A comparative study on fly ash, geopolymer and faujasite block for Pb removal from aqueous solution

A comparative study on fly ash, geopolymer and faujasite block for Pb removal from aqueous solution

Fuel 185 (2016) 181–189 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article A compar...

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Fuel 185 (2016) 181–189

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

A comparative study on fly ash, geopolymer and faujasite block for Pb removal from aqueous solution Yi Liu a,b,c, Chunjie Yan a,b,⇑, Zuhua Zhang c,⇑, Hongquan Wang a, Sen Zhou a, Wei Zhou a,b a

Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China Engineering Research Center of Nano-Geomaterials of Education Ministry, China University of Geosciences, Wuhan 430074, China c Centre for Future Materials, University of Southern Queensland, Toowoomba, QLD 4350, Australia b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Geopolymer and its deriving zeolite–

faujasite–was prepared from fly ash.  Both geopolymer and its deriving

faujasite can effectively remove Pb.  Geopolymer has the same adsorption

mechanisms as faujasite or zeolite materials.  Shows geopolymer technology is a feasible process in adsorbent manufacturing.

a r t i c l e

i n f o

Article history: Received 4 April 2016 Received in revised form 21 June 2016 Accepted 28 July 2016

Keywords: Adsorption Absorbent Fly ash Geopolymer Faujasite Lead

a b s t r a c t This work aims to evaluate the efficiencies of fly ash, fly ash-based geopolymer and faujasite block, which is transformed from geopolymer, as sorbents for lead (Pb) from aqueous solutions. Comparative experiments were performed to examine the mineralogical features of the fly ash, geopolymer and faujasite block and their adsorption capacities. Equilibrium isotherms and thermodynamic parameters were obtained through systematic investigation of parameters including pH, initial Pb concentration, temperature and contact time. The adsorption kinetics of geopolymer and faujasite block fit well to the pseudosecond-order kinetic model, while the adsorption of fly ash fit to the pseudo-first-order kinetic model. The adsorption equilibrium data of fly ash, geopolymer and faujasite can be expressed using Langmuir model. The maximum adsorption capacities of fly ash, geopolymer and faujasite block at pH = 3 were determined to be 49.8, 118.6 and 143.3 mg/g, respectively. Through this study we demonstrate that both geopolymer and faujasite can effectively remove Pb from wastewater. Most importantly, we prove that geopolymer has the same adsorption mechanisms as faujasite or similar zeolite materials. This finding suggests geopolymer technology being an energy-saving, low cost and environmentally friendly process in adsorbent manufacturing. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding authors at: Engineering Research Center of Nano-Geomaterials of Education Ministry, China University of Geosciences, Lu Mo Road 388, Wuhan 430074, China (C. Yan). E-mail addresses: [email protected] (C. Yan), [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.fuel.2016.07.116 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

Lead (Pb) is one of the most toxic heavy metal pollutants discharged into environment (soil, groundwater and air) through industrial wastes [1]. The sources include petroleum refining industry, lead-containing pesticides, discarded batteries and

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painting materials [2,3]. Lead is nondegradable and persistent, and is detrimental to humans and other living beings [1,4]. A large number of techniques have been applied to remove such heavy metals (particularly from aqueous solution), such as chemical precipitation followed by coagulation and ion exchange, solvent extraction, ultrafiltration, reverse osmosis, electro-dialysis, evaporative recovery and adsorption [4–6]. Among these techniques adsorption is particularly effective by using a variety of adsorbents. In industrial application the selection of a ‘suitable’ adsorbent is critical because it deals with the cost and efficiency. Adsorbents with low-cost and high pollutant-removal efficiency, such as activated carbon [7,8], and gasified granulated scrap tires [9] recommended in the literature, are always favored. Fly ash has been found to be a potential and versatile adsorbent for removal of toxic metals from aqueous solutions [10] and capture metals in gas phase [11]. Fly ash is collected from coal combustion at power station and petroleum industry, and is cheap and ready available in large quantities (world generation is over 500 Mt annually) [12]. However, the problem of raw fly ash is the low adsorption capacity [13], which usually needs proper physical and/or chemical treatments [14,15]. The conversion of fly ash into crystalline zeolites has gained significant interests in the last 30 years [16,17]. Zeolites are crystalline aluminosilicates with three-dimensional frameworks of SiO4 and AlO4 tetrahedral units. The negative charge is balanced by the interchangeable positive ions such as: Na+, K+, Ca2+ and Mg2+. A particular zeolite usually has fixed-sized pores and paths that allow certain heavy metals to pass through, which means selective separation. Faujasite is such a type of zeolite which can be used for the adsorption of Pb2+ heavy metals from wastewaters [6]. Nevertheless, synthesize zeolites from fly ash normally cause alkaline pollution [18]. Moreover, the conversion procedures have problems of low yields and high costs [19]. Recently a new process of converting fly ash into geopolymers is attracting more and more interests because of the effective absorption [15,20–22] and immobilization [23,24] of heavy metals. Geopolymers are a class of amorphous materials formed by reaction of aluminosilicate source with alkaline solution at ambient or higher temperatures [25]. They consist of SiO4 and AlO4 tetrahedral units linked alternatively in three directions by sharing all oxygen atoms between two tetrahedral units, and positive ions, such as Na+ and K+, in the frame work cavities balance the negative charge of Al [26]. The formation of geopolymer follows almost the same way as that for most zeolites, and the difference between zeolites and geopolymers is that geopolymer gel has no sufficient time and space to grow into a well crystallized structure. The mixture of geopolymer usually contains a much lower water content than the reaction system of zeolite synthesis. However, many geopolymers can also be further transformed into zeolites through hydrothermal curing and ageing [27–29]. Hence, geopolymers are regarded as the precursor of zeolites with almost the same structure at atomic scale, and are paid great attention as an emerging group of sorbents. Studies on fly ash-based geopolymers as adsorbents for the removal of Cu2+ [20], Pb2+ [22], Cd2+ [23] have been reported, which have clearly shown the advantage ofgeopolymer technique in improving the adsorption capacity offly ash. However, comparative study of the difference between geopolymer and its deriving zeolite or the same type of commercial zeolite in adsorption of heavy metal ions and relevant mechanisms is rarely reported. Wang et al. [15] investigated the adsorption capacity of fly ash, geopolymer and natural zeolite, and they found the synthetized geopolymer adsorption capacity outclass the natural zeolite and raw ash for Cu2+ removal. However, their study did not

