3.1. Adsorbent Characterization
Elemental analysis does not provide enough information to determine the presence of phenyl groups. The residual surfactant and the alcohol adsorbed used in the extraction procedure can interfere in the carbon content (%C). Therefore, the phenyl incorporation cannot be determined by CHN elemental analysis. However, the increasing %C when phenyl groups are grafted was observed. Thus, for the extracted materials in which the amount of surfactant present is residual, the observed differences in the %C are remarkable, being this C % higher for material Φ5
-PPH (9.04%) and reduced for Φ50
-PPH (2.62%), as displayed in Table 1
The concentration of phenyl groups in the synthesized material was determined by UV-VIS spectroscopy at λ = 210 nm in hexane. As seen in the UV-VIS spectra (Figure 1
), the absorption due to the phenyl groups decreases with the TEOS/Φ molar ratio. A calibration curve was performed by hydrolysis of different amounts of phenyl triethoxysilane and the amount of phenyl groups incorporated in each material was determined (Table 2
). As expected, the highest observed incorporation of phenyl groups occurred with sample Φ5
-PPH, although the found value (0.047 mmol/g) was below the theoretical one if all the phenyl groups added were incorporated to the structure. This fact may be due to steric effects and to the non-polar character of phenyl groups which prevents their presence in large amounts in the interlayer space, giving a low incorporation when the silica galleries are formed upon hydrolysis and condensation of PTES, together with the TEOS.
This also justifies the found number of functional groups incorporated into the material Φ25-PPH that is only slightly lower than that found for Φ5-PPH, despite the fact that the amount of functionalized alkoxide added to the latter was five times the added amount in the case of Φ25-PPH. However, in spite of the increase in the number of phenyl groups incorporated, the final absorbance remained constant. Finally, as expected, in the case of material Φ50-PPH, the incorporation of phenyl groups in this material is very low.
After removing the surfactant used as a template, the inner silica galleries are free and a porous material is obtained. For hybrid Φ×-PPH, the surface phenyl groups are directed to the inner porous material. This is true if the inorganic framework is preserved after surfactant removal.
This fact can be determined by studying the corresponding XRD diffractograms. The results show a single broad signal at a low angle corresponding to the d001
reflection, showing the separation of the layers of zirconium phosphate (Figure 2
A). This confirms the presence of silica galleries in the interlayer region of zirconium phosphate, which keep these layers separate when the surfactant has been removed.
-PPH, the diffraction peak appears at a slightly lower angle so that the basal spacing is 32.9 Å, the intensity is weak and the definition of the peak poor. This indicates that the presence of phenyl groups hinders the arrangement of the internal phase material. This low crystallinity can be justified by the steric hindrance of the phenyl groups, due to their non-polar character, since the surfactant micelles which act as structural elements of the silica galleries present a polar end placed in this environment which hinders the correct formation of the inorganic structure, mainly for the precursors which contain the phenyl group. This fact is reflected, as discussed in the previous section, in the incorporation of organic groups where the amount of phenyl groups is lower than the other functionalized PPH studied [35
The evaluation of the typical textural parameters such as surface area and pore size distribution will confirm the formation of silica in the galleries of the interlayer region. Textural parameters obtained from the corresponding Nitrogen adsorption-desorption isotherms of N2
at −196 °C correspond to type IV (IUPAC) [36
], characteristic of mesoporous materials (Figure 2
B, Table 2
). The surface area values (SBET
) obtained are quite high (above 500 m2
/g), indicating a large accessible surface within the material due to the presence of silica galleries in the interlayer region being lower than the pure heterostructures (620 m2
], showing that the incorporation of phenyl groups decreases the surface area of the material. Regarding the volume of pores (Vp
), these materials have very high values, even more than the pure silica heterostructures (0.543 cm3
/g). This may be due to the presence of larger pore diameters as shown in the distribution of pores.
