Removal of safranin from aqueous solutions by NaOH‐treated rice husk: thermodynamics, kinetics and isosteric heat of adsorption

Safranin is a widely used, cationic, azine synthetic dye, and because of various adverse properties any effluent streams require treatment prior to discharge. The correct disposal of synthetic dye wastewater after proper treatment is difficult because of the dye properties and their resistance to degradation processes. Adsorption is often an effective, economical and useful process for the treatment of dye effluents. The potential use of rice husk, a plentiful waste agricultural product, to remove safranin from aqueous solution by a batch adsorption process was evaluated. Rice husk was transformed into an effective adsorbent material by treating with sodium hydroxide (NaOH). During the treatment process, the effects of stirring rate, temperature, pH, initial dye concentration and sorbent dose were studied. The maximum dye adsorption was obtained at pH 8. The Langmuir isotherm model was a good representation of the equilibrium data, and the maximum monolayer adsorption capacity obtained was 37.97 mg g−1 at 303 K. The mean free energy (E) estimated from the Dubinin–Radushkevich (D–R) model indicated that the main mechanism governing the sorption process was chemical ion‐exchange. The kinetic data were shown to be represented by the pseudo‐second‐order model and the intraparticle diffusion model. The Arrhenius and Eyring equations were used to evaluate the activation parameters. The activation energy was estimated to be 46.57 kJ mol−1. Gibbs' free energy was spontaneous for all interactions, and the adsorption process exhibited endothermic enthalpy values. The isosteric heat of adsorption, determined using the Clausius–Clapeyron equation, increased directly in relation to the surface loading showing its strong dependence on surface coverage. Results suggest that NaOH‐treated rice husk is a potential low‐cost adsorbent for safranin removal from aqueous solution.