clarify the relation between zeolite and its analogue - geopolymer. This is a widely concerned question in industry: is it necessary or cost-effective to further convert geopolymer into well crystalline zeolite to increase the adsorption efficiency? In this work, raw fly ash, a geopolymer manufactured from the fly ash and a faujasite block material derived from the geopolymer were comparatively evaluated by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Their adsorption behavior of Pb under different conditions was investigated to understand the adsorption model and relevant thermodynamic parameters of adsorption process. 2. Materials and methods 2.1. Materials Fly ash was obtained from Shenhua Junggar Energy Corporation in Junggar, Inner Mongolia, China. Analytical grade sodium hydroxide and hydrochloric acid were supplied by Sinopharm Chemical Reagent Co., Ltd. Commercial sodium water glass was obtained from Shenghuai Chemical Technology Co. Ltd., Foshan, with original modulus of 3.25. The Pb2+ solutions with different concentrations were prepared by dissolving analytical grade Pb(NO3)2 (Sinopharm Chemical Reagent Co., Ltd) in distilled water. 2.2. Characterization of materials The chemical compositions of fly ash, geopolymer and faujasite block were determined by X-ray fluorescence (AXIOSmAX, PANalytical Netherlands). X-ray diffractions of these materials were recorded on a D8-Focus type X-ray powder diffractometer (BrokerAXS Germany). The scanning range was from 5° to 50° 2h at a scanning rate of 0.5°/min using Cu Ka radiation (generator voltage of 40 kV and current of 40 mA). The morphologies of the sample fracture surfaces were analyzed using a scanning electron microscopy (SU8010, Hitachi Japan) at an acceleration voltage of 10 kV. The FTIR spectra of the samples were obtained using Nicolet iS50 spectrometer (Thermo Scientific America) in the range of 450–4000 cm1 using the KBr pellet technique [30]. Pore size distribution is obtained from N2 adsorption branch of nitrogen isotherms by BJH method by using an automatic ASAP2020 surface area and porosimetry system (Micromeritics, America), and the specific surface area was calculated by the Brunauer–Emmet–Tell er (BET) method. 2.3. Synthesis of geopolymer and faujasite block A mixture of sodium hydroxide and the commercial sodium water glass mixed at a mass ratio of 1:4.15 was used as alkaline activator. Fly ash was then mixed with the alkaline activator solution at a mass ratio of 1:1.12 to make geopolymer paste. After thoroughly mixed for 3–5 min, the paste was cast into a 20 mm  20 mm  20 mm mold, followed by air curing for 12 h and sealed curing at 80 °C in an oven for another 12 h. Thin geopolymer specimens (20 mm  20 mm  2.0 mm; an average mass of 2.5 g) were made by the steps described above. Faujasite blocks were fabricated from geopolymer via in-situ hydrothermal method. Geopolymer specimens were placed into a 100 ml Teflon bottle containing 20 ml 1.0 mol/L NaOH solution, under the hydrothermal condition of 70 °C for 24 h. The effects of various factors, such as the SiO2/Al2O3 molar ratio of the geopolymer, the alkalinity, crystallization time and crystallization

