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Tailoring fly ash activated with bentonite as adsorbent for complex wastewater treatment
Applied Surface Science 263 (2012) 753–762
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Tailoring fly ash activated with bentonite as adsorbent for complex wastewater treatment Maria Visa ∗ Transilvania University of Brasov, Department Renewable Energy Systems and Recycling, Eroilor 29, 500036 Brasov, Romania
a r t i c l e
i n f o
Article history: Received 6 August 2012 Received in revised form 26 September 2012 Accepted 28 September 2012 Available online 9 October 2012 Keywords: Bentonite Fly ash Pollutants Adsorption Wastewater treatment
a b s t r a c t Used as adsorbent, alkali fly ash represents a low cost solution for advanced wastewater treatment. The alkali treatment raises sustainability issues therefore, in this research we aim to replace alkali fly ash with washed fly ash (FAw). For improving the adsorption capacity of washed fly ash, bentonite powder (B) was added, as a natural adsorbent with a composition almost identical to the fly ash. The new adsorbent was characterized by AFM, XRD, FTIR, SEM, EDS and the surface energy was evaluated by contact angle measurements. For understanding the complex adsorption process on this mixed substrate, preliminary tests were developed on synthetic wastewaters containing a single pollutant system (heavy metal), binary (two-heavy metals) and ternary (dye and two heavy metals) systems. Experiments were done on synthetic wastewaters containing methylene blue, cadmium and copper, using FAw, B and their powder mixtures. The pseudo-second order kinetics could well model all the processes, indicating a good adsorbent material which can be used for the pollutants removal from wastewater. After adsorption the substrates loaded with pollutants, annealed at 500 ◦ C can be reused for padding in stone blocks. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Anywhere and anytime, water plays an important role for human beings, natural environment, and social development, but the subsequences of used water are represented by polluted municipal and industrial wastewaters (e.g. resulted from mining, metallurgy, metal plating, smelting flux electrolysis, battery manufactories, petroleum refineries, paint and pigments manufacture, printing, photographic industries and the glass production [1]. In all wastewater resulted from textile dyeing, dyes are found along with surfactants, whitening agents, anti-foaming agents even heavy metals, thus complete treatment becomes complicated. One of the most bio-toxic heavy metal from wastewater is cadmium, while dyes and pigments are affecting water transparency, reducing light penetration and gas solubility in water [2], also being mutagenic to aquatic organisms and humans. The most popular method to treat heavy metals in wastewaters is chemical precipitation, followed by filtration or other solid/liquid separation processes. Although the chemical precipitation method is quite effective for heavy metal removal, the resultant heavy metal sludge is classified as a hazardous solid waste and needs to be adequately treated. Other methods such as electrodialysis [3] nanofiltration, coagulation/co-precipitation, reverse osmosis [4] biosorption including some economic bio-adsorbent [5],
∗ Tel.: +40 729 109355; fax: +40 268 410525. E-mail address: [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.156
adsorption [2,6] and ion exchange process are developed to remove heavy metals from industrial wastewaters [7]. Among these, adsorption technologies have several advantages: easy operation and well known technology, inexpensive equipments, less sludge, the adsorbents materials can be selected from a broad variety of bio-materials, mineral oxides, activated carbon, resins and even waste materials. Plenty of research was devoted to the investigation of the adsorption capacity of less expensive materials such as sugar beet pulp [8], red mud [9], natural zeolite, clay and diatomite [10], scrap rubber, peat [11] or bone char [12], fly ash [13] etc. for heavy metals (cadmium, copper, zinc, nickel, iron) and dyes removal [14,15]. For a sustainable industry, low-cost adsorbents are intensively studied, mainly based on natural compounds or on organic artificial or synthetic wastes such as fly ash. We proposed to obtain a low-cost adsorbent combining bentonite (B) – a material widespread in nature and fly ash (FA) – a waste. Bentonites are the most abundant argillaceous materials which can be used in wastewater treatment [16]. They are reported as low cost efficient adsorbents for some heavy metals (copper, lead, cadmium and zinc) while modified bentonite was used for removing 60 Co radionuclide from radioactive waste solutions. The outstanding adsorption capability is due to main mineral component as montmorillonite, smectite and clay. The fly ash represents a major pollution source for air, soil, and ground water but can also be used as adsorbent; large amounts of FA
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M. Visa / Applied Surface Science 263 (2012) 753–762
Table 1 Summary of adsorption of Cd2+ , Cu2+ , Ni2+ , Pb2+ cations and dye on fly ash and bentonite [18]. Metal
Adsorbent
Adsorption capacity (mg/g)
References
Cd2+
Coal fly ash Fly ash Fly ash zeolite FA-Z Bagasse fly ash Bentonite
18.98 198.2 95.6 30.21 1.24–2.0 2.8
[70] [79] [80] [19]* [95] [20]*
18.83 9.69 17.87
[21]* [14]* [22]*
Cu2+
Fly ash Fly ash Bentonite
Pb2+
Fly ash washed Fly ash Bentonite
Ni2+
Bentonite
13.97
[22]*
Congo red
Bentonite
7.27
[23]*
Methylene blue
Fly ash+TiO2
0.37–0.52
[14]*
483.4 18.8 22
[79] [87] [20]*
[ ]* are included in references.
are not re-used, as for example the 50% of the 2010 EU production (over 100 million tons) that is not valorised currently. Brasov City has a CPH plant which works with two boilers (420 tons/h capacity) and two turbo-aggregates which generate 50 MW/each. Every year 6 × 105 tons of coal is burned and the fly ash resulted amounts 2 × 105 tons/year out of which only 1/3 is reused as a cement additive. One possible application is to develop alternative processes (like wastewater treatment), involving the use of this waste. Previous studies proved a good efficiency of alkali modified fly ash for heavy metals (cadmium, copper, zinc, nickel) and for dyes (methylene blue (MB) and methyl orange (MO)) removal, from mono-, bi- or three-components (Cu2+ , Cd2+ , Ni2+ and MB) wastewaters, resulted from the dye finishing industry [14]. The results showed that raw fly ash has limited efficiency, thus the surface of fly ash was modified with NaOH solution or other reagents. Although effective, the alkali treatment results in supplementary wastewaters, therefore, this paper investigates efficient solutions for the FA re-use by combining the washed FA with a natural adsorbent, bentonite, resulting in a low-cost, efficient substrate. There is a growing interest in using low cost adsorbents, fly ash, bentonite or their mixture. If these materials are characterized and tested for to remove the heavy metals and dyes, the adsorption process will be a promising technology. The Fe2+ cation can be removed from wastewater resulted at a galvanized pipe manufacturing industry using bentonite with efficiency up 90% [17].Table 1 summarizes some of the recent results of Cd(II), Cu(II), Ni(II), Pb(II) ions and dyes removal from wastewater using fly ash and bentonite.
2. Materials and methods 2.1. Adsorbents materials Bentonite was provided from NW Romania area. The raw fly ash was supplied by the CHP Plant Brasov, Romania directly from the electro-filters, sifted, choosing grains with diameter between 40 and 100 m. The major oxide components in fly ash and bentonite, with a certain influence in heavy metals and dyes removal are presented in Table 2. Using emission spectrometry other metals were also identified in small amounts (Ba, Cu, Sn, Pb, Cr, Ni, V, Zn, Ti).
Table 2 Adsorbent materials composition. Major oxides [wt%] SiO2
Al2 O3
Fe2 O3
CaO
FA composition [%] 53.32 22.05 8.97 5.24 Bentonite composition [%] 72.11 14.92 2.62 2.31
MgO
K2 O
Na2 O
TiO2
MnO
LOI*
2.44
2.66
0.63
1.07
0.08
1.58
2.12
1.34
1.91
0.62
0.05
2.06
According to the ASTM standards, the raw FA pertains of class F because the sum of the SiO2 , Al2 O3 and Fe2 O3 is above 70% [24], while bentonite is of Na type [25]. Both substrates were separately sieved and the 40–100 m fractions were selected as substrates for experiments. Raw adsorbent materials contain soluble compounds, with pollution potential therefore washing before use is compulsory. By mixing 200 g of powder FA and separate 200 g of powder bentonite) with 2000 mL ultra pure water, followed by stirring at room temperature (20–22 ◦ C), for 48 h till constant pH value. The pH and conductivity values were evaluated for FA (pH 10.2 and for bentonite pH 9.9, respectively = 1.710 mS for both). The washed FA and washed bentonite were further dried at 105–120 ◦ C, till constant mass. These substrates are further denominated as FAw and B. Structural and crystallographic properties of individual FAw and B particles were evaluated by XRD (Bruker D8 Discover Diffractometer) and for morphology studies the AFM images were used (Ntegra Spectra, NT-MDT model BL222RNTE). The images were taken in semi-contact mode with Golden silicon cantilever (NCSG10), with constant force 0.15 N/m, having the tip radius of 10 nm. Scanning was conducted on three or more different places with a certain area 10 m × 10 m or 5 m × 5 m for each position, chosen randomly at a scanning grate of 1 Hz. Further surface investigations were done using scanning electron microscopy (SEM) operated with SEM S-3400N-Hitachi at an accelerating voltage of 20 kV. Compositions were measured using energy-dispersive X-ray spectroscopy (EDS Thermo Scientific Ultra Dry). The surface area of the fly ash and bentonite was measured by using the Tri Star II 3020 Analyser (Micromeritics). The information related to the functional groups on the surface was provided by the FTIR data (PerkinElmer BX II 75548). 2.2. Adsorption experiments The adsorption tests were performed by batch experiments, under stirring up to 240 min at room temperature (20–22 ◦ C), at the natural pH of the dispersion, Table 3. Aliquots were taken each at 15, 30, 45, 60, 90, 120 180, 240 min, when stirring was briefly interrupted and the substrate was removed by vacuum filtration. The supernatant was further analyzed by AAS (Analytic Jena, ZEEnit 700), at: Cd = 228.8 nm, Cu = 324.75 nm. The absorbance measurements were recorded in the range of 200–900 nm, using a UV–vis spectrophotometer (PerkinElmer Lambda 950 UV/VIS) at the maximum absorption wavelength registered at 664 nm. Three series of experimental tests were done on mono component solution of Cd2+ (cCd = 0.750 mg/L, using CdCl2 ·2.5 H2 O, Scharlau Chemie), two heavy metals solution (Cd2+ and Cu2+ , cCu = 0.400 mg/L, using CuCl2 ·2H2 O Scharlau Chemie), and threecomponents solution (Cd2+ , Cu2+ and methylene blue (C16 H18 N3 S) cMB = 0.05 mM, using MB Fluka AG, reagent grade). The lowest admissible discharge concentration are, in most national regulations set for cadmium therefore, the optimized adsorption conditions set for cadmium were extended also for copper in multicomponent solutions, in the tests performed using FAw, B and their mixtures, as shown in Table 4.
