Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003 Adsorption of Mercury onto Fly Ash Karl Schroeder, Mike Schoffstall, and Ann Kim U.S. Dept. of Energy, NETL, P.O. Box 10940, Pittsburgh PA 15236 KEYWORDS: mercury, adsorption isotherm, Langmuir isotherm, coal utilization by- product (CUB) ABSTRACT Regulating gas-stack emissions of mercury will shift the environmental burden from the flue gas to the solids formed as by-products of the combustion and flue-gas clean up processes. Those coal utilization by-product (CUB) uses that may allow for transport of the mercury into surface or ground water may be jeopardized if the captured Hg is released. For this reason, it is important to understand the chemistry at the CUB-water interface, to be able to predict the environmental fate of the CUB-bound Hg, and to be able to anticipate the effect of additional Hg loads in the CUB material. Here we present a study of Hg(II) adsorption isotherms. Mercury concentrations relevant to US coals were used. The range included not only currently found Hg loadings but also the maximum attainable if all of the Hg in the coal were to be captured in the CUB. The effect of pH was studied by pre-equilibration of the CUB at the desired pH prior to introduction of a Hg(II) solution of the same pH. The results indicated that complete equilibrium was not attained, even after one month, although the changes were small. Prior leaching of the fly ash to remove materials soluble at a pH of 2 did not eliminate the problem. Because the trend was toward increasing Hg adsorption at longer times, the measured values may under estimate the Hg retention capacity of the fly ashes. The data were analyzed using the Langmuir adsorption isotherm equation.
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Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Adsorption of Mercury onto Fly Ash Karl Schroeder, Mike Schoffstall, and Ann Kim U.S. Dept. of Energy, NETL, P.O. Box 10940, Pittsburgh PA 15236 KEYWORDS: mercury, adsorption isotherm, Langmuir isotherm, coal utilization by-product (CUB) ABSTRACT
Regulating gas-stack emissions of mercury will shift the environmental burden from the flue gas to the solids formed as by-products of the combustion and flue-gas clean up processes. Those coal utilization by-product (CUB) uses that may allow for transport of the mercury into surface or ground water may be jeopardized if the captured Hg is released. For this reason, it is important to understand the chemistry at the CUB-water interface, to be able to predict the environmental fate of the CUB-bound Hg, and to be able to anticipate the effect of additional Hg loads in the CUB material. Here we present a study of Hg(II) adsorption isotherms. Mercury concentrations relevant to US coals were used. The range included not only currently found Hg loadings but also the maximum attainable if all of the Hg in the coal were to be captured in the CUB. The effect of pH was studied by pre-equilibration of the CUB at the desired pH prior to introduction of a Hg(II) solution of the same pH. The results indicated that complete equilibrium was not attained, even after one month, although the changes were small. Prior leaching of the fly ash to remove materials soluble at a pH of 2 did not eliminate the problem. Because the trend was toward increasing Hg adsorption at longer times, the measured values may under estimate the Hg retention capacity of the fly ashes. The data were analyzed using the Langmuir adsorption isotherm equation.
• Isotherms Hg adsorption onto other materials: Soils and Minerals
• Coals, Activated Carbons, Soot• Ion Exchange Resins
• Langmuir Equation
• Freundlich Equation
][1][
max][ SKeqSKeqAA +=
nSmA ][][ =
Presented at Air Quality IV, Sept. 22-24, 2003
Hg Analysis
Techniques
• Solids via DMA-80• Leachate via ICP-AES or
CVAA (DL = 1 ng/L)
DMA-80 Mercury Analyzer
Presented at Air Quality IV, Sept. 22-24, 2003
Change in pH with Time, FA24, Initial pH = 7
6.95
7.00
7.05
7.10
7.15
7.20
7.25
0 10 20 30 40 50 60 70
Time (Days)
pH
Liquid / Solid = 5
Liquid / Solid = 10
Presented at Air Quality IV, Sept. 22-24, 2003
Variation of [Hg] in Solution with TimeFly Ash 24, pH = 7, Liquid / Solid Ratio = 5
0
50
100
150
200
250
0 10 20 30 40 50 60 70
Time (Days)
[Hg]
in so
lutio
n (u
g/L)
Hg Initial = 2923
Hg Initial = 4060
Hg Initial = 1299
Hg Initial = 1949
Hg Initial = 325
Hg Initial = 650
Hg Initial = 162
Hg Initial = 81
Variation of [Hg] in Solution with TimeFly Ash 24, pH = 7, Liquid / Solid Ratio = 10
0
50
100
150
200
250
0 10 20 30 40 50
Time (Days)
[Hg]
in so
lutio
n (u
g/L
)
Hg Initial = 1740
Hg Initial = 1450
