Effects of organic acids on the dissolution of orthoclase at 80°C and pH 6

The University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1063, USA
Chemical Geology (Impact Factor: 3.52). 10/1996; 132(1-4):91-102. DOI: 10.1016/S0009-2541(96)00044-7


The dissolution of K-rich feldspar (orthoclase), quartz and Al(OH)3 was investigated at 80°C and pH 6 in buffered solutions of organic acids. Previous studies of the effects of organic acids (OA) on feldspar dissolution have typically been conducted in acidic, low-ionic strength solutions often under conditions which preclude isolation of effects of pH from those due to organic acids. Our experiments were conducted at constant pH, temperature, ionic strength, and buffer composition to allow direct comparison of experiments with and without OA. The dissolution experiments were conducted under closed-system conditions to: (1) determine the magnitude of mineral solubility enhancement by OA; (2) examine changes in reaction stoichiometry as equilibrium is approached; and (3) investigate the effects of OA on secondary mineral precipitation.The carboxylic acid species, oxalate and citrate, significantly enhanced the dissolution of orthoclase at pH 6. The concentrations of Si and Al in 10 m M oxalate and citrate solutions were nearly 3 times that in solutions of acetate buffer without oxalate or citrate. Aluminum was below the limit of detection ( < 0.007 mM) in the acetate buffer alone. Citrate increased the release of Si and Al from orthoclase more than did oxalate at the same concentration. Equilibrium modeling indicates that solutions with oxalate and citrate attained supersaturation with respect to gibbsite, kaolinite, and smectite, and saturation with respect to quartz. Nevertheless, orthoclase dissolution remained congruent with respect to Si and Al release.Separate experiments using pure quartz and Al(OH)3 suggest that a mechanism other than formation of Si-organic complexes may be involved in the OA-enhanced release of Si from orthoclase. The increase in dissolved silica was modest in 10 mM oxalate and 10 mM citrate solutions reacted with quartz, and did not increase with increasing OA concentration. The solubility of quartz was similar in solutions of oxalate and citrate. In contrast, Al(OH)3 dissolution was 50% greater in citrate than in oxalate, similar to the behavior of orthoclase. Citrate may be more effective in dissolving orthoclase than oxalate due to a stronger interaction between citrate and Al, rather than due to a synergistic effect of Al-citrate and Si-citrate complexes. These results have implications for the mass transport of Si and Al in diagenetic environments.

