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

Heavy metal chemical stabilization with soluble PO43- was assessed for bott om ash from combustion of municipal solid waste. Bottom ash can contain hea vy metals (e.g. Pb) that can leach. An experimental dose of 0.38 mols of so luble PO43- per kg of residue was used without optimizing the formulation f or any one heavy metal. The reduction in the fraction available for leachin g 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 concentration s by 0.5 to 3 log units for these heavy metals. Bulk and surface spectrosco pies showed that both crystalline and amorphous precipitates were present a s insoluble metal phosphate reaction products. Dominant reaction products w ere calcium phosphates, tertiary metal phosphates, and apatite family miner als. Observed phases included, beta-Ca-3(PO4)(2) (tertiary calcium phosphat e); Ca-5(PO4)(3)OH (calcium hydroxyapatite); Pb-5(PO4)(3)Cl (lead chloropyr omorphite); and Pb-5(PO4)(3)OH (lead hydroxypyromorphite). These are consid ered to be very geochemically stable mineral phases. The geochemical thermo dynamic equilibrium model MINTEQA2 was modified to include both extensive p hosphate minerals and simple ideal solid solutions in order to better model pH-dependent leaching. Both end members [e.g, Pb-5(PO4)(3)Cl, beta-Ca-3(PO 4)(2)] and ideal solid solutions [e.g. (Pb-2,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 effec tively stabilized by treatment with soluble PO43-. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.