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Abstract

In this study, adsorption capacities of uremic toxins over Faujasite (HFAU) and Beta (HBEA) have been evaluated by varying the composition of solvent by using water, physiological, and sodium chloride solutions. HFAU was found to be more efficient in adsorption of these molecules. The adsorption results over HFAU were compared in various conditions to understand the adsorption mechanism. Thus, the adsorption mechanism was confirmed also by Fourier transform infrared and X-ray diffraction analysis, and it is found to be through the interaction of creatinine by hydrogen bonding on two types of sites on zeolites. Pseudo-second-order equation described well the adsorption kinetics data. Equilibrium isotherms were determined by Fowler–Guggenheim model. Finally, hydrophobic HFAU zeolite seems to be an efficient adsorbent; it is able to be easily regenerated under air, through retention of these initial adsorption properties.

Keywords

Adsorption uremia creatinine physiological solution dealuminated zeolite

Introduction

Kidney is a critical organ for the human organism, acting as a filter for all of the fluids in our body. Nevertheless, kidney failure or uremia has become a worldwide public health problem. It is the reason for thousands of deaths every year. Kidney failure is characterized by an accumulation of human metabolism products (uremic toxins) in blood unable to eliminate them down to a vital level. The artificial removal of these toxins is generally achieved by blood purification or dialysis (Brady et al., 2000; Levy et al., 1996; Thadhani et al., 1996).

Despite the progress made during the last years, the mortality of patients treated by dialysis is high. It is thus necessary to develop new methods to eliminate uremic toxins from blood. It is generally accepted that during hemodialysis uremic toxins are mainly eliminated from blood by diffusion through the pores of dialysis membranes. However, Lesaffer et al. (2000) found that during hemodialysis uremic toxins removal is only 29% for p-cresol versus 75% for urea and 66% for creatinine. Fortunately, recent studies had shown that uremic toxins may also be adsorbed onto dialysis membranes (Tijinka et al., 2013).

Materials efficient for adsorption in such application should possess high surface area, stability, toxicological safety, and selectivity. Inorganic microporous materials such as zeolites have been proved to be good adsorbent for uremic toxins. They are supposed to be non-toxic, stable, have channel systems corresponding to the dimensions of small uremic toxin molecules, and show selective adsorption with adsorption capacities potentially high (Koide et al., 1991; Malik et al., 2004; Yushin et al., 2006).

Several materials were investigated for the adsorption of uremic toxins. In vitro experiments have shown that some materials like activated charcoal (Lee and Hsu, 1990), hydrous alumino-silicates such as the phyllosilicates clays, zeolite (Wernert et al., 2005), and sepiolite (Grynpas et al., 1984) could be used as adsorbers of uremic toxins.

Wernert et al. (2005) mentioned the use of activated carbon as adsorbent for different uremic toxins. Activated carbon is characterized by its highly developed porosity, large surface area, variable characteristics of surface chemistry, and high degree of surface reactivity (Koubaissy et al., 2014).

Recently, silica-based mesoporous materials prepared by templating methods have shown interesting properties for biotechnologies. MCM-41, for example, presents high adsorption capacities for several small molecules like amino acids (Ernst et al., 2001), but also for larger ones like vitamin B12 (Diaz and Balkus, 1996), cytochrome C, and lysozyme (Hartmann, 2005). Nevertheless, their stability as well as their non-toxicity are not yet proved and their pore size is not selective toward small molecules (Bass et al., 2007).

Ernst et al. (2001) investigated the adsorption of amino acids including creatinine on silica-based mesoporous materials. These materials are highly ordered and have large surface area, which increase the probability of reactant to come into contact with the surface and interact.

Wernert et al. (2005) studied the adsorption of different uremic toxins on different types of zeolites. Bergé-Lefranc et al. (2009) showed in their study the efficiency of creatinine adsorption on mordenite framework type zeolites with different Si/Al ratios and H⁺ as charge compensating cation. Namekawa et al. (2014) studied the fabrication of zeolite–polymer composite fibers for the adsorption of creatinine. The zeolite with the highest creatinine adsorption capacity was found to be the beta type.

