Relationships between the structure of natural organic matter and its reactivity towards molecular ozone and hydroxyl radicals

Oxidation reaction rate parameters for molecular ozone (O-3) and hydroxyl ( HO) radicals with a variety of hydrophobic organic acids (HOAs) isolated fr om different geographic locations were determined from batch ozonation stud ies. Rate parameter values, obtained under equivalent dissolved organic car bon concentrations in both the presence and absence of non-NOM HO radical s cavengers, varied as a function of NOM structure. First-order rate constant s for O-3 consumption (k(O3)) averaged 8.8 x 10(-3) s(-1). ranging from 3.9 x 10(-3) s(-1) for a groundwater HOA to > 16 x 10(-3) s(-1) for river HOAs with large terrestrial carbon inputs. The average second-order rate consta nt (k(HO.DOC)) between HO radicals and NOM was 3.6 x 10(8) l(mol C)(-1) s(- 1); a mass of 12 g C per mole C was used in all calculations. Specific ultr aviolet absorbance (SUVA) at 254 or 280 nm of the HOAs correlated well (r > 0.9) with O-3 consumption rate parameters, implying that organic pi-electr ons strongly and selectively influence oxidative reactivity. HO radical rea ctions with NOM were less selective, although correlation between k(HO.DOC) and SUVA existed. other physical-chemical properties of NOM. such as aroma tic and aliphatic carbon content from C-13-NMR spectroscopy, proved less se nsitive for predicting oxidation reactivity. than SUVA. The implication of this study is that the structural nature of NOM varies temporally and spati ally in a water source, and both the nature and amount of NOM will influenc e oxidation rates. (C) 1999 Elsevier Science Ltd. All rights reserved.