3.1 Characterization of adsorbents

The morphology of xerogel adsorbents was portrayed by SEM, and the images are illustrated in Figure 2.

It very well may be obviously observed that all hybrid materials evinced wrinkled surface and irregular shaped particles with pore diameter over 1.5 nm, indicating the mesoporous structure. The textural properties of the synthesized materials, including specific surface area, total mesoporous volume, and average pore diameter were assessed by N2 adsorption-desorption isotherms and BJH method. Interestingly, all samples recorded typical type IV adsorption-desorption isotherms which is a characteristic pattern of mesoporous composites as stated in IUPAC classification. Moreover, the hysteresis loop is of type H2 which is usually tied to the ink-bottle pores with bulky orifice of the border inner parts (Figure 3).

A clearly defined step is eventuated roughly at P/P° = 0.4 characteristic of the mesopore filling owing to capillary condensation [24]. Evenly, the pore size curve obtained from the isotherm branch showed a tight distribution centered at about 3.5 nm.

Based on data in Table 2, all xerogels showed relatively high specific surfaces and pore volumes. More convincing underpins for the successful anchor of organic moieties into the siliceous network emanate from the 13C CP MAS NMR analysis. The 13C NMR spectra of the synthesized materials are shown in Figure 4. These spectra recorded resonance peaks at 156.3–166.4 ppm (C=N) typical of sp2 carbon atoms which characterized the cross links of the organic functional groups in the inorganic network, providing the possibility of creating Si-N and Si-S covalent bonds. The peaks at 13–58 ppm were attributed to CH3CH2O-Si- groups (TEOS). This outcome portrayed the incorporation of the organic bi-functional compounds in the inorganic network, providing the formation of Si-N and Si-S covalent bonds. So as to confirm the attachment of the organic precursors onto the skeleton of

) Vpor (cm<sup>3</sup>

.g<sup>1</sup>

) dmoy (Å)

N2 absorption-desorption isotherms of (a) M1 and (b) M2 and BJH pore size distribution of (c) M1

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from…

M1 290 0.4 34.80 M2 310 0.2 20

.g<sup>1</sup>

BET surface area, pore volume, and average pore size of xerogels M1 and M2.

All samples showed a typical band related to Si-O bonds, at 434–440 cm<sup>1</sup>

the stretching vibration of Si-O-Si groups. Besides, the broad and strong bands detected at 3383–3325 cm<sup>1</sup> were attributed to the stretching vibration of OH groups which are associated to Si-OH groups ensuing from the TEOS hydrolysis. The signals revealed at 1557–1576 cm<sup>1</sup> were ascribed to the stretching vibration of C=N groups (heterocyclic part). Another indicative band of the covalent Si-N bond

, and strong bands at 1061–1134 cm<sup>1</sup> assigned to

,

silica species, FT-IR spectroscopy was carried out.

bending of O-Si-O at 754–760cm<sup>1</sup>

Sample SBET (m<sup>2</sup>

DOI: http://dx.doi.org/10.5772/intechopen.86802

Figure 3.

Table 2.

133

and (d) M2.

The FT-IR spectra of all samples are depicted in Figure 5.

Figure 2. SEM micrographs of pristine adsorbents: (a) M1 and (b) M2.

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from… DOI: http://dx.doi.org/10.5772/intechopen.86802

#### Figure 3.

and parched in the air for the forthcoming experiment. Successive sorptiondesorption cycles are rehashed 10 times to build up the genuine application and the

The effluent specimens are gathered from the discharge exits of electroplating plant, Yaroslavl, Russia. The physicochemical parameters of the electroplating

Samples are stored in plastic bottles and cooled to 20°C; afterward, they are diluted ten times and alkalized with 0.5 mol.L<sup>1</sup> of NaOH solution, and finally, are filtered through a 0.35 μm membrane filter. All physicochemical features of the

The morphology of xerogel adsorbents was portrayed by SEM, and the images

It very well may be obviously observed that all hybrid materials evinced wrinkled surface and irregular shaped particles with pore diameter over 1.5 nm, indicating the mesoporous structure. The textural properties of the synthesized materials, including specific surface area, total mesoporous volume, and average pore diameter were assessed by N2 adsorption-desorption isotherms and BJH method. Interestingly, all samples recorded typical type IV adsorption-desorption isotherms which is a characteristic pattern of mesoporous composites as stated in IUPAC classification. Moreover, the hysteresis loop is of type H2 which is usually tied to the

A clearly defined step is eventuated roughly at P/P° = 0.4 characteristic of the mesopore filling owing to capillary condensation [24]. Evenly, the pore size curve obtained from the isotherm branch showed a tight distribution centered at about

ink-bottle pores with bulky orifice of the border inner parts (Figure 3).

high stability of the adsorbent.