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temperature of the hydrothermal system on the structure and morphology of the products were examined, as can be seen from our previous work [29]. Before the adsorption tests, geopolymer specimens and faujasite blocks were crushed and grinded, and all of the samples were sieved and passed through a 74 lm sieve. To remove the excess sodium hydroxide and avoid the impact of possible precipitation of Pb(OH)2 during adsorption, the samples were immersed in distilled water for at least 72 h, during this process, water was replaced every 12 h. The samples were then filtrated and dried when the pH of the wash water reach and kept constant at 7 ± 0.5 for at least 24 h. 2.4. Adsorption of Pb2+ on fly ash, geopolymer and faujasite block 2.4.1. Effect of pH Pb2+ solution with concentration of 300 mg/L at different initial pH values (1.0–6.0) were added to vials to contact dried fly ash, geopolymer or faujasite at mass ratio of 25.0 ml:0.1 g, shaken at 150 rpm at 25 °C for 300 min. The pH was adjusted with diluted nitric acid (HNO3) or sodium hydroxide (NaOH). The adsorption of Pb2+ was determined by using the inductively coupled plasma mass spectroscopy (ICP-MS), its value was calculated by the following equation:

qe ¼ ðC 0  C e Þ

V m

ð1Þ 2+

where qe is the amount of Pb adsorbed at equilibrium (mg/g), C0 and Ce are the initial and equilibrium concentrations of Pb2+ (mg/L). m is the mass of fly ash, geopolymer or faujasite (g), and V is the volume of solution (L). In this study all of the equilibrium concentration was measured at the contact time of 300 min. This contact period is believed long enough for the adsorption systems, given to the shorter equilibration periods of similar systems reported in literature, such as 60– 300 min for Pb removal by fly ash; 120 min for Pb and Cu removal by fly ash based geopolymer; 100–150 min for Pb removal by zeolite-NaX [2,5,10,22,31].

2.4.2. Effect of contact time In order to study the effect of contact time on the adsorption of Pb2+, the same mixing procedure was performed using 300 mg/L Pb2+ solution at initial pH of 3.0 but different shaking periods of time (10–300 min). The solutions were contact with dried sorbent at mass ratio of 25.0 ml:0.1 g. The adsorption capacities were determined via Eq. (1) for each contact time.

2.4.3. Effect of initial concentration The effect of initial concentration on the adsorption of Pb2+ was investigated. Pb2+ solutions with concentrations of ranging from 100 to 1000 mg/L at initial pH = 3.0 were used. The shaking was kept constant at 150 rpm at 25 °C for 300 min. The solutions contacted with dried sorbent at mass ratio of 25.0 ml:0.1 g.

3. Results and discussion 3.1. Characterization of materials 3.1.1. XRF analysis The chemical compositions of the raw fly ash, synthesized geopolymer and the deriving faujasite block are shown in Table 1. The fly ash is classified to be Class F fly ash according to the American Society for Testing Materials (ASTMC618) [32]. Dai et al. [33] point out that boehmite (AlOOH), which is one of the important carriers of Al, is rich in the Inner Mongolian coal. This may explain the high concentration of alumina present in the fly ash from Inner Mongolia, China. The content of sodium and the loss on ignition (LOI) are increased in the geopolymer sample, which indicates that sodium and AOH group being part of the synthesized geopolymer structure [22]. The added alkaline activator is the sources of the increased relative concentrations of SiO2 content and the decreased Al2O3 content in the synthesized geopolymer as compared to the raw fly ash. The faujasite block sample had a higher content of Al2O3 and lower content of SiO2 and Na2O as compared to the geopolymer sample. This is probably because during the hydrothermal process, the content of SiO2 dissolved in the NaOH solution was higher than the content of Al2O3, and the dissolved Al2O3, SiO2 and extra Na2O were removed after being thoroughly washed.

3.1.2. SEM and XRD analysis Fig. 1 shows the SEM images and the corresponding XRD patterns of fly ash, geopolymer and faujasite block. Fig. 1a demonstrates the irregular and rough shape of fly ash particles, which is totally different from the normal spherical shape [30,34], and Fig. 1d reveals mullite and anatase being the major crystalline constituents. After geopolymerization process, as illustrated in Fig. 1b, it is difficult to distinguish non-reacted fly ash particles from the very compact matrix, and this indicates a high extent of geopolymerization. In Fig. 1e, the sharp diffractions of mullite and anatase disappear as compared to Fig. 1d. This indicates the dissolution of mullite and anatase phases and the formation of amorphous structure in the geopolymer. After hydrothermal process, the geopolymer are successfully transformed into faujasite, as shown in Fig. 1c. The faujasite crystals are dense and have particle size of around 1 lm. The deriving products also show a typical pure faujasite shape that is octahedral [35]. The XRD results (Fig. 1f) demonstrates that the major part of geopolymer has been transformed into well crystallized faujasite after the hydrothermal process. The diffraction peaks of the faujasite match well with those of FAU-type zeolite (JCPDS card No. 120228).

