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Removal of mercury in aqueous solution by fluidized bed plant fly ash
Fuel 82 (2003) 153–159 www.fuelfirst.com
Removal of mercury in aqueous solution by fluidized bed plant fly ashq Se´bastien Rio, Arnaud Delebarre* Energetics and Environmental Engineering, Ecole des Mines de Nantes, 4, rue Alfred Kastler, BP 20722, F-44307 Nantes Cedex 3, France Received 7 March 2002; revised 22 July 2002; accepted 23 July 2002; available online 2 September 2002
Abstract Coal combustion in power plant produces fly ash. Fly ash may be used in water treatment to remove mercury (Hg2þ) from water or to immobilize mercury mobile forms in silts and soils. Experiments were carried out on two kinds of fly ashes produced by two circulating fluidized bed plants with different chemical composition: silico-aluminous fly ashes and sulfo-calcic fly ashes. For the two kinds of fly ashes, adsorption equilibrium were reached in 3 days. Furthermore, removal of mercury was increased with increasing pH. Sulfo-calcic fly ashes allow us to remove mercury more efficiently and more steady. The chemical analysis of fly ash surface was carried out by electron spectroscopy. The results show that mercury is bound to ash surface thanks to several chemical reactions between mercury and various oxides (silicon, aluminium and calcium silicate) of the surface of the ashes. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fly ash; Fluidized bed; Mercury; Adsorption; Leaching; Electron spectroscopy for chemical analysis
1. Introduction Mercury is known for its toxicity towards the aquatic environment. The discharge of effluents containing mercury in the environment can constitute a threat to the aquatic life and has serious repercussions on the food chain. This extreme toxicity, the bioaccumulation of this element in soil and sediments, but also numerous important ecological accidents, notably between 1953 and 1956 in Minamata’s bay in Japan [1] have contributed to the development of treatment processes of effluents containing mercury. The adsorption of metallic ions at fluid/solid interface has been studied for several years, as well as the use of some so-called low cost sorbents. Moreover, fly ashes produced by coal combustion are considered in numerous studies aimed at their valorization. Different applications (cements, roads and backfill) already allow a recycling of an important part of fly ash production which, for instance, reached 450,000 tons in 1997 in France [2]. The use of fly ashes for metallic ions removal from aqueous solution was studied in numerous works [3 – 5,17] and some experiments have * Corresponding author. Tel.: þ 33-2-51-85-82-53; fax: þ33-2-51-85-8299. E-mail address: [email protected] (A. Delebarre). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
showed that fly ashes might be benefic for mercury removal [6,7]. The widespread occurrence of mercury pollution from industrial sources and the availability of power station fly ashes as a high tonnage industrial byproduct suggest the potential for remediation techniques and particularly the immobilization of mobile forms of mercury by fly ash in silts and soils. Moreover, two studies about mercury contamination of a river system showed the mercury to be strongly bound to ash deposits into the river from a thermal power station. For instance the Nura river in Kazakhstan is suspected to have had mercury concentration up to 50 mg l21 between 1950 and 1999 and that around 140 ton of mercury is present in the bed of this river due to local discharge of calcium hydroxide by a nearby industrial production plant [8,15]. The aim of the present work was to study the removal of mercury (II) in aqueous solution by fly ashes withdrawn from two industrial circulating fluidized bed boilers. At first, the adsorption of mercury present in aqueous solutions onto fly ashes was studied in static reactor. Then a leaching test was also carried out to estimate the capacity of solids to retain durably the mercuric ions. Finally, the surface of spent ash samples after the adsorption experiments were investigated to understand mechanisms involved by mercury adsorption.
0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 2 3 7 - 5
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Table 1 Chemical composition of silico-aluminous and sulfo-calcic fly ashes (mass%) Constituent
Silico-aluminous fly ash
Sulfo-calcic fly ash
SiO2 Al2O3 Fe2O3 TiO2 CaO MgO MnO K2 O Na2O P2O5 SO3 Hg (mg kg21)
44.9 22.0 8.4 0.8 5.7 3.5 0.2 4.0 0.3 0.1 4.0 ,0.5
43.5 18.0 7.4 0.7 16.9 1.4 0.1 2.2 0.3 0.4 9.1 ,0.03
2. Materials and methods 2.1. Characteristics of fly ashes Two types of fly ashes were tested in this study: fly ashes from a circulating fluidized bed power plant located in the Northeast of France, and fly ashes from another circulating fluidized bed power plant situated in the Southeast of France. The sampling was carried out at the flue gas dedusting systems of the two plants. The chemical composition of these two fly ashes is given in Table 1. The first type of fly ashes is considered to be silico-aluminous because the cumulated percentage of silica and alumina is above 65% and the amount of calcium oxide is poor, whilst the second type fly ashes are called sulfocalcic because of their large amount of sulfur trioxide and calcium oxide. Their carbon content is low according to their loss of ignition, which is in the order of 1%. The Hg content of the silico-aluminous ashes is below 0.5 £ 1023 mg g21 whilst the one of the sulfo-calcic ashes is less than 0.03 £ 1023 mg g21. Moreover, the X-photon spectrophotometry (XPS) analysis detected neither mercury
nor carbon on the surface of the ash samples before their use in the sorption experiments. These two fly ashes have a specific area of 12 m2 g21 and their particles median diameter, i.e. the diameter that share the particle size distribution in two equal parts, is about 20 mm. 2.2. Adsorption experiments Two series of experiments were carried out: an Hgadsorption kinetic series and an Hg-adsorption isotherms series. The first one was aimed at completing a set of experiments of adsorption previously done with the same fly ashes and other metals in the given experimental conditions [9]. For that reason, same mercury concentration, pH and temperature conditions were chosen. A second objective of the first series was to determine the time necessary to reach the steady state and the fly ash removal efficiency. For the first series of Hg-adsorption tests, the initial mercuric ion concentration was chosen equal to 3 £ 1023 mol Hg l21 (602 mg Hg l21) and the ash concentration to 100 g l21. Solutions and ashes were stirred at 250 rpm in a thermostated bath at 30 8C. During the experiments, pH of solutions was constantly controlled to three set values 3– 5 by the addition of HCl. Samples of 10 ml of reaction were taken every day and then filtered by the mean of a membrane (porosity 0.45 mm). The objective of the second series of Hg-adsorption experiments was to draw the adsorption isotherms. They were carried out in 250 ml glasses where aqueous solutions of mercury were mixed with fly ashes at a concentration of 100 g l21. Mercury concentration was set to values from 5 £ 1027 mol l21 (100.3 mg l21) to 5 £ 10 23 mol l 21 (1.003 g l21) and pH of the suspension was controlled at the value 5. These experiments were carried out at 30 8C and 250 rpm. Samples of the solution were regularly withdrawn and filtered on a membrane. In order to estimate the stability of the adsorption, the
Fig. 1. Kinetic curves of mercury removal onto silico-aluminous and sulfo-calcic fly ashes. [Hg2þ]0 ¼ 602 mg l21; m ¼ 100 g l21; T ¼ 30 8C; V ¼ 500 ml.
