Ferrihydrite as a powerful scavenger of As: A thermochemical and a field study
Final Report Abstract
Within this work, we were investigating the capacity of ferrihydrite, a poorly crystalline hydrous ferric oxide, to adsorb and retain arsenic. It was known before that ferrihydrite scavenges arsenic and other toxic metals ferociously. However, the thermodynamic aspects of this behavior were not known and the competitive adsorption capacity of ferrihydrite versus organic matter in the field was rarely addressed. We have found that ferrihydrite (hydrous ferric oxide, HFO) with adsorbed arsenate is significantly stabilizes with respect to pure HFO. The stabilization energy steadily increases with the increasing As/Fe ratio. It is possible that HFO with adsorbed arsenate may be thermodynamically stable with respect to some crystalline iron arsenates, e.g., scorodite. In other words, a poorly crystalline or almost X-ray amorphous compound would be stable with respect to a crystalline phase. Because of the interesting consequences that these findings may have upon natural systems, we extended our study to HFO with adsorbed sulfate, phosphate, and antimonate, and found a repeating pattern. Adsorbtion of these oxoanions stabilizes HFO to a great extent. The field portion of this study was performed at the locality Kyjov in eastern Slovakia. The waste impoundment located here contains fly ash, fine-grained industrial waste, and arsenic-containing organic compounds (pesticides). Using X-ray techniques with micro-resolution, we have found that As fractionates onto the surfaces of newly-formed iron oxides already within the body of the impoundment. Arsenic here is mainly As5+, rarely As3+. Some portions of the waste contain Zn; this element shows only vague correlation with As. However, we did find some newlyformed Zn-As phases whose identification is still uncertain.The solutions that flow out from the impoundment are rich in arsenic and generate secondary reservoirs of Asrich HFO's in the sediments of the near-by streams. Therefore, the remediation strategy should not only concentrate on the impoundment itself but also on the stream sediments in its vicinity.
Publications
- 2007: Arsenic mobility in stream sediments and impoundment material as evaluated by column and batch experiments. Journal of Hydrology and Hydromechanics 55, 223-235
Hiller, E., Veselská, V., Majzlan, J.