3.1.3. Infrared analysis (IR) The FTIR spectra of the fly ash, geopolymer and faujasite are given in Fig. 2. The main IR characteristic bands and their corresponding species are listed in Table 2.

Table 1 Chemical compositions of fly ash (FA), geopolymer (GEO) and faujasite block (FAU) by XRF analysis, mass%, LOI is loss on ignition. Sample

Al2O3

SiO2

Fe2O3

TiO2

K2O

Na2O

CaO

MgO

P2 O5

MnO

LOI

FA GEO FAU

51.39 29.95 39.73

29.12 33.41 27.28

2.22 1.33 1.73

1.80 1.04 1.39

0.34 0.41 0.24

0.052 16.22 9.86

5.14 3.085 4.030

0.16 0.094 0.12

0.18 0.11 0.075

0.033 0.022 0.027

9.39 14.23 15.38

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Fig. 1. SEM images and XRD patterns of fly ash, geopolymer and faujasite block (a) (d) fly ash; (b) (e) geopolymer; (c) (f) faujasite block.

Table 2 IR bands and corresponding species of fly ash, geopolymer and faujasite. Materials

Bands, cm1

Assignments

References

FA

1640; 3443 1410 1099 813

Stretching and deformation vibrations of OH and HAOAH groups OACAO stretching vibration Si(Al)AOASi asymmetric stretching Symmetric SiAOASi bond stretching and stretching AlAO in mullite-like structures Symmetric stretch vibration of SiAOAAl in mullite or mullite-like structure SiAOASi and OASiAO bending vibration

[30,36]

Stretching and deformation vibrations of OH and HAOAH groups from the water molecules OACAO stretching vibration Asymmetric AlAO/SiAO stretching vibrations AlAO symmetric stretching in tetrahedral SiAOASi/SiAOAAl bending band Bending vibration of SiAOAAl AlAO/SiAO bending vibration

[37,40,41]

Stretching and deformation vibrations of OH groups or the water present in the zeolite channels OACAO stretching vibration TO4 (T = Si or Al) tetrahedral asymmetric stretch vibration SiAOASi/SiAOAAl bending band TO4 (T = Si or Al) tetrahedral symmetric stretch vibration Bending vibration of SiAOAAl, double six-member rings (D6R) AlAO/SiAO bending vibration

[44–46]

563

472 GEO

1640; 3436 1440 1018 880

Fig. 2. Infrared analysis of fly ash, geopolymer and faujasite block. 747 565 452

For fly ash, typical bands of stretching and deformation vibrations of OH and HAOAH groups from the water molecules at 1640 and 3443 cm1 respectively are very weak infly ash, as shown in Fig. 2. While for geopolymer and faujasite, the broad bands are clearly, and these bands belong to the weakly bound water molecules which were adsorbed on the surface or trapped in the large cavities between the rings of geopolymeric products [40] or trapped in faujasite channels [45,46]. In the raw fly ash sample, the relatively weak peak at 1410 cm1 was due to OACAO stretching vibration [37]. For NaOH-rich geopolymer and faujasite, the bands around 1440 cm1 were attributed to OACAO stretching vibration [38], and these bands represent the presence of sodium carbonate [41], which is due to the efflorescence or carbonation of the samples [49].

FAU

1646; 3461 1442 984 755 677 565 452

[37] [30,38] [30]

[30,39]

[38]

[37,38,41,42] [38,41,42] [42] [40] [37,43] [37,38,42]

[37,38] [45,46] [47] [45] [45–48] [45,48]

In the fly ash sample, the bands at 1099 cm1, 813 cm1 and 563 cm1 are indicative of mullite [30]. For the geopolymer and faujasite, the band 1099 cm1 which is due to SiAOASi asymmetric

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stretching in fly ash sample, shifts to lower wavenumbers (1018 cm1 and 984 cm1 respectively) as a consequence of polycondensation with alternating SiAO and AlAO bonds [42]. The band at 880 cm1 linked to AlAO symmetric stretching in tetrahedral also indicates the formation of the geopolymer [37]. As compared to the geopolymer, in the faujasite sample, the band at about 677 cm1 arose and the intensity of the band 565 cm1 increase. The bands at 984 and 677 cm1 corresponded to TO4 (T = Si or Al) tetrahedral asymmetric and symmetric stretch vibration respectively, the peak at 565 cm1 is assigned to the building blocks of the octahedral structures [47], and these bands indicate the formation of faujasite from the geopolymer.

