Aging of iron (hydr)oxides by heat treatment and effects on heavy metal binding

Amorphous iron (hydr)oxides are used to remove heavy metals from wastewater and in the treatment of air pollution control residues generated in waste incineration. In this study, iron oxides containing heavy metals (e.g., Pb, Hg, Cr, and Cd) were treated at 50, 600, and 900 °C to simulate their transformations caused by heat treatment prior to disposal or aging at a proper disposal site. The transformations were investigated by XRD, SEM, XANES, EXAFS, surface area measurements, pH static leaching tests, and extractions with oxalate and weak hydrochloric acid. It was found that at 600 and 900 °C the iron oxides were transformed to hematite, which had a greater thermodynamic stability but less surface area than the initial products. Heat treatment also caused some volatilization of heavy metals (most notably, Hg). Leaching with water at pH 9 (L/S 10, 24 h) and weak acid extraction showed that heat treatment caused a part of the metals bound in the oxides to be released, thus increasing metals leachability by 1-2 orders of magnitude depending on the metal. Pb and Cd were released in particularly significant concentrations, suggesting less incorporation into the iron oxides after heat-induced transformation. For Pb, this transformation of the chemical state of the bound metal was clearly supported by the X-ray absorption fine structure (XAFS) studies. A fraction of the bound Cr remained stable even after treatment at the highest temperature used in the study. It was concluded that the heat treatment of iron oxides may be advantageous to improve the thermodynamic stability of the product but that thermal treatment at both 600 and 900 °C significantly reduced the binding capacity for heavy metals.