Water Chemistry

effluent are enlisted in Table 1.

3. Results and discussion

are illustrated in Figure 2.

3.5 nm.

Figure 2.

132

SEM micrographs of pristine adsorbents: (a) M1 and (b) M2.

3.1 Characterization of adsorbents

2.2.6 Adsorption test with the electroplating wastewaters

effluent are determined by a conventional procedure [23].

N2 absorption-desorption isotherms of (a) M1 and (b) M2 and BJH pore size distribution of (c) M1 and (d) M2.


#### Table 2.

BET surface area, pore volume, and average pore size of xerogels M1 and M2.

Based on data in Table 2, all xerogels showed relatively high specific surfaces and pore volumes. More convincing underpins for the successful anchor of organic moieties into the siliceous network emanate from the 13C CP MAS NMR analysis.

The 13C NMR spectra of the synthesized materials are shown in Figure 4. These spectra recorded resonance peaks at 156.3–166.4 ppm (C=N) typical of sp2 carbon atoms which characterized the cross links of the organic functional groups in the inorganic network, providing the possibility of creating Si-N and Si-S covalent bonds. The peaks at 13–58 ppm were attributed to CH3CH2O-Si- groups (TEOS). This outcome portrayed the incorporation of the organic bi-functional compounds in the inorganic network, providing the formation of Si-N and Si-S covalent bonds.

So as to confirm the attachment of the organic precursors onto the skeleton of silica species, FT-IR spectroscopy was carried out.

The FT-IR spectra of all samples are depicted in Figure 5.

All samples showed a typical band related to Si-O bonds, at 434–440 cm<sup>1</sup> , bending of O-Si-O at 754–760cm<sup>1</sup> , and strong bands at 1061–1134 cm<sup>1</sup> assigned to the stretching vibration of Si-O-Si groups. Besides, the broad and strong bands detected at 3383–3325 cm<sup>1</sup> were attributed to the stretching vibration of OH groups which are associated to Si-OH groups ensuing from the TEOS hydrolysis. The signals revealed at 1557–1576 cm<sup>1</sup> were ascribed to the stretching vibration of C=N groups (heterocyclic part). Another indicative band of the covalent Si-N bond

3.2 Effect of pH solution

(Figure 6).

Figure 6.

Figure 7.

; adsorbent dosage 0.4 g.L<sup>1</sup>

L<sup>1</sup>

135

Zeta potential of xerogels at different pH values.

adsorbent surface metal binding sites.

DOI: http://dx.doi.org/10.5772/intechopen.86802

The pH of the solution is regarded as one of the foremost adsorption factors,

As illustrated in Figure 6, when pH ˂ pHZps, the adsorbent surface charge is positive by virtue of the amino and thiol group protonation. Therefore, electrostatic repulsive force emerges between heavy metal ions and the adsorbent surface, inciting abatement in the adsorption capacity. Be that as it may, at pH > pHZps, all samples earn a negative surface charge, promoting electrostatic attraction between metal cations and adsorbent and therefore, the absorption efficiency enhancement. It ought to be stressed that beyond pH 5 [25], the uptake yield of heavy metal ions diminishes through the metallic hydroxide precipitation which thwarts the diffusion of the metal ions into the adsorbent active site. Therethrough, the pH at 7 was

Effect of pH on adsorption of metal ions onto both xerogels ((a)M1 and (b)M2) (metal concentration 20 mg.

, contact time 1 h).

selected as the ideal incentive in the resulting experiments (Figure 7).

since it has a prominent impact on the metal ion solubility, as well as on the

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from…

The effect of pH on the removal efficiency of heavy metal ions by hybrid materials was assessed inside a scope of 2–10. Zeta potential is the best and reliable method to determine the adsorbent surface charge, which was characterized by point of zero charge (pHpzc). The pH of zero charge (pHZps) of the as-prepared xerogels M1 and M2 is recognized to be 4.6 0.2 and 4.2 0.2, respectively

Figure 4. 13C NMR CP MAS spectra of M1 (a) and M2 (b).

Figure 5. IR spectra of: (a) M1 and (b) M2.

created between 1,2,4-thiadiazole heterocyclic molecules and the polysiloxane backbone emerged at 1175 cm<sup>1</sup> . The bands located at 780 and 2680 cm<sup>1</sup> , attributed to Si-S links and S-H groups, were observed in the M2 spectrum. In contrast, the signals at 1614 and 3343 cm<sup>1</sup> recognize the presence of Si-N links, NH and NH2 groups, respectively, in M1 [7].

