Removal of 4-Picoline from Aqueous Solution by

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teristics for many organic and inorganic compounds in the aqueous solutions. Bagasse fly ash ... 2Professor, Dept. of Chemical Engineering, Indian Institute of Techno- logy, Roorkee, 247667, India. .... Gilchrist 1985). We studied the effect of ...
Removal of 4-Picoline from Aqueous Solution by Adsorption onto Bagasse Fly Ash and Rice Husk Ash: Equilibrium, Thermodynamic, and Desorption Study Dilip H. Lataye1; Indra M. Mishra2; and Indra D. Mall3 Abstract: This paper deals with the adsorption of 4-picoline (4Pi) from aqueous solutions onto bagasse fly ash (BFA) and rice husk ash (RHA) as adsorbents. We discuss the effect of several parameters, such as initial pH, adsorbent dose, contact time, initial concentration, and temperature, on the batch adsorption. We collected equilibrium sorption isotherm data, which could be well-represented by the Redlich-Peterson, Toth, and Radke-Prausnitz isotherm equations. The maximum removal of 4Pi was found to be 46% and 96% by BFA, and 55% and 95% by RHA for lower (50 mg/L) and higher (600 mg/L) concentration of 4Pi, respectively, with sorbent dosages of 5 g/L for BFA and 20 g/L for RHA. Adsorption was found to be very fast, and about 72–90% of 4Pi removal was achieved in the initial 5 min of contact time. We found the sorption of 4Pi on BFA and RHA to be endothermic in nature. The spent adsorbents can be dried and used as a cofuel in boiler furnaces/incinerators. DOI: 10.1061/(ASCE)EE.1943-7870.0000423. © 2011 American Society of Civil Engineers. CE Database subject headings: Adsorption; Desorption; Equilibrium; Fly ash; Thermodynamics. Author keywords: Adsorption; Desorption; Equilibrium; Isotherm; 4-picoline; Bagasse fly ash (BFA); Rice husk ash (RHA); Adsorption isotherms.

Introduction 4-picoline (4Pi), also known as γ-picoline, is a derivative of pyridine. It is a colorless liquid with an intense disagreeable odor and is used in the production of the antituberculosis agent isoniazid and also to make 4-vinylpyridine and subsequent polymers. It is soluble in water, diethyl ether, and acetone. Its vapors are a fire and explosion hazard when exposed to a flame or a spark. 4Pi emits highly toxic fumes of NOx when it is heated to decompose in an oxidative atmosphere. Through all routes of exposure to humans, it is toxic, and it affects the kidneys, liver, and central nervous system. Chronic exposure to 4Pi results in the deterioration of general health because of biomagnification. Severe overexposure may result in death (Kirk and Othmer 1996; Lewis 2004). 4Pi is generally found in the wastewaters of industrial units manufacturing pyridine and its derivatives and in pharmaceutical units, among other places. Normal concentration of 4Pi in wastewaters produced from a typical plant manufacturing α-picoline, β-picoline, γ-picoline, 4-aminopyridine, and other pyridine derivatives is found to be in the range of 20–300 mg=L (Lataye 2007). 1 Assistant Professor, Dept. of Civil Engineering, Visvesvaraya National Institute of Technology, Nagpur, 440010, India (corresponding author). E-mail: [email protected] 2 Professor, Dept. of Chemical Engineering, Indian Institute of Technology, Roorkee, 247667, India. E-mail: [email protected] 3 Professor, Dept. of Chemical Engineering, Indian Institute of Technology, Roorkee, 247667, India. E-mail: [email protected] Note. This manuscript was submitted on September 25, 2008; approved on May 2, 2011; published online on May 5, 2011. Discussion period open until April 1, 2012; separate discussions must be submitted for individual papers. This paper is part of the Journal of Environmental Engineering, Vol. 137, No. 11, November 1, 2011. ©ASCE, ISSN 0733-9372/2011/111048–1057/$25.00.

Various treatment methods such as adsorption (Zhu et al. 1988; Mall et al. 2003; Liu et al. 2005; Lataye et al. 2006, 2008a, b, c, 2009a, b) and biodegradation (Sims and Sommers 1986) can be used for the treatment of wastewater containing pyridine and its derivatives. Adsorption is a simple and economical technique for the removal of various organics or inorganics from wastewater. Agricultural wastes generally exhibit excellent adsorption characteristics for many organic and inorganic compounds in the aqueous solutions. Bagasse fly ash (BFA) and rice husk ash (RHA) are waste materials collected from the flue gases released from boilers/ furnaces that use bagasse and rice husk as fuel. These wastes are collected by using particulate collection equipment such as multiclones and bag filters and are available almost free of cost. These materials have excellent sorption characteristics and have been used by several investigators (Swamy et al. 1997, 1998; Mall et al. 2003; Lataye et al. 2006; Srivastava et al. 2006, 2007; Lataye et al. 2008a, b, c, 2009a, b; Lakshmi et al. 2009) for the removal of organics, metals, and dyes. The aim of the present paper is to report adsorption equilibrium and thermodynamics of adsorption of 4Pi from aqueous solutions onto BFA and RHA. The effect of such process parameters as adsorbent dose (m), initial pH of the solution (pH0 ), contact time (t), initial 4Pi concentration (C 0 ), and temperature (T) on the adsorption process were studied and reported. The paper also reports the adsorption equilibrium data and tests the validity of various two-parameter (Freundlich, Langmuir, and Temkin) and three-parameter [Redlich-Peterson (R-P), Toth, and RadkePrausnitz) isotherm models. Error analysis has been done to test the fitment of the isotherm equations with the experimental equilibrium sorption data. Adsorption thermodynamic study has also been examined. We carried out the desorption experiments with various solvents to test the stability of the adsorbate-loaded adsorbents and the leaching characteristics of the adsorbate.

