Environment

[PY1-22] Suppression of anionic pollutants released from fly ash by different Ca additives

○Shingo Nakama¹, Binglin Guo¹, Tsuyoshi Hirajima¹, Keiko Sasaki¹ (1. Kyushu University)

Keywords:Fly ash, Anionic pollutants, Cement, Ca additives, Immobilization

Fly ash (FA) is known as a by-product in coal power plants, since it is utilized for blending in cement to improve the mechanical strength of concrete. Thus, the use of coal fly ash has the economic and environmental significance. On the other hand, there is a risk of pollutant species release considering the long life period of the building structures, especially arsenate, selenite, chromate, borate and fluoride which are originally included to a greater or lesser extent in FA depending on coal types. To suppress the release of these anionic pollutants, it is known some Ca additives effectively function when FA is solidified in cement. In the present work, the effect of different Ca additives on release of anionic pollutants was systematically investigated. To extract the key reactions, FA was simply mixed with various Ca additives (e.g. lime, gypsum, and slag) excluding cement, and the released anionic pollutant concentrations and XRD patterns of the solid products were monitored over 8 weeks. Based on the comprehensive results of water chemistry and the solid characterization, the 60 % hydroxylated calcinated dolomite (HCD 60) was the most efficient Ca additives for immobilization of different anionic pollutants as long as investigated. The important mechanism would be the formation of ettringite via monosulfate for immobilization of chromate and borate, and the precipitation of calcium salts with low solubility products for immobilization of fluoride, arsenate and selenite, which are dependent on their contents in FA. Furthermore, transmission electron microscopy- energy dispersive X-ray spectroscopy (TEM-EDX) observation can provide the supporting evidence. Understanding the suppression mechanism of anionic pollutants released from FA by different Ca additives contributes to the practical utilization for waste management and cement and concrete industries.