Bull Environ Contam Toxicol (2014) 93:549–554 DOI 10.1007/s00128-014-1381-8
Removal of Phenolic Compounds from Aqueous Phase by Adsorption onto Polymer Supported Iron Nanoparticles Bhupendra K. Sen • Dhananjay K. Deshmukh Manas K. Deb • Devsharan Verma • Jolly Pal
•
Received: 19 June 2013 / Accepted: 8 September 2014 / Published online: 26 September 2014 Ó Springer Science+Business Media New York 2014
Abstract The removal of phenolic compounds, i.e., o-cresol, m-cresol, and p-cresol from aqueous solution have been evaluated employing activated carbon (AC) coated with polymer supported iron nanoparticles (FeNPs). The synthesized FeNPs were characterized by scanning electron microscope and X-ray diffraction analysis. High correlation coefficient values indicated that the adsorption of phenolic compounds onto AC coated with polyvinylpyrrolidon (PVP) supported FeNPs obey Freundlich and Langmuir adsorption isotherms. Higher Freundlich and Langmuir constant values for AC coated with PVP supported FeNPs indicated its greater efficiency than AC. The adsorption data are well represented by both the Freundlich and Langmuir isotherms, indicating favourable adsorption of cresols by the adsorbents. Cresols were effectively removed (90 %) by adsorption process from aqueous solution using AC coated with FeNPs. The percentage removal of above phenolic compounds was studied under varying experimental conditions such as pH, temperature, adsorbent dosage, and contact time. The adsorption of phenolic compounds is quite sensitive to pH of the suspension and optimum uptake value was found at pH 7.0. Temperature also has a favorable effect on adsorption
B. K. Sen M. K. Deb J. Pal School of Studies in Chemistry, PT. Ravishankar Shukla University, Raipur 492010, India D. K. Deshmukh (&) Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan e-mail: [email protected] D. Verma Central Ground Water Board, Central Region, Nagpur 440001, India
when varied from 20 to 50°C. On the contrary, beyond 30°C, a decrease in the adsorption was noticed. Keywords Adsorption Phenolic compounds Adsorption isotherm Iron nanoparticles Polyvinylpyrrolidone
The disposal of waste is of widespread international concern (Rezaee et al. 2008). Increased industrialization and human activities have impacted the environment through the disposal of waste containing organic pollutants (Djebbar et al. 2012). Phenol and its derivatives are typical organic pollutants and have been identified as priority pollutants by United States Environmental Protection Agency (USEPA) because of its highly toxic, carcinogenic, and mutagenic properties (USEPA 2002). Industrial process such as oil refineries, petrochemical industries, coal conversion processes, phenolic resin industries, and pharmaceutical industries generate phenolic compounds (Jasper et al. 2010). Phenols are widely used for the commercial production of a variety of resins including phenolic resins, epoxy resins, adhesive and polyamides for various applications (Seddigi 2010). Phenolic compounds impart objectionable taste and odor to drinking water at concentration as low as 0.005 mg dm-3 (USEPA 2002). EPA regulations call for lowering phenol content in wastewater to \1 ppm (USEPA 2002). Cresols are one of the major organic pollutants belonging to the class of phenolic compounds, and its sources include the discharges of chemical process industries such as coal gasification, polymeric resin production, fungicides production, intermediate of manufacture of pesticides, and dyes. Phenolic compounds are harmful even at low concentrations and many of them have been classified as hazardous pollutants
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Bull Environ Contam Toxicol (2014) 93:549–554
Table 1 Comparative features of some adsorbents reported in previous studies Adsorbents
Advantage
Disadvantage
Reference
Activated carbon
High adsorption capacity and versatility
Quite expensive
Babel and Kurniawan 2003
Straw and rice husk
Inexpensive and require little processing
Inefficient desorption
Babel and Opiso 2007
Biosorbent
High selectivity and good removal performance
Effective only in favorable condition
Shah et al. 2012
Nanosorbent (FeNPs)
Stable, eco-friendly, easy to prepare and recyclable
Present work
due to their potential harmful effects to human health. Phenolic compounds can be absorbed by the body through the respiratory organ and alimentary canal. Phenolic compounds can cause allergic dermatitis and skin irritation. It can also restrain the central nervous system and interact with the liver and nephridium. Therefore, the removal of phenolic compounds is necessary from the drinking water to protect the human health. There are many methods such as chemical and electrochemical oxidation, ion exchange process, and adsorption that have been used for the removal of phenol and its derivatives from the aqueous solution (Jiang et al. 2006; Djebbar et al. 2012). Adsorption is a well-established and powerful technique for removing wide variety of hazardous materials such as dyes and organic compounds from the aqueous solution. A number of adsorption materials have been used to remove organic compounds including ion exchange resins, crushed coals, straws, and activated carbon (Rezaee et al. 2008). Some of these materials such as ion exchange resins are quite effective but expensive, and others such as coal and straw are inexpensive but not very effective. Table 1 shows some of the comparative features on different adsorbents reported earlier. Nowadays, activated carbon (AC) is the most widely used adsorbent that has high adsorption capacity for organic compounds (Fei and Bei 2007; Jasper et al. 2010; Wu et al. 2010). Recent investigations have shown that metal nanoparticles such as Fe2O3, TiO2 and SiO2 extremely increase the surface adsorption capacity of conventional adsorbents with respect to many organic compounds. Surface of nanoparticles with organic polymers coatings help to prevent particle agglomeration caused by electrostatic and magnetic attractions that may produce embolism. Polymers such as PVP have been widely used as protective media for colloidal dispersions (Pal and Deb 2014). The combined technique of adsorption and magnetic separation holds the advantages of eco-friendly characteristics and economic
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viability. The inclusion of PVP supported FeNPs in the adsorbent increases the removal efficiency of AC. Therefore, in the present study, an attempt has been made to the removal of phenolic compounds, i.e., o-cresol, m-cresol and p-cresol using AC coated with PVP supported FeNPs.