S. Rio, A. Delebarre / Fuel 82 (2003) 153–159
Fig. 2. Speciation diagram of mercury established with SPECIA software for an initial concentration of Hg2þ equal to 5 £ 1023 mol l21. Constants of hydrolyses were extracted from Ref. [16].
spent adsorption cakes were submitted to a leaching test based on the protocol of the French standard AFNOR X31210 after a 4-month drying. Five grams of solid were mixed with 50 ml of demineralized water of pH 7 with a magnet rotation controlled at 150 rpm during 24 h. A sample of the leaching solution was then taken, filtered on a membrane and then analyzed.
155
Fig. 4. Adsorption isotherms of mercury on silico-aluminous and sulfocalcic fly ashes. [Hg2þ]0 was comprised between 100.06 mg l21 and 1.003 g l21, m ¼ 25 g, T ¼ 30 8C, V ¼ 250 ml, pH ¼ 5.
done using the 1s layer electron peaks for oxygen and of the 2p layer electrons for aluminium, silicon, calcium and sulfur to optimize their intensity.
3. Results and discussion 3.1. Adsorption kinetic
2.3. Analytical methods The analysis of mercuric ions in solution was made by a Perkin – Elmer atomic absorption spectrophotometry apparatus (Perkin – Elmer, 1994). An atomic fluorescence spectrometer (Axe 10023) [10] was used when mercury concentrations were low. XPS analysis was also carried out by the means of a Leybold LHS 12 spectrometer. The XPS analysis was aimed at measuring the type of bounds between mercury and the fly ash surface. At the first stage, a preliminary spectrum with an incident photon energy ranging from 0 to the maximum was recorded to identify the most interesting regions. Then, a detailed spectrum was carried out on these specific areas. The interpretation of the peak spectrum was
Fig. 3. Adsorption isotherms of mercury on silico-aluminous and sulfocalcic fly ashes. [Hg2þ]0 varied from 100.3 mg l21 to 20.06 mg l21, m ¼ 25 g, T ¼ 30 8C, V ¼ 250 ml, pH ¼ 5.
Results of adsorption kinetic experiments are presented in Fig. 1. It can be seen that the equilibrium of adsorption reaction was nearly reached after 72 h for both types of ashes. The rate of adsorption is faster and the steady-state removal is higher for the sulfo-calcic fly ashes than for silico-aluminous fly ashes whatever the pH value of the aqueous solution is. The maximum adsorption capacities at equilibrium were equal to 3.2 mg g21 for silico-aluminous ashes and 4.9 mg g21 for sulfo-calcic ashes, corresponding to removal efficiencies of 53 and 81%, respectively. This difference in efficiency between the two fly ashes might be related to their chemical composition: silico-aluminous fly ashes contain less of sulfur trioxide than sulfo-calcic ashes. As a matter of fact, mercury has a strong affinity for sulfur and these two compounds bind themselves easily as it is the case for the cinnabar HgS [1]. Furthermore, sulfo-calcic fly ashes was containing 17% of lime, and the dissolution of this oxide during mixing of fly ashes with aqueous solutions would have allowed to enhance the number of available adsorption sites for mercuric ions [3]. According to mercury speciation diagram (Fig. 2), HgCl2 is the major mercury compound that was present in the solution, in the initial conditions of the experiments, i.e. pH ¼ 5 and initial Hg concentration ¼ 5 £ 1023 mol Hg l21. In other respects, the surface charges of the ashes became negative when the pH of the solution increased from 3 to 5, because the pH of the zero point charge, (pHZPC) of silico-aluminous fly ashes is 2.7 and that of sulfo-calcic fly ashes is 2.9. Nevertheless, considering that HgCl2 has no electrostatic charge, the mercury removal might have taken place by chemical
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Fig. 5. Speciation diagram of mercury established with SPECIA software for an initial concentration of Hg2þ equal to 5 £ 1027 mol l21. Constants of hydrolyses were extracted from Ref. [16].
reactions with molecules constituting the surface of fly ashes (chemisorption) rather than by electrostatic interactions between mercury and the surface of ashes (physisorption) [11]. This situation might have prevailed as long as the concentration of mercury as well as the unchanged pH were imposing a major neutral speciation in the aqueous solution.
initial concentration is higher (C0 $ 100 mg l21), isotherms are concave indicating a less favorable adsorption of mercury (Fig. 4). Finally, when the initial concentration in mercury is even higher, the adsorption capacities remain constant. This first stage might have corresponded to the saturation of the adsorption sites by surface complexation phenomenon. Then, a second layer of mercury might have been adsorbed by hydrophobic interactions between atoms of mercury present in solution and mercuric complexes already adsorbed on the surface of fly ashes [12]. When Hg2þ concentration was weak, the predominant mercury 2 species in solution were HgCl22 4 and HgCl3 (Fig. 5). During the adsorption experiments at pH equal to 5, the surface of ashes was negatively charged. Attraction phenomenon between surface sites negatively charged and the ion Hg2þ complexed by the ions Cl2 might explain the removal of these negatively charged complexes in solution [13]. When mercury concentration was increased, the major compound in solution became HgCl2 whilst the other minor species 22 were HgCl2 3 and HgCl4 (Fig. 2). After the saturation of the surface sites presenting strong affinities for mercury, mercury removal would occurred by chemical bonds between this compound and the mercury species already adsorbed at fly ash surface [13].
