1046
Energy & Fuels 1999, 13, 1046-1050
Influence of Coal Properties on Mercury Uptake from Aqueous Solution Janos Lakatos,† Stephen D. Brown,‡ and Colin E. Snape*,‡ Research Institute of Applied Chemistry, Miskolc University, 3515 Miskolc-Egyetemva´ ros, POB 2, Hungary, and Department of Pure and Applied Chemistry, Thomas Graham Building, 295 Cathedral Street, University of Strathclyde, Glasgow G1 1XL, Scotland, U.K. Received January 25, 1999. Revised Manuscript Received June 3, 1999
The uptake of mercury (II) from aqueous solution by a range of coals has been studied and the results have been compared to those for a number of other sorbents, including commercial active carbons and cation-exchange resins. At pH 5 in a buffer medium, the capacities for mercury removal of the low-rank coals and the oxidized bituminous coals investigated are comparable to those of the other sorbents tested. For the lignites investigated, a high content of organic sulfur does not markedly affect the capacity for mercury uptake in relatively neutral and low chloride media, owing to redox reactions being the most likely mechanism involved. However, in highly acidic solutions, the capacities for mercury uptake are considerably greater for the high-sulfur coals investigated than for their low-sulfur counterparts due to chelation being the major sorption process involved.
Introduction Mercury is a highly mobile pollutant that is toxic at extremely low concentrations. Anthropogenic emissions account for around 60% of mercury in the environment,1 with the main sources being coal-fired power plants, waste incinerators, nonferrous metal (Pb, Zn) production, and chloro-alkali plants.2 Increased acceptance of the dangers posed by mercury in the environment and better industrial practices are lowering mercury discharges. However, there is still a need for the development of effective, low-cost methods to treat mercurycontaminated wastes. Mercury is discharged into the environment in three distinct chemical formssnamely, as atoms, ions, and organic compounds. The literature indicates that atomic and ionic mercury are the most prevalent forms and are present in roughly equal amounts.3,4 Atomic mercury [Hg(0)] will modify the global mercury balance, whereas for cationic Hg(II) contamination will occur close to the emission source.5 As a consequence of the different dispersion mechanisms and toxicities of atomic, ionic, and organically bound mercury, different technologies are required for environmental cleanup. The capture of mercury in the gas phase has been achieved by the use of fixed beds of active carbon, sulfur-containing resins, and silica or zeolite sorbents impregnated with sulfur.6-8 * Author to whom correspondence should be addressed. † Miskolc University. ‡ University of Strathclyde. (1) Niragu, J. O. Nature 1989, 338, 47. (2) Pacyna, J. M.; Munch, J. Water, Air, Soil Pollut. 1991, 56, 51. (3) Hall, B.; Lindquist, O.; Ljungstrom, E. Environ. Sci. Technol. 1990, 24, 108. (4) Vog, H.; Braun, H.; Metzge, M.; Sneider, J. Chemosphere 1987, 16, 21. (5) Capri, A. Water, Air, Soil Pollut. 1997, 98, 241. (6) Meij, R. Water, Air, Soil Pollut. 1991, 56, 117.
The removal of mercury from solution is often achieved in parallel with gas purification where Hg(II) is first reduced and stripped with air before the gas is cleaned. The use of iron powder or felt as a reducing agent,9 electrochemical methods,10,11 ion-exchange,12,13 and sorption14,15 have all been investigated to remove Hg(II) from aqueous solution. A number of naturally occurring materials, including coals, agricultural wastes, and biomass, have been investigated as sorbents for mercury.16-22 Previous studies have indicated that coals can act as low-cost sorbents for mercury wastes.16-18 Pandey and Chaudhuri17 found that a bituminous coal was just as effective as a commercially available active carbon for removing mercury from water at levels typical of those found in industrial wastes. However, a significant portion of the sorbed mercury could not be recovered by the procedures commonly applied to regenerate ion-exchange resins which is clearly a factor that must be overcome. (7) Daza, L.; Mendioroz, S.; Pajares, J. A. Appl. Catal. B: Environ. 1993, 2, 277. (8) Otani, Y.; Emi, H.; Kanaoka, C.; Uchijima, I.; Nishino, H. Environ. Sci. Technol. 1988, 22, 708. (9) Grau, J. M.; Bisang, J. M. J. Chem. Technol. Biotechnol. 1995, 62, 153. (10) Matlosz, M.; Newman, J. Electrochem. Soc. 1986, 133, 1850. (11) Gardiner, W. C.; Munoz, F. Chem. Eng. 1971, Aug 23, 57. (12) Oliveira, S. F.; Airoldi, C. Mikrochim. Acta 1993, 110, 95. (13) Pall, G. Res. Ing. (G. B.) 1986, 31, 55; Chem. Abstr. 1986, 105, 139096r. (14) Namasivayam, C.; Periasamy, K. Water Res. 1993, 27, 1663. (15) Ma, X.; Subramanian, K. S.; Chakrabarti, C. L.; Guo, R.; Cheng, J.; Lu, Y.; Pickering, W. F. Sci. Eng. 1992, 27, 1389. (16) Cyril, T. J. Canadian Patent 1034686, 1978. (17) Pandy, M. P.; Chaudhurri, M. Prog. Water Technol. 1980, 12, 697. (18) Pandy, M. P.; Chaudhurri, M. Water Res. 1982, 16, 1113. (19) Masri, M. S.; Friedman, M. Environ. Sci. Technol. 1973, 7, 951. (20) Friedman, M.; Waiss, A. C. Environ. Sci. Technol. 1972, 6, 745. (21) Macchi, G. Environ. Technol. Lett. 1986, 7, 431. (22) Barkley, N. P. J. Air Waste Manage. Assoc. 1991, 41, 1387.
