The chemical cross-linking of carboxymethyl chitosan (O-CM-chitosan), as a method for its modification, was performed using trimellitic anhydride isothiocyanate to obtain novel cross-linked O-CM-chitosan hydrogel. Its structure was proven using FTIR, XRD and SEM. Its adsorption capacity for the removal of Methylene Blue (MB) dye from aqueous solution was studied. The effects of different factors on the adsorption process, such as the pH, temperature and concentration of the dye, in addition to applications of the kinetic studies of the adsorption process, adsorption isotherm and thermodynamic parameters, were studied. It was found that the amount of adsorbed MB dye increases with increasing temperature. A significant increase was obtained in the adsorption capacities and removal percentage of MB dye with increasing pH values. An increase in the initial dye concentration increases the adsorption capacities, and decreases the removal percentage. It was found that the pseudo-second-order mechanism is predominant, and the overall rate of the dye adsorption process appears to be controlled by more than one step. The Langmuir model showed high applicability for the adsorption of MB dye onto O-CM-chitosan hydrogel. The value of the activation energy (Ea) is 27.15 kJ mol−1 and the thermodynamic parameters were evaluated. The regeneration and reuse of the investigated adsorbent was investigated.

O‐Carboxymethyl chitosan (O‐CM‐chitosan) hydrogel, prepared in our previous study, was investigated in this work for the first time as adsorbent for Basic Red 12 (BR 12) dye. With rising temperature (from 25°C to 55°C) and decreasing solution pH (from 9 to 4), a lowering in both adsorption capacity (from 703.47 to 400.79 mg/g) and removal efficiency (from 70.34% to 40.08%) was observed. A significant increase in adsorption capacity (from 342.75 to 621.39 mg/g) was observed with increasing initial dye concentration (from 400 to 1000 mg/L), while a decrease in removal efficiency (from 85.69% to 62.14%) was noted. Pseudo‐second‐order model fitted well the adsorption. Isotherms of adsorption conform to Langmuir model with a maximum monolayer adsorption capacity of 625 mg/g. The value of ∆H° (−23.99 kJ/mol) implied exothermic nature of adsorption process. Values of ΔG° indicated that the adsorption was spontaneous at low temperature and nonspontaneous at high temperature. The value of activation energy is equal to 13.071 kJ/mol. The dye removal percentage from the hydrogel was 88% at the first cycle. Thus, O‐CM‐chitosan hydrogel has promising and effective properties for removing BR 12 and similar dyes from aqueous media.