HDPE Geomembranes

Geosynthetics International, 2004, 11, No. 2

Effect of acidic mine drainage on the polymerproperties of an HDPE geomembrane

S. B. Gulec1, T. B. Edil2 and C. H. Benson3

1Senior Staff Engineer, GeoSyntec Consultants, 1100 Lake Hearn Driver, Suite 200, Atlanta,

GA 30342, USA, Telephone: +1 404 705 9500, Telefax: +1 404 705 9400, E-

mail: [email protected] of Geological Engineering and Professor, Department of Civil and Environmental

Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, WI 53706, USA,

Telephone: +1 608 262 3225, Telefax: +1 608 263 2453, E-mail: [email protected], Department of Civil and Environmental Engineering, University of Wisconsin-Madison,

1415 Engineering Drive, Madison, WI 53706, USA, Telephone: +1 608 262 7242,

Telefax: +1 608 263 2453, E-mail: [email protected]

Received 10 September 2003, revised 8 February 2003, accepted 11 February 2004

ABSTRACT: A laboratory exposure programme was conducted to assess the effects of acidic mine

drainage (AMD) from metallic mine wastes on the bulk polymer properties of a 1.5-mm-thick HDPE

geomembrane. Coupons of the geomembrane were immersed in tanks containing a synthetic AMD

maintained at 20, 40 and 608C. Two other solutions (acidic water and deionised water at 20, 40, and

608C) were also used. Specimens were periodically removed from the tanks and tested to determine

whether changes were occurring in the melt flow index (MFI), polymer structure (as determined by

Fourier transform infrared spectroscopy, or FTIR), and oxidation induction time. Only modest

effects of AMD exposure were observed in the MFI and FTIR tests over the 22-month exposure

period. The OIT changed appreciably during exposure, and followed a first-order (exponential in

time) degradation pattern. Greater depletion rates occurred at higher temperatures and in the

chemical solutions. An Arrhenius model was used to estimate the antioxidant depletion time for

HDPE geomembranes immersed in AMD. Conservative estimates of the depletion time range from

46 years to 426 years depending on field temperature, initial OIT, and exposure condition.

KEYWORDS: Geosynthetics, Acid mine drainage, AMD, Compatibility, Durability, HDPE

geomembrane, Lifetime prediction, Mine waste, OIT, Tailings, Waste rock

REFERENCE: Gulec, S. B., Edil, T. B. & Benson, C. H. (2004). Effect of acidic mine drainage on

the polymer properties of an HDPE geomembrane. Geosynthetics International, 11, No. 2, 60–72

1. INTRODUCTION

Acid mine drainage (AMD), the leachate from waste

rock and tailings from mining and beneficiation of

sulfide-rich ores, poses a threat to groundwater and

surface water at mine sites worldwide. AMD is formed

when sulfide-rich mine wastes oxidise in the presence of

water, and is characterised by low pH and elevated

concentrations of metals, some of which are toxic. The

most common elements found in AMD are sulfur, iron,

copper, zinc, silver, gold, cadmium, arsenic and uranium

(Ripley et al. 1996).

Modern mining operations deal with the risks of

groundwater contamination by placing mine wastes in

engineered waste containment facilities designed using

principles employed for solid waste landfills (Almeida

et al. 2002; Brennecke and Corser 1998; Van Zyl and

Simpson 1997). These mine waste containment facilities

make extensive use of geosynthetics, such as geomem-

branes, geotextiles and geocomposite drainage layers, to

prevent leakage and to collect leachate for treatment.Because geosynthetics play a pivotal role in the effec-tiveness of these waste containment facilities, the long-term compatibility of geosynthetic materials in contactwith mine waste leachates (and AMD in particular) isimportant. However, little information currently exists inthe literature regarding how mine waste leachates affectgeosynthetic materials (Gulec 2003).

This paper describes part of a study conducted toevaluate how exposure to AMD from metallic minewastes affects the properties of geosynthetic materialsused in mine waste containment. A geomembrane,geotextile and drainage geocomposite were immersed intanks containing synthetic AMD, acidic water anddeionised (DI) water at 20, 40 or 608C. Samples wereremoved from the tanks periodically and tested todetermine whether exposure to the test liquids affectedtheir bulk properties. Effects on the polymer propertiesof the geomembrane are described in this paper. Polymerproperties were investigated because the durability of

1072-6349 # 2004 Thomas Telford Ltd 60

geosynthetic materials depends largely on the composi-tion of the polymers from which they are made as well ason conditions during the fabrication process (Cassidyet al. 1991). Effects of exposure on the mechanical(trapezoid tear strength, puncture strength, and wide-strip tensile strength) and hydraulic (permittivity,transmissivity) properties of the geosynthetics aredescribed by Gulec et al. (2004). In general, nostatistically significant changes in the mechanical orhydraulic properties of the geosynthetics occurredduring the 22-month exposure period.

2. BACKGROUND

2.1. Degradation of geosynthetics

The following mechanisms can degrade geosynthetics,depending on the nature of exposure and the type ofpolymer: UV light, radiation, elevated temperatures,chemical disintegration, mechanical damage and oxida-tion (Haxo and Nelson 1984; Koerner et al. 1990).Degradation generally begins as changes in the polymerstructure and eventually is manifested as changes inengineering properties (e.g. mechanical and hydraulicproperties) of geosynthetics (Hsuan and Koerner 1998).Common changes in the polymer include embrittlement,reduction or gain in molecular weight, generation of freeradicals, loss of additives and plasticizers, and impair-ment of transparency (Kulshreshtha 1992).

Identification of a single dominant degradationmechanism for geosynthetics used in mine waste linersystems is not possible because, in most cases, more thanone of the listed degradation mechanisms is encountered.However, some of the aforementioned degradationmechanisms are unlikely for mine waste applications.Geosynthetics are buried in liner applications, whicheliminates UV degradation except during construction.Radiation generally is not important because radioactivematerials generally do not exist in appreciable quantitiesin most metallic mine wastes (except those associatedwith uranium mining). Thermal degradation (i.e. degra-dation of a polymer at elevated temperatures without theinvolvement of another compound) is negligible atcommon environmental temperatures (Schnabel 1981)except near seams, and microbiological attack is unlikelyowing to the high molecular weight of polymer resinsused in geosynthetics (Koerner et al. 1990; Rebenfeldand Cooke 1989). Consequently, the most likelymechanisms for degradation of geosynthetics in minewaste facilities are mechanical degradation (not ad-dressed in this paper), chemical degradation andoxidative degradation.

