Preparation of activated carbon sorbent from highly saline waste phytomass

The study presented here solved the problem of very low specific surfaces, which are achieved when using the standard method of physical activation of brown macroalgae (Phaeophyceae) waste. The material consist-ing of crushed thallus residues from the processing of macroalgae for food purposes contained 14.6% of ash, which consisted mostly of Na, K, Mg and Ca salts. This material was first subjected to slow pyrolysis at a rate of 10 °C.min–1 to a final temperature of 450 °C, at which 19% condensate, 40% gas, and 41% solid residue were obtained on a dry basis. The pyrolysis residue then served as an input raw material for the subsequent activation step. Three alter-native activation procedures performed by steam in a ver-tical fixed-bed reactor were compared. In the first of them, the pyrolysis residue was heated to 800 or 900 °C in an inert atmosphere (N2), and at this temperature it was exposed to steam. Under the given activation conditions, which are otherwise fully satisfactory for other types of phytomass and dendromass, the salts present in the sam-ple led to an underdeveloped porous system. It was thus possible to prepare only products with a specific surface area (BET) of a maximum of 15 m2 g‒1. In the second procedure, the pyrolysis residue was washed with a larger volume of distilled water and, after filtering under vacuum and drying, was steam activated. After activation, the product inside the reactor was washed with water, removed and then washed again un-der vacuum. This procedure led to a product with a spe-cific surface area of 400 m2.g-1. In the third activation procedure, the pyrolysis resi-due was heated to 300 °C in an inert atmosphere and hy-drated with steam at this temperature for 30 minutes. This was followed by heating to the final activation tempera-ture and re-dosing of steam. The product obtained was washed with a large volume of distilled water and vac-uum filtered. By applying the described method, at an ac-tivation temperature of 800 °C combined with steam dos-ing for 30 minutes, the highest specific surface area (BET) of 675 m2.g-1 was achieved, and at the same time the product yields were higher than in the above proce-dure. If the same procedure used a temperature of 900 °C combined with only 2 min of steam introduction, a BET surface area of 636 m2.g‒1 was achieved, but the overall yield of activated product increased from 17 to 25% (cal-culated for the entire process including pyrolysis). This procedure was therefore found to be the good compro-mise between BET surface area and yield.