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Coupling adsorption with photocatalysis process for the Cr(VI) removal
Desalination 270 (2011) 166–173
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Coupling adsorption with photocatalysis process for the Cr(VI) removal M. Kebir a, M. Chabani a, N. Nasrallah a, A. Bensmaili a, M. Trari b,⁎ a b
Laboratory of Engineering Reaction Faculty of Engineering Mechanic and Engineering Processes USTHB, BP 32, Algiers, Algeria Laboratory of Storage and Valorization of Renewable Energies Faculty of Chemistry USTHB, BP 32, Algiers, Algeria
a r t i c l e
i n f o
Article history: Received 12 August 2010 Received in revised form 22 November 2010 Accepted 23 November 2010 Available online 5 January 2011 Keywords: Adsorption Cr(VI) Kinetic Isotherm Photoreduction
a b s t r a c t The adsorption of Cr(VI) onto red peanut skin and its photoelectrochemical reduction into Cr3+ were investigated. The material has been characterized by infrared spectroscopy and scanning electron microscopy. It was successfully applied to the removal of chromium in aqueous air equilibrated powder suspension. The influence of the contact time, initial concentration, pH and temperature has been studied. The best performance occurred at 50 °C in the acidic solution (200 mg L− 1, pH 2) with an uptake removal of 85% in less than 30 min and the adsorption follows a pseudo second order kinetic. The experiments have been carried out in a batch reactor; the data were evaluated using the linear isotherms of Langmuir and Freundlich models. The maximal adsorption capacity (44.05 mg g− 1) has been determined from the Langmuir model. The thermal variation obeys to the Arrhenius type law from which the thermodynamic parameters ΔH°, ΔS° and ΔG° have been deduced. The chromate adsorption is spontaneous and endothermic in nature. Red Peanut Skin is used as a post treatment technique and the adsorption is coupled to light driven catalysis over the hetero-system CuAl2O4/TiO2. A reduction of more than 58% of HCrO− 4 after ~ 2 h is achieved under optimal conditions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the solution, the chromium presents different oxyanionic forms depending on the pH profile according to the potential-pH diagram. It is widespread in many industrial applications like electroplating and pigments, leather tanning, corrosion control and ferrochrome industry [1]. Because of its negative impact on the aquatic ecosystem, the hexavalent state is more concerned by the research. It is highly toxic because of its oxidizing properties and tends to accumulate in living organisms causing serious damages for bacteria, plants and animals (i.e. reduction of fish production) [2]. Cr(VI) is classified by the International Agency for Research on Cancer (IARC) at the top priority list of toxic pollutants. The maximum authorized in water is restricted at 5 mg L− 1 and the threshold concentration is a challenge for the −2 water quality [3]. Below pH 3, both HCrO− predominate 4 and Cr2O7 and the industrial effluents can contain up to 200 mg L− 1 [4]. Various techniques are currently used for the Cr(VI) removal among which are chemical precipitation, reverse osmosis, electrochemical reduction and ion exchange [5,6]. Such techniques are expensive and some of them are inefficient at low concentrations. So, the search of the methods concerning the water treatment becomes a topic of high priority. The adsorption is an inexpensive technique which does not require any special set up. A literature survey shows that some scientists have already studied different ways for the removal of
⁎ Corresponding author. Tel.: +213 21 24 79 50; fax: +213 0826 32 46 02. E-mail address: [email protected] (M. Trari). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.11.041
heavy metals and hazardous waste management [7]. The carbon based materials offer an attractive alternative to high cost techniques and have generated a renewed interest over the last decade providing energy safety, economic growth and environmental advantages (non toxicity) [8–12]. The adsorption processes appear to be multi determined and depend on both the characteristic of the adsorbent and the operating conditions. The objective of the present work is to evaluate the adsorption ability of Red Peanut Skin (RPS) for the chromate removal and to determine the influence of operating parameters on the adsorption mechanism. The RPS is a versatile material which has a large adsorption capacity. It consists of lignin and cellulose in hydroxylic and carboxylic groups playing an important role in binding ions through several mechanisms [13]. However, this study is mainly focused on the equilibrium isotherms and kinetic study of Cr(VI) adsorption. The adsorption greatly reduces the pollution but is not enough to assess its effectiveness at high concentration and does not comply with the standards of the world health organization [3]. At this level, the photocatalysis appears to be quite promising because of its simplicity. Cr(VI) is photo electrochemically reduced to a less harmful state (Cr3+) which can be safely recovered by precipitation or chelation on various substrates [14,15]. The hetero-system CuAl2O4/ TiO2, allows the extension of the spectral photo response of wide band semi conductors (SCs) toward the visible region. The enhancement is attributed to the electron transfer from the activated CuAl2O4 sensitizer to the conduction band of TiO2 (TiO2-CB) resulting in the reduction process. There were only few studies on the combined
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Fig. 1. A schematic diagram of the coupled adsorption/photocatalysis system and the corresponding set up.
systems (adsorption/photocatalysis) concerning the water decontamination. Considering this, the present work aimed at studying the isotherms and kinetic models in linear form to describe the sorption process on the RPS. In a second step, the system is directed to spinel CuAl2O4 which is presented as having the required photocatalytic parameters [16]. Its synthesis by co precipitation has proved its usefulness in the hydrogen photo evolution and the band gap (~1.7 eV) matches well with the solar spectrum which permits an absorption of ~ 90% of sunlight.
2. Materials and methods 2.1. Preparation and characterization of adsorbent Each RPS possesses its own specificity and because of the great variety of natural adsorbents, it is necessary to study the physicochemical characterization of our specimen. The RPS (origin, south Algeria) was washed to remove the dust, and then dried at 40 °C until a constant weight was reached. The sample was crushed in a mechanical grinder and after that the powder was preserved in a glass bottle for further use. The determination of the size distribution was performed on the powder by a laser size analysis (Mastersizer 2000 ver. 2.00, Malvern Instruments Ltd.). The FTIR spectrum was recorded with a Perkin-Elmer spectrometer (Perkin-Elmer spectrum One FTIR model) over the range (4000– 400 cm− 1). ~ 4 mg of powder was mixed with 100 mg of dried spectroscopic KBr and pressed into pellets. Particle size and morphology were observed by the scanning electron microscopy
(JSM-6301F SEM). The chemical analysis was carried out by using Xray fluorescence (Philips MagiX Pro spectrometer). The stock solution (1000 mg L− 1) was prepared by dissolving K2Cr2O7 (Assay, 99%) in distilled water and diluted to the concentration of the working solution (25–400 mg L− 1). The adsorption tests were carried out in a laboratory-scale cylindrical batch Rushthon reactor (Fig. 1) at a variable adsorbent concentration (2–6 g L− 1). The RPS was placed in the reactor and the mixture was homogeneously dispersed by mechanical stirring (350 rpm). The temperature was controlled up to 50 °C using a thermostated bath (Julabo). Above 50 °C, the water evaporation makes the analysis inaccurate. The pH was adjusted by HCl and NaOH and monitored by a pH-meter (Hanna pH 211). The Cr(VI) concentration was determined by spectrophotometry with a double beam UV-Visible spectrophotometer (Shimadzu UV 1800) using 1 cm path length quartz cell. The aliquots were withdrawn at regular times in order to determine the Cr(VI) concentration. The solution was then filtrated on a Wattman filter paper in order to separate the grains from the solution and systematically acidified by H2SO4. Cr(VI) was complexed with 1,5-
Table 1 Chemical composition of RPS. Composition
C.O.H
K
Ca
Mg
Si
Al
Zn
Fe
% Wt
96.98
1.02
0.78
0.53
0.22
0.11
0.01
0.08
Fig. 2. The particle size distribution of RPS.
