Clean Techn Environ Policy (2015) 17:1619–1631 DOI 10.1007/s10098-014-0895-4
ORIGINAL PAPER
Primary sewage sludge-derived activated carbon: characterisation and application in wastewater treatment Anirudh Gupta • Anurag Garg
Received: 21 October 2014 / Accepted: 18 December 2014 / Published online: 4 January 2015 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract The present study deals with the characterization of primary sewage sludge-derived activated carbons which were used for phenol adsorption from the synthetic wastewater. The waste-derived adsorbents were prepared by a two-step process: chemical activation (with ZnCl2 and KOH) followed by pyrolysis. The mesoporous adsorbents were found to have BET surface area comparable to commercial activated carbon (surface area = 495–515 m2/g). The adsorption runs were performed in orbital shaker with synthetic phenolic wastewater (phenol concentration = 50–250 mg/L) to investigate the effect of various reaction parameters. The equilibrium phenol adsorption data could be described by Redlich–Peterson isotherm model and the pseudo first-order kinetic model exhibited the best fit to time-based adsorption data. The phenol adsorption was endothermic for all adsorbents. The performance of the thermally regenerated waste-derived adsorbents in the wastewater treatment was found to reduce significantly. Keywords Sewage sludge-derived adsorbents Phenol adsorption Adsorption kinetics Equilibrium isotherm Adsorption thermodynamics Adsorbent regeneration
Electronic supplementary material The online version of this article (doi:10.1007/s10098-014-0895-4) contains supplementary material, which is available to authorized users. A. Gupta A. Garg (&) Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India e-mail:
[email protected]
Introduction Sewage sludge (SS) is a major by-product generated in conventional sewage treatment plants. The annual production of sewage sludge is continuously increasing all across the world. Due to the presence of high volatile carbon, several researchers have used SS (primary and biological both) for the production of activated carbon to find substitute for expensive commercially available activated carbon (Martin et al. 2003; Monsalvo et al. 2011; Wen et al. 2011). The sludge-based adsorbents have been tested for the aqueous adsorption of different dyes (Rozada et al. 2007), organic compounds (Przepiorski 2006), antibiotics (Gupta and Garg 2013) and heavy metals (Seredych and Bandosz 2006; Merrikhpour and Jalali 2012) present in wastewater. In developing countries, the waste sludge is generally either landfilled or applied on agricultural land. The presence of heavy metals in the sludge poses a high risk to the crop and sub-surface water resources. Moreover, the expensive adsorption process (using commercial activated carbon (CAC) produced by virgin materials) is not affordable for small and medium scale industries which are generating effluent containing recalcitrant compounds. The development of low-cost adsorbent will motivate such industries to treat their effluents properly before the final discharge into water bodies. Therefore, the present investigation was focussed on the characterization of primary SS-derived adsorbents and their performance for the purification of phenolic wastewater. The carbon-rich sewage sludge can be activated by thermal or chemical activation methods. The latter method involves the impregnation of waste sludge with a salt, acid or base. It has been reported that a good quality product can be manufactured by chemical activation process (Smith
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et al. 2009). The chemical activation with ZnCl2 (a dehydrating agent) improves the porosity of material by promoting the aromatization of the carbon skeleton, while suppressing the tar formation. On the other hand, KOH, a strong base, directly reacts with carbon atoms and the dehydration reaction results in the increase of carbon content along with porosity. Several authors have used KOH (Ros et al. 2006; Monsalvo et al. 2011; Reddy et al. 2014) and NaOH (Nethaji and Sivasamy 2014) as activating agents for activated carbon preparation from biomass or other waste materials. The adsorbent generated as a result of KOH activation of SS had surface area and pore volume of 950 m2/g and 0.4 cm3/g (Monsalvo et al. 2011). In other research studies, the adsorbents synthesized by ZnCl2 and H3PO4 activation of SS showed lower surface area and pore volumes (Wen et al. 2011; Boualem et al. 2014). The previous studies indicate that the surface properties of the product can vary widely possibly due to the changed activation sequence, characteristics of the precursor and activation temperature. Mohan and Pittman (2007) have reviewed the characteristics of different wastederived adsorbents and their performances in water and wastewater treatment. Some studies have been performed on the adsorption of phenolic compounds with low-cost activated carbon derived from SS (Tay et al. 2001; Otero et al. 2003; Li et al. 2012) and biomass residues (Mohan et al. 2011; Amin et al. 2012). Phenol is a versatile organic compound either used as a raw material in several chemical industries (e.g. petrochemicals, pharmaceuticals, etc.) or appears as a byproduct. It is a weak acid with a pKa value of 9.92 and dissociates slightly in aqueous solution. The high phenol concentrations are difficult to degrade biologically due to its toxic and persistent nature. United States Environmental Protection Agency (USEPA) has listed phenol and its derivatives among the priority pollutants. In the present study, the characteristics of primary SSderived activated carbons produced by chemical activation using ZnCl2 and KOH were studied. Subsequently, the performance of waste-derived adsorbents was compared with commercial activated carbon (CAC) for the removal of phenol from wastewater. The equilibrium isotherms, kinetics and thermodynamic aspects of the adsorption process were also studied. The spent adsorbents were reused for phenol adsorption after regeneration and characterization.
