Apr 2, 2012 - study of monochlorinated phenols onto various sorbents. Waste. Manage, 1, 695-702 (1999). 2. Alinnor I. J., Adsorption of heavy metal ions ...
Alinnor et al. Int. J. Res. Chem. Environ. Vol.2 Issue 2 April 2012(124-129)
International Journal of Research in Chemistry and Environment Vol. 2 Issue 2 April 2012(124-129) ISSN 2248-9649 Research Paper
Removal of Phenol from Aqueous Solution onto Modified Fly Ash * 1
Alinnor I.J1 and Nwachukwu M.A.2
Department of Pure and industrial Chemistry, Federal University of Technology, P.M.B. 1526, Owerri, Imo State, NIGERIA 2 Department of Environmental Technology, Federal University of Technology, P.M.B. 1526, Owerri, Imo State, NIGERIA
Available online at: www.ijrce.org (Received 30th January 2012, Accepted 27th February 2012) Abstract- The purpose of this study was to investigate the possibility of the utilization of coal fly ash as a low cost adsorbent. Experiments were conducted to evaluate the removal of phenol from aqueous solution by fly ash under various conditions of contact time, pH and temperature. The influence of pH on the phenol uptake by the fly ash was carried our between pH4 and pH9. The level of uptake of phenol by fly ash increased at higher pH value. The effect of temperature on the uptake of phenol was carried out between 30 and 55 0C. The result indicates that adsorption increases at low temperature. This study revealed that citric acid modification of fly ash surface enhances adsorption process. The distribution coefficient of phenol for modified and unmodified fly ash were 1.580 and 0.995, respectively. The adsorption data were best fitted for Freundlich adsorption model than Langmuir model. Keywords: Fly ash, phenol, modification, adsorption kinetics, mechanism. Introduction Fly ash is particulate material produced from the combustion of coal. The removal of heavy metals from effluents and wastewaters by adsorption on fly ash has been studied by a number of researchers. Fly ash is strong alkali materials, which exhibits pH of 10-13 when added to water, and its surface is negatively charged at high pH. In view of this, it can be expected that metal ions or organic materials can be removed from aqueous solutions by precipitation or electrostatic adsorption. Bayat (2002, a and b), studied the effectiveness of removal of Ni, Cu, Zn, Cr and Cd by fly ash. Alinnor (2007) worked on the removal of Pb2+ and Cu2+ from aqueous solution using fly ash as adsorbent. Mavros et al (1993) used two different types of fly ash (from the coal fields of Kardia and Megalopolis in Greece) to remove Ni from wastewater. Weng and Huang (1994) investigated that fly ash can be used as an effective adsorbent for Zn and Cd to clean up dilute industrial wastewaters. Heechom et al (2005) reported removal characteristics of heavy metals from aqueous solution by fly ash. Removal of toxic solvents from polluted environment can be achieved by several techniques but the adsorption technique is widely used due to its high rate, high uptake capacity, effective treatment in dilute solution, low cost and regeneration[20]. Adsorption is a reliable technique that achieves rapid results. Adsoprtion of organic
compounds on the surface of carbon has been studied extensively [14,22,23,26,27]. A lot of studies were conducted to show the effectiveness of fly ash in the removal of organic materials from aqueous solutions [3,12,17,24]. The adsorption of phenol and its analogoues from contaminated water onto fly ash has been investigated by Sarkar and Acharya (2006). The potential of fly ash as a substitute for activated carbon was examined by Aksu and Yener (1999). The result obtained showed that the capacity of fly ash for absorption of phenol depends on the initial pH and phenol concentration. Kumari et al. (1988) have reported the use of fly ash for the removal of carbofuran a pesticide in soil. The most important characteristics of fly ash are calcium content that provides alkalinity in the system raising pH to strongly alkaline values (~12) and the (SiO2 + Al2O3+Fe2O3) content [11]. The effect of surface modification of carbon on the adsorption of organic compounds has been studied by earlier researches (Kaneko et al, 1988, Asakwa et al, 1985, Zawadzki, 1988) [4,10,28] and it has been observed that surface modification has a pronounced effect on adsorption. But work on the removal characteristics of phenol from aqueous solution by fly ash modified with citric acid is very scanty. The aim of present study was to investigate the use of modified and unmodified fly ash as a law- cost adsorbent for the removal of phenol from aqueous solution.