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Materials and Methods

Table 1. Properties of 4-Picoline S. No.

Adsorbents BFA was collected from Deoband Sugar Mills, Ltd. (Deoband, U.P., India) and RHA was obtained from Bhawani Paper Mills (Raebareli, U.P., India). Both the adsorbents were throughly washed with hot water (70°C), dried, and sieved by using standard sieves (IS 437 1979). The mass fraction of the adsorbents between 30 and 80 mesh (equivalent U.S. standard test sieves) was used for the adsorption studies. The physicochemical characteristics of the desired mass fraction of the adsorbents were determined by using standard methods. Proximate analysis of the adsorbents was carried out using standard procedure (IS 1350 1984). Bulk density was determined using a MAC bulk density meter. Scanning Electron Microscopy (SEM) micrographs were obtained from a scanning electron microscope (LEO 435 VP). The specific surface area and the pore diameter of BFA particles were determined by using the Brunaer-Emmett-Teller (BET) method from the nitrogen adsorption isotherm and with an ASAP 2010 Micromeritics instrument and the Micromeritics software. Nitrogen was used as a cold bath (77.15 K). The Barrett-Joyner-Halenda (BJH) method was used to calculate the mesopore distribution (Barrett et al. 1951). The elemental analysis (carbon, hydrogen, and nitrogen) was carried out using Elementar Vario EL III (Elementar Analysensysteme GmbH, Germany). The high heating value (HHV) of the adsorbents was determined using a standard calorimeter. The lower heating value (LHV) was calculated from the HHV and the total weight percent of hydrogen in the adsorbent (from elemental analysis) was calculated by using the equation LHV ¼ HHV  218:13 H

ð1Þ

in which LHV = lower heating value (J=g), HHV = higher heating value (J=g), and H = the total weight percent of hydrogen in the adsorbent. Adsorbate We used only analytical reagent-grade chemicals in the experiments. The 4Pi (4-methylpyridine; chemical formula = C6 H7 N; formula weight = 93.14; density = 0:95 g=mL) was procured from Acros Organics. Table 1 gives the properties of 4Pi. An accurate volume of 4Pi was taken and mixed well with doubledistilled water (DDW) to prepare a 4Pi stock solution of 1;000 mg=L concentration. The stock solution was successively diluted with DDW to obtain the desired test concentration of 4Pi. Analytical Measurements We found the aqueous solution of 4Pi to be stable over the concentration range of 50–600 mg=L used in the present study. We observed no change in the concentration as a result of perceived evaporation of 4Pi when kept in the stoppered/capped glass containers during the experiments and the analysis. The concentrations of 4Pi in aqueous solutions were determined with a Perkin Elmer Lambda 35 double-beam spectrophotometer (λmax ¼ 264 nm) by using the linear portion of the calibration curve between absorbance and 4Pi concentration (mg=L) in aqueous solution. Samples having higher concentrations of 4Pi were diluted with DDW to a concentration falling in the linear range of the calibration curve for the accurate determination of its concentration.

Property

4-picoline γ-picoline, 4-methylpyridine C6 H 7 N 93.14 Colourless liquid; disagreeable odour 0.95 145 3.6 134 water, alcohol, ether Not known TWA 4 (8 h exposure) 5 (STEL, 15 min) 2.2

14

Synonyms Chemical formula Molecular weight Physical properties Density (g=mL) Boiling point (°C) Freezing point (°C) Flash point (°C) Soluble in IDLH (mg=L) OSHA PEL (mg=L) ACGIH TLV (mg=L) NFPA hazard classification Safety profile

15

Health effects

1 4 3 4 5 6 7 8 9 10 11 14 13

Poison by intraperitorial route, moderately toxic by ingestion, skin contact, intravenous, and subcutaneous routes. Mildly toxic by inhalation, skin, and severe eye irritant Can cause CNS depression, gastrointestinal upset, and liver and kidney damage

For each experimental run, 0.05 L of 4Pi solution of known initial concentration (C 0 ), pH0 , and m was collected in a 250 mL stoppered conical flask and agitated in a temperature-controlled orbital shaker at a constant speed of 150  5 RPM. Samples were withdrawn at appropriate time intervals and centrifuged by using a research centrifuge (Remi Instruments, Mumbai), and the residual 4Pi concentration (C e ) of the centrifuged supernatant was then determined. The effect of pH0 (2 ≤ pH0 ≤ 12) on 4Pi removal was studied by adjusting the pH0 , with the addition of either 0.1 N H2 SO4 or 0.1 N NaOH solutions. The optimum m was determined by contacting 50 mL of 4Pi solution with different amounts of adsorbents (m) until the equilibrium was attained. The effect of contact time on the removal of 4Pi was studied by sampling the solution at different time intervals. The sorption equilibrium data were obtained over a C 0 range of 50–600 mg=L and contacted with a known amount of adsorbent m until equilibrium was achieved. The effect of temperature on equilibrium adsorption was studied in the temperature range of 283–323 K. Blank runs, with only the adsorbent in 50 mL of DDW, were taken simultaneously at similar experimental conditions to account for any leaching of 4Pi by BFA and RHA and the adsorption by glass containers. Similarly, we also conducted blank experimental runs with 4Pi solution (C 0 ¼ 50 mg=L) and without the adsorbent. We observed no change in the 4Pi concentration over a time period of 12 h. For desorption experiments, an accurate amount of 4Pi-loaded BFA and RHA was mixed with 0.05 L of the solvent, and the mixture was agitated in an orbital shaker for 5 h at a constant temperature of 288 K and 303 K. After 5 h, the mixture was centrifuged, the supernatant was analyzed for 4Pi concentration, and the amount desorbed was determined. The amount of 4Pi adsorbed/desorbed by the adsorbent at equilibrium was calculated as follows:

