A disturbing map: Occurrence of dimethylmonothioarsenate around the world.

[NNadir]

Arsenic poisoning from eating rice has been described as the greatest mass poisoning in world history in Bangladesh. The decrease in the flows of the Ganges River in its delta, which practically constitutes the entire nation of Bangladesh, has caused farmers to rely on groundwater, which percolates through natural arsenic containing rocks.

The toxicity of arsenic is very much connected to its oxidation state, the +3 state is far more toxic than the +5 state. (In Bangladesh it is present in the more soluble and more toxic +3 state in ground water.

However some compounds are toxic in the +5 state.

Here's a paper on one of these compounds: Widespread Occurrence of the Highly Toxic Dimethylated Monothioarsenate (DMMTA) in Rice Globally Jun Dai, Zhu Tang, A-Xiang Gao, Britta Planer-Friedrich, Peter M. Kopittke, Fang-Jie Zhao, and Peng Wang Environmental Science & Technology 2022 56 (6), 3575-3586.

Some text from the introduction:

Synopsis
Dimethylated monothioarsenate (DMMTA), a potentially highly toxic arsenic species, is found in rice worldwide, but has so far been being mistaken as a relatively harmless, nonregulated arsenic species.

Introduction
ARTICLE SECTIONSJump To
Inorganic arsenic (iAs) is classified as a nonthreshold (Class I) carcinogen. (1,2) Human exposure to iAs can cause a wide range of health problems, such as cancers of the bladder, lung, and skin, as well as cardiovascular and metabolic disorders. (3−5) Arsenic is ubiquitous in paddy soils, threatening the production of rice (Oryza sativa). (6,7) Paddy rice accumulates more As in the grain than other major cereal crops because arsenite is mobilized under anaerobic conditions in flooded paddy soils, (8) and arsenite is taken up efficiently by the highly expressed silicon uptake pathway in rice roots. (9) Understanding the accumulation of As in rice is of critical importance─rice grain is the main staple food for more than half of the world’s population, resulting in the consumption of rice being a major source of iAs. (10−14) As a carcinogen, iAs is regulated by food safety regulations. For example, it has been recommended that the limit of iAs in polished rice should be 200 μg/kg, while for children/infant foods, it should be 100 μg/kg. (15) Apart from iAs species, methylated As species such as dimethylated arsenate (DMA) are also commonly reported in rice grains. (6,16−19) It is generally recognized that DMA, with As in the pentavalent oxidation state, is much less toxic to humans, (20−22) and is therefore exempt from food safety considerations.

Although iAs and DMA are the two most commonly reported species of As in paddy systems, methylated thioarsenate species, predominantly dimethylated monothioarsenate [DMMTA, (CH3)2AsSOH], have recently been detected at considerable levels in porewaters of paddy soils. (24−26) Concentrations of DMMTA in the paddy porewaters varied from 0.2 to 36.2 μg/L, accounting for up to 58% of its precursor DMA and up to 34% of the total dissolved As. (24−27) DMMTA can be taken up by rice roots where it is efficiently translocated to the shoots. (28) In contrast to DMA(V), which is considered to be of low toxicity, numerous in vitro studies have shown that DMMTA(V) (also sometimes referred to as thio-DMA(V)) is by far the most toxic As species to human cells, with cytotoxicity being 3 to 10 times higher than that of iAs. (14,20,21,29−31) Furthermore, DMMTA has frequently been detected in human urine samples following exposure to inorganic arsenic and arsenosugars. (32−35) It remains unclear whether DMMTA is produced as a metabolite following ingestion of the arsenite- or arsenosugar-containing foods or is derived directly from dietary exposure...

