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Journal of Molecular Liquids 232 (2017) 367–381
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Comparing adsorption properties of NH4Cl-modified activated carbon towards chlortetracycline antibiotic with those of commercial activated carbon Ahmad Alahabadi a, Ahmad Hosseini-Bandegharaei a,b,⁎, Gholamreza Moussavi c, Bahareh Amin d, Ayoob Rastegar a, Hamidreza Karimi-Sani a, Mojtaba Fattahi d, Mohammad Miri a a
Wastewater Division, Faculty of Health, Sabzevar University of Medical Sciences, PO Box 319, Sabzevar, Iran Department of Engineering, Kashmar Branch, Islamic Azad University, PO Box 161, Kashmar, Iran c Department of Environmental Health Engineering, Tarbiat Modares University, Tehran, Iran d Faculty of Medicine, Sabzevar University of Medical Sciences, Sabzevar, Iran b
a r t i c l e
i n f o
Article history: Received 10 September 2016 Accepted 8 February 2017 Available online 27 February 2017 Keywords: Adsorption Chlortetracycline NAC SAC Adsorption properties
a b s t r a c t In this work, kinetics, equilibrium, thermodynamic and other adsorption properties of a novel activated carbon with regular-shaped pores (NH4Cl-modified activated carbon, NAC) for removal of chlortetracycline antibiotic from aqueous solutions was compared with those of a commercial standard activated carbon (SAC). Effects of temperature, on chlortetracycline adsorption by both adsorbents were carefully investigated and compared. Then optimum conditions were used for comparing the adsorption properties of adsorbents. Among the isotherm models studied, the equilibrium experimental data of both activated carbons were followed by Redlich-Peterson model, and the parameters calculated from the studied models showed more favourability of chlortetracycline adsorption onto NAC via the physical π-π electron-donor-acceptor interaction. The experimental data were fitted to several kinetic models and, based on calculated respective parameters such as rate constants, equilibrium adsorption capacities, correlation coefficients and statistical indices, both the Ritchie second order and pseudo second-order model showed the best in describing the adsorption process for both activated carbons, and the kinetic performance of NAC was significantly higher than that of SAC. The calculated thermodynamic parameters of the adsorption process revealed that, under the conditions used in this study, the adsorption of chlortetracycline onto NAC is more feasible, spontaneous and endothermic than its adsorption onto SAC. Overall, the results indicated that, compared to SAC, NAC is a more suitable choice for removal of chlortetracycline antibiotic from aqueous solutions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, the consumption of pharmaceuticals, including antibiotics, is being continuously increased worldwide and, since these chemicals are often released directly into the aquatic environment without change or as their metabolic compounds, they are regarded as a new emerging class of harmful organic environmental pollutants [1– 4]. Because of low removal of some pharmaceutical compounds at usual water and wastewater treatment plants and their strong hydrophilic character, some pharmaceuticals have even reached drinking water and groundwater sources, and many studies have revealed their
⁎ Corresponding author at: Wastewater Division, Faculty of Health, Sabzevar University of Medical Sciences, PO Box 319, Sabzevar, Iran. E-mail address:
[email protected] (A. Hosseini-Bandegharaei).
http://dx.doi.org/10.1016/j.molliq.2017.02.077 0167-7322/© 2017 Elsevier B.V. All rights reserved.
presence in such samples [5–9]. For instance, ineffective removal of antibiotics by conventional wastewater treatment processes and their presence in surface and ground waters has raised a great concern about their potential impacts on the environment and public health, because they can cause adverse effects such as biological toxicity and the development of resistant genes in human pathogens [10–12]. Therefore, the development of efficient technologies for removing antibiotics is of environmentally critical importance, and different novel technologies, like advanced oxidation processes [13], osmosis [14], nanofiltration [15] or adsorption [16–19], are under discussion to remove antibiotics from waters and wastewaters. Among these mentioned technologies, adsorption appears to be the superior strategy for the removal of pharmaceuticals and other organic pollutants, because of its cost-effectiveness, simplicity and other advantages which are useful for efficient removal of pollutants from water sources [16–34]. In the last decades, due to porosity, specific surface area and high adsorption capacity, large efforts have been made to synthesize different
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Nomenclature
α
area of plunger surface (m2) Redlich-Peterson empirical constant average relative error (%) Tempkin constant related to the heat of adsorption Langmuir constant related to the free energy of adsorption (L mg−1) Langmuir constant related to the free energy of adsorpbM tion (L mol−1) equilibrium concentration of the adsorbate in the bulk Ce solution (mg L−1) E mean adsorption energy estimated from DubininRadushkevich (J mol−1) Bangham constant (mL g−1 L−1) K0 distribution coefficient (mL g−1) Kd Freundlich constant indicative of the relative adsorption Kf capacity of the adsorbent (mg1−(1/n) L1/n g−1) film diffusion rate constant Kfd intra-particle diffusion constant (mg g−1 min−1/2) Kid Redlich-Peterson affinity constant (L mg−1) KR-P equilibrium binding constant, Tempkin-Pyzhev conKT-P stant (L g−1) pseudo-first order rate constant (min−1) k1 pseudo-second order rate constant (g mg−1 min−1) k2 Ritchie second order rate constant (min−1) kR I intercept in the intraparticle diffusion model (mg g−1) L sample height (m) m adsorbent dose, weight of adsorbent per liter of solution (g L−1) N number of measurements n Freundlich constant indicative of the heterogeneity factor Redlich-Peterson constant indicative of the heterogenenR-P ity factor maximum adsorption capacity based on Dubininq0 Radushkevich model (mol g−1) amount of adsorbate adsorbed per unit weight of adsorqe bent at equilibrium (mg g−1) theoretical qe values obtained from the kinetic or isoqe.cal therm models(mg g−1) experimental qe values (mg g−1) qe.exp maximum adsorption capacity; Langmuir constant qmax (mg g−1) qmax,exp maximum experimental adsorption capacity (mg g−1) amount of adsorbate adsorbed at any time t (mg·g−1) qt R universal gas constant (J mol−1 K−1); electrical resistance (Ω) correlation coefficient R2 mean radius of the adsorbent particles (m) r0 R% removal efficiency (%) dimensionless separation factor RL RMSE root mean square error (%) T temperature (K) t time (min) time for half adsorption of adsorbate onto the adsorbent t1/2 particles (min) V solution volume (L or mL) W weight of adsorbent (mg) Greek letters ΔG° Gibb's free energy change (J·mol−1) ΔH° enthalpy change (J·mol−1) ΔS° entropy change (J·mol−1·K−1) Δq% normalized standard deviation (%)
β
A aR-P ARE B b
δ σ ε
Elovich constant indicative of the initial adsorption rate (mg g−1 min−1) Elovich constant indicative of the desorption constant (g mg−1) Dubinin-Radushkevich constant related to the adsorption energy (mol2 J−2) electrical conductivity (S m−1) Polanyi potential
types of activated carbon for removing environmental pollutants from waters and wastewaters [34–38]. In addition, to reduce the costs and optimization of adsorption capacity, various modification methods, such as chemical treatment by potassium hydroxide, zinc chloride, sulfuric acid, sodium hydroxide, etc., have been utilized for improving the performance of activated carbon samples [39,40]. However, the synthesized activated carbons usually consist of highly irregular-shaped pores and closed structures which can cause low adsorption capacities and/or slow adsorption kinetics. Recently, our research group has devised and efficient procedure for chemical modification of activated carbon using ammonium chloride which is of immense value for this purpose and our previous studies have proved its superiority for improving the adsorption properties of activated carbon in the removal of some organic pollutants from aqueous solutions [41–45]. The results of previous works indicated that, in addition to other advantages, the explosive properties of ammonium at high temperatures make parallel and long channels in the structure of activated carbon, which can cause the better availability of carbon surface for adsorption process [45]. From the other hand, such regular-shaped pores in NH4Cl-modified activated carbon (NAC) along with its high carbon content can result in large graphitic sheets which are of great ability to involve in strong interactions, such as π-π electron-donor-acceptor interactions, with highly aromatic pollutants [46,47]. As mentioned above, introducing advantageous adsorbents for pharmaceuticals and throwing light on their adsorption properties is of great importance from environmental protection point of view. Therefore, in the follow up of our previous group works and owning to the specific properties of NH4Cl-modified activated carbon (NAC), the performance of this novel activated carbon was evaluated for removal of antibiotic chlortetracycline (CTCN, structure shown in Fig. 1) as a representative of highly aromatic pollutants. The adsorption properties of NAC were compared with those of commercially standard activated carbon (SAC), using batch adsorption experiments. The adsorption mechanism of this new pollutant was clarified by performing more characterization studies like XRD patterns and electrical conductivity. In addition, the insight on the difference between the performance of NAC and SAC for this new purpose was gained by evaluating the effects of various physicochemical factors, such as pH, agitation speed, sorbent dose, contact time and temperature, and kinetics, equilibrium, thermodynamic and other adsorption properties of both activated carbons towards chlortetracycline were carefully investigated.