Materials and methods Materials The analytical grade chemicals such as phenol, ZnCl2, KOH, ammonia and HCl were purchased from Merck
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Chemicals, Mumbai. The CAC was also supplied by the same vendor. The primary sewage sludge employed to produce adsorbents was collected from the sewage collection chamber located at Indian Institute of Technology (IIT) Bombay, Mumbai, India. The stock solution of phenol (concentration = 1,000 mg/L) was prepared in distilled water and used for the adsorption studies after appropriate dilutions to achieve phenol concentration of 50–250 mg/L. Methods Preparation of sewage sludge-based adsorbents The adsorbents were synthesized using the chemical activation method described by Smith et al. (2009). The ovendried SS was mixed with activating agent (ZnCl2 or KOH) in the desired mass ratio and agitated with magnetic stirrer for 7 h. The mixed slurries were oven dried at *110 °C temperature for 10 h before subjecting to pyrolysis (at 600 °C temperature) in a muffle furnace for a duration of 1 h. The resulting materials were crushed and rinsed with 500 mL of 1.2 M HCl to remove acid-soluble inorganic species. Additionally, the mixtures were washed with distilled water so that any residual activating agent and HCl could be eliminated. Finally, the wet materials were again oven dried (at 105–110 °C temperature) for 2 h before detailed characterisation. The different steps used for the preparation of activated carbons are shown in Fig. S1. The different synthesized activated carbon samples were designated as ACZn2, ACZn2.5, ACK2 and ACK3 where AC represents activated carbon, Zn and K represents the activating chemical (i.e. ZnCl2 and KOH) and the last digit indicates the activating chemical to SS weight ratio. The ratio of the activating chemical to SS weight ratios (i.e. impregnation ratios) was chosen based on the information in literature (Martin et al. 2003; Wang et al. 2008; Wen et al. 2011; Lin et al. 2012). Batch adsorption experiments For a typical experimental run, 100-mL capacity conical flasks were placed on a shaker for 3 h duration. The adsorption study was started with the determination of equilibrium time, i.e. the time needed to achieve the maximum adsorbate removal from the solute. For the equilibrium runs, an adsorbent concentration of 5 g/L was added to the conical flasks containing synthetic wastewater with a phenol concentration of 50 mg/L. The shaker speed was maintained at 50 rpm. To record the extent of phenol adsorption with time, the conical flasks were removed from the shaker periodically. Further runs were performed to study the effect of various operating parameters such as
Primary sewage sludge-derived activated carbon
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shaking speed (50–150 rpm), wastewater pH (3–11) and initial phenol concentration (50–250 mg/L) on the adsorption efficiency. The kinetics of adsorption process was determined using the phenol removal data with time, whereas the equilibrium phenol concentration data were used to study the isotherm models. The adsorbate removal and the equilibrium adsorption uptake or the amount adsorbed at equilibrium conditions (qe) were calculated by applying the following formulae: Phenol removal ð%Þ ¼ ½ðC0 Ce Þ= C0 Þ 100;
ð1Þ
qe ðmg/gÞ ¼ ðC0 Ce Þ ðV=wÞ;
ð2Þ
where C0 and Ce initial and final (or equilibrium) adsorbate concentration in aqueous phase, respectively (mg/L); V volume of the solution (L); w adsorbent mass (g). Analytical methods CHNS analyser (LECO CHNS-932, Michigan, USA) was used for the rapid determination of carbon, hydrogen, nitrogen and sulphur in the synthesized waste-derived activated carbon samples and CAC. To determine surface area and pore volume of the adsorbents, nitrogen adsorption/desorption isotherms were obtained at 77 K temperature using an automated adsorption analyser (Micromeritics ASAP2020 sorption analyzer, Georgia, USA). Brunauer–Emmet–Teller (BET) surface area was calculated using the adsorption data at a relative pressure (i.e. p/p°) ranging from 0.06 to 0.20. The total pore volume was estimated using liquid nitrogen at high relative pressure of 0.975, whereas the average pore diameter was obtained by assuming a straight, cylindrical and uninterrupted pore shape. Scanning electron microscopy (SEM) was performed with an electron microscope (LEO-1530VP, Oxford, UK) to study the morphology of the synthesized materials. The presence of major elements in the SS-derived adsorbents and CAC could be established using Energy-dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) patterns of the activated carbon samples were recorded on X-ray diffractometer (Shimadzu XRD-6000, Kyoto, Japan) with Cu-Ka radiation (Electric potential = 40 kV and current = 30 mA). The samples were scanned through the scanning angle (2h) ranging from 10° to 60°. The peaks in XRD spectra were identified using JCPDS (Joint Committee on Powder Diffraction Standards) files. The waste-derived and commercial activated carbon samples were also subjected to Fourier transform infrared (FTIR) to recognize the major functional groups present in these materials spectroscopy which was performed on IFS 66 Spectrometer (Bruker, USA).
