Table
4 summarizes results of the hydraulic conductivity tests. The pore volumes of flow (PVF) reported in Table
4 are based on the initial pore volume of each GCL before permeation. At the time this paper was prepared, the GCLs had been permeated for up to 2.8 years with RSL and NSL. The CS, GS, CPL, GPL1, and GPL2 GCLs permeated with RSL or NSL met all of the termination criteria in ASTM D6766 (
ASTM 2012), and the supplemental criteria regarding pH and the concentrations of major cations in the influent and effluent. The CPH, GPH, and BPC GCLs permeated with RSL or NSL had not met all of the termination criteria because of their very low hydraulic conductivity. Computations based on the current data suggest that these tests will require at least another 5 years of permeation to reach chemical equilibrium. Tests on these GCLs are still ongoing.
Temporal Behavior and Chemical Analysis
Typical data from a hydraulic conductivity test are shown in Fig.
3 for the GS GCL. The hydraulic conductivity increases by approximately a factor of 10 during the initial 40 PVF due to cation exchange processes [Fig.
3(a)]. At approximately 40 PVF, the EC and pH of the effluent level off and fall within the range associated with the EC and pH termination criteria [Fig.
3(b)]. However, the hydraulic conductivity continues to increase very slowly until 80 PVF, which reflects slow rate-limited cation exchange processes, as described by Jo et al. (
2001,
2006). Similar behavior was observed for all other GCLs for which the hydraulic conductivity increased over time. For those GCLs that retained low hydraulic conductivity throughout the test, the hydraulic conductivity decreased initially and then remained essentially constant.
Major cation concentrations shown in Fig.
4 are from the test illustrated in Fig.
3.
concentrations in the effluent decreased rapidly for approximately 40 PVF and then gradually dropped to the influent concentration by approximately 80 PVF, which was due to slow rate-limited cation exchange reaction (
Jo et al. 2006). In contrast, the concentrations of
,
, and
in the effluent increased over the testing period until reaching the inflow concentration at approximately 80 PVF. These observations indicate that
,
, and
replaced Na in the exchange complex of the bentonite. Comparison of Figs.
3 and
4 shows that that hydraulic conductivity and major cations in inflow and effluent both reach equilibrium at approximately 80 PVF. Similar findings were obtained for all other tests.
Effect of Radionuclides on Hydraulic Conductivity
Hydraulic conductivities of the GCLs exposed to RSL or NSL are compared in Fig.
5. Essentially, the same hydraulic conductivities were obtained using both leachates, with all of the data falling within a band corresponding to a factor of 2, which Daniel et al. (
1997) indicate is the reproducibility of hydraulic conductivity tests on GCLs. The similarity in hydraulic conductivities to RSL and NSL reflects the small impact of radionuclides on ionic strength and RMD of RSL (
). Similar swell indices were also obtained with NSL and RSL for each Na-B or B-P GCL (Fig.
6). Based on these findings, RSL and NSL were considered comparable in terms of their effect on hydraulic conductivity of GCLs, and subsequent tests were only conducted with NSL to reduce safety concerns and to simplify disposal of testing waste.
Hydraulic conductivity of the GCLs permeated with RSL or NSL are compared to the hydraulic conductivity to DI water in Fig.
7. Hydraulic conductivities of the CS and GS Na-B GCLs to RSL or NSL are approximately 5–20 times higher than the hydraulic conductivity to DI water. Hydraulic conductivity of the B-P GCLs fell into four categories. The GPL2 GCLs were 100 times more permeable to NSL than to DI water, and had the highest hydraulic conductivity of all GCLs in the study. Hydraulic conductivity of the CPL and GPL1 GCLs permeated with RSL or NSL were comparable to conventional GCLs, being approximately 5–25 times more permeable to RSL or NSL than to DI water. The CPH and GPH GCLs had the lowest hydraulic conductivity to RSL or NSL, and had nearly the same hydraulic conductivity as to DI water. The BPC GCL had the lowest hydraulic conductivity of all GCLs (
for NSL, RSL, or DI) and was 1.4–2.5 times less permeable to NSL and RSL than to DI water. Fewer PVF passed through the CPH, GPH, and BPC GCLs than the Na-B GCLs due to the very low flow rate associated with the very low hydraulic conductivity of these GCLs.
Cation Exchange, Swelling, and Hydraulic Conductivity
The RSL and NSL leachates contain divalent cations that have propensity to replace the monovalent
originally bound to the mineral surface. In NSL and RSL, the total molarity of divalent cations is 30% higher than of monovalent cations, and the total charge associated with the divalent cations is 2.6 times the charge associated with the monovalent cations. Cation exchange is evident in Fig.