Batch Adsorption Study All the batch adsorption experiments (except those to study the effect of temperature) were carried out at a temperature of 303 K.

qe ¼

ðC 0  C e ÞV W

ð2Þ

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in which C 0 and C e = initial and equilibrium concentrations (mg=L) of 4Pi in the solution, V = volume (L), W = weight (g) of the adsorbent, and qe = amount of 4Pi adsorbed by the adsorbent at equilibrium (mg=g).

these compounds as Py 2, 3-DA; Py 2, 4-DA; and Py 2, 5-DA, respectively. Effect of Initial pH of the Solution The structure of 4-picoline (γ-picoline; Kirk and Othmer 1996; Lewis 2004) is as follows:

Results and Discussion Characterization of the Adsorbent Table 2 gives the physicochemical properties of BFA and RHA. The average particle size of BFA and RHA used in the study were found to be 381.45 μm and 412 μm, respectively, and the bulk density (kg=m3 ) was 133.3 and 175.3, respectively. Figs. 1(a) and 1(b) show the SEM micrographs of BFA and RHA. Virgin BFA is found to have a fibrous structure with a large pore size and strands in each fiber, whereas RHA has a rough surface texture with fewer pores [Figs. 1(a) and 1(b)]. Fig. 2 shows X-ray diffraction (XRD) patterns for blank and loaded BFA and RHA. The major components identified in blank BFA are silica (SiO2 ), wollastonite (CaSiO3 ), aragonite (CaCO3 ), and akdalaite ½ðAl2 O3 Þ4 · H2 O. The components identified in blank RHA are cristoballite (SiO2 ), margaritasite ½ðCs; K; H3 OÞ2 ðUO2 Þ2 V2 O8 · ðH2 OÞ, and macedonite (PbTiO3 ). Major pyridine compounds found in 4Pi-loaded BFA and RHA are pyridine 2, 3-diacarboxylic acid or quinolinic acid (C7 H5 NO4 ), pyridine 2, 4-dicarboxylic acid hydrate (C7 H5 NO4 · H2 O), pyridine 2, 5-diacarboxylic acid hydrate (C7 H5 NO4 · H2 O), and pyridine picrate (C12 H8 N4 O7 ), identified at 2θ of 14.43, 28.99, 17.26, and 21°, respectively. Fig. 2 lists Table 2. Physicochemical Characteristics of Adsorbents Characteristics Proximate analysis Moisture (%) Volatile matter (%) Ash (%) Fixed carbon (%) Bulk density (kg=m3 ) Lower heating value (MJ=kg) Higher heating value Average particle size (μm) CHNSO analysis C (%) H (%) N (%) S (%) O (%) Surface area of pores (m2 =g) (i) BET (ii) BJH (a) Adsorption cumulative (b) Desorption cumulative BJH cumulative pore volume (cm3 =g) (i) Single point total (ii) BJH adsorption (iii) BJH desorption Average pore diameter (Å) (i) BET (ii) BJH adsorption (iii) BJH desorption