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Table 3 The natural pH values of the solution with adsorbent. FAw:B [g:g] pH
1:0 7.4
0:1 9
0.25:0.75 6.2
0.50:0.50 5
1.00:1.00 5.1
0.5:1.5 6.5
Table 4 Substrates and pollutant systems experimentally tested. Substrates (g) FAw:B Substrate Pollutant systems
1:0 (FAw) Cd2+
0:1 (B) Cd2+
0.25:0.75 (1) Cd2+ +Cu2+
0.50:0.50 (2) Cd2+ +Cu2+
1.00:1.00 (3) Cd2+ +Cu2+
1.5:0.5 (4) Cd2+ +Cu2+ +MB
The optimal adsorbent mass: wastewater volume was evaluated based on cadmium750 mg/L solutions, using a contact duration of 90 min, while the optimal contact time was evaluated on suspensions of 0.5 g FAw in 100 mL multicomponent solutions of Cd2+ (750 mg/L), Cu2+ (400 mg/L), respectively methylene blue 0.05 mM. Preliminary experiments proved that heavy metals and MB losses due to adsorption onto the container walls and filter system were negligible. 2.3. The adsorption kinetics For the kinetic studies, the metal uptake qe (mg/g) was evaluated using the initial and current, t, heavy metal concentrations (c i n+ and c t n+ ), in a given solution volume, (V = 100 mL), as preM M sented in Eq. (1): qe =
(c i n+ −c t n+ )×V M M ms
.
(1) Fig. 1. XRD patterns of (1) raw FA, (2) FAw, and (3) bentonite.
The kinetics of the heavy metals adsorption was modelled by the following equations: the pseudo first-order equation Lagergren, 1898 [26]: log(qe − qt ) = log(qe ) −
KL t. 2.303
(2)
where KL is the Lagergren constant, qe is the equilibrium uptake value and qt the current metal uptake; the pseudo-second order kinetic equation Ho and McKay [27] and a comparison of linear and non-linear methods [28]: 1 t t = + qt qe k2 qe 2
(3)
where, k2 is the equilibrium rate constant for the pseudo secondorder adsorption which can be evaluated from the slope of the plot. Another possible kinetic model which can be applied in adsorption processes on porous materials is the interparticle diffusion model. In this case, the amount of heavy metals ions adsorbed can be calculated with the Eq. (4), [29]: q = kid t 1/2 + C
The main constituent of bentonites is montmorillonite, which is a 2:1 mineral with one octahedral sheet and two silica sheets, forming a layer. Layers are held together by van der Waals forces. Because of these weak forces and some charge deficiencies in the structure, water can easily penetrate these layers and cations balance the charge deficiencies [2]. The data also show that major components, quartz and graphite, are not affected during FA modification. The crystalline degree of (B) is 79.5% the rest being represented by amorphous phases. The crystallites size, calculated with the ˚ Scherer formula ranges from 171.0 to 653.2 A. Supplementary information was obtained using the FTIR spectra. The FTIR spectra of FAw and Bentonite are presented in Fig. 2. The absorption band observed at 3622–3624 cm−1 was attributed to the hydroxyl group stretching/vibration in Si OH, Al OH Al, Mg OH Al and/or Fe OH Al units in octahedral layer [30]. The
(4)
3. Results and discussions 3.1. Characterization of the adsorbent material The diffractograms (Fig. 1) show that some crystalline phases of FA (quartz, philipsite cristobalite, hematite) are mostly present in raw FA and new crystalline phases appear (mullite) in FAw. The XRD data show that during washing soluble compounds concentrated on the fly ash surface are dissolute, leaving the core, with a different composition of non-soluble crystalline compounds (Fig. 1). In bentonite the new crystalline phases are: sodium aluminium silicate NaAlSiO4 , silimanite (Al2 (SiO4 )O, sodium aluminium silicate hydrate (phillipsite) (Na6 Al6 Si10 O32 ·12H2 O), silicate hydroxide hydrate (illite) (KH3 O)Al2 Si3 AlO10 (OH)2 , chabazite Na NaAlSi2 O6 ·3H2 O, stelerite Na, Na2 Al2 Si7 )O18 ·7H2 O faujasite silimanitul and other aluminosilicates phases, in lower amounts.
Fig. 2. IR spectrum (a) bentonite, (b) FAw.
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Fig. 3. The AFM topography and phase distribution.
asymmetric stretching mode of Si O Si in bentonite and FAw was suggested by the absorption band at 1010 cm−1 for bentonite and 1016 cm−1 for fly ash, both with sharp peak. In bentonite, water molecules are associated with the cations and are in some extent hydrogen bonded to the oxygen ions of the framework, explaining the peak with less intensity, recorded at 1639 cm−1 which is characteristic of the bended mode in the water molecules. This observation indicates the similarity between bentonite and fly ash. These data can be corroborated with the surface roughness which shows a strong decrease after washing of the fly ash Fig. 3. The roughness can give the different information versus the level of investigation (the millimeter scale usually permits one to distinguish the main surface treatments) [31]. Part of the oxides in raw FA and bentonite are water soluble. These chemical and structural changes are mirrored in morphology modifications, Figs. 3(b), 4(a) and (b) resulting in large differences
in the substrates’ affinity for heavy metals. On phases distributions images there can be seen less agglomerates in new material adsorbent so more mesopores ready to lodge the cations of heavy metals or dye molecules. These AFM images were used to characterize the surface morphology: the uniformity, grain size and pore size distribution [32] of the samples (Figs. 6 and 7). The surface area of the FAw and Bentonite were analyzed and the results show a strong increase in the specific area surface, and a decrease in the average pores diameter; the mixed substrate (FAw + B) has average values showing that mechanical mixing is the most likely occurring process (Fig. 5). Adsorption capacity and cation exchange depend on the chemical nature of the sorbent surface, pore structure, size of the aggregates, crystallinity degree of the particles, thus the textural and structural features, but also on the cation present in the cell layer, the solution pH and temperature and the solution–adsorbent
Fig. 4. The AFM topography and phase distribution.
M. Visa / Applied Surface Science 263 (2012) 753–762
Fig. 5. The interparticle voids distribution.
contact duration. In this view, bentonite adsorption strength due to fine particles is large, exhibiting a large contact surface. The surface characteristics for FA are presented in Table 4 and are characteristic to mezzo-porous solids (Table 5). The SEM images show significant differences between the substrates: most FAw particles are spherical with diameters between 3.61 and 111 m. The bentonite particles have a stratified structure with hexagonal large agglomerates, with sizes between 2.23 and 8.08 m.
757
Fig. 7. Scanning electron microscopic images after adsorption (a) Faw + B loaded with Cd2+ , Cu2+ and MB, (b) Faw + B-particle loaded with Cd2+ , Cu2+ and MB.
The surface morphology of FAw and bentonite appears as corn flake crystals with fluffy appearance revealing the fine platy structure. The results of the EDS analysis (Fig. 8) show that FAw and B have a similar composition that abounds in hydrous aluminosilicates. The EDS spectra of the materials FAw and B shown in Fig. 8 offered semi-quantitative ratio of present elements: O, Na, K, Ca, Mg, Al, Si, Ti, Fe. The analysis shows that the rims have a similar composition and abound the hydrous aluminosilicates. Bentonites are composed of hydrated largely of sodium aluminosilicate (chabazite Na NaAlSi2 O6 ·3H2 O) what are capable of adsorption and ion exchange. Bentonites can form colloidal suspensions, gels and water molecules are adsorbed between the lattice planes in the movement of ions [33]. Cation adsorption involves mainly electrostatic forces therefore the surface energy of the new substrate (FAw:B) can strongly influence the adsorption process. The polar and disperse contributions to the surface energy of FAw, B and their mixtures were calculated according to the model developed by Owens, Wendt, Robel and Kaelble and are presented in Table 6. The data show a high global surface energy and a large polar component, recommending the material as a good adsorption substrate for heavy metals cations and for methylene blue (Table 6).
3.2. Uptake kinetics of the heavy metals
Fig. 6. Scanning electron microscopic images of (a), (c) bentonite, (b), (d) FAw.
