## 3 Results and Discussion
### Adsorption of Ammonium Nitrogen on Biochar
#### 3.2.4 Adsorption Kinetics  
The adsorption kinetics of KOH-activated biochar (produced at 750 \({}^{\circ}\)C) at various adsorption temperatures are presented in Figure 3a. It was found that the adsorption trends for different temperature were similar. Specially, the first 2 h were attributed to the rapid adsorption stage for NH4+-N, which was owing to the exposed surface, porous network structure, and the high amounts of vacant adsorption sites. The adsorption capacity (\(Q_{\mathrm{J}}\)) increased from 11.58 to 14.34 mg/g when the adsorption temperature increased from 25 to 45 \({}^{\circ}\)C. [PERSON] et al. [34] investigated the adsorption of NH4+-N onto progressively oxidized maple wood biochar and found that the maximum adsorption capacity was 10.1 mg/g. [PERSON] et al. [20] prepared biochar from anaerobic digestion residue, with an  
Figure 2: Effect of solution pH on NH4+-N adsorption and the zeta potential of biochar (adsorption conditions: NH4+-N, 100 mg/L; biochar dosage, 4 g/L; temperature, 25 \({}^{\circ}\)C). NH4+ adsorption capacity of 8.37 mg/g. [PERSON] et al. [PERSON] et al. (2019) prepared activated carbon from avocado seeds through methanesulfonic acid activation and found that its NH4+-N adsorption capacity was 5.4 mg/g. The adsorption process was further fitted using the pseudo-first-order and pseudo-second-order models, which are expressed in Equations (2) and (3), respectively:  
\[Q_{t}=\ Q_{e}(1-exp-k_{1}t) \tag{2}\]  
Figure 3: (**a**) Adsorption kinetics of NH4+-N onto biochar (NH4+-N: 100 mg/L, biochar dosage: 4 g/L, temperature: 25–45 \({}^{\circ}\)C), as well as the fitting curves using pseudo-first-order and pseudo-second-order models. (**b**) Langmuir and Freundlich equilibrium adsorption isotherms for the adsorption of NH4+-N onto biochar (adsorbent dose: 4 g/L, temperature: 25–45 \({}^{\circ}\)C). \[Q_{t}=\frac{k_{2}Q_{e}^{2}t}{1+k_{2}Q_{e}t} \tag{3}\]  
where \(Q_{e}\) (mg/g) is the equilibrium amount of NH4+-N adsorbed by the biochar, \(Q_{t}\) (mg/g) is the amount of NH4+-N adsorbed by the biochar at time \(t\) (min), and \(k_{1}\) (1/min) and \(k_{2}\) (g/(mg-min)) are the rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively. The plots of \(Q_{t}\) against \(t\) are presented in Figure 3a, and their fitting curves using the two models are also given in Figure 3a. Table 1 shows the results of the nonlinear regression analysis of the two kinetic models. The results in Figure 3a show that the theoretical adsorption capacities (\(Q_{e,\mathrm{cal}}\)) using the pseudo-second-order kinetic model were closer to the experimental adsorption capacities (\(Q_{e}\)) than those using the pseudo-first-order kinetic model. The pseudo-second-order kinetics model had higher correlation coefficient values (\(R^{2}\)) than the pseudo-first-order model (Table 1). These results suggest that the adsorption of the NH4+-N on KOH-activated biochar followed a pseudo-second-order kinetics model. Besides this, the values of both \(Q_{e,\mathrm{cal}}\) and \(k_{2}\) increased when the temperature was raised from 25 \({}^{\circ}\)C to 45 \({}^{\circ}\)C, suggesting that the adsorption process was intensified by elevating the adsorption temperature. The relationship between temperature and the rate constant can be described by the Arrhenius equation, which is expressed as follows:  
\[lnk_{2}=lnA-\frac{E_{a}}{RT} \tag{4}\]  
where \(k_{2}\) (g\(\cdot\)mg\({}^{-1}\cdot\)min\({}^{-1}\)) is the adsorption rate constant of the pseudo-second-order model, \(A\) (min\({}^{-1}\)) is the pre-exponential factor, and \(R\) (8.314 J/(K\(\cdot\)mol)) and \(T\) (K) are the ideal gas constant and the absolute temperature, respectively. The activation energy (\(E_{a}\)) magnitude can provide an insight into the adsorption type [35]. In this work, the value of \(E_{a}\) for the adsorption of NH4+-N on KOH-activated biochar was 47.01 kJ/mol after calculation, indicating that the adsorption is a chemical adsorption dominant type owing to the high \(E_{a}\) value (\(>\)40 kJ/mol).