Zinc Leaching from Tire Crumb RubberEmily P. Rhodes,† Zhiyong Ren, and David C. Mays*

Department of Civil Engineering, University of Colorado Denver, Campus Box 113, PO Box 173364, Denver, Colorado, UnitedStates

*S Supporting Information

ABSTRACT: Because tires contain approximately 1−2% zinc by weight, zincleaching is an environmental concern associated with civil engineeringapplications of tire crumb rubber. An assessment of zinc leaching data from14 studies in the published literature indicates that increasing zinc leaching isassociated with lower pH and longer leaching times, but the data display a widerange of zinc concentrations, and do not address the effect of crumb rubbersize or the dynamics of zinc leaching during flow through porous crumbrubber. The present study was undertaken to investigate the effect of crumbrubber size using the synthetic precipitation leaching procedure (SPLP), theeffect of exposure time using quiescent batch leaching tests, and the dynamicsof zinc leaching using column tests. Results indicate that zinc leaching from tirecrumb rubber increases with smaller crumb rubber and longer exposure time.Results from SPLP and quiescent batch leaching tests are interpreted with asingle-parameter leaching model that predicts a constant rate of zinc leachingup to 96 h. Breakthrough curves from column tests displayed an initial pulse of elevated zinc concentration (∼3 mg/L) beforesettling down to a steady-state value (∼0.2 mg/L), and were modeled with the software package HYDRUS-1D. Washing crumbrubber reduces this initial pulse but does not change the steady-state value. No leaching experiment significantly reduced thereservoir of zinc in the crumb rubber.

1. INTRODUCTION

According to the U.S. Environmental Protection Agency (EPA),290 million tires are disposed annually in the United States,1

nearly one tire per person per year. Although the waste tirestockpile has been reduced by over 87% since 1990, the RubberManufacturers Association reports that the states of Alabama,Arizona, Colorado, Massachusetts, Michigan, New York, andTexas have over 85% of the remaining waste tire stockpile.2

Landfilling tires is problematic because tires tend to rise to thesurface, which can harm landfill covers. Stockpiled tires cancreate breeding grounds for mosquitoes and other pests, andalthough tires are not categorized as hazardous material, wastetire facilities that have caught fire have been categorized asSuperfund sites. These tire fires generate toxic emissions thatdamage both human and environmental health.1

To avoid the problems associated with waste tire disposal, tirecrumb rubber is used in various applications including infill forturf fields, mulches, crumb rubber modified asphalt and othercivil engineering applications, as well as molded and extrudedproducts.3 Tire crumb rubber is produced when tires are groundand the fiber and steel belts are removed. Tire crumb rubber has agranular texture and ranges in size from very fine powder to 1 cmpieces.4 Tire crumb rubber has a density between 1.13 and 1.16kg/L,5 and 4−5 kg of tire crumb rubber can be derived from onepassenger tire.6

Applications utilizing tire crumb rubber are steadily increasing;however, crumb rubber can have negative impacts on air quality7

and tire leachate contains several chemicals of concern,8 the most

significant of which is zinc.9−15 Zinc is added to tires during thevulcanization process and represents approximately 1−2% oftires by weight.16−19 At elevated concentrations, zinc has beenshown to cause a range of reproductive, developmental,behavioral, and toxic responses in a variety of aquaticorganisms.20 Under the Clean Water Act, the EPA’s freshwaterquality criteria specify a maximum zinc concentration (bothacute and chronic) of 0.12 mg/L for protection of aquatic lifeassuming hardness of 100 mg/L.21 Similarly, the EPA’sstormwater discharge benchmark value for zinc is set to 0.117mg/L for multisector general permits for industrial activities.22

Accordingly, recent research has focused on the transport of zincthrough soils23−25 and the importance of zinc in urban waterquality.26,27

Wik andDave28 provide a review of the ecotoxicological effectsof tire wear particles in the environment. Their review focuses onthe toxicity of tire leachate using various leaching procedures andtest organisms. Other studies also research the effects of tireleachate on differing aquatic organisms.10,12−14,29−34 Thesestudies indicate a link between tire-derived zinc and toxicity, sothe primary focus in the present study will be zinc leaching.Evidence in the literature suggests that zinc leaching from tire

crumb rubber depends on zinc oxide levels in tires, particle size

Received: February 9, 2012Revised: September 14, 2012Accepted: November 12, 2012Published: November 12, 2012

