Composite inorganic membranes containing nanoparticles of hydrated zirconium dioxide for electrodialytic separation
 Yuliya S Dzyazko^{1}Email author,
 Yurii M Volfkovich^{2},
 Valentin E Sosenkin^{2},
 Nadejda F Nikolskaya^{2} and
 Yurii P Gomza^{3}
DOI: 10.1186/1556276X9271
© Dzyazko et al.; licensee Springer. 2014
Received: 6 December 2013
Accepted: 6 May 2014
Published: 29 May 2014
Abstract
The aim of the work was to elucidate the nature of chargeselective properties of macroporous composite inorganic membranes modified with nanoparticles of hydrated zirconium dioxide. The membranes have been investigated using methods of standard contact porosimetry, potentiometry, electron microscopy and smallangle Xray scattering. The ion exchanger has been found to deposit inside pores of ceramics. Differential curves of pore volume distribution have been resolved using Lorentz functions; each maximum has been related to structure elements of the matrix and ion exchanger by means of calculations according to homogeneous and heterogeneous geometrical models. It was found that the voids, the radius of which is 4 to 8 nm, are responsible for charge selectivity of the composite membranes. These pores are formed due to blocking of macropores of ceramics with aggregates of nanoparticles of the ion exchanger; the radius of these aggregates is 20 to 24 nm. The membranes were applied to desalination of the solution containing NaCl. The removal degree of the salt from the solution reached 95% and 9% for the composite and unmodified membranes, respectively.
Keywords
Composite ion exchange membranes Hydrated zirconium dioxide Nanoparticles Standard contact porosimetry ElectrodialysisBackground
Inorganic membranes can operate at high temperatures and in aggressive media; moreover, they are stable against fouling with organic matters [1, 2]. Since these materials possess remarkable properties, they are attractive for separation processes particularly for electromembrane techniques [3]. However, application of ceramic separators to electromembrane processes is limited by an absence of charge selectivity in spite of a nanoporous active layer. This is due to extremely low ion exchange capacity (low surface charge density) of ceramics, since these materials are produced at high temperature [4], which does not provide retention of functional groups.
The conditions of thermal treatment of the membranes provided ion exchange ability of HZD.
Pores of 190 nm dominated in pristine ceramics. After modification, their size decreased to 80 nm [6, 7] indicating formation of an active layer inside the pores of ceramics, opposite to known inorganic materials for baromembrane separation [1, 2]. This location of the active layer provides its mechanical durability.
Predominant pores of the composite membranes [6, 7] cannot provide overlapping of intraporous diffusion double electric layers. In spite of this, the membranes were shown to possess charge selectivity. They demonstrate membrane potential in rather concentrated acid solutions [6]. When the composite separators are applied to electrodialysis, the ion transport through these separators is due to migration of counter ions and electrolyte diffusion during electrodialysis [7]. At the same time, no migration of coions through these separators was found.
Many types of ceramics contain larger pores (up to several microns) in comparison with the material investigated in [6, 7]. The aims of the work involve formation of the inner active layer in coarsepored membranes, ascertainment of the cause of their charge selectivity and application of these materials to electromembrane separation.
A method of standard contact porosimetry (SCP) was applied to membrane investigation. The method allows us to obtain pore size distribution in a wide interval of 1 nm to 300 μm as well as total volume of micropores of 0.3 to 1 nm [8–11]. The SCP method is nondestructive, since it does not require high pressure compared to mercury porosimetry. Thus, small pores can be determined more exactly. Moreover, analysis of integral pore size distribution gives a possibility to determine particle size using geometrical models [12–14]. However, in the case of composites, the particle size of their components can be close to each other; as a result, the constituents cannot be recognized. Thus, the next important task of the work is to develop an approach for analysis of pore size distributions for composite materials.
