Separation study of Mg þ 2 from seawater and RO brine through a facilitated bulk liquid membrane transport using 18-Crown-6

A facilitated bulk liquid membrane transport approach is studied for Mg(II) extraction from seawater and reversed osmosis brine simulated media using 18-Crown-6 and dibenzo (DB)-18-Crown-6. The work is based on investigating the experimental parameters affecting the transport ef ﬁ ciency, such as pH of feed and receiving phase, type of membrane solvent, temperature, type and concentration of the carrier, and stripping solution conditions. The transported amount of magnesium ions from feed phase (Mg(II) ¼ 0.059 M, NaCl ¼ 0.01 M, pH ¼ 3.3) across a chloroform membrane (18C6 ¼ 0.001 M) into the receiving phase (SCN (cid:2) 0.1 M pH 3) was found to be %97 ( ± 0.7) after 2.5 hr. The selectivity of the method was evaluated by performing competitive transport experiments on the mixtures containing Ca 2 þ , Na þ , K þ , and Mg 2 þ ions.


INTRODUCTION
Magnesium is the third most plentiful element dissolved in seawater, with an average of 0.13%. Most magnesium and magnesium compounds are produced from seawater, well and lake brines and bitterns (Al-Mutaz & Wagialla ), as well as from minerals such as dolomite, magnesite, brucite, carnallite. Elemental magnesium alloys are increasingly used as a lightweight metal in the manufacture of engine parts, jet engines, missiles, and rockets. Likewise, dry magnesium salts and solutions are widely used in industrial, agricultural, medical, and environmental contexts (Kramer ).
The most common method for the initial separation of magnesium from seawater and brines is solar evaporation which induces a chemical precipitation. Solar evaporation is usually applied for processing highly concentrated brines such as the Dead Sea and Great Salt Lake waters (Epstein ; Tripp ). This method is based on the fact that MgCl 2 is more soluble than NaCl. The chemical precipitation method is typically applied to low concentrated brines, such as seawater (Bhatti et al. ).
Traditional methods of magnesium production, i.e., processing of the hydromineral sources, despite their profitability, do not satisfy increasing ecological standards (Mero ). Extraction techniques for metal separation (Mohite & Khopkar ) are of increasing interest due to the importance of environmental protection problems. In recent years, the use of various types of liquid membranes has attracted increasing attention due to their ability for selective metal ions separation in aqueous solutions (Danesi et al. ; Kamiński & Kwapiński ). Transport of metal ions across a membrane has an important role in industrial processes and has useful practical applications in separation science (Jafari et al. ). This type of membrane has been widely utilized for carrier facilitated separations and acquired significant importance for its use in separation, purification, or analytical application in diverse areas like water and waste water treatment It seems that the main challenge to do an efficient liquid membrane transport process with unlimited feed like seawater (containing the macro components of Mg 2þ , Ca 2þ , K þ , and Na þ ) is finding alternative carriers, and membrane conditions (Rounaghi et al. ), such as solvent type, stripping reagent, etc. Effective transport requires the carrier, which is distributed primarily to the membrane phase, to provide a selective complex with the desired metal ion (Kikuchi & Sakamoto ). Macrocyclic ligands, i.e., crown ethers as carrier (Izatt et al. ), have been developed to be used in several transport procedures for their selectivity to cation ions. In some studies (Heo ), selectivity of crown ethers to alkaline and alkaline earth ions is mentioned.
In this study, an attempt has been made to use facilitated bulk liquid membrane (BLM) transport using crown ethers 18-Crown-6 and DB-18-Crown-6 to find optimum conditions, such as pH, temperature, type and concentration of the carrier, solvent and ionic strength for extraction of magnesium ion from the simulated seawater media. Results show that the best extraction of magnesium was carried out at an acidic feed and received phase using 18-Crown-6 dis-

