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).

Chemical degradation of 2,2-bis(bromomethyl)propan-1,3-diol (DBNPG) in alkaline conditions.

The mechanism and kinetics of the spontaneous decomposition of 2,2-bis(bromomethyl)propan-1,3-diol (DBNPG) and its decomposition daughter products were determined in aqueous solution at a temperatures range between 30 and 70 degrees C and pH from 7.0 to 9.5. DBNPG decomposition in basic aqueous solutions involves release of bromide ions through a sequential formation of 3-bromomethyl-3-hydroxymethyloxetane (BMHMO) and 2,6-dioxaspiro[3.3]heptane (DOH). DBNPG decomposition into BMHMO is a two-stage reaction. The first stage is an acid/base equilibrium, in which an alkoxide is formed. In the second stage, DBNPG predominantly undergoes an intramolecular nucleophilic substitution to form the BMHMO. The transformation rate increases with the pH and the energy barrier for the degradation is 98 kJ mol(-1). Good agreement was found between the rate coefficients derived from variations in the organic molecules concentrations and those determined from the changes in the Br(-) concentration. DBNPG is one of the most abundant pollutants in a studied polluted aquitard underneath industrial park in the northern Negev, Israel, and together with its by-products pose an environmental hazard. DBNPG half-life is estimated to be about 65 years. This implies that high concentrations of DBNPG will persist in the aquifer long after the elimination of all its sources.

High naturally occurring radioactivity in fossil groundwater from the Middle East.

High levels of naturally occurring and carcinogenic radium isotopes have been measured in low-saline and oxic groundwater from the Rum Group of the Disi sandstone aquifer in Jordan. The combined 228Ra and 226Ra activities are up to 2000% higher than international drinking water standards. Analyses of the host sandstone aquifer rocks show 228Ra and 226Ra activities and ratios that are consistent with previous reports of sandstone rocks from different parts of the world. A compilation of previous data in groundwater from worldwide sandstone aquifers shows large variations in Ra activities regardless of the groundwater salinity. On the basis of the distribution of the four Ra isotopes and the ratios of the short- to long-lived Ra isotopes, we postulate that Ra activity in groundwater is controlled by the balance of radioactive decay of parent Th isotopes on aquifer solids, decay of the dissolved radium isotopes, and adsorption of dissolved Ra on solid surfaces. The availability of surface adsorption sites, which depends on the clay content in the aquifer rocks, is therefore an important constraint for Ra activity in sandstone aquifers. These findings raise concerns about the safety of this and similar nonrenewable groundwater reservoirs, exacerbating the already severe water crisis in the Middle East.

Dissolution kinetics of synthetic zeolite NaP1 and its implication to zeolite treatment of contaminated waters.

The effect of pH on the dissolution kinetics of NaP1 zeolite, which was produced from the alkaline treatment of coal fly ash and may be used for decontamination of acid mine waters, is studied. The sample contains considerable amounts of accessory phases that partly dissolve during the experiment. Therefore, the dissolution rate was estimated during a stage in which the Al/Si ratio was equal to that of NaP1 (0.6). The release rate of these elements is controlled by the dissolution of the zeolite itself during this stage. The dissolution rate of NaP1 slows down with increasing pH in the acidic range, becomes constant at an intermediate pH, and increases with increasing pH in the basic range. The observed changes in rates were described using a rate law based on a surface speciation model. Using this rate law, we calculated the half-life of NaP1 to be about 2 years at near neutral pH and less than 10 days at pH below 3. For the utilization of NaP1 in the treatment of wastewaters or acid mine waters, these short half-lives bear two implications: (1) The treated waters must be kept at near neutral pH, and NaP1 should be added periodically to the treated waters in order to compensate for zeolite loss. (2) In water treatment applications that require a relatively short reaction time, the zeolite removed from the effluents should be kept dry in order to avoid its decomposition and the consequent release of the adsorbed metal to the environment.

Chemical transformation of 3-bromo-2,2-bis(bromomethyl)-propanol under basic conditions.

The mechanism of the spontaneous decomposition of 3-bromo-2,2-bis(bromomethyl)propanol (TBNPA) and the kinetics of the reaction of the parent compound and two subsequent products were determined in aqueous solution at temperatures from 30 to 70 degrees C and pH from 7.0 to 9.5. TBNPA is decomposed by a sequence of reactions that form 3,3-bis(bromomethyl)oxetane (BBMO), 3-bromomethyl-3-hydroxymethyloxetane (BMHMO), and 2,6-dioxaspiro[3.3]-heptane (DOH), releasing one bromide ion at each stage. The pseudo-first-order rate constant of the decomposition of TBNPA increases linearlywith the pH. The apparent activation energy of this transformation (98+/-2 KJ/mol) was calculated from the change of the effective second-order rate constant with temperature. The pseudoactivation energies of BBMO and BMHMO were estimated to be 109 and 151 KJ/mol, respectively. Good agreement was found between the rate coefficients derived from changes in the organic molecules concentrations and those determined from the changes in the Br- concentrations. TBNPA is the most abundant semivolatile organic pollutant in the aquitard studied, and together with its byproducts they posess an environmental hazard. TBNPA half-life is estimated to be about 100 years. This implies that high concentrations of TBNPA will persist in the aquifer long after the elimination of all its sources.

Surface protonation data of kaolinite-reevaluation based on dissolution experiments.

The aim of the present study is to compare available surface titration curves of kaolinite, to explain the differences between them, and to constrain their interpretation based on predictions of surface protonation that emerged from dissolution experiments. Comparison of six surface titration curves obtained at 25 degrees C reveals significant discrepancies, both in the shape of the curves and in the pH of the point of zero net proton charge (pH(PZNPC)). Based on an analysis of the different sites available for adsorption on kaolinite surfaces we conclude that different kaolinite samples are expected to have similar pH(PZNPC). Therefore, the major reason for the differences in the observed surface protonation is related to the different ways in which the pH(PZNPC) was determined. To compare the titration curves, some of the curves were recalculated so that the proton surface concentrations of all the titration curves would be zero around pH 5. As a result, we obtained a good agreement between the titration curves. A prediction of the molar fraction of protonated sites was retrieved from modeling of kaolinite dissolution reaction and was compared to the protonation data obtained from surface titration. The model successfully predicts the surface protonation data of most of the surface titrations.