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Fly ash based geopolymer for heavy metal removal: A case study on copper removal
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JECE 677 1–9 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
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Fly ash based geopolymer for heavy metal removal: A case study on copper removal
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3 Q1 4 5 6 7
Mohammad S. Al-Harahsheha,* , Kamel Al Zboonb , Leema Al-Makhadmehc , Muhannad Hararahc, Mehaysen Mahasnehb a b c
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Department of Chemical Engineering, Jordan University of Science and Technology, Irbid 22110, Jordan Al-Huson University College, Al-Balqa’ Applied University, Jordan Faculty of Engineering, Al-Hussein Bin Talal University, Ma’an 71111, Jordan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 March 2015 Received in revised form 14 May 2015 Accepted 5 June 2015
This work aims to evaluate the effectiveness of an amorphous geopolymer, synthesized from waste fly ash, as sorbent material for copper from aqueous solutions. The obtained geopolymer was found to be highly amorphous in structure due to the dissolution of fly ash glass phases. It has much higher removal efficiency compared to the raw fly ash. The removal efficiency was affected by solid/liquid ratio, temperature, reaction time, and Cu2+ initial concentration (C0). The highest Cu2+ removal capacity was obtained at pH 6. The kinetic data were found fit to the pseudo second order kinetic model. Furthermore, it was found that Langmuir model is better than Freundlich model within the temperature range (25– 45 C). The maximum sorption capacity (qm) occurred at 45 C, reaching a value of 152 mg/g. The sorption process by the obtained geopolymer is endothermic in nature and more favorable at higher temperatures with enthalpy of adsorption of 39.49 kJ/mol. The activation energy of the sorption process at the optimum conditions was found to be 34.9 kJ/mol. ã2015 Published by Elsevier Ltd.
Keywords: Geopolymer Heavy metals Copper removal Langmuir model
8
Introduction
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Copper as a heavy metal is hazardous to human health and the environment [1]. It is widely used in industry and is accumulating in their waste streams. Adsorption of Cu2+ has short and long term effects on human health [1–3]. Many effective methods are used for copper removal from wastewater including chemical precipitation [4], ion exchange, reverse osmosis, electrochemical treatment, evaporative recovery, and adsorption [5]. Recently, natural materials with low cost treatment such as silica, mud, agricultural wastes, ash, and solid wastes were used as sorbent materials. Some researchers used palm fruit, activated carbon prepared from palm oil empty fruit bunches [2], palm kernel shell based activated carbon [1], and oil palm leaf powders [6] for Cu2+ removal from aqueous solutions. Other plants such as Cassia angustifolia [7], Tridax procumbens [8], Pleurotus cornucopiae [9], Sorgum vulcaris dust [10], spruce sawdust [11], mangrove barks [12], polar and oak leaves [13] have been used for the same purpose. The conducted studies investigated the effects of different conditions (pH, contact time, and Ci, and adsorbent dosage) on adsorption capacity. Arivoli et al. [14] prepared activated carbon
10 11 12 13 14 15 Q3 16 17 18 19 20 21 Q4 22 23 24 25 26 27
* Corresponding author. E-mail address: [email protected] (M.S. Al-Harahsheh).
from solid waste called Terminalia catappa Linn shell and studied its effectiveness in adsorbing copper ion from aqueous solution. Fixed-bed tests were conducted to investigate the capabilities of a modified silica with respect to the selective removal of copper ions from multi-metal solutions [15]. Sthiannopkao and Sreesai [16] used lime mud and recovered boiler ash to remove the heavy metals from metal finishing wastewater through sorption and precipitation process. A high removal efficiency of Cu2+ (99%) and lower leachability were obtained. Fly ash based zeolites for the removal of heavy metal ions were used by Solanki et al. [17]; the synthesized zeolite obtained has high removal efficiency of Cu2+ up to 100%. In addition to the synthetic zeolites, Gabai et al. [18] tested the adsorption capability of chelating resins and activated carbons for Cu2+ removal. There is a significant trend in recycling of waste materials and converting it to usable and valuable materials. One of these waste materials is coal fly ash. The disposal of the large amount of fly ash has become a serious environmental and economic problem [19,20]. One approach to deal with fly ash waste is to convert it to geopolymer, which is not only effective for heavy metals removal, but also helps in solving the problem of ash accumulation as an industrial waste product. Geopolymer is amorphous material produced by reacting solid aluminosilicate with highly alkali hydroxide. It consists of a polymeric silicon–oxygen–aluminum framework with alternating silicon and aluminum tetrahedral
http://dx.doi.org/10.1016/j.jece.2015.06.005 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: M.S. Al-Harahsheh, et al., Fly ash based geopolymer for heavy metal removal: A case study on copper removal, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.005
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joined together in three directions by sharing all the oxygen atoms [21]. Geopolymers have been synthesized from fly ash [19], natural zeolite [22], kaolinite [23] and volcanic ash [24]. They have been used as adsorbent materials for removal of Cd, Ni, Pb(II), Cu(II), phosphate, NOx, boron, fluoride, radionuclide of 137Cs and 90Sr, as well as dyes [19]. The aim of this research is to determine the capability of fly ash based geopolymer in removing copper from aqueous solution as compared to other sorbents. Fly ash was characterized before and after geopolymerization by X-ray diffraction (XRD) and X-ray fluorescence (XRF). The behavior of sorption process under different conditions (solid/liquid ratio, copper concentration, pH, temperature, and contact time) was investigated. Langmuir and Freundlich models were used to study copper adsorption isotherm on the produced geopolymer. The thermodynamic parameters of the adsorption process were also evaluated.
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Materials and methods
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Materials
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Coal fly ash samples were collected from Rajhi Cement Plant, Jordan. Fly ash was homogenized by grinding using pestle and mortar. After sieving the fraction of less than 45 mm was used to prepare the geopolymer. The synthesized geopolymer was then crushed and ground to 100% less than 200 mm [25]. The chemicals used in this study were of reagent grade. Copper solutions with different concentrations were prepared from 1000 ppm standard (Cu2+) solution (Merk1).
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Instrumentation
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X-ray diffraction (XRD) analysis was carried out using (SHIMADZU-XRD-6000) diffractometer. The scanning range of 2u was between 2.0 and 65.0, with a scan speed of 2.0 /min and a receiving slit width of 0.30 mm. The chemical composition of the ash sample and the geopolymer was analyzed using X-ray fluorescence (XRF) spectrometer (SHIMADZU-XRF-1800). Inductively coupled plasma (ICP-OES) (SHIMADZU-ICPS-7510) was employed for the measurement of copper concentration in aqueous solutions.
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Synthesis of geopolymer
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Theoretically, any alkali can be used in geopolymerization reactions; however most of the studies have focused on the effect of sodium (Na+) and potassium (K+) ions [26]. Both NaOH and KOH could be used in the activation process, but the extent of dissolution was higher when NaOH is used; this is due to the smaller size of Na+ which can better stabilize the silicate monomers and dimmers present in the solution, enhancing the minerals dissolution rate [27]. For this reason NaOH is used in this study for geopolymer synthesis. A specified quantity of fly ash sample was reacted with NaOH solution (14 M) at a mass ratio of 4 to 3, respectively. The obtained paste was then shacked for 5–10 min. The obtained mixture was poured in a plastic cylinder and vibrated for 30 s in an Elma–Digital S1 ultrasonic bath (2 40 W, 120% of ultrasonic power, 40 kHz) to de-foam the paste. It is reported that ultrasonification can improve the dissolution of alumina–silicate components of fly ash in caustic soda. The obtained product was then transferred into a cylindrical vessel and aged for 2 h at a temperature of 105 C. The obtained product was then left for 3 days at room temperature to develop enough strength.
