Widespread Occurrence of the Highly Toxic Dimethylated Monothioarsenate in Rice

Arsenic is ubiquitous in soils globally and occurring at elevated concentrations in irrigation water in south and south-east Asia, threatening the production of food crops. Paddy rice accumulates more arsenic in the grain than any other cereal because arsenite is mobilized from soil minerals under anaerobic conditions in flooded paddy fields. Since inorganic arsenic is classified as a carcinogen, arsenic accumulation in rice is a global concern for human health, resulting in regulation for maximal tolerable concentrations for inorganic arsenic. Besides inorganic arsenic iAs, dimethylated arsenate (DMA) is often found in rice, which however because of much reduced toxicity is exempt from food safety considerations. In general, arsenic toxicity is depending on the species being present, with inorganic arsenic considered to be the most toxic.

Anyhow, during the last 15 years, some organic arsenic species have been recognized to show toxicity that should not be ignored. Dimethylated monothioarsenate (DMMTA) is such a highly toxic species that has been found in pore water from paddy fields at considerable concentrations.  

Figure: Thio-arsenic species found in pore water from paddy soils

An international group of researchers now have investigated the occurrence of DMMTA in rice grains from different locations worldwide.  In order to clarify whether DMMTA is present in rice grains globally or from some regions, the researchers analysed more than 100 rice samples from the main rice-producing regions of China as well as 140 rice samples from 16 countries across six continents.  

Total arsenic in rice was determined by inductively coupled plasma-mass spectrometry (ICP-MS) after microwave-assisted digestion.  For speciation analysis, the researchers compared two different sample pretreatment methods, using either 1 % HNO₃ extraction or enzymatic extraction.  For the chromatographic separation of arsenic species two different methods were compared, using either an anion exchange column (AEC) or a reversed-phase column (RPC). Arsenic species were identified by retention time match with standard compounds and quantified by ICP-MS via external calibration, leading to limits of detection between 0.2 and 0.4 µg/kg for the different species.

The comparison of the two separation methods revealed that only the RPC showed the presence of DMMTA in rice. The authors also excluded that DMMTA was produced from other species during enzymatic extraction. By further investigating the reasons for such differences, it was established that DMMTA is converted to DMA during nitric acid extraction and therefore cannot be found by this often applied method. Even if DMMTA had survived the extraction procedure, DMMTA was strongly retained in the AEC column and therefore cannot be detected under the near-neutral pH elution conditions.

The analysis of more than 100 rice samples from China showed the presence of DMMTA in all samples, with concentrations between 2.10 and 34.8 µg/kg representing 2-19% of the total As. When looking at the relation between DMMTA concentrations and geographical origin of the rice, a clear pattern was observed, with rice produced in temperate regions showing higher concentrations than rice from tropical regions. Such trend was confirmed for rice samples from other countries, showing higher DMMTA concentration in rice samples from Europe and North America than from Asia.  

By statistical analysis of all results, the authors concluded that the DMMTA concentration is about 30% of the DMA concentration in the enzymatic extraction. This relationship allows to estimate the concentration of DMMTA for samples analysed by acid extraction, during which DMMTA was degraded.

The authors concluded, that because of the high toxicity of DMMTA, its widespread presence in rice from different countries and its stability during cooking, DMMTA likely poses a serious threat to human health that should be considered in food safety regulations.

Related studies (newest first)

J. Dai, C. Chen, A.X. Gao, Z. Tang, P. Wang, Dynamics of dimethylated monothioarsenate (DMMTA) in paddy soils and its accumulation in rice grains. Environ. Sci. Technol., 55 (2021) 8665−8674. DOI: 10.1021/acs.est.1c00133

A.E.C. Blanco, C.F. Kerl, B. Planer-Friedrich, Detection of thioarsenates in rice grains and rice products. J. Agric. Food Chem.. 69 (2021) 2287−2294. DOI: 10.1021/acs.jafc.0c06853

C. Chen, B. Yang, Y. Shen, J. Dai, Z. Tang, P. Wang, F.-J. Zhao, Sulfate addition and rising temperature promote arsenic methylation and the formation of methylated thioarsenates in paddy soils. Soil Biol. Biochem., 154 (2021) No. 108129. DOI: 10.1016/j.soilbio.2021.108129

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last time modified: September 18, 2024