The surface chemical composition of the adsorbent Φ5
-PPH before and after contact with AB113 (sample Φ5
-PPH + AB113) is shown in Table 3
. Upon contact with the dye, the percentages of C and N increase and S are now detected. However, Na was not detected indicating that the dye anion was taken up. The empiric formula of the dye anion is C32
and the theoretical N/S atomic ratio is 2.5. The observed N/S atomic ratio after incorporation of the dye is 2.6, a value very near the theoretical one indicating the incorporation of the dye. Sulfur is the only element of the dye that is not present in the adsorbent Φ5
-PPH and the S 2p
core level spectrum for sample Φ5
-PPH + AB113 shows a S 2p3/2
contribution at 168.2 eV assigned to the presence of S(VI) as the sulfonic group of the dye [43
3.2. Adsorption Isotherms
From the results obtained after chemical, structural and textural characterization, Φ5
-PPH material was selected as adsorbent for AB113, given that it presents the highest phenyl group surface density (Table 1
). Also, to evaluate the effect of the phenyl group on the AB113 adsorption, the pristine PPH material was used as a reference. AB113 adsorption isotherms were investigated in triplicate at 25 °C and pH 6.5.
shows the fitting of the experimental data to the adsorption isotherms for PPH and Φ5
-PPH respectively. In the case of pristine PPH, the isotherm was fitted to the SIPS isotherm with a saturated adsorption capacity of 0.04 mmol/g (Figure 3
A). The adsorption data from Φ5
-PPH were fitted to a Langmuir isotherm model, showing a saturated adsorption capacity (Qo
) of 0.0967 mmol/g and a Langmuir constant (KL
) of 613 L/Kg (Figure 3
B). Other model fittings are shown as supplementary information
as Figure S1
The adsorption effectiveness was also evaluated. The retention effectiveness was close to 100% for Φ5
-PPH material, when the adsorption capacity was lower than 0.040 mmol/g. From this point it decreases while increasing the amount of AB113 in the solution. For the pure PPH material, the highest retention percentage reached was 55% for the first fit, decreasing in the next points. In the literature, AB113 adsorption on mesoporous activated carbon is reported [44
], where the observed adsorption capacities are in the order obtained for Φ5
-PPH. In this case the adsorption capacities for an activated carbon obtained from a rubber tyre and for a commercial activated carbon obtained from the Langmuir isotherm (Qo
), were 0.014 and 0.011 mmol/g, respectively, although for both activated carbons the specific surface areas are higher than that observed for Φ5
-PPH with values of 562 and 1168 m2
/g. Another adsorbent based in red mud obtained a Qo
= 0.112 mmol/g [45
], demonstrating the feasibility of the proposed system to be applied.
The difference in the adsorption capacities of PPH and Φ5
-PPH can be explained by the difference in the intermolecular forces between the different surface groups and the dye monomer. In the case of PPH, the main forces involved are strong hydrogen bonding with the AB113 (Si-OH and P-OH) and the presence of aromatic rings on the surface of phenyl-functionalized Φ5
-PPH which offer a superior degree of delocalization due to the π-π stacking of the phenyl surface groups of the adsorbent (Figure 4
). The different mechanisms of adsorption are also indicated by the fit of sorption data to two distinct isothermal models. The better fit of the Φ5
-PPH sorption data to the Langmuir isotherm reveals that the sorption sites are identical and energetically homogeneous, suggesting that the coordination between AB113 and the aromatic rings of Φ5
-PPH is the main mechanism of adsorption involved. However, the better fit of PPH sorption data to the SIPS isotherm indicates that there is more than one type of site involved in this process, each characterized by distinct energies and distinct affinities to adsorb. In this case, it is particularly related to the adsorption on the Si-OH and P-OH groups of PPH. To demonstrate the mechanism described, decreasing a specific area in our material does not limit increased dye adsorption.
The various applications of phenyl functionalized adsorbent have been reported in recent literature [46
]. However, the current work is unique in terms of the use of phenyl-PPH for dye adsorption, which is reported here for the first time.