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Alinnor et al. Int. J. Res. Chem. Environ. Vol.2 Issue 2 April 2012(124-129) Material and Methods The fly ash used in this study was obtained from Nigeria coal corporation, Enugu. The fly ash samples were dried at 1050C for 2h before tests. The fly ash samples were ground and sieved to a particle size of 250µm before use (Alinnor, 2007). Table 1 shows the chemical composition of the fly ash sample used in this study. SiO 2 and Al2O3 contents make up about 80% of the fly ash, while Fe2O3 and CaO compose about 11%. 0.5g portions of fly ash were taken in different Erlenmeyer flasks. 100cm3 of phenol solution was added to each flask having concentrations (30100mg/l). The flask containing fly ash and phenol was agitated in a water bath for different durations of time at 30 0 C using magnetic stirrer. The pH of the mixture was noted before and after agitation with pH meter. The starry formed was then filtered through ordinary filter paper. The clear filtrate was then analyzed for phenol content by Spectrophotometer (Spectronic 2ID) at wavelength 269.4nm. Table 1 Chemical composition of the fly ash Constituent SiO2 Al2O3 Fe2O3 CaO MgO SO3 TiO2 K2O Others
Wt.% 57.25 22.03 8.36 2.97 0.97 0.76 0.68 0.52 6.49
period, the phenol adsorbed by fly ash was determined as described above. Modification of fly ash surface: The powdered fly ash was steeped in 3L of 0.1M NaOH solution and stirred for 1hr at 290C. The fly ash was washed until it was base free[18]. Base extracted fly ash and non – base extracted fly ash were mixed with 0.6m citric acid in a ratio of 1g fly ash to 7.0ml citric acid at 300C, for 3hr. The citric acid modified fly ash was filtered and washed several times to remove excess citric acid. The modified fly ash was air – dried [15] and stored. All the chemicals used were of analytical reagent grade.
Result and Discussion Reaction kinetics: Figure 1 shows the amount of phenol adsorbed onto fly ash as a function of contact time. It was observed that the rate of uptake of phenol onto fly ash increases rapidly in the first 20min as phenol concentration increases. Figure 1 indicates that equilibrium was established within 100min in both concentrations of phenol studied. At equilibrium 30.9mg/L or 49.88% and 72.0 mg/L or 72.0% were removed from the initial concentrations of phenol by fly ash. The result indicates that level of removal of phenol by the fly ash depends on the initial concentration of phenol. It has been reported that greater equilibrium time of 4h was established on adsorption of phenol onto activated charcoal [9], the result was attributed to greater micro – porosity of activated charcoal. Rengaraj et al. (2002) reported equilibrium contact time of 2h for the adsorption of phenol onto palm seed coat activated carbon. The rate constant for adsorption of phenol onto fly ash were determined using first order kinetics [2,6]: …………………………………... (2)