Chemical degradation can occur when geosyntheticsare exposed to strong chemicals (e.g. acids, bases,solvents, reactive gases) that alter the polymer byprocesses such as swelling, extraction and dissolution.Oxidative degradation is also a form of chemicaldegradation, but is considered separately owing to itssignificance in polymer degradation, even in environ-ments where limited oxygen exists (Grassie and Scott

1985; Klemchuk 2002). Oxidative attack has beenreported to have particularly severe impact on poly-olefins (e.g. polyethylene and polypropylene) (Horrockset al. 1997; Sangam and Rowe 2002).

2.2. Oxidation of polyethylene

Oxidation of polyethylene is an auto-accelerating pro-cess, meaning that the rate is slow at first, but graduallyaccelerates. The period before oxidation accelerates (andmeasurable degradation takes place) is called the‘induction period’ (Hsuan and Koerner 1998). Whenpolymer degradation due to auto-oxidation chain reac-tions becomes severe, alteration of the physical andmechanical properties of polymers occurs (Hsuan andKoerner 1998).

Oxidation of polyethylene is caused by an auto-oxidation chain mechanism that involves two interactingcyclical processes, shown as Circle A and Loop B inFigure 1. Circle A consists in the formation ofhydroperoxide (ROOH) and free radicals, and Loop Bconsists in decomposition of the hydroperoxide to formmore free radicals (Grassie and Scott 1985). Thedegradation reactions start with generation of a freeradical (R�) (any species that contains one or moreunpaired electrons, which make the species reactive)from the polymer chain (RH):

RH ! R� þH� ð1Þ

Catalysts, heat or UV light can initiate chain scission inthe polymer, generating free radical species (Fay andKing 1994). After the free radical is generated, it reactswith oxygen and forms peroxy radical (ROO�) (CircleA):

R� þO2 ! ROO� ð2Þ

The peroxy radical (ROO�) then abstracts a hydrogenatom from the surrounding polymer chain to form

ROOHRH

(b)(d)

O2

Start ofreactions

RH (Polymer chain)

(c)

Circle A(R- and ROOH

(generation)

Loop B(ROOH

decomposition)RO. + .OH

R. ROO.

(a)

Figure 1. Chain mechanisms involved in auto-oxidation of

polyethylene (adapted from Grassie and Scott 1985)

Effect of acidic mine drainage on the polymer properties of an HDPE geomembrane 61


hydroperoxide (ROOH) and creates another free radical(Fay and King 1994):

ROO� þRH ! ROOH þR� ð3Þ

The amount of ROOH generated by Reaction 3 is small,which results in slow oxidation. When the concentrationof ROOH reaches a critical level, decomposition ofROOH begins, as shown in Loop B in Figure 1:

ROOH ! RO� þOH� ð4Þ

Decomposition of ROOH causes a substantial in-crease in the amount of free radicals, which attack thepolymer chain and accelerate the chain reactions. TheRO� radical reacts with the polymer chain (RH) andforms more free radicals and alcohol (ROH):

RO� þRH ! ROHþR� ð5Þ

The OH� radical reacts with the polymer chain to formfree radicals and water:

OH� þRH ! H2OþR� ð6Þ

Metal ions in mine waste liquids may affect the rate ofthese reactions because oxidation of polymers isaccelerated by metals or metallic compounds (Osawaand Ishizuka 1973). One of the main functions of ametallic catalyst during oxidation is the breakdown ofhydroperoxides to free radicals. Osawa and Ishizuka(1973) studied the effects of various metals on oxidativedegradation of polypropylene and found that thecatalytic effect of acid metal salts followed the order(high to low): Co > Mn > Cu > Fe > V > Ni > Zn >Al > Mg. Fe, Cu and Zn are commonly found elementsin metallic mine waste liquids, and therefore mayaccelerate the degradation of geosynthetics exposed tothese liquids.

2.3. Antioxidants

Antioxidants are added to polymeric materials to inhibitoxidation and extend the induction period (Al-Malaika1998; Grassie and Scott 1985). Because geosynthetics aremanufactured at high temperatures (2008C and 2808C),antioxidants are needed that function at the hightemperatures associated with manufacturing as well asthe lower temperatures associated with in-service appli-cations. Consequently, manufacturers generally use acombination of two or more types of antioxidant toprovide overall stability (Fay and King 1994).

The main function of antioxidants is to retardoxidation by breaking links in the auto-oxidation chains,shown as a–d in Figure 1. The following antioxidants arecommonly used (operating temperature in parentheses):hindered phenols (0–3008C), hindered amines (HALS)(0–1508C), phosphites (150–3008C) and thiosynergists(0–2008C). Hindered phenols and hindered amines areprimary antioxidants; phosphites and thiosynergists aresecondary antioxidants. Hindered amines are also usedas secondary antioxidants (Fay and King 1994). Primaryantioxidants are also known as ‘chain-breaking antiox-idants’. They act by either accepting an electron ordonating an electron. Secondary antioxidants are known

as ‘preventive antioxidants’ because their major functionis to interfere with the formation of free radicals bydecomposing hydroperoxides.

2.4. Depletion of antioxidants

Antioxidants are depleted by chemical reactions toprevent oxidative degradation of polymers and physicallosses to the surrounding media by volatilisation or byextraction (Gedde et al. 1994). Volatilisation is not amajor concern for HDPE geomembranes at typicaltemperatures in waste containment systems (<608C)(Hsuan and Koerner 1998). However, extraction can besignificant (Calvert and Billingham 1979). For example,Smith et al. (1992) found that approximately 75% of theantioxidants in medium-density polyethylene pipes ex-posed to water (internally) and air (externally) were lostby extraction.

