FTIR Study for the Cr(VI) Removal from Aqueous Solution Using Rice Waste
Tarun Kumar Naiya1+, Biswajit Singha2 and Sudip Kumar Das2 1Department of Petroleum Engineering, Indian School of Mines, Dhanbad – 826004, India
2Department of Chemical Engineering, University of Calcutta, 92, A. P. C. Road, Kolkata - 700 009, India
Abstract. Rice waste - low cost agricultural waste has been utilized as the adsorbent for the removal of hexavalent chromium from aqueous solution. Adsorption capacity of rice wastes and interaction of the different functional groups were studied. Batch adsorption technique was employed to optimize the process parameter. A detailed FTIR characterization of the fresh adsorbents and Cr(VI) loaded adsorbents were carried out. The results show that the different functional groups such as surface hydroxyl, alkene, aromatic nitro, carboxilate anion and silicon oxide etc were responsible for the adsorption process. Keywords. FTIR, Chromium (VI), Rice straw, Rice bran, Rice husk
1. Introduction The use of Cr(VI) in several industrial processes such as mining, tanning, cement, production of steel
and other metal alloys, electroplating operations, photographic material, corrosive painting etc leads to contamination of natural waters mainly due to improper disposal methods [1, 2]. It is carcinogenic, mutagenic and toxic. Its presence in environment poses a significant threat to aquatic life and as well as public health [3].
The maximum permissible limit of Cr(VI) for discharge into inland surface water is 0.1 mg/L and in portable water is 0.05 mg/L [4, 5]. The Ministry of Environment and Forest (MOEF), Government of India has set minimal national standards (MINAS) of 0.1 mg/L for safe discharge of effluent containing Cr(VI) in surface water [6].
In order to comply with permissible limit, industries have to treat their effluents to reduce Cr(VI) concentration in wastewater to acceptable levels. In waste water treatment various technologies are developed such as chemical precipitation, ion exchange, electrochemical precipitation, solvent extraction, membrane separation, concentration, evaporation, reverse osmosis, emulsion per traction, adsorption etc. [7]. Among these technologies, adsorption is an user-friendly for removal of heavy metal. This process includes selective transfer of solute components in the fluid phase to the surface or onto the bulk of solid adsorbent materials.
In recent years, several natural or agricultural waste have been used for the removal of heavy metal from industrial waste water. In general natural or agricultural waste contains different functional groups like hydroxyl, aldehyde, aliphatic acid, alkene, amide, aromatic nitro and silicate etc. The present paper deals with the identification of functional groups which are responsible for Cr(VI) ion adsorption in the rice waste.
2. Experimental
2.1. Adsorbent
+ Corresponding author. Tel.: 09471191367(T. K. Naiya)
Rice straw, rice bran and rice husk were used as low cost agricultural wastes for Cr(VI) removal from aqueous solution. All the adsorbents were collected from local area near Kolkota, West Bengal, India. Rice wastes were boiled with water for 6 hr. to remove color materials. Finally adsorbents were washed with distilled water several times and dried at 1050C for 6 hr to remove the adherent moisture. After drying, all the adsorbents were sieved to obtain particle size of 250-350 μm prior to use for adsorption studies. This size of particle was chosen for suitable in subsequent column operations as it lower sized particle plug the filter in column studies.
2.2. Adsorbate The stock solution containing 1000 mg/L of Cr(VI) was prepared by dissolving 3.73 g of A. R. grade
K2CrO4, 2H2O in 1000 ml double distilled water. The stock solution was diluted with double distilled water to obtain desired concentration.
2.3. Analysis All the necessary chemicals used in the study were of analytical grade and obtained from E. Merck
Limited, Mumbai, India. Characterization of adsorbents was carried out by surface area analysis, bulk density, scanning electron microscope (SEM) and FTIR studies. The surface area was measured on Micromeritics Surface Area Analyzer (ASAP 2020). The moisture content determination was carried out with a digital microprocessor based moisture analyzer (Metteler LP16). Scanning electron microscope (S-3400N, Hitachi, Japan) study was conducted to observe the surface texture. The pH of the solution was measured with a EUTECH make digital microprocessor based pH meter previously calibrated with standard buffer solutions. The Cr(VI) concentration in the experimental solution was determined from the calibration curve prepared by measuring absorbance at λmax 540 nm using a UV-Spectrophotometer (U-4100 spectrophotometer, Hitachi, Japan) [8]. FTIR (Jasco FT/IR-670 Plus) studies were carried out to determine the type of functional group responsible for Cr(VI) adsorption. The point of zero charge (pHPZC) was determined by solid addition method [9].
