Formation and control of calcium sulfate dihydrate (gypsum) crystallization on RO membranes and surrogate polymeric surfaces

Mineral salt scaling and antiscalant suppression of gypsum scaling in brackish water RO desalting were investigated via experimental studies of bulk crystallization, RO membrane scaling and surface crystallization on surrogate surfaces. Bulk crystallization studies assessed antiscalant effectiveness in retarding gypsum crystallization in the presence of Al3+ and Si. RO membrane surface scaling and crystallization onto surrogate surfaces illustrated the development of surface crystals as affected by surface chemical functionality and topology. Real-time gypsum growth kinetics, as impacted by antiscalants, was monitored in a batch crystallization system, via calcium depletion and particle light-scattering, to signal the onset of crystallization. Gypsum crystallization on RO membranes and surrogate surfaces was studied in a membrane diagnostic system and a novel microbalance crystallizer cell, respectively. Surface scaling was characterized via flux decline, optical and scanning electron microscopy, atomic force microscopy and contact angle measurements.; Progressive axial development of gypsum surface crystals on RO membranes demonstrated an increase in crystal size with the corresponding increase in the degree of concentration polarization. Gypsum crystallization on surrogate surfaces revealed a higher scale density for rougher surfaces with the same charge polarity (+/-), and the crystal morphology and growth rate varied significantly for different chemical functionalities. Kinetic modeling of surface crystallization, based on surface nucleation and single crystal growth model, suggested that surfaces with a high affinity to gypsum scale can be represented by a higher kinetic parameter---which accounts for both chemical functionality and surface roughness. Surface scaling was successfully suppressed with antiscalants. However, for the range of antiscalants tested, the crystallization induction times varied by more than an order of magnitude depending on the specific antiscalant used. In the presence of aluminum ions, an exponential decay of the induction time was observed with increased aluminum concentration of 0-100mug/L. In the presence of silica, gypsum crystallization induction time increased 9-fold for Si concentration range of 0 - 625 mg/L. Addition of colloidal silica in the system offset the adverse impact of aluminum on antiscalant effectiveness.