RESULTS AND DISCUSSION

where a represents the activities of a target counter-ion, i, and a driving counter-ion, j, and zi and zj are, respectively, their charges. The subscripts feed and stripping refer to, respectively, the feed and stripping solutions.

performed under the best conditions, i.e. fixing 0.1 M NaCl solution as a receiving phase.

Table 2.2.2. Effect of the stripping composition on the SLM efficiency for As transport Stripping solution As transported (%) (after 5 h)

0.1 M NaCl 95

0.1 M NaClO4 16

0.1 M NaNO3 26

0.1 M HCl 40

Feed phase: [As(V)]=10 mg L^-1, pH 7. SLM: 0.5 M Aliquat 336 in dodecane and 4% dodecanol.

Under these new stripping conditions, the influence of the pH of the feed solution on As(V) transport was evaluated for pH values ranging between 3 and 13. The results are presented in Figure 2.2.1, where the time course of the As concentration in the stripping phase is given for each pH.

Time (h)

0 5 10 15 20 25 30

[As(V)] stripping (mg L-1 )

0 2 4 6 8 10 12

pH 7 pH 10

pH 3

pH 13

Fig. 2.2.1. Effect of the pH of the feed solution on As(V) transport in SLM system. Feed phase:

[As(V)] = 10 mg L^-1. SLM: 0.5 M Aliquat 336 in dodecane and 4% dodecanol. Stripping phase:

0.1 M NaCl.

As it can be observed, better results were achieved when using feed phases at pH 7 and pH 10, since the As recovery was quantitative after 6 h of experiment. Thus, H2AsO4-

, HAsO42-

are believed to be the most labile species of As when aqueous solutions are contacted with a supported liquid membrane containing Aliquat 336. In Figure 2.2.2, arsenic permeability values are depicted together with the different possible As(V) species present in water against pH. As can be seen, higher arsenic permeability values were obtained at pH 10 when the species HAsO42-

is predominant.

These results are in agreement with our previous work where, from distribution data, we postulated that the species with 1:2 stoichiometry was preferably extracted into the organic phase [5].

pH

2 4 6 8 10 12 14

Molar fraction

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Permeability (cm/min)

0.0 0.1 0.2 0.3 0.4 0.5

H3AsO4 H2AsO4-

HAsO42-

AsO43-

Fig. 2.2.2. Variation of SLM permeability to As(V) (closed circles) versus pH together with the speciation diagram for As(V) in water.

2.2.4.2. ANION-EXCHANGE MEMBRANES

In Donnan dialytic transport studies, two anion-exchange membranes with different anion permselectivities: multi-valent and mono-valent, respectively were investigated.

To evaluate the influence of the dominant anionic species on the overall As transport, a set of experiments was carried out working with 10 mg L^-1 As(V) at a pH 5 or 7 for both

comparison is straightforward since for achieving identical pHs, equal amounts of pH- regulating agents (HCl or NaOH) were added to the feed solution. Thus, the inevitable effect of changing the feed ionic composition on the transport driving force for arsenic to the stripping solution can be disregarded if the process behaviour is compared for the same pH. Figure 2.2.3 shows the As(V) concentration profiles in the two solutions at these pHs using the different types of membranes. It can be observed that at pH 7 the multi-valent anion permselective membrane allows for an almost quantitative arsenic transport after 6 h of experiment (Fig. 2.2.3a), whereas longer time periods are needed for the mono-valent permselective anion-exchange membrane (Fig. 2.2.3c).

Additionally, at pH 5 arsenate transport is slower for both membranes (Fig. 2.2.3b, d).

The percentage of As transported after 24 h of experimental time was only 60% for the mono-valent anion permselective membrane (Fig. 2.2.3d) even though the mono- charged species H₂AsO₄^- predominates at this pH.

Fig. 2.2.3. Effect of the AEM permselectivity over As(V) transport at different pH. Feed phase:

[As(V)] = 10 mg L^-1. Stripping phase: 0.1 M NaCl.

