Surface-functionalized silica gel adsorbents for efficient remediation of cationic dyes

Pure Appl. Chem. 2014; 86(7): 1177–1188

Conference paper

Aleeza Farrukh, Attia Akram, Abdul Ghaffar, Eylül Tuncel, Zehra Oluz, Hatice Duran*, Habib ur Rehman and Basit Yameena,*

Surface-functionalized silica gel adsorbents for efficient remediation of cationic dyes

Abstract: The toxic and non-biodegradable nature of organic dyes necessitates the design and synthesis of novel adsorbents for their effective removal from the environment. This study reports an effective remedia-tion behavior of surface-functionalized silica gel against water-soluble cationic dyes (up to 98 % removal). Thiol groups were functionalized at the surface of silica gel (SiO2–SH). The surface-tethered –SH groups were further oxidized to sulfonic acid groups to generate the negatively charged moieties at the surface of silica gel (SiO2–SO3H). The morphology of the developed adsorbents and the surface modifications were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Uptake study of three cationic dyes, namely, rhodamine B (Rh B), rhoda-mine 6G (Rh 6G), and crystal violet (CV) with SiO2–SH and SiO2–SO3H adsorbents was performed by varying the adsorbent amount, contact time, pH of solution, and temperature. The presence of negatively charged species at the surface of SiO2–SO3H results in an increased electrostatic interaction with the cationic dyes, which leads to better remediation characteristics for SiO2–SO3H as compared to SiO2–SH. The reusability of the developed adsorbents was also assessed by investigating adsorption/desorption of dyes. The simple fab-rication process provides a facile avenue to the adsorbents with efficient remediation towards cationic dyes.

Keywords: adsorption; cationic dyes; environmental chemistry; interfaces; IUPAC Congress-44; silica gel; surface chemistry; surface modification; waste; water remediation.

DOI 10.1515/pac-2014-0105

IntroductionWater pollution is a matter of great environmental concern where the increasing industrial activities are among the major contributors [1]. Water soluble organic dyes are widely used in the industry and about

Article note: A collection of invited papers based on presentations on the Environmental Chemistry theme at the 44th IUPAC Con-gress, Istanbul, Turkey, 11–16 August 2013.

aCurrent address: Laboratory of Nanomedicine and Biomaterials, MIT-Harvard Center for Cancer Nanotechnology Excellence, BWH, 75 Francis Street, Boston, MA 02115, USA*Corresponding authors: Hatice Duran, Department of Materials Science and Nanotechnology Engineering, TOBB University of Economics and Technology, Sogutozu Cad. 43, 06560 Ankara, Turkey, e-mail: [email protected]; and Basit Yameen, Depart-ment of Chemistry, SBA School of Science and Engineering (SSE), Lahore University of Management Sciences (LUMS), Lahore 54792, Pakistan, e-mail: [email protected] Farrukh and Habib ur Rehman: Department of Chemistry, SBA School of Science and Engineering (SSE), Lahore Univer-sity of Management Sciences (LUMS), Lahore 54792, PakistanAttia Akram and Abdul Ghaffar: Department of Chemistry, University of Engineering and Technology (UET), Lahore, PakistanEylül Tuncel and Zehra Oluz: Department of Materials Science and Nanotechnology Engineering, TOBB University of Economics and Technology, Sogutozu Cad. 43, 06560 Ankara, Turkey

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1178 A. Farrukh et al.: Water remediation

