Adsorptive removal of chromium(VI) from aqueous solution using binary bio-polymeric beads made from bagasse
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In this study, bio-polymeric gel beads were made from synthetic and laboratory-made CMC (bagasse). Calcium chloride cross-linked with sodium alginate (Na-Alg) added to CMC displayed great affinity for the removal of hexavalent chromium (Cr(VI)) ions present in an aqueous solution. Activated carbon obtained from bagasse was also used for adsorptive removal of Cr(VI) ions from aqueous solution. The effect of different adsorption parameters such as pH, contact time and adsorbent dosage was studied. Bio-polymeric gel beads and activated carbon were prepared and characterized by SEM, FTIR and XRD. The maximum percentage removal for synthetic and bagasse bio-polymeric gel beads reaches 94.56% and 98.42% values at a pH of 4.0 at 25 °C and for activated carbon 64.79% value at a pH of 6.0 at 25 °C. Higher degree of substitution results in an increase in the percentage removal of Cr(VI) ions due to the increase in the surface area and the binding sites of the adsorbent. Our study suggests that bio-polymeric gel beads made from laboratory-made CMC (bagasse) can be used in a more cost-effective and efficient way for the removal of harmful chromium ions.
KeywordsCarboxymethyl cellulose (CMC) Sodium alginate Gel beads Activated carbon
Heavy metals, widely used in industrial wastewater, are extremely toxic to human kidneys, liver, lungs and intestines. Discarded chromium (Cr) is a general toxic heavy metal pollutant present in wastewater, where it basically exists in two stable oxidation states, i.e., trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Chromium is widely used in industries like electroplating, leather tanning, ceramics, pigment manufacturing, ceramics, wood preservation and manufacturing of paper. Chromium is used in leather tanning process in large quantity to stop water diffusion inside leather pores. Cr(VI) is primarily present in the form of chromate (CrO42−) and dichromate (CrO72−) ions. On the other hand the Cr(VI) is also 500 times more toxic than the trivalent form (Garg et al. 2007; Fahim et al. 2006). Cr(VI) is a very soluble and toxic chromate anion and is a distrusted carcinogen and mutagen. The conventional physical and chemical methods used for the chromium removal from wastewater include reduction, solvent extraction, precipitation, ion exchange, membrane filtration, reverse osmosis and adsorption (Sathish et al. 2015). Precipitation process is favored, but the major drawback is sludge formation. Ion exchange is a better technique, but its operating cost is high. Cellulose is a linear and high molecular weight polymer, and due to the presence of inter- and intra-molecular –OH bond, it neither melts nor dissolves easily in common solvents and it can be chemically modified to increase its metal-binding ability (Yang et al. 2011; Selvi et al. 2001; Khezami and Capart 2005). Cellulose is highly crystalline in nature, and this high crystallinity results in low adsorption capacity for heavy metal ions such as chromium. Adsorption on bio-polymeric adsorbent made from CMC which is extracted from cellulose is an extremely effective way for the removal of heavy toxic metals because of low cost and high feasibility. Activated carbon is also efficient in the removal of heavy metals because of its low cost and easy regeneration of the carbon. Cr(VI) adsorption is dependent on pH, and maximum removal takes place between pH 5 and 6 for activated carbon. It is an amorphous solid involving of microcrystallites with a graphite lattice, and they are nonpolar, highly porous, usually equipped in powder form (Abdulrazak et al. 2017; Fahim et al. 2006). Natural bio-polymeric beads offer number of advantages such as nontoxic, inexpensive, renewable, biodegradable, modifiable, etc. This is an efficient method for the removal of heavy metals from wastewater. In this study, we focus on bio-polymeric gel beads made from carboxymethyl cellulose (CMC). CMC is a water-soluble polysaccharide formed by mercerization and etherification process. CMC displays alkaline solubility when the degree of substitution is about 0.3 and displays water solubility when the degree of substitution is above 0.4 (Kumar et al. 2018). CMC is highly amorphous in nature and has high adsorption capacity for heavy metal ions removal. CMC is widely used in oil exploration, detergents, cosmetics, paper products, food and textile industries. Synthetic CMC obtained directly from market offers 0.51 degree of substitution (DS), and laboratory-made CMC from bagasse has a DS of 0.65 (Gulati et al. 2014; Joshi et al. 2015; Mohkami and Talaeipour 2011). This higher DS of laboratory-made CMC results in better use in the commercial products (Yeasmin and Mondal 2015). Na-Alg is a natural polysaccharide bio-polymer mainly composed of mannuronic and guluronic acid and contains free carboxyl groups which make it attractive addition to CMC. Alginate can be cross-linked with several divalent and trivalent cations (i.e., Ca, Ba and Fe) to form a stable gel (Pourjavadi et al. 2006; Ren et al. 2016). The main objective of the current study is to calculate the adsorption capacity of bio-polymeric gel beads made from synthetic CMC and laboratory-made CMC (bagasse) and activated carbon obtained from bagasse waste for the efficient removal of Cr(VI) from the aqueous solution by varying the pH, adsorbent dosage and contact time. The bio-polymer gel beads obtained were characterized and analyzed by infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction analysis.
Materials and methods
The chemicals used to make bio-polymeric beads were synthetic CMC having high molecular weight (Mw = 10,000 Da, DS = 0.51) that was obtained from Molychem. We manufactured the CMC from bagasse (DS = 0.65). All the other chemicals used were of analytical grade.