On the other hand, the zeolites have been used to transport biomolecules to viable cells, a performance that seems to show their non-toxicity of these solids (Dahm and Eriksson, 2004). Tsai and Syu (2005, 2011) studied creatinine adsorption on moleculary imprinted polymers (MIP), which are polymers that contain highly selective recognition sites as a result of polymerization around a template molecule bound covalently or non-covalently to functional monomers. MIPs are characterized by their high selectivity.

The adsorption of uremic toxins on dialysis membranes have been only poorly investigated, as general interest has always been focused on their dialysis properties. Moreover, the need to gather detailed insights into the behavior of zeolite with the sorbate molecules on the molecular scale has influenced the usage of molecular simulation studies. Thus, the present study was undertaken to introduce the application of zeolite for artificial kidney and adsorption property of this material for eliminating uremic toxins. The present work was devoted to evaluate the efficiency of the adsorption of some uremic toxins on zeolites and the effect of various parameters on the efficiency of adsorption. Furthermore, we have tried to highlight the influence of the structure of the zeolite on adsorption of uremic toxins.

Experimental part

Adsorbents

Table 1 illustrates the various properties of zeolites purchased from Zeolyst international used during the study.

Table 1.

Properties of zeolites used.

Adsorbent	Formula	Pore size (Å)	Particle size (µm)	Si/Al	S_BET (m²/g)	Pore volume (cm³.g⁻¹)
Adsorbent	Formula	Pore size (Å)	Particle size (µm)	Si/Al	S_BET (m²/g)	Micropore	Mesopore
HBEA₁₉	H_3.2Al_3.2Si_60.8O₁₂₈	6.6 × 6.7 and 5.6 × 5.6	0.4 and 22	19	650	0.224	0.083
HFAU₁₉	H_9.6Al_9.6Si_182.5O₃₈₄	7.4 × 7.4	10	19	740	0.245	0.11

Chemicals

Creatinine was purchased from Uni-Chemicals. Urea was purchased from BDH chemicals. The physiological solution used was the Dulbecco phosphate-buffered saline (D-PBS) purchased from Sigma-Aldrich. Uremic toxins solutions were prepared using distilled water. The main physical and chemical properties of uremic toxins studied are illustrated in Table 2.

Table 2.

Physical and chemical properties of uremic toxins.

Solute	Structure	MW (g.mol⁻¹)	pK_a	λ_max (nm)	Size (nm)
Solute	Structure	MW (g.mol⁻¹)	pK_a	λ_max (nm)	x	y	z
Creatinine C₄H₇N₃O		113	4.4	235	0.71	0.81	0.30
Urea H₂NCONH₂		60	0.1	200	0.56	0.63	0.30

Analysis

X-ray diffraction (XRD)

Powder XRD is one of the most powerful techniques for qualitative and quantitative analysis of crystalline compounds. The adsorbent structure before and after adsorption was determined by Powder XRD on Brucker D8 Advanced diffractometer with CuKα radiation at λ = 1.5406 Å.

Infra-red analysis

Fourier transform infrared (FTIR) spectroscopy is a technique, which is used to identify the surface functional groups and adsorptive form of organic compounds. The measurements were recorded using a Bruker IFS66S with a Bruker SpectraTech Baseline DRIFT module, for doing DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectra. DRIFT spectra were recorded between 4000 and 350 cm⁻¹.

Adsorption experiments

Experiments were performed using a batch equilibration technique. Uremic toxins solutions were prepared in the concentration range of 0.1–5 mM in distilled water and once for creatinine in a physiological solution. The composition of physiological solution used is NaCl (8 g/L), CaCl₂ (0.1 g/L), MgCl₂ (0.1 g/L), KCl (0.2 g/L), KH₂PO₄ (0.2 g/L), and NaH₂PO₄ (2.16 g/L).

The experimental isotherms were measured at room temperature and for creatinine at 30 and 37℃ by using a thermostat shaker heated at temperature. Moreover, further measurements were done for creatinine at different pH values; the pH of the solution was adjusted using 1 M HCl or NaOH solutions. Measurements done in the presence of NaCl were prepared from NaCl purchased from Uni-chemicals by adding it directly to the creatinine samples.

