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Membranes and Membrane Technologies

, Volume 1, Issue 3, pp 153–167 | Cite as

Diagnostics of the Structural and Transport Properties of an Anion-Exchange Membrane MA-40 after Use in Electrodialysis of Mineralized Natural Waters

  • V. I. Vasil’evaEmail author
  • E. M. Akberova
  • D. V. Kostylev
  • A. A. Tzkhai
Article
  • 18 Downloads

Abstract

Changes in the structural and transport characteristics of MA-40 anion-exchange membranes after operation in industrial electrodialysis apparatuses have been assessed. Causes for the deterioration of operational properties by the action of various factors in the process of demineralization and concentration of natural waters have been revealed. Samples of the anion-exchange membrane after long-term operation in the working stack of an electrodialysis concentrator, as well as samples taken out from the electrode compartment of the electrodialysis reversal apparatus, have been studied. The most significant change in the structure of the membrane taken out from the electrodialyzer concentrator is an increase in macroporosity, which is the main reason for the growth in the electrical conductivity and water content against the background of a loss of ion-exchange capacity and selectivity. The formation of poorly soluble carbonates and hydroxides both on the surface and in the bulk of a membrane from the electrode compartment of the reversal electrodialyzer leads to the blocking of the functional groups and transport channels, decrease in electrical conductivity, and complication of transport processes.

Keywords:

anion-exchange membrane electrodialysis natural waters microstructure scaling 

INTRODUCTION

Electrodialysis plays a significant role in water desalination industry. This method demonstrated high efficiency in the production of both high-purity water and maximally concentrated solutions [1]. Electrodialysis desalination of natural mineralized waters got widespread application due to the technical simplicity, low energy intensity, and environmental cleanliness [2]. The advantages of electrodialysis desalination in comparison with reverse osmosis are high speeds of water treatment and longer membrane operation time due to their high chemical and mechanical stability, as well as lower membrane fouling. In addition, electrodialysis is characterized by a smaller number of stages of preliminary water treatment and flexibility of the process for the production of drinking water of different quality [3, 4, 5].

The performance of electromembrane water treatment systems is mainly limited by the decrease in the electrochemical activity of membranes by polarization and temperature effects and by scaling. Modern heterogeneous ion-exchange membranes have service life from two to five years. The cost in the case of replacement of membranes upon the completion of operation is the second electrodialysis expenditure item after the cost of energy consumption [6]. In the case of demineralization of natural waters, the probability of scaling in an electrodialyzer is directly associated with the pH value [7, 8, 9] and is not the same in all of its zones. The ratio of the scale-forming ions is an additional factor affecting scaling due to their competition during the migration from the compartment with a diluate to the compartment with a concentrate and due to the cross-influence of different scalant ions on the formation and growth of crystals [7, 8, 10, 11]. Comparing the scale formation on homogeneous and heterogeneous membranes and its prevention has been shown in [12, 13, 14]. It was found in [14] that a gypsum (CaSO4 · 2H2O) scale mainly grows in the bulk of an MA-40 heterogeneous membrane, while in the case of an AMV homogeneous membrane, it mainly grows on the membrane surface facing the concentrate compartment. On the MA-40 membrane surface, the scale is localized on the current-conducting sections, and its amount is higher in comparison with a homogeneous membrane.

The precipitation of crystalline inorganic compounds such as hardly soluble salts and hydroxides on the surface of membranes in the concentration compartment from the side of the anion-exchange membrane significantly deteriorates their operational characteristics in the desalination of waters with a high value of hardness [15, 16, 17, 18, 19, 20]. Scales formed on ion-exchange membranes mainly contain calcium carbonate, magnesium hydroxide, calcium sulfate, clay particles, and iron oxides. A study of the formation kinetics of calcium sulfate and carbonate [16] and the influence of scaling on the physicochemical properties of membranes [17] showed that these scales exhibit the passivating capacity, which manifests itself in a significant increase in the ohmic resistance of the system has [14, 21, 22].

In addition to the preliminary softening of water to desalinated, the main methods for preventing scale formation during electrodialysis are the periodic variation of the direction of direct electrical current (electrodialysis reversal), use of pulsed electrical current [7, 23, 24], and modification of the membrane surface with a layer that is selectively permeable for singly charged ions [25]. The electrodialysis reversal technology makes it possible to decrease the precipitation of a salt on the surface of membranes due to the periodic and simultaneous variation of the directions of current and mass transport [26]. Scales formed on a membrane at one direction of the current dissolve when the polarity of the electrodes is changed. The current reversal technique is successfully used for the desalination of weakly mineralized waters [27] because it provides their effective desalination upon minimum preliminary treatment and low operational expenditures. However, the efficiency of the process decreases with time because of changes in the structural and transport properties of membranes induced by harsh operational conditions [28]. Disadvantages of the method include the loss of finished product in the case of change in the polarity.

