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Removal of Pb2+ from water by using Na-Y zeolites prepared from Egyptian kaolins collected from different sources
Journal of Environmental Chemical Engineering 2 (2014) 723–730
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Removal of Pb2+ from water by using Na-Y zeolites prepared from Egyptian kaolins collected from different sources Doaa M. EL-Mekkawi *, Mohamed M. Selim Physical Chemistry Department, National Research Center, Dokki, Cairo, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 May 2013 Received in revised form 9 November 2013 Accepted 12 November 2013
This article aims to study the effect of the chemical composition of the Egyptian kaolins on the performance of the low cost kaolin-based zeolites toward Pb2+ removal. In this work, different Egyptian kaolins collected from Saint Catherine, Kalabsha and Dehessa (KS, KK and KD, respectively) were used in the preparation of the corresponding Na-Y zeolites (ZS, ZK and ZD). X-ray Fluorescence (XRF), X-ray diffraction (XRD) and atomic absorption analyses were recorded. XRF and XRD indicate that the zeolitic contents in the prepared samples are proportional to the kaolinite contents in the corresponding kaolins. The difference in the removal efficiency between zeolites depends on the experimental conditions. Generally, the order of Pb2+ loading and Na+ release is ZS > ZK > ZD, which is in accordance with the zeolitic contents in each sample. From sorption isotherms, maximum sorbed amounts of Pb2+ were 299.6, 299.3 and 260.6 meq/100 g for ZS, ZK and ZD, respectively. Comparison with previous studies shows that the maximum loading capacities of zeolites are higher than the commonly used; and it is also comparable to ion exchange resins. For all zeolites, in Pb2+ loading experiments, the relation between released and loaded equivalents of positive charges was not stoichiometric. We concluded that the loading process involves competition between three processes: (a) precipitation of some Pb2+as hydroxides, (b) ion exchange and (c) chemisorption of Pb2+on zeolite surfaces. During Pb2+ loading processes, the equivalent amount of loaded Pb2+ is always higher than that of the released Na+ suggesting the predominance of the chemisorption mechanism. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Zeolite Y Egyptian kaolin Pb2+ Sorption Heavy metal Ion exchange
Introduction Removal of Pb2+ from aqueous solutions using zeolites has been widely introduced as an efficient method for the treatment of wastewater contaminated with this highly toxic inorganic
Abbreviations: ZS, Zeolite prepared from Egyptian kaolin collected from Saint Catherine; ZK, Zeolite prepared from Egyptian kaolin collected from Kalabsha; ZD, Zeolite prepared from Egyptian kaolin collected from Dehessa; KS, Egyptian kaolin collected from Saint Catherine; KK, Egyptian kaolin collected from Kalabsha; KD, Egyptian kaolin collected from Dehessa; XRF, X-ray fluorescence; XRD, X-ray diffraction; AAS, atomic absorption spectrophotometer; ZFA, cancrinite-type zeolite; MEL, maximum exchange level; qe, equivalent amounts of adsorbed Pb2+ per 100 grams of adsorbent (meq/100 g); Ce, unadsorbed Pb2+ concentration in solution at equilibrium (mg/l); qm, maximum amount of the Pb2+ bound per unit weight of adsorbent to form a complete monolayer on the surface at high Ce (meq/ 100 g); b, constant related to the affinity of the binding sites (l/mg); KF, empirical constant that refers to the relative adsorption capacity; n, adsorption intensity under different experimental conditions. * Corresponding author. Tel.: +20 1224086609. E-mail addresses: [email protected], [email protected] (D.M. EL-Mekkawi), [email protected] (M.M. Selim). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.11.014
pollutant [1–10]. For example, Kragovic et al. studied the sorption of Pb2+ by the natural and Fe(III)-modified zeolite (clinoptilolite) [1]. They found that at lower initial concentrations of Pb2+, ion exchange of inorganic cations in zeolites with Pb2+, together with uptake of hydrogen dominated, while at higher initial Pb2+ concentrations beside these processes, chemisorption of Pb2+ occurred. O’Connor and Townsend [11] examined the ionexchange properties of Pb2+ ions in a range of synthetic sodium faujasites of varying framework silica to alumina ratios. Ionexchange isotherms were constructed for all systems, but nonstoichiometric behavior was observed in all cases. They found that the more siliceous the zeolite, the lower the preference for Pb2+. Qiu and Zheng prepared a cancrinite-type zeolite (ZFA) from Class C fly ash via the molten-salt method [12]. They studied the adsorption equilibriums of Pb2+, Cu2+, Ni2+, Co2+, and Zn2+ on ZFA in aqueous solutions. The increase of pH levels during the adsorption process suggests that the uptake of heavy metals on ZFA was subjected to an ion exchange mechanism. It is found that the maximum exchange level (MEL) follows the order: Pb2+ > Cu2+ > Ni2+ > Co2+ > Zn2+. The removal efficiency of zeolite (clinoptilolite) and sepiolite from Pb2+ containing aqueous
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solutions was investigated by Turan et al. [13]. The removal of Pb2+ is found to be a sum of adsorption induced by ion exchange and precipitation of lead hydroxide. Castaldi et al. evaluated Pb2+, Cd2+ and Zn2+ adsorption capacity of a natural zeolite in batch tests [14]. The zeolite adsorption capacity for the three cations was Zn > Pb > Cd. The main mineralogical phase involved in the adsorption process was clinoptilolite. Synthetic aluminosilicate zeolites act as efficient porous solid exchange media where the ‘‘compensating’’ zeolitic cations (normally alkali metal ions) are not rigidly fixed at specific locations within the hydrated unit cell and readily exchange with external cations in solution. The application of the large pore high surface area zeolite Y, (Na58Al58Si134O384260H2O) to the removal of Pb2+, from water has been reported previously [2]. Clay minerals have been used as a combined source for A12O3 and SiO2 for the synthesis of zeolites [15,16]. Kaolinite (A12O32SiO22H2O) with a Si/A1 ratio of 1, is the principal mineral in clay and is ideally suited for the synthesis of zeolites [17]. Since clays are produced from natural deposits formed during various long geological processes, they are complex mixtures of different minerals. Besides kaolinite, they usually contain some quantities of quartz, anatase, feldspars, smectites, micas, etc. [18,19]. Previously, we carried out many studies with the faujasite zeolites prepared from Egyptian kaolins. These studies almost concerned the utilization of zeolites in various applications having environmental, industrial or medical concerns, e.g., reduction of nitrophenols [20,21] preparation of nanosized spinels [22] removal of heavy metals and organic pollutants from waters [23], etc. The relations between the chemical composition of clay and the performance of the prepared zeolites were not extensively studied. The necessity of better understanding of the Egyptian kaolins’ properties and their appropriateness for the synthesis of zeolites according to their applications becomes our main concern. Comparing the cost of commercially available faujasite zeolites and prepared kaolin-based zeolites according to our preparation methods, feasibility studies indicate that the later costs 200 $ per ton which is approximately 15 times cheaper than the former (see Supporting Information, Table S1). Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.jece.2013.11.014. The aim of the present article is to assess the effect of the chemical composition of the Egyptian kaolins, for the preparation of zeolite Y, on the performance of the prepared zeolites toward Pb2+ removal from simulated wastewater. In the present study, we introduce comparative analyses of the Pb2+ loading capacities of zeolite Y samples prepared from different Egyptian kaolins. The influence of the zeolite dose, the initial Pb2+ concentrations and the number of loading cycles on loading of Pb2+ from aqueous solution by each zeolite was investigated. Experimental Zeolites preparation Three Egyptian kaolins, KS, KD and KK, were obtained from three different areas in Egypt. KS and KD were collected from two different areas at Sinai Peninsula (Saint Catherine and Dehessa), which is considered as the main source of kaolin in Egypt. On the other hand, KK was collected from Kalabsha at the Upper Egypt. The corresponding Y zeolites; ZK, ZD and ZS; were prepared by hydrothermal treatment of the Egyptian kaolins with alkali solution and sodium silicate at 100–120 8C for 8 h according to the submitted Egyptian Patent No. (165/2008).
