## 3 Results and Discussion
### The Influence of Operating Parameters on Column Sorption
#### 3.3.3 The Effect of Residence Time on Cu(II) Removal  
The effect of residence time was investigated by varying residence time (8, 10 and 12 min) at a constant bed depth (20 cm) and a constant Cu(II) concentration of 0.5 mg/L. The breakthrough curve of \(C/C_{0}\) versus time with varying residence time is shown in Figure 9. The results indicated that increasing the residence time at a constant bed depth increased breakthrough time (\(t_{\text{b}}\)). The breakthrough times were 2.05 h, 6.98 h and 15.10 h at residence times of 8, 10 and 12 min, respectively. The bed exhaustion time (\(t_{\text{e}}\)) decreased from 174.04 h to 134.23 h with the residence time decreasing from 12 min to 8 min. With a shorter residence time, the contact between Cu(II) and the lime sand bricks was quicker, thus, the diffusion of copper ions into brick pores is insufficient. When the residence time of solution is shorter in the column, the equilibrium is not reached, therefore, Cu(II) solution leaves column before reaching equilibrium. On the other hand, a higher residence time allows Cu(II) to diffuse and reach the active sites of the sorbent. The finding is in agreement with the results obtained by [PERSON] et al. on neodymium removal using the low cost composite material of chitosan/Iron(III) hydroxide [ChiFer(III)] [46], who conclude that the external film diffusion reduced under the higher superficial velocity, thus, resulting in a faster breakthrough. The Cu(II) adsorption capacity was 59.65, 84.98 and 98.16 mg/kg with the increasing residence times for 8, 10 and 12 min, respectively. These results may be considered that Cu(II) adsorption in lime sand bricks columns requires longer contact times and lower flow rates.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Initial/Inlet Concentration**} & **0.5 mg/L** & **1.0 mg/L** & **2.0 mg/L** \\ \hline \hline \multirow{2}{*}{sorption capacity} & batch system & 0.098 mg/g & 0.114 mg/g & 0.1775 mg/g \\  & column system & 0.088 mg/g & 0.104 mg/g & 0.1696 mg/g \\ \hline \multirow{2}{*}{ratio} & Sorption capacity—Batch & \multirow{2}{*}{1.101} & \multirow{2}{*}{1.096} & \multirow{2}{*}{1.046} \\ \cline{2-2} \cline{5-5}  & Sorption capacity—Column & & & \\ \hline \hline \end{tabular}
\end{table}
Table 6: The comparison of sorption capacity obtained from the batch system and column system.  
Figure 8: The breakthrough curve for varying inlet concentration (mg/L) at a bed depth of 20 cm and a residence time of 10 min.  
Compared with the adsorbent such as activated carbon, although the adsorption capacity is much different, the construction waste bricks are used as a kind of garbage for treating heavy metals in the aqueous solution, and the effect of wastes treatment can be achieved. Moreover, the activated carbon resources are limited, the lifespan is short, and the cost of regeneration operations is high. Compared with other adsorbents, such as biomass, gravel, zeolite, etc., although the adsorption capacity of construction wastes bricks is relatively low, the amount of construction wastes is large and with large economic advantages.