Hsuan and Koerner (1995, 1998) studied degradationand antioxidant depletion of a 1.5-mm-thick HDPEgeomembrane. Four different incubations were used:water immersion, water immersion with the geomem-brane loaded to 30% of the yield stress, air immersion,and one-sided exposure (water above and air below)under a compressive stress of 260 kPa. Samples wereexposed in water at 55, 65, 75 and 858C and in air at 55,65, 75, 95 and 1158C. Samples were retrieved periodicallyand tested to determine the oxidation induction time(OIT). The OIT data showed that the depletion rateincreased with increasing exposure temperature, and washigher for water immersion than for one-sided exposure.Depletion rates were not reported for air exposure andwater immersion with the geomembrane loaded to 30%of the yield stress.

The OIT data were used to make lifetime predictionsusing the Arrhenius method. The time for antioxidantdepletion at 258C was approximately 40 years for waterimmersion and 120 years for water above, air belowexposure. Air exposure and water immersion with thegeomembrane loaded to 30% of the yield stress were notincluded in the lifetime predictions. Additional testsshowed that no major changes in density, melt flow index(MFI) or tensile properties occurred during the 24-month exposure period (Hsuan and Koerner 1998).

Sangam and Rowe (2002) studied antioxidant deple-tion in a 2-mm-thick HDPE geomembrane exposed tosynthetic municipal solid waste (MSW) leachate, wateror air at 22, 40, 55, 70 and 858C. The synthetic MSWleachate had pH 6 and contained the surfactantIGEPAL CA-720 (5000 mg/l), acetic acid (4000 mg/l),propionic acid (3000 mg/l), butyric acid (500 mg/l),HCO3

7 (4876 mg/l), Cl7 (4414 mg/l), Na+ (1615 ml/g),Ca+2 (1224 mg/l), and trace metals. OIT of the exposedgeomembrane was measured by standard (Std) and high-pressure (HP) OIT methods. The antioxidant depletionrate in the synthetic MSW leachate was 3.8–7.0 timesfaster than in air and 2.3–3.2 times faster than in water.Predictions of the time for antioxidant depletion weremade using the Arrhenius method. The predictedantioxidant depletion time was at least 40 years at338C and over 150 years at 138C when exposed to

62 Gulec, Edil and Benson


MSW leachate in a liner application (Sangam and Rowe2002).

3. MATERIALS AND METHODS

3.1. Geomembrane

A commercially available 1.5-mm-thick smooth HDPEgeomembrane meeting the criteria in GeosyntheticResearch Institute (GRI) (GRI GM13) was used forthis study. The geomembrane was manufactured as asingle sheet (no co-extrusion) using flat die extrusion.The unexposed geomembrane had a density of 0.95 g/ml,MFI=0.25 g/10 min (mean from four tests per ASTMD 1238; standard deviation=0.01 g/10 min), an Std-OIT of 208 min (average of duplicate tests per ASTM D3895), and an HP-OIT of 484 min (average of duplicatetests per ASTM D 5885). The geomembrane consisted ofa blend of polyethylene, carbon black (2%), and anantioxidant package. Other details of the formulationwere not divulged by the manufacturer.

3.2. Exposure solutions and temperatures

Geomembrane samples were immersed in three differentliquids: synthetic AMD, acidic water, and DI water. Thesample length ranged between 260 and 410 mm and thewidth ranged between 460 and 510 mm. Periodically,samples were removed from the tanks for testing. Afterremoval, the samples were rinsed with deionised waterand dried with a paper towel. Test specimens were cutfrom the samples immediately prior to testing. Theremaining sample was discarded.

A detailed discussion of the solutions is reported byGulec (2003). The synthetic AMD contained Fe(1500 mg/l), Zn (350 mg/l), Cu (35 mg/l), SO4 (4500mg/l) and Ca (200 mg/l), and was prepared usingFeSO4�7H2O, ZnSO4�7H2O, CuSO4 and CaSO4 alongwith H2SO4 to achieve pH 2.1. The acidic water wasprepared using DI water and H2SO4 to pH 2.1, and wasused to distinguish the effects of metals and acidity ongeosynthetic degradation. DI water was used as thereference solution.

Composition of the synthetic AMD was based on areview of the composition of a variety of AMDs reported

in the literature for metallic mine wastes. This reviewidentified the most abundant metals, their concentra-tions, and the relative ratios in AMD (Gulec 2003).Composition of the synthetic AMD is shown in Table 1along with the mean, maximum and minimum concen-trations of key elements in AMDs reported in theliterature. Concentrations of Fe, Zn and Cu wereestimated based on the mean concentrations of Fe, themean Fe :Zn ratio, and a Zn :Cu ratio of 10. Concentra-tions of Ca and SO4 were chosen so that the solutionwould have pH around 2 in equilibrium. The geochem-ical equilibrium speciation model MINTEQA2 (USEPA1991) was used to estimate the Ca and SO4 concentra-tions as well as the final composition and pH of thesolution (Gulec 2003).

Insulated stainless steel tanks equipped with heatersand mixers were used for immersion. The tanks weremaintained at 20, 40 or 608C (one tank for each solutionand each temperature). All metallic items (frames forhanging samples, heaters, and mixers) were made ofstainless steel to prevent corrosion. Lids were placed onthe tanks to minimise evaporation and to minimisediffusion of oxygen so as to prevent iron precipitates. Tofurther limit diffusion of oxygen into the solution, theheadspace was continuously purged with nitrogen andthe surface of the liquid was covered with plastic balls(diameter=15 mm). The solution in each tank was alsoreplaced monthly.

Composition of each solution was monitored (firstweekly and later twice a month) by measuring the pHand electrical conductivity (EC) and by measuring themetals’ concentrations using atomic adsorption spectro-metry. Results of the monitoring showed that thecomposition of the solutions remained essentially con-stant over time. pH was maintained within 2.1� 0.15and EC was maintained within 0.82� 0.05 S/m. Inaddition, the coefficient of variation for the metalsconcentrations was less than 10%. A compilation of allof the analytical data is reported by Gulec (2003).

