Phytochemicals, i.e., flavonoids, phenolics, and anthocyanin, extracted from red cabbage, were crosslinked with alginate to prepare biocomposite hydrogel beads (BHB). The preparation of BHB involved three consecutive steps: (1) extraction and solvent reduction of phytochemicals from red cabbage, (2) crosslinking of phytochemicals into alginate matrix using glutaraldehyde, and (3) formation of the hydrogel beads in CaCl2 solution. The resulting BHB sorbents were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analyses. The cross-section structure of the BHB was confirmed from the SEM images. The alteration of FTIR peaks implied the success of the crosslinking of phytochemical compounds into the alginate. The adsorption equilibrium and kinetic studies of BHB were conducted using basic blue 9 (BB9) as the model adsorbate. FTIR characterization of the BHB post-adsorption reveals the functional groups of the adsorbent involved in the dye adsorption. The calculated adsorption isotherm, kinetics, and thermodynamic parameters show good agreement with the characterization results of adsorbate post-adsorption. The adsorption isotherm is being in congruence with the Langmuir model, and the highest adsorption capacity recorded was 1442.0 mg g−1 at 323 K and pH of 11.0. Adsorption kinetics was better fitted to the pseudo 1st order model than the pseudo 2nd order and Elovich models, which further support the dye physisorption behavior. The initial adsorption rate was influenced by the rapid surface adsorption followed by intraparticle diffusion. The thermodynamic parameters show the spontaneity of the adsorption, and the adsorption proceeds endothermically. The cost analysis shows the economic feasibility of BHB sorbent production for adsorption applications.