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from activated carbon
Chemical Engineering Journal 168 (2011) 35–43
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Studies on adsorption/desorption of nitrobenzene and humic acid onto/from activated carbon Megha Syam Rauthula, Vimal Chandra Srivastava ∗ Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e
i n f o
Article history: Received 2 September 2010 Received in revised form 8 December 2010 Accepted 9 December 2010 Keywords: Activated carbon Nitrobenzene Humic acid Desorption Kinetics
a b s t r a c t This paper reports results of the studies on adsorption of nitrobenzene (NB) and humic acid (HA) from aqueous solution onto activated carbon commercial (ACC) grade. Characterization of ACC showed its meso-porous nature. Fourier transform infra-red (FTIR) spectra of the ACC indicated presence of various types of functional groups on its surface. Thermo-gravimetric analysis exhibited the thermal stability of the ACC up to 300 ◦ C. The adsorption kinetics of NB and HA onto ACC could be represented by pseudosecond-order kinetic model. The adsorption processes could be well described by a two-stage diffusion model. Thermal regeneration showed that ACC could be used for five desorption–adsorption cycles with good efficiency for NB and HA in each cycle. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nitrobenzene (NB) and its derivatives are considered to be very toxic. The evidence of carcinogenicity of NB in experimental animals has been established by the IARC [1]. NB is soluble in water (1.9 g/l) and gets biodegraded completely only after 7–10 days. This makes it incompatible for treatment in simple wastewater treatment plants [2]. The European Union Directive 2001/59/EC states that “NB is toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment”. Humic substances are the major organic constituents of river, lake and pond waters. They derive from soils where they get formed through the breakdown of biological tissues by chemical and biological processes [3]. Humic substances including humic acid (HA) are an important disinfection by-products (DBPs) precursor such as chloroform and bromo-dichloromethane which are suspected carcinogenic compounds [4]. It has been shown that humic substances often reduce the efficiency of removal of the target substances due to their adsorption onto adsorbents and/or due to formation of complexes with the target substances [5]. Elevated levels of humic substances in water produce color making the water aesthetically unpleasing to the consumer [6]. Various techniques such as activated sludge process, trickling filter, biofiltration, ozone degradation, and electro-coagulation are used for the removal of NB and HA from waste water [7]. Majumder and Gupta [8] used hybrid reactor, comprising of trickling filter and activated sludge process, in treating NB wastewater. Eggins
∗ Corresponding author. Tel.: +91 1332 285889; fax: +91 1332 276535/273560. E-mail address: [email protected] (V.C. Srivastava). 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.12.026
et al. [9] demonstrated use of photo-catalysis for the removal of HA from potable water using titanium dioxide. It took approximately 12 min to reduce the HA concentration by half; however 50% complete mineralization took 60 min. The ability of TiO2 to remove humic substances has been confirmed by other workers also [10,11]. Hiraide et al. [12] separated HA from fresh water by coprecipitation and flotation in 1 h. Reungoat et al. [13] investigated adsorption of NB onto zeolites followed by regeneration of zeolites with ozone. NB was completely removed from water and the initial adsorption capacity of zeolite was totally restored after desorption with ozone. Few investigators have theoretically modeled the phenomena of HA adsorption/desorption at the solid/water interface [14]. The presence of humic substances in water resources is of major concern in the water supply. Humic substances react with chlorine during water treatment to produce trihalomethanes which are known human carcinogens [15]. Among the various treatment processes, adsorption onto activated carbon is one of the most important processes. Activated carbon is primarily used for the treatment of potable water (24% of all use); wastewater (21%) and groundwater remediation (4%) [16]. It has large surface area (100–2000 m2 /g) and pore volume that gives it high adsorption capacity [17]. The aims and objectives of the present work are: (i) to characterize activated carbon commercial grade (ACC) for its physico-chemical and adsorption properties before and after adsorption of NB and HA, (ii) to study the effect of various parameters for the removal of NB and HA from the aqueous solution in batch study, (iii) to perform kinetic study of NB and HA adsorption onto ACC and to analyze the experimental data using various kinetic models and (iv) to perform desorption study for the possible regeneration of ACC.
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2. Materials and methods 2.1. Adsorbent and its characterization Coconut-based ACC supplied by ZeoTech Adsorbents Pvt. Ltd., New Delhi, India. Detailed physico-chemical characteristics of the ACC have already been presented elsewhere [17]. Bulk density of ACC was determined using bulk density meter supplied by Macro Scientific Works, Delhi. Bulk density meter consisted of two cylinders screwed on holding brackets by Knurled rings. First, standard powder (of known bulk density) and ACC of known weights were filled in the cylinders up to 50 ml mark. Knurled rings were tightened from the top to keep the cylinders firmly on the brackets. Both cylinders were then subjected to 100 strokes vibrations in 3 min. Thereafter, the volumes of both cylinders were noted and bulk density calculated. Particle size analysis of ACC was done as per Indian standard (IS) 2720 (Pt 4): 1985 using IS sieves of sizes 90 m, 180 m, 250 m, 500 m, 1000 m, 1700 m, 3350 m and 4000 m. Phillips (Holland) diffraction unit (Model PW1140/90) was used for finding X-ray diffraction (XRD) data of ACC. Copper was used as the target with nickel as the filter media. K radiation was maintained constant at 1.542 A˚ whereas goniometric speed was maintained at 2◦ /min. LEO 435 VP scanning electron microscope was used for taking scanning electron micrographs (SEM) of ACC. Thermo Nicolet (Model Magna 760) Fourier transform infrared (FTIR) spectrometer was employed to determine the presence of surface functional groups in ACC, before and after the adsorption of NB and HA, at room temperature over a spectral wave number range of 4000–400 cm−1 . The point of zero charge (pHPZC ) of the ACC was determined by the solid addition method [18]. The thermal decomposition of ACC was carried out non-isothermally in a pyris diamond PerkinElmer instrument. The degradation runs were taken at heating rate of 100 K/min with air flow rate kept constant at 200 ml/min. 2.2. Adsorbates NB supplied by HiMedia Laboratories Pvt. Ltd., Mumbai, India; and HA supplied by S.D. Fine Chemicals Pvt. Ltd., Mumbai, India were used as adsorbates. Stock solutions of 1000 mg/l were prepared by dissolving accurate quantity of NB and HA in distilled water. Solutions of required concentration were prepared by diluting the previously prepared stock solutions with distilled water whenever required. 2.3. Analytical measurement The concentration of NB and HA was determined by finding out the characteristic maximum absorbance wavelength using UV/vis spectrophotometer (PerkinElmer lambda 35, Schimadzu, Japan). A standard solution of known concentration was taken and the absorbance was determined at different wavelengths to obtain a plot of absorbance versus wavelength. The wavelength corresponding to maximum absorbance (max ) was determined. The max for NB and HA were found to be 268 and 337 nm. Calibration curve was plotted between the absorbance and the concentration of NB and HA solution. Linear portion of this curve was used for determining the unknown concentration of NB and HA solution. 2.4. Batch study Batch experiments were conducted to study the effect of various parameters like adsorbent dose (m), initial pH (pH0 ), contact time (t) and temperature (T) on the adsorptive removal of NB and HA by ACC. For performing an experimental run, 250 ml stoppered conical
flask containing 100 ml of NB or HA solution with known C0 , pH0 and an optimum m were agitated at 303 ± 1 K in a temperaturecontrolled orbital shaker at a constant speed of 150 rpm. Samples were withdrawn from the shaker after appropriate time. These samples were centrifuged (Research Centrifuge, Remi Scientific Works, Mumbai) at 10,000 rpm for 5 min and analyzed for the residual NB and HA concentrations. Blank experimental runs were conducted simultaneously at similar conditions with only adsorbent in 100 ml of distilled water. Also, stability of the adsorbates was checked by agitating blank aqueous solution of adsorbates without any adsorbent. No change in concentration of adsorbates was observed during the experimental runs and the adsorbates were found to be stable. Effect of pH0 on both NB and HA removal was studied over a pH0 range of 2–12. pH0 was adjusted by addition of 0.1 N H2 SO4 or NaOH. Optimum m, was found by contacting different amounts of ACC with 100 mg/l NB and HA solutions till equilibrium was attained. The percentage removal of adsorbate and the equilibrium adsorption uptake in solid phase, qe (mg/g), were calculated as follows: Percent removal = qe =
100(C0 − Ce ) C0
(C0 − Ce )V w
(1) (2)
where C0 is the initial concentration (mg/l), Ce is the equilibrium concentration (mg/l), V is the volume of the solution (l) and w is the mass of the adsorbent (g). 2.5. Adsorption kinetic theory 2.5.1. Pseudo-first-order- and pseudo-second-order-model Pseudo-first-order equation is given as [19]: qt = qe [1 − exp(−kf t)]
(3)
where qt is the amount of adsorbate adsorbed at time t (mg/g), qe is the amount of the adsorbate adsorbed on the adsorbent at time under equilibrium condition, kf = (ka Cs + kd ) is the pseudo-first order rate constant and Cs is the adsorbent concentration in the solution. The pseudo-second-order model is represented as [20]: qt =
tks q2e 1 + tks qe
(4)
The initial sorption rate, h (mg/g min), at t → 0 is defined as h = ks q2e . Where, ks is the pseudo-second order rate-constant. 2.5.2. Intra-particle diffusion study Intra-particle diffusion model is given as [21]: qt = kid t 1/2 + I
(5)
where kid is the intra-particle diffusion rate constant and values of I give an idea about thickness of the boundary layer. 2.6. Desorption studies Desorption study was performed using solvent and thermal methods. In the solvent desorption method, the NB-loaded ACC (0.2 g) was agitated in a series of 250 ml conical flasks containing 50 ml of aqueous solution of HCl, H2 SO4 , HNO3 , distilled water, CH3 COOH, C2 H5 OH, acetone and NaOH of known concentration at 303 K at 150 rpm for 24 h in the orbital shaker [22]. The same procedure was followed for HA-ACC system. In the thermal desorption method, the NB or HA loaded ACC was separated from the solution after adsorption study. It was dried in oven at 373 K for 2 h
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Fig. 1. X-ray diffraction of blank ACC.
