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Transport of Mg2+ 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, KClO3, NaNO3, 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, magnesium transport is higher. However, due to economic reasons and the abundant presence of Cl in seawater and RO brine, which are our target samples, Cl 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.
Table 3

Effect of type of the counter ion

%EContour ion (0.01 M)
16.59 KCl 
16.59 KI 
20.34 KClO3 
15.25 NaNo3 
22.03 NaBr 
20.34 NaCl 
%EContour ion (0.01 M)
16.59 KCl 
16.59 KI 
20.34 KClO3 
15.25 NaNo3 
22.03 NaBr 
20.34 NaCl 

Experiment conditions: [Mg+2] = 0.059 M; membrane = 0.001 M of 18 crown 6 in chloroform; temperature = 25 °C, feed pH = 4; stripping reagent = 0.1 M of SCN; pH of the receiving solution = 3; extraction time = 2 hr.

Figure 2

Effect of Cl concentration in feed phase, experiment conditions: [Mg+2] = 0.059 M; membrane = 0.001 M of 18-Crown-6 in chloroform; temperature = 25 °C, feed pH = 4; stripping reagent = 0.1 M of SCN; pH of the receiving solution = 3; extraction time = 2 hr.

Figure 2

Effect of Cl concentration in feed phase, experiment conditions: [Mg+2] = 0.059 M; membrane = 0.001 M of 18-Crown-6 in chloroform; temperature = 25 °C, feed pH = 4; stripping reagent = 0.1 M of SCN; pH of the receiving solution = 3; extraction time = 2 hr.

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