Fig. 1. Chemical structures of CTCN.
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2. Experimental 2.1. Materials All chemical reagents used in this study, unless otherwise specified, were purchased from Merck (Darmstadt, Germany) and were of AR grade. Chlortetracycline (≥99%, Sigma–Aldrich Co., St. Louis, MO, USA) was used as received. The NH4Cl-modified activated carbon (NAC) was synthesized from dried pomegranate wood according the procedure introduced in our earlier group works [42–45]: After heating the small particles of wood at 700 °C for 1 h under N2 atmosphere, the obtained carbonized granules were soaked in the 2.0 w% NH4Cl solution for 24 h and then oven-dried at 105 °C. The pre-treated granules were activated by oven-heating at 800 °C for 2.5 h under N2 atmosphere and, then, the performance of granules with the mesh size of 16–20 for sorption of CTCN from aqueous solutions was compared with that of standard activated carbon (SAC; Merck) of the same size. All dilutions throughout this work were made by utilizing highly pure deionized water (Milli-Q Millipore, 18.2 MΩ cm−1 resistivity). All the working solutions were prepared by diluting the stock solution of CTCN (500 mg L−1) which was prepared daily by dissolving the appropriate amounts of antibiotic in deionized water. To protect CTCN from photodegradation, the stock solution was well sealed and wrapped with aluminium foil, and stored in a cool and dark place prior to use. The 1.0 M HCl or 1.0 M NaOH solution was used for adjusting CTCN solutions. 2.2. Apparatus The laboratory glassware was kept overnight in a 5% (v/v) nitric acid solution and then was rinsed thoroughly with deionized double distilled water. A PHS-3BW Model pH-meter (Bel, Italy) with a combined glass– calomel electrode was employed for measuring pH values in the aqueous solutions. A Gallenkamp automatic shaker model BKS 305-010, UK, was used for the batch experiments. The activated carbon samples were gold-palladium sputter coated by a sputter coater instrument (Model SC 7620) and, subsequently, scanning electron microscopic (SEM) micrographs were taken by a VEGA//TESCAN instrument at an accelerating voltage of 15 kV. X-ray diffraction (XRD) patterns in the Bragg's angle (2θ) range from 10 to 80° were recorded in a Philips instrument (Model: X'Pert MPD) with steps of 0.02°/s. Two copper made plungers and a digital multimeter (DM-442B Model, Korea) were used to measure the electrical conductivity of the samples. The concentration of remained chlortetracycline in the aqueous phase after suitable was determined using a high performance liquid chromatography (HPLC) instrument (Waters, USA) with a 996 photodiode array detector set at 254 nm and a Nova-Pak C18 column, using a mixture of 0.01 mM phosphoric acid and methanol (60/40 vol.) as the mobile phase at a flow rate of 1 mL min−1 (see supplementary data). 2.3. Adsorption experiments All adsorption experiments in this study were conducted in 250-mL Erlenmeyer flasks at a known pH and constant temperature, using batch method. In a typical experiment, a certain portion of NAC or SAC was placed in an Erlenmeyer flask containing 100 mL solution of known CTCN concentration and agitated at the shaking speed of 150 rpm, using a reciprocating shaker. After a prescribed time period, the solution was filtered and the remained concentration of antibiotic was measured carefully. The effect of various variables, including pH (2−10), adsorbent dose (0.05–0.50 g L−1), contact time (2–60 min), initial concentration of CTCN (20–200 mg L− 1), and temperature (283–323 K) were investigated in such a way that the considered factor changed and all other factors were kept constant. Isotherm and thermodynamic experiments were performed at five different temperatures of 283, 293, 303, 313 and 323 K. Kinetic experiments were performed at four
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different CTCN concentrations of 50, 75, 100 and 150 mg L−1 at 293 K. The adsorption capacity of the adsorbent at any time (qt, mg g−1) and at equilibrium (qe, mg g−1), the removal percentage (R%) and the distribution coefficient (Kd, mL g− 1) were computed from the following equations: qt ¼
ðC0 −Ct ÞV W
ð1Þ
qe ¼
ðC0 −Ce ÞV W
ð2Þ
R% ¼
Kd ¼
ðC0 −Ce Þ 100 Ce
ð3Þ
ðC0 −Ce Þ V Ce W
ð4Þ
In the above equations qt (mg g−1) is the amount of CTCN adsorbed onto the activated carbon at time ‘t’, qe, (mg g−1) is the amount of CTCN antibiotic adsorbed onto the activated carbon at equilibrium, qmax,exp. (mg g−1) is the amount of CTCN antibiotic adsorbed onto the activated carbon after several adsorption equilibrium cycles,C0 (mg L−1) is the initial concentration of CTCN in the aqueous phase, Ct (mg L−1) is the CTCN antibiotic concentration remaining in the aqueous phase at time ‘t’, Ce(mg L−1) is the equilibrium concentration of CTCN in the solutions, V (L; mL) is the volume of the solution and W (g) is the weight of the activated carbon used in the adsorption experiments. Comparison of different models and goodness-of-fit evaluation in the kinetic and isotherm studies was performed by fitting experimental data with different models and exploiting correlation coefficient (R2) and several statistical indices. These statistical indices, including normalized standard deviation (Δ q(%)),Chi-square (χ2), average relative error (ARE(%)) and root mean square error (RMSE), were calculated using the following relations and their values should be as close to ‘zero’ as possible: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2 u u 1 N q −q exp cal t Δqð%Þ ¼ 100 ∑ qexp N−1 i¼1
ð5Þ
i
0 2 1 B qexp −qcal C χ ¼ ∑@ A qcal i¼1 2
N
AREð%Þ ¼
100 N qexp −qcal ∑ qexp N−1 i¼1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 1 N RMSE ¼ ∑ qexp −qcal i N i¼1
ð6Þ !2 ð7Þ i
ð8Þ
where qexp and qcal respectively are the experimental value and the calculated value of the adsorption capacity of each activated carbon for chlortetracycline at time ‘t’ or equilibrium concentration ‘Ce’ and N is the number of measurements made. 3. Results and discussion The main objective of this study is to compare the validity of NAC with that of SAC in the removal of chlortetracycline antibiotic from aqueous solutions, which was examined by batch sorption experiments. To ensure repeatability, each adsorption experiment was replicated three times and the results were averaged. The obtained relative standard deviations (RSDs, n = 3) for the experiment conducted in this work were lower than 3.9%.
