EDTA analogues – unconventional inhibitors of gypsum precipitation

Abstract

In the current study, EDTA (ethylenediamine-N,N,N′,N′-tetraacetic acid) and three further structurally related analogues with various spacing between the amino groups - PDTA (1,3-Diaminopropane-N,N,N′,N′-tetraacetic acid), HDTA (1,6-Diaminohexane-N,N,N′,N′-tetraacetic acid) and EGTA (Ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid) - were tested as inhibitors of gypsum crystallization. The precipitation reactions were performed at high initial supersaturation and calcium ions were present in large excess relative to the amount of the inhibitor. This way the crystallization reaction was facilitated and the degree of supersaturation was not affected by the complexation. It was found that the structural differences between the inhibitors (i.e., the spacing between the amino groups) seem to exert only a secondary effect on the efficiency of the inhibition. The optimum pH for inhibition was found to be strongly dependent on the inhibitor used. The optimum conditions for inhibition were explained in terms of the concentration of the uncomplexed ligand, which is determined by the stability of the calcium complexes with 1:1 composition as well as with the extent of proton competition. The structure, composition and morphology of the precipitating solids isolated from equilibrium mixtures at the completion of the precipitation process were also studied and the crystal faces where adsorption is most likely to happen were identified.