Modeling Arsenic (V) Adsorption by Fly Ash Using a Speciation-Based Adsorption Model

Received for publication March 22, 2007. The leaching characteristics of selenium from several bituminous and subbituminous coal fly ashes under different pH conditions were investigated using batch methods. Results indicated that pH had a significant effect on selenium leaching from bituminous coal ash. The minimum selenium leaching occurred in the pH range between 3 and 4, while the maximum selenium leaching occurred at pH 12. The release of selenium from subbituminous coal ashes was very low for the entire experimental pH range, possibly due to the high content of calcium which can form hydration or precipitation products as a sink for selenium. The adsorption results for different selenium species indicated that Se(VI) was hardly adsorbable on either bituminous coal ashes or subbituminous coal ashes at any pH. However, Se(IV) was highly adsorbed by bituminous coal ashes under acidic pH conditions and was mostly removed by subbituminous coal ashes across the entire pH range. This result suggests that the majority of selenium released from the tested fly ashes was Se(IV). A speciation-based model was developed to simulate the adsorption of Se(IV) on bituminous coal fly ash, and the pH-independent adsorption constants of HSeO3- and SeO32- were determined. The modeling approach is useful for understanding and predicting the release process of selenium from fly ash.