Selection of membrane solvent
The main objective of this study is identification of the where, C 0f , C f are initial and final concentration of Mg 2þ in the feed phase and C R is concentration of Mg 2þ in the receiving phase after the transport process.  The type and concentration of the carrier The type and concentration of the carrier for metal ion transporting is another important factor that should be selected properly. In facilitated BLM transport, the carrier forms a complex with the guest cation and helps it to transfer through the hydrophilic membrane (Ajwani et al. ).
For this purpose, the behavior of two crown ethers (18-Crown-6 and DB-18-Crown-6) was studied and the results are shown in Table 2. It seems the greater flexibility of 18-Crown-6 works better than the hydrophobicity of DB-18-Crown-6 (possessing two benzenes) in the chloroform membrane for magnesium transport. It should be noted that DB-18-Crown-6, due to possessing two benzene groups, is more hydrophobic than 18-Crown-6. For the same reason, it is easier for 18-Crown-6 ether to adjust its structure for better reacting with Mg 2þ , and this leads to its greater flexibility.
In the next stage of this study, the concentration of 18-Crown-6 as carrier in the range of 0.0001 to 0.1 M was evaluated. Results show that 0.001 M with an extraction of 20.33% has the best performance. It seems that at higher concentration of carriers, due to increasing irreversible interaction between metal cations and 18-Crown-6 in membrane, the extraction efficiency was decreased. In the facilitated BLM transport process, the complexation reaction between the carrier and the cation should not be so strong, since its delivery is not done in receiving phases. A high concentration of carrier ligand leads to irreversible reaction with the metal ions and the transfer process will be stopped.

Effect of type and concentration of the counter ion
Transport of Mg 2þ across the membrane via the predicting mechanism of Figure 1, needs to apply a contour ion. For this purpose, different types of 0.01 M salt solution, such as KCl, KI, KClO 3 , NaNO 3 , NaBr, and NaCl, were studied against 0.059 M of magnesium ions to find a better contour ion. According to the results of Table 3, in the presence of Br À , magnesium transport is higher. However, due to economic reasons and the abundant presence of Cl À in seawater and RO brine, which are our target samples, Cl À was selected as the contour ion. In order to determine the optimum concentration of contour ion, NaCl at a concentration range of 0.0001-2 M was tested in the membrane containing 18-Crown-6. As a result (Figure 2), the maximum extraction percent of magnesium ion was found at the concentration of 0.01 M of NaCl.

Effect of pH of feed solution on extraction efficiency
At this stage, the pH of the feed phase was adjusted in the range of 1-10, using concentrated NaOH and HCl solutions. Results of magnesium ions transport in different pH values are plotted in Figure 3. As is clear, at pH ¼  maximum compared with the other points. This is due to the ability of H þ to pull crown ether to the boundary zone of the membrane, and the increase of Cl À for the contour ion role. Therefore, this pH was selected as the optimum point for future work.

3.3, the extraction percent of magnesium (22.8%) is
The effects of type and concentration of stripping reagent in receiving phase Transport of Mg 2þ in the mentioned membrane is based on reaction between 18-Crown-6 as a chelating agent and this cation. The complex after passing through the aqueous/organic interface is transferred to the receiving phase, but in the absence of stripping reagent, transport is negligible. For this purpose, the effect of several stripping reagents (0.1 M of EDTA, SCN À , sodium citrate, nitric acid, oxalic acid, and citric acid) on the stripping percent (%S) was studied. Figure 4 presents the results of this experiment, which accordingly show SCN À with the best behavior as stripping reagent. To find the optimum concentration of SCN À , its concentration was investigated in a range of 0.0001-2 M and a concentration of 0.1 M was chosen as the optimum amount.

Effect of pH of receiving phase on stripping process
Here, due to the importance of the role of pH of the receiving phase on the stripping process, this parameter was adjusted at different amounts by using concentrated NaOH and HCl solutions in the presence of 0.1 M SCN À as the optimum stripping reagent. Results of this study are shown in Figure 5, and as is clear, the best stripping percent was obtained at pH ¼ 3. Therefore, this pH was selected as the optimum pH of the receiving phase. It seems that the main role of acidic media is establishing a tendency for the metal complex to be present between the aqueous/ organic boundary zone.

Effect of time and cell temperature on transport
Since BLM transport is time-consuming, finding the optimal time for the work is very important. Figure 6 shows the   Temperature of the transport cell is the next factor whose effect was investigated at the three points of 25, 30, and 35 W C. To do this work, the transport cell was inserted into a water bath and the results of this study are plotted in Figure 7 for the feed phase as extraction percent and receiving phase as stripping percent.

Selectivity of BLM system
As the aim of this study was separation and extraction of magnesium ion from seawater and RO brine, the selectivity of the BLM system was evaluated by Mg 2þ transport studies in the presence of Ca 2þ (450 mg/L), Na þ (13,680 mg/L), and K þ (800 mg/L) ions in the feed solution close to the reported concentration of these ions in seawater. Table 4 shows the percentage transport of Mg 2þ (0.059 M) and these cations into the receiving phase. As is observed, in the absence of   any masking agents, the interfering effects of these foreign metals ions are low.

CONCLUSIONS
As mentioned before, with the increasing consumption of metals such as magnesium, extraction has become important, especially from the easy to access source, seawater.
Here, a facilitated BLM transport method is presented for