10.1021/ef990012+ CCC: $18.00 © 1999 American Chemical Society Published on Web 07/13/1999
Mercury Uptake from Aqueous Solution and Coal Properties
Energy & Fuels, Vol. 13, No. 5, 1999 1047
Table 1. Characteristics of the Coals Used in This Study coala
ash d.b.b %
C d.m.m.f.c %
H d.m.m.f.c %
N d.m.m.f.c %
Oe d.m.m.f.c %
S d.b.b %
Sorg. d.b.b %
Visonta (Hungary)d Skye peat (U.K.)d Mequinenza (Spain)d Hambach (Germany) Can (Turkey)d North-Dakota (U.S.A.) Tuncbilek (Turkey)d Illinois No. 6 (U.S.A.) Daw Mill (U.K.) Gedling (U.K.) Mecsek (Hungary)d Gascoigne Wood (U.K.) Pittsburgh #8 (U.S.A.) Point of Air (U.K.) Pocahontas (U.S.A.) Taff Merthyr (U.K.) Cynheidre (U.K.)
40.8 2.5 13.0 4.3 5.8 9.7 15.0 15.5 4.7 2.2 19.5 21.2 9.0 10.1 4.8 4.0 1.8
56.7 58.0 65.8 67.5 68.8 74.1 76.7 80.7 81.3 81.6 82.6 84.5 84.9 87.2 91.8 92.4 95.2
5.5 6.6 5.4 4.4 5.0 4.9 5.4 5.2 4.8 5.2 4.8 4.9 5.4 5.8 4.48 4.2 2.9
0.2 1.5 0.8 0.03 1.6 1.2 2.5 1.43 1.3 1.7 1.7 1.8 1.7 1.6 1.34 1.5 1.0
36.5 33.8 17.9 27.2 20.7 19.1 14.3 10.1 11.5 10.3 9.3 7.7 6.9 4.6 1.66 1.2 0.3
1.1 trace 10.3 0.8 3.9 0.8 1.0 4.83 1.5 1.0 1.6 1.4 2.2 1.7 0.66 0.7 0.63
n.d.f n.d.f 9.9 0.73 n.d.f 0.63 n.d.f 2.01 1.12 0.89 n.d.f 0.76 0.81 0.63 0.48 0.67 0.59
a The U.K. coals are from the British Coal Sample Bank, and the U.S.A. coal is from the Argonne Premium Coal Sample programme. The Hungarian coals are composite samples taken from a particular basin. b d.b. ) dry basis. c d.m.m.f. ) dry mineral matter free basis. d 0.1 HCl treated coals. e by difference. f n.d. ) not determined.
Nevertheless, the current interest in the reduction of mercury levels prior to combustion and the need to deal with heavy metals from power station wastes in an environmentally sensitive manner23,24 affords considerable opportunities for the development of low-cost sorbent technologies, based on naturally occurring materials, such as coals. This work forms part of an investigation into the application of coals and related materials to the cleanup of wastewater and their use in the containment of heavy metals in localized spills. This contribution describes the influence of coal properties, particularly organic sulfur and oxygen functionality, on mercury uptake from aqueous solution. The effects of pH, chloride ion concentration, and the type of associated anion are also covered, and comparisons are made with peat, commercially available active carbons, and cation-exchange resins. Experimental Section The characteristics of the coals and the peat sample used in this investigation are summarized in Table 1. In an attempt to increase the Hg uptake for bituminous coals, two of the samples investigated (Gedling and Taff Merthyr, Table 1) were oxidized in air for approximately 1 day. Experience had indicated that a higher temperature of 300 °C was appropriate for the higher rank coal investigated (Taff Merthyr, cf. 250 °C for Gedling). The model cation exchangers (R-COOH and R-SH) were Amberlite IRP-64 (Aldrich) and Duolite GT-73 (Supelco) with exchange capacities of ∼10 and ∼1.8 meq g-1, respectively. The activated carbons, namely Norit C and Norit AZO (American Norit Co.) and Chemviron-HGR (Chemviron Carbon, Neuizenburg) were industrial grade materials and were used as received. The sulfur-containing active carbon investigated (MIS-II, supplied by Montecatini Technology) had a sulfur content of 16% w/w. Reagent grade Hg(II) salts and analytical grade calcium salts were used for the metal uptake experiments. The experimental conditions used were similar to those employed in an earlier study.25 Prior to the sorption experi(23) Lafferty, C. J.; Robertson, J. D.; Parekh, B. K.; Huggins, F. E. Coal Science, Vol. 24; Pajares J. A., Tascon, J. M. D., Eds.; Elsevier: New York, 1995; p 1601. (24) Keener, T. C.; Gieske, A. C.; Khang, S. J. Coal Science, Vol. 24; Pajares, J. A., Tascon, J. M. D., Eds.; Elsevier: New York, 1995; p 1605.