Experimental
Synthesis of the composite membranes
Planar ceramic membranes (matrix) based on TiO_{2} (TAMI GmbH, Hermsdorf, Germany), which contain no active layer, were used. Sol of insoluble zirconium hydroxocomplexes was prepared by adding a NH_{4}OH solution (1,000 mol m^{−3}) to a 1 M ZrOCl_{2} solution (1,000 mol m^{−3}) at 353 K followed by boiling for 48 h and storage for 48 days at 298 K [6, 7]. Sol was analysed with a dynamic lightscattering method using a Zetasizer Nano ZS device (Malvern Instruments, Worcestershire, UK). Stability of particle distribution has been found after longterm storage.
The membrane was impregnated with sol, treated with a NH_{4}OH solution (1,000 mol m^{−3}), dried at ≈ 298 K and heated at 423 K [6, 7]. A layer of the ion exchanger was removed from the outer surface of the membrane with ultrasonic activation at 30 kHz. The procedure, which involves impregnation, HZD deposition, drying, heating and ultrasonic treatment, was repeated two and seven times. The samples were marked as TiO_{2} (matrix), TiO_{2}HZD2 and TiO_{2}HZD7 (modified membranes). Similar growth of HZD content (2.2 to 2.4 mass%) was reached both for TiO_{2}HZD2 (in comparison with the matrix) and TiO_{2}HZD7 (in comparison with TiO_{2}HZD2).
Electron microscopy
After dehydration of sol at room temperature, its solid constituent was investigated using a JEOL JEM 1230 transmission electron microscope (JEOL Ltd., Tokyo, Japan). Finely dispersed powders obtained both from initial and modified membranes were also researched. Before the investigations, the powders of ceramics were treated with a CH_{3}COOH solution (100 mol m^{−3}) to shade the modifier particles.
Transverse section of the membranes was investigated using a Zeiss EVO 50XVP scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany).
Smallangle Xray scattering
Finely dispersed powders of the membranes were inserted into cuvettes, the thickness of which was 0.1 to 0.2 mm, with 17μmthick Mylar windows. Smallangle Xray scattering (SAXS) curves were obtained in a vacuum Kratky camera using a Cuanode tube. Recording of SAXS data has been carried out under the conditions of multiple scanning of a scintillation detector at scattering angles of 0.03° to 4.0°. The first treatment of the SAXS data was carried out by means of the FFSAXS11 program. The exclusion of parasitic scattering by the camera and cuvette windows, normalization of the scattered intensity to absolute units, and the introduction of the collimation correction were performed.
Standard contact porosimetry
The membranes were heated at 423 K before the measurements. Octane was used as a working liquid [8–11]. The curves of differential pore volume (V) distribution ($\frac{\mathit{dV}}{\mathit{d}\left(log\mathit{r}\right)}$, where r is the pore radius) were resolved by Lorentz components using the PeakFit v. 4.12 program. Treatment of the curves involved resolution within the intervals of pore radius of 1 to 100 nm and 1 to 10^{5} nm and comparison of the data for peaks with a maximum at ≈ 100 nm. Data adequacy is confirmed by coincidence of these maxima in two diapasons and high correlation coefficient (0.99). This procedure was necessary because the $\frac{\mathit{dV}}{\mathit{d}\left(log\mathit{r}\right)}$ values are rather low at 1 to 100 nm.
The particle density of the membranes (ρ_{p}) was determined using a pycnometer method [15], and the bulk density (ρ_{b}) was estimated from geometrical parameters.
Sorption capacity and potentiometric measurements
Ion exchange capacity of the membranes has been determined by their treatment with a HCl solution (100 mol m^{−3}), washing with deionized water followed by treatment with a NaOH solution (100 mol m^{−3}) and analysis of the eluate using an I160 M potentiometer and Cl^{−}selective electrode. The solution was neutralised with HNO_{3} before the measurements.
The difference of ${\mathit{E}}_{\u043c}\frac{\mathit{RT}}{\mathit{F}}ln\frac{{\mathit{a}}_{1}}{{\mathit{a}}_{2}}$ was found, and then its dependence on a_{±} (i.e. on activity of more concentrated solution, a_{2}) was plotted. At last, the transport number was calculated from a slope of the curve.
