International Journal of Physical Sciences Vol. 4 (4), pp. 172-181, April, 2009
Available online at http://www.academicjournals.org/IJPS ISSN 1992 - 1950 © 2009 Academic Journals
Full Length Research Paper
Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto activated phosphate rock Atef S. Alzaydien* and Waleed Manasreh Department of Chemistry, Mutah University, P. O. Box 7, Jordan. Accepted 13 March, 2009
Phosphate rock being locally abundant and cheap material in Jordan can be easily activated to become a promising adsorbent for phenol removal from aqueous solution. The phosphate rock before and after activation was characterized using XRD and IR techniques. The effects of various experimental parameters, such as initial phenol concentration, temperature, pH, contact time and adsorbent dose on the adsorption extent were investigated. Langmuir adsorption model was used for the mathematical description of the adsorption equilibrium and the equilibrium data fixed very well with this model. The activated phosphate rock had the monolayer adsorption capacity equal to 38.34 mg/g at pH value of 8.0 and 20°C, adsorption measurements show that the process is very fast and physical in nature. The extent of the phenol removal increased with decrease in the initial concentration of the phenol and temperature of solution. The results showed that as the amount of the adsorbent was increased, the % of phenol removal increased accordingly. Adsorption data were modeled using the pseudo-first and pseudo-second-order kinetic equations, Elovich and intra-particle diffusion models. It was seen that the pseudo- second-order kinetic equation could best describe the sorption kinetics. Thermodynamic parameters showed that the adsorption of phenol on activated phosphate rock was endothermic and spontaneous in nature. Key words: Phenol, activated phosphate rock, adsorption isotherm, thermodynamic parameters, kinetics of adsorption, intra-particle diffusion, rate constants. INTRODUCTION Organic contaminants from industrial waste streams that seriously threaten the human health and the environment, has been recognized as an issue of growing importance in recent years. Phenol pollution is a serious problem in many countries. The major sources of phenolic waste are petroleum refineries, petrochemical, steel mills, coke oven plants, coal gas, synthetic resins, pharmaceuticals, paints, plywood industries and mine discharge (Patterson, 1975). Phenolic waste imparts a carbolic odour to river water and is also toxic to fish and human beings. Total phenol concentration in the wastewater of a typical Jordanian refinery processing 3 million tons of crude per year is around 85 mg/L. The concentration of phenolic compounds in the wastewater from resin plants is typically in
*Corresponding author. Email:
[email protected]. Tel: +962-796785513. Fax: +962-32375073.
the range of 12 - 300 mg/L. The wastewater with the highest concentration of phenol (> 1000 mg/L) is typically generated from coke processing (Patterson, 1975). The discharge of phenolic waste into waterways may adversely affect human health as well as that of flora and fauna. Ingestion of a small amount of phenol (TLV of 5 ppm) by human beings may cause nausea, vomiting, paralysis, coma, greenish or smoky colored urine and even death from respiratory failure or cardiac arrests. Phenols also impart undesirable taste to water even at extremely low concentration (USEPA recommends a maximum allowable limit of 0.001 ppm). Fatal poisoning may also occur by adsorption of phenol by skin, if a large area of it is exposed (Zumriye and Yener, 2001; Banat et al., 2004). It is therefore necessary to remove phenol completely from wastewater (general effluent discharge standard of phenol is 0.05 ppm), before being discharged into waterways. There are many methods for the removal of phenol from aqueous solutions, such as adsorption, chemical
Alzaydien and Manasreh
precipitation, ion exchange, membrane processes, reverse osmosis, chemical oxidation, precipitation, distillation, gas-stripping, solvent extraction, complexation and bio-remediation. Adsorption is the most popular method in which activated carbon or ion exchange resins are usually applied. Activated carbon adsorption has been recommended by the USEPA as one of the best available technologies (BAT) (Adams and Watson, 1996) in removal of organic compounds, but it is highly expensive especially for developing countries like Jordan. In recent years, there has been a continuous search for locally available and cheaper adsorbents for the replacement of activated carbon for removal of a variety of organic compounds such as phenol (Keerthinarayan and Bandopadhayay, 1993; Swamy et al., 1997; Khanna and Malhotra, 