10–15 % of the employed dyes are released as effluent [2]. Dyes generally exhibit high tinctorial values [3] and the resulting water coloration leads to the reduced penetration of light through water streams ultimately affecting the photosynthesis process of the aquatic plants [4]. There are approximately 40 000 synthetic dyes and pigments that are being used commercially. Azo dyes, anthraquinones, indigoids, phthalocyanines, xanthenes, benzodifuranones, sulfur and nitro dyes represent their main chemical classes [5, 6]. Depending on the charge on the molecule, dyes can be classified as anionic, nonionic and cationic. The major anionic dyes are the direct, acid and reactive dyes which are mostly water-soluble, and the most problematic ones are the brightly coloured reactive and acid dyes [6]. The major nonionic dyes are disperse dyes that do not ionize in an aqueous environment, while most cationic dyes are azo basic, anthraquinone disperse and reactive dyes [1]. Depending on the class of the dye, the losses in wastewaters can vary from 2 % for basic dyes to as high as 50 % for reactive dyes, leading to severe con-tamination of surface and ground waters in the vicinity of dyeing industries [7]. Most of these dyes are toxic and non-biodegradable, which has prompted Ecological and Toxicological Association of the Dye-stuffs Manufacturing Industry (ETAD), US Environmental Protection Agency (EPA), Environment Agency (EA) for England and Wales, and Scottish Environment Protection Agency (SEPTA) to implement strict legislation for the treatment of dye effluents and release of dyes in the water bodies [1, 8, 9]. Conse-quently, the development of effective methodologies for the remediation of dyes is an urgent necessity. Among the various remediation methodologies available [2, 3, 10, 11], adsorption, is the most commonly employed strategy [2]. A wide range of natural and synthetic adsorbents have been employed for the treatment of dyes-based effluents. The natural adsorbents include rice husk, wheat straw wood, bark, banana and orange peels, whereas activated carbon, silica (SiO2), and alumina are among the most widely employed synthetic adsorbents [4, 12, 13]. The synthetic adsorbents such as magnetic nanoparticles, gra-phene oxide nanosheets, and multi–walled carbon nanotubes (CNTs) have also been explored for their remediation potential towards aqueous dye contaminants [2, 8, 14]. In context of silica based absorbents, the functionalized mesoporous SiO2, SiO2 nano/micro-particles and bentonite have been employed for the remediation of dyes from water samples [15–18]. The low cost and commonly available silica gel has been reported for effective remediation of dyes [14]. The surface modification of silica based materials via silanization provides a very convenient route to a variety of functional materials [19]. Capitalizing on this opportunity, we explore the potential of tuning the surface properties of silica gel for the remediation of dye contaminated water samples. Incorporating the negatively charged functionalities at the surface of silica gel would increase its electrostatic interaction with the cationic species. Based on this hypothesis, we report a straightforward surface modification protocol for improving the removal capacity of silica gel against cationic dyes. The silica gel was functionalized with (3–mercaptopropyl)triethoxysilane (MPTES) for incorporating thiol (–SH) moieties at the surface (SiO2–SH), which were subsequently oxidized to the sulfonic acid groups (–SO3H) to introduce the negatively charged moieties on the surface of silica gel (SiO2–SO3H) [20, 21]. Both adsorbents (SiO2–SH and SiO2–SO3H) were evaluated for their remediation effi-ciency towards electrostatically charged dye contaminants (xanthene dyes: rhodamine 6G and rhodamine B, and crystal violet). The adsorption process is further characterized by adsorption isotherms estima-tion, and by evaluating the pertinent kinetic and thermodynamic parameters.

Experimental section

Thiol-functionalized silica gel (SiO2–SH)

A 10 % MPTES solution in dry toluene (80 mL) was heated up to 80 °C and dry activated silica gel (ASG) (3 g) was added to it [20]. The suspension was refluxed for 20 h under inert atmosphere, cooled to room tempera-ture (rt) and the silica gel was separated by centrifugation (4000 rpm, 10 min). Finally, SiO2–SH was washed twice with each toluene, n-hexane, and methanol, and dried in vacuo overnight.



A. Farrukh et al.: Water remediation 1179

Sulfonic acid-immobilized silica gel (SiO2–SO3H)

SiO2–SH particles (3 g) were dispersed in H2O2 (60 mL, 33 %) and heated for 4 h at 60 °C [21, 22]. The SiO2–SO3H particles were separated by centrifugation (4000 rpm, 10 min) and washed three times with deionized water, and dried under vacuum at 60 °C for 24 h.

