2.1. Ion-Exschange Membranes
In this work, heterogeneous AEMs made on the basis of strongly basic anion-exchange aminated polystyrene resins cross-linked with divinylbenzene (samples 1, 2, 5,
Table 1) were used, as well as a heterogeneous cation-exchange membrane (CEM) based on sulfonated polystyrene cross-linked with divinylbenzene (sample No. 3). In addition, a weakly basic AEM (sample No. 4) and a thin dialysis membrane (sample No. 6) were studied.
The membranes had different thicknesses, reinforcing mesh, ionogenic group types and ion-exchange capacities (
Table 1). Samples № 1-3 were provided by MEGA a. s. (Prague, Czech Republic), samples № 4, 5 by LLC Innovation Enterprise Shekinoazot (Tula, Russia), sample № 6 by Tianwei Membrane Corporation Ltd (Weifang High-tech Zone, Shandong, China). Samples 1-5 were conditioned according to a standard procedure, after which CEMs were converted to H
+-form and AEMs were converted to OH
--form. At this stage, IEM samples were separated to determine the exchange capacity. Sample 6, which was used for dialysis, had not been treated with alkaline solution and had remained in Cl
--form after the conditioning. Next, the IEMs were converted into the form of sulfate anions as a result of 48 hours exposure to 1 M sulfuric acid solution and then were washed with deionized water. Ion-exchange capacity was measured by acid-base back titration. For this, sample 6 had been previously converted to OH
--form. Moisture content and thickness in the
Table 1 are given in the H
+-form for CEMs and in the form of sulfate anions for AEMs.
2.2. Method of Dialisys
The study of DD separation process was carried out in a dual chamber flow dialyzer (
Figure 1a). To ensure flowability, an inert polyethylene mesh-separator was arranged in each chamber. Circulation of the solution was ensured by a Heidolph Pumpdrive 510 peristaltic pump (Heidolph Instruments GmbH & Co, Schwabach, Germany). The feed tank contained the processed solution, as which a real waste solution of an electroplating facility, containing mostly sulfuric acid and nickel (
Table 2) was used. Processed solution was pumped through the first chamber. The total volume of the processed solution was 0.5 L. Deionized water circulated through the second chamber. Since the driving force behind the dialysis process is the concentration gradient between solutions separated by an IEM, portions of deionized water were changed two times per day at the initial stage and once per day at the final stage to sustain it. The volume of each portion of deionized water was 5 L. At certain intervals during the experiment concentrations of sulfuric acid and nickel sulfate in the processed solution and dialysate were measured, as well as volume of the processed solution. Sulfuric acid concentration was measured by EasyPlus Automated Titrator (N.V. Mettler-Toledo S.A., Zaventem, Belgium), using acid-base potentiometric titration. Ni
2+ ions concentration was measured using complexometric titration. The solutions’ densities during the dialysis were measured with an areometer.
Fluxes of nickel sulfate and sulfuric acid through the IEM were determined based on the experimental results according to the formula:
where
i was sulfuric acid or nickel sulfate, Δ
ni was the change in the amount of the
i substance in the processed solution over a period of time Δ
t,
was the amount of the
i substance taken with a sample for solution analysis,
S was the IEM area. Recovery of sulfuric acid and loss of nickel sulfate (
) calculated according to the equation
where
t was the time by which the value
was reached,
and
were the amount of the substance of the
i-component in the processed solution at the initial time and at time
t, respectively. Average flux of sulfuric acid and nickel sulfate (
) in time
t were calculated by
During the dialysis process, a diffusion flux of an electrolyte (
jdif) which was directed to an area with a lower concentration occurs due to a chemical potential gradient (
Figure 1b). The drag flux of water was transferred with the electrolyte (
jh). So, (
jh) has the same direction as a diffusion flux of an electrolyte. In addition, as two solutions with different concentrations were separated by an IEM, an osmotic water flux (
jos), opposite to diffusion flux, occurs. Thus, the total volume water flux (
) to the processed solution chamber was defined as the difference between the osmotic flux and the drag flux
Thereby, a change of the processed solution volume was defined by a decrease through a diffusion flux of dissolved substance, which was connected to a transfer of water as a part of hydration shells (drag water flux), and an increase as a result of an osmotic water flux.
