Reduction of azo dyes with zero-valent iron

PII: S0043-1354(99)00331-0

Wat. Res. Vol. 34, No. 6, pp. 1837±1845, 2000 # 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter

www.elsevier.com/locate/watres

REDUCTION OF AZO DYES WITH ZERO-VALENT IRON SANGKIL NAM* and PAUL G. TRATNYEK{M Department of Environmental Science and Engineering, Oregon Graduate Institute of Science and Technology, P.O. Box 91000, Portland, OR 97291-1000, USA (First received 1 February 1999; accepted in revised form 1 July 1999) AbstractÐThe reduction of azo dyes by zero-valent iron metal (Fe0) at pH 7.0 in 10 mM HEPES bu€er was studied in aqueous, anaerobic batch systems. Orange II was reduced by cleavage of the azo linkage, as evidenced by the production of sulfanilic acid (a substituted aniline). Adsorption of the dyes on iron particles was less than 4% of the initial concentration, and >90% mass balance was achieved by summing aqueous concentrations of dye and product amine. All of the 9 azo dyes tested were reduced with ®rst-order kinetics. The kinetics of decolorization at the lmax of each dye were rapid: a typical kobs was 0.35 2 0.01 minÿ1 for Orange II at 130 rpm on an orbital shaker, corresponding to a surface area normalized rate constant (kSA) of 0.21 2 0.01 L mÿ2 minÿ1. The rate of reduction of Crocein Orange G varied with initial dye concentration in a way that suggests saturation of surface sites on the Fe0, and varied with the square-root of mixing rate (rpm) in a manner indicative of mass transfer limited kinetics. Correlation analysis using kobs for all of the azo dyes, estimates of their di€usion coecients, and calculated energies of their lowest unoccupied molecular orbitals (ELUMO), gave no strong trends that could be used to derive structure-activity relationships. Using an authentic sample of wastewater from a dye manufacturing operation and construction-grade granular Fe0, rapid decolorization was achieved that was consistent with reduction of azo dyes. # 2000 Elsevier Science Ltd. All rights reserved Key wordsÐiron metal, decolorization, kinetics, mass transport, correlation analysis

NOMENCLATURE

ra lmax o A C, C0 D ELUMO kobs kobs ' kSA krxn K1/2 Vm

INTRODUCTION 2

ÿ1

iron surface area concentration (m L ) wavelength of maximum absorbance (nm) rotation rate (rpm) empirical coecient describing the kinetics of mass transport (rpm1/2 minÿ1) concentration of substrate at time t, and at time 0 (mM) di€usion coecient (cm2 sÿ1) energy of the lowest unoccupied molecular orbital (eV) pseudo ®rst order disappearance rate constant (minÿ1) ®rst order appearance rate constant (minÿ1) surface-area normalized rate constant (L mÿ2 minÿ1) ®rst order rate constant for chemical reaction (minÿ1) concentration of substrate at half of maximum reaction rate (mM) maximum reaction rate (mM minÿ1)

*Present address: H. Lee Mott Cancer Center and Research Institute, 12902 Magnolia Drive, Tampa, FL 33612, USA. {Author to whom all correspondence should be addressed. Fax: +1-503-690-1273; e-mail: [email protected]

The decolorization of azo dyes is one of many useful reduction reactions that can be e€ected by zerovalent iron (Fe0). In the past, most investigations of the environmental applications of reduction by Fe0 have focused on remediation of groundwater contaminated with chlorinated solvents (e.g. Gillham and O'Hannesin, 1994; Matheson and Tratnyek, 1994), although there is now a substantial body of literature on the potential for using Fe0 to treat materials contaminated with chlorinated aromatic compounds (Chuang et al., 1995; Grittini et al., 1995), nitro aromatic compounds (Agrawal and Tratnyek, 1996; Burris et al., 1996; Singh et al., 1998a), nitrate (Cheng et al., 1997; Chew and Zhang, 1998; Huang et al., 1998), and various metals (Cantrell et al., 1995; Blowes et al., 1997; Pratt et al., 1997; Fiedor et al., 1998; Gu et al., 1998). More recently, it has been suggested that Fe0 can be used to initiate remediation of more complex anthropogenic chemicals by reduction of critical functional groups. Pesticides that may be subject to this treatment include alachlor and metolachlor (Eykholt and Davenport, 1998); DDT, DDD and DDE (Sayles et al., 1997); and atrazine (Monson et al., 1998; Singh et al., 1998b). Dyes are another category of complex chemicals that may be labile to