3.2. Adsorption isotherms
3.3. Leaching test
Adsorption isotherms carried out with sulfo-calcic and silico-aluminous ashes are presented in Figs. 3 and 4. Adsorption capacity of sulfo-calcic fly ashes is more important than that of silico-aluminous ashes as during the kinetic experiments and they were equal to 5.0 and 3.2 mg g21, respectively. Furthermore, the shapes of the adsorption isotherms are similar for both types of fly ashes indicating that identical mechanisms would have been responsible for mercury removal. When different orders of magnitude of concentration, adsorption isotherms are considered, different steps and processes of the adsorption seem to have occurred. On the one hand, when mercury concentration was weak, the ascending convex shape of the curves indicates a favorable adsorption of mercury onto the surface of fly ashes (Fig. 3). On the other hand, when the
The results of the leaching test carried out on adsorption cakes resulting from adsorption experiments are presented in Table 2. For both fly ash types, a pH fixed to the value 5 during adsorption experiments yielded a minimum percentage of leaching. The pH conditions maximizing the adsorption of the mercury onto fly ashes were thus identical to those minimizing the leaching of this compound. Sulfocalcic fly ashes, which were more effective than silicoaluminous one to remove mercury in aqueous solution, also present a higher stability towards this metal. When the pH is fitted to the value 5, only 7% of mercuric ions adsorbed onto sulfo-calcic fly ashes were released into leaching solution while about 16% of mercury removed by silico-aluminous ashes returned to the leaching solution after 24 h of contacting time.
Table 2 Measurements of Hg removed from aqueous solutions by ashes (%), measurements of leached Hg during leaching tests of cakes (V ¼ 50 ml, m ¼ 5 g, reaction time ¼ 24 h) related to the initial Hg concentration in ashes (%), calculated net abatement after Hg removal from aqueous solutions and the leached quantities (%) Fly ashes
pH of adsorption experiments
Hg2þ removal (%)
Hg2þ leached (%)
Hg2þ net abatement (%)
Silico-aluminous
3 4 5
42.1 43.4 52.7
22.3 19.2 16.3
32.8 35.0 44.1
Sulfo-calcic
3 4 5
70.2 78.6 81.2
9.6 8.1 7.2
63.4 72.3 75.3
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Table 3 Positions, intensities, length of middle height (LMH) and percentage of peaks obtained after experimental peak decomposition of silico-aluminous and sulfocalcic fly ashes Element
Sulfo-calcic fly ash
Silico-aluminous fly ash
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
O 1s O 1s O 1s
532.12 534.72 536.12
4733 19,098 24,543
2.20 2.20 2.20
57.3
533.12 535.25 536.36
6090 34,227 27,985
2.20 2.20 2.20
59.5
Si 2p Si 2p
105.92 107.02
3679 1840
2.00 2.00
13.1
106.13 107.13
5476 2738
2.12 2.12
14.2
Al 2p
77.98
2078
2.10
6.3
78.02
3856
2.15
7.5
Ca 2p Ca 2p
351.56 355.11
4059 2030
2.52 2.52
4.8
351.95 355.50
1723 861
2.36 2.36
2.0
S 2p S 2p
172.82 174.00
963 482
2.39 2.39
1.4
Not detected
When the pH increased, the dissolution of calcium oxide was observed. Fly ashes, which were mainly siliceous and aluminous products, were able to react with lime to form hydrated calcium silicate or hydrated calcium aluminate having binder properties. It allows to obtain a pouzzolanic material which would have increased the stability of the mercury adsorption [3]. 3.4. XPS analysis XPS spectrums were recorded for the silico-aluminous and sulfo-calcic fly ashes as received and also after the so-called adsorption tests. The results are presented in Table 3 together with the peak characteristics. In the case of silico-aluminous fly ashes before adsorption, the main detected peaks were those of oxygen, carbon, calcium, aluminium, silicon and potassium. The sulfo-calcic exhibited the same peaks excepted the peak of potassium
which was not present whilst a sulfur peak was identified. Concerning the speciation of these elements on the ‘as received’ fly ash surfaces, the XPS peak of sulfur revealed the presence of sulfate on sulfo-calcic fly ashes surface [14]. Furthermore, the position of the aluminium peak showed that the compound Al2O3 would not have been the species present on the surface but rather an aluminium halogenous AlX3. From the combined observation of the oxygen, silicon and aluminium peaks, the compounds SiO2 a quartz, mullite and/or sillimanite (Al2SiO5) were also identified on the surface of both types of ashes. Furthermore, the study of the calcium peak proved the existence of calcium silicate hydrate Ca3Si3O9 although it was not identified by a combination with the silicon and oxygen peaks. The position of Ca and O peaks suggested the existence of the compound Ca(OH)2 on fly ashes surface even if lime is not listed in XPS data [14].
