Speciation of Metals in Natural Waters

Abstract

The reactions of many trace metals in natural waters are affected by their speciation or form. This will affect the biological uptake (Anderson and Morel 1982) and toxicity (Sunda and Ferguson 1983) as well as the solubility (Liu and Millero 1999). For example, Fe(II) and Mn(II) are biologically available for marine organisms, while Fe(III) and Mn(IV) are not normally available. Although the form of an element in natural waters can be four phases (solid, gas, colloid and dissolved), we will only consider the form of a metal in the dissolved state. The definition of a dissolved metal is defined by the filter size used to separate solid and colloidal phases from the soluble form. In the past, this separation was made with a 0.45 μm filter, and in more recent work smaller size filters are used (~o.2 μm). The speciation of metals is controlled by ionic interactions of the metals with inorganic (Cl-, OH-, CO32-, etc.) and organic (fulvic and humic acids) ligands. The dissolved forms of a metal like Fe in sea water can include:Free ions: Fe2+, Fe3+Inorganically Complexed: Fe(OH)+, Fe(OH)2, FeCO3 and Fe(CO3)22-, FeCl2+, FeCl2+, FeSO4+, Fe(SO4)22-, Fe(OH)2+, Fe(OH)3, FeCO3+, and Fe(CO3)22-Organically Complexed: FeL, where L can be a wrde range or unKrnown nlatural llgands (fulvic and humic acids, siderifores, etc.)