The particle size distribution was obtained by passing known amount of a sample through a stack of BS (British standard) sieves numbered from 150 (mesh size = 104 lm) to 52 (mesh size = 295 lm). The mass retained at each sieve was used to calculate the fraction of particles (by weight) in a particular size range. The concentration of phenol in the unknown treated wastewater sample was determined in terms of absorbance at 270 nm wavelength, measured using a UV/Vis spectrophotometer (model UV 210 A, Shimadzu, Japan). The pHPZC (pH of the point of zero charge) value of the sludge-based adsorbents was determined using pH titration procedures (Orfao et al. 2006). As per the procedure, 50 mL of 0.01 M NaCl solution was poured into several conical flasks. The pH of the solution was adjusted between 2 and 9 by the addition of 0.1 M HCl or NaOH solution, as the case may be. Afterwards, 0.15 g of adsorbent was added to each flask and mixed completely. The final pH of the resulting solutions was measured after 48 h. The pHPZC is defined as the point at which pH of the above solution remains unchanged. Due to the amphoteric nature of activated carbon, the surface of carbon was neutral, positively charged and negatively charged at a pH equal to, lesser than or greater than pHPZC, respectively. Batch isotherm and kinetic studies To find out the best-fitting isotherm for phenol adsorption in the current study, four models were used: Langmuir, Freundlich, Temkin (based on two parameters) and Redlich–Peterson (three parameters model). The equations for different isotherms are as follows Foo and Hameed (2010): Langmuir isotherm : qe ¼ Q KL Ce =ð1 þ KL Ce Þ;
ð3Þ
where Q monolayer adsorption capacity (mg/g); KL = binding sites affinity (L/mg) Freundlich isotherm : qe ¼ KF Ce1=n ;
ð4Þ 1/n
where KF adsorption capacity [(mg/g)/(mg/L) ]; n heterogeneity factor Temkin isotherm : qe ¼ B1 lnKT þ B1 lnCe ;
ð5Þ
where B1and KT (L/mg) are the Temkin constants Redlich-Peterson isotherm: qe ¼ KR Ce 1 þ aR Ceb ; ð6Þ where KR (L/g), aR (L/g) and b are the Redlich–Peterson constants The validity of isotherm models was determined using the coefficient of determination (R2) and Chi-square (v2) values from linear and non-linear regressions, respectively
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(Ho 2004). R2 considers the percentage of variability to analyse the goodness of experimental data fitting in a model. The value of R2 can vary from 0 to 1 showing the best fit at the maximum value (i.e. 1). On the other hand, v2 function is an error function which takes the difference between the experimental and predicted data into account. Lower v2 value shows the better fit to a model. The mathematical formula used for the non-linear Chisquare analysis was as follows: v2 ¼
n h X
i ðqe qe;m Þ2 =qe;m ;
ð7Þ
DG ¼ R T lnK;
ð12Þ
where R is the universal gas constant and T is temperature in Kelvin, DG° is also related to the DS° and DH° according to the following expression:
DG ¼ DH T DS ;
ð13Þ
Using Eqs. 12 and 13, DG° can be eliminated and the following equation can be obtained which can be used to determine DS° and DH°:
lnK ¼ ðDS =RÞðDH =R TÞ:
ð14Þ
i¼1
where qe and qe,m are the corresponding equilibrium capacity from experimental study and calculated from model. In order to analyse the time-based phenol adsorption data, the Lagergren pseudo first- and second-order kinetic models were used. The coefficient of determination was used to obtain the best fit model for the kinetic data. The intra-particle diffusion model was used to find the effect of particle diffusion on the adsorption process and the ratecontrolling step (Weber and Morris 1962). In addition, Boyd’s model was also employed to check the rate-controlling step (Boyd et al. 1947). The equations for different kinetic models are expressed as Albadarin et al. (2012):
ln K was obtained by drawing a plot between ln (qe/Ce) versus qe at different temperatures (Khan and Singh 1987). Regeneration studies The best performing adsorbents were subjected to thermal treatment process as described by Sabio et al. (2004). The regeneration process was carried out in two steps: oven drying (at 105 °C temperature and 24 h duration) followed by heating under inert atmosphere at 700 °C temperature for 1 h. The resulting material was characterized before being used for phenol adsorption. The various activities performed from the preparation of adsorbents to their use in phenol adsorption are summarised in the flow chart (Fig. S2).
