fenton oxidation of 2,4,6-trinitrotoluene in contaminated soil slurries

ENVIRONMENTAL ENGINEERING SCIENCEVolume 14, Number 1, 1997Mary Ann Liebert, Inc.

Fenton Oxidation of 2,4,6-Trinitrotoluene inContaminated Soil Slurries

Z. M. LI, P. J. SHEA, and S. D. COMFORTSchool of Natural Resource Sciences

University of Nebraska-LincolnLincoln, Nebraska 68583-0758

ABSTRACT

Numerous acres of soil are contaminated with trinitrotoluene (TNT) at sites where munitions were

formerly manufactured, stored, or demilitarized. Our objective was to determine the potential forremediating TNT-contaminated soil by direct Fenton oxidation of contaminated soil slurries. TheFenton reagent (Fe2+ + H2O2) effectively oxidized TNT in a soil slurry (1:5 w/v soihH^O) contain-ing 4200 mg TNT kg-1, but TNT destruction was affected by temperature, dissolved organic mat-ter (DOM), and clay mineralogy. Greater TNT destruction occurred at 45°C than 23°C and whenthe Fenton reagent was added sequentially rather than in a single-batch addition. Experiments per-formed with aqueous TNT indicated little effect of DOM on total TNT destruction within 24 h, buttransformation rates were affected. While the TNT transformation rate was increased by fulvic acid(20 mg C L_1), destruction rates were similar in humic acid solution and pure H20. Although bothhumic and fulvic acid were shown to reduce Fe(III) to Fe(II), more Fe(II) was regenerated in thepresence of fulvic acid and may explain the higher TNT destruction rate. TNT mineralization ratewas not greatly affected by fulvic or humic acid in the treatment solution. Small amounts of Ca2+-montmorillonite (0.1-1.0% w/v) also increased the TNT transformation rate. Adsorption of Fe(III)and TNT by montmorillonite may have enhanced Fenton oxidation of the TNT near the montmo-rillonite surface. In contrast, kaolinite had little affinity for TNT or Fe(III) and TNT transforma-tion rate decreased as kaolinite concentration was increased from 0.1 to 2.0% (w/v). Although tem-perature, soil, and solution variables affected the efficiency of TNT destruction, our results indicatethat Fenton oxidation is an effective abiotic treatment for remediating TNT-contaminated soils.

Key words: Contamination; Fenton oxidation; munitions; remediation; TNT; soil

INTRODUCTION

Wastewaters generated at former munitions pro-duction facilities often contained nitrated organ-

ics, particularly 2,4,6-trinitrotoluene (TNT) and hexahy-dro-l,3,5-trinitro-l,3,5-triazine (RDX) (Urbanski, 1964).Past disposal of these wastewaters to the surrounding en-

vironment has resulted in numerous acres of contami-nated soils and groundwater (Comfort et al., 1995; Spald-

ing and Fulton, 1988). Munitions compounds and some

of their degradation products may be mutagenic (Kaplanand Kaplan, 1982; McCormick et al., 1981), carcinogenic(Stayner et al., 1993), or otherwise toxic to aquatic andterrestrial life (Klausmeier et al., 1973; Liu et al., 1976;

. Schott and Worthley, 1974; Smock et al., 1976). Soilshighly contaminated with TNT need to be remediated to

prevent ground and surface water contamination and en-

sure public safety.

55

56 LI ET AL.

Advanced oxidation processes (AOPs), based on gener-ation of reactive radicals to destroy organic pollutants, are

commonly used for remediating wastewaters contaminatedwith synthetic organic compounds (Sato and Leung, 1991;Sedlack and Andren, 1991; Venkatadri and Peters, 1993;Watts et al., 1990). The Fenton reaction (Fenton, 1894) isone of the oldest, most powerful oxidation treatments avail-able. The Fenton reagent is a mixture of ferrous iron andhydrogen peroxide that generates reactive radicals, primar-ily the hydroxyl radical (HO-). Hydroxyl radicals (E° =

—2.8 V) react nonspecifically with organic compounds ata rate of 107 to 109 L mol"1 s"1 (Buxton et al., 1988). Theprimary reactions in the Fenton process are:

H202 + Fe2+ -* HO- + HO" + Fe3+ (1)H202 + Fe3+ -> H02- + H+ + Fe2+ (2)

HO-+ Fe2+ -+ OH' + Fe3+ (3)H02- + Fe3+ -* 02 + H+ + Fe2+ (4)

H202 + HO- -^ H20 + H02- (5)

where HO- is the hydroxyl radical and H02- is theSuperoxide radical. The efficiency of the Fenton reactionis highly pH dependent.

