I could not help mentioning duckweed, an aquatic plant, because many macrophytes are very rich in heavy metals and because many years ago, in the 1970s, I worked on a doctorate in aquatic plant bio-geochemistry at the EC Joint Research Centre (JRC) in Ispra1 (italy).
At the time, my lab got involved in the preparation of a large and homogeneous sample of aquatic plant powder to be used as one of the first Reference Materials (RM) produced. The idea was to have several laboratories across Europe measure the contents of trace metals in the RM, hopefully come up with comparable results and eventually certify the Standard Reference Material for its chemical composition. The Material would then be made available to institutions measuring trace metals for the purpose of food safety2, environmental monitoring etc. The activity grew considerably and Interlaboratory comparisons are now routinely carried out throughout the world and the Reference Materials are regulated by ISO norms.
The EC/JRC now regularly updates it’s catalogue of Certified Reference Materials. The 2025 issue has two aquatic plants, including the Common duckweed with concentrations for 26 elements, of which a partial3 list follows. The elements are formatted as Name (symbol): abundance in ppb (uncertainty in ppb). Here is the list: Cadmium (Cd): 75.5 (2.5), Cerium (Ce): 990 (40), Dysprosium (Dy): 79 (7), Erbium (Er): 44.0 (2.8), Europium (Eu): 23.2 (1.5), Holmium (Ho): 15.8 ( 1.8), Lanthanum (La): 487 (20), Lutetium (Lu): 6.3 (0.5), Neodymium (Nd): 473 (15), Praseodymium (Pr): 121 (6), Scandium (Sc): 191 (11), Samarium (Sm): 94 (7), Terbium (Tb): 14.0 (1.1), Thorium (Th): 159 (18), Thulium (Tm): 5.7 (0.7), Yttrium (Y): 460 (60) and Ytterbium (Yb): 40 (4).
When considering the abundance in the earth’s crust, the rarest element in the list is Cadmium (Cd, at 0.16 ppm) , which itself is more abundant than gold (Au, 0.004 ppm), platinum (Pt, 0.005 ppm), bismuth (Bi, 0.009 ppm), Selenium (Se, 0.05 ppm), Silver (Ag, 0.075 ppm) and Mercury (Hg, 0.085 ppm). This is to say that all the “exotic” elements above are more common than some metals well known to humans, such as gold (Au).
Notes
- With an EC grant, for which I’ll be grateful forever! However, as soon as I completed my doctorate, I changed direction completely and all there is left of this period of my life is a handful of published reports and papers, most of which cannot be found on the web. My main focus was on heavy metals and I very much resented the fact that my job was mainly collecting heavy metal concentrations like others collect butterflies. There was little theory and little system. Sometimes, like with manganese (Mn) and iron (Fe), there was some logic in the environment-plant interactions, but most of the time I would collect analytical results. I do not know to what extent the situation has changed now. ↩︎
- Food safety deals with the health aspects of food, contrary to Food security, which focuses mainly on availability. But these are complex issues: refer to the remarkable website Hunger explained and especially this document for definitions and concepts. ↩︎
- Here is the complete list of elements in the Reference Material (in ppb), together with the uncertainty (in ppb as well!): Arsenic (As): 1980 ( 190), Cadmium (Cd): 75.5 ( 2.5), Cerium (Ce): 990 ( 40), Chromium (Cr): 2050 ( 100), Caesium (Cs): 77 ( 10), Copper (Cu): 1820 ( 300), Dysprosium (Dy): 79 ( 7), Erbium (Er): 44.0 ( 2.8), Europium (Eu): 23.2 ( 1.5), Gadolinium (Gd): 98 ( 8), Holmium (Ho): 15.8 ( 1.8), Lanthanum (La): 487 ( 20), Lutetium (Lu): 6.3 ( 0.5), Molybdenum (Mo): 560 ( 70), Neodymium (Nd): 473 ( 15), Lead (Pb): 2060 ( 120), Praseodymium (Pr): 121 ( 6), Scandium (Sc): 191 ( 11), Samarium (Sm): 94 ( 7), Terbium (Tb): 14.0 ( 1.1), Thorium (Th): 159 ( 18), Thulium (Tm): 5.7 ( 0.7), Uranium (U): 82 ( 8), Yttrium (Y): 460 ( 60), Ytterbium (Yb): 40 ( 4), Zinc (Zn): 24000 ( 2100). The determination of the uncertainty is itself a complex issue (Pauwels et al., 1998, and this link) ↩︎
List of publications and references
Muntau (Herbert), René Gommes 1974 Kritischer Vergleich verschiedener Methoden zur Bestimmung von Kadmium in Luft. Proc. Internat. Symposium on Problems of the contamination of man and his environment by mercury and cadmium, Luxembourg, 3-5 July 1973. EUR 5075, EEC Directorate General on scientific and technical information and information management, Luxembourg, pages 191-201. (Critical comparison of different methods to determine cadmium concentrations in air)
Ravera (Oscar), René Gommes, Herbert Muntau 1974 Cadmium distribution in aquatic environments and its effects on aquatic organisms. Proc. Internat. Symposium on Problems of the contamination of man and his environment by mercury and cadmium, Luxembourg, 3-5 July 1973. EUR 5075, EEC Directorate General on scientific and technical information and information management, Luxembourg, pages 317-330.
