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Mechanisms and pathways of arsenic, antimony and mercury biomethylation in soil

An EU-funded initiative has provided a better understanding of the biogeochemical cycle of arsenic (As), antimony (Sb) and mercury (Hg) by developing new techniques for studying biomethylation and biovolatilisation processes in the environment.
Mechanisms and pathways of arsenic, antimony and mercury biomethylation in soil
Arsenic (As) is ubiquitous in the environment and can be found in soils in both its inorganic and organic form. Organic forms found in soils are mostly mono- and dimethylarsenic acid (MMA(V) and DMAV(V)). However, the process known as biomethylation that leads to these compounds is not well understood.

Microorganisms in soil transform inorganic As to less toxic oxidized methylated compounds, mostly MMA(V) and DMA(V). However, they also produce reduced methylated compounds MMA(III) and DMA(III), which are even more toxic than inorganic As. Biomethylation is linked to another mechanism called biovolatilisation, which leads to the production of four highly volatile As compounds (arsine, mono, di- and trimethylarsine).

The BIOMETA (Biomethylation and biovolatilisation of arsenic in soils: Using carbon and hydrogen isotopes to unravel the mechanisms and pathways involved) project was initially established to study the biomethylation and biovolatilisation of As. Firstly, researchers found that lichens represent an important source of As to the atmosphere. Furthermore, the rate at which they emit volatile As could prove useful in revealing biomethylation pathways in incubation experiments.

Furthermore, a series of experiments was also conducted on the biomethylation and biovolatilisation of Sb (a sister element of As) and Hg, another highly relevant pollutant. New cost-efficient, user-friendly techniques were developed to measure methylated and volatile species of Sb and Hg, using high-performance liquid chromatography combined with inductively coupled plasma mass spectrometry (HPLC-ICP-MS).

Researchers also developed a new method to measure volatile Sb species at trace levels using liquid traps and a new quantitative extraction technique that uses oxalic acid to measure methylated Sb species in soils and organisms. The extraction and analysis methods were combined to show that trimethylantimony (up to 1.8ppm) can be found in shooting ranges. The researchers also found out that when shooting range soil is flooded, up to 10 % of the antimony in the pore water is trimethylantimony.

Furthermore, hydroponic plant experiments showed that plants growing on shooting ranges, such as rye grass, could take up trimethylantimony and translocate it to the shoots. These may be eaten by livestock as shooting ranges are still used as pastures.

A new technique was also developed for extracting methylmercury (MeHg) from soil and biota using hydrochloric acid followed by extraction with dichloromethane and L-cysteine. The extract was then measured using HPLC-ICP-MS. Scientists used the technique to investigate the presence of MeHg in a contaminated floodplain in Switzerland. In addition, they conducted a soil incubation experiment to show the impact of flooding and addition of organic matter on Hg levels.

BIOMETA work will enable better management and remediation of polluted sites through improved understanding of the behavior of these toxic elements. It can also help policymakers to create new laws based on specific compounds.

Related information

Keywords

Arsenic, soil, antimony, mercury, biomethylation, biovolatilisation, BIOMETA
Record Number: 188688 / Last updated on: 2016-10-18
Domain: Environment
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