Mobility and speciation of arsenic in the coal fly ashes collected from the Savannah River Site (SRS)

• Concerted methodology (batch, column, and speciation tests) to assess As mobility in fly ash.
• Considerable fraction of mobilizable As, though limited mobile As present in fly ash.
• Occurrence of species conversion from AsV to AsIII during column leaching process.

Arsenic (As) leaching from coal fly ash stockpiled at waste disposal sites is a source of environmental concern. An array of techniques including batch extraction and column leaching tests, in combination with speciation analysis of chemically specific As species, was employed to evaluate the mobility of As in fly ashes collected from the U.S. DOE Savannah River Site. The results obtained using the U.S. EPA Toxicity Characteristic Leaching Procedure (TCLP), a two-step sequential extraction technique, and continuous column leaching experiments suggest that only a small portion of total As in the fly ashes was mobile, but mobilizable As could be a considerable fraction (3.1–43%), varying inversely with alkalinity of fly ash. Speciation analysis by using phosphate extraction suggests that arsenate (AsV) was the major extractable species in the fly ash samples. During the column leaching experiment, however, it was observed that arsenite (AsIII) was an important species leached out of the fly ashes, indicating species conversion during the leaching process. The matrix-bound AsV within the fly ash, once being released from the solid matrix, could be converted to AsIII during its transport inside the column. The pHs of leachates and fly ashes (both acidic in column leaching experiments here) could be related to the dominance of AsIII in the effluents.