Materials and Methods Ferric chloride (FeCl37H2O, [99 %) was obtained from Himedia. Sodium borohydride and PVP were procured from Merck. Cresols, i.e., o-cresol, m-cresol and p-cresol ([99 %) were purchased from Merck. All aqueous solutions were prepared in Milli-Q water. Glasswares were cleaned with ultrasonic cleaning bath (UCB-50, Spectra Lab India) using mild detergent and after proper washing rinsed with Milli-Q water. Cleaning normally lasts between 3 and 6 min. Ultrasonic cleaning uses cavitation bubbles induced by high frequency sound waves (usually from 20 to 400 kHz) to agitate a liquid. The agitation produces high forces on contaminants adhering to substrates. This action also penetrates blind holes, cracks, and recesses. 5 MLH Magnetic stirrer was used for the homogenous stirring of the reaction mixture. Absorption spectra were recorded in a Varian Carry 50 UV–Visible spectrophotometer equipped with a peltier temperature controller unit. The size and morphology of the synthesized PVP supported FeNPs were characterized by Morgagni 268D scanning electron microscope (SEM) operating at 80 KB (Mega view III Camera CCD). The XRD measurements were carried out using Bruker D8 Advance X-ray diffractometer. The PVP stabilized FeNPs were synthesized by the borohydride reduction approach by PVP solution (1 %) in water as a pre-agglomeration stabilizer. Dried AC were dipped into the synthesized PVP supported FeNPs, and heated gently at 50°C till dryness to complete removal of moisture. PVP provides a polymeric support to FeNPs over the surface of AC. AC coated with PVP supported FeNPs was then taken for adsorption studies. Aliquots of o-cresol, m-cresol, and p-cresol solutions with known concentration were introduced individually into the columns containing accurately weighed amount of AC to perform the adsorption experiments. The equilibrium adsorption capacities of o-cresol, m-cresol, and p-cresol by the AC coated with PVP supported FeNPs were determined. A 20 mL of adsorbate solution of different concentrations were placed in columns (1200 length 9 1.500 diameter). The flasks were sealed with stopper and kept for 24 h, and adsorbate equilibrium concentration was measured by UV–Visible spectrophotometer. The adsorption capacities of o-cresol, m-cresol, and p-cresol by AC were also measured for comparison purpose. The amounts of o-cresol, m-cresol, and p-cresol adsorbed were calculated by the following mass balance equation:
Bull Environ Contam Toxicol (2014) 93:549–554
Percentage removal ¼
ðCi Ce Þ 100 Ci
where, Ci is initial concentration and Ce is equilibrium or final concentration of phenolic compounds in ppm.
Results and Discussion Morphological features of synthesized PVP supported FeNPs were studied by SEM. The SEM image of PVP supported FeNPs is shown in Fig. 1. It showed that PVP supported FeNPs has a considerable number of pores, where there is a good possibility for adsorbate to be trapped and adsorbed. The XRD patterns of the AC and PVP supported FeNPscoated-AC are shown in Fig. 2. The peaks appeared at 2h = 25.7, 29.5, and 39.5 for AC. AC shows low crystallinity, while in PVP supported FeNPs-coated-AC, considerable changes were observed in XRD graph, and new peaks appeared at 2h = 22.2, 36.2, 39.9, 56.9, and 58.6, suggesting the impregnation of FeNPs into AC. The XRD patterns of PVP supported FeNPs-coated-AC indicated that the crystallinity of AC increases due to the presence of nanoparticles onto the surface of AC. The crystallinity of AC loaded with PVP supported FeNPs was also determined by the degree of crystallinity. The numerical formula used to calculate the percentage of crystallinity of PVP
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supported FeNPs-coated-AC is shown by the following equation: Ac Xc ¼ þ Ac 100 Aa where, Ac and Aa are the area of crystalline and amorphous phase, respectively. The present crystallinity of AC loaded with PVP supported FeNPs was calculated to be 39.6 %. An adsorption isotherm describes the relationship of the amounts of adsorbate and the concentration of remaining adsorbate in the solution at equilibrium. Two important isotherms selected for this study were Freundlich and Langmuir isotherm models. The Freundlich isotherm is a widely used model for adsorption of organic compounds from aqueous medium and may be represented as: 1 1 a ¼ Kf Cne or log a ¼ log Kf þ logCe n where, Kf and 1/n are Freundlich constants indicating the adsorption capacity and adsorption intensity, respectively. Values of 1/n [ 1 signify that the solute has a low affinity for the adsorbent at low concentrations. Likewise, the values of 1/n \ 1 are indication of favorable adsorption and a high affinity of the solute with the solid phase. The Langmuir isotherm is an empirical isotherm derived from a proposed kinetic mechanism. This model has found successful application in many adsorption processes and it could be used to explain the adsorption of o-cresol, mcresol, and p-cresol onto AC and PVP supported FeNPscoated-AC. The Langmuir isotherm may be presented as: Ce b Ce ¼ þ Qe Q0 Ce
