A biodegradable filter for the removal of various pollutants, contaminants, or harmful substances from the surrounding air, and a method of fabrication. The filter includes 30% by weight of cellulose nanofibers, 30% by weight of activated carbon, and 20% by weight of polyethylene glycol. The cellulose nanofibers and activated carbon are isolated from date palm biomass, making the filter cost-effective and environmentally friendly.

A method for transforming a black liquor, comprising the following steps: a) supplying a black liquor, b) gasifying the black liquor by means of supercritical water gasification in the presence of a hydrogenated carbon-free fuel, by means of which reaction products are formed, c) cooling the reaction products, by means of which a gaseous mixture is obtained which comprises at least methane and a green liquor comprising at least sulphur in the form of NaHS, Na.sub.2S and H.sub.2S.