Pseudo first-order : logðqe qt Þ ¼ logqe ½ðk1 =2:303Þ t; ð8Þ
Results and discussion
where k1 = pseudo 1st-order rate constant Characterization of the adsorbents
Pseudo second-order : qt ¼ t k2 q2e =ð1 þ t k2 qe Þ; ð9Þ where k2 = pseudo 2nd-order rate constant Intra-particle diffusion model : qt ¼ kid t0:5 þ C;
ð10Þ
where kid = intra-particle diffusion rate constant and C = intercept on y-axis Boyd model : Bt ¼ 0:4977lnð1 FÞ;
Elemental analysis
ð11Þ
where F = qt/qe, Bt = Boyd value (function of F) Estimation of thermodynamic parameters The thermodynamic properties, Gibbs’ free energy change (DG°), entropy change (DS°) and the heat of adsorption (DH°), of a system are affected by the transfer of solute from the bulk solution to the solid/liquid interface. The equilibrium adsorption constant (K) is related to DG° according to the following relation:
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The physical and chemical properties, such as elemental composition, ash content, BET surface area, morphology, phase identification, particle size distribution and the attached surface groups, of the commercial as well as the synthesized activated carbons were determined.
The elemental analysis results showed that the carbon content in CAC (i.e. *92 %) was much higher compared to the synthesized activated carbon samples (Table 1). Various SS-derived activated carbon samples can be arranged in the increasing order of carbon content as follows: ACK2 (*56 %) \ ACZn2 (*62 %) \ ACK3 (*65 %) \ ACZn2.5 (*70 %). Generally, the carbon content in sewage sludge varies from 20 to 45 %. During the synthesis, some hydrogen and oxygen containing structures could have lost resulting in the enrichment of synthesized activated carbons (Martin et al. 2003). The ash
Primary sewage sludge-derived activated carbon Table 1 Elemental analysis and surface properties of the commercial and sewage sludgederived adsorbents
Adsorbents
1623
ACZn2
ACZn2.5
ACK2
ACK3
C (%)
62.25
H (%)
2.6
N (%)
69.85
55.65
64.75
92.06
2.9
2.5
2.8
2.1
1.5
1.3
1.8
1.6
1.2
S (%)
0.6
0.9
1.1
0.9
0.46
O (%)
19.85
13.7
23.15
18.65
4.2
Ash (%)
20.25
16.8
21.35
17.5
5.4
2.7
2.8
2.4
2.7
3.5
502.65
510.80
495.75
505.65
514.12
Moisture (%) BET surface area (m2/g)
CAC
Total pore volume (cm3/g)
0.289
0.297
0.279
0.288
0.296
Average pore diameter (nm)
4.58
4.67
4.45
4.56
4.67
content in the synthesized adsorbents (*15–25 %) was higher compared to CAC (*5 %). Surface area, porosity and particle size distribution The BET surface area of the synthesized activated carbon samples was marginally lower to that of CAC (*495–515 m2/g). The specific surface area of the dried sludge is generally very low (*3 m2/g) (Martin et al. 2003). An improvement in surface area after chemical activation indicates the removal of organic matter and opening of pores within the sludge texture. The lower diffusional resistance to an activating agent results in the rapid release of the volatile matter ensuing in the development of porous structure. The total pore volume of different samples was also comparable (0.279–297 cm3/g) (Table 1). The average pore diameter (Dave) in the range of 4.45–4.67 nm suggested the mesoporous nature of adsorbents which is highly desirable for the adsorption of organics in aqueous phase. A comparison of the present and previously reported work is shown in Table 2. There is no consistent trend to be observed. The properties of the adsorbents depend upon the nature of precursor, sludge to impregnation ratio and calcination temperature. The sieve analysis of the synthetic and commercial activated carbons showed that around 50 % of the particles (by weight) were passed through the sieve of 178 lm mesh size, while 40–46 % were in the range of 178–295 lm. The remaining *4–7 % particles were retained on 295 lm mesh. SEM–EDX analysis It is evident from the SEM images that all the synthesized activated carbons possessed porous structure. The visual appearance indicates that the activated carbons prepared by ZnCl2 impregnation have better porosity (Fig. 1a, b) compared to KOH impregnated carbons (Fig. 1c, d). The
circular and oval-shaped pores of varied sizes can be observed in all the synthesized samples though the pores in ZnCl2-treated activated carbons were distributed uniformly. The formation of highly porous structure should be due to the significant reduction of organic and inorganic matter during pyrolysis and washing steps (Wang et al. 2008). On the other hand, CAC was highly porous with almost no spacing between the pores. EDX patterns informed about the presence of metals and non-metals in the prepared adsorbents. In the EDX image of CAC, a high peak of carbon could be seen, while the peaks corresponding to other inorganic species (such as Al, Si and Ca) were relatively extremely small. In the synthesized activated carbons, the significant height of Al, Si and Ca peaks could also be observed. This indicates the presence of aluminosilicates such as illite and feldspar along with silica. In addition, the EDX images of the samples treated by ZnCl2 showed the peaks of Zn and Cl, whereas the peak of potassium could be detected in the KOH-treated samples. The EDX images of various activated carbon samples are presented in the supporting information (Fig. S3). XRD analysis The XRD patterns of various activated carbons are shown in Fig. S4. The mineral phase transformations are expected to occur during the sewage sludge conversion into adsorbents (Ros et al. 2006). The presence of minerals, such as illite (I), feldspars (F) and quartz (Q), was confirmed from the XRD spectra of the adsorbents. Illite and feldspars are formed due to the pyrolysis of sewage sludge-derived adsorbents resulted with calcium, magnesium and iron as exchangeable cations (Seredych and Bandosz 2006). In the XRD spectrum for CAC, a major broad peak was observed showing its amorphous nature though the other wastederived activated carbons have sharp peaks which resemble to the crystalline structure.
