Metal organic complexes in natural waters: control of distribution by thermodynamic, kinetic and physical factors

Two organic compounds, nitrilotriacetate (NTA) and citrate, which may conceivably appear in natural waters affected by industrial societies, have a potential to modify the existing distributions of ionic species in waters. The two compounds for strong complexes with such metal ions as Cu, Pb, Fe, Ni, Co, and Zn, and somewhat weaker complexes with such major constituents of natural waters as Ca and Mg. A study of thermodynamic equilibria in a model fresh water (similar in composition to an average river and lake water) shows that NTA and citrate complex virtually all available Cu, Fe, Pb, Ni, Co, and Zn. When no more of these metal ions are available for complexation but the ligand concentration continues to rise owing to input from outside, then NTA and citrate form complexes with Ca and, to a lesser extent, Mg. Complexing of Ca becomes significant at the total ligand concentration in the range from 2 mg/l to 15 mg/l. In fresh waters, such concentrations of NTA or citrate can significantly affect the concentration of Ca in ionic form, and they may therefore be potentially damaging to many fresh water environments. These results may be generalized by stating that any complexing agent with strong affinities to the major and minor metal ions in solution can perturb the existing distribution of ionic species in natural waters. In the case of NTA, the equilibrium constant of the CaNTA- complex is of the order of 10⁶. If for some other complexing agent the equilibrium constant were higher, its effect on the concentration of Ca ion in solution may have been pronounced at concentrations lower than those of NTA in the NTA fresh water system. If an organic complexing agent is removed from solution by such processes as decay, oxidation or biodegradation, the preexisting distribution of metal ions may be restored. In large bodies of water characterized by longer residence time (of the order of years to decades), a relatively fast decay (half lives of organic ligands of the order of weeks to months) assures that the total concentration of the complexing agent is low by comparison with its input concentration. On the other end of the half life scale, input of a slowly decaying or stable complexing agent into the same system would result in relatively much higher concentrations in water. Uptake of the ligand containing species from solution by solid mineral phases reduces, analogously to decay, the ligand concentration in water. Such uptake mechanisms as equilibrium exchange and adsorption by sediment particles may very strongly retard the rise in concentration of a new complexing agent introduced into the water system by input from outside. The immediate effects of equilibrium exchange or adsorption in a transient state are lower concentrations in solution. The final steadystate concentration, however, is not affected by exchange and adsorption processes: provided the conditions of input to, and removal from the water system are such that a steady state can exist, chemical exchange and adsorption from solution only slow down the rate of approach to steady state, without affecting the steady state concentration value.