Blank determinations were performed under similar experimental conditions. The amount of phenol adsorbed by the fly ash at equilibrium was calculated using the following expression [7]: …………………………………..(1) Where qe is the amount of phenol adsorbed at equilibrium (mg/kg), V the sample volume (ml). Co the initial phenol concentration (mg/L), Ce the equilibrium phenol concentration (mg/L) and M is the dry weight of the fly ash (g). The reported values of phenol adsorbed by fly ash in each test were the average of at least three measurements[9]. The effect of pH on the uptake of phenol by fly ash were determined by adjusting the pH of the slurry in the range of pH 4 to pH9. At the end of agitation period, the phenol uptake by fly ash was determined as earlier described. The effect of temperature on the adsorption of phenol on fly ash was carried out between 30 and 55 0C using thermostated water bath. At the end of agitation
Where Co is the initial phenol solution concentration, Ct is the concentration at time t, and k is the rate constant. Figure 2 shows that the initial rate of phenol uptake conforms to first – order kinetics as shown in equation (2). A plot of In C o/Ct versus t should yield a straight line from the slope of which the rate constant, K was calculated to 5.60 x 10-1 s-1 and 6.29x10-1s-1 for phenol concentrations 100mg/L and 80mg/l, respectively. Effect of pH: The adsorption isotherms of phenol at different pH ranges are shown in figure 3. The range of concentration varied from 30 to 80mg/L and the pH ranges were pH 4 to pH 9. Figure 3 indicates that the amount of phenol adsorbed from aqueous solution is significantly high at pH 9 when compared to pH 6 and pH 4, respectively. At low pH 4 the adsorption of phenol onto the surface of fly ash increases rapidly. Whereas there was a gradual increase on adsorption of phenol onto fly ash at pH 6. The increase in adsorption of phenol onto fly ash at pH 4 when compared to pH 6, may be attributed to phenol being
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Alinnor et al. Int. J. Res. Chem. Environ. Vol.2 Issue 2 April 2012(124-129) partially converted to positively charged phenolium ions. These phenolium ions will rapidly adsorb onto the negatively charged surface of fly ash.
alkaline value, thereby increasing the adsorption of phenol onto fly ash. 16 14
Amount of phenol adsorbed (mg/kg)
However, the decrease in the uptake of phenol by fly ash at pH 6 may be attributed to dissociation of phenol into phenolate anions. Thus repulsion accurs between the negatively charged fly ash surface and negative phenolate anions, this might probably resulted in the lower adsorption of phenol at pH 6. Moreover, phenolate ions have more affinity for an aqueous solution than neutral phenol and this provides another contributing factor for low adsorption rate at pH 6. Figure 3 shows that the removal of phenol from aqueous solution was more at pH 9 when compared to pH 4. At pH 9 the surface of the adsorbent acquires negative charge due to the adsorption of OH-,
12 10 8 6 4
pH 4.0 pH 6.0 pH 9.0
2 0 30
40
50
60
70
Initial phenol concentration (mg/L)
80
Fig. 3 Effect of pH on asdsorption of phenol on fly ash
15
Amount of phenol adsorbed (mg/kg)
14
A schematic representation of the electrostatic repulsion between the phenolate ion and the negatively charged fly ash surface is shown in fig. 4
13 12 11 10 9
However, phenolate and phenolium ions have more affinity for an aqueous solution than neutral phenol and this provides another factor to low adsorption of phenol at pH 4 and pH 6. Generally solubility of phenol changes with changes in the pH of the medium and thus would have its own contributing role in adsorption.
