EPR investigations of structure of humic acids from compost, soil, peat and soft brown coal upon oxidation and metal uptake |
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Authors: | A Jezierski F Czechowski M Jerzykiewicz J Drozd |
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Institution: | (1) Department of Biophysics, School of Pharmacy and Laboratory Medicine, Medical University of Silesia in Katowice, Jedności 8, 41-200 Sosnowiec, Poland;(2) Artificial Heart Laboratory, The Zbigniew Religa Foundation of Cardiac Surgery Development, Wolności 345a, 41-800 Zabrze, Poland |
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Abstract: | Free radical concentration and theirg-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°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 free 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. β-Diketone groups present in HA react with NO2 yielding iminoxy radicals. In the HA-metal complexes these structural units are engaged in metal coordination which lowers
effectiveness of the iminoxyl synthesis. |
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