Abstract <p>The aim of the study was to compare the effectiveness of flax shive modification with alkaline solutions of sodium bisulfite and hydroxymethylsulfinate in relation to solving current problems of sorption treatment of wastewater from divalent metal ions and cationic organic compounds. Using methods of selective extraction and photometry of polycarbohydrates, scanning electron microscopy, and low-temperature nitrogen adsorption, changes in the polymer composition of the shives, the state of the xylem cell walls, and the distribution of the specific surface area by pore size were assessed. The elements of the development of the pore system of the substrate are shown with the achievement of a specific surface area of more than 170 m²/G. Using the method of Fourier-transform IR spectroscopy with decomposition of overlapping signals, the functionalization of lignin macromolecules was traced, and the complex occurrence of reactions of reduction, hydrolysis, sulfonation and demethylation under the action of sodium hydroxymethylsulfinate was confirmed, which provide a content of grafted sulfo groups four to five times higher than treatment with a sodium bisulfite solution, with an increase of 6.6–8.6 times in the absorption of phenolic hydroxyls. The sorption properties of native and modified botrytis samples were studied in a steady state with respect to Cu²⁺ ions, methylene blue dye, and the cationic detergent cetylpyridinium chloride. The most adequate description of the sorption kinetics is provided by the pseudo-second-order kinetic model. The chemisorption indices on native pectin and on transformed lignin in modified samples were differentiated and compared, confirming the significant advantages of replacing the sorption-active biopolymer when treating the shives with a sodium hydroxymethylsulfinate solution.</p>