Performance and mechanism of 1-hydroxy ethylidene-1,1-diphosphonic acid and 2-phosphonobutane-1,2,4-tricarboxylic acid in the inhibition of calcium carbonate scale

The performance and mechanism of typical scaling inhibitors, i.e., 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), in inhibiting calcium carbonate scaling were investigated. Results of the static experiment at 30 °C and 80 °C revealed that at the optimal concentration, the scaling inhibition efficiency of PBTCA was superior to that of HEDP. Additionally, X-ray powder diffraction evaluation demonstrated that in comparison with HEDP, PBTCA inhibited the growth of the (1 0 4) crystal plane of calcite more evidently. Scanning electron microscopy analysis indicated that the addition of scaling inhibitors induced the crystal transformation of CaCO₃ from calcite to aragonite. Molecular dynamics simulation also confirmed that the interaction of PBTCA with the (1 0 4) crystal surface of calcite is more apparent than that of HEDP. This observation was a consequence of the PBTCA being more negatively charged, as determined via a quantum chemical calculation. The calculation further indicated that the spatial distance of PBTCA to the (1 0 4) face of calcite was less due to stronger chelating ability and better suitability of the structure. As a result, in the tested temperature range, PBTCA exhibited the more effective inhibitory effect against calcium carbonate scaling.