Table 4 Positions, intensities, LMH and percentage of peaks obtained after experimental peak decomposition of fly ashes adsorption cakes Element
Sulfo-calcic fly ash
Silico-aluminous fly ash
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
O 1s O 1s O 1s
532.32 534.64 535.98
4189 11,240 23,661
2.25 2.25 2.25
54.3
533.25 535.55 536.13
5189 28,131 12,464
2.20 2.20 2.20
56.2
Si 2p Si 2p
105.78 107.10
2165 1083
2.10 2.10
11.1
105.95 107.02
3599 1799
1.95 1.95
13.8
Al 2p
78.65
1279
2.14
8.0
78.80
2727
2.28
9.1
Ca 2p Ca 2p
351.75 355.32
4374 1687
2.42 2.42
5.1
352.12 355.84
1309 655
2.48 2.48
1.6
S 2p S 2p
172.74 173.90
1003 502
2.39 2.39
1.6
Not detected
Hg 2p Hg 2p
103.35 107.40
811 435
3.05 3.05
0.7
103.56 108.27
654 212
3.16 3.16
0.5
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Adsorption cakes of ashes resulting from the adsorption kinetic tests at pH equal to 5 were also analyzed by XPS. The aim was to understand the behavior of oxides of the fly ashes surface after their contact with a mercury aqueous solution and particularly the nature of the bonds between mercury and surface oxides. The XPS spectrums are presented in Table 4. The peak intensities obtained with the adsorption cake (Table 4) were compared to those of the as received fly ash spectrums (Table 3). Oxygen relative intensities of the adsorption cakes were lower than those observed on the fly ash spectrums that might result from a partial dissolution of silica and calcium oxides [3]. The silicon relative intensities also decreased that would confirm this hypothesis. Unlike the case of the fly ash spectrums, the aluminium peak of the cakes was undoubtedly the one of oxides according to Ref. [14], and not the aluminium halogenous (AlX3) that was the most probable for the ashes. Besides, Ricou also observed the same transformation of aluminium halogenous in aluminium oxides during metal adsorption on fly ashes from pulverized coal power plants. Furthermore, the sulfur peak analysis carried out to find out the different sulfur species did not confirm the formation of bonds between mercury and sulfur surface sites of ashes. On the other hand, different attempts to analyze the mercury peaks were not convincing because of the partial superposition of the silicon and mercury peaks. Therefore, no mercury compound was identified with certainty. However, the known mercury species that might give the peak Hg 2p at 103.35 ^ 0.2 eV are HgCl2 and HgO, the first one being the mercury species that was used in the prepared aqueous solutions. The compound SiO2 a quartz was identified as it was the case for the fly ash spectrums before adsorption tests from the same silicon and oxygen peak analysis, but not the mullite and sillimanite that were detected on the as received fly ash surface. It could be explained by the partial dissolution of these compounds during mixing of fly ashes with aqueous solutions of mercury. Then, the deconvolution of the calcium peaks showed that the compound Ca(OH)2 was present on adsorption cakes surface as well as Ca3Si3O9. The calcium peaks also suggested that a part of the lime has remained free during the adsorption experiments and the drying of the adsorption cakes. Therefore, these analyses showed that silicon and aluminium oxides were responsible for mercury adsorption on fly ashes surface, whilst the role of sulfur has not been revealed although the affinity of mercury for this element is well known at the natural state. The existence of hydrated calcium silicate Ca3Si3O9 in adsorption cakes was detected and would result from pouzzolanic reactions occurring during the drying phase of the cakes [3]. However, this hydrate did not bring an increase in fly ashes adsorption capacity but would rather enhance the adsorbed mercury
stability by the development of a hydrated calcium silicate layer at the surface.
4. Conclusions Mercury adsorption from aqueous solution onto silicoaluminous fly ashes and sulfo-calcic fly ashes produced by two circulating fluidized bed boilers was studied. Kinetic experiments first showed that, accounting for the pH and temperature of the test, steady state was reached in 3 days. It was also observed that sulfo-calcic fly ashes were more effective than silico-aluminous fly ashes to remove mercury. Sulfo-calcic fly ashes and silico-aluminous fly ashes adsorption capacities were equal to 5.0 and 3.2 mg g21, respectively. Then, a leaching test has showed that the adsorbed mercury was more stably adsorbed by the sulfocalcic fly ashes. This screening experiment plan has thus proved that these circulating fly ashes might behave as an efficient low cost sorbent for the mercury. This property will also be studied in the frame of a project of the European funding programme INTAS where Kazakhstan laboratories will study the remediation of rivers polluted by mercury using fly ashes from old ash lagoons or fresh fly ashes from power plants. Fly ash surface analysis revealed the existence of silicon and aluminium oxides (silica, mullite and sillimanite), aluminium halogenous and calcium oxide. The XPS analyses showed that adsorption mechanisms of mercury were corresponding to its chemical reactions with different oxides present on fly ash surface. Hydration of silica, presence of aluminium oxides and formation of hydrated calcium silicate are phenomena that could take place during mercury adsorption. Finally, pouzzolanics reactions development into adsorption cakes and surface covering by hydrated calcium silicate could be responsible for resulting cakes stability.
References [1] Picot A, Proust N. Le mercure et ses compose´s: de la spe´ciation a` la toxicite´. L’actualite´ Chimique 1998;4:16– 24. [2] Blondin J. CFBC ash valorisation routes by the French SNET company. Rapport CERCHAR; 2000. p. 15, personal communication. [3] Ricou P. Proce´de´s d’e´limination d’ions me´talliques sur cendres volantes de charbon et sur me´langes cendres/chaux. Spe´cialite´ physico-chimie des interfaces. The`se de l’universite´ de Nantes; 1998. p. 302. [4] Apak R, Tutem E, Hugul M, Hizal J. Heavy metal cation retention by unconventional sorbents (red muds and fly ashes). Water Res 1998; 32(2):430–40. [5] Ayala J, Blanco F, Garcia P, Rodriguez P, Sancho J. Asturian fly ash as a heavy metals removal material. Fuel 1998;77(11):1147–54. [6] Sen AK, De AK. Adsorption of mercury (II) by coal fly ash. Water Res 1987;21(8):885–9. [7] Kapoor A, Viraraghavan T. Adsorption of mercury from wastewater by fly ash. Environ Sci Technol 1992;9(3):130–47.