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from… DOI: http://dx.doi.org/10.5772/intechopen.86802

#### 3.2 Effect of pH solution

The pH of the solution is regarded as one of the foremost adsorption factors, since it has a prominent impact on the metal ion solubility, as well as on the adsorbent surface metal binding sites.

The effect of pH on the removal efficiency of heavy metal ions by hybrid materials was assessed inside a scope of 2–10. Zeta potential is the best and reliable method to determine the adsorbent surface charge, which was characterized by point of zero charge (pHpzc). The pH of zero charge (pHZps) of the as-prepared xerogels M1 and M2 is recognized to be 4.6 0.2 and 4.2 0.2, respectively (Figure 6).

As illustrated in Figure 6, when pH ˂ pHZps, the adsorbent surface charge is positive by virtue of the amino and thiol group protonation. Therefore, electrostatic repulsive force emerges between heavy metal ions and the adsorbent surface, inciting abatement in the adsorption capacity. Be that as it may, at pH > pHZps, all samples earn a negative surface charge, promoting electrostatic attraction between metal cations and adsorbent and therefore, the absorption efficiency enhancement. It ought to be stressed that beyond pH 5 [25], the uptake yield of heavy metal ions diminishes through the metallic hydroxide precipitation which thwarts the diffusion of the metal ions into the adsorbent active site. Therethrough, the pH at 7 was selected as the ideal incentive in the resulting experiments (Figure 7).

Figure 6. Zeta potential of xerogels at different pH values.

#### Figure 7.

Effect of pH on adsorption of metal ions onto both xerogels ((a)M1 and (b)M2) (metal concentration 20 mg. L<sup>1</sup> ; adsorbent dosage 0.4 g.L<sup>1</sup> , contact time 1 h).

created between 1,2,4-thiadiazole heterocyclic molecules and the polysiloxane

uted to Si-S links and S-H groups, were observed in the M2 spectrum. In contrast, the signals at 1614 and 3343 cm<sup>1</sup> recognize the presence of Si-N links, NH and NH2

. The bands located at 780 and 2680 cm<sup>1</sup>

, attrib-

backbone emerged at 1175 cm<sup>1</sup>

IR spectra of: (a) M1 and (b) M2.

13C NMR CP MAS spectra of M1 (a) and M2 (b).

Figure 4.

Water Chemistry

Figure 5.

134

groups, respectively, in M1 [7].

#### 3.3 Adsorption kinetics

The contact time is well recognized as the dwelling time of sorbate uptake at the superficial adsorbent surface. To study the effect of contact time, 0.015 g of hybrid materials was thoroughly mixed in 25 mL of initial metal concentration 20 mg.L�<sup>1</sup> and was shaken at a rotational speed of 150 rpm.

As portrayed in Figure 8, the heavy metal adsorption onto the three xerogels rapidly increased in the first 40 min; thereafter, it becomes slower and in the later stage reaches to saturation (equilibrium). Further increase in contact time did not ameliorate the uptake efficiency; this trend may be attributed to the fact that padding of void active sites becomes impossible owing to the electrostatic repulsion between the solute ions of the adsorbent and bulk phases [26]. Thus, 60 min is seen fit to attain equilibrium in ulterior trials.

In order to acquire an insight into the adsorption mechanism and reveal the rate controlling steps, three kinetic models including pseudo-first-order, pseudosecond-order, and intra-particle diffusion were checked.

The adsorption rates were first examined by Lagergren's pseudo-first-order [27] and its linearized integral form is spoken to as follows in Eq. (4):

$$
\ln \left( q\_e - q\_t \right) = \ln q\_e - k\_1 t \tag{4}
$$

capacities ((qth)) evaluated from the pseudo-second-order equation were very close to experimental (qe (ex)) values; besides, the correlation coefficients related to the above-mentioned model were found to be higher than those ascertained from the pseudo-first-order equation. These unequivocally propose that the pseudo-secondorder model, as opposed to the pseudo-first-order model, is more suitable to depict

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from…

By the by, the aforementioned kinetic models cannot identify the diffusion mechanism and also, the rate controlling step of the adsorption kinetic process. In this regard, it is important to apply the Weber and Morris intra-particle diffusion model. This later assumes that the adsorption process might be controlled either by one of the resulting steps, namely film diffusion, pore diffusion, and adsorption onto the inner sites of adsorbent or a mix of a few stages (multi-step process) [29]. The rate parameter for intra-particle diffusion is displayed as follows Eq. (6):

qt ¼ Kidt

boundary layer thickness. If the rate of adsorption is controlled only by the intraparticle diffusion, the value of C should be zero (C = 0) and the plots of qt against