Table 1 Chemical composition of raw kaolins and the prepared zeolites (in mass%). Oxide
KD
KK
KS
ZD
ZK
ZS
SiO2 TiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O L.O.I.
49.97 2.73 33.83 0.79 0.01 0.09 0.42 0.01 11.73
45.00 3.50 35.00 1.50 0.10 0.60 0.30 0.05 14.00
47.00 1.30 37.00 0.20 0.02 0.20 0.15 0.04 13.4
41.94 2.04 22.14 0.52 0.10 0.37 13.60 0.10 18.87
39.30 2.25 21.68 0.25 0.22 1.36 13.24 0.08 21.28
40.44 0.84 22.34 0.18 0.11 0.64 13.21 0.09 21.57
Samples characterization The chemical compositions of natural Egyptian kaolins and the prepared zeolites are determined using X-ray fluorescence (see Table 1). (XRF Axios, sequential WD_XRF spectrometer, PANalytical 2005.) X-ray diffractograms of kaolin and zeolite samples were obtained using a Bruker D8 advance instrument with CuKa1 target with secondary monochromator 40 kV, 40 mA. Pb2+ loading experiments The Pb2+ loading experimental studies were conducted using synthetic solutions prepared using distilled water with constant metal concentration for each of Pb2+. The uptake of Pb2+on the three prepared zeolites was carried out using the batch method at room temperature (25 0.1 8C) in 150-ml glass bottles. The bottles were shaken in a rotary shaker at 269 rpm from 5 min to 24 h. The influence of zeolites dose on loading of Pb2+ at the constant initial Pb2+ concentration – 3931 mg/l (Pb(NO3)2, p.a., Aldrich) was studied with the following protocol: in 50 ml of Pb solution, 0.02, 0.2, 0.4, 0.6, 1, 1.2 g of either the ZD, ZS, or ZK zeolite was added and the mixtures were shaken at room temperature for 2 h. To study the effect of the initial Pb2+ concentrations, 1 g of each zeolite was mixed with 50 ml of aqueous solutions, containing various initial concentrations of Pb(NO3)2 (1045–8312 mg/l) for a period of 2 h. Another series of loading experiments dealing with the effect of repetition on the loading behavior of zeolite Y for Pb2+ has been carried out. The initial concentration of Pb2+ solution is 1045 mg/l and fresh salt solution is used for each repetition. In all Pb2+ loading experiments, after the reaction time, suspensions were centrifuged 2 times at 5500 rpm for 10 min to separate the solution and solid. The initial and non-loaded concentrations of Pb2+ in supernatants were determined by the atomic absorption spectrophotometer (AAS). The concentrations of Ca, Mg, Na, Ti, K and Fe in supernatants released from both zeolites were determined by AAS (Thermo Elemental, Solar S4, Atomic Absorption Spectrophotometer). Only Na+ cations are found during the exchange process. In order to study the adsorption behavior of Pb2+ on zeolite surfaces, the Langmuir isotherm and Freundlich isotherm models were verified. The Langmuir and Freundlich equations are commonly used to describe adsorption isotherms at a constant temperature for water and wastewater treatment applications. The Langmuir model is valid for monolayer sorption onto a surface with a finite number of identical sites. The distribution of Pb2+ ions between the solid solution interface equilibrium has been described by the Langmuir equation [24,25]. The well known expression of the Langmuir model is given by Eq. (1): C e =qe ¼ 1=qm b þ C e =qm
(1)
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where qe (meq/100 g) and Ce (mg/l) are the equivalent amounts of adsorbed Pb2+ per 100 grams of adsorbent and unadsorbed Pb2+ concentration in solution at equilibrium, respectively. qm is the maximum amount of the Pb2+ bound per unit weight of adsorbent to form a complete monolayer on the surface at high Ce, and b is a constant related to the affinity of the binding sites (l/mg). Freundlich model explains the relation between qe and equilibrium Pb2+ concentration Eq. (2): ln qe ¼ ln K F þ 1=n ln C e
(2)
The Freundlich adsorption isotherm usually is applied to the highly heterogeneous surfaces [26,27]. The empirical constant KF refers to the relative adsorption capacity and ‘n’ represents adsorption intensity under different experimental conditions. Results and discussion Characterization of the raw kaolins and the prepared zeolites X-ray diffractograms of the natural kaolin and synthetic zeolites are shown in Fig. 1a and b. The figures indicate the presence of
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kaolinite and quartz as the crystalline phases in all kaolin samples with some minor anatase. These results are confirmed by the XRF results (Table 1) where the main constituents of the starting raw materials are Al2O3 and SiO2 in addition to 2.73%, 3.50% and 1.30% of TiO2 in KD, KK and KS, respectively. All kaolin samples KS, KK, and KD have relatively low amounts of SiO2, and high amounts of Al2O3 with minute amounts of alkali and alkaline earth oxides, typical of kaolins [28]. The slightly higher Al2O3 content in KS and KK indicates its higher kaolinite content. The silica to alumina ratios of KS, KK, and KD are 1.27, 1.29 and 1.48. The data for KS and KK approaches that of pure kaolinite (1.18) [29], while the analytical composition of KD might indicate the presence of either silica-rich clay minerals and/or free quartz. The quartz reflections in KD were more significant in agreement with its higher SiO2 content. On the other hand, the prepared zeolites contain a significant amount of Na, if compared to Ca, K, Fe and Mg. The hydrothermal treatment of the natural kaolins with NaOH during the preparation of zeolites results in the high Na content. Besides, the treatment of kaolins under strongly basic conditions leads to a significant change in Si and Al content in the corresponding zeolite samples.
[(Fig._1)TD$IG]
Fig. 1. X-ray difractograms of: (a) raw kaolins and (b) prepared zeolites [A = anatase, k = kaolinite, q = quartz, z = zeolite].
[(Fig._2)TD$IG]
[(Fig._3)TD$IG]
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Fig. 2. The effect of the contact time on the Pb2+ removal percentages. Fig. 3. Effect of zeolites dose on the uptake of Pb ions.
Comparing reflection lines in Fig. 1a and b, one can observe that the zeolite contents in the prepared samples are proportional to the kaolinite contents in the corresponding raw kaolins.
conditions, at initial concentrations in the range 1045–5890 mg/l, the removal percentages decreased from 100% to approximately 95% in all zeolites which are not big differences. Further increase in the initial Pb2+ concentrations (from 7500 to 8300 mg/l) results in decrease in the removal percentages from approximately 80% to 60% in the removal percentages. Pb2+ sorption isotherms of the three zeolites were determined and the results are presented at Fig. 4a. As shown in Fig. 4a, for all zeolites, loading of Pb2+ increases with increasing the initial Pb2+ concentration. The prepared zeolites have satisfactory loading capacities and they are capable of sorbing Pb2+ from aqueous solution. However, at very high initial Pb2+ concentrations (8312 mg/l), the amount of loaded Pb2+ decreased. This could be attributed to the difficulty of Pb2+ cations to diffuse toward zeolite surfaces at higher concentrations. The equilibrium adsorption data of Pb2+ onto zeolites was analyzed using Langmuir and Freundlich models. The obtained experimental data were fitted with these two models (plots are not given here). Table 2 summaries the Langmuir and Freundlich isothermal parameters for the adsorption of Pb2+ on zeolites. As shown in Table 2, Langmuir isotherm was found to be linear over the whole concentration range studied with R > 0.99 suggesting that the adsorption of the Pb2+ was favorable by Langmuir isotherm model (i.e. adsorption through monolayer). On the other hand, the experimental data did not agree with Freundlich model (R < 0.85) suggesting the absence of multilayer formation. Under our experimental conditions, the maximum amounts of loaded Pb2+ estimated from Langmuir isotherms were 299.6, 299.3 and 260.6 meq/100 g for ZS, ZK and ZD, respectively. Table 3 shows the comparison of the maximum loading capacities of zeolites and other adsorbents reported in previous studies. A comparison between Table 3 and the obtained data shows that ZS, ZK and ZD had exceptionally high adsorption capacities than several natural and synthetic zeolites, and comparable loading capacities to ion exchange resins.
