Removal of a cationic dye from contaminated water using poly(phenylenediamine-modified kaolin composite: Box–Behnken optimization, nonlinear kinetic modelling, and thermodynamic studies

Abstract

In this study, kaolin was modified with poly(phenylenediamine) to prepare a new composite (PPDA@K), which was used as a low-cost, environmentally friendly adsorbent for removing gentian violet (GV) from contaminated water. The synthesis of this composite was characterized using several analytical methods, including X-ray diffraction, Infrared Spectroscopy, BET, \(\text{pH}_{\text{pzc}}\), and scanning electron microscopy (SEM). Besides, optimizing the factors governing the PPDA@K dose (\({D}_{\text{c}}\)), sorbent particle size (\({P}_{\text{s}}\)), medium pH (pH), contact time (t), and initial GV dose (\({C}_{\text{GV}}\)) was performed using a Box–Behnken plan to minimize the time and number of experiments. The optimal conditions for the removal of GV were determined using a response surface methodology with a composite objective function, which were as follows: \({C}_{\text{GV}}\) =100 mg L⁻¹, pH = 10, \({D}_{\text{c}}=\) 25 mg L⁻¹, \({P}_{\text{s}}\)= 231.8 μm, and t = 55.96 min to obtain a maximum adsorption amount (Q_e) of 175.55 mg g⁻¹. The kinetic modeling indicates that the adsorption of GV is well described by the pseudo-second order model, with an R² value of 0.9930 and a calculated adsorption capacity (Q_{e cal}) of 178.47 mg g⁻¹. On the other hand, thermodynamic study shows that the negative ΔG° and ΔH° values suggest that gentian violet removal is an exothermic and spontaneous process, and the ΔS° value confirms the stability of the system.