1837

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Sangkil Nam and Paul G. Tratnyek

reduction by Fe0, as has recently been shown for several azo dyes (Cao et al., 1999). The reduction of azo dyes by Fe0 is of interest, not only for its potential application in the decolorization of wastewaters from dye use and manufacturing, but also because it provides a convenient model system for investigating (i) the abiotic reduction of azo linkages and (ii) the kinetics of fast reactions with granular iron metal. The abiotic reduction of azo linkages in environmental media has been inferred from experiments done with anaerobic sediments (Weber and Wolfe, 1987; Yen et al., 1991; Weber and Adams, 1995), but this system is so complex that it is dicult to fully characterize the processes involved (Smolen et al., 1999). The kinetics of fast reactions with granular iron in batch experiments appear to be in¯uenced by mass transport of the oxidant to the metal surface (Agrawal and Tratnyek, 1996), although the signi®cance of this behavior under environmental conditions is still not entirely clear (Scherer et al., 1997, 1999). To advance our understanding of the reduction of azo dyes in the environment and the reduction of environmental contaminants by iron metal, we have investigated the reaction of 9 azo dyes with granular Fe0 in batch systems.

MATERIALS AND METHODS

Chemicals The food dyes Allura Red, Tartrazine, and Sunset Yellow FCF were obtained from Warner Jenkinson, St. Louis, MO. Orange I was purchased from TCI America, Portland, OR. Amaranth, Naphthol Blue Black, Crocein Orange G, Orange II, and Acid Blue 113 were obtained from Aldrich, Milwaukee, WI. Properties of the dyes, and their structures, are summarized in Table 1 and Fig. 1. All dyes were purchased in the highest purity that was commercially available and used as received without further puri®cation. Reagent grade, granular iron metal was obtained from Fluka (>99.9%, Cat. No. 44905), sieved to obtain the 16±32 mesh size fraction, and then used without any further treatment. The speci®c surface area of this iron was determined to be 0.0071 m2 gÿ1 by BET gas adsorption with Krypton (cf. previously reported values summarized by Johnson et al., 1996). Decolorization of the authentic dye wastewater was done with construction grade, granular iron from Master Builders (Beachwood, OH) and Peerless Metal Powders and Abrasives (Detroit, MI). Typical speci®c surfaces areas for these metals have been reported previously (Johnson et al., 1996; Su and

Puls, 1999). The sample of dye wastewater was e‚uent from a manufacturing facility in South Carolina, and was known to contain disperse azo dyes. Batch experiments All experiments were done in pH 7.0, 10 mM HEPES [N-(2-hydroxyethyl)-piperazine-N'-(2-ethanesulfonic acid)] bu€er. The bu€er was prepared with deionized and deoxygenated water. All individual dye reduction experiments were done in 7 mL scintillation vials containing 1.000 2 0.002 g Fe0, resulting in an iron surface area concentration (ra) equal to 1.42 m2 Lÿ1. To decolorize the wastewater sample, more of the construction grade iron was used: 2.5 g of iron from Master Builders and 2.0 g of the material from Peerless. In an anaerobic glove box (H2/N2), stock solutions of dye and HEPES bu€er were combined with the Fe0, and the vials were sealed with Te¯on-lined caps and Para®lm1. In all cases, the ®nal reaction volume was 5 mL. The resulting initial concentrations were 10 mM HEPES, and 60 mM, 300 mM, 1 mM, or 3 mM dye. During the reaction, the vials were mixed on an orbital shaker (New Brunswick Scienti®c) at room temperature. The aqueous phase was sampled periodically and analyzed by spectrophotometry or by high-performance liquid chromatography (HPLC). Analytical methods The reduction of Orange II and appearance of reaction products were determined by HPLC using a C-18 reverse phase column (0.46 i.d.  25 cm, Separation Group, Hesperia, CA) and detection of absorbance at 254 nm. The mobile phase gradient was 1 mL minÿ1 of 100 mM, pH 7.0 phosphate bu€er for 5 min, followed by a 5 min ramp to 100% methanol and 10 min at 100% methanol. Under these conditions, retention times were 28.2 min for Orange II and 3.8 min for sulfanilic acid. Routine analysis of dye decolorization was done by measuring absorbance at the lmax for each dye (Table 1) using a UV-visible spectrophotometer (Model UV-265, Shimadzu, Kyoto). Samples were diluted 2 to 10 fold with deionized water so that readings never exceeded 1 AU. For the experiments with textile wastewater, decolorization was determined by taking full scans of absorbance from 350 to 700 nm, vs a blank consisting of deionized water.