As depicted in Figure 9, all the plots show multi-linear uptake revealing three adsorption stages. The first steep-sloped portion corresponds to the transport of

Pb2+ 285 5.7.10�<sup>4</sup> 178 0.958 17.3.10�<sup>4</sup> 278 0.998 Cd2+ 234 2.8.10�<sup>4</sup> 167 0.953 8.02.10�<sup>4</sup> 38 0.999 Zn2° 410 7.5.10�<sup>4</sup> 249 0.936 19.9. 10�<sup>4</sup> 380 0.998

Pb2+ 261 4.6.10�<sup>4</sup> 185 0,926 14.8.10�<sup>4</sup> 265 0.998 Cd2+ 214 2.4.10�<sup>4</sup> 150 0,942 7.3. 10�<sup>4</sup> 199 0.997 Zn2+ 360 6.6.10�<sup>4</sup> 210 0,912 17.8. 10�<sup>4</sup> 320 0.999

Pseudo-first-order and pseudo-second-order parameters for the adsorption of Pb (II), Cd (II), and Zn (II) onto

Pseudo-first-order kinetic Pseudo-second-order kinetic

) rk2(g mg�<sup>1</sup> min�<sup>1</sup>

where Kid is the intra-particle rate constant (mg:g:min<sup>0</sup>:<sup>5</sup>

) qe cal (mg.g�<sup>1</sup>

Intra-particle diffusion plots model for metal ions adsorption onto (a) M1 and (b) M2.

<sup>0</sup>:<sup>5</sup> provide a straight line passing through the origin.

k1(min�<sup>1</sup>

<sup>0</sup>:<sup>5</sup> <sup>þ</sup> <sup>C</sup> (6)

<sup>0</sup>:<sup>5</sup> and C is the intercept corresponding to the

Þ obtained from the

) qe cal (mg g�<sup>1</sup>

) r

the adsorption procedure.

DOI: http://dx.doi.org/10.5772/intechopen.86802

slope of the straight line qt versus t

t

Metal ions

M1

M2

Table 3.

Figure 9.

137

qe exp (mg g�<sup>1</sup> )

two adsorbents at different temperatures.

where qt and qe (mg.g�<sup>1</sup> ) are the adsorption capacities at equilibrium (mg.g�<sup>1</sup> ) and t (min), respectively, and k1 is the rate constant of the equation (min�<sup>1</sup> ). The rate constant, k1, equilibrium adsorption capacity, qe, and the correlation coefficient R2 , are determined experimentally by plotting ln qe � qt versus t. The allied parameters are listed in Table 1.

The pseudo-second-equation provided by Ho [28] can be stated by the pursuing equation Eq. (5):

$$\frac{t}{q\_t} = \frac{1}{k\_2 q\_2^2} + \frac{t}{q\_2} \tag{5}$$

where k<sup>2</sup> (g.mg�<sup>1</sup> min�<sup>1</sup> ) is the rate constant and q2 is the amount of adsorption equilibrium capacity (mg.g�<sup>1</sup> ). The values of qe and k2 can be determined graphically from the slope and the intercept of the plot t/qt versus t at different temperatures.

The kinetic parameters gained from pseudo-first-order and pseudo-secondorder are introduced in Table 3. It is obvious that the theoretical adsorption

#### Figure 8.

Effect of contact time and temperature on adsorption of metal ions by the two adsorbents (metal concentration: 20 mg.L�<sup>1</sup> ; adsorbent dosage: 0.4 g.L�<sup>1</sup> ; pH: 5): (a) M1 and (b) M2.

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from… DOI: http://dx.doi.org/10.5772/intechopen.86802

capacities ((qth)) evaluated from the pseudo-second-order equation were very close to experimental (qe (ex)) values; besides, the correlation coefficients related to the above-mentioned model were found to be higher than those ascertained from the pseudo-first-order equation. These unequivocally propose that the pseudo-secondorder model, as opposed to the pseudo-first-order model, is more suitable to depict the adsorption procedure.

By the by, the aforementioned kinetic models cannot identify the diffusion mechanism and also, the rate controlling step of the adsorption kinetic process. In this regard, it is important to apply the Weber and Morris intra-particle diffusion model. This later assumes that the adsorption process might be controlled either by one of the resulting steps, namely film diffusion, pore diffusion, and adsorption onto the inner sites of adsorbent or a mix of a few stages (multi-step process) [29]. The rate parameter for intra-particle diffusion is displayed as follows Eq. (6):

$$q\_t = K\_{id}t^{0.5} + \mathcal{C} \tag{6}$$

where Kid is the intra-particle rate constant (mg:g:min<sup>0</sup>:<sup>5</sup> Þ obtained from the slope of the straight line qt versus t <sup>0</sup>:<sup>5</sup> and C is the intercept corresponding to the boundary layer thickness. If the rate of adsorption is controlled only by the intraparticle diffusion, the value of C should be zero (C = 0) and the plots of qt against t <sup>0</sup>:<sup>5</sup> provide a straight line passing through the origin.