The structure and chemical composition of the surface have critical importance in the determination of the electrochemical behavior of ion-exchange membranes under the electrodialysis conditions. Deterioration of the performance and electrochemical activity of membranes was noted in the process of electrodialysis treatment of natural waters and model solutions [18, 19, 20, 28, 29, 30, 31] and liquid media of food industry [32, 33, 34, 35, 36, 37, 38] as a result of the action of temperature and currents as well as a result of the contact of the surface of membranes with acids, alkalis, and organic components. The understanding of the mechanisms of degradation of membranes during the demineralization of natural waters is necessary for searching the ways for prolonging their usable life expectancy and regeneration techniques. The aim of this study is to reveal the changes in the structural and transport properties of the MA-40 anion-exchange membrane in the process of long-term operation in electrodialysis apparatuses during the desalination and concentration of mineralized natural waters.

EXPERIMENTAL

Objects of Study

Membranes. The objects of the study were samples of heterogeneous MA-40 anion-exchange membranes after a standard conditioning procedure and after long-term operation in industrial electrodialyzers in the mode of desalination/concentration of natural waters. The MA-40 membrane is a composite of the polyfunctional (mixed-basicity) anion-exchange resin EDE-10P, high-density polyethylene, and a reinforcing fabric (Kapron). The polymer matrix of EDE-10P is obtained via polycondensation of polyethylene polyamines with epichlorohydrin. The fixed groups are secondary \({\text{NH}}({\text{C}}{{{\text{H}}}_{{\text{3}}}})_{{\text{2}}}^{ + }\) and tertiary NH2(CH3)+ amino groups, as well as quaternary –N+(CH3)3 ammonium groups in an amount of up to 20% [39]. Physicochemical, transport, and structural characteristics of the membranes after operation were compared with MA-40 membrane samples conditioned according to standard procedures [40].

A sample of the MA-40 membrane from the anode compartment of a reversal electrodialyzer after 1000-h operation in the desalination of the high-hardness natural waters of the Aral region was studied. Sides 1 and 2 of the surface of the MA-40 membrane from a reversal electrodialyzer faced the electrode and the neighboring compartments respectively. The second sample under study was the MA-40 membrane, the duration of operation of which in the middle of the working stack of a concentrator apparatus was about 500 h (the current density was 2.5 A/dm2). Side 1 of the membrane was from the side of the anode in the concentration compartment, and side 2 was in the deionization compartment and faced the cathode.

Electrodialyzers

Modern electrodialyzers are based on the principle of alternation of cation-exchange and anion-exchange membranes in multicompartment apparatuses, where the number of compartments reaches several hundreds. When applying an electrical potential difference, a general result of the process is an increase in the concentration of ions in the odd chambers (concentration compartment) and a simultaneous decrease in their concentration in the even chambers (desalination compartment).

An electrodialyzer–concentrator with nonflow concentration chambers is presented in Fig. 1. It was used without current reversal to get brines with a concentration of 180–200 g/dm3 from underground water of the chloride class. The total concentration of salts was 17.29 g/dm3 at a hardness of 52 mmol/dm3 and pH 7.3. The composition of the water under treatment included 5.732 g/dm3 sodium, 0.381 g/dm3 magnesium, 0.417 g/dm3 calcium, 0.189 g/dm3 hydrocarbonates, and 10.582 g/dm3 chlorides. The type of electrodialyzer used provides enhanced reliability of operation due to the prevention of the burn-through of the membranes, increase in their working area, and elimination of current leakages [42]. The operational life tests of the concentrating apparatus were performed by TOO Membrannye tekhnologii S.A. [41] in Tyumen oblast; the samples for the studies were also provided by it.

Fig. 1.

Photographic image of the electrodialyzer–concentrator. Adapted from [41].

A reversal electrodialyzer. The process flow diagram of an electrodialysis desalination unit included three-stage desalination apparatuses (ED-300) with the productivity of 8 m3/h. Part of the diluate with the total salt content of 1.5 g/dm3 was used for technical purposes and irrigation, and the other part, for desalination to the level of drinking water in an EDS-300 electrodialyzer with the productivity of 2 m3/h. The concentrate with the total salt content of 4 g/dm3 was returned to the inlet of the unit and mixed with the feed water. During the course of the process, natural mineralized waters of the Aral region were fed to all the compartments of the electrodialyzer. Waters to be treated had a high concentration of chlorides and sulfates [43]. At a total salt content of 12.87 g/dm3 and a water hardness of 49 mmol/dm3, the concentrations were 3.911 g/dm3 sodium, 0.023 g/dm3 potassium, 0.313 g/dm3 calcium, 0.400 g/dm3 magnesium, 0.183 g/dm3 hydrocarbonates, 1.368 g/dm3 sulfates, and 6.671 g/dm3 chlorides; pH was 7.5.

Methods for Investigation of the Equilibrium, Transport, and Structural Properties of the Membranes

The water content W of the membranes was controlled via hot air drying, thickness d was determined with an accuracy of up to 1 μm using a micrometric method, and density ρ was measured using a pycnometric method [40].

The total ion-exchange capacity of the membranes Q was evaluated using acid–base titration under static conditions by determining the total amount of counterions that entered the ion exchange according to the neutralization reaction.

