Barriers composed of sodium bentonite (Na-bentonite) are used tocontrol flow and contaminant transport because Na-bentonite haslow hydraulic conductivity to water. Montmorillonite, a member ofthe smectite group of clay minerals (Grim 1968), is the primarymineral within Na-bentonite. Effectiveness of bentonite as a con-tainment barrier depends on the osmotic absorption of water mol-ecules between montmorillonite platelets, which is manifest asswelling at the macroscale (McBride 1994). These bound watermolecules constitute an immobile water phase that reduces the sizeand number of hydraulically active pores that conduct flow andtransport (Jo et al. 2001; Kolstad et al. 2004).
Conditions that promote osmotic absorption of water in Na-bentonite result in greater swell and lower hydraulic conductivity(Jo et al. 2005; Guyonnet et al. 2005; Benson andMeer 2009; Scaliaand Benson 2011). The extent of osmotic swelling depends on the
ionic strength of the hydrating solution and the preponderance ofmonovalent cations, such as sodium (Na1), in the exchange complex(i.e., the collection of cations adsorbed to the mineral surface) (Joet al. 2001; Kolstad et al. 2004). If multivalent cations (i.e., valence$ 21) dominate the exchange complex, or if the hydrating solutionhas high ionic strength, osmotic swelling does not occur, and thehydraulic conductivity is higher (Shackelford et al. 2000; Jo et al.2001, 2005; Kolstad et al. 2004; Katsumi et al. 2008). In contrast,low hydraulic conductivity is realizedwhen the exchange complex iscomposed predominantly of monovalent cations and the ionic strengthis low to modest.
The Na1 in the exchange complex of Na-bentonite is thermo-dynamically unstable in environments where multivalent cations(e.g., Ca21) are present (Sposito 1984), including most naturally oc-curring pore waters and many leachates (Scalia and Benson 2010;Bradshaw et al. 2013). When present, multivalent cations replacemonovalent cations originally comprising the exchange complex,thereby reducing or eliminating osmotic swell and increasing hy-draulic conductivity (Jo et al. 2001, 2005; Vasko et al. 2001; Kolstadet al. 2004; Lee and Shackelford 2005; Scalia and Benson 2010,2011).
Numerous laboratory studies on thin (∼10mm) Na-bentonitebarrier layers, viz, geosynthetic clay liners (GCLs), have demon-strated that Ca21-for-Na1 exchange results in reduced swelling, asquantified by the swell index test (ASTMD5890; ASTM2006c) andhigher hydraulic conductivity (Lin and Benson 2000; Jo et al. 2001,2005; Egloffstein 2001; Shackelford and Lee 2003; Kolstad et al.2004; Lee and Shackelford 2005; Lee et al. 2005; Meer and Benson2007; Benson and Meer 2009; Scalia and Benson 2010, 2011;Bradshaw et al. 2013). Field studies have shown that GCLs placedin contact with soils generally will undergo Ca21-for-Na1 exchange(Aboveground Tank Update 1992; James et al. 1997; Benson et al.2004, 2007; Meer and Benson 2007; Scalia and Benson 2011), as
1Senior Associate, Exponent, 15375 SE 30th Pl., Suite 250, Bellevue,WA 98007 (corresponding author). E-mail: [email protected]
2Wisconsin Distinguished Professor and Chair, Geological Engineering,Univ. of Wisconsin, Madison, WI 53706. E-mail: [email protected]
3University Fellow, Univ. of Wisconsin, Platteville, WI 53818. E-mail:[email protected]
4Professor Emeritus, Geological Engineering, Univ. of Wisconsin, Mad-ison, WI 53706. E-mail: [email protected]
5Professor, Civil and Environmental Engineering, Colorado State Univ.,Fort Collins, CO 08532. E-mail: [email protected]
Ca21 in the surrounding soil pore water migrates into the GCL inresponse to hydraulic and/or chemical gradients (Meer and Benson2007; Bradshaw et al. 2013). Dissolution of calcite (CaCO3) as anaccessory mineral in bentonite (Shackelford et al. 2000; Kolstadet al. 2004; Jo et al. 2005; Guyonnet et al. 2005) can also be a sourceof Ca21 for exchange (Guyonnet et al. 2005), as can leachatesencountered during service (Bradshaw et al. 2013). Cation exchangeoften occurs slowly because the rate of exchange is controlled by therate at which multivalent cations diffuse from the bulk pore waterinto the interlayer space (Jo et al. 2005, 2006). Regardless of the sourceof divalent cations (soil pore water, native calcite, or leachate), ex-change of multivalent cations for Na1 is inevitable in Na-bentonite inthe long term and can result in increases in hydraulic conductivityunder certain conditions.
The sensitivity of Na-bentonite to chemical interactions hasspurred development of amended bentonites for hydraulic barrierapplications [Onikata et al. 1996, 1999; B. Flynn and G. Carter,“Waterproofing material and method of fabrication thereof,” U.S.Patent No. 6,537,676 B1 (1998); Boels and Van Der Wal 1999;Trauger and Darlington 2000; Katsumi et al. 2001, 2008; Schroederet al. 2001;Ashmawy et al. 2002;Kolstad et al. 2004;Guyonnet et al.2009; Di Emidio et al. 2010, 2011; Mazzieri et al. 2010; Naismithet al. 2011; Scalia et al. 2011; Bohnhoff et al. 2013]. Most amendedbentonites are hydrated with an organic solution such as propylenecarbonate (Onikata et al. 1996; Onikata et al. 1999; Katsumi et al.2001; Kolstad et al. 2004; Katsumi et al. 2008) or carboxylmethylcellulose and methanol [B. Flynn and G. Carter, “Waterproofingmaterial and method of fabrication thereof,” U.S. Patent No.6,537,676 B1 (1998); Schroeder et al. 2001; Katsumi et al. 2008;Di Emidio et al. 2010, 2011; Mazzieri et al. 2010] to activate andmaintain osmotic swelling in solutions with ionic strengths thatwould preclude osmotic swell in unmodified bentonite.
