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

Rhodochrosite is the main Mn mineral phase in neutral to alkaline anoxic environments and is likely the initial precipitation phase when Mn 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 (Pco:)Precipitation rates were measured in supersaturated solutions of rhodochrosite in the presence of seeds of the mineral and Pmi 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 PC02 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 PCOi was increased. The most likely causes of the increase in the precipitation rate when Pfm increases are an increase in the negative surface charge and an increase in the activity of MnHCOI. 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. M IS THE SECOND MOST ABUNDANT HEAVY METAL in the crust after Fe, to which it has many geochemical similarities (Crerar et al., 1980). Manganese is also an essential micronutrient for plants, and its cycle in oxic-anoxic boundaries is associated with several soil biogeochemical processes, such as microbial respiration, scavenging and transport of heavy metals, and oxidation of humic substances (Balistrieri and Murray, 1982; Friedl et al., 1997). The oxidation state of Mn in natural environments varies from +2 to +4; however, due to solubility considerations only Mn-containing species are expected in the soil solution across the range of pE and pH of most soils (Norvell, 1988). Manganese (II) and its complexes constitute the principal transport species (Wolfram and Krupp, 1996) and are the predominant forms taken up by plant roots (Marschner, 1988). In arid-zone regions, where calcareous soils with high pH and high content of calcite (CaCO3) are abundant, low Mn availability may be a limiting factor for plant growth. Addition of Mn directly to the irrigation water has been proposed and is practiced as a component of fertigation. Formation of rhodochrosite (MnCO3) is typically associated U.S. Salinity Lab., USDA-ARS, 450 W. Big Springs Rd., Riverside, CA 92507. Contribution from the U.S. Salinity Lab. Received 27 Apr. 1998. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 63:561-568 (1999). with CaCO3 as a (Mn, Ca)CO3 solid solution, as shown in numerous studies Pederson and Price, 1982; Jackobsen and Postma, 1989; Wartel et al., 1990; Okita, 1992, Boyle, 1983, Friedl et al., 1997). Under anoxic conditions and independent of the specific mineral phase, Mn chemistry in soils is constrained by the carbonate solution chemistry. Many efforts have been devoted to the study of the carbonate chemistry in the last 50 years. This attention is justified if we consider that in neutral to alkaline environments, carbonates control all the pH-dependent processes, such as the surface mineral-water interactions, transport, and bioavailability of micronutrients to plants. Predicting the rate of carbonate precipitation is essential in any attempt to model solute transport in soils and to prevent undesired precipitation in irrigation systems. The concentration of Mn and CO3 in sedimentary environments frequently yields saturation index (£1) values >1 for rhodochrosite (O = lAP/Ksp, where IAP is the ionic activity product, and Ksp is the solubility product of pure mineral phase at 25°C). The results of Jackobsen and Postma (1989) showed slow rhodochrosite precipitation in porewaters from the Baltic Deeps, indicating that the solid solutions of (Mn, Ca)CO3 are metastable and kinetically regulated (Sternbeck, 1997). These studies suggest that equilibrium assumptions may not be satisfactory for predicting Mn in solution, and that kinetics processes may be dominant. The only reported experiment on the kinetics of rhodochrosite precipitation is that performed by Sternbeck (1997). Sternbeck studied the crystal growth of rhodochrosite at 25°C and applied the surface speciation model of van Cappellen et al. (1993) to describe the process. Nevertheless, it has been shown for the calcite system that small amounts of DOC inhibit the precipitation rate of carbonates by blocking crystal growth (Kitano and Hood, 1965; Reddy, 1977; Reynolds, 1978; Reddy and Wang, 1980; Inskeep and Bloom, 1986; Dove and Hochella, 1993; Gratz and Hillner, 1993; Katz et al. 1993, Paquette et al., 1996; Lebron and Suarez, 1996). Inhibition occurs as a result of adsorption of DOC molecules onto the surface of the carbonate crystals (Inskeep and Bloom, 1986; Lebron and Suarez, 1996). Fulvic acids have been found to be more efficient in inhibiting calcite precipitation than the DOC obtained from a soil extract. However, fulvic and humic acids from a soil extract are more efficient than small organic molecules like citric, gallic, syringic, adipic, and azealic acids in the reduction of hydroxyapatite precipitation (Inskeep and SilverAbbreviations: DIW, deionized water; DOC, dissolved organic carbon; IHSS, International Humic Substances Society; pHZPC, pH of zero point of charge. 561 Published May, 1999