The integral coefficient of diffusion permeability of the membranes was determined by measuring the amount of the electrolyte transported y the action of diffusion forces in the NaCl/MA-40/H2O system from a solution of the salt with a defined concentration through the membrane to water [44]. When measuring diffusion permeability, to eliminate the influence of the diffusion layers of the solution at the boundaries with the membrane, the required speed of feeding of solutions V > 9 × 10−5 m/s was preliminarily determined, at which the dependence of the integral coefficient of permeability of the membranes under measurement on the flow rate disappears. The contact-difference method [45, 46] for the measurement of specific electrical conductivity consisted in the measurement of the impedances of one and two membranes in a cell with platinum electrodes. Then, their vector difference was found, which was considered as the true electrical resistance of the membrane. The experimental measurements were performed using a Tesla BM-507 impedance meter at a working ac frequency of 5 kHz.

The fraction of the intergel phase f2 was found by plotting the concentration dependences of the specific electrical conductivity of the membranes and equilibrium solution in logarithmic coordinates. According to the microheterogeneous model [47], the value of f2 corresponds to the fraction of the electroneutral solution that occupies the centers of pores in the membrane. The techniques of using a single set of structural and kinetic parameters of the microheterogeneous model for describing the kinetic properties of ion-exchange membranes are detailed in [44, 48, 49].

Prior to the determination of specific electrical conductivity, the membrane samples were rinsed with distilled water. Then the membranes, swollen in water after operation, were divided into several samples, each of which was equilibrated with one of sodium chloride solutions in the concentrations range of 0.01 to 0.10 mol/L. To estimate the diffusion properties of the membranes after conditioning and operation in electrodialyzers, solutions of NaCl in the concentration range of 0.05–1.00 mol/L were used as feed solutions. The transport numbers of the counterions in the membranes were determined potentiometrically from the experimentally measured difference of the membrane potentials of a cell that contained reversible electrodes, a membrane, and solutions with different concentrations of the electrolyte on both sides of the membrane [50].

The surface of dry and swollen samples of the membranes was studied by scanning electron microscopy (SEM). A JSM-6380 LV microscope (Japan) was equipped with an energy dispersive elemental composition analyzer and a pressure controller in the chamber with the sample under study. SEM images of the MA-40 membrane from the concentrator apparatus were obtained for surface 1 from the anode side and surface 2 facing the cathode. The SEM images of the surface of a membrane from the reversal electrodialyzer are presented for sides 1 and 2 facing the electrode and the neighboring compartment, respectively. The possibility for pressure control in the chamber with the sample under study made it possible to study membranes in the swollen state in the low-vacuum mode [51, 52]. The information about the chemical composition of the surface or section of the membrane was obtained by mapping the elemental composition using an energy dispersive spectrometer. The color coding for X-ray spectrum imaging allowed combining data on several elements in one image.

The proportion of pore composition on the surface and section of the membranes was quantified using a customized software complex [53]. The automated analysis of morphology of the membranes was performed by digital processing of their electron microscopy images. In preliminary experiments, the optimum value of the relative magnification factor of 500× was chosen for the simultaneous determination of the fraction and size of microphases with allowance for the “size effect.” The proportion of macropores on the membrane surface was determined according to P =Si/S) × 100%, where ΣSi is the total area of the macropores and S is the area of the section scanned. By pore radius r is meant the effective radius of the round section modeled by the program which was equivalent in area to a real pore with an arbitrary shape. When calculating the average radius, a average weighted value was used to take into account different fractions of pores with different sizes [51].

The surface roughness of the membranes was studied by atomic force microscopy (AFM) [54]. Scanning with a NSG20 cantilever of a of 90 ± 5 μm length with a tip curvature radius of 10 nm was performed on dry samples in the tapping mode on a SolverP47 Pro microscope (Russia, Zelenograd). The resonance frequency was 260–630 kHz. The scanning area was 20 × 20 μm. The experiments were performed in air at 25 ± 1°C. The obtained AFM images were processed using the Solver P47 Pro Nova RC1 AFM software. The following surface roughness parameters were analyzed: peak-to-peak height Ry and arithmetic average roughness Ra (ISO 4287/1). The obtained height distribution histograms characterized the relief surface topography of the entire sample of the membrane.

All the measurement results were processed using mathematical statistics [55]. The values of the average measurement results \(\bar {x}\) and standard deviations were found using microstatistics. The value of the confidence interval of the average measurement results ∆x was found using the t-test (Student coefficient) at a confidence probability P = 0.95.

