Speciation and Cycling of Arsenic, Cadmium, Lead and Mercury in Natural Waters

The cycling of the elements in natural waters is driven by a combination of chemical and biological processes. This paper will focus on two key features of the natural water chemistry of As, Cd, Pb and Hg, chemical speciation and mean oceanic residence time. Calculation of equilibrium speciation in solution is heavily dependent on the accuracy of both the complex stability constants used and of the procedure used for correcting them to the required ionic strength. Some progress has been made in this respect following the publication of critical data-bases, but there remains considerable uncertainty for lead in particular as to the relative significance of different complexes. Treatment of organic complexation is further complicated by our lack of understanding of the structure of natural organic matter. Calculations and measurements based on thermodynamic equilibrium are, however, only the first stage in the study of chemical speciation. Biological processes can give rise to metastable chemical species with sufficiently long lifetimes to be important components in an element's speciation. Arsenic, mercury and lead alkyls, for instance, fall into this category, and lead alkyls also have an anthropogenic source. Redox speciation must also be considered in the case of arsenic; arsenic (V), which is the stable oxidation state in oxic waters, can be reduced to arsenic (III) in anoxic systems. Arsenic (III) released to oxic waters can then be reoxidized to arsenic (V) within a timescale of some days. The mean oceanic residence time gives a measure of the tendency of an element in seawater to be incorporated into particulate phases: a very short residence time « 1000years) indicates that the element is removed very rapidly to the sediments while a very long residence time (108 years; Na, Br, CI) indicates that the element is very unreactive towards particulate matter. Residence times for the elements c.onsidered here range from about 105 years (As) to 103 years or less (Pb). Each of the processes considered above relates to the chemical properties of the