Dyes uptake study

Uptake study of three cationic dyes, namely, rhodamine B (Rh B), rhodamine 6G (Rh 6G), and crystal violet (CV) (Fig. 1) for SiO2–SH and SiO2–SO3H adsorbents was performed by varying the adsorbent amount. The silica gel adsorbents (5–20 mg) were added to 15 ppm aqueous dye solutions (10 mL) and shaken at 400 rpm at rt for optimized time (10 min for Rh 6G and CV at pH 6 while 15 min for Rh B at pH 4). The particles were subsequently separated by filtration and filtrates were analyzed by UV–vis spectrophotometer at λmax of 554 nm for Rh B, 530 nm for Rh 6G and 590 nm for CV [23, 24]. The concentration of the dyes was determined from respective calibration curves and the removal uptake was calculated by the following expression:

Dye removal efficiency (%) 100i r

i

C CC−

= ×

(1)

where Ci is the initial concentration of the dye in water before treatment with adsorbent and Cr is the concen-tration of the dye after treatment. The effects of the adsorbent/adsorbate contact time, temperature, pH and dye concentration were also studied in the same manner.

Results and discussion

Preparation and characterization of thiol and sulfonic acid-functionalized silica gel

The ASG was refluxed with MPTES in dry toluene under inert atmosphere to give SiO2–SH [20–25]. The surface thiol moieties were oxidized with hydrogen peroxide to achieve SiO2–SO3H (refer to Scheme 1) [21].

The surface modifications were characterized by XPS analysis (see Fig. S1, Supplementary Information). In the survey scan of SiO2–SH, the binding energies for the C1s (285 eV), S2s (228 eV) and S2p (164 eV) orbitals established the thiol functionalization on the surface of silica gel (see Fig. S1a, Supplementary Information) [20]. The high-resolution S2p scan for SiO2–SH showed signals for S2p3/2 (163.6 eV) and S2p1/2 (164.8 eV) [26]. The transformation of the surface –SH groups to –SO3H groups was confirmed by an increase in the surface oxygen content for SiO2–SO3H and the associated shift of the signals for the S2s (233 eV) and S2p (169 eV) orbitals to higher binding energies as compared to the SiO2–SH (see Fig. S1c, Supplementary Information)

Fig. 1 Chemical structures of the employed cationic dyes.




[27]. The high-resolution S2p scan for SiO2–SO3H reveals binding energies for sulfur S2p3/2 and S2p1/2 at 169.0 and 164.3 eV. The shift of the binding energies associated with the sulfur content towards the higher values is in agreement with the transformation of C–SH to C–SO3H linkage [28]. The comparative peak intensities indicated a substantial transformation of the surface thiol groups to the sulfonic acid groups [29]. In order to quantify this transformation, the XPS spectra were normalized according to the peak intensity of Si2s (155 eV) and C1s (285 eV) orbitals, which are considered to be invariant during the transformation. The estimation based on the relative O1s (533 eV) orbital peak ratios for SiO2–SH and SiO2–SO3H revealed ∼52 % conversion for the transformation of the surface –SH groups to the –SO3H groups.

The size of functionalized commercial silica particles ranged between 0.5–8 μm as indicated by SEM images (Fig. 2a and b). The evaluation of surface functionalization by the TEM (Fig. 2c and d) was limited by

a b

c d

Fig. 2 SEM images of SiO2–SO3H (a, b) and TEM images of SiO2–SH (c), and SiO2–SO3H (d).

Scheme 1 Schematic illustration of the preparation of SiO2–SH and SiO2–SO3H silica gel adsorbents.




the large variation in size and relatively thin layer of the surface-immobilized silane monolayer. However, in one of our previous publications [30] we have proved the deposition of monolayer of APTES under analo-gous experimental conditions. Furthermore, the effective surface functionalization is evident from the XPS analysis.