RESULTS AND DISCUSSION

Transport and Structural Properties of an Anion-Exchange Membrane from the Concentrator Apparatus

The main physicochemical characteristics of samples of the MA-40 anion-exchange membrane after conditioning and operation in electrodialyzers of different types are presented in Table 1. A decrease in the total ion-exchange capacity by 10% and a growth in the water content and thickness of the membrane in the process of operation in the working stack of the concentrator apparatus have been revealed. The loss of the ion-exchange capacity of the MA-40 membrane is associated with the degradation of the active groups of the EDE-10P anion exchanger. On the basis of thermographic data, it was found [56, 57] that the strongly and weakly basic groups of the EDE-10P anion-exchange resin differences stability and the decrease in ion-exchange capacity is mainly associated with the behavior of the strongly basic groups. In the case of heating in water, the deamination reaction with the detachment of a quaternary ammonium from the polymer matrix and a degradation reaction yielding tertiary amino groups that can be then transformed to secondary amino groups occur [58, 59]. It should be noted that for the anion exchanger in the OH form, the transformation of strongly basic to weakly basic groups occurs without the loss of total exchange capacity. In addition, thermal degradation of the EDE-10P polycondensation polyamine anion exchanger, on the basis of which the MA-40 membrane is fabricated, can occur according to the same mechanism as in the case of branched polyethylene polyamine [60], namely, with the carbon–tertiary nitrogen bond rupture and the migration of the mobile methylene hydrogen to the place of breaking occurring at the initial step.

Table 1.

Physicochemical characteristics of swollen MA-40 anion-exchange membrane samples after conditioning and operation in electrodialyzers of different types

Membrane sample

Q, mmol/g

Water content W, %

Thickness, μm

Density, g/cm3

Conditioned

2.71 ± 0.09

38 ± 3

 560 ± 20

1.19

Concentrator

2.47 ± 0.09

43 ± 2

 580 ± 10

1.10

Reversal electrodialyzer

2.20 ± 0.08

35 ± 1

505 ± 5

1.20

The results of studying the transport characteristics of the MA-40 membrane from the working stack of the electrodialyzer–concentrator showed that the specific electrical conductivity increased by 40% (Fig. 2) against the background of the loss of exchange capacity and the growth of water content. The decrease in the concentration of fixed groups (loss of exchange capacity) should lead to a decrease in electrical conductivity due to the decrease in the diffusion coefficient of counterions. A probable reason for the increase in the electrical conductivity is the increase in pore size due to the degradation of the ion exchanger and the inert binder polyethylene as a result of long-term operation. The data on the quantification of the pore composition of the membrane surface (Table 2) give evidence for the changes in the microstructure, which are associated with an increase in macroporosity and occurrence of structural defects. The growth in the size of macropores leads to an increase in the electrical conductivity of the membranes at concentrations in solutions which exceed the concentration at the isoconductivity point of the gel part of the membranes and solution. In this case, the main contribution to the electrical conductivity of the membranes is made by the solution of the pore space of the membrane, the concentration of which is equal to the concentration of the external solution. The increase in the parameter f2, which characterizes the volume fraction of the electroneutral solution that occupies the centers of pores in the membrane, is the proof of the growth in the number and size of the pores in the bulk of the test membrane. After operation in the concentrator apparatus, f2 increased by 19% (Table 2). A growth in the fraction of ion-exchange sections on the surface of swollen samples by 18% due to the increase in the volume of the ion-exchanger particles has also been found from the analysis of the SEM images of the surface of the membranes after operation. In the region of low concentrations, the increase in the water content of the gel phase of the membrane due to the growth in the water uptake causes a growth in the diffusion coefficients of counterions and coions.

Fig. 2.

Concentration dependences of the specific electrical conductivity of MA-40 membrane samples: (1) conditioned, (2) from the working stack of the concentrator apparatus, and (3) from the anode compartment of the reversal electrodialysis apparatus.

Table 2.  

Structural characteristics of the swelled samples of the MA-40 anion-exchange membrane after conditioning and operation in electrodialyzers of different types

Membrane sample

P, %

\(\bar {r}\), μm

f 2

Conditioned

2.2 ± 0.2

2.53

0.21

Concentrator

2.8 ± 0.2

2.90

0.25

Reversal electrodialyzer

2.4 ± 0.1

2.41

0.31

P is the proportion of macropores on the surface, \(\bar {r}\) is the average weighted radius of macropores on the surface, and f2 is the volume fraction of the electroneutral solution in the membrane.

After operation in the concentrator apparatus for 500 h, a drop in the transport numbers in the membrane by 18% was found (Fig. 3). The main reasons for the decrease in the selectivity of the MA-40 membrane sample are the loss of exchange capacity and the increase in the pore size of the membranes. The lower exchange capacity of MA-40 membranes after operation facilitates the transport of coions through the membrane as a result of weakening of the Donnan exclusion effect. It was shown in [61] using the model of a charged pore and the electrical double layer theory that selectivity increases with a decrease in membrane pore size. Coions cannot penetrate into micropores (r ≤ 1 nm) because of the strong action of the electrostatic field. The experimental data presented in [62] confirm that industrial heterogeneous membranes that have a phase of intergel pores possess lower selectivity in comparison with homogeneous membranes in not very dilute solutions.

Fig. 3.

Potentiometric transport numbers of sodium ions in MA-40 membrane samples: (1) conditioned, (2) from the concentrator apparatus, and (3) from the anode compartment of the reversal electrodialysis apparatus.