3.3. Melt flow index (MFI) test

MFI is a simple method to assess the molecular weight ofpolymeric materials and is commonly used as an index ofmolecular weight in chemical compatibility studies of

Table 1. Composition of synthetic AMD and AMDs reported in the literature for metallic mine wastes

Property Units

Synthetic

AMD

Literature AMDs

Mean Maximum Minimum

pH – 2.1 2.4 2.8 2.0

Fe mg/l 1500 1404 5000 198

Zn mg/l 350 747 2400 80

Cu mg/l 35 183 700 8

SO4 mg/l 4500 9013 20000 2700

Ca mg/l 200 421 460 382

Fe :Zn – 4 5 13 0

Zn :Cu – 10 24 206 1

Literature sources for AMD are as follows: Avnesen and Iversen (1997), Christensen and Laake (1996), Crandon

Mining Company (1996), Eidsa et al. (1997), Gauld (1999), Gray (1998), Gusek (1995), Horan (1999), Ing and

Heinrich (1997), Kashir and Yanful (2000).



geosynthetics (Hsuan and Koerner 1998; Maisonneuveet al. 1997; Surmann et al. 1995). The MFI protocoldefined by ASTM D 1238 consists in measuring theamount of molten polymer (1908C) that is extrudedthrough an orifice in 10 min under a constant load of2.16 kg. Chain scission reactions, which are one of themost important consequences of degradation, producesmaller polymer molecules (Grassie and Scott 1985).This change in molecular size is reflected as higher MFI.

The MFI tests were conducted by Dickten & MaschInc. of Nashotah, WI. Typically one specimen (30 g) wastested per sample. However, the tests were repeated (aduplicate test was run) when unusual MFIs wereobtained (e.g. the MFI differed from the mean by 3 ormore standard deviations, a large jump in MFI occurredof a short period of exposure, or the MFI deviatedappreciably from the trend).

3.4. Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared spectroscopy (FTIR) is aspectroscopic method used to detect structural changesin polymeric materials at the molecular level. A polymerspecimen is subjected to infrared radiation in succes-sively decreasing frequencies. The amount of infraredradiation absorbed at each frequency is indicated in aspectrum. Peaks in the FTIR spectrum generallycorrespond to functional groups [e.g. methylene(–CH2–) or methyl (–CH3–)] that vibrate in a specificmode at a particular frequency. Polymer structure isdetermined by identifying the peaks using catalogues.Changes observed in the spectrum can be used asindicators of degradation (Dudzik and Tisinger 1990;Duquennoi et al. 1995; Maisonneuve et al. 1997; Tisinger1989).

The FTIR analyses were conducted by Dickten &Masch Inc. of Nashotah, WI. A small specimen (20–30 mm square) was cut from an exposed sample foranalysis. The specimen was sealed in a plastic bag andshipped to the laboratory. The sample was further cutacross its thickness by Dickten & Masch prior to FTIRanalysis. The FTIR analysis was conducted with aNicolet Magna FTIR 550 spectrometer equipped with amicro attenuated total reflectance (ATR) lens and aMagna 550 infrared (IR) bench.

3.5. Oxidation induction time (OIT) by differential

scanning calorimetry (DSC)

OIT was measured with differential scanning calorimetry(DSC), a common thermoanalytical method used tomeasure OIT (Dudzik and Tisinger 1990; Hsuan andKoerner 1995, 1998; Maisonneuve et al. 1997; Sangamand Rowe 2002; Surmann et al. 1995; Tisinger 1989).Most of the OIT analyses were conducted using thestandard method (Std-OIT) in accordance with ASTMD 3895. Some were conducted using the high-pressuremethod (HP-OIT) in accordance with ASTM D 5885.Specimens approximately 20–30 mm square were cutfrom an exposed sample for OIT testing.

Std-OIT was measured at the University of Wisconsin-Madison (UW) using a NETZSCH DSC 200 differential

scanning calorimeter (DSC). The only deviation fromASTM D 3895 was that the gas flow rate was limited to35–40 ml/min owing to the capacity of the DSC (D 3895requires 50 ml/min). A 5–10 mg specimen was heated to2008C at a rate of 208C/min in a nitrogen atmosphere.After 2008C was reached, the specimen was maintainedin an isothermal state for 5 min. Then the purge gas waschanged from nitrogen to oxygen, and the change inenthalpy was recorded. The test was terminated when anexothermal peak was detected.

To evaluate the importance of the non-standard flowrate, one sample of unexposed geomembrane was sent tothe Geosynthetic Research Institute (GRI) at DrexelUniversity for testing at the standard flow rate. Tworeplicate tests were conducted on the sample by GRI.Companion tests were conducted on the same sample atUW. GRI reported OITs of 204 and 211 min for theunexposed geomembrane, whereas the OIT determinedat UW was 266 min. Comparison of these tests indicatesthat use of the non-standard flow rate resulted in anoverestimate of the Std-OIT by approximately 20%.

Additional tests were conducted with HP-OIT toverify that the higher temperature employed in the Std-OIT tests was not outside the effective range of theantioxidants in the geomembrane (the antioxidants usedin the geomembrane are confidential and were notdivulged by the manufacturer). For example, hinderedamines and thiosynergists will volatilise or degrade in anStd-OIT test (Hsuan and Guan 1997), which isconducted at temperatures above their effective range(0–1508C for hindered amines and 0–2008C for thiosy-nergists). Standard OIT and HP-OIT tests are similar,except that relatively high gas pressure and lowtemperature are used in HP-OIT tests (specimens areoxidised at 2008C and 35 kPa in Std-OIT, whereas 1508Cand 3500 kPa are used in HP-OIT). The HP-OIT testswere conducted in accordance with ASTM D 5885 byDickten & Masch, Inc. of Nashotah, WI.

Std-OIT and HP-OIT were both conducted on sixspecimens of geomembrane (two unexposed and fourexposed to AMD at 608C for various durations). Therelationship between HP-OIT and Std-OIT for thesespecimens is shown in Figure 2. The relationship islinear, as was observed by Hsuan and Koerner (1998)and Sangam and Rowe (2002) for other HDPEgeomembranes. A linear relationship between HP-OITand Std-OIT indicates that the high temperature in theStd-OIT test does not destroy the antioxidants, and thateither method can be used to monitor depletion ofantioxidants (Hsuan and Koerner 1998). The linearrelationship also suggests that the antioxidant packageused in the geomembrane is phosphites and phenols,which have effective temperature ranges of 150–3008Cand 0–3008C respectively (Hsuan and Koerner 1998).

