Biomass (Hazel Sterculia seed-HS) was functionalized with chitosan and sodium alginate polymers to synthesize novel sorbent materials for the extraction of anionic reactive red 120 dye molecules from aqueous streams. Sodium alginate functionalized biomass (HSSA) and chitosan functionalized biomass (HSCS) were generated using a 1:1 ratio throughout the functionalization process. Characterization techniques (XRD, FTIR, SEM, and BET) showed that the functionalization process effectively modified the morphological properties of the raw biomass, including an increase in surface area from 1.03 m2/g to 7.81 m2/g (HSCS) and an increase to 5.58 m2/g (HSSA). Similarly, batch adsorption studies were conducted with varying starting dye concentrations (10–1000 mg/L), adsorbent dosages (0.05–0.15 g), contact times (0–360 min), agitation speeds (100–500 rpm), and pH (1−10). The Langmuir and Freundlich isotherm models were used to learn more about the adsorption process. Maximum adsorption capacities of 79.35 mg/g (HSCS) and 60.27 mg/g (HSSA) were observed for Langmuir monolayer adsorption, respectively. In addition, kinetic modeling studies showed that pseudo-second order kinetics, with a regression coefficient of R2 > 0.99, was a good fit for RR120 adsorption onto HSCS, HSSA. The thermodynamic properties showed that the adsorption of RR120 onto the selected adsorbents was an endothermic, spontaneous process. Finally, regeneration studies verified that HSCS and HSSA sorbents could be recycled for three, and two cycles respectively.