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Fig. 3. FT-IR spectra of RPS in crude form.
diphenylcarbazide and the absorbance was measured at 540 nm [17,18]. The titrations were performed in triplicate and the mean values were reported. Details concerning the experimental protocol for the CuAl2O4 synthesis were previously described [19]. The X-ray diffraction revealed a homogeneous single phase with a brown color. The solgel is convenient for preparing the active variety of TiO2 (anatase). Photoelectrochemical experiments were done in a standard three electrode cell. A saturated calomel electrode (SCE) and Pt foil
(Tacussel) were used as reference and counter electrodes. The Mott Schottky characteristic was plotted by a PGZ301 potentiostat (Radiometer). Photocatalytic tests were performed in a double walled Pyrex reactor regulated at 25 °C (Fig. 1). The point of zero charge (pzc) has been accurately determined by measuring the equilibrium pH of an aqueous solution containing suspension of finely powdered RPS, which in our case occurs in 24 h with intermittent manual shaking. The solubility of salicylic acid, used as hole scavenger to prevent the photo driven corrosion of CuAl2O4 is weak and was fixed at 10− 4 M. 3. Result and discussion 3.1. Characterization of the adsorbent Even though the catalytic processes are related to the extrinsic properties of the adsorbent, a detailed understanding of the fundamental properties is required to get a high-quality material. The RPS is an abundant and non toxic waste product, stable over the pH range (1–8). It is composed mainly of organic matter and its composition, established by X-ray fluorescence, is given in Table 1. The mean size of the RPS powder, determined from the particle size distribution, works out to be 412 μm (Fig. 2). Measurements of specific surface area and pore volume were obtained from nitrogen adsorption isotherms at 77 K using a surface area analyzer (ASAP 2010 Micromeritics apparatus) which indicated the Langmuir area of 7.35 m2 g− 1 and pore volume of 0.0362 cm3 g− 1. The FTIR displays a number of fundamental absorption bands (Fig. 3). The broad intense
30
qe (mg g-1)
25 20 15 10 5 0 0
1
2
3
4
5
6
7
8
pH Fig. 4. (a) SEM of native RPS before Cr(VI) adsorption. (b) SEM of native RPS after Cr(VI) adsorption.
Fig. 5. Effect of pH on the Cr(VI) adsorption of 200 mg L− 1 onto RPS; adsorbent dosage 5 g L− 1; temperature 22 °C and speed agitation 350 rpm.
M. Kebir et al. / Desalination 270 (2011) 166–173
30
Table 2 Isotherm parameters of Langmuir and Freundlich model.
25
pH
qt (mg/g)
169
Langmuir model
20 1 2 3 4 5 6 7
15 10 5 0 0
5
10
15
20
25
30
Freundlich model
R2
b (L g− 1)
qo (mg g− 1)
R2
n
kf
0.973 0.992 0.998 0.992 0.996 0.998 0.999
0.400 0.594 0.116 0.131 0.230 0.309 0.504
44.05 30.96 26.59 21.78 18.80 19.96 18.80
0.968 0.975 0.989 0.951 0.959 0.984 0.963
0.323 0.297 0.097 0.114 0.075 0.116 0.126
15.60 12.51 7.07 11.27 12.25 11.26 10.60
35
Time (mn) Fig. 6. Effect of the initial concentration and contact time on the Cr(VI) removal onto RPS (+) 25 mg L− 1), (•) 50 mg L− 1, (*) 75 mg L− 1, (×) 100 mg L− 1, (▲) 200 mg L− 1, (■) 300 mg L− 1, (♦) 400 mg L− 1; adsorbent dosage 5 g L− 1 temperature 22 °C; speed agitation 350 rpm and pH 5.35.
peak at 3408.8 cm− 1 corresponds to O–H stretching vibrations of alcohols and carboxylic acids. The peaks 2918.7, 1440.5 and 1373.42 cm− 1 are attributed to C–H stretching vibrations of CH3 groups whereas those centered at 1739.8 and 1630.9 cm− 1 are assigned to the stretching vibration of C=O bond of carboxylic groups. The morphology of the RPS before adsorption (Fig. 4a) reveals a furrowed and asymmetric porous structure with large cavities that are able to host foreign ions. After chromate adsorption, the micrograph shows a closed structure which became more compact, due to Cr(VI) binding (Fig. 4b). The decrease of active surface after adsorption is due to the partial filling by HCrO− 4 species inside the cavities. The Cr(VI) uptake efficiency has been calculated from the relation: ðCo Ce Þ × 100 Co
Table 3 Adsorption capacity of different adsorbents for hexavalent chromium. Adsorbent
Optimal Initial Optimum dosage concentration pH (g L− 1) (mg L− 1)
Hazelnut shell 2.5 Almond shell 2.4 Saw dust 2.4 Wool 2.4 Maple waste 50 Boiled sunflower 4.0 steam Formaldehyde-treated 4.0 sunflower stem wood charcoal ash 5 wood charcoal 5 RPS 5.0
qe Ref (mg g− 1)
100 100 100 100 10 50
1.0 2.0 2.0 2.0 5.0 2.0
170 10.62 15.82 41.15 5.1 4.9
[24] [25] [25] [25] [26] [27]
50
2.0
3.6
[27]
588 588 100
2.0 2.5 1.0
30.10 46.17 44.05
[28] [28] This work
ð2Þ
been performed at 22 °C by varying the pH in the range (1–8) and the concentration Co from 25 to 400 mg L− 1. Cr(VI) exists in various oxyanion forms where the specie HCrO− 4 predominates at low pH. The surface is charged positively and this should favor the electrostatic interaction of chromate, thanks to pzc (6.15) and the free potential Uf (0.380 VSCE) of the RPS. The cationic Cr(VI) adsorption increases with the decreasing pH and peaks at pH 2 (Fig. 5), the mechanism of adsorption has already been proposed by Bayat [20]:
V represents the volume of the solution (L) and m the mass of adsorbent (mg). Repeated results have shown nearly the same results.
2H + 2HCrO4 ↔ 2H2 CrO4 ↔2H2 O + Cr2 O7 ↔ 2CrO3 + H2 O: ð3Þ
ð1Þ
where Co and Ce are the initial and equilibrium concentrations. The adsorbed amount qe (mg g− 1) is determined from the mass balance: qe ¼
ðCo Ce Þ × V: m
þ − 2H
þ
þ 2− 2H
3.2. Effect of pH on the Cr(VI) adsorption 3.3. Effect of initial concentration and contact time Concerning the optimization, only one parameter has been varied at a time while keeping the others constant. Experiment series have
It was noticed that the efficiency of Cr(VI) removal strongly depends on the concentration Co. At pH 5.35 and an adsorbent dosage
25 8.0
20
t/qt (min g mg-1)
qt (mg/g-1)
7.0 15 10 5 0
6.0 5.0 4.0 3.0 2.0 1.0
0
5
10
15
20
25
30
35
Time (mn) Fig. 7. Effect of adsorbent dose on the Cr(VI) adsorption by RPS;(♦) 2 g, (■) 3 g, (Δ) 4 g, (×) 5 g, (*) 6 g initial concentration 100 mgL− 1; temperature 22 °C; speed agitation 350 rpm and pH 5.35.
0.0
0
5
10
15
20
25
30
35
Time (mn) Fig. 8. Pseudo second order kinetic for Cr(VI) (◊)25 mg L− 1; (□)50 mg L− 1 (Δ) 75 mg L− 1, (*) 100 mg L− 1, (□) 200 mg L− 1 pH 5.35, temperature 25 °C and speed agitation 350 rpm.
