Adsorption of methylene blue on natural clay: Characterization, isotherms, kinetics, and thermodynamics

In this study, an abundantly available regional natural clay was investigated as a low-cost adsorbent without modification for the removal of methylene blue (MB). Characterization revealed that the clay consisted predominantly of smectite rich minerals (60%). The high smectite content provides swelling behavior, interlayer adsorption, a high surface area, and a high cation exchange capacity, all of which favor the adsorption of cationic dyes such as MB. Following the characterization analyses, the effects of stirring rate (100–400 rpm), adsorbent dose (0.025–0.100 g/100 ml), initial pH (3.0–8.0), background ionic strength (0–0.1 M), initial dye concentration (25–500 mg/L), and temperature (20–60 °C) were investigated. Under selected optimum operation conditions, an adsorption capacity of 116.9 mg/g and MB removal efficiency of 87.7% were achieved (250 rpm, m:0.075 g, C0:100 mg/L, pH:5.95, NaCl:0, 20°C). The adsorption efficiency increased with increasing adsorbent dose, stirring rate, pH, ionic strength, and temperature, but decreased with increasing initial dye concentration. Complete MB was achieved at a higher adsorbent dose (m:0.100 g). The adsorption kinetics were best described by pseudo-second-order model (R2>0.998), while the strong agreement between the experimental and calculated adsorption capacities (R2>0.99) further confirmed the suitability of the model. The adsorption equilibrium was best described by the Langmuir isotherm model (R2>0.999), resulting in maximum adsorption capacity of 144.93 mg/g and indicating monolayer adsorption on a homogeneous surface. The activation energy was 11.45 kJ/mol, and thermodynamic analysis indicated that adsorption process was endothermic, predominantly physical, and spontaneous (Enthalpy: 64.5 kJ/mol, Entropy: 0.24 kJ/mol, Gibbs free energy: - 4.52 – 13.95). Overall, the naturally available yellow clay exhibited effective performance for MB removal, and shows promise for further investigation at larger scales.