For each equilibrate isotherm, 15 mg of the adsorbent was added to 20 mL of solution to equilibrate for 24 hours in a batch equipment. Then, the zeolite was separated from the solution by centrifugation.

The amount adsorbed of the adsorbates can be calculated by the following equation:

Q = V (C_{0} - C) / m

(1)

where m: mass of zeolite (g); V: volume of solution (L); C: concentration after adsorption (mg/L); C₀: initial concentration (mg/L); Q: amount adsorbed (mg/g of zeolite).

Kinetics study

A solution of creatinine with zeolite samples was placed on a multishaker shaken at 210 rpm and at room temperature. Samples were withdrawn at different time intervals and the zeolite was separated from the samples by centrifugation. Then absorbance was measured for each sample at λ = 236 nm using UV-spectrometer.

The kinetics of the adsorption of creatinine on zeolite was successfully described by the pseudo-second-order kinetic model that was proposed by Blanchard et al. (1984) and is also based on the sorption capacity of the solid phase and is expressed as:

\frac{{dq}_{t}}{dt} = K (q_{e} - q_{t})

(2)

Theoretical model

The adsorption of creatinine on zeolite adsorbents has been successfully described by Fowler and Guggenheim (1965) following the equation:

KC = \frac{θ}{1 - θ} \exp (\frac{2 θ W}{RT})

(3)

Linearized Ln [\frac{C (1 - θ)}{θ}] = - Lnk + [\frac{2 θ W}{RT}]

where K: equilibrium constant for adsorption of the adsorbate on an active site; C: concentration at equilibrium adsorption; W: empirical interaction energy between two molecules adsorbed on nearest sites; R: ideal gas constant (8.314 J.mol⁻¹.K⁻¹); T: thermodynamic temperature.

Fowler–Guggenheim equation is taking into account the lateral interaction. This model is based on the hypothesis that interaction energy is constant and independent of fractional coverage of the surface θ, and hence the number and distribution of adsorbed molecules.

Results and discussion

Adsorption of creatinine

Effect of adsorbent type

Two different zeolites, HFAU and HBEA, were investigated for the adsorption of creatinine at the same conditions (room temperature, contact time t = 24 hours, V/m = 1.33). The results are shown in Figure 1.

Figure 1.

Isotherms of creatinine adsorption on different zeolite types at room temperature.

Figure 1 shows that creatinine adsorption is favored by using HFAU zeolite although it has the same Si/Al ratio as HBEA zeolite. This result can be explained by the difference in size between the adsorbents’ pores. Creatinine has large size (7.1 × 8.1 × 0.3 Å) and the size of HBEA pores (5.6 × 5.6 and 6.7 × 6.7 Å) are smaller than HFAU pores (7.4 Å). Therefore, HFAU has pore size more adequate for creatinine dimensions, and thereby has adsorption efficiency better than HBEA.

The adsorption isotherms were described by Fowler–Guggenheim model and the different parameters are listed in Table 3. The plot of Ln [C(1 − θ)/θ] against 2θ/T give a linear relationship, from which W and K can be determined. The Fowler–Guggenheim model obtained for creatinine sorption over HBEA and HFAU showed excellent correlation R²= 0.9937 and 0.9979, respectively (Figure 2).

Table 3.

Values of parameters of Fowler–Guggenheim model of creatinine over HFAU and HBEA zeolites.

pH	HFAU	HBEA
K (L/g)	4.2	2.7
W (KJ/mol)	−3.4	−3.1
Qads (mg/g)	70	38

Figure 2.

Fowler–Guggenheim linearized isotherm model for creatinine adsorption on HBEA and HFAU.