Fig. 1 SEM image of PVP supported FeNPs
where, Ce is the equilibrium phenolic compound concentration in ppm, Qe is the equilibrium phenolic compound concentration on the adsorbent (mg g-1), Q0 is the maximum absorption capacity of the phenolic compound per unit weight of adsorbent (mg g-1), and b is a Langmuir constant related to the affinity of the binding sites (L mg-1). The essential characteristics of the Langmuir isotherm can be expressed by a separation factor (RL), which is defined in the following equation:
Fig. 2 XRD patterns of AC and PVP supported FeNPs-coatedAC
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RL ¼
Bull Environ Contam Toxicol (2014) 93:549–554
1 1 þ bC0
where, RL indicates the type of isotherm to be favorable (0 \ RL \ 1), linear (RL = 1), unfavorable (RL [ 1) or irreversible (RL = 0). The results obtained by the application of Freundlich and Langmuir models are presented in Table 2. From this table, it could be observed that the correlation coefficients for Freundlich isotherm are significantly higher than that of Langmuir isotherm. The analysis of KF and Qo, which are measures of adsorption capacity, showed that the adsorption isotherm of cresols fits Freundlich isotherm model much better as compared to Langmuir isotherm model and can be approved to the effective physical adsorption of cresols onto AC coated with PVP supported FeNPs. The results showed that the value of 1/n is less than unity indicating that the cresols are favorably adsorbed by the AC coated with PVP supported FeNPs. According to the values of RL, all the systems showed favorable adsorption of phenolic compounds, i.e., 0 \ RL \ 1. The low values of RL indicated high and favorable adsorption of phenolic compounds onto AC coated with PVP supported FeNPs. The effect of solution pH on the removal of phenolic compounds could be explained by considering the presence of ionic and molecular forms of phenolic compounds in aqueous solution. These compounds proceed as weak acid in aqueous solution resulting in the dissociation of hydrogen ions from phenolic compounds, which strongly Table 2 Freundlich and Langmuir isotherms constants for the adsorption of cresols by AC and PVP supported FeNPscoated-AC
Adsorbents
Activated carbon
Fig. 3 Effect of pH and temperature on the percentage removal of cresols by PVP supported FeNPs-coated-AC
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Freundlich model -1
KF (mg g ) ACa
FeNPs-coated-AC
a
Adsorbate
depends on the initial pH of the solution. The molecular form dominates in acidic solution, whereas in alkaline medium the anionic form is the leading species. Therefore, in order to study the effect of pH on the removal of ocresol, m-cresol, and p-cresol by PVP supported FeNPscoated-AC, the adsorption experiments were performed at different pH levels ranging from 3 to 11 using 0.1 N HCl and 0.1 N NaOH (Fig. 3). The removal of o-cresol, mcresol, and p-cresol by PVP supported FeNPs-coated-AC increases in the pH range from 3 to 7, and decreases at higher pH value from 8 to 12. The amount of cresols adsorbed showed a declining trend with higher as well as with lower pH, and maximum removal of cresols (*80 %) at neutral pH. The pH of aqueous solution of cresols affects its uptake on adsorbent and the uptake decreases at lower as well as at higher pH values. At lower pH value the presence of hydrogen ions suppresses the ionization of cresols, and hence its uptake on adsorbent is reduced. At higher pH range, cresols form salts that readily ionize leaving the negative charges on the phenolic group. At the same time the presence of hydroxyl ions on the adsorbent prevents the binding of phenolate ions that leads to low adsorption of phenolic compounds. The pH of the initial cresols concentration was maintained for all the experiments at 7 because of the reasons mentioned above. The temperature has an important effect on the process of adsorption. The adsorption experiments were performed at different temperatures in the range from 20 to 50°C
1/n
Langmuir model R
2
Qo (mg g-1)
b (L mg-1)
RL
R2
o-cresol
1.13
0.92
0.88
0.57
0.11
0.57
0.83
m-cresol
1.03
0.99
0.90
0.58
0.13
0.56
0.89
p-cresol
1.19
0.86
0.92
0.56
0.11
0.59
0.86
o-cresol
5.02
0.36
0.99
1.02
0.22
0.31
0.92
m-cresol
3.99
0.38
0.98
1.03
0.26
0.28
0.93
p-cresol
5.09
0.39
0.99
1.02
0.19
0.33
0.92
Bull Environ Contam Toxicol (2014) 93:549–554