TraugerandDarlington(2000) developed an amended bentonite forhydraulic barrier applications by polymerizing an organic monomer inNa-bentonite slurry and referred to this material as bentonite-polymeralloy (BPA). The BPA slurry was embedded in a needle-punchednonwovengeotextile (GT) andpermeatedwith seawater (ionic strengthand cation composition not provided). The hydraulic conductivity ofthe BPA-GT was 53 10212 m=s, compared with 23 1028 m=s fora GCL containing natural Na-bentonite permeated with the samesolution. X-ray diffraction (XRD) of dried BPA showed an increase ininterlayer spacing from 0.35 nm (typical for dry montmorillonite) to1.0–1.5 nm, indicating that the polymer was intercalated in themontmorillonite interlayer. The anionic polymer in BPA was hy-pothesized to bond with Na1 ions in the montmorillonite interlayer.
A second generation of BPA was investigated in this study. Na-bentonite was modified by intercalation and in situ polymerizationof acrylic acid to form Na-polyacrylate within and outside the in-terlayer. Polyacrylate was chosen because polyacrylate was expec-ted to interact with Na-bentonite via hydrogen bonding and isrelatively inexpensive, not readily biodegradable, and similar to thehighly effective superabsorbent and superswelling polymers used in
baby diapers (Buchholz and Graham 1998). This bentonite-polymercomposite (BPC), which is also known as bentonite-polymernanocomposite (BPN) (Scalia et al. 2011; Scalia 2012; Bohnhoffet al. 2013), was subjected to a series of tests to evaluate BPCsefficacy in inorganic solutions known to affect Na-bentonite ad-versely. Similar tests were conducted on Na-bentonite and super-absorbent polypolyacrylate (SAP). Long-term hydraulic conductivitytests and swell index tests were conducted using aqueous solutionshaving a range of Ca21 concentrations (5, 20, 50, 200, and 500 mMCaCl2) and extreme pH (0.3 and 13.1). Hydraulic conductivity testswere conducted on a thin layer (,6mm dry thickness) of granularmaterial (BPC, Na-bentonite, or SAP) mimicking a GCL underlower stress (20-kPa effective stress).
Materials
BPC
The BPC was created using a method similar to that used for con-ventional polymer composites (Muzny et al. 1996). Powdered Na-bentonite, referred to as the base bentonite (BB), was mixed intoa monomer solution prepared by dissolving acrylic acid in water,followed by slow neutralization with sodium hydroxide (NaOH) toallow dissipation of the heat of neutralization. The BB was added tothe neutralized solution in concentrations between 30 and 50% byweight to form a bentonite-monomer slurry and vigorously agitatedto increase the surface area available for polyacrylate adsorption.Sodium persulfate (Na2S2O8) was added as a thermal initiator.Laboratory safety procedures were followed commiserate with thematerial safety data sheets (MSDSs) of each material.
Once the bentonite-monomer slurry was prepared, polymeriza-tion was initiated by raising the slurry temperature above the de-composition temperature of the initiator, causing the initiator todecompose and form free radicals. During polymerization, freeradicals (R•) attack the double bond of the acrylic acid monomer toform new free radicals (RM•), which then react with additionalmonomers to propagate the polymer chain (RMMM•). After poly-merization, the solution was oven dried at 105�C to remove freewater, milled, and screened to match the gradation of Na-bentonitegranules in theGCL described in Shackelford et al. (2000). The BPCand BB used in this study were provided by Colloid EnvironmentalTechnologies Company (CETCO) of Hoffman Estates, Illinois.Properties of both are shown in Tables 1 and 2. The mineralogicalcomposition of the BB determined by XRD is 73–77% montmo-rillonite, 15–17% quartz, 4–5% plagioclase feldspar-andesine, and,3% illite/mica, heulandite, clinoptilolite, and calcite.
Na-Bentonite
A natural Na-bentonite used for GCLs was used in this study for com-parative testing. Properties of theNa-bentonite are in Tables 1 and 2.The Na-bentonite was provided by CETCO, is used in Bentomat
Table 1. Properties of Na-Bentonite (Na-B) Used in Hydraulic Conductivity Tests, Na-Bentonite Used to Produce BPC (BB), SAP, and BPC
Material
Swell index (mL=2 g)(ASTM D5890;ASTM 2006c)
Water content (asreceived) (%) (ASTMD2216; ASTM 2005)
GCLs, and is sold in bulk as Volclay CG-50. The granule-size dis-tribution is similar to that of the GCL bentonite in Shackelford et al.(2000), but with a smaller fraction of fine sand-size particles.The mineralogical composition, determined by XRD, is 85–91%montmorillonite, 0–5% augite, 2–4% quartz, and ,3% cristobalite,plagioclase feldspar-andesine, calcite, illite/mica, heulandite, gyp-sum, ferroan dolomite, and K-feldspar–microcline. The Na-bentonite and BB are not the same clay, and BB is not currentlyused in GCLs. This Na-bentonite was used to simulate GCLs in thisstudy for comparative hydraulic conductivity testing.
Na-Polyacrylate (SAP)
Na-polyacrylate (½C3H3NaO2�n), or SAP, was tested to explore theswelling and hydraulic properties of the polymer component ofBPC.The charge on the carboxyl group (CH3COO2) of the SAP issatisfied by a sodium ion (Na1). This configuration is representativeof SAP produced in base Na-bentonite slurry during the productionof BPC, in which the solution has a preponderance of Na1 cations.SAP also is readily available commercially and is used commonly asthe active component of baby diapers and other water-absorbingproducts (Buchholz and Graham 1998). For hydraulic conductivitytesting, granular SAP was used with granule sizes matching thegranule size distribution of the GCL described in Shackelford et al.(2000).
Liquids
To investigate the impact of divalent cations and ionic strength onthe swelling and hydraulic behavior of the bentonite and polymermaterials, deionized water (DW) and 5, 20, 50, 200, and 500 mMCaCl2 were used as permeant solutions (Table 3). Jo et al. (2001) andLee et al. (2005) show that permeating GCLs containing Na-bentonite with dilute divalent solutions (5 or 20 mM CaCl2)results in a relatively low hydraulic conductivity (∼23 10210 m=s)even after near complete exchange of Ca21 for native Na1, althoughthe hydraulic conductivity to these solutions was higher thanobtained with DW. In contrast, permeation of GCLs originally
containing Na-bentonite with more aggressive divalent cation so-lutions such as 50, 200, or 500 mM CaCl2 results in a hydraulicconductivity on the order of 1028e1027 m=s (Jo et al. 2001; Leeet al. 2005). Thus, 5 and 20 mM CaCl2 were selected as permeantsolutions that would induce gradual cation exchange and alterationsin hydraulic conductivity of conventional Na-bentonite, whereassolutions containing 50, 200, and 500 mM CaCl2 were chosen aspermeant solutions that adversely affect Na-bentonite quickly. DWwas used as a control.
