Characteristics of mPHEMA beads
mPHEMA beads (in the size range of 80–120 μm) carrying alizarin yellow were prepared as a specific affinity adsorbent for removal of Al(III) from the water which was used for preparation of dialysis solution. mPHEMA beads used in this study were prepared and characterized in our earlier study [24
]. The main criteria of selection of PHEMA is due to its mechanical strength and chemical stability. With the goal of testing the mechanical stability of the magnetic beads, a sample of these magnetic beads was treated in a ball mill for 60 min. Negligible percentage of the sample was broken. The dry density of the magnetic beads was measured as 1.27 g/cm3
. The magnetic beads are crosslinked hydrogels. They do not dissolve in aqueous media, but do swell, depending on the degree of cross-linking and on the hydrophilicity of the matrix. The equilibrium swelling ratio (the ratio of the volumes of the microbeads before and after swelling) of the beads used in this study is 34%. The simple incorporation of water weaken the secondary bonds within the hydrogels. This enlarges the distance between the polymer chains and causes the uptake of water. It should be mentioned that the water uptake properties of the mPHEMA beads did not change after Alizarin Yellow attachment.
After the attachment of the dye (i.e., alizarin yellow) the size of the swollen beads did not change, but the colour became dark yellow, which is a clear indication of the incorporation of the dye molecules in the structure of the mPHEMA microbeads. As shown in our previous paper, the dye molecules were attached to the mPHEMA beads by covalent bonding via hydroxyl groups [24
]. The mPHEMA beads containing 208 μmol alizarin yellow/g polymer, which was the maximum amount of dye-attachment that we have reached, were used in this study. Alizarin Yellow release from the mPHEMA beads was also monitored continuously. There were no dye release in any of the adsorption and desorption media, which assured that the cleaning procedure used for removal of physically adsorbed alizarin yellow molecules from the mPHEMA beads was satisfactory.
The adsorption capacity at different flow-rates are given in Figure . The adsorption capacity decreased significantly from 17.2 mg/g to 6.9 mg/g polymer with the increase of the flow-rate from 0.5 ml/min to 3.0 ml/min. One of the explanation for such phenomenon would be a faster ligand-metal ion (i.e., alizarin yellow) dissociation rate compared to the association rate. Hence, the adsorbate (i.e., Al(III) ions) would pass through the magnetically stabilized column without adsorption at high flow-rate. Second explanation could be that the increased nonideal flow hydrodynamics of liquid phase and the solid phase for magnetically stabilized fluidized bed. These phenomena can be summarized by the increase of the axial dispersion coefficient in the axial dispersion model [25
Effect of flow-rate on Al(III) adsorption. Alizarin yellow loading: 208 μmol/g; Al(III) concentration: 50 mg/L; pH: 5.0; T: 25°C.
Figure shows the adsorption profile of Al(III) ions. The amount of Al(III) ions adsorbed per unit mass of the polymer (i.e. adsorption capacity) increased first with the initial concentration of Al(III) ions then reached a plateau value at about an initial Al(III) ions concentrations of 50 mg/L, which represents saturation of the active attachment sites (which are available for Al(III) ions) on the beads. The maximum adsorption capacity of Al(III) ions was of 647 μmol/g (17.5 mg/g). Unit mass of the mPHEMA beads carries 208 μmol alizarin yellow which was found by elemental analysis. From the mass-stoichiometry, it seems that one attached alizarin yellow molecule interacts with around three Al(III) ions. Since alizarin yellow has seven coordinating sites of a single sulphur and six oxygen atoms, it can form a ternary complex which is coordinated with water molecules at vacant coordination sites of metal-alizarin yellow complexes.
Effects of Al(III) concentration on Al(III) adsorption. Alizarin yellow loading: 208 μmol/g; Flow-rate: 0.5 ml/min; pH: 5.0; Adsorption time: 60 min; T: 25°C.
It should be noted that the nonspecific adsorption (adsorption on plain mPHEMA beads) of Al(III) ions was relatively low (0.63 mg/g). mPHEMA beads do not contain ion exchange or chelating groups. Preferred coordination structure and preferred coordinating ligand atom may be utilized for this adsorption. Al(III) ions may interact with Oxygen atoms as the ligand. Diffusion of Al(III) ions into the swollen polymeric structure and retention in the pores may also contribute to this nonspecific Al(III) adsorption.
An adsorption isotherm is used to characterize the interactions of each molecule with the adsorbent. In this case it provides a relationship between the concentration of the Al(III) ions in the solution and the amount of Al(III) ions adsorbed on the solid phase when the two phases are at equilibrium. The Langmuir adsorption model assumes that the species are adsorbed at a fixed number of well-defined sites, each of which is capable of holding only one molecule. These sites are also assumed to be energetically equivalent, and distant from each other so that there are no interactions between molecules adsorbed on adjacent sites.
