Environ. Sci. Technol. 2002, 36, 784-789
Removal and Sequestration of Iodide Using Silver-Impregnated Activated Carbon JAY S. HOSKINS AND TANJU KARANFIL* Department of Environmental Engineering and Science, Clemson University, 342 Computer Court, Anderson, South Carolina 29625 STEVEN M. SERKIZ Waste Processing Technology Department, Savannah River Technology Center, Aiken, South Carolina 29808
Two silver-impregnated activated carbons (SIACs) (0.05 and 1.05 wt % silver) and their virgin (i.e., unimpregnated) granular activated carbon (GAC) precursors were investigated for their ability to remove and sequester iodide from aqueous solutions in a series of batch sorption and leaching experiments. Silver content, total iodide concentration, and pH were the factors controlling the removal mechanisms of iodide. Iodide uptake increased with decreasing pH for both SIACs and their virgin GACs. The 0.05% SIAC behaved similarly to its virgin GAC in all experimental conditions because of its low silver content. At pH values of 7 and 8 there was a marked increased in iodide removal for the 1.05% SIAC over that of its virgin GAC, while their performances were similar at a pH of 5. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses prior to reaction with iodide showed the presence of metallic silver agglomerates on the 1.05% SIAC surface. After the reaction, elemental mapping with EDX showed the formation of silver iodide agglomerates. Oxidation of metallic silver was observed in the presence of oxygen, and the carbon surface appears to catalyze this reaction. When the molar ratio of silver to iodide was greater than 1 (i.e., MAg,SIAC > MI,TOTAL), precipitation of silver iodide was the dominant removal mechanism. However, unreacted silver leached into solution with decreasing pH while iodide leaching did not occur. When MAg,SIAC < MI,TOTAL, silver iodide precipitation occurred until all available silver had reacted, and additional iodide was removed from solution by pH-dependent adsorption to the GAC. Under this condition, silver leaching did not occur while iodide leaching increased with increasing pH.
Introduction In aqueous environments, iodine exists primarily as the anions iodide (I-) and iodate (IO3-) depending on redox conditions and pH (1). At low to neutral pH values and positive redox potentials, iodide is the dominant species in freshwater environments. 129I is a long-lived (t1/2 ) 1.57 × 107 yr) radionuclide present in contaminated groundwater and process wastes at many U.S. Department of Energy (DOE) production facilities (2, 3). Furthermore, increased levels of * Corresponding author phone: (864)656-1005; fax: (864)656-0672; e-mail:
[email protected]. 784
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129 I in northern European precipitation and runoff were recently reported (4). One potential risk mitigation strategy associated with 129I in the environment is to minimize its release by using sequestration agents, also called getters. Getter materials, the engineering term applied to materials capable of sequestering specific contaminants, should ideally be capable of sorbing large amounts of the targeted contaminant, stable during treatment processes (i.e., must not produce or release other unwanted materials), and suitable for long-term waste disposal. Numerous materials, including silver-impregnated sorbents (e.g., zeolite and alumina), activated carbons, activated carbon fibers, and anion-exchange resins have been evaluated for use in removal of iodide from the aqueous phase (5, 6). Carbon fibers showed higher uptakes than activated carbon, silica gel, alumina, and silver-impregnated zeolite (silver zeolite). Compared to activated carbon, the silver zeolite was generally found to be more effective. Silver zeolites, however, are generally more expensive than activated carbon and may leach silver into solution under acidic conditions (7). Ho and Kraus (8) found that pretreatment of activated carbon with silver and chloride caused a large increase in the uptake of iodide. Column experiments with tracer iodide solutions as low as 10-7 M showed over 98% removal after treatment of several thousands of bed volumes. It was postulated that anion exchange of chloride for iodide occurs in the silver precipitate on the carbon surface as a result of solubility differences between silver chloride and silver iodide (Ksp ) 10-10 and 10-17 for AgCl(s) and AgI(s), respectively):
AgCl(s) + I-(aq) f AgI(s) + Cl-(aq) The researchers used X-ray diffraction (XRD) to monitor changes in the silver halide crystallites on the carbon surface before and after reaction with iodide. After reaction with iodide, the presence of silver iodide crystallites on the carbon surface was confirmed.