The ratio of Std-OIT to HP-OIT for the unexposedgeomembrane was also calculated and compared withratios reported in the literature for HDPE geomem-branes. The Std-OIT of the unexposed geomembrane(208 min) measured at GRI was used to precludeambiguities caused by the non-standard flow rate used



at UW. The ratio for the data in Figure 2 is 2.3, whichis comparable to ratios (2.1 and 2.8) reported by Hsuanand Guan (1997), Hsuan and Koerner (1998) andSangam and Rowe (2002) for five HDPE geomembraneswith hindered phenols and phosphites as antioxidants.

4. RESULTS AND DISCUSSION

4.1. Melt flow index (MFI)

MFIs are presented in Figure 3 for the geomembraneexposed to DI water and the simulated AMD solution.Each point in Figure 3 corresponds to a single MFI test.Geomembranes exposed to acidic water were not testedowing to budget limitations. The mean MFI of theunexposed geomembrane is shown as the solid horizon-tal line. One and two standard deviations from the meanare shown as dashed horizontal lines. For normallydistributed data, one standard deviation from the meanencompasses 68% of the dispersion in the data, and twostandard deviations from the mean encompass 95% ofthe dispersion. MFIs falling within the lines correspond-ing to two standard deviations generally are consistentwith the scatter associated with the tests on unexposedgeomembrane, and thus are not statistically differentfrom the MFI for unexposed conditions.

Nearly all of the MFIs for DI water exposure fallwithin the lines for two standard deviations, regardlessof the temperature (Figure 3a). The only exception is onereplicate at 208C. This indicates that exposure in DIwater had no statistically significant effect on the bulkpolymer properties over the exposure period. However,the data do exhibit an upward trend between 12 and 21months. Thus MFIs outside the normal scatter mighthave been obtained had the geomembrane been im-mersed longer, or if tests were conducted on polymerclose to the surface of the geomembrane (i.e. whereinteractions between the solution and polymer may begreatest). Temperature appears to have little effect on the

MFI as well. Slightly higher MFIs were obtained at 608Cduring the first 12 months, but the MFIs for 208C areslightly higher than those at 608C after 12 months.

Several MFIs of the geomembrane exposed to AMDfall above the line corresponding to two standarddeviations above the mean (Figure 3b). However,duplicate tests on these samples yielded lower MFIs aswell. Thus these large MFI do not necessarily imply thatchanges in the polymer occurred due to AMD exposure.However, the MFIs for AMD exposure generally arehigher than those for exposure to DI water, suggestingthat exposure to AMD caused some alteration of thepolymer.

Analysis of variance (ANOVA) was used to furtherevaluate whether any of the treatments had statisticallysignificant effects on MFI. In ANOVA, the F-statistic isused to determine whether any of the treatments had asignificantly significant effect (Box et al. 1978). Acomparison is made between the F-statistic computedusing the MFI data for each treatment and a criticalvalue (Fa) corresponding to a significance level a. IfF < Fa, then the null hypothesis (no difference betweenthe means of the groups being compared) is accepted.

0 50 100 150 200 250 300

Standard pressure-OIT (min)

500

400

300

200

100

Hig

h pr

essu

re-O

IT (

min

)

Figure 2. Comparison of OIT of HDPE geomembrane

determined by high-pressure (ASTM D 5885) and standard

(ASTM D 3895) OIT methods

0.20

0.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.30

0 3 6 9 12 15 18 21 24

0.20

0.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.30

0 3 6 9 12 15 18 21 24

(a)

(b)

Time (months)

Mel

t flo

w in

dex

(g/1

0 m

in)

Mel

t flo

w in

dex

(g/1

0 m

in)

Time (months)

20°C40°C60°C

20°C40°C60°C

m + 2s

m + s

m

m − s

m − 2s

m + 2s

m + s

m

m − s

m − 2s

Figure 3. MFI against time for geomembrane samples exposed

to: (a) DI water at 20, 40 and 608C; (b) AMD at 20, 40 and 608C



For this analysis, each of the six treatments (AMD at 20,

40 and 608C and DI water at 20, 40 and 608C) was

treated as a group, and the MFIs for each treatment

were compared with each other. The significance level, a,was set to 0.05, which is the conventional significance

level used in hypothesis testing (Box et al. 1978). The

ANOVA yielded F=2.085, whereas F0.05=2.43

(F<Fcr). That is, no statistically significant difference

exists between the mean MFIs corresponding to the six

different treatments (DI water and AMD exposure at 20,

40 or 608C).Linear regression of MFI on time was used to evaluate

whether a significant temporal trend existed in the MFI

for any of the exposure conditions. In a regression

analysis, the p-statistic is used to determine whether the

slope of the regression (i.e. the temporal trend) is

statistically different from zero. A p-statistic less than

the significance level a indicates a statistically significant

temporal trend. Results of the linear regression are

presented in Table 2 for a=0.05. p-statistics higher than

0.05 (no temporal trend in MFI) exist for all exposure

conditions except DI water at 208C and AMD at 408C.However, for each of these two treatments, the last data

point (21 months) is largely responsible for the inference

of statistical significance. Thus a definitive inference

cannot be made that the trends in these two data sets are

truly significant.

4.2. FTIR spectra

Spectra for the geomembrane samples exposed to AMD

at 608C for 6, 12 and 18 months are shown in Figure 4

along with the spectrum of the unexposed geomembrane.

AMD exposure at 608C was the most aggressive

treatment and therefore was chosen for FTIR testing.

Other exposure times and treatments were not tested

owing to budget limitations.