S. Rio, A. Delebarre / Fuel 82 (2003) 153–159 [8] Heaven S, Ilyushchenko MA, Tanton TW, Ullrich SM, Yanin EP. Mercury in the River Nura and its floodplain, Central Kazakhstan. I. River sediments and water. Sci Total Environ 2000;260:35–44. [9] Rio S, Delebarre A, He´quet V, Le Cloirec P, Blondin J. Metallic ion removal from aqueous solutions by fly ashes: multicomponent studies. In: Nugteren H, editor. Morella International Workshop 2001. p. 189– 96. [10] Sanjuan J, Cossa D. Dosage automatique du mercure total dans les organismes marins par fluorescence atomique. IFREMER, Nantes; De´cembre 1993. [11] Carrott PJM, Ribeiro-Carrott MML, Nabais JMV. Influence of surface ionization on the adsorption of aqueous mercury chlorocomplexes by activated carbons. Carbon 1998;36(1/2):11– 17. [12] Desauziers V, Castre N, Le Cloirec P. Sorption of methylmercury by clays and mineral oxides. Environ Technol 1997;18(1):1009 –18.
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[13] Semu E, Singh BR, Selmer-Olsen AR. Adsorption of mercury compounds by tropicals soils (III-Adsorption isotherms). Water, Air Soil Pollution 1987;32:11–16. [14] Molder JF, Stickle WF, Sobol PE. In: Chastain J, King RC Jr., editors. Perkin-Elmer Corporation, USA. Handbook of X-ray photoelectron spectroscopy. 1995. [15] Heaven S, Ilyushchenko MA, Kamberov IM, Politikov MI, Tanton TW, Ullrich SM, Yanin EP. Mercury in the River Nura and its floodplain, Central Kazakhstan. II. Floodplain soils and riverbank silt deposits. Sci Total Environ 2000;260:45 –55. [16] Smith RM, Mortell AE. Critical stability constants. Inorganics complexes, vol. 4. New York: Plenum Press; 1989. [17] Panday KK, Prasad G, Singh VN. Copper (II) removal from aqueous solutions by fly ash. Water Res 1985;19(2):869– 73.
Removal of mercury in aqueous solution by fluidized bed plant fly ashq Se´bastien Rio, Arnaud Delebarre* Energetics and Environmental Engineering, Ecole des Mines de Nantes, 4, rue Alfred Kastler, BP 20722, F-44307 Nantes Cedex 3, France Received 7 March 2002; revised 22 July 2002; accepted 23 July 2002; available online 2 September 2002
Abstract Coal combustion in power plant produces fly ash. Fly ash may be used in water treatment to remove mercury (Hg2þ) from water or to immobilize mercury mobile forms in silts and soils. Experiments were carried out on two kinds of fly ashes produced by two circulating fluidized bed plants with different chemical composition: silico-aluminous fly ashes and sulfo-calcic fly ashes. For the two kinds of fly ashes, adsorption equilibrium were reached in 3 days. Furthermore, removal of mercury was increased with increasing pH. Sulfo-calcic fly ashes allow us to remove mercury more efficiently and more steady. The chemical analysis of fly ash surface was carried out by electron spectroscopy. The results show that mercury is bound to ash surface thanks to several chemical reactions between mercury and various oxides (silicon, aluminium and calcium silicate) of the surface of the ashes. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fly ash; Fluidized bed; Mercury; Adsorption; Leaching; Electron spectroscopy for chemical analysis
1. Introduction Mercury is known for its toxicity towards the aquatic environment. The discharge of effluents containing mercury in the environment can constitute a threat to the aquatic life and has serious repercussions on the food chain. This extreme toxicity, the bioaccumulation of this element in soil and sediments, but also numerous important ecological accidents, notably between 1953 and 1956 in Minamata’s bay in Japan [1] have contributed to the development of treatment processes of effluents containing mercury. The adsorption of metallic ions at fluid/solid interface has been studied for several years, as well as the use of some so-called low cost sorbents. Moreover, fly ashes produced by coal combustion are considered in numerous studies aimed at their valorization. Different applications (cements, roads and backfill) already allow a recycling of an important part of fly ash production which, for instance, reached 450,000 tons in 1997 in France [2]. The use of fly ashes for metallic ions removal from aqueous solution was studied in numerous works [3 – 5,17] and some experiments have * Corresponding author. Tel.: þ 33-2-51-85-82-53; fax: þ33-2-51-85-8299. E-mail address: [email protected] (A. Delebarre). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
showed that fly ashes might be benefic for mercury removal [6,7]. The widespread occurrence of mercury pollution from industrial sources and the availability of power station fly ashes as a high tonnage industrial byproduct suggest the potential for remediation techniques and particularly the immobilization of mobile forms of mercury by fly ash in silts and soils. Moreover, two studies about mercury contamination of a river system showed the mercury to be strongly bound to ash deposits into the river from a thermal power station. For instance the Nura river in Kazakhstan is suspected to have had mercury concentration up to 50 mg l21 between 1950 and 1999 and that around 140 ton of mercury is present in the bed of this river due to local discharge of calcium hydroxide by a nearby industrial production plant [8,15]. The aim of the present work was to study the removal of mercury (II) in aqueous solution by fly ashes withdrawn from two industrial circulating fluidized bed boilers. At first, the adsorption of mercury present in aqueous solutions onto fly ashes was studied in static reactor. Then a leaching test was also carried out to estimate the capacity of solids to retain durably the mercuric ions. Finally, the surface of spent ash samples after the adsorption experiments were investigated to understand mechanisms involved by mercury adsorption.
