5 research outputs found
Monothioarsenate Uptake, Transformation, and Translocation in Rice Plants
Thioarsenates
form under sulfur-reducing conditions in paddy soil
pore waters. Sulfur fertilization, recently promoted for decreasing
total arsenic (As) grain concentrations, could enhance their formation.
Yet, to date, thioarsenate toxicity, uptake, transformation, and translocation
in rice are unknown. Our growth inhibition experiments showed that
the toxicity of monothioarsenate (MTA) was similar to that of arsenate
but lower than that of arsenite. Higher toxicity of MTA with lower
phosphate availability might imply uptake through phosphate transporters
similar to arsenate. To demonstrate direct uptake of MTA by rice plants,
a species-preserving extraction method for plant samples was developed.
When plants were exposed to 10 ÎŒM MTA for 72 h, up to 19% and
4% of total As accumulated in roots and shoots, respectively, was
MTA. Monothioarsenate was detected in xylem sap and root exudates,
and its reduction to arsenite in rice roots and shoots was shown.
Total As uptake was lower upon exposure to MTA compared to arsenate,
but root to shoot translocation was higher, resulting in comparable
As shoot concentrations. Thus, before promoting sulfur fertilization,
uptake and detoxifying mechanisms of thioarsenates as well as potential
contribution to grain As accumulation need to be better understood
Experimental Confirmation of Isotope Fractionation in Thiomolybdates Using Ion Chromatographic Separation and Detection by Multicollector ICPMS
Molybdenum <sup>98</sup>Mo/<sup>95</sup>Mo isotope ratios are a
sediment paleo proxy for the redox state of the ancient ocean. Under
sulfidic conditions, no fractionation between seawater and sediment
should be observed if molybdate (MoO<sub>4</sub><sup>2â</sup>) is quantitatively transformed to tetrathiomolybdate (MoS<sub>4</sub><sup>2â</sup>) and precipitated. However, quantum mechanical
calculations previously suggested that incomplete sulfidation could
be associated with substantial fractionation. To experimentally confirm
isotope fractionation in thiomolybdates, a new approach for determination
of isotope ratios of individual thiomolybdate species was developed
that uses chromatography (HPLC-UV) to separate individual thiomolybdates,
collecting each peak and analyzing isotope ratios with multicollector
inductively coupled plasma mass spectrometry (MC-ICPMS). Using commercially
available MoO<sub>4</sub><sup>2â</sup> and MoS<sub>4</sub><sup>2â</sup> standards, the method was evaluated and excellent
reproducibility and accuracy were obtained. For species with longer
retention times, complete chromatographic peaks had to be collected
to avoid isotope fractionation within peaks. Isotope fractionation
during formation of thiomolybdates could be experimentally proven
for the first time in the reaction of MoO<sub>4</sub><sup>2â</sup> with 20-fold or 50-fold excess of sulfide. The previously calculated
isotope fractionation for MoS<sub>4</sub><sup>2â</sup> was
confirmed, and the result for MoO<sub>2</sub>S<sub>2</sub><sup>2â</sup> was in the predicted range. Isotopic fractionation during MoS<sub>4</sub><sup>2â</sup> transformation with pressurized air was
dominated by kinetic fractionation. Further optimization and online-coupling
of the HPLC-MC-ICPMS approach for determination of low concentrations
in natural samples will greatly help to obtain more accurate species-selective
isotope information
Potential of high pH and reduced sulfur for arsenic mobilization:insights from a Finnish peatland treating mining waste water
Abstract
Peatlands, used for purification of mining waste waters, have shown efficient solid-phase sequestration of contaminants such as arsenic (As). However, contaminant re-mobilization may occur related to management changes or chemical alteration of original peatland conditions. For a treatment peatland in Finnish Lapland, we here confirm efficient As retention in near-surface peat layers close to the mining waste water inflow, likely due to binding to FeIII-phases. Seven years into operation of the treatment peatland, there appears to be further retention potential, as large areas downstream still had solid-phase As concentrations at background levels. However, via depth-resolved pore water analysis we observed a hotspot 170 m from the inflow at 10â50 cm depth, where As pore water concentrations exceeded input concentrations by a factor of 20, indicating substantial As re-mobilization. At the same spot, a peak of reduced sulfur (S) species was found. Arsenic species detected were arsenite and up to 26% methylated oxyarsenates, 15% methylated and 7.9% inorganic thioarsenates. We postulate that As mobilization is a result of short-term re-equilibration to a changed inflow chemistry after installation of a process water treatment plant and a long-term consequence of changing pore water pH from acidic to near-neutral, releasing reduced S and As. We infer that the co-occurrence of reduced S and As leads to formation of methylated and/or thiolated As species with known low sorption affinity, thereby further enhancing As mobility. Laboratory incubation studies with two peat cores confirmed a high S-induced As mobilization potential, especially when As-Fe-rich, oxic surface layers were incubated anoxically at near-neutral pH. Highest risk of As re-mobilization from this treatment peatland is expected in a scenario in which mining waste water inflow has stopped but the peatland remains flooded, and near-surface layers transition from oxic to anoxic conditions