Electrodialysis
A solution containing NaCl (10 mol m^{−3}), the volume of which was 50 cm^{3}, circulated from the desalination compartment with a flow velocity of 1 cm^{3} s^{−1} (first liquid line). The second line provided circulation of the solution, which contained initially K_{2}SO_{4} (1,000 mol m^{−3}), through the cathode and anode compartments (second line). At last, a H_{2}SO_{4} solution (100 mol m^{−3}) circulated through the concentration compartment. The content of Cl^{−} and Na^{+} species in the solution being purified was controlled by means of ionselective electrodes. The removal degree of NaCl from the solution was calculated as $\frac{{\mathit{C}}_{\mathrm{i}}\mathit{C}}{{\mathit{C}}_{\mathrm{i}}}\times 100\mathrm{\%}$, where C is the concentration at time τ and C_{i} is the initial concentration. The current efficiency was calculated as $\frac{\mathit{zFn}}{\mathit{iA\tau}}\times 100\mathrm{\%,}$ where z is the charge number, n is the amount of electrolyte removed from the solution, i is the current density and A is the membrane area.
Discussion
Sol of zirconium hydroxocomplexes
During sol formation, fragmentation and defragmentation of nanoparticles occur simultaneously [18]. As a result, sol can contain several types of particles [19]. The first one is nonaggregated particles; their merging leads to formation of larger ones.
Structure of membranes
Porous structure of the composite membranes
Sample  Increase of mass,%  S_{m}, m^{2}kg^{−1}  Density, kg m^{−3}  Volume of micropores, cm^{3}g^{−1}  ϵ_{m},${\mathit{\u03f5}}_{\mathrm{m}}^{\mathbf{/}}$  

Total  Micropores  ρ _{p}  ρ _{b}  
TiO_{2}  820  80  4,260  3,270  3.0 × 10^{−4}  0.23  
TiO_{2}HZD2  2.4  3,340  990  4,260  3,350  1.5 × 10^{−3}  0.21 
TiO_{2}HZD7  4.6  10,430  5,120  4,260  3,420  5.0 × 10^{−3}  0.20 
where Δρ is the difference of electron densities between the particle and its environment, and R_{g} is the gyration radius, which has been determined from the slope of the linear part of lnI − q^{2} curve at q = 1.1 to 1.6 nm^{−1} (inset of Figure 5). The particle radius (r_{p}) was calculated as 1.29R_{g}[21, 22]. It was found, that r_{p} = 3 nm.
The logI − logq curve (where I is the intensity, q is the scattering vector), which has been obtained for pristine ceramics, is characterized by a long straight part within the interval of scattering vector of 2.82 × 10^{−2} to 1.1 nm^{−1}. This interval corresponds to particles II of the ceramic matrix. The slope of the curve is −4; this indicates smooth surface of these particles, which include no constituents [21, 22]. The curves demonstrate deviation from linearity under low q values; thus, the order of particle size is about 100 nm. Larger particles cannot be determined with a SAXS method.
Regarding the modified membranes, a small change of the slope of the linear part (q = 2.82 × 10^{−2} to 1.1 nm^{−1}) has been found. Thus, deposition of the modifier on particles II is inconsiderable. However, a change of slope of the lnI − q^{ 2 } curve at wider angles indicates the presence of HZD particles, which are smaller, than particles I of the matrix.
Porosity measurements
The results obtained with a pycnometer method allow us to determine porosity of the samples. Modification of the matrix causes an increase of bulk density of the membranes; however, no change of particle density has been found. Thus, the particle densities of the ion exchanger and matrix are equal. Porosity (ϵ_{m} for the initial matrix and ${\mathit{\u03f5}}_{\mathrm{m}}^{/}$ for the modified membranes) has been calculated as $1\frac{{\mathit{\rho}}_{\mathrm{p}}}{{\mathit{\rho}}_{\mathrm{b}}}$[15]. The porosity decreases in the order: TiO_{2} > TiO_{2}HZD7 > TiO_{2}HZD2.