The models described in paragraph 2.3 were tested for the Cd2+ , Cu2+ removal by adsorption on FAw, B and on their mixtures. The kinetic parameters are presented in Table 7. The results proved that the pseudo-second order kinetic well describes the adsorption mechanism for both cations, on bentonite and on all theirs the mixtures investigated. The data also prove that copper adsorption can follow more parallel mechanisms, which could be expected considering the FAw: B composition and/or the pores distribution with active sites of various energies. The values of the adsorption capacity on system FAw:B is found to decrease in the order Cu2+ > Cd2+ on mixtures 0.5:0.5 and 1:1. The amount of bentonite in mixture increase the adsorption capacity as expected considering the differences in the specific surface values between FAw and B because in aluminosilicates are more groups Si OH that in FAw. The adsorption efficiency, , and adsorption capacity, qm , were evaluated based on the optimal contact time, the mass balance and initial cations’ concentration of adsorption for the most
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Table 5 Surface properties of FAw and bentonite. Sample
Surface area BET [m2 /g]
Micropores volume (t-plot) [cm3 /g]
Micropores surface (t-plot) [m2 /g]
Average pores diameter [nm]
FAw B
6.14 21.33
0.0004 0.0030
2.25 14.09
27.2 15.42
Fig. 8. EDS spectra of FAw and bentonite.
Table 6 Surface energy data for the substrates. Substrate
Surface energy [mN/m]
Disperse contribution [mN/m]
Polar contribution [mN/m]
FAw B FAw:B (4)
187.42 107.5 330.79
32.61 3.85 89.62
154.81 103.65 241.17
Table 7 Kinetic parameters of the heavy metal removal on FAw:B mixed substrate. FAw:B [g:g]
Pseudo first-order kinetics −1
KL [min
]
2
Pseudo-second order kinetics
Interparticle diffusion 2
R
k2 [g/mg min]
qe [mg/g]
R
Kid (mg/g min1 /2 )
R2
C
Cadmium 1:0 0.50:0.50 0.25:0.75 0:1 1:1 1.5:0.50
– – – – 0.007 –
0.413 0.627 0.023 0.026 0.954 0.236
– 0.147 0.049 0.059 0.408 0.092
– 31.646 30.395 42.553 19.841 28.011
0.854 0.998 0.998 0.999 0.997 0.998
– – – – – –
– – – – – –
0.801 0.535 0.409 0.446 0.592 0.454
Cd2+ (Cd2+ +Cu2+ ) 0.50:0.50 1:1 1.5:0.5
– 0.022 0.013
0.415 0.843 0.967
1.613 0.379 7.330
24.752 10.341 20.877
0.948 0.998 0.982
1.505 – –
0.851 – –
0.942 0.547 0.867
Cu2+ (Cu2+ +Cd2+ ) 0.50:0.50 1:1 1.5:0.5
0.016 0.026 0.012
0.886 0.962 0.931
1.348 0.477 0.092
25.839 14.771 16.78
0.976 0.999 0.999
1.035 0.274 0.092
7.456 10.603 15.425
0.962 0.852 0.903
M. Visa / Applied Surface Science 263 (2012) 753–762
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Fig. 9. Cd2+ immobilization; (a) efficiency vs. substrate dose, (b), (c) efficiency vs. contact time. Fig. 10. Cd2+ , Cu2+ and MB immobilization from multi-pollutant systems; efficiency vs. contact time from mixture.
toxic heavy metal, cadmium. The efficiency was calculated using the following Eq. (5). =
(c i n+ −c t n+ )×100 M M c i n+ M
(5)
where c i n+ and c t n+ are the initial and equilibrated cations’ conM M centrations (mg/L).
The dynamic adsorption results are presented in Fig. 9a–c for the Cd2+ cations and for all pollutants (Cd2+ , Cu2+ and MB) from multi-pollutant systems are presented in Fig. 10a–c. The data allow optimising the adsorbent amount, considering cadmium as reference, in a single pollutant system.
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3.3. Adsorption isotherms and mechanism The Langmuir and Freundlich isotherms equations are selected for calculated adsorption parameters of these adsorbents materials. The Langmuir isotherm – linearization is: Ceq Ceq 1 = + qeq qmax a qmax
(9)
where qmax (mg/g) represents the monolayer adsorption capacity, a is a constant related to the free energy of adsorption, qeq and Ceq show the amount of metal adsorbed on. The Freundlich isotherm – linearization: ln qeq = ln kF +
Fig. 11. Efficiency of adsorption at different initial concentration of Cd2+ and of Cu2+ .
Adsorption efficiency of cadmium on FAw is low because the surface of FAw is not activated and the specific surface area is low compared to bentonite. The adsorption equilibrium for cadmium and copper needs 90 min to be settled therefore, this was the set as optimal contact time in all further experiments. Also, for these two ions, higher substrate dosage is required, almost similar removal efficiencies (close to 70%) being registered at 2 g of FAw:B dispersed in 100 mL of solution. The removal efficiency increases with amount of bentonite from mixture, proving that adding B to FAw represents a path to fulfil the set target: the use of washed FA in advanced wastewater treatment. The Cd2+ and Cu2+ cations can adsorb by chemically bonding with the active site ( SiO− ), and ( AlO− ) and can form complexes on the surface as presented by [30]. The metal compounds are the hydrated cations which can adsorb with partial or total dehydration (Eq. (6)): substrate + Me(H2 O)n z+ ⇔ substrate–Me(H2 O)n−x z+ + xH2 O
(6)
where 0 ≤ x ≤ n [21]. Or: The Cd2+ and Cu2+ cations can also be adsorbed by the silanol group (Si OH) of the layers, but their numbers are distinctly smaller (Eqs. (7) and (8)): Si OH + Cd2+ (H2 O)n ⇔ Si OCd+ (H2 O)n−x + H+ (H2 O)x
(7)
Si OH + Cd2+ (H2 O) ⇔ (Si O)2 Cd(H2 O)n−x + 2H+ (H2 O)x
(8)
The data show that cadmium and copper have a much lower affinity for the bentonite substrate comparing to the MB dye, showing an efficiency decrease for contact times longer than 240 min, supporting the assumption of an adsorption process without dehydration. The FAw:B substrate is highly efficient ( > 90%) for the removal of all cations at concentrations below 100 mg/L Fig. 11, thus for advanced wastewater treatment, another preliminary process (e.g. precipitation) could be necessary.
1 ln Ceq n
(10)
where kF is Freundlich adsorption constants, being indicators of adsorption capacity and 1/n dimensionless parameter is a measure of the adsorption density. Table 8 presents the adsorption parameters for the heavy metal ions (Cd2+ , Cu2+ ) calculated from the slope of the plots. The Langmuir model is better than the Freundlich model in simultaneous adsorption of cadmium and copper cations from multicomponent system. The mechanisms of adsorption is a hybrid one and corroborated with kinetic parameters can follow – chemisorption mechanism or ion exchange for the cadmium and cooper cations, according to Eqs. (7) and (8). This suggests that the heterogeneity, loading on the surface or pores of substrate (FAw–B) play a role in heavy metals adsorption. The adsorption studies carried out to estimate the heavy metal removal from wastewater, using fly ash modified with bentonite, showed that the efficiency follows the order Cu2+ ≥ Cd2+ , according with the dimensionless separation factors are: Cu 2+ = 0.243 > Cd 2+ = 0.151. Supplementary, after adsorption and filtration the substrates loaded with pollutants were subject of pelletization. Round tablets (pellets) with 1 g mass, were annealed at different temperatures (100 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C and 500 ◦ C. The five annealed samples, plus one which was not subject to heat treatment were introduced in 100 mL bidistilled water (each pellet in one glass). The ratio mpellet :Vbidistilled water = 1:100. The stability of the pellets strongly depends on the annealing temperature, and the best stability corresponds to the tablets annealed at 400 ◦ C and 500 ◦ C. After 48 h, the methylene blue, cadmium and copper cations concentration were evaluated to test the desorption processes. The results show that, after 48 h, there are no desorption of cadmium or copper and maybe methylene blue was decompose or burn. Mechanical properties and composite environmental behavior can be tailored by dispersing different oxide powders (TiO2 , raw fly ash and these annealed pellets). Efficient dispersing occurs when the powders are in the range of nano- or mezzo-scale, resulting hybrid inorganic–organic composites. This can be a suggestion for reused the spent adsorbent for padding in the stone blocks. The data also show that cadmium and copper have a much lower affinity for the bentonite substrate, showing an efficiency decrease for contact times longer than 240 min, supporting the assumption of an adsorption process without dehydration. The FAw:B substrate is highly efficient (>90%) for the removal of all cations at concentrations below 100 ppm, thus for advanced wastewater treatment, another preliminary process like precipitation. The FAw:B mixture is an alternative adsorbent material to remove in a single step more cations and dye and to decrease the amount of fly ash from environment.