Objective and Approach A review of literature indicates that sorbent surfaces may be tailored to enhance removal of specific environmental contaminants (e.g., cyanide, arsenic, and metals) (9-19). In this research, it was hypothesized that an oxidized form of silver in a sorbent could be used to sequester iodide from solution through silver iodide precipitation (i.e., Ag+ + I- f AgI(s)). Considering the very low solubility of silver iodide, silver-impregnated activated carbon (SIAC) has been proposed as a suitable getter material for controlling iodide contamination and for minimizing risk associated with disposal of 129I-containing waste. Therefore, a systematic investigation was undertaken to examine the removal of iodide by SIAC under representative waste treatment and disposal conditions. The main objective of this research was to evaluate the efficacy of SIAC as a potential getter material for removal and sequestration of iodide. Experiments were also conducted to gain insight into possible mechanisms of iodide removal by SIAC. A mechanistic understanding of iodide sequestration by SIAC is expected to be instrumental for the design of more effective sorbents. Two commercial SIACs with different silver contents and their virgin granular activated carbon (GAC) precursors were employed in this study. Therefore, it was possible to quantitatively evaluate the impact of silver impregnation on the iodide uptake. Sorbents were characterized to determine selected physicochemical properties (i.e., surface area, pH of point of zero charge (pHPZC), and surface functional groups). 10.1021/es010972m CCC: $22.00
2002 American Chemical Society Published on Web 01/17/2002
TABLE 1. Physicochemical Characteristics of Adsorbents
carbon type
surface area (m2/g)
silver contenta (%)
Nucon virgin GAC Nucon SIAC Calgon virgin GAC Calgon SIAC
1002 982 950 941
0.00 0.05 0.00 1.05
pHPZC
strong carboxylic groupsb (mequiv/g)
weak carboxylic & lactonicc (mequiv/g)
hydroxyl & carbonyl groupsd (mequiv/g)
total acidic groupse (mequiv/g)
total basic groups f (mequiv/g)
9.0 8.5 9.0 10.0
0.03 0.03 0.02 ndg
ndg ndg ndg ndg
ndg 0.09 0.09 0.22
ndg 0.10 0.10 0.22
0.61 0.62 0.45 0.49
a Percent wt of Ag/wt of GAC supplied by the manufacturers. uptake. f HCl uptake. g Not detectable.
b
NaHCO3 uptake. c Na2CO3-NaHCO3 uptake.
Constant-dose equilibrium isotherms were conducted for all sorbents in oxic aqueous iodide solutions at three pH values (5, 7, and 8) over a range of total silver to total iodide mole ratios (MAg,SIAC:MI,TOTAL). Subsequently, leaching experiments were performed under acidic (pH 4.5) and basic (pH 9.0) conditions to evaluate the stability of the iodide-loaded SIACs under possible disposal conditions. During both adsorption and leaching experiments, silver and iodide concentrations in the liquid phase were monitored in order to assess iodide sorption and silver leaching. To better understand mechanisms for iodide sorption to SIAC, thermodynamic speciation calculations were made, and scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) measurements were performed. In addition, experiments investigating the interactions between the individual system components (i.e., Ag0 and GAC) were also completed.