Article

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distribution, and surface area of rubber granulates.35 As pHdecreases, zinc leaching increases.12,17 Zinc leaching alsoincreases from rubber with greater surface area per mass,28

although for small particles, increasing the solid-to-liquid ratiocan decrease zinc leaching due to aggregation of tire wearparticles that decreases the effective surface area.12,36,37 Assalinity increases, zinc leaching decreases17 and tire leachatebecomes less toxic.32 Decreasing zinc leaching rates with timehave been observed (in order of decreasing size) from tires,10,31

tire shreds,18 tire crumb rubber,19 and tire wear particles,17 andthe mortality of aquatic organisms has been observed to decreaseafter sequential leaching periods.32,36 In contrast, a moreaggressive test caused approximately 50% of the zinc in the tirecrumb rubber to be leached during a HNO3

−-H2O2 digestion testusing microwave heating followed by centrifuge separation.37

Increased leaching during more aggressive leaching tests werealso noted when comparing the EPA’s synthetic precipitationleaching procedure (SPLP), which specifies 18 h of end-over-endagitation at lowered pH, to a column leaching test in which 30 cmof simulated rainfall passed through a 5.1 cm column of the samecrumb rubber; in this case the SPLP caused over 16 times moreleaching than the column test.38

A new comparison of tire zinc leaching data from the literature(Table 1) highlights a large degree of variability. Accordingly, thepresent study reports (1) a statistical meta-analysis of the zincleaching data in Table 1, (2) new leaching experiments, and (3)interpretation of results using a kinetic leaching model.Specifically, the SPLP was performed to measure how zincleaching depends on crumb rubber size, quiescent batch leachingtests were performed to measure how zinc leaching depends ontime, and column leaching tests (with unwashed and washedcrumb rubber) were performed to measure the dynamics of zincleaching. To our knowledge, this is the first study to synthesizesuch a broad collection of zinc leaching data from the literature,to report zinc leaching as a function of crumb rubber size, time,and flow dynamics, and to interpret results with a kinetic leachingmodel.

2. MATERIALS AND METHODS2.1. Statistical Methods. For each study in Table 1, a linear

model regressing the available variables (e.g., pH) on zincconcentration was performed. For each model, a one-sided testwas used to determine overall significance of the association of aselected variable on zinc concentration (i.e., a nonzero slope forthe variable in the linear model). Then, p-values from multiplestudies were combined in a meta-analysis to obtain an overall p-value for the selected variable using the Stouffer test (ref 39, page45, footnote 15). All calculations were implemented in thestatistical software package R 2.7.1.40

2.2. Experimental Methods.While it is recognized that thechemical composition of waste tires composed primarily of autotires will vary from that composed primarily of truck tires, for thisstudy a representative sample was used to study a typical auto−truck waste tire mix from a temperate climate in the continentalU.S. The crumb rubber used in this study was provided by thenow-defunct tire recycling company AcuGreen (CommerceCity, CO), who donated 54 kg of tire crumb rubber, from whichsubsamples were randomly removed. The crumb rubber had ad60 of 1.8 mm (viz., 60% of the material by weight was smallerthan 1.8 mm) and a coefficient of uniformity Cu = d60/d10 = 3.3(Figure S1 in the Supporting Information). When washed, tirecrumb rubber was rinsed with tap water between no. 16 and no.200 sieves, and then air-dried. Tap water used in this study had

0.033 mg/L of background zinc. Zinc concentrations weremeasured by the private contract laboratory Colorado Analytical(Brighton, CO) or by the analytical laboratory at Denver’s MetroWastewater Reclamation District (Denver, CO). Both labo-ratories measured zinc by inductively coupled plasma massspectrometry (ICP-MS) following EPA Method 200.8 Rev.5.4.41

