Hydrothermal carbonization (HTC) of carbohydrates and biomass is a straightforward method for preparing hydrochars at low temperatures of 180-250 °C. Hydrochars are more carbonized than their precursors. Increasing the carbonization degree of hydrochars at hydrothermal temperatures is a scientific quest that is addressed in this thesis. Hydrochars are known to have a spherical or irregular morphology. Here we address thin film hydrochars for the first time.  Hydrochars themselves are carbon precursors for preparing activated carbons. Activated carbons are porous materials that can be used for gas adsorption applications. In this thesis, enhanced adsorption of VOCs at low pressures is addressed by using iron phosphate impregnated activated carbons. Finaly, any chemical process or product including those in this thesis such as HTC, activation, hydrochar and activated carbons may contribute to the issue of environmental degradation positively or negatively. Such environmental impacts are addressed by life cycle assessment of processes of HTC and activation and their related products in the last paper of this thesis. Briefly mentioned, in my first study (Paper I), I focused on the HTC of glucose in the presence of iron (II) sulfate. By changing the concentration of iron (II) sulfate, with a catalytic amount, blue hydrochars were formed at the bottom of the autoclave. The blueness was related to thin film interference. The thin film hydrochars were more carbonized than spherical hydrochars and the yield of HTC has increased in the presence of iron (II) sulfate. The second study (Paper II) is focused on the activation of hydrochars with H₃PO₄ and H₃PO₄+FeCl₃. We showed that ultramicroporosity and impregnated iron phosphate species enhance the adsorption of VOCs at low pressure. The ACs were impregnated with Fe (PO₃)₂ and it was shown that Fe (PO₃)₂ acts as an activation agent which opens up for future studies. In the third study (Paper III), H₃PO₄-activated carbons were prepared and modified with FeS and FeSe and it was shown that the ACs were also impregnated with Fe₂P, in the case of AC-FeS/Fe₂P. FeSe and FeS were not detected by XRD. Only large crystals of Fe₂P were detected in the sample AC-FeS/Fe₂P. In the last study (Paper IV), prickly pear seed biomass from the agro sector in Tunisia was hydrothermally carbonized. The hydrochars were then activated into ACs by CO₂ activation. The life cycle assessment of the HTC and activation process was investigated.