The interaction of manganese(II) with the surface of calcite in dilute solutions and seawater

Abstract

The interaction of Mn 2+ with the surface of calcite in aqueous solutions is complex. In dilute solutions the Mn 2+ is rapidly absorbed, MnCO3 3 nucleates on the calcite surface and then grows by a first order reaction with respect to the initial Mn 2+ concentration. At higher ionic strengths in NaCl solutions, the rate of these processes is slower, but the same general pattern persists. In solutions containing Mg 2+, at the concentration of seawater and in seawater, the nucleation phase of the uptake process does not appear to occur. The long-term uptake rate of Mn 2+ on the surface of calcite in seawater is first order with respect to the dissolved Mn 2+ concentration. The rate constant is over three orders of magnitude smaller than that found in dilute Mg 2+-free solutions. A probable explanation for the slower growth rate in seawater is that MnCO 3 is not nucleated on the calcite surface due to the presence of high Mg 2+ concentrations. The Mg 2+, through site competition, prevents enough Mn 2+ from being adsorbed to reach a critical concentration for MnCO 3 nucleation. This behavior is similar to that found for orthophosphate with calcite surfaces in dilute solutions and seawater. It indicates that rhodochrosite cannot nucleate in carbonate-rich recent sediments unless the Mg 2+ concentration is lowered below that of seawater. Measurements of the solubility of rhodochrosite in seawater gave results from an undersaturation approach to equilibrium in excellent agreement with those found in previous studies in dilute solutions. When equilibrium was approached from supersaturation, approximately fifty times more calcium was precipitated than Mn 2+. The measured solubility was over twice that determined from undersaturation. It is possible that a Mn—calcite containing 25 to 30 mol% MnCO 3 formed on the rhodochrosite from the supersaturated solutions. Consequently, it is doubtful that pure rhodochrosite controls the concentration of Mn 2+ even in calcium carbonate-poor marine environments.