Effect of adsorbent dosage

The removal of creatinine by HFAU zeolite at different adsorbent doses (15–30 g in 20 mL) for two creatinine concentrations (2 and 3 mM) was studied. The other parameters were kept fixed (contact time = 24 hours, room temperature). The results in Figure 3 show that the adsorption capacity of creatinine decreased rapidly with increase in the dose of zeolite. However, the percentage removal of creatinine increased with the increase in HFAU zeolite dose. The increased removal percentage of creatinine with increasing zeolite dosage may be due to the availability of greater amounts of active sites of the adsorbent.

Figure 3.

Variation in the sorption capacity and percent of removal of creatinine versus the adsorption dose of HFAU zeolite for two different creatinine concentrations at room temperature (graphs a: 2 mM; graphs b: 3 mM).

Furthermore, higher adsorbent dose will result in lower adsorption capacity (qe). At low zeolite dose, all types of sites are entirely exposed and the adsorption on the surface is saturated faster, showing a higher qe value (qe = 71 mg/g at 15 mg zeolite for 3 mM creatinine). But at higher adsorbent dose, the availability of higher energy sites decreases with a larger fraction of lower energy sites occupied, resulting in a lower qe value (qe = 50 mg/g at 30 mg zeolite for 3 mM creatinine). Similar results were obtained by Kara et al. (2007) for adsorption of reactive textile dyes by fly ash. It is noticed from this study that the optimum mass of zeolite is 15 mg for 20 ml solution. Accordingly, the liquid/solid ratio used in the following studies is taken as 1.33.

Kinetics study

Figure 4 shows the adsorption kinetics of creatinine removal at room temperature by plotting the creatinine uptake capacity, q, versus time at 4.5 mM initial creatinine concentration. For the given concentration, the amount of creatinine adsorbed by zeolite increased rapidly with time in the beginning (first 300 minutes), then progressively at a slower rate and finally a plateau has been reached after approximately 24 hours, which is called the equilibrium time.

Figure 4.

The pseudo-second-order adsorption kinetics of creatinine, represented by the solid line, with the initial concentration of 4.5 mM over HFAU at room temperature.

The plot of t/q against t give a linear relationship, from which qe and K can be determined from the slope and intercept of the plot. The pseudo-second-order kinetic model obtained for creatinine sorption showed excellent correlation R²= 0.9987 (Figure 5). So the change in the sorption capacity with time is found to fit the pseudo-second-order equation relationship, which is based on the adsorption capacity of solid phase.

Figure 5.

The pseudo-second-order linearized kinetics model of creatinine adsorption on HFAU.

Furthermore, it is observed that the time needed for adsorption to reach equilibrium is about 24 hours. Consequently, this time is taken into consideration being the optimum contact time in further studies done.

Creatinine adsorption in a biological system

Next step was to mimic biological system conditions by investigating creatinine adsorption at different conditions resembling the conditions of the blood:

At body temperature 37℃.

From physiological solution (D-PBS) similar to the composition of plasma in blood

Effect of temperature

Creatinine adsorption experiments were carried out at 20, 30, and 37℃ in thermostat shaker while keeping all other parameters constant, such as adsorbent concentration (V/m = 1.33) and contact time (24 hours). The result is shown in Figure 6. It is observed in the figure that the adsorption capacity decreases with the increase of temperature.

Figure 6.

Isotherms of creatinine adsorption on HFAU at different temperatures.

That is also observed with the parameters of the Fowler–Guggenheim model (Table 4). K is the equilibrium constant for adsorption of the adsorbate on an active site. It represents the interaction between the adsorbate and the adsorbent. Therefore, the highest K value at room temperature (K = 2.7) than that at T = 30℃ (2.3) and T = 37℃ (2.1) indicates the most important interaction between creatinine molecules and HFAU zeolite at lower temperature. The equilibrium constant K was determined using Excel software.

Table 4.

Parameters of Fowler–Guggenheim model of creatinine over HFAU at different temperatures.

T (℃)	20	30	37
K (L/g)	2.7	2.3	2.1
W (KJ/mol)	−3.4	−3.8	−4.0
Qads (mg/g)	70	40	30

As the adsorption process is spontaneous, the free energy change (ΔG) must be less than zero. Moreover, ΔH is also negative since the adsorption is exothermic (Table 5). According to equilibrium shift principle, the increase of temperature encumbers adsorption since it causes a decrease in the value of the free energy change and thus a decrease in the spontanousity of the adsorption process.