(Fig. 3). It was observed that the adsorption of o-cresol, m-cresol, and p-cresol by PVP supported FeNPs-coatedAC increases from 20 to 30°C, whereas, beyond 30°C, a sudden fall in the adsorption occurred. By the increase in temperature, a large number of active sites may be generated on the AC, which clearly bring about an increase in the adsorption of cresols by PVP supported FeNPs-coatedAC. However, at higher temperature (above 30°C), the observed decrease in adsorption was due to the weakening of binding force of cresols and active sites on AC. The adsorption experiments were performed with different o-cresol, m-cresol, and p-cresol concentrations ranging from 10 to 50 ppm to observe the effect of initial
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concentration on the removal of cresols by AC and PVP supported FeNPs-coated-AC (Fig. 4). The adsorption experiments were conducted in optimum conditions at pH 7 and 30°C temperature. The percentage removal of o-cresol, m-cresol, and p-cresol decreases with increase in the concentration of respective phenolic compounds. The results showed that the maximum percentage removal of cresols was at 10 ppm initial concentration of respective phenolic compounds for both AC and PVP supported FeNPs-coated-AC. At higher concentrations (above 50 ppm), the percentage removal of cresols reached to a constant value that may be attributed to the saturation of adsorption sites. The decrease in the percentage removal of
Fig. 4 Effect of initial concentration on the percentage removal of cresols by activated carbon and synthesized PVP supported FeNPs-coatedAC
Fig. 5 Effect of contact time on the percentage removal of cresols by activated carbon and synthesized PVP supported FeNPs-coated-AC
Fig. 6 Effect of adsorbent dosage on the percentage removal of cresols by activated carbon and synthesized PVP supported FeNPs-coated-AC
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cresols with increase in the initial concentration was mainly due to the limited number of available active sites on the surface of AC and PVP supported FeNPs-coated-AC to accommodate higher concentration of phenolic compounds. The adsorption experiments were performed at different duration of time ranging from 3 to 24 h to study the effect of contact time on the adsorption of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC (Fig. 5). Aqueous solutions containing a definite initial concentration of o-cresol, m-cresol, and p-cresol were passed through the column and the pH of the solution was adjusted to 7.0 by the addition of 0.1 N HCl or 0.1 N NaOH. The effect of contact time on adsorption of cresols was performed at a temperature 30°C. The results showed that the percentage removal of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC increases with increase in contact time, and reaches to a constant value because at the initial stage the rate of removal of phenolic compounds was higher due to the availability of more than required number of active sites on the surface of AC and PVP supported FeNPs-coated-AC which becomes lower at the later stages of contact time due to the decreased or lesser number of active sites. Therefore, the maximum removal of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC was obtained within 20 h. Based on these results, a contact time of 20 h was assumed to be suitable for subsequent experiments of adsorption isotherm. Above the contact time of 20 h, the adsorption of cresols reached to a constant value. Adsorbent dose is also an important parameter in the determination of adsorption capacity. The dependence of o-cresol, m-cresol and p-cresol sorption on adsorbent dose was studied by varying the amounts of adsorbents from 0.5 to 3.0 g with fixed volume of adsorbates keeping other parameters such as pH (7.0), temperature (30°C), initial concentration (10 ppm) and contact time (20 h) stable (Fig. 6). The removal efficiency of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC from the aqueous solution increases with increase in the adsorbent dosage. This can be attributed to the increased adsorbent surface area and availability of more adsorption sites, resulting in an increase in the amount of cresols adsorbed per unit mass of AC and PVP supported FeNPs-
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coated-AC. It was observed that the maximum removal of o-cresol, m-cresol, and p-cresol attained with 2.0 g adsorbent and 10 mL of o-cresol, m-cresol, and p-cresol solutions of 10 ppm concentration. The results clearly indicated that the removal efficiency increases up to an optimum dose of 2.0 g and beyond which the removal of o-cresol, m-cresol, and p-cresol remained to be constant. Acknowledgments The authors thank All India Institute of Medical Science (AIIMS), New Delhi, India for SEM analysis, and UGC-DAE Consortium for Scientific Research Centre Indore, India for the XRD analysis.
