Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons

MOLLIQ-04966; No of Pages 9 Journal of Molecular Liquids xxx (2015) xxx–xxx

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Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons Dimitrios A. Giannakoudakis a, George Z. Kyzas a, Antonis Avranas b, Nikolaos K. Lazaridis a,⁎ a b

Laboratory of General & Inorganic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR 54124, Greece Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, GR 54124, Greece

a r t i c l e

i n f o

Article history: Received 5 June 2015 Received in revised form 30 June 2015 Accepted 3 July 2015 Available online xxxx Keywords: Commercial activated carbons Reactive Black 5 Ionic strength Temperature Adsorption capacity Desorption

a b s t r a c t The aim of this study is to adsorptively evaluate the ability of some commercial activated carbons for the removal of a reactive dye from synthetic wastewaters. The model dye molecule used is Reactive Black 5 (RB5), while the activated carbon samples (fine powders after chopping) are: (i) Norit Darco 12 × 20 (DARCO), (ii) Norit R008 (R008), and (iii) Norit PK 1–3 (PK13). An extensive study about the influence of ionic strength to adsorption was also done, given that a crucial parameter in real dyeing wastewaters is the salinity. At first, pH-effect experiments showed the optimum pH-conditions (10). Then, kinetic (fitting to pseudo-first and -second order equations) and equilibrium experiments (fitting to Langmuir, Freundlich and Langmuir–Freundlich equations) indicated the crucial time-point in which the process ends and the maximum theoretical adsorption capacities (Qm) for all carbon samples at 25 °C (348, 527, 394 mg/g for DARCO, R008, PK13, respectively). The increase of the ionic strength improved the capacity of each carbon sample used. Moreover, the effect of temperature on dye adsorption was tested and found to significantly enhance the removal of dye. The use of carbon samples in grain form (instead of fine powders) showed that the decrease of particle size improved the dye removal. The desorption ability of materials was low using pH-adjusted deionized water as eluant (pH = 2–10), while testing a surfactant as eluant at 25 °C (sodium dodecyl sulfate), higher desorption percentages were observed for all carbons, which further increased at high temperatures (85 °C: 83, 67, 42% for DARCO, PK13, R008, respectively). Based on the above adsorption results and with those exported from characterization techniques (BET, FTIR, SEM, Boehm titrations), some adsorption interactions were proposed. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Wastewaters discharged from dye-houses can be one of the biggest contributors to aquatic pollution. The most studied dye classes, in dye bearing effluent treatment, are reactive and basic [1]. However, the larger amount of the dye loss from the dyeing process to the effluent is estimated to be 10–50% for reactive dyes [2]; therefore, it is necessary to firstly treat/remove these reactive dyes. Furthermore, wastewaters containing dyes are very difficult to be treated, since the dyes are recalcitrant organic molecules, resistant to aerobic digestion, and stable to light, heat and oxidizing agents [3]. The dyeing process of cotton textiles using reactive dyes involves unit operations, such as (i) desizing, (ii) scouring, (iii) bleaching, (iv) dyeing, and (v) finishing [4]. The waste streams from each individual sub-operation are collected to an “equalization tank”, where they are mixed and homogenized. In particular, the wastewater produced by the dyeing bath-reactor contains hydrolyzed reactive dyes, dyeing auxiliaries, and electrolytes (60–100 g/L of NaCl and Na2CO3). The latter are responsible for the high saline content of the wastewater, which exhibits ⁎ Corresponding author. E-mail address: [email protected] (N.K. Lazaridis).

high pH values (10–11) [4]. In a typical dyeing procedure with reactive dyes, 0.6–0.8 kg NaCl, 30–60 g dyestuff, and 70–150 L water are required for the dyeing of 1 kg of cotton [5]. So, the large volume of colorized effluents, after the dyeing process, has to be treated in some manner. Additionally, the research on textile effluent decolorization has often been focused on reactive dyes for three main reasons [4]: (i) reactive dyes represent an increasing market share, because they are used to dye cotton fibers, which makes up about half of the world's fiber consumption; (ii) a large fraction, typically around 30% of the applied reactive dyes, is wasted due to the dye hydrolysis in alkaline dye bath; (iii) conventional wastewater treatment plants have a low removal efficiency for reactive and other anionic soluble dyes, which leads to colored waterways [5,6]. A typical effluent treatment is broadly classified into preliminary, primary, secondary, and tertiary stages [1,6,7]. The preliminary stage includes equalization and neutralization [7]. The primary stage involves screening, sedimentation, flotation, and flocculation. The secondary stage reduces the organic load and facilitate the physical/chemical separation (biological oxidation), while the tertiary stage is focused on decolorization [7]. In the latter stage, the adsorption onto activated carbon is broadly used to limit the concentration of color in effluents [8]. Adsorption has been applied either in a single mode, mainly for dyes

http://dx.doi.org/10.1016/j.molliq.2015.07.010 0167-7322/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

