In current research work, chitosan (Chi) was subjected to subsequent physical and chemical modifications by incorporating kaolin clay (KA) into its polymeric structure, and crosslinking process with a covalent cross-linker namely epichlorohydrin (ECH) respectively. The final product of crosslinked chitosan-epichlorohydrin/kaolin (Chi-ECH/KA) composite was successfully applied for color removal and chemical oxygen demand (COD) reduction of textile dye namely reactive blue 19 dye (RB19) from aqueous environment. The influence of pertinent parameters, i.e. A: Chi-ECH/KA dose (0.02–0.1 g), B: pH (4–10), and C: time (5–30 min) on the RB19 color removal and COD reduction were statistically optimized by using response surface methodology with Box–Behnken design (RSM-BBD). The experimental data of the adsorption kinetic and the adsorption isotherm demonstrated a better fitness to pseudo-second order model and Langmuir isotherm model respectively. Excellent absorption ability of 560.9 mg/g was recorded for Chi-ECH/KA composite. The calculated thermodynamic functions clarified that the RB19 adsorption process was endothermic and spontaneous in nature. The mechanism of RB19 adsorption onto the Chi-ECH/KA may include electrostatic interactions, hydrogen bonding, Yoshida H-bonding, and n-π interactions. This study introduces Chi-ECH/KA composite as an eco-friendly, potential and multi-function composite bio adsorbent for removal of textile dye and COD reduction from aqueous environment. •Chitosan-epichlorohydrin/kaolin (Chi-ECH/KA) composite was synthesized.•The Chi-ECH/KA was applied for color removal and COD reduction of reactive blue 19 (RB19) dye.•Box–Behnken design was applied to optimize the adsorption input variables.•The adsorption capacity for RB19 was 560.9 mg/g.•The adsorption isotherm follows Langmuir model.