The osmotic water flux may be defined as
where
was the water permeability of the IEM,
и
were the osmotic pressures of the processed and dialysate solutions, respectively. The Van't Hoff equation or the equation (6) can be used to calculate the osmotic pressure
where π was the osmotic pressure,
R was universal gas constant,
T was temperature,
and
were partial molar volume and molecular weight of the solvent,
ν was the Van’t Hoff coefficient,
m was molality of electrolyte (amount of a solute divided by the mass of the solvent),
φ was molal osmotic coefficient [
16].
However, the task of defining osmotic pressure values in concentrated solutions, in which properties deviate from ideal solutions greatly, was quite complex. It included the problem of determination of activity coefficients and osmotic coefficients of electrolytes. Thermodynamic properties of many electrolyte solutions were defined experimentally, but this data cannot be used for solutions containing more than one electrolyte. Furthermore, theoretical derivation was extremely difficult for all cases but solutions containing either one electrolyte or two binary electrolytes. Thus, work [
17] was dedicated to model ion composition of sulfuric acid and defining osmotic coefficients of solutions with concentrations up to 6 mol/L. The authors used method based on modified Debye-Hückel theory, developed in a famous series of studies including applied to multicomponent systems [
18], [
19], [
20], [
21]. It was shown, that results obtained using this model agree well with the experimental data. Wherein, this model used a number of fitting parameters and virial coefficients representing forces of ion-ion interactions for a range of electrolytes. However, application of this method was not verified for a system considered in this study. In [
22] osmotic pressures were defined according to the Van’t Hoff equation and model [
17] for a solution containing copper and nickel sulfates given that their ratio was a constant value. It was shown that the experimental data of osmotic pressure values in these solutions agrees well with the results obtained using model [
17] and did not agree with the results calculated according to the Van’t Hoff equation. The problem of defining osmotic coefficients and activity coefficients for individual electrolyte solutions, taking hydration and ion association into account, were solved in [
23], [
24]. In some studies, the authors disregarded the presence of the second component in the solution and considered the parameters for a solution with one solute [
11].
In general, a precise calculation of all the ion forms in concentrated solutions containing several components was a quite difficult task requiring separate consideration. Thus, total concentrations of hydrogen ions, which present as a proton and in a hydro-sulfuric anion in solution, and nickel ions concentration were defined in this study. Thermodynamic properties of the system, such as osmotic pressure or degree of dissociation were not determined.
The estimation of the total water flux was calculated based on the following reasoning. The weight loss of the processed solution was equal to the sum of masses of water, sulfuric acid and nickel sulfate, which transferred to the dialysate. The total water flux was calculated using the formula
where Δ
V was the change of processed solution volume during the time Δ
t,
ρs was the processed solution density,
was the water density (1000 kg/m
3),
and
were the masses of sulfuric acid and nickel sulfate which transferred to the dialysate during the time Δ
t. The water drag flux was calculated using the formula
where
,
и
were the solvation number of the
H+,
and
Ni2+ ions. A similar approach was used to estimate the water drag flux carried in the hydration shell of ions was used in [
11]. The adopted solvation numbers were 2 for H
+, 5 for
and 5 for Ni
2+ [
25], [
26], [
27], [
28]. The osmotic flux was calculated as a difference between total water flux and drag water flux using the formula (4).
Formula (5) and Van’t Hoff equation could be used to estimate
value
where Δ
C was the difference of ions concentration between the processed solution and dialysate, which was calculated using the sulfuric acid and nickel sulfate concentration and the Van’t Hoff coefficients. According to [
29] the
was determined by formula
where
was the water diffusion coefficient in the membrane,
was the water concentration inside membrane at the feed interface. Formula (8) showed that the water permeability included membrane thickness. It allowed to compare the membrane water permeability for the reverse osmosis membranes, ultra- and nanofiltration membranes, which had a similar thickness [
29]. However, in presented study IEMs had different thicknesses. It was well known, that the diffusion flux might be determined using formula
where
Pi was the coefficient of membrane diffusion permeability [
29]. Then the water permeability coefficient
was proposed to compare the properties of membranes with different thicknesses.
could be calculated by the formula