Heavy Metal, Total Arsenic, and Inorganic Arsenic Contents of Algae Food Products

Overview

Almela and colleagues measured total arsenic, inorganic arsenic, lead, cadmium, and mercury in 18 processed edible-algae food products on retail sale in Spain, using a hydride-generation method for arsenic speciation validated against three macroalgal reference materials. Across the products, total arsenic ranged 2.3–141 mg/kg dry weight and inorganic arsenic 0.15–88 mg/kg dw, with the brown alga Hizikia fusiforme (hijiki) standing apart: its inorganic arsenic (83–88 mg/kg dw) constituted 60–72% of total arsenic, two orders of magnitude above every other product, while 84% of all samples held inorganic arsenic below 1 mg/kg dw. Cadmium reached 1.9 mg/kg dw in wakame and exceeded the French (0.5 mg/kg dw) and Australian (0.2 mg/kg dw) limits in 10 and 14 samples respectively. The study is a foundational speciated-arsenic occurrence survey for the seaweed/kelp food category.

Key numbers

Concentration ranges (mg/kg dry weight unless noted), n=18 products, mean of three independent analyses each:

• Total arsenic: 2.3–141 mg/kg dw
• Inorganic arsenic: 0.15–88 mg/kg dw
• Lead: <0.05 (LOD)–1.33 mg/kg dw
• Cadmium: 0.03–1.9 mg/kg dw
• Mercury: 4–42 µg/kg dw (0.004–0.042 mg/kg dw)

Per-product values (Table 3; mean ± SD; arsenic (As)/lead (Pb)/cadmium (Cd) in mg/kg dw, mercury (Hg) in µg/kg dw):

Additional source-reported facts:

• In Hizikia fusiforme, inorganic arsenic was 83–88 mg/kg dw, equal to 60–72% of total arsenic in those samples; the authors note literature values of 50–70% inorganic arsenic (iAs) in H. fusiforme.
• 84% of samples had inorganic arsenic <1 mg/kg dw (0.15–0.57 mg/kg dw).
• The three H. fusiforme samples exceeded the French/US inorganic-arsenic limit (3 mg/kg dw) and the Australia/New Zealand limit (1 mg/kg dw).
• 10 samples exceeded the French Cd limit (0.5 mg/kg dw); 14 exceeded the Australian limit (0.2 mg/kg dw).
• Method limits of detection (mg/kg): total As 0.025, inorganic As 0.014, Pb 0.05, Cd 0.003, Hg 0.003.
• Exposure note: a daily consumption of 1.7 g of H. fusiforme would reach the WHO Provisional Tolerable Weekly Intake for inorganic arsenic (15 µg/kg body weight/week) at 68 kg body weight.

Methods (brief)

Eighteen dried algae food products (12 brown, 4 red, 2 green) bought in Valencia were milled and analyzed in triplicate. Total arsenic and inorganic arsenic were determined by flow-injection hydride-generation atomic absorption spectrometry (FI-HG-AAS); inorganic arsenic [As(III)+As(V)] was isolated beforehand by overnight HCl solubilization followed by chloroform solvent extraction and HCl back-extraction (the Muñoz/Vélez/Montoro seafood method, here assessed for algae). Lead and cadmium were determined by graphite-furnace AAS with Zeeman correction using the standard-additions method after microwave HNO3/H2O2 digestion; mercury by cold-vapor atomic fluorescence spectrometry. Quality control used reference materials Fucus sp. (IAEA), BCR 060 (Lagarosiphon major), and BCR-279 (Ulva lactuca); recoveries for inorganic arsenic spikes were 95–101%. Mercury was reported as total Hg (no speciation); arsenic was speciated to the inorganic fraction. All values are dry weight. The authors note the absence of certified reference materials for inorganic arsenic in this matrix as a limitation.

Implications

• Certification (HMTc): supplies per-sample speciated inorganic-arsenic, cadmium, and lead occurrence data on the dry-weight, as-sold basis for the Category 6 seaweed-kelp-foods row, where iAs/total arsenic (tAs)/Cd are the platform analytes. The data establish a strongly bimodal inorganic-arsenic distribution: most edible algae sit below 1 mg/kg dw iAs, while Hizikia fusiforme (hijiki) is a categorical outlier at >80 mg/kg dw iAs, supporting the within-row species sub-split flagged in the Category 6 Step 0 lock. Cadmium occurrence (up to 1.9 mg/kg dw, exceeding several national algae limits) is independently relevant.
• App: contributes to the contamination_profile for the seaweed ingredient on inorganic arsenic, total arsenic, cadmium, lead, and total mercury.

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