Enhanced Biodegradation of Azo Dyes Using an Integrated Elemental Iron-Activated Sludge System: I. Evaluation of System Performance Jennie Perey Saxe, Brian L. Lubenow, Pei C. Chiu, Chin-Pao Huang, Daniel K. Cha
ABSTRACT: The objective of this research is to evaluate an integrated system coupling zero-valent iron (Fe0) and aerobic biological oxidation for the treatment of azo dye wastewater. Zero-valent (elemental) iron can reduce the azo bond, cleaving dye molecules into products that are more amenable to aerobic biological treatment processes. Azo dye reduction products, including aniline and sulfanilic acid, were shown to be readily biodegradable at concentrations up to approximately 25 mg/L. Batch reduction and biodegradation data support the proposed integrated iron pretreatment and activated sludge process for the degradation of the azo dyes orange G and orange I. The integrated system was able to decolorize dye solutions and yield effluents with lower total organic carbon concentrations than control systems without iron pretreatment. The success of the bench-scale integrated system suggests that iron pretreatment may be a feasible approach to treat azo dye containing wastewaters. Water Environ. Res., 78, 19 (2006). KEYWORDS: zero-valent iron, azo dyes, reduction, wastewater treatment, aerobic biodegradation. doi:10.2175/106143005X84477
Introduction Azo dyes are a group of chemicals that are largely resistant to aerobic biodegradation and persist in wastewater treatment processes. The electron-withdrawing nature of the azo bond makes these compounds less susceptible to oxidative biological processes. Activated sludge processes are ineffective for removal of azo dyes from wastewaters by means other than sorption to flocs (Doha´nyos et al., 1978; Hitz et al., 1978; Shaul et al., 1991). Chemical treatment methods, involving Fenton’s oxidation (Arslan and Balcioglu, 1999), photocatalytic oxidation (Chun and Yizhong, 1999), and ozonation (Liakou et al., 1997), are effective in decolorizing dye solutions, and, in some cases, complete oxidation to carbon dioxide (CO2). Anaerobic biological treatment methods also decolorize dye solutions through cleavage of the azo bond (Chung and Stevens, 1993), yielding aromatic amines as products that are generally not biodegraded under anaerobic conditions (Zissi and Lyberatos, 1996). Physical treatment methods, such as adsorption of dyes to granularactivated carbon, merely serve the purpose of phase transfer, and transformation of the contaminant does not occur. The high cost of chemical treatment methods, the often-incomplete anaerobic biodegradation of dyes, and the cost of treatment or disposal of a sorbent material illustrate the need for another technology for treatment of azo dyes in wastewater. Zero-valent iron (ZVI) has proved successful in treatment of several classes of recalcitrant compounds, including explosiveJanuary 2006
contaminated soil and groundwater (Singh et al., 1998), halogenated organic compounds (Johnson et al., 1996; Orth et al., 1998), and nitroaromatic compounds (Agrawal and Tratnyek, 1996; Devlin et al., 1998). Previous studies indicated that iron metal will also reduce azo functions in a dye molecule (Cao et al., 1999; Weber, 1996), yielding aromatic amines as products (Nam and Tratnyek, 2000). Perey et al. (2002) showed that treatment of azo dyes orange G and orange II with ZVI produces aromatic amines as products that are readily biodegraded by an aerobic, mixed culture. Iron-treated dye solutions exerted a higher biochemical oxygen demand (BOD) than the solutions containing the untreated dye solution, demonstrating that the recalcitrant azo dyes can be aerobically biodegraded after iron pretreatment. The objective of this research is to evaluate the feasibility of an integrated iron pretreatment and activated sludge process for treatment of dye-containing wastewaters. Batch biodegradation experiments were performed with two common products of azo dye reduction: aniline and sulfanilic acid. Respirometric assessments were performed to assess the biodegradability of an iron-treated orange G solution relative to a solution containing aniline, one of two orange G reduction products. Batch reduction and biodegradation data were considered in the construction of a bench-scale, integrated iron pretreatment and activated sludge system, consisting of an iron column upstream of an aeration basin and clarifier. The results indicate that implementation of the iron pretreatment step significantly enhances dye removal from the wastewater, that