To investigate effect of crumb rubber size, the SPLP wasperformed on each sample in accordance with EPA StandardMethod 1312.41 In brief, unwashed tire crumb rubber wasleached for 18 h in an end-overend agitator with an extractionfluid selected in accordance with the waste type and location. Theextraction fluid used in the present study had pH 5,representative of locations west of the Mississippi River. TheSPLP was selected because its low pH and fixed durationprovided results in a short time frame, and using unwashedcrumb rubber provided an upper bound on the expected degreeof leaching. To prepare for the SPLP, the tire crumb rubber wassieved into four different sizes with diameters of 0.42, 0.84, 1.67,and 3.35 mm, where each size is the geometric mean of theopening space on the passing and retaining sieves. Tests wereperformed in duplicate.To investigate the effect of time, the required long-duration

tests were conducted in quiescent water, which better simulatesenvironmental leaching compared to the end-over-end SPLP.These quiescent batch leaching tests used washed crumb rubberin order to isolate leaching effects from the crumb rubber itself,rather than from any associated tire wear particles. 600 mLbeakers were filled at ambient temperature with 500 mL tapwater (1st replicate at pH 6.3) or deionized water (2nd replicateat pH 6.7), to which 25 g of crumb rubber was added. Deionizedwater used in this study was derived from tap water, and thereforehad less than 0.033 mg/L of background zinc. The six beakerswere not stirred, and samples were taken after 6, 18, 24, 48, and96 h.To investigate the effect of flow dynamics, column leaching

tests were performed using an acrylic column 152 cm tall with aninner diameter of 10.2 cm. In each experiment, the column wasdry-packed with crumb rubber to a depth of 51.4 cm. Dry-packing allowed a stainless steel wire mesh to be placed atop thecrumb rubber, which was required to prevent floating. To ensuresaturation, the column was then filled with tap water from thebottom at a rate of 50 mL/min, which required 46.9 min. Oncethe water level reached 2.5 cm above the crumb rubber, thecolumnwas filled with tap water from the top at a rate of 540mL/min. Once the water reached an overflow valve placed to impose91.4 cm of ponding above the crumb rubber, release valves at thebase were opened to allow the water to flow through the columnat a steady rate of 540 mL/min confirmed by a flow meter.Column leaching tests 1 and 2 were conducted with unwashedcrumb rubber, and column leaching test 3 was conducted withwashed crumb rubber. The pH was 6.4 in column test 1, 6.0 incolumn test 2, and 6.5 in column test 3.

3. RESULTS3.1. Statistical Results. Statistical meta-analysis of the data

in Table 1 identifies three variables with significant association tozinc concentration, based on linear associations for individualstudies: pH has a negative association with zinc concentration (p= 2.8 × 10−5), based on significant negative associations (p ≤0.05) in two studies37,42 and nonsignificant negative associationin one study.12 The solid-to-liquid ratio (i.e., mass of crumbrubber per volume of water) has a positive association with zinc

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Table 1. Zinc Leaching from Tire Crumb Rubber in 14 Studies in the Literature

study T [oC] pHd50[μm]

solidliquidratio[g/L] t [hr]

C[mg/L]

Bocca 11 25 5 3500a 62.5 24 2.55↓ n/a ↓ ↓ ↓ 0.966

California 46 37 2.3 n/a 200 21 17Davis 47 20b 4.3 n/a 1 24 3.4Gualtieri 12 20b 3 45a 50 24 44.73

↓ ↓ ↓ ↓ 48 14.4772 13.396 8.49240 28.57

100 24 35.28↓ 48 13.02

72 12.3796 9.26240 20.15

4 50 24 10.55 ↓ ↓ 6.56 5.27 1.2

Hartwell 31 23 ± 2 4.9 10000 50 168 0.026↓ ↓ ↓ ↓ 336 0.021

504 0.034Kanematsu 37 10 5 590 50 72 12.483

↓ 7 ↓ ↓ ↓ 4.1779 2.544

25 5 18.93↓ 7 5.597

9 2.54440 5 27.839↓ 7 3.263

9 2.082Mattina 48 23 ± 2 4.2 3000 50 18 2.8

↓ 7.0 ↓ ↓ ↓ 1.05Nelson 10 20b 8.36 n/a 181 744 0.755New York 38 23 ± 2 4.2 n/a 50 column 0.291