The DW used in the permeant solutions and for swell indexes wasType II water perASTMD1193 (ASTM 2006b). The CaCl2 solutionswere prepared by dissolving reagent grade dihydrate-calcium chloride(CaCl2×2H2O) in DW. Concentrations of Ca21 were verified by in-ductively coupled plasma-optical emission spectroscopy (ICP-OES)(MPX ICP-OEX; Varian Inc., Palo Alto, California) following EPAMethod 6010 B (EPA 2007). The concentration of Ca21 in DW wasbelow the ICP-OES method detection limit (MDL) of 0.05 mM.Solutionswere stored in collapsible carboyswith nohead space to limitinteraction with atmospheric CO2.
A hyperacidic solution (1 M HNO3, pH 0.3) and a hyperalkalinesolution (1 M NaOH, pH 13.1) (Table 3) were also used as permeantsolutions to investigate the effect of extreme pH that might be en-countered in containment systems for mineral processing and minewastes (Benson et al. 2008, 2010;Bouazza2010).Acidic leachates areof particular concern for GCLs because bentonite is thermodynam-ically unstable at pH, 3e4, resulting in dissolution of montmoril-lonite (Gates et al. 2002; Jozefaciuk and Matyka-Sarzynska 2006).Similarly, hyperalkaline solutions can dissolve montmorillonite, aswell as other accessory minerals in GCLs (Gates and Bouazza 2010).The concentrated acidic solutionwas prepared by diluting trace-metalgrade 70% HNO3 stock solution with DW. The 1 M NaOH wasprepared by dissolving reagent grade NaOH in DW.
Electrical conductivity (EC) and pH of the permeant solutionswere monitored on a monthly basis. EC was measured using anelectrical conductivity probe (Con 5 series, Cole-Parmer InstrumentCo., Vernon Hills, Illinois) and pH was measured using a pH probe(Accumet pH meter 50, Fisher Scientific Co., Waltham, Massa-chusetts). The EC and pH of the solutions are reported in Table 3.
Methods
Swell Index
Swell index tests were performed on BPC and the Na-bentonite ingeneral accordancewithASTMD5890 (ASTM2006c).Materialswereground to 100% passing a standard No. 200 woven wire sieve (ASTME11; ASTM 2013) with a mortar and pestle. Swell index tests wereconducted using DW; 5, 20, 50, 200, and 500 mM CaCl2 solutions;1MHNO3; and 1MNaOH. Swell indexes are summarized in Table 4.
A scaled-up swell index test incorporating a 2-L graduated cyl-inderwas used to determine swell indexes of SAP inDWand5 and 20mMCaCl2. A modified testing methodology was used because of thehigh (.100mL) swelling capacity of 2 g of SAP. The 2-L graduatedcylinder was filled with 1.8 L of liquid (DW, 5 mMCaCl2, or 20 mMCaCl2), SAPwas added following themethodology inASTMD5890(ASTM 2006c), and the graduated cylinder was topped off to 2.0 L.The test then was allowed to equilibrate for 16 h (per ASTMD5890).The modified swell indexes of SAP toDW, 5mMCaCl2, and 20mMCaCl2 are reported in Tables 1 and 4. Swell indexes of SAP in 50, 200,and 500 mM CaCl2, 1 M NaOH, and 1 M HCl were determinedfollowing ASTM D5890 and are reported in Table 4.
Colorado State University (CSU) conducted duplicate swell indextests on BPC andNa-bentonite inDWand 5, 50, and, 500mMCaCl2.
Table 2. Soluble Cations, Bound Cations, and Cation Exchange Capacity(CEC) of Na-Bentonite (Na-B) Used in Hydraulic Conductivity Tests, Na-BUsed to Produce BPC (BB), and BPC
Data from CSU are included in Tables 1 and 4. Interlaboratory swellindexes are compared in Fig. 1(a). The standard deviation of theswell index was within 4% of the mean for both BPC and Na-bentonite in DW, 5 mM CaCl2, and 50 mM CaCl2. For thests in500 mM CaCl2, the standard deviation was within 14% of the meanswell index [for Na-bentonite, swell index 5 6:7mL=2 g at CSU,8:0mL=2 g at University of Wisconsin-Madison (UW); for BPC,swell index 5 8:5mL=2 g at CSU, 7:0mL=2 g at UW]. A t-testperformed on the paired data yielded a P-statistic of 0.15, indicatingno statistically significant difference between the swell indexes ofboth data sets at the 5% level (P5 0:15. 0:05). No bias was evidentin the paired swell index data.
Hydraulic Conductivity
Hydraulic conductivity tests were conducted using flexible wallpermeameters in general accordance with ASTM D5084 (ASTM2003). The falling headwater-constant tailwater method was used.The permeameters were equipped with larger 4.3-mm-inside-diameter tubing to preclude clogging by salt precipitates during long-term hydraulic conductivity tests, as recommended in Jo et al.(2005). Glass tubing with a 5.2-mm inside diameter was used as thefalling headwater reservoir. The tubing was fixed at 5� from hori-zontal to minimize the change in hydraulic gradient during testing,but still allow sufficient influent solution for convenient and accuratelong-term testing. Gravity heads were used to apply the cell andinfluent pressure; backpressure was not applied to minimize chemicalalteration of the permeant solution and to allow for convenient col-lection of effluent for chemical analyses (i.e., pH and EC).
These testing conditionswere chosen to allow for comparisonwithexisting data on long-term hydraulic conductivity of GCLs to in-organic salt solutions (Lee and Shackelford 2005; Lee et al. 2005; Joet al. 2005). Tests were conducted under an average effective stress of20 kPa and an average influent head of 1.5 m, corresponding to anaverage hydraulic gradient of 200 for a 7.5-mm-thick specimen. Thishydraulic gradient is typical for GCL testing (Shackelford et al. 2000)but is considerably higher than hydraulic gradients conventionallyused to test soils.