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6
Table 4 Second order adsorption rate constants for different initial concentrations. Pseudo second order model qe (mg g− 1)
K2 (g mg− 1.min− 1)
R2
h (mg g− 1 min− 1)
25 50 75 100 200
3.872 9.132 13.368 14.814 27.247
1.335 0.991 0.074 0.079 0.027
1 0.999 0.998 0.998 0.990
20.014 82.642 13.224 17.336 20.044
4
of 5 g L− 1, the adsorbed amount qt increases with the increasing Co to reach a maximum at 200 mg L− 1 at a contact time of 30 min and under an agitation speed of 350 rpm. Fig. 6 shows that both the equilibrium time and qt are concentration dependent. The adsorption increases rapidly at the beginning and tends to saturation. The equilibrium time for low concentration averages 6 min, a value 2.5 fold greater than that obtained at high concentrations with a maximum capacity of 26.08 mg g− 1. The high adsorption capacity obtained at high initial concentrations can be attributed to the increased rate of mass transfer due to the increased driving force [21]. By contrast, at low concentrations, the ratio of the available surface to the Cr(VI) concentration Co is larger, so the amount is lesser (the removal is higher) and the equilibrium is rapidly reached. 3.4. Effect of adsorbent mass It has been found that the Cr(VI) adsorption depends also on the mass of the RPS; the amount qe increases from 13.62 to 20.08 mg− 1 g when increasing the dosage from 2 to 5 g L− 1, under the experimental condition (Co: 100 mg L− 1, 25 °C, pH 5.35) afterwards the efficiency remains nearly constant (Fig. 7). This may be due to the availability of adsorption sites per unit mass of adsorbent [22]. 3.5. Adsorption isotherms To get a deeper understanding of the chromate uptake, the adsorption isotherms were plotted to determine the nature of the solute–surface interaction. To reach this purpose, various models have been used; some of them have either a theoretical foundation or an empirical nature. In the present work, only the Langmuir and Freundlich models have been tested [23]: Ce 1 C = + e bqo qe qo
3 3.05
3.1
3.15
40 35
3.25
3.3
3.35
3.4
3.45
Fig. 10. Arrhenius plot for Cr(VI) adsorption onto RPS.
where qo and b are the Langmuir constants related to the adsorption capacity (mg g− 1) and the energy of adsorption (L mg− 1) respectively in the same manner as Kf and n are for the Freundlich constants. The constants (Table 2) have been calculated from the slopes and intercepts of the isotherms. The correlation coefficients (R2) are higher in the Langmuir isotherm, indicating a good relationship between the pH and concentrations. The Langmuir model confirms the monolayer coverage for the adsorption of Cr(VI) onto the RPS surface, where the maximal adsorption capacity (44.05 mg g− 1) occurs at pH 1. Comparing the results of the literature (Table 3), it is clearly observed that except Hazelnut shell, RPS exhibits the highest performance among the adsorbents investigated so far [24–28]. 3.6. Adsorption kinetics The adsorption processes require the knowledge of both the kinetic and the design parameters. That is why, the pseudo first and second order models [29] have been applied for the experimental data. Pseudo-first order kinetic model dqt = k1 ðqe qt Þ dt
ð6Þ
k1 being the adsorption rate constant. After applying the boundary conditions (t = 0 to t and q = 0 to q = qt), Eq. (6) can be written as follows: logðqe qt Þ = logðqe Þ
ð5Þ
3.2
103 (K-1)
ð4Þ
lnqe = lnKf + nlnCe
k1 t 2:303
ð7Þ
Pseudo-second order kinetic model The pseudo second order kinetic is given by: dqt 2 = k2 ðqe qt Þ dt
30
qt (mg/g-1)
5
LnKc
Initial concentration (mg L− 1)
ð8Þ
where k2 is the rate constant (g mg− 1 min− 1). The integration of Eq (8) at the boundary conditions is linearized:
25 20 15
t 1 t = + qt qe k2 q2e
10
ð9Þ
5 0
0
5
10
15 20 Time (mn)
25
30
35
Fig. 9. Effect of temperature of the Cr(VI) adsorption onto RPS(×) 20 °C, (Δ) 30 °C, (□) 40 °C, (◊); initial concentration 100 mg L− 1; adsorbent dosage 5 g L− 1; speed agitation 350 rpm; pH 5.35.
Table 5 Thermodynamics parameters. ΔHo (kJ moL− 1)
ΔSo (J moL− 1 K− 1)
25.235
119.20
ΔGo(kJ moL− 1) 293 K
303 K
313 K
323 K
− 34.900
− 36.092
− 37.284
− 38.476
M. Kebir et al. / Desalination 270 (2011) 166–173
1.8
due to a greater mobility of ionic species from the bulk solution to the interface [30]. In addition, this increase can either accelerate the adsorption steps or generate new active sites on the RPS surface. The thermodynamic parameters ΔH°, ΔG° and ΔS° are determined from the distribution coefficient Kdis which obeys to the Arrhenius type law:
1.5
10-7 (ηhν) ν)2 (eV2)
171
1.2 0.9 E g =1.70 eV
0.6
K = dis
0.3 0.0 1.6
2.0
2.4
2.8
lnKdis =
hν ν (eV) Fig. 11. Determination of the indirect optical transition of CuAl2O4.
The rate constants have been used to calculate the initial adsorption rate h (mg g− 1 min− 1) given by: 2
ð11Þ
3.2
h = k2 qe :
Cad;eq CL;eq
ð10Þ
The correlation coefficients indicate the best fit between the calculated and experimental data and the Cr(VI) adsorption is well described by the pseudo second order model. The Cr(VI) removal by the RPS is concentration dependent. With increasing Co from 25 to 40 mg L− 1, the amount of Cr(VI) adsorbed at equilibrium increased from 3.872 to 27.247 mg g− 1. The rate h has no specific role while k2 decreases with the increase of the concentration Co. The values of k2, qe and h determined from the intercepts and the slopes of plots t/qt versus t (Fig. 8) are shown in Table 4. 3.7. Thermodynamics for adsorption Cr(VI) onto RPS The Cr(VI) adsorption has been studied in the temperature range (20–50 °C), above which the loss of water through vaporization becomes an important problem. Fig. 9 shows that the adsorption capacity increases with temperature. In term of uptake removal, an 85% of Cr(VI) was obtained at 50 °C in less than 30 min. This is mainly
ΔS ΔH : R RT
ð12Þ
As a result, ΔH° and ΔS° obtained from the slope and the intercept of the plots lnKdis versus 1/T (Fig. 10) are listed in Table 5. Cad,eq and CL,eq are, respectively the equilibrium concentrations of Cr(VI) on the adsorbent and in the solution. The positive enthalpy (ΔH° =25.235 kJ moL− 1) indicates an endothermic nature of Cr(VI) adsorption (physical sorption) which is thermodynamically favorable as an evidence from the negative ΔG° values. The entropy (ΔS° =119.20 J moL− 1 K− 1) reflects an increase of the randomness at the solid/solution interface. The uptake of metal ions greatly lowers the pollution but not enough to comply with the standards of the water quality. Consequently, it can be used as post treatment for photocatalytic process. The photocatalysis is an emerging field for the recovery of heavy metals at low concentrations [31]. Using colloidal semi conductor particles as light absorbing units is simple and does not need any sophisticated device. The first step is to generate electronhole (e−/h+) pairs by photons with energy higher than the band gap (Eg). Cr(VI) is photo-electrochemically converted to trivalent state. With a smaller size and a positive charge, Cr3+ is repelled by the surface and released in a solution in which it can be recovered as insoluble hydroxide (Ks ~10− 30). In this respect, the spinel CuAl2O4 shows light absorption of wavelengths shorter than 730 nm. The precise Eg value of the band gap (1.70 eV) has been determined from the plot of (αhν)2 vs. hν (Fig. 11) where α is the optical absorption coefficient and hν is the incident light. The oxide is chemically stable in acidic solutions and has been elaborated via nitrate route in order to -4.74
Vacuum (0) Solution
n-TiO2
p-CuAl2O4 e-
≈
-1.05 V e-
e- 0.8 V
Eg= 1.77 eV - 0.35
H+/H2 ΔE= 0.2 eV
Eg= 3.2 eV
Cr2O72-/Cr3+
+ 0.58
C7H5O3*/C7H6O3 h+
0.721
2.4 V
Energy (eV)
Potential (VSCE)
Fig. 12. The energy band diagram of CuAl2O4/TiO2/electrolyte junction. Scheme of the photoelectrochemical cell.