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D.A. Giannakoudakis et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx

removal from synthetic wastewaters, or in a combinational mode for total cleaning of real dyeing wastewaters. Among the technologies used for water and wastewater treatment, adsorption proved to be one of the most efficient separation methods for water purification. Numerous works have been recently published with primary goal which is the investigation of the removal of different pollutants (either in gas or liquid medium) using adsorbent materials [9–24]. In particular, adsorption was also found to be an effective treatment method for the removal of synthetic dyes from water and wastewaters that are toxic and cannot be efficiently decolorized by traditional methods. The release of dyes into environment constitutes an important proportion of water pollution [25]. The adsorption of synthetic dyes onto adsorbents is considered as a simple and economical method, especially when applied low-cost adsorbents [26–28]. The choice of the appropriate adsorbent materials is a very important issue. Carbon materials are well known for their use as excellent adsorbents [29,30]. In this study, some commercial carbon samples were tested as potential adsorbent materials for the removal of (one of the most widely-known dyestuffs existed in wastewaters) Reactive Black 5. The novelty of the present work is that many adsorption parameters affecting the process (i.e., pH of adsorption, initial dye concentration, temperature, ionic strength, particle size, contact time) were investigated along with the desorption potential of those materials (effect of pH on desorption). Some basic isotherm models (Langmuir, Freundlich, Langmuir–Freundlich) were used for fitting to equilibrium experimental results, in line with some kinetic equations (pseudo-first, -second order) for the respective of kinetic results. 2. Materials and methods 2.1. Carbon samples and model dye Three carbon samples used in this study: (i) Norit Darco 12 × 20 (denoted as DARCO), (ii) Norit R008 (denoted as R008), and (iii) Norit PK 1–3 (denoted as PK13). The samples were purchased by Mead Westvaco (U.S.A.). The adsorbents were initially in granular form as purchased (DARCO, 1000 μm; R008, 800 μm; PK13, 1300 μm), but after chopping and sieving were used as fine powders (75–125 μm). A commercial reactive dye (anionic and anthraquinonic) was used as a model dye molecule for adsorption experiments. The reactive dye used is Reactive Black 5 — C.I. 20505 (denoted as RB5, supplied by Kahafix), presents the following characteristics: C26H21N5Na4O19S6, MW = 991.82 g/mol, λmax = 603 nm, purity = 55% w/w. The dye purity was taken into account for all calculations. The chemical structure of the dye used is given in Fig. 1. 2.2. Characterization techniques Scanning electron microscopy (SEM) images were performed with an electron microscope (model Zeiss Supra 55 VP, Jena, Germany) equipped with an energy dispersive X-ray (EDX) Oxford ISIS 300 micro-analytical system. The accelerating voltage was 15.00 kV, while the scanning was performed in situ on a sample powder.

NaO3SO

O H2 H2 C C S

NH2 N

N

The FTIR spectra of the samples were taken with a Nicolet 560 (Thermo Fisher Scientific Inc., MA, USA) FTIR spectrometer. The spectra were recorded in transmission mode using potassium bromate wafers containing 0.5 wt.% of carbon. Pellets made of pure potassium bromate were used as the reference sample for background measurements. The spectra were recorded from 4000 to 400 cm−1 at a resolution of 4 cm−1 and are baseline corrected. For the measurement of the surface pH of carbon samples, 0.4 g of dry carbon sample was added to 20 mL of water, and the suspension was stirred overnight to reach equilibrium. Then the pH of the solution was measured. This method provided information about the acidity or basicity of the material's surface. The determination of surface functional groups was based on the Boehm titration method [31]. Aqueous solutions of NaHCO3 (0.05 mol/L), Na2CO3 (0.05 mol/L), NaOH (0.10 mol/L), and HCl (0.10 mol/L) were prepared. 25 mL of these solutions was added to vials containing 0.5 g of adsorbents, let to be shaken (140 rpm) until equilibrium (24 h), and then filtered. Five blank solutions (without adsorbent) were also prepared. In this way, the number of the basic sites was calculated from the amount of HCl which reacted with adsorbent. The various free acidic groups were derived using the assumption that: (i) NaOH neutralized carboxyl, lactonic and phenolic groups; (ii) Na2CO3 neutralized carboxyl and lactonic groups, and (iii) NaHCO3 neutralized only carboxyl groups. The excess of base or acid was then determined by back titration using NaOH (0.10 mol/L) and HCl (0.10 mol/L) solutions [32]. Nitrogen isotherms were measured using AS1Win (Quantachrome Instruments, FL, USA) at liquid N2 temperature (77 K). The samples were degassed at 150 °C in a vacuum system at 10−4 Torr before the analysis. The specific surface area (SBET) was calculated from the isotherm data using the Brunauer, Emmet and Teller (BET) model. The micropore volumes (Vmic) were obtained with the accumulative pore volume using the density functional theory (DFT) method. The total pore volumes (Vtot) were obtained from the volumes of nitrogen adsorbed at a relative pressure of 0.99 cm3/g. The mesopore volumes (Vmes) were calculated by subtracting Vmic from Vtot. The pore size distribution curves were also obtained using DFT method.