1977; Kumar et al., 1987; Jan et al., 1993; Singh and Mishra, 1990; Bhat et al., 1983). There is scope for developing adsorbents from low-cost materials. Viraraghavan and Alfaro (1992) investigated the adsorptive capacity of phenol by peat, fly ash and bentonite with an initial phenol concentration of 1000 mg/L. They found that in comparison to activated carbon, the adsorption capacity of these adsorbents was much lower. Kinetic study results revealed that a long equilibrium time (15 h) was needed for the adsorption of phenol by the materials. The use of sawdust for the removal of phenol from aqueous solution has been studied by Sivanandam and Anirudhan (1995). Yapar and Yilmar explored the adsorptive capacity of some clays and natural zeolite materials found in Turkey for removal of phenol. They found calcined hydrotalcite was the best among the studied adsorbents. It can adsorb 52% of phenol from a solution with an initial phenol concentration of 1000 mg/L at the adsorbent/phenol ratio of 1:100 while the others could adsorb only 8% of phenol for the same operation conditions. Alemany et al. also studied the removal of phenol from aqueous solution by adsorption on coal fly ash. The phenol adsorption capacity was found to depend on the solubility or hydrophobicity of the adsorbate and on the pH-modified fly ash. They reported that temperature has a relatively minor influence on the adsorption process. Das and Patnaik utilized blast furnace flue dust (BFD) and slag to investigate phenol adsorption through batch experiment. They observed that after 8 h of contact time, the equilibriums of phenol adsorption with BFD and slag were attained. They achieved 75 and 90% removal efficiencies for slag and BFD respectively. Both Freundlich and Langmuir isotherm models well fitted to their adsorption data. The aim of the presented study was to investigate the phenol adsorption characteristics of activated phosphate rock taking into account equilibrium, kinetic and thermodynamic aspects. The Langmuir isotherm was applied to describe and predict the adsorption equilibrium. 4 simplified kinetic models including pseudo-first-order equation, pseudo-second-order equation, Elovich and intraparticle diffusion models were used to determine the mechanism of adsorption. The thermodynamic parameters such as
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standard free energy, enthalpy and entropy were also evaluated. MATERIALS AND METHODS The phosphate rock (PR) sample used in this study comes from the Al-Hisa phosphate mines located in the southern region of Jordan, about 136 km south of the capital Amman. The choice of this material is based on its low cost, considering its abundance in the Jordanian ores. Only the fraction between 100 - 400 m grain size was used in this study. This fraction was activated with nitric acid, 100 g of (PR) was added to 400 ml of 1M HNO3 solution and stirred for 2 h. The sample was finally filtrated, washed with distilled water, dried at 105°C and crushed to obtain activated phosphate rock (APR) . Purity of the phosphate rock samples before and after activation was tested by IR spectral analysis. An IR transmittance spectrum of the ground samples was obtained in the 4000 - 500 cm−1 range with a SHIMATZU IR 470 spectrometer. The spectra were taken from thin KBr pellets prepared by compacting an intimate mixture obtained with 1.5 mg of phosphate rock and 300 mg of KBr. Phases present in the samples were analyzed using an X-ray diffracto-meter (PANalytical X-ray, Philips Analytical, Germany) with Ni-filtered Cu K _radiation ( = 0.154 nm), 40 kV/40 mA, divergent and scattering slits of 0.02 mm and receiving slit of 0.15 mm, with stepping of 0.01 . Scans were conducted from 10 to 80 at a rate of 2 min−1. Adsorption studies Adsorption experiments were carried out by agitating 1.0 g of APR with 100 ml of phenol solution of desired concentration and pH at 180 rpm, 20°C in a thermostated mechanical shaker (ORBITEK, Chennai, Germany). Concentration of phenol was estimated spectrophotometrically by monitoring the absorbance at 270 nm using UV-Vis spectrophotometer (Hitachi, model U-3210, Tokyo). The pH was measured using pH meter (3151 MWT GmbH, Germany). The samples were withdrawn from the shaker at predetermined time intervals and the phenol solution was separated from the adsorbent by centrifugation at 10,000 rpm for 15 min. The absorbance of supernatant solution was measured. Effect of pH was studied by adjusting the pH of phenol solutions using dilute HCl and NaOH solutions. Effect of adsorbent dosage was studied by agitating 100 ml of 40 mg/l phenol solutions with different adsorbent doses (0.2 - 1.0 g) at equilibrium time. Effect of temperature was studied using 40 mg/l of