PII S0016-7037(00)00487-7 The dissolution kinetics of amorphous silica into sodium chloride solutions: Effects of temperature and ionic strength JONATHAN P. ICENHOWER ² and PATRICIA M. DOVE* School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340, USA (Received August 23, 1999; accepted in revised form June 29, 2000) Abstract—The kinetics of amorphous silica, SiO 2 (am), dissolution was quantified in deionized water and NaCl solutions. By using two sources of pure SiO 2 glass (fused purified quartz and pyrolyzed SiCl 4 ), rates were measured at 40°C to 250°C by applying three types of reactor systems to assess kinetic behavior over the full temperature range. Dissolution rates of the two materials are similar within experimental error. Absolute rates of amorphous silica dissolution in deionized water exhibit an experimental activation energy, E a,xp , of 81.9  3.0 and 76.4  6.6 kJ/mol for the fused quartz and pyrolyzed silica, respectively. These values are similar to estimates for quartz within experimental errors. Absolute dissolution rates of SiO 2 (am) in deionized water are 10 faster compared to quartz. Amorphous silica dissolution rates are significantly enhanced with the introduction of NaCl to near-neutral pH solutions such that 0.05 molal sodium ion enhances rates by 21 compared to deionized water. The new kinetic data are combined with previous measurements of SiO 2 (am) dissolution rates in ‘pure’ water to evaluate the temperature dependence of dissolution. The comprehensive data set spans 25°C to 250°C and yields the Arrhenius expression log k + = 0.82191 - 3892.3/T(K) to give an apparent activation energy for dissolution of 74.5  1.4 kJ/mol. These findings step toward the larger goal of understanding silica polymorph reactivity in the complex fluid compositions of natural systems. Copyright © 2000 Elsevier Science Ltd 1. INTRODUCTION Silica polymorphs are prevalent within diverse marine and terrestrial environments and comprise a significant fraction of the Earth’s crust. Of the nine recognized polymorphs, the crystalline varieties of SiO 2 have received the greatest scrutiny to date. Numerous experimental studies have investigated the dissolution kinetics of -quartz (e.g., Knauss and Wolery, 1988; Bennett et al., 1988; Brady and Walther, 1990; Dove and Crerar, 1990; House and Orr, 1992; Tester et al., 1994) and a coherent, although incomplete, picture has emerged regarding the mechanisms of dissolution. In contrast, relatively little is understood regarding the dis- solution kinetics of amorphous forms of silica. In natural en- vironments, amorphous silica, SiO 2 (am), has widespread oc- currence as a result of numerous inorganic and biologically mediated processes. Combined with the recognition that this solid-phase silica reservoir is more reactive than its crystalline counterparts, a knowledge of the fundamental controls on the dissolution mechanisms of SiO 2 (am) is prerequisite to accurate models of silica movement between mineral and aqueous res- ervoirs within the silicon biogeochemical cycle. Rationale for understanding the reactivity of silica poly- morphs are also found on a broader scale in the context of understanding the physical and chemical phenomena that gov- ern the reactivity of simple Si-O bonded phases. These minerals and materials constitute the end-member composition of highly complex silica phases. Because the SiO 4 moiety is the funda- mental structural component of silicate minerals and glasses found in nature, the chemical durability of the Si-O bond has a strong bearing on the overall dissolution behavior of earth materials. It is recognized that the extent to which SiO 4 tetra- hedra are cross-linked in a mineral or glass impacts the disso- lution rates and leaching behavior of silicate minerals and glasses (e.g., review in Casey and Bunker, 1990). For example, Grambow (1985) found that the rate-limiting step in the overall breakdown of the glass is governed by dissolution of the SiO 4 network. Thus, the corrosion resistance of the SiO 4 network is reflected in the aggregate reactivity of both crystalline and amorphous materials. This theme is echoed in studies of the complex engineered glass compositions that are proposed worldwide to host nuclear wastes. A central issue in amorphous or glassy systems is the mag- nitude of SiO 4 reactivity in a framework that lacks long-range order. Students of the Earth intuitively recognize that the amor- phous forms of silica dissolve more quickly than their crystal- line counterparts, and there is evidence to corroborate this, as detailed below. The contrast in dissolution behavior can be attributed to structural differences between SiO 2 (am) and crys- talline silica, but the physical nature of this variance remains elusive. Therefore, dependence of SiO 2 (am) dissolution rates on solution chemistry and temperature cannot be inferred with full confidence from the more comprehensive quartz dissolu- tion studies. 1.1. Previous Investigations and Rationale for this Study Studies that quantify the kinetics of amorphous silica disso- lution are few and the findings, although important, show significant gaps in our quantitative understanding of SiO 2 (am) reactivity. First, previous experimental investigations of SiO 2 (am) reactivity report few data as summarized in Table 1 * Corresponding author and new address: Dept. Geological Sciences, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061 USA (dove@vt.edu). ² Present address: Pacific Northwest National Laboratory, Applied Geology and Geochemistry Group, Richland, WA 99352. Pergamon Geochimica et Cosmochimica Acta, Vol. 64, No. 24, pp. 4193– 4203, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 + .00 4193