RESULTS AND DISCUSSION

Pathway and kinetics The reduction of azo dyes by Fe0 is illustrated in Fig. 2 for Orange II. These data, obtained by HPLC, show that degradation of Orange II produces stoichiometric amounts of sulfanilic acid. The appearance of this product is consistent with reductive cleavage of the azo group as shown in Eq. (1).

Table 1. Dyes and their properties No. 1 2 3 4 5 6 7 8 9

Name Acid Blue 113 Allura Red Amaranth Crocein Orange G Naphthol Blue Black Orange I Orange II Sunset Yellow FCF Tartrazine

Synonym FD&C Red #40 Acid Red 27 Acid Orange 12 Acid Black 1 Acid Orange 7 FD&C Yellow #6 FD&C Yellow #5

C.I. No.

CAS No.

M.W.

lmax

26360 16035 16185 15970 20470 14600 15510 15985 19140

3351-05-1 25956-17-6 915-67-3 1934-20-9 1064-48-8 523-44-4 633-96-5 2783-94-0 1934-21-0

637.68 452.45 538.52 328.34 574.54 328.34 328.34 408.40 468.41

566 500 521 484 618 476 484 482 424

Reduction of azo dyes

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…1† Although this process presumably occurs with the hydrazo (±N(H)±N(H)±) derivative as an intermediate, and produces 1-amino-2-naphthol as a product, no standards were available for these compounds, so they were not matched with product peaks on the chromatograms. Even if a peak were assigned

to 1-amino-2-naphthol, it may have been dicult to obtain a quantitative mass balance with respect to this product due to its autoxidation (Kudlich et al., 1999). Similar results were expected for all the azo dyes, although the only additional product studies that were performed involved qualitative veri®ca-

Fig. 1. Structures of the nine dyes used in this study.

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Sangkil Nam and Paul G. Tratnyek

tion that sulfanilic acid was formed from the reduction of Orange I, Sunset Yellow FCF and Tartrazine. Additional evidence that aromatic azo compounds are reduced by Fe0 to give amines is available from two previous studies: Weber identi®ed the products formed from immobilized 4-aminoazobenzene using HPLC (Weber, 1996) and Cao et al. found shifts in UV absorbance spectra for Orange II that are consistent with Eq. (1) (Cao et al., 1999). Reduction of Orange II by Eq. (1) can also be e€ected by other reductants, as has recently been shown with potassium borohydride (Laszlo, 1997). In contrast, Fe0 in the presence of H2O2 produces a Fenton-like reaction that degrades azo dyes largely by oxidation (Tang and Chen, 1996). Figure 2 also illustrates the kinetics of azo dye reduction by Fe0. The disappearance of Orange II and appearance of sulfanilic acid ®t the ®rst order kinetic models C . C0 eÿkobs t

…2†

0

…3†

C . C0 …1 ÿ eÿkobs t †,

where C is the concentration of analyte at time t, C0 is the initial or ®nal concentration, kobs is the pseudo ®rst order disappearance rate constant and kobs ' is the ®rst order appearance rate constant. Nonlinear regression using Eq. (2) and the data for decolorization of Orange II gives kobs=0.35 2 0.01 minÿ1 and C0=25723 mM, whereas ®tting Eq. (3) to the data for formation of sulfanilic acid gives kobs '=0.60 2 0.03 minÿ1 and C0=235 2 3 mM. The two-fold di€erence between kobs and kobs ' produces a small excess mass balance at two minutes, but after 2 min the mass balance is remarkably good.