The following steps were followed to prepare specimens withgranular BPC, Na-bentonite, or SAP for permeation. A latex mem-brane was attached to a 154-mm-diameter base pedestal with O-rings,and a 0:75-kg=m2 nonwoven needle-punched GT was placed on thebase pedestal and toppedwith a nonwoven calendared andheat bondedGT(nonwovenGTswereused in lieuof porous stones andfilter paper).Using the calendared and heat-bonded GT permitted removal ofspecimenswithminimal disturbance at termination of permeation. The
Table 4. Swell Index and Hydraulic Conductivity of Na-Bentonite (Na-B), SAP, and BPC in CaCl2 Solutions, 1 M NaOH, and 1 M HNO3
Note: All data are the average of two or more duplicate tests, with the exception of hydraulic conductivities at CSU and hydraulic conductivities with 5 mMCaCl2 at UW (one test each).
Fig. 1. Comparison of (a) swell indexes and (b) hydraulic conduc-tivities from paired replicate tests conducted at UW and CSU; error barsshow the range of measured hydraulic conductivities
BPC or Na-bentonite was placed with a dry mass per unit area of4:8 kg=m2 [average for GCLs previously studied with inorganic saltsolutions by Jo et al. (2005)]. Test specimens prepared with SAP hada drymass per unit area of 0:48 kg=m2 because of their very high swellpotential.Duringpreliminary testing, SAPspecimenswith highermassper area resulted in excessive swell such that the membrane wasseparated from the pedestal. Two GTs and a top pedestal were placedatop the BPC, Na-bentonite, or SAP specimen, and the latex mem-brane was secured to the top and bottom pedestals with O-rings.
After the permeameter was assembled and connected to thefalling headwater apparatus, cell pressurewas applied, and all tubingwas saturated with the permeant liquid. The inflow valve on thepermeameter was left open to allow the specimen to hydrate whilethe effluent valve remained closed. After 48 h, the lines were flushedto remove any air bubbles, and flow was initiated by opening theeffluent valve. During testing, effluent was collected in evacuatedfluorinated ethylene propylene (FEP) bags (Jensen Inert Products,Coral Springs, Florida) to minimize interactions with the atmo-sphere and sampled for chemical analysis.
All hydraulic conductivity tests were required to meet the fol-lowing termination criteria: (1) no systematic trend in hydraulic con-ductivity over time (per ASTMD5084; ASTM2003), (2) at least fourconsecutive hydraulic conductivity readings within 625% of themean (per ASTM D5084), (3) at least four consecutive outflow-to-inflow ratios within 1:06 0:25 (per ASTM D5084), and (4) estab-lishment of chemical equilibrium. Chemical equilibrium was definedas the ratio of EC of the effluent solution to EC of the influent solutionbeingwithin 1:06 0:1 and the ratioofpHof the effluent solution topHof the influent solution within 1:06 0:1 [both per ASTM D6766(ASTM 2009)]. In addition to these criteria, tests with BPC werecontinued for at least 2 years to investigate long-term behavior.
All tests met the termination criteria except for those conductedwith DW, 1 M NaOH, and 1 M HCl, which did not meet the ECcriterion even at 2 years of permeation. Not reaching the EC ter-mination criteria for the DW tests was deemed acceptable, as thehydraulic conductivity to DW was intended only as a baselinecomparative measurement. Jo et al. (2005) also report Na1 beingeluted from GCLs permeated with DW after 2.5 years under similartesting conditions. The EC outflow-to-inflow ratios for tests with1 M NaOH and 1 M HCl are discussed with the results.
TheCSUconducted duplicate hydraulic conductivity tests onBPCandNa-bentonite with DWand 5 and 500mMCaCl2. Data from CSUare included in Table 4. Interlaboratory hydraulic conductivity dataare compared in Fig. 1(b). The values of hydraulic conductivity formost tests were within a factor of 2, which is consistent with thereproducibility typically associated with GCL hydraulic conductivitytesting (Petrov et al. 1997; Shackelford et al. 2000).The exceptionwasthe test with BPC and 500 mM CaCl2; however, for this test, bothlaboratories reported very low hydraulic conductivities on the sameorder of magnitude (i.e., ,9:73 10213 m=s). A t-test was performedon the data at a 5% significance level to confirm that the two sets ofpaired hydraulic conductivities were statistically similar. The datawere transformed logarithmically prior to testing so that the as-sumption of normality in the t-test would be satisfied. A P-statistic of0.69 was calculated, indicating that there was no statistically sig-nificant difference between the interlaboratory hydraulic conductivitydata. Thus, the data collected at the University ofWisconsin and CSUare pooled in the remainder of this paper.
Soluble Cations, Bound Cations, and CationExchange Capacity
Soluble cations, bound cations, and cation exchange capacity (CEC)were determined for Na-bentonite, BB, and BPC following the
procedures in ASTM D7503 (ASTM 2010). BPC and Na-bentonitespecimens also were tested after permeation. Chemical analysis ofextracts was conducted by ICP-OES following EPA Method 6010 B(EPA 2007). Bound cation mole fractions of the major exchangeablecations (Na1, K1, Ca21, and Mg21) for each material (Table 2) werecalculated as the ratio of total bound cation charge per mass ofbentonite associatedwith a particular cation to theCEC. Bound cationmole fractions of monovalent cations were determined for BPCand Na-bentonite after permeation as the ratio of Na1 and K1 to theCEC. Soluble cation, bound cations, and CEC testing was attemptedon SAP following ASTM D7503 (ASTM 2010); however, SAPabsorbed all of the testing solution resulting in no effluent foranalysis.
Polymer Quantification
The polymer content of BPC before and after permeation was de-termined by loss on ignition. TheBB,BPC, and polyacrylate (producedby the BPC supplier following the same method as BPC but withoutBB, having lower average molecular weight than SAP) were ground topass a No. 20 woven wire sieve (ASTM E11; ASTM 2013) and thenoven dried at 105�C until mass ceased to change. Oven-dried materialwas ignited at 550�C for 4 h, which exceeds the decomposition tem-perature of polyacrylate (�200�C).After 4 h, thematerialwas removedfrom the furnace and allowed to cool in a water-free atmosphere.
