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Journal of Minerals & Materials Characterization & Engineering, Vol. 3, No.1, pp 41-51, 2004
jmmce.org P rinted in t he USA . A ll rig hts re ser ved
Adsorption of Hg0 on the Unburned Carbon with HF Acid Leaching
Jinjing Luo1, J.Y. Hwang2*, B. C. Greenlund3, Xiang Sun2, and Zhiyong Xu3
1Dept. of Civil & Environmental Engineering, 2Dept. of Materials Science & Engineering,
3Institute of Materials Processing, Michigan Technological University, Houghton, MI
Unburned carbons from fly ash were leached with concentrated HF acid
solutions in this study. The mercury adsorption abilities of the treated unburned
carbons were examined. Effects of temperature, contact time, preloaded mercury
emission and gaseous mercury concentration on adsorption behaviors were
investigated. Leached by HF acid solution, unburned carbons were altered both
physically and chemically. The influences of structure alteration on adsorption
behaviors were also discussed.
KEY WORDS: Unburned Carbon, HF Acid Leaching, Adsorption Capacity,
Originated from fly ash, a by-product of coal combustion power plant, unburned carbon
contains various materials, most of which are located inside the pores or on the surface. Previous
studies1,2 revealed that the impurities include trace elements and metal oxides, and among them
aluminum silicate compounds take big portions. Due to the blockage of pores and the occupation
of surfaces, it is assumed that the ability of the unburned carbon to remove Hg0 is reduced.
Removal of the impurities may be a way to increase the Hg0 adsorption capacity of the unburned
carbon samples. An acid leaching method was employed in this investigation to digest the
AEP unburned carbon and Pepco unburned carbon were extracted from AEP (American Electric
Power) fly ash, and Pepco (Potomac Electric Power) fly ash by using the froth floatation
method3, respectively. Both fly ashes belong to class F fly ash. F400 activated carbon was
purchased from Calgon Carbon Corporation, Pittsburgh, PA, USA.
Four acids were selected in this test, i.e. HCl, H
2SO4, HNO3 and HF acid. Equal portions
of representative samples were digested in same volumes and concentrations (50%) of each acid.
The leachates were analyzed by ICP for composition. SEM examination was provided by IMP,
* Auth or to who m cor res pon de nce sh ou ld be add re sse d, Tel : 1-906-487-2600, Email: email@example.com
42 J. Luo, J.Y. Hwang, R. Gree n lund , X. S u n , and Z . Xu Vol. 3, No. 1
HF Acid Leaching
AEP and Pepco unburned carbon were processed with a concentrate HF acid (49%)
solution by volume ratio of 1:2 for 2 hours. After reaction, the treated samples were filtrated and
washed by distilled water to remove residue HF acid solution and oven-dried at 105oC.
Hg Adsorption Test
Figure 1 illustrates the schematic diagram of mercury vapor adsorption apparatus. The
Hg source was a 0.5 cm long mercury permeation tube (VICI Metronics. Inc., CA). A water bath
maintained the required stable temperature. The carried gas was P.P. grade nitrogen gas. The
concentrated mercury vapor was diluted with a bypass line of nitrogen gas before being
introduced into carbon reactor. The carbon reactor was a 1cm I.D. (inside diameter), 22cm long
glass column. The mixture of carbon sample and short glass fiber was packed in the middle of
the column. The carbon bed temperature was regulated by a tube furnace. Tygon tubing from
Saint-Gobain Performance Plastics was selected as connecting materials.
Figure1. Schematic Diagram of Mercury Vapor Adsorption Apparatus
Mercury vapor was collected using the one-liter Tedlar sampling bag at the site upstream
and downstream of the carbon bed respectively. The concentration was determined by a gold
film mercury vapor analyzer (JEROME 431-X, Arizona Instrument Corp)4.
N2 1.5%KMnO4 + 10%H2SO4
Vol. 3, No. 1 Ad so r p t i o n o f H g0 on the Unburned Carbon with HF Acid Leaching 43
Exhaust vapor was introduced to the impinger solution before being expelled into the air. The
impinger solutions were prepared daily by adding 1.5% potassium permanganate in 10% sulfuric
A blank test was performed before each new adsorption experiment and after each test
the entire system was purged with pure nitrogen gas to expel leftover Hg.6 The amount of
mercury captured was determined by mass balance and normalized to the weight of sample.
