# Surfactant-Modified Construction Waste Bricks for the Removal of Diclofenac from Aqueous Solutions
## 3 Results and Discussion
### Adsorption Mechanism  
To explore the mechanism of DCF adsorption onto waste bricks, FTIR and XPS analyses were carried out before and after the adsorption. Figure 11 presents the FTIR spectra of the CCBPs and the DCF-adsorbed CCBPs. The absorption peaks observed at 2920 and 2850 cm\({}^{-1}\) correspond to the symmetric and asymmetric stretching vibrations, respectively, of the -CH2- groups in the alkyl chain [26]. Furthermore, the intensity of the alkyl chain is  
Figure 10: Adsorption thermodynamic of DCF onto (**a**) PRBPs and (**b**) CCBPs, (**c**) Van’t Hoff plot of PRBPs and CCBPs (experimental conditions: dosage = 2 g/L, T = 298 K, pH = 4.0 \(\pm\) 0.1).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Adsorbents** & \(T\) **(K)** & \(K_{d}\) **(L/mol)** & \(\Delta H\) **(kJ/mol)** & \(\Delta S\) **(J/mol\(\cdot\)K)** & \(\Delta G\) **(kJ/mol)** \\ \hline \multirow{3}{*}{PRBPs} & 298 & 0.019 & & \(-\)4.86 \\  & 308 & 0.015 & \(-\)28.64 & 112.37 & \(-\)4.67 \\  & 318 & 0.0094 & & \(-\)3.41 \\ \hline \multirow{3}{*}{CCBPs} & 298 & 0.53 & & \(-\)5.38 \\  & 308 & 0.52 & \(-\)18.49 & 80.06 & \(-\)5.46 \\ \cline{1-1}  & 318 & 0.33 & & \(-\)5.26 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Thermodynamic parameters of adsorption of DCF onto PRBPs and CCBPs.  
related to the content of CTAB, suggesting the successful modification of CTAB onto the surface of the waste bricks. After the adsorption of DCF onto the CCBPs, the main peaks of the CCBPs remained unaffected, and the new peaks of the alkyl chain were also maintained, indicating that the adsorption process did not alter the main structure of the CCBPs. However, after DCF adsorption, the spectral band at 1573 cm\({}^{-1}\) (stretching vibration of -N=N=) deformed and moved to 1554 cm\({}^{-1}\) (stretching vibration of -NO\({}_{2}\)), which suggests that one of the adsorption mechanism of DCF may be the \(\pi\)-\(\pi\) interaction [35; 43].  
The XPS of CCBPs before and after the adsorption of DCF are shown in Figure 12. The spectra of C 1s can be divided into three parts: 284.7-284.8 eV, reflecting the C-C or C-H bond, 286.1-286.2 eV, reflecting the C-O or C-N bond, and 288.6-288.7 eV, reflecting the C=O or O-C-O bond [44]. As shown in Figure 12a,b, after the adsorption of DCF, the binding energy of C-O or C-N increased from 286.10 to 286.25 eV, and the binding energy of C=O or O-C-O increased from 288.60 to 288.70 eV, indicating a decrease in the electron density in the outer layer of the C element. These results suggest that the adsorption process is mainly attributed to the chemical reaction. Furthermore, the relative content of C-O or C-N increased from 17.18% to 20.7% and the relative content of C=O or O-C-O decreased from 8.81% to 5.18%, indicating that there is a combination between the oxygen functional groups on the adsorbent and DCF.  
Figure 11: FTIR spectra of CCBPs and DCF-adsorbed CCBPs.  
Figure 12: The XPS spectra of (**a**) C 1s before adsorption, (**b**) C 1s after adsorption, (**c**) O 1s before adsorption, and (**d**) O 1s after adsorption.  
As shown in Figure 12c,d, the O 1s spectrum can be divided into two peak sections: 530.8-531.1 eV could reflect the lattice oxygen in the adsorbent, and 532.3-532.4 eV could reflect the Si-O and O-H bonds in water [45]. After the adsorption of DCF, the relative content of lattice oxygen in the adsorbents before and after adsorption decreased from 9.62% to 6.93%, suggesting that a complexation took place between the oxygen functional groups on the adsorbent and the DCF. The binding energy decreased from 531.1 eV to 530.8 eV, which suggests there may be an electrostatic interaction between the CCBPs and DCF during the adsorption process [35].