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Sorption tests
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Batch sorption tests were carried out in order to study the effects of the following parameters: geopolymer dosage, pH, contact time, temperature, and copper initial concentration (C0). 50 mL of a solution containing a specified copper concentration (10–160 ppm) of a specified pH (1–6) was added to a specified geopolymer dosage (0.6–3 mg/mL) and shaken for a specified contact time in the range between 5 and 180 min at predetermined temperature ranging from 25 to 45 C. After that, the liquid solution was removed from the solid by centrifugation followed by decantation. Copper concentration in the obtained solution was measured by ICP-OES and the removal percentage and the uptake capacity were calculated. The percentage removal efficiency (E) was calculated using the following equation [28]:
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E¼
C0 Ce 100% C0
ðC 0 C e Þ V ðmL=gÞ Ce m
ðC 0 C e Þ V m
114 115 116 117 118 119 120 121 122 123 124
126 125 127 128 129 130
(2)
where V is the solution volume (L) and m is the geopolymer mass (g). The adsorption isotherms were calculated for copper adsorption at different values of pH, temperature and concentration (10– 160 mg/L). The quantity of copper ion uptake by the geopolymer was determined as [29]: q¼
113
(1)
where C0 is the initial concentration of copper (ppm) and Ce is the remaining equilibrium copper concentration (ppm). Distribution coefficient (Kd) is a measure used to evaluate the sorption of contaminants on a solid phase. It can be calculated using Eq. (2) below [29]: Kd ¼
112
132 131 133 134 135 136 137 138
(3)
where q is the quantity of copper ion uptake by geopolymer phase (mg (metal)/g (geopolymer)).
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Results and discussion
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Fly ash characterization
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XRD analysis The XRD pattern of both fly ash and the synthesized geopolymer is shown in Fig. 1. It was found that coal fly ash sample contains the following major phases: quartz, hematite and mullite. Cristobolite and plagioclase minerals were found in trace quantities. The major part of the ash samples has an amorphous structure; such amorphousity makes fly ash reactive material [30]. After geopolymerization process, the resulting geopolymer has lost almost all crystalline phases present in the initial fly ash and transformed into amorphous structure. Mineral phase analysis of the geopolymer shows that all the identified minerals in the raw sample have disappeared (see Fig. 1). Instead broad band peaks appeared, which declare the presence of amorphous materials and disappearance of crystalline phases. This suggests that the treatment of fly ash with caustic solution has contributed to the formation of amorphous structure and the geoplymerization of Al– Si materials was successful.
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Chemical analysis of solids The composition of raw fly ash and the synthesized geopolymer, as measured by XRF analysis method, is shown in Table 1. The main components of raw fly ash are silica and alumina. Iron, calcium and
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Please cite this article in press as: M.S. Al-Harahsheh, et al., Fly ash based geopolymer for heavy metal removal: A case study on copper removal, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.005
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Fig. 1. XRD patterns of raw fly ash and the synthesized geopolymer (Q–quartz, M–mulite, H–hematite, Zeo–zeolite, Plag–plageoglase).
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magnesium are also present in the sample. It is shown that SiO2, Al2O3 and Fe2O3 represent 91.53 wt% of the sample mass. Therefore, according to the Standard Specifications for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete (ASTMC618), the obtained geopolymer may be classified as F class fly ash [19]. Although the synthesized geopolymer was thoroughly washed with water and dried at a temperature of 105 C, the sodium content was found considerably higher than that in the raw sample. The loss on ignition (L.O.I.) of the obtained material was also higher, which suggests that sodium and OH group are now part of the synthesized geopolymer structure, which clarifies the decrease of both SiO2 and Al2O3 contents in the synthesized geopolymer as compared to the initial ash sample.
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Cu
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The copper removal efficiency of the produced geopolymer and coal fly ash was compared under the following conditions: pH (6), 100 ppm initial Cu2+ concentration, 120 min contact period and 2 mg/mL sorbent dose. The results indicated that the removal efficiency was 25.15% (12.6 mg/g) and 87.7% (43.9 mg/g) for raw ash and the geopolymer material, respectively. This indicates that the geopolymer material has more sorption sites for Cu2+ ions than the raw ash. Such result can be assigned to transformation crystalline structure of ash material into amorphous one leading to formation of more porous structure encouraging Cu2+ sorption [31].
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2+
uptake
Factors affecting Cu2+ adsorption
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Effect of pH The effect of varying initial pH values (1–6) of Cu2+ solution on its removal efficiency was studied at a temperature of 25 C using a geopolymer dosage of 2 mg/mL for a contact time of 120 min. The results also suggest that the sorption effectiveness increases from 5.6% to 88.21% when pH was increased from 1 to 6 (Fig. 2). At low alkalinity, more positive H3O+ species are available in the solution, which competes with Cu2+ positive ions present as active sites on the geopolymer surface. With the increase in pH, less H3O+ is available leading to better access of Cu2+ toward the active sites [1]. This was an expected trend for the effect of pH on metal sorption Q5 were sorption increases with increasing pH values up to a certain value, and then decreases with further increase in pH [19]. It is known that increasing pH to values higher than 6 will favor the precipitation of Cu2+ as Cu(OH)2 [32].
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Effect of geopolymer dose The effect of geopolymer dose on Cu2+ sorption was studied at room temperature (25 C) and a pH value of 6 using the following doses: 0.6; 1.0; 1.4; 1.6; 2.0 and 3.0 g/L. The initial concentration of Cu2+ was 100 ppm and the contact time was 120 min (see Fig. 3). The sorbent dose is a key factor as it determines the amount of sorbent required for a particular initial concentration of sorbate. The results show that the removal efficiency increases from about
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Table 1 The chemical composition of coal fly ash and the synthesized geopolymer. Compound
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 SO3 L.O.I.
Composition (%) Raw ash
After geopolymerization
50.73 28.87 11.93 1.73 1.39 0.74 0.30 1.41 0.35 2.53
39.90 19.70 7.50 2.43 1.13 1.08 11.72 0.50 0.25 14.69
Fig. 2. Effect of pH on Cu2+ removal efficiency (100 ppm concentration, 25 C, dose: 2 g/L, and contact time: 120 min).
Please cite this article in press as: M.S. Al-Harahsheh, et al., Fly ash based geopolymer for heavy metal removal: A case study on copper removal, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.005
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Fig. 3. Effect of adsorbent dose on Cu2+ removal efficiency (initial Cu2+ concentration: 100 ppm, temperature: 25 C, pH: 6.0, and contact time: 120 min). 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228
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45% to 88.2% as the dosage rises from 0.03 g (0.6 g/L) to 0.15 g (3.0 g/L). This indicates that the available active sites at low sorbent dosage are not enough to take all available ions in the solution. A sharp increase in removal efficiency was noticed as the synthesized geopolymer dose increased from 0.6 g/L to 1.6 g/L, while insignificant increase was noticed as the dose increased from 1.6 g/L to 3 g/L. Thus, it can be concluded that a 2 g/L dose is the optimum value under such experimental conditions. The vacant sites available for binding Cu2+ ions increase as the geopolymer dosage increases, leading to a higher sorption capacity [29]. Similar results were obtained by Wang et al. [31], where the removal efficiency of their geopolymer for Cu2+ generally improved with increasing dose reaching about 75% removal efficiency when the dose was 2 g/L. Cetin and Pehlivan [33] also concluded that the removal rate of Zn ions increased with the increase in the adsorbent dose. Effect of contact time The effect of contact time on Cu2+ ions removal was considered by changing the contact time from 5 min to 180 min. The Cu2+ ions Q6 initial concentration was fixed at 100 ppm with a fixed geopolymer dosage (2 g/L) and initial pH of 6. The sorption of Cu2+ metal ions on the synthesized geopolymer is time dependent. A significant removal of Cu2+ occurred after 15 min contact time (81.26%) and only slight change in removal efficiency occurred beyond this period of time. Gupta and Ali [34] and Wang et al. [31] reported that the equilibrium contact time is 60 min for copper when using fly ash sorbents. Similar result was reported by Cetin and Pehlivan [33], where the highest removal efficiency of zinc and lead ions by coal ash was reached after 60 min contact time. Therefore, adsorption of Cu2+ metal ions by the sorbent developed in this work is relatively fast and 60 min contact time is sufficient to reach the adsorption equilibration. Nevertheless, a contact time of 120 min was applied in all following tests. Effect of temperature The effect of temperature on the sorption efficiency was evaluated at 25, 35 and 45 C at a pH value of 6, 2 g/L geopolymer dosage and a contact time of 120 min. The removal efficiency values of Cu2+ were found to be 87.98, 90.42, and 93.91%, respectively. The sorption of Cu2+ on the geopolymer increases with increasing temperature reaching a maximum value at a temperature of 45 C. At a concentration of 100 ppm and pH of 6, the uptake values increase from 44 mg/g to 46.95 mg/g when the Q7 temperature was increased from 25 C to 45 C. Panday et al. [35] showed that Cu2+ sorption on fly ash increases with increasing temperature from 20 C to 40 C. Generally, metal ion removal is enhanced as the temperature increases [31,36].
