New Findings about Nucleation and Crystal Growth of Reverse Osmosis Desalination Scales with and without Inhibitor

Nucleation and crystal growth of CaSO4·2H2O (gypsum) was studied under simulated conditions of scale formation using a reverse osmosis desalination technique. Calcium chloride and sodium sulfate were mixed with concentrated sodium chloride solution (40 g/L), and the reaction mixture turbidity was measured at different time intervals to determine the induction time of precipitated gypsum. Induction time was measured under different high supersaturation ratios ranging from 3.92 to 6.71. The induction time decreases exponentially with increasing the supersaturation. Using crystallization equations that relate induction times with supersaturation ratios, the free energy barrier and critical nuclei radius were calculated with and without addition of a scale inhibitor (trisodium phosphate, TSP). Rates of nucleation at a 4.47 supersaturation ratio are 0.4 × 1028 and 0.5 × 1028 nuclei/cm3·s with and without TSP addition, respectively.