BFA

RHA

4.91 14.63 28.30 52.16 133.30 28.73 48.90 381.45

1.10 7.36 80.58 10.96 175.30 21.76 21.80 412.00

56.03 0.78 0.00 0.00 43.19

13.1 0.19 0.00 0.00 86.67

244.54

65.36

62.29 36.19

52.35 26.62

0.134 0.053 0.051 22.50 33.78 56.12

0.057 0.057 0.039 34.66 43.27 58.34

The transition of 4Pi to 4PiHþ is pH dependent, with a maximum amount of 4PiHþ occurring in the pH range of 4.0–8.0. Surface charge of the adsorbents and the adsorption process are affected by the solution pH through the dissociation of functional groups (i.e., surface oxygen complexes having acidic character, such as carboxyl and phenolic groups, are of basic character, such as pyrones or chromens, on the active sites of the adsorbent; Gilchrist 1985). We studied the effect of pH0 on the removal of 4Pi by BFA and RHA at C 0 ¼ 100 mg=L and at 303 K after 5 h of contact with m ¼ 5 g=L for BFA and m ¼ 20 g=L for RHA. We observed an increase in 4Pi removal (> 93% by BFA and > 91% by RHA) at pH ≥ 4 and a decrease at pH ≥ 8 (< 91% by both adsorbents). We also observed a maximum 4Pi adsorption of ∼94% by BFA and ∼92% by RHA at the natural pH0 (pH0 ¼ 6:43) of the aqueous solution. However, as the pH0 is reduced (pH0 < 4), the adsorption decreases, with a minimum 4Pi removal at pH0 ∼ 2 for both the adsorbents. 4Pi contains a nitrogen atom, which is more electronegative than an SP2 -hybridized C. Therefore, 4Pi gets preferentially adsorbed on a positively charged surface (Carlos 2004; Lataye et al. 2006, 2008a, b, c, 2009a, b). At low pH (pH0 ≤ 4), the 4Pi is converted to 4PiHþ through protonation, resulting in the low adsorption of protonated 4Pi on the positively charged BFA/RHA surface. The BFA/RHA has a maximum affinity to 4Pi and 4PiHþ at pH0 ∼ 6:43. The adsorption rate of 4PiHþ is lower than that of 4Pi molecules. Effect of Adsorbent Dose (m) Fig. 3. shows the effect of m on the uptake of 4Pi by BFA and RHA for C 0 ¼ 100 mg=L and on the equilibrium uptake (qe ). We observed that the 4Pi removal increased with an increase in m up to ∼5 g=L for BFA and ∼20 g=L for RHA. The increase in 4Pi removal between 5 and 8 g=L of BFA and 20–30 g=L of RHA dosage is only marginal (1–2%). The 4Pi removal remains almost unaffected beyond 8 g=L of BFA and 30 g=L of RHA. An increase in the percent adsorption with the adsorbent dosage at fixed initial concentration C 0 can be attributed to the availability of more adsorption sites and greater surface area. The decrease in adsorption capacity with an increase in m can be ascribed to the saturation of adsorption sites through the adsorption reaction and the adsorbent particle–particle interaction, leading to solids aggregation. Such particle aggregation leads to a decrease in total surface area of the adsorbent and an increase in the diffusional path length of 4Pi (Lataye et al. 2009a). At m > 5 g=L of BFA and m > 20 g=L of RHA, the incremental 4Pi uptake is marginal as the 4Pi surface concentration and the 4Pi bulk solution concentration come to equilibrium with each other. Thus, for a C 0 of 100 mg=L, the optimum m may be taken as 5 g=L for BFA and 20 g=L for RHA. Further experimental runs were undertaken with these optimum adsorbent dosages of BFA and RHA.

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Fig. 1. SEM of BFA and RHA used in the study

800

1. RHA - Blank 2. BFA- Blank 3. RHA - 4Pi 4. BFA -4Pi

Macedonite

Cristoballite

Py 2, 5-D A

Py 2, 4- D A

1000

Py 2, 3-D A

Lin (counts)

1200

Margaritasite

1400

Silica

1600

600

4

400

.

3

200

2 1

0 10

20

30

40

50 60 2-Theta

70

80

90

100

Fig. 2. XRD pattern of blank and 4-picoline-loaded BFA and RHA

Fig. 4. Time course of removal of 4-picoline by adsorption on BFA and RHA (C 0 ¼ 100 mg=L; m ¼ 5 g=L for BFA and 20 g=L of RHA; T ¼ 303 K; pH0 ¼ 6:43)

Effect of Contact Time

Fig. 3. Removal of 4-picoline with varying dosages of BFA and RHA (C0 ¼ 100 mg=L; T ¼ 303 K; t ¼ 5 h; pH0 ¼ 6:43)

Fig. 4 shows the effect of contact time on the removal of 4Pi with the optimum adsorbent dosage. We observed very fast adsorption of 4Pi (∼90% for BFA and ∼72% for RHA) during the initial period of 5 min. The adsorption rate declined rapidly with time beyond 5 min of contact. The sorptive uptake of 4Pi by BFA and RHA seemed to be almost instantaneous. The residual 4Pi concentration after 1 h is 5.33% with BFA and about 13% with RHA. The initial rate of sorption is higher because of the availability of a large number of vacant surface sites. After some time, the remaining vacant surface sites are difficult to occupy because of repulsive forces between the sorbate molecules on the solid surface and in the bulk solution phase. Thus, the adsorption rate decreases during the later period of adsorption. The residual concentration of 4Pi remains almost constant after 2 h, 5 h, and 10 h in the case of both adsorbents. The difference in 4Pi removal at 2 h and 10 h is marginal (< 0:5% for BFA and < 5% by RHA); therefore, a steady-state

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approximation was assumed and a quasi-equilibrium state was assumed at t ¼ 2 h. However, the equilibrium experiments were performed for a contact time of 5 h. From Fig. 4, one can observe that the adsorption capacity of RHA is much lower than that of BFA, although the time of contact for equilibrium is almost the same. Effect of Initial Concentration (C 0 ) and Temperature Figs. 5 and 6 show the effect of C 0 (50 ≤ C 0 ≤ 600 mg=L) and temperature, T (283 ≤ T ≤ 343 K) on the equilibrium uptake of 4Pi by BFA and RHA at their optimum dosages and equilibrium

contact time t ¼ 5 h, respectively. We can see that the 4Pi removal decreases with an increase in C 0 but increases with an increase in temperature. The equilibrium uptake of 4Pi by BFA and RHA (qe ) increases with C 0 and T. The increase in qe with an increase in C 0 is attributed to an increase in the driving force for adsorption and a decrease in resistance to the uptake of adsorbate by the adsorbents from the solution. The increase in sorption capacity of 4Pi with an increase in temperature is caused by the endothermicity of the sorption process. With an increase in temperature, the mobility of 4Pi increases, resulting in higher uptake of 4Pi.