As depicted in Figure 9, all the plots show multi-linear uptake revealing three adsorption stages. The first steep-sloped portion corresponds to the transport of


Table 3.

3.3 Adsorption kinetics

Water Chemistry

and was shaken at a rotational speed of 150 rpm.

second-order, and intra-particle diffusion were checked.

, are determined experimentally by plotting ln qe � qt

and its linearized integral form is spoken to as follows in Eq. (4):

ln qe � qt

and t (min), respectively, and k1 is the rate constant of the equation (min�<sup>1</sup>

t qt <sup>¼</sup> <sup>1</sup> k2q<sup>2</sup> 2 þ t q2

graphically from the slope and the intercept of the plot t/qt versus t at different

The kinetic parameters gained from pseudo-first-order and pseudo-secondorder are introduced in Table 3. It is obvious that the theoretical adsorption

Effect of contact time and temperature on adsorption of metal ions by the two adsorbents (metal concentration:

; pH: 5): (a) M1 and (b) M2.

rate constant, k1, equilibrium adsorption capacity, qe, and the correlation coefficient

The pseudo-second-equation provided by Ho [28] can be stated by the pursuing

fit to attain equilibrium in ulterior trials.

where qt and qe (mg.g�<sup>1</sup>

parameters are listed in Table 1.

where k<sup>2</sup> (g.mg�<sup>1</sup> min�<sup>1</sup>

; adsorbent dosage: 0.4 g.L�<sup>1</sup>

equilibrium capacity (mg.g�<sup>1</sup>

equation Eq. (5):

temperatures.

Figure 8.

20 mg.L�<sup>1</sup>

136

R2

The contact time is well recognized as the dwelling time of sorbate uptake at the superficial adsorbent surface. To study the effect of contact time, 0.015 g of hybrid materials was thoroughly mixed in 25 mL of initial metal concentration 20 mg.L�<sup>1</sup>

As portrayed in Figure 8, the heavy metal adsorption onto the three xerogels rapidly increased in the first 40 min; thereafter, it becomes slower and in the later stage reaches to saturation (equilibrium). Further increase in contact time did not ameliorate the uptake efficiency; this trend may be attributed to the fact that padding of void active sites becomes impossible owing to the electrostatic repulsion between the solute ions of the adsorbent and bulk phases [26]. Thus, 60 min is seen

In order to acquire an insight into the adsorption mechanism and reveal the rate

The adsorption rates were first examined by Lagergren's pseudo-first-order [27]

<sup>¼</sup> ln qe � <sup>k</sup>1<sup>t</sup> (4)

versus t. The allied

)

). The

(5)

) are the adsorption capacities at equilibrium (mg.g�<sup>1</sup>

) is the rate constant and q2 is the amount of adsorption

). The values of qe and k2 can be determined

controlling steps, three kinetic models including pseudo-first-order, pseudo-

Pseudo-first-order and pseudo-second-order parameters for the adsorption of Pb (II), Cd (II), and Zn (II) onto two adsorbents at different temperatures.

Figure 9. Intra-particle diffusion plots model for metal ions adsorption onto (a) M1 and (b) M2.


Table 4.

Intra-particle diffusion adsorption rate constants of metal ions onto the two adsorbents.

metal ions from bulk solution to the adsorbent external surface via film diffusion. The second stage describes the progressive adsorption step, indicating the diffusion of adsorbate through the pores of xerogel (intra-particle diffusion). The third smallsloped section corresponds to the final equilibrium stage where the intra-particle diffusion commences progressively to slow down because of the quick abatement of metal cation concentrations. Intra-particle diffusion model parameters are enrolled in the Table 4.

The boundary layer parameter C was diverse to zero, showing that the intraparticle diffusion ought not to be the sole rate limiting step [30]. Be that as it may, it ought to be accentuated that the rate controlled step was governed by film-diffusion towards the start and afterward followed by intra-particle diffusion.

#### 3.4 Adsorption isotherms

Adsorption isotherm is viewed as a standout amongst the most critical elements for determining the mechanism between adsorbent and adsorbate. In this research, Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models were utilized to assess the equilibrium data.