2.3. Adsorption experiments 2.3.1. Effect of pH on adsorption with varying ionic strength The effect of pH was conducted by mixing 1 g/L of adsorbent with 20 mL of dye solution (C0 = 500 mg/L). The pH value, ranging between 2 and 12, was kept constant throughout the adsorption process by micro-additions of HNO3 (0.01 mol/L) or NaOH (0.01 mol/L). The pH adjustment of dye solutions in the presence of adsorbent was realized in order to study and understand the possible adsorption mechanisms in each particular pH condition. The dye removal is also occurred in free pH conditions (without readjustment to constant pH values), but in this way, it would be more difficult to examine the adsorption mechanism. The suspension was shaken for 24 h (agitation rate: N = 140 rpm) into a water bath to control the temperature at 25 °C (model Julabo SW-21C, Germany). All experiments were carried out at three ionic strength values (I = 0.001, 0.1, 1 M), adjusted by additions

OH

O N

N

S

H2 H2 C C OSO3Na

O

O HO3S

SO3H

Fig. 1. Chemical structures of Reactive Black 5 (RB5).

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

D.A. Giannakoudakis et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx

of NaCl (0.01 M). The optimum pH found (for performing the equilibrium experiments) was pH 10. 2.3.2. Effect of contact time on adsorption Kinetic experiments were performed by mixing 1 g/L of adsorbent with 20 mL of dye solution (C0 = 500 mg/L). The suspensions were shaken for 24 h at pH = 10 in water bath at 25 °C (N = 140 rpm; I = 1 M). Samples were collected at fixed intervals (5 min–48 h). The experimental kinetic data were fitted to two kinetic models; pseudo-first [33] (Eq. (1)) and -second order [34] (Eq. (2)) (their selection is based on the fact that they are the two most widely-used kinetic models in adsorption works [34,35]).   Ct ¼ C0 −ðC0 −Ce Þ 1−e−k1 t  Ct ¼ C0 −ðC0 −Ce Þ 1−

 1 1 þ k2 t

ð1Þ

ð2Þ

where k1 and k2 (min−1) are the rate constants for the pseudo-first and second order kinetic equations, respectively; C0, Ct, and Ce (mg/L) are the initial, transient and equilibrium concentrations of dye in the aqueous solution, respectively. 2.3.3. Effect of initial dye concentration with varying ionic strength The effect of initial dye concentration on equilibrium was observed by mixing 1 g/L of adsorbents with 20 mL of dye solutions of varying initial concentrations (100–800 mg/L). The suspensions were shaken for 24 h at pH = 10 in water bath at 25 °C (N = 140 rpm). All experiments were carried out at three ionic strength values (I = 0.001, 0.1, 1 M), adjusted by additions of NaCl (0.01 M). The experimental equilibrium data were fitted to the Langmuir (Eq. (6)) [36], Freundlich (Eq. (7)) [37] and Langmuir–Freundlich (L–F) (Eq. (8)) [38] isotherm equations expressed by the following equations:

Qe ¼

Q m KL Ce 1 þ KL Ce

Q e ¼ K F Ce 1=n

ð6Þ

ð7Þ

3

different initial concentrations (100–800 mg/L). The suspensions were shaken for 24 h at pH = 10 in water bath at 25, 45, and 65 °C (N = 140 rpm, I = 1 M). 2.3.5. Effect of particle size The effect of particle size on adsorption was observed by mixing 1 g/L of adsorbents with 20 mL of dye solutions of varying initial concentrations (100–800 mg/L). The suspensions were shaken for 24 h at pH = 10 in water bath at 25 °C (N = 140 rpm). All experiments were carried out at I = 1 M, adjusted by additions of NaCl (0.01 M). The adsorbents were in granular form (as purchased: DARCO, 1000 μm; R008, 800 μm; PK13, 1300 μm) and fine powders (75–125 μm, after chopping and sieving). 2.4. Desorption Desorption experiments were performed in batch mode using the optimum adsorption conditions found (pH = 10; C0 = 500 mg/L; T = 25 °C; t = 24 h). At first, after the end of adsorption stage, the adsorbent particles were separated from supernatant using filtration membranes. Then, the adsorbent particles were placed in flasks using various eluants. The first attempt was done by adjusting deionized water at different pH values (2–10). The volume of eluant was fixed to be the same as that of adsorbate (20 mL). However, it was found that the greatest desorption percentage was extremely small (~ 6%) and therefore it was decided to use sodium dodecyl sulfate (SDS) at two different temperatures (25, 85 °C). The desorption step (as in adsorption step) lasted 24 h. The quantitative evaluation of desorption was done using desorption percentages, calculated from the difference between the loaded amount of dye on adsorbent after adsorption and its amount in solution after desorption. 3. Results and discussion 3.1. SEM images The surface of the carbon samples was investigated by taking SEM micrographs. All carbons seem to have similar surface properties with many cavities and channels (Fig. 2). The effect of porosity on their surface is clear and caused many “tunnels” which can help the diffusion of dyes into them. SEM images were also taken for the initial granular carbons (Fig. 2d–f) along with simple photos (Fig. 2g–i), in order to compare the effect of particle size on adsorption. 3.2. Effect of pH on adsorption, FTIR spectroscopy, surface chemistry

