TY - JOUR
T1 - Smectite dissolution kinetics at 80°C and pH 8.8
AU - Cama, Jordi
AU - Ganor, Jiwchar
AU - Ayora, Carles
AU - Lasaga, C. Antonio
N1 - Funding Information:
Fruitful commentaries during the course of the study from I. MacInnis, J. L. Mogollón, V. Metz, K. Nagy, G. Kacandes, and J. Cuadros are greatly appreciated. The smectite sample from Cabo de Gata was selected and supplied by J. Linares. Analytical support from E. Pelfort, S. Matas, R. Fontarnau, and X. Alcover of Serveis Cientı́fico-Tècnics of Barcelona University and technical assistance of T. Vujic, S. Riney, and A. Goodhue from Yale University are also gratefully acknowledged. We also would like to acknowledge Susan Carroll, Paul Wersin, and an anonymous reviewer for their constructive comments that increased the quality of the paper. J.C. and C.A. wish to thank the support of ENRESA (Spanish Nuclear Waste Management Company) and the Spanish Government CICYT AMB96-1101-C02-02 research contract. J.G. wishes to thank the Ministry of National Infrastructures grant 97-17-010 and the Belfer Foundation for Energy and Environmental Research for financial support.
PY - 2000/9/13
Y1 - 2000/9/13
N2 - The kinetics of dissolution of smectite from the Cabo de Gata volcanic deposit was investigated in the present study. Assuming that the sample is composed solely of smectite, the structural formula of the treated smectite was calculated to be K0.19Na0.51Ca0.195Mg0.08(Al2.56Fe0.42Mg1.02)(Si7. 77Al0.23)O20(OH)4 Two types of experiments were carried out: batch experiments to obtain equilibrium data and stirred-flow-through experiments to measure the smectite dissolution rate. All experiments were carried out at a temperature of 80°C and pH of 8.8. After more than 2 yr smectite was still dissolving in the batch experiments, but at a very slow rate. The slow dissolution rate indicates that the system is reasonably close to equilibrium with respect to smectite dissolution. Therefore, the average ion activity product (5 ± 4 x 10-53), obtained from the last samples of the batch experiments, is used as a proxy for the equilibrium constant of the smectite dissolution reaction at 80°C given as Smectite + 20H2O → 0.51Na+ + 0.19K+ + 0.195Ca2+ + 1.1Mg2+ + 0.42Fe(OH)4- + 2.79Al(OH)4- + 7.77H4SiO4 + 0.08OH- In the flow-through experiments at steady state, the average Al/Si (0.33 ± 0.03) and Mg/Si (0.15 ± 0.03) ratios were in very good agreement with these molar ratios of the whole rock analysis (0.35 and 0.14, respectively). The major achievements and conclusions of the present study are as follows: For the first time we present a full stoichiometric dissolution of smectite (i.e., stoichiometric dissolution was observed for Al, Si, and Mg), and show that the obtained dissolution rate is a good measure of the smectite dissolution rate. Pretreatment of the smectite surfaces is necessary to obtain reliable and stoichiometric kinetic results. The dissolution rate of the sample reflects the dissolution rate of the montmorillonitic layers. Under the experimental conditions smectite dissolution rate is not inhibited by aluminum. The dissolution rate of smectite decreases as a function of the silicon concentration. This observation may be explained both by the effect of deviation from equilibrium on dissolution rate and by silicon inhibition, expressed as respectively. The current data set cannot be used to differentiate between these two possible reaction mechanisms. Copyright (C) 2000 Elsevier Science Ltd.
AB - The kinetics of dissolution of smectite from the Cabo de Gata volcanic deposit was investigated in the present study. Assuming that the sample is composed solely of smectite, the structural formula of the treated smectite was calculated to be K0.19Na0.51Ca0.195Mg0.08(Al2.56Fe0.42Mg1.02)(Si7. 77Al0.23)O20(OH)4 Two types of experiments were carried out: batch experiments to obtain equilibrium data and stirred-flow-through experiments to measure the smectite dissolution rate. All experiments were carried out at a temperature of 80°C and pH of 8.8. After more than 2 yr smectite was still dissolving in the batch experiments, but at a very slow rate. The slow dissolution rate indicates that the system is reasonably close to equilibrium with respect to smectite dissolution. Therefore, the average ion activity product (5 ± 4 x 10-53), obtained from the last samples of the batch experiments, is used as a proxy for the equilibrium constant of the smectite dissolution reaction at 80°C given as Smectite + 20H2O → 0.51Na+ + 0.19K+ + 0.195Ca2+ + 1.1Mg2+ + 0.42Fe(OH)4- + 2.79Al(OH)4- + 7.77H4SiO4 + 0.08OH- In the flow-through experiments at steady state, the average Al/Si (0.33 ± 0.03) and Mg/Si (0.15 ± 0.03) ratios were in very good agreement with these molar ratios of the whole rock analysis (0.35 and 0.14, respectively). The major achievements and conclusions of the present study are as follows: For the first time we present a full stoichiometric dissolution of smectite (i.e., stoichiometric dissolution was observed for Al, Si, and Mg), and show that the obtained dissolution rate is a good measure of the smectite dissolution rate. Pretreatment of the smectite surfaces is necessary to obtain reliable and stoichiometric kinetic results. The dissolution rate of the sample reflects the dissolution rate of the montmorillonitic layers. Under the experimental conditions smectite dissolution rate is not inhibited by aluminum. The dissolution rate of smectite decreases as a function of the silicon concentration. This observation may be explained both by the effect of deviation from equilibrium on dissolution rate and by silicon inhibition, expressed as respectively. The current data set cannot be used to differentiate between these two possible reaction mechanisms. Copyright (C) 2000 Elsevier Science Ltd.
UR - http://www.scopus.com/inward/record.url?scp=0033846454&partnerID=8YFLogxK
U2 - 10.1016/S0016-7037(00)00378-1
DO - 10.1016/S0016-7037(00)00378-1
M3 - Article
AN - SCOPUS:0033846454
SN - 0016-7037
VL - 64
SP - 2701
EP - 2717
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 15
ER -