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Table 2 Comparison of BET surface area and pore volume of the sewage sludge-derived activated carbon samples with that reported in previously studies References
Activation
Carbonisation
Reagent
Sludge: reagent (by mass)
Temperature (°C)
Properties Time (h)
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm) 18.27
Yu and Zhong (2006)
ZnCl2 and H2SO4
1:2.5
550
2
633.39
0.10
Wen et al. (2011)
ZnCl2
1:1
750
2
509.88
0.301
4.85
Monsalvo et al. (2011)
KOH
1:3
750
0.5
1,832
0.75
–
Lin et al. (2012)
KOH
1:1
600
0.33
130.7
0.13
4
ZnCl2
1:1
124.8
0.12
3.6
Zhuang et al. (2014)
ZnCl2
1:3
700
1
398.6
0.362
3.725
Present study
ZnCl2
1:2
600
1
502.65
0.289
4.58
ZnCl2
1:2.5
510.80
0.297
4.67
KOH
1:2
495.75
0.279
4.45
KOH
1:3
505.65
0.288
4.56
FTIR analysis In the FTIR spectra of different materials (Fig. 2), a strong and broad peak was observed near 3,450 cm-1 wavelength which can be assigned to hydroxyl functional group (Jung and Chun 2009). Some other peaks detected in the region around 1,000 cm-1 and 400–800 cm-1 can be related to the vibration of C–O functional group (Ping et al. 2010). The olefinic C=C stretching band is generally located at around 1,650 cm-1 and its shifting to the lower wavenumbers is possible if it is conjugated with another C=C or C=O bond (Jung and Chun 2009). Based on the observation, it can be summarised that the main surface functional groups such as O–H, C=C, C=O and C–O were found on the surface of commercial as well as synthetic activated carbons. Phenol adsorption studies The performance of various synthetic adsorbents for phenol removal from the synthetic wastewater was determined and compared with that of CAC. Initially, the equilibrium time (the time after which almost no adsorption occurs) for the phenol adsorption was obtained. Determination of equilibrium time All the batch experiments were conducted for a contact period of 3 h to find the equilibrium time for phenol adsorption on various activated carbons. For a typical test, an adsorbent dose of 5 g/L was added to the synthetic phenolic wastewater (initial phenol concentration (C0) = 50 mg/L). The removal of phenol with time during the adsorption process is shown in Fig. 3, and the
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adsorption patterns were similar for all the adsorbents. This can be stated that the adsorption effectively occurred up to 2 h in two phases: fast step during first 30 min followed by slower adsorption rate for 90 min. No significant phenol adsorption could be noted in last 60 min. Based on the above results, the equilibrium time for phenol adsorption could be suggested as 2 h. This result is also in agreement with the earlier investigations (Otero et al. 2003; Srivastava et al. 2006). Moreover, the results suggested that the higher impregnation ratio of activating agent during adsorbent preparation favoured better phenol removal from the synthetic wastewater though the performance was slightly inferior compared to CAC. The result may be attributed to the superior pore and surface characteristics of the adsorbents. A ‘control’ run was also performed to account for any reduction in phenol concentration due to evaporation loss. In the run, no adsorbent was added and the synthetic wastewater was agitated for 2 h duration at a speed of 50 rpm. After the run, there is negligible phenol removal (*1 %) which was not accounted. Effect of shaking speed The effect of shaking speed (50, 100, 120 and 150 rpm) on the adsorption of phenol (C0 = 100 mg/L) was also found. With an increase in shaking speed, meagre enhancement in the equilibrium adsorption uptake was recorded. The phenol adsorption was improved from 8.2 to 8.4 mg/g for CAC with increase in shaking speed from 50 to 150 rpm, while the corresponding phenol uptake on ACZn2 and ACZn2.5 was ranged from 7.1 to 7.4 mg/g and 7.25 to 7.6 mg/g. On KOH-based adsorbents, the phenol equilibrium uptake was also around 7 mg/g. Since there was no significant
Primary sewage sludge-derived activated carbon
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Fig. 1 SEM images of synthetic and commercial adsorbents: a ACZn2, b ACZn2.5, c ACK2, d ACK3 and e CAC
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Transmittance
(e) (d) (c) (b) (a)
0
1000
2000
3000
4000
5000
Wavenumber (cm-1)
Fig. 2 FTIR spectra of synthetic and commercial adsorbents: a ACZn2, b ACZn2.5, c ACK2, d ACK3 and e CAC
from 7.20 to 7.42. The adsorption of phenol on AC can be explained via a ‘‘donor–acceptor complex’’ mechanism in which the carbonyl surface oxygen groups and aromatic ring of phenol act as electron donor and acceptor, respectively (Mattson et al. 1969). In this study, the phenol adsorption was decreased on either side of pH 7. The pKa value for phenol is 9.92, so a major fraction of anionic phenolate ions should be present above this pH. The repulsion between surface layer and anionic ions may have reduced the degree of adsorption. In acidic range (i.e. pH \ 7.0), additional protons are introduced in the solution which will compete with un-dissociated phenol molecules for the carbonyl sites and may lessen the efficiency of adsorption process (Snoeyink et al. 1969). Effect of initial phenol concentration