The Langmuir isotherm model expects the formation of monolayer coverage of adsorbate on the external surface of adsorbent and a finite number of equipotential sites [31]. The Langmuir model can take the following linear form Eq. (7):

$$\frac{C\_{\epsilon}}{q\_{\epsilon}} = \frac{1}{q\_{\max}K\_{L}} + \frac{C\_{\epsilon}}{q\_{\max}}\tag{7}$$

Freundlich equilibrium constants kf and 1/n were determined from the slopes and intercepts of the linear plot of log qe versus log Ce . The values for Freundlich

Langmuir, Freundlich, and D-R parameters for Pb (II), Cd (II), and Zn (II) adsorption onto mesoporous

parameters

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from…

Pb2+ 523 0.07 0.998 75.85 0.38 0.845 3.73.10�<sup>4</sup> 13.1 0.993 Cd2+ 507 0.05 0.997 128 0.27 0.871 2.64.10�<sup>4</sup> 13 0.991 Zn2+ 578 0.1 0.999 149 0.42 0.832 4.69.10�<sup>4</sup> 13.63 0.994

Pb2+ 509 0.09 0.997 135 0.27 0.836 4.51.10�<sup>4</sup> 12.82 0.995 Cd2+ 493 0.04 0.998 67 0.41 0.883 3.34.10�<sup>4</sup> 13.33 0.992 Zn2+ 549 0.09 0.998 140 0.37 0.842 4.57.10�<sup>4</sup> 13.22 0.993

) R2 KF 1/n R2 qm (mol.g�<sup>1</sup>

Langmuir parameters Freundlich

) KL(L mg�<sup>1</sup>

DOI: http://dx.doi.org/10.5772/intechopen.86802

As can be seen, the adsorption capacity is higher for M1 than M2. This pattern could be related to the specific surface area as well as to the structure of the amino groups which displayed a high chelating ability to heavy metal ions. However, the isotherm parameters, together with the correlation coefficients showed that the Langmuir model gives a good fit to the adsorption isotherm. Additionally, the Freundlich adsorption capacity kf is higher for M1 than M2. The n values which mirror the adsorption intensity also presented the same trend. The acquired values n for both adsorbates model that the adsorption process onto mesoporous material was favorable at considered conditions. However, compared to the R2 values, with that obtained from the Langmuir model, it can be remarkably noted that the Lang-

The Dubinin-Radushkevich (D-R) model is a semi-hypothetical equation which

is by and large allied to a sorption induced by a pore padding mechanism. This model gives valuable information on the nature of the adsorption process (chemisorption or physisorption) [33]. The linear presentation of the D-R isotherm equa-

) for the adsorption process are also

D-R parameters

) E(KJ mol�<sup>1</sup>

) R2

log qe <sup>¼</sup> log qm � β ε<sup>2</sup> (9)

), β is the activity coefficient which gives an idea about the mean

), and <sup>ε</sup> is the Polanyi potential [<sup>ε</sup> <sup>¼</sup> RTLn <sup>1</sup> <sup>þ</sup> <sup>1</sup>

ffiffiffiffiffiffiffiffiffi

of qm and β, can be generated from the slope and the intercept of the plot ln qe versus

<sup>E</sup> <sup>¼</sup> <sup>1</sup>

adsorption mechanism, either physical or chemical. The maximum adsorption

), q<sup>m</sup> is the maximum adsorption

Ce

) can be calculated using Eq. (10):

) value provided data about the type of

�2<sup>β</sup> <sup>p</sup> (10)

� �]. The values

constants and correlation coefficients (R2

muir isotherm model better fits the equilibrium data.

where q<sup>e</sup> is the adsorption capacity (mol.g�<sup>1</sup>

ε2. The adsorption mean free energy (E, kJ.mol�<sup>1</sup>

In addition, the magnitude of E (kJ.mol�<sup>1</sup>

.J�<sup>2</sup>

recorded in Table 5.

materials M1 and M2.

qm (mg.g�<sup>1</sup>

M1

M2

Table 5.

tion [34] is granted as Eq. (9):

capacity (mol.g�<sup>1</sup>

free energy (mol2

139

where q<sup>e</sup> is the amount of metal cations adsorbed by unit weight of adsorbent (mg. g�<sup>1</sup> ), C<sup>e</sup> is the equilibrium concentration of adsorbate in the solution (mg. L�<sup>1</sup> ), qmax is the maximum adsorption capacity at monolayer coverage (mg.g�<sup>1</sup> ), and KL is the Langmuir adsorption constant (L.mg�<sup>1</sup> ). Linear plots of Ce/qe against Ce were used to determine the value of qmax (mg�g�<sup>1</sup> ) and KL (L�mg�<sup>1</sup> ). The data obtained with the correlation coefficients (R<sup>2</sup> ) are reported in Table 5.

