Alkali assisted hydrophobic reinforcement of coconut fiber for enhanced removal of cationic dyes: equilibrium, kinetics, and thermodynamic insight

Abstract

The present study illustrates enhanced removal of methylene blue (MB) and malachite green (MG) from water using alkali-activated coconut fiber (ACF) as adsorbent. Alkali activation effectively reduces the lignocellulosic components present within coco-fiber which in turn reinforces the coco-fiber to become more water-stable. The material was characterized by FTIR, SEM-EDS, BET, XRD, and pH_ZPC. BET surface area was found to be 10.901 m² g⁻¹, whereas pH_ZPC of the material is 6.05. FESEM images reveal rod-like morphology. Batch experiments were optimized with respect to contact time (0–120 min), temperature (288–308 K), pH (3–10), dose (1–5 g) and input dye concentration (10–50 mg L⁻¹). The maximum adsorption coefficient was found to be 133.11 and 110.74 mg g⁻¹ for MB and MG respectively. Adsorptions are best described by pseudo-second-order kinetics (k_MB = 1.712, R² = 0.999; k_MG = 1.399, R² = 0.999) and Langmuir isotherm model (R² = 0.999). Thermodynamic data suggests a spontaneous (ΔG, −14 kJ mol⁻¹) and feasible process. Spent material could be regenerated by using 0.5 M HCl. Up to 50% retention of activities was seen after five cycles. It can be concluded that alkali-activated coconut fiber is an economic and sustainable choice for dye removal.

Novelty statement: Spent coconut was converted into an effective biosorbent by simple alkali activation under ambient conditions to increase the hydrophobicity of the fibers by reducing the lignocellulosic components. Two cationic dyes; methylene blue and malachite green have been efficiently removed with adsorption capacities of 133.11 and 110.74 mg g⁻¹. The operation is simple, economically viable, and partially fulfills the principles of green engineering. Comparing with contemporary adsorbents, this material offers higher adsorption capacities with multi-cycle reusability and enhanced water stability.

Keywords:

• Malachite green
• methylene blue
• coconut fiber
• alkali
• hydrophobicity

Acknowledgment

J.M. and M.A.Q. acknowledge the Central University of Jharkhand, Ranchi, for fellowship.

Disclosure statement

No potential conflict of interest was reported by the author(s).