followed by heating in a furnace at 623 K for 3 h. The sample was then taken out of the furnace and kept in a desiccator. This sample was again used for adsorption. This adsorption–desorption procedure was repeated till adsorption capacity got reduced to half of its adsorption capacity as compared to that in the first adsorption study. 3. Results and discussion
aldehydes and ketones. This band may also be due to conjugated hydrocarbon bonded carbonyl groups. The FTIR spectra also show transmittance around ∼1085 cm−1 region due to the vibration of the CC group in lactones and due to SiOSi and –COH stretching and –OH deformation. Peak at ∼790 cm−1 indicates the presence of SiH. Fig. 3 also shows the FTIR spectra of NB- and HA-loaded ACC. Many peaks get shifted in NB- and HA-loaded ACC. This shift signifies the participation of these functional groups in the adsorption process.
3.1. Characterization of adsorbent
3.2. Effect of initial pH (pH0 )
The average particle size of ACC was 1.67 mm. Bulk density and heating value of ACC were determined as 599.32 kg/m3 and 18.81 MJ/kg, respectively. XRD pattern for ACC is shown in Fig. 1. It shows the presence of moganite (SiO2 ), akdalaite [(Al2 O3 )4 ·H2 O], tamarugite [NaAl(SO4 )2 ·6H2 O], fersilicate (FeSi) and majorite [Mg3 (Fe,Al,Si)2 (SiO4 )3 ] as major components in ACC [17]. The broad peak in the XRD indicates the presence of amorphous form of silica. Diffraction peaks corresponding to crystalline carbon was not seen in ACC [23]. The BET surface area of ACC was found to be 171.05 m2 /g, whereas the BET average pore diameter ˚ Thus, ACC exhibits meso-porous of ACC was found to be 31.03 A. ˚ [24] which is desirnature (20 A˚ < average pore diameter < 500 A) able for the liquid phase adsorption of organic compounds. Fig. 2 shows the SEM of the ACC before and after adsorption at various magnifications. SEM at lower magnification (50×) does not show any difference in the texture of the blank and NB- and HA-loaded ACC. It only shows that the ACC is crystalline in nature and with varying particle size. However, at higher magnification (10,000×), some difference in the surface texture of the blank and NB- and HA-loaded ACC can be envisaged. Blank ACC shows porous structure with pores of varying sizes. Numbers of pores observed in the loaded ACC are less as compared to that in blank ACC. Chemical structure of the adsorbent surface affects the adsorption nature. The functional groups often found on activated carbons are carboxyl, phenolic hydroxyl, carbonyl (e.g. quinone-type) and lactone (e.g. fluorescein-type) groups [25]. The FTIR spectrum (Fig. 3) of ACC shows a broad band between 3100 and 3700 cm−1 which is indicative of the presence of both free and hydrogen bonded OH groups on the ACC surface. This stretching is due to both the silanol groups (Si–OH) and adsorbed water on the surface. Broad peak in the region of 1500–1700 cm−1 with peak at 1580 cm−1 indicates the presence of CO group stretching from
There are mainly two types of interactions, electrostatic and dispersive, during adsorption of aromatic compounds onto activated carbons [26,27]. The hydrogen ion and hydroxyl ions are quite strongly adsorbed, and therefore, the adsorption of other ions is affected by the pH of the solution. Acidic electrolytes are going to be dissociated into negative ions at pH > pKa . A similar phenomenon occurs with the carbon surface which has an amphoteric character. If pH > pHZPC , then the carbon surface is negatively charged, the opposite occurs when the pH < pHZPC [28]. Adsorption of cations is favored at pH > pHZPC , while the adsorption of anions is favored at pH < pHZPC . At low pH (e.g., pH < pHZPC ), most carbons are positively charged, at least in part as a consequence of donor/acceptor interactions between the graphene layers and the hydronium ions [29]. pHZPC for ACC was found to be 8.5 [17]. In this study, the performance of HA adsorption by ACC was examined at pH values ranging from 2 to 12. From the results shown in Fig. 4, the capacity for HA adsorption is found to generally increase with decreasing pH. HA adsorption onto ACC was highest at pH ∼2 and there was hardly any adsorption of HA at pH ∼12. HA is a subclass of humic substances that is soluble in water at pH > 2. This macromolecule has a complex structure containing phenolic and carboxylic groups, thus, carrying negative charges in natural waters for all pH > 2 [30,31]. HA molecules generally gain an excess negative surface charge as the pH is increased due to the deprotonation, first of the carboxylic groups at pH values of 4–6, followed by the dissociation of phenolic groups at higher pH [32,33]. At pH0 ∼2, the zeta potentials of ACC and HA are of opposite sign to each other. Hence, the interactions between the granules and the HA were electro-statically attractive, which led to significant adsorption of the HA onto the ACC. The large decrease of adsorption between pH0 2 and 3 may be attributed to an increase in the size of the HA molecules at higher pH [34]. Larger molecular sizes
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Fig. 2. SEM of blank and NB- and HA-loaded ACC.
cause lesser molecules to be adsorbed onto the ACC surface. Also as the pH0 increases, the ACC becomes less positively charged, and thus, electrostatic attraction between the HA and ACC decreases leading to lesser adsorption at higher pH. Since both the ACC and the HA are negatively charged at pH0 > 8.5, the electrostatic repulsion between the HA molecules to be adsorbed and the surfaces of the ACC most probably prevented HA from approaching close enough to the surface of the granules for adsorption to take place. The functional group linked to the adsorptive aromatic ring can activate or deactivate it, delocalizing its electronic charge [35]. Activating groups are electron-deactivating groups, creating a partial positive charge in the ring (nitrobenzene), while withdrawing groups produce the opposite effect, creating a partial negative charge (phenol, aniline). Because the aromatic ring has a much
larger size than the functional group, the interaction of the aromatic ring with the basal planes of the carbon is more effective [36]. Substitution of a nitro group on a benzene ring results in withdrawal of o-electron density from the ring, especially from the ortho and para positions (see Scheme 1). Substituents that are more powerful electron attractors than hydrogen are said to exhibit a negative inductive effect and vice versa. Groups that withdraw electron density from conjugated systems by resonance effects are designated as −R in character [37]. Thus, NB is a very weak Lewis acid that possesses the electron-withdrawing NO2 group. The effect of pH0 on NB adsorption was studied with blank solutions of C0 = 100 mg/l having natural pH0 = 5.6. The results in Fig. 4 show that the NB removal is maximum at natural pH0 . However, the adsorption of NB is appreciable in the whole pH range. NB is
Fig. 3. FTIR of blank and NB- and HA-loaded ACC.
