oxidation of aniline and other primary aromatic amines by manganese

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Environ. Sci. Technol.

Gagosian, R.; Smith, S. 0.;Nigrelli, G. E. Geochim. Cos-mochim. Acta 1982,46, 163-1172.Lefebvre, G.; Germain,P.;Schneider, F. Bull. SOC.him.Fr . 1980, -2, 11-96-11-97,

Stohs, S . J.;Haggerty, J. A.Phytochemistry 1973, 12 ,2869-2872.

I 9 9 0 , 2 4 , 3 6 3 - 3 7 3

(40) Fauve, A.; Kergomard,A. etrahedron 1981,37,899-901.

Received for review March 20,1989. Accepted N ovember 9,198 9.Financial suppor t fr om EROS-2000 Project is gratefully a c -knowledged. W e thank J. . Gomez-Belinchon, R. Llop, and M.Valls fo r technical assistance.

Oxidation of Aniline and Other Primary Aromatic Amines by Manganese

Dioxide

Shonali Laha and Rlchard G. Luthy"

Department of Civil Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

Thi s investigation evalua ted the redox reaction betweena m anganese dioxide, 6 -Mn 02, and anilines and otheraromatic reductants in aqueous suspensions at pH valuesranging from 3.7 to 6.5. Th e reaction with manganesedioxide may represe nt a pathway for transformation ofaniline and other prim ary arom atic amines in acidic min-eralogical and soil/water environ men ts in the absence ofoxygen and substantial m icrobial activity. Th e reactionrate with aniline is pH-dependent, increasing with de-creasing pH, and firs t order with respect to 6-M n02andorganic solute. Aniline and p-toluidine are demo nstratedto be 2-equiv reductants, as is believed to be the case forth e other a roma tic solutes considered in this study, in-cluding the s ubstituted anilines, and hy droquinone andcatechol and their alkyl substituents. Ring-bound nitro-gen-containing arom atic solutes (methylimidazole, quino-line, and 5,5-dimethylhydantoin) were unreactive w ithmanganese dioxide a t pH 6.4. Th e order of th e reactivityof para-substituted anilines was methoxy >> methyl >chloro > carboxy >> nitro ; the relative reac tivity of thesecom poun ds correlated with th e solute's half-wave potentialand Ham me tt constant. Th e principal oxidation productsof aniline and p-toluidine with manganese dioxide at p H4 were azobenzene and 4,4'-dimethylazobenzene, respec-

tively, which agreed with a po stulated oxidative-couplingreaction mechanism . Th e abiotic redox reactions of pri-mary arom atic amines and azo compounds may result invarious respective oxidative-coupling and reductive-de-coupling reactions. The se processes may be significantwith regard to th e persistance an d transforma tion of theseclasses of organic contam inants in envir onm ental systems.

Introduction

Th e purpose of this investigation was to assess the rateof the redox reaction between a manganese dioxide, 6-

M n02 (s) , nd various aromatic reductants including severalnitrogen-containing compounds. Th e initial rate and orderof th e reaction with respect to the re ducta nt was deter-

mined as well as the effects of manganese dioxide con-centration and pH . Th e principal reaction products, underacidic cond itions, resulting from th e oxidatio n of anilineand p-toluidine were determined.

In n atura l waters, M n(II1) and Mn(1V) are usuallypresent in the form of sparingly soluble oxides and hy-droxide s, whereas Mn(I1) is the soluble phase. In soil/sediment environments, manga nese oxide is believed to beamong the strongest oxidizing agents that may be en-countered in the a bsence of molecular oxygen. M anganeseoxide can be redu ced and dissolved by organic compou nds,increasing th e m obility of m anganese a nd its availabilityto organisms ( 1 , 2 ) . These oxidative processes involvingmanganese oxides may constitute an important abioticdegradative pathway for organic com pounds in subsurface

environments. Earlier studies by Stone and Morgan (2)demo nstrated some features of the reductive dissolutionreaction between manganese oxide and organic solutes andsome of th e factors that influence the r ate of the reaction.Stone (3) considered the red uctive dissolution of manga-nese(III/IV ) oxides by substituted phenols. Those in-vestigations were performed with various ma ngan ese oxidesuspensions, one of which was primarily th e min eral-p hasefeitknechtite, P-MnOOH(s), with some am ount of m an-

ganite, y-MnO OH(s). In this study, a hydrous manganesedioxide susp ension was prepared according to Murray (4) ,for which his stoichiometric and X-ray diffraction analysesindicated th at this synthetic manganese dioxide is struc-turally similar to the naturally occurring min eral birnessite,6-Mn02(s).

Th e organic reductants investigated in this study wereaniline an d various subs tituted anilines, hydroquinone andcatechol and some of their alkyl substitu ents , and severalring-sub stituted nitrogen-containing aromatics. Figure 1shows the structures of the compounds discussed in thisstudy. Anilines and other aro matic amines may originateas environmental contam inants from the use of pesticidesand herbicides, as well as from chem ical man ufacturin g

residues, and from byproducts of energy technologies.Along with phenol, aniline is listed as a high-prioritycom poun d in the stud y of pollutants from coal-conversionprocess wastes ( 5 ) . Aniline residues are formed in the soilas a result of microbial and plan t metabolism of phenyl-urea, acylanilide, pheny lcarbam ate, an d nitroaniline her-bicides (6). Chlorin ated aniline s such as 2,4,5-trichloro-aniline, 4-chloroaniline, 3,4-dichloroaniline, and 2,6-di-ethylaniline may be released as degradation products andintermediates of various phenylurea and phenylcarbam atepesticides (7, 36). Aniline derivatives occur as interme-diates in dye-stuff manufacture an d this con stitutes an-other possible source of environmental aniline contami-nation. A number of substituted anilines may be carci-nogenic (8).Aniline and toluidine, i.e., methylaniline, a nd

other a rom atic amines are generally toxic and can inducevarious adverse physiological responses (8,9). For thesereasons it is of considerable interest t o explore the physical,chemical, and microbial transformations that may alter thetoxicity, mobility, and bioavailability of aniline an d relatedcompounds in the environment.

Aniline and other aromatic amines are subject to com-plex environmental transformations. Lyons, Katz, andBartha (6, 10, 11) performed studies on the microbialpathways for aniline elimination from aquatic environ-ments, from which they concluded that biodegradationmay be th e most significant mechanism for the removalof aniline from pond water. Hwang and Lee (7 ) concludedth at photochem ical processes were primarily responsiblefor m ineralization of 2,4,5-trichloroaniline in surface wate r

00 3-936X/90/0924-0363$02.50/0 @ 1990 American Chemical Society Environ. Sci. Technol., Vol. 24,No. 3, 1990 363



I

OCH, CH3 c1Ani l ine p-Anis id ine p-Toluidine p-Chloroani l ine

NHz OH

I

COOH NO 2 CH3

p-Aminobenzo ic p -N l t roan i li ne Catecho l 4 -Met hy l ca te cho l

OH OH OH

Ac id

OH OH OH

Hydroquinone Methyl hydroquinone 2,3-Dimethylhydroquinone

OH OH

OH OH CH3

Tr imethy lhydroqu inone te r t -Bu ty lhydroqu inone Methy l im id azo le

Quinol ine 5,5-Dimethylhydantoin

Flgure 1. Structure of the anilines and other aromatic compoundsemployed in this study.

from an eutrophic lake. Zepp and Schlotzhauer ( 12 )showed th at th e photolytic degradation of aniline is en-hanced in the presence of algae.