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Table 8 Parameters of Langmuir and Freundlich isotherms of various adsorbents (FAw:B). FAw:B [g:g]/parameters
Langmuir isotherm
Freundlich isotherm
qmax [mg/g]
a [L/mg]
R2
1/n
KF
R2
Cd2+ (0.5:0.5) (1:1) (1.5:0.5)
25.907 27.174 27.101
0.025 0.007 0.006
0.979 0.983 0.998
– – –
– – –
0.751 0.024 0.665
Cd2+ /(Cd2+ +Cu2+ ) (0.5:0.5) (1:1) (1.5:0.5)
27.781 27.701 7.485
0.005 0.015 0.196
0.977 0.988 0.981
0.281 0.275 0.281
0.098 0.056 0.004
0.923 0.917 0.936
Cu2+ /(Cu2+ +Cd2+ ) (0.5:0.5) (1:1) (1.5:0.5)
11.389 24.096 7.299
0.019 0.008 0.193
0.991 0.939 0.948
0.280 0.255 0.268
0.005 0.153 0.003
0.924 0.933 0.906
4. Conclusions The removal efficiency of heavy metals and MB increases with amount of bentonite from mixture, because the surface area of bentonite is large that of FAw. The adsorption capacity depends on the chemical nature of the sorbent surface, pore structure, crystallinity degree of the particles, thus the textural and structural features, and the solution–adsorbent contact duration. In this view, the adsorption on bentonite strength due to fine particles is large, exhibiting a large contact surface. An important characteristic of mixture FA:B (1.5:0.5) is its large cation adsorption capacity and has a convenient efficiencies after 90 min of contact time and is not strongly influenced by the MB existent in the solution. Copper has a higher affinity for the active sites comparing to cadmium. The reason may be the higher mobility and lower volume of the hydrated complexes of the cations in water (hexa-complexes for Cd and tetra-coordination for Cu), at the working pH of 4.8 . . . 5.3. Due to the large porosity of FA, the efficiency of the heavy metal adsorption does not depend on the FA fraction used. This becomes significant for the adsorption of large molecules, as it is MB. The pseudo-second order kinetics describe well all the processes. Large adsorption capacities are registered for copper, confirming the higher affinity of the ion for this mixture (FAw:B). The Langmuir model is better than the Freundlich model in simultaneous adsorption of cadmium and copper cations from multicomponent system and the mechanisms of adsorption is a hybrid one and can follow – chemisorption mechanism or ion exchange for the cadmium and cooper cations. High adsorption efficiencies are registered for heavy metals concentrations below 100 ppm, recommending this substrate for simultaneous removal of heavy metals and MB from wastewater resulted in the dyes finishing industry. Using washed fly ash and bentonite as substrate, the pollutants can be removed at low cost by adsorption. Adsorption of heavy metals ions by bentonite depends on charge characteristics of the adsorbent, including surface charge magnitude, type of charge (permanent or variable) and point of origin. The spent adsorbent annealed at 500 ◦ C can be included in stone blocks resulting hybrid inorganic–organic composites. This can be a suggestion for reused the spent adsorbent for padding in the stone blocks. Acknowledgements This paper is supported by the Sectorial Operational Programme Human Resources Development (SOP HRD), financed from the
European Social Found and by the Romanian Government under the contract number POSDRU ID 59323.
References [1] M.E. Argun, S. Dursun, C. Ozdemir, M. Karatas, Heavy metal adsorption by modified oak sawdust: thermodynamics and kinetics, Journal of Hazardous Materials 141 (2007) 77–85. [2] L.J. Yu, S.S. Shukla, K.L. Dorris, A. Shukla, J.L. Margrave, Adsorption of chromium from aqueous solutions by maple saw dust, Journal of Hazardous Materials 100 (2003) 53–63. [3] T. Mohammadi, A. Razmi, M. Sadrzadeh, Effect of operating parameters on Pb2+ separation from wastewater using electrodialysis, Journal of Desalination 167 (2004) 379–385. [4] Z. Ujang, G.K. Anderson, Performance of low pressure reverse osmosis membrane (LPROM) for separating mono- and divalent ions, Journal of Water Science and Technology 38 (1998) 521–528. [5] C.R.T. Tarley, M.A.Z. Arruda, Biosorption of heavy metals using rice milling byproducts. Characterisation and application for removal of metals from aqueous effluents, Journal of Chemosphere 54 (2004) 987–995. [6] H. Choi, S.D. Kim, S.H. Noh, S.J. Oh, W.J. Kim, Adsorption behaviors of nano-sized ETS-10 and Al-substituted-ETAS-10 in removing heavy metal ions, Pb2+ and Cd2+ , Journal of Microporous and Mesoporous Materials 87 (2006) 163–174. [7] I. Lee, Y.C. Hsein Kuang, J.M. Chern, Equilibrium and kinetics of heavy metal ion exchange, Journal of the Chinese Institute of Chemical Engineering 38 (2007) 71–84. ˜ [8] Y.N. Mata, M.L. Blázquez, A. Ballester, F. González, J.A. Munoz, Sugar beet pulp pectin gels as biosorbent for heavy metals: Preparation and determination of biosorption and desorption characteristics, Journal of Chemical Engineering 150 (2009) 289–301. [9] W. Shaobin, Y. Boyjoo, A. Choueib, Z.H. Zhu, Removal of dyes from aqueous solution using fly ash and red mud, Journal of Water Research 39 (2005) 129–138. ˇ ´ I. Smiˇciklas, S. Pejanovic, ´ I. Plecaˇ ´ s, Comparative study of Cu2+ adsorp[10] M. Sljivi c, tion on a zeolite, clay and a diatomite from Serbia, Journal of Applied Clay ScienceV 43 (2009) 33–40. [11] Y.S. Ho, G. McKay, Sorption of Cooper(II) from aqueous solution by peat, Journal of Water Air and Soil Pollution 158 (2004) 79–97. [12] C.W.J. Cheung, F. Porter, G. McKay, sorption kinetic analysis for the removal of cadmium ions from effluents using bone char, Journal of Water Research 35 (2001) 605–612. [13] S. Chaizasith, P. Chaizasith, C. Septhum, removal of cadmium and nickel from aqueous solutions by adsorption onto treated fly ash from Thailand, Journal of Science and Technology 11 (2006) 13–19. [14] M. Visa, L. Andronic, D. Lucaci, A. Duta, Concurrent dyes adsorption and photo-degradation on fly ash based substrates, Journal of Adsorption 1 (2011) 101–108. [15] M. Visa, L. Andronic, A. Duta, Photocatalytic properties of titania–fly ash thin films, Environmental Engineering and Management Journal 8 (2009) 633–638. [16] S. Ding, J.S.B. Xu, Q. Liu, Y. Sun, The factors on removal of zinc cation from aqueous solution by bentonite, Journal of Natural Resources 2 (2011) 107–113. [17] S.S. Tahir, N. Rauf, Removal of Fe(II) from the wastewater of a galvanized pipe manufacturing industry by adsorption onto bentonite clay, Journal of Environmental Management 73 (2004) 285–292. [18] M. Ahmaruzzaman, A review on the utilization of fly ash, Progress in Energy and Combustion Science 36 (2010) 327–363. [19] M. Visa, L. Isac, A. Duta, Fly ash adsorbents for multi-cation wastewater treatment, Applied Surface Science 258 (2012) 6345–6352. [20] S. Andini, R. Cioffi, F. Montagnaro, F. Pisciotta, L. Santoro, Simultaneous adsorption of chlorophenol and heavy metal ions on organophilic bentonite, Journal Applied Clay Science 31 (2006) 126–133. [21] M. Visa, A. Duta, Adsorption behavior of cadmium and copper compounds on mixture FA:TiO2 , Revue Roumaine de Chimie 55 (2010) 167–173.
762
M. Visa / Applied Surface Science 263 (2012) 753–762
[22] L. Zhi-rong, Z. Shao-qi, Adsorption of copper and nickel on Na-bentonite, IChemE 88 (2010) 62–66. [23] V. Vimonses, S. Lei, B. Jin, C.W.K. Chow, C. Saint, Adsorption of congo red by three Australian kaolins, Journal ofApplied Clay ScienceV 43 (2009) 465–472. [24] M. Visa, C. Bogatu, A. Duta, Simultaneous adsorption of dyes and heavy metals from multicomponent solutions using fly ash, Applied Surface Science 256 (2010) 5486–5491. [25] W.K.W. Lee, J.S.J. Van Deventer, Structural reorganization of class F fly ash in alkaline silicate solutions, Journal of Colloid and Surfaces 211 A (2002) 49–66. [26] S. Lagergren, About the theory of so-called adsorption of soluble substances, Kungliga Svenska Vetenskapsakademiens Handlingar 24 (4) (1898) 1–39. [27] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Journal of Process Biochemistry 34 (1999) 51–465. [28] Y.S. Ho, G. McKay, Second-order kinetic model for the sorption of cadmium onto tree fern: a comparison of linear and non-linear methods, Journal of Water Research 40 (2006) 119–125.