The SEM images of the MA-40 membrane surfaces are presented in Fig. 4. After use of the membrane to concentrate the natural waters of Tyumen oblast, mineral scales are barely seen. The X-ray spectral analysis data (Table 3) and distribution maps of chemical elements show insignificant scaling at ion-exchange regions on the membrane surface. The analysis of the composition of the natural waters under concentration and the scale formed showed that the scale is carbonate-based with the predominant concentration of calcium in comparison with magnesium and iron.

Fig. 4.

SEM images of the surface of dry samples of the MA-40 membrane from the electrodialyzer–concentrator under magnifications of (a) 500× and (b) 1500×.

Table 3.  

The elemental composition of the sides of the surface and section of the samples of the MA-40 membrane (a magnification of 200×)

Element

Concentration, wt %

membrane sample

conditioned

concentrator

reversal electrodialyzer

surface

section

surface

section

surface

section

1

2

1

2

C

O

S

Ca

Mg

Fe

Si

91.76

7.62

0.00

0.00

0.00

0.00

0.00

92.12

7.88

0.00

0.00

0.00

0.00

0.00

94.25

3.44

0.00

0.04

0.01

0.04

0.05

93.40

3.38

0.00

0.05

0.01

0.02

0.02

85.23

7.83

0.00

0.12

0.02

0.09

0.00

30.67

53.69

0.35

1.98

12.21

0.47

0.11

27.10

55.83

0.30

0.48

14.15

1.24

0.31

70.64

19.02

0.00

6.86

1.51

0.17

0.38

The precipitation of poorly soluble carbonates CaCO3 and MgCO3 is possible in the case of an increase in the pH of feed water as a result of the disturbance of the carbonic acid equilibrium in the system [7]:

$${\text{HCO}}_{{\text{3}}}^{-} + {{{\text{H}}}_{{\text{3}}}}{{{\text{O}}}^{ + }} \to {{{\text{H}}}_{{\text{2}}}}{\text{C}}{{{\text{O}}}_{{\text{3}}}} \to {{{\text{H}}}_{{\text{2}}}}{\text{O}} + {\text{C}}{{{\text{O}}}_{{\text{2}}}} \uparrow ,$$
(1)
$${\text{HCO}}_{{\text{3}}}^{ - } + {\text{O}}{{{\text{H}}}^{ - }} \to {{{\text{H}}}_{{\text{2}}}}{\text{O}} + {\text{CO}}_{{\text{3}}}^{{{\text{2}} - }},$$
(2)
$${\text{CO}}_{{\text{3}}}^{{{\text{2}} - }} + {\text{ 2}}{{{\text{H}}}_{{\text{3}}}}{{{\text{O}}}^{ + }} \to {{{\text{H}}}_{{\text{2}}}}{\text{C}}{{{\text{O}}}_{{\text{3}}}} + {\text{2}}{{{\text{H}}}_{{\text{2}}}}{\text{O}}.$$
(3)

The concentration of carbonate ions is a function of concentration of hydrogen ions which is determined by the value of the second dissociation constant of carbonic acid K2 = 4.8 × 10−11. Increasing the acidity of the medium leads to the binding of carbonate ions to weakly dissociating carbonic acid (3), which decreases the concentration of scale-forming carbonate anions. Taking into account the calcium carbonate precipitation conditions [Ca2+]\(\left[ {{\text{CO}}_{3}^{{2 - }}} \right]\)Ks, the concentration of hydrogen ions at which the formation of CaCO3 occurs is given by

$$\left[ {{{{\text{H}}}_{{\text{3}}}}{{{\text{O}}}^{{\text{ + }}}}} \right]{\text{ < }}\,\,\frac{{\left[ {{\text{C}}{{{\text{a}}}^{{{\text{2 + }}}}}} \right]\left[ {{\text{HCO}}_{{\text{3}}}^{ - }} \right]{{K}_{{\text{2}}}}}}{{{{K}_{{\text{s}}}}}}{\text{.}}$$
(4)

For concentrations of calcium ions of 10.2 mmol/dm3 and hydrogen carbonate ions of 3.1 mmol/dm3 in the natural water of Tyumen oblast, the pH value for the onset of precipitation calculated according to relation (4) is 6.50. At a magnesium concentration of 15.8 mmol/dm3 in natural water, the pH corresponding to the condition of precipitation of MgCO3 is above 9.95. It should be noted that the conditions of formation of crystalline particles of calcium and magnesium carbonates in the concentrate flux can strongly differ from the conditions in the feed water due to the increase in the total concentration of dissolved substances and disturbance of the balance of cations, hydrocarbonates, and free CO2. The iron hydroxide deposit on the anode-facing side of the membrane surface is apparently determined by a local increase in the pH as a result of the transport of the OH ions formed as a result of the water splitting reaction intensively proceeding in the limiting regimes of electrodialysis through the membrane. Due to the fact that the fraction of the conducting surface of the MA-40 membrane is about 20% [54, 63], the generation of H+ and OH ions on individual resin beads can start at currents that are significantly lower than the calculated values of limiting current.