Th e principal degradative processes for aniline and otherprimary arom atic amines in subsurface environments areeither m icrobially induced rea ctions or abiotic degradationprocesses through redox reaction with soil constituents,for which the oxidative degradation of aniline by manga-

nese dioxide is reported here.Materia ls a nd M ethods

T he inorganic reagents were analytical grade (FisherChemicals) and the organic compounds were used as re-ceived (purity >99%, Aldrich). All stock solutions of or-ganic substrates were used w ithin 24 h to m inimize thepossibility of deg radation by photolysis or oxidation byoxygen. D eionized water was used in the exper imen ts an dall glassware was soaked in 1:5 HN 03 /w ate r and th or-oughly rinsed with deionized water prior to use. Otherreagent solutions were also made up in acid-cleanedglassware with deionized water; these solutions were dis-carded within 2 or 3 days t o prevent microbial activity.Reagents an d reaction solutions were foil-wrapped,and the

more reactive nitrogen aromatics were main tained und erinert atmosphere.

Preparation of Oxide Suspension. Th e preparationof approximately 0.01 M b-MnOz(s) was performed asdescribed by M urray ( 4 ) . Murray's analysis of th e man -ganese dioxide produced by his method indicated it to bestructurally similar to the naturally occurring mineralbirnessite, with a surface area of 263 f 5 m2/g by the BE Tmethod a nd a m aximum p H (zpc) of 2.4 by extrapolatingelectrophoretic mobilities. T he manganese dioxide usedin this study may be similar to th at described by M urray,as the same method of prepara tion was employed. Th eprocedures for prepa ration of the m anganese dioxide were

364 Environ. Sci. Technol., Vol. 24, No. 3, 1990

as follows: 1. Combine 40 mL of 0.1 M KM n0 4 and 80mL of 0.1 M N aOH with stirring; adjust volume to 500 mL.2. Add 60 mL of 0.1 M MnC1,. 3. Allow 6-MnOZsus-pension to settle out. 4. Decant supern atant; wash oxideseveral times an d adjust t o 1-L volume w ith water havingthe s ame ionic strength and buffer as to be utilized in theexperiments.

The reactivity of a manganese oxide suspension maychange with time (1,2). To account for the effect of aging,all suspensions were used 24 h after preparation.

Tes ts with Catechols and Hydroquinones. Hydro-quinone, m ethylhydroquinone, 2,3-dimethylhydroquinone,trimethylhydroquinone, tert-butylhydroquinone, catechol,and 4-methylcatechol were evaluated in dissolution reac-tions with manganese dioxide. T he reaction rate wasmon itored by increase in Mn(I1) with atomic adsor ptionspectrometry (Perkin-Elmer M odel 703). T he reaction wasperformed w ith continuous stirring under nitrogen purgewith no oxygen. Sam ples of 5 mL were withdra wn bysyringe at predetermined time intervals and expressedthrough a syringe -tight me mbr ane filter (0.22 wm pore,1-in. diam eter; Millipore Millex-GS). It was verified tha tthe filtration procedure stopped th e reaction. Th e aliquotswere acidified with 4 N HN 0 3and analyzed for dissolvedmanganese, [Mn(II)ldisS.Where th e reaction proceededvery fast, the reacting solutions were flash mixed in a

beaker, and sam ples were withdrawn by syringe imm edi-ately and then filtered. T he reaction time included theresidence time in the syringe, and filtration m arked th eend of the time interval. Th e minimum reaction intervalby this proced ure was about 12-15 s, which represe nts apractical limit of such a technique.

Th e [Mn(II)]d, is an opera tional definition of dissolvedmanganese as employed earlier in Stone and Morgan'swork ( I ) . It is th at am ount of manganese not retained bythe m embrane filter; and an increase in manganese beyondany th at m ay be present in a reaction blank is taken tomean soluble manganese, Mn(I1).

The pH of the reaction mixture was maintained at6.4-6.5 for these tests with a 0.01 M sodium bicarbo nate

buffer. Ionic strength was kept a t 0.1 M NaN03. Blankswere prepared without organic solute to assess any changesin oxide and Mn(I1) concentrations due to reasons otherthan reductive dissolution by reaction with the organicsolute. Th e Mn(I1) reported represents net manganeseoxide dissolved, accounting for any mangan ese th at maypass th e filter initially. Th us, a value of zero is take n forthe initial concentration of M n(I1) produced by reaction.Generally blanks were found n ot to have significant dis-solved manganese. Initial reaction rate s were determ inedfrom th e slope of the manganese dissolution curve by usingthe first few da ta points for which th e reactant concen-trations did not change significantly.

Tests w ith Aniline and Substituted Anilines. Thetest protocols were similar to those fo r the hydroquinones,

bu t in addition to evaluating th e reaction ra te by increasein M n(II), the organic concentration was also monitoredby use of high-pressure liquid chromtographic (HPLC)techniques. Tests were performed a t pH 4 with an acetatebuffer as well as at pH 6 with a bicarbonate buffer.Chromatographic analyses were performed w ith a P er-kin-Elmer Series 3 liquid chromatograph equipped w itha Rheodyne 7105 injector and a Perkin-Elmer LC-15 UVdetector a t wavelength 254 nm. T he column used was a15-cm Supelco LC-P AH column w ith a particle size of 5pm. T he solvent used was 50:50 acetonitrile/w ater, a t aflow r ate of 1 mL/min. Injection volumes were between4 an d 9 pL. Where higher concentra tions of reacta nts were



used the buffer concentration was also increased, e.g., 0.1M acetate for 0.005 M reactants a t pH 4, and 0.1 M bi-carbon ate for pH values of 6. Sam ples were withdrawnat predeterm ined time intervals by syringe and th en fil-tered (0.2-pm pore, Millipore Millex-FG Teflon filters) into5-mL vials that were sealed, covered with foil, and re-frigerated until analyzed by HPL C techniques. Repre-sentative aqueous stock s olutions of organics were analyzedboth before and after filtration to ensure lack of organicadsorption on th e Teflon filters. Reaction blanks were alsostudied to observe any change in aniline concentration inthe ab sence of bo th oxygen and 6-M n02. A niline degra-

dation was negligible under such conditions in the timeperiod of the experiment.

Pea k areas for the organic analysis from H PL C deter -minations were divided by the quantity injected to yieldresponse factors from which calibration curves were con-stru cte d with known solute concentrations. Grap hs ofsolute concentration rem aining versus time were plottedand t he initial slopes were used to determine t he initialreaction rates.

Identification of Reaction Products. Th e principalreaction products for aniline and p-toluidine were deter-mined by GC-MS techniques. Th ese samples were pre-pared for reaction by mixing 0.01 M organic and 0.01 M6-Mn02(s) , t pH 4 with 0.1 M ac etate buffer and 0.1 M

NaN03 onic strength , in a separatory flask. Th e flask wasshaken intermittently in a gentle manner to ensure propermixing. L-Ascorbic acid (1M) was added to stop the re-action by dissolving th e remaining m anganese oxide (1).

T he dissolution of residual mangan ese oxide also providedfor release of any adsorbed reaction products. T he pH ofthe mixture was then adjusted to 12 by the addition ofsodium hydroxide pellets, and the organic compounds wereextracted with methylene chloride by shaking vigorouslyfor 10 min, allowing the CH2C12 o s epara te, and with-drawing throu gh a Teflon stopcock. Th e methylenechloride extract was concentrated by use of a Kuderna-Danish evaporator concen trator to volatilize the solvent.In these te sts filtration was avoided in th e event tha t areaction product may have been retained on the filters.