TheBBlost1:66 0:1%mass on ignition (10 replicate tests), whichcorresponds to removal of strongly bound water from the montmo-rillonite interlayer (Grim 1968). Polyacrylate lost 74:76 0:0% masson ignition (10 replicate tests), and BPC lost 22:46 0:0% on ignition(10 replicate tests). From the relative mass loss of BB and poly-acrylate, the initial polymer content in the BPC was calculated to be28.5% by mass. The same procedure was used to determine thepolymer content of BPC after the tests were terminated.
Results
Swell Index
Swell index ofNa-bentonite, SAP, andBPC in solutions with varyingCaCl2 concentration are shown in Fig. 2(a) and are presented inTable 4. Swell index of BPC in DW was more than two times theswell index of Na-bentonite in DW (73.0 versus 30:5mL=2 g).However, similar to both Na-bentonite and BPC, the swell index de-creased with increasing CaCl2 concentration, reaching a swell indextypical of Ca-bentonite in 200 and 500 mM CaCl2 (,10 mL=2 g).
Swell index of the Na-bentonite decreased with increasing CaCl2concentration, as shown by others (Jo et al. 2001;Kolstad et al. 2004;Lee et al. 2005; Katsumi et al. 2008), with most of the loss occurringbetween 5 and 50 mM CaCl2 (28:7e10:2 mL=2 g). At CaCl2concentrations $50mM, the swell index of Na-bentonite wastypical of Ca-bentonite (8e10mL=2 g).
Decreasing swell with increasing CaCl2 concentration also wasexhibited by SAP. In DW, SAP swelled to 1,790mL=2 g, or 58 timesgreater than that of Na-bentonite, whereas in 500mMCaCl2, the swellindex of SAPwas greater, albeit only slightly, than that ofNa-bentonite(13.2 versus 8:0mL=2 g) and 135 times less than in DW. In contrast,for Na-bentonite, the reduction in swell index in 500 mM CaCl2 wasonly 3.8 times lower than that in DW; that is, SAP swell was muchmore sensitive to CaCl2 concentration than was Na-bentonite swell.This behavior for SAP was anticipated, as ionic crosslinking by Ca21
ions stitches carboxyl groups together, causing catastrophic collapse ofthe polymer gel (Buchholz and Graham 1998).
The swell index of Na-bentonite, SAP, and BPC are shown atvarious pH in Fig. 2(b) (circumneutral data are for tests in DW).
Swell of Na-bentonite in 1 M NaOH and 1 M HNO3 is in the rangetypical of bentonite not undergoingosmotic swell (e.g.,Ca-bentonite).Low swell indexes also were exhibited by BPC at both pH extremes.Similar swell indexes for Na-bentonite are shown by Jo et al. (2001)for pH 1 HCl and a pH 13 NaOH (0.1 M NaOH).
The swell index of SAP was reduced relative to DW by a factorof 22.8 in 1 M NaOH and by a factor of 66.3 in 1 M HNO3 [Fig. 2(b); Table 4]. These data are consistent with trends reported byElyashevich et al. (2009) for sodium-polyacrylate hydrogels. Atlow pH, Na1 in the SAP is replaced byH1, reducing polyelectrolytedissociation and consequently reducing swelling (Elyashevichet al. 2009). At high pH, ion screening from the high concentrationof Na1 from the dissociation of NaOH results in decreasedconcentration gradient and less osmotic swell (Buchholz andGraham 1998).
Temporal Hydraulic Behavior with DW
Hydraulic conductivity versus average cumulative flow (ACF) fortests conductedwithDWareshown inFig. 3. TheACFwas calculatedas themean of the cumulative inflowand the cumulative outflow. Porevolume of flow was not used as an indication of the amount of flowbecause the pore volume changed temporally because of elution ofpolymer solid during testing (discussed subsequently).
In tests with BPC, hydraulic conductivity initially dropped dra-matically to �13 10212 m=s, at which time all permeameter lineswere flushed to remove possible clogs. Tests were then reinitiated,and hydraulic conductivity of �83 10211 m=s was measured,followed again by a drop apparently caused by clogging. On re-flushing, precipitated polyacrylate was observed in the brass fit-tings on the permeameter (Fig. 4). All brass fittings subsequentlywere replaced with flexible acrylic tubing with pinch-clamps, andpermeation was reinitiated. Removal of brass fittings did noteliminate elution of polyacrylte or clogging but did limit completeimmobilization of flow by precipitating polymer in constrictions.Eluted polyacrylate was observed in both the inflow (from backdiffusion) and outflow lines during flushing.
The existence of polyacrylate in BPC that could be elutedwas theresult of early termination of polyacrylate chains during manufac-ture. Gel permeation chromatography showed that the eluted poly-mer had a weight-average molecular weight (Mw) of 280,000 g=moland a polydispersivity index (weight-average molecular weight overnumber-average molecular weight) of 6.0, indicating that a widerange of polyacrylate molecular weights were present. In contrast,a Mw greater than 1,000,000 g=mol is typical for commercial SAP(Buchholz and Graham 1998).
To expedite flushing of mobile polymer and attaining of equilib-rium, the hydraulic gradient of one BPC test permeated with DWwasincreased from 75 to 500 after 380 days. Because of the thicknessof BPC specimens in DW (�18 mm), an increase of the average
Fig. 2. Swell index versus solution CaCl2 (a) concentration and (b) pHfor Na-bentonite (Na-B), SAP, and BPC; swell indexes in DW areshown at the MDL of Ca21 (mM) [data from Jo et al. (2001), Lee andShackelford (2005), and Katsumi et al. (2008) for Na-B included forcomparison]
Fig. 3. Hydraulic conductivity versus average cumulative flow for(a) BPCand (b) SAPandNa-bentonite (Na-B) permeatedwith deionizedwater at varying hydraulic gradients (i); dup.5 duplicate test conductedunder identical conditions
effective stress to 55 kPa was required to achieve a hydraulic gradientof 500 while maintaining the pore water pressure at the inflow endlower than the cell pressure to retain membrane integrity. The du-plicate BPC test was maintained at a hydraulic gradient of 75 for theduration of testing. Hydraulic equilibrium was achieved rapidly ata hydraulic gradient of 500 [Fig. 3(a)] and yielded a similar equi-librium hydraulic conductivity as the test maintained at a hydraulicgradient of 75. To explore the impact of a higher hydraulic gradient oninitial test clogging, a BPC specimen was assembled and permeatedwith DW at a gradient of 500. Both reduced clogging and attainmentof hydraulic equilibrium in a shorter durationwere observed, althoughhydraulic equilibrium occurred at a similar ACF [Fig. 3(a)]. Theequilibrium hydraulic conductivity of BPC tested at a high gradientwas slightly lower than tests initially permeated at a hydraulic gradientof 75. These data illustrate the potential minor sensitivity of BPC toeffective stress during hydration and permeation.