Fig. 2. Time course for disappearance of the azo dye Orange II and appearance of the product sulfanilic acid. Reaction was with 200 g Lÿ1 (ra=1.42 m2 Lÿ1) of unpretreated, 16±32 mesh Fluka Fe0 in pH 7.0,10 mM HEPES bu€er. C0 of the dye was 300 mM, mixing rate was 130 rpm, and the temperature was ambient.

At the end of the experiment, extraction of the Fe0 (after removing the aqueous phase) with methanol recovered <4% of the original dye, which apparently was adsorbed but not reduced. Previous studies with chlorinated ethenes have found that adsorption to nonreactive sites on Fe0 can in¯uence the overall disappearance kinetics of substrate (Burris et al., 1995; Allen-King et al., 1997), but there is no indication of this complication in the data shown in Fig. 2. Qualitatively similar kinetics for azo reduction by Fe0 were obtained by Weber for 4-aminoazobenzene and its four-electron reduction product aniline (Weber, 1996). In contrast to the above, only absorbance of the dye (at an appropriate lmax) was monitored for most experiments in this study. In these experiments, kobs was determined from plots of the natural logarithm of absorbance vs time and linear regression of these data to the integrated form of Eq. (2). The results, summarized in Table 2, re¯ect the e€ects of dye structure, initial concentration, and mixing rate. All other experimental variables were held constant, including the type and amount of bu€er and iron. Bu€er e€ects are possible in reactions with Fe0 (Schreier and Reinhard, 1995; Johnson et al., 1998; Zawaideh and Zhang, 1998), Table 2. Summary of decolorization experiments No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Dye

C0 (mM)

o (rpm)

kobs (minÿ1)

Acid Blue 113 Allura Red Allura Red Allura Red Allura Red Amaranth Amaranth Amaranth Amaranth Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Crocein Orange G Naphthol Blue Black Orange I Orange I Orange I Orange II Orange II Orange II Orange II Sunset Yellow FCF Sunset Yellow FCF Sunset Yellow FCF Tartrazine

300 300 300 300 300 300 300 300 300 60 60 60 60 60 300 300 300 300 300 300 1000 1000 1000 3000 3000 3000 3000 300 300 300 300 300 300 300 300 300 300 300 300

100 100 100 120 140 100 100 120 140 90 100 120 140 164 90 100 100 120 140 160 120 140 160 100 100 124 158 100 100 120 140 100 120 130 140 100 120 140 100

0.06420.002 0.14420.004 0.14120.027 0.15920.007 0.22620.017 0.14920.004 0.13720.003 0.19420.008 0.27920.000 0.17420.012 0.19720.000 0.29120.011 0.35620.010 0.53020.015 0.16720.016 0.20720.006 0.17720.007 0.27620.007 0.38020.007 0.45820.002 0.23920.009 0.30320.006 0.39320.010 0.11920.014 0.10420.006 0.17020.013 0.22520.015 0.02320.002 0.13820.005 0.21820.002 0.27420.013 0.16120.003 0.24520.011 0.30620.009 0.38020.011 0.14820.006 0.24120.004 0.30420.018 0.11620.038

Reduction of azo dyes

but we decided it was more important to control pH for this particular study. We also chose not to investigate the e€ect of iron surface area concentration (ra) in this study, because Cao et al. (1999) had shown that the rate of Orange II reduction is linearly dependent on ra, and this relationship has been found for many other compounds that are subject to reduction by Fe0, including nitrobenzene (Agrawal and Tratnyek, 1996) and various chlorinated solvents (e.g. Matheson and Tratnyek, 1994; Johnson et al., 1996). Surface area normalized ®rst order rate constants (kSA) can be estimated from any of the values of kobs reported in this study, by dividing by ra (which is 1.42 m2 Lÿ1 for all data in Table 2). The data in Fig. 2, for example, give kSA=0.2120.01 L mÿ2 minÿ1 for the disappearance of Orange II. This value is greater than would be expected for any chlorinated solvent (Scherer et al., 1998a) and is about 10 times larger than values reported previously for nitro aromatic compounds (Agrawal and Tratnyek, 1996). E€ect of initial concentration The e€ect of initial dye concentration on the kinetics of dye reduction was investigated with Crocein Orange G, and is shown in Fig. 3. Over the range of C0 studied (60 mM to 3 mM), decolorization rates (kobs  C0) increased monotonically but nonlinearly with C0 (at each mixing rate). Similar trends have been observed for the reduction of carbon tetrachloride (Scherer and Tratnyek, 1995; Johnson et al., 1996; Tratnyek et al., 1997; Scherer et al., 1998b) and tetrachloroethene (Arnold and Roberts, 1997) in batch systems containing granular Fe0. All of these systems appear to give a transition from pseudo ®rst-order to zero-order kinetics with