[29] S.J. Allen, G. Mc Kay, K.Z.H. Khader, Interparticle diffusion of a basic dye during adsorption onto sphagnum peat, Journal of Environmental Pollution 56 (1999) 39–50. [30] D.M. Manohar, B.F. Noeline, T.S. Anirudhan, Adsorption performance of Alpillared bentonite clay for the removal of cobalt(II) from aqueous phase, Journal of Applied Clay and Science 31 (2006) 194–206. [31] A. Garbacz, L. Courard, K. Kostana, Characterization of concrete surface roughness and its relation to adhesion in repair systems, Journal of Materials Characterization 56 (2006) 281–289. [32] I. Horcas, R. Fernandez, J.M. Gomez-Rodriquez, J. Colchero, J. Gomez- Herrero, A.M. Baro, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Review of Scientific Instruments 78 (2007) 013705. [33] S. Ding, I. Sun, Z. Yu, C. Yang, B. Xu, Removal of copper from aqueous solutions by bentonites and the factors affecting it, Mining Science and Technology (China) 19 (2009) 489–492.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Tailoring fly ash activated with bentonite as adsorbent for complex wastewater treatment Maria Visa ∗ Transilvania University of Brasov, Department Renewable Energy Systems and Recycling, Eroilor 29, 500036 Brasov, Romania
a r t i c l e
i n f o
Article history: Received 6 August 2012 Received in revised form 26 September 2012 Accepted 28 September 2012 Available online 9 October 2012 Keywords: Bentonite Fly ash Pollutants Adsorption Wastewater treatment
a b s t r a c t Used as adsorbent, alkali fly ash represents a low cost solution for advanced wastewater treatment. The alkali treatment raises sustainability issues therefore, in this research we aim to replace alkali fly ash with washed fly ash (FAw). For improving the adsorption capacity of washed fly ash, bentonite powder (B) was added, as a natural adsorbent with a composition almost identical to the fly ash. The new adsorbent was characterized by AFM, XRD, FTIR, SEM, EDS and the surface energy was evaluated by contact angle measurements. For understanding the complex adsorption process on this mixed substrate, preliminary tests were developed on synthetic wastewaters containing a single pollutant system (heavy metal), binary (two-heavy metals) and ternary (dye and two heavy metals) systems. Experiments were done on synthetic wastewaters containing methylene blue, cadmium and copper, using FAw, B and their powder mixtures. The pseudo-second order kinetics could well model all the processes, indicating a good adsorbent material which can be used for the pollutants removal from wastewater. After adsorption the substrates loaded with pollutants, annealed at 500 ◦ C can be reused for padding in stone blocks. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Anywhere and anytime, water plays an important role for human beings, natural environment, and social development, but the subsequences of used water are represented by polluted municipal and industrial wastewaters (e.g. resulted from mining, metallurgy, metal plating, smelting flux electrolysis, battery manufactories, petroleum refineries, paint and pigments manufacture, printing, photographic industries and the glass production [1]. In all wastewater resulted from textile dyeing, dyes are found along with surfactants, whitening agents, anti-foaming agents even heavy metals, thus complete treatment becomes complicated. One of the most bio-toxic heavy metal from wastewater is cadmium, while dyes and pigments are affecting water transparency, reducing light penetration and gas solubility in water [2], also being mutagenic to aquatic organisms and humans. The most popular method to treat heavy metals in wastewaters is chemical precipitation, followed by filtration or other solid/liquid separation processes. Although the chemical precipitation method is quite effective for heavy metal removal, the resultant heavy metal sludge is classified as a hazardous solid waste and needs to be adequately treated. Other methods such as electrodialysis [3] nanofiltration, coagulation/co-precipitation, reverse osmosis [4] biosorption including some economic bio-adsorbent [5],
∗ Tel.: +40 729 109355; fax: +40 268 410525. E-mail address: [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.156
adsorption [2,6] and ion exchange process are developed to remove heavy metals from industrial wastewaters [7]. Among these, adsorption technologies have several advantages: easy operation and well known technology, inexpensive equipments, less sludge, the adsorbents materials can be selected from a broad variety of bio-materials, mineral oxides, activated carbon, resins and even waste materials. Plenty of research was devoted to the investigation of the adsorption capacity of less expensive materials such as sugar beet pulp [8], red mud [9], natural zeolite, clay and diatomite [10], scrap rubber, peat [11] or bone char [12], fly ash [13] etc. for heavy metals (cadmium, copper, zinc, nickel, iron) and dyes removal [14,15]. For a sustainable industry, low-cost adsorbents are intensively studied, mainly based on natural compounds or on organic artificial or synthetic wastes such as fly ash. We proposed to obtain a low-cost adsorbent combining bentonite (B) – a material widespread in nature and fly ash (FA) – a waste. Bentonites are the most abundant argillaceous materials which can be used in wastewater treatment [16]. They are reported as low cost efficient adsorbents for some heavy metals (copper, lead, cadmium and zinc) while modified bentonite was used for removing 60 Co radionuclide from radioactive waste solutions. The outstanding adsorption capability is due to main mineral component as montmorillonite, smectite and clay. The fly ash represents a major pollution source for air, soil, and ground water but can also be used as adsorbent; large amounts of FA
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Table 1 Summary of adsorption of Cd2+ , Cu2+ , Ni2+ , Pb2+ cations and dye on fly ash and bentonite [18]. Metal
Adsorbent
Adsorption capacity (mg/g)
References
Cd2+
Coal fly ash Fly ash Fly ash zeolite FA-Z Bagasse fly ash Bentonite
18.98 198.2 95.6 30.21 1.24–2.0 2.8
[70] [79] [80] [19]* [95] [20]*
18.83 9.69 17.87
[21]* [14]* [22]*
Cu2+
Fly ash Fly ash Bentonite
Pb2+
Fly ash washed Fly ash Bentonite
Ni2+
Bentonite
13.97
[22]*
Congo red
Bentonite
7.27
[23]*
Methylene blue
Fly ash+TiO2
0.37–0.52
[14]*
483.4 18.8 22
[79] [87] [20]*
[ ]* are included in references.
are not re-used, as for example the 50% of the 2010 EU production (over 100 million tons) that is not valorised currently. Brasov City has a CPH plant which works with two boilers (420 tons/h capacity) and two turbo-aggregates which generate 50 MW/each. Every year 6 × 105 tons of coal is burned and the fly ash resulted amounts 2 × 105 tons/year out of which only 1/3 is reused as a cement additive. One possible application is to develop alternative processes (like wastewater treatment), involving the use of this waste. Previous studies proved a good efficiency of alkali modified fly ash for heavy metals (cadmium, copper, zinc, nickel) and for dyes (methylene blue (MB) and methyl orange (MO)) removal, from mono-, bi- or three-components (Cu2+ , Cd2+ , Ni2+ and MB) wastewaters, resulted from the dye finishing industry [14]. The results showed that raw fly ash has limited efficiency, thus the surface of fly ash was modified with NaOH solution or other reagents. Although effective, the alkali treatment results in supplementary wastewaters, therefore, this paper investigates efficient solutions for the FA re-use by combining the washed FA with a natural adsorbent, bentonite, resulting in a low-cost, efficient substrate. There is a growing interest in using low cost adsorbents, fly ash, bentonite or their mixture. If these materials are characterized and tested for to remove the heavy metals and dyes, the adsorption process will be a promising technology. The Fe2+ cation can be removed from wastewater resulted at a galvanized pipe manufacturing industry using bentonite with efficiency up 90% [17].Table 1 summarizes some of the recent results of Cd(II), Cu(II), Ni(II), Pb(II) ions and dyes removal from wastewater using fly ash and bentonite.
2. Materials and methods 2.1. Adsorbents materials Bentonite was provided from NW Romania area. The raw fly ash was supplied by the CHP Plant Brasov, Romania directly from the electro-filters, sifted, choosing grains with diameter between 40 and 100 m. The major oxide components in fly ash and bentonite, with a certain influence in heavy metals and dyes removal are presented in Table 2. Using emission spectrometry other metals were also identified in small amounts (Ba, Cu, Sn, Pb, Cr, Ni, V, Zn, Ti).
Table 2 Adsorbent materials composition. Major oxides [wt%] SiO2
Al2 O3
Fe2 O3
CaO
FA composition [%] 53.32 22.05 8.97 5.24 Bentonite composition [%] 72.11 14.92 2.62 2.31
MgO
K2 O
Na2 O
TiO2
MnO
LOI*
2.44
2.66
0.63
1.07
0.08
1.58
2.12
1.34
1.91
0.62
0.05
2.06
According to the ASTM standards, the raw FA pertains of class F because the sum of the SiO2 , Al2 O3 and Fe2 O3 is above 70% [24], while bentonite is of Na type [25]. Both substrates were separately sieved and the 40–100 m fractions were selected as substrates for experiments. Raw adsorbent materials contain soluble compounds, with pollution potential therefore washing before use is compulsory. By mixing 200 g of powder FA and separate 200 g of powder bentonite) with 2000 mL ultra pure water, followed by stirring at room temperature (20–22 ◦ C), for 48 h till constant pH value. The pH and conductivity values were evaluated for FA (pH 10.2 and for bentonite pH 9.9, respectively = 1.710 mS for both). The washed FA and washed bentonite were further dried at 105–120 ◦ C, till constant mass. These substrates are further denominated as FAw and B. Structural and crystallographic properties of individual FAw and B particles were evaluated by XRD (Bruker D8 Discover Diffractometer) and for morphology studies the AFM images were used (Ntegra Spectra, NT-MDT model BL222RNTE). The images were taken in semi-contact mode with Golden silicon cantilever (NCSG10), with constant force 0.15 N/m, having the tip radius of 10 nm. Scanning was conducted on three or more different places with a certain area 10 m × 10 m or 5 m × 5 m for each position, chosen randomly at a scanning grate of 1 Hz. Further surface investigations were done using scanning electron microscopy (SEM) operated with SEM S-3400N-Hitachi at an accelerating voltage of 20 kV. Compositions were measured using energy-dispersive X-ray spectroscopy (EDS Thermo Scientific Ultra Dry). The surface area of the fly ash and bentonite was measured by using the Tri Star II 3020 Analyser (Micromeritics). The information related to the functional groups on the surface was provided by the FTIR data (PerkinElmer BX II 75548). 2.2. Adsorption experiments The adsorption tests were performed by batch experiments, under stirring up to 240 min at room temperature (20–22 ◦ C), at the natural pH of the dispersion, Table 3. Aliquots were taken each at 15, 30, 45, 60, 90, 120 180, 240 min, when stirring was briefly interrupted and the substrate was removed by vacuum filtration. The supernatant was further analyzed by AAS (Analytic Jena, ZEEnit 700), at: Cd = 228.8 nm, Cu = 324.75 nm. The absorbance measurements were recorded in the range of 200–900 nm, using a UV–vis spectrophotometer (PerkinElmer Lambda 950 UV/VIS) at the maximum absorption wavelength registered at 664 nm. Three series of experimental tests were done on mono component solution of Cd2+ (cCd = 0.750 mg/L, using CdCl2 ·2.5 H2 O, Scharlau Chemie), two heavy metals solution (Cd2+ and Cu2+ , cCu = 0.400 mg/L, using CuCl2 ·2H2 O Scharlau Chemie), and threecomponents solution (Cd2+ , Cu2+ and methylene blue (C16 H18 N3 S) cMB = 0.05 mM, using MB Fluka AG, reagent grade). The lowest admissible discharge concentration are, in most national regulations set for cadmium therefore, the optimized adsorption conditions set for cadmium were extended also for copper in multicomponent solutions, in the tests performed using FAw, B and their mixtures, as shown in Table 4.
