Comparative study on salinity removal methods: an evaluation-based stable isotopes signatures in ground and sea water

This research aims to attain the optimal method of removing the high salinity concentrations without its effect on the balance or accuracy of stable isotopes measurement of deuterium and oxygen-18 (δ18O, δ2H). Four treatment methods (i.e., distillation, vacuum distillation, electro dialysis and ion exchange) were applied for nine samples, which were obtained from different water sources (sea, groundwater, river).l Worth to notice that the samples have Electrical Conductivity (EC) ranged (1000–60,000 µs/cm). Liquid–Water Isotope Analyzer used to measure the isotope concentration of δ18O, δ2H. The research findings of the four applied methods revealed their effectiveness with various percentages (normal distillation: 92.37%; vacuum distillation: 88.31%; electro dialysis: 94.85%; ion exchange: 99.62%). In addition, the investigation was conducted a clear correspondence measurement of (δ18O, δ2H) isotopes before and after treatment. The four methods results indicated that samples with EC ranged (1000–5000 µs/cm) have no effect on stable isotope readings. Whereas, samples with EC higher than 10,000, have substantial influence on the stable isotope readings. Finally, vacuum distillation method attained the best results among the treatment methods for EC ranged (10,000–60,000 µs/cm) without affecting the isotopic content of (δ18O, δ2H). There is a clear correspondence of the stable isotopic measurements before and after treatment, for all the selected samples.


