Análisis de la ventana de oportunidad

In analogy with previous chapter, i.e. when no OM is contained in the saline solution, salt and water transfer is studied in this chapter for desalination of saline water containing organic solute. Then, the results with and without neutral OM are put in parallel for discussion.

Firstly, variations of the quantities of salt and water transferred versus time for different current intensities are shown in Figure IV-1.

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Figure IV-1 Variation of salt (a) and water (b) quantities transferred versus time:

influence of the current

Glucose / NaCl system, [OM] = 0.1 mol.L-1 _{and [S] = 0.8 eq.L}-1_{; C for Concentrate, D for} diluate

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As expected, the quantity of salt (a) and water (b) transferred versus time are positive in the concentrate compartment while negative in the diluate compartment. It indicates that the direction of salt and water transfer is from diluate towards concentrate under current, seen in Figure IV-1. The mass balance is respected, with the difference below 5% for salt and 2% for water, calculated according to Eq.(II-1).

One can further observe that the quantities of salt and water transferred increase with the current intensity applied. Under each current, the salt and water transfer vary linearly versus time, which is in agreement with previous results [43,52-53,64]. Then, the salt and water fluxes are deduced from the corresponding slopes, and further plotted on Figure IV-2 versus current.

Figure IV-2 Variation of salt and water flux versus current

Glucose / NaCl system, [OM] = 0.1 mol.L-1 _{and [S] = 0.8 eq.L}-1

Figure IV-2 shows that the variations of both salt and water flux are proportional to the current (R2 _{> 99%). This observation confirms that under current, electrical migration} dominates the salt transfer as indicated by Eq.(II-5), and electro-osmosis dominates water transfer as indicated by Eq.(II-7). Then, the contribution of diffusion to the salt flux and that

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of osmosis to the water flux are negligible [43]. These findings are observed for any condition investigated.

Then, deduced from the linear variation of the salt and water transfer versus current, the corresponding coefficients 𝛼 and β can be obtained, according to Eq.(II-5) and (II-7). Values of current coefficients 𝛼 for all the solutions investigated are reported in Table IV-1 and illustrated in Figure IV-3.

α

×10-4 _eq.m-2_.s-1_.A-1 acetic acid phenol glucose no OM

MgCl2 4.7 4.5 5.1 4.9

NaCl 5.1 4.5 5.0 4.6

Na2SO4 4.8 4.4 4.9 4.8

Table IV-1 Current coefficient (𝛼, eq.m-2_.s-1_.A-1_{) in different OM / S systems} [OM] = 0.1 mol.L-1 _{and [S] = 0.8 eq.L}-1_{; calculated according to Eq.(II-5)}

Figure IV-3 Histogram of current coefficient (𝛼, eq.m-2_.s-1_.A-1_{) in different OM / S systems} [OM] = 0.1 mol.L-1 _{and [S] = 0.8 eq.L}-1_{; calculated according to Eq.(II-5)}

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One can find that for a given solute, the current coefficient 𝛼 has very small variation according to the salt. This was already observed in case with no organic solute. This result is mainly due to the constant current applied, which fixes the total quantity of salt transferred .

Concerning the different solutes, values obtained with acetic acid and glucose are very close, and also close to those obtained without OM. For any salt composition, slightly lower values are obtained with phenol, compared to the two other solutes.

Then, the obtained electro-osmotic coefficients, characterising the water transfer, in different systems are reported in Table IV-2, and illustrated in Figure IV-4 .

β

×10-8 _m3_.m-2_.s-1_.A-1 acetic acid phenol glucose no OM

MgCl2 7.0 6.1 6.6 6.8

NaCl 6.3 5.5 5.9 5.8

Na2SO4 5.9 5.1 5.6 5.6

Table IV-2 Electro-osmotic coefficient (β , m3_.m-2_.s-1_.A-1_{) in different systems} [OM] = 0.1 mol.L-1 _{and [S] = 0.8 eq.L}-1_{; calculated according to Eq.(II-7)}

Figure IV-4 Histogram of electro-osmotic coefficient (β , m3_.m-2_.s-1_.A-1_{) in different} OM / S systems

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For a given solute, one can observe that β varies according to the salt composition (variation ca. 30%). Moreover, for any solute, the same order of magnitude of β is observed according to the salt as:

𝛽_{𝑀𝑔𝐶𝑙2} > 𝛽_{𝑁𝑎𝐶𝑙} > 𝛽_{𝑁𝑎2𝑆𝑂4}

Concerning different OMs, Table IV-2 shows that the values with acetic acid and glucose are close, and approximating those obtained in case without OM (difference below 8%). Again, the values obtained with phenol are systematically slightly lower (difference below 12%).

As demonstrated in Chapter III, the coefficient β characterizes the water transfer due to electro-osmosis, i.e. the water accompanying the salt migration. Then, the ratio between water and salt transfer, calculated as β / α, characterizes the salt hydration, as explained in Chapter III. The values of this ratio obtained for the different systems are reported in Table IV-3.

β / α

×10-4 _m3_.eq-1 acetic acid phenol glucose no OM

MgCl2 1.5 1.3 1.3 1.4

NaCl 1.2 1.2 1.2 1.3

Na2SO4 1.2 1.2 1.2 1.2

Table IV-3 Ratio of water and salt transfer (β / α, m3_.eq-1_{) in different OM / S systems} [OM]=0.1 mol.L-1 _{and [S]=0.8 eq.L}-1

It shows that for a given salt, very close values of β / α are obtained for the different solutes. Even if the respective value 𝛼 and β with phenol is slightly lower, as aforementioned, the ratio is found close to results for the two other solutes. Moreover, the results indicate that the salt hydration is constant with and without the neutral OM and for any organic solute.

Thus, these findings of salt and water transfer in presence of OM coincide with the results obtained without OM. It suggests that the presence of organic solute (of different nature, i.e. size, hydrophibility, solution pH) doesn’t affect singificantly the salt and water transfer in the conditions investigated here.

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