Scaling of an Anion-Exchange Membrane from the Electrode Compartment of the Reversal Electrodialyzer

Membranes located in the electrode compartments operate under the harshest conditions because, in this case, they directly contact with the products formed during the electrodialysis of mineralized water. For the MA-40 membrane from the electrode compartment of the reversal electrodialyzer, a drop in the total exchange capacity by 20% and a decrease in the water content and thickness of the sample were found (Table 1).

The investigation of the transport characteristics of the membrane showed that in comparison with the conditioned sample, long-term operation led to a twofold decrease in the specific electrical conductivity (Fig. 2) and diffusion permeability (Fig. 5). This is determined by the fact that during the operation, not only partial loss of exchange capacity by the membrane because of the destruction of the ionogenic groups as a result of the action of the electrolysis products and overheating, but also significant changes in the microstructure of their surface and volume associated with the formation of mineral scales occurred.

Fig. 5.

Concentration dependences of the integral coefficient of diffusion permeability of (1) a conditioned sample of the MA-40 membrane and (2) a sample of the MA-40 membrane from the anode compartment of the reversal electrodialysis apparatus.

The SEM images of the MA-40 membrane surface sides that faced the electrode (side 1) and neighboring (side 2) compartments of the electrodialyzer are presented in Figs. 6 and 7, respectively. The formation of scale is visualized on both sides of the membrane surface, which is localized not only in the good-conductivity regions where ion-exchanger particles are located, but covers almost the entire surface as a film.

Fig. 6.

(a) The SEM image and (b–d) corresponding distribution maps of (b) Mg, (c) Ca, and (d) Fe on side 1 of the surface of a dry sample of the MA-40 membrane from the reversal electrodialyzer under a magnification of 200×. The distribution of the elements is marked in white.

Fig. 7.

(a) The SEM image and (b–d) corresponding distribution maps of (b) Mg, (c) Ca, and (d) Fe on side 2 of the surface of a dry sample of the MA-40 membrane from the reversal electrodialyzer under a magnification of 200×. The distribution of the elements is marked in white.

The AFM histograms of the density of height distribution on the membrane surface show that scaling significantly changed the relief of the surface (Fig. 8). The maximum of height distribution shifted from 700–800 nm to 1.0–1.2 μm. The arithmetic mean roughness Ra and peak-to-peak height Ry on the surface increased by factors of 2.7 and 1.5, respectively. The weakly pronounced diffused maximum that corresponds to the membrane after operation demonstrates a macroscopically developed and morphologically nonuniform surface coated with a continuous scale film in comparison with the conditioned sample.

Fig. 8.

Histograms of the density of height distribution on the surface of a heterogeneous MA-40 anion-exchange membrane after (1) conditioning and (2) operation in the anode compartment of the reversal electrodialyzer at a scanning area of 20 × 20 μm.

The nature of the scale on the ion-exchange membrane depends on the pH and the rate of migration of ions through the membrane. The distribution maps of elements (Figs. 6 and 7) and the results of the X-ray spectral microanalysis of the component composition of the scale on the surface (Table 3) reveal a predominant concentration of Mg and the presence of elements such as Ca, Fe, and Si. The supposed composition and the calculated pH values of the onset of precipitation of slightly soluble compounds are presented in Table 4.

Table 4.  

The composition, solubility product constants Ks, and pH of the onset of formation of slightly soluble compounds

Composition of the sediment

Ks [75]

pH*

CaSO4

9.1 × 10–6

CaCO3

4.8 × 10–9

6.63

Mg(OH)2

1.8 × 10–11

9.52

MgCO3

2.1 × 10–5

9.94

Ca(OH)2

5.5 × 10–6

11.56

Fe(OH)3

3.2 × 10–38

* Calculated for the corresponding concentration of the components in feed water .

By reversing the polarity of the electrodes in the electrodialyzer, the functions of its electrode and working chambers simultaneously change. The electrode compartment alternately becomes either cathode or anode. The schemes of scale formation via different mechanisms during the operation of the reversal electrodialyzer are presented in Fig. 9. An acidic solution of the anolyte is formed in the anode compartment, and an alkaline solution of the catholyte, in the cathode compartment. The acidic reaction of the medium in the electrode compartment prevents scaling in the form of magnesium and calcium hydroxides and carbonates. However, in this case, problems are generated by the precipitation of calcium sulfate, iron oxides, and silica SiO2. The calcium sulfate formation process slightly depends on the pH of solutions and is determined by reaching and surpassing the solubility limit \({\text{[C}}{{{\text{a}}}^{{{\text{2}} + }}}][{\text{SO}}_{{\text{4}}}^{{{\text{2}} - }}] \geqslant 9.1 \times {{10}^{{ - 6}}}.\) It is known that a quite high concentration of CaSO4 in a solution can be maintained in electrodialysis reversal units. The upper limit of supersaturated solutions of calcium for systems in the absence of chemical additives is 175% saturation with respect to CaSO4 [65]. For the composition of the Aral water subjected to purification, the product of the concentrations of calcium ions and sulfate ions is 1.1 × 10−4, which exceeds their permissible concentration in water by two orders of magnitude and causes precipitation (Fig. 9a).

Fig. 9.

