Effect of Fe2+ ions on gypsum precipitation during bulk crystallization of reverse osmosis concentrates.

In reverse osmosis desalination, the concentrate is a saline solution that may become supersaturated. Heterogeneous nucleation of salts occurs at the membrane surface, resulting in the buildup of inorganic deposits on the membrane. The inorganic nucleation process, however, is complex in natural waters. Most studies focused primarily on single salt fouling of membranes, and related treatment for single solute systems. However, scale formation, especially gypsum, is affected by the presence of different salts and metals. In this wok, for the first time, we investigate the mixed precipitation of iron oxides and gypsum. The role of citric acid in the inhibition of precipitation was studied for different concentrations in both the absence and the presence of Fe2+. Conductivity and ion concentration measurements were used to estimate the formation time of gypsum. Scanning electron microscopy, X-Ray Diffraction (XDR) analysis, and Infra-Red spectroscopy analysis were used to provide structural information. Collected data showed that the presence of Fe2+ accelerates gypsum precipitation and shortens its induction time. Analytic results showed that gypsum crystals are greatly affected by the presence of Fe2+ ions, which generated needle shaped crystals. Citric acid can delay the induction time of gypsum precipitation. It also affects the morphology of gypsum crystals through adsorption mechanism. XDR diagrams revealed that the presence of citric acid (20 mg/L) can stabilize the bassanite phase (CaSO4·½H2O) for much longer periods. In the presence of Fe2+ ions, citric acid extends the induction time of calcium sulfate and minimizes the acceleration effect of Fe2+ ions. SEM images showed that the presence of ferrous ions during the chemical inhibition generates the β-hemihydrate form of calcium sulfate.