The effect of carboxylic acids on the dissolution of calcite in aqueous solution. Part 1.—Maleic and fumaric acids

Abstract

The effects of maleic and fumaric acids and their mono- and di-anions on the dissolution of calcite in aqueous solution have been studied over a wide pH range (3–8.5) by (i) the channel flow cell method, (ii) etch pit morphology measurements and (iii) the determination of adsorption isotherms. It is established that the mono-anion of maleic acid reacts with CaCO₃via a first order heterogeneous process of rate constant 0.014 cm s^–1 indicating a significant inhibition of the dissolution reaction as compared to dissolution in HCl of comparable pH. The inhibition is attributed to adsorption of the dianion and the shapes of the etch pits are used to infer the crystallographic environment of the adsorption sites on the (100) surface. In contrast the monoanion of fumaric acid shows no such inhibition. Under conditions where both maleic mono- and di-anions are present it is demonstrated that a Langmuirian adsorption of the latter occurs at dislocations greatly reducing the rate of dissolution. Again the fumaric acid system is impotent. Around pH 8 where the dissolution reaction is controlled by a process involving H₂O, rather than a proton source, strong inhibition is observed in the presence of Langmuirian adsorption of trace amounts of the maleate dianion (10^–6 mol dm^–3). Much higher levels (10^–3 mol dm^–3) of fumarate dianion are required for a comparable effect.