General scheme of scale formation during the operation of the electrodialyzer in the (a) forward and (b) reverse current modes. The asterisk (*) denotes the reactions occurring at the solution/membrane interface. Numbers 1 and 2 denote the sides of the membrane.

It should be taken into account that the MA-40 membrane is characterized by a high catalytic activity of the ionogenic groups in the water splitting reaction in the solution at the interface [66, 67, 68]. In the case of occasional violation of the current regime or exceeding the local limiting current density in the desalination compartment, hydroxyl ions are formed near the surface (side 2) of the anion-exchange membrane and migrate to the electrode compartment (Fig. 9a). On one hand, the transport of hydroxyl ions through the membrane can shift the balance of weakly acidic anions. For example, in the case of a local increase in pH in the solution at the interface and in the pore space of the membrane, the balance shifts towards the formation of carbonate ions according to reaction (2), which leads to the precipitation of carbonates [69]. On the other hand, the migration of hydroxyl ions through the anion-exchange membrane can lead to the precipitation of hydroxides of multicharged cations [69] as a result of a local increase in pH of the solution in the near-membrane region. Magnesium hydroxide precipitates when the product of ion concentrations in the solution exceeds the solubility product constant [Mg2+][OH]2 ≥ 1.8 × 10−11. During the electrodialysis of the natural water of the Aral region with a concentration of magnesium ions of 16.7 mmol/dm3, this condition corresponds to pH > 9.52. Calcium hydroxide can precipitate at pH > 11.56 under the condition of [Ca2+][OH]2 ≥ 5.5 × 10−6 and at a concentration of calcium ions of 7.8 mmol/dm3. It should be noted that in the electrode concentration compartment, the trend to scaling by CaSO4 is higher, and the pH value of the onset of precipitation of hydroxides is somewhat lower because of the higher concentration of the scale-forming ions.

In the case changing the polarity of the electrodes, the probability of deposition of hydroxides on the electrode-faced side of the MA-40 membrane (side 1) significantly increases as a result of the increase in the pH of the diluate due to the electrode reaction (Fig. 9b). In this case, the solution near the membrane surface facing the neighboring concentration compartment (side 2) is also alkalized, since hydroxyl ions formed on the cathode during the electrolysis migrate to the neighboring compartment. The elemental composition of the surface corresponds to crystals of Mg(OH)2, with their content prevailing on the both sides of the membrane. A significant increase in the pH of the waters under treatment is accompanied by the precipitation of slightly soluble calcium carbonate as a result of the disturbance of the carbonic acid equilibrium in system (2) and an increase in the concentration of scale-forming carbonate ions. The influence of magnesium ions on the deposition of calcium carbonate on an anion-exchange membrane during electrodialysis was first indicated in [70]. In the case of absence of magnesium in a model salt solution, the mineral scale was identified as calcium hydroxide; in solution containing magnesium ions, the scale consisted of a mixture of CaCO3 and Ca(OH)2. The formation of magnesium hydroxide and calcium carbonate on the surface of a cation-exchange membrane from the side of the electrode compartment at a high pH value of solution (pH 12) has been confirmed in [7] by X-ray diffraction and energy dispersive X-ray elemental analysis data. The presence of iron ions in the composition of the scale on the surface of the MA-40 membrane under study according to the results of X-ray spectral microanalysis (Table 3) suggests the exceedance of \({\text{[F}}{{{\text{e}}}^{{{\text{3}} + }}}]{{[{\text{O}}{{{\text{H}}}^{ - }}]}^{3}} \geqslant {\text{3}}{\text{.2}} \times {\text{1}}{{{\text{0}}}^{{ - {\text{38}}}}}\) and the formation of a compact stable iron hydroxide deposit. It should be noted that no silica precipitates at high values of pH.

It was found that scaling affects not only the surface, but also the bulk of the membrane. Scaling is promoted by an increase in the pH of the internal pore solution as a result of the migration of hydroxyl ions to the membrane phase. Comparing the data on the elemental composition of the section (Table 3) and distribution map of elements (Fig. 10) leads to the conclusion that the amount and localization of the scale are uneven over the area of the cross-section of the MA-40 membrane. Insoluble Mg compounds were predominantly formed on the surface, whereas the formation of a mixture of CaCO3 and Ca(OH)2 was observed in the membrane phase on both sides. The prevalence of magnesium ions on the membrane surface is explained by the influence of pH of the medium on the precipitation of slightly soluble hydroxides. Magnesium hydroxide is less soluble than calcium hydroxide, and a lower value of pH ≥ 9.52 is required for the onset of its precipitation (Table 4). The value of pH of the onset of precipitation of calcium hydroxide is 11.56. Thus, precipitation from the external solution onto the surface of the membrane is characteristic of Mg(OH)2. The pH value of the internal solution of anion-exchange membranes is higher by 2 to 3 units than the pH of the external solution. In this connection, more favorable conditions for scaling by Ca(OH)2 are created in the solution of the pore space of the membrane. Unlike hydroxides, the poorly soluble salt CaCO3 can start precipitating in a neutral medium at pH > 6.63 (Table 4), a fact that contradicts the prevalence of calcium ions in the bulk of the membrane. This fact is explained by its higher mobility (equivalent conductivity) in comparison with magnesium ions in aqueous solutions (λ(Ca2+) = 59.50 versus λ(Mg2+) = 53.05 Ω−1 mol−1 cm2 [71]). Hydrated calcium ions are characterized by a smaller radius (rst(Ca2+) = 3.5 Å) in comparison with hydrated magnesium ions rst(Mg2+) = 4.0 Å [72]. The hydration numbers of Ca2+ and Mg2+ ions are 8 and 10, respectively [73]. These parameters facilitate the migration of calcium ions in the solution of the pore space of the membrane in comparison with magnesium ions [7, 74, 75].