Materials and Methods Sorbents. Two bituminous coal-based SIACs with different silver contents [(1.05 wt % (TOG-NDS-20x50, Calgon Corporation) and 0.05 wt % (Nusorb A 20x40, Nucon International, Inc.)] and their GAC precursor materials were used as sorbents in this study. These silver contents are comparable with 1-2 wt % loadings reported for silver-impregnated ionexchange resins and zeolites in previous studies (5). In selected experiments, Ag0 metal powder (Aldrich 32,708-5) was used alone or with virgin GACs. All materials were used as received from the manufacturers. Sorbent Characterization. Silver contents of the impregnated materials were reported by the manufacturers. The Brunauer-Emmett-Teller (BET) surface areas were determined using a computer-controlled nitrogen gas adsorption analyzer (ASAP 2010, Micromeritics Inc.) at 77 K. A SEM (Hatachi S3500N) was used to examine the surface characteristics of the sorbent materials before and after reaction with iodide. Elemental mapping of the sorbent surfaces was conducted with EDX (Oxford Inca 400). The pHPZC for the sorbents was determined by using the pH equilibration method as described by Muller et al. (20) and by Summers (21). Surface functional group distributions of the carbons were evaluated by acid and base adsorption experiments known as the Boehm technique (22). This technique is a selective neutralization of surface acidic groups by varying strength of bases and basic groups by a strong acid and is described in Karanfil (23). Solution Analysis. Iodide and silver concentrations were measured using ion-selective electrodes. The potential of each electrode was measured on a pH meter (Accumet pH meter 50). Iodide standards were prepared from a 0.1 M NaI stock solution (Orion 945306), and silver standards were prepared from a 1000 ppm AgNO3 stock solution (Fisher SS457-100). The method detection limits (MDLs) for the iodide (Orion ISE 9653BN) and the silver (Orion ISE 9616BN) ion-selective electrodes (0.02 and 0.05 µM, respectively) were determined according to Standard Methods (24). The MDLs were consistent with the detection limits of the electrodes reported by the manufacturer. A 0.3-mL aliquot of ionic
d
NaOH-Na2CO3 uptake. e NaOH
strength adjuster (Orion 940011) was added to each beaker before measuring the silver ion and iodide concentrations using the ion-selective electrodes. Adsorption and Leaching Experiments. Isotherm experiments were conducted using completely mixed batch reactors (CMBRs) and the constant-dose bottle-point method. To eliminate waste disposal and handling problems as well as to facilitate the experimental requirements, stable iodine isotopes were used in the experiments. Oxic iodide stock solutions were prepared from a stable sodium iodide standard solution. Potassium mono and/or dibasic phosphate buffers were used to maintain constant pH (25). The ionic strength ranged between 0.05 and 0.09 M as NaCl. A total of 50 mL of various concentrations of iodide stock solutions (8-1576 µM) was added in polypropylene centrifuge tubes (i.e., CMBRs) that contained a constant sorbent dose of 1 g/L. The CMBRs were placed on a rotary tumbler for 1 week. Preliminary rate experiments showed this period to be satisfactory for reaching equilibrium (25). After being equilibrated, the sorbents were separated from solution by settling. A 30-mL supernatant sample was withdrawn from each CMBR using a plastic syringe and was split to conduct duplicate I and Ag measurements. Leaching experiments were performed after iodide sorption isotherms. Two independent sets of sorption isotherms were conducted, and these samples were subsequently leached under acidic (pH 4.5) or basic (pH 9.0) conditions. Solution remaining in each CMBR after sorption or leaching was discarded, and the carbon was retained. The CMBRs were then refilled with mono or dibasic phosphate buffered solutions that did not contain measurable iodide at an ionic strength of 0.05 M as NaCl. A total of three 1-week leaching cycles were conducted for each condition (25). Isotherm Modeling. Sorption isotherms were modeled using the Freundlich isotherm model:
qe ) KFCen where qe is the mass of the contaminant sorbed per unit mass of sorbent at equilibrium; KF is the Freundlich parameter for a sorbent, which is also the amount adsorbed at a value of equilibrium concentration equal to unity; Ce is the equilibrium solution-phase concentration of the contaminant; and n is the sorption intensity factor, which is related to the adsorbent site energy distribution. Linear geometric mean functional regression of the log-transformed experimental data was used to fit the Freundlich model and obtain log KF and n values. Confidence intervals at the 95th percentile were also determined for each parameter based on the regression of log-transformed data.