Effect of contact time on removal capacity

The influence of contact time is a key parameter in ascertaining the remediation efficiency of the adsor-bents. The effect of adsorbent/adsorbate contact time on the dye uptake characteristics of the employed adsorbents was studied for different time intervals (5–20 min, Fig. 3). The optimized amounts of the SiO2–SH and SiO2–SO3H adsorbents (5 mg for CV at pH = 6, 10 mg for Rh 6G at pH = 6, and 20 mg for Rh B at pH = 4) were added to 15 ppm water solutions (10 mL) of the dyes and shaken at 400 rpm at rt. The dye uptake was observable instantly upon the addition of adsorbent, indicating a ready availability of active sites for the dye remediation. An increase in the uptake of CV was observed as the contact time was increased and up to 98 % dye remediation was obtained with both the adsorbents within 10 min. Rh 6G also showed an increase in removal with contact time and maximum adsorption up to 96 and 94 % were observed in 10 min with SiO2–SH and SiO2–SO3H adsorbents. In the case of Rh B, a rapid increase was observed at 15 min for SiO2–SH adsorbent with 93 % removal. The SiO2–SO3H adsorbent gave a 95 % removal of Rh B dye within 15 min. Based on the maximum removal capacities, the optimized contact time selected for Rh 6G and CV was 10 min, and 15 min for Rh B. These optimized contact times were used for rest of remediation experi-ments. Further increase in contact time does not show any significant improvement in removal uptake due to adsorption equilibrium [31].

Effect of adsorbent amount on removal capacity

The effect of adsorbent amount on the removal capacity was investigated by varying the amounts of SiO2–SH and SiO2–SO3H absorbents (5, 10, 15, and 20 mg). The specified amounts of the adsorbents were added to a 15 ppm solution of different dyes (10 mL) and shaken at 400 rpm at rt at optimized contact time (10 min, pH = 6 for Rh 6G, and CV, and 15 min, pH = 4 for Rh B). The adsorption capacity, generally, increased with increas-ing the amount of adsorbent (Fig. 4). The optimized adsorbent amounts for CV, Rh 6G, and Rh B were 5, 10, and 20 mg, respectively. The results indicated higher uptake capacity of SiO2–SH for removal of Rh 6G and CV, while SiO2–SO3H exhibited relatively higher remediation for Rh B. Among all the dyes employed, a maximum

Fig. 3 Effect of contact time on the uptake capacity of Rh 6G, Rh B and CV employing SiO2–SH (a) and SiO2–SO3H (b) as adsor-bents. The study was conducted for mentioned time periods with 10 mL of 15 ppm aqueous dye solutions at rt for 5 mg of CV at pH = 6, 10 mg of Rh 6G at pH = 6, and 20 mg of Rh B at pH = 4.




removal was observed for CV for both the adsorbents (∼98 %). Both the adsorbents also exhibited high reme-diation tendency towards Rh 6G (97 and 95 % removal with SiO2–SH and SiO2–SO3H) and Rh B (92 and 95 % removal with SiO2–SH and SiO2–SO3H). The slight variations in remediation capacities can be attributed to the density of charges, both on the dye molecules and on the adsorbents surface.

In a control experiment, 5 mg of ASG added to 10 mL of 15 ppm aqueous dye solutions showed a removal of 80.98, 60.59, and 2.56 % for CV (10 min at pH = 6), Rh 6G (10 min at pH = 6), and Rh B (15 min at pH = 4), respectively. This control experiment highlights the influence of the surface –SH and –SO3H functional groups on the remediation of the cationic dyes (see Fig. S2, Supplementary Information). The SiO2–SH and SiO2–SO3H adsorbents exhibited analogous adsorption behaviors towards CV, while there was approximately 18 % difference between the adsorption of CV by ASG and thiol or sulfonic functionalized silica adsorbents. On the other hand, for Rh 6G there is approximately 35–40 % difference in adsorption of dye for ASG and SiO2–SH or SiO2–SO3H. Among all the dyes used in present study, the developed adsorbents showed remark-able improvement in the adsorption of Rh B.

