Adsorption and Thermal Stabilization of Pb2+ and Cu2+ by Zeolite

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Adsorption and Thermal Stabilization of Pb2+ and Cu2+ by Zeolite Xingwen Lu, Fei Wang, Xiao-yan Li, Kaimin Shih, and Eddy Y. Zeng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00896 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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Industrial & Engineering Chemistry Research

Adsorption and Thermal Stabilization of Pb2+ and Cu2+ by Zeolite

Xingwen Lua,b, Fei Wangc,b,d *, Xiao-yan Lib, Kaimin Shihb, and Eddy Y. Zengc a

Institute of Environmental Health and Pollution Control, and School of Environmental

Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China b

Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong

c

School of Environment, Guangzhou Key Laboratory of Environmental Exposure and Health,

and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, China d

Guangdong Provincial Key Laboratory of Environmental Pollution Control and

Remediation Technology, Guangzhou 510275, China

*Corresponding Author. Tel: +86-18898607769; E-mail address: [email protected]

ABSTRACT In recent years, zeolite has been proven to be a cost-effective adsorbent for treating heavy metal ions-containing wastewater. However, few studies have investigated the subsequent treatment of metals-laden zeolites after adsorption, which may become a source of secondary pollution if not properly dealt with. To fill the knowledge gap, the adsorption behavior of Pb2+ and Cu2+ on zeolite was examined in the present study. The adsorption process was fast and reached equilibrium within 60 min, with adsorption data fitted better with Langmuir equation than Freundlich equation. After thermal treatment for metaladsorbed zeolites, XRD results showed that Pb2+ and Cu2+ adsorbed on zeolites can be readily stabilized into PbAl2Si2O8 and CuAl2O4, and the best temperatures for formation of

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PbAl2Si2O8 and CuAl2O4 were 950 and 1000 °C, respectively. Finally, a prolonged leaching test (at pH 2.9 for 22 d) demonstrated that the Pb2+ and Cu2+ concentrations in the leachate of calcined samples were approximately two orders of magnitude lower than those of untreated samples, validating the high stability of treated zeolites. All these findings pointed to the potential for use of zeolites as a promising and sustainable strategy to treat and stabilize Pb2+ and Cu2+ in wastewater. Keywords: Heavy metal ion; Zeolite; Adsorption; Stabilization; Leaching. 1.

Introduction Rapid industrialization and intensive human activities have generated large amounts of

metal-laden wastewater. Lead and copper are two heavy metal ions commonly found in industrial wastewaters from metal plating, mining, smelting, and battery manufacturing, etc. 1. Since most metals are not biodegradable and can be accumulated to cause diseases and disorders of human beings 2, metal contaminated water poses great environmental and human health concerns. The potential of environmental pollution motivate the researchers to investigate the technologies of removing heavy metal ions from wastewater. Numerous techniques have been developed and used to remove heavy metal ions from wastewater, such as chemical precipitation, ion-exchange, chemical coagulation, membrane processes, and adsorption 3-6. Among these techniques, adsorption is recognized as one of the simplest and most effective method due to its low initial cost, simplicity of design, and easiness of operation 7.

The most commonly used solid adsorbent is activated carbon, which can be applied in various types of wastewater. For heavy metal ion containing wastewater, activated carbon sometimes performs poorly and requires chelating agents to enhance its performance 8. This would greatly increase the cost of wastewater treatment. Therefore, searching for other alternative 2 Environment ACS Paragon Plus

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low-cost and effective adsorbents is significant in an effort to reduce wastewater-treatment cost 9-13. Zeolite is one potential inexpensive adsorbent that is widely used in water treatment. It is an abundant source of aluminosilicate with a three dimensional framework consisting of silica and alumina tetrahedral units linked by shared oxygen atoms 14. The porous structure of zeolite allows it to accommodate a wide variety of cations, such as Na+, K+, Ca2+, and Mg2+. These cations are rather loosely held and can readily be exchanged for heavy metal ions in a contact solution. Previous studies have recognized zeolite as an effective adsorbent for removal of various heavy metal ions 10, 13-17. On the other hand, if used zeolites are not properly treated, adsorbed heavy metal ions would desorb and leach out to cause secondary pollution. So far, the issue of satisfactorily treating used zeolites with adsorbed heavy metal ions has not been adequately addressed. Previous studies showed that nickel and copper can be thermally stabilized into aluminum-rich ceramic matrices, reducing the metal leachability through the formation of spinel properties 18-21. In addition, recent studies demonstrated that incorporating lead into lead feldspar decreased the leaching out of lead 22-24. Because metal-adsorbed zeolites contain both heavy metal ions and aluminosilicate precursors, spinel and feldspar products can be theoretically formed upon thermal treatment 18-24. The present study investigated the adsorption behavior of lead and copper on one commercial zeolite, and obtained the adsorption kinetics and isotherms. Lead (or copper)adsorbed zeolites were further thermally treated in the temperature range of 800−1000 °C. In addition, the powder XRD technique was utilized to determine the formation of copper spinel and lead feldspar and the optimal temperature for producing the target products. Finally, a prolonged leaching procedure modified from the toxicity characteristic leaching procedure