Removal of Pb2+ by zeolites The effect of the contact time on the loading rates of Pb2+ on the three zeolites is shown in Fig. 2. It was found that ZS was the fastest in the competitive loading of Pb2+ ions. Its loading rate reached the highest value among the other zeolites (99.2% within 1 min). Whereas, the loading rates on ZK and ZD reached 95.2% and 86.15%, respectively in 1 min. For all zeolites, the loading of Pb2+ has gradual increase with contact time until 120 min then the removal percent became constant (i.e. equilibrium is attained). Removal percents after 24 h show a slight increase percent removal (0.05– 0.1% more than removal percent after 120 min). Consequently, the contact time of all zeolites with solution was set at 120 min. The effect of the adsorbent dose on the uptake of Pb2+ is also shown in Fig. 3. The results indicate that an increase in dosage results in an increase in the Pb2+ uptake percentages; this is because with an increase in mass/dosage there is an introduction of more adsorption sites which adsorb more cations from the solution. In all zeolites, the lower optimum dose was 1.0 g since it was the lower dose at which the uptake percentage of Pb2+ reached 100%. From this figure it can be also seen that at zeolites doses (less than 1 g), the percentages of Pb2+ removal are slightly different for the three zeolites. ZD shows the lowest tendency toward Pb2+ removal while ZS shows the highest tendency. Regarding these observations, it can be concluded that at low zeolite dose, the percentage of Pb2+ uptake depends on the type of zeolite. The effect of the initial concentrations of Pb2+ on the removal efficiencies of Pb ions has been studied using relatively high concentration values (i.e. from 1045 to 8300 mg/l). Generally, the affinity of zeolite Y toward the adsorption of Pb ions is very high. Zeolite Y has a uniform pore size distribution with pore sizes in the range 0.9–1.2 nm. However, the hydrated ionic radius of Pb2+ equals to 0.4 nm. Therefore, under the given experimental
Table 2 Adsorption isotherm parameters of zeolite samples. Samples
ZS ZK ZD
Langmuir isotherm model
Freundlich isotherm model
qm (meq/100 g)
b (L/mg)
R
KF (meq/100 g)
n
R
299.6 299.3 260.6
0.049 0.049 0.102
0.9936 0.9960 0.9972
85.7 84.2 95.6
6.50 6.07 7.63
0.7379 0.8417 0.8465
[(Fig._4)TD$IG]
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Fig. 4. (a) Lead sorption isotherms for the prepared zeolites. (b) The amounts of Pb2+ loaded and amounts (solid symbols) of Na+ released (empty symbols) as a function of the initial lead concentrations for the prepared zeolites.
One of the main mechanisms involved in heavy metals sorption is ion exchange which is defined as stoichiometric replacement of one equivalent of an ion in solid phase by equivalent of another ion in liquid phase. Ion exchange reaction is also referred as out sphere complexation and does not involve formation of bonds between metal ions and zeolitic surface. Another mechanism involved in heavy metals uptake is adsorption. This mechanism is often referred to as chemisorption or inner sphere complexation [35]. Fractions of amounts of heavy metal sorbed to the zeolitic surface and exchanged with zeolite cations depend mainly on the type of heavy metal cation [1,36,37]. In order to consider these two processes, the relationship between Na+ released from zeolitic structure and Pb2+ ions removed from aqueous solution at different sorbents doses are studied. For any given initial Pb2+ concentration, the concentrations of Na+ released from the three prepared zeolites are shown in Fig. 4b.
For all zeolites, the relation between released and loaded equivalents of positive charges was not stoichiometric, for all the initial Pb2+ concentrations. For all the initial Pb2+ concentrations, during Pb2+ loading into zeolite surfaces, the amount of loaded Pb2+ was always higher than the released amount of Na+. Under our experimental conditions, the maximum amounts of released Na were 77.69, 76.51, 66.00 meq/100 g for ZS, ZK and ZD, respectively. During these experiments, the pH of the initial Pb2+ solutions was not adjusted, it was only measured. Thus, the pH was in the range from 4.5 0.2 for the lowest initial Pb2+ concentration to 4.0 0.2 for the highest initial Pb2+ concentration. The pH of the suspended zeolite solution having the same experimental w/v ratio (1 g zeolite per 50 ml of distilled water) equals to 8.5 0.2. During the experiment, pH of the Pb-zeolite mixture solutions decreased from pH 8.5 0.2 to 6.8 0.2 and to 4.2 0.2 for the lowest and the
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highest initial Pb2+ concentrations, respectively. This may indicate that the loading process may involve also precipitation of some Pb2+ cations as hydroxides. Considering the entire above findings one can conclude that the loading process involves competition between three processes: (a) precipitation of some Pb2+ cations as hydroxides, (b) ion exchange and (c) chemisorption of Pb cations on zeolite surfaces. Another series of loading experiments dealing with the effect of repetition on the loading behavior of zeolites for Pb2+ has been carried out. The initial concentration of Pb2+ solutions is 1045 mg/l, respectively. The amount of Pb2+ loaded on zeolite surfaces increases steadily upon increasing the number of repetitions. At 2+ the [(Fig._5)TD$IG] early four repetitions, Pb loading percentages are 100% for all
zeolites. However, the loading percentages decrease at further repetitions. In the case of ZD, after nine repetitions for 120 min, using fresh salt solutions for each repetition, the amount of Pb cation loading increased from 50.3 to 316 meq/100 g of zeolite. Whereas, after eleven repetitions, the amount of loaded Pb2+ increased from 50 to 357 and 351 meq/100 g of zeolites ZS and ZK, respectively (see Fig. 5a). Further repetitions for all zeolites lead to no additional Pb2+ loading. Here it worth to mention that the maximum amounts of loaded Pb2+ upon loading through successive cycles using relatively initially diluted solutions are much higher than that obtained by using concentrated solutions for one cycle. These observations have been also recognized during our work while loading zeolites with metal ions and even upon
Fig. 5. The effect of the number of repetitions on the amount of: (a) loaded Pb2+ and (b) released Na+ per 100 g of the prepared zeolites.