The Freundlich isotherm is an experiential equation which presumes various affinities for the binding sites on the surface of the adsorbent accompanied by the interactions between adsorbed molecules. The linear form of the Freundlich adsorption isotherm [32] can be communicated as follows Eq. (8):

$$
\log \mathbf{q}\_{\mathbf{e}} = \log \mathbf{k}\_{\mathbf{f}} + \frac{\mathbf{1}}{\mathbf{n}} \log \mathbf{C}\_{\mathbf{e}} \tag{8}
$$

where kf is a constant related to the bonding energy and n is a measure of the deviation from linearity and the heterogeneity degree of adsorption sites. The

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from… DOI: http://dx.doi.org/10.5772/intechopen.86802


Table 5.

metal ions from bulk solution to the adsorbent external surface via film diffusion. The second stage describes the progressive adsorption step, indicating the diffusion of adsorbate through the pores of xerogel (intra-particle diffusion). The third smallsloped section corresponds to the final equilibrium stage where the intra-particle diffusion commences progressively to slow down because of the quick abatement of metal cation concentrations. Intra-particle diffusion model parameters are enrolled

<sup>2</sup> (mg.g�<sup>1</sup>

.min�0.5) K<sup>0</sup>

<sup>3</sup> (mg.g�<sup>1</sup>

.min�0.5)

The boundary layer parameter C was diverse to zero, showing that the intraparticle diffusion ought not to be the sole rate limiting step [30]. Be that as it may, it ought to be accentuated that the rate controlled step was governed by film-diffusion

Adsorption isotherm is viewed as a standout amongst the most critical elements for determining the mechanism between adsorbent and adsorbate. In this research, Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models were

The Langmuir isotherm model expects the formation of monolayer coverage of adsorbate on the external surface of adsorbent and a finite number of equipotential

where q<sup>e</sup> is the amount of metal cations adsorbed by unit weight of adsorbent

), qmax is the maximum adsorption capacity at monolayer coverage (mg.g�<sup>1</sup>

The Freundlich isotherm is an experiential equation which presumes various affinities for the binding sites on the surface of the adsorbent accompanied by the interactions between adsorbed molecules. The linear form of the Freundlich

where kf is a constant related to the bonding energy and n is a measure of the deviation from linearity and the heterogeneity degree of adsorption sites. The

1

), C<sup>e</sup> is the equilibrium concentration of adsorbate in the solution (mg.

þ Ce qmax

(7)

),

). The data

). Linear plots of Ce/qe against

) and KL (L�mg�<sup>1</sup>

<sup>n</sup> log Ce (8)

) are reported in Table 5.

sites [31]. The Langmuir model can take the following linear form Eq. (7):

<sup>¼</sup> <sup>1</sup> qmaxKL

Ce qe

adsorption isotherm [32] can be communicated as follows Eq. (8):

log qe ¼ log kf þ

and KL is the Langmuir adsorption constant (L.mg�<sup>1</sup>

Ce were used to determine the value of qmax (mg�g�<sup>1</sup>

obtained with the correlation coefficients (R<sup>2</sup>

towards the start and afterward followed by intra-particle diffusion.

in the Table 4.

Table 4.

Sample K<sup>0</sup>

Water Chemistry

<sup>1</sup> (mg.g�<sup>1</sup>

.min�0.5) K<sup>0</sup>

Intra-particle diffusion adsorption rate constants of metal ions onto the two adsorbents.

M1–Pb 10.252 4.792 2.103 M2–Pb 12.348 6.453 3.501 M1–Cd 11.457 5.395 3.424 M2–Cd 12.567 6.637 3.667 M1–Zn 10.684 5.138 2.735 M2–Zn 12.729 6.841 3.829

(mg. g�<sup>1</sup>

L�<sup>1</sup>

138

3.4 Adsorption isotherms

utilized to assess the equilibrium data.

Langmuir, Freundlich, and D-R parameters for Pb (II), Cd (II), and Zn (II) adsorption onto mesoporous materials M1 and M2.

Freundlich equilibrium constants kf and 1/n were determined from the slopes and intercepts of the linear plot of log qe versus log Ce . The values for Freundlich constants and correlation coefficients (R2 ) for the adsorption process are also recorded in Table 5.

As can be seen, the adsorption capacity is higher for M1 than M2. This pattern could be related to the specific surface area as well as to the structure of the amino groups which displayed a high chelating ability to heavy metal ions. However, the isotherm parameters, together with the correlation coefficients showed that the Langmuir model gives a good fit to the adsorption isotherm. Additionally, the Freundlich adsorption capacity kf is higher for M1 than M2. The n values which mirror the adsorption intensity also presented the same trend. The acquired values n for both adsorbates model that the adsorption process onto mesoporous material was favorable at considered conditions. However, compared to the R2 values, with that obtained from the Langmuir model, it can be remarkably noted that the Langmuir isotherm model better fits the equilibrium data.

