Adsorption of Mercury onto Fly Ash · KEYWORDS: mercury, adsorption isotherm, Langmuir isotherm, coal utilization by-product (CUB) ABSTRACT Regulating gas-stack emissions of mercury

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.

http://www.flyash.info


The poster that follows has 6 parts.

1. The first slide shows the overall poster with the position of each of the individual slides.

2. The second 2 slides are the header slides that appear at the top of the poster: a title slide and a conclusions slide.

3. The next 7 slides are goal and objective slides that appear in the first 2 columns of the poster.

4. The next 4 slides are the Experimental slides shown in column 3 of the poster

5. The next 7 slides show the change in pH and the concentration of Hg with time that appear in columns 4 and 5 of the poster.

6. The final set of 4 slides shows the adsorption isotherms of Hg onto 2 fly ashes that appear in column 6 of the poster.


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 Air Quality IV, Sept. 22-24, 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.


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 CUBPresented at Air Quality IV, Sept. 22-24, 2003

1. 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.

Experimental Procedure


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 effectsPresented at Air Quality IV, Sept. 22-24, 2003

Fly Ash 24 Adsorption Isotherm pH=7, Both Liquid / Solid Ratios

0

4,000

8,000

12,000

16,000

20,000

0 10 20 30 40 50 60[Hg] in Solution (ug/L)

[Hg]

in F

ly A

sh (u

g/kg

)

L/S Ratio = 5

L/S Ratio = 10

Langmuir Fit

Fruendlich Fit


Goal - To determine the chemistry leading to the stability of Hg in CUB

• Provide mechanistic insight• CUB properties• Receiving environment

Soil Amendment

Acid Mine Drainage ControlWallboard

Cement/Concrete/

Grout


Objective –To determine the mechanism(s) of Hg retention under end-use conditions.

Hg Adsorbed on CCB

Hg in Solution


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


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


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)


Test to blockFly Ash 24, pH = 2

-

1,000

2,000

3,000

4,000

5,000

6,000

0 500 1,000 1,500 2,000 2,500Hg in Solution, ug/L

Hg

on F

ly A

sh, u

g/kg

Data Both L/S

Langmuir

Fruendlich


Mechanism of Hg Capture in Flue Gas• All Hg is converted to Hg0 at combustion temperatures

• Hg0 adsorption displays Langmuir isotherm behavior

• Hg+2 formed in situ

• Hg (TOTAL) adsorbed on carbon

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4 5 6

Gaseous Hg (mg Hg / cu m)

Solid

s Hg

(ppm

)

Serre, S.D. Silcox, G.D. 2000. Ind. Eng. Chem. Res. 39, 1723-1730.

177 oCHg+2 is the

predominate captured species


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 ][][ =


Hg Analysis

Techniques

• Solids via DMA-80• Leachate via ICP-AES or

CVAA (DL = 1 ng/L)

DMA-80 Mercury Analyzer


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


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


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


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, Desorption

Langmuir Fit to FA 24 Data


• 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


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


Amax = 25


KEQ=1 vs KEQ= 10


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


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


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)


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


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.


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


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


Mechanism of Hg Capture in Flue Gas• All Hg is converted to Hg0 at combustion temperatures

• Hg0 adsorption displays Langmuir isotherm behavior

• Hg+2 formed in situ

• Hg (TOTAL) adsorbed on carbon

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4 5 6

Gaseous Hg (mg Hg / cu m)

Solid

s Hg

(ppm

)

Serre, S.D. Silcox, G.D. 2000. Ind. Eng. Chem. Res. 39, 1723-1730.

177 oCHg+2 is the

predominate captured species


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


Objective –To determine the mechanism(s) of Hg retention under end-use conditions.

Hg in Solution

Hg Adsorbed on CCB


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 ][][ =


Langmuir Adsorption Isotherm


Amax = 25


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


Amax = 25


KEQ=1 vs KEQ= 10


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.


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


Hg Analysis

Techniques

• Solids via DMA-80• Leachate via ICP-AES or

CVAA (DL = 1 ng/L)

DMA-80 Mercury Analyzer


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


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


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


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


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.)


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)



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


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


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)


Fly Ash 24 Adsorption Isotherm pH=7, Both Liquid / Solid Ratios

0

4,000

8,000

12,000

16,000

20,000

0 10 20 30 40 50 60[Hg] in Solution (ug/L)

[Hg]

in F

ly A

sh (u

g/kg

)

L/S Ratio = 5

L/S Ratio = 10

Langmuir Fit

Fruendlich Fit


Test to blockFly Ash 24, pH = 2

-

1,000

2,000

3,000

4,000

5,000

6,000

0 500 1,000 1,500 2,000 2,500Hg in Solution, ug/L

Hg

on F

ly A

sh, u

g/kg

Data Both L/S

Langmuir

Fruendlich


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

)


Langmuir Fit To FA 17 Data


Fly Ash 24, Desorption

Langmuir Fit to FA 24 Data


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

Documents

Adsorption of Mercury onto Fly Ash · KEYWORDS: mercury, adsorption isotherm, Langmuir isotherm, coal utilization by-product (CUB) ABSTRACT Regulating gas-stack emissions of mercury