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Table 3 The natural pH values of the solution with adsorbent. FAw:B [g:g] pH
1:0 7.4
0:1 9
0.25:0.75 6.2
0.50:0.50 5
1.00:1.00 5.1
0.5:1.5 6.5
Table 4 Substrates and pollutant systems experimentally tested. Substrates (g) FAw:B Substrate Pollutant systems
1:0 (FAw) Cd2+
0:1 (B) Cd2+
0.25:0.75 (1) Cd2+ +Cu2+
0.50:0.50 (2) Cd2+ +Cu2+
1.00:1.00 (3) Cd2+ +Cu2+
1.5:0.5 (4) Cd2+ +Cu2+ +MB
The optimal adsorbent mass: wastewater volume was evaluated based on cadmium750 mg/L solutions, using a contact duration of 90 min, while the optimal contact time was evaluated on suspensions of 0.5 g FAw in 100 mL multicomponent solutions of Cd2+ (750 mg/L), Cu2+ (400 mg/L), respectively methylene blue 0.05 mM. Preliminary experiments proved that heavy metals and MB losses due to adsorption onto the container walls and filter system were negligible. 2.3. The adsorption kinetics For the kinetic studies, the metal uptake qe (mg/g) was evaluated using the initial and current, t, heavy metal concentrations (c i n+ and c t n+ ), in a given solution volume, (V = 100 mL), as preM M sented in Eq. (1): qe =
(c i n+ −c t n+ )×V M M ms
.
(1) Fig. 1. XRD patterns of (1) raw FA, (2) FAw, and (3) bentonite.
The kinetics of the heavy metals adsorption was modelled by the following equations: the pseudo first-order equation Lagergren, 1898 [26]: log(qe − qt ) = log(qe ) −
KL t. 2.303
(2)
where KL is the Lagergren constant, qe is the equilibrium uptake value and qt the current metal uptake; the pseudo-second order kinetic equation Ho and McKay [27] and a comparison of linear and non-linear methods [28]: 1 t t = + qt qe k2 qe 2
(3)
where, k2 is the equilibrium rate constant for the pseudo secondorder adsorption which can be evaluated from the slope of the plot. Another possible kinetic model which can be applied in adsorption processes on porous materials is the interparticle diffusion model. In this case, the amount of heavy metals ions adsorbed can be calculated with the Eq. (4), [29]: q = kid t 1/2 + C
The main constituent of bentonites is montmorillonite, which is a 2:1 mineral with one octahedral sheet and two silica sheets, forming a layer. Layers are held together by van der Waals forces. Because of these weak forces and some charge deficiencies in the structure, water can easily penetrate these layers and cations balance the charge deficiencies [2]. The data also show that major components, quartz and graphite, are not affected during FA modification. The crystalline degree of (B) is 79.5% the rest being represented by amorphous phases. The crystallites size, calculated with the ˚ Scherer formula ranges from 171.0 to 653.2 A. Supplementary information was obtained using the FTIR spectra. The FTIR spectra of FAw and Bentonite are presented in Fig. 2. The absorption band observed at 3622–3624 cm−1 was attributed to the hydroxyl group stretching/vibration in Si OH, Al OH Al, Mg OH Al and/or Fe OH Al units in octahedral layer [30]. The
(4)
3. Results and discussions 3.1. Characterization of the adsorbent material The diffractograms (Fig. 1) show that some crystalline phases of FA (quartz, philipsite cristobalite, hematite) are mostly present in raw FA and new crystalline phases appear (mullite) in FAw. The XRD data show that during washing soluble compounds concentrated on the fly ash surface are dissolute, leaving the core, with a different composition of non-soluble crystalline compounds (Fig. 1). In bentonite the new crystalline phases are: sodium aluminium silicate NaAlSiO4 , silimanite (Al2 (SiO4 )O, sodium aluminium silicate hydrate (phillipsite) (Na6 Al6 Si10 O32 ·12H2 O), silicate hydroxide hydrate (illite) (KH3 O)Al2 Si3 AlO10 (OH)2 , chabazite Na NaAlSi2 O6 ·3H2 O, stelerite Na, Na2 Al2 Si7 )O18 ·7H2 O faujasite silimanitul and other aluminosilicates phases, in lower amounts.
Fig. 2. IR spectrum (a) bentonite, (b) FAw.
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Fig. 3. The AFM topography and phase distribution.
asymmetric stretching mode of Si O Si in bentonite and FAw was suggested by the absorption band at 1010 cm−1 for bentonite and 1016 cm−1 for fly ash, both with sharp peak. In bentonite, water molecules are associated with the cations and are in some extent hydrogen bonded to the oxygen ions of the framework, explaining the peak with less intensity, recorded at 1639 cm−1 which is characteristic of the bended mode in the water molecules. This observation indicates the similarity between bentonite and fly ash. These data can be corroborated with the surface roughness which shows a strong decrease after washing of the fly ash Fig. 3. The roughness can give the different information versus the level of investigation (the millimeter scale usually permits one to distinguish the main surface treatments) [31]. Part of the oxides in raw FA and bentonite are water soluble. These chemical and structural changes are mirrored in morphology modifications, Figs. 3(b), 4(a) and (b) resulting in large differences
in the substrates’ affinity for heavy metals. On phases distributions images there can be seen less agglomerates in new material adsorbent so more mesopores ready to lodge the cations of heavy metals or dye molecules. These AFM images were used to characterize the surface morphology: the uniformity, grain size and pore size distribution [32] of the samples (Figs. 6 and 7). The surface area of the FAw and Bentonite were analyzed and the results show a strong increase in the specific area surface, and a decrease in the average pores diameter; the mixed substrate (FAw + B) has average values showing that mechanical mixing is the most likely occurring process (Fig. 5). Adsorption capacity and cation exchange depend on the chemical nature of the sorbent surface, pore structure, size of the aggregates, crystallinity degree of the particles, thus the textural and structural features, but also on the cation present in the cell layer, the solution pH and temperature and the solution–adsorbent
Fig. 4. The AFM topography and phase distribution.
M. Visa / Applied Surface Science 263 (2012) 753–762
Fig. 5. The interparticle voids distribution.
contact duration. In this view, bentonite adsorption strength due to fine particles is large, exhibiting a large contact surface. The surface characteristics for FA are presented in Table 4 and are characteristic to mezzo-porous solids (Table 5). The SEM images show significant differences between the substrates: most FAw particles are spherical with diameters between 3.61 and 111 m. The bentonite particles have a stratified structure with hexagonal large agglomerates, with sizes between 2.23 and 8.08 m.
757
Fig. 7. Scanning electron microscopic images after adsorption (a) Faw + B loaded with Cd2+ , Cu2+ and MB, (b) Faw + B-particle loaded with Cd2+ , Cu2+ and MB.
The surface morphology of FAw and bentonite appears as corn flake crystals with fluffy appearance revealing the fine platy structure. The results of the EDS analysis (Fig. 8) show that FAw and B have a similar composition that abounds in hydrous aluminosilicates. The EDS spectra of the materials FAw and B shown in Fig. 8 offered semi-quantitative ratio of present elements: O, Na, K, Ca, Mg, Al, Si, Ti, Fe. The analysis shows that the rims have a similar composition and abound the hydrous aluminosilicates. Bentonites are composed of hydrated largely of sodium aluminosilicate (chabazite Na NaAlSi2 O6 ·3H2 O) what are capable of adsorption and ion exchange. Bentonites can form colloidal suspensions, gels and water molecules are adsorbed between the lattice planes in the movement of ions [33]. Cation adsorption involves mainly electrostatic forces therefore the surface energy of the new substrate (FAw:B) can strongly influence the adsorption process. The polar and disperse contributions to the surface energy of FAw, B and their mixtures were calculated according to the model developed by Owens, Wendt, Robel and Kaelble and are presented in Table 6. The data show a high global surface energy and a large polar component, recommending the material as a good adsorption substrate for heavy metals cations and for methylene blue (Table 6).
3.2. Uptake kinetics of the heavy metals
Fig. 6. Scanning electron microscopic images of (a), (c) bentonite, (b), (d) FAw.
