are not in equilibrium with the seawater medium in which they are dis- 

 solved. For this reason, it is important that the chemical forms of natu- 

 rally occurring trace elements be identified and their concentrations 

 measured. 



Laboratory studies and model calculations of trace-element associa- 

 tions should include the major inorganic ligands in seawater: chloride, 

 sulfate, bicarbonate, carbonate, hydroxide, fluoride, and borate. The 

 measurement of association between trace elements and organic com- 

 pounds is important, but these studies require a knowledge of the most 

 important naturally occurring organic ligands, which is not available 

 at present. 



The kinetics and mechanisms of the reactions of trace metals with 

 both inorganic and organic ligands is important for a complete under- 

 standing of trace-element chemistry. Some knowledge is available on 

 the inorganic systems, but it must be extended to include the effects of 

 the seawater medium, temperature, and pressure. The study of the 

 organic systems must await a more detailed knowledge of important 

 potential ligands in the organic composition of seawater (see pp. 61- 

 62). 



Laboratory studies of trace element speciation should be made in 

 solutions that approximate the composition of seawater as closely as 

 possible and must include the effects of temperature, pressure, redox 

 potential, and hydrogen ion concentration. 



Heterogeneous Reactions 



Several types of heterogeneous reactions can be identified as being 

 significant in the marine environment. The adsorption of substances such 

 as dissolved metals and phosphate on suspended matter is beUeved to 

 be important in the removal of some minor elements from seawater. 

 Ion exchanges with adsorbed materials as it is subjected to compositional 

 gradients in coastal and estuarine regions, or to pH gradients within 

 the ocean, may produce variations in the concentrations of the minor 

 constituents. The precipitation and dissolution of solid phases in sea- 

 water alter the distribution and sedimentation of some of the minor 

 elements. The formation and entrapment of gas bubbles not only produce 

 variations in dissolved gas concentrations, but also alter the acoustical 

 properties of the environment, and may provide sites for chemical 

 reactions that would not otherwise occur. 



Adsorption — Existing knowledge of adsorption reactions in seawater 

 has been acquired by observing the removal of specific minor elements 

 such as zinc and phosphate from a solution by a particulate solid phase 

 such as a clay mineral or natural sediment. This process depends on 



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