Effect of diphosphonates on the precipitation of calcium carbonate in aqueous solutions

Abstract

The effect of two diphosphonates, ethane-1-hydroxy-1, 1-diphosphonic acid (EHDP) and 1,2-dihydroxy-1,2-bis(dihydroxyphosphonyl)ethane (DDPE) on the precipitation of calcium carbonate has been investigated. EHDP and DDPE were found to reduce markedly the rate of crystallization at solution concentrations as low as 10^–7 mol dm^–3, both in cases where precipitation was spontaneous and when it took place on well characterized calcite crystals introduced in the supersaturated solutions. In spontaneously precipitating supersaturated solutions, small concentrations of the additives caused a marked increase in the induction period preceding precipitation. Inhibition was mainly at the crystal-growth step by adsorption at the active-growth sites. The adsorption isotherms of EHDP and DDPE showed that these compounds adsorb extensively on to calcite surfaces. Interpretation of the kinetics and adsorption data in terms of the Langmuir isotherm gave good agreement for the ‘affinity’ parameter obtained by these two methods.