Table 5.

Thermodynamic parameters for the adsorption of creatinine over HFAU.

T (℃)	K	ΔG° (J/mol)	ΔH° (KJ/mol)	ΔS° (J/mol.K)
20	2.7	−2419	−11.2	−30.1
30	2.3	−2098	−11.2	−30.1
37	2.1	−1912	−11.2	−30.1

Effect of solution composition

The adsorption isotherms of creatinine onto HFAU are given in Figure 5 for aqueous and physiological media (D-PBS). Apparently, L-type adsorption isotherms are obtained in both cases, but with different affinities. In aqueous solution, high affinity of the molecule toward the adsorbent is found with a maximum adsorption capacity of about 70 mg/g. The observed adsorption capacity is high compared with that observed on activated carbon and carbon nanotubes about (28 mg/g and 25 mg/g, respectively, each) (Ye et al., 2007)) or compared with selectively adsorbing imprinted polymers (about 11.31 mg/g) (Tsai et al., 2005). In the buffered and isotonic media D-PBS, the initial affinity of creatinine onto Faujasite is reduced. The adsorption capacity at high concentrations is around (50 mg/g).

The two isotherms of creatinine adsorption on HFAU in distilled water and in D-PBS are presented in Figure 7. A change of behavior between aqueous and D-PBS media can be observed. This decrease of affinity may indicate that there is a competition for adsorption between creatinine and other components of D-PBS, in fact zeolites become cation exchanger and a competition between creatinine and cations takes place. To understand this influence of ion presence, adsorption isotherms were determined from sodium chloride solutions, because sodium represents the main cation in physiological solutions. NaCl was added to creatinine solution and adsorption experiments were carried out with the same conditions on different concentrations. The results are presented also in Figure 5.

Figure 7.

Isotherms of creatinine adsorption on HFAU in distilled water, in NaCl, and in D-PBS media at room temperature.

The amount of creatinine adsorbed onto HFAU in the presence of sodium chloride is systematically lower than that determined in pure water. Nevertheless, adsorbed amounts are higher than those observed in the presence of D-PBS, indicating the influence of other components constituting D-PBS. The adsorption isotherms were described by Fowler–Guggenheim model and the different parameters are listed in Table 6.

Table 6.

Parameters of Fowler–Guggenheim model of creatinine on different solutions.

Type of solution	Water	Physiological solution	Water + NaCl
K (L/g)	2.7	2.45	2.55
W (KJ/mol)	−3.4	−3.3	−3.6
Qads (mg/g)	70	52	60

Effect of pH

It is well known that pH could affect the protonation of the functional groups on the adsorbent as well as the adsorbate. The effect of pH on creatinine adsorption capacity is shown in Figure 8. As the pH of the creatinine solution decreased from 7 to 4, the maximum adsorption capacity of creatinine increased from 70 to 112 mg/g. At pH higher than 7, i.e. pH = 11, the adsorption capacity decreased from 70 to 22 mg/g. The results showed strong pH dependence of adsorption. Creatinine has two pKa values: 4.83 and 9.2. The predominant species in each range are shown in Table 7.

Figure 8.

Effect of pH on creatinine adsorption on HFAU zeolite at room temperature.

Table 7.

Creatinine pKa values and the predominant species at different pH ranges.

	pH < 4.83 (pka₁)	(pka₁) 4.83 < pH < 9.2 (pka₂)	pH > 9.2 (pka₂)
Predominant species

At pH = 7, creatinine is found mostly in its molecular form, which are adsorbed over zeolite by H-bond between the oxygen of the zeolite and the hydrogen of the NH-group of creatinine. However, at low pH, the predominant species of creatinine are in protonated form. Protonated creatinine molecules which are more acidic would form stronger H-bonds explains the higher value of adsorption capacity in this case of acidic medium.

Whereas in basic medium, creatinine is predominantly found in its deprotonated form, which is more basic and thus form weaker H-bond with the oxygen of the HFAU zeolite. As a result, lower Q values will be obtained, which is clearly shown in Figure 8.