Effect of pH on removal capacity

The pH of the solution considerably influences the adsorption/desorption capability of an adsorbent [31]. In the present study, the impact of pH on the adsorption capacity of adsorbents towards the employed dyes was determined by performing adsorption studies at six different pH values (pH = 2–12). The pH values of the solutions were adjusted with aqueous 0.1 M HCl and 0.1 M NaOH solutions. The SiO2–SH and SiO2–SO3H adsorbents were separately added to 10 mL of 15 ppm solutions of the dyes and shaken at rt under optimized conditions (5 mg for CV, 10 mg for Rh 6G for 10 min, and 20 mg for Rh B for 15 min). The pH of the solution substantially affects the removal tendency of the developed adsorbents, which depends on the nature of adsorbent in connection with the electrostatic charges on the employed dye molecules.

The removal capacity for both the adsorbents towards Rh B dye decreases with increasing the pH and maximum uptake was obtained at pH values of the 2 and 4 for SiO2–SH and SiO2–SO3H, respectively (Fig. 5). The gradual decrease in the uptake of Rh B with the increase in pH for SiO2–SH adsorbent was attributed to the ionization of carboxylic moiety of Rh B molecules at higher pH. The developing negative charge on Rh B dye molecules may result in a repulsive effect towards the negative charge developing on the SiO2–SH adsor-bent surface. The ionization of the carboxylic acid groups can also lead to enhanced intermolecular electro-static interactions. These effects also apply to the rapid decrease in the remediation capacity of Rh B in case of SiO2–SO3H adsorbent. The effect is more prominent in case of SiO2–SO3H, which is attributed to the higher ionization tendency of the surface –SO3H groups. Among the set of dyes used in this study, the adsorption behavior of the Rh 6G exhibited the most drastic variations with the change in the solution pH. The maximum

Fig. 4 Effect of adsorbent amount on the removal of Rh 6G, Rh B and CV dyes with SiO2–SH (a) and SiO2–SO3H (b) adsorbents. The stated amounts of the adsorbents were added to 10 mL of 15 ppm aqueous dye solutions at rt under optimized time and pH (10 min, pH = 6 for Rh 6G, and CV, and 15 min, pH = 4 for Rh B).




Fig. 5 Influence of pH on the adsorption of Rh 6G, Rh B and CV with SiO2–SH (a) and SiO2–SO3H (b) adsorbents. The effect was investigated at stated pH values employing 10 mL of 15 ppm aqueous dye solutions at rt for optimized adsorbent amount and contact time.

adsorption tendency of Rh 6G with adsorbents SiO2–SH and SiO2–SO3H was observed at pH 6. The low adsorp-tion capacity at acidic pH for the SiO2–SH adsorbent was attributed to the protonation of surface thiol groups, leading to the formation of sulfonium ions [2]. A decrease in the adsorption capacity of SiO2–SH adsorbent towards the Rh 6G dye was also observed with the increase in pH. This trend may also have originated from the development of negative charge due the carboxylic acid moieties, which may form by the hydrolysis of ester linkage present in Rh 6G dye. The similar adsorption behavior of Rh 6G dye was also observed with SiO2–SO3H, but the influence of acidic pH was less significant due to the more persistent negative charge of sulfonate groups as compared to the thiolate groups. The investigation of influence of pH on adsorption of CV revealed maximum removal ability of both adsorbents at pH 6. The impact of pH change was less influential for the removal of CV probably due to the relatively invariable charge on the dye molecules. A decrease in the adsorption capacity of SiO2–SH in acidic pH was attributed to the low acidity of thiol accompanied. The pH shows no observable impact on uptake capacity of SiO2–SO3H, rationalized by the higher stability of sulfonic acid conjugate base.

Effect of temperature

Temperature variations significantly influence the adsorbent/adsorbate interaction [32]. The effect of temper-ature was studied by varying the solution temperature (25–65 °C), whilst keeping rest of variables constant. The SiO2–SH and SiO2–SO3H adsorbents were added to a 10 mL of 15 ppm aqueous solutions of the employed dyes under optimized conditions (5 mg for CV, 10 mg for Rh 6G for 10 min at pH = 6, and 20 mg for Rh B for 15 min at pH = 4). The gradual decrease in the uptake capacity with an increase in the temperature sug-gests an exothermic nature of the interaction between dye molecules and the adsorbent surface (Fig. 6). The optimum temperature selected for investigation for all three dyes was 25 °C. The decrease in adsorption with the increase in temperature can be attributed to a decrease in the Van der Waals and electrostatic interactions between adsorbent and adsorbate [33–35].