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3.1. Characterization studies The earlier works exhibited that NH4Cl-modified activated carbon (NAC) can be considered as an excellent adsorbent for some organic pollutants in terms of high sorption capacity, cost, rapid adsorption, etc. [42–45]. According to the characteristic data of NAC and the Merck traditional standard carbon (SAC) which are reported in Table 1, the newly introduced activated carbon (NAC) is of a high specified surface area (1029 m2 g−1) and mean pore diameter of 2.46 nm, and it can be categorized as an mesoporous adsorbent, according to the IUPAC size classification in which microporous materials have pore diameters of b2 nm, mesoporous materials have pore diameters of 2–50 nm and macroporous materials have pore diameters of N50 nm. In addition to higher carbon content of NAC (93.4%) respect to SAC (75.0%), as can be observed from SEM micrographs in Fig. 2, characterization studies in the earlier works have indicated that the form of NAC is like compressed and porous fibers with a series of parallel and long channels, as compared with SAC which its porous structure includes scattered holes and smooth surface [42,45]. Although the higher carbon content of NAC is a clear reason for superiority of its adsorption properties over the traditional standard carbon, the specific porosity structure of NAC, which is resulted from the explosive properties of ammonium at high temperatures [42–45], makes its surface more available and, as mentioned earlier, large graphitic sheets in its structure may increase its adsorption performance towards organic species. However, to ensure from larger graphitic sheets and more crystallinity in the NAC structure, further characterization studies were performed, including X-ray diffraction (XRD) patterns and electrical conductivities. 3.1.1. X-ray diffraction (XRD) patterns Traditional carbon samples in general possess amorphous pore walls. The production of porous carbons with regular-shaped pores and higher percent of graphitization (more crystalline pore walls) are very desirable for many purposes. The powder X-ray diffraction (XRD) patterns of both SAC and NAC were recorded and are shown in Fig. 3. The patterns exhibit two peaks at 2θ = 26 and 43 which are respectively related to (002) and (101) diffraction peaks from graphitic pore walls [48]. As can see from Fig. 3, the more intensive and sharper peaks of NAC indicate that this sample possess more graphitic pore walls (and higher crystallinity) than SAC which has more amorphous pore walls. These results confirm that chemical activation of carbon samples with ammonium chloride bring up generating highly ordered pores with more crystallinity, probably because of its explosive properties. 3.1.2. Comparing electrical conductivity of SAC and NAC The DC electrical conductivity (σ) was measured using the fourprobe method utilized by Sa´nchez-Gonza´lez et al. [49]. For this purpose, 0.1 g activated carbon dried over night at 100 °C and was compressed in a hollow PVC cylinder with an inner diameter of 1.0 cm between two copper made plungers at suitable compression pressure of 756.2 kPa which is high enough to get a good electrical contact between the sample particles and the plungers and, on the other hand, is
Table 1 Characteristic data of the NAC and SAC. Parameter
NAC
SAC
Carbon content (%) Surface area (m2 g−1) Total pore volume (P/P0 = 0.990; cm3 g−1) Monolayer volume (Vm; cm3 g−1) Mean pore diameter (nm) BET C constant pHzpc
93.4 1029 0.633 236.4 2.46 1088.7 7.4
75.0 1024 0.572 235.5 2.23 691.8 6.6
Fig. 2. SEM micrographs of SAC and NAC.
not too high to cause the crushing and breaking of the carbon granules [50]. The conduction was considered to be ohmic in nature and the electrical conductivity (σ; S·m−1) was calculated by the following relation [51]: σ¼
L RA
ð9Þ
where R is the electrical resistance (Ω), A is the area of the plunger surface (m2) and L is the sample height (m). The electrical conductivity of both SAC and NAC samples were determined at room temperature and a same compression pressure (756.2 kPa) to be 155.1 and 196.7 S·m−1, respectively. Since both SAC and NAC are almost of the same porosity, the greater electrical conductivity of NAC can be mainly assumed to be due to its higher graphitization degree and, hence, more sp2 carbon structures in its matrix. 3.2. Effect of pH From the basic science point of view, the pH-induced changes in both the physiochemical properties of adsorbate and electrostatic
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Fig. 3. XRD patterns of SAC and NAC.
charge of adsorbent surface can involve in the pH-dependent interaction of adsorbate with the adsorbent. Therefore, investigation of effect of pH on the performance of both NAC and SAC against the removal of chlortetracycline is of critical importance. For this purpose, the chlortetracycline solutions with different pHs in the range of 2–10 and initial concentration of 100 mg L−1 were contacted for 60 min at 293 K, and the effect of pH on the adsorption capacity of both NAC and SAC was shown in Fig. 4 which indicates that the adsorption process of chlortetracycline onto both activated carbons is clearly pH-dependent. It was found that as the pH value increases the adsorption capacities for
371
antibiotic increase, and the maximum adsorption capacities of 204.83 and 356.41 mg L− 1, respectively for SAC and NAC, were obtained in the pH range of 5.5–6.5 in which the majority of chlortetracycline molecules are present as zwitterions. As mentioned above, the pH values of working solutions affect the chemistry of both antibiotic molecule and the surface of activated carbons. At low pHs, in addition to protonation of tetracycline molecules, the charge of carbons surface is positive and, therefore, an electrostatic repulsion can be happened between the positively charged adsorbent surface and the positively charged chlortetracycline. Also, the competition between H+ and antibiotic ion for the limited active surface sites lead to poor interaction between antibiotic and carbon surface and diminishes adsorption capacities. By decreasing the solution acidity, the number of H+ ions and positive charge of carbons surface decrease drastically and the pH values become closer to the pHZPC values of both NAC and SAC (Table 1), in which the zeta potential of activated carbons are zero, so higher adsorption capacities are resulted due to more interaction between antibiotic and adsorbent surface at higher pH values. However, at the very high pHs, the dissociation of antibiotic molecule converts it to anionic molecule and, therefore, the weak interaction between the adsorbent surface and the negatively charged chlortetracycline molecules as well as the competition between OH− and antibiotic anion for the limited active surface sites again result in poor adsorption capacities. Overall, based on the obtained results from the study of pH effects, it can be concluded that adsorption affinity is almost correlated with neutrality of carbons surface and chlortetracycline molecule. Therefore, because chlortetracycline molecule has multiple groups/moieties (phenol, amino, chloride, alcohol, enone) that are capable of electronic coupling, and since this antibiotic is a quinine containing structure which is π-electron-acceptors [52], the adsorption affinity of carbons surface towards chlortetracycline molecule can be attributed to the physical π-π electron-donor-acceptor interactions which can be existed between the multiple functionalities of chlortetracycline molecule and the corresponding structures on the surface of both activated carbons. In the usual granular porous materials, there are many transitional pores which connect the outer pores to the pores existing in the inner parts of particles body. These transitional pores, especially those of micro size, can limit the accessibility of inner surfaces for adsorption of large-molecule pollutants. From this point of view, the parallelshaped pores existing in the structure of an engineered porous material can partially overcome the limitations of pollutant adsorption onto the inner surfaces of adsorbent particles, depending on the several conditions such as size of pollutant molecule and adsorption mechanism. Therefore, since both the NAC and SAC are of the same mesh size of 16–20, the difference between the external surfaces of these two carbons is not too high, and the higher adsorption capacity of NAC in Fig. 4 can be mainly attributed to (i) more crystallinity and its higher graphitization degree which poses more adsorption sites for π-π electron-donor-acceptor interactions and (ii) its regular-shaped pores which simplify the accessibility of its inner surfaces for CTCN adsorption. 3.3. Effect of ionic strength
Fig. 4. pH-dependency of CTCN removal by SAC and NAC (Volume, 100 mL; activated carbon dose, 0.020 g; initial concentration, 100 mg L−1; contact time, 60 min; temperature, 293 K; agitation speed, 150 rpm).