References Babel S, Kurniawan TA (2003) Low-cost adsorbents for heavy metal uptake from contaminated water: a review. J Hazard Mater 97:219–243 Babel S, Opiso ME (2007) Removal of Cr from synthetic waste water by sorption into volcanic ash soil. Int J Environ Sci Technol 4:99–107 Djebbar M, Djafri F, Bouchekara M, Djafri A (2012) Adsorption of phenol on natural clay. Appl Water Sci 2:77–86 Fei Q, Bei W (2007) Single- and Multi-component Adsorption of Pb, Cu, and Cd on Peat. Bull Environ Contam Toxicol 78:265–269 Jasper A, Sahil HH, Sorial GA, Sinha R, Krishnan R, Patterson CL (2010) Impact of nanoparticles and natural organic matter on the removal of organic pollutants by activated carbon adsorption. Environ Eng Sci 27:85–93 Jiang HL, Tay JH, Maszenan AM, Tay ST (2006) Enhanced phenol biodegradation and aerobic granulation by two coaggregating bacterial strains. Environ Sci Technol 40:6137–6142 Pal J, Deb MK (2014) Microwave green synthesis of PVP stabilized gold nanoparticles and their adsorption behavior for methyl orange. J Exp Nanosci 9:432–443 Rezaee A, Pourtaghi GH, Khavanin A, Mamoory RS, Ghaneian MT, Godini H (2008) Photocatalytic decomposition of gaseous toluene by TiO2 nanoparticles coated on activated carbon. Iran J Environ Health Sci Eng 5:305–310 Seddigi ZS (2010) Removal of alizarin yellow dye from water using zinc doped WO3 catalyst. Bull Environ Contam Toxicol 84:564–567 Shah B, Tailor R, Shah A (2012) Equilibrium, kinetics, and breakthrough curve of phenol sorption on zeolitic material derived from BFA. J Dispers Sci Technol 33:41–51 US Environmental Protection Agency (USEPA) Washington D.C. (2002) Toxicol Rev Phenol. EPA-635-R-02-006 Wu B, Zhang Y, Zhang X, Cheng S (2010) Health risk from exposure of organic pollutants through drinking water consumption in Nanjing, China. Bull Environ Contam Toxicol 84:46–50
Removal of Phenolic Compounds from Aqueous Phase by Adsorption onto Polymer Supported Iron Nanoparticles Bhupendra K. Sen • Dhananjay K. Deshmukh Manas K. Deb • Devsharan Verma • Jolly Pal
•
Received: 19 June 2013 / Accepted: 8 September 2014 / Published online: 26 September 2014 Ó Springer Science+Business Media New York 2014
Abstract The removal of phenolic compounds, i.e., o-cresol, m-cresol, and p-cresol from aqueous solution have been evaluated employing activated carbon (AC) coated with polymer supported iron nanoparticles (FeNPs). The synthesized FeNPs were characterized by scanning electron microscope and X-ray diffraction analysis. High correlation coefficient values indicated that the adsorption of phenolic compounds onto AC coated with polyvinylpyrrolidon (PVP) supported FeNPs obey Freundlich and Langmuir adsorption isotherms. Higher Freundlich and Langmuir constant values for AC coated with PVP supported FeNPs indicated its greater efficiency than AC. The adsorption data are well represented by both the Freundlich and Langmuir isotherms, indicating favourable adsorption of cresols by the adsorbents. Cresols were effectively removed (90 %) by adsorption process from aqueous solution using AC coated with FeNPs. The percentage removal of above phenolic compounds was studied under varying experimental conditions such as pH, temperature, adsorbent dosage, and contact time. The adsorption of phenolic compounds is quite sensitive to pH of the suspension and optimum uptake value was found at pH 7.0. Temperature also has a favorable effect on adsorption
B. K. Sen M. K. Deb J. Pal School of Studies in Chemistry, PT. Ravishankar Shukla University, Raipur 492010, India D. K. Deshmukh (&) Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan e-mail: [email protected] D. Verma Central Ground Water Board, Central Region, Nagpur 440001, India
when varied from 20 to 50°C. On the contrary, beyond 30°C, a decrease in the adsorption was noticed. Keywords Adsorption Phenolic compounds Adsorption isotherm Iron nanoparticles Polyvinylpyrrolidone
The disposal of waste is of widespread international concern (Rezaee et al. 2008). Increased industrialization and human activities have impacted the environment through the disposal of waste containing organic pollutants (Djebbar et al. 2012). Phenol and its derivatives are typical organic pollutants and have been identified as priority pollutants by United States Environmental Protection Agency (USEPA) because of its highly toxic, carcinogenic, and mutagenic properties (USEPA 2002). Industrial process such as oil refineries, petrochemical industries, coal conversion processes, phenolic resin industries, and pharmaceutical industries generate phenolic compounds (Jasper et al. 2010). Phenols are widely used for the commercial production of a variety of resins including phenolic resins, epoxy resins, adhesive and polyamides for various applications (Seddigi 2010). Phenolic compounds impart objectionable taste and odor to drinking water at concentration as low as 0.005 mg dm-3 (USEPA 2002). EPA regulations call for lowering phenol content in wastewater to \1 ppm (USEPA 2002). Cresols are one of the major organic pollutants belonging to the class of phenolic compounds, and its sources include the discharges of chemical process industries such as coal gasification, polymeric resin production, fungicides production, intermediate of manufacture of pesticides, and dyes. Phenolic compounds are harmful even at low concentrations and many of them have been classified as hazardous pollutants
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Bull Environ Contam Toxicol (2014) 93:549–554