4 nm and 6 nm on its PSD curve and the Dave of faujasite decrease to 9.69 nm. BET surface area is a crucial property for the adsorbing material. The fly ash sample has a BET surface area of 16.45 m2/g, which increases to 20.48 and 174.35 m2/g after geopolymerization and hydrothermal transformation process. The significantly increased surface area partially explains the higher adsorption capacities in geopolymer and zeolite blocks, which will be discussed in the following sections.

3.1.4. BET analysis Fig. 3d shows the total pore volume (Vtotal), t-plot micropore area (St-plot), BET surface area (SBET), average pore diameter (Dave) and pore size distribution (PSD) of fly ash, geopolymer and faujasite. Fly ash has a Vtotal of 0.025 cm3/g and Dave of 17.33 nm, but there is no featured peak in the PSD curve. After geopolymerization process, the Vtotal increase to 0.070 cm3/g, which is 180% higher than that of fly ash. The PSD curve of geopolymer centers at around 14 nm with a wide distribution, indicating that geopolymer has a wide pore distribution. The Dave of geopolymer is 19.62 nm. After hydrothermal transformation process, the Vtotal further increased to 0.14 cm3/g. There are two relatively narrow peaks center at

3.2.1. Effect of pH The effect of solution pH on the adsorption of Pb2+ ions on fly ash, geopolymer and faujasite inthe initial pH range of 1–6 is shown in Fig. 3a. The equilibrium pH values were also measured as they indeed affect the adsorption efficiency. The result indicates that the adsorption capacities of fly ash, geopolymer and faujasite all increase as pH increases. All of the equilibrium pH values increase when compared to the initial pH. The alkaline nature of lime (CaO) in these sorbents can account for the increase. The predominant lead specie is Pb2+ at pH < 6 [50], thus the Pb2+ sorption takes places at equilibrium pH < 6 (corresponding to initial pH 6 3.0) can be attributed to the

3.2. Adsorption performance of fly ash, fly ash based geopolymer and faujasite

Fig. 3. Adsorption performance and BET analysis of fly ash, fly ash based geopolymer and faujasite (a) effect of pH; (b) effect of contact time; (c) effect of initial Pb2+ concentration and (d) BET analysis results.

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competition between H3O+ and Pb2+ on the surface sites. At the conditions of initial pH = 1, all of the samples had low adsorption capacities. This is because at low pH, more H3O+ are available in the solution, the adsorption capacities for Pb2+ are lowed by the competition of H3O+. As pH increases, the concentration of the H3O+ decreases and it leads to an increase adsorption of the amounts of Pb2+ ions on the sample surface. At the conditions of pH = 2, the results of adsorption capacities by fly ash, geopolymer and faujasite are 12.15, 25.32 and 30.59 mg/g, respectively. The results show that the adsorption efficiency follows the order faujasite > geopolymer > fly ash. This indicates that the faujasite material has more sorption sites than the geopolymer materials, and both of them have many more sorption sites for Pb2+ ions than the raw ash. Pb2+ is drastically adsorbed at pH of 3–6 where Pb2+ cations predominate in the solution. The adsorption amount by geopolymer and faujasite is 74.83 and 74.32 mg/g when initial pH = 3, equilibrium pH  6; in comparison, the adsorption amount by fly ash is only 47.83 mg/g. Visa et al. [51] found the optimum pH for adsorption Pb2+, Zn2+ and Cd2+ by alkaline activated fly ash was between 5.5 and 6.6. Al-Zboon et al. [22] found the initial pH under which maximum uptake capacity for Pb by fly ash-based geopolymer is 5, but the authors did not mention the final pH. The pH of Pb2+ cations begin to precipitate at 7.86 at initial concentration of 300 mg/L (calculated from the precipitation constant of Pb(OH)2(s), 1.2  1015 [52]). At initial pH > 3, all of the equilibrium pH values are higher than 7.0 but lower than 7.8. Thereby, the precipitation of Pb2+ is negligible under all of the equilibrium conditions. The adsorption amounts by geopolymer and faujasite almost keep constant at 74 mg/g because the Pb2+ cations in the solutions are less saturated. The adsorption amount by fly ash increases slightly when the initial pH is higher than 4. 3.2.2. Effect of contact time Kinetic adsorption experiments are performed in order to calculate the adsorption equilibrium time, with the results shown in Fig. 3b. The adsorption of Pb2+ on all of the samples increases with the increase of contact period. Geopolymer and faujasite have rapid adsorption in the first 10 min (49.0 and 60.41 mg/g, respectively), while for the raw ash, the adsorption amount is 12.63 mg/g. The adsorption rates of geopolymer and faujasite materials keep relatively high at the first 60 min, then start to slow and reach maximum value after 150 min. It takes much longer time (240 min) for the raw ash to reach the adsorption equilibrium. For Pb2+ removal, thegeopolymer-deriving faujasite is slightly more efficient than thegeopolymer material itself, and both of them are far superior to the raw ash. Geopolymer has higher sorption capacity than raw ash material. This can be attributed to the fact that during the geopolymerization process, the glass structure of ash material is transformed into amorphous, and specific surface area and pore volumes of the resulting geopolymer increase, thus leading to more sorption sites and the higher sorption capacity [15,53]. After hydrothermal process, geopolymer is transformed into faujasite. Faujasite has nano-sized pore structures (aperture of channels close to 7.4 Å) [6,18], and it has been proven to be efficient in the adsorption of heavy metals [6]. However, according to our result, it seems that turning geopolymer into faujasite by post hydrothermal process does not increase the efficiency too much. 3.2.3. Effect of initial Pb2+ concentration Fig. 3c presents the adsorption capacity of fly ash, geopolymer and faujasite at different initial Pb2+ concentration. The Pb2+ adsorption capacities depend on initial Pb2+ concentration. When the initial Pb2+ concentration increases, the accessible sorption sites of the materials become insufficient, therefore, most of the ions still remain in the solution.