Fig. 3. E€ect of initial dye concentration on initial decolorization rate (kobs  C0) of Crocein Orange G by Fe0 (unpretreated Fluka 16-32 mesh, ra=1.42 m2 Lÿ1) at various rotation rates on an orbital shaker. Data for 10 mM, pH 7.0 HEPES bu€er at room temperature. Curves are from ®ts to Eq. (4).

1841

increasing C0, which presumably is due to saturation of reactive surface sites. This site-saturation behavior was ®rst modeled using an empirical formulation analogous to the Michaelis±Menton equation (Johnson et al., 1996, 1998), and subsequently described more rigorously in terms surface site concentrations, reaction rate constants and complexation constants (Arnold and Roberts, 1997; Scherer et al., 1998b). Since the two formulations are mathematically equivalent, and the former requires fewer explicit assumptions about the system, we have applied it to the data obtained in this study. The model is ÿ

dC Vm C ˆ , dt K1=2 ‡ C

…4†

where Vm is the maximum reaction rate for a particular experiment (type and amount of iron, rotation rate, temperature, etc.) and K1/2 (C at Vm/2) re¯ects the anity of the metal surface for the organic reactant. It has been shown previously that K1/2 is roughly constant for a particular substrate (presumably because all iron tends to be coated with similar oxides (Scherer et al., 1998b)), but Vm varies (due to di€erent concentrations of reactive sites, etc. (Johnson et al., 1996, 1998; Scherer et al., 1998b)). Therefore, we have ®t Eq. (4) to all the data in Fig. 3 by treating K1/2 as a global parameter (one value common to all four rotation rates) and Vm as a local parameter (one value for each rotation rate). The resulting values are K1/2=3.1 2 0.2 mM, and Vm=0.56 2 0.08, 1.02 2 0.04, 1.24 2 0.06 and 1.5620.05 mM minÿ1 for 90, 120, 140 and 160 rpm, respectively. The agreement between the ®tted curves and the data is apparent from Fig. 3. Since Eq. (4) is an empirical model, the absolute values of the ®tted parameters are of little general

Fig. 4. E€ect of mixing rate on initial decolorization rate (kobs  C0) of Crocein Orange G by Fe0 (unpretreated Fluka 16-32 mesh, ra=1.42 m2 Lÿ1) at various initial concentrations. Data for 10 mM, pH 7.0 HEPES bu€er at room temperature. Curves are from ®ts to Eq. (5).

1842

Sangkil Nam and Paul G. Tratnyek

signi®cance, but the qualitative agreement with previously reported trends is remarkable. Again, the anity of organic reactant for the metal surface (K1/2) appears to be relatively una€ected by experimental conditions, and the controlling variables apparently are represented by Vm. It has been recognized previously that Vm depends primarily on the concentration of reactive surface sites and the intrinsic rate constant for reaction at these sites (Johnson et al., 1996; Scherer et al., 1998b), but the apparent increase in Vm with rpm suggests that Vm is confounded with e€ects of mass transport under the conditions of this study. E€ect of mixing To clarify the in¯uence that mass transport had on the kinetics observed in this study, the data for Crocein Orange G have been plotted vs mixing rate in Fig. 4. The rate of decolorization clearly increases with the rate of mixing, and, at micromolar values of C0, the trend is linear with the square root of rpm. At higher C0, the data suggest some tailing, presumably due to the saturation e€ect that was discussed in the previous section. Qualitatively, these results indicate that azo dye reduction in this system was limited by mass transport at low dye concentrations and by the saturation of surface sites at high dye concentrations. Evidence for mixed kinetic control has been found for other contaminants and other model systems containing granular Fe0 (e.g. Agrawal and Tratnyek, 1996), but the relative importance of mass transport vs chemical reaction in these systems has not been quanti®ed. Scherer et al. (1997) were able to do this for the reduction of carbon tetrachloride at an Fe0 rotating disk electrode, because the rotating disk electrode provides a well characterized ¯ow regime. The electrochemical model they used can be simpli®ed to the semi-empirical model: 1 1 1 ˆ ‡ kobs C krxn C Ao1=2 C

