Chemical recovery of magnesium from the Dead Sea and its use in wastewater treatment

Mg(OH)₂ and MgCl₂ are important and valuable commercial resources because they can be used in many applications, including products used to reduce dust, materials used for drilling, ion exchange resins, cements containing oxy-chlorine, coagulants, and fertilizers. Seawater is one of the primary sources of magnesium, particularly MgCl₂ and Mg(OH)₂. There are significant MgCl₂ reserves in the Dead Sea (DS), approximately 52,000 mg/l, while seawater concentrations in other areas are approximately 1,400 mg/l (Weber et al. 2022). Magnesium is an alkaline earth metal and the eighth most abundant element in the Earth's crust; however, it is formed in combination with other elements making it difficult and expensive to extract (Kim 2011).

One of the environmental challenges is waste brines, which were generated from seawater desalination, and the potential of using rejected brines as a valuable source for mineral recovery. Specifically, it emphasizes the significance of magnesium, classified as an essential raw material, found in these brines, and the need for a pretreatment method to optimize salt production (Morillo et al. 2014; Loganathan et al. 2017).

Various methods have been developed for magnesium recovery, such as cationic selectrodialysis (Ghyselbrecht et al. 2019) and precipitation processes using ammonium to react with MgCO₃ (Mohammad et al. 2019). Selective precipitation, crystallization, and electrodialysis technologies have demonstrated efficiency in magnesium recovery, aligning with zero liquid discharge initiatives and environmental pollution reduction goals (Zhang et al. 2021).

The chemical separation method for precipitating Mg(OH)₂ from seawater is outlined, followed by the challenges associated with impurities in the precipitate. Various techniques, including the use of Ca(OH)₂ and sulfuric acid (H₂SO₄), have been explored to enhance purity, with notable successes reported in the literature, which are discussed here.

Continuing the exploration of magnesium recovery methods, Bevacqua et al. (2015) have used a sophisticated method to improve Mg(OH)₂ purity, achieving 99.5% Mg(OH)₂ purity using inexpensive reactants such as Ca(OH)₂. In addition, Quist-Jensen et al. (2016) discovered that the magnesium salts recovered from bittern of seawater were feasible low-cost and effective coagulants for removing both organic and inorganic substances from wastewater.

Transitioning to applications, Young et al. (2021) demonstrated the effectiveness of Mg(OH)₂ and MgCl₂ in the flocculation process for wastewater treatment, removing various organic contaminants, inorganic compounds such as ammonia and phosphorous, and chemical oxygen demand (COD) from wastewater by adjusting the pH value. The recovery of MgCl₂ from Mg(OH)₂, another detailed study by Peeva et al. (2022), presented its cost-effectiveness compared to traditional coagulants, particularly in removing organic and inorganic contents from wastewater sludge.

The objective of this research was to develop a comprehensive process for recovering MgCl₂ and Mg(OH)₂ from DS water by adding NaOH and HCl. The recovered materials were used to remove total suspended solids (TSS) and COD from domestic wastewater. DS water has a very high concentration of magnesium ions; therefore, it is vital to evaluate recovering magnesium for use in industrial applications and the coagulation process, which is more efficient than recovering magnesium from regular seawater due to higher concentrations as investigated by Reiss et al. (2020). Chemically recovering salts, such as magnesium, reduces the load on other desalination technologies, allowing stakeholders to mitigate rejected brines through reverse osmosis, which is becoming popular for water desalination in Jordan and elsewhere since it conserves natural resources and protects the environment. Chemical treatments also reduce a large amount of TSS and COD during the pretreatment process, which makes the secondary treatment process more economically viable (Walschot et al. 2020).

Thirty DS water samples with a pH of 7.2, total dissolved solids (TDS) concentration of 358.4 g/l, total alkalinity ((HCO₃)⁻, (CO₃)⁻², and (OH)⁻) of 182 mg/l, and an approximate magnesium ion concentration of 38 g/l were collected. Samples were obtained from the DS beach during Spring – April 2020 and contained high-density polyethylene (1,000 ml). Also, more than 30 l of municipal wastewater samples were collected from the Jordan University of Science and Technology's treatment plant primary sedimentation tank effluent and filled in high-density polyethylene bottles with an initial magnesium ion concentration of approximately 8 mg/l and a calcium ion concentration of approximately 21 mg/l. The municipal water's pH value was 7.3, and the TSS and TDS concentrations were 450 and 680 mg/l, respectively. The phosphorus concentration was 9.2 mg/l, nitrate nitrogen was 35 mg/l, and the COD was 620 mg/l. The samples were used for the purpose of magnesium recovered from DS water and used for wastewater treatment experiments using the magnesium recovered from DS water.