Fenton oxidation has been used to treat wastewaterscontaining recalcitrant compounds, including nitroaro-matics and azo dyes (Fenerstein et al., 1981; Ho, 1986;Kitao et al., 1982; Mohanty and Wei, 1993; Pignatelloand Day, 1996). We previously showed that the Fentonreaction could effectively destroy and mineralize TNT (Liet al. 1997a, 1997b). Adding the Fenton reagent to a 0.31mM TNT (70 mg L~') solution resulted in complete de-struction within 8 h, and 40% mineralization within24 h. Oxalate, N03~, H20, and C02 were the primary endproducts of TNT oxidation by Fenton reagent in the dark(Li et al., 1997a). Although these results demonstrated thepotential of the Fenton reaction to destroy TNT, the ef-fects of soluble organic matter and clay minerals on Fen-ton oxidation of TNT must be understood before scale-upof this treatment can be realized for on-site remediation.To date, only a few researchers have investigated Fentonoxidation in soil systems (Gauger et al., 1991; Li et al.,1997a, 1997b; Pignatello and Day, 1996; Ravikumar andGurol, 1992; Tyre et al., 1991; Watts et al., 1990, 1991,1993), and little information is available on the effects ofslurry temperature, DOM, and clay minerals on Fentonoxidation of soil contaminants. Our objective was to de-termine the potential of the Fenton reaction to destroyTNT in slurries of TNT-contaminated soil. By varying theconcentrations of humic acid or fulvic acid and clay min-erals in the reaction mixtures, the utility of this approachfor remediating various soil types was evaluated.

METHODS

MaterialsPure crystalline 2,4,6-trinitrotoluene (TNT) was ob-

tained from the U.S. Biomédical Research and Develop-ment Laboratory (Fort Detrick, Frederick, MD). Carbon-14 ring-labeled TNT was custom-synthesized (NENResearch Products, Boston, MA). Montmorillonite andkaolinite were obtained from Ward's Natural ScienceEstablishment, Inc. (Rochester, NY). Analytical gradeH202, FeS04-7H20, FeCl3-6H20 and H2S04 (Fisher Sci-entific, Pittsburgh, PA), and kaolinite were used as ob-tained. The montmorillonite (<2 pm) was washed fourtimes with 1 M CaCl2 to prepare Ca2+-saturated clay.The clay was washed repeatedly with double-deionizedwater andcentrifuged after each wash to remove the Cl^.After oven drying and light grinding with a mortar andpestle, clay powders were stored in airtight plastic bagsuntil used. Standard reference humic acid IR103H andfulvic acid IR103F were obtained from the InternationalHumic Substance Society (IHSS, Colorado School ofMines, Golden, CO). The humic acid contained 56.8%carbon (C), 4.0% hydrogen, 34.9% oxygen, 3.7% nitro-gen, 0.7% sulfur, and 0.1% phosphorus (total 100.2% byweight); fulvic acid contained 48.0% C, 3.4% hydrogen,41.4% oxygen, 2.3% nitrogen, 0.7% sulfur, and <0.1%phosphorus (total 95.9% by weight).

General ProceduresTechnical grade and 14C-ring labeled TNT (137 MBq

mmol-') were used in all experiments. Unless otherwiseindicated, the Fenton reagent is defined as a mixture of0.29 M H202 (1%) and 1.43 mM Fe2+ (80 mg L"1 addedas FeS04-7H20). Carbon-14 TNT was added to TNT so-

lutions prepared from technical grade TNT in distilleddeionized water. Initial TNT concentration was 0.31 mM(550 Bq 14C-TNT mL"') and reaction solutions were ad-justed to pH 3.0 with 1.0 N H2S04. All reactions were con-

ducted in duplicate in the dark on a reciprocating shaker.TNT concentration was determined by high perfor-

mance liquid chromatography (HPLC) (Shimadzu, Kyoto,Japan) using a Keystone Betasil NA column (KeystoneScientific Inc., Bellefonte, PA) with an isocratic mobilephase containing 55:45 (v/v) CH3OH:H20 at flow rate of1.0 mL min-1. TNT was detected spectrophotometricallyat 254 nm and quantified by comparison with high puritystandards. The limit of quantification was 0.44 pM TNT(0.1 mg L_1). Carbon-14 activity was determined by mix-ing 0.5 mL of the sample solution with 6 mL Ultima GoldCocktail (Packard, Downers Grove, IL) and liquid scintil-lation counting (LSC). Solution pH was measured using a

pH meter equipped with an Accu-pHast micro-combina-

FENTON OXIDATION OF 2, 4, 6-TRINITROTOLUENE 57

tion glass electrode (Fisher Scientific, Pittsburgh, PA). To-tal soluble iron concentration was determined by atomicabsorption (Model 460, Perkin-Elmer, Norwalk, CT). Soilorganic matter was quantified by the Walldey-Blackmethod (Nelson and Sommers, 1982). X-ray diffraction ofpowder-mounted soil was used to determine changes in in-terlayer spacing following TNT sorption (Soil SurveyStaff, 1982). Samples were analyzed with a Rigaku Geiger-flex X-ray diffractometer (Rigaku/USA, Inc., Danvers,MA), with a strip chart recorder. The unit was set at 40kV and 30 mA across the X-ray tube, which emitted Cu-Karadiation at 0.15405 nm; scan speed was 8° 20min"1.