The influence of ionic strength on the removal of chlortetracycline was surveyed at the presence of sodium nitrate within the concentration range of 0.0–0.3 M. For this purpose, 100 mL aliquots of CTCN solution (different ionic strengths, pH 6.0) having a concentration of 50 mg·L−1 were equilibrated with 0.02 g of activated carbon samples at 293 K. The effect of sodium nitrate on the removal percentages obtained by both NAC and SAC are shown in Fig. 5 which indicates that, with increasing ionic strength, adsorption decreases almost significantly. This behavior is almost predictable since by increasing the ionic strength hydrophobic interactions between the antibiotic molecules increase and result in aggregation of the CTCN molecules. As a result of aggregation, the access of CTCN molecules to the adsorption sites in the
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Fig. 5. Effect of ionic strength on the CTCN removal by SAC and NAC (Volume, 100 mL; activated carbon dose, 0.020 g; initial concentration, 50 mg L−1; contact time, 60 min; temperature, 293 K; agitation speed, 150 rpm).
pores is limited and, depending to the ionic strength of solution, the adsorption diminishes to lower levels. 3.4. Effect of agitation speed Since the increase of shaking speed decrease the thickness of the boundary film surrounding adsorbent particles and diminish the resistant offered by film diffusion, the effect of agitation speed on the removal process was investigated by conducting sorption experiments at varying shaking speeds of 0–250 rpm. The results showed that the increase of agitation speed up to 150 rpm increased the removal percentage for both SAC and NAC, and thereafter the increase in the removal efficiency was marginal. Therefore, shaking speed of 180 rpm was used for further experiments. 3.5. Effect of adsorbent dose Since increasing the NAC dose increases the number of available adsorption sites, higher doses would result in higher removal percentages (R%) of chlortetracycline. Therefore, to maximize the interactions between the CTCN antibiotic and adsorption sites of the activated carbon in the solution media, and gaining higher extent of removal, investigation on the effect of NAC and SAC dose is important. Therefore, the effect of activated carbon dose for chlortetracycline adsorption was investigated using various amounts of NAC and SAC from 0.05 to 0.50 g L−1 and 100 mL-portions of 100 mg L−1 antibiotic solution at pH 6.0 and 293 K. The samples were agitated by a reciprocating shaker at 150 rpm and, after 100 min of contact time, the removal efficiencies for both carbons were calculated and summarized in Fig. 6 which shows that the removal percentage of CTCN antibiotic is largely dependent on the carbon dose, so that the removal efficiency of chlortetracycline increases by increasing the activated carbons dose. This trend can be explained as the activated carbon dose increases, the available surface also increases, facilitating more adsorption sites for involving in adsorption process. However, as the activated carbon dose increases, the antibiotic/adsorbent ratio reduces, resulting in less adsorption capacity. The results also show that, compared to the SAC, a small amount of NAC has ability to remove large amounts of chlortetracycline antibiotic, indicating that NAC is a good potential candidate of chlortetracycline removal for use in a full-scale treatment plant. Anyway, the dose of 0.20 g L−1 was used for further investigations on both activated carbons.
Fig. 6. Effect adsorbent dose on the CTCN removal by SAC and NAC (Volume, 100 mL; pH, 6.0; initial concentration, 100 mg L−1; contact time, 60 min; temperature, 293 K; agitation speed, 150 rpm).
3.6. Effect of initial CTCN concentration The mass transfer resistances existing between the solution and adsorbent phases can be affected by the initial concentration of adsorbate and, therefore, effect of chlortetracycline initial concentration (in the range of 20–200 mg L−1) on the removal percentage and adsorption capacity of both NAC and SAC was studied at pH value of 6.0 and 293 K. The results showed that, for both activated carbons, the antibiotic removal percentages decrease with increasing initial concentration (Fig. 7), while the adsorption capacities significantly increase. These results indicate that the initial concentration of chlortetracycline provides an important driving force in the adsorption process, and the higher adsorption capacities at the higher initial concentrations may be due to the higher ratio of the initial number of moles of chlortetracycline to the available adsorption sites existing on the surface activated carbons. It should be mentioned that the high removal percentages obtained in the case of NAC use, especially in the lower initial antibiotic concentrations, is of great practical importance for using this activated carbon as
Fig. 7. Effect of initial concentration on the removal extent of CTCN from aqueous solutions by SAC and NAC (volume, 100 mL; pH, 6.0; activated carbon dose, 0.020 g; contact time, 60 min; temperature, 293 K; agitation speed, 150 rpm).
A. Alahabadi et al. / Journal of Molecular Liquids 232 (2017) 367–381
adsorbent for the removal of chlortetracycline in the full-scale treatment plants.
3.7. Effect of contact time Contact time usually plays an important role in the adsorption of an adsorbate onto adsorbents, and its effect is dependent on the adsorbate type, nature of used adsorbent and the adsorption mechanism. Thus, investigation of the effect of contact time is of critical importance to compare the efficiency and feasibility of NAC with SAC for practical use in chlortetracycline removal from aqueous solutions. For this purpose, 100 mL-portions of solutions containing different initial concentrations of antibiotic ranging from 50 to 150 mg L−1 at pH 6.0 and 293 K were contacted with 20 mg of NAC or SAC, and the results were reported as Fig. 8. As can be seen from this figure, the removal rate is rapid in initial stages for both activated carbons and, during the removal of chlortetracycline by the adsorption onto NAC and SAC surface, the adsorption capacities respectively increased up to 35 and 50 min and then they became constant, and the equilibrium times are almost independent of the initial concentration of antibiotic. It should be mentioned that
373
the faster rate of chlortetracycline adsorption onto NAC surface is due to better accessibility of its surface, which is resulted from its parallelshaped pores, and the stronger interaction between antibiotic molecules and the adsorption sites existing on the surface of this adsorbent. 3.8. Effect of temperature and maximum experimental capacities The maximum experimental adsorption capacity (qmax,exp) of an adsorbent is is of critical importance for industrial plants and, therefore, the maximum experimental adsorption capacity at different temperatures was determined for both activated carbons. For this purpose, 10 mg of each activated carbon was equilibrated with the 100-mL aliquots of CTCN solution (100 mg L−1, pH 6.0) at known temperatures, repeatedly till the saturation of activated carbon was arrived. In each cycle, after agitating the mixture at 150 rpm for 100 min, the aqueous phase was analyzed for determining the adsorbed amount of CTCN antibiotic and, then, the activated carbon particles were further equilibrated with fresh antibiotic solution. This process was repeated several times till the activated carbon was fully saturated and, finally, the maximum experimental adsorption capacity (qmax, exp) was calculated by the formula given in the following equation: q max; exp ¼ ∑
ðC0 −Ce ÞV W
ð10Þ
where the summation is carried out over all the repeated cycles until saturation of the carbon particles was achieved. The maximum experimental adsorption capacities (qmax,exp) for both activated carbons at 283, 293, 303, 313, and 323 K were indicated in Fig. 9 which shows that the qmax,exp values of NAC at all the studied temperatures are significantly higher than those of SAC, indicating the advantage of NAC for using in CTCN adsorption systems. Also, these results indicate that the adsorption of CTCN antibiotic onto both NAC and SAC is more favourable at higher temperatures, and the adsorption process is endothermic. 3.9. Adsorption equilibrium In all adsorption systems, equilibrium state is achieved after a necessary period of contact time when adsorbate molecules are settled onto adsorbent surface. At the equilibrium conditions, the equilibrium capacity (qe) of the adsorbent is a function of equilibrium concentration (Ce)
Fig. 8. Effect of contact time on adsorption of CTCN onto the SAC and NAC at different initial concentrations (volume, 100 mL; initial concentration, 50, 75, 100 and 150 mg L−1; activated carbon dose, 0.020 g; temperature, 293 K; agitation speed, 150 rpm).