Cellulose isolation from bagasse waste
Removal of moisture and reduction in size
First, half kilogram of bagasse was dry in distilled water for 24 h to eliminate water-soluble extractives and dried in an oven at 75 °C for 2 days. Afterward, bagasse was grind to pass through a 60-mesh size screen.
Dilute sulfuric acid (4% v/v) can lead to the enzymatic biomass hydrolysis to eliminate hemicelluloses sugars through stirred at the temperature of 80 °C for 2 h. The treated fibers were washed with Millipore water to attain pH 7. The filtered bagasse fibers were dried in an oven at 70.0 °C temperature.
2% NaOH was fed to the reactor at 80.0 °C to temperature for 2 h to enhance the swelling of the inner surface and reduces crystallinity and lignin structure disturbance. The treated fibers were filtered and washed with Millipore water to attain pH 7. The filtered bagasse fibers were dried in an oven at 70.0 °C temperature.
Whiteness was obtained by using sodium chlorite (4% w/v) at 80.0 °C for 4 h. The pH maintained during bleaching process was within 3–4 adjusted by adding acetic acid drop by drop. After washing, cellulose fibers were dried in an oven at 70 °C temperature.
Production of CMC from cellulose
The cellulose produced from bagasse was dried in an oven for 1 h at 70.0 °C temperature, and after that, it was placed in the desiccator to avoid moisture. There are two steps for the production of CMC mercerization followed by etherification.
In total, 100 ml isopropanol in 100 ml of 20% w/v NaOH solution was mixed with 10 grams of cellulose sample that helps to break the inter- and intra-molecular –OH bonding. The mixture was stirred for 2.5–4 h and increased the temperature from 50.0 to 60.0 °C. The washed alkali cellulose fibers were filtered and dried in an oven for 3–4 h s at 70.0 °C, and then, it was sent for etherification process.
Preparation of bio-polymeric beads
Preparation of activated carbon from bagasse waste
Activated carbon was prepared from sugarcane bagasse waste under nitrogen atmosphere. The pyrolysis of waste was done at 800 °C. After pyrolysis, the char was soaked with potassium hydroxide (KOH) and mixture was dried overnight at 90 °C. The produced activated carbon was transferred to a beaker containing hydrochloric acid (0.1 mol/l) (to remove inorganic compounds), stirred for 1 h and then washed with warm deionized water to remove residual HCl, until the pH of the solution neutralized and then filtered. The filtered activated carbon was dried in an oven at 90 °C overnight (Eslami et al. 2018).
Preparation of the chromium solution
Chromium solutions of 1000 mg/dm−3 were prepared by dissolving 0.245 mg K2Cr2O7 in 1000 ml of double-distilled water. The required operational solutions of different concentrations were prepared by proper dilution of the stock solution. The reagent was prepared by dissolving 0.2 g diphenyl carbazide in ethyl alcohol and sulfuric acid (1:2) concentration. After that the reagent was stored in amber bottle to avoid light.
Batch mode adsorption studies
Characterization of adsorbent
Fourier Transform infrared spectroscopy (FTIR) indicates chemical occurring in the bio-polymer. For analysis the samples were crushed with KBR to make pellets, and spectra were taken on PerkinElmer BX 11-FTIR Spectrophotometer, in the range of 4000–400 cm−1. The surface morphology of the bio-polymeric beads and activated carbon was examined using scanning electron microscopy (SEM). The dried samples were layered with a thin layer of palladium gold alloy after mounting on a twice sided carbon tape (Zeiss 1555 VP). The dried samples were compressed into the disks and then measured with an X-ray diffractometer models recorded with Rigaku D/MAX-2400 X-ray diffractometer system using a Cu Ka radiation (l ¼ 1.5406 Å) in the 2q range from 5° to 70°, operated at 40 mA and 40 kV, and a scanning speed of 10°/min.
Results and discussion
Effect of pH on removal
Effect of time on removal
Effect of adsorbent dosage
Functional group of CMC, polymeric gel bead and activated carbon
Wave number (cm−1)
Bands indication of CMC
Bands indication of polymeric gel
Bands indication of activated carbon
Carboxylic –OH stretching
–CH aliphatic stretching
–CH aliphatic stretching
C–H aliphatic stretching
C–O stretching of carboxyl group
Asymmetrical stretching –COO−
C=C stretching band
Symmetrical stretching –COO−
–C–H aliphatic bending
C–O carboxylic acid, alcohols, esters
In this study bio-polymeric gel beads manufactured from synthetic and laboratory-made CMC (bagasse) and sodium alginate show great promise as an adsorbent for the removal of chromium ions. The pH also affects degree of ionization of the adsorbent, solubility of metal ions and charge present on the adsorption sites. An increase in pH hikes the adsorption capacity due to de-protonation (reduction of H+ ions) of the metal-binding sites. Activated carbon made from bagasse also displays high chromium percentage (64%) removal, but it is much lower than synthetic gel beads and bagasse gel beads (94.56%) and (98.45%). Gel beads concentration also effects the Cr(VI) removal. Bagasse bio-polymeric gel bead shows higher percentage removal than synthetic bio-polymer gel bead and activated carbon because laboratory-made CMC (bagasse) has higher degree of substitution that increases surface area and porosity and hence provides more active sites for adsorption. This porosity is due to the electrostatic repulsions among the carboxylate anions (COO−) which increases the size of pores in the gel beads. This study provides an economical, easily available adsorbent for the removal of chromium ions from the aqueous solutions.
We greatly acknowledge the support of CIR, MNNIT, Allahabad, and department of physics, Allahabad University, for permitting us to use analytical instruments.
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