Adsorption of lanthanoid ions on calcite

Abstract

The adsorption of 14 trivalent lanthanoid ions and yttrium ion (denoted by Ln³⁺) on calcite surfaces was investigated under various solution conditions of pH (pH = 6.8–7.8) and calcium ion concentration (pCa = −log[Ca²⁺] = 2.0 and 3.0), and different surface conditions of calcite crystals (well-developed and rough surfaces). The lanthanoid ions were equilibrated in a solution of ionic strength 0.1 mol dm⁻³ (NaCl) saturated with calcite at 25.0 °C using excess (solid) calcite crystals suspended in solution. The concentrations of the lanthanoid ions on the calcite crystals (C_cry/ mol kg⁻¹) and in solution (C_soln/ mol dm⁻³) were determined by means of inductively coupled plasma-mass spectrometry (ICP-MS). It is found that the distribution ratio (D = C_cry/C_soln) decreases as the atomic number of the lanthanoid increases showing the so called “Tetrad Effect”. D values increase with increasing pH, whereas they are independent of the calcium ion concentration (i.e., carbonate ion concentration). These results indicate that lanthanoid ions are adsorbed on the calcite surface together with hydroxide ions, i.e., the adsorption of hydroxo-complexes. The heavy lanthanoid ions (Er³⁺ to Lu³⁺) are adsorbed as monohydroxo-complexes, (Ln(OH)²⁺), whereas those of the light lanthanoids are predominantly adsorbed as dihydroxo-complexes (Ln(OH)₂⁺). Other lanthanoids show competitive adsorption reactions of mono- and dihydroxo complexes. Both successive adsorption constants of hydroxo complexes increase with decreasing atomic number of the lanthanoid. The rough surface of calcite is quite active and the distribution ratio of the lanthanoid ions on the rough surface is much higher than that on the well-developed crystalline surface. Rates of adsorption of lanthanide ions were measured and mechanisms are being discussed.