In specific, for the raw ash sample, the Pb2+ adsorption capacity increases from 24.9 mg/g at 100 mg/L to 49.8 mg/g at 200 mg/L, then with the increase of initial Pb2+ concentration, the Pb2+ adsorption capacity remains almost constant (about 50 mg/g). This suggests that the raw ash reached the saturation level at the initial concentrations above 200 mg/L. For thegeopolymer, the Pb2+ adsorption capacity increases from 24.6 mg/g at 100 mg/L to 118.6 mg/g at 600 mg/L, then reaches saturation level. For the faujasite, the Pb2+ adsorption capacity increases from 24.9 mg/g at 100 mg/L to 143.3 mg/g at 800 mg/L. 3.3. Adsorption kinetics and isotherms 3.3.1. Adsorption kinetics In this study the pseudo-first-order equation and the pseudosecond-order equation are adopted to describe the kinetics of Pb2+ adsorption on fly ash, geopolymer and faujasite. The pseudo-first-order rate (Lagergren’s equation) [54] is one of the most widely used models for describing sorption kinetics from aqueous solutions. The expression of this kinetic model is:

lnðqe  qt Þ ¼ lnqe  k1 t

ð2Þ

in which, k1 is the Lagergren constant (1/min), qe (mg/g) and qt (mg/ g) are the amounts of metal ions adsorbed on the adsorbent at equilibrium and at the time t (min), respectively. The pseudo-second-order equation [55] is expressed as Eq. (3):

t 1 t ¼ þ qt k2 q2e qe

ð3Þ

in which, k2 is the pseudo-second-order rate equilibrium constant [g/(mg min)], qe (mg/g) and qt (mg/g) are the amounts of metal ions adsorbed on the adsorbent at equilibrium and at the time t (min), respectively. The linear fitting results of the adsorption data with the pseudofirst-order and pseudo-second-order kinetic models for the adsorptions of Pb2+ are shown in Fig. 4a and b, respectively. In Table 3, k1, k2, qe,calc,1, and qe,calc,2 are calculated kinetic parameters, R2 is the corresponding correlation coefficient. For the raw ash sample, the experimental data fit to the pseudofirst-order model better than the pseudo-second-order model. Besides, the adsorption capacities predicted by pseudo-first-order model (qe,calc,1 = 44.68 mg/g) and obtained experimentally (qe,exp = 48.03 mg/g) are closer than the one predicted by pseudosecond-order model (qe,calc,1 = 60.61 mg/g), which also suggests the suitability of using pseudo-first-order model. For the geopolymer and faujasite materials, the experimental data fit the pseudo-second-order model much better than the pseudo-first-order. Moreover, the theoretical adsorption capacities predicted by pseudo-second-order model (qe,calc,2 = 77.52 mg/g, 75.76 mg/g) are very close to the experimental data (qe,exp = 74.48 mg/g, 74.50 mg/g). The pseudo-second-order model is based on the assumption that the chemical reaction is rate controlling, and this chemisorption involves vacancy forces through sharing or exchange of electrons between the sorbent and solutes [56,57]. This suggests that the sorption mechanisms of geopolymer and faujasite materials is predominantly governed by chemisorption. In Table 3, all of the adsorption rate constants (k2) are relatively low, which means that the adsorption of Pb2+ by geopolymer and faujasite need a long time (at least 150 min, see Fig. 3b) to reach equilibrium [21]. The adsorption rate constant of faujasite (13.50 g/(mg min)) is lower than the adsorption rate constant of geopolymer (26.73 g/(mg min)). However, it can be seen from Fig. 3b that faujasite is slightly more efficient than the geopolymer in the adsorption of Pb2+ and it takes almost the same time to reach equilibrium. This is probably due to the faujasite material has a