Results and Discussion Sorbent Characterization. The results of sorbent characterization experiments are summarized in Table 1. The BET surface areas are similar for the two SIACs and their virgin GACs. These results indicate that the impregnation process VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Influence of pH on the uptake of iodide by 0.05 wt % SIAC and its virgin GAC (Nucon).
FIGURE 2. Influence of pH on the uptake of iodide by 1.05 wt % SIAC and its virgin GAC (Calgon). did not significantly change the texture and pore structure of virgin GAC. SEM examination of the 1.05% SIAC showed the presence of small silver agglomerates on the carbon surface. Apparently, these agglomerates had minimal impact on BET surface area. The pHPZC values of the virgin GACs and SIACs were between 8.5 and 10.0, indicating basic surface characteristics as supported by surface functional groups analyses with the Boehm method. The hydroxyl and carbonyl groups were the dominant acidic groups on both virgin GACs and SIAC surfaces. As indicated by the increase in the hydroxide uptake, the presence of weak acidic groups (e.g., in the phenol pKa range) increased significantly after impregnation for each carbon type. However, other pKa ranges were not affected by impregnation. Overall, the two SIACs and virgin GACs showed very similar physicochemical characteristics but significantly different silver contents. Therefore, they represent good candidates for examining the role of silver content of the SIAC on iodide uptake. Sorption Isotherms. Equilibrium iodide isotherms at pH values of 5, 7, and 8 for the two SIACs and their virgin GACs
are presented in Figures 1 and 2. Table 2 shows the Freundlich coefficients for the isotherms performed in this study. Because impregnation did not significantly affect surface area, it is reasonable to compare the isotherms on a mass basis. Over the range of experimental conditions, the 0.05% SIAC exhibited similar sorption behavior to that of its virgin GAC (Figure 1). This is attributed to the small silver content of this SIAC relative to total iodide concentration (i.e., MAg,SIAC , MI,TOTAL). At a reaction stoichiometry of 1 Ag:1 I, the available silver (4.6 µM) is capable of precipitating 4.6 µM iodide. Therefore, the small silver content is expected to have a minimal impact on the removal of iodide from solution over the concentration range of the isotherms (MI,TOTAL: 8-1500 µM). Furthermore, iodide uptake increased with decreasing pH. This observation is consistent with a more positively charged surface and less competitive sorption from hydroxide ions at lower pH values. In contrast to the 0.05% SIAC, the 1.05% SIAC exhibited markedly greater iodide uptake than its virgin GAC at pH values of 7 and 8 (Figure 2). However, enhanced iodide uptake did not occur at pH 5. For a 1 g/L dose of this SIAC, the available silver (97.3 µM) can precipitate approximately 97.3 µM iodide. Therefore, silver iodide precipitation, should it occur, can make a significant difference on the iodide concentration remaining in solution. To examine silver iodide precipitation, SEM and EDX analyses were conducted with 1.05% SIAC before and after sorption experiments. Before iodide uptake, silver agglomerates were present on the SIAC surface (Figure 3a). Within these agglomerates, no counteranion was identified by EDX, suggesting that the original SIAC contains silver in the metallic state (i.e., Ag0). After iodide uptake, a significant change in the size and shape of the agglomerates was observed with SEM analysis with the reacted agglomerates being generally larger in size and more diffuse in shape (Figure 3b). Furthermore, EDX analysis of the agglomerates showed a nearly 1 Ag:1 I for all SIAC samples reacted with iodide. These observations suggest that the precipitation of silver iodide occurs on SIAC surfaces at all three pH values tested in this study. On the other hand, the presence of agglomerates was not observed on the virgin GAC (25). Figure 4 shows the iodide uptake by 1.05% SIAC as a function of the initial iodide concentration (CT) in sorption experiments. It is noteworthy that the iodide uptake is independent of pH until about 120 µM Ag, a value near the iodide concentration (i.e., 97 µM) that can react with all available silver on this SIAC. However, at concentrations greater than 120 µM Ag, the iodide uptake increased with decreasing pH. These findings suggest that in the presence of silver on activated carbon, the uptake is initially controlled by precipitation of silver iodide (i.e., MAg,SIAC > MI,TOTAL). After all the available silver is consumed by precipitation with iodide, additional uptake will occur as a result of adsorption by the GAC, a pH-dependent phenomenon. Therefore, the iodide sorption experiments were designed to monitor the uptake behavior of SIAC and virgin GAC for both MAg,SIAC < MI,TOTAL and MAg,SIAC > MI,TOTAL conditions.
