Oxidation of arsenic bearing fly ash as pretreatment before solidification

When a waste fly ash, containing large amounts of As 2O 3, is solidified using cement and lime, the arsenic concentration in the leachate (extraction test DIN 38 414 S4) is determined by the solubility of CaHAsO 3 and can be lowered to a value of ca. 5 mg/l, in a saturated solution of Ca(OH) 2. One of the criteria for landfilling of hazardous waste is, however, that the arsenic concentration in the leachate must be lower than 1 mg/l. In this paper, it is shown that oxidation of the waste before solidification, whereby As(III) is oxidised to As(V) using H 2O 2, lowers the leaching of arsenic, and other contaminants, from the solidified product. With the speciation program MINTEQA2, it is calculated that the solubility of As(V) in the presence of a pure Ca 3(AsO 4) 2 precipitate is lower than the solubility of As(III) in the presence of a pure CaHAsO 3 precipitate. The arsenic concentration in the presence of both a Ca(OH) 2 and a Ca 3(AsO 4) 2 precipitate can even be lowered to 0.47 mg/l (pH 12.5). The As concentration in the leachate of the extraction test on an oxidised S/S sample was indeed lowered to ca. 0.5 mg/l, which is a reduction by a factor of 10 compared to the concentration of ca. 5 mg/l, obtained in the leachate of the extraction test on a non-oxidised S/S sample. This is in very good agreement with the calculated value of 0.47 mg/l. Also, the pretreatment decreased the cumulative fraction of arsenic released over the entire test period of a semi-dynamic leach test by a factor of 7. At all times during the test, the As concentration did not exceed the norm of 1 mg/l.