Journal of Hazardous Materials A125 (2005) 45–61 Modelling the leaching of Pb, Cd, As, and Cr from cementitious waste using PHREEQC Cheryl E. Halim a , Stephen A. Short b , Jason A. Scott a , Rose Amal a,∗ , Gary Low c a ARC Centre for Functional Nanomaterials, School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia b Ecoengineers Pty. Ltd., 9 Sunninghill Circuit, Mount Ousley, NSW 2519, Australia c Department of Environment and Conservation, Analytical and Environmental Chemistry Section, Lidcombe, NSW 2141, Australia Received 19 January 2005; received in revised form 30 May 2005; accepted 31 May 2005 Available online 25 July 2005 Abstract A leaching model was developed using the United States Geological Survey public domain PHREEQC geochemical package to simulate the leaching of Pb, Cd, As, and Cr from cementitious wastes. The model utilises both kinetic terms and equilibrium thermodynamics of key compounds and provides information on leachate and precipitate speciation. The model was able to predict the leaching of Pb, Cd, As, and Cr from cement in the presence of both simple (0.1 and 0.6 M acetic acid) and complex municipal landfill leachates. Heavy metal complexation by the municipal landfill leachate was accounted for by the introduction of a monoprotic organic species into the model. The model indicated Pb and As were predominantly incorporated within the calcium silicate hydrate matrix while a greater portion of Cd was seen to exist as discrete particles in the cement pores and Cr (VI) existed mostly as free CrO 4 2- ions. Precipitation was found to be the dominant mechanism controlling heavy metal solubility with carbonate and silicate species governing the solubility of Pb and carbonate, silicate and hydroxide species governing the solubility of Cd. In the presence of acetic acid, at low pH values Pb and Cd acetate complexes were predominant whereas, at high pH values, hydroxide species dominated. At high pH values, the concentration of As in the leachate was governed by the solubility of Ca 3 (AsO 4 ) 2 with the presence of carbonate alkalinity competing with arsenate for Ca ions. In the presence of municipal landfill leachate, Pb and Cd organic complexes dominated the heavy metal species in solution. The reduction of As and Cr in municipal landfill leachate was crucial for determining aqueous speciation, with typical municipal landfill conditions providing the reduced forms of As and Cr. Published by Elsevier B.V. Keywords: Cementitious waste; PHREEQC; Geochemical modelling; Lead; Cadmium; Arsenic; Chromium; Municipal landfill; Leachate 1. Introduction In recent years there have been increased concerns regard- ing the leaching of hazardous substances from landfills into local surroundings. As a result, an extensive array of leach- ing tests has been developed to assess the hazards of heavy metal-containing wastes prior to disposal. Failure to pass a regulatory leaching test typically necessitates some form of waste stabilisation, such as cement addition. Batch leach- ∗ Corresponding author. Tel.: +61 2 93854361; fax: +61 2 93855966. E-mail address: r.amal@unsw.edu.au (R. Amal). ing tests are the preferred choice for regulatory assessment due to their simplicity, improved reproducibility, and shorter time requirements. However, as batch leaching tests are typ- ically run over short time frames, it is debatable whether the compounds of interest behave similarly in the long term. Modelling can potentially predict the long term leaching of wastes providing a solution to problems inherent to batch pro- cedures. A model capable of describing contaminant leaching from wastes can assist in improving the development of man- agement options [1]. This study focuses on the leaching of Pb, Cd, As, and Cr from cement-stabilised waste. The leaching of heavy metal ions from cementitious waste has been extensively investi- 0304-3894/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jhazmat.2005.05.046