D.M. EL-Mekkawi, M.M. Selim / Journal of Environmental Chemical Engineering 2 (2014) 723–730 Table 3 Comparison of the maximum loading capacities with other adsorbents. Adsorbent
Maximum loading capacity (meq/100 g)
Reference
ZS ZK ZD Na-X zeolite (synthesized from Egyptian kaolin) Na-A zeolite (synthesized from Egyptian kaolin) Na-PI zeolite (synthesized from fly ash) Natural zeolite (clinoptilolite) modified Fe (III) natural zeolite (clinoptilolite) Natural zeolite-kaolin-bentonite Synthetic polyvinylcalix[4]arene resin Humified insoluble solid (HIS) HCl treated activated red mud
299.6 299.3 260.6 206.0
[10]
176.0
[10]
68.1 63.7 128.3
[30] [1] [1]
136.0 364.0 145.8 5.99
[31] [32] [33] [34]
using zeolites in the adsorption of organic adsorbates [17,18]. We suggest that loading zeolites with Pb2+ using initial low concentrated Pb2+ solutions permits the Pb2+ ions to reach the deeply inner pores and cages inside zeolites. Therefore, at each repetition, Pb2+ ions are capable to occupy more adsorption sites. Contrary, loading using initial concentrated solutions probably leads the Pb2+ ions to occupy the windows and cages located near the external surface. The adsorption of large amount of Pb2+ at the external surface may block the entrances of the inner pores which in turn inhibit further adsorption processes. On the other hand, the amount of Na+ released from all zeolites remains constant till four repetitions (21 meq/100 g for each repetition). Further repetitions lead to decrease on Na+ releasing. Under the given experimental conditions, the amount of released Na+ ions reached zero after seven repetitions in case of ZD and after 8 repetitions in case of ZS and ZK. The total amount of released Na+ ions was 130, 126 and 122 meq/100 g of ZS, ZK and ZD, respectively (see Fig. 5b). As can be seen from Fig. 5a and b, as in the previous experiments, for all zeolites the relation between released and loaded equivalents of positive charges was not stoichiometric. At the first cycles, the amount of loaded Pb2+ was similar (50 meq/ 100 g) whereas, the amounts of Na+ released was 21 meq/100 g. During the experiment, initial pH decreased from pH 8.5 0.2 to 4 0.2 after three repetitions. This may indicate that the loading process at the early repetitions involves also precipitation of some Pb2+ cations as hydroxides. At the early stages, the lower amounts of Na+ released from zeolites than the amount of loaded Pb2+ and decrease of pH indicated that ion exchange and uptake of hydroxide (only the first three cycles), along with of chemisorption of Pb2+, occurred. However after several repetitions, loading Pb2+ via both ion exchange and chemisorption steadily decreases until the loading process is driven via only chemisorption at the final stages. Conclusion In this work, efficient removal of Pb2+ has been successfully achieved by using low cost Na-Y zeolites prepared from Egyptian kaolins collected from different sources. The hydrothermal treatment of the natural kaolins with NaOH during the preparation of zeolites results in the predominance of Na+ as an exchangeable cation. The zeolite contents in the prepared samples are proportional to the kaolinite contents in the corresponding raw kaolins. The data for KS and KK approaches that of pure kaolinite. KS shows slightly higher content of kaolinite. However, the analytical composition of KD might indicate the presence of either silica-rich clay minerals and/or free quartz. Consequently, the
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order of zeolitic contents in the prepared zeolites is ZS > ZK > ZD since the zeolitic contents are proportional to the kaolinite portions in the raw kaolin samples. At low Pb2+ initial concentrations the amount of loaded Pb cations and the released Na+ are always similar for the three prepared zeolites, where the removal percentages of Pb2+ are approximately 100%. In case of loading with initial high Pb2+ concentrations or loading through successive cycles, the difference in both Pb2+ loading and Na+ exchange capacities appears. Besides, with the same sense, at low zeolite dose, the percentage of Pb2+ uptake depends on the type of zeolite where the removal percentages of Pb2+ are less than 100%. The order of Pb2+ loading and Na+ release is then ZS > ZK > ZD, which is in accordance with the zeolitic contents in each sample. The maximum amounts of loaded Pb2+ and released Na+ upon loading through successive cycles using relatively initially diluted solutions are much higher than that obtained by using concentrated solutions for one cycle. Comparison with previous studies shows that the maximum loading capacities of zeolites are higher than the commonly used natural and synthetic zeolites; and it is also comparable to ion exchange resins. For all zeolites, in Pb2+ loading experiments, the relation between released and loaded equivalents of positive charges was not stoichiometric. One can conclude that the loading process involves competition between three processes: (a) precipitation of some Pb2+ cations as hydroxides, (b) ion exchange and (c) chemisorption of Pb cations on zeolite surfaces. During Pb2+ loading processes, the amount of loaded Pb2+ is always higher than the released Na+ suggesting the predominance of the chemisorption mechanism. References [1] M. Kragovic, A. Dakovic, Z. Sekulic, M. Trgo, M. Ugrina, J. Peric, G.D. Gattac, Removal of lead from aqueous solutions by using the natural and Fe(III)-modified zeolite, Appl. Surf. Sci. 258 (2012) 3667–3673. [2] S. Ahmed, S. Chughtai, M.A. Keane, The removal of cadmium and lead from aqueous solution by ion exchange with Na-Y zeolite, Sep. Purif. Technol. 13 (1998) 57–64. [3] M. Panayotova, B. Velikov, Kinetics of heavy metal ions removal by use of natural zeolite, J. Environ. Sci. Health A 37 (2002) 139–147. [4] A. Cincotti, N. Lai, R. Orru, G. Cao Sardinian, Natural clinoptilolites for heavy metals and ammonium removal: experimental and modeling, Chem. Eng. J. 84 (2001) 275–282. [5] V. Inglezakis, M. Loizidou, H. Grigoropoulou, Equilibrium and kinetic ion exchange studies of Pb2+, Cr3+, Fe3+ and Cu2+ on natural clinoptilolite, Water Res. 36 (2002) 2784–2792. [6] M. Kragovic, A. Dakovic, M. Markovic´, J. Krstic´, G.D. Gatta, N. Rotiroti, Characterization of lead sorption by the natural and Fe(III)-modified zeolite, Appl. Surf. Sci. 283 (2013) 764–774. [7] G.Sh. Sultanbayeva, R. Holze, R.M. Chernyakova, U.Zh. Jussipbekov, Removal of Fe2+-, Cu2+-, Al3+- and Pb2+-ions from phosphoric acid by sorption on carbonatemodified natural zeolite and its mixture with bentonite, Microporous Mesoporous Mater. 170 (2013) 173–180. [8] H. Liu, S. Peng, L. Shu, T. Chen, T. Bao, R.L. Frost, Magnetic zeolite NaA: synthesis, characterization based on metakaolin and its application for the removal of Cu2+, Pb2+, Chemosphere 91 (2013) 1539–1546. [9] T.S. Jamil, H.S. Ibrahim, I.H. Abd El-Maksoud, S.T. El-Wakeel, Application of zeolite prepared from Egyptian kaolin for removal of heavy metals: I. Optimum conditions, Desalination 258 (2010) 34–40. [10] H.S. Ibrahim, T.S. Jamil, E.Z. Hegazy, Application of zeolite prepared from Egyptian kaolin for the removal of heavy metals: II. Isotherm models, J. Hazard. Mater. 182 (2010) 842–847. [11] J.F. O’Connor, R.P. Townsend, Exchange of lead (II) ions in synthetic faujasitic zeolites: the effect of framework charge, Zeolites 5 (1985) 158–164. [12] W. Qiu, Y. Zheng, Removal of lead, copper, nickel, cobalt, and zinc from water by a cancrinite-type zeolite synthesized from fly ash, Chem. Eng. J. 145 (2009) 483–488. [13] M. Turan, U. Mart, B. Yu¨ksel, M.S. C¸elik, Lead removal in fixed-bed columns by zeolite and sepiolite, Chemosphere 60 (2005) 1487–1492. [14] P. Castaldi, L. Santona, S. Enzo, P. Melis, Sorption processes and XRD analysis of a natural zeolite exchanged with Pb2+, Cd2+ and Zn2+ cations, J. Hazard. Mater. 156 (2008) 428–434. [15] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular sieves, Academic Press, London, 1978.
