Calcite interaction with acidic sulphate solutions: a vertical scanning interferometry and energy-dispersive XRF study

Abstract

Calcite dissolution on (10 (1) over bar4) cleavage surface was investigated by means of ex situ vertical scanning interferometry (VSI) measurements using flow-through and batch experiments at ambient pCO(2), pH 1-7 and room temperature in metal (Fe(II), Zn, Cu and Cd) and metal-free chloride solutions, and metal sulphate and metal-free sulphate solutions, undersaturated with respect to calcite and undersaturated or in equilibrium with respect to gypsum.Based on the VSI measurements, surface retreat was quantified at different pH and Delta G(r) < -8.1 kcal mol(-1), yielding intrinsic dissolution rate constants of calcite. These rates were used to derive an empirical rate law that takes into account the pH effect on calcite dissolution at room temperature and far from equilibrium with respect to calcite and is expressed asRate(calcite)(mol . cm(-2) . s(-1)) = 3.3 +/- 1. 8 x 10(-6) . (a(H+))(0.94+0.08) + k(2)where a(H+) is the proton activity and k(2) equals 2.5 +/- 0.8 x 10(-11) mol cm(-2) s(-1). The rhombic etch pit morphology changes at very low pH (pH < 3) due to the catalytic effect of pH on calcite dissolution, yielding rounded pseudo-rhombic etch pits.The presence of Fe(II), Cu, Zn and Cd inhibits calcite dissolution at pH 3, in contrast to the presence of sulphate, which does not affect calcite dissolution. The presence of Zn affected the etch pit morphology, yielding triangular etch pits. Under gypsum saturation conditions, gypsum (or other metal-bearing sulphate phases) precipitated on calcite as calcite dissolved, yielding a decrease in dissolution rate with time due to loss of calcite reactivity. Based on energy-dispersive X-ray fluorescence (EDXRF) measurements the precipitation rates of Zn4SO4(OH)(6) center dot 5H(2)O and Cu4SO4(OH)(6) center dot 2H(2)O were calculated to be between 1 x 10(-7) and 6 x 10(-7) mol m(-2)s(-1).