TNT Oxidation in a Soil SlurryFenton oxidation was conducted in an aqueous slurry

of TNT-contaminated soil obtained from the former Ne-braska Ordnance Plant (NOP) near Mead, NE. Soils inthe vicinity of the sampled area are classified as Sharps-burg (Typic Argiudoll). The contaminated soil contained4200 mg kg"1 CH3CN-extractable TNT. The sample soilhad a pH of 6.5 (1:1, soiLFLiO). Particle size analysis(Soil Survey Staff, 1982) indicated 25% sand, 52% silt,and 23% clay. Organic matter was 2.9% as determinedby the Walkley-Black method (Nelson and Sommers,1982). Total iron in the soil was 16600 mg kg-1, as de-termined by EPA Method 6010.

Soil slurries were prepared by shaking a 1:5 soil:water(w/v) suspension for 24 h in a 50-mL Teflon tube. Solu-tion pH was then adjusted to 3.0 with 1.0 N H2S04 be-fore initiating the Fenton reaction. Fenton oxidation ofTNT in the soil slurry was conducted at 23 and 45°C, withthe 45°C temperature controlled by a circulating waterbath. At selected samplings, two reaction tubes from eachtreatment were removed and centrifuged. Supernatant (0.5mL) was mixed with 30/xL of concentrated H2SO4 to ter-minate the reaction (Li et al., 1997a) and TNT concen-

tration was determined by HPLC. The remaining super-natant was decanted and residual TNT was extracted bysonicating with 15 mL CH3CN for 18 h at 30°C. TNTconcentration in the extract was analyzed by HPLC. Step-wise Fenton oxidation was conducted by either addingfour additions of Fenton reagent at 4-h intervals (adding0.29 M H202 + 1.43 mM Fe2+ at each step), or two ad-ditions at 8-h intervals (adding 0.58 M H202 + 2.86 mMFe2+ at each step) during the first 16 h of a 24-h treat-ment. A single-dose treatment of 1.18 M H2O2 + 5.73mM Fe2+ was used for comparison with step-dose treat-ments. After 24 h, the tubes were centrifuged, and TNTconcentrations in the solution and soil were determinedas previously described. Dissolved organic carbon in theslurry solution ranged from 1.67 to 2.5 mM C (20 to 30mg C L_1), as determined by a carbon autoanalyzer.

Effect of Dissolved Organic Matter (DOM)Standard reference humic and fulvic acids were dis-

solved in 0.1 N NaOH to prepare stock solutions. Aque-ous solutions containing humic or fulvic acid (0, 0.83,1.67 and 3.33 mM C) and TNT (0.31 mM, spiked with550 Bq 14C-TNT mL-1) were prepared in duplicate, ad-justed to pH 3.0, and Fenton reagent was added. Reac-tions were conducted at 30°C in 18-mL glass tubeswrapped with aluminum foil to prevent light from con-

tacting the solution.Reduction of Fe(III) by DOM was determined by equi-

librating aqueous solutions (50 mL) containing fulvic andhumic acid (1.67 mM C, adjusted to pH 3.0 with 1 NH2S04) with 1.36 mM Fe(III) added from FeCl3-6H20stock solution. The solutions were equilibrated in 250-mL Pyrex flasks on a reciprocating shaker in the dark.Subsamples (2.5 mL) were periodically withdrawn andFe(II) was measured using the Iron Reflectoquant TestKit (EM Science, Germany).

Influence of Clay Minerals

Adsorption of 14C-TNT to kaolinite and Ca2+-satu-rated montmorillonite was determined by batch equili-bration. Clay (0.3 g) was mixed with 15 mL aqueous TNT(0, 0.04, 0.09, 0.18 and 0.31 mM TNT spiked with 550Bq 14C-TNT mL"1), adjusted to pH 3.0, and shaken ina 50-mL Teflon tube for 24 h at 23 or 45°C on a recip-rocating shaker. All sorption measurements were con-ducted in triplicate. After equilibration, tubes were cen-

trifuged to separate supernatant from the clay. Solution14C and adsorbed (CH3CN-extractable) 14C were deter-mined. Average 14C recovery was 98%. Initial and equi-librium TNT concentrations were used to construct sorp-tion isotherms using the linear form (N = 1) of theFreundlich equation:

Q = KdCN (1)where Q is the quantity of TNT adsorbed per mass of ad-sorbent (mg kg"1), C is the equilibrium TNT concentra-tion (mg L_1), and K¿ is the sorption distribution coeffi-cient (L kg-1).

Adsorption of Fe(III) to clays was determined by equi-librating Ca2+-montmorillonite (0.5, 1.0, 1.5, 2.0, and2.5%, w/v) with 15 mL of 1.21 mM Fe(III) (68 mg L"1,added from FeCl3-6H20 stock solution) solution at pH3.0 in 50-mL Teflon tubes. After 24-h equilibration on a

reciprocating shaker, the tubes were removed and cen-

trifuged. The Fe(III) as determined by mixing 1 mL ofthe supernatant with 2 mL 10% ascorbic acid for 1 minto reduce Fe(III) to Fe(II), which was measured as pre-viously described. Preliminary results with kaolinite in-dicated insignificant Fe(III) adsorption.