In order to examine the adsorption capabilities of MgCl₂ obtained from DS water as well as various commercial coagulants, another raw wastewater sample, with high levels of TSS at 1,150 mg/l and COD at 950 mg/l, was taken from the same source for analysis.

For robust environmental data, we implement stringent quality control measures during sample transportation and storage. This includes using clean, inert containers, maintaining temperature control, establishing a clear chain of custody, and taking duplicate samples. We carefully choose transportation conditions and controlled storage environments. Regular instrument calibration, meticulous documentation, and periodic quality control checks ensure the reliability of our laboratory measurements for parameters such as pH, temperature, TDS, magnesium ion concentration, total alkalinity, phosphorus concentration, nitrate, COD, Biochemical Oxygen Demand (BOD), and TSS. Table 1 shows all measured parameters.

Mg(OH)₂ was recovered from DS water using an alkali solution (NaOH) at different dosages. The recovered Mg(OH)₂ was separated and then dissolved in distilled water to convert it to MgCl₂ by adding HCL. The recovered MgCl₂ and Mg(OH)₂ salts were assumed to be mostly pure. The recovery procedure of Mg(OH)₂ from DS water was carried out in a jar tester, six beakers of 1 l at 25 °C to obtain magnesium precipitation after rapid mixing and followed by slowing mixing. Each dose of caustic soda was added to the beaker, after which the stirrer immediately began mixing for a minute rapid mixing and then slowing mixing at 40 rpm for 20 min (Bagastyo et al. 2021). The following recovery plan was implemented:

• Mg(OH)₂ was precipitated by adding NaOH to 250 ml of DS water to increase pH value from 7.2 to 9.5, 10.5, and 11. The precipitated Mg(OH)₂ was removed using filter paper 45 μm and purified with distilled water to obtain high purity. The recovered Mg(OH)₂ is called liquid bittern. Different doses of NaOH were added, from 8 to 35 ml, to increase the pH values to 8.5–11 and precipitate Mg(OH)₂.

• A stock solution was prepared for magnesium measurement after the Mg(OH)₂ was precipitated. The extracted liquid bittern was placed in 1 l of distilled water, and then the amount of magnesium present was examined and measured. Mg(OH)₂ samples with known magnesium concentrations were dissolved by adding HCL to convert it to MgCL₂. pH was adjusted to 4 for each Mg(OH)₂ sample, and then the dissolved MgCl₂ was placed inside an oven at 104 °C for 24 h to produce the dry MgCL₂ bittern. The dry bittern was added to 1 l of distilled water for sedimentation experiments.

Chemical coagulation experiments were performed by adding pure Mg solution from the recovered MgCL₂ and Mg(OH)₂. Commercial coagulants, including Al₂(SO₄)₃, Fe₃Cl₃, Fe₂SO₄, and polyacrylamide (PAM), were used to compare their effectiveness when removing TSS and COD from wastewater. The jar test apparatus was used to investigate the optimal removal of TSS and COD using the magnesium recovered from the DS water. The conventional jar test apparatus consisted of six beakers of wastewater, where 1 l at room temperature was used to mix the samples for each experiment. A known dose of magnesium was then added to the six jars of wastewater. The samples were mixed rapidly at 250 rpm for 1 min. The rapid mixing was followed by gentle mixing at 20 rpm. TSS and COD were measured in the bulk after the settling period. The experiments were performed with prior pH adjustments and at a fixed dose of 0.4 g/l for each coagulant, Al⁺³, Fe⁺³, Fe⁺², and Mg⁺², to compare the performance of the magnesium recovered from the DS water and the other coagulants. Each sample was promptly stirred at a speed of 180 rpm for 3 min after the dose was added to the solution, and then the solution was mixed at a slow speed of 50 rpm for 15 min. Stirring was then stopped to obtain the sludge that was precipitated after 1 h.

Table 1 lists the concentration of the magnesium extracted from the DS water at different pH values.

Table 2 lists Mg concentration in the solution recovered from 250 ml DS water at different pH values. Table 3 includes the mass of Mg(OH)₂ and MgCl₂ in solids after performing calculation.

The results demonstrated that the magnesium salts recovered from DS water were effective coagulants for the two main treatment objectives, a decrease in both TSS and COD. The precipitation of the colloidal particles after 10 min of coagulation using MgCl₂ in wastewater is depicted in Figure 2(a). Another interesting result was the residual color change (Figure 2(b) and 2(c)); the coagulation process resulted in increasingly clearer water after 2 and 10 min; however, color clarity was not a part of our study.