Qe ¼

Q m KL F ðCe Þ1=b 1 þ KL F ðCe Þ1=b

ð8Þ

where Qm (mg/g) is the maximum amount of adsorption; KL (L/mg) is the Langmuir adsorption equilibrium constant; KF (mg1–1/n L1/n/g) is the Freundlich constant representing the adsorption capacity; n (dimensionless) is the constant depicting the adsorption intensity; KLF ((L/mg)1/b) is the L–F constant; and b (dimensionless) is the L–F heterogeneity constant. The equilibrium amount in the solid phase (Qe, mg/g) was calculated according to the following equation (where C0 and Ce are the initial and equilibrium concentrations of dye, respectively; V (L) is the volume of aqueous solution; m (g) is the mass of particles used): Qe ¼

ðC0 −Ce ÞV : m

ð9Þ

2.3.4. Effect of temperature To study the effect of temperature on equilibrium, experiments were performed by mixing 1 g/L of adsorbent with 20 mL of dye solutions of

A series of pH-effect experiments was carried out in order to evaluate the adsorption behavior of the studied carbon samples at different pH values (2, 4, 7, 10, 12). Fig. 3 illustrates the aforementioned curves for increasing values of ionic strength (0.001, 0.1, 1 M). At first, it is necessary to begin the discussion for the case of nearly zero (0.001 M) ionic strength in order to compare the other removal percentages. It is clear that all carbons presented the same pH-effect behavior; increasing the pH of the solution, an improvement of the RB5 removal is observed. Another finding is that the increase of RB5 removal was relatively lower at the highly acidic values (pH = 2, 4) than that of the highly alkaline value (pH = 10, 12). Especially, a strong increase was observed between pH = 7 and 10. This was not observed for all carbon samples. In the case of DARCO (Fig. 3a), the RB5 removal increased from 27 to 37% (10% change) for this pH step (from pH = 7 to 10). The respective values (which caused the differentiation) for the other carbon samples were 1% for PK13 and 4% for R008 (Fig. 3b, c). The effect of ionic strength at various pH values was also illustrated in the same figure (Fig. 3). The reason for which this factor was selected to be studied is that the real dyeing effluents produced by dyeing bathreactors bear high concentration of electrolytes and exhibits high pH

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Fig. 2. (a) SEM of DARCO (fine powder); (b) SEM of R008 (fine powder); (c) SEM of PK13 (fine powder); (d) SEM of DARCO (grain); (e) SEM of R008 (grain); (f) SEM of PK13 (grain); (g) photo of DARCO; (h) photo of R008; (i) photo of PK13.

H+ and OH− is theoretically equal at neutral pH conditions, but at pH = 10, excess of OH− occurs which supports the adsorption of RB5 molecules onto carbon samples. In general, the existence of salinity in the aqueous solution significantly favors and enhances the adsorption of dye molecules onto carbon. This increase (after NaCl addition) can

values (10–11) [4,39]. In the case of our study, the increase of ionic strength from 0.001 to 1 M caused the improvement of RB5 removal percentages in all carbon samples. Similar trend was observed for 0.001 M (it was discussed in the previous paragraph), but the sudden jump from pH = 7 to 10 was even more intense. The concentration of

90

70

60

60

DARCO

1M 0.1 M 0.001 M

R008

1M 0.1 M 0.001 M

80

PK13

1M 0.1 M 0.001 M

50

50

40

Removal (%)

Removal (%)

Removal (%)

70 60 50

40

30

40 20

30 30

10

20

20 2

4

6

8

10

12

2

4

6

8

10

12

2

4

6

8

pH

pH

pH

(a)

(b)

(c)

10

12

Fig. 3. Effect of pH on adsorption of RB5 onto (a) DARCO, (b) R008, (c) PK13 at various values of ionic strength.