TABLE 2. Freundlich Isotherm Parametersa pH 5 carbon type
KFb
Calgon virgin GAC 91.0 (101.5-81.6) Calgon SIAC 148.5 (153.7-143.6) Nucon virgin GAC 31.9 (37.4-27.3) Nucon SIAC 39.9 (50.0-30.7) a
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pH 7
nc
KFb
0.43 (0.45-0.40) 3.84 (3.90-3.79) 0.29 (0.30-0.29) 111.8 (120.4-103.9) 0.72 (0.78-0.65) 5.52 (7.75-3.93) 0.50 (0.58-0.42) 5.89 (8.98-3.82)
pH 8
nc
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nc
0.81 (0.82-0.81) 1.31 (1.36-1.27) 0.69 (0.71-0.68) 0.13 (0.16-0.10) 88.1 (93.4-83.0) 0.13 (0.14-0.12) 0.74 (0.83-0.65) 2.12 (2.48-1.82) 0.63 (0.67-0.59) 0.64 (0.72-0.55) 3.49 (3.93-3.10) 0.48 (0.51-0.45)
Values in the parentheses for KF and n are the 95% confidence intervals. *(µmol/g)/(µM)n. § Dimensionless.
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KFb
silver leaching decreased significantly with increasing pH. The results of SEM and EDX analyses indicate that silver initially exists as metallic silver (i.e., Ag0) on the SIAC surfaces (25). Therefore, it appears that metallic silver is oxidized to silver ions before iodide precipitation occurred. It is postulated that metallic silver is oxidized in aqueous solutions according to the following reaction (26):
O2(aq) + 4H+(aq) + 4Ag0(s) f 4Ag+(aq) + 2H2O(l) ∆G0 ) -182.2 kJ/mol
FIGURE 3. (a) Silver agglomerates on the SIAC surface (before reacting with iodide). (b) Silver iodide agglomerates on the SIAC surface (after reacting with iodide).
FIGURE 4. Iodide uptake as a function of initial iodide concentration (CT) at pH 5, 7, and 8 by 1.05 wt % SIAC. To further provide insight to iodide sorption by SIAC and virgin GAC, silver concentrations were also monitored during sorption isotherms. As expected, there was no silver leaching from virgin GAC. Silver leaching was observed in all SIAC control (i.e., MI,TOTAL ) 0) CMBRs. This was particularly significant at the pH 5 condition, such that nearly all available silver was leached from the carbon surface after 24 h. The leaching decreased markedly with increasing pH. However, silver leaching was observed in the CMBRs only if MAg,SIAC > MI,TOTAL (25). It was found that at the pH 5 condition, the amount of silver leached was almost equivalent to the molar excess of silver after precipitation according to a 1 Ag:1 I stoichiometric reaction ratio. Similar to the control samples,
To verify the occurrence of this reaction in the system evaluated in this study, experiments were conducted with Ag0 powder at three pH conditions in the presence and absence of oxygen. These experiments were designed to yield a 222.5 µM silver ion concentration in the CMBR at equilibrium (MINEQL+ V. 4.01). However, results showed minimal silver oxidation after a 24-h contact time for all experimental conditions (Table 3). In contrast, silver leaching from SIAC was significant over the same contact time, especially under oxic and acidic conditions. Because the literature documents several carbon-catalyzed metal oxidation reactions (27), it was hypothesized that the activated carbon surface serves as a catalyst for Ag0 oxidation. This hypothesis was tested by conducting experiments with Ag0 powder and virgin GAC under oxic and anoxic conditions. Results showed that in the presence of virgin GAC and Ag0 powder