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Scheme 1. Resonance structure of nitrobenzene.
in the molecular form in the whole pH range. Because of this, the dispersive interactions will be the most important in the adsorption process. It seems that adsorption of NB takes place primarily via diffusion into the meso-pores of the ACC. At pH < pHZPC , adsorption of NB to ACC may be through negative charge on nitro group whereas for pH > pHZPC adsorption may be via the partial positively charged aromatic ring. Since 5.6 is the natural pH of the solution, therefore, further experiments with NB were conducted without adjusting the pH. 3.3. Effect of adsorbent dosage (m) The effect of m on the removal of NB and HA by ACC at C0 = 100 mg/l is shown in Fig. 5. NB and HA removal increases up to m < 10 g/l, and then it remains almost constant for m ≥ 10 g/l. The adsorbent surface becomes saturated with adsorbates for m < 10 g/l, and the residual concentration in the solution is large. An increase in adsorption with an increase in m for m < 10 g/l can be attributed to greater surface area and availability of more adsorption sites. Further, in all cases, equilibrium was found to be attained more rapidly at lower m. However, for m ≥ 10 g/l, the surface NB and HA concentrations and the solution NB and HA concentrations come to saturation limit with each other and the incremental removal becomes very low. Thus, optimum m was found to be 10 g/l for both NB and HA. 3.4. Effect of contact time Fig. 6 shows the effect of t on the removal of NB and HA by the ACC at C0 = 50–1000 mg/l for m = 10 g/l. During first 1 h, adsorp-
Fig. 4. Effect of pH0 on the adsorption of NB and HA by ACC. T = 303 K, t = 24.0 h, C0 = 100 mg/l, and ACC dose = 10 g/l.
tion of NB and HA was found to be quick for all C0 and thereafter, the adsorption rate decreased gradually. This is due to the fact that the adsorbates get adsorbed into the meso-pores during initial stages, thereafter, adsorbates have to travel deeper into the pores encountering larger resistance. This results in the slowing down of the adsorption during the later period of adsorption. For C0 ≤ 100 mg/l, the residual concentrations at 5 h contact time were found to be higher by a maximum of ∼1% than those obtained after 12 h contact time. Therefore, after 5 h contact time, a steady state approximation was assumed and a quasi-equilibrium situation was accepted for C0 ≤ 100 mg/l. For 100 mg/l < C0 ≤ 500 mg/l, a quasiequilibrium situation may be assumed after 12 h. Accordingly all the batch experiments were conducted with a contact time of 12 h under vigorous shaking conditions for all C0 ≤ 500 mg/l.
3.5. Adsorption kinetic study 3.5.1. Pseudo-first-order- and pseudo-second-order-model The best-fit values of kf ; h, qe and ks along with correlation coefficients for the pseudo-first-order and pseudo second-order models are shown in Table 1. The qe,exp and the qe,cal values for the pseudofirst-order model and pseudo-second-order models are shown in Table 1. The calculated correlation coefficients are also closer to unity for pseudo-second-order model. Also, qe,exp and the qe,cal values of NB and HA from the pseudo-second-order kinetic model are very close to each other. Therefore, the sorption can be approximated more appropriately by the pseudo-second-order kinetic model than the pseudo-first-order kinetic model for the adsorption of NB and HA by ACC. The fit of pseudo-second-order kinetic model is shown by solid line in Fig. 6.
Fig. 5. Effect of adsorbent dose on the adsorption of NB and HA by ACC. T = 303 K, t = 24.0 h, and C0 = 100 mg/l.
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Table 1 Kinetic parameters for the removal of NB and HA by ACC (t = 24.0 h, C0 = 50–1000 mg/l, m = 10 g/l, and T = 303 K). Equations
NB-ACC adsorption system
HA-ACC adsorption system
50 mg/l
100 mg/l
250 mg/l
500 mg/l
1000 mg/l
50 mg/l
100 mg/l
250 mg/l
500 mg/l
1000 mg/l
Pseudo-first-order kf (min−1 ) qe,cal (mg/g) qe,exp (mg/g) R2 (non-linear) MPSD
0.0201 5.1147 5.1770 0.9926 43.2800
0.0280 12.4091 12.8150 0.9881 34.6300
0.0089 25.6179 26.6281 0.9767 84.6200
0.0070 47.2732 51.8020 0.9900 52.8800
0.0069 82.0281 92.0990 0.9579 109.9000
0.0066 3.6769 3.7390 0.9480 113.750
0.0101 7.0451 7.7270 0.9460 105.2400
0.0059 16.6943 21.7910 0.9825 129.6700
0.0043 47.1859 47.1270 0.9725 86.0700
0.0035 86.4211 89.3180 0.9792 118.3000
Pseudo-second-order ks (g/mg min) h (mg/g min) qe,cal (mg/g) R2 (non-linear) MPSD
0.0050 0.1443 5.3551 0.9890 47.5500
0.0025 0.4263 13.0884 0.9889 31.8500
0.0006 0.4289 26.1595 0.9738 68.5600
0.0002 0.5483 51.9736 0.9962 42.5400
0.0069 1.2274 89.338 0.9344 80.9600
0.0077 0.1099 3.7809 0.9729 102.560
0.0020 0.1328 8.0107 0.9538 87.1100
0.0004 0.2084 21.0072 0.9700 105.8300
0.0002 0.3967 49.0790 0.9565 99.5300
0.00007 0.69675 97.3512 0.9556 117.62
Weber–Morris kid,1 (mg/g min1/2 ) I1 (mg/g) R2 kid,2 (mg/g min1/2 ) I2 (mg/g) R2
0.5684 0.8703 0.9392 0.0243 4.3468 0.7430
0.9498 1.4096 0.9311 0.0420 11.2680 0.9790
1.1672 1.3862 0.9731 0.1040 22.4650 0.6584
2.8179 3.0843 0.9707 0.5549 30.0960 0.9425
3.6641 16.5020 0.8032 1.6169 33.8930 0.9046
0.1647 0.4407 0.7456 0.0113 3.2975 0.9831
0.2934 1.034 0.8883 0.0384 6.2921 0.9666
1.0023 0.5261 0.8966 0.1909 14.8180 0.8239
2.3369 4.2937 0.8833 0.1411 41.6840 0.9166
2.8808 4.7441 0.9383 0.6305 65.9550 0.8491
3.5.2. Intra-particle diffusion model The possibility of intra-particle diffusion was explored by using the intra-particle diffusion model. Fig. 7 presents plot of qt versus t1/2 (Weber–Morris) at various C0 for NB and HA adsorption onto ACC. The plots are multi-linear indicating that more than one process is controlling the adsorption process. First portion (line not drawn for the clarity of figure) gives the diffusion of adsorbate through the solution to the external surface of adsorbent or boundary layer diffusion [38]. Further two-linear portions depict intra-particle diffusion. Second linear portion (first linear line in Fig. 7) is attributed to the gradual equilibrium stage with intraparticle diffusion dominating. Third portion (second linear line in Fig. 7) is the final equilibrium stage for which the intra-particle diffusion starts to slow down due to the extremely low adsorbate concentration left in the solution [39,40]. The slope of the linear portions are defined as a rate parameters (kid,1 and kid,2 ) and are characteristics of the rate of adsorption in the region where intraparticle diffusion is rate controlling. It can be inferred from Fig. 7 that the diffusion of adsorbate from the bulk phase to the external surface of ACC, which begins at the start of the adsorption process, is the fastest. It seems that the intra-particle diffusion of adsorbate into micro-pores (third portion) is the rate controlling step in the adsorption process. The third portion of the plots is nearly parallel (kid,2 ≈ 0.001–0.013 mg/g min0.5 ), suggesting that the rate of adsorption adsorbate into the micro-pores of ACC is comparable at all C0 . Slopes of second and third portions (kid,1 and kid,2 ) are higher for higher C0 , which corresponds to an enhanced diffusion of adsorbate through meso- and micro-pores. This is due to the greater driving force at higher C0 . Extrapolation of the linear portions of the plots back to the y-axis gives the intercepts (values of I in Table 1) that provide the measure of the boundary layer thickness. The deviation of straight lines from the origin indicates that the pore diffusion is not the sole rate-controlling step. Therefore, the adsorption proceeds via a complex mechanism [41] consisting of both surface adsorption and intra-particle transport of NB and HA within the pores of ACC. 3.6. Desorption study
Fig. 6. Effect of contact time on the adsorption of (a) NB and (b) HA by ACC. Experimental data points given by the symbols and the lines predicted by the pseudo-second-order model. T = 303 K and m = 10 g/l.
