ELECTRON-SPIN-RESONANCE INVESTIGATIONS OF MN COMPOUNDS AND FREE-RADICALS IN PARTICLES FROM THE SEINE RIVER AND ITS ESTUARY

Paramagnetic species associated with particles in water samples from the Seine river and its estuary (in Northern France) were studied by electron spin resonance (ESR) spectroscopy. The ESR spectra obtained for these particles show the presence of. ( 1 ) Mn (II) embedded in calcite lattices in the form of a solid solution of Mn2+-CaCO3 (MnxCa1-xCO3): (2) Mn(III) ions associated with Fe(III) oxides; (3) organic radicals identified as semiquinone-type compounds. Because the solubility product for MnCO3 is never exceeded in these systems, the broad resonance of rhodochrosite is not detected. To confirm the importance of the role of iron on the paramagnetic behaviour of particulate Mn(III), we carried out several experiments with Mn(OH)2-Fe(OH)3 (co)precipitates. The detailed study of the ESR spectra obtained for these Mn-Fe oxides indicates that excess iron (with respect to manganese) stabilizes Mn(III) ions in the precipitates: the Fe/Mn weight ratio found in the particles from the Seine river is in the 60-75 range. When these particles were heated or treated with acid, the ESR spectra changed significantly. The results indicate that valuable information can be obtained from ESR spectroscopy, particularly on Mn(II), Mn(III) and Mn(IV).