Birgit Daus

Sorption of aqueous antimony and arsenic species onto akaganeite.

Two akaganeite materials were tested for the removal of antimonate, trimethyl antimonate, arsenate, arsenite, and dimethyl arsenate from water: a commercial product (GEH) and a synthesized akaganeite. The two materials show similar q(max) values, but differ in their K(L) values. This could be a result of their different crystal sizes indicated by sharper XRD reflections of the synthesized akaganeite compared with GEH. Batch experiments were carried out using all species to investigate the influence of the pH on their sorption onto the commercial material. The best results for the removal of antimonate and arsenate were achieved under acidic conditions, while the sorption of arsenite has an optimum at pH 7. The maximum loadings vary from 450 mg g(-1) (antimonate at pH 2.2.) to 2 mg g(-1) (trimethyl antimonate at pH 7). Competition reactions (up to a 10-fold excess of the competitor ion) were studied with antimonate, arsenate, and phosphate. The sorption capacity of arsenate decreases up to 12.5% by adding phosphate (ratio 1:10), but the addition of antimonate did not influence the sorption of arsenate. Conversely, the sorption of antimonate decreases due to the addition of 10-fold concentration of arsenate (31%) or phosphate (27%).

Investigation on stability and preservation of arsenic species in iron rich water samples

Important for the accurate and reproducible determination of inorganic redox forms of arsenic in iron-rich waters is their conservation prior to analysis. Species and trace element analysis methods are commonly laboratory based. Stabilisation of samples is necessary for subsequent laboratory analysis in order to preserve the information about the system from which the samples were taken. Pre-treatment procedures based on complexation of metal ions and moderate acidification of the samples are presented. The concentration of the stabilisation agent and the storage temperature were optimised. The most satisfactory results were obtained with 0.01 mol l(-1) phosphoric acid and a storage at 6 degrees C.

Fate of Arsenic during Microbial Reduction of Biogenic versus Abiogenic As–Fe(III)–Mineral Coprecipitates

Fe(III) (oxyhydr)oxide minerals exhibit a high sorption affinity for arsenic (As) and the reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides is considered to be the primary mechanism for As release into groundwater. To date, research has focused on the reactivity of abiogenic Fe(III) (oxyhydr)oxides, yet in nature biogenic Fe(III) (oxyhydr)oxides, precipitated by Fe(II)-oxidizing bacteria are also present. These biominerals contain cell-derived organic matter (CDOM), leading to different properties than their abiogenic counterparts. Here, we follow Fe mineralogy and As mobility during the reduction of As-loaded biogenic and abiogenic Fe(III) minerals by Shewanella oneidensis MR-1. We found that microbial reduction of As(III)-bearing biogenic Fe(III) (oxyhydr)oxides released more As than reduction of abiogenic Fe(III) (oxyhydr)oxides. In contrast, As was immobilized more effectively during reduction of As(V)-loaded biogenic than abiogenic Fe(III) (oxyhydr)oxides during secondary Fe mineral formation. During sterile incubation of minerals and after microbial Fe(III) reduction stopped, As(V) was mobilized from biogenic Fe(III) (oxyhydr)oxides probably by sorption competition with phosphate and CDOM. Our data show that the presence of CDOM significantly influences As mobility during reduction of Fe(III) minerals and we suggest that it is essential to consider both biogenic and abiogenic Fe(III) (oxyhydr)oxides to further understand the environmental fate of As.

Kinetics of the arsenite oxidation in seepage water from a tin mill tailings pond

The kinetic of the oxidation of trivalent arsenic was investigated in synthetic as well as in natural samples of a tin mill seepage water. The influence of ferric ions and solid MnO(2) on the process was studied. To determine the time dependence of the concentrations of the arsenic species, a series of samples were taken sequentially and analysed by coupling of ionchromatographic separation and ICP-MS detection. To investigate the naturally occurring oxidation reaction, original seepage water samples were filtered and spiked with As(III) (1 mg l(-1)) and Fe(II) (10 mg l(-1)) and shaken providing intensive contact with the air. Additional synthetic samples buffered with carbonate were used in similar experiments to simplify the system. The reaction was incomplete in the presence of excess of iron for both types of samples. The oxidation of As(III) was complete within 8 h in the presence of MnO(2) (10 mg), but there was a difference in oxidation rates between the natural and the synthetic samples. The results are discussed with respect to the redox potentials and equilibrium constants.

