Response Surface Optimization, Isotherm, Kinetic and Thermodynamic Studies of Methylene Blue Adsorption onto Conductive Poly(3-butylthiophene)

Abstract

Conductive poly(3-butylthiophene) (P3BT) was synthesized via oxidative polymerization and investigated as an adsorbent for methylene blue (MB) removal from aqueous solutions. Structural characterization using FTIR, SEM, XRD, and BET analyses confirmed the formation of a porous semi-crystalline polymer with a BET surface area of 392.5 m² g⁻¹ and abundant adsorption-active sites. Adsorption parameters, including pH, adsorbent dosage, initial dye concentration, contact time, and temperature, were optimized using Response Surface Methodology (RSM) based on Box–Behnken Design. The developed quadratic model was statistically significant (p < 0.0001) with excellent predictive capability (R² = 0.9987), and the adjusted R² and predicted R² were in agreement, with no significant lack of fit. Under optimized conditions of pH 7.5, 0.55 g L⁻¹ adsorbent dosage, 105 mg L⁻¹ dye concentration, 70 min contact time, and 40 °C, a maximum adsorption capacity of 270.46 mg g⁻¹ and 99.6% methylene blue removal efficiency were achieved. Adsorption equilibrium followed the Langmuir isotherm model (R² = 0.9939), indicating monolayer adsorption on homogeneous active sites, while adsorption kinetics followed the pseudo-second-order model (R² = 0.9978), suggesting adsorption was governed predominantly by surface interaction processes. Thermodynamic studies revealed spontaneous, endothermic adsorption, characterized by a negative Gibbs free energy (ΔG° < 0) and a positive enthalpy change (ΔH° = 60.01 kJ mol⁻¹). Regeneration studies showed that P3BT retained approximately 59% adsorption efficiency after five adsorption–desorption cycles, demonstrating moderate reusability and structural stability. The findings establish that conductive P3BT is a promising high-capacity adsorbent for the remediation of dye-contaminated wastewater.