Kaolin clay (KN) was employed as an inorganic filler to modify a cross-linked chitosan-glyoxal as Schiff’s-based chitosan composite derivative (CTS-GLY). The resulting (CTS-GLY/KN) was found to be a promising composite synthetic biopolymer that can be potentially utilized for color removal as well as COD reduction of an industrial anionic dye (remazol brilliant blue R, RBBR). The surface porosity, crystallinity, morphology, functionality, charge, and amine content of the CTS-GLY/KN were studied using BET, XRD, SEM, FTIR, pHpzc and pH-potentiometric titration analyses, respectively. Response surface methodology-Box-Behnken design (RSM-BBD) was used to optimize the impact of the main input factors on the color removal and COD reduction of RBBR. The adsorptive performance CTS-GLY/KN towards RBBR was well-defined by both Langmuir and Freundlich isotherm models with highest adsorption capacity of 447.1 mg/g at 30 ˚C. This finding reveals that CTS-GLY/KN can be utilized as a promising, feasible, and environmentally friendly composite-biosorbent for color removal and COD reduction of textile dyes from aqueous medium.