RESULTS & DISCUSSION
Effect of Acid Leaching
Table 1 presents leaching test result for AEP carbon. Hydrofluoric acid produced the best
digestion among four tested acids. It readily removed the aluminum silicate compounds from
Table 1 Laboratory Leaching Tests of AEP Unburned Carbon
Leachate Impurity Composition (ppm)
Acid Ca Mn Si Mg Na Zn Al
HCl 3.82 0.04 1.62 0.93 0.29 0 9.89
H2SO4 3.75 0.05 0.34 1.38 0.46 0.02 14.19
HNO3 4.94 0.03 1.89 1.11 0.57 0 8.9
HF 1.38 0.08 117.5 2.92 0.04 49.1
SEM examinations of the AEP carbon before and after HF acid leaching were performed
and the results are displayed in Figure 2 through Figure 5. The SEM photo taken at 250X is
shown in figure 2. The light colored particles are the impurities. The LOI of this specimen is
69.75%. The same material is magnified to 3500X and presented in Figure 3. The photos show
that the spherical fly ash was physically locked into the pores of the carbon matrix. The photo
taken at 250X of HF leached AEP sample is pictured in Figure 4, and shows a significant
reduction of impurity particles. The leached material possesses an assay of 97.34% LOI. The
same material was magnified to 3500X and is shown in Figure 5. A noticeable absence of fly ash
spheres is observed when compared with Figure 3.
Hg Adsorption Test
Adsorption curves of AEP-HF and Pepco-HF are presented in Figure 6. AEP, Pepco and
F400 activated carbon are shown as references. With HF acid leaching, both unburned carbons
improved their adsorption performance over their virgin carbons. The average adsorption rate of
44 J. Luo, J.Y. Hwang, R. Gree n lund , X. S u n , and Z . Xu Vol. 3, No. 1
AEP-HF is 2.26 times higher than that of AEP carbon. And although Pepco carbon did not show
positive adsorption capacity, Pepco-HF indicated satisfactory performance that was near to that
of AEP carbon. Furthermore, Pepco-HF and AEP-HF displayed almost similar adsorption
behaviors and capacities within first 225 minutes of testing. This is important because the
reaction time is very short if absorbents are used in ESP (electrostatic precipitator), which is the
most popular pollutant control device in coal-fired power plants. Based on this point,
hydrofluoric acid leaching may be a promising method to diminish the influence on Hg
adsorption capacity from variations in carbon sources at the initial contact time. More unburned
carbon samples will be examined in a future study. From Figure 7, the breakthrough profiles,
AEP-HF did not reach its sorption equilibrium after approximately 2700 minutes of testing,
where it still possessed 30% adsorption ability. Pepco-HF had reached its maximum adsorption
capacity at the end of the experiment.
The adsorption behaviors of unburned carbons with HF acid leaching were compared
with that of F400 activated carbon and the curves are presented in Figure 6. Activated carbon
performed the best among all samples, and its adsorption rate was around two times that of AEP-
HF and about three times of that of Pepco-HF.
Effect of Temperature
Influence of temperature on the adsorption capacity of Pepco-HF carbon at a
concentration of 0.05mg/m3 is displayed in Figure 8. With temperature increasing from 20oC to
150oC, its adsorption capacity was reduced over twelve times. With HF acid leaching treatment,
Pepco-HF carbon still obeyed the physisorption theory. But AEP-HF carbon revealed a
contradictory result. Figure 9 shows temperature effect for AEP-HF at the concentration of
0.05mg/m3. In the first 270 minutes, AEP-HF increased its adsorption capacity with temperature
decreasing. It followed the physisorption theory during this period. But afterwards the adsorption
rate of AEP-HF at 150oC was found to be around 1.5 times greater than that at 20oC. The
physisorption theory cannot explain this phenomenon, which means the chemisorption may be
the dominant factor. The adsorption mechanism of AEP-HF was controlled by physisorption and
chemisorption respectively during the entire adsorption test. It can be concluded that with HF
acid leaching the AEP carbon surface was altered both physically (more empty pores supplied)
Effect of Influent Hg Concentration
The influence of gaseous Hg concentration on the adsorption capacity of the samples was
studied and results are displayed in Figure 10. Pepco-HF and F400 activated carbon showed a
greater adsorption rate with an increasing feed of Hg concentration, F400 sample having the
faster rate. AEP-HF demonstrated similar behavior during the first 4 hours of experiment, but its
adsorption rates were almost equal to each other afterwards, no matter whether the Hg
concentration was high or low. Moreover, the equilibrium capacity of AEP-HF at a low influent
Hg content was much better than that of AEP-HF at a high gaseous Hg content. These results
imply that AEP-HF is more suitable for use at low Hg influent content condition.
Effect of Preloaded Mercury
A previous study6 demonstrated that the emission of preloaded mercury from carbon
surface at 150oC impaired the Hg capturing ability of unburned carbons. In this study, the
Vol. 3, No. 1 Ad so r p t i o n o f H g0 on the Unburned Carbon with HF Acid Leaching 45
amount of preloaded Hg held by HF acid leached unburned carbon was tested. Figure 11 presents
the desorption curve and desorption amount of Pepco-HF carbon. Being purged mercury-free
vapor at 150oC, Pepco carbon desorbed 0.2µgHg/gCarbon and this would be its entire preloaded
mercury value since its desorbed mercury concentration reached zero at the end of experiment.
