Mechanisms and precipitation rate of rhodochrosite at 25 degrees C as affected by P-CO2 and organic ligands

Rhodochrosite is the main Mn mineral phase in neutral to alkaline anoxic en vironments and is likely the initial precipitation phase when Mn2+ is added to irrigation wafer. Solutions supersaturated with respect to rhodochrosit e that was detected in various natural environments suggest that equilibriu m assumptions may not be satisfactory and kinetic processes may be dominant . This study was conducted to evaluate the precipitation mechanisms of rhod ochrosite in natural environments where DOC is present and there are variat ions in partial pressure of CO2 (P-CO2) Precipitation rates were measured i n supersaturated solutions of rhodochrosite in the presence of seeds of the mineral and P-CO2 0.035 kPa, 5 kPa, and 10 kPa and in a concentration rang e of DOC of 0.02 to 3.2 mM of Suwannee River fulvic acid. Precipitation rat es were measured in the absence and presence of 1 mM leonardite humic acid. Precipitation rates increased when the P-CO2 increased and decreased when the concentration of the fulvic acid increased at constant levels of supers aturation. However, higher concentrations of DOC were needed to produce the same reduction in precipitation rates when P-CO2 was increased. The most l ikely causes of the increase in the precipitation rate when P-CO2 increases are an increase in the negative surface charge and an increase in the acti vity of MnHCO3+. No significant change in the precipitation rate of rhodoch rosite was measured when the leonardite humic acid was added to the reactio n vessels. The lack of inhibition of leonardite humic acid on rhodochrosite precipitation is explained by its molecular configuration in solution.