aromatic amine products are at least partially degraded, and that the effluent total organic carbon (TOC) from the integrated system was lower than that from the control system. Materials and Methods Chemicals. Orange G dye (C.I. 16230, 86% purity), sulfanilic acid, beef extract, magnesium sulfate, manganese chloride, and sodium molybdate were purchased from Sigma (St. Louis, Missouri). Orange I dye (C.I. 14600, purity unknown) was purchased from Fluka (Milwaukee, Wisconsin). Aniline, glycine, yeast extract, potassium phosphate (mono- and di-basic), ammonium chloride, and calcium chloride were purchased from Fisher Scientific (Pittsburgh, Pennsylvania). Sodium acetate and ferric chloride were purchased from Aldrich (Milwaukee, Wisconsin). The structures of orange G, orange I, and their reduction products are shown in Figure 1. Iron. Zero-valent iron used in this study was cast iron filings obtained from Master Builders, Inc. (Aurora, Ohio) and was used as 19
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Figure 1—Chemical structures of the azo dyes tested and their reduction products. received. Master Builders iron is known to contain 3.2% carbon, 0.65% manganese, and 0.6% sulfur (Reardon, 1995). The size of the iron particles was non-uniform (up to a few millimeters), and the specific surface area was previously determined to be 1.28 6 0.06 m2/g (Perey et al., 2002). Microorganisms. Acclimated activated sludge cultures used for batch biodegradation and respirometry studies were established in bench-scale sequencing batch reactors (SBRs) on a synthetic wastewater. The constituents of the synthetic wastewater and operating parameters of the SBRs are described in detail in Perey et al. (2002). Laboratory SBRs were initially seeded with mixed liquors from an industrial wastewater treatment plant (Seaford, Delaware) and a domestic wastewater treatment plant (Wilmington, Delaware). Batch Biodegradation Experiments. To determine the rate of aniline and sulfanilic acid biodegradation, acclimated seed culture collected from the SBR was suspended in phosphate-buffered, mineral-salt medium (Perey et al., 2002) and spiked with the test compound. The mineral salt solution contained (per liter) 182 mg dibasic potassium phosphate (K2HPO4), 104 mg monobasic potassium phosphate (KH2PO4), 20 mg ammonium chloride (NH4Cl), 5 mg calcium chloride (CaCl22H2O), 0.5 mg magnesium sulfate (MgSO47H2O), 0.2 mg manganese chloride (MnCl22H2O), 0.1 mg ferric chloride (FeCl36H2O), and 0.005 mg sodium molybdate (Na2MoO4H2O). Batch-biodegradation experiments were conducted in 250-mL Erlenmeyer flasks with a liquid volume of 150 mL and an initial seed concentration of 700 mg total suspended 20
solids (TSS)/L. Flasks were covered with aluminum foil and shaken continuously at 150 rpm on a platform shaker. Abiotic control flasks were established in parallel to confirm that there were no losses of reduction products resulting from physicochemical processes. Sterile controls containing autoclaved biomass were also established to quantify the extent of test compound sorption to biomass. Samples (2 mL) from bioreactors were collected at predetermined time intervals, filtered through a GF/A glass fiber filter (Whatman, Kent, United Kingdom), with a nominal pore size of 1.6 lm, and analyzed by high-pressure liquid chromatography (HPLC). Respirometric Analysis. To compare the biodegradability of iron-treated orange G (aniline concentration 5 2.5 mg/L) and aniline alone (4 mg/L), respirometric experiments were performed with a BI-1000 electrolytic respirometer (Bioscience Inc., Bethlehem, Pennsylvania). Respirometer reactor vessels (1000 mL) containing iron-treated orange G and aniline (1-L total liquid volume) were prepared according to Standard Methods (APHA et al., 1992) and seeded with acclimated culture (50 mg/L). Seed blanks were prepared identically to samples but without test chemicals. Abiotic controls contained deionized water, which had been run through a column packed with Master Builders cast iron. Respirometric analyses were run for 120 hours at 258C at an oxygen generation rate of 100 mg/h. Reactor content was mixed vigorously and continuously with a stir bar, and cumulative oxygen uptake data was recorded every 0.5 hours. Operation of Integrated System. This system was assessed with two model azo dyes: orange G and orange I. Removal of dyes, Water Environment Research, Volume 78, Number 1
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Table 1—Constituents of synthetic dye wastewater and nutrient solution.
