Geologic Membrane Controls on Saturated Zone Heavy Metal Transport
When a clay membrane partially rejects solute, a concentration polarization layer (CPL) forms at the higher-pressure membrane interface. Solute concentrations at the membrane may be from several to more than 100 times that of background concentration. Therefore, even though the concentration outside the CPL may be below saturation, concentrations within the CPL nearest the membrane may reach supersaturation and precipitation may occur.
As an experiment test of this concept, seven hyperfiltration experiments were performed with sedimented clay membranes; three with undersaturated copper carbonate solutions, two with undersaturated lead chloride solutions, one with an undersaturated copper chloride solution, and two with undersaturated cobalt chloride solutions. Heavy metal precipitates were present during scanning electron microscopic examination of the clay membranes for six out of the seven experiments, and in two of these experiments heavy metal precipitate was visible to the naked eye through the clear acrylic cell before the experiments ended.
One problem with geologic membrane research is in obtaining measured values of the membrane coefficients quickly and efficiently. Several experiments were performed with two litho-osmometers. These instruments were designed to measure the membrane coefficients of shales. Preliminary results suggest that neither design works rapidly enough for routine commercial use.
An analytical steady-state model was developed to predict when heavy metal precipitation due to membrane processes in the subsurface is possible. This model is adaptable for use at specific contaminated sites.