Adsorbate-loaded ACC was stirred with 50 ml of various eluents and the results are shown in Fig. 8. A very low desorption is obtained for NB with the mineral acids (HNO3 and HCl), alkali (NaOH) and water than compared with adsorption percent obtained with
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Fig. 9. Thermal desorption efficiencies of NB and HA. T = 303 K, t = 24 h, C0 = 1000 mg/l and m = 10 g/l.
Fig. 7. Weber and Morris intra-particle diffusion plot for the removal of (a) NB and (b) HA by ACC. T = 303 K and m = 10 g/l.
organic acid (CH3 COOH). The solvent desorption of HA loaded ACC is very low for all solvents. Very low desorption of adsorbates indicates that some complex formation takes place between the active sites of ACC and the adsorbates. Spent ACC was also tested for thermal desorption by keeping NB- and HA-loaded ACC first in hot air-oven for 2 h at 373 K and then in furnace for 4 h at 623 K. Thermal desorbed ACC was again used for adsorption for C0 = 1000 mg/l, m = 10 g/l, T = 303 K and t = 24 h. After that, it was again thermally desorbed by procedure as given above. This cycle of adsorption–desorption was repeated five times. Percent removal of NB and HA in those cycles is shown in Fig. 9. It is seen that the spent-ACC can be used for five number of adsorption–desorption cycles when adsorption capacity gets reduced to nearly half of its capacity during first cycle. It may be seen that percent removal of HA was higher than that of NB in each cycle. It may be due to the more intricate attachment of NB with ACC. This could be attributed to the strong chemical attachment between ACC and adsorbed NB and HA. This prevented the adsorbed NB and HA from being effectively cleaned by thermal desorption, and caused lower removal efficiency after each cycle. 3.7. Thermal oxidation and disposal of spent ACC
Fig. 8. Desorption efficiencies of NB and HA by various desorbing agents. T = 303 K, t = 24 h, C0 (desorbing agents) = 0.1 N, and m = 4 g/1.
The TG, DTA and DTG curves of the blank and NB- and HA-loaded ACC are shown in Fig. 10a–c, respectively. The TG traces for the blank and NB- and HA-loaded ACC show that the loss of moisture and the evolution of some light weight molecules including water take place (7–11% weight loss) from 25 to 500 ◦ C. Higher temperature drying (100–500 ◦ C) occurs due to loss of the surface tension bound water of the particles. Samples do not show any endothermic transition between room temperature and 400 ◦ C, indicating lack of any crystalline or other phase change during the heating process [42]. The rate of weight loss was found to increase between ∼500 and ∼690 ◦ C (61% weight loss) for blank ACC, between ∼500 and ∼670 ◦ C (48% weight loss) for NB-ACC, and ∼52% weight loss between ∼500 and 645 ◦ C for HA-ACC. In these temperature ranges the blank and NB- and HA-loaded ACC oxidize and lose their weight gradually. The strong exothermic peak centered between 450 and 700 ◦ C is due to the oxidative degradation of the sample. This broad peak as that observed from the first derivative loss curve (DTG) may be due to the combustion of carbon species. At higher temper-
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Fig. 10. TGA-DTA graphs of blank and NB- and HA-loaded ACC.
atures (third zone), the samples present a gradual weight loss up to 1000 ◦ C. This weight loss has been reported to be associated in part with the evolution of CO2 and CO. Major differences between thermal behaviors of blank ACC and the NB- and HA-loaded ACC are: first, in the second degradation zone, amount of weight loss was found to be highest for ACC (61% weight loss) followed by HA-ACC (∼52% weight loss) and NB-ACC (48% weight loss); second, rate of weight was found to be highest for ACC (1.08 mg/min at ∼640 ◦ C) followed by NB-ACC (1.00 mg/min at ∼600 ◦ C) and HAACC (0.58 mg/min at ∼550 ◦ C); and last amount of ash left after third degradation in ACC, NB-ACC and HA-ACC was found to be 30.3%, 41.3% and 46.7%, respectively. TGA and DTA curves were used to deduce drying and thermal degradation characteristics. The distribution of volatiles and oxidation products evolved in different temperature ranges show irregular pattern. ACC has a heating value of about 18.81 MJ/kg [17]. Thus, the exhausted ACC along-with the adsorbed NB and HA can be dried and used as a fuel in the boilers/incinerators, or can be used for the production of fuel-briquettes. The bottom ash may be blended with clay to make fire bricks, or with cement–concrete mixture to make colored building blocks thus disposing of NB and HA through chemical and physical fixation. This approach of ACC disposal entails energy recovery from the ACC and the safe disposal of the adsorbed NB and HA.
4. Conclusions The present paper aimed to investigate the suitability of using commercial activated carbon (ACC) for individual adsorption of nitrobenzene (NB) and humic acid (HA) from aqueous solution.
The XRD spectra of the ACC reflected the presence some characteristic components in ACC. FTIR spectra of the ACC indicated the presence of various types of functional groups. The influence of various experimental parameters viz. initial pH (pH0 ), adsorbent dose, contact time and temperature on the adsorptive removal of NB and HA from aqueous solution by commercial grade activated carbon (ACC) were investigated. Equilibrium contact time for both the adsorbates were found to be 5 h for C0 ≤ 100 mg/l and 12 h for 100 mg/l < C0 ≤ 500 mg/l. Optimum removal of NB and HA onto ACC occurred at respective natural pH of 5.6 and 2, respectively. Optimum adsorbent dosage was found to be 10 g/l for both the adsorbates. The NB and HA adsorption was found to follow the pseudo-second-order kinetics. NB- and HA-loaded ACC regeneration was studied using various solvents as well as by heating the spent ACC at 350 ◦ C. Solvent aided regeneration of ACC was found to be very less with maximum desorption efficiency shown by acetone (around 25%) for NB-ACC system. However with thermal regeneration, ACC showed good efficiency for NB and HA removal in five subsequent desorption–adsorption cycles. References [1] IARC, International Agency for Research on Cancer, Nitrobenzene IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 65, 1996, pp. 380–408. [2] ATSDR, Agency for Toxic substances and Disease Registry, US Public Health Service Toxicological Profile for Nitrobenzene, ATSDR, Atlanta, GA, 1990. [3] S. Boggs, D. Livermore Jr., M.G. Seitz, Humic substances in natural waters and their complexation with trace metals and radionuclides: a review report, Macromol. Chem. Phys. 25 (1985) 599. [4] L. Ruiping, L. Huijuan, W. Dongjin, M. Yang, Characterization of the Songhua River sediments and evaluation of their adsorption behavior for nitrobenzene, J. Environ. Sci. 20 (2008) 796–802.
