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Mechanisms and Precipitation Rate of Rhodochrosite at 25°C as Affected by P_CO2 and Organic Ligands

U.S. Salinity Lab., USDA-ARS, 450 W. Big Springs Rd., Riverside, CA 92507

^* Corresponding author (lebron{at}ucracl.ucr.edu).

ABSTRACT

Rhodochrosite is the main Mn mineral phase in neutral to alkaline anoxic environments and is likely the initial precipitation phase when Mn²⁺ is added to irrigation water. Solutions supersaturated with respect to rhodochrosite that was detected in various natural environments suggest that equilibrium assumptions may not be satisfactory and kinetic processes may be dominant. This study was conducted to evaluate the precipitation mechanisms of rhodochrosite in natural environments where DOC is present and there are variations in partial pressure of CO₂ (P_CO2). Precipitation rates were measured in 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 range of DOC of 0.02 to 3.2 mM of Suwannee River fulvic acid. Precipitation rates 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 supersaturation. However, higher concentrations of DOC were needed to produce the same reduction in precipitation rates when P_CO2 was increased. The most likely 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 activity of MnHCO⁺₃. No significant change in the precipitation rate of rhodochrosite was measured when the leonardite humic acid was added to the reaction vessels. The lack of inhibition of leonardite humic acid on rhodochrosite precipitation is explained by its molecular configuration in solution.

Contribution from the U.S. Salinity Lab.

Received for publication April 27, 1998.

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