Environ. Sci. Techno/. 1995, 29, 1748-1753
Cation Exchange Properties of Hydrothermally Treated Coal Fly Ash A R I E H S I N G E R * A N D V A D I M BERKGAUT Seagram Centre for Soil and Water Science, Hebrew University of Jerusalem, P.O. Box 12, Rehouot 76100, Israel
Two samples of fly ash were treated for 2-48 h in 3.5 M NaOH a t 100 "C. Powder XRD patterns of resulting products were obtained, and their CEC was determined. Zeolite P and/or hydroxysodalite formed during the treatment from the glassy part of fly ash, while quartz gradually dissolved and mullite remained stable. Approximately 50% of fly ash could be converted to zeolites with the CEC of resulting products reaching 2.5-3 mequiv/g. Concentrations of extractable B, Mo, and Se in fly ash considerably decreased upon treatment. Adsorption isotherms of lead on treated fly ash suggested that at low initial lead concentrations and at pH lower than 6 precipitation of lead compounds IS not likely. A desorption experiment indicated incomplete reversibility a t higher concentrations, suggesting that part of the adsorption may not have been cation exchange related. Treated fly ash displayed high selectivity for Pb2- > Sr2* > Cu2- > Cd2' > Zn2' > Cs in competition with Na-, especially a t low concentrations of these cations, and was effective in removing Pb and Zn from industrial wastewaters. It was not selective for Ni2- and U022-. In a column test, 160 bed vols of NHq--contaminated fish-pond water was filtered through treated fly ash until "4' breakthrough occurred.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY 'VOL. 29, NO. 7, 1995
Introduction More than 150 million t of fly ash are produced annually in the world from the combustion of coal in power plants. At least a half of it finds no application and is disposed of by landfill, which becomes increasingly expensive and brings about environmental pollution. Therefore, much research work is now directed toward finding new ways of its utilization. The main constituents offly ash are aluminosilicateglass, mullite, and quartz with smaller amounts of residual coal and ore minerals. The glass accounts for 60-80'76 of fly ash (1) and makes a readily available source of Si and Ai for zeolite synthesis. In several papers (2-61, conversion of a significant fraction of fly ash to zeolites was reported as a result of hydrothermal treatment in NaOH medium. The reaction products contained hydroxysodalite, zeolite P. zeolite X, or zeolite A, while their cation exchange capacity (CEC) ranged up to 360 mequivl100 g. The objectiveof this workwas to study products resulting from hydrothermal treatment of fly ash derived from two different coals and to examine cation exchange properties of treated fly ash as well as the effect of this treatment on concentrations in fly ash of extractable toxic elements, particularly B, Mo, and Se.
Experimental Section Samples of fly ash collected by electrostatic precipitation and derived from the burning of Colombian and South African coal in a Babcock and Wilcox vacuum tower boiler were obtained from the Hadera Power Plant, Israel. They will be further referred to as Colombian fly ash and South African fly ash. Chemical compositions of fly ash as well as particle-size distribution are shown in Table 1. Fractions ('25 pm) of fly ash were used to study mineral transformations during hydrothermal treatment. Following the procedure of Henmi (3),20 g of fly ash was boiled with reflux for a specified time with 160 mL of 3.5 N NaOH solution, then repeatedly washed with deionized water and dried. Powder XRD patterns were obtained using a Philips Model 1010 diffractometer with Fe-filtered CoKa radiation. SEM observations were made using a Jeol JSM-35Cinstrument. Total elemental composition of untreated fly ash was determined by RFA, and that of synthetic products was determined by ICP/AES on a Spectroflame spectrometer produced by Spectro; samples were dissolved in HF in polyethylene bottles with the subsequent addition of H i BO3 ( 7 ,and standards with the same matrix were used. The procedure of Polemio and Rhoades (8), which is particularly suited for carbonate- and zeolite-containing materials, was employed to determine CEC with the followingmodifications. 