EPR investigations of structure of humic acids from compost, soil, peat and soft brown coal upon oxidation and metal uptake

Free radical concentration and their g-values in humic acids (HA) isolated from various sources were studied by quantitative EPR technique. EPR data f or HA formed during composting and natural humification processes occurring in soil, peat and brown coal are given. In more detail the EPR data were a nalyzed for brown coal HA under carbonization, air oxidation (150 degrees C ) as well as metal uptake (Ca(II), Zn(II), Cd(II), Hg(II), Co(II), Ni(II) a nd Cu(II)) and NO2 reaction. Two groups of metal complexes were distinguish ed on the basis of their interaction with free radicals in HA. Ca(II), Zn(I I), Cd(II) and Hg(II) ions increase Gee radical concentration, while Co(II) , Ni(II) and Cu(II) ions quench the radicals compared to the raw HA. This p henomenon can be explained either by the strong interaction of the metal io ns with active centres responsible for the quinone-hydroquinone-semiquinone equilibria. and/or by the antiferromagnetic interaction between radical sp ins and metal d orbitals. Gaseous ammonia was found to be a very useful bas e easily penetrating the solid matrix of HA and strongly influencing the eq uilibria. beta-Diketone groups present in HA react with NO2 yielding iminox y radicals. In the HA-metal complexes these structural units are engaged in metal coordirices. These radicals are sensitive to various physical and ch emical agents, e.g., radiation, redox reactions, acid-base reactions, etc. [2]. Due to the various functionalities in humic substances they readily fo rm simple humates and/or metal chelate complexes [3]. It was found that met al content in FA or HA is an additional factor which has influence on the f ree radical concentration in FA- or HA-metal complexes in aqueous solution [2-4]. Solid HA-metal systems are studied to a lesser extent. This paper presents various factors influencing the g-value and free radica l concentration in solid HA, e.g., the overall oxidation effects in compost , soil, peat and brown coal and reactions with gaseous ammonia, nitrogen di oxide as well as actions of various solvents. The main emphasis concerns th e role of metal binding to soft-brown-coal HA on the characteristics of org anic radicals present in these HA-metal systems.