Dynamics of turbidity in gypsum-precipitating brines: The case of the Red Sea - Dead Sea project

Authigenic processes in aqueous environments, such as mineral precipitation, can create turbidity which may have undesired effects on the natural environment and in industrial processes. Turbidity is often used to monitor such environments, as a mean to determine water quality and to follow the industrial processes. However, turbidity develops and grows or dissipates with time as the processes underlying its development advance. This dynamic nature of turbidity has yet to be studied. The proposed pilot Red Sea - Dead Sea project (RSDSP) is to desalinate seawater from the Gulf of Aqaba/Eilat and convey the reject brine, with or without additional seawater, to the Dead Sea to slow down the rate of its water level decline. The pilot is considered environmentally safe and will be used as a mean to determine if increased inflow volumes to stabilize the Dead Sea level will not negatively affect the lake. The mixing of the two very different solutions will lead to gypsum precipitation in the Dead Sea. In a large-scale project, if this gypsum remains in suspension, it may result in increased turbidity and whitening of the Dead Sea's surface water, thereby impacting the lake's appearance, its energy balance, and its touristic and mineral industries. We have studied the dynamic nature of turbidity as gypsum crystals form, grow and sink out of the water column in enriched mixtures of Dead Sea brine with seawater from the Red Sea. Our laboratory experiments suggest that precipitation from simple mixtures is likely to proceed without creating a significant spontaneous increase in turbidity. Turbidity did however develop in sulfate-enriched mixtures that had higher initial oversaturation. In these enriched solutions increased turbidity was observed, which developed faster and to higher values with increasing initial oversaturation. A linear relationship was found between the mass of gypsum precipitated and turbidity. However, this relationship was not universal; a unit mass of precipitated gypsum resulted in higher turbidity when the gypsum precipitated from mixtures having higher %wt of Dead Sea. This study shows that under laboratory conditions, mixtures of Dead Sea - seawater or Dead Sea - reject brine, do not develop turbidity due to gypsum precipitation. However, precipitation process in large scale natural systems can differ from those in the lab. Therefore, our findings cannot unequivocally conclude whether a whitening of the Dead Sea would develop following the implementation of the full scale RSDSP. Nevertheless, it does set forth the factors that need to be monitored during the pilot stage. Moreover, the study also demonstrates that: 1) authigenic processes do not lead to a one-to-one relationship between particulate matter and turbidity; and 2) turbidity readings must first be calibrated before used as a monitoring tool to identify and quantify gypsum formation (e.g., in desalination plants) or for the determination of induction times (e.g., in experiments).