The concentrated extract was injected into a 5985Hewlet-Packard gas chromatograph-mass spectrophotom-eter with a 25 m X 0.2 mm SB-Sm ectic column. Th einjection tem per atu re was 280 "C, the carrier gas heliuma t 1 mL/min, the ionization potential 70 eV, and thevoltage 2400 V. Th e program was 1min at 40 "C , 40-220"C at 20 "C/m in and 220-280 "C at 4 OC/min. Th e massspectra indexes used were obtained from th e literature (13,14) . Stan dard s were also run under t he same conditionsto confirm th e iden tity of the reactants an d major prod-ucts. Th e confirmation was done by matching the chro-matographic retention time and comparing the principalmass ion distribution of stan dards an d samples with thosereported in the compendia of mass spectra.

Stock solutions of aniline and p -toluidine were analyzedby GC-MS to ensure purity and the absence of possibledegrad ation products. Control samples having organicsolute and buffer, but no 6-Mn02(s),were carried throughthe mixing and extraction procedure, including the ad-dition of ascorbic acid to verify the lack of reaction prod-ucts and to dem onstrate th at identified organic productswere present as a result of redox reaction with 6-M n02 (s) ,rather than as a spurious experimental artifact.

Additional Tests. Rate tests were performed w ith thefollowing ring-su bstituted nitrogen-containing organiccompounds: methylimidazole, quinoline, and 5,5-di-methylhydantoin. Th e pH was maintained at 6-6.4 an d

Exper imenta l Da ta

React ion Rote

---- s t imate of I n i t i a l

1/' Initial Rate=

.- ,,* , 6 1 ,/ .5x10-5mo1es/~-min

'Itl

C pH =4.4G 1 [S-MnO ],=6.8 IO+ Mz

I / [AnilineIo = 2 x I O + M

00 10 20

Time, minutes

Figure 2. Reductive dissolutionof manganese dioxide by aniline at pH4.4.

the reaction was monitored by the concentration of dis-solved manganese (using atomic absorption sp ectrometrytechniques) over a period of days.

Results

Initial Reaction Rates with Aniline. Rate tests wereperformed to evaluate the dissolution of manganese dioxideby several nitrogen-containing aromatic compounds. Th ecomp ounds considered were aniline, p-anisidine, quinoline,methylimidazole, and 5,5-dimethylhydantoin. Th e ob-jective was to obse rve th e rat e of dissolution of m anganesedioxide by representative nitrogen-containing aromaticsin order to identify those compounds tha t are reactive withmangan ese dioxide. T he reaction was monitored by in-crease in the concentration of dissolved manganese.

A t pH 6.4 and millimolar concentrations of organic andoxide, dissolution of 6-Mn02(s)by methylimidazole was

extremely slow, and unambiguous results were not ob-tained over 1 mo nth. Similarly, quinoline and 5,5-di-methylhydantoin did not exhibit significant dissolutionrates under these tes t conditions.

The initial rate with aniline and 6-M n02(s)was relativelyslow at a pH value of 6.5, and th e reaction r ate increasedwith decreasing pH. Figure 2 shows tha t a t pH 4.4 dis-solution of m anganese dioxide by aniline gave an initialreaction rate of 4.5 X mol/L.min expressed in termsof rate of dissolution of manganese, and that the reactionappeared to proceed at a slower rate after -20 min.Subsequent experiments with aniline were monitored bydetermining the concentration of aniline remaining insolution with HPLC techniques, for which a typical resultgave an initial reaction rate of 8 X mol/L.min ex-

pressed in terms of consumption of aniline for test con-ditions similar to those shown in Figure 2, except for asmall proportiona l increase in the initial aniline concen-tration. T he similarity of the initial reaction rates undercomparab le test conditions as given by either manganes edissolution or consum ption of aniline, in conjunction withidentification of aniline reaction products as discussedlater, suggests an eq uivalent stoichiometry in th e red uctionof aniline by m anganese dioxide. An analogo us conclusionregarding stoichiometry was obtained by Stone andMorgan (1,2) for th e case of redu ctive dissolution of p-

MnO OH(s) by hydroquinone, which was applied in theirwork for the case of 15 aromatic and 12 aliphatic com-

365nviron. Sci. Technol., Vol. 24 , No. 3, 1990



-3 - /

lo g { Ar~iline]~moles/J}

Flgure 3. Estimation of the order of initial reaction rate with respectto aniline concentration for oxidation of aniline by manganese dioxide.

pounds behaving as 2-equiv reductants with manganeseoxide. A similar inference regarding th e com pounds re-acting as 2-equiv reductants was used in this study toassess the rate of the reaction for 6-Mn 02(s )with anilineand substituted anilines, as well as for catechols and hy-droquinones. Hence, the am ount of organic compoundsremaining co uld be expressed in ter ms of dissolution of

manganese:

(1)

where [organic], represents t he molar concentration oforganic solute at time t , and [Mn(I I)], represents th edifference in molar conc entratio n of dissolved manganesebetween th e reaction mixture an d blanks at time t. Whilesubsequent identification of principal reaction productsin acidic samples suggests th at aniline and p-toluidine areprimarily 2-equiv reductants, additional work is neededto determine if the anilines are exactly 2-equiv reductantsunder various conditions.

Reaction Order wit h Aniline. T he reaction kineticsof aniline and manganese dioxide were investigated to

determ ine the order of the reaction with respect to theinitial organic solute and oxide concentrations and thedependence of th e initial reaction rate on the pH of thereaction mixture.

Th e order of th e initial reaction rate with respect toaniline was evaluated by tests in which the oxide concen-tratio n was employed a t a relatively high value of 5 X

M , while maintaining constant pH and buffer strengthsthrough comparative tests and varying the aniline con-centration by an order of m agnitude from 0.5 X to 5X M. In this approach , the initial reaction rate iscom puted over th e first few time incre men ts of the reac -tion, for which it may be assumed that the change inconce ntration of the reacta nts is negligible. T he resultsshown in Figure 3 demonstrate that the reaction rate is

first order with respect t o aniline.Similarly, the order of the reaction with respect to 6 -

M n0 2( s) was determ ined by varying the initial oxideconcentration from 1.25 X to 5 X M, whilemaintaining th e initial aniline concentration a t 2.5 X lom3

M and pH at 4. Figure 4 indicates th at the reaction orderwith respect to manganese dioxide concentration is ap-proximately unity.

