Elsevier

Desalination

Volume 496, 15 December 2020, 114638
Desalination

The effect of phosphonate-based antiscalant on gypsum precipitation kinetics and habit in hyper-saline solutions: An experimental and modeling approach to the planned Red Sea – Dead Sea Project

https://doi.org/10.1016/j.desal.2020.114638Get rights and content

Highlights

  • ATMP in hyper-saline solutions retards nucleation but does not affect the rate of crystal growth.

  • ATMP affects the precipitation rate of gypsum via altering the surface area of the crystals created during nucleation.

  • Fewer and larger crystals precipitate in the presence of ATMP.

  • The properties of the crystals affect the reaction order of gypsum growth.

Abstract

The effect of phosphonate-based antiscalant on the precipitation kinetics of gypsum under the hyper-saline solutions relevant to the Red Sea-Dead Sea desalination project was studied. Batch experiments were carried out to determine the effect on the induction times and rates of nucleation and gypsum crystal growth. A wide range of ionic strength (5.5–9 m), pH (6.33–6.91) and oversaturation with respect to the mineral (1.69 ≤ Ωgypsum ≤ 2.11) were covered in the experiments.

Induction times in solutions that contained the antiscalant were longer by about 20–500% compared to solutions with a similar composition that did not contain the antiscalant. In the presence of the antiscalant fewer crystals precipitated that were larger in size. The specific surface area of the crystals that precipitated in solutions that contained the antiscalant were smaller in about 20–500% than those that precipitated in solutions that did not contain the antiscalant, and the relative specific surface area was in agreement with the induction time. We conclude that increased interfacial tension in the presence of the antiscalant, retards nucleation and reduces the surface area available for growth. Subsequent rates of crystal growth were impervious to the presence of the antiscalant.

Introduction

Gypsum precipitation during industrial and commercial processes is a common and undesirable phenomenon that can lead among others to clogging of pipes and membranes during desalination [[1], [2], [3], [4], [5]]. In the subsurface it may decrease the permeability and other important properties of reservoirs and aquifers during oil [6,7] and geothermal fluid production [8] and CO2 geological sequestration [9]. To avoid such undesired mineral precipitation, commercial scale inhibitors (henceforth, antiscalants) are routinely added.

Precipitation of a mineral from solution involves two distinct processes: a) nucleation, in which dissolved ions in solution react to form crystal nuclei, and b) a phase of crystal growth during which nuclei and crystals existing in solution grow via a reaction between the solid and dissolved phases. Different antiscalants are known to affect either nucleation of minerals, or their crystal growth. Some antiscalant may affect both [[10], [11], [12]]. In addition to the effect antiscalants have on the kinetics of precipitation, their presence in solution may change crystal morphology. Gypsum crystals precipitated from solutions containing antiscalants have been reported to be more tabular in comparison to crystals that precipitated from solutions of similar composition but without antiscalants [10,11,13].

One of the most widely used groups of antiscalants is based on phosphonate (C-PO(OH)2) as the active functional group. Although phosphonate-based antiscalants have been found to be effective as inhibitors, the exact mechanisms by which they inhibit precipitation is not completely clear [12,14]. However, it has been proposed that phosphonate-based antiscalants reduce the rate of nucleation by adsorbing on to nuclei or binding metal ions to create a ‘poisoned’ nucleus that cannot grow beyond the critical size in which it becomes a stable crystal. The mechanism by which the antiscalant reduces the rate of crystal growth is commonly attributed to adsorption of the negative phosphonate species onto the crystal [1,12,15] (and references within).

During desalination by reverse osmosis, the saline water is passed through semi-permeable membranes that block most of the ions. The antiscalants are added to the feed water before it reaches these membranes and are retained in the reject brine (RB) - the by-product of desalination that contains the dissolved ions from the feed water. In seawater-desalination plants these reject brines are routinely discharged into the marine environments where, due to their relative stability and low biodegradation rates, the phosphonate-based antiscalants remain stable for long periods of time [[16], [17], [18]].

