Batch adsorption studies on the removal of malachite green from water by chemically modified Azolla pinnata

Azolla pinnata (AP), a common water fern found in rice fields in Asia and cultivated for biological nitrogen fixation, was studied as a potential adsorbent for the removal of malachite green in a batch adsorption system. The batch adsorption studies, involving unmodified AP (UAP) and two chemically modified AP—H3PO4-treated AP (PAP) and NaOH-treated AP (NAP)—included the effects of adsorbent dosage, pH, ionic strength, contact time, thermodynamics and kinetics studies, estimation of activation energy and regeneration experiments. Three isotherm models namely the Langmuir, Freundlich and Dubinin–Radushkevich models were used and the Langmuir model best represented all the three adsorption systems with maximum adsorption capacity (qm) of UAP, PAP and NAP at 25°C, to be at 87.0, 292.1 and 109.6 mg g−1, respectively. The kinetics modelling included the pseudo-first-order, pseudo-second-order, Weber–Morris intraparticle diffusion and the Boyd models. Thermodynamic studies showed that all the three adsorption systems are endothermic and spontaneous in nature. All three adsorbents were regenerated with 0.1 mol L−1 NaOH and were effective even after five cycles.