Fig. 4. Effect of the initial concentration on Cu2+ removal efficiency (temperature: 25 C, pH 6, dose: 2 g/L, contact time: 120 min).
Effect of initial Cu2+ concentration The effect of initial copper concentration was studied at a temperature of 25 C using a geopolymer dosage of 2 g/L for a 120 min contact time. Fig. 4 presents the Cu2+ sorption effectiveness of the synthesized geopolymeric material at various Cu2+ concentrations. The Cu2+ removal efficiency remains higher than 80% when initial Cu2+ concentrations are less than 140 mg/L. It was found that the adsorption efficiency drops from 88% at 100 mg/L to 73.1% at 160 mg/L, however, the Cu2+ uptake increases from 44 mg/ g at 100 mg/L to 57.9 mg/g at 160 mg/L, then remains almost constant (58.5 mg/g) at 160 mg/L. This suggests that the geopolymer reached the saturation level at the initial concentrations above 140 mg/L. Wang et al. [31] found that the Cu2+ adsorption efficiency on geopolymer depends on concentration, i.e., the higher the initial concentration, the lower the removal efficiency. According to those authors, the efficiency changed from 90% at 100 mg/L to 30% at initial Cu2+ concentration 250 mg/L. Similarly, Gundogan et al. [37] found that the percent of Cu2+ removal by herbaceous peat decreases with increasing initial Cu2+ concentration. When the initial Cu2+ ion concentration increases, the accessible sites become inadequate to adsorb them, therefore, a major part of the ions remain in the solution without being sorbed by the sorbent.
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Sorption isotherm
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Sorption isotherms are essential in order to explore the nature of the interaction between metal ion and the synthesized geopolymer and are useful in optimizing its use as a sorbent. Two isotherm models were used in this study to quantify the sorption capacity of the synthesized geopolymer.
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Langmuir adsorption isotherm Langmuir isotherm is one of the most widely used models to study the adsorption process. The assumptions of this model include: the adsorbent surface is in contact with the adsorbate present in solution, the surface of the adsorbent contains a number of active sites on which the adsorbate adsorbed, and, finally, the adsorption comprises the attachment of only one molecular monolayer on adsorbate surface [34]. The most commonly used form of this model is given by the following expression [38]:
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1 1 1 1 ¼ þ qe qm K L C e qm
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(4)
where KL is constant related to the heat of adsorption and qm is the maximum monolayer adsorption capacity. According to the above equation, a plot of 1/qe versus 1/Ce will yield a straight line with a slope of (1/qm KL) and an intercept of (1/qm).
Please cite this article in press as: M.S. Al-Harahsheh, et al., Fly ash based geopolymer for heavy metal removal: A case study on copper removal, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.005
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Table 2 Langmuir and Freundlich parameters for different parameters and pH values. Parameters of Langmuir model T ( C)
25 35 45
pH 4
pH 5
pH 6
R2
qm (mg/g)
KL (L/mg)
R2
qm (mg/g)
KL (L/mg)
R2
qm (mg/g)
KL (L/mg)
0.9844 0.9879 0.9913
50.81 56.08 68.91
0.062 0.061 0.059
0.9903 0.9915 0.9732
83.13 77.63 75.97
0.032 0.047 0.099
0.9941 0.9864 0.9863
96.84 109.10 152.31
0.061 0.072 0.072
Parameters of Freundlich model T ( C)
25 35 45
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322 323 324 325 326
pH 5
pH 6
KF
n
R2
KF
n
R2
KF
n
0.9815 0.9897 0.9828
6.09 6.50 7.16
2.09 2.03 1.89
0.9787 0.985 0.9442
4.73 6.45 11.30
1.60 1.75 2.03
0.9855 0.9934 0.993
8.47 10.25 12.20
1.627 1.577 1.380
Langmuir model was tested at temperatures of 25, 35, and 45 C and different pHs (4–6). It was found that the adsorption data fits splendidly Langmuir model with correlation coefficients between 0.973 and 0.994 for the investigated conditions (temperature range: 25–45 C and pH range: 4–6) as shown in Table 2 and Fig. 5. As both temperature and pH increase, the adsorption capacity (see Table 2) increases reaching the highest value of 152.3 mg/g (2.38 mmol/g) at a pH value of 6 and a temperature of 45 C. It is known that high adsorption temperatures increase the kinetic energy of the Cu2+ and, hence, enhance the mobility of metal ion (Cu2+), leading to a higher chance of the metal being sorbed onto the sorbent and an increase in its sorption capacity [39]. Langmuir isotherm may be characterized by a dimensionless parameter called separation factor. The dimensionless separation factor (RL) was calculated from the Langmuir isotherm using the following equation [34]: RL ¼
320 319 321
pH 4 R2
1 1 þ K L Ce
(5)
where KL is the Langmuir adsorption constant and Ce is the equilibrium concentration at qm. Separation factor suggests that isotherm will be shaped according to the following adsorption characteristics: RL > 1 unfavorable; RL = 1 corresponds to linear; 0 < RL < 1 is favorable and RL = 0 is irreversible. The values of RL were found to be 0.401, 0.448 and 0.583 for temperatures of 25, 35 and 45 C,
Fig. 5. Comparison between experimental and theoretical Langmuir values of qe as a function of Ce at different pH values and at a temperature of 25 C.
respectively. The separation factor (RL) less than 1 suggests a favorable adsorption [34].
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Freundlich adsorption isotherm The Freundlich isotherm is an empirical model used to describe the adsorption in aqueous systems. It involves heterogeneous adsorption surface and active sites with different energies. The mathematical expression of this model is shown in the following equation [40]:
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qe ¼ K F C e 1=n
330 331 332 333 334
(6)
where qe is the mass of adsorbate per unit mass of adsorbent at equilibrium, Ce is the equilibrium concentration of the adsorbate in solution, KF (mg11/n g1 L1/n) and n is constants for a given adsorbate and adsorbent at a specified temperature [29]. The logarithmic form of the above equation gives a linear relationship as shown below: 1 ln qe ¼ ln K F þ ln C e n
328
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(7)
The plot of ln qe versus ln Ce will yield an intercept of to ln KF and a slope of 1/n. Similar to Langmuir model, Freundlich isotherm was tested at temperatures of 25, 35, and 45 C and different pHs (4–6). The plot of the experimental and the calculated values of qe in Freundlich isotherm is shown in Fig. 6. Parameters of Freundlich model at
Fig. 6. Comparison between experimental and theoretical Freundlich values of qe as a function of Ce at different pH values and at a temperature of 25 C.
Please cite this article in press as: M.S. Al-Harahsheh, et al., Fly ash based geopolymer for heavy metal removal: A case study on copper removal, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.005
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different conditions are listed in Table 2. The R2 values varied between 0.944 and 0.993 suggesting that Freundlich model is also acceptable to describe the adsorption of Cu2+ on the synthesized geopolymer. Furthermore, the adsorption intensity (1/n) is >1 (1.38–2.09) suggesting favorable adsorption. It was reported [41] that the magnitude of the exponent 1/n gives an indication of the favorability and capacity of the sorbent/sorbate system. When n > 1 suggests preferable adsorption onto microporous adsorbent [39]. Generally, if the exponent lies between 1 < n < 10, it indicates advantageous adsorption. With regards to the KF, it can be seen from Table 2 that it increases with increasing temperature suggesting good adsorbent affinity toward Cu2+. As shown above both Langmuir and Freundlich models could explain the adsorption of Cu2+ on the synthesized geopolymer, although the values of correlation coefficient obtained by Langmuir model are slightly better than those obtained for Freundlich plots. This suggests that the adsorption of Cu2+ is monolayer [42]. Similar results were also obtained by Wang et al. [31].
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Modelling of kinetic data
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377 378 379 380 381 382 383 384 385 386 387
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Q10 389 390
Effectiveness of Cu2+ removal by various sorbents
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The sorption capacity of various sorbents for copper ions was reviewed and is summarized in Table 3. The capacity of the geopolymer obtained in this study is the highest after chitosan based materials prepared by Lee et al. [47] (qm = 248.5 mg/g) and Wu et al. [48] (qm = 174.8 mg/g). Other common adsorbents like, kaolinite [49], clinoptilolite [50], sea nodule residue [51], blast furnace slag [52] and activated carbon [39] have adsorption capacities of 10.8, 20, 22, 34 and 54 mg/g, respectively. Considering the availability and cost of coal fly ash, as well as the ease and the cost of geopolymer preparation, it can be put forward that the synthesized geopolymer has high capacity (152.3 mg/g) for Cu2+ removal from aqueous solutions under the optimum conditions determined in this work.
