Environ. Sci. Technol. 1997, 31, 1218-1223
Anion Sequestering by the Formation of Anionic Clays: Lime Treatment of Fly Ash Slurries E. J. REARDON* AND S. DELLA VALLE Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
The addition of hydrated lime and dolomite to fly ash slurries results in the development of anionic clays. These minerals are comprised of stacked sequences of doublemetal hydroxide layers. Unlike most clays, the hydroxide layers have a net positive charge. As a result, anionicform elements in fly ash leachate waters are scavenged into the interlayer positions to neutralize the positive charge. In our experiments, leachate borate, sulfate, and arsenate concentrations were reduced by up to 2 orders of magnitude. High leachable boron is the chief limitation on the agricultural use of fly ash in soil remediation, and in this study lime treatment reduced leachable boron to levels below those toxic to plants. Under the oxygenated conditions of surface and near-surface waters, many elements of environmental concern occur as oxyanions, such as borate, arsenate, chromate, selenate, and molybdate. Anions are typically mobile under the neutral to alkaline pH conditions typical of most groundwaters, and little reliance can be given to the geosphere for their attenuation. The design of geochemical conditions to precipitate anionic clay minerals is a promising means to sequester and immobilize anionic-form elements from wastewaters. However, it is not known whether high pH conditions, which promote the formation of these anionic clays, will also be necessary to ensure their stability and thus the long-term immobilization of sequestered anions.
1. Introduction The boron concentration of fly ash produced from coal-fired electric generating stations can vary widely (10-5000 mg/kg) (1). Most are present as borate condensates on the surface of ash particles. This surface enrichment occurs because borates condense at temperatures lower than those required for the solidification of the molten droplets of aluminosilica glass, which constitutes the bulk of fly ash material. The borate condensates form sometime during the transport of the ash particles from the furnace to the electrostatic precipitators. Because of this surface enrichment and the high solubility of most borate minerals at pH values below 9.0, Fruchter et al. (2) found that 50% or more of the total boron content can be leached within minutes after the addition of water (3-5). At low water/ash mixing ratios, leachates easily develop boron levels that are toxic to most plants (>5 mg/L). For example, at a 2:1 water/ash mixing ratio, leachable boron concentrations from the fly ash used in this study were nearly 60 mg/L. On a molar basis, boron was the third highest concentration element in the leachates, surpassed only by calcium and sulfate. A high leachable boron * Corresponding author telephone: (519) 888-4567, ext. 3234; fax: (519) 746-0183; e-mail:
[email protected].
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concentration is recognized as the principal limitation to the agricultural use of fly ash as a soil conditioner (6, 7). Reardon et al. (8) proposed the addition of dolomite, CaMg(CO3)2, and lime, CaO, or hydrated lime, Ca(OH)2, to alkaline industrial solid wastes such as fly ash or oil shale solid wastes to reduce boron concentrations in leachates. Through thermodynamic calculations and in laboratory experiments, they showed that hydrated lime and dolomite are incompatible phases at ambient temperatures and with time convert to an assemblage of calcite, CaCO3, and brucite, Mg(OH)2. The reaction can be described as a dedolomitization reaction:
Ca(OH)2(s) + CaMg(CO3)2(s) f 2CaCO3(s) + Mg(OH)2(s) and has a calculated ∆G°r of -31.0 kJ/mol indicating that the reaction should proceed to the right until either all dolomite or hydrated lime is exhausted. Reardon et al. (8) postulated that the progressive precipitation of calcite accompanying the above reaction should serve as an in situ chemical pump to reduce leachate concentrations of any anionic or cationic constituents capable of substituting for carbonate or calcium ions in the calcite structure. Although their principal interest was in applying the technique to reduce leachate boron concentrations (BO33is structurally similar to CO32- and can substitute into the calcite crystal lattice), they suggested that concentrations of other elements could be reduced as well. The initial objective of this follow-up study was to evaluate the effectiveness of the proposed treatment on a fly ash from the Lakeview thermal generating station in southern Ontario and to quantify the impact on the concentration of boron and other trace elements in the leachates from this fly ash.
2. Experimental Section 2.1. Materials. The fly ash sample used in this study was collected in 1993 from a dry landfill at Ontario Hydro’s Lakeview thermal generating station. The coal used at this generating station is bituminous and derived from the eastern United States. The ash consists predominantly of spherical particles ranging from 0.5 to 150 µm with a mean diameter of 6-9 µm (9). Table 1 records the major and minor elemental composition of the fly ash. The dolomite used in the experiment was collected from the Eramosa formation (Manitoulin Group), southern Ontario, and crushed to the grain size of Portland cement (
