Organoarsenic Species Research Articles

Behaviour of arsenic on the continental shelf off the Gironde estuary: Role of phytoplankton in vertical fluxes during spring bloom conditions

The arsenic (As) cycle in the marine environment is known to be sensitive to biological activity. Within the scope of the National Coastal Oceanography Program, we undertook a specific study of the behaviour of this element in a water column on the continental shelf of the Bay of Biscay off the Gironde estuary during two oceanographic cruises conducted in May 1994 and May 1995. Various chemical forms were measured: arsenate (As3), arsenate (As5), monomethylarsenic (MMA), dimethylarsenic (DMA) and dissolved total As after ultraviolet mineralization. The net flux of total As from the Gironde estuary was evaluated as well as its effect on the concentration in surface marine waters. The vertical profiles in the coastal zone during a period of active primary production confirmed a certain analogy between arsenates and phosphates as well as the formation of As3 and DMA. The importance of organoarsenic species not directly accessible to the formation of volatile hydrides is demonstrated. Their presence modifies assessments, reducing the deficit of dissolved total As in the euphotic layer to just 5%. For particles, the As content in phytoplankton was estimated at 6 μg·g −1 compared to 20 to 30 μg·g −1 in iron- and aluminium-rich terrigenous particles. These results do not invalidate the role of phytoplankton in the speciation of dissolved As but indicate that their involvement in vertical transfers was apparently not predominant in the zone under study.

First order speciation of As using flow injection hydride generation atomic absorption spectrometry with in-situ trapping of the arsine in a graphite furnace

A simple method is described to distinguish between As species that react with sodium tetrahydroborate (III) to form AsH 3 and the naturally occurring As species that are unreactive. Results for this rudimentary or “first order” speciation scheme are reported for biological tissue, aquatic plant material, urine and natural water samples. Biological tissue and aquatic plant samples were briefly solubilized in a mixture of 50% nitric acid, no sample preparation was required for the urine or natural water samples. Organoarsenic species which do not react with sodium borohydride under acidic conditions such as arsenobetaine, arsenocholine and tetramethylarsenic, are converted to As(V) by on-line photo-oxidation or microwave heating in a mixture of 0.5 M NaOH and 0.05 M K 2S 2O 8. The sample is subsequently acidified, reduced with sodium borohydride and the generated arsine is trapped in a heated graphite furnace prior to atomization. The superior detection limit (0.14 ng) of the trapping technique permits the dilution of most types of samples, minimizing or eliminating interference effects. Without photolysis or microwave heating a combined result for As(III), As(V), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) is obtained. Results are reported for the first order speciation of As in a suite of certified reference materials (CRMs) including National Research Council (NRC) biological tissues and natural water samples, Community Bureau of Reference (BCR) aquatic plant materials and the National Institute of Standards and Technology (NIST) SRM 267ON urine sample. The determination of a non-hydride forming As fraction in untreated urine and natural water certified reference materials (CRMs) has revealed a species of As previously undetected in NRC seawater CRMs.

Developments in the electrospray mass spectrometry of inorganic species

The development of new ion sources for mass spectrometry capable of providing more information about solution samples is an ongoing challenge for the analytical chemist. Recently there has been interest in the development of inorganic Electrospray Mass Spectrometry as this technique offers a direct approach to the determination of solution ions. In addition to identifying the type of ion present, the technique also offers information on an ion's oxidation state and molecular form. The exact form of such species as alkylated tin complexes or organoarsenic species in solution may then be deduced. Some important fundamental aspects of electrospray and a brief overview of this technique for inorganic solutions are provided.

The fate of arsenic in a laminar diffusion flame

An arsenic compound (cacodylic acid) has been added to laminar diffusion flames burning in a co-flowing stream of air. Two fuels were used viz. ethene (C2H4) and a mixture of hydrogen and nitrogen. Samples of aerosols and gaseous species were collected within the flames and above the flames with filters and sorbent tubes. The hydrogen flame showed the conversion of arsenite to arsenate with increased residence time through the flame. The conversion of arsenite to arsenate in the ethene flames was inhibited by the emission of soot, which apparently competed with the arsenic for oxygen in the postflame gas. Arsenic was not found to be incorporated into soot particles. The carbon and arsenic were present in the posflame gases as two distinct aerosol phases. The addition of arsenic did not have a significant effect on the formation and oxidation of soot. The addition of arsenic did not lead to the production of organoarsenic species; only common polycyclic aromatics were found in the sooting flames.

Arsenic speciation in soil porewaters from the Ashanti Mine, Ghana

At the Ashanti Au mine in southwest Ghana, arsenopyrite constitutes an important part of the hypogene ore assemblage, often hosting significant concentrations of Au. The oxidation of arsenopyrite either through lateritic weathering or during mineral processing releases As into the environment. The speciation of As in soil porewaters both from uncontaminated soils and those overlain by mine waste has been assessed. In aerobic soils overlying, weathered Au As mineralization in the bedrock, As was present in concentrations of 189–1025 ppm in soil and porewater concentrations ranged from 86.2 to 557 μg l −1. Arsenic in these waters was found to be largely present as arsenate (approximately 78–95% of the total As) with arsenite, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) also present. Where arsenite concentrations are greater than arsenate, no organoarsenical species were observed. In the uncontaminated soils overlying unmineralized bedrock, background As concentrations were in the range of 12.5–20.2 ppm and porewater concentrations ranged from 11.2 to 20 μg l −1. In these porewaters arsenate was the dominant species. No organoarsenical species were observed in these waters, suggesting that there is a possible threshold level of total As concentration required, above which methylation of As by soil microorganisms takes place. In the acidic soils contaminated by mine tailings, arsenite was the major species present (up to 45% of total As in the aerobic soils and 79% of total As in the anaerobic soils). Although the soils were underlain by the unmineralized bedrock, soil As concentration ranged from 40.5 to 1290 ppm and As concentration in extracted porewaters from anaerobic soils ranged from 70.2 to 110 μg l −1 and in aerobic soil from 70.8 to 599 μg l −1. In the aerobic soils MMAA, DMAA, and arsenate were also present but only arsenate was found, along with arsenite, in the anaerobic soil porewaters.

Characterization of Arsenic in Sea Water by Hydride Generation-Atomic Absorption Spectrometry

A simple digestion procedure with potassium persulfate in alkaline medium is described for the determination of the total concentration of arsenic species in sea water. Combined with the previously proposed method for the differential determination of As(III) and As(V) by hydride generation-atomic absorption spectrometry, arsenic species in sea water were determined as As(III), As(V) and organoarsenic species. Sub ppb level of arsenic in sea water could be detected with a sample volume of 100ml. In sea water samples of the Japan Sea and the northwestern North Pacific Ocean, an average of total concentrations of arsenic was found to be about 2.0μg l-1 irrespective of locality. Total concentrations were rather constant from surface to deep water. As(III) and organic species were found only in surface water and the concentration of As(V) was about 0.2 to 0.3μg l-1 lower in the surface water than in the deep water.