## 3 Results and Discussion
### Adsorption Kinetics  
Kinetics data for the adsorption of MB onto both the raw and modified hydrochars versus contact time are presented in Figure 6a and b, respectively.  
Sorption of MB onto HC (Figure 6a) increased rapidly during the initial 2 and 4 h of contact time for 190 HC and 220 HC, respectively. For both samples, sorption increased a little more until 6 h of contact time and then slowly, reaching an asymptotic value: 41.8 mg g\({}^{-1}\) and 41.2 mg g\({}^{-1}\) for 190 HC and 220 HC, respectively. The trend for sample 250 HC differs significantly from those discussed above. The adsorption of MB onto sample 250 HC occurs very slowly with a q\({}_{\text{t}}\) that increases from 10.2 to 29.4 mg g\({}^{-1}\) during 8 h of contact time, finally reaching a value of 37.5 mg g\({}^{-1}\) at 24 h. Thus, this observation is consistent with the adsorption isotherm results, where sample 250 HC showed a lower adsorption capacity compared to samples 190 HC and 220 HC.  
Considering the modified hydrochars (Figure 6b), the adsorption of MB onto MHC increased very rapidly. Samples 190 MHC and 220 MHC reached both a q\({}_{\text{t}}\) value equal to 48.0 mg g\({}^{-1}\) after only 15 min of contact time and this value remained quite stable up to 24 h. Sample 250 MHC shows an adsorption of MB slightly slower than the other two samples; the q\({}_{\text{t}}\) value increased up to 50 mg g\({}^{-1}\) during the initial 2 h of contact time and then remained stable until 24 h. Thus, at the maximum contact time tested all the samples show approximately the same adsorption capacity accounting for 50.0, 52.5 and 51.5 mg g\({}^{-1}\) for 190 MHC, 220 MHC and 250 MHC, respectively. It is evident that the KOH-treated hydrochars reach a higher value of q\({}_{\text{t}}\) compared to the raw hydrochars. The adsorption of MB onto MHC, especially for samples 220 MHC and 250 MHC, is to some extent comparable to that of the activated carbon. In all cases, the adsorption of MB was very rapid. Moreover, the adsorption capacity of the activated carbon resulted equal to 64.3 mg g\({}^{-1}\), that is about 22% higher than that of the MHC samples.  
The trends of 190 HC, 220 HC, 190 MHC, 220 MHC and 250 MHC are similar; in all the samples the sorption of MB onto hydrochars increased during the first period of contact time and then slowed down significantly until an asymptotic value was reached, most probably related to an equilibrium stage. This can be explained considering that there are available active sites on the adsorbent that are  
Figure 6: MB adsorption kinetics on (**a**) raw and (**b**) modified hydrochars.  
reduced gradually with the adsorption progress [28]. The main difference between raw and modified hydrochars is the adsorption rate; what happens for the raw hydrochars within 8 h of contact time occurs, instead, for the modified hydrochars within 15 min or at most 2 h. This means that the treatment with KOH causes a change in the hydrochar structure and surface, speeding up the adsorption process.  
The pseudo-first order and pseudo-second order kinetic models were used to evaluate the kinetics of the MB adsorption process. Data and related linear regression lines for the two models are reported in Figure 15 (see Appendix A) while Table 3 reports the calculated kinetics parameters.  
The correlation coefficients of the second order kinetic model are greater and closer to unity (R\({}^{2}\) > 0.95) than those of the first order kinetic model for both the raw and modified hydrochars obtained at all the HTC treatment temperatures. These results reveal that the pseudo second-order equation is preferable to describe the adsorption kinetics, and it is very accurate for five samples (R\({}^{2}\)\(\geq\) 0.999) out of six. Thus, the adsorption process involved chemical interactions between MB and the polar functional groups present at the surface of HC and MHC [34].