Introduction
Isotope technology has an important role in the assessment and management of water and the protection of its sources (Sharma et al. 2015;Nguyen et al. 2021). Stable isotope analysis has become more rigorous through recent advances that provide: (i) signature determination of microscopic organisms such as microalgae, (ii) analysis of dissolved organic carbon, and (iii) improved quantification of relative source contributions (Pitt et al. 2008). Stable isotopes have successfully showed that the quality of the surface water is being affected by slight mixing with sea water. The interpretation of the stable isotope signatures of water (δ 2 H and δ 18 O) from soils in many research disciplines relies on accurate and high-precision measurements (Wassenaar et al. 2012). The slight enrichment showed by the surface water ( 18 O ranges from − 0.68 to − 1.82 and Deuterium 2 H ranges from − 2.2 to − 4.9) could be due to a possible slight mixing of sea water (Denutsui et al. 2012).
It is an important tool for studying different water sources and types to determine the behavior of water through the use of stable and radioactive isotopes as extractors by which water is evaluated (Ehleringer and Dawson 1992;Yang et al. 2010;Khan et al. 2019). It allows the measurement of stable isotope concentrations of oxygen and deuterium (δ 18 O, δ 2 H). In rainwater, the concentration of these two isotopes during the hydrological cycle, and when the water moves from one phase to another, the concentration of the isotope's changes, which is called isotopic fractionation. Isotopic fractionation gives an idea of the geochemical or hydrological process that occurred, for example, the difference in isotopic concentration of isotopes (δ 18 O, δ 2 H) in rainwater according to latitude, altitude, climate and time of year (Mook 2001).
Measuring the composition of stable hydrogen and oxygen (δ 18 O, δ 2 H) isotopes in salt water is analytically difficult, unlike fresh water, where the high salt concentration in water is an obstacle to direct stable isotope analysis and the results obtained may require correction in order to improve analytical accuracy (Shmulovich et al. 1999). The analytical problem arises to a high degree due to the large fragmentation that occurs between the free water molecules and those associated with the salt molecules in the water solution (Skrzypek and Ford 2014). There are some methods used in the treatment of highly saline water, such as ordinary distillation where brine is boiled in a water tank without pressure. Worth to mention, some research over the literature confirmed its reliability in the desalination plants with minimal production capacity (Grigg 2004).
In recent decades several researches presented in the literature, which concerns the technical issues of determination of water isotopic composition in concentrated aqueous salt solutions (Sofer andGat 1972, 1975;Stewart and Friedman 1975;Horita and Gat 1988;Porowski and Kowski 2008). The application of simple vacuum distillation allows full extraction of water and dehydration of remaining salts in a temperature range from 300 to 350 °C without hydrogen and oxygen isotope fractionation. The precision and accuracy of δ 18 O and δ 2 H determination of saline waters and brines with prior application of AgF desalination procedure is comparable with that usually obtained for fresh waters (Porowski 2019). Vacuum distillation of water is one of the most environmentally friendly and safe methods for separating the hydrogen and oxygen isotopes (Magomedbekov et al. 2021).
Vacuum distillation method is relied on the liquid boiling point which is proportional to the pressure applied to them. The pressure reduction on the liquid resistance a reduction in the liquid boiling point. This method is used in desalination plants with large production capacity of 30,000 cubic meters, approximately 8 million gallons of water per day (Cath et al. 2005). Electrodialysis known commercially since the sixties, as a cost-effective method for desalinating salty well water and gave way to interest in this regard (Chehayeb et al. 2017). The electrodialysis technique relies on the general principles that most of the dissolved salts in water are ionized. Positive (CATHODIC) or negative (IONIC), These ions are attracted to the electrode (ELECTROD) due to their electrical charge (ELETRIC CHARGE), membranes can be created that selectively allow the passage of ions according to their electrical charge (negative or positive), the contents of dissolved ions in the saline solution such as sodium ( +) chloride (−) Calcium (+ +) and Carbonate (−), they remain scattered in the water to take charge of the equation of their own charges. When the electrodes are connected to an external current source, such as a battery connected to water, the ions move toward the opposite charges of their charges in the solution, through the electric current in the solution in an effort to neutralize. In order to desalinate salt water through these phenomena, membranes that allow the passage of ions of only one type (not both types) are placed between two electrodes (Jiang et al. 2019).
Another treatment method exhibited over the literature is the ion exchange resins. The first organic ion exchanger was prepared in by (Adams and Holmes 1935), where the sulfurization of some types of coal resulted with positive ion exchanger "mechanism of action of resins". Solid with ions of similar type of charge present in the mobile liquid ionic phase passing through the exchanger (Small 2013). There are two main types of these resins namely cations, which are used in the removal of hard water usually occurs as a result of the presence of calcium and magnesium compounds. The second resin is anions, which are used in ion exchange systems to remove most of the negative ions in water, the installation of such systems increases the efficiency of the water produced (Solanki et al. 2012).
In nature, there are different water sources that presented as surface water, subsurface water and underground water. The establishment of investigating the hydrogen and oxygen isotopes for different source of water can be varied from one method to another. Hence, conducting different treatment methods could be the essential methodology to find the optimal removal method with high salinity concentrations without its effect on the balance or accuracy of stable isotopes measurement of (δ 18 O, δ 2 H). Therefore, the main contribution of this research was to test different treatment methods (i.e., distillation, vacuum distillation, electro dialysis and ion exchange) for nine samples collected from different water sources (sea, groundwater, river) for removing high salinity concentrations. The ultimate goal of the research is to find a robust and reliable methodology for mearing water salinity removal with accurate procedure that does not interrupt isotopic measurements.

