Modification of banana stem fibers for efficient mercury (II) adsorption

Abstract

The potential of banana stem fiber (BSF) and its modifications as a low cost adsorbent for Hg(II) adsorption was evaluated. In this study, three types of adsorbents were prepared; chemically treated BSFs, grafted BSFs and amine functionalized grafted BSFs. Chemically-treated BSFs were modified by HCl and NaOH pre-treatments. Grafted BSFs were prepared by grafting methacrylic acid (MAA) onto BSF using ß-radiation, microwave-radiation and conventional chemical initiation grafting methods. Subsequently, the grafted BSF was further functionalized using three types of amine, namely ethylenediamine (EDA, primary amine), N,N’- methylenebisacrylamide (MBA, secondary amine) and triethylamine (TEA, tertiary amine). Prior to adsorption, all the adsorbents were activated in vacuo at 373 K. The adsorbents were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, nitrogen physisorption, thermogravimetric analysis (TGA), field emission scanning electron microscopy-energy dispersive X-ray (FESEM-EDX), electron spin resonance (ESR) spectroscopy and pHzpc. Results of the study showed that HCl pre-treatment on BSF increased the cellulose accessibility, while activation process generated a large amount of structural defects on the BSF. Moreover, BSF grafted via ß-radiation (BSF-ß) was proved to have a higher grafting yield, which led to a higher Hg(II) adsorption capacity. In addition, the introduction of amines into BSF-ß significantly enhanced the Hg(II) uptake due to the stronger affinity towards Hg(II) ions. The functionalization of ethylenediamine onto BSF-ß (EDABSF) showed the highest Hg(II) adsorption capacity followed by MBA-BSF and TEABSF. These results indicate that the position of amine functional groups on BSF plays an important role in the adsorption process. Fitting of the adsorption data with the nonlinear Langmuir isotherms produced the maximum adsorption capacity of 372, 484 and 843 mg g-1 for activated BSF-HCl, BSF-ß and EDA-BSF, respectively. Besides, activation of BSF-HCl altered the activation energy from 3.5 to 76.9 kJ mol-1, which showed the increasing of Hg(II) chemisorption. On the other hand, the adsorption of Hg(II) onto BSF-ß and EDA-BSF were mainly via ion-exchange process, where their activation energy falls in the range of 13.7 to 19.2 kJ mol-1. It should be noted that BSF-HCl, BSF-ß and EDA-BSF adsorbents were effectively regenerated with 0.1 M HCl solution and exhibited good recyclability and reusability for a few cycles of Hg(II) adsorption. Furthermore, BSF-ß and EDA-BSF possessed an excellent adsorption capacity for cationic heavy metals (Hg2+, Pb2+ and Cd2+), offering potential applications in the recovery of cationic heavy metals from multi-metal aqueous systems. This study demonstrates an inexpensive yet effective material such as BSF from agricultural waste, has a great potential as adsorbent for the removal and recovery of Hg(II) ions from aqueous solutions.