Time (h)

0 5 10 15 20 25 30

[As(V)] (mg L-1 )

0 2 4 6 8 10 12

feed stripping

Time (h)

0 5 10 15 20 25 30

[As(V)] (mg L-1)

0 2 4 6 8 10 12

feed stripping

Time (h)

0 5 10 15 20 25 30

[As(V)] (mg L-1 )

0 2 4 6 8 10 12

feed stripping

Time (h)

0 5 10 15 20 25 30

[As(V)] (mg L-1)

0 2 4 6 8 10 12

feed stripping

a) multi-valent membrane (pH 7) b) multi-valent membrane (pH 5)

c) mono-valent membrane (pH 7) d) mono-valent membrane (pH 5)

The effect of the pH of the feed solution was further examined by testing the arsenic transport through the multi-valent AEM at pH 10 and 13 and the results are presented in the Table 2.2.3. As can be seen, after 24 h experiment, the percentage of As transported was nearly quantitative for the first case, whereas at pH 13 was only of 30%. However, as stated before, both pH 7 and pH 5 give better results in terms of arsenic transport. In this case, the explanation of the results obtained should relate to the Donnan exclusion theory. According to this theory, in an AEM the ion H⁺ is excluded by the fixed positively charged functional groups (e.g. N(CH3)3+

) in the membrane.

Therefore, the pH value in the membrane interstitial solution is higher than in the bulk solution.

Table 2.2.3. Effect of the pH of the feed solution for the multi-valent AEM.

pH % As transported

after 6h after 24h

5 78 100

7 89 95

10 42 92

13 12 30

Feed phase: [As(V)] = 10 mg L^-1. Stripping phase: 0.1 M NaCl.

Zhao et al. [20] have developed a model to predict As(V) removal by Donnan dialysis and they calculated pH values in the membrane interstitial solution, as well as the corresponding dominant As(V) species in the membrane under different experimental conditions. For a neutral bulk solution, these authors calculated a membrane pH ~ 9 at which HAsO42-

is the dominant species; moreover, working at a bulk pH of 7 a higher membrane permeability to As was obtained. Thus, taking into account the pH dependent character of the As speciation in aqueous medium, it can be inferred that the dominant species of As(V) in the anion-exchange membrane is not the same as that in the external bulk solution. Additionally, due to their differences in ionic valence and hydration enthalpies, the intermembrane mobility of As(V) should be higher for HAsO₄^2- ions than for H₂AsO₄^- and AsO₄^3-. The hydrophilicity of the H₂AsO₄^- anions is weaker and their motion in the hydrophilic zone of the membrane

would be relatively difficult, whereas the trivalent AsO₄^3- anions require three adjacent available positively charged functional groups, thus making the movement of AsO₄^3- anions in the membrane more restricted [20]. It can therefore be concluded that for AEM as well as for SLM, HAsO42-

is the main responsible for As transport.

2.2.4.3. COMPARISON OF SLM AND AEM FOR SEPARATION OF As(V) AND As(III)

Besides the total As removal, in some cases an efficient separation of As(V) from As(III) could be of extreme importance due to the different toxicity of these two inorganic arsenic forms. Moreover, an easy quantitative separation would facilitate the analytical determinations of As(V) and As(III) in inorganic arsenic-containing aqueous samples.

Experiments were done using as a feed solution As(III) at pH 7, and the obtained concentration profiles compared to the ones for As(V) are presented in Figure 2.2.4a, for the SLM, and Figure 2.2.4b for the multi-valent anion permselective AEM. As can be seen in Figure 2.2.4a, transport of As(III) through a SLM containing Aliquat 336 was not possible. This behaviour can be attributed to the fact that at pH 7 As(III) is present in its neutral form H3AsO3, and, consequently, it cannot be extracted via an anion exchange mechanism. However, in the case of arsenate the transport is quantitative.

This result demonstrates the feasibility of using this membrane system for separation of inorganic arsenic containing species in natural water samples.

Fig. 2.2.4. As(V) and As(III) transport efficiency through (a) SLM and (b) AEM .Feed phase:

[As] = 10 mg L^-1, pH 7. Stripping phase: 0.1 M NaCl.