This result suggests that preloaded mercury could be emitted from carbon surface during the
adsorption test at 150oC, which causes a poorer mercury capturing ability.
SUMMARY AND CONCLUSION
With HF acid leaching, the pore structure on unburned carbon surface was changed. The
impurity spheres, including mostly the aluminum silicate compound, were dissolved by HF acid
solution, leaving more empty pores on carbon surface.
In addition to physically altering the surface of the unburned carbon, HF acid leaching
may have also changed the surface chemistry of the unburned carbon. This is supported by the
observation that adsorption performance of AEP-HF at high temperature was better than that at
low temperature. Pepco-HF carbon obeyed the physisorption mechanism, which is consistent
with the performances of AEP, Pepco and F400 activated carbon6.
Both AEP-HF and Pepco-HF demonstrated better adsorption behaviors than the virgin
unburned carbon. Pepco-HF even increased its capturing capacity from negative to close to that
of AEP unburned carbon. During the initial adsorption time, AEP-HF and Pepco-HF did not
show significant difference between their adsorption behaviors and capacities. With HF acid
leaching, the adsorption behavior depending on the carbon source was reduced in some extent.
F400 activated carbon demonstrated the best adsorption behavior at 150oC temperature and Hg
concentration of 0.05mg/m3.
The influence of gaseous Hg content on adsorption behavior of carbon samples varied.
Pepco-HF carbon and F400 activated carbon improved their adsorption behaviors with Hg
influent concentration increasing. But AEP-HF carbon did not indicate significant difference in
adsorption rate with mercury influent concentration changing.
The preloaded mercury desorbed from Pepco-HF at 150oC when purged by mercury-free
vapor. The desorption of preloaded mercury from carbon surface may be a reason that unburned
carbon shows less adsorption capacity than the activated carbon.
46 J. Luo, J.Y. Hwang, R. Gree n lund , X. S u n , and Z . Xu Vol. 3, No. 1
Figure 2. AEP Carbon Product, 250X magnification.
Figure 3. AEP Carbon Product, 3500X magnification
Vol. 3, No. 1 Ad so r p t i o n o f H g0 on the Unburned Carbon with HF Acid Leaching 47
Figure 4. Leached AEP Carbon Product, 250X magnification
Figure 5. Leached AEP Carbon Product, 3500X magnification.
48 J. Luo, J.Y. Hwang, R. Gree n lund , X. S u n , and Z . Xu Vol. 3, No. 1
Fig. 6 HF Acid Effect on Hg Adsorption at 150 oC, 0 . 0 5 m g / m 3
0200400 600 800 1000 1200 1400 1600 1800 2000 22002400 2600
Vol. 3, No. 1 Ad so r p t i o n o f H g0 on the Unburned Carbon with HF Acid Leaching 49
Fig. 7 Breakthrough Profiles of Carbon Samples at 150oC, 0.05 mg/m3
0200400 6008001000 12001400 1600 1800 2000 2200 2400 2600 2800
Fig. 8 Influence of Temperature on Hg adsorption
50 J. Luo, J.Y. Hwang, R. Gree n lund , X. S u n , and Z . Xu Vol. 3, No. 1
Fig. 9 Temperature Effect on Hg Adsorption for AEP-HF at 0.05mg/m
01000 20003000 4000 5000 6000 700080009000 1000
Fig. 10 Effect of Gaseous Hg Concentration on Adsorption Capacities at
0200 400600 800 1000 1200 14001600 1800 2000 2200 2400 2600
Vol. 3, No. 1 Ad so r p t i o n o f H g0 on the Unburned Carbon with HF Acid Leaching 51
Fig.11 Hg Preloaded in Pepco-HF Carbon
0200400 600 80010001200 1400
1 J.Y.Hwang, X.Sun, Z.Li, “Unburned Carbon from Fly Ash for Mercury Adsorption: I.
Separation and Characterization of Unburned Carbon”, Journal of Minerals & Materials
Characterization & Engineering, Vol1. No.1, p39-60
2 Dewey,M.VPI/ALRC Coal Refuse Leach Project Physical Characterization. U.S. Bureau of
Mines. Albany, OR, Research Center, 1995
3 J.Y.Hwang, “Wet Process for Fly Ash Beneficiation”, U.S. Patent 5,047,145 (1991)
4 JEROME 431-X Mercury Vapor Analyzer Manual
5 Shendrikar, A.D.; Damle, A.; Gutknect, W.F. “Collection Efficiency Evaluation of Mercury
Trapping Media for the SASS Train Impinger System”, U.S. Environmental Protection Agency.
U.S. Government Printing Office: Washing, DC, 1984, EPA-600/7-84-089
6 J.Luo, J.Y.Hwang, “Adsorption of Vapor Phase Mercury on Various Carbons”, Journal of
Minerals & Materials Characterization & Engineering, Vol.3, No.1, 2004