Abstract
Calcium carbonate scale formation was simulated by the development in situ of supersaturation in the vicinity of a stainless steel electrode immersed in a stable calcium carbonate solution. The initiation of the growth of the supercritical crystals was accompanied by a decrease of the absolute value of the cathodic current, which at the end of crystallization was constant. The absolute final value of the cathodic current was the measure of the extent of crystallization. In the absence of any additive the only phase identified on the electrode was calcite. In the presence however of additives (organic compounds) vaterite was stabilized for long periods and was identified. The compounds tested included a commercial organophosphorous inhibitor (B206G), poly-acrylate copolymers: Poly acrylic acid (MW 2000, PAA), Poly (acrylic acid 2-acrylamido-2-methyl propane sulfonic acid) (PAS), Poly (acrylic acid: 2-acrylamido-2-methyl propane sulfonic acid: sulfonated styrene) (PSS) and Humic acid (HA). The order of inhibition efficiency was: B206G> PAA, PAS> PSS >> HA. In all cases inhibition was almost complete with inhibitors concentration up to 10 ppm, with the exception of HA which was effective for concentrations of 80 ppm. The additives acted by adsorption onto the active growth sites of the growing crystals of calcium carbonate. The supersaturation developed at the electrode was sufficiently high to develop vaterite which was converted into calcite, in the absence of the additives. The presence of all additives, with the exception of HA, stabilized the formation of vaterite. The dissolution of calcium carbonate at alkaline pH, 8.25, in undersaturated solutions, close to equilibrium showed that the mechanism was a surface diffusion controlled process. The presence of (1-hydroxyethylidene) - 1,1 diphosphonic acid (HEDP) inhibited drastically the rates of dissolution, even at 1μM concentrations. The inhibition was caused because of the adsorption of the HEDP ions onto the active growth sites by electrostatic interactions as demonstrated by surface charge measurements. Additional interactions including hydrogen bonding and lateral interactions between adsorbed molecules is also a possibility according to the analysis of a surface complexation model.