ENVIRON ENG SCI, VOL. 14, NO. 1, 1997

58 LI ET AL.

Fenton oxidation of TNT was conducted in aqueous sus-

pensions of kaolinite and Ca2+-saturated montmorillonite.The clays (0.1, 0.5, 1, and 2% w/v) were equilibrated with14C-TNT solution (0.31 mM spiked with 550 Bq 14C-TNTmL-1, pH 3.0) by shaking in 50-mL Teflon tubes for 24h. After equilibration, the suspension was readjusted to pH3.0 and the oxidation treatment was initiated by adding theFenton reagent. At selected times, two reaction tubes were

removed from each treatment, centrifuged, and TNT con-

centrations and 14C activity were determined.

RESULTS

Fenton Oxidation of TNT in Soil SlurriesFenton oxidation effectively destroyed TNT in a soil

slurry (1:5 w/v soil:H20, Fig. 1). Increasing slurry tem-

perature to 45°C significantly increased the rate of TNToxidation. Within 24 h, TNT concentration in soil solu-tion decreased 95% at 45°C, compared to 53% at 23°C(Fig. 1A). Remediation goals for contaminated soil are

set by the U.S. Environmental Protection Agency (EPA)and are based on CH3CN-extractable TNT. In this ex-

periment, CH3CN-extractable TNT was reduced approx-imately 96% after 24 h following treatment with Fentonreagent at 45°C (Fig. IB). At 23°C, the same treatmentresulted in a 22% decrease in CH3CN-extractable TNT.Overall, 95% of the initial TNT mass in the soil-H20mixture (solution TNT + soil-extractable TNT) was de-stroyed at 45°C, compared to 30% at 23°C (Fig. 1C).

Greater TNT destruction observed at 45°C may have re-

sulted in part from enhanced solubility, since we found theaqueous solubility of TNT increased from 0.54 mM TNTat 25°C to 1.38 mM TNT at 45°C (Li et al., 1997b). In-creasing slurry temperature also increased soluble iron con-

centration in the Fenton-slurry system and the amount of or-

ganic matter oxidized. Total soluble iron concentration was

twofold higher at 45°C than at 23°C following addition of1.18 M H202 + 1.43 mM Fe2+ (Table 1). Our previous re-

search (Comfort et al., 1995; Hundal et al., 1997) indicatedthat a large quantity of the TNT added to suspensions ofsimilar soils was found in the soil organic matter fractions;approximately 51% more soil organic matter was oxidizedby Fenton reagent at 45°C than at 23°C (Table 1).

A factorial test of H202 and Fe2+ concentrations on

TNT destruction in soil slurries was conducted. IncreasingH202 concentration up to 1.18 M increased the amount ofTNT destroyed in the soil slurry (Fig. 2). No effect of Fe2+concentration on TNT destruction was observed, exceptfor the 1.76 M H202 treatment, where each increase inFe2+ concentration resulted in greater TNT destruction(Fig. 2). At 45°C, the amount of TNT destroyed was op-

500

Time (min)FIG. 1. TNT concentration in solution (A), CH3CN-ex-tractable TNT (B), and total TNT (C) in slurries of TNT-con-taminated soil (3 g soil + 15 mL H20) treated with Fentonreagent (1.18 M H202 + 1.43 mM Fe2+), at pH 3.0 and 23 or

45°C. Initial TNT concentration in soil was 4200 mg kg" '. Barson symbols represent standard errors; where absent, bars fallwithin symbols.

timal at 1.18 M H202; this treatment also produced thehighest iron concentration after 24 h of treatment by Fen-ton oxidation (1.18 M H202 + 0 mM Fe2+ and 1.18 MH202 + 1.43 mM Fe2+; Table 1). We also noted that in-creasing the Fe2+ concentration reduced the total iron re-

leased into the system. Net iron release was 1.91, 1.63,1.36, 0.63, and -0.02 mM total soluble iron followingtreatment with 1.18 M H202 and the corresponding initialFe2+ additions (0, 0.71, 1.43, 2.86 and 5.73 mM Fe2+).

Because none of above treatments achieved the EPAremediation goal of 17.2 mg TNT kg-1 for the highlycontaminated NOP soil (4200 mg TNT kg-1), we soughtto improve the efficiency of Fenton oxidation by step-dose addition of Fenton reagent, as suggested by Potterand Roth (1993). By adding Fenton reagent to the soilslurry (45°C, pH 3.0) at 4-h intervals during the first 16h of a 24-h Fenton oxidation (0.29 M H202 + 1.43 mM


Table 1. Soluble Iron, Organic Matter Content, CHsCN-Extractable TNT, and Total TNT Removed24 h After Treating a Slurry of Soil Containing 4200 mg TNT kg-1 with Fenton Reagent