phenol and 1.0 g of adsorbent at 20, 35 and 50°C. All the chemicals used in the study were of analytical reagent grade. For kinetic studies, volumes of 100 ml of phenol solutions with a concentration of 40 mg/l and 1 g of APR were placed in 250 ml flasks. The mixtures were shaken at 150 rpm for 180 min at 20°C in a temperature controlled shaker. Samples of 5 ml were withdrawn from the solution at predetermined time intervals until the end of the experiment. All experiments were performed in duplicate at least and mean values were presented with a maximum deviation of 4% in all the cases studied. Blank samples were run under similar experimental conditions but in the absence of adsorbent. It was not detected chemical precipitation and losses of phenol to the containers wall. The amounts of phenol sorbed by the adsorbent were calculated using the following equation, Q = (C0 – Ce) V/M
(1)
Where Q (mg/g) is the amount of phenol sorbed by APR, C0 and Ce (mg/L) are the initial and equilibrium liquid-phase concentration of phenol, respectively, V (L) the initial volume of phenol solution and
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M (g) the weight of the APR. The % adsorption was calculated using the following equation, % Adsorption = [(C0 – Ce ) / C0] X 100%
(2)
Theory Isotherm models Adsorption isotherm is basically important to describe how solutes interact with adsorbents and is critical in optimizing the use of adsorbents. The Langmuir isotherm model was used to describe the relationship between the amount of phenol adsorbed and its equilibrium concentration in solutions. The Langmuir isotherm is valid for monolayer adsorption on a surface containing a finite number of identical sites. The model assumes uniform adsorption on the surface and no transmigration in the plane of the surface (Hall et al., 1996). The linear form of the Langmuir isotherm can be represented by the following equation, Ce/qe =1/QoKL + Ce/Qo
(3)
Where Ce (mg/L) is the equilibrium concentration of the adsorbate, qe (mg/g) is the amount of adsorbate adsorbed per unit mass of adsorbent, Qo and KL are Langmuir constants related to adsorption capacity and rate of adsorption, respectively. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless separation factor (RL) which is defined by RL =1 / (1 + KLC0)
Where is the initial sorption rate (mol/g min) and is the desorption constant (g/mol). To simplify the Elovich equation, it is assumed that t 1), linear (RL = 1), favorable (0 < RL pKa. Therefore, the adsorption decrease at high pH values due to ionization of adsorbate molecules. The reason could be also due to the electrostatic repulsions between the negative surface charge and the phenolate– phenolate anions in solution (Moreno-Castilla, 2004). While pH < 8 phenol was undissociated and the dispersion interaction predominated. Effect of adsorbent dose The effect of varying the APR dose for a fixed volume (0.1 l) of phenol solution at constant concentration (40
The linearized Langmuir adsorption isotherm model was used to describe the adsorption equilibrium. The adsorption isotherms were studied at 20, 35 and 50°C. A plot of linear Langmuir equation Ce/qe versus Ce was drawn. The value of isotherm constants, KL and equilibrium monolayer capacities, Q0, are given in Table 1. The monolayer capacities of APR increased with decreasing temperatures and were determined as 38.34, −1 28.41 and 25.64 mg g at 20, 35 and 50°C, respectively. The RL values given in Table 1, showed that the adsorption behavior of APR were extremely favorable for the phenol (RL < 1). The low values of RL indicated that adsorption tend to be weakly irreversible (RL = 0). Effect of contact time and adsorption kinetics The adsorption of phenol at affixed concentration on APR was studied as a function of contact time to determine the equilibrium time (Figure 6). Nearly 180 min were required for the equilibrium adsorption. Therefore, equilibrium time was set conservatively at 240 min for further experiments. Adsorption of phenol was fast initially and nearly 30% of phenol were adsorbed in 20 min. Equilibrium −1 phenol uptake of APR was determined as 22.3 mg g at -1 40 mg initial phenol concentration. The effect of contact time on adsorption kinetics had been studied with initial concentration of 40 mg/L at 20°C and pH 8.0. Adsorption kinetics was modeled by the pseudo first-order Lagergren, pseudo second-order rate equation, Elovich and intra-particle diffusion equations. Linear plots of all considered kinetic models and the adsorption kinetic rate constants are shown in Figures 7 10 and Table 2, respectively. All kinetic models show a good correlation values. Although the Lagergren plot was linear, it did not fulfill
Int. J. Phys. Sci.
