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Faculty for Biology, Chemistry, and Earth Sciences

Environmental Geochemistry Group - Prof. Dr. Britta Planer-Friedrich

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Research Modul

Synthesis of methylthioarsenates

Harold Suaznabar (02/2018-03/2018)

Support: Britta Planer-Friedrich, Carolin Kerl

Background. Rice is the major staple food worldwide and increasingly popular also in Europe, e.g. as baby food or in alternative diets. However, rice also accumulates arsenic, a human carcinogen which is why in 2016 an arsenic limit for rice grains was introduced in the European Union. The high arsenic accumulation is related to high arsenic mobility under anoxic conditions which form because rice is typically grown under flooded conditions in organic-rich soil. In the wake of investigating arsenic mobility in paddy soils and its uptake in the rice plant in cooperation with colleagues from major rice-growing areas in Italy and France we discovered novel arsenic species in the rice field soils. These species are arsenic-sulfur species, so called thioarsenates. Both inorganic and methylated thioarsenates exist. In our group we have been investigating these species, e.g. in hydrothermal areas, for years, but no one has looked for them in the context of rice, yet.

Given the global threat of chronic poisoning by consumption of As-contaminated rice as staple food and the ubiquitous natural occurrence of reduced S in paddy soils, there is an urgent need to know under which conditions and to which extent thioarsenates form and how they affect As mobility in the rhizosphere, uptake and translocation in the plant as well as accumulation in the rice grain. Both aspects are part of 2 current PhD projects in our group.

Clarifying the role of thioarsenates is all the more important as they are expected to form in significant quantities especially in degraded paddy soils low in Fe, in poorly drained organic soils, in acid sulfate soils or upon sulfate fertilization which has recently been advertised to reduce grain As concentrations. If intensification of land use proceeds as it has in the past, increasing soil degradation and compaction, over-fertilization, and reclamation of new land (e.g. mangrove mud rich in sulfate) will increase the share of such soils used for rice cultivation. Furthermore, it is important to understand how water management affects thiolation versus methylation because both to save water consumption and reduce greenhouse gas emissions, more and more rice cultivation in the up-coming years will rely on alternate wetting-drying strategies instead of growing rice under continuously flooded conditions.

Task. Methylated thioarsenates are As-S species that contain 1 or 2 CH3 groups and 1 or 2 SH groups (monomethyl-monothioarsenate, monomethyl-dithioarsenate, dimethyl-monothioarsenate, and dimethyldithioarsenate). No one has tested their uptake in plants, yet. Two major problems typically impede researchers from advancing on the field of thioarsenate analysis: 1.) a lack of the right instrumentation (IC-ICP-MS) and 2.) a lack of standards. None of the thioarsenates is commercially available. During a master thesis in 2018 we will test uptake, tolerance, and toxicity of two methylthiolated arsenates in rice. However, before being able to do so, these substances have to be synthesized. There are different recipes in the literature. The task of this research module will be to test in close cooperation with master student Alina Schindele different syntheses methods and determine purity of the substances by IC-ICP-MS. A further step will be to test the stability of the produced standards under different storage and experimental conditions and in different test solutions, required for later plants experiments.

Preferred time: 4 weeks in February/March/April

All research modules will be done in close cooperation with a PhD Student or Postdoc in our group. The modules can typically either be done in a 4 weeks block during the semester break (mid of February to mid of April) or as individual hours during the summer semester (exact schedule will be discussed with the direct supervisor). During that time you will have a specific topic you are working on but you are also free to have a look around in our lab, participate in other people´s projects or analyses whenever it fits in your and their schedule.

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