The spectra at 6, 12 and 18 months of AMD exposure

at 608C (Figures 4b, c and d) are consistent with the

spectrum of the unexposed geomembrane (Figure 4a),

except for one small absorption peak below 1750 cm71

Table 2. Slopes and p-statistics from linear regression analysis of

MFI data

Treatment

Slope p-valueExposure

solution

Exposure

temperature

(8C)

DI 20 0.0016 0.047

DI 40 0.0005 0.076

DI 60 0.0005 0.185

AMD 20 0.0010 0.133

AMD 40 0.0012 0.039

AMD 60 0.0005 0.556

(a)

(b)

(c)

(d)

4000 3000 2000 1000 500

0.2

0.6

0.4

0.8

0.0

0.2

0.6

0.4

0.8

0.0

0.2

0.6

0.4

0.8

0.0

0.2

0.4

0.0

Abs

orba

nce

Wave number (cm−1)

Figure 4. FTIR spectra of: (a) unexposed geomembrane; (b) geomembrane exposed to AMD at 608C for 6 months; (c) geomembrane

exposed to AMD at 608C for 12 months; (d) geomembrane exposed to AMD at 608C for 18 months. Arrows indicate extra peaks

observed in geomembrane exposed for 18 months



in the 6-month spectrum and four additional smallabsorption bands (the most significant of which appearnear 3200 cm71; other bands appear near 1100, 900 and800 cm71) in the 18-month spectrum. The small peakobserved in the 6-month spectrum corresponds to acarbonyl (C=O) linkage, which suggests that oxidationhas occurred in the polymer structure. However, thispeak was not observed in the longer exposure times (i.e.12 and 18 months). The extra peak at 6 months may be alocal effect, but a definitive conclusion cannot be madebecause replicate FTIR analyses were not conductedowing to budget limitations. Nevertheless, the absence ofmajor changes in the spectra suggests that exposure toAMD at 608C did not significantly change the bulkpolymer structure over 18 months.

The small absorption bands observed at 3200, 1100,900 and 800 cm71 in the 18-month spectrum are notassociated with the wavelengths at which polymeroxidation peaks occur, and are believed to be associatedwith mixed metal oxides that precipitated on the surfaceof the geomembrane during exposure to the AMD. TheAMD solution was rich in metals, and precipitation ofsome metal oxides probably occurred despite the meas-ures taken to prevent the ingress of oxygen. However,types of metal oxides cannot be identified by FTIR. Thusthe AMD solution cannot be confirmed as the source ofthese peaks.

4.3. Oxidation induction time

OITs of the geomembrane exposed to the AMD at 20, 40and 608C and DI water and acidic water at 608C areshown in Figure 5. Most of the data in Figure 5 are forsamples exposed at 20, 40 or 608C for up to 21 months.However, an additional set of samples was exposed at808C towards the end of the study to assess the effects ofhigher temperature. The samples at 808C were onlyexposed for 10 weeks.

OIT decreases with exposure time and at a greater rateat higher temperature (Figure 5a), as has been observedby others (Hsuan and Guan 1997; Hsuan and Koerner1995; Sangam and Rowe 2002). Comparison of anti-oxidant depletion curves for AMD, acidic water and DIwater exposures at 608C in Figure 5b indicate that thelargest and most rapid reductions in OIT occurred in theAMD, followed by the acidic water, and DI water.

None of the treatments completely depleted theantioxidants during the exposure period, which isconsistent with the lack of oxidation products in theFTIR spectra. Moreover, as described in a companionpaper (Gulec et al. 2004), no statistically significantchange in the bulk mechanical properties of thegeomembrane (trapezoid tear strength, puncturestrength, and wide-strip tensile strength) occurred duringthe 22 months of exposure. The absence of change in themechanical properties is also consistent with incompletedepletion of the antioxidants and the lack of oxidationproducts in the FTIR spectra. Theoretically, degradationof mechanical properties caused by alterations in thepolymer occurs after the antioxidants are depleted(Hsuan and Guan 1997).

Graphs of lnOIT against time are shown in Figure 6.The relationship between lnOIT and time is approxi-mately linear, suggesting that depletion of the antiox-idants can be approximated as a first-order process (i.e.exponential depletion in time). Hsuan and Koerner(1995) and Sangam and Rowe (2002) also observed first-order degradation in their antioxidant depletion experi-ments on HDPE geomembranes. The first-order reactionrate (S) is the slope of the line of lnOIT against time.First-order reaction rates (S) for each exposure con-dition were obtained by linear regression. A summary ofdepletion rates and the corresponding coefficients ofdetermination (R2) is given in Table 3.

Comparison of the depletion rates in Table 3 indicatesthat temperature had a significant effect on antioxidantdepletion. For example, the depletion rate for AMD at608C is 2.2 times higher than that at 408C and 17.6 timeshigher than that at 208C. The chemistry of the solution isalso influential. At 608C the depletion rate of AMD is

0

50

100

150

200

250

300

0 3 6 9 12 15 18 21 24

0

50

100

150

200

250

300

0 3 6 9 12 15 18 21 24

(a)

(b)

Exposure time (months)


Std

-OIT

(m

in)

Std

-OIT

(m

in)

Fit of first-order degradationequation, OITt = OIT0 e−St

Fit of first-order degradationequation, OITt = OIT0 e−St

DI - 60°C

Acid - 60°C

AMD - 60°C

20°C

40°C

60°C

80°C

Figure 5. Std-OIT against time for geomembrane exposed to: (a)

AMD at 20, 40 and 608C; (b) AMD, acidic water and DI water

at 608C. OIT0=OIT of unexposed geomembrane. OITt=OIT

of geomembrane after exposure time t



1.8 times higher than the depletion rate for acidic waterand 1.3 times higher than that for DI water. Tempera-ture had a stronger effect than the solution chemistrybecause of the broad range of the temperatures that wereused. However, temperature is expected to have less

effect in a mine waste containment application wheretemperature variations are tempered by burial beneaththe waste.

The higher depletion rate for AMD exposure relativeto that for acidic water (1.3 times) may be due to themetals in the AMD solution. As indicated in Osawa andIshizuka (1973), metals have a catalytic effect on theoxidative degradation of polypropylene. However, de-pletion of antioxidants by extraction is believed to be thedominant depletion mechanism in this study owing tothe limited amount of oxygen in the tanks. The absenceof extensive carbonyl peaks in the FTIR spectra (Figure4) also suggests that oxidation was not appreciable.

