Release of soluble ions and heavy metal during fly ash washing by deionized water and sodium carbonate solution

Two municipal solid waste incineration fly ashes were selected for washing by deionized water and Na2CO3 solution for comparison. Results showed that the benefits of washing were two folds: (1) Washing was able to reduce the contents of Cl− and SO42− while increased the contents of CaO, SiO2, Al2O3 etc.; (2) Washing by Na2CO3 solution showed increased stability of heavy metals (Cr, Ni, Cd, Cu, Zn and Pb) and fly ash was safe for later reuse. Release of Cl− was high at more than 90% regardless of washing solution. SO42− and Ca2+ removal was highly dependent on the dissolution and precipitation equilibriums. Na2CO3 washing promoted the formation of CaCO3. Thus SO42− was washed off instead of precipitating as CaSO4 and retained in fly ash solid. SO42− removal was raised to more than 80% by Na2CO3 washing as compared with about 30% by deionized water. At the same time, Ca removal by Na2CO3 dropped to 1–2%. In addition, the basicity of fly ash was important as high basicity helped SO42− removal. Overall, washing by Na2CO3 appears to be a promising option for fly ash treatment. [Display omitted] •Dissolution and precipitation equilibrium affected the release of sulfate and calcium.•Basicity of fly ash played a significant role in soluble ions removal.•Na2CO3 washing could greatly enhance sulfate removal.•Leaching of heavy metal was below the limit after Na2CO3 washing.•Washing was able to enrich contents such as Ca, Al, Mg and SiO2.