0.5M ammonium acetate solution adjusted to pH 8.2 was used in the replacing step, and the replacement of Na- by NH4- was first carried out four times at 25 "C, and then, since cation exchange is known !o be restricted in sodalite at room temperature, four times at 80 "C. Na was determined by flame emission spectrometry, and C1 was determined on a Jenway Model PCLM3 chloridometer. Cation exchange properties were examined for unseparated Colombian fly ash hydrothermally treated for 24 h
0013-936X/95/0929-7748$09 OO/O
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1995 American Chemical Society
TABLE 1
Chemical Composition and Patticle=Size Distribution of Fly Ash (% on Ignited Basis) fly ash
Si02
AI203
F&
CaO
Ti02
KzO
Colombian South African particle sizes ( p m ) cumulative (%)
61.6 44.7 250.8 5.6
21.3 32.3 40.3 12.0
6.5 4.0 32 19.1
3.2 9.4 25.4 27.5
1.0 1.5 20.2 36.0
1.6 0.9 16 45.8
a
so3
P205
LOP
ND
0.5 2.3 10.1 66.1
10.9 6.9
0.8
12.7 56.2
6.3 82.3
8
74.5
5 88.8
4 93.8
3.2 97.5
2.5 100
Loss on ignition.
(henceforth to be referred to as ZFA, zeolitized fly ash). To remove traces of calcite, it was first washed with 0.5 M sodium acetate-acetic acid buffer solution at pH 5. XRD revealed that this procedure removed traces of calcite without affecting the existing zeolite. Cation exchange isotherms were obtained at pH 5 to avoid precipitation of heavy metals. A 30-mL sample of solutions containing sodium acetate-acetic acid buffer and a soluble salt of respective cation were added to 50 mg of fly ash and equilibrated for 12 h at constant shaking. The total concentration of cations in the initial solutions in all cases was 0.01 N, and the equivalent fraction of a cation competing with Na varied between 1I15 and 113. The pH of solutions after interaction with fly ash did not change more than 0.3 unit. NH4+ was measured with an ionselective electrode, Cs was measured by AAS, other elements were measured by ICPIAES. Equilibrium pH dependency of adsorbance was determined by shaking 100 mg of air-dry ZFA with 25 mL of M Pb2+in NaC104 0.01 M. The equilibrium solution pH was measured after the removal of the adsorbent by centrifugation. Batch sorption experimentswere performed in triplicate using 50-mL centrifugetubes. Lead perchlorate salt was used because perchlorate is a weak ligand that does not form complexes with metal ions (9). Metal concentrations ranged between and 5 x M at pH 5. Sorption was determined by the difference between initial and final Pb2+concentrations. Desorption of Pb2+ was carried out by repeated replacements with NaC104 following adsorption. Equilibration times were 24 h. The ionic strength of NaC104was 0.01 M at room temperature (approximately25 "C). Extractable B, Mo, Se, Cu, Ni, and Cd in ZFA and untreated fly ash were determined in the supernatant after neutralizing to pH 7 the suspension of 6 g of solid in 30 mL of water and shaking it for 24 h. The column filtration experiment was carried out with NH4+-contaminatedfishpond water passed through a 0.45-pm filter and a 7-mL column packed with ZFA flow rate was maintained at 2 mLImin by a peristaltic pump. Samples of industrial wastewaters were received from two companies producing electric cells (VulkanAccumulators andTadiran). In batch experiments, these wastewaters were allowed to interact for 3 h with varying amounts of ZFA at constant shaking and then were separated by centrifugation. NH4+in the effluent from the column was measured colorimetrically by the indophenol blue method (10);other elements were determined by ICP/AES.
Results and Discussion Mineral Transformations during the Treatment. Quartz and mullite were the two major crystalline phases, with magnetite as an accessory mineral, identified by X-ray diffractometry in untreated fly ash (Figure 1). In ZFA, two
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2 theta (Co)
FIGURE 1. XRD patterns of source fly ash and treated fly ash. (A) Colombian fly ash, fraction