Fig. 9. The maximum experimental adsorption capacities of NAC and SAC at different temperatures.
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and equilibrium temperature (T), i.e. qe (Ce, T). When the adsorption process is conducted at a constant temperature, the change in qe against the value of Ce is called the adsorption isotherm, i.e. qe (Ce) at constant T [53]. Since investigation of the characteristics of adsorption isotherm is of great importance in the development of any system design that involves adsorption processes, study of adsorption isotherms for both CTCN/NAC and CTCN/SAC systems was performed at different temperatures of 283, 293, 303, 313 and 323 K. For this purpose, equilibrium studies were conducted by contacting the NAC and SAC with antibiotic solutions (pH 6.0) with various initial concentrations (0–200 mg L−1) and shaking the mixtures at 150 rpm for 60 min, and the adsorption isotherms of CTCN antibiotic at five different temperatures were shown in Fig. 10 and 1S–4S. To find the best isotherm model for interpreting the experimental data in these figures, five well-known adsorption isotherms viz. Langmuir [54], Freundlich [55], Tempkin-Pyzhev (T-P) [56], Dubinin–Radushkevich (D-R) [57] and Redlich-Peterson (R-P) [58] isotherm were used to fit the experimental data and describe the adsorption mechanism obeyed in antibiotic removal by NAC and SAC. The Langmuir isotherm model is generally utilized to calculate the maximum adsorption capacity corresponding to the complete monolayer coverage of a homogenous adsorbent surface without any interaction between the adsorbed pollutant molecules. The Langmuir model is commonly represented by the following equation: qe ¼
qmax bCe 1 þ bCe
ð11Þ
where Ce (mg L−1) is the equilibrium concentration of pollutant, qe (mg g−1) is the adsorption capacity of the adsorbent at the equilibrium, qmax (mg g−1) is the theoretical maximum capacity, and b (L mg−1) is the Langmuir constant related to the affinity of adsorption sites. The Freundlich isotherm assumes that the adsorption process is non-ideal, reversible and multilayer. The Freundlich equation can be given by the following relation: qe ¼ K f C1=n e
ð12Þ
where KF(mg1−(1/n)·L1/n·g−1) is roughly an indicator for the adsorption capacity and n is related to the adsorption intensity which has a numerical value and varies with heterogeneity. The magnitude of n for a favourable adsorption is greater than unity. The Tempkin-Pyzhev (T-P) isotherm model assumes that the heat of adsorption of all the adsorbates in the layer decreases linearly with
coverage due to adsorbent–adsorbate interactions and, also, the adsorption process is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The T-P isotherm is given as Eq. (13): qe ¼ BlnKT−P Ce
ð13Þ
where KT (L g−1) is the equilibrium binding constant corresponding to the maximum binding energy and constant B is related to the heat of adsorption (B = RT/b). The Dubinin–Radushkevich (D-R) isotherm is generally applied to express the adsorption mechanism with a Gaussian energy distribution onto a heterogeneous surface and assumes that the characteristics of adsorption curves relate to the porous structure of the adsorbent. The non-linear form of isotherm is generally expressed by the following equation: 2
qe ¼ ðq0 Þexp−δε
ð14Þ
where qe (mg g−1) is the amount of CTCN adsorbed on the adsorbent at equilibrium, q0 (mg g−1) is the theoretical saturation capacity based on the D-R isotherm, δ (mol2 J−2) is the constant related to the adsorption energy and ε is Polanyi potential which is related to the equilibrium concentration as follows: 1 ε ¼ RTln 1 þ Ce
ð15Þ
where Ce (mol L− 1) is the equilibrium concentration, R (8.314 J mol−1 K−1) is the gas constant and T (K) is the absolute temperature. The D-R isotherm model is generally used to distinguish the physical and chemical adsorption of pollutants with their mean adsorption energy (E, J mol−1) which is calculated from the following relation: 1 E ¼ pffiffiffiffiffiffi 2δ
ð16Þ
The Redlich-Peterson (R-P) isotherm is a three-parameter equation which is of high versatility and can be applied to both homogenous and heterogeneous systems. This model incorporates features of both the Langmuir and Freundlich models. It is approaches the Freundlich model at high concentrations and is in agreement with the low concentration limit of the Langmuir equation. The Redlich-Peterson isotherm is generally given by the following equation: qe ¼
KR‐P Ce 1 þ aR−P Cne R‐P
ð17Þ
where KR-P (L g−1) and αR-P (L mg−1) are the R-P isotherm constant, nRP is the exponent which lies between 0 and 1. The nonlinear plots of aforementioned isotherm models for adsorption of chlortetracycline onto both NAC and SAC surface at different temperatures are given in Fig. 10 and 1S–4S, and the parameters of all models calculated from the plots and the statistical indices are reported in Tables 2 and 1S–3S. As can be seen from these tables, the R-P isotherm with higher correlation coefficient (R2) values and lower statistical indices gave the best fit to experimental data, indicating the progress of adsorption process via interaction between CTCN and energetically heterogeneous adsorption sites in the surface of both activated carbons. Separation factor or equilibrium parameter (RL), which is an essential characteristic of Langmuir isotherm model, is a dimensionless constant and can be used for predicting whether an adsorption system is favourable or unfavourable [59]. RL is usually given by the following relation: Fig. 10. Non-linear plots of different equilibrium models for adsorption of CTCN onto the SAC and NAC (T = 293 K).