↓ ↓ ↓ ↓ column 0.214118 1.94718 1.15

Park 49 20b 6.9 50000 n/a 0.017 1.3↓ ↓ ↓ ↓ 0.10 0.14

Ronchak 42 20b 3 25400 428.9 24 18.6↓ ↓ ↓ 437.4 ↓ 23.5

5 469.0 8.53↓ 469.0 3.808 250.0 0.005↓ 250.0 0.005

San Miguel 50 20 6.2 420c 50 48 0.57820b 7 ↓ 0.1 24 0.55

Stephensen 51 20b 7.69 n/a 100 n/a 2.29Wik 36 20 ± 2 7.5 n/a 0.1 120 0.07

↓ ↓ ↓ ↓ ↓ 0.080.07

168 0.47↓ 0.4

0.09216 0.25↓ 0.16

0.17264 0.12

study T [oC] pHd50[μm]

solidliquidratio[g/L] t [hr]

C[mg/L]

↓ 0.10.08

480 0.41↓ 0.38

0.41 120 1.14↓ ↓ 0.13

0.440.090.50.1

168 0.54↓ 0.32

0.16216 1.49↓ 0.39

0.42264 0.18↓ 0.12

0.16480 1.44↓ 0.47

0.6610 120 1.46↓ ↓ 1.75

2.240.522.340.71

168 2.57↓ 1.08

1.6216 2.56↓ 2.28

3.66264 2.16↓ 0.82

1.44480 4.46↓ 2.92

5.01

aArithmetic average of given range. bAssumed ambient temperature.cMaximum size given.

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concentration (p = 2.2 × 10−3), based on significant positiveassociation in one study36 and nonsignificant negativeassociation in two studies.12,42 Leaching time is positivelyassociated with zinc concentration (p = 2.0 × 10−3), based onsignificant positive association in one study,36 nonsignificantpositive association in two studies,31,38 and nonsignificantnegative association in one study.12 No significant associationwas identified between zinc concentration and temperature overthe temperature ranges studied. No study in Table 1 variedcrumb rubber diameter.3.2. Experimental Results. The SPLP test indicates that the

amount of zinc leaching from the tire crumb rubber decreaseswith increasing tire particle radius (Figure 1). This is consistent

with the fact that smaller tire crumb rubber has greater surfacearea per mass than larger rubber particles. Therefore, the smallerthe tire particle, the greater the concentration of zinc leached.The highest concentration of zinc leached in the SPLP tests was1.3 mg/L.The quiescent batch leaching tests show that zinc concen-

tration increased linearly with time, reaching a maximum of 2.7mg/L after 96 h (Figure 2). This linear increase with timeindicates similar leaching kinetics throughout the quiescent batchleaching test, suggesting that zinc release was not limited bysource depletion or by accumulation of zinc ions near the crumbrubber surface.In column test 1 (Figure 3), the maximum concentration of

zinc leached was 2.63 mg/L after 5.0 min. After 2 h and 30 min,the zinc concentration approached the EPA’s freshwater criteriaof 0.12 mg/L. In column test 2, the leaching again spiked at 2.55

mg/L after 5.0 min. After 20 h, the zinc concentration fell to0.115 mg/L. Column test 3 was conducted to explore thedifference in leaching from washed and unwashed rubber.Compared to column leaching tests 1 and 2, the peak in columntest 3 was much lower, 0.60 mg/L, suggesting that washing thetire crumb rubber can reduce the initial leaching concentration.However, washing the tire crumb rubber did not reduce leachingat late time: Zinc concentration in column test 3 was 0.149 mg/Lafter 24 h, similar to column tests 1 and 2.

4. MODELINGZinc leaching and transport are modeled with the advection-dispersion equation (ADE) with an additional term for kineticmass transfer and a corresponding expression for the solidconcentration:

αρθ

∂∂

= ∂∂

− ∂∂

− −Ct

DC

xv

Cx

K C S( )bd

2

2 (1)

and

α∂∂

= −St

K C S( )d (2)

where t is time [min], x is position [cm], C is aqueous zincconcentration [μg/mL] = [mg/L], S is solid phase zincconcentration [μg/g] = [mg/kg], D is the dispersion coefficient

Figure 1. Synthetic precipitation leaching procedure (SPLP) results: (a)Zinc concentration versus crumb rubber diameter; (b)mass transfer ratecoefficient α in eq 6 versus crumb rubber surface area. Data labelsindicate sieve size, for example, R-50 is the sample retained on a no. 50sieve. Error bars are one standard deviation calculated from duplicatemeasurements, and are smaller than the plotting symbol for R-16.

