Tetra-$n$-butylammonium bromide modified sugar beet pulp (TBAB-SBP) with good biosorption and regeneration potential was prepared for removal of dye from contaminated solutions. The decolorization performance of TBAB-SBP was investigated using Reactive Yellow 2 (RY2) as a model dye. The characterization of the modified biosorbent was investigated by IR, SEM, zeta potential, and BET surface area analysis to discuss the decolorization mechanism. The effects of initial pH, biosorbent amount, contact time, dye concentration, and flow rate on the treatment process were examined. The experimental kinetic data were well predicted by the pseudo-second-order kinetic model and Langmuir isotherm with a maximum monolayer biosorption capacity up to 7.20 $\times $ 10$^{-5}$ mol g$^{-1}$. The modified biosorbent was also successfully used in dynamic flow treatment mode. Moreover, the prepared biosorbent could be regenerated using 0.01 M NaOH during 15 biosorption/desorption cycles. Experimental results indicated that the electrostatic interaction, complexation, and ion exchange were possible interactions in the decolorization process. The real wastewater treatment application showed that TBAB-SBP is a suitable green-type biosorbent for the decolorization of reactive dye-contaminated aquatic media.