there was an appreciable amount of silver ions in oxic solutions after a 24-h contact time (Table 3). The silver concentration increased with decreasing pH, a trend consistent with SIAC test results and the model oxidation reaction. These results clearly indicate that the carbon surface catalyzes silver oxidation in the presence of oxygen and that the reaction is more favorable under acidic conditions. However, it is noted that the silver powder was only partially oxidized in the presence of virgin GAC. A possible explanation is that only a limited amount of carbon surface (i.e., external) area was available for the reaction. Additional iodide sorption experiments were conducted with SIAC that was initially treated with acidic buffer solution (pH 4.5) until nearly all the available silver was leached from the surface. The acid-treated SIAC exhibited very similar uptake as compared to virgin GAC at all three pH values (Figure 5). Therefore, the capacity of SIAC should always be higher than its virgin GAC if both full precipitation and adsorption capacity are reached. However, there is a negligible difference in uptake between the SIAC and its virgin GAC at pH 5 (Figure 2). The difference in behavior is attributed to silver iodide precipitation in the carbon pore space, as it is postulated that the precipitate prevented iodide from reaching all available activated carbon adsorption sites. Additionally, the uptake decreased with increasing pH for virgin GAC, while iodide uptake by SIAC was less pH- and concentration-dependent (Figure 2). Calculations using pH 7 and pH 8 model parameters show that, at low equilibrium iodide concentrations, the difference in iodide uptake from SIAC and virgin GAC is near the amount of iodide that may be precipitated. However, at high equilibrium concentrations of pH 7 condition, similar capacities of virgin GAC and SIAC suggest that some pore blockage may be occurring as a result of precipitation. Further studies are required for a better understanding of the phenomena occurring inside of the carbon pores. Leaching Experiments. After sorption, leaching experiments were conducted for the 1.05% SIAC and its virgin GAC under basic or acidic conditions. For both leaching conditions, iodide concentrations were not measurable in CMBRs when MAg,SIAC > MI,TOTAL during sorption. On the other hand, iodide leaching was measurable when MAg,SIAC < MI,TOTAL. In contrast, iodide leaching was always quantifiable from the VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Silver Ion Concentration after 24 h pH 5
pH 7
pH 8
material
oxic condition (µM)d
anoxic condition (µM)e
oxic condition (µM)d
anoxic condition (µM)e
oxic condition (µM)d
anoxic condition (µM)e
silver powdera Calgon SIACb Calgon Virgin GAC w/silver powderc
0.71 96.9 13.6
ndf 0.63 0.87
ndf 19.94 9.38
ndf ndf 0.47
ndf 1.97 2.68
ndf ndf ndf
a Ag0 dose 222 µM. b 1.05% SIAC dose 1 g/L c 297 µM Ag0 dose with 2 g/L activated carbon dose. d DO measurement indicated complete saturation e Anoxic conditions were created by bubbling nitrogen into the stock solution. DO measurements indicated less than 1% of saturation. Bottles were filled in an anaerobic chamber (25). f Not detectable.
FIGURE 5. Iodide uptake by acid-treated 1.05 wt % SIAC as compared to its virgin GAC.