Th e effect of p H on th e initial reaction rate was de-termined a t six different pH values from 4.0 to 6.5 at a ninitial aniline concen tration of 2 X M and a n initialmanganese oxide concentration, [6 -Mn 02]o, f 5 X M.Th e results are shown in Figure 5, from which it is seen

[organic], = [organiclo- [Mn(II)] ,

' 13.5

.-E9 /

slopen=I

pH.4

: /-.-c.-

Y+Cn

-5-3 -2.5 -2

l og { [8-Mn02]o, moles/Q}

Figure 4. Estimation of the order of initial reaction rate with respectto manganese dioxide concentration for reduction of 6-Mn0, by aniline.

c

Cn0

-6I I I I I

4 5 6

PH

Figure 5. Effect of pH on the initial reaction rate for oxidation/reduction

reaction between aniline and manganese dioxide.

tha t pH has a marked effect on reaction rate with the ratedecreasing by an order of m agnitude over this pH range.The reaction order is not constant with respect to pH,increasing from -0.3 at pH 4 to -0.6 a t pH 6.5. AlthoughpH values generally encountered in na tural system s arehigher, a pH of 4 was chosen in subs equen t tests to assessreaction products an d to evaluate the effect of sub stituentson the reaction rate of anilines. This was done for purposesof experim ental convenience because of th e ease of gen-eration of data in a short period of time; also the blankvalues were negligible the shor ter the reaction tim e con-sidered. It is appreciated , however, th at th e reaction ratemay dim inish substantially at higher pH , as suggested by

data in Figure 5.Substituent Effect and Reaction Order for H y-

droquinone and Catechol. Hydroquinines and catecholsare benzyl compounds with two hydroxy s ubs tituen ts, a tpara and ortho positions, respectively. A study of thedissolution of manganese was performed with hydro-quinone, catechol, and several alkyl substituents: me-thylhydroquinone, 2,3-dimethylhydroquinone,rimethyl-hydroquinone, tert-butylhydroquinone, and 4-methyl-catechol. T he initial conc entration s of the organic soluteswere varied from 0.5 X to 5.0 X M.

Mconcentration with pH maintained at 6.5 by using 0.01 M

6-Mn02was used in these tests at (1.7-2.3) X

366 Environ. Sci. Technol., Vol. 24, No. 3, 1990



Table I. Initial Reaction Rates for Hydroquinone and Alkyl-Substituted Hydroquinones at pH 6. 5

init concn, mol/L init reaction rate: mol/L.min

[6-MnO*Io [organiclo HQ methyl-HQ 2,3-dimethyl-HQ tert-butyl-HQ trimethyl-HQ

1.7 x 10-3 0.5 x 10-3 2.4 X lo4 2.1 x 10-4 2.0 x 10-4 1.6 X lo4 1.4 X 10”1.7 x 10-3 1.5 x 10-3 1.5 x 10-3 1.4 x 10-3 5.2 x 10-4 5.2 x 10-4 4.4 x 10-42.3 x 10-3 2.5 x 10-3 1.5 x 10-3 2.0 x 10-3 1.0x 10-3 1.0x 10-3 5.9 x 10-4

a HQ, hydroquinone.

1.9 x 10-3 1.9 x 10-3 1.8 x 10-3.9 x 10-3 5.0 x 10-3 2.3 x 10-3 2.2 x 10-3

- 6 . 3

-6 .4

hIW> -6 .5

-E-- 6.6dFca 6 .7*C

9

-6.8

-6.9

pH.6.5

[8-MnO2],= 1 .7~Om3M

[0rgonic]~=2.5 I O - ~ M

- rimethyl hydroquinone

I 2

Time,minutes

Figure 6. Reductive dissolution of manganese dioxide by hydroquinoneand alkylsubstituted hydroquinones at pH 6.5.

NaHCO , buffer. Dissolution of 6-M n02was measured overtime. Th e results were expressed as the na tural logarithmof the oxide remaining in th e reaction mixture versus timesince the start of the reaction, for which a typical set ofresults obtained for th e hydroquinones is shown in Figure6. The se dat a indicate that t he reaction may slow with

time, and thu s only the initial reaction rates were con-sidered for comparison, as summarized in Table I.

I t is observed that reaction rate increases with the initialconcentration of organic red uc tan t, and results shown inFigure 7 for dimethylhydroquinone and trimethylhydro-quinone indicate tha t the reaction order with respect toinitial organic loading is close to unity. Th is is consisten twith the d ata presented in Table I, for which the initialreaction rates for the hydroquinones increase by approx-imately an or der of magnitud e as initial organic concen-tration increases from 5 X to 5 X M. Methyl-hydroquinone and hydroquinone are the most reactiveamong the five compounds. However, the sub stituen teffect on reaction rate fo r the hydroquinones is not par-ticularly significant. As the reaction proceeds, the d atain Figure 6 show deviation from linearity, suggesting th atthe reaction processes are more complex tha n implied bycomparison of first-order initial reaction rates.

Results in Table I1 show the initial reaction rates forcatechol and 4-m ethylcatechol. T he initial reaction ratesare in th e range of those observed for the hydroquinones.As with the hydroquinones, it was observed th at t he dis-solution reaction with catechol was rapid, with th e reactionslowing appreciably after 2 min for these t est conditions.T he initial reaction rates increase for 4-methylcatechol inproportion to the organic concentration. However, at aconcentration of 5 x M organic th e reaction rate forboth catechol and 4-methylcatechol is no different from

7J//,3 -dim ethyl hydroquinone

- 4 - 3 -2

lo g { ~rganic]~moies/l}

Figure 7. Estimation of the order of the redox reaction with respectto 2,3-dimethylhydroquinone and trimethylhydroqulnone for reductionof manganese dioxide.

Table 11. Initial Re action Rates for Catechol and4-Methylcatechol at pH 6.4

init concn, mol/L init reaction rate, mol/L.min

[6-MnO 2Io [organic],, catech ol 4-me thylca techo l

1.7 x 10-3 0.5 x 10-3 1.0x 10-3 4.5 x 10-41.9 x 10-3 1.5 x 10-3 1.5 x 10-3 1.7 x 10-3

1.7 x 10-3 2.5 x 10-3 2.6 x 10-3 2.7 x 10-32.7 x 10-3.3 x 10-3 5.0 x 10-3 2.7 x 10-3

th at at organic concentration 2.5 X lo-, M. Stone ( 3 )reported similar behavior for the case of p-me thylphen oland m anganese(III/IV) oxide in which the reaction ratebegins to level out as th e pH is decreased below 5.0. Thisis explained through a reaction schem e in which precursorcomplex formation and electron transfer are rate-limitingsteps and electron back-transfer is negligible. In th at case,the reaction rate resembles a transition from pseudo firstorder to zero order with respect to the conce ntration of theorganic compound. In Stone’s work it was explained th at,at lower concentrations of p-m ethylphenol, the reactionrat e increases linearly with organic concentration, bu t asthe concentration of the organic compound increases, apoint is reached where the oxide surface is saturated withorganic compound and electron transfer becomes ratelimiting, and the reaction rate remains unaffected byhigher concentrations of organic. In this stud y the re sultsfor catechol and 4-methylcatechol indicate that as theinitial concentration of th e orga nic solute increases beyondthe initial concentration of 6-Mn02, the proportionalitybetween reaction rate and organic concentration no longerholds, probably through a change in rate-limiting mecha-nism as suggested by Stone ( 3 ) .

The formation of Mn(I1)-organic complexes was notinvestigated. Stab ility consta nts reported for catechol by

367nviron. Sci. Technol., Vol. 24, No. 3, 1990



Table 111. Initial Reaction Rates for Aniline and Substituted Anilines in the Presence of 6- Mn02

Hammettb [organiclo,compound EljZ? v const mol/L

p-anisidine 0.393 -0.78 6.2 X

p-toluidine 0.537 -0.31 2 x 10-3aniline 0.625 2.5 x 10-3p-chloroaniline 0.675 0.11 5 x 10-3aminobenzoic acid 0.794 0.42 5 x 10-3p-nitroaniline 0.935 0.79 2 x 10-3

" E , , , values from Su atoni e t al. (28). u+ values from March (22).

