SPECIATION AND LEACHABILITY OF ZINC IN ENERGY-FROM-WASTE AIR POLLUTION CONTROL RESIDUES AND EFFECTS OF PARTIAL NEUTRALISATION

Abstract

Seven UK air pollution control residues (APCR) from municipal solid waste combustion were examined to understand the speciation of potentially ecotoxic zinc, before and after partial APCR neutralisation using acid wastes, as is sometimes conducted before disposal. Fe K-edge XAS showed Zn-containing magnetite (spinel) is an important phase in APCR, along with ferrihydrite. Linear combination fitting of X-ray absorption near edge structure (XANES) Zn K-edge spectra strongly concluded that the 2600-7300 mg/kg of zinc in the APCR is speciated mainly as spinel, hemimorphite, a glassy phase, zinc phosphate and hydrozincite, with statistically consistent findings both within and between APCRs. The extended X-ray absorption fine structure (EXAFS) spectra of the APCR mainly show the first Zn-O shell, which is consistent with Zn in solid solution, glass and poorly crystalline phases. Presented in the context of a full review of previous studies, these results suggest changes in Zn speciation under modern operating regimes. pH-dependent leaching behaviour of the raw APCRs was consistent with solubility control by secondary Zn(OH)2 over the alkaline range, and also Zn5(OH)8Cl and hydrozincite at mid-alkaline pH. Partial neutralisation of the APCRs with concentrated HCl formed secondary reaction products that agglomerated the APCRs, but the same zinc species were found after neutralisation, and lower zinc leachability is attributable only to decreasing the pH to 10.0-10.7. Since this pH is unlikely to be stable in the environment (e.g., in interaction with landfill leachate at pH 5-8), industrial “treatment” by partial neutralisation does not reduce the environmental risk associated with zinc in APCRs.