Why study the geochemistry and environmental mineralogy of the iron-sulphur-arsenic system? In the minds of most people, "arsenic" and "poison" are almost synonyms. It has been available to dissatisfied spouses and the politically ambitious for thousands of years, a cheap and effective solution to many of the awkward situations that develop in human affairs (quote from Penrose, 1974). Presently, arsenic is recognised as one of the most serious inorganic contaminants in drinking water on a worldwide basis. An alarming example of arsenic problems in groundwater is Bangladesh, where many of the recently installed millions of drinking water wells contain high concentrations of arsenic (Smedley and Kinniburgh, 2002). These wells withdraw water from suboxic aquifers. The processes behind the effectively high arsenic mobility in this and other suboxic and anoxic environments are poorly understood (Smedley and Kinniburgh, 2002; Harvey et al., 2002). Knowledge of such processes is essential to understand and predict the behaviour of arsenic in these environments. The solubility of arsenic oxides and sulphides is relatively high under a wide range of pH and redox conditions. The most important process reducing arsenic mobility in the environment is sorption onto other, less soluble, oxides and sulphides. Iron oxides and sulphides are ubiquitous phases in sedimentary environments. While arsenic sorption onto iron oxides has been studied intensively over the past few years, its sorption onto Fe(II) sulphides has not been widely investigated.

The scope of the thesis is to study arsenic sorption onto disordered mackinawite, FeSam, and the association of As with Fe(II) sulphides during the formation of pyrite, FeS2. At ambient temperatures and pressures, several iron-sulphide phases can be formed. However, in anoxic sulphidic environments at pH values higher than 5, pyrite is the most stable and ubiquitous phase formed. Generally, pyrite formation is preceded by the precipitation of metastable disordered mackinawite. Since, in sedimentary settings, arsenic is present as dissolved As(III) or As(V), the behaviour of both As(V) and As(III) in the presence of FeSam and during the reaction to pyrite is studied.

As a background to the thesis, a brief overview of the literature on (disordered) mackinawite, pyrite and arsenic is provided in Chapter 1. In Chapters 2 and 3, the bulk characteristics, crystallinity and surface properties of synthetic FeSam are determined. The constructed surface model is applied in Chapter 4 to experimental arsenic sorption data. In Chapter 5, the behaviour of arsenic during the transformation of FeSam to pyrite is studied in batch experiments and results are interpreted in relation to the sorption reactions proposed in Chapter 4. Subsequently, pyrite formation in the presence of As(III) at concentrations approaching those in ambient environments is explored in Chapter 6, using a continuous-flow reaction system. Lastly, Chapter 7 is the synthesis of the thesis, in which conclusions are discussed and environmental implications of the study considered.

(10) Wolthers, M. , 2012. Building Biominerals. Monthly meeting of the London Palaeoclimate Group, Imperial College, London, U.K..

(9) Wolthers, M. , Nehrke G., Van Cappellen P., 2011. Calcite growth rate and solution composition. C-Seminar cycle of the AWI, Bremerhaven, Germany.

(8) Wolthers M., Charlet L., Van Cappellen P., 2010. Carbonate mineral surface chemistry: a surface structural model. Invited replacement of keynote lecture, Goldschmidt conference 2010, Knoxville, U.S.A.

(7) Wolthers M., Charlet L., Van Cappellen P., 2009. Carbonate mineral surface charge and potential reevaluated. American Chemical Society Meeting , Salt Lake City, U.S.A., March 2009.

(6) Wolther M., Mineral surface reactivity of iron sulphides and divalent metal carbonates. Seminar Series, Institut für Nukleare Entsorgung, Forschungszentrum Karlsruhe, Karlsruhe, Germany, May 2008.

(5) Wolthers M., Langezaal A.M. , Meysman F. , van Lith Y. , Ernst S. , Van der Zwaan G.J. 2007. Microhabitat distribution of benthic foraminifera. Darwin Days, The Netherlands, April 2007.

(4) Wolthers M., Rickard D., Charlet L., 2006. Structure and reactivity of FeS. American Chemical Society Fall Meeting, San Fransisco, U.S.A., September 2006.

(3) Wolthers M., Charlet L., Van Cappellen P., 2004. The surface chemistry of carbonates. Talk within the C-Seminar cycle of the Alfred Wegener Institute, Bremerhaven, Germany, September 2004.

(2) Wolthers M., 2004. Mackinawite surface chemistry. Journée scientifique de l'IMBG 'Le soufre et les métaux: Importance dans les sciences de l'univers et du vivant.' Grenoble, France, March 2004.

(1) Wolthers M., Rickard D., van der Weijden C.H., 2000. Arsenic incorporation into pyrite at low temperature. Talk at the meeting of the Kring Aardse Materialen of the Royal Geological and Mining Society of the Netherlands KNGMG, Utrecht, the Netherlands, December 2000.

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©Mariëtte Wolthers
This webpage, initially developed in HTML Kit, was last updated February 2013.