FIGURE 6. Iodide leaching from 1.05 wt % SIAC and its virgin GAC at pH 9. The legend refers to sorption isotherm conditions. qr (mmol/g of GAC) is the iodide retained by activated carbons; Ce,d (µM) is the equilibrium iodide concentration after 2 weeks of leaching experiments. virgin GAC. These observations are consistent with the conclusion that silver iodide precipitation takes place until all available silver is reacted, after which additional uptake occurs as a result of adsorption. The effect of basic pH (i.e., 9.0) on leaching of iodide is shown in Figure 6. For SIAC, despite significant differences in the extent of iodide uptake at three pH conditions, approximately 118 µmol of I/g was retained after two leaching cycles. This amount is near a molar ratio of 1 Ag:1 I, suggesting that silver iodide was sequestered in the carbon pores. In contrast to SIAC, an appreciable amount of iodide was released from virgin GAC after two leaching cycles. This was attributed to the increased net negative surface charge on GAC and competitive ion effects with hydroxide, which created a larger repulsive force for iodide anions. A smaller amount of iodide leaching was observed from acidic conditions, a behavior consistent with increasing iodide 788
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FIGURE 7. Iodide leaching from 1.05 wt % SIAC and its virgin GAC at pH 4.5. The legend refers to sorption isotherm conditions. qr (mmol/g of GAC) is the iodide retained by activated carbons; Ce,d (µM) is the equilibrium iodide concentration after 2 weeks of leaching experiments. uptake by SIAC and virgin GAC with decreasing pH. The difference between SIAC and virgin GAC leaching behavior is consistent with the iodide removal mechanisms (i.e., precipitation and adsorption) that were postulated for the sorption experiments (Figure 7). Independent of sorption pH, SIAC retained a minimum of approximately 118 µmol of I/g, an amount similar to that retained during basic leaching conditions. Data points in the lower equilibrium concentration region come from sorption isotherms at pH 7 and pH 8, where silver iodide precipitation is responsible for the increased iodide uptake. In contrast, data points in the higher equilibrium concentration region come from pH 5 sorption isotherm samples where the higher uptake is probably due to both iodide precipitation and adsorption. Because silver iodide precipitation is not affected by pH and adsorption of iodide is favorable at pH 5, iodide release from SIAC and virgin GAC is minimized. During leaching, silver did not leach from SIAC where the available silver had fully reacted with iodide. Furthermore, basic leaching conditions (pH 9.0) helped to retain unreacted silver in the SIAC. However, silver that was not used in the formation of silver iodide (i.e., where there was a molar excess of Ag in the sorption reaction) leached from SIAC during the pH 4.5 leaching cycles. On the basis of the results obtained during this research, the removal and sequestration of iodide by SIAC is an interplay between iodide concentration, silver content of activated carbon, and pH. First, metallic silver on SIAC is oxidized under oxic aqueous conditions, and the carbon surface acts as a catalyst in this reaction. Then, as silver ions become available, silver iodide precipitation can occur. As silver ions are removed by precipitation with iodide, a driving force for additional silver oxidation is created until all available silver reacts with iodide.
If MAg,SIAC > MI,TOTAL, precipitation of silver iodide is the dominant removal mechanism. However, excess silver will increasingly leach into solution with decreasing pH. During the disposal of SIAC, iodide leaching is not expected to occur under either acidic or basic conditions. The extent of silver leaching will increase with decreasing pH and will depend on the amount of unreacted silver remaining on the SIAC. If MAg,SIAC < MI,TOTAL, silver iodide precipitation will occur until all available silver has reacted, and additional iodide will be removed from solution by pH dependent adsorption. During sorption and subsequent disposal of SIAC, silver leaching is not expected to occur under either acidic or basic conditions. The extent of iodide leaching will increase with increasing pH and will depend on the amount of iodide adsorbed by the SIAC.
Acknowledgments This work is funded in part by the South Carolina Universities Research and Education Foundation (SCUREF) under U.S. Department of Energy Contract DE-FC09-00SR22184. This work does not necessarily reflect the views of the Foundation, and no official endorsement should be inferred. The assistance from personnel at the Savannah River Technology Center and Clemson University Electron Microscope Laboratory is gratefully recognized. The authors acknowledge the comments on the manuscript by Drs. Elizabeth Carraway and Mark Schlautman.