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(TCLP) was carried out to evaluate the long-term stabilization of copper and lead in the product phases.

2.

Materials and methods

2.1. Chemicals Na-A zeolite (Molecular sieve 4A) was purchased from Aladdin Chemistry Co. (Shanghai, China). The particle sizes of zeolites generally ranged from 4 to 6 mm, and the bulk density ranged from 0.69 to 0.75 g/mL. CuSO4·5H2O and Pb(NO3)2 were purchased from BDH Ltd. (Poole, Dorset, UK), and dissolved in water to obtain stock solutions (10 g/L).

2.2. Adsorption experiments All adsorption experiments were conducted in 250-mL beaker flasks containing 1 g of zeolite and 200 mL of solution with different lead (or copper) concentrations. In the kinetic adsorption experiments, an initial lead or copper concentration of 1000 mg/L was adopted. Samples were collected after 5, 10, 20, 30, 60, 90, 120, and 180 minutes of agitation. The adsorption isotherm experiments were carried out with lead or copper concentrations ranging from 100 to 1000 mg/L. The pH of all adsorption experiments was controlled at pH 5.0±0.1. The glass flasks were shaken at 150 rpm and maintained at 25 °C for 4 h. The heavy metal ion concentrations in the aqueous phase were determined by a flame-type Perkin Elmer model 3300 atomic absorption spectrometer (Perkin Elmer, Norwalk, Conn.). All measurements were taken in triplicates, and the average values are reported here.

2.3. Thermal experiments and XRD analysis


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Adsorption experiments were first conducted with 4 g/L of zeolites in a lead (or copper) solution of 1000 mg/L. After adsorption, the mixture was filtered with 0.45 µm cellulose membranes and metal-adsorbed zeolites were collected. The metal-adsorbed zeolites were thermally dried at 60 °C. One gram of dried zeolites was introduced into a furnace preheated from 800 to 1000 °C. After 10 min of sintering, each sample at different temperatures was taken from furnace and cooled in the air, and zeolite particles were grounded to less than 10 µm for X-ray powder (XRD) diffraction analysis. The XRD data of each powder sample were recorded on a D8 Advance Diffractometer (Bruker AXS) equipped with a Cu Kα X-ray tube and a LynxEye detector. The system was calibrated by Standard Reference Material 660a (lanthanum hexaboride, LaB6), obtained from the United States National Institute of Standard and Technology. The diffractometer was operated at 40 kV and 40 mA, and the 2θ scan range was 10-80° with a step size of 0.02° and a scan speed of 0.5 s per step for data collection. Qualitative phase identification was performed with Eva XRD Pattern Processing software (Bruker, City, Country???) by matching powder XRD patterns with those retrieved from the standard powder diffraction database of the ICDD PDF-2 Release 2008.

2.4. Leaching test The leachability of untreated and thermally-treated metal-adsorbed zeolites was tested with a leaching procedure modified from a method in the United States Environmental Protection Agency’s SW-846 Method 1311-TCLP, with a pH 2.9 acetic acid solution (extraction fluid #2) as the leaching fluid (USEPA, Method 1311). Each leaching vial was filled with 200-mL TCLP extraction fluid and 0.8 g of zeolite particles. The leaching vials were rotated end-over-end at 200 rpm for 1−22 days. At the end of each agitation, the pH



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was measured and the leachates were filtered with 0.2-µm syringe filters. The concentrations of lead and copper were determined with a flame-type Perkin Elmer model 3300 atomic absorption spectrometer within the satisfactory calibration range (1−10 ppm; R2 > 0.999).

3.