The presence of a small amount of calcite in the sample has been identified at a 2-theta value with a peak at 29.4°, which matched with the XRD pattern of calcite studied by investigators (Rioyo et al. 2018). Although Mg(OH)₂ presence inhibits the precipitation of calcite, a small quantity of calcite is shown in the SEM-energy-dispersive X-ray spectroscopy analysis as shown in Figure 4. The main reason for the impure magnesium and presence of calcite is due to not using H₂SO₄ to release CO₂ from solution; thereafter, CaCO₃ was precipitated (Yoo et al. 2022).

The removal efficiency increased using the same concentration when the wastewater's pH value reached 11, after which the removal rate is constant. Mg(OH)₂ precipitates at a pH greater than 10.5 due to the solubility constant. When the concentrations of either Mg ions or hydroxide ion increase, the Mg(OH)₂ are formed. Thus, Mg(OH)₂ particles, which have a strong affinity for pollutants attract and bind with the organic compounds in the water, form larger particles that are easier to remove through sedimentation. Raising the magnesium concentrations in wastewater was found to be an effective coagulant and in an alkaline state, which increases the precipitation of Mg(OH)₂.

The highest COD removal of about 57% was achieved at pH 11.5 and 117 mg/l dose at 30 min coagulation time. Therefore, pH 11.5 is deemed to be the most suitable for wastewater treatment using Mg(OH)₂ recovered from DS water.

Efficient coagulation can be achieved by increasing the dosage of magnesium concentration, as it can neutralize the negative charge of organic particles and flocs (Zhao et al. 2015). Due to the rise in precipitated Mg(OH)₂, which has a large absorbent and positive surface that attracts negative particles, thus reducing colloidal particles from wastewater (Stumm & Morgan 2012). Magnesium began to precipitate in the form of Mg(OH)₂ when the wastewater had a pH of 8.5 by inducing the adsorption and agglomeration process. Thus, COD removal efficiency increased with an increase in Mg(OH)₂ dosage. The presence of Mg(OH)₂ in wastewater with a high pH value of less than or equal to 10.5 aids the coagulation process, which leads to better removal efficiency.

The greatest COD removal about 44% was achieved at pH 10.5 and 71 mg/l dose at 30 min coagulation time. In addition, the solution's alkalinity increases by adding NaOH containing MgCl_2, leading to the formation of precipitates of Mg(OH)₂. However, as the pH level goes beyond 11, the rate of precipitation slows down (Kajjumba et al. 2021).

The rate of precipitation of TSS reduction, after treatment using precipitated Mg(OH)₂ as Mg, slows down with the increasing pH above 11 due to the solubility equilibrium of magnesium hydroxide (Mg(OH)₂) in water.

The precipitation reaction is favored at lower pH levels because at higher pH, the hydroxide ions (OH-) in solution increase. As the pH rises, the equilibrium shifts toward the dissolved forms of magnesium, i.e., magnesium hydroxide becomes more soluble.

At pH values above 11, the concentration of hydroxide ions is high, and the equilibrium shifts to the right, leading to the dissolution of Mg(OH)₂ back into Mg²⁺ and OH⁻ ions. This means that as you move to a higher pH, the precipitation of magnesium hydroxide becomes less favorable, and the rate of TSS reduction slows down.

The efficiency of MgCL₂ and Mg(OH)₂ in the TSS and COD treatment process became more effective when the pH was raised due to a higher precipitation rate, allowing for a higher rate of coagulation and flocculation. The findings suggested that TSS and COD removal was highly efficient when using MgCL₂ and Mg(OH)₂ at a high pH value due to an increase in magnesium ions. The removal rate when using Mg(OH)₂ was more efficient than MgCl₂ at the same pH values and dosages due to the greater quantity of magnesium ions present, which improves the coagulation process.

The influence of different coagulants such as Al₂(SO₄)₃, FeCl₃, Fe SO₄, and cationic PAM on the removal of TSS and COD from wastewater was determined at different pH values. The coagulant doses were chosen based on the optimized doses of MgCl₂ and Mg(OH)₂ recovered from DS water, which achieved optimal removal of TSS and COD in wastewater, aiming to identify the most effective coagulant.