5
qt (mg/g)
4 ACZn2
3
ACZn2.5
2
ACK2 ACK3
1
CAC
0
0
30
60
90
120
150
180
t (min) Fig. 3 Effect of contact time on phenol adsorption
improvement in phenol adsorption uptake with increase in shaking speed, further adsorption runs were performed at a shaking speed on 50 rpm. Effect of initial pH (pH0) Generally, pH of a solution influences the surface charge of an adsorbent and the degree of ionization of a pollutant. A pH change may result in the dissociation of specific functional groups present on the adsorbent surface, thereby affecting the performance of adsorption process (Srivastava et al. 2006). The removal of various ionic species by adsorption process depends on the competitive adsorption of H? and OH- ions with the adsorbate molecules. Usually, it is observed that the effective adsorption of anions occurs at lower pH due to the presence of H? ions, whereas the cation adsorption is better at higher pH in the excessive OH- ions presence. The adsorption of phenol was increased with change in pH from 3 to 7 and was dropped when the initial pH was higher than 7 (Fig. S5). It should be noted that the pHPZC of the commercial and waste-derived adsorbents was ranged
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The effect of initial phenol concentration on the performance of different adsorbents was determined at an optimum pH of 7.0 by varying the phenol concentration from 50 to 250 mg/L. The percent phenol removal was decreased with increase in initial phenol concentration. The highest phenol removal was achieved with CAC (*90 and 70 % from the wastewater containing 50 and 250 mg/L phenol). However the adsorption capacity for initial phenol concentration of 250 mg/L was the highest (4.5 and 17.3 mg/g for phenol concentrations of 50 and 250 mg/L). The sewage sludge-derived activated carbon samples showed 78–84 % phenol reduction from the wastewater having 50 mg/L phenol (adsorption capacity = 4.0–4.2 mg/g), while the phenol removal was in the range of 56–60 % for phenol concentration of 250 mg/L (adsorption capacity = 14–15 mg/g). According to the results, ACZn2.5 showed the best phenol adsorption among the synthesized samples. The better performance of CAC compared to waste-derived adsorbents can be attributed to the higher surface area and porosity of the former. With an increase in initial phenol concentration, the mass transfer driving force of phenol towards the adsorbent would increase as the phenol molecules pass from the bulk solution to the particle surface (Amin et al. 2012). Hence, the higher phenol adsorption capacity can be expected despite lower percentage of phenol removal. The results are illustrated in Fig. S6 provided in the supporting information. Adsorption isotherms studies The equilibrium phenol concentration (Ce) data at different starting phenol concentrations (C0) were fitted to Langmuir, Freundlich, Temkin and Redlich–Peterson isotherm models. The coefficient of determination (R2) and nonlinear Chi-square test (i.e. v2) was calculated to determine
Primary sewage sludge-derived activated carbon
1627
the best fit isotherm. The experimental data fitting in various models are presented in Fig. 4, while the model parameters are presented in Table 3. Redlich–Peterson isotherm can be suggested the best model based on the lowest v2 values though R2 was near to 1.0 for all isotherm models. According to the results obtained from Langmuir isotherm, the maximum monolayer adsorption capacity was *17–18 mg/g which was found slightly higher than the experimental values. The affinity between phenol and CAC (i.e. KL) was found to be the highest (i.e. 0.065 L/mg) compared to the other waste-derived adsorbents. The dimensionless separation factor (RL) was between 0 and 1 showed the favourable adsorption of phenol on different adsorbents. The 1/n value obtained from Freundlich 20
Adsorption kinetics studies The data of phenol adsorption with time obtained from the equilibrium study were fitted in pseudo first- and secondorder kinetic models. The rate constants were calculated by 20
(a)
(b)
16
12
qe (mg/g)
qe (mg/g)
16
E F
8
L T
4
12
E F
8
L 4
T
RP 0
0
50
100
0
150
RP 0
50
Ce (mg/L) 20
20
(c)
150
(d)
qe (mg/g)
16
12
E 8
F L
4
RP 0
50
100
12
E F
8
L 4
T
0
150
T RP 0
50
100
150
Ce (mg/L)
Ce (mg/L) 20
(e)
16
qe (mg/g)
0
100
Ce (mg/L)
16
qe (mg/g)
Fig. 4 Equilibrium data fitting to the adsorption isotherm models for various adsorbents: a ACZn2, b ACZn2.5, c ACK2, d ACK3 and e CAC (E experimental, F Freundlich, L Langmuir, T Temkin, RP Redlich–Peterson)
isotherm model (ranging from 0.50 to 0.55) confirmed the favourability of phenol adsorption. Redlich–Peterson isotherm combines elements from both Freundlich and Langmuir isotherms. The value of b was obtained between 0.54 and 0.57. The relative adsorption capacity (i.e. KR) and aR suggested the good affinity of phenol towards CAC and ZnCl2 activated adsorbents compared to the other waste-derived activated carbons.
