Arsenic Species in Seaweeds Commercially Available in the United States

Arsenic was speciated in 46 seaweed samples purchased from the U.S. market, i.e., brown (arame, hijiki, kombu, knotted wrack, oarweed, wakame), green (sea lettuce), and red (dulse, Irish moss, laver, nori) macroalgae. The total arsenic (tAs) concentrations varied from 3000 to 105 000 ng/g (dry weight). Speciation analysis based on the extraction of water-soluble and nonpolar arsenic showed that, among the variety of arsenicals identified, arsenosugars were dominant in most seaweeds. Inorganic arsenic (iAs) did not exceed 350 ng/g except for knotted wrack (1760 ng/g), oarweed (8900-16 000 ng/g), and hijiki (50 800- 56 900 ng/g). While arsenate (As5+) was the predominant form of iAs, arsenite (As3+) was also detected in some samples such as oarweed and hijiki. Nonpolar arsenic accounted for 2-59% of the tAs; the highest percentages were in sea lettuce and wakame. Unknown arsenic species were found representing 0.1-12% of the tAs in the seaweeds. Reference materials and spike recovery tests were used for quality control. While the regulations are not applicable to products in the U.S., the iAs concentrations in the seaweeds (except hijiki and oarweed) are lower than the guidance levels established by several countries.