Adsorption isotherms

The adsorption isotherm facilitates in understanding the interrelation between the amounts of adsorbate on the adsorbent surface to its concentration. Langmuir, Freundlich, and Temkin isotherm models are most frequently employed for evaluating the adsorption behavior of the adsorbents [24, 32].

Langmuir isotherm reveals the adsorbent/adsorbate interaction and is based on the assumptions that at maximum adsorption, a monolayer of adsorbate will deposit on the surface of adsorbent and adsorbate molecules do not interact with each other [31, 32]. The linear form of isotherm can be expressed as:




e e

e max max

1C Cq q b q

= +

(2)

where Ce (mg L–1) is the equilibrium concentration of the adsorbate, qe (mg g–1) represents the amount of adsorbate adsorbed on the adsorbent surface at equilibrium, qmax (mg g–1) is the maximum adsorption capac-ity of the adsorbent, and b (L mg–1) is a Langmuir constant representing the affinity of binding sites. The qmax values (Table 1) indicate higher adsorption tendency of SiO2–SO3H with all three dyes as compared to SiO2–SH. The maximum adsorption capacity calculated for CV was 93.28 ± 3 mg g–1 with SiO2–SH while 1215 ± 20 mg g–1 with SiO2–SO3H adsorbent. See Fig. S3, Supplementary Information, for the plots of Ce/qe against Ce for both adsorbents, and linear Langmuir model constants, i.e., separation factor (RL) and regression coefficient (R2) are summarized in Table 1. The RL is a dimensionless constant calculated by the following expression:

L

o

11

RbC

=+

(3)

where Co represents the initial concentration of adsorbent (15 mg L–1 for the present investigation). The values of RL indicate the feasibility of the adsorption process and RL > 1, 0 < RL > 1, RL = 0, RL = 1 correspond to the unfavorable, favorable, irreversible, linear behaviors, respectively [24, 32]. The RL values obtained in the present study (Table 1) reveal a favorable uptake of the employed dyes by both the adsorbents.

Fig. 6 Effect of temperature on the removal of Rh 6G, Rh B and CV with SiO2–SH (a) and SiO2–SO3H (b) adsorbents. The study was conducted at stated temperature range with 10 mL of 15 ppm aqueous dye solutions employing optimized adsorbent amount, contact time, and pH.

Table 1 Parameters of Langmuir, Freundlich, and Temkin isotherm models calculated for the adsorption of dyes on SiO2–SH and SiO2–SO3H adsorbents.

Langmuir model

Freundlich model

Temkin model

qmax (mg/g) b (L mg–1) RL R2 Kf (mg/g) 1/n R2 KT bT (kJ/mol) R2

SiO2–SH Rh 6G 45.455 0.328 0.169 0.8278 12.065 0.548 0.9758 7.079 0.259 0.8040 Rh B 41.710 0.158 0.297 0.9839 5.764 0.608 0.9674 7.901 0.548 0.8793 CV 149.254 0.486 0.121 0.8638 27.324 0.521 0.9156 11.271 0.143 0.8912

SiO2–SO3H

Rh 6G 76.923 0.388 0.147 0.9164 2.213 0.973 0.9981 11.055 0.142 0.9205 Rh B 25.757 0.196 0.254 0.9575 1.000 1.000 1.0000 13.072 0.328 0.9780 CV 1495.117 0.105 0.389 0.9695 4.062 1.020 0.9903 11.987 0.067 0.9691