Results and discussion

3.1. Adsorption of lead and copper on zeolite The solute uptake rate controls the residence time of adsorbate uptake at the solid– solution interface, and is therefore important for understanding the adsorption kinetics of adsorbates in general. The adsorption kinetics of Pb2+ and Cu2+ on zeolite (Figure 1(a)) indicated that the adsorption process was fast and reached equilibrium within 60 min. To better understand the adsorption kinetics, three commonly used kinetic models, i.e., pseudo first-order (Eq. 1), pseudo second-order (Eq. 2), and intra-particle diffusion equations (Eq. 3) 25-26

were used to fit the kinetic data:

ln(qe − qt ) = ln qe − k1t

(1)

qt = ki t1/ 2 + C

(2)

qt = ki t 1/ 2 + C

(3)

where qe (mg/g) and qt (mg/g) are the amounts of a heavy metal ion adsorbed per unit mass of the adsorbent at equilibrium and at any time (t), respectively; and k1 and k2 are the rate constants of the pseudo first and second-order models, respectively; ki is the intra-particle diffusion speed, and C is a constant. The constants of the kinetic models along with the regression coefficients (R2) are listed in Table 1. It is clear that both the pseudo first-order and second-order models were fitted well with the experimental data, judging from the values of R2 (Table 1). In contrast, the data were not fitted well by the intraparticle model. The rate constant k1 (pseudo first-order) was 6 Environment ACS Paragon Plus

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0.057 and 0.066 L/min, respectively, for adsorption of Pb2+ and Cu2+ on zeolite, comparable to recently reported values for lead adsorption by natural zeolite (0.047–0.025 L/min) 27 and lead biosorption onto Rhizopus nigricans (0.0560 – 0.0654 L/min) 28. The rate constant k2 (pseudo second-order) was 5.3 × 10-4 and 8.2 × 10-4 g/mg•min for adsorption of Pb2+ and Cu2+on zeolite, respectively. A previous study showed that the rate constant k2 was in the ranges of 7.0 × 10-4 and 8.14× 10-3 g/mg•min for adsorption of lead on clinoptilolite (one zeolite), respectively 27, which were generally higher than our results. The reason for the difference can be attributed to the use of different initial concentrations in the two studies. Gunay et al. 27 found that k2 decreased with increasing Pb2+ initial concentration. Hence, higher initial concentrations used in the present study resulted in a lower k2 value than that of Gunay et al. 27. The experimental adsorption capacity of Pb2+ and Cu2+ on zeolite were around 93.5 and 124.3 mg/g, which are comparable with the calculated values from the kinetic models. Adsorption isotherms of Pb2+ and Cu2+ are shown in Figure 1(b). Two commonly used isotherm models, the Langmuir and Freundlich equations 29-30 (Eq. 4 and 5), were adopted to describe the experimental data with the obtained constants presenting in Table 1:

Langmuir model:

Freundlich model:

qe =

KLqmCe 1+ KLCe

qe = KFCe

(4) 1 n

(5)

where qe is the amount of an adsorbate on the surface of the adsorbent at equilibrium (mg/g), Ce is the equilibrium concentration of the adsorbate in solution (mg/L), qm is the maximum adsorption capacity (mg/g), KL is the Langmuir adsorption constant (L/mg), KF is the Freundlich adsorption constant [(mg/g)(mg/L)-n)], which suggests the adsorption capacity, and n represents the measure of the nonlinearity involved.



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The adsorption isotherms of Pb2+ and Cu2+ were fitted better with the Langmuir equation than with the Freundlich equation, judging from the correlation coefficients (R2) (Table 2). In fact, the adsorption of heavy metal ion on zeolite should be mostly governed by ion-exchange mechanism occurred inside the cavities of the zeolite with rarely electrostatic interaction on the surface, although the Langmuir isotherm concerned with the monolayer and homogenous adsorption. The maximum adsorption capacities of Pb2+ and Cu2+ on zeolite were 135.5 mg/g and 115.5 mg/g, respectively (Table 2). As the same adsorbent and solution compositions were applied, the difference in the plateau adsorption levels may be attributed to the different adsorption affinities of Pb2+ and Cu2+. It was reported that the adsorption affinities of heavy metal ions on zeolite depend on the first hydrolysis constant of the metals (MOH+ formation) and solubility product of the metal hydroxides 17. The proportion of MOH+ increased with the decrease of the first hydrolysis constant, and the MOH+ has stronger adsorption capability than M2+ in solution. High solubility product favors precipitation of metals, especially on the surface of adsorbents which can occur at a pH lower than the pH of precipitation in solutions 31. Lead hydroxide has higher solubility product and lower first hydrolysis constant than copper hydroxide; consequently lead has higher adsorption capacity on zeolite than copper 32. To further understand the adsorption thermodynamic behavior, the adsorption isotherms of Pb2+ and Cu2+ on zeolite at 25 °C, 35 °C, and 45 °C were subsequently performed, and the fitted isotherms and calculated adsorption thermodynamic parameters were shown in Figure S2-S3 and Table S2-S3. 3.2. Thermal stabilization of lead and copper Sintering temperature has been found to have significant effects on the association of lead and copper with aluminosilicate precursors such as kaolinite and mullite 23, 33. To investigate the effect of temperature on the incorporation of Pb2+ and Cu2+ with zeolite into


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PbAl2Si2O8 and CuAl2O4, a 10-min short sintering scheme at temperatures ranging from 800 to 1000 °C was conducted. Figure 2 shows the XRD patterns of products after sintering lead-adsorbed zeolite at 800 to 1000 °C for 10 min. It is clear that only small amounts of lead feldspar (PbAl2Si2O8) (ICDD PDF #89-6435) were formed at 800 °C, and zeolite remained as the main phase. However, the peak intensity of the lead feldspar phase increased with the elevated temperatures, with a substantially higher lead feldspar signal at 850 °C. Such behavior was different from that observed in a previous study, i.e., the initial formation temperature of lead feldspar was 750 °C for a thermal reaction between PbO and kaolinite 23. This difference was attributed to the different sintering times employed in these two studies. To lower energy consumption, shorter sintering temperature was selected (10 min) in the present study than that used in the previous studies (3 h) 22-24. As a result, a relatively higher temperature was needed to initiate the reaction between Pb2+ and zeolite in the present study. The database for the lead feldspar phase indicates that the two major peaks located at 2θ = 13.58 and 25.75° are associated with the diffraction planes (020) and (220), respectively. Therefore, two 2θ ranges of the XRD patterns (2θ = 13.30–13.80° and 24.45–26.10°) were selected to further assess peak intensities representing lead feldspar formation at elevated temperatures (Figure 3). The intensities of lead feldspar peaks were weak in the samples sintered at 800 °C, i.e., substantial formation of lead feldspar in the system was not detected until the temperature reached 850 °C. This temperature range may have approximately reflected the energy needed to overcome the major diffusion barrier in the system. The continuing formation of lead feldspar by lead-adsorbed zeolite was observed from 800 to 950 °C. When the temperature was increased to 1000 °C, the peak intensity of lead feldspar decreased to the production level at 900 °C.



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Figure 4 shows the powder XRD patterns of copper-adsorbed zeolites sintered at temperatures from 800 to 1000 °C for 10 min. At 800 °C, no CuAl2O4 spinel (ICDD PDF # 33-0448) was observed in the sintered products, and zeolites had no phase change at this stage. However, the zeolites were transformed to carnegieite and quartz, and CuAl2O4 spinel began to form when temperature was increased to 850 and 900 °C. The substantial formation of CuAl2O4 was observed at 950 and 1000 °C, respectively. Jacob and Alcock 34 reported the formation of CuAl2O4 in equilibrium thermal experiments (for 24 h) and found the spinel formation initiated at 612 °C. However, the sintering temperature should be at least above 900 °C for effectively incorporating copper into CuAl2O4 in the short sintering experiment. Because solid-state reactions are usually both thermodynamically- and diffusion-controlled, this result may further suggest that spinel formation at temperatures below 900 °C is largely limited by slow diffusion although it is thermodynamically feasible at temperatures above 612 °C. Two 2θ ranges of the XRD pattern (2θ = 36.3−37.3° and 64.8−65.9°) were selected to further monitor the formation of CuAl2O4 spinel at elevated temperatures (Figure 5). The results indicated that the best temperature to incorporate copper ion with zeolite into CuAl2O4 spinel was around 1000 °C, which is similar to that of a previous study on the thermal reaction of CuO and alumina 20.