[MnOzlo,

3.3 x 10-52. 5 x 10-35 x 10-35 x 10-35 x 10-35 x 10-3

mol/Linit rate, k ex ?

p~ mol/L.min L/moiimin

4.4 6. 5 X lo4 320004 1.2 x 10-3 2404 1.3 x 10-4 10.44 1.1x 10-3 444 3.1 x 10-3 3.444 - 1 x 10-7 -0.01

5 4 x ' 6 5 t

//' I n i t i a l R a t e =

/e 6 5 x 1 0 - s m o l e s / l - m i n

p H . 4 . 4

- Mno,] 0=3.5 x I0-4

[ p - A n i ~ i d m e ] ~ = 65 ~ 1 0 ~ ~

--- Exp e r i me n ta l Da ta- s t i m a t e of In i t ia lRe a ct i o n Ra te

I

0 50 I O 0

T i m e , s e c on ds

Figure 8. Reaction of p-anisid ine and manganese dioxide at pH 4.4.

Sillen and Martell (37), e.g., log K1 = 7.52, suggest th atMn(I1)-organic com plex formation in th e aqueous phasemay have an effect on the reaction mechanism and rate.Sm ith and Martell (38)reported no stability constants forMn(I1) with a niline, p-toluidine, or p-anisidine. However,

reported log K values are sm all for Cd(I1) with aniline (logK = O . l ) , p-toluidine (log K = 0.261, and p-an isidin e (logK = 0.45), and by inference, complexation of the anilineswith M n(I1) may be insignificant in comparison to catechol.

Substituent Effect for Aniline. Th e initial reactionrate was evaluated for various substituents of aniline:p-anisidine (i.e,, p-meth oxyaniline), p-toluidine &e., p-methylaniline), p-nitro anilin e, 4-aminobenzoic acid, andp-chloroaniline. The se tests were performed a t pH 4withace tate buffer. An initial oxide concentration of 5 X

M was employed except for the te sts with p-toluidine a ndp-anisidine, for which [6-Mn02]o= 2.5X and 3.25 X

lo4 M, respectively. Th e organic solute concentration was(2-5) X M, except for tes ts with p-anisidine, for whichi t was 6.25 X M. p-Anisidine dissolved mangane seoxide so rapidly th at it was not possible to measure thedissolution ra te a t millimolar concentrations of bot h oxidean d organic. F igure 8 shows a test resu lt for increase inMn(I1) fo r p-anisidine where th e initial concentrations oforganic and 6-M n02 are 6.25 X and 3.25 X M ,respectively. In this case the reaction was complete in -2min, as the amount of manganese dissolved approachedthe stoichiometric amoun t of p-anisidine. For the testswith the other compounds, the rate was substantiallyslower and the course of the reaction was assessed bydisappearance of reductant.

Table I11 summarizes the initial reaction rates of th evarious subs tituted anilines. Th e experimental rate con-

368 Environ Sci Technol, Vo l 24 , No 3, 1990

p H = 4 ,T i m e ? O mins115 I0 I5 20 25 30

Reaction Blank

" , I

5 25 305 200

Time, minutes

Figure 9. Chromatogr am from GC-MS analysis for identification ofreaction product for 10-min redox reaction of aniline and manganesedioxide.

stants, kexp, are also presented in the table. Th e rateconstants were computed as

kexp= (initial reaction rate)/[6-MnOz]o[organic]o (2)

where psuedo-first-order dependence has been assumedwith respect to both organic compound an d manganesedioxide for the initial reaction rate as discussed earlier.

p-Nitroaniline was found t o be particularly unreactive;after a reaction period of 1 2 h the re was only -5% dis-

appearance of the compound at pH 4. As with p-anisidine,p-tolu idine reacted readily with visible color production;the oxide suspension became w ine-red on a dditio n of or-ganic, an d dissolution was pronounced. p-Chloroanilinean d 4-aminobenzoic acid also reacted rap idly with visiblecolor changes, the product being a deep peach for theformer and orange for the latter.

Reaction Products. The identity of the reactionproducts was investigated for the reductive dissolution ofmanganese dioxide by aniline and p-toluidine. A set oftest s with aniline was conducted by mixing 5 mL ofM an iline with 5 mL of M 6-M n02 n acetate buffera t p H 4 n a centrifuge tube. Th e reaction was stoppeda t 10min by th e add ition of 30 mL of 0.05 M L-ascorbicacid, which dissolved the remaining oxide suspension im-mediately. Aside from stopping the reaction, the purposeof this st ep was to release any reaction pro duc t that mayhave adsorbed on unreacted manganese dioxide and toprevent problems of solids separation a nd em ulsificationduring subsequent extraction of the reaction mixture. Theextraction was performed by adjusting the pH of the re-action solution t~ 12 with sodium hydroxide and extractingwith 10mL of methylene chloride for 10min with vigorousshaking. Th e extract was concentrated by a factor of 13and t he concentra ted extract was analyzed by GC-MSprocedures. A typical chromatograph ic result is presentedin Figure 9, which shows the presence of aniline a t reten-tion time 4.3 min an d azobenzene at retention time 19.8



Reaction of Anilinewith 8 - M n 0 2pHz4,Time 30 mins

100

80

6 0 -.-.z 4 0 -

5 2 0-

c 0 -

~ loo-.

$ 8 0 -

2 6 0 -

4 0 -

20

+

H

5 10 15 20 25

Time, minutes

Figure 10 . Chromatogram f rom GC-MS analysis for identification ofreaction products for 30-min reaction of aniline and m anganese dioxideshowing azobenzene as th e prin cipal product with various minorproducts including 4-aminodiphenyiamine.

-. Azobenzene Standa rd- -

, I I , II I , ' I I I ' I ' I I I 1 1 1 1 L

20 40 60 80 100 120 140 160

100-. AzobenzeneReaction Time: IOmin

80 -.60 -.

-. 40 -.

min. Th e chromatogram shows no apparen t reactionproducts other than azobenzene. Sample blanks wereprepared for analysis in the sam e manner with the om is-sion of th e ma nganese dioxide, in which the mang anesesuspensio n volume was replaced by distilled water. Figure9 also shows the chromatogram for the reaction blank,which indicates that sample manipulation and preparationfor analysis, including th e a dditio n of ascorbic acid, re-sulted in no reaction p roduct fo rmation, confirming th atthe a zoben zene is a prod uct of the reaction of aniline withmanganese dioxide, rather th an being attributa ble to otherdegradative processes or impurities in th e aniline.

Figure 10shows a chromatogram of produ cts for reactionof 6-Mn02and aniline a t equimolar concentrations of 5 X

in a s eparatory flask in which the reaction was allowedto proceed for 30 min prior to analysis. In this test , largervolumes (100 mL) of reac tants w ere used so t h a t t h esubseque nt extract would result in a greater c oncentrationfactor. In this case of the longer reaction time, azobenzene

0 I , ' I 1 20180 200

loo-.

80

60

40

20

was still the principal reaction product, althou gh a numbe rof minor species were app are nt also. p-Am inodiphe nyl-amin e was identified as one of the minor reaction products.