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Removal of Pb2+ from water by using Na-Y zeolites prepared from Egyptian kaolins collected from different sources Doaa M. EL-Mekkawi *, Mohamed M. Selim Physical Chemistry Department, National Research Center, Dokki, Cairo, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 May 2013 Received in revised form 9 November 2013 Accepted 12 November 2013
This article aims to study the effect of the chemical composition of the Egyptian kaolins on the performance of the low cost kaolin-based zeolites toward Pb2+ removal. In this work, different Egyptian kaolins collected from Saint Catherine, Kalabsha and Dehessa (KS, KK and KD, respectively) were used in the preparation of the corresponding Na-Y zeolites (ZS, ZK and ZD). X-ray Fluorescence (XRF), X-ray diffraction (XRD) and atomic absorption analyses were recorded. XRF and XRD indicate that the zeolitic contents in the prepared samples are proportional to the kaolinite contents in the corresponding kaolins. The difference in the removal efficiency between zeolites depends on the experimental conditions. Generally, the order of Pb2+ loading and Na+ release is ZS > ZK > ZD, which is in accordance with the zeolitic contents in each sample. From sorption isotherms, maximum sorbed amounts of Pb2+ were 299.6, 299.3 and 260.6 meq/100 g for ZS, ZK and ZD, respectively. Comparison with previous studies shows that the maximum loading capacities of zeolites are higher than the commonly used; and it is also comparable to ion exchange resins. For all zeolites, in Pb2+ loading experiments, the relation between released and loaded equivalents of positive charges was not stoichiometric. We concluded that the loading process involves competition between three processes: (a) precipitation of some Pb2+as hydroxides, (b) ion exchange and (c) chemisorption of Pb2+on zeolite surfaces. During Pb2+ loading processes, the equivalent amount of loaded Pb2+ is always higher than that of the released Na+ suggesting the predominance of the chemisorption mechanism. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Zeolite Y Egyptian kaolin Pb2+ Sorption Heavy metal Ion exchange
Introduction Removal of Pb2+ from aqueous solutions using zeolites has been widely introduced as an efficient method for the treatment of wastewater contaminated with this highly toxic inorganic
Abbreviations: ZS, Zeolite prepared from Egyptian kaolin collected from Saint Catherine; ZK, Zeolite prepared from Egyptian kaolin collected from Kalabsha; ZD, Zeolite prepared from Egyptian kaolin collected from Dehessa; KS, Egyptian kaolin collected from Saint Catherine; KK, Egyptian kaolin collected from Kalabsha; KD, Egyptian kaolin collected from Dehessa; XRF, X-ray fluorescence; XRD, X-ray diffraction; AAS, atomic absorption spectrophotometer; ZFA, cancrinite-type zeolite; MEL, maximum exchange level; qe, equivalent amounts of adsorbed Pb2+ per 100 grams of adsorbent (meq/100 g); Ce, unadsorbed Pb2+ concentration in solution at equilibrium (mg/l); qm, maximum amount of the Pb2+ bound per unit weight of adsorbent to form a complete monolayer on the surface at high Ce (meq/ 100 g); b, constant related to the affinity of the binding sites (l/mg); KF, empirical constant that refers to the relative adsorption capacity; n, adsorption intensity under different experimental conditions. * Corresponding author. Tel.: +20 1224086609. E-mail addresses: [email protected], [email protected] (D.M. EL-Mekkawi), [email protected] (M.M. Selim). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.11.014
pollutant [1–10]. For example, Kragovic et al. studied the sorption of Pb2+ by the natural and Fe(III)-modified zeolite (clinoptilolite) [1]. They found that at lower initial concentrations of Pb2+, ion exchange of inorganic cations in zeolites with Pb2+, together with uptake of hydrogen dominated, while at higher initial Pb2+ concentrations beside these processes, chemisorption of Pb2+ occurred. O’Connor and Townsend [11] examined the ionexchange properties of Pb2+ ions in a range of synthetic sodium faujasites of varying framework silica to alumina ratios. Ionexchange isotherms were constructed for all systems, but nonstoichiometric behavior was observed in all cases. They found that the more siliceous the zeolite, the lower the preference for Pb2+. Qiu and Zheng prepared a cancrinite-type zeolite (ZFA) from Class C fly ash via the molten-salt method [12]. They studied the adsorption equilibriums of Pb2+, Cu2+, Ni2+, Co2+, and Zn2+ on ZFA in aqueous solutions. The increase of pH levels during the adsorption process suggests that the uptake of heavy metals on ZFA was subjected to an ion exchange mechanism. It is found that the maximum exchange level (MEL) follows the order: Pb2+ > Cu2+ > Ni2+ > Co2+ > Zn2+. The removal efficiency of zeolite (clinoptilolite) and sepiolite from Pb2+ containing aqueous
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solutions was investigated by Turan et al. [13]. The removal of Pb2+ is found to be a sum of adsorption induced by ion exchange and precipitation of lead hydroxide. Castaldi et al. evaluated Pb2+, Cd2+ and Zn2+ adsorption capacity of a natural zeolite in batch tests [14]. The zeolite adsorption capacity for the three cations was Zn > Pb > Cd. The main mineralogical phase involved in the adsorption process was clinoptilolite. Synthetic aluminosilicate zeolites act as efficient porous solid exchange media where the ‘‘compensating’’ zeolitic cations (normally alkali metal ions) are not rigidly fixed at specific locations within the hydrated unit cell and readily exchange with external cations in solution. The application of the large pore high surface area zeolite Y, (Na58Al58Si134O384260H2O) to the removal of Pb2+, from water has been reported previously [2]. Clay minerals have been used as a combined source for A12O3 and SiO2 for the synthesis of zeolites [15,16]. Kaolinite (A12O32SiO22H2O) with a Si/A1 ratio of 1, is the principal mineral in clay and is ideally suited for the synthesis of zeolites [17]. Since clays are produced from natural deposits formed during various long geological processes, they are complex mixtures of different minerals. Besides kaolinite, they usually contain some quantities of quartz, anatase, feldspars, smectites, micas, etc. [18,19]. Previously, we carried out many studies with the faujasite zeolites prepared from Egyptian kaolins. These studies almost concerned the utilization of zeolites in various applications having environmental, industrial or medical concerns, e.g., reduction of nitrophenols [20,21] preparation of nanosized spinels [22] removal of heavy metals and organic pollutants from waters [23], etc. The relations between the chemical composition of clay and the performance of the prepared zeolites were not extensively studied. The necessity of better understanding of the Egyptian kaolins’ properties and their appropriateness for the synthesis of zeolites according to their applications becomes our main concern. Comparing the cost of commercially available faujasite zeolites and prepared kaolin-based zeolites according to our preparation methods, feasibility studies indicate that the later costs 200 $ per ton which is approximately 15 times cheaper than the former (see Supporting Information, Table S1). Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.jece.2013.11.014. The aim of the present article is to assess the effect of the chemical composition of the Egyptian kaolins, for the preparation of zeolite Y, on the performance of the prepared zeolites toward Pb2+ removal from simulated wastewater. In the present study, we introduce comparative analyses of the Pb2+ loading capacities of zeolite Y samples prepared from different Egyptian kaolins. The influence of the zeolite dose, the initial Pb2+ concentrations and the number of loading cycles on loading of Pb2+ from aqueous solution by each zeolite was investigated. Experimental Zeolites preparation Three Egyptian kaolins, KS, KD and KK, were obtained from three different areas in Egypt. KS and KD were collected from two different areas at Sinai Peninsula (Saint Catherine and Dehessa), which is considered as the main source of kaolin in Egypt. On the other hand, KK was collected from Kalabsha at the Upper Egypt. The corresponding Y zeolites; ZK, ZD and ZS; were prepared by hydrothermal treatment of the Egyptian kaolins with alkali solution and sodium silicate at 100–120 8C for 8 h according to the submitted Egyptian Patent No. (165/2008).