Solution Synthetic-dye wastewater
Nutrient solution
Constituents Sodium acetate Glucose Orange G/Orange I Glycine Yeast extract Beef extract KH2PO4 NH4Cl MgSO47H2O FeCl36H2O CaCl22H2O MnCl22H2O Na2MoO4H2O
Table 2—Operating parameters for integrated ironcolumn and activated sludge system and control activated sludge system for azo dye treatment.
Concentration (mg/L) 266 133 100 6180 600 200 480 2000 20 4 200 8 0.2
aromatic amines, and TOC were compared to a control system without the iron pretreatment column. The integrated system consisted of a cylindrical activated sludge reactor with a liquid volume of 2 L and a 300 mL Imhoff settling cone as a clarifier. The aeration basins were stirred by impeller mixers and aerated with porous stone diffusers connected to aquarium air pumps. The reactors were seeded with the biomass from the same source as described above. The iron pretreatment column of the integrated system was packed with a mixture of Ottawa sand (20 to 30 mesh, Fisher Scientific) and Master Builders iron at a 2:1 (w/w) ratio. The glass column (Ace Glass, Vineland, New Jersey) had an effective volume of 175 ml (30 cm L 3 2.5 cm i.d.) and Teflon end fittings with Swagelok connections to tubing. The porosity of the sand-and-iron mixture was determined to be 0.38 by gravimetric analysis. The constituents of the synthetic dye wastewater and nutrient solution are listed in Table 1. Peristaltic pumps were used to supply the synthetic wastewater to either the iron column (integrated system) or the aeration basin (control system) at a rate of 4 L/day and the nutrient solution to the aeration basin at a rate of 100 mL/ day. The settled activated sludge was continuously recycled to aeration basin at a recirculation ratio of 1. The operating parameters are listed in Table 2. A schematic of the integrated system is illustrated in Figure 2. Reactors were operated for 3 to 4 weeks, until steadystate conditions were reached. This condition was determined by
Parameter Mean cell residence time (MCRT), day Mixed-liquor suspended solids (MLSS), mg/L VSS/TSS Food-to-microorganism ratio (F:M), mg TOC/mg VSS/day pH (aeration basin) Temperature, 8C HRT: Aeration basin, h HRT: Iron column, min
Integrated system
Control system
15
15
1800 0.7
1000 0.9
0.54 6.8 21 12 24
0.53 6.5 21 12 Not applicable
constant values over at least 3 days for dye concentration, reductionproduct concentration, and TOC at each of the sampling locations (synthetic wastewater, column effluent, integrated system effluent, and control effluent). Analytical Protocol. All samples were filtered through Whatman GF/A filters before UV and visible absorption spectroscopy (UV-VIS), HPLC, and TOC analyses. Quantification of dyes was performed with a Hach DR/2010 spectrophotometer (Loveland, Colorado). Orange G and orange I were found to have maximum absorption wavelengths (kmax) at 475 nm. A Varian HPLC model 2510 pump (flowrate 5 1 mL/min) with a model 2550 variable wavelength detector (Varian, Walnut Creek, California) was equipped with a guard column and an Alltima C-18 column (Alltech, Deerfield, Illinois) for the analysis of dye reduction products (aniline and sulfanilic acid). Sulfanilic acid (eluent: 50/50 vol/vol methanol/water) required a detection wavelength of 254 nm and a retention time of 1.7 minutes. Analysis of aniline required the an eluent containing 40/60 vol/vol pH 3.5 phosphate buffer/ acetonitrile, a detection wavelength of 254 nm, and a retention time of approximately 3.9 minutes. Total organic carbon was measured with a Tekmar-Dohrmann DC-190 TOC analyzer (Cincinnati, Ohio). Total suspended solids were measured according to Standard Methods 2540 D and the ratio of TSS to volatile suspended solids (VSS) was measured according to Standard Methods 2540 E (APHA et al., 1992).
