The leaching behavior of arsenic, selenium and other trace elements in coal fly ash

Trace elements in coal fly ash have long been an environmental concern in terms of their toxicity and mobility. Among them, arsenic and selenium are two oxyanionic elements of greatest concern, the regulation for arsenic in drinking water is even stricter with MCL of 10 ug/L since January 2006. Therefore, understanding the leaching process of these trace elements in fly ash during ash disposal and reuse is important in developing novel methods to control their leaching and protecting water quality. The goals of this study are four fold, they are: (1) to investigate the leaching behavior of arsenic and selenium from fly ash as well as their affecting factors; (2) to analyze the speciation of arsenic and selenium in fly ash leachate; (3) to develop a modeling approach to quantify the availability and stability of trace cationic elements in fly ash; (4) to apply this model to describe the leaching/adsorption behavior of arsenic and selenium on fly ash, as well as the calcium effect on arsenic (V) adsorption process. This research shows that pH is one of the most important factors affecting arsenic and selenium leaching from fly ash, which is also dependent upon the types of fly ash, solid-to-liquid ratio and leaching time. The leaching of arsenic and selenium from fly ash is governed by adsorption/desorption process in samples with low calcium concentration, but likely controlled by the calcium phase in samples with high calcium concentration. The total leachable mass and adsorption constant of trace cationic elements in fly ash can be determined with the modeling approach developed in this study. The adsorption of arsenic and selenium on bituminous coal fly ash was also successfully quantified with a speciation-based adsorption model. The speciation of arsenic and selenium in fly ash leachate varied with samples and was affected by the S/L ratio and leaching time.