TY - JOUR
T1 - The effects of organic acids on the dissolution of silicate minerals
T2 - A case study of oxalate catalysis of kaolinite dissolution
AU - Cama, Jordi
AU - Ganor, Jiwchar
N1 - Funding Information:
This research was supported by Grant # 94-00089 from the United States-Israel Binational Science Foundation and by a seed money grant from the Research and Development Authority at Ben-Gurion University of the Negev. We thank Lisa Stillings and two anonymous reviewers for their thorough reviews that significantly improved the quality of the manuscript. The handlings of the manuscript by the associate editor, Carrick Eggleston, as well as his comments are gratefully acknowledged. Ellen Faller from the Yale Peabody Museum and John Yang from the Clay Mineral Society kindly supplied the kaolinite samples. We wish to express our gratitude to A. C. Lasaga, A. Sivan, and V. Metz for fruitful discussions and to E. Shani, A. Avital, E. Roueff, G. Ronen, O. Halabi, and V. Pavlov for their technical assistance.
PY - 2006/5/1
Y1 - 2006/5/1
N2 - 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:{A formula is presented}The possible effect of oxalate on the proton-promoted dissolution rate was examined by comparing the results of the present study to literature observations on the effects on kaolinite dissolution rate of deviation from equilibrium and Al inhibition, respectively. The comparison indicates that the effect of oxalate on kaolinite dissolution rate is not related to Al inhibition or saturation state. Therefore, we suggest that oxalate catalyzes kaolinite dissolution through an oxalate-specific mechanism. The oxalate-promoted dissolution is best described using a quadratic rate law, i.e., a rate law in which the oxalate-promoted dissolution rate depends on the square of the oxalate surface concentration. A quadratic rate law may represent a mechanism in which the dissolution is catalyzed by the simultaneous adsorption of two ligands on two neighboring edge aluminol sites. This mechanism is supported by the observation that on saturation, the amount of adsorbed oxalate is similar to the amount of available Al surface sites on the kaolinite edge, and is much smaller than the amount of available Al surface sites on the basal planes. This observation indicates that the adsorption of oxalate occurs mainly on edge aluminol sites, and suggests that the formation of Al-oxalate complexes on two neighboring edge aluminol sites must be reasonably common above a threshold oxalate concentration.
AB - 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:{A formula is presented}The possible effect of oxalate on the proton-promoted dissolution rate was examined by comparing the results of the present study to literature observations on the effects on kaolinite dissolution rate of deviation from equilibrium and Al inhibition, respectively. The comparison indicates that the effect of oxalate on kaolinite dissolution rate is not related to Al inhibition or saturation state. Therefore, we suggest that oxalate catalyzes kaolinite dissolution through an oxalate-specific mechanism. The oxalate-promoted dissolution is best described using a quadratic rate law, i.e., a rate law in which the oxalate-promoted dissolution rate depends on the square of the oxalate surface concentration. A quadratic rate law may represent a mechanism in which the dissolution is catalyzed by the simultaneous adsorption of two ligands on two neighboring edge aluminol sites. This mechanism is supported by the observation that on saturation, the amount of adsorbed oxalate is similar to the amount of available Al surface sites on the kaolinite edge, and is much smaller than the amount of available Al surface sites on the basal planes. This observation indicates that the adsorption of oxalate occurs mainly on edge aluminol sites, and suggests that the formation of Al-oxalate complexes on two neighboring edge aluminol sites must be reasonably common above a threshold oxalate concentration.
UR - http://www.scopus.com/inward/record.url?scp=33645918314&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2006.01.028
DO - 10.1016/j.gca.2006.01.028
M3 - Article
AN - SCOPUS:33645918314
SN - 0016-7037
VL - 70
SP - 2191
EP - 2209
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 9
ER -