Heavy metal stabilization in municipal solid waste combustion bottom ash using soluble phosphate Bradley S. Crannell a , T. Taylor Eighmy a, *, James E. Krzanowski b , J. Dykstra Eusden Jr. c, , Elisabeth L. Shaw d , Carl A. Francis e a Environmental Research Group, A115 Kingsbury Hall, University of New Hampshire, Durham, NH 03824, USA b Mechanical Engineering Department, 134 Kingsbury Hall, University of New Hampshire, Durham, NH 03824, USA c Geology Department, Carnegie Hall, Bates College, Lewiston, ME 04240, USA d Analytical Shared Experimental Facility, Center for Material Science and Engineering, Room 13-4137, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA e Harvard University Mineralogical Museum, 24 Oxford Street, Cambridge, MA 02138, USA Accepted 4 October 1999 Abstract Heavy metal chemical stabilization with soluble PO 4 3 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 PO 4 3 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, b-Ca 3 (PO 4 ) 2 (tertiary calcium phosphate); Ca 5 (PO 4 ) 3 OH (calcium hydro- xyapatite); Pb 5 (PO 4 ) 3 Cl (lead chloropyromorphite); and Pb 5 (PO 4 ) 3 OH (lead hydroxypyromorphite). These are considered to be very geochemically stable mineral phases. The geochemical thermodynamic equilibrium model MINTEQA2 was modi®ed to include both extensive phosphate minerals and simple ideal solid solutions in order to better model pH-dependent leaching. Both end members [e.g. Pb 5 (PO 4 ) 3 Cl, b-Ca 3 (PO 4 ) 2 ] and ideal solid solutions [e.g. (Pb 2 ,Ca)(PO 4 ) 2 ] were observed as controlling solids for Ca 2+ , Zn 2+ , Pb 2+ , and Cu 2+ . Controlling solids were not identi®ed for Cd 2+ because pH dependent concentrations were generally below detection limits. The divalent metal cations in bottom ash were eectively stabilized by treatment with soluble PO 4 3 . # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Municipal solid waste combustion bottom ash; Lead; Heavy metals; Phosphate; Stabilization 1. Introduction Chemical stabilization is one method of reducing the leachability of heavy metals in waste materials. The principal aim of stabilization is to form new, less soluble mineral phases that are more geochemically stable in leaching environments. One stabilization agent of recent interest, particularly for Pb 2+ , is orthophosphate (PO 4 3 ) [1±4]. Over 30 elements can react with PO 4 3 to form about 300 dierent naturally-occurring minerals [5]. Metal phosphate compounds are found as secondary minerals in the oxidized zones of lead ore deposits and as assem- blages around ore bodies [5]. Their presence in soils, sediments, and phosphatic beds [5] demonstrates that they are stable with respect to pH, Eh, and mineral diagenesis. Divalent cations and oxyanions very often undergo isomorphic substitution in these minerals; such as Pb 2+ substituting for Ca 2+ or AsO 4 3 substituting for PO 4 3 [5]. Research has shown that phosphate minerals are common controlling solids for Ca 2+ , Cd 2+ , Cu 2+ , Pb 2+ and Zn 2+ in natural soil systems [6±8]. PO 4 3 is 0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(99)00312-8 Waste Management 20 (2000) 135±148 www.elsevier.nl/locate/wasman * Corresponding author. Tel.: +1-603-862-2206; fax: +1-603-862- 2364. E-mail address: taylor.eighmy@unh.edu (T. Taylor Eighmy).