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M. Kebir et al. / Desalination 270 (2011) 166–173
a
9 6 V
3 0
6 5
12
0.0
0.1
0.2
0.3
fb
= + 0.45 V
0.4
0.5
Potential (V)
104 C (Mol L-1)
C-2 (mF-2 cm4)
15
4 3 2 1
Fig. 13. The Mott–Schottky plot of CuAl2O4 in H2SO4 medium (0.05 M).
0 60
40
P = 4:75 + eVfb + ΔE–Eg
ð13Þ
where ΔE is the difference between the bottom of the conduction band and the Fermi level of the semiconductor. A value of 0.17 eV was determined from the conductivity measurements performed on sintered pellet [19]. The flat band potential Vfb is obtained from the Mott–Schottky plot (Fig. 13). The difference (Vfb − Ered) measures the driving force of the junction and for a zero (e−/h+) pairs recombination, a value of at least ~ 0.3 V is required. However, for CuAl2O4 the difference is too large (~ 1.6 V) and only a weak percentage of chromate is reduced. The visible light-induced chromium conversion is mediated by photo electrons via TiO2. The improved activity is attributed to the electron injection from the activated CuAl2O4-CB, acting as electron pump, into TiO2-CB (− 0.8 V) resulting in the Cr(VI) reduction. The powder was kept overnight in the solution to ensure a complete dark HCrO− 4 adsorption before illumination. The heterosystem acts in a short circuited configuration through the electrolytic acid solution. The photocatalytic tests are carried out with various amounts of catalyst and the results gave an optimal value of 1.25 mg mL− 1. A reduction of more than 58% of HCrO− 4 after ~2 h is achieved under optimal conditions (pH ~ 2 at 25 °C) with a mass concentration CuAl2O4/TiO2 of (1/1). Photocatalytic oxidation of organic pollutants has been already studied for water purification [32]. It contributes significantly to the metal recovery because of the longer lifetime of the photo carriers. Indeed, the chromate reduction has been greatly improved in presence of salicylic acid, due to the separation of (e−/h+) pairs located in CuAl2O4-VB (+0.72 V). The p-CuAl2O4/n-TiO2 junction catalyses the downhill reaction: 2
þ
3þ
2Cr2 O7 + 3C7 H6 O3 + 18O2 + 16H →4Cr
b
Reduction %
increase the active surface. To perform the chromate reduction, the conduction band (CB) of CuAl2O4 (−1 V) must have a potential Ered 3+ below that of the HCrO− couple (Fig. 12). The energetic position 4 /Cr of CuAl2O4-CB is given by:
80
100
120
Time (min) 60
40
20
0
no salicylic acid
salicylic acid
Fig. 14. The Cr(VI) reduction in the presence of salicylic acid (a), the effect of the salicylic acid(b).
4. Conclusion The main goal of this work is to develop a low cost material for the chromate removal in aqueous media. The high capacity of the RPS to adsorb Cr(VI) has been demonstrated and the results reveal that more than 85% are removed. The Cr(VI) adsorption is dependent on the physical parameters like the initial concentration, time contact, pH and temperature. The process is well described by the Langmuir model with monolayer adsorption coverage, which is comparatively larger than most adsorbents reported to date. The thermodynamic
+ 21CO2 + 17H2 O ð14Þ
with a free enthalpy of −557 kcal moL− 1. It is helpful to outline that the reduction percentage decreases down to 25.2% in the absence of salicylic acid which acts as hole scavenger and favors the charge separation (Fig. 14a) The removal process follows a first order kinetic with a half life of 105 min where a 58% of Cr(VI) was reduced to trivalent state upon visible illumination (Fig. 14b). The product formed after 2 h illumination in a buffered chromate solution turned out to be Cr3+. This feature has found an experimental support, the visible band intensity due to Cr(VI) at 352 nm decreased and a new band attributed to the formation of Cr3+ appeared at 298 nm (Fig. 15). The same reaction will be tested over a fixed bed. New experiments will be carried out and reported in the next paper.
Fig. 15. UV-Visible spectra of (a) initial solution Cr(III), (b) 2 h illuminated solution Cr(VI).
M. Kebir et al. / Desalination 270 (2011) 166–173
parameters reveal that the adsorption process is spontaneous and endothermic. The adsorption is rapid and follows a pseudo second order kinetic. The results indicate that the coupled system for wastewater may be a viable alternative and a new way to increase the Cr(VI) removal. The photocatalyst CuAl2O4 elaborated by co precipitation, has been tested successfully. The photoreduction obeys to the first order kinetic and the concentration is lowered down to the threshold required by the international regulations concerning pollution. Acknowledgement The authors gratefully acknowledge the financial support from the Faculty of Engineering Mechanic and Engineering Processes. They would like to express their gratitude to Pr R. Maachi. References [1] D.E. Cummings, S. Fendorf, N. Singh, B.M. Peyton, T.S. Magnuson, Reduction of Cr (VI) under acidic conditions by the facultative Fe(III)-reducing bacterium Acidiphilium cryptum, Environ. Sci. Technol. 41 (2007) 146–152. [2] E. Cooney, B. Luo, J.W. Patterson, C. Petropoulou, Toxicity reduction methodologies, in: D.L. Frod (Ed.), Water Quality Management Library, vol. 3, 1992, pp. 109–1818, Rechnomic, Lancaster. [3] World Health Organization international Agency for Research on Cancer, IARC Monographs on the Evaluation of Carcinogenic Risks to Human: Chromium, Nickel and Welding, Lyon France, 1990. [4] S. Baral, N. Das, T.S. Ramulu, S.K. Sahoo, S.N. Das, G. Roy Chaudhury, Removal of Cr (VI) by thermally activated weed Salvinia cucullata in a fixed-bed column, J. Hazard. Mater. 161 (2009) 1427–1435. [5] Y. Wu, B. Li, S. Feng, X. Mi, J. Jiang, Adsorption of Cr(VI) and As(III) on coaly activated carbon in single and binary systems, Desalination 249 (2009) 1067–1073. [6] S. Gupta, B.V. Babu, Removal of toxic metal Cr(VI) from aqueous solutions using sawdust as adsorbent: equilibrium, kinetics and regeneration studies, Chem. Eng. J. 150 (2009) 352–365. [7] S.E. Baily, T.J. Olin, R.M. Bricka, D.D. Adrian, A review of potentially low-cost sorbents for heavy metals, Water Res. 33 (1999) 2469–2479. [8] V. Vinodhini, N. Das, Packed bed column studies on Cr (VI) removal from tannery wastewater by neem sawdust, Desalination 264 (2010) 9–14. [9] D. Mohan, C.U. Pittman Jr., Activated carbons and low cost adsorbents for remediation of tri-and hexavalent chromium from water, J. Hazard. Mater. B137 (2006) 762–811. [10] E.I. El-Shafey, Removal of Se(IV) from aqueous solution using sulphuric acid treated peanut shell, J. Environ. Manage. 84 (2007) 620–627. [11] A. El-Hamouz, H.S. Hilal, N. Nassar, Z. Mardawi, Solid olive waste in environmental cleanup: oil recovery and carbon production for water purification, J. Environ. Manage. 84 (2007) 83–92.