Table 1 Comparative features of some adsorbents reported in previous studies Adsorbents
Advantage
Disadvantage
Reference
Activated carbon
High adsorption capacity and versatility
Quite expensive
Babel and Kurniawan 2003
Straw and rice husk
Inexpensive and require little processing
Inefficient desorption
Babel and Opiso 2007
Biosorbent
High selectivity and good removal performance
Effective only in favorable condition
Shah et al. 2012
Nanosorbent (FeNPs)
Stable, eco-friendly, easy to prepare and recyclable
Present work
due to their potential harmful effects to human health. Phenolic compounds can be absorbed by the body through the respiratory organ and alimentary canal. Phenolic compounds can cause allergic dermatitis and skin irritation. It can also restrain the central nervous system and interact with the liver and nephridium. Therefore, the removal of phenolic compounds is necessary from the drinking water to protect the human health. There are many methods such as chemical and electrochemical oxidation, ion exchange process, and adsorption that have been used for the removal of phenol and its derivatives from the aqueous solution (Jiang et al. 2006; Djebbar et al. 2012). Adsorption is a well-established and powerful technique for removing wide variety of hazardous materials such as dyes and organic compounds from the aqueous solution. A number of adsorption materials have been used to remove organic compounds including ion exchange resins, crushed coals, straws, and activated carbon (Rezaee et al. 2008). Some of these materials such as ion exchange resins are quite effective but expensive, and others such as coal and straw are inexpensive but not very effective. Table 1 shows some of the comparative features on different adsorbents reported earlier. Nowadays, activated carbon (AC) is the most widely used adsorbent that has high adsorption capacity for organic compounds (Fei and Bei 2007; Jasper et al. 2010; Wu et al. 2010). Recent investigations have shown that metal nanoparticles such as Fe2O3, TiO2 and SiO2 extremely increase the surface adsorption capacity of conventional adsorbents with respect to many organic compounds. Surface of nanoparticles with organic polymers coatings help to prevent particle agglomeration caused by electrostatic and magnetic attractions that may produce embolism. Polymers such as PVP have been widely used as protective media for colloidal dispersions (Pal and Deb 2014). The combined technique of adsorption and magnetic separation holds the advantages of eco-friendly characteristics and economic
123
viability. The inclusion of PVP supported FeNPs in the adsorbent increases the removal efficiency of AC. Therefore, in the present study, an attempt has been made to the removal of phenolic compounds, i.e., o-cresol, m-cresol and p-cresol using AC coated with PVP supported FeNPs.
Materials and Methods Ferric chloride (FeCl37H2O, [99 %) was obtained from Himedia. Sodium borohydride and PVP were procured from Merck. Cresols, i.e., o-cresol, m-cresol and p-cresol ([99 %) were purchased from Merck. All aqueous solutions were prepared in Milli-Q water. Glasswares were cleaned with ultrasonic cleaning bath (UCB-50, Spectra Lab India) using mild detergent and after proper washing rinsed with Milli-Q water. Cleaning normally lasts between 3 and 6 min. Ultrasonic cleaning uses cavitation bubbles induced by high frequency sound waves (usually from 20 to 400 kHz) to agitate a liquid. The agitation produces high forces on contaminants adhering to substrates. This action also penetrates blind holes, cracks, and recesses. 5 MLH Magnetic stirrer was used for the homogenous stirring of the reaction mixture. Absorption spectra were recorded in a Varian Carry 50 UV–Visible spectrophotometer equipped with a peltier temperature controller unit. The size and morphology of the synthesized PVP supported FeNPs were characterized by Morgagni 268D scanning electron microscope (SEM) operating at 80 KB (Mega view III Camera CCD). The XRD measurements were carried out using Bruker D8 Advance X-ray diffractometer. The PVP stabilized FeNPs were synthesized by the borohydride reduction approach by PVP solution (1 %) in water as a pre-agglomeration stabilizer. Dried AC were dipped into the synthesized PVP supported FeNPs, and heated gently at 50°C till dryness to complete removal of moisture. PVP provides a polymeric support to FeNPs over the surface of AC. AC coated with PVP supported FeNPs was then taken for adsorption studies. Aliquots of o-cresol, m-cresol, and p-cresol solutions with known concentration were introduced individually into the columns containing accurately weighed amount of AC to perform the adsorption experiments. The equilibrium adsorption capacities of o-cresol, m-cresol, and p-cresol by the AC coated with PVP supported FeNPs were determined. A 20 mL of adsorbate solution of different concentrations were placed in columns (1200 length 9 1.500 diameter). The flasks were sealed with stopper and kept for 24 h, and adsorbate equilibrium concentration was measured by UV–Visible spectrophotometer. The adsorption capacities of o-cresol, m-cresol, and p-cresol by AC were also measured for comparison purpose. The amounts of o-cresol, m-cresol, and p-cresol adsorbed were calculated by the following mass balance equation:
Bull Environ Contam Toxicol (2014) 93:549–554
Percentage removal ¼
ðCi Ce Þ 100 Ci
where, Ci is initial concentration and Ce is equilibrium or final concentration of phenolic compounds in ppm.