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Thermodynamics of copper sorption
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In order to explore the mechanism of the sorption process, the kinetic data presented in Section “Effect of contact time” were fitted by the pseudo first order and pseudo second order kinetic models. The expression of these kinetic models is listed below [43]:
Thermodynamic parameters including enthalpy of adsorption (DH ), free energy (DG ) and entropy of adsorption (DS ) for Cu2+ adsorption by the geopolymer were determined using the following equations [34].
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First order : ln ðqe qt Þ ¼ lnqe k1 t
lnK d ¼
Second order : 375 374 376
assumption that the rate limiting step might be chemisorption [44,45]. Therefore one can conclude that the sorption process using the synthesized geopolymer is governed by chemisorption [44,46].
t 1 1 ¼ þ t qt k2 q2e qe
(8)
(9)
where qe and qt (mg/g) are the values of amount sorbed per unit mass at equilibrium and at time t; k1 and k2 are the pseudo-first order and pseudo-second-order rate constants, respectively. The experimental data fit well to the second order model much better than the first order model with excellent correlation coefficient (R2 = 0.999) (see Fig. 7). The rate constant for the first order model was found to be 0.042 min1, whereas, it was 0.018 g (mg min)1 for the pseudo-second-order model. On the other hand, the equilibrium capacity or uptake obtained from both models is comparable and close to that of the experimental values (qe corresponding to experimental, calculated from pseudo-first order and pseudo-second-order are 44.31, 44.28 and 44.01 mg/g, respectively). The pseudo-second-order model is based on the
Fig. 7. Comparison between experimental and calculated dynamic data of Cu2+ adsorption (initial Cu2+ concentration: 100 ppm, temperature: 25 C, dose:2 g/L, initial pH 5).
DS R
DH RT
394 395 396 397 398 399 400 401 402 403 404
407 408 409
(10)
where T is temperature in Kelvin and R is the universal gas constant. The plot of (ln Kd) versus (1/T) will yield a straight line (as presented in Fig. 8), and the values of (DH /R) and (DS /R) can obtained from the slope of the straight line and the intercept, respectively. The Gibbs free energy change (DG ) then can be calculated using Eq. (11) [34]:
DG ¼ DH T DS
393
411 410 412 413 414 415 416 417
(11) 418
The calculated thermodynamic parameters are displayed in Table 4. It is shown that the values of DH and DS are positive and the DG decreases with temperature increase, which suggests the endothermic behavior of the sorption and that the process is favorable at higher temperatures [62]. Furthermore, DG values are negative at all temperatures investigated (Table 4), indicating the spontaneous nature of Cu2+ adsorption on the synthesized geopolymer [63]. The endothermic behavior indicates the solvation of Cu2+ ions [62] and that the Cu2+ ions have strong interaction with geopolymer [64]. For the Cu2+ ions to be adsorbed, part of their heat of hydration has to be lost which requires additional energy, because the dehydration energy of the system supersedes the energy of the ions needed for attachment on the surface. Energy needed to bond Cu2+ ions to geopolymer matrix exceeds the energy of dehydration of Cu2+ ions [29,62,65]. The endothermic behavior of the adsorption process is also supported by the fact that the values of DS are positive indicating that the system entropy increases after adsorption process. Adsorption of Cu2+ ion the geopolymer surface leads to the separation of hydrated water molecules before they attach to the surface of geopolymer leading to an increase of the system disorder, thus increases its entropy [62]. The values of DG (mainly >15 kJ/mol but lower than 30 kJ/mol) indicate that the chemical interactions (both ionic and covalent) have the chief role in controlling adsorption rate in the adsorption mechanism [63].
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Table 3 Adsorption capacities (qm, mmol/g) for copper ions of various adsorbents. Adsorbent material
qm (mg/g)
Conditions
Ref.
Activated carbon Lignite Various agricultural based adsorbents Peanut Hulls Peanut pellets
54 6.36 5.7–53.3 8.9 12.1
T = 60 C
[39] [53] [39] [54] [54]
Modified activated carbon
12.1
Prawn shell Crosslinked chitosan beads (polymer) Chitosan flakes Chitosan powder Chitosan flakes Anatase-type titanium
16.9 248.3 174.6 45.1 21 1.5
Kaolinite Clinoptilolite Blast furnace residue Sea nodule residue Sewage sludge ash Harbaceous peat Phosphate rock Geopolymer Geopolymer (present study)
10.8 20 34 22 3.2–3.8 5.1 10.8 98.4 96.8 152.3
Size: 568 mm; Room T; 0–400 ppm Cu2+ Dose: 5.5 g/L Room T; 0–400 ppm Cu2+ Dose: 50 g/L Cu2+ conc. 60–1000 ppm Room T pH 6 pH 5 pH 4.7–5.4 pH 5 pH 6 pH 6, T = 30 C, period: 20 h, C0: 10 Cu2+ mg/L Dose: 10 g/L, T = 25 C: pH 7.25, T = 25 C, Ci = 400 ppm pH 4.2, T = 25 C pH 5.5, T = 30 C Dose: 30 g/L, Ci = 50 ppm Ci = 20–40 ppm, T = 21 C, pH 5.5 Dose: 5 g/L, Ci = 15–100 mg/L, pH 5 T = 40 C, Dose: 0.06 g/L, pH 6.2 pH 6, T = 25 C pH 6, T = 45 C
Fig. 8. Plot of ln(Kd) versus (1/T) for copper adsorption on the geopolymer at different pH values.
[55]
[56] [47] [48] [57] [58] [59] [49] [50] [52] [51] [60] [37] [61] [31] This work
Fig. 9. Modified Arrhenius plot of ln(1 u) against 1/T used to calculate Ea of Cu adsorption the geopolymer at pH 6.
453
Conclusions 445 446 447 448 449 450 451 452
The activation energy (Ea) of the adsorption process, at pH value of 6, was calculated using the modified Arrhenius equation, by plotting of ln(1 u) against 1/T (u is the surface coverage and estimated as (1 C0/Ceq)) as shown in Fig. 9. The slope of the plot yields Ea/R. A value of Ea = 34.9 kJ/mol was obtained suggesting a chemical nature of adsorption process as well as confirming that there is a strong dependence of adsorption process on temperature as shown above.
454 The present work focused on the synthesis of fly ash—based 455 geopolymer and its adsorption characteristics toward Cu2+ removal. 456 The obtained geopolymer was found to be highly amorphous. It was 457 also found that the adsorption capacity of the synthesized geo458 polymer is high compared to other sorbents used in previous work. Q11 459 From the economical and environmental point of view, this 460 application provides an adequate method to solve the problem of
Table 4 Thermodynamic parameters of copper adsorption on geopolymer. pH
DG (kJ/mol)
4 5 6
T (K) 298
308
318
8.00 17.21 19.15
17.37 18.48 21.12
18.57 19.75 23.09
R2
pH
DH (kJ/mol)
DS (J/mol K)
0.958 0.961 0.984
4 5 6
19.49 20.63 39.49
119.66 126.98 196.78
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accumulation of fly ash waste material. Therefore, it is highly recommended to be a good alternative of many other sorbents. The sorption characteristics of Cu2+ ions from aqueous solutions were examined under different conditions using batch technique. It is concluded that the synthesized geopolymer in this study can be used as a sorbent with high efficiency for Cu2+ adsorption from aqueous solutions. The influence of the synthesized geopolymer dosage on Cu2+ metal ions uptake showed that the uptake increases as the synthesized geopolymer dose increases. Additionally, the maximum sorption efficiency was achieved at pH of 6 under experimental conditions (temperature: 25 C, contact time: 120 min, dose: 2 g/L). The equilibrium for Cu2+ ions on the geopolymer surface occurs rapidly and a curing time of 120 min is enough to attain maximum uptake level. The sorption capacity of Cu2+ on the synthesized geopolymer increases as the temperature increases and the adsorption efficiency decreases as the initial concentration increases. The kinetic data was found to fit well to the pseudo second order kinetic model with a correlation coefficient of 0.999. Langmuir isotherm model was found to be more applicable than Freundlich model and the maximum sorption capacity (qm) was 152.3 mg/g at a temperature of 45 C. The calculated thermodynamic parameters suggest that the sorption of Cu2+ on the synthesized geopolymer is spontaneous endothermic. Furthermore, the activation energy (Ea = 34.9 kJ/mol) further supports higher solution temperatures for better copper removal.
