Modification of kenaf fibers by single step radiation functionalization of 2-hydroxyl methacrylate phosphoric acid (2-HMPA)
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Phosphorylated adsorbent is commonly used in the extraction of rare earth metals and radioactive waste because of its high affinity in acidic condition. In this study, adsorbent containing phosphoric acid groups was synthesized by one-step radiation-induced grafting of 2-hydroxyl methacrylate phosphoric acid monomer (2-HMPA) onto kenaf fibers. The kenaf fibers were pre-treated with sodium chlorite prior grafting process. The incorporation of poly (2-HMPA) in the bleached kenaf fibers was confirmed by Fourier transform infrared spectroscopy (FTIR), FT-IR chemical imaging and scanning electron microscopy (SEM). The structural property was investigated using X-ray diffraction (XRD). The result revealed optimum grafting yield of 91.3% with 10% 2-HMPA and 50 kGy at 40 °C for 3 h. The performance of the synthesized adsorbent showed almost complete thorium adsorption. The removal efficiency of 99.9% thorium from aqueous solution was achieved using fibrous adsorbent at an initial thorium concentration of 10 mg/L, a reaction time of 3 h, and pH of 3 at room temperature. This single step combining grafting and functionalization to modify kenaf fibers showed sufficient property for application of natural fibers as adsorbent.
KeywordsRadiation-induced grafting Kenaf fibers Electron beam Emulsion 2-HMPA
Content of cellulose, hemicellulose and lignin (%) in kenaf fiber
Removal of hazardous metal ions and radioactive elements from the waste waters have gained increasing attention due to the adverse effects of these pollutants to all living organisms. Various methods have been reported for the removal of thorium form solution, including chemical precipitation , solvent extraction  and adsorption . Among all the treatment methods, adsorption is the most effective method for separation and decontamination of thorium from polluted water . Typically, the concentration of thorium in waste water is far below the value that is suitable for separation using solvent extraction and chemical separation, but can be removed effectively by adsorption technique. In addition, more efforts are placed towards improving adsorption technique because of its good operational flexibility, high efficiency, and excellent reusability.
Adsorbents developed using the naturally occuring based materials have gained increasing attention as they are abundant resources that offer cheap and facile modification to impart desired functional groups for specific binding property of the synthesized adsorbent for removal of pollutants from waste water. The kenaf fiber has been utilized as an adsorbent for removal of various pollutants in water such as copper , nickel , lead, zinc , oil removal , dye  and fluoride . However, it has not been investigated for removal of thorium from solution.
The most common method reported on the preparation of radiation grafted adsorbent for removal of metal ions in solution involves two steps, which are graft polymerization followed by chemical modification with desired moiety groups . However, single step modification offers better economy over two-step modification with less consumption of chemicals, time and advantage for up-scaling or industrial production.
In this study, a one-step grafting technique was introduced to synthesize phosphorylated adsorbent by radiation-induced graft polymerization of 2-hydroxyl methacrylate phosphoric acid monomer (2-HMPA) onto kenaf fiber. The effect of grafting parameters to achieve optimum grafting yield was evaluated. The morphology, structure and chemical properties of the synthesized kenaf adsorbent with reference to the original fiber were evaluated. The prepared kenaf adsorbent was tested for thorium adsorption ability in a batch adsorption experiments mode.
2.2 Pre-treatment of kenaf fibers
The pre-treatment was done according to the method in our previous study . The pH of the sodium chlorite (NaClO2) solution of 0.7% was adjusted to 4 and the dried kenaf bast fibers were treated with sodium chlorite solution at 70 °C for 6 h under continuous stirring. Then, the kenaf fibers were drained and washed with distilled water and dried in an oven at 60 °C for 12 h. The lignin content of treated and raw kenaf fiber were determined according to TAPPI T222 om-02 Standard Method for determination of acid-insoluble lignin in wood and pulp.
2.3 Radiation grafting of 2-HMPA onto kenaf fibers
2.5 Adsorption of thorium using kenaf fibers
3 Results and discussion
3.1 Effect of reaction time and monomer concentration on grafting yield
3.2 Evaluation of 2-HMPA radiation-grafted kenaf fibers properties
Crystallinity index (%) from XRD using Segal’s empirical method
Crystallinity index (%)
2-HMPA grafted kenaf
3.3 Adsorption of thorium using 2-HMPA radiation-grafted kenaf fibers
2-hydroxyl methacrylate phosphoric acid (2-HMPA) grafted kenaf fiber has been successfully synthesized by electron beam irradiation by single step grafting. Factors affecting the grafting reaction, i.e. reaction time and monomer concentration were investigated. Based on the overall findings, the grafting of 2-HMPA onto kenaf fibers strongly dependent on 2-HMPA monomer concentration and reaction time. The morphology of ungrafted and grafted fiber was characterized by SEM. The incorporation of 2-HMPA was also confirmed by FTIR and EDX. XRD spectra showed that grafting process has decreased the crystallinity index of kenaf fiber. The grafted kenaf fiber suceesfully removed 99.9% of thorium ions after 180 min of adsorption reaction.
5 Supplementary information
See supplementary information for the complete data of lignin content (Table S1) and thorium uptake capacities (Table S2).
This work was financially supported by Malaysian Nuclear Agency, Ministry of Energy, Science, Technology, Environment and Climate Change (MESTECC) (FP0214D052(DSTIN)) received by N. A. F Othman. The corresponding author also would like to thank Universiti Teknologi Malaysia for providing additional financial and consultation support under Research University Grant (Q.J130000.2546.16H46) awarded to A.K Zulhairun.
Compliance with ethical standards
Conflict of interest
No potential conflict of interest was reported by the authors relevant to this article.
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