Towards the establishment of a general rate law for gypsum nucleation

Gypsum nucleation kinetics from a wide range of chemical compositions (1.45 < Ca2+/SO42- < 115), ionic strengths (I = 2.5–10 m) and saturation state with respect to gypsum (Ωgyp = 1.07–8.4) were examined in batch experiments containing mixtures of Ca2+-rich Dead Sea brine and SO42--rich seawater with or without addition of extra Na2SO4 and CaCl2·2H2O. The induction times attained in the present study were compiled together with literature values from experiments carried out under significantly different conditions (synthetic NaCl solutions; I = 0.09–6.3 m; Ca2+/SO42- = 1; Ωgyp:1.59–7.76). Despite the variability in the experimental solutions, a single rate law based on classic nucleation theory was formulated to describe the induction times from more than 80 experiments:logTind=log13.17·Cs·exp-7.08ln2Ω+0.072·Cs·exp-1.426ln2Ωwhere Tind is the induction time, Cs is the solubility of gypsum and Ω is the saturation state with respect to gypsum.The rate law provides Tind for gypsum precipitation from aqueous solutions at 25 °C, containing no synthetic antiscalants or catalysts, within a 95% confidence interval within a factor of 5.Based on this rate law, we show that at present most of the precipitation of gypsum from the Dead Sea brine occurs following significant evaporation in the industrial evaporation ponds and not in the Dead Sea itself. Whereas Tind in Dead Sea brines is very long (on the order of 3 years), the evaporation of brine in the industrial ponds leads to increased Ω values, and thus to short Tind in the order of a few days. However, if seawater or reject brine from seawater desalinization will be introduced to the Dead Sea to restore its declining level, Tind will be significantly reduced and gypsum nucleation and precipitation will occur. For evaporated seawater, the proposed rate law predicts that even though saturation is obtained when seawater is evaporated by a factor of 2.8, gypsum will nucleate at reasonable times (few years) only when seawater are evaporated by a factor of ∼3.3.