25
20
Amount adsorbed (mg/g)
15
10
5
0 0
50
100
150
200
250
Time (min)
Figure 6. Plot of the amount of phenol adsorbed versus time , initial phenol concentration = 40 mg/L, T= 20 C, pH = 8.0. 1.2
1
0.8
0.6
log ( qe-qt)
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0.4
0.2
0 0
20
40
60
80
100
120
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-0.2
-0.4
Time ( min )
Figure 7. Lagergren-first-order kinetic plot for the adsorption of phenol onto APR initial phenol concentration = 40 mg/l, APR dose 1.0g , pH 8.0, at 20°C Table 2. Kinetic parameters for the adsorption of phenol onto APR.
Models Pseudo-first order Pseudosecond order Intra-particle diffusion Elovich equation
Coefficients k1 =0.016 min−1 Qe = 12.03 mg/g Qe = 22.32 mg/g -1 -1 k2 = 0.00312 g mg min t1/2 = 14.28 min −1 -1/2 ki = 0.7884 mg g min −1
= 0.3635 mg g min -1 = 3.8711 g mg
−1
2
R
0.9778 0.9988 0.9641 0.9746
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10 9 8 7
t/qt
6 5 4 3 2 1 0 0
20
40
60
80
100
120
140
160
180
200
Time (min)
Figure 8. Pseudo-second-order kinetic plot for the adsorption of phenol onto APR initial phenol concentration = 40 mg/l, APR dose 1.0 g , pH 8.0, at 20°C.
25
20
q t (m g /g )
15
10
5
0 0
1
2
3
4
5
6
ln t . Figure 9. Plot of Elovich equation for adsorption of phenol onto APR initial phenol concentration = 40 mg/l, APR dose 1.0g , pH 8.0, at 20°C.
the essential condition of yielding the same Qe values as given by experiments. The pseudo second-order reaction rate model adequately described the kinetics of sorption 2 of phenol with high correlation coefficient (R > 0.998) in Figure 7, and its calculated equilibrium capacities (Qe
cal) fit well the experimental data. These suggested that the pseudo second-order adsorption mechanism was predominant and that the overall rate of the phenol adsorption process appeared to be controlled by chemical process (Gupta, 1998; Ajmal et al., 2003). The
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qt (mg/g)
20
15
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0 0
2
4
6
8
10
12
14
16
(Time)1/2, (min)1/2
Figure 10. Intraparticle diffusion kinetics for adsorption of phenol onto APR initial phenol concentration = 40 mg/l, APR dose 1.0 g , pH 8.0, at 20°C.