The ratio of ${\mathit{V}}_{\mathrm{micr}}^{/}{\mathit{V}}_{\mathrm{micr}}$ values is 1:3.9 for TiO_{2}HZD2 and TiO_{2}HZD7 membranes, respectively (here, V_{micr} and ${\mathit{V}}_{\mathrm{micr}}^{/}$ are the volume of micropores for pristine and modified membranes, respectively). The ratio of $\frac{{\mathit{m}}^{/}\mathit{m}}{\mathit{m}}$ (here, m and m^{ l } are the mass of matrix and modified membrane, respectively) is 1:1.9. This is evidently due to different porous structures of HZD: more compact structure is attributed to the TiO_{2}HZD2 sample.
Parameters of globular model for the matrix and ion exchanger layer
Parameter  Homogeneous model  Heterogeneous model  

Matrix  Ionexchanger  Spheres  Matrix  Ionexchanger  
TiO_{2}HZD2  TiO_{2}HZD7  TiO_{2}HZD2  TiO_{2}HZD7  
ϵ, $\overline{\mathit{\u03f5}}$  0.23  0.29  0.46        
S, m^{2} kg^{−1}  820  1.05 × 10^{5}  2.09 × 10^{5}         
ϵ _{p}        I    0.03  0.42 
II  0.02  0.26  0.04  
III  0.21  
Packing  CFC or HXG  CBC  SC  I    CBC  SC 
II  CFC or HXG  
III      
$\mathit{S}\frac{\mathit{\u03f5}}{{\mathit{\u03f5}}_{\mathrm{p}}}$, $\mathit{S}\frac{\overline{\mathit{\u03f5}}}{{\mathit{\u03f5}}_{\mathrm{p}}}$, m^{2} kg^{−1}        I  7.77 × 10^{5}  2.27 × 10^{5}  
II  8,176  3.06 × 10^{4}  3.88 × 10^{4}  
III  201      
r_{ g }, nm  859  7  4  I    5  3 
II  86  23  20  
III  3,500    (≈400)  
r_{ n }^{a}, nm  133 (204)  1 (≤1)  1 (≤1)  I    1 (≤1)  1 (≤1) 
II  13 (8)  5 (8)  8 (4)  
III  542 (204)    (190)  
r_{ c }^{a}, nm  355 (1,730)  2 (2)  2 (2)  I    2 (2)  2 (2) 
II  36 (39)  9 (8)  13 (6)  
III  1,449 (1730)    (331) 
Calculation of porous structure according to globular models
Both homogeneous and heterogeneous globular models were applied to relate the maxima either to the matrix or to ion exchanger. The models have been developed by A.P. Karnaukhov; their main principles are described in [12–14]. Parameters of the models are radii of globules (r_{p}), pore necks (r_{n}) and pore cavities (r_{c}); the values of surface and porosity are also used. The magnitudes of r_{n} and r_{c} are calculated using special factors for each type of globule packing: r_{n} = 0.41r_{p} and r_{c} = 0.73r_{p} for simple cubic (SC), r_{n} = 0.22r_{p} and r_{c} = 0.29r_{p} for bodycentred cubic (BCC), and r_{n} = 0.15r_{p} and r_{c} = 0.41r_{p} for hexagonal (HXG) or facecentred cubic packing (FCC). A packing type is determined from the porosity (see Table 2).
According to the homogeneous model, the effective particle size was calculated as ${\mathit{r}}_{\mathrm{p}}=\frac{3}{{\mathit{\rho}}_{\mathrm{p}}\mathit{S}}$. The heterogeneous model provides analysis of integral pore size distributions [12–14]. Porosity caused by different types of particles is determined according to each semiwave. In the case of composite materials, it is difficult to recognize their components, when sizes of the particles are close to each other. We have proposed resolution of differential pore size distributions by Lorentz components; these functions provide the best agreement of experimental and calculated curves. The globular model was assumed to give pairs of peaks: the first maximum corresponds to narrowing of pores between globules (pore necks), and the second one is related to their widening (pore cavities). Then, the porosity, which is attributed to the peak, was found by means of peak integration. The surface of each type of pores was found as $\mathit{S}\frac{\mathit{\u03f5}}{{\mathit{\u03f5}}_{\mathrm{p}}}$ (matrix) and $\mathit{S}\frac{\overline{\mathit{\u03f5}}}{{\mathit{\u03f5}}_{\mathrm{p}}}$ (ion exchanger), where ϵ or $\overline{\mathit{\u03f5}}$ are the total porosity, and ϵ_{ p } is the porosity due to each type of particles.