Figure 2. Quiescent batch leaching results showing zinc concentrationincreasing linearly with time. Error bars are one standard deviationcalculated from duplicate measurements, and are smaller than theplotting symbol at t = 48 h. No replicate was taken at t = 24 h.

Figure 3.Column leaching results for unwashed crumb rubber (test 1 inwhite diamonds, and test 2 in gray squares) and washed crumb rubber(test 3 in black triangles), showing an initial spike in effluentconcentration before steady-state leaching. The EPA freshwatercriterion of 0.12 mg/L is shown as a dashed horizontal line, and theinset shows detail of first 60 min. The first sample in column test 3, at t =0, was discarded due to contamination.

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[cm2/min], v is pore water velocity [cm/min], θ is dimensionlesswater content, α is a mass transfer rate coefficient (min−1), ρb isthe bulk density of the crumb rubber [g/mL] = [kg/L], and Kd isa linear equilibrium partitioning coefficient [mL/g] = [L/kg].The pore water velocity is v = q/θ, where q is volumetric flux[cm/min]. These equations are simplified by assuming plug flow(D = 0), and that leaching is far from equilibrium (KdC ≪ S).With these assumptions, (1) and (2) simplify to

αρθ

∂∂

= − ∂∂

Ct

Sv

Cx

b(3)

and

α∂∂

= −St

S(4)

Equation 3 is further simplified for quiescent batch leaching tests(v = 0). Furthermore, by assuming S ≈ constant, the samesimplification (that v = 0) can be applied to the steady phase (t≥12 h) of column leaching tests by adopting a moving coordinateframe in which v = 0. With these assumptions, eq 3 simplifies to

αρθ

=Ct

Sdd

b(5)

For initially zinc-free water, this gives a simple linear model forzinc leaching:

αρθ

=CS

tb(6)

This model will be used to interpret the zinc leaching results forthe SPLP (Figure 1), the quiescent batch leaching tests (Figure2), and the column leaching tests (Figure 3).In the SPLP, 100 g of unwashed crumb rubber was added to

make a total volume of 2000 mL. Assuming crumb rubber soliddensity 1.13 kg/L,43 this gives bulk density ρb = 0.050 kg/L andwater content θ = 0.956. Assuming crumb rubber is 1.6% zincoxide by weight (Chris Madden, Michelin Americas ResearchCorporation, personal communication, 2010) gives an initialsolid concentration of S = 12 775 mg/kg. Using the standardSPLP agitation time of 18 h, the mass transfer rate parameter αwas then calculated for each of the concentration measurementsin Figure 1a, and ranged from aminimum of 5.1× 10−7 min−1 forthe largest crumb rubber to a maximum of 1.8 × 10−6 min−1 forthe smallest crumb rubber. These results are plotted versuscrumb rubber surface area in Figure 1b, where surface area iscalculated by assuming monodisperse spherical crumb rubber:

ρ=A

md

6

s (7)

where A is surface area [m2],m is the mass of crumb rubber [kg],ρs is solid density [kg/m3], and d is the geometric mean of theopening space of the passing and retaining sieves [m]. Figure 1bshows that the mass transfer rate parameter depends linearly oncrumb rubber surface area.In the quiescent batch leaching tests, 25 g of washed crumb

rubber was added to 500 mL of water. Assuming solid density of1130 kg/m3 as above gives total volume of 522 mL, bulk densityρb = 0.048 kg/L, and water content θ = 0.958. Linear regressionof the (t,C) data in Figure 2, where the regression line was forcedto pass through the origin, gives a slope of dC/dt = 0.0281 (mg/L)/h, from which the average mass transfer rate coefficient α =7.3× 10−7 min−1 was calculated from (5). Figure 2 shows that themass transfer rate coefficient is constant over time.

