Iron and manganese surface complex formation with extracted lignin. Part 2: characterisation of magnetic interaction between transition metal and quinonic radical by EPR microwave power saturation experiments

Abstract

EPR spectra of a Lignocellulosic Substrate (LS) extracted from wheat straw, were obtained after surface complexation by the metal cations Fe(III), Cu(II) and Mn(II). For each system, we determined the geometrical environment of the metal centre at 15 K, 77 K and room temperature for Fe(III), Cu(II), and Mn(II) complexes respectively. Power saturation experiments were also performed in order to show the interactions between metal ions and organic radicals. Unfortunately, it was not possible to apply this technique to the Cu(II)–LS system due to the superimposition of the metal and radical signals. The fitting of the curves of iron and manganese systems led to the determination of the half-saturation power value (P_1/2), which is dependent on the spin–lattice and spin–spin relaxation time. The presence of a sorbed metal induces a change in the relaxation time of the radical, which is evidence of interactions between these two paramagnetic species. Comparisons of the two metal centres Fe(III) and Mn(II) behaviour towards LS were made. This study gives useful information about interaction processes, which can be interpreted in terms of lignin degradation in soil influenced by transition metal ions.