Review Leaching mechanisms of oxyanionic metalloid and metal species in alkaline solid wastes: A review Geert Cornelis a, , C. Anette Johnson b , Tom Van Gerven a , Carlo Vandecasteele a a Laboratory of Applied Physical Chemistry and Environmental Technology, K.U. Leuven, W. De Croylaan 46, B-3001 Leuven, Belgium b Swiss Federal Institute for Environmental Science and Technology (EAWAG), Box 611, Ueberlandstrasse 133, CH-8600 Du ¨ bendorf, Switzerland Received 13 November 2006; accepted 12 February 2008 Editorial handling by R.N.J. Comans Available online 10 March 2008 Abstract An overview is presented on possible mechanisms that control the leaching behaviour of the oxyanion forming elements As, Cr, Mo, Sb, Se, V and W in cementituous systems and alkaline solid wastes, such as municipal solid waste incinerator bottom ash, fly ash and air pollution control residues, coal fly ash and metallurgical slags. Although the leachability of these elements generally depends on their redox state, speciation measurements are not common. Therefore, experimental observations available in the literature are combined with a summary of the thermal behaviour of these elements to assess possible redox states in freshly produced alkaline wastes, given their origin at high temperature. Possible redox reactions occurring at room temperature, on the other hand, are reviewed because these may alter the initial redox state in alkaline wastes and their leachates. In many cases, precipitation of oxyanions as a pure metalate cannot provide a satisfactory explanation for their leaching behaviour. It is therefore highly likely that adsorption and solid solution formation with common minerals in alkaline waste and cement reduce the leachate concentration of oxyanions below pure-phase solubility. Ó 2008 Elsevier Ltd. All rights reserved. Contents 1. Introduction ..................................................................... 956 2. Redox chemistry .................................................................. 957 2.1. Possible redox states .......................................................... 957 2.2. Predominant redox states in alkaline wastes.......................................... 957 2.3. Oxidation and reduction processes in alkaline wastes ................................... 961 3. Solubility ....................................................................... 963 3.1. Metalate precipitation ......................................................... 963 3.2. Adsorption and solid solution formation ............................................ 966 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.02.001 Corresponding author. Tel.: +32 16 322343; fax: +32 16 322991. E-mail address: Geert.Cornelis@cit.kuleuven.be (G. Cornelis). Available online at www.sciencedirect.com Applied Geochemistry 23 (2008) 955–976 www.elsevier.com/locate/apgeochem Applied Geochemistry