Cation Exchange Properties of Hydrothermally Treated Coal Fly Ash

Abstract

Two samples of fly ash were treated for 2-48 h in 3.5 M NaOH at 100 °C. Powder XRD patterns of resulting products were obtained, and their CEC was determined. Zeolite P and/or hydroxysodalite formed during the treatment from the glassy part of fly ash, while quartz gradually dissolved and mullite remained stable. Approximately 50% of fly ash could be converted to zeolites with the CEC of resulting products reaching 2.5-3 mequiv/g. Concentrations of extractable B, Mo, and Se in fly ash considerably decreased upon treatment. Adsorption isotherms of lead on treated fly ash suggested that at low initial lead concentrations and at pH lower than 6 precipitation of lead compounds is not likely. A desorption experiment indicated incomplete reversibility at higher concentrations, suggesting that part of the adsorption may not have been cation exchange related. Treated fly ash displayed high selectivity for Pb 2+ > Sr 2+ > Cu 2+ > Cd 2+ > Zn 2+ > Cs + in competition with Na + , especially at low concentrations of these cations, and was effective in removing Pb and Zn from industrial wastewaters. It was not selective for Ni 2+ and UO 2 2+ . In a column test, 160 bed vols of NH 4 + -contaminated fish-pond water was filtered through treated fly ash until NH 4 + breakthrough occurred.