Figure 2—Schematic diagram of an integrated iron-column and activated sludge system. January 2006
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Figure 3—Result of batch biodegradation of aniline. Data points are averages of triplicate samples from replicate reactors, and error bars represent standard deviations. Results and Discussion Batch Biodegradation Experiments. The batch biodegradation of aniline and sulfanilic acid by an aerobic, mixed culture are shown in Figures 3 and 4. Aniline was completely degraded within 24 hours, whereas 92.4% of added sulfanilic acid was degraded in 30 hours. This agrees with data from a similar experiment by Perey et al. (2002), in which aniline and sulfanilic acid were produced via reduction of azo dyes, then degraded by a similar mixed culture. Aerobic biodegradation of aniline and sulfanilic acid has been reported by several researchers (Coughlin et al., 2003; Gheewala and Annachhatre, 1997; O’Neill et al., 2000; Tan et al., 2000). No sorption or volatilization of the test compounds were observed in these batch experiments. Although not all organic carbon was removed in the course of these experiments (76.8% TOC removal for aniline and 58.6% removal for sulfanilic acid), the residual likely results from soluble microbial products that passed through the GF/A filter during sample filtration. The origin of these carbonaceous molecules is likely the SBR seed because the initial, measured organic carbon (OC) is higher than the initial calculated OC based on aniline and sulfanilic acid concentrations. The residual OC, carbon not accounted for in
Figure 5—Biochemical oxygen demand data from respirometric analysis of aniline and iron-treated orange G. The orange G data have been corrected against an abiotic control. aniline or sulfanilic acid molecules, was calculated using the following equation: residual OC ¼ TOC OC from test compound ðmeasuredÞ ðcalculatedÞ
From their molecular formulas, aniline and sulfanilic acid were calculated to be 77.3% and 41.6% OC, respectively. Figures 3 and 4 show that there is no significant increase in the residual OC values over the course of the experiment, suggesting that there is not an accumulation of metabolites in the system and that mineralization of the test compounds is possible. Respirometric Analysis. Figure 5 illustrates the BOD data from analysis of iron-treated orange G and aniline. The data for irontreated orange G has been corrected for oxygen demand from the oxidation of ferrous ion in the iron-column effluent. Although the seed blank (endogenous) BOD has not been corrected for, this oxygen demand will be equal in both samples. Because the concentration of aniline was higher (by 60%) in the aniline solution than in the irontreated orange G solution, if only aniline was degraded but not 1-amino-2-hydroxy-6,8-naphthalenedisulfonate (AHND, the other reduction product of orange G), the BOD curve of iron-treated orange G should fall below that of aniline solution. However, as shown in Figure 5, the BOD curves of these two samples are essentially the same, indicating that AHND was at least partially oxidized. Calculation of theoretical oxygen demand (ThOD), neglecting nitrogenous oxygen demand, also suggests that the second product of orange G may be exerting oxygen demand. C6 H5 NH2 þ 7O2 ! 6CO2 þ NH3 þ 2H2 O ) ThOD ¼ 2:4 g O2 =g aniline
Figure 4—Result of batch biodegradation of sulfanilic acid. Data points are averages of triplicate samples from replicate reactors, and error bars represent standard deviations. 22
ð1Þ
ð2Þ
Based on these calculations, 4.0 mg/L of aniline should exert a BOD of 9.6 mg/L, and the 2.5 mg/L of aniline in iron-treated orange G solution should exert an oxygen demand of 6.0 mg O2/L. Correcting the measured BOD in the aniline and iron-treated orange G samples for seed blank BOD (cumulative oxygen uptake 5 14 mg/L), the corrected BOD values for the aniline solution and irontreated orange G are 10 and 11 mg/L, respectively. These correspond to BOD/ThOD ratios of 1.0 for the aniline sample and 1.8 for iron-treated orange G solution. Water Environment Research, Volume 78, Number 1