Na-bentonite permeated with DW maintained a stable hydraulicconductivity between 1 and 23 10211 m=s regardless of hydraulicgradient [Fig. 3(b)]. SAP specimens absorbed water for the first 50–60 days of testing, during which the hydraulic conductivity droppedsignificantly, and then stabilized at approximately one order ofmagnitude lower than the hydraulic conductivity of either Na-bentonite or BPC (Fig. 3). All duplicate tests yielded similar hy-draulic behavior (Scalia 2012).
Temporal Behavior with 50 mM CaCl2
For tests conducted with 50 mM CaCl2, hydraulic conductivity isshown versusACF in Fig. 5(a) and versus time in Fig. 5(b). TheBPC
permeated with 50mMCaCl2 initially exhibited a clog/flush patternsimilar to BPC tests with DW. However, after an ACF of 275 mL, thehydraulic conductivity of BPC to 50 mM CaCl2 equilibrated at8:13 10211 m=s in both tests [Fig. 5(a)]. Similar clogging followedby eventual increase to an equilibrium hydraulic conductivity wasexhibited by BPC in 5 and 20 mM CaCl2. The hydraulic gradient ofthe duplicate BPC test permeated with 50 mM CaCl2 was increasedto 500 at an ACF of 2,000 mL to expedite polymer flushing andinvestigate if the hydraulic gradient would impact hydraulic con-ductivity (Fig. 5). No long-term change in hydraulic conductivitywas observed with the elevated hydraulic gradient (Scalia 2012).
Na-bentonite and SAP permeated with 50 mM CaCl2 maintainedhigh hydraulic conductivity (.73 1028 m=s) for the duration oftesting (Fig. 5). To ensure that the lower equilibrium hydraulic con-ductivity of BPC was not an artifact of permeameter clogging, aftereach BPC test was terminated, the test specimenwas removed, and thepermeameter reassembled with the geotextiles but without the BPCspecimen. The system was then repermeated under the same effectivestress andhydraulic headusedduring long-termhydraulic conductivitytests. In all cases, no polymerwas observed in the permeameter effluentand the rate of flow through the system was in excess of five orders ofmagnitude greater than the flow rate for the highest hydraulic con-ductivity observed during BPC testing (8:13 10211 m=s), indicatingclogging of the system did not govern the measured hydraulic con-ductivity (Scalia 2012). The rate of flow during these diagnostic testswas at the upper limit possible with these permeameters.
Temporal Behavior with 500 mM CaCl2
Hydraulic conductivity is shown versus ACF in Fig. 6(a) and versustime in Fig. 6(b) for tests conductedwith 500mMCaCl2. Permeating
Fig. 4. Photograph of precipitated polymer being removed from cloggedbrass fitting (scale in millimeters; photograph credit: Joseph Scalia IV)
Fig. 5. Hydraulic conductivity versus (a) ACF and (b) time for Na-bentonite (Na-B), SAP, and BPC, permeated with 50 mM CaCl2 atvarying hydraulic gradients (i); dup. 5 duplicate test conducted underidentical conditions
with 500 mMCaCl2 resulted in the greatest polymer clogging. Evenafter increasing the hydraulic gradient of one BPC test to 500, thepattern of clogging until flushing continued for approximately 400days [Fig. 6(b)]. Eventually, however, clogging ceased and the BPCtests equilibrated to a hydraulic conductivity ,6:93 10213 m=s.Similar behavior was observed when BPC was permeated with200 mM CaCl2.
BPC had an equilibrium hydraulic conductivity lower than thehydraulic conductivity to DW with 200 and 500 mM CaCl2. Incontrast, Na-bentonite and SAP exhibited stable and high hydraulicconductivity (.3:53 1027 m=s) from the onset of testing (Fig. 6).
Temporal Behavior with Hyperalkaline andHyperacidic Solutions
Hydraulic conductivity versus ACF is shown in Fig. 7(a) for testsconducted with 1 M NaOH. After only two clogging cycles, thehydraulic conductivity of BPC steadily increased to a final equi-librium of 1:83 10211 m=s. This increase in hydraulic conductivitymay have been the result of mobile polymer flushing from the BPClayer, thereby increasing the pore space open for flow. Although thehydraulic conductivity became steady, the ratio of outflow EC to in-flow EC remained at approximately 0.40 throughout the tests. Thisdrop in EC may be caused by precipitation of complexes within thespecimen, as was proffered by Benson et al. (2008, 2010) for long-term permeation of GCLs with concentrated NaOH.
Permeation of Na-bentonite with 1 M NaOH resulted in a steadyand high hydraulic conductivity (.13 1028 m=s). The high hy-draulic conductivity observed from the onset of testing indicates thatthe high ionic strength of the permeant solution suppressed osmoticswell rather than dissolved the mineral, which would occur slowly
(Benson et al. 2010). These findings are consistent with Ruhl andDaniel (1997), who showed that a nonprehydrated GCL permeateddirectly with 0.1MNaOH (pH5 13) had a hydraulic conductivity ofapproximately 13 1028 m=s. SAP was also permeated with 1 MNaOH but dissolved during testing and is therefore not shown inFig. 7(a). Dissolutionwas likely the result of hydrolysis catalyzed byOH2 ions in the hyperalkaline permeant solution.
