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A hydrothermal mixed flow reactor has been developed to study the reaction kinetics of a wide variety of mineral/solution systems. The reactor is constructed of commercially pure titanium to minimize corrosion and operates at temperatures of 25 to 300°C and pressures up to 124 bars. This system is used to measure the dissolution rates of quartz at near-neutral pH in 0.0 to 0.15 m solutions of NaCl, KCl, LiCl, MgCl2 over a temperature range of 100 to 300°C. In all cases, small concentrations of electrolytes increase the rate, some by as much as 1.5 orders of magnitude above the values measured for deionized water. The effect is greatest for solutions of NaCl and KCl where reaction rates increase with increasing electrolyte concentrations up to 0.05 molal and become constant at higher molalities. Smaller rate increases are observed for LiCl and MgCl2 solutions. The first-order rate equation for quartz dissolution in pure water at temperatures of 100 to 300°C is given by r H 4 sio 4 = k + (a sio 2 )(a H 2 o ) 2 (1 — Q K) for a standard system of 1 m2 of surface area and 1 kg of solution. The addition of electrolytes to reacting solutions at near-neutral pH accelerates the rate according to a Langmuir adsorption model and has the form r H 4 sio 4 = (k + + k ad K me + 1 + k me + )(a sio 2 )(a H 2 o ) 2 (1 − Q K) . mme + Analysis of the data indicates that the observed rate increases are controlled by the identity and concentration of the cation where alkali cations coordinate with the surface to increase the reactivity of siloxane groups by disrupting the structure of the mineral-solution interface. The rate-limiting step for the dissolution mechanism is described by (Si — O — Si) + H2O = (Si —O —Si · OH2)† → 2(Si —O —H) where the intermediate species is probably the same in deionized water and electrolyte solutions, but the reaction frequency is higher in electrolyte solutions due to increases in the accessibility of water to the mineral surface structures. Transition state calculations support this mechanism by showing that the rate increases are caused by an increase in the pre-exponential factor, A, where the activation entropy, Δs†, becomes more positive and/or the mole fraction of water at the reactive sites, XH2o, increases. The activation enthalpy, ΔH†, remains constant. Increased reactivity of the surface in the presence of adsorbed cations is also demonstrated by ab initio molecular orbital calculations of possible surface intermediate species, which show that the presence of certain cations increase the Si — O — Me angle. These results suggest that the mobility of quartz in geologic settings may be quite different from predictions made based on experiments conducted in pure water. |