Adsorption Equilibrium We used the experimental equilibrium adsorption data to test the validity of various two-parameter and three-parameter models such as Langmuir, Freundlich, Temkin, and Redlich-Peterson for different adsorbate–adsorbent systems in the literature. Tables 3 and 4 give these equilibrium isotherm equations. These models have been used to test their fitment with the experimental data for 4Pi-BFA and 4Pi-RHA systems. The parameters for various isotherm equations were determined by using the solver add-in function of MS Office Excel for the fitting of the experimental equilibrium data. The linear coefficient of determination and a nonlinear Chi-square test have been used for the purpose. The Chi-square test is the sum of the squares of the differences between the experimental data and those calculated by using isotherm equations divided by the corresponding calculated data. The Chi-square error function is given by χ2 ¼

Fig. 5. Removal and adsorptive uptake, qe , of 4Pi by BFA at different temperatures (m ¼ 5 g=L; t ¼ 5 h; pH0 ¼ 6:43)

X ðqe;exp  qe;calc Þ2 qe;calc

ð3Þ

in which qe;exp = experimental equilibrium adsorption capacity or the equilibrium uptake, and qe;calc = adsorption capacity (mg=g) calculated by using isotherm equations. Tables 3 and 4 present the estimated values of the isotherm parameters, regression coefficients, and Chi-square error functions. Figs. 7 and 8 show the comparative fit of the predicted values from all the equilibrium isotherm equations with the experimental data. By comparing the results for the Chi-square values and the correlation coefficients for the isotherms for a particular adsorbent, we find that the 4Pi adsorption onto BFA and RHA can be best represented by the Radke-Prausnitz isotherm equation. However, the Toth and Redlich-Peterson isotherm equations also give reasonable fit and may be used.

Estimation of Thermodynamic Parameters The classical van’t Hoff equation relates the Gibbs free energy change (ΔG0 ) of the adsorption process and the adsorption equilibrium constant (K ad ; Faust and Aly 1987): ΔG0 ¼ RT ln K ad

ð4Þ

ΔG0 also relates the change in entropy, ΔS0 , and the heat of adsorption (ΔH 0 ) at a constant temperature as given in the following: ΔG0 ¼ ΔH 0  TΔS0 Fig. 6. Removal and adsorptive uptake, qe , of 4Pi by RHA at different temperatures (m ¼ 20 g=L; t ¼ 5 h; pH0 ¼ 6:43)

ð5Þ

Therefore, K ad can be obtained from Eqs. (4) and (5), as follows:

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Table 3. Isotherm Parameters for the Adsorption of 4-Picoline onto BFA at Different Temperatures (m ¼ 5 g=L; t ¼ 5 h; pH0 ¼ 6:43) Temperatures (K) Isotherms

Constants

Langmuir (1918), qe ¼ ðqm K L Ce Þ=ð1 þ K L C e Þ

1=n

Freundlich (1906), qe ¼ K F Ce

Temkin (Temkin and Pyzhev 1940), qe ¼ ðRTÞ=ðbÞ lnðK T C e Þ

Redilich-Peterson (1959), qe ¼ ðK R C e Þ=ð1 þ aR Cβe Þ

Toth (1971), 1=Th qe ¼ ðqTh C e Þ=½1=ðK Th þ C Th e Þ

Radke-Prausnitz (1972), qe ¼ ðK RP k rp Ce Þ=ð1 þ K RP C Pe Þ kRP

ln K ad ¼

K L (L=mg) qm (mg=g) R2 (linear) R2 (nonlinear) χ2 K F (L=mg) 1=n n R2 (linear) R2 (nonlinear) χ2 B K T (L=mg) R2 (linear) R2 (nonlinear) χ2 aR (L=mg) K R (L=mg) β 2 R (linear) R2 (nonlinear) χ2 Th qeinf (mg=g) K Th ðmg=LÞTh R2 (linear) R2 (nonlinear) χ2 P K RP (L=g) ½ðmg=gÞ=ðmg=LÞ1=P  R2 (linear) R2 (nonlinear) χ2

ΔG0 ΔS0 ΔH 0 1 ¼  RT R R T

ð6Þ

in which ΔG0 = free energy change (kJ=mol), ΔH 0 = change in enthalpy (kJ=mol), ΔS0 = entropy change (kJ=mol=K), T = absolute temperature (K), and R = universal gas constant (8:314 J=mol=K). ΔH 0 can be determined from the slope of the semilogarithmic plot, ln K ad , versus (1=TÞ as   d ln K ad 0 ð7Þ ΔH ¼ R dð1=TÞ ΔH 0 corresponds to the isosteric heat of adsorption (ΔH st;0 ) with zero surface coverage (i.e., qe ¼ 0; Suzuki 1990). We obtained K ad at qe ¼ 0 from the intercept of the lnðqe =C e Þ versus qe plot. Fig. 9 shows the van’t Hoff plot from which we calculated the values of ΔH 0 , ΔS0 , and ΔG0 for BFA and RHA (Table 5). ΔH 0 and ΔS0 have positive values, whereas ΔG0 has negative values (Table 5). Thus, the overall sorption process is endothermic in nature. The positive value of ΔS0 suggests increased randomness

283

293

303

313

323

0.05 52.08 1.00 1.00 0.76 7.74 0.34 2.91 0.94 0.97 4.27 8.87 1.06 0.99 1.00 0.61 0.10 3.71 0.94 1.00 1.00 0.03 0.57 61.52 0.28 1.00 1.00 0.14 0.83 0.57 18.59 1.00 1.00 2.07

0.05 54.35 1.00 1.00 1.87 8.43 0.34 2.96 0.94 0.97 3.86 8.99 1.13 1.00 1.00 0.13 0.16 4.95 0.90 1.00 1.00 0.01 0.59 62.72 0.29 1.00 1.00 0.04 0.85 0.37 22.08 1.00 1.00 0.62

