Zein as an adsorbent for removing methylene blue dye from water

Abstract

The adsorption-based dyes removal from wastewater requires efficient, and sustainable processes and adsorbent materials. This study aimed to investigate the adsorption of methylene blue dye using zein as an adsorbent material. The influence of adsorbent weight and solution pH, as well as the adsorption kinetics and equilibrium, were evaluated in batch systems containing zein and the dye, with controlled shaking speed and temperature. The adsorption process was analysed by fitting kinetic data to pseudo-first-order, pseudo-second-order, and Weber-Morris intraparticle diffusion models, and by fitting equilibrium data to Langmuir and Freundlich isotherm models. The pseudo-first-order kinetic model provided the best fit under the experimental conditions, yielding an adsorption rate constant (k) of 4.42 g mg⁻¹ min⁻¹. The Weber-Morris model indicated that intraparticle diffusion is the rate-limiting step of the process. The equilibrium study was conducted across a range of temperatures, and the Langmuir isotherm model yielded the best fit. Zein showed a high potential for methylene blue adsorption, achieving a maximum removal capacity (q_max) of 0.831 mg g⁻¹ at 25°C, which could reach up to 4.24 mg g⁻¹ at 70°C. The Gibbs free energy (ΔG < 0), enthalpy range (ΔH = -41.44 kJ mol⁻¹), and entropy range (ΔS = -0.068 kJ mol⁻¹ K⁻¹) highlight the efficiency of methylene blue removal by zein, confirming that the process is both spontaneous and exothermic.