The Freundlich isotherm interprets the non-ideal and multilayer adsorption at heterogeneous surfaces. The linear form of Freundlich model is expressed as:

e f e

1log log logq K Cn

= +

(4)

where qe (mg g–1) is the amount of adsorbate adsorbed at equilibrium, Kf (mg g–1), Freundlich adsorption equi-librium constant, is a measure of the adsorption capacity and 1/n is the Freundlich intensity factor that repre-sents the feasibility of an adsorption process [2, 31]. See Fig. S4, Supplementary Information, for the obtained Freundlich isotherms. The Kf calculated by the above expression showed a different trend as predicted by qmax in Langmuir model (Table 1). The CV showed the highest Kf value of 27.32 with SiO2–SH while 4.06 with SiO2–SO3H. The plot log qe against log Ce enables the calculation of Kf, 1/n and regression coefficient (R2) (Table 1). The value of 1/n in the range of 0.1 < (1/n) < 1.0 and (1/n) > 2 indicates favorable and unfavorable adsorption process, respectively [32]. The comparison of regression coefficient (R2) values from both isotherms revealed that the adsorption of cationic dyes by adsorbents was best described by the Freundlich model. Thus, the functionalized silica gel adsorbents primarily show heterogeneous adsorption behavior [32].

Temkin isotherm model was used to evaluate the changes in the heat of sorption during adsorption process. It is based on the assumption that adsorption is a spontaneous process occurring with the release of heat. The heat of adsorption is initially high when adsorption sites are free, which decreases linearly during the latter stages of adsorption process with increasing surface coverage [31]. The linear form of the adsorption isotherm is expressed as:

e T eln lnq B K B C= + (5)

T

RTBb

= (6)

where B (kJ mol–1) Temkin constant represents variation in adsorption energy, bT (kJ mol–1) shows heat of sorp-tion, KT (L mg–1) indicates Temkin equilibrium constant, while Ce (mg L–1) and qe (mg g–1) represent concentration of adsorbate and amount of adsorbate adsorbed at equilibrium [36]. See Fig. S5, Supplementary Information, for the corresponding Temkin isotherms. Higher heat of adsorption predicts stronger adsorbent/adsorbate interac-tion. The bT values for the adsorption of dyes by SiO2–SO3H and SiO2–SH suggested an electrostatic interactions between adsorbent and adsorbate (Table 1). The KT values for SiO2–SH were lower than SiO2–SO3H, which can be attributed to the stronger electrostatic interaction of the SiO2–SO3H adsorbent with the cationic adsorbates.

Adsorption kinetics

Understanding of the rate of adsorption is crucial to estimate the operational effectiveness of the adsorbents. Sorption kinetic facilitates in analyzing the rate of adsorption, adsorption mechanism, and rate limiting steps [2, 32]. Adsorption kinetics were investigated by pseudo first-order and pseudo second-order models by com-paring the experimental and theoretical calculation. The Lagergren pseudo first-order and pseudo second-order equations employed for studying reaction kinetics are expressed as:

Pseudo first-order:

1

e elog( ) log2.303t

kq q q t− = −

(7)

Pseudo second-order:

2

e2 e

1 1

t

t tq qk q

= +

(8)




where qe (mg/g), and qt (mg/g) express the amount of adsorbate adsorbed per unit mass of adsorbent at equi-librium and at time t, while k1 (s–1) and k2 (g mg–1 s–1) represent equilibrium rate constants of pseudo first-order and pseudo second-order reaction [8]. The kinetic rate constants (k1 and k2) and regression coefficients (R2) calculated from both plots of kinetic equations provide comparison of adsorption kinetics. The data illustrate that adsorption of all cationic dyes proceed through pseudo second-order mechanism as supported by value of correlation coefficient (R2) (see Table S1, Supplementary Information). The agreement between experimental and calculated values of qe verified the order of adsorption kinetics [32]. The adsorption rate of SiO2–SH and SiO2–SO3H adsorbents for the removal of dyes were comparable, as indicated by pseudo second-order rate constant (k2), therefore both adsorbents were active in remediation as explained in study of contact time.