Table 1 Chemical composition of raw kaolins and the prepared zeolites (in mass%). Oxide
KD
KK
KS
ZD
ZK
ZS
SiO2 TiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O L.O.I.
49.97 2.73 33.83 0.79 0.01 0.09 0.42 0.01 11.73
45.00 3.50 35.00 1.50 0.10 0.60 0.30 0.05 14.00
47.00 1.30 37.00 0.20 0.02 0.20 0.15 0.04 13.4
41.94 2.04 22.14 0.52 0.10 0.37 13.60 0.10 18.87
39.30 2.25 21.68 0.25 0.22 1.36 13.24 0.08 21.28
40.44 0.84 22.34 0.18 0.11 0.64 13.21 0.09 21.57
Samples characterization The chemical compositions of natural Egyptian kaolins and the prepared zeolites are determined using X-ray fluorescence (see Table 1). (XRF Axios, sequential WD_XRF spectrometer, PANalytical 2005.) X-ray diffractograms of kaolin and zeolite samples were obtained using a Bruker D8 advance instrument with CuKa1 target with secondary monochromator 40 kV, 40 mA. Pb2+ loading experiments The Pb2+ loading experimental studies were conducted using synthetic solutions prepared using distilled water with constant metal concentration for each of Pb2+. The uptake of Pb2+on the three prepared zeolites was carried out using the batch method at room temperature (25 0.1 8C) in 150-ml glass bottles. The bottles were shaken in a rotary shaker at 269 rpm from 5 min to 24 h. The influence of zeolites dose on loading of Pb2+ at the constant initial Pb2+ concentration – 3931 mg/l (Pb(NO3)2, p.a., Aldrich) was studied with the following protocol: in 50 ml of Pb solution, 0.02, 0.2, 0.4, 0.6, 1, 1.2 g of either the ZD, ZS, or ZK zeolite was added and the mixtures were shaken at room temperature for 2 h. To study the effect of the initial Pb2+ concentrations, 1 g of each zeolite was mixed with 50 ml of aqueous solutions, containing various initial concentrations of Pb(NO3)2 (1045–8312 mg/l) for a period of 2 h. Another series of loading experiments dealing with the effect of repetition on the loading behavior of zeolite Y for Pb2+ has been carried out. The initial concentration of Pb2+ solution is 1045 mg/l and fresh salt solution is used for each repetition. In all Pb2+ loading experiments, after the reaction time, suspensions were centrifuged 2 times at 5500 rpm for 10 min to separate the solution and solid. The initial and non-loaded concentrations of Pb2+ in supernatants were determined by the atomic absorption spectrophotometer (AAS). The concentrations of Ca, Mg, Na, Ti, K and Fe in supernatants released from both zeolites were determined by AAS (Thermo Elemental, Solar S4, Atomic Absorption Spectrophotometer). Only Na+ cations are found during the exchange process. In order to study the adsorption behavior of Pb2+ on zeolite surfaces, the Langmuir isotherm and Freundlich isotherm models were verified. The Langmuir and Freundlich equations are commonly used to describe adsorption isotherms at a constant temperature for water and wastewater treatment applications. The Langmuir model is valid for monolayer sorption onto a surface with a finite number of identical sites. The distribution of Pb2+ ions between the solid solution interface equilibrium has been described by the Langmuir equation [24,25]. The well known expression of the Langmuir model is given by Eq. (1): C e =qe ¼ 1=qm b þ C e =qm
(1)
D.M. EL-Mekkawi, M.M. Selim / Journal of Environmental Chemical Engineering 2 (2014) 723–730
where qe (meq/100 g) and Ce (mg/l) are the equivalent amounts of adsorbed Pb2+ per 100 grams of adsorbent and unadsorbed Pb2+ concentration in solution at equilibrium, respectively. qm is the maximum amount of the Pb2+ bound per unit weight of adsorbent to form a complete monolayer on the surface at high Ce, and b is a constant related to the affinity of the binding sites (l/mg). Freundlich model explains the relation between qe and equilibrium Pb2+ concentration Eq. (2): ln qe ¼ ln K F þ 1=n ln C e
(2)
The Freundlich adsorption isotherm usually is applied to the highly heterogeneous surfaces [26,27]. The empirical constant KF refers to the relative adsorption capacity and ‘n’ represents adsorption intensity under different experimental conditions. Results and discussion Characterization of the raw kaolins and the prepared zeolites X-ray diffractograms of the natural kaolin and synthetic zeolites are shown in Fig. 1a and b. The figures indicate the presence of
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kaolinite and quartz as the crystalline phases in all kaolin samples with some minor anatase. These results are confirmed by the XRF results (Table 1) where the main constituents of the starting raw materials are Al2O3 and SiO2 in addition to 2.73%, 3.50% and 1.30% of TiO2 in KD, KK and KS, respectively. All kaolin samples KS, KK, and KD have relatively low amounts of SiO2, and high amounts of Al2O3 with minute amounts of alkali and alkaline earth oxides, typical of kaolins [28]. The slightly higher Al2O3 content in KS and KK indicates its higher kaolinite content. The silica to alumina ratios of KS, KK, and KD are 1.27, 1.29 and 1.48. The data for KS and KK approaches that of pure kaolinite (1.18) [29], while the analytical composition of KD might indicate the presence of either silica-rich clay minerals and/or free quartz. The quartz reflections in KD were more significant in agreement with its higher SiO2 content. On the other hand, the prepared zeolites contain a significant amount of Na, if compared to Ca, K, Fe and Mg. The hydrothermal treatment of the natural kaolins with NaOH during the preparation of zeolites results in the high Na content. Besides, the treatment of kaolins under strongly basic conditions leads to a significant change in Si and Al content in the corresponding zeolite samples.
[(Fig._1)TD$IG]
Fig. 1. X-ray difractograms of: (a) raw kaolins and (b) prepared zeolites [A = anatase, k = kaolinite, q = quartz, z = zeolite].
[(Fig._2)TD$IG]
[(Fig._3)TD$IG]
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Fig. 2. The effect of the contact time on the Pb2+ removal percentages. Fig. 3. Effect of zeolites dose on the uptake of Pb ions.
Comparing reflection lines in Fig. 1a and b, one can observe that the zeolite contents in the prepared samples are proportional to the kaolinite contents in the corresponding raw kaolins.
conditions, at initial concentrations in the range 1045–5890 mg/l, the removal percentages decreased from 100% to approximately 95% in all zeolites which are not big differences. Further increase in the initial Pb2+ concentrations (from 7500 to 8300 mg/l) results in decrease in the removal percentages from approximately 80% to 60% in the removal percentages. Pb2+ sorption isotherms of the three zeolites were determined and the results are presented at Fig. 4a. As shown in Fig. 4a, for all zeolites, loading of Pb2+ increases with increasing the initial Pb2+ concentration. The prepared zeolites have satisfactory loading capacities and they are capable of sorbing Pb2+ from aqueous solution. However, at very high initial Pb2+ concentrations (8312 mg/l), the amount of loaded Pb2+ decreased. This could be attributed to the difficulty of Pb2+ cations to diffuse toward zeolite surfaces at higher concentrations. The equilibrium adsorption data of Pb2+ onto zeolites was analyzed using Langmuir and Freundlich models. The obtained experimental data were fitted with these two models (plots are not given here). Table 2 summaries the Langmuir and Freundlich isothermal parameters for the adsorption of Pb2+ on zeolites. As shown in Table 2, Langmuir isotherm was found to be linear over the whole concentration range studied with R > 0.99 suggesting that the adsorption of the Pb2+ was favorable by Langmuir isotherm model (i.e. adsorption through monolayer). On the other hand, the experimental data did not agree with Freundlich model (R < 0.85) suggesting the absence of multilayer formation. Under our experimental conditions, the maximum amounts of loaded Pb2+ estimated from Langmuir isotherms were 299.6, 299.3 and 260.6 meq/100 g for ZS, ZK and ZD, respectively. Table 3 shows the comparison of the maximum loading capacities of zeolites and other adsorbents reported in previous studies. A comparison between Table 3 and the obtained data shows that ZS, ZK and ZD had exceptionally high adsorption capacities than several natural and synthetic zeolites, and comparable loading capacities to ion exchange resins.
