KOH-activated lightweight expanded clay aggregate (LECA) for malachite green adsorption: activation mechanism and adsorption process assessment

Removing organic dyes soluble in water is of paramount importance for managing and protecting water resources. This study investigated the removal of malachite green (MG) by lightweight expanded clay aggregate (LECA) as a green adsorbent and improved its removal using KOH activation. Alkaline-activated LECA had a pore volume of 0.03 cm/g and a specific surface area of 3.43 m/g. The zeta potential of LECA was decreased with alkaline activation, which ranged between − 10.2 mV for raw LECA and − 24 mV for alkaline-activated LECA. The adsorption of MG on raw and alkaline-activated samples was studied in a batch process to determine the effects of initial dye concentration (10–350 mg/L), pH of the solution (2–10), contact time (5–400 min), and adsorption temperature (15–55 °C). The optimum conditions for MG adsorption were 180 min of contact time, 2.5 g/L of adsorbent, and a pH of 8. The maximum MG removal efficiency at optimal conditions was 56.2% and 90.6%, respectively, for raw and alkaline-activated LECA. Results showed that MG adsorption isotherm was well correlated using a linear Langmuir equation, with maximum adsorption capacities of 14.70 mg/g for raw and 43.10 mg/g for activated samples. Raw and KOH-activated samples were consistent with the pseudo-second-order model with R > 0.99. The standard Gibbs free energies of MG uptake at 15, 25, 35, and 55 °C were negative. The values of ΔH° and ΔS° were 12.783 kJ/mol and 59 J/mol K, respectively. As a result, MG adsorption on KOH-activated LECA was endothermic and spontaneous.