Crystallography of Environmentally Relevant Arsenic Oxosalts

Arsenic is an environmental pollutant that stems both from natural and anthropogenic sources. It ranks first on the international list of hazardous substances. Under environmental conditions, arsenic is most commonly bounded to oxygen and found in the III (arsenite) and V (arsenate) oxidation states. In order to understand the fate of As in the environment, one has to investigate structural features and stability of naturally occuring As compounds. In addition, a study of mineral-related synthetic phases should be very helpful, because they can occur as a result of human activities. Within the last fifteen years, there has been an unravelling activity in the field of systematic treatment of various classes of inorganic oxosalts, in particularly those associated with either technological or environmental impact, such as uranium, neptunium and beryllium compounds. Systematic crystal chemical reviews were published on borates, sulfates and phosphates. However, there were no systematic overviews provided on natural arsenites and arsenates, despite of their importance as extremely harmfulenvironmental agents. Compilation of such a review is one of the major theoretical topics of the planned activity. A detailed review of the structural chemistry of secondary arsenic minerals (arsenites and arsenates) would be very useful to a broad scientific community dealing with arsenic and its role in natural and technological systems. The aim of this project is to elaborate a detailed structural classification of natural arsenates and arsenites on the basis of current knowledge of their structural and spectroscopic characteristics, and systematic experimental studies of particular classes of arsenic-bearing environmentally relevant compounds. From the experimental viewpoint, the research will be focused on the M1O-M2O-As2O5-(H2O) (M1 = Sr2+, Ba2+, Cd2+; M2 = Mg2+, Co2+, Ni2+, Cu2+, Zn2+) arsenate system and arsenite compounds in the M1O-As2O3-(H2O), M2O-As2O3-(H2O) and M1O-M2O-As2O3- (H2O) (M1 = Na+, K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+; M2 = Mg2+, Mn2+,3+, Fe2+,3+, Co2+, Ni2+, Cu2+, Zn2+) systems. Some previously known, but poorly characterized, secondary arsenic minerals occuring in the contaminated environments will be re-investigated.

In order to create a structural overview of environmentally relevant mineral-related arsenic bearing compounds, the Elise Richter project V 203-N19 aimed at hydrothermal and ionothermal synthesis of various metal arsenic oxosalts and their structural and spectroscopic characterizations. More than 28 novel phases have been isolated from mostly hydrothermal and partly ionothermal reactions, of which more than 21 adopt mineral-related structures. The effectiveness of the ionothermal method to produce crystalline material suitable for the examination of crystal-chemical relationships among arsenic oxosalts was tested for the first time. The successful preparation dozens of metal arsenates in ionic liquids demonstrates not only that many more metal-arsenates with unique or mineral-related structures could be obtained but also that the ionothermal method is a promising method to synthesize new kinds of arsenic containing materials. Knowing the crystal-structural features of synthetic arsenic oxosalts, one can predict their stabilization in contaminated soils, sediments or drainage ponds. Synthesis and characterization of these phases brought us one step further to the understanding of how the ions behave in each others presence under differing laboratory conditions (e.g., pH, type of sorbent, solid:solution ratio, concentration, temperature, ionic strength), knowledge which could be directly or indirectly transferred to the environmental conditions.