Kinetics of gypsum crystal growth from high ionic strength solutions: A case study of Dead Sea - seawater mixtures

Gypsum precipitation kinetics were examined from a wide range of chemical compositions (11<Ca2+/SO42-<115), ionic strengths (4.75-10m) and saturation state with respect to gypsum (1.16-1.74) in seeded batch experiments of mixtures of Ca²⁺-rich Dead Sea brine and SO42--rich seawater. Despite the variability in the experimental solutions, a single general rate law was formulated to describe the heterogeneous precipitation rate of gypsum from these mixtures:Ratehet=k1·(Ω0.5-1)10+k2·(Ω0.5-1)2mol m-2s-1, where k₁ and k₂ are heterogeneous rate coefficients (mols^-1m^-2) that vary as a function of the solution compositions, and is the saturation state with respect to gypsum. It is suggested that two parallel mechanisms control the heterogeneous precipitation rate. Under closer-to-equilibrium conditions, the reaction is dominated by a mechanism best described as a 2nd order reaction with respect to Ω^0.5-1, which fits to the predictions of both the Burton Cabrera and Frank (BCF) crystal growth theory (Burton et al., 1951) and other layer-by-layer growth mechanisms (Goto and Ridge, 1967; Van Rosmalen et al., 1981; Bosbach and Rammensee, 1994). Under further-away-from-equilibrium conditions, the reaction is dominated by an apparent 10th order reaction. A conceptual model for gypsum growth kinetics is presented. The model is based on the 2nd order kinetic coefficients determined in the present study and data from the literature and is valid under a wide range of ionic strengths and Ca2+/SO42- ratios. According to this model, the integration of SO42- to kinks on the surface of the growing crystals is the rate-limiting step in the precipitation reaction. At ionic strengths above 8.5. m the precipitation rate of gypsum is enhanced, possibly due to the formation of CaSO4° ion pairs and/or a decrease in hydration frequencies.