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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Fly ash based geopolymer for heavy metal removal: A case study on copper removal
1 2
3 Q1 4 5 6 7
Mohammad S. Al-Harahsheha,* , Kamel Al Zboonb , Leema Al-Makhadmehc , Muhannad Hararahc, Mehaysen Mahasnehb a b c
Q2
Department of Chemical Engineering, Jordan University of Science and Technology, Irbid 22110, Jordan Al-Huson University College, Al-Balqa’ Applied University, Jordan Faculty of Engineering, Al-Hussein Bin Talal University, Ma’an 71111, Jordan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 March 2015 Received in revised form 14 May 2015 Accepted 5 June 2015
This work aims to evaluate the effectiveness of an amorphous geopolymer, synthesized from waste fly ash, as sorbent material for copper from aqueous solutions. The obtained geopolymer was found to be highly amorphous in structure due to the dissolution of fly ash glass phases. It has much higher removal efficiency compared to the raw fly ash. The removal efficiency was affected by solid/liquid ratio, temperature, reaction time, and Cu2+ initial concentration (C0). The highest Cu2+ removal capacity was obtained at pH 6. The kinetic data were found fit to the pseudo second order kinetic model. Furthermore, it was found that Langmuir model is better than Freundlich model within the temperature range (25– 45 C). The maximum sorption capacity (qm) occurred at 45 C, reaching a value of 152 mg/g. The sorption process by the obtained geopolymer is endothermic in nature and more favorable at higher temperatures with enthalpy of adsorption of 39.49 kJ/mol. The activation energy of the sorption process at the optimum conditions was found to be 34.9 kJ/mol. ã2015 Published by Elsevier Ltd.
Keywords: Geopolymer Heavy metals Copper removal Langmuir model
8
Introduction
9
Copper as a heavy metal is hazardous to human health and the environment [1]. It is widely used in industry and is accumulating in their waste streams. Adsorption of Cu2+ has short and long term effects on human health [1–3]. Many effective methods are used for copper removal from wastewater including chemical precipitation [4], ion exchange, reverse osmosis, electrochemical treatment, evaporative recovery, and adsorption [5]. Recently, natural materials with low cost treatment such as silica, mud, agricultural wastes, ash, and solid wastes were used as sorbent materials. Some researchers used palm fruit, activated carbon prepared from palm oil empty fruit bunches [2], palm kernel shell based activated carbon [1], and oil palm leaf powders [6] for Cu2+ removal from aqueous solutions. Other plants such as Cassia angustifolia [7], Tridax procumbens [8], Pleurotus cornucopiae [9], Sorgum vulcaris dust [10], spruce sawdust [11], mangrove barks [12], polar and oak leaves [13] have been used for the same purpose. The conducted studies investigated the effects of different conditions (pH, contact time, and Ci, and adsorbent dosage) on adsorption capacity. Arivoli et al. [14] prepared activated carbon
10 11 12 13 14 15 Q3 16 17 18 19 20 21 Q4 22 23 24 25 26 27
* Corresponding author. E-mail address: [email protected] (M.S. Al-Harahsheh).
from solid waste called Terminalia catappa Linn shell and studied its effectiveness in adsorbing copper ion from aqueous solution. Fixed-bed tests were conducted to investigate the capabilities of a modified silica with respect to the selective removal of copper ions from multi-metal solutions [15]. Sthiannopkao and Sreesai [16] used lime mud and recovered boiler ash to remove the heavy metals from metal finishing wastewater through sorption and precipitation process. A high removal efficiency of Cu2+ (99%) and lower leachability were obtained. Fly ash based zeolites for the removal of heavy metal ions were used by Solanki et al. [17]; the synthesized zeolite obtained has high removal efficiency of Cu2+ up to 100%. In addition to the synthetic zeolites, Gabai et al. [18] tested the adsorption capability of chelating resins and activated carbons for Cu2+ removal. There is a significant trend in recycling of waste materials and converting it to usable and valuable materials. One of these waste materials is coal fly ash. The disposal of the large amount of fly ash has become a serious environmental and economic problem [19,20]. One approach to deal with fly ash waste is to convert it to geopolymer, which is not only effective for heavy metals removal, but also helps in solving the problem of ash accumulation as an industrial waste product. Geopolymer is amorphous material produced by reacting solid aluminosilicate with highly alkali hydroxide. It consists of a polymeric silicon–oxygen–aluminum framework with alternating silicon and aluminum tetrahedral
http://dx.doi.org/10.1016/j.jece.2015.06.005 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: M.S. Al-Harahsheh, et al., Fly ash based geopolymer for heavy metal removal: A case study on copper removal, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.06.005
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joined together in three directions by sharing all the oxygen atoms [21]. Geopolymers have been synthesized from fly ash [19], natural zeolite [22], kaolinite [23] and volcanic ash [24]. They have been used as adsorbent materials for removal of Cd, Ni, Pb(II), Cu(II), phosphate, NOx, boron, fluoride, radionuclide of 137Cs and 90Sr, as well as dyes [19]. The aim of this research is to determine the capability of fly ash based geopolymer in removing copper from aqueous solution as compared to other sorbents. Fly ash was characterized before and after geopolymerization by X-ray diffraction (XRD) and X-ray fluorescence (XRF). The behavior of sorption process under different conditions (solid/liquid ratio, copper concentration, pH, temperature, and contact time) was investigated. Langmuir and Freundlich models were used to study copper adsorption isotherm on the produced geopolymer. The thermodynamic parameters of the adsorption process were also evaluated.
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Materials and methods
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Materials
71
Coal fly ash samples were collected from Rajhi Cement Plant, Jordan. Fly ash was homogenized by grinding using pestle and mortar. After sieving the fraction of less than 45 mm was used to prepare the geopolymer. The synthesized geopolymer was then crushed and ground to 100% less than 200 mm [25]. The chemicals used in this study were of reagent grade. Copper solutions with different concentrations were prepared from 1000 ppm standard (Cu2+) solution (Merk1).
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Instrumentation
80
X-ray diffraction (XRD) analysis was carried out using (SHIMADZU-XRD-6000) diffractometer. The scanning range of 2u was between 2.0 and 65.0, with a scan speed of 2.0 /min and a receiving slit width of 0.30 mm. The chemical composition of the ash sample and the geopolymer was analyzed using X-ray fluorescence (XRF) spectrometer (SHIMADZU-XRF-1800). Inductively coupled plasma (ICP-OES) (SHIMADZU-ICPS-7510) was employed for the measurement of copper concentration in aqueous solutions.
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Synthesis of geopolymer
90
Theoretically, any alkali can be used in geopolymerization reactions; however most of the studies have focused on the effect of sodium (Na+) and potassium (K+) ions [26]. Both NaOH and KOH could be used in the activation process, but the extent of dissolution was higher when NaOH is used; this is due to the smaller size of Na+ which can better stabilize the silicate monomers and dimmers present in the solution, enhancing the minerals dissolution rate [27]. For this reason NaOH is used in this study for geopolymer synthesis. A specified quantity of fly ash sample was reacted with NaOH solution (14 M) at a mass ratio of 4 to 3, respectively. The obtained paste was then shacked for 5–10 min. The obtained mixture was poured in a plastic cylinder and vibrated for 30 s in an Elma–Digital S1 ultrasonic bath (2 40 W, 120% of ultrasonic power, 40 kHz) to de-foam the paste. It is reported that ultrasonification can improve the dissolution of alumina–silicate components of fly ash in caustic soda. The obtained product was then transferred into a cylindrical vessel and aged for 2 h at a temperature of 105 C. The obtained product was then left for 3 days at room temperature to develop enough strength.