0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 2 3 7 - 5
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S. Rio, A. Delebarre / Fuel 82 (2003) 153–159
Table 1 Chemical composition of silico-aluminous and sulfo-calcic fly ashes (mass%) Constituent
Silico-aluminous fly ash
Sulfo-calcic fly ash
SiO2 Al2O3 Fe2O3 TiO2 CaO MgO MnO K2 O Na2O P2O5 SO3 Hg (mg kg21)
44.9 22.0 8.4 0.8 5.7 3.5 0.2 4.0 0.3 0.1 4.0 ,0.5
43.5 18.0 7.4 0.7 16.9 1.4 0.1 2.2 0.3 0.4 9.1 ,0.03
2. Materials and methods 2.1. Characteristics of fly ashes Two types of fly ashes were tested in this study: fly ashes from a circulating fluidized bed power plant located in the Northeast of France, and fly ashes from another circulating fluidized bed power plant situated in the Southeast of France. The sampling was carried out at the flue gas dedusting systems of the two plants. The chemical composition of these two fly ashes is given in Table 1. The first type of fly ashes is considered to be silico-aluminous because the cumulated percentage of silica and alumina is above 65% and the amount of calcium oxide is poor, whilst the second type fly ashes are called sulfocalcic because of their large amount of sulfur trioxide and calcium oxide. Their carbon content is low according to their loss of ignition, which is in the order of 1%. The Hg content of the silico-aluminous ashes is below 0.5 £ 1023 mg g21 whilst the one of the sulfo-calcic ashes is less than 0.03 £ 1023 mg g21. Moreover, the X-photon spectrophotometry (XPS) analysis detected neither mercury
nor carbon on the surface of the ash samples before their use in the sorption experiments. These two fly ashes have a specific area of 12 m2 g21 and their particles median diameter, i.e. the diameter that share the particle size distribution in two equal parts, is about 20 mm. 2.2. Adsorption experiments Two series of experiments were carried out: an Hgadsorption kinetic series and an Hg-adsorption isotherms series. The first one was aimed at completing a set of experiments of adsorption previously done with the same fly ashes and other metals in the given experimental conditions [9]. For that reason, same mercury concentration, pH and temperature conditions were chosen. A second objective of the first series was to determine the time necessary to reach the steady state and the fly ash removal efficiency. For the first series of Hg-adsorption tests, the initial mercuric ion concentration was chosen equal to 3 £ 1023 mol Hg l21 (602 mg Hg l21) and the ash concentration to 100 g l21. Solutions and ashes were stirred at 250 rpm in a thermostated bath at 30 8C. During the experiments, pH of solutions was constantly controlled to three set values 3– 5 by the addition of HCl. Samples of 10 ml of reaction were taken every day and then filtered by the mean of a membrane (porosity 0.45 mm). The objective of the second series of Hg-adsorption experiments was to draw the adsorption isotherms. They were carried out in 250 ml glasses where aqueous solutions of mercury were mixed with fly ashes at a concentration of 100 g l21. Mercury concentration was set to values from 5 £ 1027 mol l21 (100.3 mg l21) to 5 £ 10 23 mol l 21 (1.003 g l21) and pH of the suspension was controlled at the value 5. These experiments were carried out at 30 8C and 250 rpm. Samples of the solution were regularly withdrawn and filtered on a membrane. In order to estimate the stability of the adsorption, the
Fig. 1. Kinetic curves of mercury removal onto silico-aluminous and sulfo-calcic fly ashes. [Hg2þ]0 ¼ 602 mg l21; m ¼ 100 g l21; T ¼ 30 8C; V ¼ 500 ml.
S. Rio, A. Delebarre / Fuel 82 (2003) 153–159
Fig. 2. Speciation diagram of mercury established with SPECIA software for an initial concentration of Hg2þ equal to 5 £ 1023 mol l21. Constants of hydrolyses were extracted from Ref. [16].
spent adsorption cakes were submitted to a leaching test based on the protocol of the French standard AFNOR X31210 after a 4-month drying. Five grams of solid were mixed with 50 ml of demineralized water of pH 7 with a magnet rotation controlled at 150 rpm during 24 h. A sample of the leaching solution was then taken, filtered on a membrane and then analyzed.
155
Fig. 4. Adsorption isotherms of mercury on silico-aluminous and sulfocalcic fly ashes. [Hg2þ]0 was comprised between 100.06 mg l21 and 1.003 g l21, m ¼ 25 g, T ¼ 30 8C, V ¼ 250 ml, pH ¼ 5.
done using the 1s layer electron peaks for oxygen and of the 2p layer electrons for aluminium, silicon, calcium and sulfur to optimize their intensity.
3. Results and discussion 3.1. Adsorption kinetic
2.3. Analytical methods The analysis of mercuric ions in solution was made by a Perkin – Elmer atomic absorption spectrophotometry apparatus (Perkin – Elmer, 1994). An atomic fluorescence spectrometer (Axe 10023) [10] was used when mercury concentrations were low. XPS analysis was also carried out by the means of a Leybold LHS 12 spectrometer. The XPS analysis was aimed at measuring the type of bounds between mercury and the fly ash surface. At the first stage, a preliminary spectrum with an incident photon energy ranging from 0 to the maximum was recorded to identify the most interesting regions. Then, a detailed spectrum was carried out on these specific areas. The interpretation of the peak spectrum was
Fig. 3. Adsorption isotherms of mercury on silico-aluminous and sulfocalcic fly ashes. [Hg2þ]0 varied from 100.3 mg l21 to 20.06 mg l21, m ¼ 25 g, T ¼ 30 8C, V ¼ 250 ml, pH ¼ 5.
Results of adsorption kinetic experiments are presented in Fig. 1. It can be seen that the equilibrium of adsorption reaction was nearly reached after 72 h for both types of ashes. The rate of adsorption is faster and the steady-state removal is higher for the sulfo-calcic fly ashes than for silico-aluminous fly ashes whatever the pH value of the aqueous solution is. The maximum adsorption capacities at equilibrium were equal to 3.2 mg g21 for silico-aluminous ashes and 4.9 mg g21 for sulfo-calcic ashes, corresponding to removal efficiencies of 53 and 81%, respectively. This difference in efficiency between the two fly ashes might be related to their chemical composition: silico-aluminous fly ashes contain less of sulfur trioxide than sulfo-calcic ashes. As a matter of fact, mercury has a strong affinity for sulfur and these two compounds bind themselves easily as it is the case for the cinnabar HgS [1]. Furthermore, sulfo-calcic fly ashes was containing 17% of lime, and the dissolution of this oxide during mixing of fly ashes with aqueous solutions would have allowed to enhance the number of available adsorption sites for mercuric ions [3]. According to mercury speciation diagram (Fig. 2), HgCl2 is the major mercury compound that was present in the solution, in the initial conditions of the experiments, i.e. pH ¼ 5 and initial Hg concentration ¼ 5 £ 1023 mol Hg l21. In other respects, the surface charges of the ashes became negative when the pH of the solution increased from 3 to 5, because the pH of the zero point charge, (pHZPC) of silico-aluminous fly ashes is 2.7 and that of sulfo-calcic fly ashes is 2.9. Nevertheless, considering that HgCl2 has no electrostatic charge, the mercury removal might have taken place by chemical
156
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Fig. 5. Speciation diagram of mercury established with SPECIA software for an initial concentration of Hg2þ equal to 5 £ 1027 mol l21. Constants of hydrolyses were extracted from Ref. [16].
