The vaterite saturation index can be used as a proxy of the S&DSI in sea water desalination by reverse osmosis process

The Stiff and Davis solubility index (S&DSI) is widely used to predict the precipitation of calcium carbonate on seawater reverse osmosis membranes. Although this index is defined as the pH of the solution minus the value of the pH of the solution in equilibrium with calcium carbonate, the actual polymorph of calcium carbonate to which this index refers is not specified. In the present work, the geochemical significance of the S&DSI was examined by geochemical modelling. The pH of a number of solutions minus the pH of these solutions in equilibrium with different polymorphs of calcium carbonate (calcite, aragonite, vaterite and monohydrocalcite) was determined using PHREEQC software and then compared with S&DSI indices for the same solutions. This index was found to correspond to the pH of the solution minus the value of the pH in equilibrium with vaterite. The possibility that the pH, and concentration of Ca2+ and HCO3− ions, measured in the original experiment of Stiff and Davis were controlled by the solubility of vaterite is discussed. Further, it is proposed that in seawater reverse osmosis desalination plants, the saturation index of vaterite may be used as a proxy of the S&DSI index.