Banana (Musa acuminata L) rachis fibres were obtained from the banana rachis, which was collected from rural area Modhupur, a place of Kushtia, in Bangladesh. Kaolinite clay was collected from Bijoypur of Netrokona district in Bangladesh. Industrial wastewater was collected from Kumarkhali, Kushtia, Bangladesh. It contained heavy metal ions Pb(II) and Cr(III) as well as a basic yellow2 dye. The chemicals used in this study, such as sodium hydroxide (NaOH), sulphuric acid (H2SO4), sodium acetate (CH3COONa), acetic acid (CH3COOH), sodium chlorite (NaClO2), and tri-ethyl aniline ((C2H5)3N), were analytical grade and purchased from British Drug Houses (BDH), England.
2.2.1 Preparation of cellulose nano-crystal (CNC) from banana rachis fibre banana rachis fibres were extracted from banana rachis by retting process. The banana rachis was cut about one foot and put into normal water in a container for two weeks. Thereafter, fibres were separated and washed with fresh water several times and finally dried and store in a polybag. The dried fibres were then bleached with 1% NaClO2 (1 g fibres: 80 ml NaClO2) solution at 90-95ºC temperature and pH 4 for 90 minutes with constant stirring. The pH of the liquor was maintained using acetic acid and sodium acetate buffer solution (1 ml buffer solution: 10 ml chlorite solution) to ensure the proper bleaching of the fibres. The bleached fibres were then treated with 17.5 wt.% NaOH solution to obtain α-cellulose by removing hemicellulose. The α-cellulose fibres were then hydrolyzed by 60 wt.% sulphuric acid solution at 45°C for 45 minutes (50 ml acid solution/1 g of α-cellulose) as shown in Figure 1b. After hydrolysis, the nanocrystals were washed with distilled water by centrifugation at 10000 rpm to remove free acid. Washing of crystals by centrifugation process was repeated 5 times. Finally, CNC was collected from the vial of a centrifuge and preserved in ethanol.
2.2.2 Treatment of clay Clay collected from Bijoypur was treated with 5% tri-ethyl amine [(CH3)3N] for 3 hours for removing organic materials and increasing the hydrophobicity of clay. The pH of the solution was maintained between 11.8-11.4 during treatment of the clay. After treatment, the clay was filtered and dried at a temperature of 105°C temperature. Figure 2b shows a photograph of treated clay.
2.2.3 Preparation of composite and column CNC dispersed clay composite filter was prepared by simple blending method. CNCs were blended with the treated clay in a mortar by a spatula which was then soaked into ethanol. Composite filters of 5 g of different clay/CNC compositions 90/10, 80/20, 70/30, and 60/40 as reported in Table 1 were prepared for investigating their performance. A filter of plain clay was also made and used as a control. At first, a 0.5 cm cotton bed was placed at the bottom of a filter column having 1 cm diameter and 80 cm length as shown in Figure 3. Then a sand bed of 0.5 cm thickness was prepared. prepared. After completing the preparation of the sand bed the clay/CNC composite was placed into the column. The thickness of the nanocomposite filter layer was 2 cmAfter completing the preparation of the sand bed the clay/CNC composite was placed into the column. The thickness of the nanocomposite filter layer was 2 cm.
The thickness of the composite layer was very important. If the thickness exceeded that limit then it would take a long time to pass the water through the filter layer, whereas if the layer was lower than the desired scale then the filter layer would lose the resistance properties and allow to pass all the dye molecule through it. 2.3 Characterization techniques 2.3.1 Fourier transform infrared (FTIR) spectroscopy chemical modification of clay, as well as available functional groups present in clay and CNC, were investigated by FTIR spectroscopy using ATR-FTIR (Attenuated Total Reflectance/Fourier Transforms Infrared) spectrophotometer (Model-FTIR8400S spectrophotometer, SHIMADZU Corp, Japan) to functional group detection. The samples were scanned in the frequency range 500-4000 cm-1 at a resolution of 4 cm-1.2.3.2 X-ray diffraction (XRD) spectroscopyXRD analysis of α-cellulose, CNC, untreated and treated clay was carried out by a Rigaku Ultima IV diffractometer operating at 40 kV and 30 mA, using the radiation (λ = 0.1546 nm). The crystalline degree of the samples was determined as the ratio of the areas of crystalline reflections to the whole area (after subtraction of background) in the 2θ range. The XRD method is based on Bragg’s Law [39]: nλ = 2d sinθ.2.3.3 Scanning electron microscopy (SEM)SEM images of treated and untreated clay were taken at 20 kV by a Scanning Electron Microscope, JSM-6490LA, Jeol, Japan. The surface morphology of the treated and untreated clay was investigated from the SEM images.2.3.4 UV spectroscopyUV spectroscopy was used to determine the concentration of dye in wastewater and purified water. UV spectroscopy obeys the Beer-Lambert law. The analysis was carried out on UV-1601. The wavelength was fixed at 520 nm.2.3.5 Atomic absorption spectroscopy (AAS)AAS was used to determine the heavy metal ions (lead and chromium) concentration in wastewater and purified water. The atomizer in which the analyte was atomized was a flame type. For the identification of heavy metal, Varian AA 240 FS atomic absorption spectrophotometer was utilized. The efficiency of banana rachis CNC and clay composite for Pb(II) and Cr(III) ions removal from aqueous solutions was calculated quantitatively by using the following equation: