JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH Part A—Toxic/Hazardous Substances & Environmental Engineering Vol. A39, No. 9, pp. 2275–2291, 2004
Adsorption of Divalent Lead Ions by Zeolites and Activated Carbon: Effects of pH, Temperature, and Ionic Strength Kelly B. Payne and Tarek M. Abdel-Fattah* Department of Biology, Chemistry and Environmental Science, Christopher Newport University, Newport News, Virginia, USA
ABSTRACT Lead alloy bullets used at the 2600 military small arm ranges and 9000 nonmilitary outdoor shooting ranges in the United States are a source of mobilized lead ions under conditions of low pH, significant changes in ionic strength, changes in the reduction oxidation potential (redox), and through binding metal ions to soil organic matter. Once mobile, these lead ions can contaminate adjacent soil and water. Batch adsorption kinetic and isotherm studies were conducted to compare and evaluate different types of adsorbents for lead ion removal from aqueous media. The effects on lead ion absorption from pH changes, competing ions, and temperature increases were also investigated. Adsorbent materials such as activated carbon and naturally occurring zeolites (clinoptilolite and chabazite) were selected because of their relative low cost and because the zeolites are potential point-of-use materials for mitigating wastewater runoff. Molecular sieves, Faujasite (13X) and Linde type A (5A) were selected because they provide a basis for comparison with previous studies and represent well-characterized materials. The relative rate for lead ion adsorption was: 13X > chabazite > clinoptilolite > 5A > activated carbon. Modeling lead ion
*Correspondence: Tarek M. Abdel-Fattah, Department of Biology, Chemistry and Environmental Science, Christopher Newport University, Newport News, VA 23606, USA; E-mail:
[email protected]. 2275 DOI: 10.1081/LESA-200026265 Copyright & 2004 by Marcel Dekker, Inc.
1093-4529 (Print); 1532-4117 (Online) www.dekker.com
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Payne and Abdel-Fattah adsorption by these adsorbents using the Langmuir and Freundlich isotherm expressions determined the adsorbents’ capacity for lead ion removal from aqueous media. 13X, 5A, and activated carbon best fit the Langmuir isotherm expression; chabazite and clinoptilolite best fit the Freundlich isotherm. Applications of chabazite would require pH values between 4 and 11, clinoptilolite between 3 and 11, while activated carbon would operate at a pH above 7. Ionic competition reduced lead ion removal by the zeolites, but enhanced activated carbon performance. Increasing temperature improved adsorption performance for the zeolites; activated carbon lead ion adsorption was temperature independent. Key Words: Adsorbents; Lead removal; Zeolites; Activated carbon.
INTRODUCTION Human lead exposure results from lead plumbing materials such as solder, brass fittings, and service lines, crumbling and chalking lead-based paint, airborne lead particulates released by industrial or waste elimination emissions, and fallout of decades of leaded gasoline emissions.[1,2] An additional anthropogenic lead source is from bullets. Currently, the Environmental Protection Agency (EPA) has estimated 4% of all lead made in the United States (72,575 metric tons annually) is made into lead alloy bullets containing lead primers. Much of this lead finds it way into one of 2600 military small arm ranges or 9000 nonmilitary outdoor shooting ranges in the United States.[3,4] These bullets, as well as soluble microscopic and fragmented lead, accumulate in earthen berms permitting soluble lead fraction release due to stormwater erosion.[5,6] Stormwater samples from the Barksdale Air Force Base small arms ranges averaged 541 mg L1 lead over a five year period with a maximum discharge of 2350 mg L1 lead.[7] The major route of lead absorption in children is the gastrointestinal tract.[8] Lead has a high degree of accumulation upon continued exposure and a slow rate of removal when exposure ceases. A strong link has been established between low-dose lead exposure and intellectual deficit in children.[9] Above this level, adverse health effects range from upset stomachs, irritability, constipation, and damage to organs, including the nervous system and brain; high lead levels can be fatal. A common method for heavy metal removal from water is ion exchange. Naturally occurring zeolites may be an effective and economical adsorbent alternative. High lead ion removal efficiencies have been reported using zeolites.[10–15] Additional minerals have been used for lead ion removal: francolite,[16] phillipsite (Jordanian zeolite tuff),[17] and pyrophyllite.[18] Olin and Bricka[19] screened 12 sorbents including activated carbon and acid washed reagent-grade zeolite and found that the zeolite demonstrated the highest capacity for lead ion removal. Removal of lead ions from contaminated soils and wastewater by activated carbon has been reported.[20–24] The objective of this study is to screen and evaluate a series of candidate adsorbents, including zeolites and activated carbon, for the removal of lead ions from aqueous media. This study compares multiple adsorbents exposed to one
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pollutant (lead) under the same conditions. Numerous studies have been conducted on a single adsorbent exposed to multiple pollutants (soluble metals). One potential application of adsorbents for lead ion removal is point-of-use treatment devices hence the selection of activated carbon and the zeolite materials which are relatively low in cost. Zeolites have the added potential of being used as lead adsorbing barriers to prevent heavy metal laden groundwater runoff due to stormwater erosion. Two synthetic molecular sieves were included in the study to provide a basis for comparison with previous studies and represent a well-characterized material. Results of the initial screening determined the adsorbents used in further kinetic and isotherm studies as well as investigating the effects on adsorptive performance from pH changes, competing ions, and temperature increases.
MATERIALS AND METHODS Materials Zeolites are naturally occurring, porous, hydrated aluminosilicates with exchangeable cations such as sodium and potassium. Chabazite and clinoptilolite (volcanic in origin, mined in Arizona, and obtained from GSA Resources, Inc.) were studied (Table 1) due to their availability at a very low cost. Faujasite (13X; sodium potassium aluminosilicate) and Linde type A (5A; sodium calcium aluminosilicate) are synthetic versions of naturally occurring zeolites with well documented behavior. 13X and 5A were obtained from Aldrich. X-ray powder diffraction (XRD) analysis was performed on chabazite, clinoptilolite, 13X, and 5A to confirm the crystal structure and mineral identity of the zeolites. X-ray powder diffraction (XRD) patterns were obtained on a Siemens diffractometer equipped with a rotating anode and Cu–K radiation ( ¼ 0.15418 nm). The activated carbon used in this study is randomly stacked microcystallines of graphite produced from coal. The activated carbon was obtained from Calgon Carbon Corporation.
Adsorbent Screening A series of zeolites and activated carbon adsorbents were initially screened for the removal of lead ions from aqueous media. Adsorbent screening was performed using 100.00 mL of solution with an initial lead concentration of 50 mgL1 lead (from Pb(ClO4)2) and 0.1000 g of adsorbent in 125 mL nalgene bottles. Bottles were agitated at 125 rpm on a reciprocating shaker at constant temperature (23 1 C). Samples were withdrawn for analysis by selective ion electrode (Orion model 9682 ionplus series lead electrode) at periods of 3, 24, 48, and 144 h. The electrode was calibrated by serially diluting a 1000 mgL1 lead standard (from Pb(ClO4)2) to 100, 50, 30, 10, 1, and 0.1 mgL1 and using these solutions to develop a calibration curve (r2 ¼ 0.99 or higher). This curve was used to determine lead concentrations in the samples and controls. Electrode measurements were
Mesh
CABSORB—ZS500A (Batch 2) Molecular Sieves (Aldrich) 5A 13X
CABSORB—ZS500RW (Batch 1)
CABSORB—ZS403H (Batch 2) CABSORB—ZS403TM (Batch 3) Chabazite (GSA Resources, Inc.)
Sodium calcium aluminosilicate Sodium potassium aluminosilicate
8X20 12 12
Natural clinoptilolite; hydrous sodium aluminosilicate (sodium and potassium in exchangeable cation positions) Natural clinoptilolite Natural clinoptilolite
Bituminous coal Bituminous coal
Description
Adsorbent characterization.
Natural sodium chabazite (herschelite) Thermally activated chabazite
8X20
8X20 20X35
Activated Carbon (Calgon Carbon Corporation) 8X40 Filtrasorb (400) 8X30 Filtrasorb (300) Clinoptilolite (GSA Resources, Inc.) 8X12 CABSORB—ZS403H (Batch 1)
Adsorbent
Table 1.
520 730
1:1 1.3:1
4:1
4:1
6:1 6:1
6:1
N/A N/A
Si:Al
2278
5.4 7.4
520
520
4.3 4.3
40 40
40
4.0
4.0 4.0
1,200 1,200
Surface area (m2g1)
2–2,000 2–2,000
Pore diameter (A˚)
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periodically checked to ensure electrode stability. A 1:1 solution to methanol formaldehyde ratio and 50:1 solution to ionic strength adjuster NaClO4H2O (5 M) ratio were added to the samples and standards. All standards, controls, and samples were measured at constant temperature (23 1 C).
Kinetic Study The kinetic study was performed using 100.00 mL of solution with an initial lead concentration of 50 mgL1 and 0.1000 g of adsorbent in 125 mL nalgene bottles. Bottles were agitated at 125 rpm at constant temperature (23 1 C). Samples were withdrawn for analysis by selective ion electrode at periods of 1, 2, 4, 6, 16, 24, 48, and 72 h. The integrated rate law models how reactant and product concentrations vary with time. The linearized integrated rate law for a first-order reaction is shown: Equation 1. Linearized 1st Order Rate Law log½C t ¼ ðk=2:303Þt þ log½C½25 o where [C ]t ¼ concentration of C at any time t, [C ]o ¼ original concentration of C at some initial time, and k ¼ rate constant.
Isotherm Study The isotherm study was performed using different weights of adsorbents (0.0500, 0.1000, 0.1500, 0.3000, or 0.5000 g) to which a constant volume (100.00 mL) of solution (50 mgL1 lead) was applied in 125 mL nalgene bottles. Bottles were agitated at 125 rpm while allowed to achieve equilibrium (72 h) at constant temperature (23 1 C). Samples were analyzed by selective ion electrode. Comparison with Langmuir and Freundlich isotherm models was made using the following equations: Equation 2. Langmuir adsorption isotherm qe ¼ ðXm KL Ce Þ=ð1 þ KL Ce Þ½26 Equation 3. Equilibrium Uptake qe ¼ ððCi Ce ÞVÞM where qe ¼ mg of adsorbate per g of adsorbent at equilibrium, Xm ¼ mg of solute adsorbed per g of adsorbent, KL ¼ Langmuir constant (liter of adsorbent per mg of adsorbate), Ce ¼ equilibrium concentration of adsorbate in solution (mgL1), Ci ¼ adsorbate initial concentration (mgL1), V ¼ volume of solution (l), and M ¼ amount of adsorbent used (mg).
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Equation 4. Linear form of Langmuir isotherm Ce =qe ¼1=ðXm KL Þ þ ð1=Xm ÞCe Equation 5. Freundlich isotherm qe ¼ Kf Ce1=n where qe ¼ mg of adsorbate per g of adsorbent at equilibrium, Kf ¼ Freundlich constant, n ¼ a Freundlich constant, which is always greater than one, and Ce ¼ concentration of adsorbate in solution (mgL1). Equation 6. Linear form of Freundlich isotherm ln qe ¼ ln Kf þ ð1=nÞðln Ce Þ Results of experimental data were compared to Eqs. (4) and (6) to determine which model most accurately described adsorption by the adsorbent.
pH Study Measurements for the effects of varied pH on adsorbent performance used 0.1000 g of each adsorbent added to 50.00 mL of 50 mgL1 lead solution from Pb(ClO4)2. The varied pH solutions were prepared by adding dilute NaOH or HNO3 dropwise to achieve pH values of 2.0 through 12.0. Bottles were shaken at 125 rpm for 48 h at constant temperature (23 1 C). Samples were serially diluted to mgL1 and atomic adsorption measurements were made using a Spectra AA 220 Graphite Furnace Atomic Adsorption spectrometer (GFAAS) (Varian Australia Pty Ltd, Mulgrave, Australia) with a Varian Graphite Tube Analyzer (GTA) 110. Calibration curves were completed before each sample run with 0, 15, 30, and 45 mgL1 solutions and %RSD < 5 for triplicate sample sets.
Ion Competition Study Measurements for the effects of ionic strength on adsorbent performance used 0.1000 g of each adsorbent added to 50.00 mL of 50 mgL1 lead solution from Pb(ClO4)2. Solutions contained varied amounts of KNO3 (0, 0.01, or 0.1 M). Bottles were shaken at 125 rpm for 48 h at constant temperature (23 1 C). Samples were serially diluted to mgL1 and analyzed by GFAAS.
Temperature Study Measurements for the effects of increased temperature on adsorbent performance used 0.1000 g adsorbent added to 50.00 mL of 50 mgL1 lead solution from Pb(ClO4)2. Bottles were shaken for 48 h at 23 1, 35 1, or 45 1 C in American
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Shaking Water Bath (Model #YB-531, Japan) at a shake speed of 5. Samples were serially diluted to mgL1 and analyzed by GFAAS. Equilibrium thermodynamic parameters were calculated according to LopezDelgado et al.[27]: Equation 7. Fraction of metal ions adsorbed Kc ¼ F=ð1 FÞ where F (the fraction of metal ions adsorbed at equilibrium) ¼ (Fi Fe)/Fi Fi ¼ initial fraction, Fe ¼ fraction at equilibrium. Equation 8. Gibbs free-energy exchange at equilibrium G ¼ RT ln Kc Equation 9. Gibbs free-energy exchange (linear form) ln Kc ¼ ðH=RÞð1=TK Þ þ S=R where H ¼ enthalpy, S ¼ entropy, TK ¼ temperature in Kelvin, and R ¼ universal gas constant (8.131451 J mol1 TK1 ).
Statistical Analysis Comparison between adsorbents and other parameters was completed by ANOVA. Percent relative standard deviations were computed for all replicate samples.
RESULTS AND DISCUSSION Adsorbent Screening Zeolites (chabazite and clinoptilolite) and molecular sieves (13X and 5A) removed greater than 95% of lead from the sample solution while activated carbon removed more than 60% (Fig. 1). 13X, 5A, chabazite (batch 2), clinoptilolite (batch 3), and activated carbon (Calgon Carbon Filtrasorb 300) were chosen as adsorbents on which to perform kinetic and isotherm studies. The effects on lead absorption from pH changes, competing ions, and temperature increases were also investigated for chabazite (batch 2), clinoptilolite (batch 3), and activated carbon (Calgon Carbon Filtrasorb 300) due to their potential use as low-cost sorbents for mitigating lead pollution at small arms ranges.
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Payne and Abdel-Fattah 99.6%
98.8%
98.0%
96.9%
96.3%
96.1%
95.6%
100%
63.6%
% Pb Removal
60.4%
50%
13 X
Ch ab az it e
(B at ch
(B at ch
2)
1)
1) az ab Ch
op t Cl in
ite
ilo lit e(
Ba tc h
5A
3) Ba tc h
2) ilo
lit e(
Ba tc h op t Cl in
Cl
at iv ct A
in op t
Ca ed
ed at iv ct A
ilo lit e(
rb o
Ca rb o
n
n
(3
(4 0
00
0)
)
0%
Adsorbent Figure 1.
13X
Percent lead ion removal with 144 h exposure to adsorbents.
5A
Chabazite
Clinoptilolite
Activated Carbon
Lead Remaining in Solution (ppm))
50 40 30 20 10 0 0
10
20
30
40
50
60
70
80
Time (h)
Figure 2.
Lead ion removal vs. time for lead ion sorption by adsorbents.
Kinetic Study The relative rate for lead ion adsorption was: 13X > chabazite > clinoptilolite > 5A > activated carbon (Fig. 2). Lead ion removal followed a single straight line relationship for each adsorbent (Fig. 3) with first order rate constants listed in Table 2. The rate constant for 13X was 50% higher than for chabazite, the
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Adsorption of Divalent Lead Ions by Zeolites and Activated Carbon 13X
5A
Chabazite
Clinoptilolite
2283 Activated Carbon
log([C]o /[C]t )
3
2
1
0 0
10
20
30
40
50
60
70
80
Time (h)
Figure 3.
Lead ion concentration vs. time for lead ion sorption by adsorbents.
Table 2.
Sorption rate constants for adsorbents.
Adsorbent
Sorption rate constant (10,000 k (h1)) 340 256 174 149 90
13X Chabazite Clinoptilolite 5A Activated carbon
adsorbent with the second highest rate constant; this may be attributed to the large pore size in 13X (0.74 nm).
Isotherm Study Sorption is the term used to describe metallic or organic materials attaching to an adsorbent (the chemical and physical adsorption processes which take place). Equilibrium is achieved when the capacity of the adsorbent material is reached and the rate of adsorption equals the rate of desorption. The theoretical adsorption capacity of an adsorbent can be calculated with an adsorption isotherm.[28] The Langmuir and Freundlich models used for this study are the most common models used for aqueous solutions. The Langmuir constant (KL) and the Freundlich constant (Kf) are indicators of sorption capacity.[29] Langmuir, the simplest type of isotherm, is based on the view that every adsorption site is equivalent and independent; the ability of a molecule to bind is
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Payne and Abdel-Fattah 13X Clinoptilolite Log. (Activated Carbon) Log. (Chabazite)
5A Activated Carbon Log. (13X)
Chabazite Log. (5A) Log. (Clinoptilolite)
140 120
qe (mg g−1)
100 80 60 40 20 0 −20
0
5
10
15
20
25
30
35
40
45
−1
Ce (mg l )
Adsorption isotherms of lead ions by zeolites and activated carbon.
Figure 4.
Table 3.
Parameters of Langmuir and Freundlich models. Langmuir model
Adsorbent 13X 5A Activated carbon
KL 3.905 1.371 0.938 Kf
Chabazite Clinoptilolite
11 3,917
Xm
r2
121.951 37.594 54.945
0.960 0.981 0.996
Freundlich model n 0.349 6.361
r2 0.970 0.863
independent of whether or not neighboring sites are occupied. The molecular sieves and activated carbon isotherms best fit the Langmuir model (Fig. 4; Table 3). The Langmuir model assumes that adsorption sites on the adsorbent surface are occupied by the adsorbate in the solution therefore the Langmuir constant (KL) represents the degree of sorption affinity the adsorbate has to the adsorbent. The maximum adsorption capacity (Xm) associated with complete monolayer cover is typically expressed in mg g1.[26] KL is nearly three times greater for 13X than 5A and four times greater than activated carbon, indicating a much stronger affinity for Pb adsorption by 13X. 13X has the largest pore diameter, 0.74 nm, and silicon to aluminum ratio (1.3:1).[30]
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Adsorption on nonuniform sites, either preexisting in the different adsorption sites or caused by repulsive forces between adsorbed atoms or molecules, is the basis of the Freundlich model. Chabazite and clinoptilolite best fit the Freundlich model. The Freundlich model assumes an infinite supply of unreacted adsorbent sites and tends to represent heterogeneous materials better than other models. The higher the Freundlich constant (Kf), the higher the potential to characterize the adsorbent as more reactive, although the constant tends to be site and adsorbent specific. The constant n is an associated empirical constant which is dependent on the heterogeneity of sorbing sites.[22] For n values greater than 1, interactions are binding at sorbing sites, whereas n values less than 1 suggest the model does not approximate the binding interactions for the process. Clinoptilolite has a larger n value than chabazite indicating more adsorption at equilibrium for clinoptilolite (with desorption being favored more at equilibrium for the chabazite).
pH Study The hydrolysis reactions of divalent metal cations can be represented by the general reaction: o M2þ þ n H2 O MðOHÞ2n þ n Hþ n p However, based on the equilibrium constants, only the first hydrolysis reaction (n ¼ 1) is considered significant for lead ions[31]: o PbOHþ þ Hþ Pb2þ þ H2 O p The initial pH of 4.7, is low as explained by the hydrolysis reaction. Table 4 shows the effect of slurry concentration (g adsorbents per 100 mL solution) on the final pH. The increase of the slurry concentration from 0.05 to 0.5 g adsorbents per 100 mL increases the final solution pH due to the increase in the concentrations of alkaline ions which are released from adsorbents during the ion-exchange process. The chemical precipitation as lead hydroxide occurs at pH values higher than 6.0. The final pH is below 6.0 for chabazite which indicates simple ion-exchange. In the case of clinoptilolite and activated carbon, where the solution pH is only slightly above pH 6.0 at the higher slurry concentration (0.3 g and higher), precipitation is expected to be negligible. In the case of 13X and 5A, the final solution pH is rather high with values over 7.0, therefore adsorption is a combination of ion exchange and precipitation. The influence on adsorption by pH was studied over the pH range 2.0–12.0 (Fig. 5). There is a significant difference in adsorption due to the influence of pH for chabazite, clinoptilolite, and activated carbon (n ¼ 33, P < 0.001). The effective pH range for chabazite was 4 to 11, for clinoptilolite it was 3 to 11, and for activated carbon it was above 7. The zeolites would be effective lead adsorbents for mitigating the small arms ranges at Barksdale Air Force Base at which monthly soil pH samples ranged from 6 to 8.[7]
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Payne and Abdel-Fattah Table 4.
Effect of slurry concentration of final pH during lead ion removal. a
Final pH
Mass of absorbent (g) per 100 mL solution
13X
5A
Clinoptilolite
Chabazite
Activated carbon
0.05 0.1 0.15 0.3 0.5
5.17 6.08 6.44 7.30 7.94
5.06 5.25 5.80 6.89 9.45
5.77 5.69 5.97 6.77 6.79
5.34 5.41 5.62 5.72 5.84
5.08 5.26 5.48 5.97 6.14
a
Initial pH ¼ 4.7, [C]o ¼ 50 mgL1, and T ¼ 23 1 C.
Chabazite Lead Removal (%)
100 95 90 85 80 2
3
4
5
6
7 8 pH
9
10 11 12
9
10 11 12
9
10 11 12
Clinoptilolite Lead Removal (%)
100 80 60 40 20 2
3
4
5
6
7 8 pH
Activated Carbon Lead Removal (%)
100 80 60 40 20 2
Figure 5.
3
4
5
6
7 8 pH
Effects of pH on adsorbent performance.
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0.01 M KNO3
2287
0.1 M KNO3
100
Lead Removal (%)
80 60 40 20 0 Chabazite
Figure 6.
Clinoptilolite Adsorbent
Activated Carbon
Effects of ionic strength on adsorption performance.
Ion Competition Study To investigate the possibility of competition between lead ions and cations such as potassium, 0.01 M or 0.1 M KNO3 was added, and the amount of lead adsorbed in the presence of these ions was determined. There was a significant difference in adsorption performance in the presence of competing ions for chabazite, clinoptilolite, and activated carbon (n ¼ 9, P < 0.001). Activated carbon performance was enhanced in the presence of competing ions (Fig. 6). Mitigation of polluted soils would most like involve competing ions thus favoring the use of activated carbon adsorbents. Chabazite showed a slight enhancement in adsorptive performance in the presence of competing ions, but as the concentration of competing ions increased further, lead absorption decreased. Clinoptilolite adsorption performance declined in the presence of competing ions. These results may suggest zeolite lead attraction is electrostatic and hence influenced strongly by competing ions.
Temperature Study As temperature increased, adsorbent performance significantly improved for the zeolites (n ¼ 9, P < 0.001). Chabazite responded more dramatically to increased temperature while clinoptilolite showed a linear increase with increasing temperature, suggesting an endothermic adsorption process (Fig. 7). Generally sorption processes are found to be endothermic because a temperature increase aids the adsorption process through activation of adsorption sites. It has been further confirmed by calculating the value of change in enthalpy (H ) from thermodynamic parameters (Table 5) which was found positive for zeolites indicating the adsorption process by zeolites to be endothermic. Activated carbon adsorption performance was unaffected by temperature increases. The lead removal
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Lead Removed (%)
Chabazite 100 95 90 85 80 23
35
45 o
Temperature ( C)
Lead Removed (%)
Clinoptilolite 100 90 80 70 23
35 45 o Temperature ( C)
Lead Removed (%)
Activated Carbon 50 45 40 35 30 23 Figure 7.
35 45 o Temperature ( C)
Effects of temperature on adsorbent performance.
efficiency improvements with increasing temperature suggest the zeolites could operate successfully as lead adsorbents from 23 to 45 C. This is very attractive for small arms range mitigation because sunlight may elevate their temperatures to 45 C or more. The equilibrium values of G, H, and S for chabazite and clinoptilolite are presented in Table 5. The large negative values of G for the zeolites indicate the
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Adsorption of Divalent Lead Ions by Zeolites and Activated Carbon Table 5.
Thermodynamic parameters for zeolite adsorption of lead ions.a G (kJ mol1)
Adsorbent
Chabazite Clinoptilolite a
2289
23 C 5488 2811
35 C 50482 4528
H (kJ mol1)
S (kJ K mol1)
r2
631,326 644,702
2,119 2,215
0.81 0.71
45 C 52121 52121
Activated carbon not included due to its temperature independence.
reactions are strongly spontaneous. The spontaneity order of the adsorbents for all temperatures measured is chabazite >> clinoptilolite.
CONCLUSIONS Chabazite and clinoptilolite had higher sorption rate constants than 5A, but remained slower than 13X. High metal uptake can be achieved with careful selection of slurry concentration to avoid masking of sorption by chemical precipitation. Adsorption was significantly affected by pH. Applications of chabazite would require pH values between 4 and 11, clinoptilolite between 3 and 11, while activated carbon would operate at a pH above 7. Ionic effects could potentially improve performance of activated carbon but not the zeolites. Zeolite reactions were strongly spontaneous and endothermic. Increasing temperature improved adsorption performance for the zeolites while activated carbon adsorption was temperature independent. Chabazite and clinoptilolite showed promise as highly efficient, low cost adsorbents for lead removal.
REFERENCES 1. Burgoon, D.A.; Rust, S.W.; Hogan, K.A. Relationships among lead levels in blood, dust, and soil. In Lead Poisoning Exposure, Abatement, Regulation; Breen, J.J., Stroup, C.R., Eds.; Lewis Publishers: Boca Raton, 1995; 255–264. 2. Carra, J.S. The US Environmental Protection Agency’s Broad Strategy to Address Lead Poisoning. In Lead Poisoning Exposure, Abatement, Regulation; Breen, J.J., Stroup, C.R., Eds.; Lewis Publishers: Boca Raton, 1995; 71–75. 3. NSSF. Environmental Aspects of Construction and Management of Outdoor Shooting Ranges; National Association of Shooting Ranges, Facility Development Series. No.2., 1997; 125 pp. 4. EPA. Best Management Practices for Lead at Outdoor Shooting Ranges; EPA-902-B-01-001, 2001; 86 pp. 5. US Army. Implementation Guidance Handbook, Using Physical Separation and Acid Leaching to Process Small-Arms Range Soils; Battelle, 18 Sep 1997; 124 pp.
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