reactions with molecules constituting the surface of fly ashes (chemisorption) rather than by electrostatic interactions between mercury and the surface of ashes (physisorption) [11]. This situation might have prevailed as long as the concentration of mercury as well as the unchanged pH were imposing a major neutral speciation in the aqueous solution.
initial concentration is higher (C0 $ 100 mg l21), isotherms are concave indicating a less favorable adsorption of mercury (Fig. 4). Finally, when the initial concentration in mercury is even higher, the adsorption capacities remain constant. This first stage might have corresponded to the saturation of the adsorption sites by surface complexation phenomenon. Then, a second layer of mercury might have been adsorbed by hydrophobic interactions between atoms of mercury present in solution and mercuric complexes already adsorbed on the surface of fly ashes [12]. When Hg2þ concentration was weak, the predominant mercury 2 species in solution were HgCl22 4 and HgCl3 (Fig. 5). During the adsorption experiments at pH equal to 5, the surface of ashes was negatively charged. Attraction phenomenon between surface sites negatively charged and the ion Hg2þ complexed by the ions Cl2 might explain the removal of these negatively charged complexes in solution [13]. When mercury concentration was increased, the major compound in solution became HgCl2 whilst the other minor species 22 were HgCl2 3 and HgCl4 (Fig. 2). After the saturation of the surface sites presenting strong affinities for mercury, mercury removal would occurred by chemical bonds between this compound and the mercury species already adsorbed at fly ash surface [13].
3.2. Adsorption isotherms
3.3. Leaching test
Adsorption isotherms carried out with sulfo-calcic and silico-aluminous ashes are presented in Figs. 3 and 4. Adsorption capacity of sulfo-calcic fly ashes is more important than that of silico-aluminous ashes as during the kinetic experiments and they were equal to 5.0 and 3.2 mg g21, respectively. Furthermore, the shapes of the adsorption isotherms are similar for both types of fly ashes indicating that identical mechanisms would have been responsible for mercury removal. When different orders of magnitude of concentration, adsorption isotherms are considered, different steps and processes of the adsorption seem to have occurred. On the one hand, when mercury concentration was weak, the ascending convex shape of the curves indicates a favorable adsorption of mercury onto the surface of fly ashes (Fig. 3). On the other hand, when the
The results of the leaching test carried out on adsorption cakes resulting from adsorption experiments are presented in Table 2. For both fly ash types, a pH fixed to the value 5 during adsorption experiments yielded a minimum percentage of leaching. The pH conditions maximizing the adsorption of the mercury onto fly ashes were thus identical to those minimizing the leaching of this compound. Sulfocalcic fly ashes, which were more effective than silicoaluminous one to remove mercury in aqueous solution, also present a higher stability towards this metal. When the pH is fitted to the value 5, only 7% of mercuric ions adsorbed onto sulfo-calcic fly ashes were released into leaching solution while about 16% of mercury removed by silico-aluminous ashes returned to the leaching solution after 24 h of contacting time.
Table 2 Measurements of Hg removed from aqueous solutions by ashes (%), measurements of leached Hg during leaching tests of cakes (V ¼ 50 ml, m ¼ 5 g, reaction time ¼ 24 h) related to the initial Hg concentration in ashes (%), calculated net abatement after Hg removal from aqueous solutions and the leached quantities (%) Fly ashes
pH of adsorption experiments
Hg2þ removal (%)
Hg2þ leached (%)
Hg2þ net abatement (%)
Silico-aluminous
3 4 5
42.1 43.4 52.7
22.3 19.2 16.3
32.8 35.0 44.1
Sulfo-calcic
3 4 5
70.2 78.6 81.2
9.6 8.1 7.2
63.4 72.3 75.3
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Table 3 Positions, intensities, length of middle height (LMH) and percentage of peaks obtained after experimental peak decomposition of silico-aluminous and sulfocalcic fly ashes Element
Sulfo-calcic fly ash
Silico-aluminous fly ash
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
O 1s O 1s O 1s
532.12 534.72 536.12
4733 19,098 24,543
2.20 2.20 2.20
57.3
533.12 535.25 536.36
6090 34,227 27,985
2.20 2.20 2.20
59.5
Si 2p Si 2p
105.92 107.02
3679 1840
2.00 2.00
13.1
106.13 107.13
5476 2738
2.12 2.12
14.2
Al 2p
77.98
2078
2.10
6.3
78.02
3856
2.15
7.5
Ca 2p Ca 2p
351.56 355.11
4059 2030
2.52 2.52
4.8
351.95 355.50
1723 861
2.36 2.36
2.0
S 2p S 2p
172.82 174.00
963 482
2.39 2.39
1.4
Not detected
When the pH increased, the dissolution of calcium oxide was observed. Fly ashes, which were mainly siliceous and aluminous products, were able to react with lime to form hydrated calcium silicate or hydrated calcium aluminate having binder properties. It allows to obtain a pouzzolanic material which would have increased the stability of the mercury adsorption [3]. 3.4. XPS analysis XPS spectrums were recorded for the silico-aluminous and sulfo-calcic fly ashes as received and also after the so-called adsorption tests. The results are presented in Table 3 together with the peak characteristics. In the case of silico-aluminous fly ashes before adsorption, the main detected peaks were those of oxygen, carbon, calcium, aluminium, silicon and potassium. The sulfo-calcic exhibited the same peaks excepted the peak of potassium
which was not present whilst a sulfur peak was identified. Concerning the speciation of these elements on the ‘as received’ fly ash surfaces, the XPS peak of sulfur revealed the presence of sulfate on sulfo-calcic fly ashes surface [14]. Furthermore, the position of the aluminium peak showed that the compound Al2O3 would not have been the species present on the surface but rather an aluminium halogenous AlX3. From the combined observation of the oxygen, silicon and aluminium peaks, the compounds SiO2 a quartz, mullite and/or sillimanite (Al2SiO5) were also identified on the surface of both types of ashes. Furthermore, the study of the calcium peak proved the existence of calcium silicate hydrate Ca3Si3O9 although it was not identified by a combination with the silicon and oxygen peaks. The position of Ca and O peaks suggested the existence of the compound Ca(OH)2 on fly ashes surface even if lime is not listed in XPS data [14].