2.4. Adsorption experiment Batch adsorption experiments were conducted by varying pH, contact time, adsorbent dosage and initial
Cr(VI) concentration. The experiment were carried out in 250 ml stopper conical flasks and total volume of the reaction mixture were kept at 100 ml. The pH of the solution was adjusted at a desired value by adding 0.1 N HCl or 0.1 N NaOH solutions as required. The flasks were shaken for the required time period in an electrically thermostated reciprocating water bath shaker with 120-125 strokes/minute at 300C. Cr(VI) concentration were estimated by drawing conical flask from shaker at regular intervals of time to find the equilibrium when the concentration is constant against time. The range of variables for batch experiment were reported in Table 1. All the experiments were performed in triplicates. The percentage removed Cr(VI) ions (R) in solution was calculated using following equation,
0 t
0
(C C )R 100%
C−
= ×
(1)
Where C0 and Ct are the Cr(VI) concentrations (mg/L) initially and at a given time t in min.
Table 1 Range of variables for batch experiment
Adsorbent Initial pH Initial Cr(VI) conc. (mg/L) Contact time (min)
3.1. Characterization of the adsorbents The different physical characteristics of the adsorbents are shown in Table 2. Figs. 1-3 are shown the
scanning electron micrographs of adsorbents. These figures indicated that the adsorbents had irregular and porous surface. Fourier Transform Infrared Spectroscopy (FTIR) study was carried out to identify the functional groups presents in the adsorbents in the 4000-400 cm-1 range [Figs. 4-6].
Fig. 1 Scanning Electron Micrograph (SEM) of rice straw
Fig. 2 Scanning Electron Micrograph (SEM) of rice bran
Fig. 3 Scanning Electron Micrograph (SEM) of rice husk Fig. 4 FTIR spectra of (a) raw rice straw and (b) Cr(VI)
loaded rice straw
Table 2 Optimum operating condition obtaining in the batch process and different physical properties of rice waste
Adsorbent
Initial pH
Initial Cr(VI) conc.
(mg/L)
Contact time (min)
Adsorbent dosage (g/L)
Point of zero chargepHpzc
Surface area
(m2/g)
Bulk density (g/cm3)
Moisture content
(%)
Ash content
(%)
Rice straw
2.0 25 180 10 6.85 1.21 0.36 7.26 9.40
Rice bran
2.0 25 300 10 6.10 0.12 0.42 10.68 11.72
Rice husk
1.5 25 300 10 6.05 0.54 0.54 9.02 11.80
3.2. Optimum operating condition
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Metal sorption is depends on the solution pH. The range of variables investigated to obtain the optimum condition is shown in Table 1. In general adsorption of cation is favored at pH > pHpzc. At very low pH, chromium ions exist in the form of HCrO4
-, while in the increase in pH (up to pH = 6) different forms such as Cr2O7
2-, HCrO4-, and Cr3O10
2-, coexists, of which HCrO4- predominates. As the pH increases equilibrium
shifted form HCrO4- to CrO4
2- and Cr2O72- [10]. At very low pH values, the surface of adsorbent would be
surrounded by the hydronium ions which enhance the Cr(VI) interaction with binding sites of the biosorbent by greater attractive forces. As the pH increased, the overall surface charge on the biosorbents became negative and adsorption decreased [2]. Adsorption of Cr(VI) was not significant at pH values more than 6 due to dual complexation of the anions (CrO4
2-, Cr2O72- and OH-) to be adsorbed on the surface of the
adsorbents, of which OH- predominates [11]. The optimum pH, adsorbent dosage, equilibrium contact time and optimum initial Cr(VI) ion concentration were experimentally determined in batch process and the results are shown in Table 2.