12
E F
8
L 4
T RP
0 0
50
100
150
Ce (mg/L)
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A. Gupta, A. Garg
Table 3 Isotherm parameters for phenol-adsorbent systems Adsorbents
Langmuir isotherm
ACZn2
Q = 16.94; KL = 0.031; v2 = 0.353 Q = 16.67; KL = 0.041;
Kf = 1.524; n = 2;
ACZn2.5
2
v = 0.094 ACK2 ACK3 CAC
Freundlich isotherm
Temkin isotherm
Redlich–Peterson isotherm
Kf = 1.256; n = 1.927;
KT = 0.038; B1 = 4.107;
KR = 6.008; aR = 3.024;
v2 = 0.237
v2 = 0.237
b = 0.539; v2 = 0.074
KT = 0.065; B1 = 4.184;
KR = 6.024; aR = 3.126;
2
v = 0.056
v = 0.367
b = 0.558; v2 = 0.019
Q = 17.24; KL = 0.026;
Kf = 1.083; n = 1.832;
KT = 0.207; B1 = 4.257;
KR = 1.326; aR = 0.712;
v2 = 0.117
v2 = 0.025
v2 = 0.125
b = 0.548; v2 = 0.0094
Q = 18.18; KL = 0.031;
Kf = 1.368; n = 1.938;
KT = 0.262; B1 = 4.26;
KR = 2.008; aR = 1.003;
v2 = 0.109
v2 = 0.252
v2 = 0.126
b = 0.553; v2 = 0.285
Q = 18.18; KL = 0.065;
Kf = 2.08; n = 2.037;
KT = 0.18; B1 = 4.598;
KR = 6.047; aR = 3.145;
2
v = 0.419
2
v = 0.03
linear regression analysis and are reported along with correlation coefficients in Table 4. The pseudo first-order kinetic model exhibited better fit with experimental data compared to the second-order model which is contrary to the previous findings (Otero et al. 2003; Srivastava et al. 2006). The intra-particle diffusion plot drawn between qt versus t0.5 showed two linear segments. That means the film diffusion and intra-particle diffusion mechanisms were functioning simultaneously. For phenol-CAC adsorption process (C0 = 50 mg/L), the value of intra-particle rate constant (kid) was found to be the highest, i.e. 0.425 mg/g min0.5. For the waste-derived adsorbents, kid value was ranged between 0.371 and 0.406 mg/g min0.5. The Boyd plot (Bt vs. t) for various systems showed a straight line with an intercept of magnitude around -0.5. Since the linearized line did not pass through the origin, the external mass transfer (i.e. film diffusion) was rate-controlling step initially and after some time, the intra-particle diffusion controlled the rate of the adsorption process. The results from the kinetic study are provided in supporting information as Fig. S7. Adsorption thermodynamics The adsorption capacity of the various adsorbents was increased with rise in temperature. At higher temperature, the increase in the mobility of phenolate ions and the decrease in retarding force acting on the diffusing ions can be expected. The values of DG°, DH° and DS° are determined using Eqs. 12 and 14. The results from the thermodynamic study are presented in Table 5. The positive DH° indicates that the phenol adsorption onto activated carbon was accompanied by an intake of energy (i.e. endothermic). DH° was the maximum in case of phenol adsorption on CAC (DH° = 22.7 kJ/mol) compared to
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2
2
v = 0.201
b = 0.569; v2 = 0.0095
phenol adsorption on ACZn2.5 (DH° = 17.6 kJ/mol) and ACK3 (DH° = 17.7 kJ/mol). For the other two adsorbents (i.e. ACK2 and ACZn2), the corresponding heat of adsorption was 12.5 kJ/mol and 5.9 kJ/mol. The low DH° value indicates the occurrence of physical adsorption. The positive values of DS° indicate the increase in entropy which was in the range of 0.018–0.065 kJ/mol for phenolactivated carbon systems. The negative free energies suggest the spontaneity and feasibility of the adsorption process. For all waste-derived adsorbents, the positive DG° showed non-spontaneous adsorption process at the ambient temperature (i.e. 20 °C). However, the adsorption process on ACZn2.5 and ACK3 was spontaneous and feasible at higher temperatures (i.e. 40 and 60 °C). On the other hand, phenol adsorption on CAC was spontaneous at all the temperatures.
