Adsorption of toxic crystal violet dye using coffee husks: Equilibrium, kinetics and thermodynamics study

Crystal violet (CV) dye is a water-soluble, toxic, recalcitrant organic dye that causes severe health problems and environment pollution. The current study assesses the potential of Waste Coffee Husks (WCH) in adsorption of crystal violet (CV) dye. Batch experiments were designed to determine the effect of pH, dye concentration, adsorbent dose, and contact time in dye removal. Result revealed that removal of CV was highly favored by lower temperatures and acidic medium. Dye removal per unit mass of WCH increased with increase in contact time, dye concentration and surface area. At ambient temperature, the maximum adsorption efficiency of up to 94% was achieved within 10 min at pH 3. Adsorption capacity of Waste Coffee Husks was found to be 12.0360 mg/l for initial dye concentration of 12.2394 mg/l at 25 °C. The experimental data followed pseudo-second-order kinetic models indicating that dye adsorption was controlled by chemical process involving valence forces through sharing or exchange of electrons between CV dye and WCH. Adsorption equilibrium data followed Freundlich and Langmuir isotherm models. Gibbs free energy (∆G°) of the system showed increasing spontaneity with temperatures while positive entropy value (∆S°) implied random disorder of molecules featured at solution/solid interface. Negative enthalpy value (∆H°) confirmed exothermic nature of this adsorption process. These results demonstrate that Waste Coffee Husk are cost-effective, eco-friendly and low-cost bio renewable material for dye removal in industrial effluents.