Removal of Pb2+ by zeolites The effect of the contact time on the loading rates of Pb2+ on the three zeolites is shown in Fig. 2. It was found that ZS was the fastest in the competitive loading of Pb2+ ions. Its loading rate reached the highest value among the other zeolites (99.2% within 1 min). Whereas, the loading rates on ZK and ZD reached 95.2% and 86.15%, respectively in 1 min. For all zeolites, the loading of Pb2+ has gradual increase with contact time until 120 min then the removal percent became constant (i.e. equilibrium is attained). Removal percents after 24 h show a slight increase percent removal (0.05– 0.1% more than removal percent after 120 min). Consequently, the contact time of all zeolites with solution was set at 120 min. The effect of the adsorbent dose on the uptake of Pb2+ is also shown in Fig. 3. The results indicate that an increase in dosage results in an increase in the Pb2+ uptake percentages; this is because with an increase in mass/dosage there is an introduction of more adsorption sites which adsorb more cations from the solution. In all zeolites, the lower optimum dose was 1.0 g since it was the lower dose at which the uptake percentage of Pb2+ reached 100%. From this figure it can be also seen that at zeolites doses (less than 1 g), the percentages of Pb2+ removal are slightly different for the three zeolites. ZD shows the lowest tendency toward Pb2+ removal while ZS shows the highest tendency. Regarding these observations, it can be concluded that at low zeolite dose, the percentage of Pb2+ uptake depends on the type of zeolite. The effect of the initial concentrations of Pb2+ on the removal efficiencies of Pb ions has been studied using relatively high concentration values (i.e. from 1045 to 8300 mg/l). Generally, the affinity of zeolite Y toward the adsorption of Pb ions is very high. Zeolite Y has a uniform pore size distribution with pore sizes in the range 0.9–1.2 nm. However, the hydrated ionic radius of Pb2+ equals to 0.4 nm. Therefore, under the given experimental
Table 2 Adsorption isotherm parameters of zeolite samples. Samples
ZS ZK ZD
Langmuir isotherm model
Freundlich isotherm model
qm (meq/100 g)
b (L/mg)
R
KF (meq/100 g)
n
R
299.6 299.3 260.6
0.049 0.049 0.102
0.9936 0.9960 0.9972
85.7 84.2 95.6
6.50 6.07 7.63
0.7379 0.8417 0.8465
[(Fig._4)TD$IG]
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Fig. 4. (a) Lead sorption isotherms for the prepared zeolites. (b) The amounts of Pb2+ loaded and amounts (solid symbols) of Na+ released (empty symbols) as a function of the initial lead concentrations for the prepared zeolites.
One of the main mechanisms involved in heavy metals sorption is ion exchange which is defined as stoichiometric replacement of one equivalent of an ion in solid phase by equivalent of another ion in liquid phase. Ion exchange reaction is also referred as out sphere complexation and does not involve formation of bonds between metal ions and zeolitic surface. Another mechanism involved in heavy metals uptake is adsorption. This mechanism is often referred to as chemisorption or inner sphere complexation [35]. Fractions of amounts of heavy metal sorbed to the zeolitic surface and exchanged with zeolite cations depend mainly on the type of heavy metal cation [1,36,37]. In order to consider these two processes, the relationship between Na+ released from zeolitic structure and Pb2+ ions removed from aqueous solution at different sorbents doses are studied. For any given initial Pb2+ concentration, the concentrations of Na+ released from the three prepared zeolites are shown in Fig. 4b.
For all zeolites, the relation between released and loaded equivalents of positive charges was not stoichiometric, for all the initial Pb2+ concentrations. For all the initial Pb2+ concentrations, during Pb2+ loading into zeolite surfaces, the amount of loaded Pb2+ was always higher than the released amount of Na+. Under our experimental conditions, the maximum amounts of released Na were 77.69, 76.51, 66.00 meq/100 g for ZS, ZK and ZD, respectively. During these experiments, the pH of the initial Pb2+ solutions was not adjusted, it was only measured. Thus, the pH was in the range from 4.5 0.2 for the lowest initial Pb2+ concentration to 4.0 0.2 for the highest initial Pb2+ concentration. The pH of the suspended zeolite solution having the same experimental w/v ratio (1 g zeolite per 50 ml of distilled water) equals to 8.5 0.2. During the experiment, pH of the Pb-zeolite mixture solutions decreased from pH 8.5 0.2 to 6.8 0.2 and to 4.2 0.2 for the lowest and the
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highest initial Pb2+ concentrations, respectively. This may indicate that the loading process may involve also precipitation of some Pb2+ cations as hydroxides. Considering the entire above findings one can conclude that the loading process involves competition between three processes: (a) precipitation of some Pb2+ cations as hydroxides, (b) ion exchange and (c) chemisorption of Pb cations on zeolite surfaces. Another series of loading experiments dealing with the effect of repetition on the loading behavior of zeolites for Pb2+ has been carried out. The initial concentration of Pb2+ solutions is 1045 mg/l, respectively. The amount of Pb2+ loaded on zeolite surfaces increases steadily upon increasing the number of repetitions. At 2+ the [(Fig._5)TD$IG] early four repetitions, Pb loading percentages are 100% for all
zeolites. However, the loading percentages decrease at further repetitions. In the case of ZD, after nine repetitions for 120 min, using fresh salt solutions for each repetition, the amount of Pb cation loading increased from 50.3 to 316 meq/100 g of zeolite. Whereas, after eleven repetitions, the amount of loaded Pb2+ increased from 50 to 357 and 351 meq/100 g of zeolites ZS and ZK, respectively (see Fig. 5a). Further repetitions for all zeolites lead to no additional Pb2+ loading. Here it worth to mention that the maximum amounts of loaded Pb2+ upon loading through successive cycles using relatively initially diluted solutions are much higher than that obtained by using concentrated solutions for one cycle. These observations have been also recognized during our work while loading zeolites with metal ions and even upon
Fig. 5. The effect of the number of repetitions on the amount of: (a) loaded Pb2+ and (b) released Na+ per 100 g of the prepared zeolites.
