In recent years, the consumption of fruits of European chestnut has considerably enhanced due to their positive health effects. However, the chestnut peeling process generates solid residues (inner and outer shells), which account for about 10–15% of the whole chestnut weight. In the present study, an integration between a chemical and a thermochemical process is proposed as a valorization route for the chestnut residues: the extraction of polyphenols, a class of strong natural antioxidants, and the slow pyrolysis for biochar production. The chestnut residues after the polyphenols extraction are used as pyrolysis feedstock, and the produced biochars are applied as adsorbing materials to simplify the recovery of the extracted polyphenols.
The aim of this study is to evaluate how the physical and chemical characteristics of biochar from chestnut residues influence the adsorption of polyphenols. The biochar production was carried out in a slow pyrolysis reactor using two feedstocks (as received and post-extraction chestnut residues) and three pyrolysis temperatures (500 °C, 600 °C and 700 °C), thus resulting in six different biochars. Each biochar was used as an adsorbent material for the polyphenols in the aqueous extracting solution obtained from chestnut residues. Specific classes of polyphenols were considered, such as non-tannin polyphenols, hydrolysable tannins and condensed tannins. The adsorption efficiency of biochar increases in the char produced at 700 °C for both the considered feedstocks. The analysis of the specific polyphenols groups shows that, despite having an overall adsorption capacity much lower than activated carbon, biochars have a great selectivity for the adsorption of non-tannin polyphenols.