4, where
is eluted due to replacement by
and
in the leachate. The exchange complex after testing in the CS, GS, CPL, GPL1, and GPL2 GCLs with NSL or RSL (Table
5) is nearly devoid of monovalent cations, and almost completely comprised of equal amounts of
and
. None of polymers in CPL, GPL1, and GPL2 prevented cation exchange. Cation exchange resulted in nearly complete loss in swell by the end of testing, with swell indices ranging between 10 and
for the CS, GS, CPL, GPL1, and GPL2 GCLs. Prior to permeation, the swell index in leachate ranged between 16 and
(
), and in DI water between 27 and
(
).
Conversion of the Na-B to Ca-Mg-B is responsible for the higher hydraulic conductivity of the conventional Na-B GCLs (CS and GS) to NSL and RSL shown in Fig.
7. The GS Na-B GCL has slightly lower hydraulic conductivity than the CS GCL, despite cation exchange, due to the finer bentonite granule size of bentonite in the GS GCL (
Kolstad et al. 2004a).
Mechanism Controlling Hydraulic Conductivity of Bentonite–Polymer GCLs to LLW Leachate
The insensitivity of the hydraulic conductivity of the B-P GCLs to swell index suggests that the polymer is controlling the size and shape of the pores rather than swelling of the bentonite. For example, the swell index of Na-B in the CS GCL was higher than the swell index of the B-P in the CPH GCL (Fig.
6), whereas the hydraulic conductivity of CS to RSL and NSL was approximately two orders of magnitude higher than the hydraulic conductivity of CPH GCL permeated with RSL and NSL.
The hydrogel is believed to function in a manner analogous to bentonite that has undergone osmotic swell, as shown conceptually in Fig.
10. Water molecules in the hydrogel are relatively immobile compared to free water in the pore space, allowing the hydrogel polymer and water molecules to function hydraulically like a solid [Fig.
10(a)]. Polymer chains and bound water in the hydrogel block larger pores between bentonite granules, forcing water to flow in finer and more tortuous paths within the granules [Fig.
10(b)], resulting in low hydraulic conductivity. In effect, the hydrogel functions analogously to bentonite added to reduce the hydraulic conductivity to water of a more permeable soil, blocking larger pore spaces between particles that would conduct the bulk of flow in soil alone. In contrast to swollen bentonite, however, the hydrogel is a viscous liquidlike gel that is not immobile (Fig.
9), and can migrate when exposed to a hydraulic gradient. Thus, to be effective in blocking flow, the hydrogel must bind to mineral surfaces to limit elution of polymer from the pore space. Moreover, if the polymer loading is insufficient or if the polymer is eluted, not all of the pores will be filled, resulting in larger flow paths and higher hydraulic conductivity [Fig.
10(c)].
To investigate binding between the polymer and bentonite, SEM images were obtained of the CPH GCL permeated with DI water (Fig.
11). The SEM image shows that the polymer forms a three-dimensional net structure in the pores between the bentonite granules. In the saturated system, the polymer net between the granules binds water molecules to form the hydrogel shown in Fig.
9. The edge of an anionic polymer structure may attach to the mineral surface via electrostatic forces associated with cationic bridging, whereas a cationic polymer may bind via electrostatic interactions with the mineral surface (
Deng et al. 2006). Edges of the montmorillonite carrying positive charge may also play a role in binding anionic polymers (
Black et al. 1965;
Heller and Keren 2003).
Visual observation of the effluents from the B-P GCLs indicated that they had significantly higher viscosity and a sticky characteristic, suggesting that polymer was eluted during permeation and that binding between the polymer and mineral was incomplete. Scalia et al. (
2014) also report polymer eluting from BPC GCLs permeated with DI water or salt solutions. Anionic polymers, such as polyacrylate and polyacrylamide, that have been used with bentonites can interact with divalent cations (e.g.,
,
) in the solution, neutralizing charges on the polymer chains (
Schweins et al. 2003) that anchor cation bridges to the mineral surface. These hypotheses require further study before definitive conclusions can be made regarding the polymer clogging mechanism.
The relationship between hydraulic conductivity and polymer loading shown in Fig.
8 also supports the conceptual model of hydrogel clogging pore spaces shown in Fig.
10. The B-P GCLs with low polymer loading (e.g., CPL and GPL1) had high hydraulic conductivity to RSL or NSL, as also observed with the conventional CS and GS GCLs with conventional Na-B, whereas B-P GCLs with high polymer loading (
) had consistently low hydraulic conductivity (Fig.
8). This suggests that sufficient polymer hydrogel is needed to clog the pore spaces in a B-P GCL in the same manner that the hydraulic conductivity of a more permeable soil becomes impermeable to water when a sufficient amount of Na-B is added. Without sufficient polymer hydrogel, a GCL may still have large intergranular pores that control the hydraulic conductivity.
Long-term stability of the hydrogel and its effectiveness in blocking the pore space may be affected by changes in the geochemical conditions over time, as well as by abiotic and biotic degradation processes. A study addressing these issues is currently being conducted.