Mechanism of creatinine adsorption

The affinity of creatinine for HFAU can be explained both by hydrophobic interactions and hydrogen bond interactions. Since there are Bronsted acid sites on the zeolite wall, creatinine acts both as a donor and acceptor. In fact, hydrogen bonds can be formed between the zeolite surface proton and the creatinine oxygen and between the creatinine hydrogen of NH- and the zeolite oxygen atoms.

At the same time, the absence of cations in the zeolite channels implies that there is no large, local dipolar moment. Therefore, the zeolite can be seen as an overall non-polar structure interacting with non-polar molecular groups, such as the creatinine methyl group, by attractive hydrophobic interactions.

When HFAU is in medium containing NaCl, Na⁺ cations make ion exchange with the H⁺ found in HFAU. Therefore, creatinine will no more interact through the carbonyl site due to the absence of zeolite surface proton. Accordingly, interaction will decrease and so does the adsorption capacity.

In the presence of physiological solution, more cations are present including K⁺, which has higher tendency for ion exchange than Na⁺ (Breck, 1974). Therefore, more ion exchange would occur by K⁺ and Na⁺ causing less interaction and thus less adsorption capacity. These results are confirmed by FTIR and XRD analysis for HFAU before and after adsorption.

XRD analysis

XRD patterns obtained for HFAU before and after adsorption are shown in Figure 9. The XRD pattern of HFAU before and after adsorption did not show any modification of the structure or the apparition of new peak, this indicates the physical character of adsorption.

Figure 9.

XRD patterns of HFAU before and after creatinine adsorption.

FTIR analysis

The sorption mechanism of creatinine on HFAU zeolite was investigated using IR spectroscopy. Analysis was done for samples before and after adsorption. FTIR spectra of HFAU are shown in Figure 10. In HFAU spectrum before adsorption, the band at 457 cm⁻¹ corresponds to Si–O bending. The band observed at 609 cm⁻¹ is for Tetrahedron stretching vibration. Si–O–Si symmetrical stretching vibrations lead to the appearance of the band at 832 cm⁻¹. The strong absorption bands near 1200–1000 cm⁻¹ were observed and attributed to Si–O–Si asymmetric stretching vibrations. The band at 1638 cm⁻¹ corresponds to inner surface OH stretching vibrations. The broad bands within the 2900–3723 cm⁻¹ interval encompassing several not-well-resolved vibrations were ascribed to hydrogen-bonded SiOH groups. The band at 3639 cm⁻¹ corresponds to external silanol groups.

Figure 10.

Infra-red spectrums of HFAU before (a) and after creatinine adsorption (b).

By comparing the two HFAU spectra before and after creatinine adsorption, several peaks show shift in the frequency of absorption bands. This is due to the interaction of the corresponding functional groups with creatinine. Significant shifts were observed for the bands corresponding to Si–O bending, Si–O–Si symmetrical stretching, Si–O–Si asymmetrical stretching, indicating that creatinine interacts with zeolite at Si–O sites.

In the region of 1815–1479 cm⁻¹ shown in Figure 11, new peaks appear in the spectrum of HFAU after creatinine adsorption. Upon carrying out infrared analysis for creatinine before its adsorption, it was observed that these new-appearing peaks correspond to creatinine absorption bands with a certain down-shift. The band at 1491 cm⁻¹ attributes to CH₂ vibration. The band at 1552 cm⁻¹ attributed to C–NH₂ vibration mode in creatinine. Infrared spectrum shows also a number of bands at 1710 cm⁻¹ assigned to C=N vibration; 1793 cm⁻¹ is ascribed to C=O vibrations. The shifts in the peaks of creatinine after adsorption indicate its interaction with the zeolite. Moreover, after the adsorption, the bands corresponding to (Si–O–Si) between 1000 and 1200 cm⁻¹ were strengthened. This assures again that creatinine was adsorbed principally by hydrogen bonding.

Figure 11.