Material and methods
Nine samples were collected from different sources (sea, groundwater, and river) included four marine samples et al.-Faw region, southern Iraq, four random groundwater samples from high salinity wells and one sample from the Tigris River et al.-Jadriya area, Baghdad. The electric conductivity EC of samples were ranged between (1000-60,000 µs/cm). The salinity removed methods and isotope measurement were made in the Environmental Isotope Laboratory in Iraqi Ministry Science and Technology. The processing of these methods is explained as follows; • Normal distillation: All samples were distilled using a normal closed glass distillation system at a temperature of 100-105 °C. • Vacuum distillation (closed system under low pressure): Samples were distilled using a closed glass vacuum distillation system under pressure and temperature of 60-65 °C. • Electrical technology (Dialysis): The samples were processed using electrolysis system that contains of a three-room separated by two tissues of negative and positive ions using a constant current battery of a 12 V and 60 Am for a period of 2 h. • Ion exchangers: 5 gm and 10 gm of a mixed bed of an ion exchangers contained both cation exchangers and anion exchangers in one composition were used for 30 min period to remove salinity from samples conductivity ranged from (1000-5000 µs/cm), and (10,000-60,000 µs/ cm) respectively. The ion exchangers processing included removing (calcium, magnesium, sodium, chloride, carbonate, and sulfate) ions and replacing those with (H + , OH−) ions. Stable isotopes of (δ 18 O, δ 2 H) were measured by using a Liquid-Water Isotope Analyzer LWIA model DLT100. Three local standard solutions (Internal Standards) shown in Table 1, were used to calibrate the LWIA device compared with international standard solutions in Table 2.    . 1 The relationship between deuterium ( 2 H) and EC after salt removal process Fig. 2 The relationship between oxygen ( 18 O) and EC after salt removal process difference from their isotopic measurements before treatment. This can be explained due to the repetition of the distillation process for those samples in addition due to their increased salinity and thus the evaporation factor showed an effect on the isotopic ratio. In more representative manners, graphical presentation was established to reveal the relationship between EC µs/cm and δ 2 H, δ 18 O (Figs. 1, 2). Figure 1 presents in general that distillation, vacuum distillation, electro dialysis, ion exchange treatment methods were behaved almost identically the same for the EC ranged between 1000 and 5000. The diversion is started after EC value of 10,000. However, it is worth to notice that there is fluctuation in the distillation, electro dialysis, ion exchange resins method treatment methods, this is clearly shown in the ion exchange and electro dialysis methods to increase the ionic exchange rate of positive and negative ions, such as sodium (Na + ) Chlorine iodide (Cl − ) Calcium (Ca +2 ) and Carbonate (Co −2 ) in the ion exchangers, and thus the isotopic ratio of the oxygen and deuterium isotopes changes ( 18 O, 2 H) due to the replacement of hydrogen ions and oxygen ions from the assay to the sample, which could be 2 H and oxygen -18. Whereas, a remarkable diversion can be observed for the ion exchange with lower deuterium values. Figure 2 presents the relationship between oxygen (18O) and EC after salt removal process. It can be notice that again the same identical results for the four treatment methods was attained for the EC ranged between 1000 and 5000. However, after reading of 10,000 EC, the diversion can be observed. For better visualization for the influence of the four treatment methods, Fig. 3 exhibits the values of the EC for the nine samples using all treatment methods. In general, all methods presented effective salinity removal for investigated samples.

Vacuum distillation method
The distillation of saline water prior to standard preparation and isotope analysis is usually the most preferred and the most common method in the laboratory to avoid detrimental influence of dissolved salts (Porowski 2019). The analytical procedure was composed of two parts: (i) the removal of alkaline-earth cations as insoluble carbonates; and (ii) the azeotropic distillation of the water from the brine with the aid of petroleum ether. Fig. 3 The EC values of various 9 samples before and after treatment using four desalination methods The implementation of vacuum distillation treatment method was effectively worked for the EC values ranged (36-5800 µs/cm), and the percentage of the salt removal was (88.31%). Whereas, the isotopic measurements of the δ 18 O and δ 2 H showed EC values ranged (1000-60,000 µs/cm) with a significant agreement with its isotopic measurements before treatment. This is because of the temperature used in the vacuum distillation process (under pressure) is low, and therefore, there was no effect of the evaporation agent on the isotopic balance of the stable isotopes (δ 18 O, δ 2 H) as presented in Table 5.
Even the applicability and usefulness as a process of water desalination (purification) prior to isotopic analyses is still controversial. The application of the simple vacuum distillation of saline waters studied improves considerably the reproducibility and accuracy of determination of their hydrogen isotopic composition (Porowski and Kowski 2008). Nevertheless, with the possible processes of hydrocarbons transformations during high temperature water reduction (for instance, cracking, oxidation) their influence on obtained δ 2 H reproducibility should be carefully tested.

