Interaction of manganese(II) with carbonates in seawater: Assessment of the solubility product of MnCO 3 and Mn distribution coefficient between the liquid phase and CaCO 3 Particles

Abstract

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 Mn 2+-Ca 2+-CO 2− 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 Mn 2+ on CaCO 3 surfaces to form solid solutions according to xMn 2++CaCO 3(8)→Mn x Ca 1- x CO 3(8)+ xCa 2+ 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 MnCO 3 and the corresponding enthalpy have been assessed in marine environments by computational procedures: P s = 5.8×10 −12 and δH s ⋍6.1 J mol −1 . For low concentrations of dissolved Mn, calcite governs the partitioning equilibria. The distribution coefficient, D i, 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 Mn 2+ on calcite and to show the state of complete equilibrium, i.e. when the solid phase is regular. The equilibrium constant for the Mn 2+-exchange reaction as Mn 2++CaCO 3(8) ⇌ MnCO 3(8)+Ca 2+ 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).