title: Nature and abundance of organic radicals in natural organic matter: effect of pH and irradiation.
abstract: Dissolved natural organic matter (NOM) plays an essential role in freshwater geochemical and biochemical processes. A major property, its redox behavior, can be attributed to the chinone building blocks, which can form stable radicals. However, electron paramagnetic resonance (EPR) data indicating free radicals on solid NOM are sparse. Here we present EPR spectra of 23 NOM from European surface waters isolated by reverse osmosis. The organic radical concentrations of NOM ranged from 5 x 10(15) to 1.84 x 10(17) spins g(-1), and g values ranged from 2.0031 to 2.0045. Number and type of organic radicals in solid NOM are significantly influenced by the pH of raw water. EPR experiments indicate the presence of semiquinone-type radicals in coexistence with carbon-centered "aromatic" radicals, with the semiquinone-type radicals dominating at alkaline pH. Basically these processes are reversible. Organic radical concentrations in NOM adjusted to pH 6.5 before freeze-drying correlate with iron and aluminum contents. UV- and VIS-irradiation of solid NOM can lead to more than a 10-fold increase of the concentration of organic radicals. These radicals were long-lived and had the same g value as the original radical. Similar effects were not observed with isolated humic and fulvic acids, demonstrating the limited reflection of environmental properties of organic carbon by the classical isolation procedure.
authors.affiliation: Leibniz Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany. a.paul@igb-berlin.de
authors.forename: Andrea
authors.initials: A
authors.lastname: Paul
authors.forename: Reinhard
authors.initials: R
authors.lastname: Stösser
authors.forename: Andrea
authors.initials: A
authors.lastname: Zehl
authors.forename: Elke
authors.initials: E
authors.lastname: Zwirnmann
authors.forename: Rolf D
authors.initials: RD
authors.lastname: Vogt
authors.forename: Christian E W
authors.initials: CE
authors.lastname: Steinberg
mesh_terms: D001578:Benzopyrans / Q000737:chemistry / Q000528:radiation effects; D002850:Chromatography, Gel; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000032:analysis*; D006812:Humic Substances / Q000528:radiation effects*; D006863:Hydrogen-Ion Concentration; D017550:Spectroscopy, Fourier Transform Infrared; D014466:Ultraviolet Rays
chemical_list: D001578:Benzopyrans; D005609:Free Radicals; D006812:Humic Substances; C005023:fulvic acid