Azobenzene was confirm ed by compariso n of rete ntiontime a nd spectral intensities of an analytical standard.Figure 11shows an inset of mass spec tra for the reactionproduc ts corresponding to Figures 9 and 10 compared witha mass spectrum of an azobenzene standard. Th e fourlargest intensities for the azobenzene stand ard are m / z 77,182,105, and 152, which agree with th e spectral displaysfor the identification of th e comp ound as eaction p roductand which are listed as intense peaks in mass spectra in-

dexes (13, 14) .Figure 1 2 shows a chroma togram of reaction prod ucts

fo r 5 X equimo lar concen trations of p-toluidin e an d6-M n02 for a 10-min reaction time a t pH 4. The pre-dominant reaction product is clearly 4,4'-dimethylazo-benzene, although several unid entified species were alsopresent as minor reaction products. GC-MS determina-tion of 4,4'-dimethylazobenzene was confirmed with ananalytical standard by com parison of the chromatographicretention times and the largest spectral intensities, i.e.,mass ion distribu tion of m / z 91, 210, an d 119.

Discussion

For the reaction times an d conditions considered in thisinvestigation, degradation of aniline is negligible in th eabsence of manganese dioxide with exclusion of oxygen.This is consistent with th e observations of Lyons e t al. (11)who reported th at autox idation of aniline in the absenceof light co ntribu tes little to aniline removal.

Th e dissolution rate of man ganese dioxide by aromaticamine compounds depends on th e nature of the oxidant,the reactant concentrations, and t he medium composition.pH is seen to play a n im porta nt role since the dissolutionrate depends on the redox potential a nd on the e xtent andnat ure of the intera ction of the aromatic solute with theoxide surface. T he a ppare nt reaction order with respect

-.I I I.

I I I I , I I I I I I I l I l I

20 40 60 80 100 120 140 160

-. 100 -. Azobenzene-. 80 -.

- - 60 -.

-. 40 -.

Reaction Time: 30 min

I1 1 I I 20- .

loo-.

80

60

40

20

0

180 20 0

Environ. Sci. Technoi., Voi. 24, No . 3, 1990 389

I, , , I

20 40 60 80 100 120 140 160-.-.

-.- -

I I i . . / 1 . 1 I 1 ' I I , , I I I I



4,4'- D i me th y ta ro b e n ze n e

/

Reaction of p-Toluidinewith 8- nOppH.4,Time IO mtns

5 10 15

Time,minutes

Flgure 12. Chrom atogram from analysis of reaction products for10-min reaction of p-toluidine and mang anese dioxide. 4,4'-Di-methylazobenzene Is the prlncipal reaction produc t.

to [H +] s derived from th e slope of th e logarithm of th einitial reaction rates plotted as a function of pH:

initial ra te = k[organic]'[manganese oxide]"[H+]" (3)

or

log (in itial rate) = log (k)+ 1 log [organic] +m log [ma nganes e oxide] + n log [H+] (4)

The order with respect to [H+ ],n, eed not necessarily beconstant or an integer number. Figure 5 showed tha treaction rate increases as t he p H is decreased for the re-action of 2 X M aniline with 6-M n02. The appa rentorder of the reaction with respect to [H +]defines a curve,rather tha n a straight line, varying in slope from 0.32 to0.64. Stone (3) observed tha t th e reductive dissolution ofMn (III /IV) oxide by p-methylphenol varied with pH , withn decreasing from abou t 1.2-1.3 a t pH >6 to about 0.44.5a t pH 4. Th e test results with p-methylphenol, as withaniline, suggested that the observed pH dependence mayarise from specific interactions between the reductant andthe oxide surface sites, including protonation reactions tha tpromote the formation of surface precursor complexes

and/or increases in the protonation level of surface pre-cursor complexes th at increase rates of electron transf er(3) .

The experimental results suggest an equivalent stoi-chiom etry in the initial reaction of aniline with m anganesedioxide by evidence of the principal reaction products, Le.,Mn(I1) and azobenzene, and by the similarity of t he rateof disa ppea rance of th e reactants. Aniline is thu s a 2-equivreductant, fo r which the sim ilarity in initial reaction ratewith regard to appearance of M n(I1)or depletion of anilinedemonstrates that

d [ M n ( I I ) ] / d t = -d[ani l ine]/d t = reaction rate (5 )

Th e order of this reaction is demon strated to be unity withrespect to th e concentration of b oth m anganese dioxide

and aniline, and at constant pH the rate law has the formreaction rate = k,[aniline] [mangane se dioxide] (6)

Rate laws of a similar form have been proposed for reactionof aniline and su bstituted anilines as 2-equiv reductantsin aqueous acetic acid medium for oxidation by peroxy-disulfate, Sz082- (15) , thallium triacetate, Tl(OAc), (16),

and iodate, IO3-17) .Th e rate co nstants for the oxidation of th e aromatic

amin es may be correlated with th e organic solute half-wavepotential and the subs ti tuent Ham mett constant. Half-wave potentials have been reported (18) from anodicvoltamm etry experiments with para- an d meta-substituted

370 Environ. Sci. Technol., Vol. 24 , No. 3, 1990

5

4

3

2

a

W

Im0-

0

- I

-2

-3

p - Anisidine\\\

\\\\

P-MethYl Phenol \\p-Chlorophenol

\\

p- To Iu di n e 0'

@Phenolb$-Chloroani l ine

Anil ineO \

\ ocld

p-Hydrox ybenzoic acid b 0 p-Aminobenzoic

\ O\ o-Hydroxybenzoate

p-Hydroxyacetophenone@\\

0 Anilinesp H = 4 , 8 - M n 0 2

\\\\

pH.4.4, p-Nitroanil ineMn ( I I I / IV) Oxide

PPhenols

\

0 0.2 0 . 4 0.6 0.8 1.0

E b2, volts

Flgure 13. Correlation of half-wave potentials with experiment al ratecons tants for redox reacti on of substitu ted anilines wtth 6-Mn0,, andsubstituted phenols with m ang ane se(III/IV ) ox ide. Rate data for thephenols are from Stone (3).

phenols and anilines for use in polarographic identificationof oxidizable and reduced organic compounds. Th ehalf-wave reduction potentials depend upon the ch emicalspeciation and diffusion coefficients of oxidant and re-ductant as well as the thermodynam ic driving force for thereaction ( 3 , 19). The tendency to donate electrons in-creases as the solute half-wave potential decreases (18).

Logarithms of the e xperim ental rate constan ts, kexp, f

substituted anilines at pH 4 re plotted against half-wavepotentials, El 2, in Figure 13. Th e keXpvalues werepresen ted in $able I11with reporte d values of El12 .T hereaction rate is observed to generally decrease with in-creasing half-wave potential. p-An isidine has the lowesthalf-wave potential and re acts most qu ickly with ma nga-nese dioxide, while p-n itroan iline with the highest half-wave potential is the least reactive. Data from Sto ne ( 3 )for reaction of the substituted phenols with manganese-(III/IV ) oxide at pH 4.4 n acetate buffer are alsopresentedin Figure 13, and these d ata agree with the trend observedfor substituted anilines using Ellzvalues for the substitutedphenols as reported also by Su atoni et al. (18). The par-ticular manganese oxide employed by S tone ( 3 ) , or whichthe rate data are compared in Figure 13,was prepared by

reaction of MnSO, and NaMnO, at pH 6.6; this was similarto the manganese dioxide preparation procedure employedin this investigation except that Mn2+and Mn0,- werereacted un der alkaline conditions. Th e chemical conditionsare sim ilar for the two sets of tests shown in Figure 13, withpH 4 used in the tests with the su bstituted anilines andpH 4.4 n the tests with the substituted phenols. Theseresults suggest tha t the reactivities of 6 -M n02and man-ganese (III/ IV) oxide in acidic environ me nt are of the sam emag nitude with respec t to reductive dissolution of man-ganese dioxide by either substituted phenols or anilines.

