4. Column studies

It is well known that the column trials were conducted to predict the necessary residence time of effluent treatment with a specific flow rate and concentration (Figure 10).

It ought to be stressed that the breakthrough curves characterize the dynamic performance of saturated columns; delineated as the ratio of effluent concentration to influent concentration over time. The time where the pollutant concentration in effluent reached 5% is called breakthrough time (tb); however, the time taken for the effluent concentration to attain 95% of initial pollutant concentration is appealed exhaustion time. The Breakthrough curves for metal ion adsorption onto both adsorbents are portrayed in Figure 6 and their column data are tabulated in Table 6.

As portrayed in Figure 10, all breakthrough curves exhibited an S-shaped profile; besides, earlier breakthrough and exhaustion times were observed for M2

Figure 10. Breakthrough curves for metal ion adsorption onto mesoporous materials M1 (b) and M2 (a).


#### Table 6.

Column data for metal ion adsorption.

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

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

#### 4.1 Column regeneration

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 func-

It is well known that the column trials were conducted to predict the necessary residence time of effluent treatment with a specific flow rate and concentration

It ought to be stressed that the breakthrough curves characterize the dynamic performance of saturated columns; delineated as the ratio of effluent concentration to influent concentration over time. The time where the pollutant concentration in effluent reached 5% is called breakthrough time (tb); however, the time taken for the effluent concentration to attain 95% of initial pollutant concentration is appealed exhaustion time. The Breakthrough curves for metal ion adsorption onto both adsorbents are portrayed in Figure 6 and their column data are tabulated in

As portrayed in Figure 10, all breakthrough curves exhibited an S-shaped profile; besides, earlier breakthrough and exhaustion times were observed for M2

Breakthrough curves for metal ion adsorption onto mesoporous materials M1 (b) and M2 (a).

M1 Zn (II) 45 185 442

M2 Zn (II) 38 175 420

Pb (II) 42 179 410 Cd (II) 40 175 403

Pb (II) 36 172 406 Cd (II) 35 170 394

Metal Breakthrough time Exhaustion time Breakthrough capacity (mg.g<sup>1</sup>

)

tional groups) [35].

Water Chemistry

4. Column studies

(Figure 10).

Table 6.

Figure 10.

Table 6.

140

Column data for metal ion adsorption.

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

#### 4.2 Thermodynamic parameters

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 (12):

$$L\text{n}K\_L = \ \frac{\Delta \text{S}}{R} - \frac{\Delta H}{RT} \text{ (Van't Hooft equation)}\tag{11}$$

$$
\Delta G^\circ = -\text{RT} \, L n K\_L \tag{12}
$$

where KL is the Langmuir constant (L.mol�<sup>1</sup> ),T is the absolute temperature (K), 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 (Figure 12). The calculated parameters are given in Table 7.

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

Figure 11.

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

4.3 Adsorption mechanism

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

exposed to metal ions are listed in Table 8.

onto the two supports was observed.

before and after metal ion adsorption were performed.

SEM micrographs of the two adsorbents after metal-ion adsorption.

(Figure 13).

metal ions.

Figure 13.

143

SEM, FTIR, and XPS analysis have been extensively used to identify the possible

structure of the adsorbents, SEM micrographs were taken after metal ion adsorption

These micrographs indicated clearly the deformation and the presence of many shiny small particles over the surface of both supports M1 and M2 after the adsorption process. Moreover, there was also a decrease in the pore sizes after metal adsorption. This observation evidenced the surface coverage of adsorbents by

To gain further insights into the mechanism involved in the metal ions uptake process, the FTIR spectra were analyzed, and the band positions for each adsorbent

To deepen the understanding of the mechanism of metal uptake, XPS analysis

In the M1 xerogel IR spectrum, the strong band that occurred at around 3325 cm<sup>1</sup> attributed to NH and NH2 stretching vibration was shifted and becomes weaker after metal adsorption. This is likely due to the chelation between amino groups and metal ions. Besides, the peak at about 1614 cm<sup>1</sup> ascribed to NH2 and NH groups disappeared suggesting that the adsorption process is mainly dominated by the coordination of nitrogen with metal cations. However, the characteristic peak at 2680 cm<sup>1</sup> assigned to the stretching vibration of sulfhydryl group (S-H) was disappeared. This result revealed that metal ions reacted with (S-H) groups on the surface of M2 xerogel. No obvious shift of the Si-O group after lead adsorption

metal cation-adsorbent interactions. In order to examine the morphology

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

Figure 12.

Determination of thermodynamic parameters for the adsorption of metal cations onto the two adsorbents: (a) M1 and (b) M2.


#### Table 7.

Thermodynamic parameters for heavy metal adsorption onto the two adsorbents M1 and M2.

exothermic nature of the adsorption process; besides, its magnitude revealed the type of adsorption mechanism (physisorption or chemisorption). Since the ΔH° value was over 20 kJ.mol�<sup>1</sup> , this indicates that the adsorption process of metal ions onto the xerogels occurred by means chemisorption [30]. The observed negative ΔS ° reflected a lessening in the randomness at the solid/solution interface during the adsorption process [37].

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