Adsorption of cationic dye crystal violet (CV) onto a modified walnut shell (WS) adsorbent was investigated. Combined treatment of WS using urea and sodium dodecylsulfate (SDS) was carried out. Surface modification of adsorbents was confirmed by FTIR analysis, pHpzc measurements, and elemental and SEM-EDX analysis. In order to optimize the adsorption conditions, the effect of solution pH, adsorbent dose and CV concentration was studied by means of central composite face-centered design (CCFD). The highest correlation between experimental and model data was obtained for the pseudo-second-order (PSO) kinetic model, assuming an ion exchange mechanism of adsorption. A satisfactory fit of CV adsorption data was obtained from the Langmuir adsorption isotherm, supporting a single layer adsorption. According to obtained results, modified WS can be considered as a low-cost, efficient and environmentally compatible biosorbent for the removal of cationic pollutants from aqueous solutions.