Production and characterization of adsorbents from a hydrothermal char by pyrolysis, carbon dioxide and steam activation

Adsorbents for wastewater treatment were produced from industrially obtained hydrochars using pyrolysis, CO2, and steam activation. Both pyrolysis and CO2 activation were studied between 400 to 900 °C, with a holding time of 1 or 2 h. Steam activation was carried out between 600 to 900 °C for a holding time of 2 h. Pyrolysis, CO2, and steam activation yields were in the ranges of 44–72, 26–51, and 14–59 wt.%, respectively. The best performing adsorbents based on phenol and methylene blue removal were obtained at 800 °C and a 2-h holding time with the three gases (N2, CO2, steam). Steam activation of hydrochar provided the best adsorbent with maximum capacities of 179 and 167 mg g−1 toward methylene blue and phenol. The maximum Brunauer–Emmett–Teller (BET) surface area of the product was 593 m2 g−1 with an average pore size diameter of 4.38 nm and a total pore volume of 0.651 cm3 g−1. This study shows that adsorbents derived from hydrochar can be used for wastewater remediation, offering a valorization pathway of a waste stream from hydrothermal processes.