Alkali assisted hydrophobic reinforcement of coconut fiber for enhanced removal of cationic dyes: equilibrium, kinetics, and thermodynamic insight
Jhilirani Mohanta, Roshni Kumari, Md. Atif Qaiyum, Banashree Dey, Soumen Dey
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 pHZPC. BET surface area was found to be 10.901 m2 g−1, whereas pHZPC 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 (kMB = 1.712, R2 = 0.999; kMG = 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.