Infra-red spectra of HFAU before and after creatinine adsorption along with creatinine spectrum in the range of 1815–1500 cm⁻¹.

Adsorption of urea

Urea adsorption over HFAU and HBEA were done with the same conditions of contact time, adsorbent dosage, and temperature. The adsorption of urea on zeolites are illustrated in the adsorption isotherms given in Figure 12. As shown in the figure, urea adsorption has a relatively high adsorption capacity over HFAU reaching a maximum of 218 mg/g. The lower adsorption capacity found over HBEA can be principally due to BET surface area (650 m²/g versus 740 m²/g for HFAU zeolite).

Figure 12.

Adsorption isotherms of urea on HFAU and HBEA at room temperature.

The comparison of maximum adsorption capacities between urea and creatinine on HFAU is demonstrated in Figure 13. It is clearly noticed that urea has a much higher adsorption capacity than creatinine. This could be explained by the difference in size between the adsorbates. Urea has a size of 0.56 × 0.63 × 0.30 nm while creatinine 0.71 × 0.81 × 0.30 nm. So urea is smaller than creatinine and HFAU pores are of size 0.74 nm, which is more adequate for urea. Therefore, urea would easily reach the zeolite pores and get adsorbed by H bond between the hydrogen of –NH₂ and the oxygen of the HFAU zeolite, whereas creatinine faces more difficulties in adsorption over HFAU because of its larger size.

Figure 13.

Comparison between creatinine and urea adsorption on HFAU at room temperature.

Regeneration of the FAU zeolite

After adsorption, HFAU was regenerated under an air flow at 380℃ for 12 hours. Regeneration was studied after urea adsorption and successive adsorption-regeneration sequences were performed (Figure 14). After five cycles, no significant loss (around 2%), so the initial adsorption capacity of the zeolite was approximately restored. These results indicate that the regeneration is efficient and that the zeolite retains all of its properties after this treatment.

Figure 14.

Effect of thermal regeneration on the maximal adsorption capacity of urea on HFAU zeolite.

Conclusions

During hemodialysis, uremic toxins are eliminated either by diffusion or by adsorption. In this study, adsorption capacities of creatinine and urea over dealuminated HFAU and HBEA have been evaluated. Creatinine adsorption was studied on two types of zeolites. HFAU was found to be more efficient in creatinine adsorption. The adsorption results over HFAU were compared in various conditions to understand the adsorption mechanism. Adsorption capacity decreased when adsorption was done in biological conditions (T = 37℃, physiological solution). Thus, the adsorption mechanism, which was confirmed by FTIR and XRD analysis, is found to be through the interaction of creatinine by hydrogen bonding on two sites, between zeolite surface proton and the creatinine oxygen and between the creatinine hydrogen of NH- and the zeolite oxygen atoms.

The study on creatinine adsorption showed that the pH of the solution has a significant effect on adsorption. It has shown that creatinine is very poorly retained on the zeolite at very basic pH and is greatly adsorbed at an acidic one. Urea adsorption on HFAU was compared with creatinine adsorption. Urea adsorption showed an efficient and more important adsorption capacity than that of creatinine.

Removal of uremic toxin molecules such as creatinine and urea by adsorption is not a competitive method to hemodialysis, but an alternative process to eliminate one particular toxin molecule specifically. These results are a promising step in the improvement of dialysis procedure based on a specific adsorption process. Therefore, hemodialysis procedures may be completed by adsorption units containing zeolites to remove toxin molecules selectively.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Adsorption of uremic toxins over dealuminated zeolites

Abstract

Keywords

Introduction

Experimental part

Adsorbents

Chemicals

Analysis

X-ray diffraction (XRD)

Infra-red analysis

Adsorption experiments

Kinetics study

Theoretical model

Results and discussion

Adsorption of creatinine

Effect of adsorbent type

Effect of adsorbent dosage

Kinetics study

Creatinine adsorption in a biological system

Effect of temperature

Effect of solution composition

Effect of pH

Mechanism of creatinine adsorption

XRD analysis

FTIR analysis

Adsorption of urea

Regeneration of the FAU zeolite

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References