Aging of Iron (Hydr)oxides by Heat Treatment and Effects on Heavy Metal Binding

Abstract

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.

Structure and thermal stability of toxic chromium(VI) species doped onto TiO2 powders through heat treatment

“…One advantage is the decrease of the toxicity from Cr(VI) to Cr(III), another is the drop of leaching percentage of Cr 2 O 3 /TiO 2 (neo-formation). This is somewhat inconsistent with the results obtained by Sorensen et al (2000), who have found that the treatment of Cd-and Pb-doped amorphous iron oxides at 900 C causes the leachability of Cd(II) and Pb(II) to increase by about 1000-fold compared to that for the 50 C-sample. Unlike the trends of Cd(II) and Pb(II), less Cr(VI) is leached from the 900 C-sample than the 600 C-sample.…”

“…It was reported that the leaching percentages of Pb(II) and Cd(II) are considerably reduced after thermal treatment. However, Sorensen et al (2000) have used amorphous iron hydroxides to sorb Pb(II), Hg(I), Cr(VI), and Cd(III) from wastewater, which were heated at 500, 600, and 900 C to simulate the transformations caused by thermal treatment prior to disposal or aging at a disposal site. They also observed that the leachability of Pb(II) and Cd(II) increases at 600 C treatment, but that of Hg(I) and Cr(VI) reduces.…”

Long-term phosphorus effects on evolving physicochemical properties of iron and aluminum hydroxides

“…Aging (1.5 years) at 70 • C of suspensions of a Brazilian Oxisol soil containing large quantities of Fe oxides resulted in increasing Pb solubility as the Fe amorphous phases crystallized toward goethite [16]. Ferrihydrite in contact with Pb solutions at 40-70 • C for 0.5 to 1.5 months resulted in Pb exclusion from the solid because of reduced sorption sites, due to formation of goethite [19].…”

Comparison of metal release from recent and aged Fe-rich sediments

“…Since heating accelerates the phase transition of Fe-(hydr)oxides, it is sometimes used as an experimental technique to study long-term changes in Fe-(hydr) oxides. For example, heating at 900°C increased the thermodynamic stability of amorphous Fe-oxides, but the waterextractability of Cd, Zn and Pb increased significantly (Sørensen et al, 2000). During aging of ferryhydrite (pH 6, 40 and 70°C), sorption increased for Mn and Ni, while Pb and Cd showed an enhanced desorption with aging (2 to 6 weeks) (Ford et al, 1997).…”