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Studies on adsorption/desorption of nitrobenzene and humic acid onto/from activated carbon Megha Syam Rauthula, Vimal Chandra Srivastava ∗ Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e
i n f o
Article history: Received 2 September 2010 Received in revised form 8 December 2010 Accepted 9 December 2010 Keywords: Activated carbon Nitrobenzene Humic acid Desorption Kinetics
a b s t r a c t This paper reports results of the studies on adsorption of nitrobenzene (NB) and humic acid (HA) from aqueous solution onto activated carbon commercial (ACC) grade. Characterization of ACC showed its meso-porous nature. Fourier transform infra-red (FTIR) spectra of the ACC indicated presence of various types of functional groups on its surface. Thermo-gravimetric analysis exhibited the thermal stability of the ACC up to 300 ◦ C. The adsorption kinetics of NB and HA onto ACC could be represented by pseudosecond-order kinetic model. The adsorption processes could be well described by a two-stage diffusion model. Thermal regeneration showed that ACC could be used for five desorption–adsorption cycles with good efficiency for NB and HA in each cycle. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nitrobenzene (NB) and its derivatives are considered to be very toxic. The evidence of carcinogenicity of NB in experimental animals has been established by the IARC [1]. NB is soluble in water (1.9 g/l) and gets biodegraded completely only after 7–10 days. This makes it incompatible for treatment in simple wastewater treatment plants [2]. The European Union Directive 2001/59/EC states that “NB is toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment”. Humic substances are the major organic constituents of river, lake and pond waters. They derive from soils where they get formed through the breakdown of biological tissues by chemical and biological processes [3]. Humic substances including humic acid (HA) are an important disinfection by-products (DBPs) precursor such as chloroform and bromo-dichloromethane which are suspected carcinogenic compounds [4]. It has been shown that humic substances often reduce the efficiency of removal of the target substances due to their adsorption onto adsorbents and/or due to formation of complexes with the target substances [5]. Elevated levels of humic substances in water produce color making the water aesthetically unpleasing to the consumer [6]. Various techniques such as activated sludge process, trickling filter, biofiltration, ozone degradation, and electro-coagulation are used for the removal of NB and HA from waste water [7]. Majumder and Gupta [8] used hybrid reactor, comprising of trickling filter and activated sludge process, in treating NB wastewater. Eggins
∗ Corresponding author. Tel.: +91 1332 285889; fax: +91 1332 276535/273560. E-mail address: [email protected] (V.C. Srivastava). 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.12.026
et al. [9] demonstrated use of photo-catalysis for the removal of HA from potable water using titanium dioxide. It took approximately 12 min to reduce the HA concentration by half; however 50% complete mineralization took 60 min. The ability of TiO2 to remove humic substances has been confirmed by other workers also [10,11]. Hiraide et al. [12] separated HA from fresh water by coprecipitation and flotation in 1 h. Reungoat et al. [13] investigated adsorption of NB onto zeolites followed by regeneration of zeolites with ozone. NB was completely removed from water and the initial adsorption capacity of zeolite was totally restored after desorption with ozone. Few investigators have theoretically modeled the phenomena of HA adsorption/desorption at the solid/water interface [14]. The presence of humic substances in water resources is of major concern in the water supply. Humic substances react with chlorine during water treatment to produce trihalomethanes which are known human carcinogens [15]. Among the various treatment processes, adsorption onto activated carbon is one of the most important processes. Activated carbon is primarily used for the treatment of potable water (24% of all use); wastewater (21%) and groundwater remediation (4%) [16]. It has large surface area (100–2000 m2 /g) and pore volume that gives it high adsorption capacity [17]. The aims and objectives of the present work are: (i) to characterize activated carbon commercial grade (ACC) for its physico-chemical and adsorption properties before and after adsorption of NB and HA, (ii) to study the effect of various parameters for the removal of NB and HA from the aqueous solution in batch study, (iii) to perform kinetic study of NB and HA adsorption onto ACC and to analyze the experimental data using various kinetic models and (iv) to perform desorption study for the possible regeneration of ACC.
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2. Materials and methods 2.1. Adsorbent and its characterization Coconut-based ACC supplied by ZeoTech Adsorbents Pvt. Ltd., New Delhi, India. Detailed physico-chemical characteristics of the ACC have already been presented elsewhere [17]. Bulk density of ACC was determined using bulk density meter supplied by Macro Scientific Works, Delhi. Bulk density meter consisted of two cylinders screwed on holding brackets by Knurled rings. First, standard powder (of known bulk density) and ACC of known weights were filled in the cylinders up to 50 ml mark. Knurled rings were tightened from the top to keep the cylinders firmly on the brackets. Both cylinders were then subjected to 100 strokes vibrations in 3 min. Thereafter, the volumes of both cylinders were noted and bulk density calculated. Particle size analysis of ACC was done as per Indian standard (IS) 2720 (Pt 4): 1985 using IS sieves of sizes 90 m, 180 m, 250 m, 500 m, 1000 m, 1700 m, 3350 m and 4000 m. Phillips (Holland) diffraction unit (Model PW1140/90) was used for finding X-ray diffraction (XRD) data of ACC. Copper was used as the target with nickel as the filter media. K radiation was maintained constant at 1.542 A˚ whereas goniometric speed was maintained at 2◦ /min. LEO 435 VP scanning electron microscope was used for taking scanning electron micrographs (SEM) of ACC. Thermo Nicolet (Model Magna 760) Fourier transform infrared (FTIR) spectrometer was employed to determine the presence of surface functional groups in ACC, before and after the adsorption of NB and HA, at room temperature over a spectral wave number range of 4000–400 cm−1 . The point of zero charge (pHPZC ) of the ACC was determined by the solid addition method [18]. The thermal decomposition of ACC was carried out non-isothermally in a pyris diamond PerkinElmer instrument. The degradation runs were taken at heating rate of 100 K/min with air flow rate kept constant at 200 ml/min. 2.2. Adsorbates NB supplied by HiMedia Laboratories Pvt. Ltd., Mumbai, India; and HA supplied by S.D. Fine Chemicals Pvt. Ltd., Mumbai, India were used as adsorbates. Stock solutions of 1000 mg/l were prepared by dissolving accurate quantity of NB and HA in distilled water. Solutions of required concentration were prepared by diluting the previously prepared stock solutions with distilled water whenever required. 2.3. Analytical measurement The concentration of NB and HA was determined by finding out the characteristic maximum absorbance wavelength using UV/vis spectrophotometer (PerkinElmer lambda 35, Schimadzu, Japan). A standard solution of known concentration was taken and the absorbance was determined at different wavelengths to obtain a plot of absorbance versus wavelength. The wavelength corresponding to maximum absorbance (max ) was determined. The max for NB and HA were found to be 268 and 337 nm. Calibration curve was plotted between the absorbance and the concentration of NB and HA solution. Linear portion of this curve was used for determining the unknown concentration of NB and HA solution. 2.4. Batch study Batch experiments were conducted to study the effect of various parameters like adsorbent dose (m), initial pH (pH0 ), contact time (t) and temperature (T) on the adsorptive removal of NB and HA by ACC. For performing an experimental run, 250 ml stoppered conical
flask containing 100 ml of NB or HA solution with known C0 , pH0 and an optimum m were agitated at 303 ± 1 K in a temperaturecontrolled orbital shaker at a constant speed of 150 rpm. Samples were withdrawn from the shaker after appropriate time. These samples were centrifuged (Research Centrifuge, Remi Scientific Works, Mumbai) at 10,000 rpm for 5 min and analyzed for the residual NB and HA concentrations. Blank experimental runs were conducted simultaneously at similar conditions with only adsorbent in 100 ml of distilled water. Also, stability of the adsorbates was checked by agitating blank aqueous solution of adsorbates without any adsorbent. No change in concentration of adsorbates was observed during the experimental runs and the adsorbates were found to be stable. Effect of pH0 on both NB and HA removal was studied over a pH0 range of 2–12. pH0 was adjusted by addition of 0.1 N H2 SO4 or NaOH. Optimum m, was found by contacting different amounts of ACC with 100 mg/l NB and HA solutions till equilibrium was attained. The percentage removal of adsorbate and the equilibrium adsorption uptake in solid phase, qe (mg/g), were calculated as follows: Percent removal = qe =
100(C0 − Ce ) C0
(C0 − Ce )V w
(1) (2)
where C0 is the initial concentration (mg/l), Ce is the equilibrium concentration (mg/l), V is the volume of the solution (l) and w is the mass of the adsorbent (g). 2.5. Adsorption kinetic theory 2.5.1. Pseudo-first-order- and pseudo-second-order-model Pseudo-first-order equation is given as [19]: qt = qe [1 − exp(−kf t)]
(3)
where qt is the amount of adsorbate adsorbed at time t (mg/g), qe is the amount of the adsorbate adsorbed on the adsorbent at time under equilibrium condition, kf = (ka Cs + kd ) is the pseudo-first order rate constant and Cs is the adsorbent concentration in the solution. The pseudo-second-order model is represented as [20]: qt =
tks q2e 1 + tks qe
(4)
The initial sorption rate, h (mg/g min), at t → 0 is defined as h = ks q2e . Where, ks is the pseudo-second order rate-constant. 2.5.2. Intra-particle diffusion study Intra-particle diffusion model is given as [21]: qt = kid t 1/2 + I
(5)
where kid is the intra-particle diffusion rate constant and values of I give an idea about thickness of the boundary layer. 2.6. Desorption studies Desorption study was performed using solvent and thermal methods. In the solvent desorption method, the NB-loaded ACC (0.2 g) was agitated in a series of 250 ml conical flasks containing 50 ml of aqueous solution of HCl, H2 SO4 , HNO3 , distilled water, CH3 COOH, C2 H5 OH, acetone and NaOH of known concentration at 303 K at 150 rpm for 24 h in the orbital shaker [22]. The same procedure was followed for HA-ACC system. In the thermal desorption method, the NB or HA loaded ACC was separated from the solution after adsorption study. It was dried in oven at 373 K for 2 h
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Fig. 1. X-ray diffraction of blank ACC.