H202M

Fe2+mM pH

Temp.°C

Totalsoluble iron

mM

Organicmatter

TNTremoved

mg kg ' '-/,

00.290.581.181.7600.290.581.181.760.581.181.18

4 X 0.294 X 0.291.18e2 X 0.584 X 0.29

(I00001.431.431.431.431.43001.43

005.71

2 X 2.864 X 1.43

Single-Dose Treatments3

3.03.03.03.03.03.03.03.03.03.03.03.03.0

3.06.93.03.03.0

45454545454545454545232323

0.020.521.231.911.520.041.321.912.792.290.750.840.85

Step-Dose Treatments0

4545454545

2.041.074.645.144.84

2.5 (0.2)b1.2(0.1)0.8 (0.1)0.7 (0.0)0.5 (0.0)2.5 (0.2)1.4 (0.0)0.9 (0.0)0.6 (0.0)0.5 (0.0)1.4 (0.1)1.0(0.0)1.2 (0.1)

0.6(0.1)0.8 (0.0)0.5 (0.1)0.5 (0.0)0.5 (0.0)

2811 (75)1688 (112)710 (76)226 (27)300 (38)

2811 (75)1705 (73)613 (58)209 (8)218 (7)

NDCND

1602 (11)

280 (24)477 (29)111 (7)26(4)15(1)

42658595944265879696NDND67

9490979999

aH202 and Fe2+ concentrations were adjusted at the start of the experiment.bParenthetic values indicate standard deviations of means.

cNot determined.dTreatments were split into two (2X) or four (4X) applications within 24 h (i.e. 4 X 0.29 H202 indicates total H202 added was

1.18 M split into four 0.29 M doses).eA single-dose treatment for comparison with two- and four-step treatments.

Fe2+ at each step for a total of 1.18 M H202 and 5.73mM Fe2+), the EPA remediation goal of 17.2 mg TNTkg"1 was achieved for this soil. The CH3CN-extractableTNT was reduced to 25.9 mg TNT kg-1 by two-step ad-dition of Fenton reagent and to 14.8 mg TNT kg"1 bythe four-step addition, compared to 110.9 mg kg"1 by a

single addition of Fenton reagent (Table 1). AlthoughFe2+ additions were not beneficial in the single additionexperiment (Fig. 2), additions of 1.43 mM Fe2+ duringthe four-step Fenton oxidation treatment decreased ex-

tractable TNT to 14.8 mg kg" ' compared to 280 mg kg"1for a four-step addition of H2O2 alone (Table 1).

Adjusting slurry pH from 6.9 to 3.0 increased TNT de-struction during step-dose Fenton oxidation by approxi-mately 70% (0.29 M H202 or 0.29 M H202 + 1.43 mMFe2+ treatments, Table 1). The pH effect is consistentwith results obtained from Fenton oxidation of other

xenobiotics (Leung and Miller, 1992; Tyre et al., 1991;Venkatadri and Peters, 1993; Watts et al., 1990, 1991,1993). Lowering slurry pH to 3.0 nearly doubled the sol-uble iron concentration and more soil organic matter

(30%) was oxidized at pH 3.0 than at pH 6.9 (Table 1).

Effect of Dissolved Organic Matter (DOM)Adding fulvic and humic acids to the TNT reaction matrix

had little effect on the total mass of TNT destroyed follow-ing 24 h of Fenton oxidation. Slight differences in destruc-tion kinetics, however, were observed. Fulvic acid appearedto promote TNT oxidation (Fig. 3) and increased TNT trans-formation rates from 0.037 to 0.067 min-1 at 1.67 mM C(Table 2). Mineralization rates were not affected by fulvicacid. By comparison, the total amount and rate of TNT trans-formation and mineralization were not greatly affected by thehumic acid concentrations tested (Fig. 3; Table 2). At the high


60 LI ET AL.

3000

2500 +

g 2000cc

1500

ß 1000

500

OmM Fe340.72 mM Fe31.43 mMFe3

2.86 mM Fe3+5.73 mM Fe2*

-"r

Control 0.29 0.58 1.18 1.76

Hj02 Concentration (M)

FIG. 2. Acetonitrile-extractable TNT concentration in soil24 h after treating TNT-contaminated soil slurries by increas-ing amounts of H202 and Fe2+. Bars on symbols represent stan-dard errors; where absent, bars fall within symbols.

humic acid concentration (3.33 mM C), we observed floccu-lation following the pH 3.0 adjustment made prior to initiat-ing the Fenton reaction. After 24 h, however, very little 14C-activity was associated with this flocculated material.

The ability of humic and fulvic acid to alter the avail-ability of Fe(II) was determined. Although both humicand fulvic acid were shown to reduce Fe(III) to Fe(II) inthe dark, more Fe(II) was generated in the presence offulvic acid (Fig. 4), possibly explaining the enhancedTNT destruction rate observed.

Influence of Clay MineralsSmall amounts of montmorillonite increased TNT

transformation rate by Fenton oxidation, while kaolinitedecrease TNT transformation under our experimentalconditions (Fig. 5). A pseudo first-order rate equationprovided a good description (R2 > 0.90) of TNT disap-pearance in the treated clay suspension. Rate constantscalculated from the fitted curves indicate that smallamounts of montmorillonite increased TNT transforma-tion rate by as much as threefold (Table 3). Transforma-tion rate increased in suspensions containing up to 1%

?CO

<M03u

*->C

S3Ou

Ö-4-1

uessÙ

0 50 100 150 200 250 0 300 600 900 1200 1500

Time (min)FIG. 3. Effect of dissolved fulvic (FA) and humic acid (HA) on Fenton oxidation of TNT in the dark at 30°C. Initial TNT con-centration was 0.31 mM. Fenton reagent (FR) consisted of 0.29 M H202 + 1.43 mM Fe2+; solution pH was 3.0. Bars on symbolsrepresent standard errors; where absent, bars fall within symbols.