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Sorption tests
110
Batch sorption tests were carried out in order to study the effects of the following parameters: geopolymer dosage, pH, contact time, temperature, and copper initial concentration (C0). 50 mL of a solution containing a specified copper concentration (10–160 ppm) of a specified pH (1–6) was added to a specified geopolymer dosage (0.6–3 mg/mL) and shaken for a specified contact time in the range between 5 and 180 min at predetermined temperature ranging from 25 to 45 C. After that, the liquid solution was removed from the solid by centrifugation followed by decantation. Copper concentration in the obtained solution was measured by ICP-OES and the removal percentage and the uptake capacity were calculated. The percentage removal efficiency (E) was calculated using the following equation [28]:
111
E¼
C0 Ce 100% C0
ðC 0 C e Þ V ðmL=gÞ Ce m
ðC 0 C e Þ V m
114 115 116 117 118 119 120 121 122 123 124
126 125 127 128 129 130
(2)
where V is the solution volume (L) and m is the geopolymer mass (g). The adsorption isotherms were calculated for copper adsorption at different values of pH, temperature and concentration (10– 160 mg/L). The quantity of copper ion uptake by the geopolymer was determined as [29]: q¼
113
(1)
where C0 is the initial concentration of copper (ppm) and Ce is the remaining equilibrium copper concentration (ppm). Distribution coefficient (Kd) is a measure used to evaluate the sorption of contaminants on a solid phase. It can be calculated using Eq. (2) below [29]: Kd ¼
112
132 131 133 134 135 136 137 138
(3)
where q is the quantity of copper ion uptake by geopolymer phase (mg (metal)/g (geopolymer)).
140 139 141
Results and discussion
142
Fly ash characterization
143
XRD analysis The XRD pattern of both fly ash and the synthesized geopolymer is shown in Fig. 1. It was found that coal fly ash sample contains the following major phases: quartz, hematite and mullite. Cristobolite and plagioclase minerals were found in trace quantities. The major part of the ash samples has an amorphous structure; such amorphousity makes fly ash reactive material [30]. After geopolymerization process, the resulting geopolymer has lost almost all crystalline phases present in the initial fly ash and transformed into amorphous structure. Mineral phase analysis of the geopolymer shows that all the identified minerals in the raw sample have disappeared (see Fig. 1). Instead broad band peaks appeared, which declare the presence of amorphous materials and disappearance of crystalline phases. This suggests that the treatment of fly ash with caustic solution has contributed to the formation of amorphous structure and the geoplymerization of Al– Si materials was successful.
144
Chemical analysis of solids The composition of raw fly ash and the synthesized geopolymer, as measured by XRF analysis method, is shown in Table 1. The main components of raw fly ash are silica and alumina. Iron, calcium and
161
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Fig. 1. XRD patterns of raw fly ash and the synthesized geopolymer (Q–quartz, M–mulite, H–hematite, Zeo–zeolite, Plag–plageoglase).
165
magnesium are also present in the sample. It is shown that SiO2, Al2O3 and Fe2O3 represent 91.53 wt% of the sample mass. Therefore, according to the Standard Specifications for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete (ASTMC618), the obtained geopolymer may be classified as F class fly ash [19]. Although the synthesized geopolymer was thoroughly washed with water and dried at a temperature of 105 C, the sodium content was found considerably higher than that in the raw sample. The loss on ignition (L.O.I.) of the obtained material was also higher, which suggests that sodium and OH group are now part of the synthesized geopolymer structure, which clarifies the decrease of both SiO2 and Al2O3 contents in the synthesized geopolymer as compared to the initial ash sample.
166 167 168 169 170 171 172 173 174 175 176 177 178
Cu
179
The copper removal efficiency of the produced geopolymer and coal fly ash was compared under the following conditions: pH (6), 100 ppm initial Cu2+ concentration, 120 min contact period and 2 mg/mL sorbent dose. The results indicated that the removal efficiency was 25.15% (12.6 mg/g) and 87.7% (43.9 mg/g) for raw ash and the geopolymer material, respectively. This indicates that the geopolymer material has more sorption sites for Cu2+ ions than the raw ash. Such result can be assigned to transformation crystalline structure of ash material into amorphous one leading to formation of more porous structure encouraging Cu2+ sorption [31].
180 181 182 183 184 185 186 187 188
Q14
2+
uptake
Factors affecting Cu2+ adsorption
189
Effect of pH The effect of varying initial pH values (1–6) of Cu2+ solution on its removal efficiency was studied at a temperature of 25 C using a geopolymer dosage of 2 mg/mL for a contact time of 120 min. The results also suggest that the sorption effectiveness increases from 5.6% to 88.21% when pH was increased from 1 to 6 (Fig. 2). At low alkalinity, more positive H3O+ species are available in the solution, which competes with Cu2+ positive ions present as active sites on the geopolymer surface. With the increase in pH, less H3O+ is available leading to better access of Cu2+ toward the active sites [1]. This was an expected trend for the effect of pH on metal sorption Q5 were sorption increases with increasing pH values up to a certain value, and then decreases with further increase in pH [19]. It is known that increasing pH to values higher than 6 will favor the precipitation of Cu2+ as Cu(OH)2 [32].
190
Effect of geopolymer dose The effect of geopolymer dose on Cu2+ sorption was studied at room temperature (25 C) and a pH value of 6 using the following doses: 0.6; 1.0; 1.4; 1.6; 2.0 and 3.0 g/L. The initial concentration of Cu2+ was 100 ppm and the contact time was 120 min (see Fig. 3). The sorbent dose is a key factor as it determines the amount of sorbent required for a particular initial concentration of sorbate. The results show that the removal efficiency increases from about
205
Table 1 The chemical composition of coal fly ash and the synthesized geopolymer. Compound
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 SO3 L.O.I.
Composition (%) Raw ash
After geopolymerization
50.73 28.87 11.93 1.73 1.39 0.74 0.30 1.41 0.35 2.53
39.90 19.70 7.50 2.43 1.13 1.08 11.72 0.50 0.25 14.69
Fig. 2. Effect of pH on Cu2+ removal efficiency (100 ppm concentration, 25 C, dose: 2 g/L, and contact time: 120 min).
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Fig. 3. Effect of adsorbent dose on Cu2+ removal efficiency (initial Cu2+ concentration: 100 ppm, temperature: 25 C, pH: 6.0, and contact time: 120 min). 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228
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246 247 248 249 250 251 252 253 254 255 256 257 258
45% to 88.2% as the dosage rises from 0.03 g (0.6 g/L) to 0.15 g (3.0 g/L). This indicates that the available active sites at low sorbent dosage are not enough to take all available ions in the solution. A sharp increase in removal efficiency was noticed as the synthesized geopolymer dose increased from 0.6 g/L to 1.6 g/L, while insignificant increase was noticed as the dose increased from 1.6 g/L to 3 g/L. Thus, it can be concluded that a 2 g/L dose is the optimum value under such experimental conditions. The vacant sites available for binding Cu2+ ions increase as the geopolymer dosage increases, leading to a higher sorption capacity [29]. Similar results were obtained by Wang et al. [31], where the removal efficiency of their geopolymer for Cu2+ generally improved with increasing dose reaching about 75% removal efficiency when the dose was 2 g/L. Cetin and Pehlivan [33] also concluded that the removal rate of Zn ions increased with the increase in the adsorbent dose. Effect of contact time The effect of contact time on Cu2+ ions removal was considered by changing the contact time from 5 min to 180 min. The Cu2+ ions Q6 initial concentration was fixed at 100 ppm with a fixed geopolymer dosage (2 g/L) and initial pH of 6. The sorption of Cu2+ metal ions on the synthesized geopolymer is time dependent. A significant removal of Cu2+ occurred after 15 min contact time (81.26%) and only slight change in removal efficiency occurred beyond this period of time. Gupta and Ali [34] and Wang et al. [31] reported that the equilibrium contact time is 60 min for copper when using fly ash sorbents. Similar result was reported by Cetin and Pehlivan [33], where the highest removal efficiency of zinc and lead ions by coal ash was reached after 60 min contact time. Therefore, adsorption of Cu2+ metal ions by the sorbent developed in this work is relatively fast and 60 min contact time is sufficient to reach the adsorption equilibration. Nevertheless, a contact time of 120 min was applied in all following tests. Effect of temperature The effect of temperature on the sorption efficiency was evaluated at 25, 35 and 45 C at a pH value of 6, 2 g/L geopolymer dosage and a contact time of 120 min. The removal efficiency values of Cu2+ were found to be 87.98, 90.42, and 93.91%, respectively. The sorption of Cu2+ on the geopolymer increases with increasing temperature reaching a maximum value at a temperature of 45 C. At a concentration of 100 ppm and pH of 6, the uptake values increase from 44 mg/g to 46.95 mg/g when the Q7 temperature was increased from 25 C to 45 C. Panday et al. [35] showed that Cu2+ sorption on fly ash increases with increasing temperature from 20 C to 40 C. Generally, metal ion removal is enhanced as the temperature increases [31,36].