followed by heating in a furnace at 623 K for 3 h. The sample was then taken out of the furnace and kept in a desiccator. This sample was again used for adsorption. This adsorption–desorption procedure was repeated till adsorption capacity got reduced to half of its adsorption capacity as compared to that in the first adsorption study. 3. Results and discussion
aldehydes and ketones. This band may also be due to conjugated hydrocarbon bonded carbonyl groups. The FTIR spectra also show transmittance around ∼1085 cm−1 region due to the vibration of the CC group in lactones and due to SiOSi and –COH stretching and –OH deformation. Peak at ∼790 cm−1 indicates the presence of SiH. Fig. 3 also shows the FTIR spectra of NB- and HA-loaded ACC. Many peaks get shifted in NB- and HA-loaded ACC. This shift signifies the participation of these functional groups in the adsorption process.
3.1. Characterization of adsorbent
3.2. Effect of initial pH (pH0 )
The average particle size of ACC was 1.67 mm. Bulk density and heating value of ACC were determined as 599.32 kg/m3 and 18.81 MJ/kg, respectively. XRD pattern for ACC is shown in Fig. 1. It shows the presence of moganite (SiO2 ), akdalaite [(Al2 O3 )4 ·H2 O], tamarugite [NaAl(SO4 )2 ·6H2 O], fersilicate (FeSi) and majorite [Mg3 (Fe,Al,Si)2 (SiO4 )3 ] as major components in ACC [17]. The broad peak in the XRD indicates the presence of amorphous form of silica. Diffraction peaks corresponding to crystalline carbon was not seen in ACC [23]. The BET surface area of ACC was found to be 171.05 m2 /g, whereas the BET average pore diameter ˚ Thus, ACC exhibits meso-porous of ACC was found to be 31.03 A. ˚ [24] which is desirnature (20 A˚ < average pore diameter < 500 A) able for the liquid phase adsorption of organic compounds. Fig. 2 shows the SEM of the ACC before and after adsorption at various magnifications. SEM at lower magnification (50×) does not show any difference in the texture of the blank and NB- and HA-loaded ACC. It only shows that the ACC is crystalline in nature and with varying particle size. However, at higher magnification (10,000×), some difference in the surface texture of the blank and NB- and HA-loaded ACC can be envisaged. Blank ACC shows porous structure with pores of varying sizes. Numbers of pores observed in the loaded ACC are less as compared to that in blank ACC. Chemical structure of the adsorbent surface affects the adsorption nature. The functional groups often found on activated carbons are carboxyl, phenolic hydroxyl, carbonyl (e.g. quinone-type) and lactone (e.g. fluorescein-type) groups [25]. The FTIR spectrum (Fig. 3) of ACC shows a broad band between 3100 and 3700 cm−1 which is indicative of the presence of both free and hydrogen bonded OH groups on the ACC surface. This stretching is due to both the silanol groups (Si–OH) and adsorbed water on the surface. Broad peak in the region of 1500–1700 cm−1 with peak at 1580 cm−1 indicates the presence of CO group stretching from
There are mainly two types of interactions, electrostatic and dispersive, during adsorption of aromatic compounds onto activated carbons [26,27]. The hydrogen ion and hydroxyl ions are quite strongly adsorbed, and therefore, the adsorption of other ions is affected by the pH of the solution. Acidic electrolytes are going to be dissociated into negative ions at pH > pKa . A similar phenomenon occurs with the carbon surface which has an amphoteric character. If pH > pHZPC , then the carbon surface is negatively charged, the opposite occurs when the pH < pHZPC [28]. Adsorption of cations is favored at pH > pHZPC , while the adsorption of anions is favored at pH < pHZPC . At low pH (e.g., pH < pHZPC ), most carbons are positively charged, at least in part as a consequence of donor/acceptor interactions between the graphene layers and the hydronium ions [29]. pHZPC for ACC was found to be 8.5 [17]. In this study, the performance of HA adsorption by ACC was examined at pH values ranging from 2 to 12. From the results shown in Fig. 4, the capacity for HA adsorption is found to generally increase with decreasing pH. HA adsorption onto ACC was highest at pH ∼2 and there was hardly any adsorption of HA at pH ∼12. HA is a subclass of humic substances that is soluble in water at pH > 2. This macromolecule has a complex structure containing phenolic and carboxylic groups, thus, carrying negative charges in natural waters for all pH > 2 [30,31]. HA molecules generally gain an excess negative surface charge as the pH is increased due to the deprotonation, first of the carboxylic groups at pH values of 4–6, followed by the dissociation of phenolic groups at higher pH [32,33]. At pH0 ∼2, the zeta potentials of ACC and HA are of opposite sign to each other. Hence, the interactions between the granules and the HA were electro-statically attractive, which led to significant adsorption of the HA onto the ACC. The large decrease of adsorption between pH0 2 and 3 may be attributed to an increase in the size of the HA molecules at higher pH [34]. Larger molecular sizes
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Fig. 2. SEM of blank and NB- and HA-loaded ACC.
cause lesser molecules to be adsorbed onto the ACC surface. Also as the pH0 increases, the ACC becomes less positively charged, and thus, electrostatic attraction between the HA and ACC decreases leading to lesser adsorption at higher pH. Since both the ACC and the HA are negatively charged at pH0 > 8.5, the electrostatic repulsion between the HA molecules to be adsorbed and the surfaces of the ACC most probably prevented HA from approaching close enough to the surface of the granules for adsorption to take place. The functional group linked to the adsorptive aromatic ring can activate or deactivate it, delocalizing its electronic charge [35]. Activating groups are electron-deactivating groups, creating a partial positive charge in the ring (nitrobenzene), while withdrawing groups produce the opposite effect, creating a partial negative charge (phenol, aniline). Because the aromatic ring has a much
larger size than the functional group, the interaction of the aromatic ring with the basal planes of the carbon is more effective [36]. Substitution of a nitro group on a benzene ring results in withdrawal of o-electron density from the ring, especially from the ortho and para positions (see Scheme 1). Substituents that are more powerful electron attractors than hydrogen are said to exhibit a negative inductive effect and vice versa. Groups that withdraw electron density from conjugated systems by resonance effects are designated as −R in character [37]. Thus, NB is a very weak Lewis acid that possesses the electron-withdrawing NO2 group. The effect of pH0 on NB adsorption was studied with blank solutions of C0 = 100 mg/l having natural pH0 = 5.6. The results in Fig. 4 show that the NB removal is maximum at natural pH0 . However, the adsorption of NB is appreciable in the whole pH range. NB is
Fig. 3. FTIR of blank and NB- and HA-loaded ACC.
