Metal ions in natural aquatic systems

Abstract

The abundance of metal ions in natural aquatic systems varies greatly from major species (Na +, K +, Ca 2+, Mg 2+) down to very minor components such as rare earth elements. Whereas most of the natural water systems are apparently homogenous with respect to major components, the distribution patterns of minor elements reveal the impact of complex control mechanisms. The regulation of metal concentration in aquatic systems is based on the reactions ▪ (which initiate sedimentation) and their reversals (r). Formation of particulate metals (p) can proceed by (microbially mediated oxidation) (Fe(II) → Fe(OH) 3 s, Mn(II) → MnOOH (s)), by incorporation into the biomass and by absorption at inorganic particulates (Fe(OH) 3 s, MnOOH s, clay minerals). The actual extent of these reactions is dependent on the solution parameters (pH, pE, presence of ligands) i.e. the chemical speciation of the metal in the aquatic environment. Resolubilisation is caused by low redox potential (Fe(OH) 3 s → Fe(II), MnOOH s → Mn(II)), by oxidation of the biological carrier and by desorption from inorganic particulate favored by low pH-values. In addition, metals such as Hg, Sn, Pb and Tl can be remobilized by biomethylation. The usual (biologically induced) distribution of pE and pH favors p over r near the surface and r over p near the bottom of the aquatic system. The resulting concentration gradients result in eddy-assisted back diffusion. The marked increase in metal pollution (as documented by sedimentary record) gives rise to increasing concern for the impact of metal ions upon the aquatic biosphere. It is usually assumed that (some) metal ions are limiting at low concentrations and that most are toxic at higher concentrations. The sensitivity towards a given metal is largely dependent on the biological species. It is also generally agreed that availability of dissolved metals for phytoplankton is restricted to aquo ions. Hence, the biological impact of a given metal is again dependent on its chemical speciation. The situation is further complicated by the fact that some algae are able to release organic ligands (ferrichromes). Recent field studies seem to indicate that increase in metal concentration does not basically change the total amount of biota in a given aquatic system; the biological speciation is however, greatly changed.