Ham me tt constants are linear free energy param etersth at m ay be used to estimate chemical properties for or-



H-N-H H -N -H H-N-H H-N-H

I

-0.8 -0.4 0

t+ y -Aniline H

Resonance Structures ofCotion Radical which undergo

Coupling Reoctions

\ ,

m 0.4 0.8

Cat ion Rad ica lCoupling Product Final Product

-2 + p-Nitroaniline'

Flgure 14 . Correlation of Hamm ett u+ constants with experimentalrate co nstants for redox reaction of substituted anilines wtth 6-Mn0,.Nitroaniline does not follow th e co rrelation , see text for discussion onthe behavior of this compound.

ganic solutes (35),uc h as acid dissociation constan t or rateof hydrolysis (201, as well as rates for man y oth er typ es

of reactions of aromatic solutes (21). The est imationmethod is basically a substituent effect approach in whicha correlation is established between t he chem ical prop ertyof th e pare nt com pound (e.g., phenol or aniline) an d th atof the substi tuted compound. I t has been reported tha tthe Ha mm ett substi tuent constants, u, are linearly pro-portional to t he change in electron density a t the arom aticring carbon meta or para to the primary substituent group(18). Since the value of the half-wave pote ntial for sub-stituted phenols and anilines depends on the electrondensity at th e carbon atom to which either the hydroxyor amine group is attache d, the half-wave potentials ofmeta- an d para-s ubstituted phenols and anilines shouldbe linearly related to Ham mett's u constants. Th e selec-tion of the appropriate se t of Ham me tt constants (u, u,+

or a-) depen ds on the reaction m echanism within a givenseries. Hammett u+ values are used where the su bstituentgrou p can enter into direct resonance with the reaction sitein the transition sta te for a n electron-donating group (e.g.,NH,) developing a positive charge (21, 35).

Th e rate con stan ts for reductive dissolution of 6-M n0,by substituted anilines were correlated with Hammettconstants u an d u+. A bette r correlation was obtaine d withu+ values, for which Figure 14 shows a general tren d be-tween the rate constant and g+. T ha t the reaction seriesfollowed u+ better than CT suggests extensive resonanceinteraction in the transition sta te (21). T his is consistentwith th e su bsequen t discussion, which suggests the for-matio n of a cation-radical intermediate. Nitroaniline does

not follow the sequence and may interact with 6-Mn 02bya diffe rent mechanism, as suggested later.Sub stitue nt effects and their correlations with physical

and chemical properties provide a rational basis for esti-mating trend s in th e relative reactivity of pa ra-sub stitutedphenols and anilines with manganese dioxide. Th e orderof the reactivity w ith substitut ion of the parent moleculeis as follows: me thox y >> methyl > chloro > carboxy >>nitro. Those subs tituents th at are considered classicalelectron- withdra wing groups ma ke reductive dissolutionof m anganese dioxide by phenol or aniline more difficultby decreasing electro n density, while th e opp osite effecton dissolution ra te occurs for classical electron-d onatinggroups. Sim ilar orders of relative reactivity have been

head-to-tail H , N o N H o + ZH'- N ~0z H + + 2e-

4 aminodiphenylomine

benzidine

hydrozobenzene azobenzene

Flgure 15. Postulated mechanism for oxidative coupling of aniline by

reaction with manganese dioxide. The reaction proceedsfrom a cationradical through coupling products, w hich then undergo further oxidation[adapted from Sharma et al. (37)].

obtaine d for the oxidation of sub stitut ed anilines in diluteacetic acid medium by thallium triacetate and sodiumiodate (16, 17).

Oxidative Coupling of Anilines. Th e oxidation ofaniline and substituted primary aromatic amines can resultin the formation of a variety of products, depending onreaction conditions. Th e oxidation of aniline by MnOz insulfuric acid gives p-benzoq uinone (22), while o xidationwith peroxy acids, e.g., peroxyacetic acid, or nitric acid mayyield nitroso and nitro com pounds (23, 24). Polymericsubstances result from the reaction of aniline and p er-oxydisulfate in acid medium (25). Azo compounds resultfrom the oxidation of prim ary arom atic amines by leadtetraacetate in acetic acid (26), by phenyl iodosoacetatein acetic acid (27), by N-chlorobenzamide in methanol/hydrochloric acid medium (28),an d by cuprou s chlorideand oxygen when pyridine is used as solvent (29). Sub -stituted anilines can be oxidized to symmetrically sub-

stituted azobenzenes by refluxing with manganese dioxidein benzene (30).

Th e results of this work show th at th e principal oxida-tion produc ts of aniline and p-toluidine by manganesedioxide in aqueous buffered solution a t p H 4 were azo-benzene and 4,4'-dimethylazobenzene, respectively. Apostulated reaction seq uence may be inferred from cyclicvoltammetry studies of primary aro matic amines in acidicaqueous solution as sum marized in Figure 15 from thework of S harm a et al. (31), who examined th e anod ic ox-idation of aniline, 2,4- and 2,6-dimethylaniline, and to-luidines. According to this oxidation mechanism, anilineloses one electron to produce a cation radical, whichthrough resonance structures may undergo head-to-tail,

tail-to-tail, or head-to -head couplings. Th e imm ediaterespective products are p -aminodiphenylam ine, benzidine,and hydrazobenzene, all of which are more readily oxidizedthan the paren t substance and undergo further oxidationa t th e same potential. Th e overall oxidation is thus asequential, two-electron reaction. A suggested reactionpathway for p-toluidine is shown in Figure 16 in whichhead-to -head coupling is favored because of blocking atthe para position.

The results of this study confirm that the principalpathway for oxidation of aniline and p-tolu idine by man -ganese dioxide in aqueous suspension a t mod erate acidityis a two-electron process, which results in t he productionof symm etrical azobenzenes. Head-to-head co upling with

Environ. Sci. Techno l., Voi. 24 , No. 3, 1990 371



.*NH, N-N-H

CH3 c- 3

p - T o l u i d i n e Cation r a d i c a l

H 3 C o y ; + ; l o c H 3 C - . . H,c - @ - X e C H 3 + 2 H +-

H H4,4'- D i m e t h y l h y d r a z o b e n z e n e

I

4,4 ' - D l m e t h y l o z o b e n z e n e

Flgure 16. Postulated reactio n mechanism for oxidative coupling ofp -toluidine by reaction with manganese dioxide resulting in 4,4'di-methylazobenzene as the principal reaction product [adapted fromSharma et al. (37)].

formation of azo compounds is th e predom inant reaction,although a variety of minor products, such as p-amino-diphenylamine, the aniline head-to-tail intermed iate cou-pling prod uct, are evident also from reaction with m an-ganese dioxide.

The se results are cons istent with the work of Wheelerand Gonzalez (30),who demonstrated th at manganesedioxide in benzene solution oxidizes substitute d anilines

to symmetrically substituted azobenzenes. They observedtha t nitroanilines and p-aminobenzoic acid were essentiallyunreac tive over 24 h of refluxing in benzene with Mn 02 .Th e lack of reactivity of nitro- a nd carboxy-substitutedaromatic amines was attribu ted t o the nitro or carboxylgroup being preferentially adsorbed at the active oxidationcenters on th e m anganese dioxide, thu s preventing oxi-dation of the am ino group. Although the exact nature ofthe m echanisms involved in th e oxidation of the an ilineswas not explored in the investigation reported here, thegeneral features of th e reaction with regard to oxidationproducts and relative rates of reaction agree with thefindings of W heeler and Gonzalez (30)and Sharma e t al.(31).Additional studies with mu ltiple-sub stituted anilines

could help elucidate the reaction m echanism w ith MnOzin aqueous medium. For exam ple, studies with 3,5-di-chloro-4-nitroanilineor 3,5-diethyl-4-nitroaniline,n whichadsorption of the nitro group would be hindered by theneighboring groups, would indicate the degree to whichadsorption of th e nitro group impedes th e reaction.

