Depletion of Antioxidant
Fits of Eq. (
1) to the OIT data obtained by least-squares regression are shown in Figs.
3–
5. Antioxidant depletion rates for each immersion liquid and temperature are summarized in Table
2. Residual OIT in Eq. (
1) is the relatively constant OIT at the end of testing, when the remaining antioxidants are no longer effective in preventing oxidation (
Rowe et al. 2013).
in Eq. (
1) was determined to be 11 min by simultaneous least-squares fitting of the OIT depletion data for the HDPE GM exposed to RSL, NSL, and DI at 25, 50, 70, and 90°C (Fig.
6). Individual fits to each data set yielded
from 9 to 12 min for the 2-mm-thick HDPE GM immersed in RSL, 8 to 13 min for NSL, and 9 to 14 min for DI water.
OIT decreases with immersion time and decreases at a higher rate at higher temperature (Figs.
3–5, Table
2). Hsuan and Koerner (
1998), Sangam and Rowe (
2002), Gulec et al. (
2004), and Rowe et al. (
2010a,
2013) report similar results for other leachates. The depletion rate for RSL at 90°C is 0.4569
, which is 2.5 times higher than the rate at 70°C (
), 16 times higher than that at 50°C (
), and 97 times higher than that at 25°C (
). The fastest depletion occurs in RSL, followed by NSL, and DI (Fig.
6 and Table
3). At 90°C, the antioxidant depletion rate in RSL (
) is 1.06 times faster than the rate in NSL (
), and 1.50 times faster than to the rate in DI water (
). A paired t-test was used to evaluate whether the rate of OIT depletion differed statistically for RSL, NSL, and DI water at a significance level (
) = 0.05. The
-values obtained from the four aging temperatures in RSL and DI water (
to 0.015) and in NSL and DI water (
to 0.0011) were less than 0.05, indicating a statistically significant difference between the OIT depletion rate in RSL and NSL compared to DI water.
The higher depletion rate for the GM immersed in NSL or RSL relative to that for DI water is attributed to the metals and surfactant in the leachate. Transition metals (e.g., Mn, Cu, and Fe) in RSL and NSL can breakdown ROOHs via redox reactions, creating additional free radicals and additional consumption of antioxidants (
Gale et al. 1972;
Osawa and Ishizuka 1973). For example, Gulec et al. (
2004) report an antioxidant depletion rate for a 1.5-mm HDPE GM that is 1.3 times higher in synthetic acid mine drainage containing a variety of metals than in acidic water without metals, illustrating that transition metals in AMD (e.g.,
) accelerate the antioxidant depletion rate. Similarly, Rowe and Rimal (
2008) report that surfactants can increase the wetting ability of the GM, resulting in more rapid loss of antioxidants via diffusion into the leachate. Increasing the surfactant concentration from approximately
(
) to
(
) in experiments conducted by Rowe et al. (
2008) led to an increased antioxidant depletion rate.
The ratio of antioxidant depletion rates in RSL and NSL to DI water (1.1–1.9 times) is similar to the ratio (1.8 times) reported by Gulec et al. (
2004) for AMD and lower than the ratio (2.5–4.0 times) reported by Rowe et al. (
2009) for MSW leachate. This probably reflects the lower surfactant concentration in RSL or NSL (e.g.,
) relative to synthetic MSW leachate [e.g.,
(
)] (
Rowe et al. 2009). Rowe et al. (
2008) reported that a small amount of surfactant can increase the antioxidant depletion rate significantly. The AMD in Gulec et al. (
2004) had a lower pH (e.g., 2.1) and higher concentration of transition metals than RSL and NSL, but contained no surfactants. Consequently, the similar ratios observed in this study and by Gulec et al. (
2004) probably reflect compensating effects of the differences in metals and surfactant concentrations. The HDPE GMs used in previous studies may have contained different antioxidants, as reflected by the differences in initial Std-OIT [e.g., 133 min for Rowe et al. (
2009); 204 min for Gulec et al. (
2004); 197 min for the current study], which may also lead to different antioxidant depletion rates.
Effect of Radiation from RSL on Antioxidant Depletion
The antioxidant depletion rate in RSL was 9, 4, and 6% faster than in NSL when immersed at 25, 70, and 90°C (Table
3), respectively. At 50°C, the antioxidant depletion rates were the same. A paired t-test was used to determine whether the rates of OIT depletion differed statistically between RSL and NSL at each aging temperature.