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

D.A. Giannakoudakis et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx

Transmittance

R008

R008-RB5

DARCO

DARCO-RB5 PK13

PK13-RB5

2000 1800 1600 1400 1200 1000

800

-1

Wavenumber (cm ) Fig. 4. FTIR spectra of DARCO, R008, PK13 before and after RB5 adsorption.

be attributed to an increase in dimerization of RB5 molecule in solution. The effect of salt and temperature on the dimerization of reactive dyes has been extensively investigated by Alberghina and co-workers [40]. A number of intermolecular forces have been suggested to explain this aggregation as: van der Waals forces, ion–dipole, dipole–dipole, which occur between dye molecules in the solution. It has been reported that these forces increased upon the addition of salt to the dye solution [40]. Accordingly, the higher adsorption capacity of reactive dyes under these conditions can be attributed to the aggregation of dye molecules induced by the action of salt ions. The salt ions force the dye molecules to aggregate, increasing the extent of adsorption on the carbon surface. Similarly, Germán-Heins and Flury have reported an

5

increase in another reactive dye (Brilliant Blue) adsorption after adding salt to the solution [41]. The pH-behavior can be confirmed and explained with FTIR spectroscopy. For this reason, the FTIR spectra of all samples before and after RB5 adsorption were taken (Fig. 4). For all initial carbons the band at 1600 cm−1 represents the combination of C_C stretching vibration of the aromatic ring structures (1590 cm−1) and conjugated systems such as diketone, ketoester, and quinone (1550–1680 cm− 1), while the broad band from 1300 to 1000 cm−1 may be assigned to (i) C–O–C lactone structures, (ii) stretching C–O vibrations and bending O–H modes of phenol structures and (iii) ethers. The band at 1714 cm−1 may be assigned to the carboxyl C_O stretching. The adsorption of RB5 on carbons resulted in new peaks for all samples. A band at about 1550 cm−1 represents azo-bond vibrations (− N_N–) in RB5 molecule, while the broadening of the band at about 1210 cm− 1 represents the sulfonate groups of the dye. The peaks at the region 1570–1600 cm−1 (characteristic of C_C aromatic skeletal vibrations) appeared to be diminished, indicating that the interaction of dye molecule with the surface of the activated carbon materials is expected to take place between the delocalized pielectrons on the basal planes of the activated carbons and the free electrons of the aromatic rings of the dye molecule [42]. All aforementioned interactions can be briefly described in Fig. 5. The results of Boehm titration along with the surface pH measurements (Table 1) support the aforementioned conclusion. It can be seen that R008, which is the most efficient of the carbons tested, possesses the smallest number of oxygen-containing groups and its surface appeared to be basic (pH = 9.78). Thus, the oxygen-free Lewis base sites, which are related to delocalized pi-electrons on the basal planes of the carbon may have an important contribution in the interactions of RB5 with the surface of this activated carbon. These interactions may be originated from the free electrons of the dye molecule resulting from several aromatic rings and double bonds, as well as from the negatively charged ions of the dye. Oxygen-containing surface groups with acid character (carboxylic, anhydride, lactones and phenols) have a negative effect as can be observed from the adsorption results for DARCO (surface pH = 6.86), which can be attributed to the fact that these groups attract (and thus localize) pi-electrons of the condensed aromatic sheets on the surface of these adsorbents; and at the same time they may cause steric forces.

Carbon

pi pi pi-pi interactions

pi-pi interactions

SO3H

HO3S O H 2 H2 NaO3SO C C S O

pi N

pi

N

O N N

NH2

OH

H2 H2 S C C OSO3Na

O

Reactive Black 5 Fig. 5. Basic adsorption interactions between carbon-based materials and RB5. Reprinted with permission by Elsevier [42].

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

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Table 1 Surface pH of the carbon samples, Boehm titration results, parameters of the pore structure calculated from nitrogen adsorption isotherms. Adsorbent

DARCO R008 PK13

Carboxyl groups

Lactonic groups

Phenolic groups

Total acid groups

pH

mmol/g

mmol/g

mmol/g

mmol/g

6.86 9.78 9.73

0.40 0 0

0.52 0.37 0

0.81 0.12 0.75

1.73 0.49 0.75

DARCO R008 PK13

SBET cm2/g

Vmic cm3/g

Vmes cm3/g

Vtot cm3/g

773 657 696

0.18 0.60 0.23

0.41 0.16 0.24

0.59 0.76 0.47

Table 1 also presents the results after BET analysis for the carbons studied. The three carbons although appeared to possess nearly the same specific surface areas, but presented different values of pore volume. R008 (which exhibited the better adsorption performance) also presented the greater value of total pore volume, while the PK 13, although measured to have a basic surface pH = 9.78, it presented the less pore volume contributing this way to lower adsorption capacity.

