Application of Casuarina equisetifolia needle for the removal of methylene blue and malachite green dyes from aqueous solution

This study investigated the potential of Casuarina equisetifolia needle (CEN) on the removal of two important dyes, methylene blue (MB) and malachite green (MG), by batch adsorption experiments. Characterisation of CEN’s functional groups was done using Fourier Transform infrared spectroscopy while elemental analysis was carried out using CHNS analysis and X-ray fluorescence. The experiments were carried out by varying the adsorbent dosage, pH, ionic strength, contact time and initial dye concentration. The pseudo-second-order kinetics model best represented the experimental data for both CEN-MB and CEN-MG systems. The Weber–Morris intraparticle diffusion model showed that intraparticle diffusion is not the rate-limited step for both adsorbates, while the Boyd model suggested both systems could be controlled by film diffusion. The Langmuir, Freundlich and Dubinin–Radushkevich isotherm models were used for describing the adsorption process. Of these, the Langmuir model best represented both adsorbents systems (CEN-MB and CEN-MG) giving maximum adsorption capacity (qm) of 110.8 and 77.6 mg g−1, respectively, at 25 °C. Thermodynamics studies showed that both adsorption systems are spontaneous and endothermic.