Simultaneous determination of inorganic and organic antimony species by using anion exchange phases for HPLC–ICP-MS and their application to plant extracts of Pteris vittata

Antimony is a common contaminant at abandoned sites for non-ferrous ore mining and processing. Because of the possible risk of antimony by transfer to plants growing on contaminated sites, it is of importance to analyze antimony and its species in such biota. A method based on high performance liquid chromatographic separation and inductively coupled plasma mass spectrometric detection (HPLC-ICP-MS) was developed to determine inorganic antimony species such as Sb(III) and Sb(V) as well as possible antimony-organic metabolisation products of the antimony transferred into plant material within one chromatographic run. The separation is performed using anion chromatography on a strong anion exchange column (IonPac AS15/AG 15). Based on isocratic optimizations for the separation of Sb(III) and Sb(V) as well as Sb(V) and trimenthylated Sb(V) (TMSb(V)), a chromatographic method with an eluent gradient was developed. The suggested analytical method was applied to aqueous extracts of Chinese break fern Pteris vittata samples. The transfer of antimony from spiked soil composites into the fern, which is known as a hyperaccumulator for arsenic, was investigated under greenhouse conditions. Remarkable amounts of antimony were transferred into roots and leaves of P. vittata growing on spiked soil composites. Generally, P. vittata accumulates not only arsenic (as shown in a multiplicity of studies in the last decade), but also antimony to a lower extent. The main contaminant in the extracts was Sb(V), but also elevated concentrations of Sb(III) and TMSb(V) (all in microg L(-1) range). An unidentified Sb compound in the plant extracts was detected, which slightly differ in elution time from TMSb(V).

Arsenic speciation in iron hydroxide precipitates

In this study a special sequential extraction method is proposed to discriminate between arsenic adsorbed and co-precipitated in precipitates arising mainly from iron hydroxides or bound in low solubility mineral phases. Synthetic iron hydroxide precipitates were prepared to investigate the influence of the amount of arsenate, of the manganese additionally added and of the valence state of arsenic on the remobilisation of arsenic. After preparing the precipitates with arsenate no arsenic could be detected in the supernatant solution. About 82% (w/w) of the arsenate is adsorbed to the precipitate and the remaining part can be dissolved by shaking with an oxalate buffer. A significant difference between the amount of arsenic added and the amount analysed in the two steps was not found. Consequently, compounds with a low solubility, such as scorodite, were not formed in the synthesized precipitates. The valence of the arsenic and addition of manganese influence significantly the uptake of arsenic by iron hydroxides. Natural precipitate samples from a percolate water of tin mill tailings were investigated using this method.

Analytical investigations of phenyl arsenicals in groundwater.

Phenylic arsenic compounds are the main contaminants in groundwater at abandoned sites with a history of arsenic containing chemical warfare agents (CWA). A fast and sensitive HPLC-ICP-MS method was developed to determine inorganic arsenic compounds like arsenite and arsenate as well as the degradation products of the arsenic containing warfare agents (phenylarsonic acid, phenylarsine oxide, diphenylarsinic acid). Beside these arsenic species the groundwater samples contained also high iron contents (up to 23 mg/l as Fe(II)) which led to precipitates in the samples after coming into contact with the atmosphere. Preservation immediately after sampling by phosphoric acid has shown that a successful avoidance of any losses of any arsenic species between sampling and analysis was possible. The suggested analytical method was applied to groundwater samples taken from different depths at a polluted site. The main contaminant in the water samples was diphenylarsinic acid (up to 2.1 mg/l) identified by ESI-MS, but also elevated concentrations of inorganic arsenic (up to 240 microg/l) were found.