The anionic molecules of the dissolved organics interact with cations to form insoluble compounds and settle as sludge at a low pH during coagulation. The coagulation process causes organics to adsorb by metal hydroxides and precipitate out at a higher pH. As the solution has a pH value that is less than 5.5, the TSS and COD are efficiently removed using ferric chloride, probably due to the formation of iron III (Fe⁺³) with hydroxide (OH⁻) in the basic condition to form a polynuclear species. Iron III can react with more colloidal impurities in an acid, including colloidal particles and dissolved organic substances with negative charges (Pan et al. 2021). Aluminum salts interact with OH⁻ ions at higher pH levels than ferric chloride and transfer from species with a greater positive charge to lower positive charges; therefore, TSS and COD removal by Al₂(SO)₄, FeCl₃, and Fe₂SO₄ are not significantly different. Cationic PAM and magnesium were much more efficient in TSS and COD removal compared to other salt coagulants; however, the polymer was typically more expensive than inorganic coagulants, especially when compared to inorganic salts recovered from DS water, which is a rich source of salts.

The coagulation process using recovered Mg(OH)₂ from DS water achieved high removal rates of TSS (60%) and COD (54%) at a dosage of 117 mg/l and a pH of 11. However, the removal efficiency decreased to 46% for TSS and 44% for COD when the pH was lowered to 10.5 and the dosage was reduced to 71 mg/l of MgCl₂ recovered from DS water. Beyond pH of 10.5, the removal efficiency of TSS and COD was dropped by MgCl₂, while the removal was constant using Mg(OH)₂. Higher Mg doses yielded better TSS and COD removal efficiencies at all pH values; however, a small improvement in COD or TSS removal required a much higher increase in Mg dose. Reducing pH to 4.0 following magnesium recovery at high pH values did not improve COD or TSS removal efficiency. Therefore, the efficiency of MgCl₂ and Mg(OH)₂ in the TSS and COD treatment process became more effective when the pH was increased due to a higher precipitation rate, allowing for a higher rate of coagulation and flocculation.

Among the other selected coagulants, the cationic PAM exhibited the greatest effectiveness in removing TSS (with a removal efficiency of 86%) and COD (with a removal efficiency 64%). However, the polymer of PAM was used at a dosage of 0.4 g/l, while Mg(OH)₂ recovered from DS water had a great removal with a dosage of 117 mg/l and was more efficient than other commercial inorganic coagulants. In the end, it is recommended to extract MgCl₂ from DS water due to its abundance, making it suitable for utilization in the treatment of wastewater.

Verma et al. (2012) investigated TSS and COD removal using different coagulants and chemical precipitation techniques. The authors mentioned that the MgCl₂ coagulant is the cheapest, and the formed flocs have a shorter settling time than other inorganic salt coagulants. Thereafter, in the case of buying commercial coagulants, magnesium recovered from DS water for wastewater treatment is more effective and crucial.

In this study, magnesium ions from DS water through selective precipitation chemical process were investigated, yielding the highest magnesium ion concentration at a recovery pH of 11.0. The recovered magnesium compounds, (Mg(OH)₂) and (MgCl₂), demonstrated significant potential for wastewater treatment by effectively removing TSS and COD, especially by adding caustic soda with the increasing pH in enhancing Mg salt recovery; at pH 11.5. Mg(OH)₂ exhibits the highest TSS and COD removal efficiencies of 59 and 57%, respectively. In addition, comparative analyses confirmed the competitive performance of magnesium salts versus other precipitation methods, and the combination of commercial Fe₂SO₄ and MgCl₂ extracted from DS water showed significant reductions of up to 90 and 73% in TSS and COD, respectively. Overall, the study confirms the efficiency of Mg recovery and emphasizes the potential of the recovered compounds, especially Mg(OH)₂, as effective agents for sustainable wastewater treatment.

Conceptualization: M.A. and T.O.; methodology: M.A. and K.A.-Z.; software: M.A., Y.L., and I.Kh.; validation: Y.L. and A.A.s.; formal analysis: M.M. and I.Kh.; investigation: T.O. and K.A.-Z.; resources: M.A.; data curation: M.A. and T.O.; writing original draft preparation: M.A.; writing review and editing: Y.L. and I.Kh.; visualization: A.A.s and B.A.; supervision: M.A. and T.O.; project administration: M.A., Y.L., and K.A.-Z.. All authors have read and agreed to the published version of the manuscript.

The authors declare that this study was not conducted on humans.

This study is supported by Petroleum Engineering Department University of North Dakota.

The author acknowledges the Deanship of Scientific Research Petroleum Engineering Department University of North Dakota for providing support for the publication of this research project.

The authors give informed consent to participate in this study

The author hereby consents to publication of the work.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.