…5†

where krxn is a ®rst order rate constant for reaction at the surface, A is an empirical constant that depends in part on the di€usion coecient of the dye and o is the mixing rate in rpm. Eq. (5) indi-

cates that the observed rate should approach zero as o approaches zero, but our data indicate that this limit occurs around o 1/2=8 (Fig. 4). This anomaly appears to be due to inecient mixing by the table shaker used in this study because visual observation revealed a breakpoint around 60 rpm below which the Fe0 particles are not suspended. An ad hoc correction for this can be made by subtracting a constant o€set from o 1/2. Global ®tting of the corrected empirical model to the data that are not confounded with site saturation e€ects (60 and 300 mM) gives the lines shown in Fig. 4. The ®tted value for krxn is not well constrained, indicating that these results are controlled entirely by mass transport. The other ®tting parameters are: A = 0.0920.04 rpm1/2 minÿ1 and the o€set in o 1/2 is 7.9 20.5. Extrapolation with these ®tted values to mM concentrations of dye shows that the observed decolorization rates deviate from the rates predicted, as the site saturation e€ect becomes increasingly important (top two sets of data in Fig. 4). E€ect of structure Another way to assess the relative signi®cance of mass transport and chemical reaction is through correlation analysis (Tratnyek, 1998). The kinetics of mass transport limited reactions often correlate to di€usion coecients of the substrates (D ), and the kinetics of reactions limited by chemical reduction typically correlate to descriptors of electronic properties such as the energy of the lowest unoccupied molecular orbital (ELUMO). For the nine dyes used in this study, di€usion coecients were estimated using the method of Hayduk and Laudie (Lyman et al., 1982), and values of ELUMO were calculated with MOPAC (CAChe WorkSystem software, Oxford Molecular Group, Beaverton, OR) using AM1 and PM3 parameters, with and without correction for solvation using the conductor-like screening model (COSMO). The values of these ®ve descriptors are summarized in Table 3. Fig. 5 shows a matrix of scatter plots illustrating the correlations between kobs, D and two calculations of ELUMO. Only values of kobs measured at o=100 rpm and C0=300 mM are included, because they make up the largest group of experiments

Table 3. Dye descriptors No. 1 2 3 4 5 6 7 8 9

6

Name

D  10 (cm2 sÿ1)

ELUMO (eV) AM1

ELUMO (eV) PM3

ELUMO (eV) AM1/COMSO

ELUMO (eV) PM3/COMSO

Acid Blue 113 Allura Red Amaranth Crocein Orange G Naphthol Blue Black Orange I Orange II Sunset Yellow FCF Tartrazine

3.035 3.876 3.646 4.537 3.391 4.537 4.537 4.188 3.878

2.139 3.189 5.045 0.999 2.339 0.967 0.891 3.066 5.526

2.083 3.039 4.932 0.874 2.291 0.890 0.766 2.918 5.448

ÿ1.702 ÿ1.462 ÿ1.701 ÿ1.180 ÿ1.857 ÿ1.503 ÿ1.327 ÿ1.504 ÿ1.327

ÿ1.493 ÿ1.365 ÿ1.533 ÿ1.307 ÿ1.631 ÿ1.208 ÿ1.424 ÿ1.391 ÿ1.174

Reduction of azo dyes

Fig. 5. Correlation matrix of rate constants for decolorization of azo dyes (C0=0.3 mM and o=100 rpm), vs selected descriptor variables: ELUMO (AM1 and PM3, both with COSMO) and D. Coecients of correlation, r2, are given on each plot. Points marked with crosses are for dyes with two azo linkages.