Adsorption isotherms were used to evaluate adsorption properties. The Langmuir adsorption isotherm is expressed by Equation 1. The corresponding transformations of the equilibrium data for Al(III) gave rise to a linear plot, indicating that the Langmuir model could be applied in these systems and described by the equation:
Q = Qmax. b . Ceq / (1 + bCeq) (1)
where Q is the adsorbed amount of Al(III) (mg/g), Ceq is the equilibrium Al(III) concentration (mg/mL), b is the Langmuir constant (mL/mg) and, Qmax is the maximum adsorption capacity (mg/g). This equation can be linearized so that
Ceq/Q = 1/(Qmax. b) + Ceq/Qmax. (2)
The plot of Ceq versus Ceq/Q was employed to generate the intercept of 1/Qmax.b and the slope of 1/Qmax.
The maximum adsorption capacity (Qmax) data for the adsorption of Al(III) was obtained from the experimental data. The correlation coefficient (R2) was 0.989. The Langmuir adsorption model can be applied in this affinity adsorbent system. Maximum adsorption capacities determined using Langmuir isotherms show that dynamic capacity (25.3 mg/g) was somewhat higher than the batch capacity (12.6 mg/g). The dissociation constants for Al(III) were determined using the Langmuir isotherm equation to be 27.3 mM (MSFB) and 6.7 mM (batch system), indicating medium affinity, which was typical for pseudospecific affinity ligands.
Effect of pH
Metal ion adsorption onto specific adsorbents is pH dependent. In the absence of complexing agents, the hydrolysis and precipitation of the metal ions are affected by the concentration and form of soluble metal species. The solubility of metal ions is governed by hydroxide or carbonate concentration. Hydrolysis of metal ions becomes significant at approximately pH 7.5–8.5. Therefore, in the present study, we changed the pH range between 2.0–7.0. The effect of pH on the Al(III) adsorption of this alizarin yellow-attached mPHEMA beads is also shown in Figure . The magnetic mPHEMA beads exhibited a low affinity in acidic condition (pH < 4.0), a somewhat higher affinity between pH 4.0 and 7.0. High adsorption capacities at around neutral pH values imply that Al(III) ions interact with dye molecules not only through the oxygen atoms by chelating, but also electrostatically through sulfonate groups, which are ionized at neutral pH.
Effects of pH on Al(III) adsorption. Alizarin yellow loading: 208 μmol/g; Flow-rate: 0.5 ml/min; Al(III) concentration: 50 mg/L; Adsorption time: 60 min; T: 25°C.
Competitive adsorption of the metal ions from tap water in Ankara and dialysis water (reverse osmosis) was also investigated. The water containing different amounts of each metal ion was treated with dye beads in MSFB. Table and show the adsorbed amounts for each metal ion. The adsorption capacity of the dye-attached mPHEMA beads for Cu(II) and Al(III) ions was higher than that for other ions. But it should also be noted that the extent of adsorption of each type of metal ion is strongly dependent upon their relative concentrations within the medium.
Aluminium removal from the tap water.
Aluminium removal from dialysis water.
The World Health Organization (WHO) and the European Community (EC) guide values for Al(III) ions for tap water is 200 ng/ml [26
]. Al(III) concentrations both in tap water and dialysis water are below this value. It should be noted that polymer treatment (i.e, adsorption) significantly decreases the metal content and these purified waters can be used safely especially for the preparation of dialysis solutions. Magnetic dye-affinity beads exhibits the following metal ion affinity sequence: Al(III) > Cu(II) > Fe(III) > Zn(II).
Desorption and repeated use
Desorption ratios were very high (up to 97.6%) with the eluant system and under conditions used. When HNO3 is used as a desorption agent, the coordination spheres of chelated Al(III) ions are disrupted and subsequently Al(III) ions are released from the solid surface into the desorption medium. Therefore, we conclude that HNO3 is a suitable desorption agent for the dye adsorbents, and allows their repeated use. In order to show the reusability of the dye-attached mPHEMA beads, adsorption-desorption cycle was repeated ten times by using the same sample of affinity adsorbent. As can be seen from Figure , adsorption capacities did not noticeable change during the repeated adsorption-desorption cycles.
Repeated use of dye-attached mPHEMA beads. Alizarin yellow loading: 208 μmol/g; Flow-rate: 0.5 ml/min; Al(III) concentration: 50 mg/L; Adsorption time: 60 min; pH: 5.0; T: 25°C.
Comparison of magnetically stabilized fluidized bed and batch system
As can be seen in Figure , maximum Al(III) adsorption from aqueous solution is 11.8 mg/g for batch system and 17.5 mg/g for MSFB system. These results indicated that the adsorption capacity obtained in MSFB system is considerably higher than obtained in batch sytstem. This means, in equilibrium binding experiments, maximum capacity was 38.8% lower as compared to the value obtained in MSFB. This result could be explained in two ways. (i) The dye ligand-Al(III) dissociation rate in the batch system is higher than the association rate in the MSFB system. (ii) Alizarin yellow ligand is found both on the surface and in the pores of the magnetic beads. In the presence of flow, the Al(III) solution is forced from the surface into the pores thus eliminating the surface diffusion.
Comparison of MSFB and batch system. Alizarin yellow loading: 208 μmol/g; Flow-rate: 0.5 ml/min; Al(III) concentration: 50 mg/L; Adsorption time: 60 min; pH: 5.0; T: 25°C.