Plant uptake of trimethylantimony from contaminated soils

Antimony (Sb) is a hazardous pollutant that can be found at elevated concentrations in soils contaminated by mining activities or shooting ranges. While classified as a priority pollutant as well as a toxic pollutant by the US EPA, it has been much less studied than other relevant pollutants such as arsenic, lead or mercury. In soils, Sb occurs as inorganic trivalent Sb species, Sb(III), inorganic pentavalent Sb species, Sb(V), or trimethylantimony, TMSb. The latter organometallic species is a result of microbial biomethylation. In order to understand the transfer of the different antimony species into the food chain, it is important to study the first step, namely plant uptake from contaminated soil. In a review about alkylated antimony species, the author Filella states that Sb has been largely overlooked as an element of environmental concern and that the data on the physical and chemical properties of these organic compounds are fragmentary and old. Unfortunately, investigations related to antimony speciation were limited by the availability of suitable extraction and analytical methods for antimony speciation analysis.

The group of researchers introduced a new method that allows quantitative extraction of TMSb from different parts of plants using oxalic and ascorbic acid. The extraction method was combined with a HPLC-ICP-MS method for speciation analysis using ammonium tartrate as mobile phase under isocratic conditions.

The plant used for the investigations was ryegrass, grown in a growth chamber and exposed to the different Sb species via nutrient solutions, that were replaced every two days in order to reduce change of Sb speciation.  

After harvest, plants were separated into shoots and roots, washed and dried before grinding. Extraction of Sb species was performed using a newly developed method using oxalic and acetic acids in an ultrasonic bath for 30 min. This method recovered more than 70% of the total antimony. Speciation analysis was performed by using HPLC-ICP-MS using a reversed-phase column under isocratic conditions. This speciation method recovered the antimony species extracted quantitatively.



Figure: Lolium perenne (ryegrass)

Most of the antimony in the ryegrass was found in the roots, and Sb concentration were 100 times higher for Sb(III)-exposed plants than for Sb(V)- or TMSb-exposed plants. In roots and shoots of the Sb(III)-exposed plants, Sb(III) contributed to about 45 and 26 % of the total Sb. The rest of the antimony present mostly consisted of Sb(V), except in the roots, where a very small part (~0.5%) was TMSb.

In plants exposed to Sb(V) only about 60% of the Sb in the roots was found to be Sb(V) while the rest was Sb(III). Interestingly, no Sb(III) was found in the shoots. Most importantly, TMSb was found in the shoots of Sb(V) exposed plants but not in the roots, indicating fast transport from the roots and accumulation in the shoots.  

Remarkably, the roots of plants exposed to TMSb contained more than 75% of the Sb taken up as TMSb while the rest was exclusively Sb(V). This suggests that demethylation occurs at the roots or at their interface with the nutrient solution, since no inorganic Sb was present in the nutrient solution and no degradation occurred during plant extraction. In the shoots a similar part of the total Sb was TMSb while the Sb(V) made up to only 15%. An unknown species occurred in the shoots with about 8% of the total Sb that probably could be DMSb or MMSb. The researchers were not able to identify this unknown species due to missing standards and the incompatibility of the mobile phase with ESI-MS for structure elucidation.

The authors are convinced that their method, while not without some interconversion problems for the inorganic species, will be useful for studying the soil-plant system with respect to antimony species.