Hg Initial = 1015
Hg Initial = 725
Hg Initial = 580
Hg Initial = 290
Hg Initial = 145
Hg Initial = 73
The concentration of Hg in solution at a pH
of 7 also decreased with time even though the
pH was nearly constant
Presented at Air Quality IV, Sept. 22-24, 2003
Comparison of Fly Ash 17 and Fly Ash 24 at pH = 2
0
5,000
10,000
15,000
20,000
25,000
0 500 1,000 1,500 2,000 2,500 3,000Hg in Soln (ug/L)
Hg
on F
ly A
sh (u
g/kg
)
Fly Ash 17, Adsorption
Langmuir Fit To FA 17 Data
Fly Ash 24, Adsorption
Fly Ash 24, Desorption
Langmuir Fit to FA 24 Data
Presented at Air Quality IV, Sept. 22-24, 2003
• Hg is captured at low temperatures• T < 350oF = 175oC• Solidification of ash complete • No encapsulation• Hg captured on surfaces
• Captured on the carbon• LOI for higher ranked coals• Surface or pore-filling adsorption
• Captured Hg is predominately Hg(II)
Therefore (?) adsorption / desorption of Hg(II) from an aqueous phase onto (off of) fly ash will mimic higher levels of trapped Hg
Attempt to mimic higher Hg capture in the fly ash
Presented at Air Quality IV, Sept. 22-24, 2003
Langmuir Adsorption Isotherm
Provide information about extent of adsorption
Amax = 25
Provide information about strength of adsorption
KEQ=1 vs KEQ= 10Adsorption Isotherm
0
5
10
15
20
25
30
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Amount in Solution
Am
ount
Ads
orbe
d
A for Keq = 1
A for Keq = 10
Provide information about extent of adsorption
Amax = 25
Provide information about strength of adsorption
KEQ=1 vs KEQ= 10
Presented at Air Quality IV, Sept. 22-24, 2003
Mercury Mass Balancesfor Fly Ash 24
96 ± 21 %102 ± 22 %92 ± 20 %Both
108 ± 27 %115 ± 21 %100 ± 34 % 7
87 ± 8 %87 ± 8 %88 ± 7 %2
pH
Both105Liquid / Solid
Presented at Air Quality IV, Sept. 22-24, 2003
Adsorption profile for mercury on silica. Distribution diagram of mercury species, calculated using the PSEQUAD software has been added. (Walcarius, Devoy, and Bessiere, Environ. Sci. Technol., 1999, 33 (23), 4278 -4284.)
pH, speciation and adsorption
Presented at Air Quality IV, Sept. 22-24, 2003
Desorption at pH = 2 after Adsorption at pH = 7 Fly Ash 17
0
25
50
75
100
125
150
175
0 5 10 15 20 25Days
Hg
in S
oln
(ug/
L)
Presented at Air Quality IV, Sept. 22-24, 2003
Adsorption Isotherm FA 24pH 2 vs pH 7, Both Liquid / Solid Ratios
100
1,000
10,000
100,000
0.1 1.0 10.0 100.0 1000.0 10000.0[Hg] in Solution (ug/L)
[Hg]
in F
ly A
sh (u
g/kg
)
pH = 7 pH = 2Langmuir FitLangmuir Fit
Adsorption of Mercury onto Fly Ash
Poster Stand No.2 Karl Schroeder, Mike Schoffstall,
and Ann Kim
U.S. Dept. of Energy, National Energy Technology
Laboratory (NETL)
International Ash Utilization Symposium (IAUS)
Oct. 20-22, 2003
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Conclusions • The Langmuir adsorption model appears to be a
reasonable, but not perfect, approximation for Hg adsorption onto fly ash.
• However, large mass transfer effects may be affecting the results, even after a month of contact.
• No hysteresis upon desorption indicates that the adsorption is reversible at pH=2 for a pre-leached ash.
• The pH dependence is consistent with a preferential retention of oxygen containing species: HgOH+ and Hg(OH)2
• The high-carbon fly ash No. 17 has a much higher Hg adsorption capacity and adsorption energy than the low-carbon fly ash.
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Future of Coal Utilization By-products (CUB)• EPA Hg Emissions Regulations
• Control transfers Hg from gas phase to other phases
Hg
Potentially increases cost of disposal and decreases
utilization of CUB
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Goal - To determine the chemistry leading to the stability of Hg in CUB
• Provide mechanistic insight• Will depend on the CUB properties• Will depend on the environment in which the CUB is placed
Acid Mine Drainage Control
Cement/Concrete/
GroutSoil
AmendmentWallboard
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Mechanism of Hg Capture in Flue Gas• All Hg is converted to Hg0 at combustion temperatures
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Attempt to mimic higher Hg capture in the fly ash
• Hg is captured at low temperatures• T < 350oF = 175oC• Solidification of ash complete • No encapsulation• Hg captured on surfaces
• Captured on the carbon• LOI for higher ranked coals• Surface or pore-filling adsorption
• Captured Hg is predominately Hg(II)
Therefore (?) adsorption / desorption of Hg(II) from an aqueous phase onto (off of) fly ash will mimic higher levels of trapped Hg
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Objective –To determine the mechanism(s) of Hg retention under end-use conditions.
