High-field 285 GHz electron paramagnetic resonance study of indigenous radicals of humic acids

Humic substances, the largest source of carbon on Earth, contain indigenous stable free radicals that are involved in important biogeochemical environmental processes occurring in soil and water systems. Here, we present the first high-magnetic-field 285GHz electron paramagnetic resonance spectra for humic acids from various geographical origins. All humic acids irrespective of their origin contain two limiting types of indigenous stable radicals, types I and II, with distinct electronic structure. Type I, which prevails at acidic pH 5, is characterized by a g tensor with principal values g(x)(I) = 2.0032, g(y)(I) = 2.0032, and g(z)(I), = 2.0023. Type II, which prevails at alkaline pH 12, is characterized by g(x)(II) = 2.0057, g(x)(II) = 2.0055, and g(x)(II) = 2.0023. The two limiting types are correlated in a unified reversible manner with pH, irrespective of the geographic origin of the HA. Both types of radical centers are consistent with pi-type radicals. They persist not only in liquid solutions but also in humic acid powders.