The Nucleation Kinetics of Strontium Sulfate in NaCl Solutions up to 6 m and 90°C with or without Inhibitors

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The nucleation kinetics of strontium sulfate in NaCl solutions from 0 to 6 m at temperatures from 25 to 90°C was studied by measuring the induction period in the absence and presence of scale inhibitors. Employing the classical nucleation theory, the interfacial tension between strontium sulfate and supersaturated solutions with NaCl was estimated to be about 67 mJ m-2 (1 m NaCl) at 25°C. It decreases slightly with increasing temperature (to 56 mJ m-2 in 1 m NaCl solutions at 90°C). It was observed that the length of the induction period generally decreases with increasing concentrations of electrolytes up to 3 m and then starts to increase slightly to 6 m at a given temperature and saturation index. In the case of NaCl electrolyte solutions, the induction period only decreases to 1 m and then starts to increase to 6 m, probably because of substitution of small ions (Na+) into SrSO4 nuclei and the crystal lattice. The accelerated nucleation rate of SrSO4 in electrolyte solutions was a result of enhanced solubility of SrSO4 and resultant lowered interfacial tension. Threshold inhibitors 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), bishexamethylenetriaminetetra (methylene) phosphonic acid (BHTP), and phosphinopolycarboxylic acid (PPPC) are effective in inhibiting SrSO4 nucleation in NaCl solutions. Prolonged induction periods in the presence of inhibitors can be interpreted in terms of an elevation in the interfacial tension (up to 90 mJ m-2 with 10 mg liter-1 HEDP).