Fig. 10.

Distribution of Ca (white color) over the section of the MA-40 anion-exchange membrane from the anode compartment of the reversal electrodialyzer (magnification, 200×).

The nature of functional groups of the anion exchanger EDE-10P is such that coordination bonding can appear in the case of contact with metal cations. This bonding can be due to overlapping of the two-electron orbital of the nitrogen atom of the amino group of the anion-exchange resin with the unoccupied orbital of the metal ion. It is known that the sorption of Fe3+ by the MA-40 membrane is almost unaffected by the pH in the range of 0.5 to 8.0, as determined by the mechanism of their interaction. The EDE-10P anion exchanger adsorbs Fe3+ as a result of complexation with weakly charged amino groups [76]. It has been found [77] that in the case of sorption of iron by EDE-10P at pH 2, Fe3+ cations coordinate to the nitrogen atom of the amino group to form a complex of the ethylenediamine type. The examples of groups that can be bound to calcium and magnesium cations by coordination bonding are the amino groups –NH2 and =NH. In chelating agents, the groups that interact with the hardness cations via the donor–acceptor mechanism are tertiary amino groups [78]. In this connection, the possibility for the interaction of calcium and magnesium ions with the fixed groups of the MA-40 membrane is real. As a result, some ionogenic groups become transformed to the bound state and do not participate in the ion transport, which leads to an increase in the electrical resistance and growth in the diffusion permeability due to the weakening of the Donnan exclusion of the coions from the membrane phase.

It should be noted that the presence of scale inside the membrane pore space can produce the wedge effect inside the pore space and lead to an increase in f2 [37]. However, despite the increase in the volume fraction of the membrane occupied by the equilibrium solution by 48% (Table 2), the electrical conductivity and diffusion permeability of the MA-40 membrane drop after operation. The reason for the decline is that the presence of scale in the central parts of macropores leads to a decrease in the mobility of anions and cations in the intergel spaces of the membrane structure.

CONCLUSIONS

The mechanisms of the decrease in the operational characteristics of the MA-40 anion-exchange membrane in the case of the use in the processes of demineralization and concentration of natural mineralized waters via electrodialysis have been found.

The destruction of ionogenic groups inducing a decrease in the total exchange capacity of the membrane should be distinguished as the main reason for the deterioration of the properties of the MA-40 anion-exchange membrane from the membrane package after the electrodialysis concentration of the natural waters of Tyumen oblast. Despite the partial loss of exchange capacity, a growth in the specific electrical conductivity of the MA-40 membrane due to the increase in its macroporosity as a result of operation has been found. Therefore, the main reasons for the decrease in the selectivity of the membrane are the loss of exchange capacity and increase in the number and sizes of macropores. Insignificant sediment formation in the region of ion-exchange sections on the surface of the MA-40 membrane after operation in an electrodialyzer–concentrator for 500 h has a carbonate nature with the predominant concentration of calcium.

The main factor determining the deterioration of the transport properties of the MA-40 membranes from the electrode compartments of the reversal electrodialyzer in the case of demineralization of the natural waters of the Aral region is scaling, which affects the surface and bulk of the membrane. The selectivity (transport number) of ion-exchange membranes depends on the electrical conductivity and diffusion permeability. The interactions of hardness and iron cations with the fixed groups of the membrane and the formation of slightly soluble carbonates, hydroxides, and sulfates lead to the blocking of both the functional groups and transport channels of the membrane. This causes a decrease in the electrical conductivity and complication of diffusion processes. The net effect is a decrease by 25% in the selectivity of the MA-40 ion-exchange membrane that operated more than 1000 h in the anode compartment of the reversal electrodialyzer.

Notes

ACKNOWLEDGMENTS

This work was supported by the President of the Russian Federation, grant MK-925.2018.3. The micrographs and AFM images of the surface of the membranes were obtained using the equipment of the Collective Use Center of Voronezh State University (URL: http://ckp.vsu.ru).

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Copyright information

© Pleiades Publishing, Ltd. 2019

Authors and Affiliations

  • V. I. Vasil’eva
    • 1
    Email author
  • E. M. Akberova
    • 1
  • D. V. Kostylev
    • 1
  • A. A. Tzkhai
    • 2
  1. 1.Voronezh State UniversityVoronezhRussia
  2. 2.TOO Membrannye Tekhnologii S.A.AlmatyRepublic of Kazakhstan

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