## 3 Results and Discussion
### The Adsorption Kinetics of Cimetidine onto WRB, GWRB, and HWRB  
The adsorption kinetics of cimetidine onto WRB, GWRB, and HWRB are shown in Figure 4. The results showed that the adsorption of cimetidine onto three bricks was fast at the first 1 h then followed by a relatively slow process and finally reached the equilibrium at about 4 h. The rapid adsorption process of cimetidine onto bricks was probably attributed to the microstructure and abundant useful adsorption sites on the surface of bricks [29]. These results could also be observed by other reports, where the adsorption of heavy metal and basic blue onto waste bricks [15; 16]. It is interesting to notice that the adsorption capacities of cimetidine on three bricks are different at the same experimental conditions. The equilibrium adsorption capacities of cimetidine onto WRB, GWRB, and HWRB are 112.8, 194.8, and 166.6 ug/g, respectively. The differences in adsorption capacities for the different adsorbents may be related to the physicochemical characteristics of adsorbents. Figure 4: Adsorption kinetics of cimetidine onto WRB, GWRB, and HWRB. Figure 3: The FTIR of WRB, GWRB, and HWRB. In order to analyze the adsorption kinetics of cimetidine onto waste bricks further, two typical models were used to fit the experimental data. The pseudo-first-order and pseudo-second-order models were represented by Equation (2) and Equation (3), respectively [30; 31]. \[dq_{\text{t}}/dq_{\text{t}}=K_{1}(q_{\text{e}}\text{ - }q_{\text{t}}), \tag{2}\]  
\[dq_{\text{t}}/d_{\text{t}}=K_{2}(q_{\text{e}}\text{ - }q_{\text{t}})^{2}, \tag{3}\]  
where \(q_{\text{t}}\) and \(q_{\text{e}}\) were the adsorption capacity at time t and saturation (ug/g), \(K_{1}\) (s\({}^{-1}\)) and \(K_{2}\) (ug/g/s) are the adsorption rate constants of two models. As shown in Table 3, the correlation coefficients for pseudo-second-order model were relatively higher (\(R^{2}\) = 0.93-0.97), suggesting that the adsorption of cimetidine onto three bricks may be chemisorption. Furthermore, compared with experimental results, the equilibrium adsorption capacities calculated by the pseudo-second-order model were more reasonable than the pseudo-first-order model. Therefore, the adsorption mechanism was mainly attributed to the surface complex reaction between cimetidine and the useful adsorption sites of the bricks. Compared with the \(q_{\text{e}}\) of the three bricks, the incorporation of iron species could increase the amount of useful adsorption sites. The results were in accordance with the adsorption of tetracycline onto iron oxides, which shared the same functional groups (-NH\({}_{2}\) and -CH\({}_{3}\)) as cimetidine [32]. These results suggested that WRB, GWRB, and HWRB could be used as useful adsorbents for the removal of cimetidine from aqueous solutions. Especially for GWRB and HWRB, due to the microstructure and abundant useful adsorption sites, these two adsorbents have the potential for the application in the water treatment. The parameters and correlation coefficients of the two isotherms models were shown in Table 4. The results showed that Langmuir model was more appropriate for the description of the adsorption cimetidine onto WRB, GWRB, and HWRB (\(R^{2}\) = 0.98-0.98), which could be attributed to the monolayer molecular adsorption between cimetidine and bricks. These results were the same as the results that the adsorption of other micropollutants onto adsorbents [22; 23]. Furthermore, the values of 1/\(n\) were close to zero (0.32-0.41), illustrating that the adsorption of cimetidine onto WRB, GWRB, and HWRB was favorable [34]. Furthermore, as showed in Table 4, the calculated \(q_{\text{max}}\) of three bricks followed by the order of GWRB \(>\) HWRB \(>\) WRB, suggesting the adsorption efficiency of waste bricks was heightened by the incorporation of iron species. Compared with hematite, results showed that goethite could offer more useful adsorption sites for cimetidine removal.