Fig. 4. Effect of the initial concentration on Cu2+ removal efficiency (temperature: 25 C, pH 6, dose: 2 g/L, contact time: 120 min).
Effect of initial Cu2+ concentration The effect of initial copper concentration was studied at a temperature of 25 C using a geopolymer dosage of 2 g/L for a 120 min contact time. Fig. 4 presents the Cu2+ sorption effectiveness of the synthesized geopolymeric material at various Cu2+ concentrations. The Cu2+ removal efficiency remains higher than 80% when initial Cu2+ concentrations are less than 140 mg/L. It was found that the adsorption efficiency drops from 88% at 100 mg/L to 73.1% at 160 mg/L, however, the Cu2+ uptake increases from 44 mg/ g at 100 mg/L to 57.9 mg/g at 160 mg/L, then remains almost constant (58.5 mg/g) at 160 mg/L. This suggests that the geopolymer reached the saturation level at the initial concentrations above 140 mg/L. Wang et al. [31] found that the Cu2+ adsorption efficiency on geopolymer depends on concentration, i.e., the higher the initial concentration, the lower the removal efficiency. According to those authors, the efficiency changed from 90% at 100 mg/L to 30% at initial Cu2+ concentration 250 mg/L. Similarly, Gundogan et al. [37] found that the percent of Cu2+ removal by herbaceous peat decreases with increasing initial Cu2+ concentration. When the initial Cu2+ ion concentration increases, the accessible sites become inadequate to adsorb them, therefore, a major part of the ions remain in the solution without being sorbed by the sorbent.
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Sorption isotherm
282
Sorption isotherms are essential in order to explore the nature of the interaction between metal ion and the synthesized geopolymer and are useful in optimizing its use as a sorbent. Two isotherm models were used in this study to quantify the sorption capacity of the synthesized geopolymer.
283
Langmuir adsorption isotherm Langmuir isotherm is one of the most widely used models to study the adsorption process. The assumptions of this model include: the adsorbent surface is in contact with the adsorbate present in solution, the surface of the adsorbent contains a number of active sites on which the adsorbate adsorbed, and, finally, the adsorption comprises the attachment of only one molecular monolayer on adsorbate surface [34]. The most commonly used form of this model is given by the following expression [38]:
288
1 1 1 1 ¼ þ qe qm K L C e qm
260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281
284 285 286 287
289 290 291 292 293 294 295 296
(4)
where KL is constant related to the heat of adsorption and qm is the maximum monolayer adsorption capacity. According to the above equation, a plot of 1/qe versus 1/Ce will yield a straight line with a slope of (1/qm KL) and an intercept of (1/qm).
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Table 2 Langmuir and Freundlich parameters for different parameters and pH values. Parameters of Langmuir model T ( C)
25 35 45
pH 4
pH 5
pH 6
R2
qm (mg/g)
KL (L/mg)
R2
qm (mg/g)
KL (L/mg)
R2
qm (mg/g)
KL (L/mg)
0.9844 0.9879 0.9913
50.81 56.08 68.91
0.062 0.061 0.059
0.9903 0.9915 0.9732
83.13 77.63 75.97
0.032 0.047 0.099
0.9941 0.9864 0.9863
96.84 109.10 152.31
0.061 0.072 0.072
Parameters of Freundlich model T ( C)
25 35 45
303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 Q9
322 323 324 325 326
pH 5
pH 6
KF
n
R2
KF
n
R2
KF
n
0.9815 0.9897 0.9828
6.09 6.50 7.16
2.09 2.03 1.89
0.9787 0.985 0.9442
4.73 6.45 11.30
1.60 1.75 2.03
0.9855 0.9934 0.993
8.47 10.25 12.20
1.627 1.577 1.380
Langmuir model was tested at temperatures of 25, 35, and 45 C and different pHs (4–6). It was found that the adsorption data fits splendidly Langmuir model with correlation coefficients between 0.973 and 0.994 for the investigated conditions (temperature range: 25–45 C and pH range: 4–6) as shown in Table 2 and Fig. 5. As both temperature and pH increase, the adsorption capacity (see Table 2) increases reaching the highest value of 152.3 mg/g (2.38 mmol/g) at a pH value of 6 and a temperature of 45 C. It is known that high adsorption temperatures increase the kinetic energy of the Cu2+ and, hence, enhance the mobility of metal ion (Cu2+), leading to a higher chance of the metal being sorbed onto the sorbent and an increase in its sorption capacity [39]. Langmuir isotherm may be characterized by a dimensionless parameter called separation factor. The dimensionless separation factor (RL) was calculated from the Langmuir isotherm using the following equation [34]: RL ¼
320 319 321
pH 4 R2
1 1 þ K L Ce
(5)
where KL is the Langmuir adsorption constant and Ce is the equilibrium concentration at qm. Separation factor suggests that isotherm will be shaped according to the following adsorption characteristics: RL > 1 unfavorable; RL = 1 corresponds to linear; 0 < RL < 1 is favorable and RL = 0 is irreversible. The values of RL were found to be 0.401, 0.448 and 0.583 for temperatures of 25, 35 and 45 C,
Fig. 5. Comparison between experimental and theoretical Langmuir values of qe as a function of Ce at different pH values and at a temperature of 25 C.
respectively. The separation factor (RL) less than 1 suggests a favorable adsorption [34].
327
Freundlich adsorption isotherm The Freundlich isotherm is an empirical model used to describe the adsorption in aqueous systems. It involves heterogeneous adsorption surface and active sites with different energies. The mathematical expression of this model is shown in the following equation [40]:
329
qe ¼ K F C e 1=n
330 331 332 333 334
(6)
where qe is the mass of adsorbate per unit mass of adsorbent at equilibrium, Ce is the equilibrium concentration of the adsorbate in solution, KF (mg11/n g1 L1/n) and n is constants for a given adsorbate and adsorbent at a specified temperature [29]. The logarithmic form of the above equation gives a linear relationship as shown below: 1 ln qe ¼ ln K F þ ln C e n
328
336 335 337 338 339 340 341
(7)
The plot of ln qe versus ln Ce will yield an intercept of to ln KF and a slope of 1/n. Similar to Langmuir model, Freundlich isotherm was tested at temperatures of 25, 35, and 45 C and different pHs (4–6). The plot of the experimental and the calculated values of qe in Freundlich isotherm is shown in Fig. 6. Parameters of Freundlich model at
Fig. 6. Comparison between experimental and theoretical Freundlich values of qe as a function of Ce at different pH values and at a temperature of 25 C.
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different conditions are listed in Table 2. The R2 values varied between 0.944 and 0.993 suggesting that Freundlich model is also acceptable to describe the adsorption of Cu2+ on the synthesized geopolymer. Furthermore, the adsorption intensity (1/n) is >1 (1.38–2.09) suggesting favorable adsorption. It was reported [41] that the magnitude of the exponent 1/n gives an indication of the favorability and capacity of the sorbent/sorbate system. When n > 1 suggests preferable adsorption onto microporous adsorbent [39]. Generally, if the exponent lies between 1 < n < 10, it indicates advantageous adsorption. With regards to the KF, it can be seen from Table 2 that it increases with increasing temperature suggesting good adsorbent affinity toward Cu2+. As shown above both Langmuir and Freundlich models could explain the adsorption of Cu2+ on the synthesized geopolymer, although the values of correlation coefficient obtained by Langmuir model are slightly better than those obtained for Freundlich plots. This suggests that the adsorption of Cu2+ is monolayer [42]. Similar results were also obtained by Wang et al. [31].
368
Modelling of kinetic data
350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366
369 370 371 372 373
377 378 379 380 381 382 383 384 385 386 387
388
Q10 389 390
Effectiveness of Cu2+ removal by various sorbents
391
The sorption capacity of various sorbents for copper ions was reviewed and is summarized in Table 3. The capacity of the geopolymer obtained in this study is the highest after chitosan based materials prepared by Lee et al. [47] (qm = 248.5 mg/g) and Wu et al. [48] (qm = 174.8 mg/g). Other common adsorbents like, kaolinite [49], clinoptilolite [50], sea nodule residue [51], blast furnace slag [52] and activated carbon [39] have adsorption capacities of 10.8, 20, 22, 34 and 54 mg/g, respectively. Considering the availability and cost of coal fly ash, as well as the ease and the cost of geopolymer preparation, it can be put forward that the synthesized geopolymer has high capacity (152.3 mg/g) for Cu2+ removal from aqueous solutions under the optimum conditions determined in this work.
