ТЕОРЕТИЧЕСКИЕ ОСНОВЫ ХИМИЧЕСКОЙ ТЕХНОЛОГИИ, 2014, том 48, № 1, с. 66-76
УДК 66.021.2.081.3
COMPETITIVE ADSORPTION OF PHENOL AND RESORCINOL
ONTO RICE HUSK ASH
© 2014 г. C. Thakur, I. D. Mall, V. C. Srivastava
Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India
id_mall2000@yahoo.co.in Received 18.01.2012
The present study deals with the adsorptive removal of phenol and resorcinol from aqueous solution onto rice husk ash. The competitive adsorption equilibrium of the binary mixtures (phenol/resorcinol) was determined by conducting batch experiments with initial concentration varying in the range of 50 to 1000 mg/L. In order to evaluate multicomponent adsorption isotherm parameters, individual adsorption equilibrium studies were also carried out. Langmuir, Freundlich and Redlich—Peterson equilibrium isotherm models were used for single compound equilibrium sorption data modeling. All three models give almost similar fit for single compound equilibrium data. Binary equilibrium adsorption data and the parameter evaluated from single adsorption data were fitted in various multicomponent isotherm models by minimizing the sum of square of error. The extended Langmuir model gave the better fit to the experimental adsorption data of phenol and resorci-nol from binary systems onto rice husk ash. It seems that both phenol and resorcinol compete for the same adsorption sites on rice husk ash. The net interactive effect of phenol and resorcinol on the adsorption of re-sorcinol by rice husk ash was found to be antagonistic.
DOI: 10.7868/S004035711401014X
INTRODUCTION
Industrial units such as petrochemical, petroleum refinery, coke oven batteries, coal gasification plant, textile, cosmetic, chemical and pharmaceutical industries discharge phenol and its derivatives such as resorcinol in their effluents [1—3]. United States Environmental Protection Agency (USEPA) and Ministry of Environment and Forests (MoEF), Government of India, consider all phenolic compounds as toxic and consider them as priority pollutants that should be compulsorily removed before being discharged into water bodies. According to MoEF, concentration of phenols (measured as phenol) in wastewater should not be more than 1.0 mg/L for their discharge into the surface waters and 5.0 mg/L for discharge into the public sewers, on land for irrigation, and marine coastal areas [2]. The threshold value of phenols in water is 4000 ^g/L [4, 5]. Drinking of phenol-containing water above threshold value can cause protein degeneration, tissue erosion and paralysis of the central nervous system and can also damage the kidney. Removal of this substrate is necessary from wastewater. Various research efforts have been made to abate phenol and resorcinol in wastewater [3, 6—8]. Biological degradation of these substances is little rigid since it is highly toxic to the environment.
Adsorption treatment process is the one of the efficient methods for removal of various types of contaminants from effluent, especially for the moderate and low concentration effluents [9—11] and had attracted
considerable interest recently. Activated carbon has high surface area, porous structure, high adsorption capacity and high degree of surface reactivity. Therefore, it is most widely used as an adsorbent. However, it is expensive and has led to the search of various low cost adsorbents [12]. Rice husk ash (RHA) can be used as an adsorbent because of its low cost and availability in abundance.
Rice husk is a byproduct of the rice mill and is used as supplement fuel of the rice husk-fired boilers where RHA is collected from the particulate collection devices [13]. Srivastava and co-workers have used RHA for adsorptive removal of various types of adsorbates like metal ions, dyes and organics [14—17].
Aghav et al. [18] recently reported application of artificial neural network for studying adsorption of phenol and resorcinol from aqueous solution by activated carbon, wood charcoal and RHA. Suresh et al. [19, 20] recently studied removal of aniline and phenols from aqueous solution by granular activated carbon. They used the Taguchi method for optimization of parameters for binary adsorption.
The design of adsorption systems requires isotherm data. Most of the workers report equilibrium adsorption data for single component adsorption of phenolic compounds. Since actual industrial effluents contain several phenolic compounds, equilibrium adsorption data for closely related binary compounds are also important. The present paper aims to use RHA as an ad-
sorbent for the individual and simultaneous removal of phenol and resorcinol from aqueous solutions.
EXPERIMENTAL
Adsorbent. RHA obtained from a nearby paper mill (Rana paper mill, Muzaffarnagar, India) was used as such. Bulk density was determined by using bulk density meter (supplied by MAC), whereas particle size analysis was done using standard sieves. Textural characteristics were determined by nitrogen adsorption at 77.15 K to determine the specific surface area and the pore diameter of the RHA using an ASAP 2010 Mi-cromeritics instrument and by Brunauer— Emmett— Teller (BET) method [21] and Barrett-Joyner—Hal-enda (BJH) method [22]. FTIR spectrometer (Thermo nicolet, Model Magna 760) with pellet (pressed-disk) technique was used to determine the presence of functional groups in RHA.
