Cationic and anionic dyes adsorption from wastewater by clay-chitosan composite: An integrated experimental and modeling study

Water contamination with organic dyes is a significant environmental concern because it results in serious problems that threaten public health. In this study, a clay-based composite from Safi City, coated with chitosan (Cs), was used as an adsorbent (SRC/Cs) to remove co-existing dyes (methylene blue (MB) and congo red (CR)) from aqueous solution. Characterization methods such as Fourier infrared transform spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with EDX, and zero charge point determination were performed to understand the adsorption mechanisms of the co-pollutants in the adsorbent's surface. The effects of pertinent parameters, such as initial dye concentration, adsorbent's dose, reaction time, pH, and temperature, were optimized to maximize the adsorption of both anionic and cationic dyes. The Langmuir, Freundlich, and Temkin isotherm models were applied to simulate experimental data. The DFT method was used to identify the key mechanism for MB and CR adsorption. It was found that the composite with 30% (w/w) CS and 70% (w/w) clay showed a complete removal of the target dye with an initial dye concentration of 20 mg/L at optimized pH 2 (CR) and pH 4 (MB), 60 min of reaction time, and 600 rpm of agitation speed. The Langmuir model was suitable for modeling the MB dye adsorption, while the Temkin isotherm was representative to describe CR adsorption. The adsorption capacities of both MB and CR dyes were 97.08 mg/g and 80.9 mg/g, respectively. The kinetic study showed that their adsorption process followed the pseudo-second-order. Thermodynamic parameters such as ΔH⁰, ΔG⁰, and ΔS⁰ imply that the adsorption reaction was spontaneous and endothermic. Overall, this study implies the potential of the composite as a novel adsorbent for removing MB and CR in single and binary systems.