0.06 57.14 1.00 1.00 3.40 9.21 0.33 2.99 0.94 0.97 4.02 9.29 1.33 1.00 1.00 0.17 0.21 6.17 0.89 1.00 1.00 0.25 0.60 65.28 0.29 1.00 1.00 0.36 0.88 0.24 27.93 1.00 1.00 0.22

0.07 59.17 1.00 1.00 5.86 10.47 0.32 3.12 0.94 0.97 3.90 9.16 1.86 1.00 1.00 0.13 0.21 7.94 0.93 1.00 1.00 0.53 0.60 66.31 0.34 1.00 1.00 0.56 0.90 0.23 32.47 1.00 1.00 0.60

0.07 60.61 1.00 1.00 8.63 11.21 0.31 3.18 0.95 0.98 3.55 9.16 2.25 1.00 1.00 0.15 0.36 11.04 0.88 1.00 1.00 0.36 0.63 66.43 0.32 1.00 1.00 1.15 0.91 0.21 36.10 1.00 1.00 1.74

at the solid/solution interface, with some structural changes in the adsorbate, adsorbent, and 4Pi affinity to the BFA and RHA surface. The values of ΔG0 indicate the feasibility and spontaneity of the adsorption process.

Comparative Assessment of BFA, RHA, and Activated Carbons for the Removal of 4Pi The optimum adsorbent dosages for BFA and RHA for 4Pi removal were found to be 5 g=L and 20 g=L, respectively, for the initial 4Pi concentration of 100 mg=L. The same dosages were used for the initial 4Pi concentration in the range of 50–600 mg=L. The qe versus C e data of BFA and RHA indicate that the qe can be as high as 55.33 and 16:54 mg=g, respectively, at a normal temperature (30°C). Mohan et al. (2005a, b) used activated carbons (ACs) derived from coconut fibers and shells, with and without acid treatment, with a low dosage (1–3 g=L) in aqueous solutions and a low 4-picoline (γ-picoline) initial concentration range (1–100 mg=L). The ratio C 0 =m in their experiments varied from 1 to 33. In the

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Table 4. Isotherm Parameters for the Adsorption of 4-Picoline onto RHA at Different Temperatures (m ¼ 20 g=L, t ¼ 5 h, pH0 ¼ 6:43) Temperatures (K) Isotherms

Constants

Langmuir (1918), qe ¼ ðqm K L Ce Þ=ð1 þ K L C e Þ

1=n

Freundlich (1906), qe ¼ K F Ce

Temkin (Temkin and Pyzhev 1940), qe ¼ ðRT=bÞ lnðK T Ce Þ

Redilich-Peterson (1959), qe ¼ ðK R C e Þ=ð1 þ aR Cβe Þ

Toth (1971), 1=Th qe ¼ ðqTh C e Þ=½1=ðK Th þ C Th e Þ

Radke-Prausnitz (1972), qe ¼ ðK RP k rp Ce Þ=ð1 þ K RP C Pe Þ kRP

K L (L=mg) qm (mg=g) R2 (linear) R2 (nonlinear) χ2 K F (L=mg) 1=n n R2 (linear) R2 (nonlinear) χ2 B K T (L=mg) R2 (linear) R2 (nonlinear) χ2 aR (L=mg) K R (L=mg) β 2 R (linear) R2 (nonlinear) χ2 Th qeinf (mg=g) K Th ðmg=LÞTh R2 (linear) R2 (nonlinear) χ2 P K RP (L=g) ½ðmg=gÞ=ðmg=LÞ1=P  R2 (linear) R2 (nonlinear) χ2

present experiments, the ratio C 0 =m varied between 10 and 120 for BFA and between 2.5 and 30 for RHA. In their study, the maximum 4Pi adsorption capacity of ACs was found to be about 55 mg=g at a concentration of 50 mg=L at normal temperature (25°C). The maximum adsorption capacity in the present study is 55:33 mg=g for BFA and 16:54 mg=g for RHA. We found the adsorption of 4Pi by BFA and RHA to be almost spontaneous, whereas it is sluggish for ACs. The equilibrium time reported by these writers is 48 h, whereas in the present study, 2 h is sufficient for attaining equilibrium. Table 6 shows the comparative assessment of adsorbents used by Mohan et al. (2005a, b) and the low-cost adsorbents used in the present study for the removal of 4Pi from aqueous solutions. Since the leaching aspects of 4Pi from spent ACs have not been reported by these writers, the regeneration and reuse of ACs for further 4Pi sorption cannot be ascertained. Because ACs are quite costly, their regeneration and reuse will be most essential. The BFA and RHA are easily available as waste materials at almost no cost except for transportation and storage. The comparison of data for only these three parameters for all the adsorbents and the consideration of the

283

293

303

313

323

0.03 17.33 0.99 1.00 0.58 1.55 0.43 2.32 0.99 0.99 0.55 3.16 0.47 0.99 1.00 0.29 0.15 0.91 0.82 1.00 1.00 0.02 0.45 27.66 0.28 1.00 1.00 0.03 0.74 0.37 3.80 1.00 1.00 0.05

0.03 17.57 0.99 1.00 0.64 1.64 0.43 2.34 0.98 0.99 0.56 3.17 0.51 0.99 1.00 0.32 0.18 1.03 0.81 1.00 1.00 0.02 0.46 27.25 0.28 1.00 1.00 0.03 0.75 0.34 4.14 1.00 1.00 0.04