Table 2. Pseudo First-Order Rate Constants for

TNT Transformation and Mineralization by FentonOxidation (0.29 M H202 + 1.43 mM Fe2+) in the

Presence of Dissolved Organic Matter (DOM)

DOM Rate constant

Type3Concentration

mM C L~2Transformation MineralizationX 10~2 min'

FA

HA

0.000.831.673.330.000.831.673.33

3.74 (0.33)b5.49 (0.48)6.71 (0.14)5.53 (0.36)4.03 (0.26)3.66 (0.32)3.80 (0.40)3.28 (0.40)

0.25c0.220.310.260.230.180.190.18

aFA, fulvid acid; HA, humic acid.bParenthetic values indicate standard errors of the estimates.Standard errors are all<0.03 X 10~2min_1.

(w/v) montmorillonite but declined when clay was in-creased to 2% (Table 3). In contrast, kaolinite decreasedthe rate constants for TNT transformation by as much as

32% at 23°C and 52% at 45°C (Table 3).Both montmorillonite and kaolinite decreased the

amount of 14C-TNT mineralized by Fenton oxidation(Fig. 5). After 24-h of treatment with 0.29 M H202 +

1.43 mM Fe2+, about 65% of the 14C activity remainedin montmorillonite suspensions and about 60% of the 14Cwas present in the kaolinite suspensions, compared to

10 30 40

Time (h)FIG. 4. Reduction of Fe(III) in fulvic and humic acid solu-tions (1.67 mM C) in the dark. Initial Fe3+ concentration was

1.36 mM; solution pH was 3.0. Bars on symbols represent stan-dard errors; where absent, bars fall within symbols.

56% in pure aqueous solution at 23°C (Li et al., 1997a).At a clay concentration of 2% (w/v), montmorillonite de-creased TNT mineralization rate by 28% and kaolinitedecreased the rate by 20% (Table 3).

Ca2+-montmorillonite adsorbed significant amounts ofTNT (Fig. 6A), whereas kaolinite had little affinity forTNT (data not shown). TNT sorption on montmorillonitewas adequately described by the linearized Freundlichequation (R2 = 0.98). Less TNT was adsorbed at 45°Cthan at 23°C (Fig. 6A), reflecting an exothermic adsorp-tion reaction with the clay surface. Linear adsorption co-

efficients (Kd) were 48.4 L kg"l at 23°C and 45.0 L kg" '

at 45°C. Interlayer trapping may not be significant inTNT adsorption by the montmorillonite, as interlayerspacing was not affected by equilibration with TNT (datanot shown). This observation is consistent with reversibleTNT sorption on the montmorillonite, as reported by Xueet al. (1995) for TNT sorption on a Bentonite clay-sandmixture.

The Ca2+-montmorillonite also adsorbed considerableamounts of Fe(III) (Fig. 6B), whereas little Fe(III) was

adsorbed by kaolinite at pH 3.0. The linearized Freund-lich equation provided a good description of Fe(III) sorp-tion on Ca2+-montmorillonite (R2 = 0.87); the linear ad-sorption coefficient (Kd) was 319.3 L kg"1 at 23°C.

DISCUSSION

Fenton oxidation rapidly and effectively destroyed TNTin slurries of contaminated soil. TNT destruction was

greater at 45°C than at 23°C, consistent with reported Fen-ton oxidation of 2,4-dinitrotoluene (Mohanty and Wei,1993). Greater TNT destruction at 45°C can be attributedto higher TNT solubility and increased desorption fromsoil, higher soluble iron concentration, and greater oxida-tion of soil organic matter. A higher solution concentra-tion at 45°C would allow more TNT oxidation and pro-mote desorption. Pignatello and Day (1996) postulatedthat below 35°C Fenton oxidation of methyl parathion[0,0-dimethyl-0-(4-nitrophenyl)phosphorothioate] in a

1:1 soil suspension was limited by de-sorption from soil.Maintaining high soluble iron concentrations in soil solu-tion would also maximize free radical generation andmaintain a higher oxidation rate throughout the 24-h Fen-ton treatment. Increasing slurry temperature to 45°C in-creased the soil organic matter oxidized by the Fentonreagent. Oxidation of soil organic matter likely destroyedTNT associated with the matrix and increased the totalTNT destroyed by the Fenton reagent.

