## 4 Discussion
### Biochar Adsorption Mechanisms  
The biochar performance in terms of maximum adsorption capacity was not so notable, due to pH values lower than pH\({}_{\text{PZC}}\) (8.38). Under the conditions of the tests, NH\({}_{4}\)\({}^{+}\)/NH\({}_{3}\) equilibrium was mainly shifted towards NH\({}_{4}\)\({}^{+}\) and the charge distribution on the biochar surface was positive due to pH lower than pH\({}_{\text{PZC}}\). In these conditions, the ammoniacal N adsorption through cation exchange between NH\({}_{4}\)\({}^{+}\) and H\({}^{+}\) on the biochar surface was not favoured due to the high concentration of H\({}^{+}\) in the aqueous solution and the electrostatic repulsion between NH\({}_{4}\)\({}^{+}\) and the positively charged biochar surface. However, when the conditions established during the manure storage tests (T = 20 \({}^{\circ}\)C, pH in the range 8-9 and ammoniacal N concentration equal to 1 g/L) were reproduced in the NH\({}_{4}\)Cl aqueous solution (tests M and Bc), according to NH\({}_{4}\)\({}^{+}\)/NH\({}_{3}\) equilibrium at 20 \({}^{\circ}\)C, free NH\({}_{3}\) concentration ranges between 4 and 29% and when pH is higher than 8.38 the charge distribution on the biochar surface shifts from positive to negative. Different mechanisms can occur under these conditions such as: (i) physio-sorption of gaseous NH\({}_{3}\) in the biochar micropores [37], (ii) reaction of NH\({}_{3}\) at the oxygen groups on the biochar surface leading to the formation of amines and amides [38], (iii) NH\({}_{3}\) protonation at the acid functional groups on the biochar surface followed by NH\({}_{4}\)\({}^{+}\) adsorption [38], (iv) NH\({}_{4}\)\({}^{+}\) adsorption through cation exchange [37]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Include Characteristics**} & \multicolumn{6}{c}{**Include Characteristics**} & \multicolumn{6}{c}{**Lighted Street Characterization**} & \multicolumn{2}{c}{**Nift**} \\ \hline
**Feedback Origin** & \ \begin{tabular}{c} **Pridly** \\ Temperature (C) \\ \end{tabular}  & **p/p1** & **p/p/p** & **Application** & **Dose** & **Type** & \begin{tabular}{c} **T** \\ \(\text{C}\) \\ \end{tabular}  & **p/p1** & **DM** & **TEN** & **Backstation** & **Storage** & **Duration** & **Ref. **  
In this study, the physio-sorption of gaseous NH\({}_{3}\) was not observed (see Table 4) since, as suggested by [17], NH\({}_{3}\) physical interaction with the surface of the biochar is expected to be very weak due to the small size of the molecule. The results of the Bc test indicated that all the volatilized NH\({}_{3}\) is adsorbed on the biochar surface in the form of NH\({}_{4}\)+ after H+ abstraction (NH\({}_{3}\) protonation) from the acid groups on the biochar surface, whose presence is confirmed by the measured total acidity (0.0697 mmol/g). In this case, only a small reduction of pH from 8.8 to 8.4 is observed from the beginning to the end of the test, in agreement with the low amount of volatilized NH\({}_{3}\). These findings are in good agreement also with [22], which showed similar ammoniacal N adsorption (0.84 mg/g\({}_{\text{biochar}}\)) on wood shaving derived biochar produced at 500 \({}^{\circ}\)C. As observed by other authors [41; 42], the greater the number of acidic functional groups (especially hydroxyl and carbonyl groups), the greater the NH\({}_{3}\) adsorption capacity of the biochar. Finally, the results of the M test showed that, under the conditions resembling the digestate storage tests, NH\({}_{4}\)+ adsorption through cation exchange was negligible. Indeed, only 2% of the ammoniacal N lost is adsorbed on the biochar surface in form of N-NH\({}_{4}\)+, and ammoniacal N is mostly released due to NH\({}_{3}\) devolatilization. This result can be explained considering that the pH reduction from 8.8 to 7.6 determines a shift towards the NH\({}_{4}\)+ form of the ammoniacal nitrogen and a net positive charge on the biochar surface exerting an electrostatic repulsion against the NH\({}_{4}\)+ ions.