MSCC3 3rd MINERAL SCIENCES IN THE CARPATHIANS ...

Acta Mineralogica-Petrographica, Abstract Series 5, Szeged, 2006THE STRUCTURE AND AGEING OF As-RICH FERRIHYDRITE FROM PEZINOK,SLOVAKIAMAJZLAN, J. 1 , CHOVAN, M. 2 , LALINSKÁ, B. 2 , MILOVSKÁ, S. 3 & JURKOVIČ, Ľ. 41 Institute of Mineralogy and Geochemistry, Albert-Ludwig University, Albertstraße 23b, 79104 Freiburg, GermanyE-mail: Juraj.Majzlan@minpet.uni-freiburg.de2 Department of Mineralogy and Petrology, Comenius University, Mlynská dolina G, 842 15 Bratislava, Slovakia3 Geological Institute, Slovak Academy of Sciences, Banská Bystrica, Slovakia4 Department of Geochemistry, Comenius University, Mlynská dolina G, 842 15 Bratislava, SlovakiaArsenic is permanently attracting significant attentionfrom the viewpoint of its negative influence on the humanhealth, transport through the trophic chain, and consequentlyits biogeochemical cycle. It has been classified as one of thepriority pollutants, contested in its toxicity only by few otherelements. In this work, we have been investigating the formationprocess, the structure, and ageing of As-rich ferrihydritefrom the Pezinok deposit. At this site, As-ferrihydrite servesas a transport medium for As contamination.The abandoned Sb deposit Pezinok lies in the in MaléKarpaty Mts in western Slovakia. The deposit is located incarbonaceous and actinolite schists and amphibolites andcontained abundant stibnite and smaller amounts of other Sbminerals. In addition to the Sb minerals, pyrite and arsenopyritewere also common. The mineralization occurred asquartz-carbonate lenses, veinlets, nests, and impregnations inthe host rocks. The ores were milled and Sb minerals werecollected by flotation. The waste was then deposited in twolarge tailing impoundments.The fine-grained waste is currently slowly weathering andreleasing the toxic elements into the surrounding soils andstreams. The principal weathering product is As-rich ferrihydrite,a mineral almost amorphous to X-rays. Because of thelack of long-range order, techniques other than X-ray diffraction(XRD) had to be used to describe this mineral.Using X-ray absorption near-edge structure (XANES)spectroscopy, we have determined that all probed elements(Fe, As, S) are present in their highest oxidation states. Thus,the material escaping from the impoundments is already fullyoxidized, and no further changes in its oxidation state shouldbe expected.Extended X-ray absorption fine-structure (EXAFS) spectroscopygave valuable evidence about the structure of Asferrihydrite.EXAFS spectra at the Fe K edge have shownthat the particles in the studied material are extremely depolymerized.The octahedra Fe(O,OH,OH 2 ) 6 are connectedalmost exclusively by edges, forming short polyhedralchains. EXAFS spectra at the As K edge indicate that theAsO 4 tetrahedra are attached to the iron oxide particles in abinuclear-bidentate fashion. Our results agree well with previousconclusions of WAYCHUNAS et al. (1993), obtainedon a chemically similar synthetic material.Selected samples were kept at 60 ºC in a wet state for aperiod of almost 4 months and the changes in the sampleswere monitored by XRD, leaching and chemical analysis ofthe leachates, and EXAFS spectroscopy. Essentially nochanges were observed in the XRD patterns. On the otherhand, leaching the samples with 0.5 M NaHCO 3 solutionshowed that the amount of extractable Fe decreased and theamount of extractable As increased as a function of ageingtime. Therefore, the samples are indeed changing, but thesechanges cannot be detected by XRD. Fe EXAFS spectra havedocumented clearly an increase in the number of Feneighbours in the second coordination shell. In other words,the small particles polymerize during ageing and probablyevolve toward a goethite-like structure. The polymerizationand recrystallization of the Fe oxide particles is accompaniedby the release of As.The results presented here have important consequencesfor the remediation effort at this and other sites. Asferrihydriteis an excellent medium for capturing As, but anunfavourable choice for the long-term storage of this toxicelement. When As-ferrihydrite ages, it releases the arsenicback into the solution. Thus, care must be taken when the Asferrihydriteis disposed.ReferenceWAYCHUNAS, G.A., REA, B.A., FULLER, C.C. &DAVIS, J.A. (1993): Geochimica et Cosmochimica Acta,57: 2251–2269.70www.sci.u-szeged.hu/asvanytan/acta.htm