In the column leaching tests, zinc leaching is conceptualized asthe superposition of two processes. This rationale for thisconceptualization is discussed in Section 5. The first processdescribes steady-state zinc leaching at late time, implementedwith eq 6, based on a large solid concentration S1 and a smallmass transfer rate coefficient α1. The second process describesdynamic zinc leaching at early time, in which a small mass ofquickly mobilized zinc m2 gives a small solid concentration S2associated with a large mass transfer coefficient α2. The secondprocess is implemented numerically with eqs 3 and 4. Thisconceptual model was applied separately for column leachingtests with unwashed crumb rubber (column tests 1 and 2) andwashed crumb rubber (column test 3). The following areassumed to be constant in column tests 1−3: Bulk density ρb =0.495 kg/L and water content 0.562 were determined bymeasuring the mass of crumb rubber required to fill the columnof depth 51.4 cm. The volumetric flux q = 6.66 cm/min wasdetermined from the known discharge of 540 mL/min, so onepore volume is flushed every 4.34 min.To model dynamic zinc leaching, eqs 3 and 4 were

implemented using the software package HYDRUS-1D, whichwas chosen because it allows the initial spatial distributions ofaqueous concentration Co(x) and of solid concentration So(x) tobe specified independently. The spatial resolutionΔx = 2.572 cmand variable time step Δt ≥ 0.2 min were chosen to comply withthe Courant number limitation ofCre≤ 1 given in the HYDRUS-1D (3.0) user’s manual (ref 44, Section 8.4.5). The assumption ofplug flow (D = 0) prevented compliance with the correspondinggrid Peclet number limitation of Peg < 5, but sensitivity analysisshowed that results were insensitive to D. Steady flow wasmodeled by arbitrarily setting zero pressure at the upper andlower boundaries (i.e., unit gradient flow), and then setting thehydraulic conductivity in HYDRUS-1D to 6.66 cm/s in order tomatch the actual volumetric flux. All other specifications wereidentical to Example 5 in the HYDRUS-1D (3.0) user’s manual.The HYDRUS-1D model for dynamic zinc leaching wasimplemented by choosing a mass of initially mobilized zinc m2,calculating the corresponding solid concentration So(x), whichwas assumed to be uniformly distributed along the column depth,and then using HYDRUS-1D’s parameter estimation utility to fitthe mass transfer rate coefficient α2 to the difference between theeffluent concentration and the steady effluent concentration,C(t) − C(t ≥ 12 h), where the steady effluent concentration wasmodeled separately using eq 6. For unwashed crumb rubber,concentrations used for model fitting were averaged at t = 0, 5,and 10 min, where column tests 1 and 2 recorded effluentconcentrations at equivalent times. Fitting was optimized byperforming a grid search on m2 (every 0.3 mg) to identify thevalue that minimized the root mean squared error (RMSE) of thefinal superposition of the steady and dynamic zinc leachingmodels.For unwashed crumb rubber with 1.6% zinc oxide, the steady

effluent concentration, corresponding to water exposed to crumbrubber for 4.34 min, is C(t ≥ 12 h) = 0.1078 mg/L. According toeq 6 this corresponds to a steady mass transfer rate coefficient α1= 2.0 × 10−6 min−1. Fitting the mass of initially mobilized zincm2= 11.1 mg and the corresponding α2 = 0.19 min−1 gave aminimum RMSE of 0.13 mg/L (Figure 4). For washed crumbrubber, the steady effluent concentration is C(t ≥ 12 h) = 0.1595mg/L, corresponding to a steady mass transfer rate coefficient α1= 3.3 × 10−6 min−1. Choosing m2 = 7.1 mg and fitting α2 = 0.031min−1 gave a minimum RMSE of 0.056 mg/L (Figure 5).