Two additional peaks (1 to 3 nm) due to HZD are visible for modified membranes (see Figure 7b,c). Calculations give nanosized particles I, which evidently form a structure of the ion exchanger (particles I). Similar results were obtained using the homogeneous model. These particles are evidently associated into aggregates (particles II); pores between them give maxima at 8 nm for TiO_{2}HZD2 and 4 and 6 nm for TiO_{2}HZD7. Evidently, there are only HZD aggregates inside the matrix, since the SAXS data indicate no considerable change of surface of particles II of the matrix. Indeed, the size of particles II of the modifier is larger than the pores, which are formed by particles II of the matrix. In the case of TiO_{2}HZD2, the maxima for necks and cavities are overlapped with a peak attributed to the matrix and cannot be separated. A shift of the peak at 39 nm (TiO_{2}) to 52 nm (TiO_{2}HZD7) has been found. This indicates formation of larger particles III; their size can be estimated approximately from the peak at 52 nm, which is related to pore necks. These particles are evidently located in the cavities of pores, which are caused by the largest particles III of the matrix. The peaks at r > 100 nm for modified membranes are shifted towards lower r values in comparison with the matrix. This indicates HZD deposition inside macropores of the ceramics.
Potentiometric transport numbers of counter ions
Potentiometric measurements give additional information about the membrane structure. No membrane potential (E_{m}) has been registered for the matrix. E_{m} > 0 V in the case of modified samples. Since the membranes show anion exchange ability in acidic media [6, 7], Cl^{−} and H^{+} species are considered as counter and coions, respectively.
where t is the transport number of Cl^{−} in a solution, k is the shape coefficient (k = 2.8 for pores between globules), η is the surface charge density and C is the average value of concentrations of the solutions from two sides of the membranes. The surface charge density was estimated from sorption measurements as 0.07 C m^{−2} (TiO_{2}HZD2) and 0.18 C m^{−2} (TiO_{2}HZD7).
Formula (7) gives the transport number at which concentrations of the solutions from two sides of the membrane (C_{1} and C_{2}) are close to each other. The r value was plotted as a function of C_{2}C_{1}. Extrapolation of the curve to C_{2}C_{1} = 0 evidently gives the ‘real’ r magnitude, which has been estimated as 8 (TiO_{2}HZD2) and 2 (TiO_{2}HZD7) nm (Figure 8).
where ϵ_{0} is the dielectric permittivity of vacuum, and ϵ is the dielectric constant (80 for water). The concentration, which corresponds to t_{m} = 1, was found by extrapolation of t_{m} − C curves (inset of Figure 8). The r values were estimated as 7 nm (TiO_{2}HZD2) and 4 nm (TiO_{2}HZD7).
These ‘corks’ isolate macropores, which are recognized with the porosimetry method as predominant. Large particles of sol can penetrate the matrix during the first modification procedure. After blocking of the matrix pores, only the smallest particles are able to enter the membrane; moreover, they form the loosening structure of the ion exchanger.
Electrodialysis
Electrodialysis of the solution containing NaCl
Sample  After 5 min  After 30 min  After 60 min  

RD,%  CE,%  RD,%  CE,%  RD,%  CE,%  
TiO_{2}  1  5  7  5  9  3 
TiO_{2}HZD2  17  70  41  28  54  18 
TiO_{2}HZD7  23  95  75  51  95  34 
As seen from the table, the current efficiency (CE) decreased in time due to solution depletion. The highest removal degree (RD) and current efficiency were found for the TiO_{2}HZD7 membrane. This membrane is characterized by the smallest size of pores, which determine charge selectivity. Moreover, the highest surface charge density is reached for this separator.