The depletion rates for AMD are shown in Table 4with those for MSW leachate reported by Sangam andRowe (2002). At comparable temperatures, the depletionrate for exposure to MSW leachate is higher than thatfor AMD exposure (Table 4). For example, at 208C thedepletion rate for MSW is 3.7 times higher than the ratefor AMD exposure. That is, MSW leachate appearsto be more aggressive in depleting antioxidants thanAMD. Inherent in this inference is an assumption thatthe geomembrane tested in this study is identical to thegeomembrane tested by Sangam and Rowe (2002).The exact properties of both geomembranes are notknown, but the geomembranes have similar density (0.94versus 0.95 g/ml), similar Std-to-HP OIT ratios (2.1 to2.8), and similar OIT depletion rates in water (seesubsequent discussion). Thus the relative effects ofexposure to AMD and MSW leachate are believed tobe reliable.

5. PREDICTIONS OF ANTIOXIDANT

DEPLETION TIME

Predictions of antioxidant depletion time were madeusing Arrhenius modelling, which is the most widelyused method to predict the lifetime of polymericmaterials (Hsuan and Koerner 1998). The approach isdescribed in ISO Standard 11346. The Arrheniusequation is

S ¼ A e� Ea=RTð Þ ð7Þ

where S is the depletion rate, A is a constant, Ea is theactivation energy, R is the gas constant, and T is

0 3 6 9 12 15 18 21 24

0 3 6 9 12 1 5 18 21 2 4


(b)

(a)

20°C

40°C

60°C

80°C

First-order degradationequation:

ln OITt = ln OIT0 − St

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

ln O

IT (

min

)


DI - 60°C

Acid - 60°C

AMD - 60°C

ln O

IT (

min

)

6.0

5.5

5.0

4.5

4.0

3.5

Figure 6. Logarithm of Std-OIT against time for geomembrane

exposed to: (a) AMD at 20, 40 and 608C; (b) AMD, acidic water

and DI water at 608C

Table 3. Antioxidant depletion rates for HDPE geomembrane in AMD, acidic water and

DI water

Immersion

medium

Temperature

(8C)Depletion rate

(1/months) R2

20 0.0051 0.87

AMD40 0.0480 0.96

60 0.0906 0.96

80 1.2056 0.99

Acidic water 60 0.0708 0.98

DI water 60 0.0514 0.98



absolute temperature (8K). The Arrhenius equation canalso be expressed as

lnS ¼ lnA�Ea

R

� �1

T

� �ð8Þ

The depletion rates in Table 3 were used to developArrhenius plots (lnS against 1/T) for exposure to water(Figure 7a) and AMD (Figure 7b). Data reported byHsuan and Koerner (1995) and Sangam and Rowe(2002) were also used when preparing the Arrhenius plotfor water exposure. Parameters of the Arrheniusequations were obtained by linear regression of lnS on1/T, and are shown on the graphs in Figure 7. The datain the Arrhenius plot for water immersion (Figure 7a)are remarkably consistent considering that differentHDPE geomembranes were used in each study. Thesimilarity of these data sets also suggests that thegeomembrane tested in this study is similar to thegeomembranes tested by Hsuan and Koerner (1995) andSangam and Rowe (2002). The geomembrane tested byHsuan and Koerner (1995) has the same density as thegeomembrane used in this study, which also suggeststhat the geomembranes are similar.

Parameters of the Arrhenius equation for AMD wereobtained by linear regression using the data for 20, 40and 608C. The data point for 808C was not included inthe regression because of the short period of exposure(10 weeks). Nevertheless, the data point at 808C isconsistent with the rest of the data.

The Arrhenius parameters in Figure 7 were used withthe first-order degradation equation to estimate antiox-idant depletion times for water and AMD exposure atfield temperatures between 10 and 358C. The antioxidantdepletion time for MSW leachate was also estimatedusing the Arrhenius parameters reported by Sangam andRowe (2002) (lnA=13.77 and Ea/R=43.3 kJ/mol).First-order depletion and a final OIT=0.5 min (pureunstabilised HDPE; Hsuan and Koerner 1997) wereassumed. The initial OIT was set at 100 min, which fallswithin the range of the initial OITs for HDPEgeomembranes reported by Hsuan and Guan (1997)(112–156 min), Hsuan and Koerner (1997, 1998)(80.5 min), and Sangam and Rowe (2002) (133 min).The antioxidant depletion rates were assumed to beindependent of the initial OIT.

Estimated antioxidant depletion times for water,

AMD and MSW leachate are shown in Figure 8 as a

function of temperature. At a given temperature, the

antioxidant depletion time is longest for exposure to

Table 4. Comparison of depletion rates for AMD exposure to depletion rates reported in the literature for

immersion in MSW leachate

Reference

Immersion

medium

Immersion

temperature (8C)Depletion rate

(1/months)

22 0.0188

Sangam and Rowe (2002) MSW leachate40 0.0886

55 0.1504

85 0.4074

20 0.0051

Current study AMD40 0.0480

60 0.0906

80 1.2056

Hsuan and Koerner (1997)

Sangam and Rowe (2002)

Current study

ln S = 15.79 − 6228(1/T )

R2 = 0.99

SE = 0.12 min/month

Ea = 51.75 kJ/mol

A = 7.2 × 106 min/month

−1

−2

−3

−4

−5

−6

1

0

−1

−2

−3

−4

−5

−6

ln S

ln S

0.0026 0.0028 0.0030 0.0032 0.0034 0.0036

1/T (1/°K)

(a)

ln S = 19.16 − 7099(1/T )

R2 = 0.93

SE = 0.62 min/month

Ea = 58.9 kJ/mol

A = 2.1 × 108 min/month

0.0026 0.0028 0.0030 0.0032 0.0034 0.0036

1/T (1/°K)

(b)

Figure 7. Arrhenius plot of antioxidant depletion for: (a) water;

(b) AMD exposure. Data for water are from tests conducted by

Hsuan and Koerner (1997), Sangam and Rowe (2002), and

current study. SE=standard error



water and shortest for exposure to MSW. The lifetimefor AMD falls between those for water and MSWleachate. For typical environment temperatures atdepth (15–258C, Lundy 1981), the antioxidant depletiontime ranges between 45 and 105 years for AMDexposure.