Literature Cited (1) Whitehead, D. C. Environ. Int. 1984, 10, 321-339. (2) Kaplan, D.; Mattigod, S.; Parker, K.; Iversen, G. I-129 Test and Research to Support Disposal Decisions; WSRC-TR-2000-00283, REV. 0; Savannah River Technology Center: 2000. (3) Serkiz, S. M.; Reboul, S. H.; Bell, N. C.; Kanzleiter, J. P.; Bohrer, S. R.; Lovekamp, J. M.; Faulk, G. W. Proceedings of the Waste Management ‘00 Symposium, Tuscon, AZ, 2000. (4) Buraglio, N.; Aldahan, A.; Possnert, G.; Vintersved, I. Environ. Sci. Technol. 2001, 35, 1579-1586. (5) Yang, O. B.; Kim, J. C.; Lee, J. S.; Kim, Y. G. Ind. Eng. Chem. Res. 1993, 32, 1692-1697. (6) Sinha, P. K.; Lal, K. B.; Ahmed, J. Waste Manage. 1987, 17, 3337.
(7) Hilton, C. B. U.S. Patent Office, Patent 4,615,806, 1985. (8) Ho, P. C.; Kraus, K. A. J. Inorg. Nucl. Chem. 1981, 43, 583-587. (9) Cho, E. H.; Pitt, C. H. Metall. Mater. Trans. B 1979, 10B, 159164. (10) Choi, Y.; Lieuw, H. T.; Van Weert, G. Hydrometall., Pap. Int. Symp. 1994, 711-723. (11) Choi, Y.; Lieuw, H. T.; Van Weert, G. Impurity Control Disposal Hydrometallurgical Processes, Annual Meeting; 1994; pp 303313. (12) Dixon, S.; Cho, E. H.; Pitt, C. H. Fundamental Aspects of Hydrometallurgical Processes; Symposium Series; The American Institute of Chemical Engineers; 1978; pp 75-83. (13) de Jong, I.; Lieuw, H. T. Precis. Process Technol. 1993, 1, 305314. (14) Nelson, F.; Phillips H. O.; Kraus, K. A. Engineering Bulletin, Engineering Extension Series 145; Purdue University: 1974; pp 1076-1090. (15) Phillips, H. O.; Kraus, K. A. J. Chromatogr. 1965, 17, 549-557. (16) Rajakovic, L. V. Sep. Sci. Technol. 1992, 27, 1423-1433. (17) Rajakovic, L. V.; Stevanovic, S. M.; Mitrovic, M. V. J. Serb. Chem. Soc. 1995, 60, 149-159. (18) Rajakovic, L. V.; Mitrovic, M. V. Environ. Pollut. 1991, 75, 279287. (19) Rajakovic, L. V.; Mitrovic, M. V. J. Serb. Chem. Soc. 1995, 60, 161-169. (20) Muller, G.; Radke, C. G.; Prausnitz, J. M. J. Phys. Chem. 1980, 84, 369-376. (21) Summers, R. S. Ph.D. Thesis, Stanford University, Palo Alto, CA, 1986. (22) Boehm, H. P. Adv. Catal. 1966, 1, 179-271. (23) Karanfil, T. Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 1995. (24) Standard Methods for the Examination of Water and Wastewater, 19th ed.; APHA, AWWA, and WEF: Washington, DC, 1995. (25) Hoskins, J. S. M.Sc. Thesis, Clemson University, Clemson, SC, 2001. (26) Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1996. (27) Leon, C. A.; Radovic, L. R. Interfacial Chemistry and Electrochemistry of Carbon Surfaces. In Chemistry and Physics of Carbon, Vol. 24; Thrower, P. A., Ed.; Marcel Dekker: New York, 1994.
Received for review May 14, 2001. Revised manuscript received November 5, 2001. Accepted November 12, 2001. ES010972M
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