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).

Desupersaturation of RO concentrate and gypsum removal via seeded precipitation in a fluidized bed crystallizer.

The feasibility of a continuous chemically-enhanced seeded precipitation (CCESP) process was evaluated for desupersaturation of primary reverse osmosis (PRO) concentrate generated from RO desalting of inland agricultural drainage (AD) water with high gypsum scaling potential. The CCESP approach, comprised of partial lime treatment (PLT) followed by gypsum seeded precipitation (GSP), was assessed via laboratory and field tests, along with model simulations. PLT effectiveness was confirmed for residual antiscalant removal from the PRO concentrate, which otherwise would suppress gypsum crystallization. GSP was carried out in a fluidized bed crystallizer (FBC) demonstrating the feasibility of continuous PRO concentrate desupersaturation with suitable solids management. FBC operation was stable, with respect to desupersaturation performance, when operating over a sequence of periodic solids purge-only mode with intermittent seeds replenishment. The study suggests that CCESP integration with primary and secondary RO desalting (i.e., PRO-CCESP-SRO) can provide for significant enhancement of product water recovery for inland water of high gypsum scaling propensity. For example, source water of high salinity (14,347 mg/L total dissolved solid) AD water, nearly saturated with respect to gypsum, could be desalted up to a recovery of 88-96% (relative to merely 66% recovery feasible via PRO desalting. Moreover, net salt harvesting of 2.6-3.6 kg per m3 RO concentrate (with concentrate recycle) can be obtained from high recovery desalting of the above PRO concentrate.

Mobility of REE from a hyperacid brine to secondary minerals precipitated in a volcanic hydrothermal system: Kawah Ijen crater lake (Java, Indonesia).

Rare Earth Elements (REE; lanthanides and yttrium) are elements with high economic interest because they are critical elements for modern technologies. This study mainly focuses on the geochemical behavior of REE in hyperacid sulphate brines in volcanic-hydrothermal systems, where the precipitation of sulphate minerals occurs. Kawah Ijen lake, a hyperacid brine hosted in the Ijen caldera (Indonesia), was used as natural laboratory. ∑REE concentration in the lake water is high, ranging from 5.86 to 6.52 mg kg-1. The REE pattern of lake waters normalized to the average local volcanic rock is flat, suggesting isochemical dissolution. Minerals spontaneously precipitated in laboratory at 25 °C from water samples of Kawah Ijen were identified by XRD as gypsum. Microprobe analyses and the chemical composition of major constituents allow to identify possible other minerals precipitated: jarosite, Al-sulphate and Sr, Ba-sulphate. ∑REE concentration in minerals precipitated (mainly gypsum) range from 59.53 to 78.64 mg kg-1. The REE patterns of minerals precipitated normalized to the average local magmatic rock show enrichment in LREE. The REE distribution coefficient (KD), obtained from a ratio of its concentration in the minerals precipitated (mainly gypsum) and the lake water, shows higher values for LREE than HREE. KD-LREE/KD-HREE increases in the studied samples when the concentrations of BaO, MgO, Fe2O3, Al2O3, Na2O and the sum of total oxides (except SO3 and CaO) decrease in the solid phase. The presence of secondary minerals different than gypsum can be the cause of the distribution coefficient variations. High concentrations of REE in Kawah Ijen volcanic lake have to enhance the interest on these environments as possible REE reservoir, stimulating future investigations. The comparison of the KD calculated for REE after mineral precipitation (mainly gypsum) from Kawah Ijen and Poás hyperacid volcanic lakes allow to generalize that the gypsum precipitation removes the LREE from water.