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United States Department of Agriculture

Agricultural Research Service

Title: Issue Paper on the Environmental Chemistry of Metals

Authors
item Langmuir, Donald - HYDROCHEM/GOLDEN, CO
item Chrostowski, Paul - CPF ASSOC. TAKOMA PARK,MD
item Chaney, Rufus
item Vigneault, B - CANMET MINING, CANADA

Submitted to: US-EPA Applied Phytotechnologies Conference
Publication Type: Other
Publication Acceptance Date: November 8, 2003
Publication Date: April 1, 2004
Citation: Langmuir, D.L., Chrostowski, P., Chaney, R.L., Vigneault, B. 2004. Issue paper on the environmental chemistry of metals. US-EPA Risk Assessment Forum: Papers Addressing Scientific Issues in the Risk Assessment of Metals. http://cfpub.epa.gov/ncea/raf/recordisplay.cfm?deid=59052. p. 114.

Interpretive Summary: US-EPA is preparing documents to summarize the 'framework for metals risk assessment.' All manuscripts underwent professional peer review by the Workshop participants, and are now on the EPA website for public comment. This manuscript was prepared after a workshop to discuss the topics and prepare an outline for the paper. The Environmental Chemistry of metals which is reviewed in this manuscript covers all aspects of metal chemical reactions in soil and water, chemical species commonly found, and soil and plant factors which affect whether a metal may leach, be taken up by plants, or be absorbed from ingested soils. Detailed solubility diagrams (solubility vs. pH) are presented for elements which are known to be sufficiently bioavailable to plants or animals to cause toxicity in exposed organisms. Key inorganic ligands in soils which affect solubility and chemical speciation are illustrated. Soil pH and redox are key variables which influence dissolution or solubility of metal ions in soils. Examples are given of data from experiments which showed the effect of a wide range of soil pH on solubility and on plant uptake of over 50 elements. These soil processes, and the natural properties of plants provide a protection of the food-chain from most trace elements in soils. Most elements are so insoluble in soil or in roots that they do not reach plant shoots except as soil dust particles adhering to the leaf surface. Some other metals are accumulated in plant shoots but the residue in the leaves harms the plant before consumption of the leaf would harm ruminant or monogastric animals. Only a few elements are so mobile in soils and plants that consumption of plant shoots can cause adverse effects to animals, Se and Mo. Soil Cd caused harm to humans when soils where subsistence rice farmers grew their food supply on the soils contaminated by Zn-Pb mine wastes. In this case, soils are reduced during part of the growth period and CdS is formed which limits Cd uptake; but when flowering begins, fields are drained and the soils become oxidized. Different metal sulfides are oxidized at different times as the soil become aerobic, and it is clear that Cd become phytoavailable very quickly and is transported to rice grain while little Zn reaches rice grain. Other plant species do not reject Zn in this way, and plant Zn offers considerable protection to animals against plant Cd.

Technical Abstract: Assessment of potential risks from metals in the environment depend on the chemistry of the metal and of the environmental compartment of interest. The chemistry of metals in soils and water systems were summarized as part of EPA's project to develop a 'framework for risk assessment' of metals in the environment. Report includes: 1) natural occurrence of metals in the environment; 2) metal complexes; 3) metal adsorption; 4) metal solubility; 5) metal transfer to plants and the bioavailability of metals in ingested soils; and 6) relationships of metal speciation to metal toxicity. Solution chemistry is controlled both by solubility of crystalline minerals containing a metal, the presence of other ions which react with metals, and solution pH and redox; free metal ion activity can be controlled by precipitation, co-precipitation, by chelation or adsorption by organic matter, or by adsorption on surfaces of Fe and Mn oxides in soils. In soils, metal solubility is buffered more strongly than in water, but soil organic matter and metal oxide surfaces, soil pH and redox activity, and soluble ions in the soil. The manuscript includes consideration of what soil or plant factors may affect whether a metal may leach down a soil horizon, may be taken up by plants, or may be absorbed by animals which ingest soil. Detailed solubility diagrams (solubility vs. pH and redox) are presented for elements which are known to be sufficiently bioavailable to plants or animals to cause toxicity in exposed organisms under some strongly contaminated condition. Key inorganic ligands in soils which affect solubility and chemical speciation are illustrated. Soil pH and redox are key variables which influence dissolution or solubility of metal ions in soils. Examples are given of effect of a wide range of soil pH on solubility and on plant uptake of over 50 elements. These soil processes, and the natural properties of plants provide a protection of the food-chain from most trace elements in soils, and are summarized as the 'Soil-Plant Barrier' to food-chain transfer of soil metals. Most elements are so insoluble in soil or in roots that they do not reach plant shoots except as soil dust particles adhering to the leaf surface. Some other metals are accumulated in plant shoots but the residue in the leaves harms the plant before consumption of the leaf would harm ruminant or monogastric animals. Only Se and Mo are so mobile in soils and plants that consumption of the shoots of plants grown on contaminated soils can cause adverse effects to animals. Soil Cd may cause harm to humans when subsistence rice farmers grow their lifetime food supply on soils contaminated by Zn-Pb mine wastes. In this case, soils are reduced during part of the growth period and CdS is formed which limits Cd uptake; but when flowering begins, fields are drained and the soils become oxidized. Different metal sulfides are oxidized at different times as the soil become aerobic, and it is clear that Cd become phytoavailable very quickly and is transported to rice grain while little Zn reaches rice grain. Other plant species do not reject Zn in this way, and plant Zn offers considerable protection to animals against plant Cd. A few elements may comprise risk when soil is ingested (e.g., Pb, As), but not from plant accumulation of metals from soils.

Last Modified: 12/25/2014