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Available from: Ruth Blake, Nov 19, 2015
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    • "of primary minerals results in constant Si/Al ratios in the presence of organic acids over a wide range of concentrations (0.5–10 mmol L À1 citrate and 3–10 mmol L À1 oxalate) (Blake and Walter, 1996), while without organic acids, dissolution is non-stoichiometric (Blake and Walter, 1996; Gislason and Oelkers, 2003). Si/Fe ratio for the highest MA treatment (3.78) was also consistent with ratio of these elements in basaltic glass (3.82) supporting the hypothesis that this treatment resulted in stoichiometric dissolution of the glass and minimal precipitation of secondary phases. "
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    ABSTRACT: Bench-scale experiments were conducted to determine rates and patterns of coupled organic matter infusion and weathering in a San Francisco volcanic field (Flagstaff, AZ) basalt sample under experimentally-modeled biotic and abiotic condition and to inform larger-scale collaborative studies at the landscape evolution observatory (LEO), Biosphere 2 (Tucson, AZ), where the same basaltic media is being used in a synthetic hillslope experiment. We postulated that mineral transformations depend significantly on the presence of organic carbon compounds including dissolved natural organic matter (DOM), with organic C simultaneously imprinting the chemical and mineralogical properties of primary and secondary solids undergoing incongruent dissolution. The present work reports on solute releases from Flagstaff basalt (FB) along laboratory-controlled gradients in DOM type and concentration. Loamy sand textured FB was subjected to flow-through, saturated column dissolution experiments using influent solutions with and without DOM compounds. Solutions included Ponderosa pine forest soil O-horizon extracts at three target concentrations: 7, 35, and 70 mg L−1 C, malic acid (MA) solutions at 7, 35, 70, and 140 mg L−1 C, and a control without DOM but having comparable inorganic solution composition. Chemical denudation rates for FB dissolution products were calculated from the concentration difference between outflow and inflow solutions. In addition, changes in the composition of the solid phase over the course of the experiment were determined using X-ray diffraction (XRD), X-ray fluorescence (XRF), and selective dissolution (SE). Column experiments supported dissolution rates derived from the literature and indicated a potentially strong effect of plant-derived organic ligands on mineral dissolution congruency and secondary phase precipitation. Both malic acid and DOM enhanced basalt dissolution, with malic acid having larger effect on per unit C basis. The largest relative effect of organic ligands was observed for Fe and Ti. Si/Al and Si/Fe chemical denudation rate ratios indicated non-stoichiometric dissolution, consistent with the observed precipitation of secondary phases, as confirmed by XRD and SE. DOM enhanced precipitation of secondary phases but this effect decreased with increase in amount of DOM added. Stoichiometry of the outflow solutions indicated that basaltic glass was preferentially dissolved in agreement with previous modeling predictions. Results suggest that biotic colonization of FB in the large-scale, long-term LEO experiment is likely to strongly influence the rate and congruency of FB weathering reactions and their distribution across the convergent hillslopes.
    Full-text · Article · Aug 2014 · Geochimica et Cosmochimica Acta
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    • "The concentration of DOC does not directly correspond to its reactivity in terms of functional groups (e.g., Blake and Walter 1996; Ochs 1996; Vance and David 1991), and our analysis of specific biochemical reactions is limited by lack of chemical characterization of DOC. Drever (1994) has questioned whether ligand concentrations are high enough in bulk soil solution to have a measurable effect on weathering rates. "
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    ABSTRACT: Few studies of silicate mineral weathering have been conducted in carbonate-bearing temperate forest soils. With climate and vegetation held constant, we compared soil mineralogy and major element chemistry of soil waters from a carbonate-free temperate aspen forest site in the Cheboygan watershed, northern Michigan, with that from carbonate-containing soils from experimental tree-growth chambers (low- vs. high- fertility). All soils were well-drained sands (quartz, Na-rich plagioclase, and K-feldspar) with minor amounts of carbonate present only in the experimentally manipulated soils. The Na+ concentrations in soil waters corrected for atmospheric deposition (Na*) were used to compare relative rates of plagioclase feldspar weathering across sites. In natural soil water profiles, maximum concentrations of Na*, Si, and dissolved organic carbon (DOC) were observed by a depth of 15cm, a soil zone free of carbonate minerals. Mean Na* and DOC concentrations were different in the three soils, and increased in the order natural soil < low-fertility chambers < high-fertility chambers. While low pH environments are generally viewed as enhancing weathering rates, here higher Na* appears to be related to high DOC, which is consistent with observed increases in active organic functional groups as pH increases. Our results suggest that under a specific vegetative cover, the soil carbon environment affects the weathering flux observed. Our study also suggests that disturbed soils provide an enhanced physical and chemical environment for weathering. Generalized silicate weathering models may benefit from including the enhancing effects of organic anions at moderate pH in addition to precipitation and temperature.
    Full-text · Article · Feb 2007 · Biogeochemistry
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    • "A considerable volume of work assessing the catalytic effect of organic acids on silicate dissolution has been carried out since this work of Furrer and Stumm (1986). Most of these studies have focused on feldspar (Welch and Ullman, 1993; Franklin et al., 1994; Blake and Walter, 1996; Stillings et al., 1996; Welch and Ullman, 1996; Drever and Stillings, 1997; The catalytic effects of oxalic acid on kaolinite dissolution -6- 06/02/06 Stillings et al., 1998; Blake and Walter, 1999; van Hees et al., 2002). The effect of organic acids on kaolinite was studied by Chin and Mills (1991), Wieland and Stumm (1992) and Ganor and Lasaga (1994). "
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    ABSTRACT: Most studies agree that the dissolution rate of aluminosilicates in the presence of oxalic and other simple carboxylic acids is faster than the rate with non-organic acid under the same pH. However, the mechanisms by which organic ligands enhance the dissolution of minerals are in debate. The main goal of this paper was to study the mechanism that controls the dissolution rate of kaolinite in the presence of oxalate under far from equilibrium conditions (−29 < ΔGr < −18 kcal mol−1). Two types of experiments were performed: non-stirred flow-through dissolution experiments and batch type adsorption isotherms. All the experiments were conducted at pH 2.5–3.5 in a thermostatic water-bath held at a constant temperature of 25.0, 50.0 or 70.0 ± 0.1 °C. Kaolinite dissolution rates were obtained based on the release of silicon and aluminum at steady state. The results show good agreement between these two estimates of kaolinite dissolution rate. At constant temperature, there is a general trend of increase in the overall dissolution rate as a function of the total concentration of oxalate in solution. The overall kaolinite dissolution rates in the presence of oxalate was up to 30 times faster than the dissolution rate of kaolinite at the same temperature and pH without oxalate as was observed in our previous study. Therefore, these rate differences are related to differences in oxalate and aluminum concentrations. Within the experimental variability, the oxalate adsorption at 25, 50, and 70 °C showed the same dependence on the sum of the activities of oxalate and bioxalate in solution. The change of oxalate concentration on the kaolinite surface (Cs,ox) as a function of the sum of the activities of the oxalate and bioxalate in solution may be described by the general adsorption isotherm:
    Full-text · Article · May 2006 · Geochimica et Cosmochimica Acta
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