Abstract
The influence of pH on the leaching of metals from waste materials can be described by geochemical and empirical models. These equations may be integrated into dynamic leaching models in order to describe the long-term behavior of waste-derived forms or they can be used to predict the concentration of metals in equilibrium leaching tests at a given pH. The aim of this work is to describe the equilibrium concentration of the main metals (Zn, Pb, and Cr) contained in untreated and stabilized/solidified (S/S) electric arc furnace dust (EAFD) using experimental data obtained from a pH-dependence leaching test (acid neutralization capacity, ANC). EAFD is a hazardous waste generated in steel factories. Steel foundry dust coming from an electric arc furnace was characterized by acid digestion, x-ray fluorescence (XRF), and x-ray diffraction (XRD). The waste mainly contains Zn and Fe, which were identified in zincite and zinc ferrite phases. Pb and Cr were also detected at lower concentrations. Cement/EAFD formulations ranging from 7 to 20% dry wt of cement were prepared and the ANC leaching test was performed. The amphoteric behavior of Zn, Pb, and Cr was described by the geochemical model Visual MINTEQ and by an empirical model developed for these metals. Zinc and lead solubilities were well described by both models; however, Visual MINTEQ failed to describe the chromium behavior quantitatively.
Highlights
THE LEACHING OF CONTAMINANTS from waste-derived forms involves complex mechanisms which take into account the dissolution/precipitation, adsorption/desorption, or oxidation/reduction reactions together with transport of dissolved ions in the liquid phase inside the waste material
The x-ray diffraction (XRD) analysis revealed that zinc ferrite (ZnFe2O4) and zincite (ZnO) were the main crystalline phases; this means that iron is present mainly in trivalent oxidation state
Solubility data were obtained from an acid neutralization capacity (ANC) test, which is a pH-dependence leaching test
Summary
THE LEACHING OF CONTAMINANTS from waste-derived forms involves complex mechanisms which take into account the dissolution/precipitation, adsorption/desorption, or oxidation/reduction reactions together with transport of dissolved ions in the liquid phase inside the waste material. Cmo represents the equilibrium concentration of the dissolved contaminant This equilibrium concentration is pH dependent for most metals usually contained in waste forms. Equilibrium models that consider the influence of pH on the solubility of metals are required for the solution of dissolution/transport dynamic leaching models developed to describe the long-term behavior of waste-derived forms. Such equilibrium models can be used to estimate the concentration of metals at a given pH in compliance leaching tests performed under equilibrium conditions, such as the European EN12457 leaching test or the USEPA-TCLP test. The resulting pH depends on the acid neutralization capacity of the waste and the acidity/alkalinity of the leachant used in the test
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