The EPA remediation goal (17.2 mg TNT kg"1) forhighly contaminated soil from the NOP site was achievedby Fenton oxidation of a soil slurry through step-dose ad-


62 LI ET AL.

OCD

0 50 100 150 200 250 300 0 300 600 900 1200 1500

Time (min)

FIG. 5. Effects of Ca2+-montmorillonite and kaolinite on TNT transformation and mineralization by Fenton oxidation. InitialTNT concentration was 0.31 mM. Fenton reagent consisted of 0.29 M H202 + 1.43 mM Fe2+; solution pH was 3.0. TNT mass

in the clay suspension was the sum of TNT in solution and that extracted from clay with CH3CN. Bars on symbols representstandard errors; where absent, bars fall within symbols.

dition of Fenton reagent. Multiple additions of Fentonreagent at low concentrations maintained a high destruc-tion rate in the slurry throughout the reaction period andsignificantly increased the efficiency of TNT destruction.This observation indicates that intermittent injection oflow concentrations of Fenton reagent into a soil slurryreactor will maintain a high oxidation potential and max-imize treatment efficiency.

Adding Fe2+ to the soil slurry did not significantly in-crease TNT destruction in single-dose application of Fen-ton reagent but enhanced TNT destruction when appliedin a step-dose manner. Although soils may contain suf-ficient iron or other electron donors to initially react withthe added H202, multiple additions of Fe2+ would bebeneficial during treatment of highly contaminated soil.Soils containing lower TNT concentrations may not re-

quire Fe2+ addition. Tyre et al. (1991) and Watts et al.(1993) found that some iron oxyhydroxides (goethite,hematite, and magnetite) directly catalyzed pen-tachlorophenol oxidation by H202 at pH 3.0. However,

at low soil iron content and high TNT concentrations,Fe2+ addition during step-dose Fenton oxidation ap-peared critical for a higher initial oxidation rate and max-

imum treatment efficiency.Adjusting soil slurry pH to 3.0 enhanced TNT destruc-

tion by Fenton oxidation. Acidification minimizes precipi-tation of iron oxyhydroxides (Lindsay, 1985), and has beenused to optimize Fenton oxidation (Leung et al., 1992; Liet al., 1997a; Tyre et al., 1991). The pH adjustment maynot be necessary if sufficient iron is present in the soil to

generate free radicals from reaction with H202 (Sun andPignatello, 1993; Tyre et al., 1991; Watts et al., 1991).Without pH adjustment, we observed a decrease in CH3CN-extractable TNT from 4200 to 52.8 mg kg"1 after four ad-ditions of0.29 M H202 + 1.43 mM Fe2+ to the highly con-

taminated NOP soil. This indicates that the Fenton reagentcould be used to treat the soil without adjusting pH.

Although our results indicated little effect of DOM ontotal TNT destruction by Fenton oxidation after 24 h,transformation rate was affected. A number of compet-


Table 3. Pseudo First-Order Rate Constants for TNT Transformation and Mineralizationby Fenton Oxidation (0.29 M H202 + 1.43 mM Fe2+) in Clay Suspensions

Clay typeAmount

%Temperature

°C

Rate constant

Transformation MineralizationX 10- min -i

No clay

Montmorillonite

Kaolinite

0.00.00.10.51.02.00.10.51.02.00.10.51.02.00.10.51.02.0

452345454545232323234545454523232323

4.37 (0.29)a2.10(0.12)5.60 (0.20)9.25 (0.22)

18.15 (0.45)16.13 (0.17)3.39(0.11)7.80 (0.02)8.61 (0.11)7.29 (0.32)3.06 (0.31)2.63 (0.26)2.84 (0.31)2.09 (0.25)2.07 (0.16)2.02 (0.20)1.67 (0.18)1.43 (0.16)

0.15b0.120.120.120.150.110.110.120.130.090.150.140.140.140.100.100.100.09

"Parenthetic values indicate standard errors of the estimates.Standard errors are all <0.02 X 10"2 min"1.

ing processes affect the Fenton reaction in the presenceof DOM. Our experimental results indicated humic acidand fulvic acid can reduce Fe(III) to Fe(II) in the dark,but Fe(III) reduction was more rapid in the fulvic acidsolution than in the humic acid solution. This can be at-tributed to the higher reduction potential of fulvic acidthan humic acid (Tan, 1993). Fulvic acid may also pro-mote H202 production from 02 in the presence of OHradicals. Voelker and Sulzberger (1996) found thatSuwannee River fulvic acid reacts in the dark with OHradicals from the Fenton reaction to produce an organicradical that reduced oxygen to H02 /02 , which subse-quently regenerated H202 by reaction with Fe(II) (Zuoand Hoigne, 1992). Fulvic acid contains numerous car-

boxylate binding sites (Sposito, 1989) and can react withFe(II) to form Fe(II)-fulvate complexes, which react more

rapidly with H202 than Fe(II)-aquo complexes (Voelkerand Sulzberger, 1996). These characteristics may explainthe complexity of Fenton oxidation of TNT in a solutioncontaining DOM. The reaction-promoting effect of ful-vic acid may not be significant in a soil slurry where com-

peting reactions are occurring. Although dissolved fulvicacid may enhance Fenton treatment of contaminated wa-

ter where it is usually the predominant DOM fraction(Miller, 1994), the overall effect can be expected to varywith the source of fulvic acid (Wang et al., 1995).