ments, the coals were treated with 0.1 M hydrochloric acid (1:50 solid:liquid ratio, 24 h with magnetic stirring), washed with distilled water, and dried at 333 K in a vacuum. Comparative batchwise sorption experiments were conducted on the coals and other sorbents investigated in 0.1 M acetic acid/sodium acetate (1:1) buffer at pH 5 containing a 5, 50, or 100 mM concentration of HgCl2 using a substrate/solution mass ratio of 1:40. Experiments on selected samples were also carried out in 0.1 and 2 M hydrochloric and nitric acid solutions. A number of tests were also carried out using Hg(NO3)2. The resulting slurries were agitated once a day and, at the end of the required contact time, the phases were separated by centrifugation. The ion concentration in the solution phase was determined by atomic absorption spectrometry using a standard flame technique.
Results and Discussion Comparison of Mercury Uptakes. The uptakes of Hg(II) from a 5 mM HgCl2 solution by the active carbons, the cation-exchange resins, the peat, and a number of the coals used in this study are compared in Table 2 for the standard conditions used in the batchwise experiments. Table 2 indicates that, under these test conditions, the uptakes for the lignites (Hambach, Can, and Mequinenza samplesslow-rank coals containing 60-70% dmmf carbon, Table 1) are similar to those of the active carbons and cation-exchange resins investigated. Additionally, the uptakes for the bituminous coals (Gedling and Taff Merthyr) were increased considerably by the simple air oxidation treatments used here (Table 2). This effect is well-known in cationexchange studies of coals where oxidation can increase capacities by approximately 10-fold.26 The variation in Hg(II) sorption with maturity (rank) for all the coals investigated is presented in Figure 1 where the carbon content (dmmf basis) is used as the rank parameter, the initial concentrations of HgCl2 and Hg(NO3)2 being 100 mM. The sorption capacities clearly decrease as a function of rank, but the bituminous coals can still bind significant amounts of Hg(II). (25) Lakatos, J.; Goksel, A.; Brown, S. D.; Snape, C. E. Proceedings of the Sixth Symposium on Mining Chemistry, 1998, Siofok, Hungary. (26) Lakatos, J.; Brown, S. D.; Snape, C. E. Energy Fuels 1997, 11, 1101, and references therein.
1048 Energy & Fuels, Vol. 13, No. 5, 1999
Lakatos et al.
Table 2. Comparison of Hg(II) Ion Removal from 5 mM HgCl2 0.1 M Sodium Acetate-Acetic Acid (1:1) Buffer Solution by Different Substancesa
substance name
Hg(II) concentration after sorption csolution (mM)
Hg(II) remaining in soln (%)
Norit AZO Chemviron Mis-II Norit C R-SH resin R-COOH resin Skye peat (Scotland) Hambach lignite (Germany) Can lignite (Turkey) Mequinenza lignite (Spain) Visonta lignite(Hungary) Gedling bituminous coal (U.K.) Mecsek bituminous coal (Hungary) Taff Merthyr bituminous coal (U.K.) Pittsburgh #8 bituminous coal (U.S.A.) Gedling 250 °C 28 h Taff Merthyr 300 °C 24 h
0.31 0.19 0.12 0.38 0.04b 2.88c 0.61 0.21 0.21 0.12 0.61 1.85 2.07 3.66 3.56 0.11 0.82
6.2 3.8 2.4 7.6 0.8b 58c 12 4.2 4.2 2.4 1.2 37 41 73 71 2.2 16
substance type active carbons
cation-exchange resins coals
oxidized coals a
Coal/solution mass ratio 1:40. Contact time: 9 days. b Wet resin (56% moisture). c Wet resin (71.5% moisture).
Table 3. Comparison of Metal Ion Removal by Coals from 5 mM Solutions and 0.1 M Sodium Acetate-Acetic Acid (1:1) Buffer Solutions concentration of sorbed metal ionsb (mM g-1) coal
CEC (Ca)
Ca
Mn
Ni
Cu
Pd
Hg
Skye Mequinenza Gedling Oxidized Gedling (250 °C/28 h) Taff-Merthyr Oxidized Taff-Merthyr (300 °C /24 h)
0.73 0.40 0.01 0.87
