to confirm the mechanism of 4-POBN transformation. The disappearance of 4-POBN with (OH)-O-. 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)-O-. concentration ([(OH)-O-.](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 1440 mu M, 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)-O-.. Reaction of H2O2 with (OH)-O-. was an appreciable sink for (OH)-O-., but the reaction did not: contribute significantly to H2O2 depletion. Nonproductive reactions involving H2O2 (those that did not contribute to (OH)-O-. production) represented a significant source of overall reaction inefficiency, as defined.