The present study explores possibility of using sunflower seed husk agricultural biowastes from industrial pyrolysis power plant as a feedstock for preparation of activated carbon (AC). Sunflower biomass by-products contain up to 70 % of carbon with residual seed husk components, have low specific surface area (less than 21 m2 g−1) and smaller number of oxygen-containing groups in comparison with wood-based biochars. The main process parameters of AC preparation, namely a type of an activation agent (organic solvent, HNO3, NaOH), temperature and duration of the carbonization step, were optimized in respect to adsorption efficiency. We found that simple pre-treatment of sunflower biomass with tetrahydrofuran easily converts it into biochar precursor having large number of carbonyl and hydroxyl groups. Further thermal and chemical activation of the biochar precursor results in formation of AC products with specific surface area in the range from 625 to 980 m2 g−1 and from 1000 to 1755 m2 g−1, respectively. Methylene blue (MB) was used as a test molecule for characterization of adsorption properties of the obtained AC. Kinetics studies showed that MB adsorption on AC is fast and follows pseudo-second-order model. MB adsorption on biochar and AC is well described by Langmuir isotherm, assuming monolayer formation of MB on the surface of adsorbents via chemical interactions. Adsorption capacity of AC towards MB achieved by nitric acid activation is higher (379.0 mg g−1) than capacity of benchmark commercial activated carbon from Norit. The study opens up possibilities for the development of by-product-based adsorbents via the straightforward, eco-friendly approach, with specific focus on the economic effect achieved with these materials. The University of Oviedo is grateful for financial support from Spain’s Ministry for the Economy and Business - MINECO (MAT2016-78155-C2-1-R). Peer reviewed