8 7 6 5 4 3
80 mg/L 100 mg/L pH 6.8
2 1 0 20
40
60
80
100
120
140
Contact time (min) Fig. 1 Effect of contact time on the adsorption of phenol on fly ash at pH 6.8 1.0
Figure 4: Electrostatic repulsion mechanism between phenolate ion and negatively charged fly ash surface Effect of temperature
0.8
lnCo/Ct
0.6
0.4
0.2
80 mg/L phenol 100 mg/L phenol
0.0 0
20
40
60
Time (min)
80
Fig. 2 First-order kinetic plot for phenol adsorption on fly ash
as well as calcium and (SiO2 + Al2O3 +Fe2O3) content of fly ash. The increase in phenol uptake by fly ash at pH 9 may be explained in terms of electrostatic interaction between fly ash surface and neutral phenol. The increase in phenol uptake at pH 9 may be attributed to high alkalinity provided by fly ash content of calcium and (SiO2 + Al2O3 +Fe2O3). The fly ash content raises the pH to strongly
The removal characteristics of phenol from aqueous solution is temperature dependent. Figure 5 indicates the amount of phenol removed from aqueous solution as a function of temperature at phenol concentration 20 and 50mg/L, respectively. There was a gradual decrease on the uptake of phenol by fly ash from 30 to 550C at concentration 20mg/L. Also at concentration 50mg/L the removal of phenol from aqueous solution onto fly ash decreases as temperature increases from 30 to 55 0C. At initial concentration 20mg/L of phenol at 300C, the amount of phenol adsorbed was 12.20mg/L or 61%. As the temperature increases the amount of phenol uptake decreases. However, at 450C the amount of phenol removed from aqueous solution by fly ash was 11.0mg/L or 55%. At 550C the uptake of phenol from aqueous solution by fly ash was 7.80mg/L or 39%. At initial concentration 50mg/L of phenol, at 300C the phenol uptake from aqueous solution by fly ash was
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Alinnor et al. Int. J. Res. Chem. Environ. Vol.2 Issue 2 April 2012(124-129) 42.20mg/L or 84.40%. Also as the temperature increases the amount of phenol uptake by fly ash decreases. At 45 0C the amount of phenol removed from aqueous solution by fly ash decreased to 41.20mg/L or 82.40%. There was a progressive decrease on the amount of phenol uptake by fly ash from 30 to 550C. However, at 550C the amount of phenol removed from aqueous solution was 40.40mg/L or 80.80%. 9
7
10
6 5
Amount of phenol adsorbed (mg/kg)
Amount of phenol adsorbed (mg/kg)
8
The enhanced level of phenol uptake by citric acid modified fly ash, could be explained in terms of condensation product. Citric acid anhydride formed during the modification of fly ash is very reactive, and this reactive anhydride may probably combine with any of the constituents of fly ash to form citric acid anhydride–fly ash linkage. This linkage will introduce carboxyl groups to the fly ash surface, thereby increasing the negatively charged fly ash surface. The increase in the negatively charged surface of fly ash will in turn increase the uptake of neutral phenol onto fly ash. This may probably result to greater percentage of phenol removal from aqueous solution by modified fly ash.
20 mg/L phenol 50 mg/L phenol
4 3 2 1 30
35
40
45
50
55
o
Temperature C
Fig. 5 Effect of temperature on adsorption of phenol on fly ash
9 8 7 6 5 4 3 2
Unmodified fly ash Modified fly ash Conc. 80 mg/L
1 0
The result indicates that phenol adsorption onto fly ash follows a similar trend in the two concentrations studied. However, the amount of phenol uptake depends largely on the initial concentration of phenol. These results shows that the adsorption of phenol onto fly ash increases at low temperature. The decrease in adsorption of phenol onto fly ash at high temperature may be attributed to increase in average kinetic energy of the phenolate anions, thus increasing the repulsive forces between the phenolate anions and negatively charged fly ash surface. Also increase in temperature may be associated with the decrease in the stability of phenolate anionadsorbent complex. This could lead to adsorption of phenol off the surface of the fly ash instead of colliding and combining with it. Alinnor and Nwachukwu (2011) reported a decrease in adsorption of para-nitrophenol onto fly ash as temperature increase from 30 to 600C. Modification of Fly Ash Surface: Figure 6 shows the effect of contact time on adsorption of phenol onto citric acid modified and unmodified fly ash. The result indicates that at initial contact time of 20min and concentration 80mg/L the amount of phenol uptake by modified and unmodified fly ash were 22.0mg/L or 27.5% and 2.0mg/L or 3.0%, respectively. At equilibrium 39.9mg/L or 50.0% and 49.0mg/L or 61.30% of phenol were removed from aqueous solution by unmodified and modified fly ash, respectively.
