Pure and Applied Chemistry

Flat-sheet-supported liquid membranes incorporating lasalocid A (a natural ionophore), were previously shown to be permeable to metal cations (Cd2+ and Zn2+) against a proton gradient (DpH), which is the driving force of the process [1]. This transport process has been extended to other metal species such as Pb2+, Na+, and Ag+ and also to the case where two metal species compete for transport. A higher transport flux for Pb2+ as compared to Cd2+ and Zn2+ is observed and partly explained by a higher rate of interfacial complexation owing to the smaller hydration shell of this species. This effect is confirmed by the data obtained with Ag+. However, the size of the metal cation in relation to its hydration shell does not appear as the major parameter to take into account for an estimate of the trans-membrane transport efficiency, as the Na+ ions escape to this behavior, which has been considered until now as a general trend for metal cation ionophores.

Alternative coagulants based on prehydrolyzed forms of aluminium or iron can be more effective than the traditional materials in many cases, but their mode of action is not completely understood, especially with regard to the role of charge neutralization and hydroxide precipitation.

Basic principles of colloid stability and metal ion hydrolysis are briefly reviewed, and the action of hydrolyzing metal coagulants is then discussed, with some examples from recent experimental studies. Although it is possible to interpret results reasonably well in terms of established ideas, there are still some uncertainties that need to be resolved