Selenium and Molybdenum Adsorption on Kaolinite Clay Mineral Coated with Hydrous Oxides of Iron and Aluminum

Abstract

Reclamation of abandoned acid mine soils is often limited by lack of suitable soil materials for establishing self-sustaining vegetation. Fly ash has been used to create technogenic soils for reclamation of abandoned mine lands. Fly ash contains toxic trace elements such as selenium and molybdenum that can leach into the groundwater from ash treated soils. In abandoned acid mine lands, oxidation of pyrite generates acidity and dissolved Fe and Al. Neutralization of pyrite oxidation generated acidity with alkalinity from fly ash produces coatings of iron and aluminum hydrous–oxides on the surface of soil clay minerals. Kaolinite is the dominant clay mineral in these soils and Fe and Al coated kaolinite can retain Se and Mo. Laboratory investigations were conducted to determine the effect of three levels of oxide coatings on kaolinite, on Se and Mo retention capacity of coated kaolinite. Adsorption of selenite and molybdate occurred on hydrous oxide coated kaolinite above pH_o. Significant adsorption of selenate only occurred at pH values lower than pH_o of the concerned solid phase. Coating of kaolinite with Fe as well as Al hydrous oxide increased the anion retention capacity of the mineral. The effect was, however, more for Fe than Al hydrous oxide. Surface sites for the adsorption of anions were limited. Results of this investigation show that adsorption will play a very important role in controlling the mobility of oxyanions in fly ash treated mine soils containing relatively high amounts of Fe and Al hydrous oxide coatings.