3.3. Effect of contact time (kinetics) The effect of contact time on adsorption was presented in Fig. 6. The process for all carbon samples can be divided in three main steps: (i) the sharp removal of dye molecules when they come in contact with the surface of carbons; (ii) the gradual adsorption stage, when the pores of carbon begin to be saturated; (iii) the equilibrium stage, when the process is completed. The time-zone of each step depends on the system dye/carbon. The instantaneous dye adsorption (0–100 min), governed by fast external diffusion and mainly surface adsorption, was followed by a milder and gradual ascent (1.5–14 h), resulting in an equilibrium state plateau (14–48 h). In the case of all carbon studied, the kinetic behavior is similar and after 800 min the process seems to finalize. Table 2 presents the results of fitting to two kinetic equations (pseudo-first, -second order) which are widely used to carbonadsorption systems. It is clear that the fitting to pseudo-second order kinetic equation was more successful exporting higher correlation coefficients (R2ps1 = 0.905–0.942; R2ps2 = 0.604–0.810). The kinetic constants were 0.045, 0.046 and 0.087 for R008, PK13 and DARCO, respectively.

It is also noteworthy to comment on the physical significance of the so-called pseudo-first and pseudo-second order equations. It is unquestionable that since Lagergren first proposed a rate equation for the adsorption of solutes from a liquid solution, it has been used and abused without much discussion about its validity and applicability [28,43]. However, recent works not only have presented an extensive discussion of the applicability of both pseudo-first and pseudo-second order equations, but also, by means of applying distinct approaches, have provided a strong theoretical basis for them, which give them a clear physical significance [28,43]. From all of the cited works, a major conclusion can be withdrawn which is the suitability of such equations for either long or short times of adsorption, in the case of the pseudo-first and pseudo-second order equations, respectively. In other words, in the initial times, far from equilibrium, the kinetics are governed by the rate of surface reactions and, when the system approaches equilibrium, a switch takes place in the predominant mechanism from surface reactions kinetics to intraparticle diffusion [28,43]. Also, a lumped form of a single rate equation that would be suitable for correlating kinetic data recorded for the whole range of adsorption times has yet to be proposed. If the film resistance is to be considered to complete the model, a system of two differential equations is necessarily generated and its applicability to the usual types of experimental data that are currently being obtained would be impractical [28,43]. Thus, the widespread use of the simple pseudo-first and pseudo-second order equations can be easily justified by their attractive way of correlating well the experimental data strictly for either long or short adsorption times, respectively. This means that the aforementioned models provide a means to understand the dominant adsorption mechanism to a certain extent, with their practical significance being limited to the range of adsorption time being considered [28,43].

3.4. Effect of initial dye concentration 500

PK13 DARCO R008 pseudo-1st pseudo-2nd

450

Ce(mg/L)

400 350

Fig. 7 shows the isotherm curves for carbon samples at the optimum pH value (10). All isothermal parameters resulted from the fitting of experimental data to the Langmuir, Freundlich and L–F model and are reported in Table 3. The correlation coefficients obtained for Freundlich model were not as high (0.908 ≤ R2 ≤ 0.970) as those of Langmuir (0.969 ≤ R2 ≤ 0.997) or L–F model (0.983 ≤ R2 ≤ 0.999).

300 250 Table 2 Kinetic parameters for the adsorption of RB5 onto DARCO, R008, and PK13.

200

Pseudo-first order

150 0

200

400

600

800

1000

1200

1400

3000

t (min) Fig. 6. Effect of contact time on adsorption of RB5 onto DARCO, R008, PK13 (fitting to pseudo-first and -second order equations).

Pseudo-second order R2

k1 −1

Adsorbent

min

DARCO R008 PK13

0.016 0.025 0.026

min 0.648 0.810 0.604

R2

k2 −1

0.087 0.045 0.046

0.942 0.941 0.905

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

D.A. Giannakoudakis et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx 400

350

500 DARCO

R008

PK13

350

300

400

300

250

200 150

300

Qe (mg/g)

Qe (mg/g)

250

Qe (mg/g)

7

200

200 150 100

100 L-F Langmuir Freundlich

50 0 0

100

200

100

I=1M I = 0.1 M I = 0.001 M

300

400

500

600

L-F Langmuir Freundlich

0

700

0

50

100

150

Ce (mg/L)

I=1M I = 0.1 M I = 0.001 M

200

250

300

350

0

400

0

50

100

150

200

250

300

350

400

Ce (mg/L)

Ce (mg/L)

(a)

I=1M I = 0.1 M I = 0.001 M

L-F Langmuir Freundlich

50

(b)

(c)

Fig. 7. Effect of initial dye concentration on RB5 adsorption at 25 °C onto (a) DARCO, (b) R008, (c) PK13 at various values of ionic strength (I = 0.001, 0.1, 1 M).