6. PRACTICAL IMPLICATIONS

Estimates of the antioxidant depletion times of anHDPE geomembrane in a mine waste containmentfacility were made assuming that the leachate iscomparable to the synthetic AMD used in this study,the field temperature is between 15 and 258C (Lundy1981), and the geomembrane is exposed to AMD ononly one side (i.e. the interior of the containment facility)

or both sides. These depletion times can be considered as

conservative estimates of the lifetime of the geomem-

brane. The actual lifetime will likely be longer because

the induction time and degradation period have not been

included (Hsuan and Koerner 1998).

No one-sided exposure tests were conducted in this

study. Therefore the effect of one-sided exposure relative

to complete immersion was estimated by reviewing data

reported by Hsuan and Koerner (1995) and Sangam and

Rowe (2002). A comparison of depletion rates for

immersion and one-sided exposure to water is given in

Table 5. Sangam and Rowe (2002) report rates for

immersion in air and water, but not for one-sided

exposure. Thus depletion rates for one-sided exposure

based on the data by Sangam and Rowe (2002) were

computed by averaging the depletion rates for liquid and

air immersion.

The ratio of the depletion rates for one-sided exposure

to immersion in water ranges from 0.28 to 0.34 for the

data by Hsuan and Koerner (1997) and from 0.73 to 0.74

for the data by Sangam and Rowe (2002). The ratios

obtained using the data from Hsuan and Koerner (1997)

are believed to be more representative because they are

based on actual one-sided tests rather than on the

average of ratios from air and liquid immersions. Thus

the ratios for the data by Hsuan and Koerner (1995)

were used in the assessment of depletion time.

Estimated antioxidant depletion times for HDPE

geomembranes exposed to AMD in the field are shown

in Table 6. The calculations in Table 6 were made

assuming first-order depletion for initial OITs of 100

and 200 min for one-sided exposure and immersion.

Depletion rates for one-sided exposure were calcu-

lated by multiplying depletion rates for immersion in

AMD by the ratios for the data from Hsuan and

Koerner (1995) in Table 5 at the corresponding

temperatures.

The antioxidant depletion lifetimes in Table 6 range

from 46 to 426 years depending on the field temperature,

the initial OIT, and the exposure condition. These

Water

AMD

MSWleachate

5 10 15 20 25 30 35 40

Field temperature (°C)

250

200

150

100

50

0

Ant

ioxi

dant

dep

letio

n tim

e (y

r)

Figure 8. Antioxidant depletion time against field temperature of

geomembrane exposed to water, synthetic MSW leachate and

synthetic AMD. Curve for water is based on the collective data

presented in Figure 7a. Curve for MSW leachate is based on

Sangam and Rowe (2002). Curve for AMD is based on data from

the current study

Table 5. Comparison of depletion rates reported for liquid immersion and one-sided exposure

Reference Exposure method

Assumed field

temperature(a) (C)

Depletion rate

(1/months) One-sided/immersion ratio

Hsuan and Koerner

(1995)

15 0.00529158C 0.28

Water immersion 20 0.00720

25 0.00969208C 0.33

Water above/air below 15 0.00150

(under 260 kPa 20 0.00236258C 0.34

compression) 25 0.00329

Sangam and Rowe

(2002)

15 0.00295 158C 0.73

Water immersion 20 0.00428

25 0.00614208C 0.74

15 0.00216

Water–air average 20 0.00315258C 0.7425 0.00453

(a)Not an immersion temperature. The depletion rates are calculated by extrapolation.



estimates consider only antioxidant depletion time.Therefore actual degradation of the engineering proper-ties may take longer than reported in Table 6.

If one-sided exposure is assumed, then the estimateddepletion time ranges between 136 and 377 years for aninitial OIT of 100 min and between 154 and 426 years foran initial OIT of 200 min. These depletion times arelonger than those commonly assumed in design (e.g. 30to 150 years), but are shorter than the reactive lifespan ofmany mine wastes. For example, Blowes and Jambor(1990) estimate that sulfide oxidation contributing toAMD will continue at the Waite Amulet tailingsimpoundment in Quebec, Canada, for more than 500years.

7. SUMMARY AND CONCLUSIONS

This paper describes a study that was conducted toevaluate how exposure to acid mine drainage (AMD)from metallic mine wastes affects the polymer propertiesof HDPE geomembranes. Geomembrane samples wereimmersed in synthetic AMD, acidic water, and DI waterin tanks maintained at 20, 40 and 608C. Periodically,samples of each geosynthetic were removed from thesolution and tested to determine the melt flow index(MFI), Fourier transform infrared spectrum (FTIR),and oxidation induction time (OIT).

Results of the MFI and FTIR tests showed thatexposure to AMD or DI water had little effect on thepolymer properties. No consistent changes in thepolymer structure were observed. However, the MFIdid appear to increase gradually, and greater changesmight have been observed had the geomembrane beenimmersed longer.

OIT decreased with exposure time for all exposureconditions and approximately followed a first-orderdepletion rate. The depletion rate also increased as thetemperature increased, and was greatest for exposure toAMD. Comparison of antioxidant depletion rates forAMD exposure with those reported for MSW leachateindicates that the depletion rate for exposure to MSWleachate exposure is two to four times higher than therate for AMD exposure.

An analysis was conducted to estimate the antioxidantdepletion time of HDPE geomembranes in mine wastecontainment facilities. The antioxidant depletion timeranged from 46 to 426 years, depending on the field

temperature, the initial OIT, and exposure condition. Ifone-side exposure is assumed, then the antioxidantdepletion time should exceed 136 years. This antioxidantlifetime is greater than those commonly assumed duringdesign, but is shorter than the reactive lifetime of somemetallic mine wastes. However, these lifetime estimatesconsider only the antioxidant depletion time and do notaccount for the induction time and time required fordegradation of engineering properties.

ACKNOWLEDGEMENTS

This study was sponsored by the State of Wisconsin’sGroundwater Research Advisory Council, which isadministered through the Water Resources Institute atthe University of Wisconsin-Madison. The geomem-brane used in this study was donated by GSE. Y. G.Hsuan of Drexel University provided the OIT testsconducted at the Geosynthetic Research Institute. M.Fredrickson and B. Trzebiatowski are acknowledged fortheir help throughout the study.

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