Table 3. Thermodynamic parameters for the adsorption of phenol onto APR
Temperature (°C) 20 35 50
G kJ/mol) -4.93 -5.04 -5.13
H ( kJ/mol)
S (J/mol.K) 9.21
-2.15
sorption process could be ion-exchange in nature where the phenol molecules bind with the various negatively charged inorganic functional groups present on the surface of the APR. Similar phenomena have been observed in the biosorption of remazol black B on biomass (Aksu and Tezer, 2000) and adsorption of congo red (Namasivayam and Kavitha, 2002) and 2-chlorophenol (Namasivayam and Kavitha, 2003) on coir pith carbon. The Elovich equation, which has been shown to be useful in describing chemisorption on highly heterogeneous adsorbents, give a good account of adsorption of 2 phenol with R value (0.97) for the plot of qt versus ln t. 0.5 The plot for intra-particle diffusion (qt versus t ) was 2 linear with regression coefficient (R ) of 0.96, but the line did not pass through the origin, indicating that this model did not fit the adsorption process (McKay and Poots, 1980). Effect of temperature thermodynamics
and
adsorption
Temperature influenced the phenol adsorption properties of APR. Temperature effect on adsorption capacity of
−1
APR was studied at 20, 35 and 50°C using 40 mgl initial phenol concentrations at pH 8.0. Adsorption capacities of APR increased with decreasing temperatures from 50 to 20°C which indicated that the adsorption process was exothermic. The sorption capacity of APR were deter−1 mined as 38.34, 28.41 and 25.64 mg g at 20, 35 and 50°C, respectively. The optimum temperature for phenol adsorption of APR was found to the 20°C within the temperature range studied. Thermodynamic parameters Gibbs free energy change, G° was calculated using Langmuir constants (Table 3). The enthalpy change, H° and the entropy change, S°, for the adsorption process were calculated to give the values −2.15 and 9.21 J/mol K, respectively. The negative values of G° confirm the feasibility of the process and the spontaneous nature of adsorption with a high preference for phenol on APR. The value of H° is negative, indicating that the adsorption reaction is exothermic. The positive value of S° reflects the affinity of the APR for phenol and suggests some structural changes in phenol and APR interaction (McKay and Poots, 1980; Ho et al., 2005) Conclusion In batch adsorption studies, data showed that activated phosphate rock had relatively considerable potential for the removal of phenol from aqueous solution. Langmuir isotherm was fitted very well with studied temperature and concentration ranges. The RL values showed that activated phosphate rock was favorable for the adsorption of phenol. The suitability of the kinetic models for the
Alzaydien and Manasreh
adsorption of phenol on the activated phosphate rock was also discussed. It was clear that the adsorption kinetics of phenol to activated phosphate rock obeyed pseudo-second-order adsorption kinetics. Thermodynamic parameters showed that adsorption of phenol on activated phosphate rock was endothermic and spontaneous in nature. It may be concluded that activated phosphate rock may be used for elimination of phenol from wastewater. Phosphate rock is a low cost natural abundant adsorbent material in Jordan and it may be alternative to more costly adsorbent materials. REFERENCES Adams CD, Watson TL (1996). Treatability of s-altrazine herbicide metabolites Adsorpt. Sci. Technol. 13: 527-534. Ajmal M, Rao RAK, Anwar S, Ahmad J, Ahmad R (2003). Adsorption studies on rice husk: Removal and recovery of Cd(II) from wastewater. Bioresour. Technol. 86: 147–149. Aksu Z, Kabasakal E (2003). Batch adsorption of 2,4-dichlorophenoxyacetic acid (2,4- D) from aqueous solution by granular activated carbon. Sep. Purif. Technol. 35: 223–240. Aksu Z, Tezer S (2000). Equilibrium and kinetic modeling of biosorption of Remazol Black B by Rhizopus arrhizus in a batch system: effect of temperature. Process Biochem. 36: 431–439. Alemany LJ, Jiménez MC, Larrubia MA, Delgado F, Blasco JM (2005). Adsorpt. Sci. Technol. 13 (1996) 527-534. Banat F, Al-Asheh S, Al-Makhadmeh L (2004). Utilization of raw and activated date pits for the removal of phenol from aqueous solutions, Chem. Eng. Technol. 27: 80-86. Benbow J (1993). Paste Flew and Extrusion, Oxford University Press Inc, New York,. Bhat DJ, Bhargara DS, Panesar PS (1983). Effect of pH on phenol removal in moving media reactor, Indian J. Environ. Health 25: 261– 267. Chien SH, Clayton WR (1980). Application of Elovich equation to the kinetics of phosphate release and sorption in soils, Soil Sci. Soc. Am. J. 44: 265–268. Das CP, Patnaik LN (2005). Removal of phenol by industrial solid waste, Practice Period. Hazard. Toxic Radioactive Waste Manage. ASCE 9 (2):135–140. Gupta VK (1998). Equilibrium uptake, sorption dynamics, process development, and column operations for the removal of copper and nickel from aqueous solution and wastewater using activated slag, a low-cost adsorbent, Ind. Eng. Chem. Res. 37: 192–202. Halcomb DW, Young RA (1980). Thermal decomposition of human tooth enamel, Calcified Tissue Int. 31: 189–201. Hall KR, Eagleton LC, Acrivos A, Vermeulen T (1996). Pore-and soliddiffusion kinetics in fixed-bed adsorption under constant-pattern conditions, Ind. Eng. Chem. Fundam. 5: 212–223. Ho YS, McKay G (1998). Kinetic models for the sorption of from aqueous solution by wood, J. Environ. Sci. Health Part B: Process Safety Environ. Protect. 76 (B2): 183–191. Ho YS, Chiang TH, Hsueh YM (2005). Removal of basic dyes from aqueous solution using tree fern as a biosorbent, Process Biochem. 40: 119–124. Jan GP, Mulder P, Robert L (1993). Fly ash mediated reaction of phenol and monochloro phenols, Environ. Sci. Technol. 27: 1849–1863. Kandori K, Horigami N, Yasukawa A, Ishikawa T (1997). Texture and formation mechanism of fibrous calcium hydroxyapatite particles prepared by decomposition of calcium-EDTA chelates, J. Am. Ceram. Soc. 80: 1157–1164. Keerthinarayan S, Bandopadhayay M (1993). Sorption of lindane by wood charcoal, Indian J. Technol. 31: 231–238.