D.M. EL-Mekkawi, M.M. Selim / Journal of Environmental Chemical Engineering 2 (2014) 723–730 Table 3 Comparison of the maximum loading capacities with other adsorbents. Adsorbent
Maximum loading capacity (meq/100 g)
Reference
ZS ZK ZD Na-X zeolite (synthesized from Egyptian kaolin) Na-A zeolite (synthesized from Egyptian kaolin) Na-PI zeolite (synthesized from fly ash) Natural zeolite (clinoptilolite) modified Fe (III) natural zeolite (clinoptilolite) Natural zeolite-kaolin-bentonite Synthetic polyvinylcalix[4]arene resin Humified insoluble solid (HIS) HCl treated activated red mud
299.6 299.3 260.6 206.0
[10]
176.0
[10]
68.1 63.7 128.3
[30] [1] [1]
136.0 364.0 145.8 5.99
[31] [32] [33] [34]
using zeolites in the adsorption of organic adsorbates [17,18]. We suggest that loading zeolites with Pb2+ using initial low concentrated Pb2+ solutions permits the Pb2+ ions to reach the deeply inner pores and cages inside zeolites. Therefore, at each repetition, Pb2+ ions are capable to occupy more adsorption sites. Contrary, loading using initial concentrated solutions probably leads the Pb2+ ions to occupy the windows and cages located near the external surface. The adsorption of large amount of Pb2+ at the external surface may block the entrances of the inner pores which in turn inhibit further adsorption processes. On the other hand, the amount of Na+ released from all zeolites remains constant till four repetitions (21 meq/100 g for each repetition). Further repetitions lead to decrease on Na+ releasing. Under the given experimental conditions, the amount of released Na+ ions reached zero after seven repetitions in case of ZD and after 8 repetitions in case of ZS and ZK. The total amount of released Na+ ions was 130, 126 and 122 meq/100 g of ZS, ZK and ZD, respectively (see Fig. 5b). As can be seen from Fig. 5a and b, as in the previous experiments, for all zeolites the relation between released and loaded equivalents of positive charges was not stoichiometric. At the first cycles, the amount of loaded Pb2+ was similar (50 meq/ 100 g) whereas, the amounts of Na+ released was 21 meq/100 g. During the experiment, initial pH decreased from pH 8.5 0.2 to 4 0.2 after three repetitions. This may indicate that the loading process at the early repetitions involves also precipitation of some Pb2+ cations as hydroxides. At the early stages, the lower amounts of Na+ released from zeolites than the amount of loaded Pb2+ and decrease of pH indicated that ion exchange and uptake of hydroxide (only the first three cycles), along with of chemisorption of Pb2+, occurred. However after several repetitions, loading Pb2+ via both ion exchange and chemisorption steadily decreases until the loading process is driven via only chemisorption at the final stages. Conclusion In this work, efficient removal of Pb2+ has been successfully achieved by using low cost Na-Y zeolites prepared from Egyptian kaolins collected from different sources. The hydrothermal treatment of the natural kaolins with NaOH during the preparation of zeolites results in the predominance of Na+ as an exchangeable cation. The zeolite contents in the prepared samples are proportional to the kaolinite contents in the corresponding raw kaolins. The data for KS and KK approaches that of pure kaolinite. KS shows slightly higher content of kaolinite. However, the analytical composition of KD might indicate the presence of either silica-rich clay minerals and/or free quartz. Consequently, the
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order of zeolitic contents in the prepared zeolites is ZS > ZK > ZD since the zeolitic contents are proportional to the kaolinite portions in the raw kaolin samples. At low Pb2+ initial concentrations the amount of loaded Pb cations and the released Na+ are always similar for the three prepared zeolites, where the removal percentages of Pb2+ are approximately 100%. In case of loading with initial high Pb2+ concentrations or loading through successive cycles, the difference in both Pb2+ loading and Na+ exchange capacities appears. Besides, with the same sense, at low zeolite dose, the percentage of Pb2+ uptake depends on the type of zeolite where the removal percentages of Pb2+ are less than 100%. The order of Pb2+ loading and Na+ release is then ZS > ZK > ZD, which is in accordance with the zeolitic contents in each sample. The maximum amounts of loaded Pb2+ and released Na+ upon loading through successive cycles using relatively initially diluted solutions are much higher than that obtained by using concentrated solutions for one cycle. Comparison with previous studies shows that the maximum loading capacities of zeolites are higher than the commonly used natural and synthetic zeolites; and it is also comparable to ion exchange resins. For all zeolites, in Pb2+ loading experiments, the relation between released and loaded equivalents of positive charges was not stoichiometric. One can conclude that the loading process involves competition between three processes: (a) precipitation of some Pb2+ cations as hydroxides, (b) ion exchange and (c) chemisorption of Pb cations on zeolite surfaces. During Pb2+ loading processes, the amount of loaded Pb2+ is always higher than the released Na+ suggesting the predominance of the chemisorption mechanism. References [1] M. Kragovic, A. Dakovic, Z. Sekulic, M. Trgo, M. Ugrina, J. Peric, G.D. Gattac, Removal of lead from aqueous solutions by using the natural and Fe(III)-modified zeolite, Appl. Surf. Sci. 258 (2012) 3667–3673. [2] S. Ahmed, S. Chughtai, M.A. Keane, The removal of cadmium and lead from aqueous solution by ion exchange with Na-Y zeolite, Sep. Purif. Technol. 13 (1998) 57–64. [3] M. Panayotova, B. Velikov, Kinetics of heavy metal ions removal by use of natural zeolite, J. Environ. Sci. Health A 37 (2002) 139–147. [4] A. Cincotti, N. Lai, R. Orru, G. Cao Sardinian, Natural clinoptilolites for heavy metals and ammonium removal: experimental and modeling, Chem. Eng. J. 84 (2001) 275–282. [5] V. Inglezakis, M. Loizidou, H. Grigoropoulou, Equilibrium and kinetic ion exchange studies of Pb2+, Cr3+, Fe3+ and Cu2+ on natural clinoptilolite, Water Res. 36 (2002) 2784–2792. [6] M. Kragovic, A. Dakovic, M. Markovic´, J. Krstic´, G.D. Gatta, N. Rotiroti, Characterization of lead sorption by the natural and Fe(III)-modified zeolite, Appl. Surf. Sci. 283 (2013) 764–774. [7] G.Sh. Sultanbayeva, R. Holze, R.M. Chernyakova, U.Zh. Jussipbekov, Removal of Fe2+-, Cu2+-, Al3+- and Pb2+-ions from phosphoric acid by sorption on carbonatemodified natural zeolite and its mixture with bentonite, Microporous Mesoporous Mater. 170 (2013) 173–180. [8] H. Liu, S. Peng, L. Shu, T. Chen, T. Bao, R.L. Frost, Magnetic zeolite NaA: synthesis, characterization based on metakaolin and its application for the removal of Cu2+, Pb2+, Chemosphere 91 (2013) 1539–1546. [9] T.S. Jamil, H.S. Ibrahim, I.H. Abd El-Maksoud, S.T. El-Wakeel, Application of zeolite prepared from Egyptian kaolin for removal of heavy metals: I. Optimum conditions, Desalination 258 (2010) 34–40. [10] H.S. Ibrahim, T.S. Jamil, E.Z. Hegazy, Application of zeolite prepared from Egyptian kaolin for the removal of heavy metals: II. Isotherm models, J. Hazard. Mater. 182 (2010) 842–847. [11] J.F. O’Connor, R.P. Townsend, Exchange of lead (II) ions in synthetic faujasitic zeolites: the effect of framework charge, Zeolites 5 (1985) 158–164. [12] W. Qiu, Y. Zheng, Removal of lead, copper, nickel, cobalt, and zinc from water by a cancrinite-type zeolite synthesized from fly ash, Chem. Eng. J. 145 (2009) 483–488. [13] M. Turan, U. Mart, B. Yu¨ksel, M.S. C¸elik, Lead removal in fixed-bed columns by zeolite and sepiolite, Chemosphere 60 (2005) 1487–1492. [14] P. Castaldi, L. Santona, S. Enzo, P. Melis, Sorption processes and XRD analysis of a natural zeolite exchanged with Pb2+, Cd2+ and Zn2+ cations, J. Hazard. Mater. 156 (2008) 428–434. [15] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular sieves, Academic Press, London, 1978.
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