Unifying Batch-Dissolution Kinetics for Salts: Probing the Back Reaction for Gypsum and Calcite by Means of the Common-Ion Effect

Abstract

Recent success in fitting the shrinking object model for dissolution kinetics to biogenic silica, silica gel, simple salts, sucrose and gypsum prompted this study of the effects of common ions upon gypsum dissolution kinetics. Middle-ground dissolutions were mainly studied, in which shrinkage of the surface area, S, is significant, and the system approaches, but does not reach, saturation, c sat . Dissolution was monitored by conductimetry. At a constant ionic strength of 0.060 M, the net rate for gypsum dissolution is given by Net Rate ¼ k b Á S Á ðc sat À cÞ, where k b is a rate constant, and c can be expressed alternatively in terms of either [Ca 2? ], [SO 42-] and [e ± ], that part of the electrolyte concentration contributed by gypsum dissolution, or as the equivalent total concentrations of these species, for example, [SO 42-] T . The presence of either calcium or sulphate as a common ion slows dissolution, and the effect of this upon c sat , k b and k f , the forward rate constant, is discussed. Contrary to previous experience, it is emphasised that each fitting of the shrinking object model demands its own value of the Solubility of gypsum, c sat , which can be derived from the Solubility Product. This experience with gypsum is aligned with previous work on calcite, to develop a unified approach to the batch dissolution of salts. It highlights serious deficiencies in the way earlier common-ion experiments were conceived and enacted, and in particular with the rate equation of Sjöberg (Geochim Cosmochim Acta 40:441-447, 1976) for calcite above a pH of 7. Common-ion experiments are shown to be crucially important for probing the back reaction to dissolutions and might be applied to the far bigger problem of silicate-mineral dissolution, where 'non-linear kinetics' are often observed.

The role of gypsum and/or dolomite dissolution in tufa precipitation: lessons from the hydrochemistry of a carbonate–sulphate karst system

“…The resulting water from the mixing of these two distinctive hydrochemical aquifers is subsaturated in both calcite and dolomite because of the common‐ion effect (e.g. Chong and Sheikholeslami, ; Truedale, ). The SIdol at the Usp indicates that groundwater was subsaturated in dolomite confirming this water mixing process.…”

“…In this regard, calcite precipitation is known to be enhanced by the common-ion effect (e.g. Chong and Sheikholeslami, 2001;Ford and Williams, 2007;Brasier, 2011;Truedale, 2012). This implies that spring waters resulting from the mixing of sulphate and bicarbonate groundwater potentially enhance the precipitation of tufa compared to karst springs with only bicarbonate waters (Ordoñez and Benavente, 2014).…”

Response to Comments upon, “Evidence and potential implications of exponential tails to concentration versus time plots for the batch dissolution of calcite”

“…Morse and Arvidson 2002;Morse et al 2007). Turner's (2015) reservation that orders for m and n of and 1, respectively, would be problematic mechanistically was already appreciated in this laboratory (Truesdale 2012;Truesdale and Sebu 2013).…”

“…The hyperbolae also offer the possibility of reducing the dissolution properties of an entire substance to an array of hyperbolic constants (Truesdale 2009(Truesdale , 2010. The Shrinking-Object model was also validated in experiments where the reaction rate was kept pseudo-zero order in respect of surface area, when the amount of material stripped from the solid was \1 % of the mass present (Truesdale 2012;Truesdale and Sebu 2013;Truesdale 2015).…”