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[12] H. Aydin, Y. Bulut, Ç. Yerlikaya, Removal of copper (II) from aqueous solution by adsorption onto low-cost adsorbents, J. Environ. Manage. 87 (2008) 37–45. [13] A.K. Bhattacharaya, T.K. Naiya, S.N. Mandal, S.K. Das, Adsorption, kinetic and equilibrium studies on removal of Cr(VI) from aqueous solutions using different low-cost adsorbents, J. Chem. Eng. 137 (2008) 529–541. [14] S.H. Lin, C.N. Chen, R.S. Juang, Structure and thermal stability of toxic chromium (VI) species doped onto TiO2 powders through heat treatment, J. Environ. Manage. 90 (2009) 1950–1955. [15] F. Jiang, Z. Zheng, Z. Xu, S. Zheng, Z. Guo, L. Chen, Aqueous Cr(VI) photo-reduction catalyzed by TiO2 and sulfated TiO2, J. Hazard. Mater. B134 (2006) 94–103. [16] S. Boumaza, A. Boudjemaa, A. Bouguelia, R. Bouarab, M. Trari, Visible light induced hydrogen evolution on new hetero-system ZnFe2O4/SrTiO3, Appl. Energy 87 (2010) 2230–2236. [17] Ü. Daniş, B. Keskinler, Chromate removal from wastewater using micellar enhanced crossflow filtration: Effect of transmembrane pressure and crossflow velocity, Desalination 249 (2009) 1356–1364. [18] D. Park, Y.S. Yun, J.M. Park, Mechanisms of removal of hexavalent chromium by biomaterials or biomaterials-based actived carbons, J. Hazard. Mater. 137 (2006) 1254–1257. [19] S. Saadi, A. Bouguelia, M. Trari, Photoassisted hydrogen evolution over spinel CuM2O4(M = Al, Cr, Mn, Fe and Co), Renewable Energy 31 (2006) 2245–2256. [20] B. Bayat, Comparative study of adsorption properties of Turkish fly ashes. II. The case of Chromium (VI) and Cadmium (II), J. Hazard. Mater. 95 (2002) 275–290. [21] U.K. Garg, M.P. Kaur, V.K. Garg, D. Sud, Removal of chromium(VI) from dilute aqueous solution by adsorption on agricultural waste biomass using a response surface methodological approach, Bioresour. Technol. 99 (2008) 1325–1331. [22] K. Kadirvelu, C. Faur-Brasquet, P. Le Cloirec, Removal of Pb(II), Cu(II) and Ni(II) ions by adsorption onto actived carbon cloths, Langmuir 16 (2000) 8404–8409. [23] M. Chabani, A. Amrane, A. Bensmaili, Equilibrium sorption isotherms for nitrate on resin Amberlite IRA 400, J. Hazard. Mater. 165 (2009) 27–33. [24] M. Kobaya, Removal of Cr(VI) from aqueous solution by adsorption onto hazelnut shell activated carbon: kinetic and equilibrium studies, Bioresour. Technol. 91 (2004) 317–321. [25] M. Dakiky, M. Khamis, A. Manassra, M. Mer'eb, Selective adsorption of Cr(VI) in industrial wastewater using low cost abundantly available adsorbents, Adv. Environ. Res. 6 (2002) 533–540. [26] L. Yu, S. Shukla, K. Dorris, A. Shukla, J. Margrave, Adsorption of Cr from aqueous solution by maple sawdust, J. Hazard. Mater. 100 (2003) 53–63. [27] M. jain, V.K. Garg, K. Kadirvelu, Chromium(VI) removal from aqueous system using Helianthus annuus (sunflower) stem waste, J. Hazard. Mater. 162 (2009) 365–372. [28] E. Pehlivan, H. Kahraman, E. Pehlivan, Sorption equilibrium of Cr(VI) ions on oak wood charcoal (Carbo Ligni) and charcoal ash as low-cost adsorbents, Fuel Process. Technol. 92 (2011) 65–70. [29] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process. Biochem. 34 (1999) 451–465. [30] K.K. Singh, R.D.C. Rastogi, S.H. Hasan, Low cost biosorbent ‘Maize Bran’ for the removal of Cd(II) from wastewater, J. Indian Chem. Soc. 82 (2005) 392–396. [31] S. Omeiri, Y. Gabes, A. Bouguelia, M. Trari, Photoelectrochemical characterization of the delafossite CuFeO2: application to removal of divalent metals ions, J. Elect. Chem. 614 (2008) 31–40. [32] A. Idris, N. Hassan, N.S. Mohd Ismail, E. Misran, N. Mohd Yusof, A.F. Ngomsik, A. Bee, Photocatalytic magnetic separable beads for chromium (VI) reduction, Water Res. (2009) 1–6.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Coupling adsorption with photocatalysis process for the Cr(VI) removal M. Kebir a, M. Chabani a, N. Nasrallah a, A. Bensmaili a, M. Trari b,⁎ a b
Laboratory of Engineering Reaction Faculty of Engineering Mechanic and Engineering Processes USTHB, BP 32, Algiers, Algeria Laboratory of Storage and Valorization of Renewable Energies Faculty of Chemistry USTHB, BP 32, Algiers, Algeria
a r t i c l e
i n f o
Article history: Received 12 August 2010 Received in revised form 22 November 2010 Accepted 23 November 2010 Available online 5 January 2011 Keywords: Adsorption Cr(VI) Kinetic Isotherm Photoreduction
a b s t r a c t The adsorption of Cr(VI) onto red peanut skin and its photoelectrochemical reduction into Cr3+ were investigated. The material has been characterized by infrared spectroscopy and scanning electron microscopy. It was successfully applied to the removal of chromium in aqueous air equilibrated powder suspension. The influence of the contact time, initial concentration, pH and temperature has been studied. The best performance occurred at 50 °C in the acidic solution (200 mg L− 1, pH 2) with an uptake removal of 85% in less than 30 min and the adsorption follows a pseudo second order kinetic. The experiments have been carried out in a batch reactor; the data were evaluated using the linear isotherms of Langmuir and Freundlich models. The maximal adsorption capacity (44.05 mg g− 1) has been determined from the Langmuir model. The thermal variation obeys to the Arrhenius type law from which the thermodynamic parameters ΔH°, ΔS° and ΔG° have been deduced. The chromate adsorption is spontaneous and endothermic in nature. Red Peanut Skin is used as a post treatment technique and the adsorption is coupled to light driven catalysis over the hetero-system CuAl2O4/TiO2. A reduction of more than 58% of HCrO− 4 after ~ 2 h is achieved under optimal conditions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the solution, the chromium presents different oxyanionic forms depending on the pH profile according to the potential-pH diagram. It is widespread in many industrial applications like electroplating and pigments, leather tanning, corrosion control and ferrochrome industry [1]. Because of its negative impact on the aquatic ecosystem, the hexavalent state is more concerned by the research. It is highly toxic because of its oxidizing properties and tends to accumulate in living organisms causing serious damages for bacteria, plants and animals (i.e. reduction of fish production) [2]. Cr(VI) is classified by the International Agency for Research on Cancer (IARC) at the top priority list of toxic pollutants. The maximum authorized in water is restricted at 5 mg L− 1 and the threshold concentration is a challenge for the −2 water quality [3]. Below pH 3, both HCrO− predominate 4 and Cr2O7 and the industrial effluents can contain up to 200 mg L− 1 [4]. Various techniques are currently used for the Cr(VI) removal among which are chemical precipitation, reverse osmosis, electrochemical reduction and ion exchange [5,6]. Such techniques are expensive and some of them are inefficient at low concentrations. So, the search of the methods concerning the water treatment becomes a topic of high priority. The adsorption is an inexpensive technique which does not require any special set up. A literature survey shows that some scientists have already studied different ways for the removal of
⁎ Corresponding author. Tel.: +213 21 24 79 50; fax: +213 0826 32 46 02. E-mail address: [email protected] (M. Trari). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.11.041
heavy metals and hazardous waste management [7]. The carbon based materials offer an attractive alternative to high cost techniques and have generated a renewed interest over the last decade providing energy safety, economic growth and environmental advantages (non toxicity) [8–12]. The adsorption processes appear to be multi determined and depend on both the characteristic of the adsorbent and the operating conditions. The objective of the present work is to evaluate the adsorption ability of Red Peanut Skin (RPS) for the chromate removal and to determine the influence of operating parameters on the adsorption mechanism. The RPS is a versatile material which has a large adsorption capacity. It consists of lignin and cellulose in hydroxylic and carboxylic groups playing an important role in binding ions through several mechanisms [13]. However, this study is mainly focused on the equilibrium isotherms and kinetic study of Cr(VI) adsorption. The adsorption greatly reduces the pollution but is not enough to assess its effectiveness at high concentration and does not comply with the standards of the world health organization [3]. At this level, the photocatalysis appears to be quite promising because of its simplicity. Cr(VI) is photo electrochemically reduced to a less harmful state (Cr3+) which can be safely recovered by precipitation or chelation on various substrates [14,15]. The hetero-system CuAl2O4/ TiO2, allows the extension of the spectral photo response of wide band semi conductors (SCs) toward the visible region. The enhancement is attributed to the electron transfer from the activated CuAl2O4 sensitizer to the conduction band of TiO2 (TiO2-CB) resulting in the reduction process. There were only few studies on the combined
M. Kebir et al. / Desalination 270 (2011) 166–173
167
Fig. 1. A schematic diagram of the coupled adsorption/photocatalysis system and the corresponding set up.