In recent years it was shown that microbes can break the Csingle bondP bond in phosphonate molecules and utilize the phosphorous as a nutrient [19,20]. It was thus suggested that the release of phosphonate antiscalants into natural environments may contribute to eutrophication of water bodies [21,22]. The stability of phosphonates and their possible role in eutrophication of water bodies have resulted in the restriction of their use in some regulated environments (e.g., off the coast of Norway and Denmark [23]). Currently an effort is being made to develop ‘environmentally friendly’ antiscalants [17,22,23]. However, phosphonate-based antiscalants are relatively cheap and highly efficient, and therefore they remain widely used in desalination plants.

The Dead Sea is a hypersaline (~350 g/l total dissolved salts) terminal lake characterized by its calcium-chloride composition, and it is enriched in Ca2+ and depleted in SO42− compared to seawater (Table 1). Since the late 19th century the lake has been experiencing a negative water balance, which has resulted in lowering of the lake level by about 40 m to the current level (November 2019) of 433 m below sea level [24]. This water level drop has resulted in a separation between the deep northern basin and the shallow southern basin [25]. Today the area of the southern basin is utilized for industrial evaporation ponds by the Israeli and Jordanian chemical industries. Furthermore, as salinity rose the lake turned from a meromictic (stratified) to a monomictic lake that is thermally stratified during the summer [26,27]. The rise in salinity of the Dead Sea brine has also reduced the number of microorganisms in the lake [28]. Additional changes that the lake experienced include a change in the minerals that precipitate from the lake. Whereas in recent centuries aragonite (CaCO3) and gypsum were the dominant evaporites that precipitated [29], nowadays it is halite (NaCl) [[30], [31], [32]]. The receding shoreline has also impacted the environment of the lake, most notably by the development of sinkholes along its shores. The sinkhole activity endangers human lives and damages infrastructure, touristic sites and cultivated lands around the lake [[33], [34], [35]].

The Red Sea – Dead Sea Project (RSDSP) is a major project that has long been proposed and discussed between the governments of Israel and the Hashemite kingdom of Jordan [36]. The goal of the RSDSP is to supply potable water, which is greatly needed in the region, while mitigating the negative water balance experienced by the lake. According to the scheme, seawater from the Red Sea would be pumped from the Gulf of Eilat and desalinated while the reject brine created during the process will be conveyed and discharged into the Dead Sea. Discharging the reject brine into the Dead Sea would allow utilizing the large elevation gradient (>400 m) to produce electricity for the process. Depending on the volumes, discharging reject brine into the lake would reduce the negative water balance of the lake and retard the drop in the lake's level.

A pilot for a much larger project is currently being planned in which some 300 million cubic meters (MCM) of seawater would be pumped annually from the Red Sea. Some of this volume will be diverted to a desalination plant to produce 65 MCM/year of potable water while the rest will be mixed with the reject brine from the plant and discharged to the Dead Sea. Under these terms the resulting diluted reject brine would be concentrated by a factor of ~1.28 relative to the feed water. Discharging this diluted RB into the lake would both change the lake's composition and introduce antiscalant into its hypersaline brine.

The degree of saturation of a solution with respect to gypsum is described by:Ωgypsum=IAPKsp=aCa2+·aSO42·aH2O2aCa2+·aSO42·aH2O2eqwhere IAP is the ion activity product, Ksp is the solubility constant and ai is the activity of the ith constituent.

Over the last few decades the Dead Sea has been oversaturated with respect to gypsum [37] and currently Ωgypsum = 1.34. However, due to slow precipitation kinetics, no significant amount of the mineral precipitates from the lake. The main chemical parameters controlling the slow kinetics are: 1) the high Ca2+/SO42− ratio in the lake (currently 130 mol ratio). This ratio greatly affects precipitation rate with the fastest precipitation rates expected around unity [38]; 2) high concentration of cations such as Na+ and especially Mg2+.These cations adsorb onto the surface of gypsum, thereby limiting the ability of Ca2+ to do so and resulting in reduced precipitation rate [[38], [39], [40]], and; 3) an overall low concentration of SO42− [37].