Table 3 Equilibrium parameters for the adsorption of RB5 at 25 °C onto DARCO, R008, and PK13 (I = 0.001, 0.1, 1 M). Langmuir equation I

Qm

KL

Adsorbent

M

mg/g

L/mg

DARCO

0.001 0.1 1 0.001 0.1 1 0.001 0.1 1

148 263 343 380 407 537 186 268 273

0.128 0.073 0.115 0.007 0.018 0.021 0.013 0.024 4.121

R008

PK13

Freundlich equation R2

KF

L–F equation n

R2

mg1–1/n L1/n/g−1 0.971 0.990 0.993 0.989 0.991 0.997 0.969 0.982 0.980

90.19 83.41 119.27 14.37 67.13 82.85 20.59 49.17 137.28

R008 presented higher adsorption capacities (25 °C) for all values of ionic strength (284, 351, 527 mg/g for 0.001, 0.1 and 1 M, respectively). The equilibrium dye uptake was affected by its initial concentration using constant dosage of adsorbent (1 g/L). At low initial concentrations, the adsorption was very intense and reached equilibrium rapidly. This phenomenon indicated the possibility of the formation of monolayer coverage of dye at the outer interface of materials. Furthermore, for low concentrations (0–200 mg/L) the ratio of initial number of dye molecules to the available adsorption sites is low and subsequently the fractional adsorption becomes independent on initial concentration [44]. Brunauer et al. [45] divided the isotherms of adsorption into five types. Type I isotherms represented unimolecular adsorption and applies to non-porous, microporous and adsorbents with small pore sizes (not significantly greater than the molecular diameter of the adsorbate). So, the shapes of isotherm curves (Fig. 7) indicated that the isotherms for the systems studied are I-Type, according to the BET classification [45], and characterized by a high degree of adsorption at low concentrations. At higher concentrations, the available adsorption sites became lower and subsequently the adsorption depended on the initial concentration. As a matter of fact, the diffusion of dye molecules within adsorbent particles might govern the adsorption rate at higher initial concentrations. 3.5. Effect of temperature (isotherms) The temperature effect on equilibrium was also studied and presented in Table 4. The equilibrium data were only fitted to L–F model due to the best simulation found in Section 3.4 (Table 3). All adsorbents presented the same behavior; the increase of temperature from 25 to 65 °C caused an increase of the adsorption capacity (dye uptake). In particular, in the case of DARCO, it increased its Qm from 348 mg/g at 25 °C to 442 mg/g at 45 °C, and finally 564 mg/g at 65 °C. Similar adsorption behavior was revealed for R008 (from 527 mg/g at 25 °C to 599 mg/g at 45 °C, and finally 796 mg/g at 65 °C) and PK13 (from

12.99 5.07 5.57 1.99 3.47 3.28 2.87 3.61 7.41

0.908 0.969 0.943 0.970 0.966 0.972 0.936 0.936 0.995

Qm

KLF

mg/g

(L/mg)1/b

144 289 348 284 351 527 151 233 394

0.0021 0.1363 0.1422 0.0005 0.0008 0.0182 0.0003 0.0011 0.3352

b

R2

0.40 1.43 1.09 0.59 0.54 0.95 0.51 0.53 2.54

0.987 0.998 0.992 0.997 0.999 0.996 0.983 0.999 0.999

394 mg/g at 25 °C to 453 mg/g at 45 °C, and finally 474 mg/g at 65 °C). The above increase was approximately 63, 51, and 20% for DARCO, R008, and PK13, respectively. It seemed that the increase of temperature improved the diffusivity of dye molecules on water caused the increase of their movement into the pores of carbons. All above are illustrated in Fig. 8.

3.6. Effect of particle size The effect of particle size is considered to be a useful parameter about the adsorption behavior of material. For this reason, some experiments were carried out using the commercial samples as purchased (grain form). Fig. 9 shows the isotherms taken after adsorption equilibrium experiments at the optimum conditions (pH = 10; I = 1 M). The equilibrium data were only fitted to L–F model due to the best simulation found in Section 3.4 (Table 3). Table 4 Equilibrium parameters for the adsorption of RB5 at 25, 45, and 65 °C onto DARCO, R008, and PK13 (I = 1 M) (fitting to L–F model). T

Qm

KLF

Adsorbent

Form

°C

mg/g

(L/mg)1/b

DARCO

Grain Fine powder Fine powder Fine powder Grain Fine powder Fine powder Fine powder Grain Fine powder Fine powder Fine powder

25 25 45 65 25 25 45 65 25 25 45 65

274 348 442 564 287 527 599 796 195 394 453 474

0.049 0.142 0.257 0.283 0.045 0.018 0.023 0.184 0.002 0.335 0.359 0.515

R008

PK13

b

R2

1.37 1.09 2.36 1.92 1.63 0.95 0.98 2.05 0.79 2.54 2.76 2.82

0.998 0.992 0.998 0.985 0.995 0.996 0.998 0.999 0.982 0.999 0.999 0.999

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

8

D.A. Giannakoudakis et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx 400 L-F

500

L-F

DARCO

300

Qe (mg/g)

400

Qe (mg/g)

PK13

300

200

200

100 100

o

o

25 C

o

45 C

25 C

o

45 C

o

o

65 C

0 0

100

200

300

400

500

600

65 C

0 0

700

50

100

150

200

250

300

350

Ce (mg/L)

Ce (mg/L)

(a)

(b)

(c)

Fig. 8. Effect of temperature on RB5 adsorption at 25, 45, 65 °C onto (a) DARCO, (b) R008, (c) PK13 at (I = 1 M).