Hydraulic conductivity is shown versus ACF for tests with 1 MHNO3 in Fig. 7(b). BPC permeated with 1 M HNO3 did not exhibitclogging characteristic of the other permeant solutions and main-tained a hydraulic conductivity ,43 10211 m=s. In contrast, Na-bentonite permeated with 1 M HNO3 exhibited a steady and highhydraulic conductivity (.13 1027 m=s). Similar hydraulic con-ductivity (1:53 1027 m=s) is reported by Jo et al. (2001) for a GCLpermeated with pH5 1 HCl solution. SAP permeated with 1 MHNO3 also had steady hydraulic conductivity greater than 1:93 1027 m=s for the duration of testing. The high hydraulic con-ductivity of SAP is likely the result of extensive replacement of Na1
in SAP with H1, which causes weakening of the osmotic swellingpotential of the polyelectrolyte in the high ionic strength permeantsolution (Elyashevich et al. 2009).
Hydraulic Conductivity
Hydraulic conductivities of BPC, Na-bentonite, and SAP at chem-ical equilibrium are shown versus permeant CaCl2 concentration inFig. 8(a) and pH in Fig. 8(b). A summary is in Table 4. BPCmaintains low hydraulic conductivity (,8:53 10211 m=s) in allsolutions, whereas the hydraulic conductivity of Na-bentonite andSAP is strongly affected by CaCl2 concentrations and pH extremes.The hydraulic conductivity of Na-bentonite and SAP permeatedwith solutions of 20 (SAP only), 50, 200, and 500 mM CaCl2 is at
Fig. 6. Hydraulic conductivity versus (a) ACF) and (b) time for Na-bentonite (Na-B), SAP, and BPC, permeated with 500 mM CaCl2 atvarying hydraulic gradients (i); dup. 5 duplicate test conducted underidentical conditions
Fig. 7. Hydraulic conductivity profiles for Na-bentonite (Na-B), SAP,and BPC permeated with (a) 1 M NaOH and (b) 1 M HNO3; averagecumulative flow calculated as mean of cumulative inflow and cumu-lative outflow; dup.5 duplicate test initiated under identical conditions
least 2,500 times greater (.1:73 1027 m=s) than that in DW.Whenpermeatedwith 500mMCaCl2, the hydraulic conductivity ofBPC ismore than five orders of magnitude lower than that of Na-bentoniteor SAP. Similarly, the hydraulic conductivity of Na-bentonite andSAP was high (.1:73 1027 m=s) in both hyperacidic and hyper-alkaline solutions.
The data from tests with hyperacidic and hyperalkaline solutionsillustrate the potential of BPC for containment of mining wastes,where extreme pH leachates are commonplace (Benson et al. 2010;Bouazza 2010; Shackelford et al. 2010). Previous studies have ex-amined the effect of extreme pH on GCLs; however, results of thesestudies have been inconclusive (Shackelford et al. 2010). For ex-ample, Shackelfordet al. (2010) showed that both a conventionalGCLand a GCL treated for contaminant resistance had hydraulic con-ductivities to a synthetic acidic tailings leachate (pH 2.5) between1,100 and 15,000 times higher than when permeated with a dilutegroundwater. These increases in hydraulic conductivity were hy-pothesized to result from the high ionic strength of the syntheticleachate (350 mM), as dissolution was not observed. In contrast,Kashir and Yanful (2001) and Lange et al. (2007, 2009) permeatedGCLs with acid mine drainage (AMD) containing high concen-trations of metals and SO4
22 with low pH (2.5–3.3). Kashir and
Yanful (2001) indicated that XRD patterns showed that contact withAMD mineralogically altered montmorillonite. Kashir and Yanful(2001) and Lange et al. (2007, 2009) found that the hydraulicconductivity of GCLs to AMD was no higher than 13 10210 m=s.However, these tests were prehydrated with DWprior to exposure toAMD, and chemical equilibrium was not reported.
Hyperalkaline NaOH solutions (identical to the 1 M NaOH usedin this study) have been used as proxies for alumina refinery leachate(ARL) inGCLcompatibility studies (Benson et al. 2010). Typically,ARL has a pH between 11.5 and 12.5 and contains high concen-trations of Na salts and aluminum complexes (Benson et al. 2008).Benson et al. (2010) investigated the impact of 1 M NaOH solution(pH 13.1) on the hydraulic conductivity of GCLs composed ofpowdered Na-bentonite and the same Na-bentonite amended withtwo dosages of a proprietary additive intended to enhance chemicalresistance. Permeationwith 1MNaOH resulted in initial increases inhydraulic conductivity less than 50 times the hydraulic conductivityof the same bentonite to DW, followed by either stabilizing ordecreasing hydraulic conductivity with time. Benson et al. (2010)hypothesized that the trend of decreasing hydraulic conductivitywascaused by pore filling by metal precipitates, as indicated by testsusing actual ARL byBenson et al. (2008). Bentonites contactedwithrealistic ARL solutions containing high concentrations of cations inaddition to a high pH also have shown clogging of pores by mineralprecipitates (Claret 2002; Ramirez et al. 2002; Sanchez et al. 2006;Benson et al. 2008).
Swelling and Hydraulic Conductivity
Hydraulic conductivity versus swell index is shown in Fig. 9. The Na-bentonite exhibits typical behavior, where swell index and hydraulicconductivity are related inversely and the osmotic swell is responsiblefor low hydraulic conductivity. SAP exhibits a similar relationshipbetween hydraulic conductivity and swell. For both Na-bentonite andSAP, swell indexes greater than 16mL=2 g in solutions with CaCl2concentrations less than or equal to 20 mM corresponds to hydraulicconductivity less than 33 10211 m=s,whereas swell indexes less than11mL=2 g in solutions with CaCl2 concentrations greater than orequal to 50 mM corresponds to hydraulic conductivity greater than1:73 1027 m=s. The high hydraulic conductivity of Na-bentonite inextreme pH solutions also is caused by suppression of osmotic swellby the high ionic strength of both 1 M HNO3 and 1 M NaOH, asevinced by low swell indexes [Figs. 2(b) and 9].
In contrast to Na-bentonite or SAP, the hydraulic conductivity ofBPC is independent of swell index. The hydraulic conductivity ofBPC is less than 83 10211 m=s in all permeant solutions, with thelowest swell indexes (7mL=2 g) based on 500 mM CaCl2, corre-sponding to the lowest hydraulic conductivity. Thus, achieving lowhydraulic conductivity of BPC is not contingent on attainment andmaintenance of osmotic swell or high swell index. This result suggeststhat the mechanisms controlling hydraulic conductivity of BPC arefundamentally different than the classic osmotic swelling model forNa-bentonite. These data also illustrate that the swell index test isnot a reliable indicator for BPC hydraulic performance. A detailedanalysis of the mechanisms underlying the hydraulic performance ofBPC is outside the scope of this paper; however, based on the dataprovided herein, polyacrylate is hypothesized to clog pores that wouldtypically govern the hydraulic conductivity of low-swelling bentonite(such as Na-B permeated with 500 mM CaCl2) (Scalia 2012).