Electro dialysis method
For the electro dialysis method, the EC values ranged after treatment (36.4-4120 µs/cm) with average salt removal rate (92.37%) ( Table 6). The samples with EC values ranged (1000-5000 µs/cm) showed a significant affinity with their isotopic measurements of (δ 18 O, δ 2 H) before and after treatment procedure. However, the isotopic measurements of (δ 18 O, δ 2 H) showed a significant difference before and after treatment particularly for the samples with EC values ranged (10,000-60,000 µs/cm). This is due to the fact that the salts dissolved in water produce charged ions, such as sodium (Na + ) Chlorine iodide (Cl − ) Calcium (Ca +2 ) and Carbonate (Co-2). Ionic diffusion in water allows the migration of ions that are selectively determined by corresponding ion exchange membranes allow the passage of ions to be replaced by (H + , OH−) ions, which in return can be new isotopes of oxygen and deuterium ( 18 O and 2 H). In addition, through the electric current in the solution looking for neutralization, the increase in salt concentration leads to increase the ion exchange process, which negatively affects the isotopic balance of the treated sample.

Ion exchange resins method (mixed bed ion exchange)
For the ion exchange resins method, the EC values after treatment was ranged (34-160 µs/cm), and the percentage of salt removal reached (99.62%) ( Table 7). The isotopic measurements of oxygen and deuterium ( 18 O, 2 H) after treatment using the mixed bed ion exchange for some samples showed that the EC value ranged (1000-5000 µs/cm) displayed a significant agreement with its isotopic measurements after and before treatment, since the standard deviation of ( 18 O: 0.3 and 2 H:2). It is an indication of the allowable limits for measurement in the LWIA aqueous isotope analyzer. The samples that having EC values ranged (10,000-60,000 µs/ cm) showed a significant difference in its isotopic measurements after and before treatment due to the high salt concentration existed in the samples. This can increase the rate of ion exchange for positive and negative ions in ion exchangers such as sodium (Na + ), chloride (Cl − ), calcium (Ca +2 ) and carbonate (Co −2 ). Which allows it to be replaced by ions (H + , OH − ), which may build a new isotope of δ 18 O and 2H, thus changing the isotopic ratio of oxygen and deuterium ( 18 O, 2 H).

Discussion
The current research was established to proposed new methods with the aim to evaluate sea and groundwater salinity; in the condition, that isotope signatures will not be disturbed. The main contribution for the practical implication is, environmental engineers always keen to evaluate water samples salinity with the concern of the isotope signatures magnitude. Therefore, the proposition of a certain method for testing salinity removal with accurate determination in parallel with maintaining isotope signatures magnitude is the essential procedure to be targeted. Based on the current research results, in general, the four applied methods revealed an acceptable performance. However, there was some variation such as ion exchange resins and electro dialysis.

Conclusion
The high-saline water has a serious effect on the (LWIA) analyzer. It is necessary to reduce the concentration of the salts before the water stable isotopes (18O, 2H) analysis process. However, these treatments are not within the acceptable level for samples exceeding (60,000 µs/cm) which effect and change the isotope values, hence, the vacuum distillation method is considered the best method. A review of literature data that describe the current state of art in the field of process improvement was presented. The characteristics of contact devices used in distillation were analyzed, and ways to improve their efficiency were shown. Data on new developments aimed at evaluating the efficiency of using salt distillation of water for isotope separation were presented. Some aspects of the use of water distillation for concentration of heavy hydrogen and oxygen isotopes and for the preparation of water with reduced deuterium contents were considered. To the best knowledge of the current research, the salinity of nine samples collected from different sources and were treated using four desalination methods. This study was inspected the effects on the isotope measurements of (δ 18 O, δ 2 H) and to choose the appropriate method among the applied methods. For this purpose, the EC value for water samples before and after treatment was compared with the isotope measurements of (δ 18 O, δ 2 H). Significant agreement for (δ 18 O, δ 2 H) values was fond before and after the treatment, by using vacuum distillation method, the isotopic balance is constant and unchanged for all samples after processing spatially when the EC value between (1000-60,000 µs/ cm).