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

Abstract

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 (q_m) 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.

Keywords:

• Malachite green dye
• Azolla pinnata
• Adsorption isotherm
• Chemically modified adsorbent
• Kinetics
• Thermodynamics

Acknowledgements

The authors would like to thank the Government of Brunei Darussalam and the Universiti Brunei Darussalam for their financial support. A special thanks to Dr H.M. Thippeswamy of the Department of Agriculture (Soil and Plant Nutrition unit), Ministry of Industrial and Primary Resources, Brunei Darussalam for the provision of the A. pinnata sample. Lastly, appreciation to the Centre for Advanced Material and Energy Sciences (CAMES) of Universiti Brunei Darussalam for their generosity in the usage of XRF machine.