INTERACTION OF MANGANESE(II) WITH CARBONATES IN SEAWATER - ASSESSMENT OF THE SOLUBILITY PRODUCT OF MNCO3 AND MN DISTRIBUTION COEFFICIENT BETWEEN THE LIQUID-PHASE AND CACO3 PARTICLES

The chemical behavior of Mn derived from wastes discharged from a ferromanganese factory located in the harbour at Boulogne-sur-Mer (in Northern France) and its binding characteristics on particulate surfaces have been studied using furnace atomic absorption spectroscopy, plasma atomic emission (ICP), and electron spin resonance (ESR) spectroscopy. The renewal of seawater contributes to the alteration of Mn2+-Ca2+-CO2-3 systems. The evolution of ESR spectra observed with deep-water and surface-water particles accounts for these interactions and the extent of the incorporation of Mn2+ on CaCO3 surfaces to form solid solutions according to xMn2++CaCO3(8)→MnxCa1-xCO3(8)+xCa2+ At concentrations of Mn(II) close to 1.8 μM, rhodochrosite slowly appears in the solid phase. Taking into account the concentrations of several ionic and neutral species (or complexes) involved in seawater, the solubility product of MnCO3 and the corresponding enthalpy have been assessed in marine environments by computational procedures: Ps = 5.8×10-12 and δHs{reversed tilde equals}6.1 J mol-1. For low concentrations of dissolved Mn, calcite governs the partitioning equilibria. The distribution coefficient, Di, between the concentrations of Mn(II) and Ca(II) in the solid and solution phases has been used as a valuable means of assessing the extent of the surface phenomena induced by Mn2+ on calcite and to show the state of complete equilibrium, i.e. when the solid phase is regular. The equilibrium constant for the Mn2+-exchange reaction as Mn2++CaCO3(8) ⇌ MnCO3(8)+Ca2+ has also been calculated in natural media. The seawater sampling carried out near the ferromanganese factory at periods of spring tides has allowed us to propose that calcite makes an essential contribution to the stabilization of Mn(II). © 1990.