## 2 Results and Discussion
### Cd\({}^{2+}\) Adsorption with Biochar
#### 2.3.3 Cd\({}^{2+}\) Adsorption Kinetics for Both Types of Biochar Samples
Figure 6: First-order kinetics analysis for Cd\({}^{2+}\) adsorption by both sets of biochar samples. Notes: RH300, RH400, RE500, RH600, and RH700 indicate that rice husk were pyrolyzed at 300, 400, 500, 600 and 700 \({}^{\circ}\)C, respectively; CS300, CS400, CS500, CS600, and CS700 indicate that cotton straw were pyrolyzed at 300, 400, 500, 600 and 700 \({}^{\circ}\)C, respectively. The kinetics results for Cd\({}^{2+}\) adsorption revealed that the performances of these two biochar materials fit the pseudo-second-order kinetics model well, indicating that the adsorption of Cd\({}^{2+}\) by the rice-husk and cotton-straw biochar was mainly controlled by chemical adsorption, although the adsorption data for samples RH600, CS300, and CS400 were also satisfactorily fitted with the pseudo-first-order kinetics equations. The Cd\({}^{2+}\) adsorption on these samples may occur through diffusion, in addition to chemical adsorption. Some researchers [40] suggested that this adsorption may be associated with the specific surface area. The chemical adsorption of Cd\({}^{2+}\) on biochar mainly relies on an electrostatic ion-exchange mechanism. However, ion-exchange effects are not specific. When the external pH increases and more H\({}^{+}\) ions dissociate from functional groups, a larger number of negative charges remain on the surface of the biochar to adsorb Cd\({}^{2+}\). The increasing adsorbent amount increases the adsorption rate [41]. In our study, the greater the pyrolysis temperature, the higher the biochar pH became, and hence, the faster the adsorption rate became. The time taken for the biochar obtained at high temperatures (600 and 700 \({}^{\circ}\)C) to reach the adsorption equilibrium was shorter than that for the samples obtained at a moderate temperature (500 \({}^{\circ}\)C), which, in turn, was shorter than that for the samples pyrolysed at low temperatures (300 and 400 \({}^{\circ}\)C). However, the relationship between \(q_{\rm m}\) in the pseudo second-order kinetics and the amount of adsorbed Cd\({}^{2+}\) was not evident. This may be directly related to the functional groups in the synthesised biochar. A study by [PERSON] et al. [25] revealed that Cd\({}^{2+}\) adsorption by corn straw mainly occurred through surface hydroxyl (-C-OH) and carbonyl (-C=O) groups. Hence, further research is required to understand which functional groups are present in rice-husk and cotton-straw biochar, and how the pyrolysis temperature changes said functional \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{3}{c}{**First-Order Kinetics**} & \multicolumn{3}{c}{**Second-Order Kinetics**} \\ \cline{2-7}  & \(\mathbf{K_{1}\) **(min\({}^{-1}\))**} & \(\mathbf{q_{\rm m}\) **(mg\(\cdot\)g\({}^{-1}\))**} & \(\mathbf{R^{2}}\) & \(\mathbf{K_{2}\) **(mg\(\cdot\)g\({}^{-1}\)-min\({}^{-1}\))**} & \(\mathbf{q_{\rm m}\) **(mg\(\cdot\)g\({}^{-1}\))**} & \(\mathbf{R^{2}}\) \\ \hline RH300 & 1.02 & 31.08 & 0.783 & \(3.32\times 10^{-4}\) & 103.13 & 0.990 \\ RH400 & 1.43 & 29.90 & 0.880 & \(4.57\times 10^{-4}\) & 76.54 & 0.997 \\ RH500 & 4.21 & 50.24 & 0.688 & \(5.72\times 10^{-4}\) & 35.91 & 0.999 \\ RH600 & 3.06 & 49.69 & 0.908 & \(9.09\times 10^{-4}\) & 22.62 & 0.999 \\ RH700 & 2.95 & 49.00 & 0.847 & \(8.68\times 10^{-4}\) & 26.66 & 0.998 \\ CS300 & 1.50 & 29.15 & 0.939 & \(4.71\times 10^{-4}\) & 168.97 & 0.995 \\ CS400 & 1.22 & 38.07 & 0.970 & \(5.76\times 10^{-4}\) & 115.76 & 0.995 \\ CS500 & 2.74 & 51.11 & 0.872 & \(6.13\times 10^{-4}\) & 191.24 & 0.998 \\ CS600 & 3.00 & 59.90 & 0.842 & \(7.58\times 10^{-4}\) & 127.14 & 0.999 \\ CS700 & 3.09 & 62.30 & 0.859 & \(6.31\times 10^{-4}\) & 110.29 & 0.999 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Kinetic parameters of Cd\({}^{2+}\) removal by both sets of biochar samples. Figure 7: Second-order kinetics analysis for Cd\({}^{2+}\) adsorption by both sets of biochar samples.