To achieve high sorption capacity in removal of anionic pollutants, the beet waste (BW) as a cheap material was successfully functionalized by cross-linking with epichlorohydrin to attach amine groups. Before functionalization, the BW was ball milled for production of nanoparticles (NPs). The functionalized beet waste (FBW) has excellent characteristics, such as high surface area, porosity, and sorption capacity. The adsorption of reactive black 5 (RB5), as an azo dye, was evaluated by BW and FBW NPs. The removal efficiency of 95% for FBW NPs and 57% for BW NPs was achieved at sorbent concentration of 1 and 40 g L−1, respectively. The maximum sorption of 98% for removal of 100 mg L−1 RB5 by 1 g L−1 FBW NPs was rapidly achieved in 15 min and pH 6.5. Although the NPs size and surface area in BW and FBW are almost similar, FBW NPs represented high sorption performance compared with BW NPs indicating the vital role of functionalization in sorption process. The sorption kinetics and equilibrium studies were explored and the results demonstrated that experimental data was fitted properly to the pseudo-second-order kinetics and Langmuir isotherm model, respectively. Evaluating ΔH° (22.47 kJ mol−1) and Ea (10.0 kJ mol−1) for RB5 sorption by FBW suggests a physisorption which provides the possibility of frequent reuse.

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