Anion sequestering by the formation of anionic clays: Lime treatment of fly ash slurries

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 double-metal hydroxide layers. Unlike most clays, the hydroxide layers have a net positive charge. As a result, anionic-form elements in fly ash leachate waters are scavenged into the interlayer positions to neutralize the positive charge. In our experiments, leachate berate, 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 berate, 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.