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 for HA formed during composting and natural humification processes occurring in soil, peat and brown coal are given. In more detail the EPR data were analyzed 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) and Cu(II)) and NO2 reaction. Two groups of metal complexes were distinguished on the basis of their interaction with free radicals in HA. Ca(II), Zn(II), 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 phenomenon can be explained either by the strong interaction of the metal ions with active centres responsible for the quinone-hydroquinone-semiquinone equilibria. and/or by the antiferromagnetic interaction between radical spins and metal d orbitals. Gaseous ammonia was found to be a very useful base easily penetrating the solid matrix of HA and strongly influencing the equilibria. beta-Diketone groups present in HA react with NO2 yielding iminoxy radicals. In the HA-metal complexes these structural units are engaged in metal coordirices. These radicals are sensitive to various physical and chemical agents, e.g., radiation, redox reactions, acid-base reactions, etc. [2]. Due to the various functionalities in humic substances they readily form simple humates and/or metal chelate complexes [3]. It was found that metal content in FA or HA is an additional factor which has influence on the free 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 radical concentration in solid HA, e.g., the overall oxidation effects in compost, soil, peat and brown coal and reactions with gaseous ammonia, nitrogen dioxide as well as actions of various solvents. The main emphasis concerns the role of metal binding to soft-brown-coal HA on the characteristics of organic radicals present in these HA-metal systems.