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187

Fig. 4. Adsorption kinetics and isotherms: Linear fitting of experimental data to (a) pseudo-first order equation and (b) pseudo-second-order equation; (c) Langmuir isotherm; (d) Freundlich isotherm.

high adsorption rate at the beginning (adsorption amount up to 60.41 mg/g within 10 min), and then its adsorption rate decreases as the concentration of Pb2+ ions decline, while the adsorption rate of geopolymer keeps constant or decreased at a lower extent than the faujasite, and thus, it takes geopolymer and faujasite almost the same time to reach equilibrium. As indicated by the adsorption rate constants, faujasite probably will take longer time than geopolymer does to reach equilibrium if the initial Pb2+ concentration is further increased. 3.3.2. Equilibrium adsorption isotherms In this study Langmuir and Freundlich isotherm models are used to evaluate the experimental data and to obtain the isothermal adsorption curves and relevant parameters. Langmuir isotherm model assumes a monolayer adsorption onto a homogeneous surface, there are no interactions between adsorbed adsorbates and the surface has equal affinity and energy [5,58,59]. The expression of the Langmuir isotherm model is represented as Eq. (4):

Ce 1 Ce ¼ þ qe K L qmax qmax

ð4Þ

in which qe is the amount of heavy metal ions adsorbed on the adsorbents at equilibrium (mg/g); Ce is the equilibrium concentra-

tion of Pb2+ (mg/L); qmax is monolayer capacity of the adsorbent (mg/g); and KL (L/mg) is the Langmuir binding constant. Freundlich model is an empirical model, it describes the adsorption on a heterogeneous surface with nonuniform energy and is not restricted to the formation of a monolayer [60]. The linear format of Freundlich model can be expressed as follow [20]:

1 logqe ¼ logK F þ logC e n

ð5Þ

in which Ce is the equilibrium concentration of Pb2+ (mg/L); qe is the amount of Pb2+ adsorbed at equilibrium (mg/g), KF (mg11/n L1/n g1) and 1/n are Freundlich constants representing adsorption capacity and adsorption intensity. The experiment results of the Langmuir isotherm and Freundlich isotherm are shown in Fig. 4c and d, respectively. The fitted Langmuir and Freundlich parameters are listed in Table 3. It is found that the adsorption data fit more splendidly to Langmuir model, with higher correlation coefficients when compared with the Freundlich isotherm model. The R2 values suggest that Freundlich model is not suitable to describe the adsorption of Pb2+ on fly ash, geopolymer and faujasite, which is in agreement with the findings by Al-Zboon et al. [22] and Visa [61]. The validity of Langmuir isotherm can be tested by comparing the theoretical calculated adsorption capacity values with those

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Y. Liu et al. / Fuel 185 (2016) 181–189

Table 3 Kinetic parameters, adsorption isotherm constants and thermodynamic parameters for Pb2+ adsorption on fly ash, geopolymer and faujasite. Categories

Units

Materials GEO

FAU

Adsorption capacities obtained experimentally

qe,exp

mg/g

48.03

74.48

74.5

Pseudo-first-order model

k1

103; 1/min mg/g

11.42

28.09

20.54

44.68 0.9678

52.04 0.8152

21.04 0.9183

3.418

26.73

13.5

60.61 0.9599

77.52 0.9985

75.76 0.9997

mg/g L/mg

53.47 0.1769 0.9944

111.11 0.6429 0.9997

142.86 0.2966 0.9987

mg11/ L1/n g1

37.01

49.45

47.89

0.0604 0.5146

0.1691 0.5724

0.2344 0.8747

kJ/mol kJ/mol kJ/mol

12.5 13.21 14.05

19 20.46 22.33

20.49 23.54 25.49

4H°

kJ/mol

10.58

30.43

54.7

4S°

J/(mol K)

77.39

165.68

252.81

qe,calc,1 R2 Pseudo-second-order model

k2

qe,calc,2 R2 Langmuir isotherm constants

qmax KL R2

Freundlich isotherm constants

KF

104; g/ (mg min) mg/g

4. Conclusions

n

1/n R2 4G°

FA

298.15 K 308.15 K 318.15 K

obtained experimentally. The Pb2+ adsorption capacities by fly ash, geopolymer and faujasite obtained from experiment are 49.8, 118.6 and 143.3 mg/g, respectively. They are compatible with the values calculated from Langmuir isotherm, 53.47, 111.11 and 142.86 mg/g. 3.4. Thermodynamics of Pb2+ adsorption on fly ash, geopolymer and faujasite Thermodynamics of Pb2+ adsorption on fly ash, geopolymer and faujasite are investigated at pH = 3 at 298.15, 308.15 and 318.15 K in the concentration of 500 mg/L. Distribution coefficient (Kd) [22] is a standard parameter that used to evaluate the sorption and retention of the metal ion on a solid phase. It can be expressed by Eq. (6):

