OXIDATION STATE(S) OF PARTICULATE MANGANESE IN WATERS FROM THE SEINE RIVER

The Mn-Fe (co)precipitation process has been studied at controlled pH and constant oxygen pressure (pH congruent-to 8.0 and P(O2) = 1 atm) by means of potentiometry (employing a platinum electrode). Taking into account previous results about the kinetics of Mn(II) oxygenation and owing to the detailed survey of the curve E vs. time, it has been possible to appraise the surface catalytic rate, k, of this process on Fe hydroxide: k = 5.8 x 10(+17) l4 mol-4 min-1. This rate constant has been found to be much higher than that obtained in the absence of iron: k(Mn) = (4.2 +/- 1.4) x 10(+15) 14 mol-4 Min-1, which is of the same order of magnitude as that reported previously for colloidal MnO2 surface. ESR studies have shown that manganese precipitates at oxidation state IV. Qwing to the speciation of manganese, the theoretical potential of the Mn2+/MnO2 couple involved can be reached using the Nernst equation. The value is comparable with that measured potentiometrically at a Pt electrode. This suggests that under our experimental conditions manganese has a significant contribution to the redox level of the medium when this does not contain oxygen. In natural media, phenomena are more complicated because particulate Mn (and Fe) may be at oxidation states < iv (and < III). Thus, ESR and Mossbauer studies on particles from the Seine river prove the existence of Mn(III) and Fe(II). However, the stability of Mn(III) cannot be explained by E-pH diagrams. From X-ray diffraction and chemical analyses, we believe that iron in excess plays a predominant role in the stabilization of this ion.