I don't have time to go through the whole paper, but here's a scary map from it:



The caption:

Figure 4. Global rice DMMTA concentrations. (A) Mean concentrations of DMMTA in polished rice produced in different regions/countries (or continents). Data for DMMTA were (i) taken from the global basket survey in the present study (n = 140) and (ii) calculated based on eight previous studies that used the acid extraction method to extract the As species and reported DMA concentrations in rice (n = 1731) from 86 distinct main rice production regions, with DMMTA concentrations calculated to be 30% of the total DMA based on the ratio of DMMTA to DMA observed throughout Chinese field survey and global market surveys (Figures 2F and 3C). (B) DMMTA concentrations and percentages in rice produced in different countries as a function of latitude. Different letters indicate difference at P < 0.05.

On the other hand, I do eat a fair amount of rice and I'm noticeably alive.

I'm not sure I know what the implications are in their entirety, but it's probably something to keep in the back of one's mind. I am not generally all that familiar with organoarsenic chemistry, so I cannot say if boiling the rice leads to hydrolysis. Hopefully it does.

Have a nice weekend.

[eppur_se_muova]

1. Ugh, what horrible nomenclature. It's basically a monothio analog of cacodyllic acid.

Strange that substituting a sulfur atom would have such an effect on toxicity. Perhaps some interaction with disulfide linkages in proteins is involved ? Hydrolysis would certainly give cacodylate, but how easily that occurs, I don't know.

Of course I have to draw your attention to this lovely article by Seyferth.

[rog]

3. That article stinks!

That is a really fascinating article ... thanks for posting! I am not a chemist, but I couldn't stop reading ... really cool. I had to skim the last half, but I'll come back later and read the whole thing.

Of course someone thought of using such a horrible substance as a weapon.

I enjoy it when science is brought to life like that ... thanks again.

♫rog♫

[NNadir]

4. Thanks for that article. It's a fascinating piece on the history of chemistry. n/t.

[eppur_se_muova]

5. When men were men, and lucky to survive it.

Accidents with cacodyl compounds could have serious consequences. During his study of cacodyl cyanide, (CH3)2AsCN, prepared by reaction of “cacodyl oxide” with a concentrated aqueous solution of mercuric cyanide, an explosion cost Bunsen the partial sight of his right eye and, as Roscoe reports, “Bunsen was nearly poisoned, lying for days between life and death.” Bunsen recovered and completed his study of cacodyl cyanide, a most unpleasant compound. After distillation of the “cacodyl oxide”/Hg(CN)2 reaction mixture, the cacodyl cyanide formed beautiful, prismatic crystals underneath the water layer. These were quite volatile (mp 32.5 °C). They were dried by pressing them between sheets of blotting paper. Bunsen notes that it is absolutely necessary to carry out this operation in the open air while breathing through a long glass tube that extends to fresh air far beyond the volatile crystals. And well might this compound be avoided! Bunsen reports that the vapor from 1 grain (0.0648 g) of cacodyl cyanide in a room produces sudden numbness of the hands and feet, and dizziness and insensibility to the point of unconsciousness. The tongue becomes covered with a black coating. These effects, however, are only temporary, with no lasting problems. (Bunsen, it may be noted, lived to the ripe old age of 88.)

[rog]

2. "Cook rice like pasta".

Washing the rice helps, but I think there was a study that showed the best way to cook rice is in a large amount of water, like pasta, then pour off the excess water. Let's see ... INTERESTING: you lose some nutrients in the method above, but I just now learned that in 2020 they found a new method that preserves almost all of the nutrients, i.e., "parboiling with absorption". This still only removes a little more than half of the arsenic.

https://scitechdaily.com/scientists-find-new-way-of-cooking-rice-that-removes-arsenic-and-retains-nutrients/

When I found out about this arsenic thing years ago, I moved to other grains, like whole oat groats, hulled barley, buckwheat, millet, amaranth, quinoa, etc, and never looked back. Whole rye takes a long time to cook, but it is really tasty. Barley and oat groats only take about 40-45 minutes, depending on how chewy you prefer. Grains like buckwheat, millet, amaranth, and quinoa cook up in 20 minutes or so.