20
40
60 80 100 Contact time (min)
120
140
Fig. 6 Effect of contact time on phenol adsorption on fly ash
Enhancement of phenol uptake by citric acid modified fly ash could be explained further in terms of distribution coefficient. Distribution coefficient is defined as the ratio of phenol adsorbed at equilibrium to the amount in the liquid phase. This provides an estimate of adsorption efficiency. Values of the distribution coefficient lower than 1 indicates low adsorption efficiency. The distribution coefficient is calculated using relationship: ……………………….. (3) Where kd is the distribution coefficient, qe is the amount of phenol adsorbed at equilibrium and Ce is the amount of phenol in the liquid phase. The distribution coefficient of phenol for modified and unmodified fly ash when equilibrium was established were calculated to be 1.580 and 0.995, respectively. The result shows good adsorption efficiency of fly ash modified with citric acid. The value of the distribution coefficient of the phenol between the citric acid modified fly ash and the aqueous phase is dependent on the initial phenol concentration and is greater than 1. Adsorption isotherm: The adsorptive behaviour of this study can be satisfactorily described by using equation state
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Alinnor et al. Int. J. Res. Chem. Environ. Vol.2 Issue 2 April 2012(124-129) between the two phases composing the adsorption system. Figure 7 and Figure 8 shows linearized Freundlich isotherms of phenol concentrations 80mg/L and 100mg/L, respectively. Using equation (5) the adsorption capacity, KF and adsorption intensity, n for 80mg/L phenol were calculated to be 4.79mg/g and 2.0, respectively. Also at 100mg/L phenol the adsorption capacity and adsorption intensity were calculated to be 0.012mg/g and 1.20, respectively. The measured values of KF showed easy uptake of phenol with high adsorption capacity of fly ash at 80mg/L. The obtained values of n at both concentrations of phenol indicated a higher adsorption ability of phenol onto fly ash.
0.9
0.8
logqe
0.7
0.6
0.5
0.4
This study indicates that the adsorption experiment is a physical process. This means that the adsorption process obtained is a physical separation process and the adsorbed phenol is not chemical altered. This implies that the phenol is simply but effectively removed from the aqueous solution (one phase) and transferred to fly ash (another phase). The fly ash now contains the phenol. Here the chemical characteristics of the adsorbed phenol did not change, hence the use of adsorption technique in removing phenol from aqueous solution is associated with its removal from aqueous solution and transfer to the fly ash.
80 mg/L phenol 1.1
1.2
1.3
1.4
1.5
1.6
logCe Fig. 7 Freundlich isotherm plot of phenol adsorption on fly ash 1.18 1.16 1.14
logqe
1.12 1.10
Conclusion
1.08 1.06 1.04
100 mg/L phenol
1.02 1.72
1.74
1.76
1.78
1.80
1.82
1.84
1.86
1.88
logCe Fig. 8 Freundlich isotherm plot of phenol adsorption on fly ash
Langmuir and Freundlich isotherms were tested for the experimental data. The best fit was achieved using a Freundlich isotherm. Freundlich isotherm model is given by the equation: ……………….…………………… (4) Where qe (mg/g) is the amount of phenol adsorbed at equilibrium, Ce (mg/L) is the concentration of phenol in solution at equilibrium,
This study indicates that coal fly ash is good adsorbent for removal of toxic solvents from wastewater. Kinetic study indicates that phenol was adsorbed onto the fly ash within the first 20min while equilibrium was established within 100min for both concentrations of phenol studied. This investigation revealed that phenol uptake was high at pH9 due to the increase of negatively charged fly ash surface, which enhances adsorption. This study revealed that the modification of fly ash surface with citric acid enhances adsorption of phenol onto fly ash. The adsorption isotherm indicates that Freundlich model effectively fits the experimental data than the Langmuir model.
Acknowledgement The authors are grateful to Mr. Nworie Emmanuel for technical assistance in performing some measurements. The authors are also grateful to Department of Chemistry, Federal Polytechnic Nekede, Owerri, Imo State, Nigeria, for making use of some of their facilities.
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