Chemicals. The chemicals used in this study were of analytical reagent grade. Phenol (C6H6O) was procured from s.d. Fine Chemicals, Mumbai. Resorcinol (C6H6O2), was procured from E. Merck, Mumbai. Stock solutions of phenol and resorcinol were made by dissolving known amount of compounds in double distilled water. Thereafter, solutions of different concentrations (varying in the range of 50 and 1000 mg/L) were prepared from stock solutions.
Batch adsorption studies. Each experimental run was performed with RHA dosage of 20 g/L in a 250 mL conical flask containing 150 mL aqueous solution of known concentration of phenol and resorcinol or their binary mixture. These flasks were agitated at 150 rpm and 303 K in a temperature controlled orbital shaker (Remi Instruments, Mumbai). The samples were collected after the sorption process and analyzed for residual concentration of compounds using high performance liquid chromatography (HPLC) with a dual wavelength absorbance detector. A symmetry column C-18 (4.6 x 150 mm 5-micron) was used to separate the substrate using a mobile phase composed of ratio 6 : 4 : 0.1 of distilled water : methanol : acetic acid with a flow rate of 1 mL/min, and the injection volume was 20 ^L. The monitoring wavelength was 260 nm. The identification of each compound was based on the peak and retention time of the components.
The single and binary equilibrium of adsorption of phenol and resorcinol on RHA was determined at 303 K in batch mode. For single compound RHA systems, initial compound concentration was varied from 50 to 1000 mg/L. The competitive adsorption equilibrium of binary mixtures phenol/resorcinol with different ratios of initial concentrations was studied. The purpose of the use of different ratios of initial concentrations was to determine the effect of the one compound on the adsorption of the other compound onto RHA. In all cases, the pH of the solution was ~ 6.0 ± 0.3. This pH was maintained constant by use of appropriate buffer.
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Single and binary adsorption modeling. Vkrious mono- and multicomponent isotherm equations were tested in the present study. These isotherm equations are given in Table 1 [23—31]. Srivastava et al. [30, 31] have discussed the theory associated with these models.
The isotherm parameters of all the multicompo-nent models were determined by using the MS Excel for Windows by minimizing the sum of square of error S given as
n
S = £ {qe,i,exp - qe,i,cal) ■ (1)
i=1
In the above equation, the subscripts 'exp' and 'cal' show the experimental and calculated values and n is the number of measurements.
The values of qei (mmol of adsorbate/g of adsorbent), the individual adsorption yield Aj (%) and the total adsorption yield ATot (%) were calculated using the following expressions:
qei = (Co,i - Cei)V/w, (2)
4 = 100(Co,i - Ce,i)/Co,i, (3)
ATot = 100 £ (Co,i - CeJ)/£ Co,, (4)
where F is the volume of the adsorbate containing solution (l) and w is the mass of the adsorbent (g).
RESULTS AND DISCUSSION
Characterization of RHA. The average particle size of RHA was 150.5 ^m, whereas the bulk density and the heating value were 109.9 kg/m3 and 9.9 MJ/kg, respectively. The Brunauer—Emmett—Teller (BET) surface area was found to be 47.2 m2/g, whereas the Lang-muir surface area was 70.5 m2/g. Similarly, t-plot micropore area was found to be 26.9 m2/g and t-plot external surface area was found to be 20.2 m2/g. RHA was found to predominantly mesoporous. The Bar-rett—Joyner—Halenda (BJH) adsorption average pore diameter was 38.7 A, and the BJH desorption average pore diameter was 39.7 A. Single point adsorption total pore volume of pores less than 208.9 A diameter (at P/Po = 0.90) was 0.033 cm3/g. BJH analysis showed that 80% of the RHA pore area was due to the meso-pores.
Individual adsorption of phenol and resorcinol. The
equilibrium uptakes and the adsorption yields obtained for single component phenol and resorcinol solution are given in Table 2. The individual adsorption isotherm of phenol and resorcinol at 303 K is also shown in Fig. 1. Also, increasing the initial concentration up to 10.48 mmol/L for phenol and 13.51 mmol/L for resorcinol increased the equilibrium uptake and decreased the adsorption yield of both the components. At higher concentration, the driving force, which is indispensable to overcome the resistances to the mass transfer of adsorbate between the solution and the solid phases, becomes greater. Also, the inter-
Table 1. Mono- and multicomponent isotherm models
Model
Equation
Reference
Freundlich model
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