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Arsenic speciation in rice: a question of the rice plant species

(15.02.2008)


Background:
Arsenic is a carcinogen that has found high attention due to the fact that millions of people consume arsenic contaminated drinking water (see EVISA news about the biggest calamities in the world). Food is another exposure route and high concentrations of arsenic have been found recently both in rice harvested in Bangladesh as well as the US.

For a complete assessment of risks it is necessary to understand the transfer of arsenic from water/soil to the plant, the role of arsenic speciation, the role of plant species as bioaccumulators and the location of arsenic within parts of the plant.


The new study:
An international group of scientists headed by Andrew A. Meharg from the University of Aberdeen studied the local distribution of arsenic species within rice grains.   Synchrotron-based X-ray fluorescence (S-XRF) was utilized to locate arsenic (As) in polished (white) and unpolished (brown) rice grains from the United States, China, and Bangladesh. Further μ-Xray absorption near edge spectroscopy (μ-XANES) was used to assess the As speciation in rice grains. In brown rice As was found to be preferentially localized at the surface, in the region corresponding to the pericarp and aleurone layer. The localization of As in the outer grain of brown rice was confirmed by laser ablation ICP-MS. Arsenic speciation of all grains using spatially resolved X-ray absorption near edge structure (μ-XANES) and bulk extraction followed by anion exchange HPLC-ICP-MS revealed the presence of mainly inorganic As and dimethylarsinic acid (DMA). This is somehow surprising since the aleurone layer, unlike the starchy endosperm, is protein rich and As(III) has high affinity for protein and nonprotein (i.e., phytochelatin and glutathione) -SH functional group.

A wider survey of whole grain speciation of white and brown rice samples from numerous sources (field collected, supermarket survey, and pot trials) showed that brown rice had a higher proportion of inorganic arsenic present than white rice. Furthermore, the percentage of DMA present in the grain increased along with total grain arsenic.

With respect to the threat that arsenic in rice poses to the human diet, assuming that inorganic arsenic is more problematic than organic (DMA) the authors concluded that polishing rice reduces the total arsenic burden of the grain and its inorganic content.
They further warned that consumption of products such as rice milk and direct consumption of bran and germ as dietary fiber supplements, are of particular concern and need further attention.


The original study

Andrew A. Meharg, Enzo Lombi, Paul N. Williams, Kirk G. Scheckel, Jörg Feldmann, Andrea Raab, Yongguan Zhu, Rafiql Islam, Speciation and Localization of Arsenic in White and Brown Rice Grains, Environ. Sci. Technol., 42/4 (2008) 1051-1057. doi: 10.1021/es702212p


Related Resources

EVISA Link Database: XANES/EXAFS
EVISA: Tools for Speciation: LC-ICP-MS: The most often used hyphenated system for speciation analysis



Related News

EVISA News, January 31, 2008: New arsenic species detected in carrot samples
EVISA News, March 7, 2007: Elevated Arsenic Levels Found In Rice Grown In South Central States of the USA
EVISA News, September 7, 2006: Toxic inorganic arsenic species found in Japanese seaweed food
EVISA News, August 3, 2005: Surprisingly high concentrations of toxic arsenic species found in U.S. rice


last time modified: February 15, 2008



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