Arsine trap for soil emissions

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  • Published: Nov 9, 2009
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Arsine trap for soil emissions

Arsenic is one of those elements that non-scientists have come across due to its association in fiction and real life with poisoning. But arsenic is also an environmental problem and is found in contaminated soils. Research efforts at detoxification have included the use of bacteria, which convert elemental arsenic to a series of volatile methylated arsines. Fungi can operate in the same way and, in both cases, the products are dissipated in the air.

Trimethylarsine has also been detected in the outgassing emissions from geothermal waters, along with chlorinated and sulphur-containing arsines, although their mechanism of formation is said to be poorly understood.

The gold standard for analysing volatile environmental arsenic compounds is cryotrapping linked to gas chromatography-inductively coupled plasma-mass spectrometry (GC-ICPMS). However, it is not as popular as it might be because the instrumentation is not available off the shelf and the technique itself is quite tricky to operate, say a group of researchers from the UK and Bangladesh.

They remarked that trapping/preconcentration procedures in which the arsenic compounds are converted in the trap to less volatile compounds are a useful alternative but many reported systems do not differentiate between the individual arsines. Instead, they report total arsenic present. This general deficiency in sampling volatile arsenic compounds is hampering research into the volatilisation of terrestrial arsenic.

In an effort to facilitate such studies, the group has designed a chemotrap which traps and converts arsines while maintaining the identity of the individual compounds. Jorg Feldmann, Andrew Meharg and colleagues from the University of Aberdeen, UK, and the Bangladesh Agricultural University, Mymensingh, based their trap on silver nitrate.

The common arsines monomethylarsine, dimethylarsine, trimethylarsine and arsine itself react with silver nitrate to give monomethylarsonic acid, dimethylarsinic acid, trimethylarsine oxide and arsenate, respectively. So, individual compound integrity is preserved.

In initial experiments, two plugs of silver nitrate-impregnated silica gel were separated by glass wool in platinum-cured silicon tubing, with a foam plug at the opening. A mixture of the arsines in a Tedlar bag was drawn through the trap by a gas-tight syringe fitted at the other end of the tube. The arsenic products formed were eluted with boiling nitric acid for total arsenic determination, or with boiling water for speciation analysis.

The detection limits were relatively high at 17.7 ng and the team isolated the problem to the glass wool, which was high in arsenic at 146 ng/trap. Even the foam plugs contained 6.6 ng arsenic, so further experiments were conducted in the absence of the glass wool and the foam, resulting in a detection limit of 3.8 ng.

The aqueous eluate was analysed by HPLC-ICPMS which showed good separation of the oxidised arsines, allowing their quantitation. Recoveries were 80-95%.

The research team applied their new trap to a real problem, trapping arsines given off from soil collected from a Bangladeshi paddy field. The soil was either flooded or non-flooded and treated with dried cattle manure or non-manured. Samples were placed in a flask and the air was flushed to the arsenic traps over 33 days. After this time, the trap was replaced and trapping continued for a further 28 days.

The levels of arsines from non-manured soil were below the detection limit but manured soil produced measurable quantities. The most dominant was trimethylarsine oxide, followed by dimethylarsinic acid, and there were low amounts of arsenate. About 320 ng of arsenic was evolved in total over 61 days, equating to about 0.014% of the total arsenic in the soil.

This is "the first unequivocal report that arsines are evolved from paddy field soils," say the team, "confirming previous observations that arsine evolution from a range of soils can occur."

The simplicity of the trapping method, which preserves the parent arsine compound identity, may extend the measurement of arsenic species in soil and lead to a better understanding of the route to volatilisation.

The researchers declared that the method might be expanded to other arsenic compounds, such as the arsines emitted from geothermal waters and ethyldimethylarsine in landfill gas. However, their reactions with silver nitrate in the trap need to be examined first to ensure that the identities of the compounds are preserved.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.



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