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Scheme 1. Resonance structure of nitrobenzene.
in the molecular form in the whole pH range. Because of this, the dispersive interactions will be the most important in the adsorption process. It seems that adsorption of NB takes place primarily via diffusion into the meso-pores of the ACC. At pH < pHZPC , adsorption of NB to ACC may be through negative charge on nitro group whereas for pH > pHZPC adsorption may be via the partial positively charged aromatic ring. Since 5.6 is the natural pH of the solution, therefore, further experiments with NB were conducted without adjusting the pH. 3.3. Effect of adsorbent dosage (m) The effect of m on the removal of NB and HA by ACC at C0 = 100 mg/l is shown in Fig. 5. NB and HA removal increases up to m < 10 g/l, and then it remains almost constant for m ≥ 10 g/l. The adsorbent surface becomes saturated with adsorbates for m < 10 g/l, and the residual concentration in the solution is large. An increase in adsorption with an increase in m for m < 10 g/l can be attributed to greater surface area and availability of more adsorption sites. Further, in all cases, equilibrium was found to be attained more rapidly at lower m. However, for m ≥ 10 g/l, the surface NB and HA concentrations and the solution NB and HA concentrations come to saturation limit with each other and the incremental removal becomes very low. Thus, optimum m was found to be 10 g/l for both NB and HA. 3.4. Effect of contact time Fig. 6 shows the effect of t on the removal of NB and HA by the ACC at C0 = 50–1000 mg/l for m = 10 g/l. During first 1 h, adsorp-
Fig. 4. Effect of pH0 on the adsorption of NB and HA by ACC. T = 303 K, t = 24.0 h, C0 = 100 mg/l, and ACC dose = 10 g/l.
tion of NB and HA was found to be quick for all C0 and thereafter, the adsorption rate decreased gradually. This is due to the fact that the adsorbates get adsorbed into the meso-pores during initial stages, thereafter, adsorbates have to travel deeper into the pores encountering larger resistance. This results in the slowing down of the adsorption during the later period of adsorption. For C0 ≤ 100 mg/l, the residual concentrations at 5 h contact time were found to be higher by a maximum of ∼1% than those obtained after 12 h contact time. Therefore, after 5 h contact time, a steady state approximation was assumed and a quasi-equilibrium situation was accepted for C0 ≤ 100 mg/l. For 100 mg/l < C0 ≤ 500 mg/l, a quasiequilibrium situation may be assumed after 12 h. Accordingly all the batch experiments were conducted with a contact time of 12 h under vigorous shaking conditions for all C0 ≤ 500 mg/l.
3.5. Adsorption kinetic study 3.5.1. Pseudo-first-order- and pseudo-second-order-model The best-fit values of kf ; h, qe and ks along with correlation coefficients for the pseudo-first-order and pseudo second-order models are shown in Table 1. The qe,exp and the qe,cal values for the pseudofirst-order model and pseudo-second-order models are shown in Table 1. The calculated correlation coefficients are also closer to unity for pseudo-second-order model. Also, qe,exp and the qe,cal values of NB and HA from the pseudo-second-order kinetic model are very close to each other. Therefore, the sorption can be approximated more appropriately by the pseudo-second-order kinetic model than the pseudo-first-order kinetic model for the adsorption of NB and HA by ACC. The fit of pseudo-second-order kinetic model is shown by solid line in Fig. 6.
Fig. 5. Effect of adsorbent dose on the adsorption of NB and HA by ACC. T = 303 K, t = 24.0 h, and C0 = 100 mg/l.
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M.S. Rauthula, V.C. Srivastava / Chemical Engineering Journal 168 (2011) 35–43
Table 1 Kinetic parameters for the removal of NB and HA by ACC (t = 24.0 h, C0 = 50–1000 mg/l, m = 10 g/l, and T = 303 K). Equations
NB-ACC adsorption system
HA-ACC adsorption system
50 mg/l
100 mg/l
250 mg/l
500 mg/l
1000 mg/l
50 mg/l
100 mg/l
250 mg/l
500 mg/l
1000 mg/l
Pseudo-first-order kf (min−1 ) qe,cal (mg/g) qe,exp (mg/g) R2 (non-linear) MPSD
0.0201 5.1147 5.1770 0.9926 43.2800
0.0280 12.4091 12.8150 0.9881 34.6300
0.0089 25.6179 26.6281 0.9767 84.6200
0.0070 47.2732 51.8020 0.9900 52.8800
0.0069 82.0281 92.0990 0.9579 109.9000
0.0066 3.6769 3.7390 0.9480 113.750
0.0101 7.0451 7.7270 0.9460 105.2400
0.0059 16.6943 21.7910 0.9825 129.6700
0.0043 47.1859 47.1270 0.9725 86.0700
0.0035 86.4211 89.3180 0.9792 118.3000
Pseudo-second-order ks (g/mg min) h (mg/g min) qe,cal (mg/g) R2 (non-linear) MPSD
0.0050 0.1443 5.3551 0.9890 47.5500
0.0025 0.4263 13.0884 0.9889 31.8500
0.0006 0.4289 26.1595 0.9738 68.5600
0.0002 0.5483 51.9736 0.9962 42.5400
0.0069 1.2274 89.338 0.9344 80.9600
0.0077 0.1099 3.7809 0.9729 102.560
0.0020 0.1328 8.0107 0.9538 87.1100
0.0004 0.2084 21.0072 0.9700 105.8300
0.0002 0.3967 49.0790 0.9565 99.5300
0.00007 0.69675 97.3512 0.9556 117.62
Weber–Morris kid,1 (mg/g min1/2 ) I1 (mg/g) R2 kid,2 (mg/g min1/2 ) I2 (mg/g) R2
0.5684 0.8703 0.9392 0.0243 4.3468 0.7430
0.9498 1.4096 0.9311 0.0420 11.2680 0.9790
1.1672 1.3862 0.9731 0.1040 22.4650 0.6584
2.8179 3.0843 0.9707 0.5549 30.0960 0.9425
3.6641 16.5020 0.8032 1.6169 33.8930 0.9046
0.1647 0.4407 0.7456 0.0113 3.2975 0.9831
0.2934 1.034 0.8883 0.0384 6.2921 0.9666
1.0023 0.5261 0.8966 0.1909 14.8180 0.8239
2.3369 4.2937 0.8833 0.1411 41.6840 0.9166
2.8808 4.7441 0.9383 0.6305 65.9550 0.8491
3.5.2. Intra-particle diffusion model The possibility of intra-particle diffusion was explored by using the intra-particle diffusion model. Fig. 7 presents plot of qt versus t1/2 (Weber–Morris) at various C0 for NB and HA adsorption onto ACC. The plots are multi-linear indicating that more than one process is controlling the adsorption process. First portion (line not drawn for the clarity of figure) gives the diffusion of adsorbate through the solution to the external surface of adsorbent or boundary layer diffusion [38]. Further two-linear portions depict intra-particle diffusion. Second linear portion (first linear line in Fig. 7) is attributed to the gradual equilibrium stage with intraparticle diffusion dominating. Third portion (second linear line in Fig. 7) is the final equilibrium stage for which the intra-particle diffusion starts to slow down due to the extremely low adsorbate concentration left in the solution [39,40]. The slope of the linear portions are defined as a rate parameters (kid,1 and kid,2 ) and are characteristics of the rate of adsorption in the region where intraparticle diffusion is rate controlling. It can be inferred from Fig. 7 that the diffusion of adsorbate from the bulk phase to the external surface of ACC, which begins at the start of the adsorption process, is the fastest. It seems that the intra-particle diffusion of adsorbate into micro-pores (third portion) is the rate controlling step in the adsorption process. The third portion of the plots is nearly parallel (kid,2 ≈ 0.001–0.013 mg/g min0.5 ), suggesting that the rate of adsorption adsorbate into the micro-pores of ACC is comparable at all C0 . Slopes of second and third portions (kid,1 and kid,2 ) are higher for higher C0 , which corresponds to an enhanced diffusion of adsorbate through meso- and micro-pores. This is due to the greater driving force at higher C0 . Extrapolation of the linear portions of the plots back to the y-axis gives the intercepts (values of I in Table 1) that provide the measure of the boundary layer thickness. The deviation of straight lines from the origin indicates that the pore diffusion is not the sole rate-controlling step. Therefore, the adsorption proceeds via a complex mechanism [41] consisting of both surface adsorption and intra-particle transport of NB and HA within the pores of ACC. 3.6. Desorption study
Fig. 6. Effect of contact time on the adsorption of (a) NB and (b) HA by ACC. Experimental data points given by the symbols and the lines predicted by the pseudo-second-order model. T = 303 K and m = 10 g/l.
