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Conclusions
It has been shown that an interdisciplinary physical-chemical approach, using X-ray absorption spectroscopy, X-ray powder diffraction, sequential extraction and hyphenated speciation techniques, allows for unmatched unravelling of the arsenic presence in a calciner residue dump. In a soil depth profile from the dump and a mixed soil sample the following arsenic concentrations were measured: 0.287% (15-20 cm), 15.41% (20-25 cm), 8.82% (25-30 cm) and 1.081% (0-30 cm, mixed soil). Based on the results from the different physical-chemical approaches, the following general conclusions from analysis of these samples may be drawn:
- In all samples arsenic was predominantly present in pentavalent form, tightly bound to oxygen atoms in the first coordination sphere.
- In all samples the pentavalent arsenic was mostly present as amorphous or poorly-crystalline hydrous oxides of iron (probably bidentate bonding on alpha-hematite), with As in soil from the surface layer to be associated with the most amorphous or poorly-crystalline hydrous oxides of iron and thus potentially the most mobile As.
- A small fraction of the arsenic in the highest arsenic concentration samples (two deepest layers) is present as a crystalline iron arsenate phase, viz. Fe
 (As(AsO ) ). The presence of a smaller amount of this phase in the low arsenic concentration soil samples cannot be excluded.
- In all samples indications have been found for the presence of pentavalent arsenic bound to quartz, in an amorphous or poorly-crystalline fashion, with the highest arsenic concentration sample to be the least amorphous or poorly-crystalline.
Acknowledgments
The authors would like to thank Simon Camm from UEC for preparing the samples and the Slovene Ministery of Science, Education and Sport and the British Council for support in the framework of a bilateral Partnership for Science Project (PSP 2004/22). X-ray absorption measurements at HASYLAB, Hamburg were supported by the European Community - Research Infrastructure Action under the FP6 "Structuring the European Research Area" Programme, through the Integrated Infrastructure Initiative" Integrating Activity on Synchroton and Free Electron Laser Science (IHP-Contract HPRI-CT-1999-00040), and the bilateral project BI-DE/03-04-004 by Internationales Buero des BMBF. Provision of synchrotron radiation facilities by European Synchrotron Radiation Facility (proposal HE-1522) and HASYLAB (project II-01-44) are acknowledged. We would like to thank Gloria Subias Peruga of ESRF beamline BM29 and Julia Wienold of HASYLAB station X1 for expert advice on beamline operation.
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