3.7. Desorption

Fig. 9. Effect of particle size (fine powder and grain) on adsorption equilibrium of RB5 onto DARCO, R008, PK13 (fitting to L–F equation).

It is obvious that the particles of smaller size (fine powders) presented higher adsorption capacity. In particular, DARCO improved the capacity by 27% (from 274 for grain to 348 mg/g for powders). The respective changes to the other carbon samples were 84% for R008 (from 287 for grain to 527 mg/g for powders) and 123% for PK13 (from 195 for grain to 435 mg/g for powders) (Table 4).

An important parameter regarding the reusability of an adsorbent materials is their desorption percentage. Therefore, the carbons of the present work were tested about desorption. At first, it was selected to perform desorption experiments only with deionized water as eluant (after adjustment at different pH values (2–10)), because many works in literature showed great desorption percentages [42,46–48]. Fig. 10a illustrates the results of the aforementioned desorption tests. But as it is clearly observed, the higher desorption ability was only 6% for DARCO, while for PK13 and R008 were 5.5% and 2%, respectively. However, due to the extremely small desorption percentages found, an attempt was done with SDS at two different temperatures; room temperature (25 °C) and high temperature (85 °C) (Fig. 10b). The selection of SDS was based on already published works regarding dye desorption from carbons [49,50] and the procedure was according to surfactant-enhanced carbon regeneration (SECR) [51]. Briefly, SECR involves the flushing of concentrated surfactant solution (eluant) through the spent carbon. Organic adsorbate desorbs and is solubilized into micelles (surfactant aggregates typically containing 50–100 molecules) in the eluant solution. When the desorption process is complete, some residual adsorbed surfactant may be left on the activated carbon, while the eluant stream contains concentrated solute and can be further treated to recover and recycle the surfactant. A water flush is then used to remove residual surfactant from the carbon, leaving the carbon ready for reuse (for vapor phase applications, a drying step is needed). The concentration of SDS to the eluant solutions was 6 g/L. DARCO showed high desorption percentage especially in the case of high temperatures

7

100

6

SDS 85 C

90

o

SDS 25 C pH=10

80

5

70

Desorption (%)

Desorption (%)

o

DARCO PK13 R008

4 3 2

60 50 40 30 20

1

10 0

0 2

3

4

5

6

7

8

9

10

DARCO

PK13

pH

Adsorbents

(a)

(b)

R008

Fig. 10. Desorption of RB5 with (a) deionized water at various pH values, (b) SDS at 45, 85 °C.

Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010

D.A. Giannakoudakis et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx

(25 °C, 18%; 85 °C, 83%). The latter can be attributed to the fact that increasing temperature, the motion of surfactant onto carbon is favored and consequently the “push” of dye molecules out of the material increased. R008 and PK13 also presented acceptable desorption (PK13: 25 °C, 17%; 85 °C, 67%; R00813: 25 °C, 10%; 85 °C, 42%).

[19]

[20]

4. Conclusions

[21]

This study investigates the adsorption behavior of three commercial carbons (DARCO, R008, PK13) regarding the removal of RB5 from aqueous solutions. The optimum pH found for the adsorption experiments was alkaline (10). Moreover, the increase of ionic strength from 0.001 to 1 M caused an increase of the adsorption capacity of the carbons. The same was observed for the increase of temperature from 25 (348, 527, 394 mg/g for DARCO, R008, PK13, respectively) to 45 and 65 °C. The best fitting model for equilibrium results was the L–F equation, while for the kinetic ones was the pseudo-second order equation. The use of carbon samples in grain form (instead of fine powders) showed that the decrease of particle size improved the dye removal. Low desorption was found for water eluants (pH-adjusted deionized water), while higher desorption percentages were observed for all carbons using SDS (surfactant) as eluant at 25 °C, which further increased at high temperatures (85 °C). Based on the above adsorption results and with those exported from characterization techniques (BET, FTIR, SEM, Boehm titrations), the basic adsorption interaction was the pi–pi interaction between the ring of carbons and that of RB5 molecule.

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Please cite this article as: D.A. Giannakoudakis, et al., Multi-parametric adsorption effects of the reactive dye removal with commercial activated carbons, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.07.010