Time (h)

0 5 10 15 20 25

Transport efficiency(%)

0 20 40 60 80 100

As(V) As(III)

Time (h)

0 20 100 120 140 160 180

Transport efficiency (%)

0 20 40 60 80 100

As(V) As(III)

a) b)

On contrary, transport of As(III) at pH 7 through the multi-valent AEM occurred even though its rate was lower compared to that of As(V) (see Figure 2.2.4b). Arsenite could be transported, at least partially, as H2AsO3-

(pK1= 9.2) due to the increase of the pH value of the membrane interstitial solution with respect to that of the external bulk solution as pointed out in the previous section. However, the percentage of the non- dissociated H3AsO3 formeven in alkaline media remains significant, thus providing the driving force for its fickian diffusion to the stripping compartment. Therefore, in this case, one should expect two possible mechanisms of arsenic transport through the membrane, one based on the Donnan dialysis principle for the charged species and the other following the simple dialysis transport ruled by solute concentration gradient. In order to explore the validity of these statements, the system was allowed to run for an extended period of time. After 160 hours, the As(III) concentration in the two solutions separated by the membrane reached almost the same value, thus strongly supporting the importance of fickian based diffusion mechanism for the transport of As(III) containing species to the stripping compartment.

In spite of the fact that, in this case, complete As removal was not possible, optimization of the experimental conditions could make this membrane system feasible if the goal is to achieve high As removal degrees. A possible solution could be a pre- oxidation of As(III) contained in the water to As(V) prior to its feeding to the AEM system. Oxidation of As(III) to As(V) can be performed by various ways and has been widely reported in the literature as reviewed in [18].

However, if separation of As(III) from As(V) is desired, SLM is the membrane system of choice. The possible explanation of this very distinct behaviour of SLM and AEM most probably relates to the fact that in AEM the interstitial aqueous phase permits the transport of H3AsO3 since the Donnan exclusion, although high due to the high concentration of fixed charges in the membrane is ineffective towards permeation of non-electrolytes.

On the other hand, for SLM, compounds that are not negatively charged under the working conditions cannot give an ion pair with a cationic carrier and are, consequently, not extracted. Thus,transport of non-electrolytes to the stripping compartment is only possible through dynamic water clusters that might be formed in the organic phase of the extractant. However, due to its relatively high molecular size, transport of H3AsO3

would be hindered because of the increasing steric hindrance by the extractant alkyl

above mentioned aqueous phase mechanism might be expected to be significantly less important than for the case of AEM, in which the interstitial membrane phase is both aqueous and continuous.

2.2.4.4. STUDY OF THE EFFECT OF INTERFERING ANIONS

In order to compare the two types of membranes in terms of their selectivity towards As(V), transport studies for identifying possible interferences by other inorganic anions present in the aqueous medium were carried out. Taking into account that the arsenate transport is based on an anion-exchange mechanism, the presence of other anions in natural waters could affect the rate of the As transport and its removal efficiency. For this reason, MilliQ water was spiked with 10 mg L^-1 As(V) and the following anions [NO3-

] = 12.4 mg L^-1; [SO42-

] = 9.6 mg L^-1; [HCO3-

] = 12.2 mg L^-1; [H2PO4-

] = 19.4 mg L^-1. The concentrations of these anions were chosen to be the same equivalents as As(V). The results are presented in terms of arsenic concentration profiles for each membrane type (Figure 2.2.5), with or without accompanying anions in the feed solution. As it can be observed in both cases, despite the anions were co- transported with As(V) oxyanions, this did not significantly affect the performance of SLM and AEM, especially in what concerns the total arsenic removal efficiencies of these two systems. Certainly, this conclusion applies within the concentration range studied and can not be generalized for all ionic compositions and/or concentration levels. However, it does indicate that the results obtained with model bi-ionic systems can be extended to multi-ionic systems that are typical for natural water matrixes.

Fig. 2.2.5. As(V) concentration profiles in the feed and stripping solutions when alone or when accompanied in the feed solution by other anions for SLM (a) and multi-valent AEM (b). Feed phase: [As(V)] = 10 mg L^-1, pH 7, with and without anions. Stripping phase: 0.1 M NaCl.

Time (h)

0 5 10 15 20 25

[As(V)] (mg L-1 )

0 2 4 6 8 10 12

with anions without anions

Time (h)

0 5 10 15 20 25 30

[As(V)] (mg L-1 )

0 2 4 6 8 10 12

with anions without anions

a) b)

In document PDF m.dugi-doc.udg.edu (página 162-170)