ELECTROKINETIC PROPERTIES OF THE CALCITE WATER INTERFACE IN THE PRESENCE OF MAGNESIUM AND ORGANIC-MATTER

Electrophoretic mobilities of calcite particles immersed in saturated aqueous solutions (closed to atmospheric carbon dioxide) were measured as a function of solution composition at 25°C. Electrokinetic ζ potentials of calcite suspended in water are negative and slightly influenced by pH in the range 8.5 ≤ pH ≤ 10.5. At constant Ca2+ equilibrium concentration, pH does not affect ζ potential values. The value of ζ is strongly dependent on pCa2+ or pCO32- = pKso - pCa2+); below the isoelectric point of calcite, pCa2+ 2.7, ζ potentials are positive. It is demonstrated that Ca2+ and CO32- are the only potential determining ions (pdi). The effect of magnesium ions on calcite ζ potential values is heavily dependent on pH, aging time, and Mg2+ concentration. At short equilibration times, the negative charge of calcite is reverted once the solution becomes supersaturated with respect to Mg(OH)2; below these pH values, Mg2+ behaves as an indifferent ion. At long equilibration times, positive surface charge develops in conditions of undersaturation with respect to Mg(OH)2 but only at the highest studied Mg2+ concentration. This finding is interpreted in terms of the formation of a surface layer of magnesium-bearing calcite, Mg2+, Ca2+, and CO32- being the pdi. The adsorption of dodecyl sulfate anions produces a more negative surface charge. Adsorption of DS- is strongly dependent on pCa2+; this dependence is dominated by the electrostatic contribution to the overall adsorption Gibbs energy. The nonelectrostatic contribution derived from the data is indicative of a weak chemical interaction between calcium surface ions and the surfactant head group. The implications of these results for natural water systems are briefly discussed. The electrokinetic behavior of biogenic calcium carbonate secreted by the tube forming worm Ficopomatus enigmaticus is also reported. © 1992.