Table 4 Positions, intensities, LMH and percentage of peaks obtained after experimental peak decomposition of fly ashes adsorption cakes Element
Sulfo-calcic fly ash
Silico-aluminous fly ash
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
Position (eV)
Intensity (cps)
LMH (eV)
Chemical species (%)
O 1s O 1s O 1s
532.32 534.64 535.98
4189 11,240 23,661
2.25 2.25 2.25
54.3
533.25 535.55 536.13
5189 28,131 12,464
2.20 2.20 2.20
56.2
Si 2p Si 2p
105.78 107.10
2165 1083
2.10 2.10
11.1
105.95 107.02
3599 1799
1.95 1.95
13.8
Al 2p
78.65
1279
2.14
8.0
78.80
2727
2.28
9.1
Ca 2p Ca 2p
351.75 355.32
4374 1687
2.42 2.42
5.1
352.12 355.84
1309 655
2.48 2.48
1.6
S 2p S 2p
172.74 173.90
1003 502
2.39 2.39
1.6
Not detected
Hg 2p Hg 2p
103.35 107.40
811 435
3.05 3.05
0.7
103.56 108.27
654 212
3.16 3.16
0.5
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Adsorption cakes of ashes resulting from the adsorption kinetic tests at pH equal to 5 were also analyzed by XPS. The aim was to understand the behavior of oxides of the fly ashes surface after their contact with a mercury aqueous solution and particularly the nature of the bonds between mercury and surface oxides. The XPS spectrums are presented in Table 4. The peak intensities obtained with the adsorption cake (Table 4) were compared to those of the as received fly ash spectrums (Table 3). Oxygen relative intensities of the adsorption cakes were lower than those observed on the fly ash spectrums that might result from a partial dissolution of silica and calcium oxides [3]. The silicon relative intensities also decreased that would confirm this hypothesis. Unlike the case of the fly ash spectrums, the aluminium peak of the cakes was undoubtedly the one of oxides according to Ref. [14], and not the aluminium halogenous (AlX3) that was the most probable for the ashes. Besides, Ricou also observed the same transformation of aluminium halogenous in aluminium oxides during metal adsorption on fly ashes from pulverized coal power plants. Furthermore, the sulfur peak analysis carried out to find out the different sulfur species did not confirm the formation of bonds between mercury and sulfur surface sites of ashes. On the other hand, different attempts to analyze the mercury peaks were not convincing because of the partial superposition of the silicon and mercury peaks. Therefore, no mercury compound was identified with certainty. However, the known mercury species that might give the peak Hg 2p at 103.35 ^ 0.2 eV are HgCl2 and HgO, the first one being the mercury species that was used in the prepared aqueous solutions. The compound SiO2 a quartz was identified as it was the case for the fly ash spectrums before adsorption tests from the same silicon and oxygen peak analysis, but not the mullite and sillimanite that were detected on the as received fly ash surface. It could be explained by the partial dissolution of these compounds during mixing of fly ashes with aqueous solutions of mercury. Then, the deconvolution of the calcium peaks showed that the compound Ca(OH)2 was present on adsorption cakes surface as well as Ca3Si3O9. The calcium peaks also suggested that a part of the lime has remained free during the adsorption experiments and the drying of the adsorption cakes. Therefore, these analyses showed that silicon and aluminium oxides were responsible for mercury adsorption on fly ashes surface, whilst the role of sulfur has not been revealed although the affinity of mercury for this element is well known at the natural state. The existence of hydrated calcium silicate Ca3Si3O9 in adsorption cakes was detected and would result from pouzzolanic reactions occurring during the drying phase of the cakes [3]. However, this hydrate did not bring an increase in fly ashes adsorption capacity but would rather enhance the adsorbed mercury
stability by the development of a hydrated calcium silicate layer at the surface.
4. Conclusions Mercury adsorption from aqueous solution onto silicoaluminous fly ashes and sulfo-calcic fly ashes produced by two circulating fluidized bed boilers was studied. Kinetic experiments first showed that, accounting for the pH and temperature of the test, steady state was reached in 3 days. It was also observed that sulfo-calcic fly ashes were more effective than silico-aluminous fly ashes to remove mercury. Sulfo-calcic fly ashes and silico-aluminous fly ashes adsorption capacities were equal to 5.0 and 3.2 mg g21, respectively. Then, a leaching test has showed that the adsorbed mercury was more stably adsorbed by the sulfocalcic fly ashes. This screening experiment plan has thus proved that these circulating fly ashes might behave as an efficient low cost sorbent for the mercury. This property will also be studied in the frame of a project of the European funding programme INTAS where Kazakhstan laboratories will study the remediation of rivers polluted by mercury using fly ashes from old ash lagoons or fresh fly ashes from power plants. Fly ash surface analysis revealed the existence of silicon and aluminium oxides (silica, mullite and sillimanite), aluminium halogenous and calcium oxide. The XPS analyses showed that adsorption mechanisms of mercury were corresponding to its chemical reactions with different oxides present on fly ash surface. Hydration of silica, presence of aluminium oxides and formation of hydrated calcium silicate are phenomena that could take place during mercury adsorption. Finally, pouzzolanics reactions development into adsorption cakes and surface covering by hydrated calcium silicate could be responsible for resulting cakes stability.
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