Quantification of chemical states, dissociation constants, and oxygen functional groups on hydrochar

Hydrothermal carbonization (HTC), is a thermochemical process which converts organic biomass into carbon-rich solid product called hydrochar. HTC is performed at temperature around 180-260 °C at water saturation pressure. Using water as reaction medium allows wet biomass to hydrothermally carbonized without drying. Biomass, during HTC, undergo for hydrolysis, decarboxylation, dehydration, aromatization, and polymerization reactions. As a result, hydrochar possesses some exceptional functional properties. Over the years, hydrochars have been applied as catalyst, catalyst support, adsorbent, and solid fuel.

The surface functional groups on the surface of chars play a vital role to use them as a adsorbent. Some research has been done on hydrochar surface properties. However, the systematical investigation is still required to quantification of surface functional groups such as an oxygen-containing group on hydrochar. It is imperative to understand the content of such groups on the surface of hydrochar is the key for its multifunctional applications.

Therefore, the objectives of this study were to quantification of chemical states, understanding the range of dissociation constants (pKa) and content of oxygen functional group in hydrochar. A series of hydrochars from paper mill sludges at different temperature (180-260 °C) and residence time (5-30 min) were produced. ATR-FTIR spectroscopy was used to determine the presence of various oxygen functional groups on the surface of hydrochars. Acid-base titration analyzed the chemical states and pKa of functional groups. The functional groups on the surface of hydrochars vary at different experimental conditions (temperature and residence time). The values of pKa on hydrochars increases with HTC severity.