Hg in Solution
Hg Adsorbed on CCB
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Adsorption Modeling
• Isotherms Hg adsorption onto other materials: Soils and Minerals
• Coals, Activated Carbons, Soot• Ion Exchange Resins
• Langmuir Equation
• Freundlich Equation
][1][
max][ SKeqSKeqAA +=
nSmA ][][ =
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Langmuir Adsorption Isotherm
Provide information about extent of adsorption
Amax = 25
Provide information about strength of adsorption
KEQ=1 vs KEQ= 10Adsorption Isotherm
0
5
10
15
20
25
30
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Amount in Solution
Am
ount
Ads
orbe
d
A for Keq = 1
A for Keq = 10
Provide information about extent of adsorption
Amax = 25
Provide information about strength of adsorption
KEQ=1 vs KEQ= 10
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Experimental Procedure1. Mix pre-leached (to pH = 2) fly ash with known volume of H2O
2. Measure [Hg] in soln. at equilibrium (zero if no desorbable Hg in CUB)
3. Add known amount of Hg(II) to solution [HgSO4 ; Hg(NO3)2]
10-7 – 10-5 Molar
4. Allow to equilibrate (with agitation)
5. Measure Hg in solution at
several times
6. Measure total Hg in CUB at end of
equilibration
Desorption: done in a similar fashion
but with no [Hg] addition.
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Fly Ash Samples
FA17 FA24
LOI (750oC) - ASTM 2974D 12.2% 2.4%LOI (500oC) - SM 2540G 5.2% 1.3%LOI (750oC) - SM 2540G 11.3% 2.2%FeO 9.0% 3.0%CaO 1.8% 0.6%
Sand 9% 7%Silt 86% 86%Clay 5% 7%
Size Distribution
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Hg Analysis
Techniques
• Solids via DMA-80• Leachate via ICP-AES or
CVAA (DL = 1 ng/L)
DMA-80 Mercury Analyzer
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Mercury Mass Balancesfor Fly Ash 24
96 ± 21 %102 ± 22 %92 ± 20 %Both
108 ± 27 %115 ± 21 %100 ± 34 % 7
87 ± 8 %87 ± 8 %88 ± 7 %2
pH
Both105Liquid / Solid
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
pH and [Hg]SOLN were monitored for periods of up to 2 months
• Experiments at an initial pH of 7 displayed only a slight change in pH with time (total range = 0.2 s.u.).
• Experiments at an initial pH of 2 displayed increasing values to a final pH of about 3.2.
• The concentration of Hg in solution decreased with time during the adsorption at both pH values.
• Preliminary data from desorption experiments indicate a similar time dependence even though the pH values have remained essentially constant.
Conclusion: large mass transfer effects
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Change in pH with Time, FA 24, Initial pH = 2
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
0 10 20 30 40 50 60 70Time (days)
pH
Liquid/Solid = 10
Liquid/Solid = 5
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Change in pH with Time, FA24, Initial pH = 7
6.95
7.00
7.05
7.10
7.15
7.20
7.25
0 10 20 30 40 50 60 70
Time (Days)
pH
Liquid / Solid = 5
Liquid / Solid = 10
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
pH, speciation and adsorption
Adsorption profile for mercury on silica. Distribution diagram of mercury species, calculated using the PSEQUAD software has been added. (Walcarius, Devoy, and Bessiere, Environ. Sci. Technol., 1999, 33 (23), 4278 -4284.)
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Change in Concentration of Hg with Time in the Presence of FA 24 at pH = 2-3 and L/S = 5
10
100
1,000
10,000
0 10 20 30 40 50 60 70Time (Days)
Hg
in S
olut
ion
(ug/
L)
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Variation of [Hg] in Solution with TimeFly Ash 24, pH = 7, Liquid / Solid Ratio = 5
0
50
100
150
200
250
0 10 20 30 40 50 60 70
Time (Days)
[Hg]
in so
lutio
n (u
g/L
)
Hg Initial = 2923
Hg Initial = 4060
Hg Initial = 1299
Hg Initial = 1949
Hg Initial = 325
Hg Initial = 650
Hg Initial = 162
Hg Initial = 81
Variation of [Hg] in Solution with TimeFly Ash 24, pH = 7, Liquid / Solid Ratio = 10
0
50
100
150
200
250
0 10 20 30 40 50
Time (Days)
[Hg]
in so
lutio
n (u
g/L
)
Hg Initial = 1740
Hg Initial = 1450
Hg Initial = 1015
Hg Initial = 725
Hg Initial = 580
Hg Initial = 290
Hg Initial = 145
Hg Initial = 73
The concentration of Hg in solution at a pH
of 7 also decreased with time even though the
pH was nearly constant
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003
Desorption at pH = 2 after Adsorption at pH = 7 Fly Ash 17
0
25
50
75
100
125
150
175
0 5 10 15 20 25Days
Hg
in S
oln
(ug/
L)
Presented at the International Ash Utilization Symposium, Oct. 20-22, 2003