Ion Exchange
The mole fraction of monovalent bound cations in BPC and Na-bentonite specimens after long-term permeation is shown versus
Fig. 8. Equilibrium hydraulic conductivities versus permeant solutionCaCl2 (a) concentration and (b) pH for Na-bentonite (Na-B), SAP, andBPC; the hydraulic conductivity of specimens permeatedwith deionizedwater is shown at MDL of Ca21
the CaCl2 concentration of the permeant solution in Fig. 10. Ex-change of monovalent bound cations occurred in both materials andat all CaCl2 concentrations; in 50, 200, and 500 mM CaCl2, therewas near complete exchange of polyvalent cations (mainly Ca21) fornative monovalent cations (mainly Na21). Reductions in mono-valent cation mole fractions correspond to reductions in swell in-dexes [Fig. 1(a)]; both BPC and Na-bentonite exhibited swellindexes typical of Ca-bentonite in 50, 200, and 500 mM CaCl2 andwere transformed into Ca-bentonite or Ca-BPC at the end of per-meation. These data show that in situ polymerized polyacrylate doesnot prevent cation exchange in BPC and that despite near completeion exchange with 50, 200, and 500 mMCaCl2, BPC maintains lowhydraulic conductivity.
Polymer Elution
The fraction of polymer eluted from BPC specimens after long-termpermeation is shown versus CaCl2 concentration in Fig. 11(a). Anapproximately linear relationship exists between fraction of polymereluted and CaCl2 concentration in the permeant solution. BPCpermeated with DW eluted a large fraction (0.76–0.77) of the orig-inal polymer content, whereas only a small fraction (0.06–0.09) ofthe polymer was eluted from BPC permeated with 500 mM CaCl2.
Temporal evolution of polymer elution is shown in Fig. 11(b),where the fraction of polymer eluted from BPC is shown asa function of cumulative inflow. Polymer was eluted more rapidlyduring the initial stages of permeation, which is consistent withmorefrequent clogging shown in Figs. 3, 5, and 6 during the early portionof the hydraulic conductivity tests.
Equilibrium hydraulic conductivity is shown as a function of thefraction of polymer eluted in Fig. 12. The lowest hydraulic con-ductivity corresponds to the greatest polymer retention, which alsowas obtained with the highest CaCl2 concentration (500 mM). Theswell indexes for BPC at the termination of permeationwith 50, 200,and 500mMCaCl2 are in the range typical ofCa-bentonite (Table 4).Thus, the relativity low hydraulic conductivity of BPC to thesesolutions is attributed to the presence of polymer rather than swelling
Fig. 9. Equilibrium hydraulic conductivity versus swell index for Na-bentonite (Na-B), SAP, and BPC [literature data from Jo et al. (2005)included for comparison]
Fig. 10.Mole fraction of bound monovalent cations at the terminationof permeation versus permeant solution CaCl2 concentration
Fig. 11. Fraction of polymer eluted (Xp) after long-term hydraulicconductivity testing versus permeant solution CaCl2 (a) concentration(C) and (b) cumulative inflow for BPC
of montmorillonite or the prevention of ion exchange by adsorbedpolyacrylate.
Summary and Conclusions
Bentonite was modified by polymerizing acrylic acid withinsodium-bentonite slurry to create a BPC, where polymer moleculeswere formed around the montmorillonite. Long-term hydraulicconductivity and swell index tests were conducted on thin layers ofgranular BPC simulating GCLs using DW, as well as 5, 20, 50, 200,and 500mMCaCl2; 1 MNaOH (pH 13.1); and 1MHNO3 (pH 0.3).Identical tests also were conducted on GCL-grade Na-bentonite andSAP, representative of the polymeric constituent within BPC.
The following conclusions are based on thefindings of this study:• Swelling of BPC is sensitive to CaCl2 concentration and extreme
pH. BPC swellsmore thanNa-bentonite in dilute solutions, but theswelling of bothmaterials is comparable in concentrated solutions.For example, the swell indexofBPCwas 73mL=2 g inDWversus30mL=2 g for Na-bentonite, whereas in 200mMCaCl2, 500mMCaCl2, 1 M NaOH, and 1 M HNO3, the swell index of BPC wassimilar to that for calcium bentonite (i.e., 10mL=2 g).
• BPC maintains low hydraulic conductivity (,83 10211 m=s)for awide range of CaCl2 concentrations (50–500mMCaCl2), aswell as hyperacid and hyperalkaline solutions (1 M NaOH and 1M HNO3) that are known to induce high hydraulic conductivityof Na-bentonite.
• In situ polymerized polyacrylate does not prevent cation exchangeinBPC.Despite near complete ion exchangewith 50, 200, and500mM CaCl2, BPC maintains a low hydraulic conductivity.
• BPC elutes polyacrylate during permeation regardless of thepermeant solution, but polymer elution decreases as CaCl2concentration in the solution increases.
• BPC does not follow the classical model where osmotic swellingis required to achieve and maintain low hydraulic conductivity.This behavior illustrates that the mechanisms controlling thehydraulic conductivity of BPC are different from those forconventional Na-bentonite. As such, swell index tests are notan accurate indicator for the hydraulic conductivity of BPC.
• BPC may be useful for containment of concentrated leachatesthat cannot be contained effectively by GCLs comprised ofconventional Na-bentonite. However, the solution thresholds towhich BPC will maintain low hydraulic conductivity are cur-rently unknown.
Acknowledgments
Financial support for this study was provided by U.S. National Sci-ence Foundation Grant No. CMMI-0757815 with in-kind supportfrom Colloid Environmental Technologies Company (CETCO).This support is gratefully acknowledged. The authors thank CETCOfor providing the bentonites used in this study and Dr. MichaelDonovan of CETCO for his contributions during this research.The findings and recommendations that have been presented aresolely those of the authors and do not necessarily represent the opin-ions or policies of the sponsors.
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