# Adsorption of Ammonium, Nitrate, and Phosphate on Hydrochars and Biochars
## 3 Results and Discussion
### Adsorption of Ammonium  
The impact of carbonisation temperature and residence duration on the adsorption capacity and surface area of the chars was highly significant. The equilibrium isotherms and the adsorption capacity variation as a function of the char used and the initial NH4+ concentration are illustrated in Figure 3. The results of this study indicate that HC180 exhibited a higher adsorption capacity compared to HC30. However, BC700 demonstrated a lower adsorption capacity in comparison to BC500 and BC300. The results indicate that BC300 exhibited the most significant NH4+ adsorption capacity, while HC30 demonstrated the least. Upon increasing the initial concentration of NH4+ from 25 to 500 mg\(\cdot\)dm\({}^{-3}\), the observed mean values for the HCs exhibited a range of 0.63 to 2.67 mg\(\cdot\)g\({}^{-1}\), while the mean values for the BCs ranged from 2.59 to 9.05 mg\(\cdot\)g\({}^{-1}\). In terms of NH4+ adsorption capacity, the AC was surpassed only by BC300. The adsorption behaviour of the BCs was observed to be influenced by the initial concentration of NH4+ in the solution, as well as the temperature of carbonisation. The results of the observations indicate an inverse relationship between NH4+ adsorption capacity and carbonisation temperature. Specifically, as the temperature of carbonisation increased, the capacity for NH4+ adsorption decreased. Comparable results were obtained by [PERSON] et al. [14] on biochars derived from oak sawdust through pyrolysis at temperatures between 300 \({}^{\circ}\)C and 600 \({}^{\circ}\)C. The results of this study indicate that the adsorption of NH4+ is augmented by BCs that are generated at lower temperatures. As per prior research,  
Figure 3: Equilibrium isotherm plots at 20 \({}^{\circ}\)C for NH4+ sorption on BCs, HCs, and AC. Solid and dashed lines represent the Langmuir and Freundlich isotherm data models, respectively. it can be observed in Figure 1 that the BC produced at a higher pyrolysis temperature lacked the presence of aromatic C=O and C=C, -CH\({}_{2}\)-, CO, and CC functional groups. As a result, it was observed that the functional polar groups present in the material underwent a reaction with NH\({}_{4}\)+ during the adsorption process. This led to a significant increase in the adsorption capacity of the BC that was produced at lower temperatures, even its surface area was greatly lower than that produced at higher temperatures. The observed results indicate that there was no significant variation in the NH\({}_{4}\)+ adsorption capacity of the HCs that were assessed. The results suggest that varying the residence time between 30 and 180 min does not significantly impact the NH\({}_{4}\)+ adsorption capacity. In their study, [PERSON] et al. [11] conducted hydrothermal carbonisation at 250 \({}^{\circ}\)C to synthesise HC and compared it with BCs obtained from different biomass sources at low and high pyrolysis temperatures (400-450 \({}^{\circ}\)C and 600-650 \({}^{\circ}\)C, respectively). The study found that there were variations in the physicochemical properties of the char and the refining conditions. However, the NH\({}_{4}\)+ sorption capacities ranged between 105.8 and 146.4 mg\(\cdot\)g\({}^{-1}\). As per the findings of the study, it can be inferred that the NH\({}_{4}\)+ adsorption capacity is not primarily influenced by the surface area [11; 45]. The present study employed the Langmuir and Freundlich isotherm models to investigate the adsorption behaviour of NH\({}_{4}\)+, NO\({}_{3}\)-, and PO\({}_{4}\)3- on various samples of BCs, HCs, and AC. The results of the analysis are presented in Table 2. According to the existing literature, the Langmuir isotherm model posits that sorption occurs uniformly across all binding sites. Conversely, the Freundlich isotherm model suggests that the surface is heterogeneous, with an uneven distribution of adsorption capacity over the surface [46].