Gommes (René), Herbert Muntau 1975 Etude radiochimique de la cinétique d’absorption du plomb ionique et de son accumulation dans les limnophytes au moyen de 203-Pb. EEC-Euratom report 1659, Ispra, Italy, 13 pages. (The radiochemical determination of ionic lead absorption kinetics and accumulation in limnophytes using 203-Pb)
Gommes (René), Herbert Muntau 1975 La distribution de quelques métaux lourds (Zn, Cu, Cr, Ni, Mn, Co) dans la zone littorale des bassins sud et de Pallanza du lac Majeur. Memorie dell’Istituto Italiano di Idrobiologia, 32, 245-259. (The distribution of some heavy metals -Zn, Cu, Cr, Ni, Mn, Co- in the littoral zone of the southern and Pallanza basins of Lake Maggiore)
Gommes (René), Herbert Muntau 1975 Quelques aspects de la composition chimique de Trapa natans L., manganèse et fer en particulier, dans le lac Majeur (Italie). Bulletin de la Société Royale Botanique de Belgique, 108(2):209-218. (Some aspects of the chemical composition of Trapa natans L., in Lake Maggiore -Italy -, especially iron and manganese)
Gommes (René), Herbert Muntau 1976. Elementgehalt und Korrelationen zwischen Oligo- und Polyelementen in Lagarosiphon Major (Ridley) Moss im Lago Maggiore, sowie ein Versuch der ökochemischen Deutung. Memorie dell’Istituto Italiano di Idrobiologia, 33:185-200. (Chemical composition and correlations between oligo- and polyelements in Lagarosiphon major (Ridley) Moss, with an ecochemical interpretation)
Gommes (René), Herbert Muntau 1976. Cadmium levels in biota and abiota from Lake Maggiore Eur.5441e, EEC Directorate General on scientific and technical information and information management, Ispra, Italy, 31 pages
Muntau (Herbert), René Gommes 1976 Kritische Bewertung der analytischen Methoden zur Bestimmung von Blei in atmosphärischer Luft. Proc. 12ème colloque international sur les atmosphères polluées, Paris, 5-7 May 1976. 17 pages. (Critical evaluation of analytical methods used to determine lead concentrations in air)
Gommes (René) 1977 Der Mineralstoffgehalt des Sumpfbitterklees (Menyanthes trifoliata L.) im Sankt Vither Land. Zwischen Venn und Schneifel, 13, 134-137. (Mineral composition of Menyanthes trifoliata L. in the area of St.Vith)
Gommes (René), Herbert Muntau 1981 Variations de la composition chimique (polyéléments et métaux lourds) entre organes de Trapa natans L. et de Polygonum amphibium L. Memorie dell’Istituto Italiano di Idrobiologia, 38: 331-346. (Variations of the chemical composition – polyelements and heavy metals – between the organs of Trapa natans L. and Polygonum amphibium L.)
Gommes (René), Herbert Muntau 1981 Variations saisonnières de la composition chimique des limnophytes du lac Majeur. Memorie dell’Istituto Italiano di Idrobiologia, 38: 309-330. (Seasonal variations of the chemical composition of Lake Maggiore limnophytes)
Gommes (René), Herbert Muntau 1981 La composition chimique des limnophytes du lac Majeur. Memorie dell’Istituto Italiano di Idrobiologia, 38: 237-308. (The chemical composition of limnophytes in Lake Maggiore)
Gommes (René), Herbert Muntau, Roberto Cenci, Antonio Lattanzio, Giorgio Pigozzi 1981 Caractérisation du chimisme des eaux superficielles des Bassins Sud et de Pallanza du lac Majeur. Memorie dell’Istituto Italiano di Idrobiologia , 38: 19-78. (Chemical characterization of surface waters in the Pallanza and southern basins of Lake Maggiore)