RL ¼
1 1 þ bC0
ð18Þ
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Table 2 Non-linear isotherm parameters and statistical indices calculated for the adsorption of CTCN antibiotic by the NAC and SAC at the ambient temperature (293 K). Adsorbent
Non-linear parameters of isotherm models and statistical indices
Langmuir isotherm model qmax (mg g−1) NAC 482.466 SAC 377.537
b (L mg−1) 0.130 0.116
R2 0.9965 0.9988
χ2 27.400 12.920
Δq (%) 13.052 9.775
RMSE 24.039 15.944
ARE (%) 1.704 0.955
Freundlich isotherm model KF (mg1−(1/n) L1/n g−1) NAC 106.399 SAC 83.322
n 2.951 2.945
R2 0.9979 0.9988
χ2 13.621 13.635
Δq (%) 9.341 11.620
RMSE 22.459 19.186
ARE (%) 0.873 1.350
Tempkin-Pyzhev isotherm model KT-P (L g−1) NAC 2.181 SAC 1.785
B 86.618 74.017
R2 0.9983 0.9989
χ2 12.414 9.758
Δq (%) 9.702 8.847
RMSE 16.996 14.448
ARE (%) 0.941 0.783
Dubinin–Radushkevich isotherm model E (kj mol−1) NAC 17.421 SAC 16.688
q0 (mol g−1) 1.323E-03 9.972E-04
R2 0.9965 0.9971
χ2 22.740 22.283
Δq (%) 9.105 9.582
RMSE 23.938 25.183
ARE (%) 0.829 0.918
aR-P 0.758 0.457
R2 0.9993 0.9998
χ2 5.456 2.697
Δq (%) 6.023 4.443
RMSE 11.038 6.878
ARE (%) 0.363 0.197
Redlich-Peterson isotherm model KR-P (L mg−1) NAC 139.129 SAC 78.064
nR-P 0.781 0.815
where C0 (mg L−1) is the initial CTCN concentration. The value of RL can be used to indicate the nature of the isotherm is irreversible (RL = 0), linear (RL = 1), favourable (0 b RL b 1) or unfavourable (RL N 1). From the experimental data, the value of RL was found to be lied between 0 and 1 for all of initial CTCN concentrations and thereby favourability of the sorption process under the circumstances used in this study. Also, comparing to the values of b obtained for SAC, the higher values of b in the case of NAC indicate that the adsorption rate of CTCN onto NAC is of a higher adsorption rate. The value of n obtained by Freundlich model was greater than unity, indicating that the adsorption of antibiotic onto both NAC and SAC is favourable. The values of mean adsorption energy (E), which were calculated from D-R isotherm model, indicate that the adsorption process of CTCN onto NAC surface follows a physisorption process (E b 18 kJ mol−1). This is another reason for concluding that the sorption of CTCN molecule onto carbons surfaces happens via physical π-π electron-donor-acceptor interactions and, therefore, the type of sorption mechanism is independent on the functional groups of carbons. However, it should be mentioned that more electron withdrawing functional groups in the surface of activated carbon unbrace the π-π interactions by reducing the electron density of π electrons and, since SAC is of a lesser carbon content and higher oxygen content compared to NAC [52], this can be considered as an extra reason for the lower efficiency of SAC towards CTCN adsorption. 3.10. Adsorption kinetics Information on the kinetics of adsorption process of pollutants is required for gaining insight on the physical chemistry of removal process and selecting the optimum conditions for the design of a full scale adsorption system. Therefore, to find out the controlling mechanism of adsorption process, such as mass transfer, film diffusion, pore diffusion and chemical reaction, the datasets from Section 3.7 were fitted on several kinetic models and the results are discussed below. The linear form for pseudo-first order model can be given as follows [60]: k1 t logðqe −qt Þ ¼ logqe − 2:303
ð19Þ
where qe and qt are the amounts of antibiotic adsorbed (mg g−1) at equilibrium and at any instant of time t (min), respectively, t (min) is the time and k1 is the rate constant of pseudo-first order adsorption
(min−1). Values of k1 and qe for each adsorbent can be obtained from the slope and intercept of the plots of log (qe–qt) vs. t which are given in Figs. 11 and 12. The pseudo-second order kinetic model, based on equilibrium adsorption, can be expressed by the following linear equation [61]: t 1 t ¼ þ qt k2 q2e qe
ð20Þ
where k2 (g mg−1 min−1) is the pseudo-second order rate constant, t (min) is the time, qe (mg g−1) is the amount of antibiotic adsorbed at equilibrium and qt(mg g−1) is the amount of antibiotic at time ‘t’. According to the pseudo-second order model, when the interaction of antibiotic with the adsorbent surface is the rate-controlling step, the plot of t/qt against t should give a straight line. The linear plots of pseudosecond order model for both activated carbons are given in Figs. 11 and 12 from which constant k2 and qe for each adsorbent can be determined from the slopes and the intercepts of the plots. Ritchie second order model, which was proposed by Ritchie (1997), can be expressed as the following modified equation [62]: 1 1 1 ¼ þ qt qe kR qe t
ð21Þ
where kR (min−1) is the Ritchie second order rate constant, t (min) is the time, qe (mg g−1) is the amount of antibiotic adsorbed at equilibrium and qt (mg g−1) is the amount of antibiotic at time ‘t’. According to the Ritchie second order model model, when the interaction of antibiotic with the adsorbent surface is the rate-controlling step, the plot of 1/qt vs. 1/t should give a straight line. The value of rate constant and the value of qe of second order model can be calculated from the slope and intercept of the plots of 1/qt vs. 1/t which are given in Figs. 11 and 12 [61]. The linear form for Elovich model can be given as following equation [63]: qt ¼
1 1 ln ðαβÞ þ ln ðtÞ β β
ð22Þ
where α (mg g−1 min−1) and β (g mg−1) are the initial adsorption rate and the desorption constant, which are, respectively, related to the extent of surface coverage and activation energy for adsorption, qt (mg g−1) is the amount of antibiotic adsorbed at time ‘t’, and t (min)
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Fig. 11. Linear plots of different kinetic models for adsorption of CTCN onto the SAC at different initial concentrations and 293 K.
is the time. According to the Elovich model, when the chemical reaction of antibiotic with the surface of polymeric adsorbent is the rate-controlling step and removal process takes place via a multilayer adsorption, the plot of qt against lnt should give a straight line. The values of Elovich constants for each adsorbent can be obtained from the slope and intercept of the plots of qt vs. lnt which are given in Figs. 11 and 12. Intra-particle diffusion model is usually given by Weber and Morris equation [64]: qt ¼ kid t0:5 þ I
ð23Þ
where qt (mg g−1) is the amount of antibiotic adsorbed at time ‘t’, kid (mg g− 1 min−0.5) is the pore diffusion rate constant, t (min) is the time and I is constant. According to the Weber-Morris model, when diffusion into the pores of adsorbent is the rate-controlling step, the plot of qt against t1/2 should give a straight line with an intercept equal to I value which is proportional to the boundary layer. The values of intraparticle constants for each adsorbent can be obtained from the slope
and intercept of the plots of qt against t1/2 which are given in Figs. 11 and 12. The Bangham model is expressed as the following equation [65]: C0 k0 m ¼ log log log þ a logt C0 −qt m 2:303V
ð24Þ
where C0 (mg L−1) is the initial concentration of radionuclide in the solution, V (mL) is the volume of solution, m (g L−1) is the weight of adsorbent per liter of solution, qt (mg g− 1) is the amount of antibiotic adsorbed at time ‘t’, and a (b 1) and k0 (mL g−1 L−1) are Bangham constants. According to the Bangham model, when the plot of log log (C0/ C0 − qtm) against log t gives a straight line, the diffusion of antibiotic into the pores of polymeric adsorbent is the rate-controlling step. The values of Bangham constants for each activated carbon can be obtained from the slope and intercept of the plots of loglog(C0/C0 −qtm) vs. logt which are given in Figs. 11 and 12.
A. Alahabadi et al. / Journal of Molecular Liquids 232 (2017) 367–381
377
Fig. 12. Linear plots of different kinetic models for adsorption of CTCN onto the SAC at different initial concentrations and 293 K.