systems (adsorption/photocatalysis) concerning the water decontamination. Considering this, the present work aimed at studying the isotherms and kinetic models in linear form to describe the sorption process on the RPS. In a second step, the system is directed to spinel CuAl2O4 which is presented as having the required photocatalytic parameters [16]. Its synthesis by co precipitation has proved its usefulness in the hydrogen photo evolution and the band gap (~1.7 eV) matches well with the solar spectrum which permits an absorption of ~ 90% of sunlight.
2. Materials and methods 2.1. Preparation and characterization of adsorbent Each RPS possesses its own specificity and because of the great variety of natural adsorbents, it is necessary to study the physicochemical characterization of our specimen. The RPS (origin, south Algeria) was washed to remove the dust, and then dried at 40 °C until a constant weight was reached. The sample was crushed in a mechanical grinder and after that the powder was preserved in a glass bottle for further use. The determination of the size distribution was performed on the powder by a laser size analysis (Mastersizer 2000 ver. 2.00, Malvern Instruments Ltd.). The FTIR spectrum was recorded with a Perkin-Elmer spectrometer (Perkin-Elmer spectrum One FTIR model) over the range (4000– 400 cm− 1). ~ 4 mg of powder was mixed with 100 mg of dried spectroscopic KBr and pressed into pellets. Particle size and morphology were observed by the scanning electron microscopy
(JSM-6301F SEM). The chemical analysis was carried out by using Xray fluorescence (Philips MagiX Pro spectrometer). The stock solution (1000 mg L− 1) was prepared by dissolving K2Cr2O7 (Assay, 99%) in distilled water and diluted to the concentration of the working solution (25–400 mg L− 1). The adsorption tests were carried out in a laboratory-scale cylindrical batch Rushthon reactor (Fig. 1) at a variable adsorbent concentration (2–6 g L− 1). The RPS was placed in the reactor and the mixture was homogeneously dispersed by mechanical stirring (350 rpm). The temperature was controlled up to 50 °C using a thermostated bath (Julabo). Above 50 °C, the water evaporation makes the analysis inaccurate. The pH was adjusted by HCl and NaOH and monitored by a pH-meter (Hanna pH 211). The Cr(VI) concentration was determined by spectrophotometry with a double beam UV-Visible spectrophotometer (Shimadzu UV 1800) using 1 cm path length quartz cell. The aliquots were withdrawn at regular times in order to determine the Cr(VI) concentration. The solution was then filtrated on a Wattman filter paper in order to separate the grains from the solution and systematically acidified by H2SO4. Cr(VI) was complexed with 1,5-
Table 1 Chemical composition of RPS. Composition
C.O.H
K
Ca
Mg
Si
Al
Zn
Fe
% Wt
96.98
1.02
0.78
0.53
0.22
0.11
0.01
0.08
Fig. 2. The particle size distribution of RPS.
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Fig. 3. FT-IR spectra of RPS in crude form.
diphenylcarbazide and the absorbance was measured at 540 nm [17,18]. The titrations were performed in triplicate and the mean values were reported. Details concerning the experimental protocol for the CuAl2O4 synthesis were previously described [19]. The X-ray diffraction revealed a homogeneous single phase with a brown color. The solgel is convenient for preparing the active variety of TiO2 (anatase). Photoelectrochemical experiments were done in a standard three electrode cell. A saturated calomel electrode (SCE) and Pt foil
(Tacussel) were used as reference and counter electrodes. The Mott Schottky characteristic was plotted by a PGZ301 potentiostat (Radiometer). Photocatalytic tests were performed in a double walled Pyrex reactor regulated at 25 °C (Fig. 1). The point of zero charge (pzc) has been accurately determined by measuring the equilibrium pH of an aqueous solution containing suspension of finely powdered RPS, which in our case occurs in 24 h with intermittent manual shaking. The solubility of salicylic acid, used as hole scavenger to prevent the photo driven corrosion of CuAl2O4 is weak and was fixed at 10− 4 M. 3. Result and discussion 3.1. Characterization of the adsorbent Even though the catalytic processes are related to the extrinsic properties of the adsorbent, a detailed understanding of the fundamental properties is required to get a high-quality material. The RPS is an abundant and non toxic waste product, stable over the pH range (1–8). It is composed mainly of organic matter and its composition, established by X-ray fluorescence, is given in Table 1. The mean size of the RPS powder, determined from the particle size distribution, works out to be 412 μm (Fig. 2). Measurements of specific surface area and pore volume were obtained from nitrogen adsorption isotherms at 77 K using a surface area analyzer (ASAP 2010 Micromeritics apparatus) which indicated the Langmuir area of 7.35 m2 g− 1 and pore volume of 0.0362 cm3 g− 1. The FTIR displays a number of fundamental absorption bands (Fig. 3). The broad intense
30
qe (mg g-1)
25 20 15 10 5 0 0
1
2
3
4
5
6
7
8
pH Fig. 4. (a) SEM of native RPS before Cr(VI) adsorption. (b) SEM of native RPS after Cr(VI) adsorption.
Fig. 5. Effect of pH on the Cr(VI) adsorption of 200 mg L− 1 onto RPS; adsorbent dosage 5 g L− 1; temperature 22 °C and speed agitation 350 rpm.
M. Kebir et al. / Desalination 270 (2011) 166–173
30
Table 2 Isotherm parameters of Langmuir and Freundlich model.
25
pH
qt (mg/g)
169
Langmuir model
20 1 2 3 4 5 6 7
15 10 5 0 0
5
10
15
20
25
30
Freundlich model
R2
b (L g− 1)
qo (mg g− 1)
R2
n
kf
0.973 0.992 0.998 0.992 0.996 0.998 0.999
0.400 0.594 0.116 0.131 0.230 0.309 0.504
44.05 30.96 26.59 21.78 18.80 19.96 18.80
0.968 0.975 0.989 0.951 0.959 0.984 0.963
0.323 0.297 0.097 0.114 0.075 0.116 0.126
15.60 12.51 7.07 11.27 12.25 11.26 10.60
35
Time (mn) Fig. 6. Effect of the initial concentration and contact time on the Cr(VI) removal onto RPS (+) 25 mg L− 1), (•) 50 mg L− 1, (*) 75 mg L− 1, (×) 100 mg L− 1, (▲) 200 mg L− 1, (■) 300 mg L− 1, (♦) 400 mg L− 1; adsorbent dosage 5 g L− 1 temperature 22 °C; speed agitation 350 rpm and pH 5.35.