Preservation of arsenic species in water samples using phosphoric acid--limitations and long-term stability.

The preservation of arsenic species in water samples is an indispensable method to avoid their changes during storage, if it is not possible to analyse them immediately. The aim of this investigation was to demonstrate the limitations of the suggested method by using phosphoric acid as a preservation agent. The samples remain stable for 3 months, even if they show evidence of high concentrations of iron or manganese. Critical is an increasing pH>3. Theoretically, a precipitation of strengite (Fe(3)(PO(4))(2)) could occur, which should be avoided. Phosphoric acid with a final concentration of 10mM is recommended as a preservation agent, combined with keeping the samples cool (6 degrees C) and dark. Filtration of samples before preservation may be carried out with respect to the analytical aim to distinguish between the total and soluble fraction (without colloids). It was shown that filtered and non-filtered samples can be preserved by utilising the above mentioned scheme.

Biogenic Fe(III) minerals lower the efficiency of iron-mineral-based commercial filter systems for arsenic removal.

Millions of people worldwide are affected by As (arsenic) contaminated groundwater. Fe(III) (oxy)hydroxides sorb As efficiently and are therefore used in water purification filters. Commercial filters containing abiogenic Fe(III) (oxy)hydroxides (GEH) showed varying As removal, and it was unclear whether Fe(II)-oxidizing bacteria influenced filter efficiency. We found up to 10(7) Fe(II)-oxidizing bacteria/g dry-weight in GEH-filters and determined the performance of filter material in the presence and absence of Fe(II)-oxidizing bacteria. GEH-material sorbed 1.7 mmol As(V)/g Fe and was ~8 times more efficient than biogenic Fe(III) minerals that sorbed only 208.3 μmol As(V)/g Fe. This was also ~5 times more efficient than a 10:1-mixture of GEH-material and biogenic Fe(III) minerals that bound 322.6 μmol As(V)/g Fe. Coprecipitation of As(V) with biogenic Fe(III) minerals removed 343.0 μmol As(V)/g Fe, while As removal by coprecipitation with biogenic minerals in the presence of GEH-material was slightly less efficient as GEH-material only and yielded 1.5 mmol As(V)/g Fe. The present study thus suggests that the formation of biogenic Fe(III) minerals lowers rather than increases As removal efficiency of the filters probably due to the repulsion of the negatively charged arsenate by the negatively charged biogenic minerals. For this reason we recommend excluding microorganisms from filters (e.g., by activated carbon filters) to maintain their high As removal capacity.

Arsenic(V) Incorporation in Vivianite during Microbial Reduction of Arsenic(V)-Bearing Biogenic Fe(III) (Oxyhydr)oxides.

The dissolution of arsenic-bearing iron(III) (oxyhydr)oxides during combined microbial iron(III) and arsenate(V) reduction is thought to be the main mechanism responsible for arsenic mobilization in reducing environments. Besides its mobilization during bioreduction, arsenic is often resequestered by newly forming secondary iron(II)-bearing mineral phases. In phosphate-bearing environments, iron(II) inputs generally lead to vivianite precipitation. In fact, in a previous study we observed that during bioreduction of arsenate(V)-bearing biogenic iron(III) (oxyhydr)oxides in phosphate-containing growth media, arsenate(V) was immobilized by the newly forming secondary iron(II) and iron(II)/iron(III)mineral phases, including vivianite. In the present study, changes in arsenic redox state and binding environment in these experiments were analyzed. We found that arsenate(V) partly replaced phosphate in vivianite, thus forming a vivianite-symplesite solid solution identified as Fe3(PO4)1.7(AsO4)0.3·8H2O. Our data suggests that in order to predict the fate of arsenic during the bioreduction of abiogenic and biogenic iron(III) (oxyhydr)oxides in arsenic-contaminated environments, the formation of symplesite-vivianite minerals needs to be considered. Indeed, such mineral phases could contribute to a delayed and slow release of arsenic in phosphate-bearing surface and groundwater environments.