A carboxylate terpolymer as a scale inhibitor for geothermal hard water in recirculating systems.

The inhibition efficiency of polycarboxylic acid will decrease due to the weak acidity of carboxyl group in high Ca2+ geothermal circulating water. To solve this problem, we synthesized a novel carboxyl terpolymer (MA‐AEO‐7/SMAS/MA) with adipate polyoxyethylene ether maleate monoester (MA‐AEO‐7), sodium methacrylate sulfonate (SMAS) and maleic acid (MA). Its inhibition efficiency and mechanism were investigated at high Ca2+. The results indicated that the inhibition efficiency of MA‐AEO‐7/SMAS/MA was 96.84% for CaCO3 at the monomeric ratio of 0.5:1:1 and a dosage of 12 mg L−1. The X‐ray diffraction and scanning electron microscope results of CaCO3 scale showed that the presence of MA‐AEO‐7/SMAS/MA converted CaCO3 calcite to vaterite with smaller particle size. The molecular dynamics and density functional theory calculations indicated that the increased solubility of MA‐AEO‐7/SMAS/MA and the carboxylic acidity in MA‐AEO‐7/SMAS/MA enhanced the Ca2+ chelating properties. The electrostatic repulsion between the sulfonic acid groups made MA‐AEO‐7/SMAS/MA exhibit good dispersion properties. These findings explained why the inhibition efficiency of MA‐AEO‐7/SMAS/MA remained above 80% at Ca2+ concentration of 1500 mg L−1, exceeding the 30%–50% of the three commercial scale inhibitors. This study provides practical insight for the development of scale inhibitors suitable for high Ca2+ geothermal circulating water. [ABSTRACT FROM AUTHOR]