Adsorbate-loaded ACC was stirred with 50 ml of various eluents and the results are shown in Fig. 8. A very low desorption is obtained for NB with the mineral acids (HNO3 and HCl), alkali (NaOH) and water than compared with adsorption percent obtained with
M.S. Rauthula, V.C. Srivastava / Chemical Engineering Journal 168 (2011) 35–43
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Fig. 9. Thermal desorption efficiencies of NB and HA. T = 303 K, t = 24 h, C0 = 1000 mg/l and m = 10 g/l.
Fig. 7. Weber and Morris intra-particle diffusion plot for the removal of (a) NB and (b) HA by ACC. T = 303 K and m = 10 g/l.
organic acid (CH3 COOH). The solvent desorption of HA loaded ACC is very low for all solvents. Very low desorption of adsorbates indicates that some complex formation takes place between the active sites of ACC and the adsorbates. Spent ACC was also tested for thermal desorption by keeping NB- and HA-loaded ACC first in hot air-oven for 2 h at 373 K and then in furnace for 4 h at 623 K. Thermal desorbed ACC was again used for adsorption for C0 = 1000 mg/l, m = 10 g/l, T = 303 K and t = 24 h. After that, it was again thermally desorbed by procedure as given above. This cycle of adsorption–desorption was repeated five times. Percent removal of NB and HA in those cycles is shown in Fig. 9. It is seen that the spent-ACC can be used for five number of adsorption–desorption cycles when adsorption capacity gets reduced to nearly half of its capacity during first cycle. It may be seen that percent removal of HA was higher than that of NB in each cycle. It may be due to the more intricate attachment of NB with ACC. This could be attributed to the strong chemical attachment between ACC and adsorbed NB and HA. This prevented the adsorbed NB and HA from being effectively cleaned by thermal desorption, and caused lower removal efficiency after each cycle. 3.7. Thermal oxidation and disposal of spent ACC
Fig. 8. Desorption efficiencies of NB and HA by various desorbing agents. T = 303 K, t = 24 h, C0 (desorbing agents) = 0.1 N, and m = 4 g/1.
The TG, DTA and DTG curves of the blank and NB- and HA-loaded ACC are shown in Fig. 10a–c, respectively. The TG traces for the blank and NB- and HA-loaded ACC show that the loss of moisture and the evolution of some light weight molecules including water take place (7–11% weight loss) from 25 to 500 ◦ C. Higher temperature drying (100–500 ◦ C) occurs due to loss of the surface tension bound water of the particles. Samples do not show any endothermic transition between room temperature and 400 ◦ C, indicating lack of any crystalline or other phase change during the heating process [42]. The rate of weight loss was found to increase between ∼500 and ∼690 ◦ C (61% weight loss) for blank ACC, between ∼500 and ∼670 ◦ C (48% weight loss) for NB-ACC, and ∼52% weight loss between ∼500 and 645 ◦ C for HA-ACC. In these temperature ranges the blank and NB- and HA-loaded ACC oxidize and lose their weight gradually. The strong exothermic peak centered between 450 and 700 ◦ C is due to the oxidative degradation of the sample. This broad peak as that observed from the first derivative loss curve (DTG) may be due to the combustion of carbon species. At higher temper-
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M.S. Rauthula, V.C. Srivastava / Chemical Engineering Journal 168 (2011) 35–43
Fig. 10. TGA-DTA graphs of blank and NB- and HA-loaded ACC.
atures (third zone), the samples present a gradual weight loss up to 1000 ◦ C. This weight loss has been reported to be associated in part with the evolution of CO2 and CO. Major differences between thermal behaviors of blank ACC and the NB- and HA-loaded ACC are: first, in the second degradation zone, amount of weight loss was found to be highest for ACC (61% weight loss) followed by HA-ACC (∼52% weight loss) and NB-ACC (48% weight loss); second, rate of weight was found to be highest for ACC (1.08 mg/min at ∼640 ◦ C) followed by NB-ACC (1.00 mg/min at ∼600 ◦ C) and HAACC (0.58 mg/min at ∼550 ◦ C); and last amount of ash left after third degradation in ACC, NB-ACC and HA-ACC was found to be 30.3%, 41.3% and 46.7%, respectively. TGA and DTA curves were used to deduce drying and thermal degradation characteristics. The distribution of volatiles and oxidation products evolved in different temperature ranges show irregular pattern. ACC has a heating value of about 18.81 MJ/kg [17]. Thus, the exhausted ACC along-with the adsorbed NB and HA can be dried and used as a fuel in the boilers/incinerators, or can be used for the production of fuel-briquettes. The bottom ash may be blended with clay to make fire bricks, or with cement–concrete mixture to make colored building blocks thus disposing of NB and HA through chemical and physical fixation. This approach of ACC disposal entails energy recovery from the ACC and the safe disposal of the adsorbed NB and HA.
4. Conclusions The present paper aimed to investigate the suitability of using commercial activated carbon (ACC) for individual adsorption of nitrobenzene (NB) and humic acid (HA) from aqueous solution.
The XRD spectra of the ACC reflected the presence some characteristic components in ACC. FTIR spectra of the ACC indicated the presence of various types of functional groups. The influence of various experimental parameters viz. initial pH (pH0 ), adsorbent dose, contact time and temperature on the adsorptive removal of NB and HA from aqueous solution by commercial grade activated carbon (ACC) were investigated. Equilibrium contact time for both the adsorbates were found to be 5 h for C0 ≤ 100 mg/l and 12 h for 100 mg/l < C0 ≤ 500 mg/l. Optimum removal of NB and HA onto ACC occurred at respective natural pH of 5.6 and 2, respectively. Optimum adsorbent dosage was found to be 10 g/l for both the adsorbates. The NB and HA adsorption was found to follow the pseudo-second-order kinetics. NB- and HA-loaded ACC regeneration was studied using various solvents as well as by heating the spent ACC at 350 ◦ C. Solvent aided regeneration of ACC was found to be very less with maximum desorption efficiency shown by acetone (around 25%) for NB-ACC system. However with thermal regeneration, ACC showed good efficiency for NB and HA removal in five subsequent desorption–adsorption cycles. References [1] IARC, International Agency for Research on Cancer, Nitrobenzene IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 65, 1996, pp. 380–408. [2] ATSDR, Agency for Toxic substances and Disease Registry, US Public Health Service Toxicological Profile for Nitrobenzene, ATSDR, Atlanta, GA, 1990. [3] S. Boggs, D. Livermore Jr., M.G. Seitz, Humic substances in natural waters and their complexation with trace metals and radionuclides: a review report, Macromol. Chem. Phys. 25 (1985) 599. [4] L. Ruiping, L. Huijuan, W. Dongjin, M. Yang, Characterization of the Songhua River sediments and evaluation of their adsorption behavior for nitrobenzene, J. Environ. Sci. 20 (2008) 796–802.
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