Kd ¼

qe Ce

in the value of DG° with the increase of temperature indicates that the adsorption process is more favorable at higher temperatures [60]. The Gibbs free energy values offly ash are higher than 15 kJ/mol, which indicate that sorption sites and metal ion are probably physical interactions. The Gibbs free energy values ofgeopolymer and faujasite are lower than 15 kJ/mol but higher than 30 kJ/mol, indicating that the chemical interactions (both ionic and covalent) have the predominant roles in controlling adsorption rate [62]. Moreover, the magnitude of 4H° can also give insight to the type of adsorption. Since the physical interactions force is weak, the enthalpy change of fly ash is small (10.58 kJ/mol). The enthalpy change for chemisorption is more than 21 kJ/mol due to the much stronger forces of chemisorption than physisorption. The enthalpy changes of geopolymer and faujasite are 30.43 and 54.70 kJ/mol, respectively. This result indicates that the adsorption of Pb2+ onto geopolymer and faujasite is indeed a chemisorption process.

ð6Þ

The values of standard enthalpy changes (4H°) and entropy changes (4S°) and Gibbs free energy changes (DG°) of the adsorption process are calculated by using Eqs. (7) and (8) [22]:

DS DH  lnK d ¼  R RT

ð7Þ

DG ¼ DH  T DS

ð8Þ

in which T is the absolute temperature in Kelvin, R is the ideal gas constant (8.314 J/(mol K)). The plot of (lnKd) versus (1/T) will yield a straight line, the slope of the straight line will yield (4H°/R) and an intercept will be (4S°/R), thus 4H° and 4S° can be calculated. The calculated thermodynamic parameters are displayed in Table 3. The negative value of DG° indicates spontaneous nature of adsorption on fly ash, geopolymer and faujasite. The positive values of 4H° shows the adsorption process is an endothermic process. The positive values of 4S° show the existence of some structural changes at the solid-liquid interface [4]. The decrease

The effectiveness and efficiencies of fly ash, geopolymer and faujasite as sorbent materials for lead removal from aqueous solutions are evaluated in this work. We find that both geopolymer and faujasite exhibit much higher removal efficiencies than the raw ash. Faujasite has the highest adsorption capacity (143.3 mg/g) and geopolymer has almost the equivalent adsorption capacity (118.6 mg/g) at pH = 3. A period of 150 min is sufficient for geopolymer and faujasite to reach the adsorption equilibrium while 240 min is needed for the raw ash. Furthermore, a higher pH (for example, pH = 4–6) can increase the amount of heavy metal ions adsorbed on fly ash, geopolymer and faujasite material. The kinetic data of geopolymer and faujasite are found to fit well to the pseudo-second-order kinetic model, while the kinetic data of fly ash are found to fit to the pseudo-first-order kinetic model well. This reveals the chemisorption mechanisms of geopolymer and faujasite materials. The adsorption equilibrium data of fly ash, geopolymer and faujasite fit the Langmuir model better than the Freundlich model. Thus, the adsorption process of fly ash, geopolymer and faujasite can be described as a monolayer adsorption. The values of DG° and 4H° suggest that the adsorption of Pb2+ on fly ash, geopolymer and faujasite is spontaneously endothermic process. In addition, the difference of 4H° between the three materials also supports that the adsorption of Pb2+ on geopolymer and faujasite is indeed a chemisorption process while the adsorption of Pb2+ on fly ash is nearly physisorption. Acknowledgment This work was supported by the Public Service Project of the Chinese Ministry of Land and Resources (No. 201311024), the Comprehensive Utilization Demonstration Base of Ganzhou Rare Earth Resource sponsored by Chinese Ministry of Land and Resources. The participation of Z.Z. is supported by Australian Research Council linkage project (LP130101016). References [1] Jamil TS, Ibrahim HS, El-Maksoud A, El-Wakeel ST. Application of zeolite prepared from Egyptian kaolin for removal of heavy metals: I. Optimum conditions. Desalination 2010;258:34–40. [2] Pandey PK, Sharma SK, Sambi SS. Removal of lead(II) from waste water on zeolite-NaX. J Environ Chem Eng 2015;3:2604–10. [3] Barbosa R, Lapa N, Lopes H, Gunther A, Dias D, Mendes B. Biomass fly ashes as low-cost chemical agents for Pb removal from synthetic and industrial wastewaters. J Colloid Interf Sci 2014;424:27–36.

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