Biochars are sustainable carbonaceous materials prepared with biomass by means of different thermochemical processes i.e., pyrolysis, gasification, torrefaction and hydrothermal carbonization (HTC). The main advantages of the HTC process compared to the other methods are its lower energy consumption, fewer gaseous emissions and higher solids yield. HTC decreases the volatile matter and increases the heating value and carbon content of the original biomass. However, the small surface area, poor porosity and low adsorption capacity of hydrochars (HCs) can restrict their scope of application. Activation of HCs can provide the necessary characteristics to make them good adsorbents of pollutants from aqueous and gaseous environments. Depending on the operational conditions and the activation agent it is possible to obtain activated hydrochars (AHCs) suitable for use in a large variety of systems. The main objective of this work was to find an efficient way to produce AHCs. Various activation conditions of the HC including different physical and chemical treatments were studied to determine the optimal conditions for effective usage in a variety of environmental scenarios. The activated hydrochars (AHCs) were obtained by the hydrothermal treatment of pine sawdust (PS) followed by physical or chemical activation. An aqueous dispersion of PS (0.425 ¿ 1 mm) was placed in a 300¿mL stainless steel reactor and heated for 4 h up to 220 ºC. Once the HTC process was concluded, the solid product (hydrochar) was separated from the processed liquid by vacuum filtration, washed with deionized water and dried at 120 ºC overnight. For the physical activation 3 g of HTC-PS was carbonized in N2 and activated using a CO2 flow rate of 10 mL/min at 875 ºC. For the chemical activation a solution of the activation agents (KHCO3 and NH4Cl) was mixed with the hydrochar. In order to increase the N content of the AHCs melamine was applied during the activation with KHCO3. Next, any remaining water was evaporated and the resulting mixture was heat¿treated in a horizontal tube furnace for 1 h up to 850 ºC under a N2 gas flow rate of 350 mL/min. The AHCs were washed with diluted HCl to remove any inorganic salts and then with deionized water until neutral pH. The textural properties of the AHCs were analysed by means of N2 adsorption at 77 K on a Micromeritics ASAP 2420 apparatus. The BET surface area (SBET), the total pore volume at p/po = 0.96, the mesopore and the micropore volume were then calculated. It was found that activation led to a considerable increase in the surface area of the hydrochars and that the chemically activated AHCs had a larger porosity and specific surface area. The activated hydrochars showed significant improvement in their ability to adsorb contaminants.

Peer reviewed