Release of Heavy Metals from Ironite®

Ironite®, once available at many lawn and garden stores, was a common fertilizer made from mine tailings. The presence of heavy metals in Ironite® lead to its banning in Canada, and lawsuits in the United States due to the potential release of heavy metals– notably arsenic and lead. Bioavailable arsenic released from Ironite® is dependent on its mineralogical form.

Earlier research sponsored by the producer of Ironite® identified the arsenic-bearing phase as arsenopyrite, with the conclusion that arsenic in that form does not pose an ecological threat. However, a closer look with Extended X-ray Absorption Fine Structure (EXAFS) has concluded the arsenic phase within Ironite® to be scorodite-like. Scorodite is more soluble than arsenopyrite; in fact, the dissolved arsenic released from scorodite can exceed the U.S. standards for drinking water. In addition to the data collected at Argonne National Labs in February 2005 that identified arsenate sorbed to iron oxides as the dominant arsenic bearing phase, secondary identification techniques are being used to confirm this finding such as thermogravimetric analysis and Mössbauer spectroscopy.

The oxidation state of arsenic in Ironite® is a mixed combination of arsenite (As(III)) and arsenate (As(V)). The white line energy requirement for arsenite is 11.868 keV, and 11.876 keV for arsenate. The evident shoulder associated with the white line peak (first peak) illustrates the presence of arsenite, and the sharp peak is arsenate identification within Ironite®.

XANES spectra of Ironite® with arsenite (arsenopyrite and orpiment) and arsenate (scorodite) minerals. Ironite®'s claim is that all arsenic within it is present as arsenopyrite. This claim becomes evidently false upon comparing the Ironite® and arsenopyrite spectra. The Ironite® curve is a combination of arsenic species.

Contacts: Kirk Scheckel; Aaron Williams; Christopher Impellitteri; Thabet Tolaymat; James Ryan (EIMS#135367)

Office of Research & Development | National Risk Management Research Laboratory

Release of Heavy Metals from Ironite®

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