0.05 17.79 0.99 1.00 2.48 1.79 0.41 2.41 0.98 0.99 0.45 3.12 0.61 0.99 0.99 0.61 0.36 1.55 0.75 1.00 1.00 0.02 0.46 26.66 0.30 1.00 1.00 0.04 0.76 0.33 4.52 1.00 1.00 0.02

0.04 17.89 0.99 1.00 1.03 1.91 0.41 2.46 0.98 0.99 0.48 3.10 0.70 0.99 0.99 0.66 0.44 1.85 0.74 1.00 1.00 0.02 0.46 26.23 0.32 1.00 1.00 0.03 0.78 0.30 5.13 1.00 1.00 0.02

0.04 18.15 0.99 1.00 1.27 2.09 0.40 2.51 0.98 0.99 0.53 3.08 0.84 0.99 0.99 0.57 0.44 2.09 0.76 1.00 1.00 0.01 0.48 25.34 0.33 1.00 1.00 0.05 0.79 0.32 5.53 1.00 1.00 0.02

relative costs of these adsorbents and their regeneration shows the effectiveness of BFA and RHA for 4Pi removal from aqueous solution. On the basis of these experimental data and the fact that the RHA and BFA are available at low prices, we conclude that these materials can be used as effective adsorbents for 4Pi removal from aqueous solution. Although BFA seemed to be a better adsorbent than RHA, either one can be used for 4Pi removal based on its availability.

Desorption of 4-picoline We also studied the stability of 4Pi adsorbed onto BFA and RHA, the leaching of the 4Pi and, therefore, the possible recovery of the adsorbate, and the regeneration of the spent BFA and RHA by conducting desorption experiments at 288 K and 303 K. Various solvents, such as distilled water–soil slurry of varying pH (4 ≤ pH ≤ 11), 0.1 N H2 SO4 , 0.1 N HNO3 , 0.1 N NaOH, and alcohol solutions were used in the leaching experiments. An accurate amount of 4Pi-loaded BFA (qe ¼ 18:93 mg=g) and RHA

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12

10

ln (K ads)

8

6

4 BFA RHA

2

0 0.003

0.0031

0.0032

0.0033 -1 1/T (K )

0.0034

0.0035

0.0036

Fig. 9. Van’t Hoff plot for the determination of ΔH 0 , ΔS0 , and ΔG0 Fig. 7. Comparison of the fit of various equilibrium isotherm equations with the experimental sorption data for 4Pi onto BFA (T ¼ 303 K; m ¼ 5 g=L; pH0 ¼ 6:43; t ¼ 5 h)

to show a better performance in eluting 4Pi from the BFA and RHA. The distilled water-soil slurry showed a poor desorption efficiency (∼4–6% from BFA and 2–4% from RHA). Because a substantial amount of 4Pi could not be removed even with acids, and because BFA and RHA are available at low prices, desorption and regeneration of the spent adsorbents are not recommended.

Disposal of Spent Adsorbents

Fig. 8. Comparison of the fit of the various equilibrium isotherm equations with the experimental sorption data for 4Pi onto RHA (T ¼ 303 K; m ¼ 20 g=L; pH0 ¼ 6:43; t ¼ 5 h)

(qe ¼ 4:57 mg=g) were mixed separately with 0.05 L of the solvent, and the mixture was agitated in an orbital shaker for 5 h. Then the mixture was centrifuged, the supernatant was analyzed for the concentration of 4Pi, and the amount of desorbed 4Pi was determined. Fig. 10 shows the results. We found acidic solutions

The disposal of the spent adsorbents is a serious problem. The lowcost and easily available agri-wastes like BFA and RHA have considerably high heating values. The proximate analysis of 4Pi-loaded BFA shows a 4.92% moisture content, 14.02% volatile matter, 26.71% ash content, and 54.35% fixed carbon. The proximate analysis of 4Pi-loaded RHA showed 2.69% moisture content, 6.34% volatile matter, 77.85% ash content, and 13.12% fixed carbon. The blank/4Pi-loaded BFA showed higher carbon content than the blank/4Pi-loaded RHA. The thermal degradation (not shown here) of the blank and 4Pi-loaded adsorbents (BFA and RHA) also shows lower percent loss for RHA than BFA. This means that the organic matter content of RHA is lower than that of BFA. Carbon content of spent BFA and RHA also increases because of loading of the organic matter. The heating values of 4Pi-loaded BFA and RHA are 29.24 and 22:38 MJ=kg, respectively. The heating value of loaded RHA is lower than that of BFA because of its higher silica content and lower carbon content. Because of high energy content, 4Pi-loaded BFA and RHA can be used as cofuels in various industries. The spent BFA and RHA can be simply separated from the aqueous mixture by filtration, dewatered and sun-dried, and then fired as a cofuel into the boiler furnace/incinerator. The dried adsorbent can also be densified in the form of briquettes and could be used directly for firing or could be pyrolyzed to convert them into smokeless fuel and then used. The boiler furnace/incinerator ash can be mixed with the composted manure and used as a fertilizer in agricultural fields.