peak at 3408.8 cm− 1 corresponds to O–H stretching vibrations of alcohols and carboxylic acids. The peaks 2918.7, 1440.5 and 1373.42 cm− 1 are attributed to C–H stretching vibrations of CH3 groups whereas those centered at 1739.8 and 1630.9 cm− 1 are assigned to the stretching vibration of C=O bond of carboxylic groups. The morphology of the RPS before adsorption (Fig. 4a) reveals a furrowed and asymmetric porous structure with large cavities that are able to host foreign ions. After chromate adsorption, the micrograph shows a closed structure which became more compact, due to Cr(VI) binding (Fig. 4b). The decrease of active surface after adsorption is due to the partial filling by HCrO− 4 species inside the cavities. The Cr(VI) uptake efficiency has been calculated from the relation: ðCo Ce Þ × 100 Co
Table 3 Adsorption capacity of different adsorbents for hexavalent chromium. Adsorbent
Optimal Initial Optimum dosage concentration pH (g L− 1) (mg L− 1)
Hazelnut shell 2.5 Almond shell 2.4 Saw dust 2.4 Wool 2.4 Maple waste 50 Boiled sunflower 4.0 steam Formaldehyde-treated 4.0 sunflower stem wood charcoal ash 5 wood charcoal 5 RPS 5.0
qe Ref (mg g− 1)
100 100 100 100 10 50
1.0 2.0 2.0 2.0 5.0 2.0
170 10.62 15.82 41.15 5.1 4.9
[24] [25] [25] [25] [26] [27]
50
2.0
3.6
[27]
588 588 100
2.0 2.5 1.0
30.10 46.17 44.05
[28] [28] This work
ð2Þ
been performed at 22 °C by varying the pH in the range (1–8) and the concentration Co from 25 to 400 mg L− 1. Cr(VI) exists in various oxyanion forms where the specie HCrO− 4 predominates at low pH. The surface is charged positively and this should favor the electrostatic interaction of chromate, thanks to pzc (6.15) and the free potential Uf (0.380 VSCE) of the RPS. The cationic Cr(VI) adsorption increases with the decreasing pH and peaks at pH 2 (Fig. 5), the mechanism of adsorption has already been proposed by Bayat [20]:
V represents the volume of the solution (L) and m the mass of adsorbent (mg). Repeated results have shown nearly the same results.
2H + 2HCrO4 ↔ 2H2 CrO4 ↔2H2 O + Cr2 O7 ↔ 2CrO3 + H2 O: ð3Þ
ð1Þ
where Co and Ce are the initial and equilibrium concentrations. The adsorbed amount qe (mg g− 1) is determined from the mass balance: qe ¼
ðCo Ce Þ × V: m
þ − 2H
þ
þ 2− 2H
3.2. Effect of pH on the Cr(VI) adsorption 3.3. Effect of initial concentration and contact time Concerning the optimization, only one parameter has been varied at a time while keeping the others constant. Experiment series have
It was noticed that the efficiency of Cr(VI) removal strongly depends on the concentration Co. At pH 5.35 and an adsorbent dosage
25 8.0
20
t/qt (min g mg-1)
qt (mg/g-1)
7.0 15 10 5 0
6.0 5.0 4.0 3.0 2.0 1.0
0
5
10
15
20
25
30
35
Time (mn) Fig. 7. Effect of adsorbent dose on the Cr(VI) adsorption by RPS;(♦) 2 g, (■) 3 g, (Δ) 4 g, (×) 5 g, (*) 6 g initial concentration 100 mgL− 1; temperature 22 °C; speed agitation 350 rpm and pH 5.35.
0.0
0
5
10
15
20
25
30
35
Time (mn) Fig. 8. Pseudo second order kinetic for Cr(VI) (◊)25 mg L− 1; (□)50 mg L− 1 (Δ) 75 mg L− 1, (*) 100 mg L− 1, (□) 200 mg L− 1 pH 5.35, temperature 25 °C and speed agitation 350 rpm.
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6
Table 4 Second order adsorption rate constants for different initial concentrations. Pseudo second order model qe (mg g− 1)
K2 (g mg− 1.min− 1)
R2
h (mg g− 1 min− 1)
25 50 75 100 200
3.872 9.132 13.368 14.814 27.247
1.335 0.991 0.074 0.079 0.027
1 0.999 0.998 0.998 0.990
20.014 82.642 13.224 17.336 20.044
4
of 5 g L− 1, the adsorbed amount qt increases with the increasing Co to reach a maximum at 200 mg L− 1 at a contact time of 30 min and under an agitation speed of 350 rpm. Fig. 6 shows that both the equilibrium time and qt are concentration dependent. The adsorption increases rapidly at the beginning and tends to saturation. The equilibrium time for low concentration averages 6 min, a value 2.5 fold greater than that obtained at high concentrations with a maximum capacity of 26.08 mg g− 1. The high adsorption capacity obtained at high initial concentrations can be attributed to the increased rate of mass transfer due to the increased driving force [21]. By contrast, at low concentrations, the ratio of the available surface to the Cr(VI) concentration Co is larger, so the amount is lesser (the removal is higher) and the equilibrium is rapidly reached. 3.4. Effect of adsorbent mass It has been found that the Cr(VI) adsorption depends also on the mass of the RPS; the amount qe increases from 13.62 to 20.08 mg− 1 g when increasing the dosage from 2 to 5 g L− 1, under the experimental condition (Co: 100 mg L− 1, 25 °C, pH 5.35) afterwards the efficiency remains nearly constant (Fig. 7). This may be due to the availability of adsorption sites per unit mass of adsorbent [22]. 3.5. Adsorption isotherms To get a deeper understanding of the chromate uptake, the adsorption isotherms were plotted to determine the nature of the solute–surface interaction. To reach this purpose, various models have been used; some of them have either a theoretical foundation or an empirical nature. In the present work, only the Langmuir and Freundlich models have been tested [23]: Ce 1 C = + e bqo qe qo
3 3.05
3.1
3.15
40 35
3.25
3.3
3.35
3.4
3.45
Fig. 10. Arrhenius plot for Cr(VI) adsorption onto RPS.
where qo and b are the Langmuir constants related to the adsorption capacity (mg g− 1) and the energy of adsorption (L mg− 1) respectively in the same manner as Kf and n are for the Freundlich constants. The constants (Table 2) have been calculated from the slopes and intercepts of the isotherms. The correlation coefficients (R2) are higher in the Langmuir isotherm, indicating a good relationship between the pH and concentrations. The Langmuir model confirms the monolayer coverage for the adsorption of Cr(VI) onto the RPS surface, where the maximal adsorption capacity (44.05 mg g− 1) occurs at pH 1. Comparing the results of the literature (Table 3), it is clearly observed that except Hazelnut shell, RPS exhibits the highest performance among the adsorbents investigated so far [24–28]. 3.6. Adsorption kinetics The adsorption processes require the knowledge of both the kinetic and the design parameters. That is why, the pseudo first and second order models [29] have been applied for the experimental data. Pseudo-first order kinetic model dqt = k1 ðqe qt Þ dt
ð6Þ
k1 being the adsorption rate constant. After applying the boundary conditions (t = 0 to t and q = 0 to q = qt), Eq. (6) can be written as follows: logðqe qt Þ = logðqe Þ
ð5Þ
3.2
103 (K-1)
ð4Þ
lnqe = lnKf + nlnCe
k1 t 2:303
ð7Þ
Pseudo-second order kinetic model The pseudo second order kinetic is given by: dqt 2 = k2 ðqe qt Þ dt
30
qt (mg/g-1)
5
LnKc
Initial concentration (mg L− 1)
ð8Þ
where k2 is the rate constant (g mg− 1 min− 1). The integration of Eq (8) at the boundary conditions is linearized:
25 20 15
t 1 t = + qt qe k2 q2e
10
ð9Þ
5 0
0
5
10
15 20 Time (mn)
25
30
35
Fig. 9. Effect of temperature of the Cr(VI) adsorption onto RPS(×) 20 °C, (Δ) 30 °C, (□) 40 °C, (◊); initial concentration 100 mg L− 1; adsorbent dosage 5 g L− 1; speed agitation 350 rpm; pH 5.35.