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5. DISCUSSIONThis study and the literature summarized in Table 1 indicate thatpH, crumb rubber size, and leaching time all play important rolesin zinc leaching from tire crumb rubber. pH was not investigatedin this study, but linear regression of the data in Table 1 indicatesthat dC/d(pH) ranges from −4.337 to −6.3,42 confirming thewell-known result that zinc leaching increases with decreasingpH. With regard to crumb rubber size, the results in Figure 1indicate increased leaching from smaller crumb rubber, with alinear correlation between the mass transfer rate coefficient andthe crumb rubber surface area.The quiescent batch results in Figure 2 and the steady-state

results in Figure 3 are all consistent with the simple model of eq 6that indicates linear zinc leaching with time. The importance ofleaching time also reflected in the significant correlation betweenzinc leaching and time evident in the data ofWik et al.36 shown inTable 1. The leaching model of eq 6 depends on the assumptionthat the solid phase zinc concentration S is constant with time,which appears to be reasonable based on the linear leachingversus time up to 96 h in Figure 2, and the steady effluentconcentrations up to 24 h in Figure 3. The assumption ofconstant S was evaluated with a hypothetical HYDRUS-1Dmodel having So(x) = 12 775 mg/kg and α = 3.3 × 10−6 min−1

(for washed crumb rubber) which resulted in a final value of S =12 710 mg/kg, a drop of only 0.5% after 24 h of continuousleaching. The linear kinetics reported here are in contrast to theprevious study of Degaffe and Turner,17 who reported that zinc

concentration in batch samples increased as t1/2, consistent withdiffusion-limited leaching. We speculate the difference in kineticsis a consequence of our experiments being far from equilibrium,(i.e., KdC ≪ S), such that the driving force for zinc leachingremains nearly constant.Mass transfer rate coefficients fitted in this study are

summarized in Table 2. Excluding α2, these coefficients range

from aminimum of 7.3× 10−7 min−1 for quiescent batch leachingtests to a maximum of 3.3 × 10−6 min−1 for steady columneffluent fromwashed crumb rubber, with a geometric mean of 1.5× 10−6 min−1. These differ by a factor of 4.7, which is much lessthan the ratio of the maximum to the minimum zincconcentrations of 8940 reported in Table 1. One speculativeinterpretation of the results in Table 2 is that leaching in thequiescent batch samples was slower than in steady phase of thecolumn studies because the diffusion length scale is larger instagnant water than in flowing water. Similarly, one mayspeculate that leaching from washed crumb rubber in the steadyphase of the column studies was faster because washing removeddetritus from the crumb rubber surface. However, thesespeculative interpretations do not explain why the fitted leachingrate in the SPLP is within the range of other rates, rather thanbeing the fastest, which might have been expected consideringthe low pH and agitation used in the SPLP. In this light, perhaps abetter way to interpret Table 2 is that the range of zinc leachingrates is within a factor of 2 or 3 of the geometric mean of α = 1.5×10−6 min−1. The width of this range is consistent with the well-known chemical heterogeneity of tire crumb rubber.45

In contrast to the SPLP, quiescent batch leaching, and steady-state column leaching results that are all consistent with the linearmodel for zinc leaching given in (6), the initial column leachingresults required an empirically fitted additional mass of 11.1 mgfor unwashed crumb rubber and 7.1 mg for washed crumbrubber. These results are analogous to the literature cited abovethat indicate decreased leaching with time.10,17−19 The initialmasses required to fit the initial column leaching results areequivalent to 0.042% of the total mass of zinc in the unwashedcrumb rubber and 0.027% of the total mass of zinc in the washedcrumb rubber. For the case of washed crumb rubber, thisadditional mass can be explained in part as follows: To preparefor each column leaching test, and to ensure saturation withwater, the crumb rubber was filled from below at 50 mL/min,which required 46.9 min. An additional 13 min were required tofill the top of the column before the leaching test began.Therefore, at the beginning of the column leaching test, the waterat the top of the column had been exposed to crumb rubber for59.9 min. Using the steady mass transfer rate coefficient of α =3.3 × 10−6 min−1, this corresponds to an initial concentration of2.2 mg/L, with correspondingly lower initial concentrationslower in the column. Using this initial concentration profileCo(x) as an initial condition in HYDRUS-1D rather than auniformly distributed additional mass of 11.1 mg produces abreakthrough curve (Figure S2 in the Supporting Information)

Figure 4. Fittedmodel for column leaching with unwashed crumb rubber(test 1 in white diamonds, and test 2 in gray squares), with an initialpulse of 11.1 mg and a mass transfer rate coefficient α2 = 0.19 min−1,giving a root mean squared error (RMSE) of 0.13 mg/L. The insetshows detail of first 60 min.