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Table 5. Thermodynamic Parameters for the Adsorption of 4Pi onto BFA and RHA ΔG0 (kJ=mol) Adsorbents

ΔH 0 ðkJ=molÞ

ΔS0 ðkJ=mol=KÞ

283 K

293 K

303 K

313 K

323 K

19.319 11.608

0.142 0.098

20:792 16:207

22:211 17:189

23:629 18:172

25:046 19:155

26:464 20:139

BFA RHA

Table 6. Comparative Assessment of Sorption of 4Pi by BFA, RHA, and ACs

Temperature (K)

Initial concentration C0 , mg=L

C0 =m (mg=g)

Equilibrium time (h)

SAC

298

1–100

1–33.33

48

ATSAC

298

1–100

1–33.33

FAC

298

1–100

ATFAC

298

BFA RHA

303 303

Adsorbent

Maximum adsorptive uptake, qmax (mg=g)

Isotherm parameters Freundlich

Langmuir

KF

n

32

0.906

0.983

8744

48

42

1.88

0.813

2899

1–33.33

48

38

6.01

1.64

751

62.5

1–100

1–33.33

48

55

3.05

1.15

202

172.41

50–600 50–600

10–120 2.5–30

2 2

55.33 16.54

9.21 1.79

2.99 2.41

KL

qm 90.9 400

0.06 0.05

57.14 17.79

References Mohan et al. 2005a Mohan et al. 2005a Mohan et al. 2005b Mohan et al. 2005b Present study Present study

Note: SAC is activated carbon derived from coconut shells without any treatment; ATSAC is activated carbon derived from acid-treated coconut shells; FAC is activated carbon derived from coconut shell fibers, and ATFAC is activated carbon derived from acid-treated coconut shell fibers.

Fig. 10. Desorption of 4-picoline in different (qe ¼ 18:93 mg=g for BFA and 4:57 mg=g for RHA)

solvents

of 4Pi increases with an increase in initial concentration: 9:5 mg=g at C 0 ¼ 50 mg=L to 55 mg=g at C 0 ¼ 600 mg=L for BFA and 2:4 mg=g at C 0 ¼ 50 mg=L to 16:5 mg=g at C 0 ¼ 600 mg=L for RHA. Adsorption is found to be very fast, and about 72–90% removal of 4Pi is achieved in the initial 5 min of contact between BFA (m ¼ 5 g=L) and RHA (m ¼ 20 g=L) and the 4Pi solution (C 0 ¼ 100 mg=L) at 303 K. The equilibrium sorption isotherm data could be well-represented by the three-parameter equations (i.e., Redlich-Peterson, Toth, and Radke-Prausnitz). RHA is comparatively less efficient than BFA in 4Pi removal. This may be the result of lower carbon content, smaller surface area, and lower pore volume for RHA. The thermodynamic study reveals that the removal of 4Pi from aqueous solution is an endothermic process. The values of the thermodynamic parameters, ΔH 0 , ΔS0 , and ΔG0 are 19:32 ðkJ=molÞ, 0:142 ðkJ=mol=KÞ, and 20:8 to 26:5 ðkJ=molÞ, respectively, for BFA and 11:61 ðkJ=molÞ, 0:1 ðkJ=mol=KÞ, and 16:2 to 20:14 ðkJ=molÞ, respectively, for RHA. The desorption study revealed that the acidic solvents can elute ∼90% of 4Pi from the 4Pi-loaded BFA and RHA at 288 and 303 K temperatures. As BFA and RHA have good heating values and are available in large amounts at low prices, we recommend that the spent BFA and RHA be used as cofuels in furnaces/ incinerators to recover their energy value. Thus, BFA and RHA can be used as adsorbents for effective removal of 4Pi from wastewater, and the spent adsorbents can be fired in furnaces/ incinerators to recover their fuel value.

Summary and Conclusions Acknowledgments The present study shows that BFA and RHA are effective adsorbents for the removal of 4Pi from aqueous solutions as compared to ACs. The maximum removal of 4Pi was observed to be ∼96% by BFA and ∼95% by RHA in the lower concentration range (< 50 mg=L) and ∼46% by BFA and ∼55% by RHA in the higher concentration range (50–600 mg=L) by using 5 g=L of BFA and 20 g=L of RHA dosage at normal temperature. The sorptive uptake

The financial assistance provided by the Ministry of Human Resources Development, Government of India, to Dr. Dilip H. Lataye is gratefully acknowledged. Dr. Lataye also thanks the Visvesvaraya National Institute of Technology, Nagpur, India, for allowing him to undertake Ph.D. work under the Quality Improvement Program (QIP).

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Notation The following symbols are used in this paper: ACGIH = American Conference of Governmental Industrial Hygienists; aR = R-P isotherm constant (L=mg); B1 = constant in Temkin equation; b = Temkin isotherm energy constant (J=mol); C 0 = initial concentration of 4Pi (mg=L); C e = equilibrium concentration of 4Pi (mg=L); CNS = central nervous system; IDLH = immediately dangerous to life or health; K F = Freundlich constant (L=g); K L = Langmuir constant (L=g); K R = Redlich-Peterson constant (L=g); K T = constant in Temkin equation (L=g); m = adsorbent dose (g); NFPA = National Fire Protection Association; OSHA = Occupational Safety and Health Administration; PEL = permissible exposure limit; qe = amount of 4Pi adsorbed at equilibrium (mg=g); qm = monolayer adsorption capacity (mg=g); R = universal gas constant (J=mol=K); STEL = short term exposure limit; T = temperature (K); TWA = time-weighted average; TLV = threshold limit value; V = volume (L); W = weight of adsorbent (g); β = exponent in R-P equation (lies between 0–1); ΔG0 = free energy change (Kj=mol); ΔH 0 = change in enthalpy (kJ=mol); ΔS0 = entropy change (kJ=mol=K); and λmax = wavelength (nm).

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