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Khanate P, Malhotra SK (1977). Kinetics and mechanism of phenol adsorption of flyash, Indian J. Environ. Health 19 (3): 224–237. Kumar S, Upadhyay SN, Upadhyay YD (1987). Removal of phenol by adsorption on fly ash, J. Chem. Technol. Biotechnol. 37: 188–196. Lagergren S (1898). Zur theorie der sogenannten adsorption gelöster stoffe, Kungliga Svenska Vetenskapsakademiens, Handlingar 24 (4): 1–39. Low MJD (1960). Kinetics of chemisorption of gases on solids, Chem. Rev. 60: 267–312. Ma QY, Traina SJ, Logan TJ, Ryan JA (1995) Lead immobilisation from aqueous solutions and contaminated soils using phosphate rocks. Environ. Sci. Technol. 29: 1118–1126. McKay G, Poots VJP (1980). Kinetics and diffusion processes in colour removal from effluent using wood as an adsorbent. J. Chem. Tech. Biotechnol. 30(1): 279–292. Moreno-Castilla C (2004). Adsorption of organic molecules from aqueous solutions on carbon materials, Carbon 42: 83–94. Namasivayam C, Kavitha D (2002). Removal of Congo Red from water by adsorption onto activated carbon prepared from coir pith, an agricultural solid waste, Pigments 54: 47–48. Namasivayam C, Kavitha D (2003). Adsorptive removal of 2chlorophenol by low-cost coir pith carbon, J. Hazard. Mater. B98: 257– 274. Noeline BF, Manohar DM, Anirudhan TS (2005). Kinetic and equilibrium of flyash, Indian J. Environ. Health 19 (3): 224–237. Patterson JW (1975), Wastewater Treatment Technology, Ann Arbor Science Inc. Singh DK, Mishra A (1990). Removal of phenolic compound from water using chemical treated saw dust, Indian J. Environ. Health 32: 345– 351. Sivanandam AV, Anirudhan TS (1995). Indian J. Chem. Technol. 2: 137-142. Soejoko DS, Tjia MO (2002). Infrared spectroscopy and X-ray diffraction study on the morphological variations of carbonate and phosphate compounds in giant prawn (Macrobrachium rosenbergii) skeletons during its moulting period. J. Mater. Sci. 38: 2087–2093. Srivastava VC, Swamy MM, Mall ID, Prasad B, Mishra IM (2006). Adsorptive removal of phenol by bagasse fly ash and activated carbon: equilibrium, kinetics and thermodynamics, Colloids. Surf. A: Physicochem. Eng. Aspects 272: 89–104. Swamy M, Mall ID, Prasad B, Mishra IM (1997). Resorcinol removal from using powdered activated carbon, J. Environ. Eng. ASCE 122 (4): 327–330. Viraraghavan T, Alfaro J (1992). Removal of phenol from wastewater by adsorption on peat, fly ash and bentomile. J. Environ. Eng. 5: 311– 317. Yapar S, Yilmar M (2004). Removal of phenol by using montmorillonite, clinoptilolite and hydrotalcite, Adsorption 10: 287–298. Zumriye A, Yener J (2001). A comparative adsorption/biosorption study of mono-chlorinated phenols onto various sorbents, Waste Manage. 21: 695-701.