-values of 0.00046, 0.0050, and 0.000049 were obtained for 50, 70, and 90°C, indicating a statistically significant difference between antioxidant depletion rate in RSL and NSL at these temperatures. For 25°C, the
-value was 0.12, indicating no statistically significant difference. Consequently, while the effect of radionuclides in RSL generally is statistically significant, the effect on antioxidant depletion in HDPE GM relative to NSL is subtle.
The small increase in antioxidant depletion rate of the HDPE GM in RSL relative to NSL is attributed to the low radiation dosage from exposure to the LLW leachate. An
particle form
with peak energy of 4.2 MeV will penetrate only 25 μm in water, and a
particle from
with peak energy of 294 keV will penetrate approximately 0.5 mm in water (
Turner 2007). Thus, only those
and
particles emitted within a narrow zone adjacent to the GM may reach the surface of a HDPE GM. Penetration of
and
particles is strongly affected by the density of materials being penetrated (
Turner 2007), and HDPE GMs have a density (
) close to water. Therefore, the penetration of
and
particles in a HDPE GM is likely to be similar range to that in water and only impact the surface of the GM. Further study is ongoing to investigate the effect of higher levels of
and
radiation on antioxidant depletion in HDPE GMs.
Crystallinity
Crystallinity is a measure of the relative abundance of crystalline and amorphous regions in a polymer, and is an indicator of change in polymer structure (
Sperling 1992;
Kong and Hay 2002). Higher crystallinity generally corresponds to HDPE GMs with greater stiffness and lower stress crack resistance (
Rowe et al. 2009). Crystallinity of the GMs immersed in RSL and NSL at 90°C is shown in Fig.
8 as a function of immersion time. The crystallinity increased abruptly from 42.6 to 44.7–45.1% during the first three months of immersion, and then increased more gradually to 45.6–46.4% from 3 to 12 months of immersion. A very gradual decrease in crystallinity (approximately 1%) occurred during the final next nine months.
The increasing crystallinity during the first 12 months is attributed to recrystallization and/or postcrystallization as the polymer established equilibrium from a nonequilibrium state (
Petermann et al. 1976;
Wrigley 1989;
Dörner and Lang 1998). Similar increases in crystallinity of HDPE GMs immersed in leachate at 85°C have been reported by Dörner and Lang (
1998) and Rowe et al. (
2009). The slight decrease in crystallinity after 12 months, when the antioxidants were depleted, is indicative of less freedom of molecular segments to form crystallinities (
Peacock 2000;
Rowe et al. 2009) and is consistent with the crosslinking in response to oxidation suggested by the decrease in MFI. Rowe et al. (
2009) also observed a drop in crystallinity for HDPE GMs immersed in MSW leachate after antioxidants were depleted completely. Consequently, the MFI data and the crystallinity data are consistent with the HDPE GM completing Stage I (antioxidant depletion), and progressing into Stage III, when changes in polymer structure occur in response to oxidation.
One-Sided Exposure Test
One-sided exposure tests were conducted to provide a more representative condition for antioxidant depletion compared to an immersion test (
Rowe and Rimal 2008;
Rowe et al. 2010a,
2013;
Tian et al. 2014). In immersion tests, both sides of the GM are exposed to leachate, whereas only one side of a GM is exposed to leachate in a liner in a waste containment facility (
Rowe et al. 2010a;
Tian et al. 2014). Antioxidants diffuse to and are released from both sides of the GM during an immersion test, reducing the diffusion path to one-half thickness of the GM. In contrast, the diffusion path is the entire thickness of the GM in one-sided exposure. This is directly analogous to the longer time required for pore water pressure dissipation during consolidation with single drainage versus double drainage (
Budhu 2011). Because diffusion loss is a significant mechanism for antioxidant depletion (
Rimal and Rowe 2009), immersion tests underestimate the antioxidant depletion time that occurs in one-sided exposure scenario.
Ten one-sided exposure tests were conducted using RSL as the contacting liquid. Five tests were conducted at 70°C and five at 90°C. The tests were disassembled periodically and OIT was determined. OIT depletion as a function of aging time from these tests is shown in Fig.
10. Antioxidant depletion rates were calculated using Eq. (
1) and
obtained from the immersion tests. Antioxidant depletion rates for one-sided exposure to RSL were 0.0542 and
at 70 and 90°C, respectively, 3.44 and 3.38 times slower than obtained from the immersion tests. Rowe et al. (
2010a) also report antioxidant depletion rates approximately three times lower for one-sided exposure tests relative to immersion tests.