Abstract— <p>Sunflower meal, a renewable agricultural raw material, has been used to produce biochars via hydrothermal carbonization (HTC). We have assessed the influence of biomass processing steps (HTC, carbonization, and alkaline activation), the amount of the alkaline agent, and graphene oxide (GO) additions on the physicochemical and sorption properties of the biochars. According to scanning and transmission electron microscopy data, the uncarbonized material has a disordered structure, which is determined by a considerable amount of amorphous carbon with low porosity. Carbonization opens pore spaces as a consequence of the removal of amorphous organics and formation of defects, which ensure a considerable amount of meso- and micropores. According to IR spectroscopy results, the HTC process includes active removal of thermally unstable organic compounds from the sunflower meal, and alkaline activation causes alkyl groups to appear in the composition of the material, which points to an increase in defect density in the material. These results are satisfactorily correlated with Raman spectroscopy and X-ray diffraction data. We have determined the adsorption capacity of the sunflower meal-derived biochars for Zn²⁺ heavy metal ions and methylene blue (MB), a synthetic organic dye. Results of comparative sorption studies in static mode show that the highest sorption capacity for Zn²⁺ ions and MB, 58.4 and 2301.2 mg/g, respectively, is offered by the GO-containing material. To identify the assumed adsorption mechanism, experimental kinetic data have been analyzed in terms of the pseudo-first order, pseudo-second order, Elovich, and interparticle diffusion models. The results demonstrate that Zn²⁺ ion and MB molecule sorption is a mixed diffusion process contributed by a pseudo-second-order reaction between the contaminant and active centers of the sorbent.</p>