Table 5 Thermodynamics parameters. ΔHo (kJ moL− 1)
ΔSo (J moL− 1 K− 1)
25.235
119.20
ΔGo(kJ moL− 1) 293 K
303 K
313 K
323 K
− 34.900
− 36.092
− 37.284
− 38.476
M. Kebir et al. / Desalination 270 (2011) 166–173
1.8
due to a greater mobility of ionic species from the bulk solution to the interface [30]. In addition, this increase can either accelerate the adsorption steps or generate new active sites on the RPS surface. The thermodynamic parameters ΔH°, ΔG° and ΔS° are determined from the distribution coefficient Kdis which obeys to the Arrhenius type law:
1.5
10-7 (ηhν) ν)2 (eV2)
171
1.2 0.9 E g =1.70 eV
0.6
K = dis
0.3 0.0 1.6
2.0
2.4
2.8
lnKdis =
hν ν (eV) Fig. 11. Determination of the indirect optical transition of CuAl2O4.
The rate constants have been used to calculate the initial adsorption rate h (mg g− 1 min− 1) given by: 2
ð11Þ
3.2
h = k2 qe :
Cad;eq CL;eq
ð10Þ
The correlation coefficients indicate the best fit between the calculated and experimental data and the Cr(VI) adsorption is well described by the pseudo second order model. The Cr(VI) removal by the RPS is concentration dependent. With increasing Co from 25 to 40 mg L− 1, the amount of Cr(VI) adsorbed at equilibrium increased from 3.872 to 27.247 mg g− 1. The rate h has no specific role while k2 decreases with the increase of the concentration Co. The values of k2, qe and h determined from the intercepts and the slopes of plots t/qt versus t (Fig. 8) are shown in Table 4. 3.7. Thermodynamics for adsorption Cr(VI) onto RPS The Cr(VI) adsorption has been studied in the temperature range (20–50 °C), above which the loss of water through vaporization becomes an important problem. Fig. 9 shows that the adsorption capacity increases with temperature. In term of uptake removal, an 85% of Cr(VI) was obtained at 50 °C in less than 30 min. This is mainly
ΔS ΔH : R RT
ð12Þ
As a result, ΔH° and ΔS° obtained from the slope and the intercept of the plots lnKdis versus 1/T (Fig. 10) are listed in Table 5. Cad,eq and CL,eq are, respectively the equilibrium concentrations of Cr(VI) on the adsorbent and in the solution. The positive enthalpy (ΔH° =25.235 kJ moL− 1) indicates an endothermic nature of Cr(VI) adsorption (physical sorption) which is thermodynamically favorable as an evidence from the negative ΔG° values. The entropy (ΔS° =119.20 J moL− 1 K− 1) reflects an increase of the randomness at the solid/solution interface. The uptake of metal ions greatly lowers the pollution but not enough to comply with the standards of the water quality. Consequently, it can be used as post treatment for photocatalytic process. The photocatalysis is an emerging field for the recovery of heavy metals at low concentrations [31]. Using colloidal semi conductor particles as light absorbing units is simple and does not need any sophisticated device. The first step is to generate electronhole (e−/h+) pairs by photons with energy higher than the band gap (Eg). Cr(VI) is photo-electrochemically converted to trivalent state. With a smaller size and a positive charge, Cr3+ is repelled by the surface and released in a solution in which it can be recovered as insoluble hydroxide (Ks ~10− 30). In this respect, the spinel CuAl2O4 shows light absorption of wavelengths shorter than 730 nm. The precise Eg value of the band gap (1.70 eV) has been determined from the plot of (αhν)2 vs. hν (Fig. 11) where α is the optical absorption coefficient and hν is the incident light. The oxide is chemically stable in acidic solutions and has been elaborated via nitrate route in order to -4.74
Vacuum (0) Solution
n-TiO2
p-CuAl2O4 e-
≈
-1.05 V e-
e- 0.8 V
Eg= 1.77 eV - 0.35
H+/H2 ΔE= 0.2 eV
Eg= 3.2 eV
Cr2O72-/Cr3+
+ 0.58
C7H5O3*/C7H6O3 h+
0.721
2.4 V
Energy (eV)
Potential (VSCE)
Fig. 12. The energy band diagram of CuAl2O4/TiO2/electrolyte junction. Scheme of the photoelectrochemical cell.
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a
9 6 V
3 0
6 5
12
0.0
0.1
0.2
0.3
fb
= + 0.45 V
0.4
0.5
Potential (V)
104 C (Mol L-1)
C-2 (mF-2 cm4)
15
4 3 2 1
Fig. 13. The Mott–Schottky plot of CuAl2O4 in H2SO4 medium (0.05 M).
0 60
40
P = 4:75 + eVfb + ΔE–Eg
ð13Þ
where ΔE is the difference between the bottom of the conduction band and the Fermi level of the semiconductor. A value of 0.17 eV was determined from the conductivity measurements performed on sintered pellet [19]. The flat band potential Vfb is obtained from the Mott–Schottky plot (Fig. 13). The difference (Vfb − Ered) measures the driving force of the junction and for a zero (e−/h+) pairs recombination, a value of at least ~ 0.3 V is required. However, for CuAl2O4 the difference is too large (~ 1.6 V) and only a weak percentage of chromate is reduced. The visible light-induced chromium conversion is mediated by photo electrons via TiO2. The improved activity is attributed to the electron injection from the activated CuAl2O4-CB, acting as electron pump, into TiO2-CB (− 0.8 V) resulting in the Cr(VI) reduction. The powder was kept overnight in the solution to ensure a complete dark HCrO− 4 adsorption before illumination. The heterosystem acts in a short circuited configuration through the electrolytic acid solution. The photocatalytic tests are carried out with various amounts of catalyst and the results gave an optimal value of 1.25 mg mL− 1. A reduction of more than 58% of HCrO− 4 after ~2 h is achieved under optimal conditions (pH ~ 2 at 25 °C) with a mass concentration CuAl2O4/TiO2 of (1/1). Photocatalytic oxidation of organic pollutants has been already studied for water purification [32]. It contributes significantly to the metal recovery because of the longer lifetime of the photo carriers. Indeed, the chromate reduction has been greatly improved in presence of salicylic acid, due to the separation of (e−/h+) pairs located in CuAl2O4-VB (+0.72 V). The p-CuAl2O4/n-TiO2 junction catalyses the downhill reaction: 2
þ
3þ
2Cr2 O7 + 3C7 H6 O3 + 18O2 + 16H →4Cr
b
Reduction %
increase the active surface. To perform the chromate reduction, the conduction band (CB) of CuAl2O4 (−1 V) must have a potential Ered 3+ below that of the HCrO− couple (Fig. 12). The energetic position 4 /Cr of CuAl2O4-CB is given by:
80
100
120
Time (min) 60
40
20
0
no salicylic acid
salicylic acid
Fig. 14. The Cr(VI) reduction in the presence of salicylic acid (a), the effect of the salicylic acid(b).
4. Conclusion The main goal of this work is to develop a low cost material for the chromate removal in aqueous media. The high capacity of the RPS to adsorb Cr(VI) has been demonstrated and the results reveal that more than 85% are removed. The Cr(VI) adsorption is dependent on the physical parameters like the initial concentration, time contact, pH and temperature. The process is well described by the Langmuir model with monolayer adsorption coverage, which is comparatively larger than most adsorbents reported to date. The thermodynamic
+ 21CO2 + 17H2 O ð14Þ
with a free enthalpy of −557 kcal moL− 1. It is helpful to outline that the reduction percentage decreases down to 25.2% in the absence of salicylic acid which acts as hole scavenger and favors the charge separation (Fig. 14a) The removal process follows a first order kinetic with a half life of 105 min where a 58% of Cr(VI) was reduced to trivalent state upon visible illumination (Fig. 14b). The product formed after 2 h illumination in a buffered chromate solution turned out to be Cr3+. This feature has found an experimental support, the visible band intensity due to Cr(VI) at 352 nm decreased and a new band attributed to the formation of Cr3+ appeared at 298 nm (Fig. 15). The same reaction will be tested over a fixed bed. New experiments will be carried out and reported in the next paper.
Fig. 15. UV-Visible spectra of (a) initial solution Cr(III), (b) 2 h illuminated solution Cr(VI).
M. Kebir et al. / Desalination 270 (2011) 166–173
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