Introduction of SO42−-rich seawater and reject brine into the Ca2+-rich lake would result in an increase in both oversaturation with respect to gypsum and its precipitation potential (amount of gypsum required to precipitate for the system to reach chemical equilibrium) [41]. As well as affecting these thermodynamic properties of the brine, introduction of reject brine (or diluted reject brine) to the lake will supply the lake with SO42− and reduce the Ca2+/SO42− ratio. Furthermore, discharging reject brine into the lake would dilute the Dead Sea brine and reduce the concentration of the cations retarding the kinetics of gypsum precipitation. Thus, the introduction of reject brine into the lake will both increase the thermodynamic favorability of gypsum precipitation (by increasing Ωgypsum), while accelerating the kinetics of precipitation.

If gypsum expected to precipitate in the lake following mixing with reject brine (or seawater-diluted reject brine) would stay in suspension it may cause whitening of the lake's surface water. Whitening of the lake's surface water can change its albedo and thus the heat balance between the lake and its surroundings, and consequently the rate of evaporation from the lake [36]. In addition, increased turbidity, as is expected if many crystals remain in suspension, is considered to negatively change the aesthetics of water bodies and may impact tourism [42]. Given the touristic and cultural values of the Dead Sea, it is assumed that such an event is undesired. Whether the crystals will stay afloat and potentially contribute to a whitening would be determined by their size and morphology. Crystal size in turn would be determined in large by the combined rates of nucleation and crystal growth. If the process of nucleation is rapid and dominates the precipitation, many small crystals would form, whereas a low rate of nucleation and fast crystal growth would result in fewer but larger crystals.

Several studies of gypsum precipitation in Dead Sea – Seawater mixtures have been carried out in an attempt to understand the fate of the gypsum that would precipitate in the lake once the RSDSP is constructed. The focus of these studies has been the thermodynamics and kinetics of gypsum precipitation in the lake [37,39,43] and the relationship between the composition of the brine and the morphology and crystal size distribution of the mineral [44].

These studies have increased the understanding of gypsum precipitation under saline conditions and laid the basis for understanding the future of gypsum in the Dead Sea once the RSDSP is constructed. However, it is yet to be determined if the antiscalants added during desalination would affect gypsum precipitating under the hyper-saline conditions of the RSDSP. The goal of this study is therefore to study the effects a phosphonate-based antiscalant has on the kinetics and morphology of gypsum precipitating under hyper-saline conditions in general and the conditions relevant to the RSDSP desalination scheme in particular.

Section snippets

Brine sampling and preparation

Dead Sea Brine (DSB) was collected during winter (Nov. 2016), when the water column was well mixed. The brine was collected directly from the lake by lowering a jerry can from a dinghy. In order to avoid local dilution by floodwater, sampling was done at the center of the lake. The brine was filtered (0.45 μm) before every experiment.

Seawater and reject brine were collected at the Soreq desalination plant on the Israeli Mediterranean coast. Seawater pumped into the plant is flowed through a

Antiscalant and gypsum nucleation

Fig. 1A and Table 3 present the initial Ωgypsum of the mixtures between Dead Sea brine and the evaporated seawater or diluted reject brine. In the absence of crystallization seeds, a time interval exists between the mixing of the end members and the first identification of gypsum precipitation. This time interval is known as the induction time - Tind [53], and it depends on the method of observation. Yet, it is commonly accepted that Tind is inversely proportional to the rate of nucleation [54

Summary of results

The effect a phosphonate-based antiscalant with ATMP as the active phosphonate has on the precipitation kinetics of gypsum under the hyper-saline conditions of the RSDSP has been studied. Results show that the antiscalant retards the rate of gypsum nucleation. Fewer gypsum crystals have precipitated from solutions of similar composition in the presence of the antiscalant. These crystals were also larger. The slower rates of nucleation in the presence of the antiscalant were attributed to

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to thank O. Porat from BGU, Galit Sharabi and Olga Berlin from the Geological Survey of Israel for technical support. In addition, we want to thank Mr. M. Taub for arranging our visit to the Soreq desalination plant. We would also like to thank the editor and five reviewers for their insightful comments.

Funding

This research did not receive any specific grant from any funding agencies in the public, commercial, or not-for-profit sectors.

Author statement

Amit G Reiss: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Writing – Review and editing.

Jiwchar Ganor: Writing – Review and editing, Resources, Supervision.

Ittai Gavrieli: Writing – Review and editing, Resources, Supervision.

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