Results and Discussion Morphological features of synthesized PVP supported FeNPs were studied by SEM. The SEM image of PVP supported FeNPs is shown in Fig. 1. It showed that PVP supported FeNPs has a considerable number of pores, where there is a good possibility for adsorbate to be trapped and adsorbed. The XRD patterns of the AC and PVP supported FeNPscoated-AC are shown in Fig. 2. The peaks appeared at 2h = 25.7, 29.5, and 39.5 for AC. AC shows low crystallinity, while in PVP supported FeNPs-coated-AC, considerable changes were observed in XRD graph, and new peaks appeared at 2h = 22.2, 36.2, 39.9, 56.9, and 58.6, suggesting the impregnation of FeNPs into AC. The XRD patterns of PVP supported FeNPs-coated-AC indicated that the crystallinity of AC increases due to the presence of nanoparticles onto the surface of AC. The crystallinity of AC loaded with PVP supported FeNPs was also determined by the degree of crystallinity. The numerical formula used to calculate the percentage of crystallinity of PVP
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supported FeNPs-coated-AC is shown by the following equation: Ac Xc ¼ þ Ac 100 Aa where, Ac and Aa are the area of crystalline and amorphous phase, respectively. The present crystallinity of AC loaded with PVP supported FeNPs was calculated to be 39.6 %. An adsorption isotherm describes the relationship of the amounts of adsorbate and the concentration of remaining adsorbate in the solution at equilibrium. Two important isotherms selected for this study were Freundlich and Langmuir isotherm models. The Freundlich isotherm is a widely used model for adsorption of organic compounds from aqueous medium and may be represented as: 1 1 a ¼ Kf Cne or log a ¼ log Kf þ logCe n where, Kf and 1/n are Freundlich constants indicating the adsorption capacity and adsorption intensity, respectively. Values of 1/n [ 1 signify that the solute has a low affinity for the adsorbent at low concentrations. Likewise, the values of 1/n \ 1 are indication of favorable adsorption and a high affinity of the solute with the solid phase. The Langmuir isotherm is an empirical isotherm derived from a proposed kinetic mechanism. This model has found successful application in many adsorption processes and it could be used to explain the adsorption of o-cresol, mcresol, and p-cresol onto AC and PVP supported FeNPscoated-AC. The Langmuir isotherm may be presented as: Ce b Ce ¼ þ Qe Q0 Ce
Fig. 1 SEM image of PVP supported FeNPs
where, Ce is the equilibrium phenolic compound concentration in ppm, Qe is the equilibrium phenolic compound concentration on the adsorbent (mg g-1), Q0 is the maximum absorption capacity of the phenolic compound per unit weight of adsorbent (mg g-1), and b is a Langmuir constant related to the affinity of the binding sites (L mg-1). The essential characteristics of the Langmuir isotherm can be expressed by a separation factor (RL), which is defined in the following equation:
Fig. 2 XRD patterns of AC and PVP supported FeNPs-coatedAC
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RL ¼
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1 1 þ bC0
where, RL indicates the type of isotherm to be favorable (0 \ RL \ 1), linear (RL = 1), unfavorable (RL [ 1) or irreversible (RL = 0). The results obtained by the application of Freundlich and Langmuir models are presented in Table 2. From this table, it could be observed that the correlation coefficients for Freundlich isotherm are significantly higher than that of Langmuir isotherm. The analysis of KF and Qo, which are measures of adsorption capacity, showed that the adsorption isotherm of cresols fits Freundlich isotherm model much better as compared to Langmuir isotherm model and can be approved to the effective physical adsorption of cresols onto AC coated with PVP supported FeNPs. The results showed that the value of 1/n is less than unity indicating that the cresols are favorably adsorbed by the AC coated with PVP supported FeNPs. According to the values of RL, all the systems showed favorable adsorption of phenolic compounds, i.e., 0 \ RL \ 1. The low values of RL indicated high and favorable adsorption of phenolic compounds onto AC coated with PVP supported FeNPs. The effect of solution pH on the removal of phenolic compounds could be explained by considering the presence of ionic and molecular forms of phenolic compounds in aqueous solution. These compounds proceed as weak acid in aqueous solution resulting in the dissociation of hydrogen ions from phenolic compounds, which strongly Table 2 Freundlich and Langmuir isotherms constants for the adsorption of cresols by AC and PVP supported FeNPscoated-AC
Adsorbents
Activated carbon
Fig. 3 Effect of pH and temperature on the percentage removal of cresols by PVP supported FeNPs-coated-AC
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Freundlich model -1
KF (mg g ) ACa
FeNPs-coated-AC
a
Adsorbate
depends on the initial pH of the solution. The molecular form dominates in acidic solution, whereas in alkaline medium the anionic form is the leading species. Therefore, in order to study the effect of pH on the removal of ocresol, m-cresol, and p-cresol by PVP supported FeNPscoated-AC, the adsorption experiments were performed at different pH levels ranging from 3 to 11 using 0.1 N HCl and 0.1 N NaOH (Fig. 3). The removal of o-cresol, mcresol, and p-cresol by PVP supported FeNPs-coated-AC increases in the pH range from 3 to 7, and decreases at higher pH value from 8 to 12. The amount of cresols adsorbed showed a declining trend with higher as well as with lower pH, and maximum removal of cresols (*80 %) at neutral pH. The pH of aqueous solution of cresols affects its uptake on adsorbent and the uptake decreases at lower as well as at higher pH values. At lower pH value the presence of hydrogen ions suppresses the ionization of cresols, and hence its uptake on adsorbent is reduced. At higher pH range, cresols form salts that readily ionize leaving the negative charges on the phenolic group. At the same time the presence of hydroxyl ions on the adsorbent prevents the binding of phenolate ions that leads to low adsorption of phenolic compounds. The pH of the initial cresols concentration was maintained for all the experiments at 7 because of the reasons mentioned above. The temperature has an important effect on the process of adsorption. The adsorption experiments were performed at different temperatures in the range from 20 to 50°C
