Geosynthetic clay liners (GCLs) have gained growing recognition as an alternative to a conventional compacted clay liner in base seals of landfills (Rowe 1998). A typical configuration involving a GCL is shown in Figure 1. However the diffusion and sorption of heavy metals through GCLs has received very little attention in the literature. Most of the related published work focuses on sorption of radionuclides on clays including Na-bentonite. For example, Czurda et al. (1987) showed relatively high (compared to other typical materials) sorption of Sr-90 and Cs-137 for Na-bentonite.  
 
The work specifically related to GCLs has been limited, however Lo (1992) did use the program POLLUTE (Rowe & Booker 1983, 1997a) to fit the concentration profile in effluent for tests performed using a GCL and obtained a diffusion coefficient of 5.9x10^-10 m²/s and a very high linear sorption coefficient, K_d, of 6000 ml/g for lead (Pb). In contrast, the K_d of an organoclay tested was 140 ml/g. This data suggest the potential for very high sorption and retardation of heavy metals such as lead in a GCL.
 
Lo's study was unusual in that it was reported to deal specifically with a GCL, however others have examined the sorption of heavy metals in sodium bentonite. Most, like Lo (1992) used single heavy metal permeants. A limited number of investigators used multiple metal solutions to examine sorption onto bentonite. For example Czurda and Wagner (1991) examined the sorption capacity of heavy metal (Cd, Co, Cr, Cs, Cu, Pb, Sr, and Zn) chloride solutions for a number of different clayey materials and found that (a) sorption was dependent on pH and greatest at high pH, and (b) sorption of Wyoming (Na) bentonite was substantially greater than for the other clay materials examined (Figure 2).
 
However the single metal and multi-metal permanents used by investigators do not simulate the typical compositions in landfill leachate and may generally be regarded as examining "extreme worst case” scenario's when put in the context of any application of a GCL as a base liner in a landfill application. For example, Li and Li (2001) examined the sorption of the heavy metals Pb, Cu and Cd from a solution prepared by dissolving lead nitrate, copper nitrate and cadmium nitrate in distilled water and then adjusting the pH to 4 using nitric acid. These test were conducted using a 25:2 mix of sand:Na-bentonite. The pH may be relevant to some acid mine drainage applications but is much lower than typical landfill leachate pH (normally 5.5-8). Li and Li's tests showed that there was significant sorption of heavy metals (with the mobility being in the order Cd>Cu>Pb). Precipitation and complexion dominated at low Pb and Cu concentrations while cation exchange became dominant at high concentrations.
 
The concentration of heavy metals in landfill leachate is typically low and a study of metal migration (Yanful et al., 1988) from an actual landfill site in a natural clay (see Rowe et al, 1995: pp235-236) showed very slow migration with the heavy metals having precipitated predominately as carbonates although there was also an organic phase present for Fe, Zn and Cu. The precipitation was believed to have occurred at the soil pH of about 8 and the low redox potential (-150 mV) anticipated at the bottom of a landfill. Since the pH of the Na-bentonite used in GCL is considerably higher than the pH of this natural soil typically used in compacted clay liners and since carbonates and organics are abundant in landfill leachate, one would anticipate that the Na-bentoinite GCL would provide very good attenuation capacity with precipitation dominating at the typical low concentrations found in landfill leachate. In addition, the cation exchange capacity (CEC) of Na-bentonite is typically about an order of magnitude larger that of the soils commonly used in a compacted clay liner such as that examined by Rowe et al. (2000) or the natural clay examined by Yanful et al. (1988). This, given that the migration of heavy metals in the latter case was limited to about 10cm, one may anticipate that the performance of the GCL at retarding heavy metals would be as good as, if not better than, a conventional CCL.
 
The supposition that the Na-bentoinite GCL would provide very good attenuation capacity for heavy metals in landfill leachate presented in the previous paragraph is based on extrapolation from a limited data set. No tests have been performed to assess the potential for retardation of heavy metals migrating through GCLs by diffusion or advection. If this is important for a particular project, tests should be performed to assess the GCL-heavy metal interaction and the nature of the sorption processes to confirm the expected good sorption charateristics.