1843

utility of Fe0 as a reductant in authentic wastewater. For this purpose, a sample of wastewater was obtained from a dye manufacturing facility in South Carolina, U.S.A., and aliquots of the wastewater were exposed to two types of granular Fe0 that are currently used in full scale installations of permeable reactive barriers. Since the wastewater sample was believed to contain a variety of disperse azo dyes (as well as other dyes), the e€ect of treatment with Fe0 was determined by periodically collecting absorbance spectra (from 300 to 800 nm). The spectra obtained with Peerless (Fig. 6(a)) and Master Builder (not shown) Fe0 show decolorization that is somewhat variable with wavelength. This selectivity is more apparent from the di€erence spectra shown in Fig. 6(b), which were calculated by subtracting the scan at each elapsed time from the ®rst scan. Initially, decolorization around 500 to 600 nm is the most notable change, presumably due to reduction of labile dyes that have lmax in this range (cf. Table 1). After about 45 min, the samples were nearly decolorized, and the di€erence spectra

where the type of dye is the only variable (Table 2). Two smaller groups of data, corresponding to 120 and 140 rpm, are not shown because they exhibit less distinctive but similar trends to those shown in the ®rst row of Fig. 5. Correlations to the values of ELUMO calculated without correction for solvation are also not shown in Fig. 5 (because correlations to the corrected descriptors are better). No individual data for azo dyes have been excluded. The data in Fig. 5 reveal only weak correlations, and they are strongly leveraged by the two points for dyes containing two azo groups (Acid Blue 113 and Naphthol Blue Black). Correcting kobs or ELUMO for the possible formation of hydrazone tautomers (in dyes with phenolic hydroxyl groups ortho to azo linkages) is unlikely to improve the correlation because all of the dyes used are susceptible to this except for Orange I and Tartrazine. The ambiguity in the trends shown in Fig. 5 does not allow us to further resolve the role of mass transport and reaction rate control by correlation analysis, but, at the same time, it is not inconsistent the conclusions drawn from the data in Figs 3 and 4. Wastewater decolorization All of the experiments described above were conducted with solutions prepared in the laboratory from single dyes and dilute aqueous bu€ers, whereas wastewaters from dye manufacturing and application operations are known to be very complex mixtures of the colorants, conditioners, and salts (Reife and Freeman, 1996; Carliell et al., 1998). Since reduction of contaminants by Fe0 is known to be very sensitive to solution chemistry (Johnson et al., 1998), it was of interest to test the

Fig. 6. Absorbance (a) and di€erence (b) spectra for decolorization of wastewater from a dye manufacturing facility due to reaction with construction grade Fe0 (Peerless, 8± 16 mesh, 400 g Lÿ1). Data are for 160 rpm and room temperature.

1844

Sangkil Nam and Paul G. Tratnyek

appear to stabilize in the visible region. Below 400 nm, however, the di€erence spectra suggest a more sustained decrease in absorption, perhaps due to ¯occulation of dissolved organics by dissolving iron and associated adsorption to the metal-oxide coated particles of Fe0. These side e€ects deserve further investigation because they may contribute signi®cant practical advantages over other chemical reductants (e.g. dithionite, formamidine sul®nate, formaldehydesulfoxylate, and borohydride (Dubrow et al., 1996)) that have been used to decolorize wastewaters containing dyes. CONCLUSIONS

All of the nine dyes tested were rapidly decolorized by Fe0. Decolorization of the azo dyes was due to reduction of the azo groups, which resulted in formation of aromatics amines as products. The kinetics of decolorization were ®rst order, and varied with initial dye concentration, rate of mixing, and dye structure in a manner that indicates that the rates of reaction were controlled by mass transfer. These results with azo dyes are consistent with previous results obtained with other contaminants (chlorinated solvents, nitro aromatics, and various inorganics), suggesting that highly reactive compounds studied in batch model systems with gentle mixing are prone to mass transport in¯uenced kinetics, even though reaction rate controlled kinetics are the norm for most classes of contaminants and most experimental designs. With respect to the potential for application of Fe0 in the treatment of wastewaters from the dye industries, the implications of this study are mixed: although decolorization of azo dyes is very rapid, the resulting products are aromatic amines that may be of regulatory concern and require further treatment. AcknowledgementsÐThe authors would like to recognize V. Renganathan (Oregon Graduate Institute), who was S. N.'s major advisor during this and other studies of the environmental fate of azo dyes; Cikui Liang and David Gallagher (Oxford Molecular Group), who helped to perform molecular modeling calculations on the largest dyes; and Michelle Scherer (University of Iowa) and Joel Bandstra (OGI) for their input on the role of mass transport in this system. REFERENCES

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