Adsorption thermodynamics

Investigation of adsorption thermodynamics assist in assessment of energy changes associated with adsorp-tion process [8, 32]. Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) are thermodynamic parameters that facilitate in determination of adsorption mechanism and associated energy changes. The conventional equations for estimation of adsorption thermodynamics are expressed as:

o RT lnG K∆ =− (9)

o o

ln RT

S HKR

∆ ∆= −

(10)

where T represents temperature (K), and R is general gas constant (8.314 J mol–1 K–1). The thermodynamic param-eters were calculated from plot of ln K against 1/T. The negative ΔG° values endorsed that adsorption of cationic dyes on SiO2–SH and SiO2–SO3H was a spontaneous and thermodynamically feasible process (Table 2). The data shows that ΔG° falls in the range which is characteristic of adsorption predominately occurring through physisorption [8]. The negative ΔH° confirms the exothermic nature of adsorption, which is also observed in effect of temperature studies where adsorption drops with the increase in temperature. The negative ΔS° shows a decrease in the freedom of molecular movement as expected during the adsorption process [37].

Desorption and reusability

Investigation of desorption of dyes from adsorbent facilitates in evaluation of economic feasibility of an adsorbent [2]. Desorption of dyes were carried out in an aqueous solution (10 mL) with pH ranging from 2 to 12. The Rh 6G and Rh B showed maximum desorption at pH 12, whereas CV showed a maximum desorption at pH 4. The adsorbents were also washed with ethanol (10 mL) before each cycle. SiO2–SO3H displayed a good adsorption towards the employed dyes until 6 cycles without a substantial decline in the remediation capacity while uptake capacity of SiO2–SH decreased more significantly after each adsorp-tion cycle (Fig. 7).

Table 2 Thermodynamic parameters calculated for adsorption of cationic dyes on SiO2–SH and SiO2–SO3H adsorbents.

T (K) SiO2–SH SiO2–SO3H

ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (kJ/mol K) R2 ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (kJ/mol K) R2

Rh 6G 298 –8.861 –26.098 –0.0605 0.6871 –7.832 –21.663 –0.047 0.9300Rh B 298 –4.470 –28.326 –0.0798 0.9740 –5.731 –20.355 –0.049 0.9910CV 298 –12.179 –17.165 –0.0171 0.9242 –11.242 –17.533 –0.021 0.9886




ConclusionThe employed silanization process, in connection with the ready commercial availability of the silica gel, can be potentially exploited to develop low cost materials with efficient dyes removal tendency from the contami-nated water bodies. Under the optimized conditions the adsorption behavior of the SiO2–SH and SiO2–SO3H could be best described by the Freundlich model and the correlation coefficient values (R2) in the adsorption kinetics data illustrated that adsorption of the cationic dyes followed a pseudo second-order mechanism. The favorable adsorption behavior of both adsorbent was also supported by the values of the thermodynamic parameters such as ΔG°, ΔH°, and ΔS°. Furthermore, the adsorption efficiency of SiO2–SO3H was retained ( ≥ 90 %) even after 5 cycles. The current study provides a simple and cost-effective route for the removal of cationic dyes from water with functionalized silica gel, which may lead to one step ahead in efforts for a safe, clean, and healthy environment.

Supplementary Information: Materials, instrument details, XPS spectra, control remediation study, Langmuir, Freundlich and Temkin isotherms, and adsorption kinetic parameters are provided as Supplementary Informa-tion available online (http://www.degruyter.com/view/j/pac.2014.86.issue-7/pac-2014-0105/pac-2014-0105.xml).

Acknowledgments: B.Y. acknowledges financial support from Higher Education Commision (HEC) of Paki-stan (Project No. 20-1799/R&D/10-5302) and a start-up grant from LUMS. H.D. and B.Y. thank UNAM, Bilkent University for XPS, TEM, and SEM analyses.

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Fig. 7 The recycling of adsorption ability of the employed adsorbents SiO2–SH (a) and SiO2–SO3H (b) towards Rh 6G, Rh B, and CV dyes. The study was conducted with 10 mL of 15 ppm aqueous dye solution for optimized adsorbent amount, contact time and pH, followed by washing of particles with ethanol (10 mL) before each cycle.



http://www.degruyter.com/view/j/pac.2014.86.issue-7/pac-2014-0105/pac-2014-0105.xml


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Surface-functionalized silica gel adsorbents for efficient remediation of cationic dyes

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