Fig. 5 FTIR spectra of (a) raw rice bran and Fig. 6 FTIR spectra of (a) raw rice husk and
3.3. FTIR analysis for Cr(VI) adsorption The adsorption capacity of rice waste depends upon porosity as well as chemical reactivity of functional
groups at the adsorbent surface [12]. Each raw and Cr(VI) loaded adsorbents were mixed separately with KBr of spectroscopic grade and made in the form of pellets at a pressure of about 1 MPa. The pellets were about 10 mm in diameter and 1 mm thickness. Then the adsorbents were scanned in the spectral range of 4000-400 cm-1. Figs.4-6 show the FTIR spectra of different adsorbents.
Table 3 represented the shift in the wavenumber of dominant peak associated with the Cr(VI) loaded in the FTIR plots by comparing between the fresh adsorbents and Cr(VI) loaded adsorbents [Figs. 4-6]. These shifts in the wavelength showed that there was metal binding process taking place at the surface of the adsorbents [13, 14]. There were clear shifts from wavenumber of 3348. 78 cm-1 (raw rice straw) to 3417.24 cm-1 (metal loaded rice straw), 3342.03 cm-1 (raw rice bran) to 3328.53 cm-1 (metal loaded rice bran) and 3385.42 cm-1 (raw rice husk) to 3421.10 cm-1 (metal loaded rice husk) which indicated surface -OH group was one of the functional group responsible for adsorption. Unsaturated group like alkene was also responsible for adsorption of Cr(VI) on the aforesaid adsorbents which was inferred from the shift of the peak more than 10 cm-1.
Aromatic C-NO2 stretching was found to have major shift of wavenumber from 1546.63 cm -1 to 1514.81 cm-1 for the adsorption of Cr(VI) on rice bran. There were also minor shift of peak for the adsorption of Cr(VI) on rice straw and rice husk. So the aromatic nitro groups were responsible for adsorption of Cr(VI) on rice bran not for the adsorption on other adsorbents.
Rice straw FTIR spectrum also showed intense bands around 1321.00 cm-1 which shifted to 1371.14 cm-1 for Cr(VI) loaded rice straw. This was attributed that the carboxylate anion were responsible for the adsorption on rice straw. At 1072.66 cm-1(raw rice straw), 1079.94 cm-1 (raw rice bran) and 1098.26 cm-1 (raw rice husk) the band was assigned Si-O stretching. Major shift of these band indicated that Si-OH group is responsible for adsorption.
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Table 3 also indicated that the minor shift for the other band (aliphatic C-H stretching, aldehyde C-H stretching, aliphatic carboxylic acids) which showed that these groups were not involved in the adsorption process.
4. Conclusion The FTIR spectra analysis of raw and Cr(VI) loaded adsorbents indicated the complex formation of
Cr(VI) ion with different functional groups present in the bio-adsorbents. Mainly surface hydroxyl group, alkene, aromatic nitro, carboxilate anion and silicon oxide etc. were responsible for Cr(VI) adsorption.
Table 3 Wavenumber (cm-1) for the dominant peak from FTIR study for Cr(VI) adsorption
Functional groups
Raw rice straw
Rice straw loaded with Cr(VI)
Raw rice bran
Rice bran loaded with Cr(VI)
Raw rice husk
Rice husk loaded with
Cr(VI)
Surface O-H stretching
3348.78 3417.24 3342.03 3328.53 3385.42 3421.10
Aliphatic C-H stretching
2918.73 2916.81 2924.52 2924.52 2925.48 2925.48
Aldehyde C-H stretching
x x 2854.13 2854.13 2854.13 2854.13
Aliphatic acid C=O stretching
x x 1709.59 1713.44 x x
Unsaturated group like alkene
1644.09 1633.41 1655.59 1644.02 1654.62 1638.23
Aromatic C-NO2 stretching
1512.88 1505.17 1546.63 1514.81 1515.77 1509.99
Carboxylate anion C=O stretching
1321.00 1371.14 x x x x
Si-O stretching
1072.66 1058.73 1079.94 1055.84 1098.26 1075.12
5. Acknowledgement Biswajit Singha wishes to thanks the University of Calcutta for Fellowship (UPE / Science &
Technology), Ref. No. UGC/489/Fellow UPE (SC/T), dated the 16/ 4/ 2009.
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