1/n
Langmuir model R
2
Qo (mg g-1)
b (L mg-1)
RL
R2
o-cresol
1.13
0.92
0.88
0.57
0.11
0.57
0.83
m-cresol
1.03
0.99
0.90
0.58
0.13
0.56
0.89
p-cresol
1.19
0.86
0.92
0.56
0.11
0.59
0.86
o-cresol
5.02
0.36
0.99
1.02
0.22
0.31
0.92
m-cresol
3.99
0.38
0.98
1.03
0.26
0.28
0.93
p-cresol
5.09
0.39
0.99
1.02
0.19
0.33
0.92
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(Fig. 3). It was observed that the adsorption of o-cresol, m-cresol, and p-cresol by PVP supported FeNPs-coatedAC increases from 20 to 30°C, whereas, beyond 30°C, a sudden fall in the adsorption occurred. By the increase in temperature, a large number of active sites may be generated on the AC, which clearly bring about an increase in the adsorption of cresols by PVP supported FeNPs-coatedAC. However, at higher temperature (above 30°C), the observed decrease in adsorption was due to the weakening of binding force of cresols and active sites on AC. The adsorption experiments were performed with different o-cresol, m-cresol, and p-cresol concentrations ranging from 10 to 50 ppm to observe the effect of initial
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concentration on the removal of cresols by AC and PVP supported FeNPs-coated-AC (Fig. 4). The adsorption experiments were conducted in optimum conditions at pH 7 and 30°C temperature. The percentage removal of o-cresol, m-cresol, and p-cresol decreases with increase in the concentration of respective phenolic compounds. The results showed that the maximum percentage removal of cresols was at 10 ppm initial concentration of respective phenolic compounds for both AC and PVP supported FeNPs-coated-AC. At higher concentrations (above 50 ppm), the percentage removal of cresols reached to a constant value that may be attributed to the saturation of adsorption sites. The decrease in the percentage removal of
Fig. 4 Effect of initial concentration on the percentage removal of cresols by activated carbon and synthesized PVP supported FeNPs-coatedAC
Fig. 5 Effect of contact time on the percentage removal of cresols by activated carbon and synthesized PVP supported FeNPs-coated-AC
Fig. 6 Effect of adsorbent dosage on the percentage removal of cresols by activated carbon and synthesized PVP supported FeNPs-coated-AC
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cresols with increase in the initial concentration was mainly due to the limited number of available active sites on the surface of AC and PVP supported FeNPs-coated-AC to accommodate higher concentration of phenolic compounds. The adsorption experiments were performed at different duration of time ranging from 3 to 24 h to study the effect of contact time on the adsorption of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC (Fig. 5). Aqueous solutions containing a definite initial concentration of o-cresol, m-cresol, and p-cresol were passed through the column and the pH of the solution was adjusted to 7.0 by the addition of 0.1 N HCl or 0.1 N NaOH. The effect of contact time on adsorption of cresols was performed at a temperature 30°C. The results showed that the percentage removal of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC increases with increase in contact time, and reaches to a constant value because at the initial stage the rate of removal of phenolic compounds was higher due to the availability of more than required number of active sites on the surface of AC and PVP supported FeNPs-coated-AC which becomes lower at the later stages of contact time due to the decreased or lesser number of active sites. Therefore, the maximum removal of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC was obtained within 20 h. Based on these results, a contact time of 20 h was assumed to be suitable for subsequent experiments of adsorption isotherm. Above the contact time of 20 h, the adsorption of cresols reached to a constant value. Adsorbent dose is also an important parameter in the determination of adsorption capacity. The dependence of o-cresol, m-cresol and p-cresol sorption on adsorbent dose was studied by varying the amounts of adsorbents from 0.5 to 3.0 g with fixed volume of adsorbates keeping other parameters such as pH (7.0), temperature (30°C), initial concentration (10 ppm) and contact time (20 h) stable (Fig. 6). The removal efficiency of o-cresol, m-cresol, and p-cresol by AC and PVP supported FeNPs-coated-AC from the aqueous solution increases with increase in the adsorbent dosage. This can be attributed to the increased adsorbent surface area and availability of more adsorption sites, resulting in an increase in the amount of cresols adsorbed per unit mass of AC and PVP supported FeNPs-
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coated-AC. It was observed that the maximum removal of o-cresol, m-cresol, and p-cresol attained with 2.0 g adsorbent and 10 mL of o-cresol, m-cresol, and p-cresol solutions of 10 ppm concentration. The results clearly indicated that the removal efficiency increases up to an optimum dose of 2.0 g and beyond which the removal of o-cresol, m-cresol, and p-cresol remained to be constant. Acknowledgments The authors thank All India Institute of Medical Science (AIIMS), New Delhi, India for SEM analysis, and UGC-DAE Consortium for Scientific Research Centre Indore, India for the XRD analysis.
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