Interactions and chemostasis in aquatic chemical systems: role of pH, pE, solubility, and complexation

By systematic studies of chemical equilibrium models for natural aqueous systems, it was found that the speciation of the trace metal ions is usually simple: free ions, solid carbonates, sulfides or hydroxides, chloride or sulfate complexes, etc. Because the complexing or precipitating ligands are usually in large excess of the metals, they mediate few important interactions (competition) among those metals. A most notable exception to this rule is observed under reducing conditions when a sufficiently large iron concentration precipitates most of the sulfide and keeps other trace metal ions from precipitating. For infinitesimal changes in analytical concentrations, interactions among constituents of a chemical equilibrium system can be quantified by the use of 'interaction intensities'. In the case of large concentration changes such as the addition of a new ligand, simple ways were shown of predicting, in general, the new equilibrium state. For such predictions the important parameters appear to be the free concentrations of the constituents and their unbound ratios. This last parameter is, in effect, a measure of the availability of the metal for complexation by an added ligand. It has been noted that organic ligands can play a major role in natural waters by complexing trace metal ions and keeping them in solution. It was demonstrated that, in addition to this important effect on the speciation of metal ions, organic ligands are likely to mediate large interactions among those metal ions. For example, the presence of NTA couples the free concentrations of copper and cadmium. In order to predict the global effects of a given change in a natural chemical system it is thus essential to know the organic content of the system. In ecological studies, general statements are often made upon the relations between complexity and stability, diversity and homeostasis, etc. In the far simpler chemical equilibrium systems dealt with, it is clear that no such general statement can be made. To be meaningful, the quantification of the concept of chemostasis, for a given chemical system, has to be the complete matrix of interaction intensities or equivalent quantities. To say that a system is more or less chemostable than another system is, thus, somewhat abusive. It is only in the case where one is interested in the effect of a given change on a given variable that the stability of two systems can be simply compared.