The isotopic composition of oxygen in arsenate and arsenite: a novel approach to constrain the source of arsenic in groundwater
Final Report Abstract
This project was inspired by the motivation to investigate the potential utility of the isotopic signal of oxygen in arsenate and arsenite in getting new insights into the mechanisms which are responsible for the occurrence of arsenic enriched groundwaters. The underlying question was, whether autochthonous and allochthonous mobilised arsenate and arsenite can be distinguished by distinct isotopic signals. For this, a suitable mass-spectrometric method had to be worked out for the isotope analyses, a protocol had to be elaborated how to enrich and extract arsenate and arsenite from natural waters and the exchange kinetic between oxygen isotopes in water and in arsenic oxyanions had to be determined. The development of adequate extraction procedures for arsenate and arsenite and the elimination of major interfering compounds (especially of sulfate and phosphate) turned out to be an unexpectedly difficult and tedious task. After a thorough literature research, eight different materials/ procedures have been considered as promising to eliminate interfering anions and to pre-concentrate /separate the respective arsenic species. After separating arsenate and arsenite by an alumino-silicate based sorbent according to Meng et al. (1998), separate purification schemes have been worked out for the two arsenic oxyanion species at issue. These separation procedures are unique and were first described by the coworkers of this project and constitute a substantial part of the PhD thesis of Xiaohui Tang. The establishment of a procedure for measurement of the oxygen isotopes by means of isotope ratio mass spectrometry was carried out in the Isotope Laboratory of our (Institute of Mineralogy and Geochemistry of Karlsruhe Institute of Technology) and its portability was verified in cooperation with the Isotope Laboratory of the University of Lausanne, Prof. Dr. T. Vennemann. Because of the novelty of the procedure, no international standard materials for oxygen isotopes in arsenate and arsenite exist. Therefore, standards of other oxyanion compounds such as KNO3 or Ag3PO4 were used for calibration. Pure, commercially available Ag3AsO4 was used as internal standard to control the precision of the measurements. The extremely fast isotopic exchange between oxygen in water and arsenate and arsenite, respectively, turned out to be a major barrier for the utility of the approach in studying natural aquatic systems. Because of the fast exchange kinetics an undisturbed sampling in nature is practically impossible. Even at the lowest exchange rate found (at 25°C and pH 10), the halftime of equilibration was found to be 38.5 hours, whereas a steady state was reached already within 3 hours at pH 5. The ranking of 18O enrichment among different aqueous arsenate species at a given temperature was found to be H3AsO4 > H2AsO4^- > HAsO4^2- > AsO4^3-. Furthermore, equilibrium relationships for the arsenic oxyanion-water system have been calculated and are documented in this report as well as in the thesis of Xiaohui Tang. In conclusion, the project has shown that, due to fast exchange of oxygen isotopes between those oxyanion complexes and water, the oxygen isotopic composition of dissolved arsenic oxyanions is not of practical value in studying the forming conditions of arsenic rich natural waters, as it was thought initially. Nevertheless, this isotopic system could be potentially beneficial in investigating the origin of mineralizing fluids or alteration fluids out of which arsenate/ arsenite minerals precipitate in ore-forming environments. Furthermore, within this project it was developed for the first time a procedure which enables the preparative separation of arsenate and arsenite from natural waters.
Publications
- (2010): Oxygen Isotopy of Arsenate/ Arsenite: a Novel Approach to Constrain the Source of As in Groundwater. Geochimica et Cosmochimica Acta, Goldschmidt Conference 2010, Knoxville TE, Abstracts pp. A84
Berner, Z.A., Tang, X., Norra, S.
- (2011): Separation of arsenate and phosphate for the measurement of the isotope composition of oxygen in arsenate. Goldschmidt Conference 2011, Prague. Geochemica et Cosmochemica Acta, Goldschmidt Conference Abstracts pp. 1986
Tang, X., Berner, Z., Norra, S
- (2011): Separation of arsenite from other oxyanions in groundwater for the measurement of the isotopic composition of oxygen. Arsen 2011; Verhalten von Arsen in geologischen, hydrologischen und biologischen Systemen. Leipzig 12.-13. September 2011
Tang, X., Berner, Z., Norra, S.
- (2013) Method Development for the Determination of δ18O Values in Arsenic Oxyanions in natural Waters. AIG Conference, Budapest
Tang, X., Berner, Z., Norra, S., Vennemann, T.W.
- (2013): Preparative separation of arsenate from phosphate by IRA-400(OH) for oxygen isotopic work. Talanta 105: 46-51
Tang, X., Berner, Z., Khelashvilli, P., Norra, S.
(See online at https://doi.org/10.1016/j.talanta.2012.11.069) - Separating Arsenic Oxyanions from Natural Waters for Oxygen Isotope Analysis. Institute for Mineralogy and Geochemistry, KIT
Xiaohui Tang