Conclusion

Transformation of A niline and Azo Compounds.Th e present investigation demonstrated a redox pathwayfor oxidation of aniline an d primary s ubs tituted anilinesto symmetrical azo compounds by reaction with manganesedioxide in acidic aqueous systems. Th is oxidation reactionmay be an importan t transformation process for anilinecompoun ds in moderately acidic subsurface environments

in th e absence of molecular oxygen and substantial mi-crobial activity. Analogous abiotic reductive transform a-tions of azobenzene and selected azo derivatives have beenobserved by Weber and Wolfe (32) in sedimentlwatersystems. T he abiotic reduction of azobenzene was afour-electron, pH-dependent process for which GC-MSanalyses showed production of stoichiometric amoun ts ofaniline. Th e proposed scheme for the abiotic reductionof azobenzene to aniline is illustrated in Figure 17. T hereaction is postulated to be a surface-mediated reactionproceeding through hydrazobenzene, which would not bestable in a highly reducing environment. T he presentinvestigation suggests that the pattern of oxidation of

372 Environ. Sci. Technol., Vol. 24 , No. 3, 1990

2 e - 2 e-

A z o b e n z e n e Hy d r a z o b e n z e n e A n i l i n e

Figure 17. Proposed scheme for abiotic reduction of azob enzene toaniline in anaerobic sediment/water systems [Web er and Wolfe (32)].

aniline by manganese dioxide is th e reverse of th e reduc-tion of azobenzene in anaerobic sedim entlw ater systems.

T he significance of th e processes whereby manganesedioxide induced oxidative coupling of aniline and substi-tuted anilines may serve as an elimination pathway for

anilines depends on the accessibility and the form ofmangane se dioxide in natu ral system s, as well as the roleand relative rates of competing reactions. Aniline mayundergo microbial degradation th roug h dioxygenase attack,resulting in oxidative deamination to catechol, which isfurthe r degraded by ortho cleavage and mineralized (7,33).

Sub stituted anilines can be relatively resistant to m icrobialdegradation, depending on the microbial comm unity andthe presence of other growth substrates (6 ,10,34).

T he abiotic redox reaction of primary amines and azocompounds in mineralogical and sedimentlwater envi-ronments may result in various oxidative-coupling andreductive-decoupling reactions. The se processes m ay besignificant with regard t o th e persistence and transfor-matio n of thes e classes of organic contam inan ts in envi-ronmental systems.

A c k nowl e dgm e n t s

S. Shanm ugananth a assisted with a portion of th e ex-perimental studies. Curt M. White and Louise J. Douglas,Coal Science Division, Pittsburgh Energy TechnologyCenter , assisted with iden tification of reaction produc ts.

Registry No. 6-Mn02,1313-13-9;hydroquinone, 123-31-9;methyl-HQ, 95-71-6; 2,3-dimethyl-HQ, 608-43-5; ert-butyl-H Q,1948-33-0; trimethyl-HQ, 700-13-0; catechol, 120-80-9; 4-methylcatechol, 452-86-8; p-an isidine , 104-94-9; p-toluidine,106-49-0;aniline,62-53-3;p-chloroaniline, 106-47-8;aminobenzoicacid, 1321-11-5;p-nitroaniline,100-01-6;azobenzene, 103-33-3;4-aminodiphenylamine,101-54-2;1-methyl-1-imidazole, 16-47-7;

quinoline, 91-22-5; 5,5-dimethylhydantoin, 77-71-4; 4,4'-di-methylazobenzene, 501-60-0.

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Received for review December 28 , 1988. Revised manuscriptreceived August 20,1989. Accepted Novem ber 27,1989. Th iswork was supported by the U S . Department o f Energy , GrantDE-FG22-86PC90524; Richard P. Noceti , Technical ProjectOf f icer,provided useful comments.

Anaerobic Microbial Remobil ization of Toxic Metals Coprecipitated with Iron

Oxide

Aroklasamy J. Francl s" and Cleveland J. Dodge

Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973

An anaerobic N2-fixingClostr idium sp. solubilized Cd,Cr, Ni, Pb , an d Zn coprecipitated with goethite (a-F eOO H)by (i) direct action due to enzymatic reduction of ferriciron and t he release of m etals associated with iron and (ii)indirect action due to metabolic products. Th e exten t ofdissolution depended upon t he n ature of th e associationof the m etals with goethite. Substantial amo unts of Cdan d Zn, which were closely associated with iro n, were re-leased due to direc t action. Nickel was solubilized by directand indirect actions, while a small amount of Cr wassolubilized only by direct action. Th e natu re of associationof P b in the coprecipitate was not affected by the presenceof other c ations and it was solubilized by ind irect action.In th e presence of bacte ria, the conc entration of soluble

P b decreased due to biosorption. These results show tha tthe re could be significa nt remobilization of toxic metalscoprecipitated with iron oxides in wastes, contaminatedsoils, and sedime nts due to microbial iron reduction.

In troduct ionIron oxides scavenge transition and heavy metals in soils,

sediments, and energy wastes (1-8). These oxides are amajor sink for metals in th e terrestrial and aquatic envi-ronm ents and play an im porta nt role in regulating theiravailability. Sorption and coprecipitation are the pre-dom inant processes by which most of th e metals are re-tained by iron oxide. Sorp tion is a process by which m etalsare bound t o the surface of an existing solid by adsorp tionand surface precipitation (9), hereas coprecipitation is

the simultaneous precipitation of a che mical element withother elements an d includes mixed-solid formation, ad-sorption, and inclusion ( 1 0 , I I ) . Although coprecipitationhas no t been as well studied as adsor ption, it appea rs toremove trace m etals from solution more efficiently (12-14).Toxic metals such as As, Cd, Co, Cr, Cu, Hg, Ni, Pb , Se,U, and Zn from fossil- and nuclear-fue l cycle waste streams ,geothermal fluids, and electroplating w astes are curr entlyremoved by coprecipitation with ferric iron or are underconsideration for such treatmen t (3,15-17).

Th e removal of toxic metals from waste stream s by co-precipitation with iron seems to be a very efficient meth odand ec onomically feasible, but problem s remain w ith thedisposal of th e solids generated in t he process and with

the ultim ate fate of the coprecipitated metals in the en-vironm ent. Significa nt dissolution of metals from thecoprecipitate can be brought about by chemical and mi-crobiological action. In g eneral, solubility of iron ox idedepe nds upon the degree of crystallinity. A mo rphou s ironoxides are orders of ma gnitude more solub le tha n goethiteor hemati te ( I ) , while amorphous synthetic goethite is2-100 times m ore soluble th an a well-crystallized goethite(18). Microorganism s play a significant role in th e disso-lution of am orphou s and crysta lline forms of iron oxidesby direct action or by indirect action. Direct action in-volves enzym atic reductive d issolution of iron from higheroxidation state to lower oxidation state and indirect actionis due to the production of m etabolites (19-23). However,we have little inform ation on m icrobially med iated disso-

oxidation of aniline and other primary aromatic amines by manganese

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