The models described in paragraph 2.3 were tested for the Cd2+ , Cu2+ removal by adsorption on FAw, B and on their mixtures. The kinetic parameters are presented in Table 7. The results proved that the pseudo-second order kinetic well describes the adsorption mechanism for both cations, on bentonite and on all theirs the mixtures investigated. The data also prove that copper adsorption can follow more parallel mechanisms, which could be expected considering the FAw: B composition and/or the pores distribution with active sites of various energies. The values of the adsorption capacity on system FAw:B is found to decrease in the order Cu2+ > Cd2+ on mixtures 0.5:0.5 and 1:1. The amount of bentonite in mixture increase the adsorption capacity as expected considering the differences in the specific surface values between FAw and B because in aluminosilicates are more groups Si OH that in FAw. The adsorption efficiency, , and adsorption capacity, qm , were evaluated based on the optimal contact time, the mass balance and initial cations’ concentration of adsorption for the most
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Table 5 Surface properties of FAw and bentonite. Sample
Surface area BET [m2 /g]
Micropores volume (t-plot) [cm3 /g]
Micropores surface (t-plot) [m2 /g]
Average pores diameter [nm]
FAw B
6.14 21.33
0.0004 0.0030
2.25 14.09
27.2 15.42
Fig. 8. EDS spectra of FAw and bentonite.
Table 6 Surface energy data for the substrates. Substrate
Surface energy [mN/m]
Disperse contribution [mN/m]
Polar contribution [mN/m]
FAw B FAw:B (4)
187.42 107.5 330.79
32.61 3.85 89.62
154.81 103.65 241.17
Table 7 Kinetic parameters of the heavy metal removal on FAw:B mixed substrate. FAw:B [g:g]
Pseudo first-order kinetics −1
KL [min
]
2
Pseudo-second order kinetics
Interparticle diffusion 2
R
k2 [g/mg min]
qe [mg/g]
R
Kid (mg/g min1 /2 )
R2
C
Cadmium 1:0 0.50:0.50 0.25:0.75 0:1 1:1 1.5:0.50
– – – – 0.007 –
0.413 0.627 0.023 0.026 0.954 0.236
– 0.147 0.049 0.059 0.408 0.092
– 31.646 30.395 42.553 19.841 28.011
0.854 0.998 0.998 0.999 0.997 0.998
– – – – – –
– – – – – –
0.801 0.535 0.409 0.446 0.592 0.454
Cd2+ (Cd2+ +Cu2+ ) 0.50:0.50 1:1 1.5:0.5
– 0.022 0.013
0.415 0.843 0.967
1.613 0.379 7.330
24.752 10.341 20.877
0.948 0.998 0.982
1.505 – –
0.851 – –
0.942 0.547 0.867
Cu2+ (Cu2+ +Cd2+ ) 0.50:0.50 1:1 1.5:0.5
0.016 0.026 0.012
0.886 0.962 0.931
1.348 0.477 0.092
25.839 14.771 16.78
0.976 0.999 0.999
1.035 0.274 0.092
7.456 10.603 15.425
0.962 0.852 0.903
M. Visa / Applied Surface Science 263 (2012) 753–762
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Fig. 9. Cd2+ immobilization; (a) efficiency vs. substrate dose, (b), (c) efficiency vs. contact time. Fig. 10. Cd2+ , Cu2+ and MB immobilization from multi-pollutant systems; efficiency vs. contact time from mixture.
toxic heavy metal, cadmium. The efficiency was calculated using the following Eq. (5). =
(c i n+ −c t n+ )×100 M M c i n+ M
(5)
where c i n+ and c t n+ are the initial and equilibrated cations’ conM M centrations (mg/L).
The dynamic adsorption results are presented in Fig. 9a–c for the Cd2+ cations and for all pollutants (Cd2+ , Cu2+ and MB) from multi-pollutant systems are presented in Fig. 10a–c. The data allow optimising the adsorbent amount, considering cadmium as reference, in a single pollutant system.
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3.3. Adsorption isotherms and mechanism The Langmuir and Freundlich isotherms equations are selected for calculated adsorption parameters of these adsorbents materials. The Langmuir isotherm – linearization is: Ceq Ceq 1 = + qeq qmax a qmax
(9)
where qmax (mg/g) represents the monolayer adsorption capacity, a is a constant related to the free energy of adsorption, qeq and Ceq show the amount of metal adsorbed on. The Freundlich isotherm – linearization: ln qeq = ln kF +
Fig. 11. Efficiency of adsorption at different initial concentration of Cd2+ and of Cu2+ .
Adsorption efficiency of cadmium on FAw is low because the surface of FAw is not activated and the specific surface area is low compared to bentonite. The adsorption equilibrium for cadmium and copper needs 90 min to be settled therefore, this was the set as optimal contact time in all further experiments. Also, for these two ions, higher substrate dosage is required, almost similar removal efficiencies (close to 70%) being registered at 2 g of FAw:B dispersed in 100 mL of solution. The removal efficiency increases with amount of bentonite from mixture, proving that adding B to FAw represents a path to fulfil the set target: the use of washed FA in advanced wastewater treatment. The Cd2+ and Cu2+ cations can adsorb by chemically bonding with the active site ( SiO− ), and ( AlO− ) and can form complexes on the surface as presented by [30]. The metal compounds are the hydrated cations which can adsorb with partial or total dehydration (Eq. (6)): substrate + Me(H2 O)n z+ ⇔ substrate–Me(H2 O)n−x z+ + xH2 O
(6)
where 0 ≤ x ≤ n [21]. Or: The Cd2+ and Cu2+ cations can also be adsorbed by the silanol group (Si OH) of the layers, but their numbers are distinctly smaller (Eqs. (7) and (8)): Si OH + Cd2+ (H2 O)n ⇔ Si OCd+ (H2 O)n−x + H+ (H2 O)x
(7)
Si OH + Cd2+ (H2 O) ⇔ (Si O)2 Cd(H2 O)n−x + 2H+ (H2 O)x
(8)
The data show that cadmium and copper have a much lower affinity for the bentonite substrate comparing to the MB dye, showing an efficiency decrease for contact times longer than 240 min, supporting the assumption of an adsorption process without dehydration. The FAw:B substrate is highly efficient ( > 90%) for the removal of all cations at concentrations below 100 mg/L Fig. 11, thus for advanced wastewater treatment, another preliminary process (e.g. precipitation) could be necessary.
1 ln Ceq n
(10)
where kF is Freundlich adsorption constants, being indicators of adsorption capacity and 1/n dimensionless parameter is a measure of the adsorption density. Table 8 presents the adsorption parameters for the heavy metal ions (Cd2+ , Cu2+ ) calculated from the slope of the plots. The Langmuir model is better than the Freundlich model in simultaneous adsorption of cadmium and copper cations from multicomponent system. The mechanisms of adsorption is a hybrid one and corroborated with kinetic parameters can follow – chemisorption mechanism or ion exchange for the cadmium and cooper cations, according to Eqs. (7) and (8). This suggests that the heterogeneity, loading on the surface or pores of substrate (FAw–B) play a role in heavy metals adsorption. The adsorption studies carried out to estimate the heavy metal removal from wastewater, using fly ash modified with bentonite, showed that the efficiency follows the order Cu2+ ≥ Cd2+ , according with the dimensionless separation factors are: Cu 2+ = 0.243 > Cd 2+ = 0.151. Supplementary, after adsorption and filtration the substrates loaded with pollutants were subject of pelletization. Round tablets (pellets) with 1 g mass, were annealed at different temperatures (100 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C and 500 ◦ C. The five annealed samples, plus one which was not subject to heat treatment were introduced in 100 mL bidistilled water (each pellet in one glass). The ratio mpellet :Vbidistilled water = 1:100. The stability of the pellets strongly depends on the annealing temperature, and the best stability corresponds to the tablets annealed at 400 ◦ C and 500 ◦ C. After 48 h, the methylene blue, cadmium and copper cations concentration were evaluated to test the desorption processes. The results show that, after 48 h, there are no desorption of cadmium or copper and maybe methylene blue was decompose or burn. Mechanical properties and composite environmental behavior can be tailored by dispersing different oxide powders (TiO2 , raw fly ash and these annealed pellets). Efficient dispersing occurs when the powders are in the range of nano- or mezzo-scale, resulting hybrid inorganic–organic composites. This can be a suggestion for reused the spent adsorbent for padding in the stone blocks. The data also show that cadmium and copper have a much lower affinity for the bentonite substrate, showing an efficiency decrease for contact times longer than 240 min, supporting the assumption of an adsorption process without dehydration. The FAw:B substrate is highly efficient (>90%) for the removal of all cations at concentrations below 100 ppm, thus for advanced wastewater treatment, another preliminary process like precipitation. The FAw:B mixture is an alternative adsorbent material to remove in a single step more cations and dye and to decrease the amount of fly ash from environment.