Clay mineralogy may also affect Fenton oxidation ofTNT in soil slurries. At low concentrations, Ca2+-mont-morillonite promoted TNT transformation but reducedthe amount of TNT mineralized during treatment with theFenton reagent. The montmorillonite adsorbed significantamounts of Fe(III) and TNT, which may have resulted inhigher rates of Fenton oxidation of TNT near the claysurface. A lack of significant interlayer adsorption indi-cated reversible TNT sorption on the Ca2+-montmoril-lonite. Although free radical generation near particle sur-

faces could increase TNT oxidation, mineralization maybe reduced because intermediates of TNT oxidation canbe sorbed through ligand-exchange complexes withcations on the mineral surface. In contrast to montmoril-lonite, kaolinite had little affinity for TNT or Fe(III) at

pH 3.0 and TNT transformation rates decreased as thekaolinite concentration increased. The lower TNT min-eralization rate observed in kaolinite suspensions was

consistent with a lower transformation rate. In a slurry,most of the clay particles are coated with a humic film(Sposito, 1989), which would mask their effects on TNToxidation by the Fenton reagent. However, the effects ofthe clay may become significant after the humic film isoxidized by the Fenton reagent.

Our previous experiments indicated that iron-oxalatecomplexes formed during Fenton oxidation of TNT and that


64 LI ET AL.

2000

0 10 20 30 40 50

Equilibrium TNT Cone (mg L ')

0 5 10 15 20 25 30

Equilibrium Fe(III) Cone (mg L" )

FIG. 6. Sorption isotherms for TNT (A) and Fe(III) (B) on

Ca2+-montmorillonite at pH 3.0.

these complexes were resistant to mineralization when thereaction is conducted in the dark (Li et al., 1997a). Otherorganic acid complexes, such as Ca2+-oxalate, could alsohinder further oxidation and lower total mineralization. Totest this hypothesis, 50 mM Ca2+ (as CaCl2) was added toan aqueous solution containing 0.31 mM TNT (spiked with14C-TNT) and treated with Fenton reagent. After 24 h, to-tal TNT mineralized in the Ca2+-enriched solution was be-tween 5 and 10% lower than in distilled deionized water.Because Cl~ does not affect Fenton oxidation of aromaticcompounds (Lou and Lee, 1995), Ca2+ appeared responsi-ble for reduced TNT mineralization. The decrease in TNTmineralization with CaCl2 corresponded well to the differ-ence in mineralization observed between treatments withand without Ca2+-montmorillonite. These results indicatethat divalent cations in soil slurries may lower TNT miner-alization through complexation of oxidation products, suchas oxalic acid. Production of oxalate complexes may be an

acceptable endpoint in remediation, since these complexesare nontoxic and commonly occur in uncontaminated soils.

CONCLUSIONS

Our results demonstrate that Fenton oxidation can ef-fectively destroy TNT in slurries of contaminated soil.Increasing slurry temperature and step-dose additions ofFenton reagent greatly enhanced the efficiency of TNToxidation by the Fenton reagent. Because the overall ef-ficiency of TNT destruction within 24 h was not greatlyaffected by DOM or clay, Fenton oxidation should be ef-fective in remediating various soil types contaminatedwith TNT. The EPA remediation goal (17.2 mg kg"1)for TNT-contaminated soil at the NOP was achieved byFenton oxidation of a soil slurry at 45°C through four se-

quential additions of Fenton reagent. Although tempera-ture, soil, and solution variables affected the efficiencyof TNT destruction, our results indicate that Fenton ox-

idation provides an effective abiotic treatment for reme-

diating TNT-contaminated soils.

NOMENCLATURE

Bq = becquerel, units of radioactivityC = carbonC = equilibrium TNT concentration (mg L"1)14C = carbon-14°C = degrees CelsiusC/Co = reduced concentrationh = hourkg = kilogramKd = sorption distribution coefficient (L kg"1)kV = kilovoltKa = K-shell emission lineL = literM = molarMBq = megabequerelmA = milliamperemM = millimolarmL = millilitermol = moleN = normalN = Freundlich power functionnm = nanometers = secondQ = quantity of TNT adsorbed per mass of

adsorbent (mg kg-1)V = voltw/v = weight/volumepg = microgrampL = microliterpM = micromolarpm = micrometer8 = glancing angle in x-ray diffraction


ACKNOWLEDGMENTS

The authors thank Byung-Taek Oh for technical assis-tance. Support was provided through the Great Plains-Rocky Mountain Hazardous Substance Research Center(Kansas State University), NSF/ESCoR cooperativeagreement EPS-9255225, and the University of Ne-braskaWater Center. Publication no. 11696, AgriculturalResearch Division, University of Nebraska-Lincoln; pro-ject nos. NEB 12-230, 239, and 251.

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Please address correspondence to:P.J. Shea

School of Natural Resource SciencesUniversity of Nebraska-Lincoln

Lincoln, NE 68583-0758

phone: 402-472-2811fax: 402-472-7904

email: [email protected]

fenton oxidation of 2,4,6-trinitrotoluene in contaminated soil slurries

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