The Dubinin-Radushkevich (D-R) model is a semi-hypothetical equation which is by and large allied to a sorption induced by a pore padding mechanism. This model gives valuable information on the nature of the adsorption process (chemisorption or physisorption) [33]. The linear presentation of the D-R isotherm equation [34] is granted as Eq. (9):

$$
\log q\_{\varepsilon} = \log q\_{m} - \beta \left. \varepsilon^{2} \right|\_{\varepsilon} \tag{9}
$$

where q<sup>e</sup> is the adsorption capacity (mol.g�<sup>1</sup> ), q<sup>m</sup> is the maximum adsorption capacity (mol.g�<sup>1</sup> ), β is the activity coefficient which gives an idea about the mean free energy (mol2 .J�<sup>2</sup> ), and <sup>ε</sup> is the Polanyi potential [<sup>ε</sup> <sup>¼</sup> RTLn <sup>1</sup> <sup>þ</sup> <sup>1</sup> Ce � �]. The values of qm and β, can be generated from the slope and the intercept of the plot ln qe versus ε2. The adsorption mean free energy (E, kJ.mol�<sup>1</sup> ) can be calculated using Eq. (10):

$$E = \frac{1}{\sqrt{-2\beta}}\tag{10}$$

In addition, the magnitude of E (kJ.mol�<sup>1</sup> ) value provided data about the type of adsorption mechanism, either physical or chemical. The maximum adsorption

capacity qm, the adsorption free energy E, and the coefficients of linearity are computed and spoken to in Table 5. As observed from the table, the high correlation coefficients (≥ 0.99) propose that the adsorption equilibrium data fitted well the D-R isotherm model. Moreover, the mean adsorption energy values were in the range of 13–14 kJmol<sup>1</sup> for all samples. In perspective of the acquired outcomes, it tends to be reasoned that the adsorption processes of metal ions onto the asprepared xerogels might be proceeded by chemisorption (binding surface functional groups) [35].

xerogel, while M1 displayed a longer breakthrough time. It is clear that breakthrough capacities calculated from column studies were lesser than those settled from the batch method. This pattern might be because of the impact of the prolonged residence time of the sorbate as well as the agitation speed which improve the adsorption in the batch technique. It is worthy to state that the grand

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from…

The regeneration ability is an essential factor for metal recovery and the applicability of adsorbents. The metal charged column was regenerated with 0.1 M HCl

Afterwards, each column was washed with 60 mL of hot deionized water and then dried in an oven at 60°C. The adsorption efficiency of the exhausted column was checked five times. The uptake yield decreased from 96%–94% to 90%–88% for M1 and 93%–91% to 87%–86% for M2 after five adsorption-desorption cycles (Figure 11). The acquired outcomes uncovered that the as-prepared xerogels could be effortlessly regenerated and continuously used in the metal cation removal process without an obvious decrease in the total adsorption performance.

The mechanism of adsorption can be checked through determining thermodynamic parameters like Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°). These parameters can be determined from the following equations: Eqs. (11) and

and R is the gas constant. By plotting LnKL against 1/T, it is possible to determine graphically the value of ΔH° from the slope, and the value of ΔS° from the intercept

The values of Gibbs free energy change ΔG° were negative at various temperatures indicating that the adsorption of the two pollutants onto the as-synthesized adsorbent was feasible and spontaneous. Notwithstanding, the abatement of ΔG° values with temperature could be clarified by a diminishment in the mobility of the metal and the adsorption driving force [36]. The negative value of ΔG° affirmed the

RT ð Þ Van't Hoff equation (11)

),T is the absolute temperature (K),

ΔG° ¼ �RT LnKL (12)

.

breakthrough capacity of M1 is related to its longer breakthrough time.

(40 mL) and then with 0.5 M HNO3 (20 mL) at a flow rate of 7 mL.min�<sup>1</sup>

4.1 Column regeneration

DOI: http://dx.doi.org/10.5772/intechopen.86802

4.2 Thermodynamic parameters

LnKL <sup>¼</sup> <sup>Δ</sup><sup>S</sup>

where KL is the Langmuir constant (L.mol�<sup>1</sup>

(Figure 12). The calculated parameters are given in Table 7.

Adsorption-desorption efficiency of xerogels after 5 cycles: (a) M1 and (b) M2.

<sup>R</sup> � <sup>Δ</sup><sup>H</sup>

(12):

Figure 11.

141
