Hydrothermal carbonization in the synthesis of sustainable porous carbon materials for water treatment

Carbon materials are found versatile and applicable in wide range of applications. During the recent years, research of carbon materials has focused on the search of environmentally friendly, sustainable, renewable, and low-cost starting material sources as well as simple cost-efficient synthesis techniques. As an alternative synthesis technique in the production of carbon materials, hydrothermal carbonization (HTC) has shown a great potential. Depending on the application, HTC can be performed as such or as a pretreatment technique. This technique allows synthesis of carbon materials, i.e., hydrochars in closed vessel in the presence of water and self-generated pressure at relatively low temperatures (180-250°C). As in many applications, well-developed porosity and heteroatom distribution are in a key role. Therefore, in this study, different techniques, e.g., varying feedstock, templating, and posttreatment, to introduce these properties to the hydrochars structure were performed. Simple monosaccharides, i.e., fructose or glucose, and more complex compounds such as cellulose and sludge were performed as starting materials. Addition of secondary precursor, e.g., thiophenecarboxaldehyde and ovalbumin, was successfully exploited to alter heteroatom content. It was shown that well-developed porosity (S_BET 550m²/g) can be achieved via one-pot approach (i.e., exploitation of salt mixture) without conventionally used postcarbonization step. Nitrogen-enriched hydrochars indicated significant Pb(II) and Cr(VI) removal efficiency of 240mg/g and 68mg/g, respectively. Sulfur addition into carbon network was not found to have enhancing effect on the adsorption of methylene blue (MB) or change acidity of the carbon material. However, these hydrochars were found to remove 99.9% MB, and adsorption efficiency of these hydrochars remained over 90% even after regeneration. In addition to water treatment application, N-rich high temperature treated carbon materials were proven applicable as electrocatalyst and electrocatalyst support. HTC was shown to be workable technique for the production of carbon materials with variable physicochemical properties, and therefore hydrochars could be applied in several different applications, e.g., as alternative low-cost adsorbent for pollutant removal from water.