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Table 8 Parameters of Langmuir and Freundlich isotherms of various adsorbents (FAw:B). FAw:B [g:g]/parameters
Langmuir isotherm
Freundlich isotherm
qmax [mg/g]
a [L/mg]
R2
1/n
KF
R2
Cd2+ (0.5:0.5) (1:1) (1.5:0.5)
25.907 27.174 27.101
0.025 0.007 0.006
0.979 0.983 0.998
– – –
– – –
0.751 0.024 0.665
Cd2+ /(Cd2+ +Cu2+ ) (0.5:0.5) (1:1) (1.5:0.5)
27.781 27.701 7.485
0.005 0.015 0.196
0.977 0.988 0.981
0.281 0.275 0.281
0.098 0.056 0.004
0.923 0.917 0.936
Cu2+ /(Cu2+ +Cd2+ ) (0.5:0.5) (1:1) (1.5:0.5)
11.389 24.096 7.299
0.019 0.008 0.193
0.991 0.939 0.948
0.280 0.255 0.268
0.005 0.153 0.003
0.924 0.933 0.906
4. Conclusions The removal efficiency of heavy metals and MB increases with amount of bentonite from mixture, because the surface area of bentonite is large that of FAw. The adsorption capacity depends on the chemical nature of the sorbent surface, pore structure, crystallinity degree of the particles, thus the textural and structural features, and the solution–adsorbent contact duration. In this view, the adsorption on bentonite strength due to fine particles is large, exhibiting a large contact surface. An important characteristic of mixture FA:B (1.5:0.5) is its large cation adsorption capacity and has a convenient efficiencies after 90 min of contact time and is not strongly influenced by the MB existent in the solution. Copper has a higher affinity for the active sites comparing to cadmium. The reason may be the higher mobility and lower volume of the hydrated complexes of the cations in water (hexa-complexes for Cd and tetra-coordination for Cu), at the working pH of 4.8 . . . 5.3. Due to the large porosity of FA, the efficiency of the heavy metal adsorption does not depend on the FA fraction used. This becomes significant for the adsorption of large molecules, as it is MB. The pseudo-second order kinetics describe well all the processes. Large adsorption capacities are registered for copper, confirming the higher affinity of the ion for this mixture (FAw:B). The Langmuir model is better than the Freundlich model in simultaneous adsorption of cadmium and copper cations from multicomponent system and the mechanisms of adsorption is a hybrid one and can follow – chemisorption mechanism or ion exchange for the cadmium and cooper cations. High adsorption efficiencies are registered for heavy metals concentrations below 100 ppm, recommending this substrate for simultaneous removal of heavy metals and MB from wastewater resulted in the dyes finishing industry. Using washed fly ash and bentonite as substrate, the pollutants can be removed at low cost by adsorption. Adsorption of heavy metals ions by bentonite depends on charge characteristics of the adsorbent, including surface charge magnitude, type of charge (permanent or variable) and point of origin. The spent adsorbent annealed at 500 ◦ C can be included in stone blocks resulting hybrid inorganic–organic composites. This can be a suggestion for reused the spent adsorbent for padding in the stone blocks. Acknowledgements This paper is supported by the Sectorial Operational Programme Human Resources Development (SOP HRD), financed from the
European Social Found and by the Romanian Government under the contract number POSDRU ID 59323.
References [1] M.E. Argun, S. Dursun, C. Ozdemir, M. Karatas, Heavy metal adsorption by modified oak sawdust: thermodynamics and kinetics, Journal of Hazardous Materials 141 (2007) 77–85. [2] L.J. Yu, S.S. Shukla, K.L. Dorris, A. Shukla, J.L. Margrave, Adsorption of chromium from aqueous solutions by maple saw dust, Journal of Hazardous Materials 100 (2003) 53–63. [3] T. Mohammadi, A. Razmi, M. Sadrzadeh, Effect of operating parameters on Pb2+ separation from wastewater using electrodialysis, Journal of Desalination 167 (2004) 379–385. [4] Z. Ujang, G.K. Anderson, Performance of low pressure reverse osmosis membrane (LPROM) for separating mono- and divalent ions, Journal of Water Science and Technology 38 (1998) 521–528. [5] C.R.T. Tarley, M.A.Z. Arruda, Biosorption of heavy metals using rice milling byproducts. Characterisation and application for removal of metals from aqueous effluents, Journal of Chemosphere 54 (2004) 987–995. [6] H. Choi, S.D. Kim, S.H. Noh, S.J. Oh, W.J. Kim, Adsorption behaviors of nano-sized ETS-10 and Al-substituted-ETAS-10 in removing heavy metal ions, Pb2+ and Cd2+ , Journal of Microporous and Mesoporous Materials 87 (2006) 163–174. [7] I. Lee, Y.C. Hsein Kuang, J.M. Chern, Equilibrium and kinetics of heavy metal ion exchange, Journal of the Chinese Institute of Chemical Engineering 38 (2007) 71–84. ˜ [8] Y.N. Mata, M.L. Blázquez, A. Ballester, F. González, J.A. Munoz, Sugar beet pulp pectin gels as biosorbent for heavy metals: Preparation and determination of biosorption and desorption characteristics, Journal of Chemical Engineering 150 (2009) 289–301. [9] W. Shaobin, Y. Boyjoo, A. Choueib, Z.H. Zhu, Removal of dyes from aqueous solution using fly ash and red mud, Journal of Water Research 39 (2005) 129–138. ˇ ´ I. Smiˇciklas, S. Pejanovic, ´ I. Plecaˇ ´ s, Comparative study of Cu2+ adsorp[10] M. Sljivi c, tion on a zeolite, clay and a diatomite from Serbia, Journal of Applied Clay ScienceV 43 (2009) 33–40. [11] Y.S. Ho, G. McKay, Sorption of Cooper(II) from aqueous solution by peat, Journal of Water Air and Soil Pollution 158 (2004) 79–97. [12] C.W.J. Cheung, F. Porter, G. McKay, sorption kinetic analysis for the removal of cadmium ions from effluents using bone char, Journal of Water Research 35 (2001) 605–612. [13] S. Chaizasith, P. Chaizasith, C. Septhum, removal of cadmium and nickel from aqueous solutions by adsorption onto treated fly ash from Thailand, Journal of Science and Technology 11 (2006) 13–19. [14] M. Visa, L. Andronic, D. Lucaci, A. Duta, Concurrent dyes adsorption and photo-degradation on fly ash based substrates, Journal of Adsorption 1 (2011) 101–108. [15] M. Visa, L. Andronic, A. Duta, Photocatalytic properties of titania–fly ash thin films, Environmental Engineering and Management Journal 8 (2009) 633–638. [16] S. Ding, J.S.B. Xu, Q. Liu, Y. Sun, The factors on removal of zinc cation from aqueous solution by bentonite, Journal of Natural Resources 2 (2011) 107–113. [17] S.S. Tahir, N. Rauf, Removal of Fe(II) from the wastewater of a galvanized pipe manufacturing industry by adsorption onto bentonite clay, Journal of Environmental Management 73 (2004) 285–292. [18] M. Ahmaruzzaman, A review on the utilization of fly ash, Progress in Energy and Combustion Science 36 (2010) 327–363. [19] M. Visa, L. Isac, A. Duta, Fly ash adsorbents for multi-cation wastewater treatment, Applied Surface Science 258 (2012) 6345–6352. [20] S. Andini, R. Cioffi, F. Montagnaro, F. Pisciotta, L. Santoro, Simultaneous adsorption of chlorophenol and heavy metal ions on organophilic bentonite, Journal Applied Clay Science 31 (2006) 126–133. [21] M. Visa, A. Duta, Adsorption behavior of cadmium and copper compounds on mixture FA:TiO2 , Revue Roumaine de Chimie 55 (2010) 167–173.
762
M. Visa / Applied Surface Science 263 (2012) 753–762
[22] L. Zhi-rong, Z. Shao-qi, Adsorption of copper and nickel on Na-bentonite, IChemE 88 (2010) 62–66. [23] V. Vimonses, S. Lei, B. Jin, C.W.K. Chow, C. Saint, Adsorption of congo red by three Australian kaolins, Journal ofApplied Clay ScienceV 43 (2009) 465–472. [24] M. Visa, C. Bogatu, A. Duta, Simultaneous adsorption of dyes and heavy metals from multicomponent solutions using fly ash, Applied Surface Science 256 (2010) 5486–5491. [25] W.K.W. Lee, J.S.J. Van Deventer, Structural reorganization of class F fly ash in alkaline silicate solutions, Journal of Colloid and Surfaces 211 A (2002) 49–66. [26] S. Lagergren, About the theory of so-called adsorption of soluble substances, Kungliga Svenska Vetenskapsakademiens Handlingar 24 (4) (1898) 1–39. [27] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Journal of Process Biochemistry 34 (1999) 51–465. [28] Y.S. Ho, G. McKay, Second-order kinetic model for the sorption of cadmium onto tree fern: a comparison of linear and non-linear methods, Journal of Water Research 40 (2006) 119–125.
[29] S.J. Allen, G. Mc Kay, K.Z.H. Khader, Interparticle diffusion of a basic dye during adsorption onto sphagnum peat, Journal of Environmental Pollution 56 (1999) 39–50. [30] D.M. Manohar, B.F. Noeline, T.S. Anirudhan, Adsorption performance of Alpillared bentonite clay for the removal of cobalt(II) from aqueous phase, Journal of Applied Clay and Science 31 (2006) 194–206. [31] A. Garbacz, L. Courard, K. Kostana, Characterization of concrete surface roughness and its relation to adhesion in repair systems, Journal of Materials Characterization 56 (2006) 281–289. [32] I. Horcas, R. Fernandez, J.M. Gomez-Rodriquez, J. Colchero, J. Gomez- Herrero, A.M. Baro, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Review of Scientific Instruments 78 (2007) 013705. [33] S. Ding, I. Sun, Z. Yu, C. Yang, B. Xu, Removal of copper from aqueous solutions by bentonites and the factors affecting it, Mining Science and Technology (China) 19 (2009) 489–492.