3.3. Leaching capability To evaluate the metal stabilization after incorporation by different crystal structures, leaching tests were performed to assess the intrinsic leachability of metal-adsorbed and calcined metal-adsorbed zeolites. The thermal stabilization results showed that 950 and 1000 °C were the best temperatures for lead and copper ions to incorporate into lead feldspar and


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copper aluminate spinel, respectively. Thus, the calcined samples at these two temperatures were selected for leaching experiments. Figure 6 shows the temporal variability of leachate pH values upon prolonged leaching tests. It is clear that untreated lead- and copper-adsorbed zeolites substantially increased pH during the first several days of testing. In the subsequent tests, the pH of leachates was maintained at around 4.8 and 5.5, respectively. On the other hand, the pH of leachates from calcined samples remained essentially the same throughout the entire testing period. Because ion exchange usually governs metal adsorption and desorption on zeolite 35, decreases in leachate pH are likely attributed to enhanced ion exchange between Pb2+ (or Cu2+) and H+. H+ has higher adsorption affinity towards zeolites than Pb2+ and Cu2+ 36; thus, low pH (2.9) could facilitate the ion exchange between surrounded H+ in the solution and Pb2+ (or Cu2+) on zeolites. The lead and copper concentrations in the leachates of untreated and calcined metaladsorbed zeolites are presented in Figure 7. The results showed that the changes in the concentrations of Pb and Cu leached from untreated samples were similar to those of pH, further validating that the ion exchange was the dominant mechanism in the leaching process. At the end of leaching testing, the lead concentration was roughly half of the copper concentration in the leachate. This may imply that Pb2+ adsorbed on zeolite is more stable than Cu2+ under acidic condition, although Pb2+ has a higher adsorption capacity on zeolite than Cu2+. In contrast, the lead and copper concentrations in the leachates from calcined samples were approximately two orders of magnitude lower than those from untreated samples. Previous studies 20, 23 suggested that lead feldspar and copper aluminate spinel had strong resistance against acid attack. Therefore, the formation of lead feldspar and copper aluminate spinel in the calcined samples retarded the leaching of Pb2+ and Cu2+ even under acidic conditions. 11 Environment ACS Paragon Plus


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4.

Conclusions The present study suggested that the potential for use of continuous zeolite packed

columns to treat heavy metal ions-enriched wastewater, due to the fast adsorption kinetics of Pb2+ and Cu2+ on zeolites and large adsorption capacities of zeolites for Pb2+ and Cu2+. The subsequent thermally treatment of metal-laden zeolites showed the strong stabilization of adsorbed Pb2+ and Cu2+ on (or with) zeolite into PbAl2Si2O8 and CuAl2O4 crystals. The leaching tests clearly revealed the lower intrinsic lead and copper leachability of thermally treated samples over longer leaching periods, when comparing to the leachability of untreated samples. Therefore, the adsorption-stabilization technique developed in this study can transform the toxic Pb2+ and Cu2+ in water into stable ceramic products, which is a viable environmentally-friendly technology.

Supporting Information Available: Additional tables and figures as mentioned in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This research was supported by grants CRF C7044-14G and 17204914 from the Research Grants Council (RGC) of the Government of Hong Kong SAR. The research was also partially supported by the National Natural Science Foundation of China (Project No. 41503087) and the Research Fund Program of Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology.

References: 12 Environment ACS Paragon Plus

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(19) Shih, K.; White, T.; Leckie, J. O. Spinel formation for stabilizing simulated nickel-laden sludge with aluminum-rich ceramic precursors. Environ. Sci. Technol. 2006, 40, 50775083. (20) Tang, Y.; Shih, K.; Chan, K. Copper aluminate spinel in the stabilization and detoxification of simulated copper-laden sludge. Chemosphere 2010, 80, 375-380. (21) Hu, C. Y.; Shih, K.; Leckie, J. O. Formation of copper aluminate spinel and cuprous aluminate delafossite to thermally stabilize simulated copper-laden sludge. J. Hazard. Mater. 2010,181, 399-404. (22) Lu, X.; Shih, K. Phase transformation and its role in stabilizing simulated lead-laden sludge in aluminum-rich ceramics. Water Res. 2011, 45, 5123-5129. (23) Lu, X.; Shih, K. Metal stabilization mechanism of incorporating lead-bearing sludge in kaolinite-based ceramics. Chemosphere 2012, 86, 817-821. (24) Lu, X.; Shih, K.; Cheng, H. Lead glass-ceramics produced from the beneficial use of waterworks sludge. Water Res. 2013, 47, 1353-1360. (25) Ho, Y. S.; McKay, G. Pseudo-second order model for sorption processes. Process Biochem.1999, 34, 451-465. (26) Chiou, M. S.; Li, H. Y. Adsorption behavior of reactive dye in aqueous solution on chemical cross-linked chitosan beads. Chemosphere 2003 , 50, 1095-1105. (27) Günay, A.; Arslankaya, E.; Tosun, I. Lead removal from aqueous solution by natural and pretreated clinoptilolite: Adsorption equilibrium and kinetics. J. Hazard. Mater. 2007, 146, 362-371. (28) Zhang, L.; Zhao, L.; Yu, Y.; Chen, C. Removal of lead from aqueous solution by nonliving Rhizopus nigricans. Water Res. 1998, 32, 1437-1444. (29) Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 1918, 40, 1361-1403. 15 Environment ACS Paragon Plus


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Figure Captions Figure 1.