392
Thermodynamics of copper sorption
405
In order to explore the mechanism of the sorption process, the kinetic data presented in Section “Effect of contact time” were fitted by the pseudo first order and pseudo second order kinetic models. The expression of these kinetic models is listed below [43]:
Thermodynamic parameters including enthalpy of adsorption (DH ), free energy (DG ) and entropy of adsorption (DS ) for Cu2+ adsorption by the geopolymer were determined using the following equations [34].
406
First order : ln ðqe qt Þ ¼ lnqe k1 t
lnK d ¼
Second order : 375 374 376
assumption that the rate limiting step might be chemisorption [44,45]. Therefore one can conclude that the sorption process using the synthesized geopolymer is governed by chemisorption [44,46].
t 1 1 ¼ þ t qt k2 q2e qe
(8)
(9)
where qe and qt (mg/g) are the values of amount sorbed per unit mass at equilibrium and at time t; k1 and k2 are the pseudo-first order and pseudo-second-order rate constants, respectively. The experimental data fit well to the second order model much better than the first order model with excellent correlation coefficient (R2 = 0.999) (see Fig. 7). The rate constant for the first order model was found to be 0.042 min1, whereas, it was 0.018 g (mg min)1 for the pseudo-second-order model. On the other hand, the equilibrium capacity or uptake obtained from both models is comparable and close to that of the experimental values (qe corresponding to experimental, calculated from pseudo-first order and pseudo-second-order are 44.31, 44.28 and 44.01 mg/g, respectively). The pseudo-second-order model is based on the
Fig. 7. Comparison between experimental and calculated dynamic data of Cu2+ adsorption (initial Cu2+ concentration: 100 ppm, temperature: 25 C, dose:2 g/L, initial pH 5).
DS R
DH RT
394 395 396 397 398 399 400 401 402 403 404
407 408 409
(10)
where T is temperature in Kelvin and R is the universal gas constant. The plot of (ln Kd) versus (1/T) will yield a straight line (as presented in Fig. 8), and the values of (DH /R) and (DS /R) can obtained from the slope of the straight line and the intercept, respectively. The Gibbs free energy change (DG ) then can be calculated using Eq. (11) [34]:
DG ¼ DH T DS
393
411 410 412 413 414 415 416 417
(11) 418
The calculated thermodynamic parameters are displayed in Table 4. It is shown that the values of DH and DS are positive and the DG decreases with temperature increase, which suggests the endothermic behavior of the sorption and that the process is favorable at higher temperatures [62]. Furthermore, DG values are negative at all temperatures investigated (Table 4), indicating the spontaneous nature of Cu2+ adsorption on the synthesized geopolymer [63]. The endothermic behavior indicates the solvation of Cu2+ ions [62] and that the Cu2+ ions have strong interaction with geopolymer [64]. For the Cu2+ ions to be adsorbed, part of their heat of hydration has to be lost which requires additional energy, because the dehydration energy of the system supersedes the energy of the ions needed for attachment on the surface. Energy needed to bond Cu2+ ions to geopolymer matrix exceeds the energy of dehydration of Cu2+ ions [29,62,65]. The endothermic behavior of the adsorption process is also supported by the fact that the values of DS are positive indicating that the system entropy increases after adsorption process. Adsorption of Cu2+ ion the geopolymer surface leads to the separation of hydrated water molecules before they attach to the surface of geopolymer leading to an increase of the system disorder, thus increases its entropy [62]. The values of DG (mainly >15 kJ/mol but lower than 30 kJ/mol) indicate that the chemical interactions (both ionic and covalent) have the chief role in controlling adsorption rate in the adsorption mechanism [63].
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Table 3 Adsorption capacities (qm, mmol/g) for copper ions of various adsorbents. Adsorbent material
qm (mg/g)
Conditions
Ref.
Activated carbon Lignite Various agricultural based adsorbents Peanut Hulls Peanut pellets
54 6.36 5.7–53.3 8.9 12.1
T = 60 C
[39] [53] [39] [54] [54]
Modified activated carbon
12.1
Prawn shell Crosslinked chitosan beads (polymer) Chitosan flakes Chitosan powder Chitosan flakes Anatase-type titanium
16.9 248.3 174.6 45.1 21 1.5
Kaolinite Clinoptilolite Blast furnace residue Sea nodule residue Sewage sludge ash Harbaceous peat Phosphate rock Geopolymer Geopolymer (present study)
10.8 20 34 22 3.2–3.8 5.1 10.8 98.4 96.8 152.3
Size: 568 mm; Room T; 0–400 ppm Cu2+ Dose: 5.5 g/L Room T; 0–400 ppm Cu2+ Dose: 50 g/L Cu2+ conc. 60–1000 ppm Room T pH 6 pH 5 pH 4.7–5.4 pH 5 pH 6 pH 6, T = 30 C, period: 20 h, C0: 10 Cu2+ mg/L Dose: 10 g/L, T = 25 C: pH 7.25, T = 25 C, Ci = 400 ppm pH 4.2, T = 25 C pH 5.5, T = 30 C Dose: 30 g/L, Ci = 50 ppm Ci = 20–40 ppm, T = 21 C, pH 5.5 Dose: 5 g/L, Ci = 15–100 mg/L, pH 5 T = 40 C, Dose: 0.06 g/L, pH 6.2 pH 6, T = 25 C pH 6, T = 45 C
Fig. 8. Plot of ln(Kd) versus (1/T) for copper adsorption on the geopolymer at different pH values.
[55]
[56] [47] [48] [57] [58] [59] [49] [50] [52] [51] [60] [37] [61] [31] This work
Fig. 9. Modified Arrhenius plot of ln(1 u) against 1/T used to calculate Ea of Cu adsorption the geopolymer at pH 6.
453
Conclusions 445 446 447 448 449 450 451 452
The activation energy (Ea) of the adsorption process, at pH value of 6, was calculated using the modified Arrhenius equation, by plotting of ln(1 u) against 1/T (u is the surface coverage and estimated as (1 C0/Ceq)) as shown in Fig. 9. The slope of the plot yields Ea/R. A value of Ea = 34.9 kJ/mol was obtained suggesting a chemical nature of adsorption process as well as confirming that there is a strong dependence of adsorption process on temperature as shown above.
454 The present work focused on the synthesis of fly ash—based 455 geopolymer and its adsorption characteristics toward Cu2+ removal. 456 The obtained geopolymer was found to be highly amorphous. It was 457 also found that the adsorption capacity of the synthesized geo458 polymer is high compared to other sorbents used in previous work. Q11 459 From the economical and environmental point of view, this 460 application provides an adequate method to solve the problem of
Table 4 Thermodynamic parameters of copper adsorption on geopolymer. pH
DG (kJ/mol)
4 5 6
T (K) 298
308
318
8.00 17.21 19.15
17.37 18.48 21.12
18.57 19.75 23.09
R2
pH
DH (kJ/mol)
DS (J/mol K)
0.958 0.961 0.984
4 5 6
19.49 20.63 39.49
119.66 126.98 196.78
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accumulation of fly ash waste material. Therefore, it is highly recommended to be a good alternative of many other sorbents. The sorption characteristics of Cu2+ ions from aqueous solutions were examined under different conditions using batch technique. It is concluded that the synthesized geopolymer in this study can be used as a sorbent with high efficiency for Cu2+ adsorption from aqueous solutions. The influence of the synthesized geopolymer dosage on Cu2+ metal ions uptake showed that the uptake increases as the synthesized geopolymer dose increases. Additionally, the maximum sorption efficiency was achieved at pH of 6 under experimental conditions (temperature: 25 C, contact time: 120 min, dose: 2 g/L). The equilibrium for Cu2+ ions on the geopolymer surface occurs rapidly and a curing time of 120 min is enough to attain maximum uptake level. The sorption capacity of Cu2+ on the synthesized geopolymer increases as the temperature increases and the adsorption efficiency decreases as the initial concentration increases. The kinetic data was found to fit well to the pseudo second order kinetic model with a correlation coefficient of 0.999. Langmuir isotherm model was found to be more applicable than Freundlich model and the maximum sorption capacity (qm) was 152.3 mg/g at a temperature of 45 C. The calculated thermodynamic parameters suggest that the sorption of Cu2+ on the synthesized geopolymer is spontaneous endothermic. Furthermore, the activation energy (Ea = 34.9 kJ/mol) further supports higher solution temperatures for better copper removal.
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