Regeneration studies The spent adsorbents (i.e. ACZn2 and ACZn2.5) were thermally regenerated and used for phenol adsorption after characterisation. An adsorbent dosage of 5 g/L was used for synthetic phenol wastewater (C0 = 50–250 mg/L). The initial pH of the wastewater was 7.0, and the shaker speed was kept at 100 rpm. After thermal regeneration, the BET surface area of the spent adsorbents ACZn2 and ACZn2.5 was around 53 and 60 % of the surface area of corresponding fresh adsorbents (Table S1 in supporting information). The total pore volume of the spent adsorbents was also reduced significantly. The above factors were responsible for the significant loss in the adsorption capacity of the synthesized adsorbents (only *55–65 % of that obtained with fresh adsorbents). The adsorption isotherms for the fresh and spent adsorbents are shown in Fig. 5.
Primary sewage sludge-derived activated carbon
1629
Table 4 Kinetic parameters for phenol adsorption on SS-derived activated carbon and CAC Adsorbent
ACZn2
ACZn2.5
ACK2
ACK3
CAC
qe,exp (mg/g)
4
4.2
3.9
4.15
4.5
3.775
3.81
3.56
3.64
4.23
K1 (min )
0.021
0.018
0.023
0.023
0.023
R2
0.992
0.984
0.988
0.986
0.993 4.95
Pseudo first order model qe,cal (mg/g) -1
Pseudo second order model qe,cal (mg/g)
4.4
4.54
4.22
4.48
K2 (910-4) (g/mg min)
0.011
0.011
0.014
0.015
0.010
R2
0.928
0.931
0.952
0.961
0.938
0.371 0.996
0.388 0.992
0.375 0.982
0.406 0.977
0.425 0.991
(-0.067)–(1.582)
(-0.055)–(1.448)
(-0.011)–(1.776)
(0.031)–(1.98)
(-0.058)–(1.692)
0.992
0.984
0.988
0.992
0.994
Intra-particle diffusion model kid (mg/g min0.5) R2 Boyd model Bt = (-0.497)–ln (1-f) R
2
Table 5 Thermodynamic parameters for phenol-adsorbent systems
Adsorbents
DH° (kJ/mol)
DS° (kJ/mol K)
DG° (kJ/mol) 20 °C
ACZn2
5.944
0.018
40 °C
60 °C
0.533
0.466
-0.185 -1.093
ACZn2.5
17.625
0.056
1.104
-0.184
ACK2
12.529
0.037
1.574
1.051
0.119
ACK3
17.709
0.057
0.872
-0.093
-1.368
CAC
22.713
0.065
-0.789
-1.996
-3.923
Conclusions
14 12
qe(mg/g)
In the present study, a comparison of the characteristics between the sewage sludge-based adsorbents and CAC was made. The performance of different activated carbons for the phenol adsorption from synthetic wastewater was also investigated. The BET surface area and total pore volume of the waste-derived activated carbon samples (activated by ZnCl2 and KOH) were comparable to that of CAC (i.e. *500 m2/g and 0.3 cm3/g, respectively). The SEM images of the various adsorbents showed the presence of porous structure which is desirable for the adsorption process. FTIR analysis showed the presence of various functional groups such as carbonyl, ether and alcoholic which favours the adsorption of organic pollutants. The maximum phenol removal using the synthesized adsorbents was found to be in the range of 78–84 % compared to 90 % phenol removal using CAC at around neutral pH (which is close to the pHPZC). Among all isotherms, Redlich–Peterson isotherm exhibited the best fit to the adsorption equilibrium data for all the adsorbents (based on v2 value). The phenol adsorption time-based data
16
10 8 6
Fresh ACZn2
4
Fresh ACZn2.5
2
Regenerated ACZn2
0
Regenerated ACZn2.5
0
50
100
150
200
Ce (mg/L) Fig. 5 Adsorption isotherms of fresh and regenerated adsorbents
were fitted well in the pseudo first-order kinetic model. Film diffusion and intra-particle diffusion were found to be the rate-controlling steps during phenol adsorption. The thermodynamic studies suggest that the phenol removal occurred due to physical adsorption and it was endothermic
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process. The change in free energy shows that the adsorption on ACZn2.5 and ACK3 was spontaneous and feasible at elevated temperatures (at 40 and 60 °C). The thermal regeneration of the spent adsorbent caused significant reduction in the BET surface area, total pore volume and the maximum phenol adsorption capacity. Nevertheless, the study clearly demonstrates that the sewage sludge-based adsorbents can be a potential alternative to the commercially available activated carbon. From an economic point of view, the cost of the non-renewable raw material (such as coal) is eliminated by utilizing sewage sludge thus reducing the overall adsorption costs. However, the regeneration potential of such materials needs to be studied in detail and should be considered in the overall economics. Apart from this, the metal leaching in wastewater after the adsorption process should be studied in the future studies. Acknowledgments The authors would like to thank Sophisticated Analytical Instrument Facility (SAIF), Department of Chemical Engineering and Material Sciences and Metallurgical Engineering (MEMS) of Indian Institute of Technology (IIT) Bombay, Mumbai, India for their help in the analysis of adsorbents.
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