# Adsorption of Ammonium, Nitrate, and Phosphate on Hydrochars and Biochars
## 3 Results and Discussion
### Adsorption of Phosphate  
Similarly to the NO3\({}^{-}\) adsorption capacity using the studied chars, the maximum and the minimum PO4\({}^{3-}\) adsorption values were reached by the AC and HC30, respectively (Figure 5). The average values ranged from 0.12 to 1.23 mg\(\cdot\)g\({}^{-1}\), and 0.57 to 7.15 mg\(\cdot\)g\({}^{-1}\), respectively, when increasing the initial PO4\({}^{3-}\) concentration from 25 to 500 mg\(\cdot\)dm\({}^{-3}\). The adsorption capacity results for PO4\({}^{3-}\) in the BCs were found to be comparable to those observed for NH4\({}^{+}\) and NO3\({}^{-}\). It was observed that the adsorption capacity was relatively higher for BC700 as compared to BC500 and BC300. This finding further supports the earlier conclusion that a reduction in BC pyrolysis temperature results in an increase in the PO4\({}^{3-}\) adsorption capacity. Figure 4: Equilibrium isotherm plots at 20 \({}^{\circ}\)C for NO3\({}^{-}\) sorption on BCs, HCs, and AC. Solid and dashed lines represent the [PERSON] and Freundlich isotherm data models, respectively. The findings of [PERSON] et al. [16] indicate that the adsorption and desorption behaviour of PO4\({}^{3-}\) on BC derived from sugar cane bagasse and _Miscanthus x giganteus_ grass is contingent upon the initial concentration of PO4\({}^{3-}\), pyrolysis temperature, and residence duration. Likewise, [PERSON] et al. [47] found that the adsorption capacity of PO4\({}^{3-}\) was higher when utilising BC obtained through the pyrolysis of sawdust from Pinus trees at a temperature of 750 \({}^{\circ}\)C as compared to that produced at 550 \({}^{\circ}\)C.  
Regarding the HCs, it was observed that HC180 demonstrated a significantly higher adsorption capacity in comparison to HC30. The findings indicate that the augmentation of the residency period from 30 min to 180 min resulted in a rise in the quantity of adsorbed PO4\({}^{3-}\). [PERSON] et al. [20] were successful in obtaining higher values compared to our investigation by utilising an engineered HC that was synthesised through the HTC of lanthanum-pre-treated rice straw. In a pH range spanning from 2.5 to 10.5, the adsorption capacities of P on the HC in question were observed to exceed 50 mg\(\cdot\)g\({}^{-1}\). Furthermore, the impact of competing anions (such as Cl\({}^{-}\), NO3\({}^{-}\), and SO4\({}^{2-}\)) on P adsorption was found to be negligible [20]. In order to compare the properties of BC and HC, the study in [11] analysed the sorption capacities of PO4\({}^{3-}\) in BCs and HCs that were prepared at different temperatures using different types of waste materials. The PO4\({}^{3-}\) sorption capacities of these materials ranged from 0 to 30 mg\(\cdot\)g\({}^{-1}\). The study conducted by the researchers revealed that the alterations in the pyrolysis temperature of the BC and the composition of the feedstock did not have a substantial effect on the capacity of char adsorption. Additionally, the study found that the HC generally exhibited a lower adsorption capacity than the BCs. The augmentation of the initial ion concentration has been observed to intensify the adsorption process, plausibly owing to the amplified concentration gradients, thereby leading to a more comprehensive occupation of the responsive adsorption locales [45]. Previous research has indicated that the adsorption of PO4\({}^{3-}\) increased with an increase in the  
Figure 5: Equilibrium isotherm plots at 20 \({}^{\circ}\)C for PO4\({}^{3-}\) sorption on BCs, HCs, and AC. Solid and dashed lines represent the Langmuir and Freundlich isotherm data models, respectively. initial concentration of the ion. However, the efficiency of adsorption decreased, likely due to a reduction in the number of active adsorption sites available at higher initial PO4\({}^{3-}\) concentrations, as demonstrated by other studies [48].