The calculated kinetic parameters for adsorption of chlortetracycline onto the SAC and NAC at different initial concentrations are presented in Tables 3 and 4 from which it can be observed that the correlation coefficients obtained for Ritchie second order and pseudo-second order models are higher than the other models. Also, the calculated statistical indices for Ritchie second order and pseudo-second order models are the lowest ones in the studied models and, moreover, the calculated values of qe by using these two rate equations and the experimental values reported in Table 5S were close to each other for all the concentrations studied, as compared to the values obtained for the other kinetic models which showed a large deviation from the experimental values. These results indicate that the adsorption kinetics for the removal of chlortetracycline antibiotic is best described by both Ritchie second order and pseudo-second order rate equations which shows that the physical interaction between the chlortetracycline molecule and the surface of NAC is the rate-controlling step in the adsorption process. Also, the results show that, in all initial concentration studied, the values of k2 and kR obtained for NAC are greater than those of SAC, confirming
the more rapidness of CTCN adsorption onto the NAC surface. These results again show that, compared to SAC, regular pores of NAC structure improve its surface accessibility for CTCN removal and make it a better choice for industrial application.
3.11. Thermodynamic studies The adsorption isotherm studies and the results reported in the Sections 3.8 showed that an increase in the temperature results in a relative increase in adsorption capacities, indicating the bigger equilibrium constants at the higher temperatures and endothermic natural of adsorption process at the studied temperatures. However, in practical application of an adsorption system, evaluation of energetic changes during the adsorption process is of great importance to assess the feasibility and spontaneity of the adsorption process. Therefore, thermodynamic studies were carried out at 288, 298, 308 and 318 K using van't Hoff equation.
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Table 3 Parameters of different kinetic models and statistical indices for adsorption of chlortetracycline antibiotic by the SAC at different initial concentrations and 293 K. C0 (mg g−1)
Models parameters and statistical indices
Pseudo-first order model k1 (min−1) 1.068E-01 7.294E-02 6.094E-02 5.306E-02
50 75 100 150
qe,cal (mg g−1) 140.767 144.212 147.571 181.552
R2 0.9779 0.9906 0.9636 0.9702
Δq (%) 9.975 9.462 11.082 12.609
ARE (%) 0.995 0.895 1.228 1.590
RMSE 7.316 9.292 13.022 16.519
χ2 4.945 6.023 9.507 13.932
qe,cal (mg g−1) 213.220 260.892 289.687 341.997
R2 0.9989 0.9991 0.9993 0.9982
Δq (%) 2.994 3.208 3.924 3.158
ARE (%) 0.090 0.103 0.154 0.182
RMSE 3.498 3.975 4.920 5.286
χ2 0.832 1.054 1.721 1.250
kR 2.422E-01 2.997E-01 3.343E-01 3.493E-01
qe,cal (mg g−1) 211.551 252.972 283.527 336.700
R2 0.9971 0.9983 0.9975 0.9981
Δq (%) 3.041 1.809 1.609 2.177
ARE (%) 0.092 0.033 0.026 0.047
RMSE 3.658 3.768 3.269 5.606
χ2 0.893 0.577 0.434 0.434
kip (mg g−1 min-1/2) 23.715 26.938 28.870 35.169
I 59.149 84.220 104.339 114.050
R2 0.8769 0.9068 0.8858 0.8988
Δq (%) 13.691 11.419 11.032 9.950
ARE (%) 1.874 1.304 1.217 0.990
RMSE 14.081 13.686 16.430 16.308
χ2 23.715 26.938 28.870 35.169
α 0.504 0.400 0.352 0.331
R2 0.9653 0.969 0.9586 0.9601
Δq (%) 5.582 5.567 5.680 5.966
ARE (%) 0.312 0.310 0.323 0.356
RMSE 7.042 7.618 9.868 11.213
χ2 2.892 3.012 3.992 4.784
β (g mg) 2.373E-02 2.107E-02 1.950E-02 1.610E-02
R2 0.9717 0.9880 0.9803 0.9840
Δq (%) 4.912 3.852 3.984 3.760
ARE (%) 0.241 0.148 0.159 0.141
RMSE 6.143 4.901 6.610 6.357
χ2 2.186 1.304 1.827 1.659
Pseudo-second order model k2 (g mg−1 min−1) 50 1.119E-03 75 9.782E-04 100 9.480E-04 150 8.096E-04 Ritchie model 50 75 100 150 Weber-Morris model 50 75 100 150 Bangham model k0 (mL 1.283E 1.247E 1.138E 8.635E
50 75 100 150
g−1 L−1) + 02 + 02 + 02 + 01
Elovich model α (mg g−1 min−1) 1.248E + 02 2.078E + 02 2.859E + 02 2.876E + 02
50 75 100 150
Equilibrium results provide an accurate basis for calculating a correct equilibrium constant, for using in the van't Hoff equation and calculating the thermodynamic parameters [66,67]. Thermodynamic parameters of CTCN adsorption onto both SAC and NAC can be calculated by substituting the constant 55.5bM in van't Hoff equation [67,68]: ln ð55:5bM Þ ¼
ΔS ΔH − R RT
ð25Þ
where the gas constant R is defined by 8.3145 Jmol−1 K−1, 55.5 is the number of moles of water per liter of solution, bM (L mol−1) is the Langmuir constant, T is the temperature of the solution in Kelvin, Δ H (J mol−1) is enthalpy and ΔS (J mol−1 K−1) is the entropy change during the adsorption process. Therefore, the values of Langmuir constant (bM) were calculated at the different temperatures and, then, Δ H (J mol−1) and ΔS (J mol−1 K−1) for the adsorption of chlortetracycline onto both activated carbons were calculated from the slop and intercept of plots of ln55.5bM vs. 1/T, as shown in Fig. 13. The free energy change (ΔG, J mol−1) for the adsorption of chlortetracycline onto both SAC and NAC, at different constant temperatures (K), is related to the heat of adsorption, Δ H (J mol−1), and entropy change, ΔS (J mol−1 K−1), and can be determined from the following equation [69]: ΔG ¼ ΔH−TΔS
ð26Þ
The thermodynamic parameters at different temperatures were calculated for the adsorption of CTCN onto both activated carbons and
listed in Table 5. The positive values of ΔH demonstrate the endothermic nature of adsorption of CTCN onto the surface of both activated carbons. Also, the Δ H values reported in Table 5 again indicate that the adsorption process of CTCN onto both NAC and SAC is of physical type, since the Δ H value for a physical adsorption lies in the range of 2.1– 20.9 kJ mol−1 [70]. The positive values of ΔS can be contributed to the increased randomness due to the organization of the CTCN molecule at the solid/liquid interfaces and the displacement of the adsorbed water molecules by the CTCN antibiotic, which reflect the fact that CTCN have good affinities for adsorption onto the studied adsorbents. The more negative ΔG values at the higher temperatures in Table 3 indicate that, for both activated carbons, the extent of feasibility and spontaneity is proportional to the temperature and the higher temperatures are more favourable for the adsorption process and, therefore, the higher adsorption capacities can be obtained at higher temperatures. In addition, the ΔG values obtained for NAC are more negative at the studied temperatures, indicating that the adsorption process of CTCN onto the surface of NAC is more feasible and spontaneous than its adsorption onto the surface of SAC. 3.12. Comparison with other adsorbents Maximum adsorption capacities of NAC and SAC towards CTCN were compared with other solid supports reported in literature [71–76], and the results are reported in Table 6. The comparative results of adsorption capacity revealed the excellence of NAC among the other adsorbents reported, indicating that the NAC is a promising effective adsorbent from aqueous solutions.
