Bioavailability of Metal Ions

While ranges of total concentration serve to set bounds for experimentally determining effects on marine populations, the actual species of metal ion available to the biological population is of importance. Sillen, in a classic paper, has computed the stable species of many metals in sea water21). He concluded, for example, that Hg+2, Cd+2, and Pb+2 exist primarily as chloride complexes. pH determines the availability of the hydroxide ion and thereby the solubility of metal hydroxides. Sillen assumed a pH of 8.1 0.2 as representative. Significant variations could occur, however, in estuarine waters. When concentrations of trace elements were compared with calculations of their solubility products and stability constants, the observed values were considerably less than the calculated values. The implication is that the heavy metals are not in equilibrium with solid phases of their salts, but that other processes, such as chelation and adsorption, control their concentration. 

The control processes of chelation and adsorption are in turn partially dependent on the route of entry of the trace metal into the ocean system. Transport via stream or river is a complex process which varies from metal to metal. For example, analysis of trace metal transport for Fe, Cu, Co, and Mn, with respect to five possible processes was carried out22). These processes included  

a) dissolving of ionic species and inorganic associations, b) complexing with organic molecules  

Incorporation in the crystalline structure is the major mechanism for transporting Cu and Cr. Mn is carried in coatings, and Fe, Ni, and Co are distributed equally between coatings and crystalline solids. 

Another suggestion is that the natural removal cycle for Pb via river transport is an exchange between a dissolved chelated form and an insoluble precipitated form91 Exchange may occur with the precipitated form settling out on the continental shelves. Hence, while river transport mechanisms may be known in detail, removal mechanisms for oceanic systems are only known in more general terms. These removal mechanisms are  

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In aquatic systems, in addition to the complexation of metal ions by natural organic matter, metal bioavailability, bioaccumulation, and toxicity are highly affected by water hardness and alkalinity (Banks et al. 2003). This is also applicable to metal mixtures where complexation of metals can occur even at higher rates than when single chemical compounds are present. 

Bioavailability and toxicity of metal ions in aqueous systems are often proportional to the concentration of the free metal ion and thus decrease upon complexation. However, there are some metal compounds more dangerous than the metallic element itself (e.g., mercury vs. methyl mercury). 

To give a real example, have a closer look on main functions and cycle of magnesium in green plants. Control on autocatalysis depends on the principal functions of Mg, that is, on photosynthesis when substantial parts of Mg taken up by roots are allocated to chlorophyll and rubisco synthesis, less will be available for other metabolic pathways, reducing the turnovers there unless there are lots of Mg around like in marine plants. In addition, the tricarboxylate cycle (citrate cycle) requires Mg (besides Fe and Mn) to produce the enzymes hence some Mg (as well as Fe, Mn) must be invested to produce the citrate (malate, oxaloacetate (aspartate)) ions delivered by the roots to render Mg (and other metals) in turn bioavailable by means of complexation and resorption of almost neutral complex entities. Furthermore, the tricarboxylate cycle is coupled to biosynthesis of amino acids by redox transamination hence there will be both competition at the metal center(s) and possible extraction of metal ions from enzymes once NHj and electrons are... 

The toxicity and bioavailability of metals in natural waters depends on the aqueous speciation or complexation of those metals. The toxicities to aquatic life of Cu +, Cd, Zn, Ni, Hg ", and Pb are a function of the activities of the metal ions and their complexes, not of total metal concentrations (cf. Morel and Hering 1993 Manahan 1994). For example, monomethyl mercury ion (CH3Hg ) and Cu are toxic to fish, but some other Hg and Cu complexes (such as CuCO ) are far less so. The bioavailability of essential metals such as Fe, Mn, Zn, and Cu to plants is also a function of their metal speciation (Morel and Hering 1993). Until recently the U.S. Environmental Protection Agency did not recognize the importance of metal speciation in its water quality assessments (cf. Hall and Raider 1993). Metal toxicity is considered briefly near the end of this chapter. 

The speciation behavior of an element in soils profoundly affects its bioavailability. Sometimes this is not evident until a soil property is changed. For example, absorption of metal ions such as Cu and Cd by plant roots is correlated to the... 

Metal ions that would convert to insoinble precipitates are maintained in solution and enhance the bioavailability of metals. 

The references are summarized with emphasis on QSARs, as they relate to bioavailability, bioconcentration, biosoption, binding, or toxicity of metal ions. 

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