Adsorption of Crystal Violet Using Kaolin-Based Geopolymer

The removal of synthetic dyes from water resources is essential for environmental protection and sustainable water management. This study aimed to develop and evaluate a kaolin-based geopolymer (KBG) for the adsorption of crystal violet (CV) dye from aqueous solutions. Natural kaolin, an abundant aluminosilicate material in South Africa, was activated using an alkaline solution to form the geopolymer. The synthesized material was characterized using Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy, and Brunauer–Emmett–Teller (BET) surface area analysis. Batch adsorption experiments were conducted to investigate the effects of contact time (5–180 min), adsorbent dosage (0.05–1.0 g), initial dye concentration (10–150 mg/L), temperature (30–50 °C), pH (2–12), and water chemistry on CV removal efficiency. Characterization results confirmed the successful conversion of kaolin to geopolymer, exhibiting a BET surface area of 11.18 m2/g. The optimum adsorption occurred at pH 10.2, where electrostatic attraction between the negatively charged geopolymer surface and the cationic dye molecules was maximized. Kinetic data fitted best to the pseudo-second-order model, while the Langmuir isotherm provided the best description of the equilibrium data. The adsorption mechanism was attributed to electrostatic attraction, hydrogen bonding, and π–π interactions between CV molecules and the geopolymer surface. Thermodynamic analysis confirmed that the adsorption process was spontaneous and endothermic, indicating enhanced dye uptake at elevated temperatures.