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Table 4 Parameters of different kinetic models and statistical indices for adsorption of chlortetracycline antibiotic by the NAC at different initial concentrations and 293 K. C0 (mg g−1)
Models parameters and statistical indices
50 75 100 150
Pseudo-first order model k1 (min−1) 1.170E-01 1.134E-01 9.919E-02 9.072E-02
qe,cal (mg g−1) 76.913 157.761 181.134 229.087
R2 0.9411 0.9605 0.9275 0.9673
Δq (%) 14.103 10.644 12.253 9.527
ARE (%) 1.989 1.084 1.501 0.908
RMSE 14.827 11.195 19.168 18.603
χ2 11.467 10.495 13.593 15.512
qe,cal (mg g−1) 223.264 316.756 369.822 461.042
R2 0.9996 0.9995 0.9991 0.9992
Δq (%) 3.691 1.311 2.517 1.673
ARE (%) 0.136 0.017 0.063 0.028
RMSE 4.141 2.896 5.472 5.446
χ2 1.039 0.288 0.960 0.664
kR 3.701E-01 3.840E-01 3.979E-01 3.988E-01
qe,cal (mg g−1) 225.276 320.513 377.786 458.295
R2 0.9961 0.9989 0.9978 0.9987
Δq (%) 1.874 2.820 2.058 1.376
ARE (%) 0.035 0.080 0.042 0.019
RMSE 2.913 5.225 5.995 4.944
χ2 0.472 1.136 0.925 0.925
kip (mg g−1 min-1/2) 19.424 29.818 37.424 48.210
I 112.925 140.060 148.436 167.882
R2 0.7688 0.8476 0.821 0.8747
Δq (%) 11.829 10.869 13.287 11.808
ARE (%) 1.399 1.181 1.766 1.394
RMSE 15.784 18.733 25.894 27.037
χ2 12.089 13.101 21.792 20.312
α 0.393 0.388 0.384 0.361
R2 0.9205 0.9535 0.9244 0.9449
Δq (%) 6.104 5.392 7.366 6.595
ARE (%) 0.373 0.291 0.543 0.435
RMSE 8.659 10.023 17.022 18.103
χ2 3.699 3.852 8.958 8.581
β (g mg) 2.944E-02 1.964E-02 1.629E-02 1.225E-02
R2 0.9098 0.9568 0.9410 0.9706
Δq (%) 6.808 5.456 6.799 5.214
ARE (%) 0.464 0.298 0.462 0.272
RMSE 9.857 9.977 14.864 13.108
χ2 4.663 3.797 6.993 4.750
Pseudo-second order model k2 (g mg−1 min−1) 50 2.681E-03 75 1.361E-03 100 1.062E-03 150 7.124E-04 Ritchie model 50 75 100 150 Weber-Morris model 50 75 100 150 Bangham model k0 (mL 2.632E 2.154E 1.653E 1.260E
50 75 100 150
g−1 L−1) + 02 + 02 + 02 + 02
Elovich model 50 75 100 150
α (mg g−1 min−1) 6.957E + 02 6.200E + 02 5.196E + 02 5.066E + 02
3.13. Performances in the treatment of real waters Performance of the NAC and SAC in the treatment of real waters was evaluated by conducting adsorption studies on drinking water, well water and river water containing 50 mg L−1 CTCN. For this purpose,
100-mL portions of these solutions were contacted with 0.2 g of each activated carbon and, after 60 min of contact time, the residual CTCN in the solutions were determined. The results showed that, in the case of NAC, the removal percentage of aniline from drinking water, well water and river water respectively were 93.6, 89.5 and 87.3% which were higher than those obtained in the case of SAC (respectively 82.8, 80.2 and 76.6%). By remembering that higher removal percentages can be easily reachable by higher adsorbent dosages, these results indicate that the NAC is more economical than SAC for using in the full-scale treatment plants.
Table 5 Thermodynamic parameters for the adsorption of chlortetracycline antibiotic onto the SAC and NAC at different temperatures. Activated carbon
Temperature (K)
ΔG° (kJ mol−1)
ΔH° (kJ mol−1)
ΔS° (J mol−1 K−1)
NAC
283 293 303 313 323 288 298 308 318 323
−55.434 −56.226 −57.811 −58.604 −59.396 −52.940 −53.707 −55.242 −56.009 −56.777
9.789
158.498
8.738
153.476
SAC
Fig. 13. Thermodynamic plots for adsorption of CTCN onto the SAC and NAC.
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Table 6 Comparison of adsorption capacities of some adsorbents with those of NAC and SAC. Adsorbent
qmax (mg·g−1)
Ref.
Multifunctional polymeric material (HEMA/MAA) hydrogel Amino-Fe (III) functionalized SBA15 Activated pinewood biochar Graphene oxide functionalized magnetic particles Clay minerals (rectories) Granular Merck activated carbon (0.60–1.00 mm) Granular Sorbo Norit activated carbon (0.60–1.00 mm) NAC
138.0
[71]
33.1 208.3 42.6 177.7 309.9 65.1 482.466
SAC
377.537
[72] [73] [74] [75] [76] [76] This work This work
4. Conclusion This work compared the performances NAC and SAC in the adsorption of chlortetracycline, as a representative of highly aromatic pollutants, from aqueous solutions, and the following key conclusions were revealed. (i) The characterization studies performed in this study showed that NAC has more graphitic pore walls (and higher crystallinity) than SAC. (ii) Optimum values of experimental parameters such as pH, contact time, adsorbent dosage, contact time, and temperature affected on the adsorption of CTCN onto both SAC and NAC were investigated and, in all studies, the results revealed that, compared to the SAC, the NAC is of a better performance for CTCN adsorption. (iii) For both activated carbons, the adsorption isotherm of CTCN was better described by Redlich-Peterson model, and the parameters calculated from the other models showed a favourable adsorption process via the physical π-π electron-donor-acceptor interaction. Also, the equilibrium studies showed that the maximum adsorption capacity of NAC is higher than that of SAC. (iv) Different kinetic models were used to analyse the kinetic experimental data and the successfully fitting of both Ritchie and pseudo-second order models suggested that the interaction between the CTCN molecule and the surface of both SAC and NAC is the rate-controlling step in the adsorption process. Also, the kinetic studies showed that, compared to the SAC, the adsorption of CTCN onto the NAC is more rapid. (v) Thermodynamic studies indicated that, compared to the SAC, the adsorption of CTCN onto the NAC surface is more feasible, spontaneous and favourable. (vi) The result of treatment of real samples showed that, compared to SAC, NAC has better performances for the removal of chlortetracycline antibiotic from aqueous solutions. (vii) Overall, the results of this study indicated that, compared to SAC, NAC is a more suitable choice for removal of CTCN antibiotic from aqueous solutions. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.02.077. Acknowledgment The authors gratefully acknowledge the financial and technical support of the present work from the Central Research Council of Sabzevar University of Medical Sciences (Grant 3930101193). Also, the authors wish to take this opportunity to express their sincere thanks to Prof. Mohammad Mohammad-Zadeh, the research council president of Sabzevar University of Medical Sciences, for his great helps and supports during the experimental works.
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