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Because the measured BOD for iron-treated orange G is markedly higher than the theoretical value, it is most likely that the extra oxygen demand resulted from partial oxidation of AHND. C10 H4 ðOHÞðNH2 ÞðSO3 Þ2 þ 10:5 O2 ! 10CO2 þ NH3 þ 2H2 O þ 2ðSO3 Þ ) ThOD ¼ 1:06 g O2 =g AHND ð2:5 mg=L 4 93 g=mol 3 317g=mol ¼ 8:52 mg=L AHND; 8:52 mg=L 3 1:06 ¼ 9:03 mg=LÞ ð3Þ Because AHND is known to undergo autoxidation reaction (Kudlich et al., 1999) and an analytical standard is not available, this product could not be quantified and a complete carbon balance could not be obtained. However, if one assumes an aniline:AHND mole ratio of 1:1 (Figure 1) in iron-treated orange G solution and that the excess oxygen demand resulted from AHND degradation, then the excess demand was approximately one-half of the ThOD for AHND. Integrated System for Orange G Treatment. After reaching steady-state conditions, the integrated system was operated for 4 weeks for treatment of orange G-contaminated synthetic wastewater. Orange G concentration (Figure 6a), aniline concentration (Figure 6b), and TOC (Figure 6c) were monitored in both integrated and control systems. Table 2 summarizes the operating parameters of the two systems. Figure 6a clearly illustrates the color removal by the integrated system: 99% of orange G in the synthetic wastewater is reduced by the iron column in the integrated system, whereas 98% of the dye in the synthetic wastewater is detected in the effluent of the control reactor. Because spectrophotometry was used for dye quantification, the slight apparent increase in orange G concentration from the iron column effluent to the activated-sludge reactor effluent likely results from the oxidation product of AHND. Figure 6b illustrates the detection of aniline in the iron column effluent (average concentration is approximately 15.3 mg/L) and the absence of the compound in the integrated system activated sludge reactor effluent. Aniline was not detected in the control system. Combining this data with the TOC data (Figure 6c) and accounting for the dye purity, we performed a carbon balance on the system. First, we assume that all organic carbon, primarily from glucose and acetate, is completely biodegraded in both the integrated and control systems. The effluent TOC (approximately 37.2 mg/L) from the control reactor can be attributed largely to orange G (approximately 98 mg/L) in the control-system effluent, confirming the assumptions that orange G was not biodegradable and that glucose and acetate were completely mineralized or incorporated to biomass. The integrated system effluent was lower by approximately 13.7 mg/L— a value that corresponds to the TOC removed through aniline mineralization in the integrated system activated sludge reactor. The remaining TOC in the effluent of the integrated system likely results from AHND, the second reduction product of orange G. The calculated TOC from the theoretical concentration of AHND in the iron column effluent is 22.8 mg/L, corresponding well with the integrated system effluent TOC of 23.5 mg/L. Unfortunately, lack of an analytical standard for this compound and its tendency to undergo chemical oxidation (Kudlich et al., 1999) precludes its quantification. In contrast to the partial AHND degradation observed in the respirometer, the results from the integrated system suggest that the second products of orange G dye reduction are not degraded substantially. This phenomenon may be attributed to (1) substantially lower contact time in the integrated system (aeration basin hydraulic retention time [HRT] 5 12 hours) than the duration January 2006