Conclusions
The composite inorganic membranes, which contain the active layer of the HZD layer inside coarsepored ceramics, have been obtained. This has been proved by means of SEM, TEM and SAXS technique. The SCP method followed by resolution of differential pore size distribution, calculations according to homogeneous and heterogeneous geometrical models and potentiometric measurements allow us to determine structure of composite membranes. The approach, which is based on analysis of differential pore size distribution, gives a possibility to recognize each component of a composite. Application of integral pore distribution [12–14] is difficult, when the particle sizes of the constituents are close to each other.
The ceramic matrix is formed mainly with particles of micron size, which are distorted due to annealing and pressure. The ion exchanger consists of nanosized particles, the radius of which is 3 to 5 nm. The nanoparticles form aggregates (r_{p} = 20 to 23 nm). The larger particles form pores, which are responsible for charge selectivity. Radii of narrowing of these pores have been estimated as 4 to 8 nm; this is in agreement with porosimetry data. Charge selectivity is also due to ion exchange ability of HZD, which is retained under thermal treatment of the membranes. The materials can be used for electromembrane separation; the modified membranes demonstrate higher desalination degree and current efficiency in comparison with the pristine separator. Mechanical stability of the active layer is provided by its location inside pores of ceramics. As expected, the membranes can be used in aggressive media as well as for treatment of solutions containing organic substances.
Nomenclature
List of symbols
A area (m^{2})
a activity (mol m^{−3})
C concentration (mol m^{−3})
D diffusion coefficient (m^{2} s^{−1})
E_{m} membrane potential (V)
F Faraday constant (96,485 A s mol^{−1})
I intensity (cm^{−1})
i current density (A m^{−2})
i_{lim} limiting current density (A m^{−2})
k shape coefficient (dimensionless)
k_{m} mass transport coefficient (m s^{−1})
m mass of matrix (kg)
m^{ l } mass of modified membrane (kg)
n amount of species (mol)
q scattering vector (nm^{−1})
R gas constant (8.31 J mol^{−1} K^{−1})
R_{g} gyration radius (nm)
r radius of pores (m, nm)
r_{ c } radius of pore cavities (m, nm)
r_{ n } radius of pore necks (m, nm)
r_{ p } radius of globules (m, nm)
S surface (m^{2} kg^{−1})
S_{m} surface of a composite membrane (m^{2} kg^{−1})
T temperature (K)
t transport number through the solution (dimensionless)
t_{m} transport number through the membrane (dimensionless)
V pore volume (cm^{3} g^{−1})
V_{micr} volume of micropores in a matrix (cm^{3} g^{−1})
V^{/}_{micr} volume of micropores in a matrix (cm^{3} g^{−1})
z charge number (dimensionless)
Greek
ϵ porosity of a matrix (dimensionless)
ϵ^{/} porosity of a modified membrane (dimensionless)
ϵ_{d} dielectric constant (dimensionless);
ϵ_{p} porosity due to particles of chosen size (dimensionless)
ϵ_{0} dielectric permittivity of free space (8.85 × 10^{−12} F m^{−1})
η surface charge density (C m^{−2})
ν viscosity (m^{2} s^{−1})
ρ electron density (dimensionless)
ρ_{p} particle density (kg m^{−3})
ρ_{b} bulk density (kg m^{−3})
τ time (s)
ω linear flow velocity (m s^{−1})
Dimensionless criteria
Re Reynolds number (dimensionless)
Sc Schmidt number (dimensionless)
Sh Sherwood number (dimensionless)
Abbreviations
 CE:

current efficiency (%)
 HZD:

hydrated zirconium dioxide
 RD:

removal degree (%)
 SCP:

standard contact porosimetry.
Declarations
Acknowledgements
The work was supported by projects within the framework of programs supported by the government of Ukraine ‘Nanotechnologies and nanomaterials’ (grant no. 6.22.1.7) and by the National Academy of Science of Ukraine ‘Problems of stabile development, rational nature management and environmental protection’ (grant no. 3012) and ‘Fundamental problems of creation of new materials for chemical industry’ (grant no. 49/12).
Authors’ Affiliations
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