The kinetic (a) and isotherm (b) studies (25 °C) on the adsorption of Pb2+ and Cu2+on zeolite (In Figure 1(a), the dash, solid, and giant tooth lines (red for Pb2+and blue for Cu2+) are fitted curves of pseudo first-order, pseudo secondorder, and intraparticle diffusion models; in Figure 1(b), the solid and dash lines are fitted curves of Langmuir and Freundlich models, respectively).

Figure 2.

XRD patterns of lead-adsorbed zeolite sintered at temperatures between 800 °C and 1000 °C for 10 min.

Figure 3.

Comparison of XRD patterns between (a) 2θ= 13.30° and 13.80°, and between (b) 2θ= 25.45° and 26.10° of lead-adsorbed zeolite samples sintered at 800– 1000 °C for 10 min.

Figure 4.

XRD patterns of copper-adsorbed zeolite sintered at temperatures between 800 °C and 1000 °C for 10 min.

Figure 5.

Comparison of XRD patterns between (a) 2θ= 36.30° and 37.30°, and between (b) 2θ= 64.80° and 65.90° of copper-adsorbed zeolite samples sintered at 800– 1000 °C for 10 min.

Figure 6.

The leachate pH of untreated and treated lead-adsorbed (a) and copperadsorbed samples (b).

Figure 7.

The leachate lead and copper concentration of untreated and treated leadadsorbed (a) and copper-adsorbed samples (b).



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Table 1. Constants of pseudo first-order, pseudo second-order, and intraparticle diffusion models for the adsorption of Pb2+ and Cu2+ on zeolite.

Pseudo first-order

Pb2+

Cu2+

k1 (min-1)

0.057

0.066

qe (mg/g)

121.0

93.9

0.978

0.994

k2 (g/mg•min)

0.00053

0.00082

qe (mg/g)

136.5

105.0

0.979

0.984

Ki (mg/g•min )

7.56

5.54

C (mg/g)

39.6

35.1

R2

0.769

0.720

R Pseudo second-order

R Intraparticle diffusion

2

2 0.5


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Table 2. Constants of Langmuir and Freundlich equations for the adsorption of Pb2+ and Cu2+ on zeolite at 25 °C. Langmuir constants Metal Type

Freundlich constants 2

2

KF [(mg/g)(mg/L)-n)]

n

R

0.944

16.15

3.01

0.850

0.897

7.23

2.50

0.809

KL (L/mg)

qm (mg/g)

R

Pb2+

0.018

135.5

Cu2+

0.007

115.5



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Figure 1. The kinetic (a) and isotherm (b) studies (25 °C) on the adsorption of Pb2+ and Cu2+on zeolite (In Figure 1(a), the dash, solid, and giant tooth lines (red for Pb2+and blue for Cu2+) are fitted curves of pseudo first-order, pseudo second-order, and intraparticle diffusion models; in Figure 1(b), the solid and dash lines are fitted curves of Langmuir and Freundlich models, respectively).


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Figure 2. XRD patterns of lead-adsorbed zeolite sintered at temperatures between 800 °C and 1000 °C for 10 min.



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Figure 3. Comparison of XRD patterns between (a) 2θ= 13.30° and 13.80°, and between (b) 2θ= 25.45° and 26.10° of lead-adsorbed zeolite samples sintered at 800–1000 °C for 10 min.


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Figure 4. XRD patterns of copper-adsorbed zeolite sintered at temperatures between 800 °C and 1000 °C for 10 min.



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Figure 5. Comparison of XRD patterns between (a) 2θ= 36.30° and 37.30°, and between (b) 2θ= 64.80° and 65.90° of copper-adsorbed zeolite samples sintered at 800–1000 °C for 10 min.


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Figure 6. The leachate pH of untreated and treated lead-adsorbed (a) and copper-adsorbed samples (b).



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Figure 7. The leachate lead and copper concentration of untreated and treated lead-adsorbed (a) and copper-adsorbed samples (b).


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TOC Art


Adsorption and Thermal Stabilization of Pb2+ and Cu2+ by Zeolite

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