Heavy metal stabilization in municipal solid waste combustion bottom ash using soluble phosphate

Heavy metal chemical stabilization with soluble PO43− was assessed for bottom ash from combustion of municipal solid waste. Bottom ash can contain heavy metals (e.g. Pb) that can leach. An experimental dose of 0.38 mols of soluble PO43− per kg of residue was used without optimizing the formulation for any one heavy metal. The reduction in the fraction available for leaching according to the total availability leaching test was 52% for Ca, 14% for Cd, 98% for Cu, 99% for Pb, and 36% for Zn. pH-dependent leaching (pH 4, 6, 8) showed that the treatment was able to reduce equilibrium concentrations by 0.5 to 3 log units for these heavy metals. Bulk and surface spectroscopies showed that both crystalline and amorphous precipitates were present as insoluble metal phosphate reaction products. Dominant reaction products were calcium phosphates, tertiary metal phosphates, and apatite family minerals. Observed phases included, β-Ca3(PO4)2 (tertiary calcium phosphate); Ca5(PO4)3OH (calcium hydroxyapatite); Pb5(PO4)3Cl (lead chloropyromorphite); and Pb5(PO4)3OH (lead hydroxypyromorphite). These are considered to be very geochemically stable mineral phases. The geochemical thermodynamic equilibrium model MINTEQA2 was modified to include both extensive phosphate minerals and simple ideal solid solutions in order to better model pH-dependent leaching. Both end members [e.g. Pb5(PO4)3Cl, β-Ca3(PO4)2] and ideal solid solutions [e.g. (Pb2,Ca)(PO4)2] were observed as controlling solids for Ca2+, Zn2+, Pb2+, and Cu2+. Controlling solids were not identified for Cd2+ because pH dependent concentrations were generally below detection limits. The divalent metal cations in bottom ash were effectively stabilized by treatment with soluble PO43−.