As an alternative to species distribution diagrams (pM vs pH curves in aqueous solution) drawn for a fixed total metal concentration, this work has developed simple linear models for correlating the limiting pH of metal ion solubility-in equilibrium with the least soluble amorphous metal hydroxide solid phase-to the total metal concentration. Thus adsorptive metal removal processes in complex systems can be better designed once the limiting pH of heavy metal solubility (i.e., pH*) in such a complex environment can be envisaged by simple linear equations. pH* vs pM, (M-t = total metal concentration that can exist in aqueous solution in equilibrium with M(OH)(2(s))) linear curves for uranyl-hydroxide, uranyl- carbonate-hydroxide, and mercuric-chloride-hydroxide simple and mixed-ligand systems and cupric-carbonate-hydroxide complexes in equilibrium with mixed hydroxide solid phases may enable the experimental chemist to distinguish true adsorption (e.g., onto hydrous oxide sorbents) from bulk precipitation removal of the metal and to interpret some anomalous metal fixation data-usually attributed to pure adsorption in the literature-with precipitation if the pM(t) at the studied pH is lower than that tolerated by pH* vs pM(t) curves. This easily predictable pH* corresponding to a given pM(t) may aid the design of desorptive mobilization experiments for certain metals as well as their adsorptive removal with the purpose of simulating metal adsorption and desorption cycles in real complex environments with changing ground water pH. (C) 1999 Academic Press.