Machesky The adsorption of ionic species by mineral surfaces influences many geochemical phenomena including the transport and fate of contaminants, mineral dissolution and precipitation reactions, colloidal stability and various oxidation-reduction reactions. Many of the variables that influence adsorption processes have been extensively studied including pH, type of mineral surface, type and concentration of adsorption species, ionic strength, and time (kinetics). Temperature can also profoundly influence ion adsorption, but studies at other than room temperature are rare, and above 100C adsorption studies are virtually nonexistent. In contrast, other geochemically important and related phenomena such as homogenous solution equilibria and mineral solubility and dissolution kinetics have been studied over a much broader temperature range. To begin to remedy this imbalance, this study will investigate how ion adsorption by a model oxide surface (rutile) is influenced by temperature over the broad range from room temperature to 295C. After ion adsorption by rutile has been sufficiently well characterized, studies of ion adsorption by the quartz (SiO2) surface will also be performed. The specific objectives of this research are: 1) A comprehensive characterization of hydrogen ion adsorption by the metal oxides rutile and quartz as functions of temperature and electrolyte concentration and composition using a stirred hydrogen electrode concentration cell (SHECC). The success of initial efforts in this area a prime motivation for this study. 2) Direct determination of hydrogen ion adsorption enthalpies using titration calorimetry over a more limited temperature range (to 75C) but similar pH and electrolyte concentration and composition ranges as those used for the SHECC experiments. These experiments will provide direct measurements of hydrogen ion adsorption enthalpies for comparison with those obtained from the variable-temperature mea surements with the SHECC. These calorimetric measurements will be the first such experiments conducted over this broad temperature range. 3) Characterization of model cation (zinc) adsorption by rutile and quartz and model anion (oxalate) adsorption by rutile over the temperature and electrolyte composition ranges used for the hydrogen ion adsorption studies. Parallel calorimetric experiments (to 75C) to directly determine zinc and oxalate adsorption enthalpies will also accompany these variables temperature for comparative purposes. 4) The use of the surface complexation modeling approach to describe the resulting adsorption data with the goal of identifying a single model, relatively free of fitting parameters, that can adequately describe all the data generated between 25 and 295C. This will provide a consistent and powerful framework for data interpretation and will also increase the utility of the results to the geological community. The investigation is novel because the study of ion adsorption processes will be comprehensively extended into the hydrothermal regime for the first time. Moreover, the conclusions of this study will have broad applications to those areas of the geosciences concerned with reactions at the solid-liquid interface because the ion adsorption properties of many other cations and anions will be generally similar to those of zinc and oxalate. Consequently, the results of this study will hopefully stimulate and help guide additional research into this virtually unexplored area of the geosciences.