Figure 5. Fitted model for column leaching with washed crumb rubber(test 3 in black triangles), with an initial pulse of 7.1 mg and a masstransfer rate coefficient α2 = 0.031 min−1, giving a root mean squarederror (RMSE) of 0.055 mg/L. The inset shows detail of first 60 min.

Table 2. Summary of Fitted Mass Transfer Rate Coefficients(α)

unwashed washed

SPLP 1.2 × 10−6 min−1 not availablequiescent batch leaching tests not available 7.3 × 10−7 min−1

column leaching tests, steady, α1 2.2 × 10−6 min−1 3.3 × 10−6 min−1

column leaching tests, initial, α2 0.19 min−1 0.031 min−1

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that is consistent with the first measured effluent concentrationof 0.60 mg/L at 5 min, but results in a larger RMSE of 0.10 mg/L,and requires a fitted value of DL = 276 cm, which is probablyunrealistic considering that the column is only 51.4 cm tall.Similarly, for the experiments with unwashed crumb rubber,using α = 2.2 × 10−6 min−1 gives an initial concentration of 1.5mg/L for the water at the top of the column, which is too low toexplain the peak of 2.6 mg/L measured at 5 min. Based on theselines of evidence, it appears unlikely that the initial pulse ofelevated zinc concentration is simply a consequence of additionalleaching time during column filling. The additional massreflected in the peak may have resulted from mobilization ofcolloidal tire wear particles28 during the first few pore volumes ofthe column leaching test. Such particles would have been absentin the experiment using washed crumb rubber. This explanationremains speculative, however, lacking confirmation by turbiditymeasurements or other means to confirm the presence ofcolloidal tire wear particles. It is apparent, however, that washingthe tire crumb rubber reduces the zinc leaching, at least in theinitial pulse.Comparison with a similar column leaching study by Lee18

highlights several unique aspects of the current study andemphasizes the importance of EPA’s freshwater quality criteria.Lee measured zinc concentration versus time at the effluent of a20 cm column leached with deionized water at pH 4, reporting arapid drop in zinc concentration up to 10 pore volumes. Thisrapid drop in zinc concentration was modeled with an analyticalsolution of the ADE, which assumed instantaneous equilibriumbetween the solid and liquid phases, and which predicts zeroeffluent concentration at late time. In contrast, the present studyreports effluent concentrations up to 24 h, equivalent toapproximately 330 pore volumes. After an initial pulse ofelevated zinc concentrations, these long-term leaching experi-ments reveal steady-state leaching that require the kineticleaching model described above. The fact that zinc concentrationdoes not go to zero at long time emphasizes that compliance withthe EPA’s freshwater quality criteria for zinc is more difficult thancompliance with the EPA’s maximum contaminant level (MCL)of 5 mg/L for zinc.Although steady zinc leaching concentrations approached the

EPA’s 0.12 mg/L freshwater criteria, it should be stated that thiscorrespondence is likely coincidental. Had the time required totransmit a pore volume been greater than 4.34 min, presumablythe steady zinc leaching concentration would have been higher.The time required to transmit a pore volume, in turn, depends onthe hydraulic head gradient and on the hydraulic conductivity,the latter of which depends on crumb rubber size. The hydraulicconductivity also depends on the degree of physical, chemical, orbiological clogging, any of which could reduce flow rates, increasedetention times, and consequently increase the concentration ofzinc leached into the environment.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information provides two figures showing thegrain size distribution for the tire crumb rubber used, and resultsfrom an alternative HYDRUS-1D model. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +1-303-352-3933; fax: +1-303-556-2368; e-mail: david.mays@ucdenver.edu.

Present Address†URS Corporation, 8181 East Tufts Avenue, Denver, CO 80237-2579.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We thank Jeffrey Gee and Phil Robinson for assistance with thelaboratory experiments, Randy Ray for assistance withmachining, Katerina Kechris for the statistical meta-analysis,AcuGreen for donating the tire crumb rubber used in this study,and three anonymous reviewers for constructive feedback.Funding for this work was provided by the ColoradoDepartmentof Public Health and Environment through their AdvancedTechnology Grant Program.

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