Figure 6—(a) Orange G concentration, (b) aniline concentration, and (c) TOC in the integrated and control systems. of batch respirometric study and (2) the presence of glucose and acetate in the integrated system, resulting in the preferential use of these readily biodegradable organic compounds in the aeration basin. We will further investigate each of these possibilities in our future studies. The carbon balance demonstrated that aniline is mineralized or incorporated to biomass in the integrated system and that TOC removal is enhanced in the integrated system because of the pretreatment of the dye wastewater with elemental iron. Integrated System for Orange I Treatment. The orange I treatment system was operated for 6 weeks, after steady-state conditions were reached. We monitored the same operational parameters as the orange G treatment systems; these parameters 23
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Table 3—Summary of performance data from integrated systems and controls for azo-dye treatment. Concentrations at various sampling points Measured parameter (mg/L)
Integrated system effluent
Control effluent
94.2 6 3.5 1.3 6 0.1 — 15.3 6 0.4
4.9 6 0.5 0
95.4 6 0.9 —
Orange I 115.2 6 5.7 5.9 6 2.8 Sulfanilic Acid — 29.1 6 2.3
1.9 6 1.3 0
87.8 6 8.1 —
Synthetic wastewater
Column effluent
Orange G System: Orange G Aniline Orange I System:
are also summarized in Table 3. Figure 7a illustrates the color removal achieved through the iron pretreatment process, as was demonstrated with the orange G system. Sulfanilic acid was detected at an average concentration of 29.1 mg/L in the iron-column effluent and was completely degraded by the biological process of the integrated system (Figure 7b). The sulfanilic acid data was used to backcalculate the dye purity, assuming 100% reduction of orange I by elemental iron, and lack of sulfanilic acid sorption within the column; both assumptions are supported by orange II data from Perey et al. (2002). This calculation also assumes that sulfanilic acid is not introduced to the system as an orange I impurity or as the reduction product of an impurity. If orange I was 100% in purity, a concentration of 100 mg/L should yield 49.4 mg/L of sulfanilic acid. Because only 29.1 mg/L was detected in the column effluent, the dye purity can be estimated as approximately 60%. Again, a carbon balance was performed on both systems with the estimated orange I purity. Theoretically, sulfanilic acid should contribute 12.9 mg/L organic carbon, while the second product of orange I reduction product (4-amino-1-naphthol) should contribute 20.4 mg/L TOC. Compared to the control system (effluent TOC 5 35.8 mg/L), the TOC removal in the integrated system (effluent TOC 5 8.3 mg/L) was enhanced by more than the theoretical amount from sulfanilic acid, suggesting that 4-amino-1-naphthol is partially degraded in the biological treatment process.
Summary This study has extended the findings of earlier studies, and the data supports the conclusion that zero-valent iron pretreatment is a feasible option for enhancing the aerobic biodegradability of azo dyes in wastewater. Separate experiments have demonstrated the rapid reduction of azo dyes by Fe0 and the aerobic biodegradability of the reduction products. The construction and successful operation of the integrated system prove that this technology can be applied for treatment of a continuous stream of dye-laden wastewaters. Work continues to assess the conditions under which zero-valent iron pretreatment will be effective in enhancing aerobic azo dye biodegradability. Investigations are being made into physical– chemical parameters that affect the reduction rate of azo dyes, the effect wastewater impurities will have on column performance, and the life span of a given iron column. Although the design of the integrated system will likely be wastewater specific, the data presented herein support zero-valent iron pretreatment as a feasible option for treatment of azo dyes in wastewater. 24
Figure 7—(top) Orange I concentration, (middle) sulfanilic acid concentration, and (bottom) TOC in the integrated and control systems. Acknowledgments Credits. We gratefully acknowledge the Water Environment Research Foundation (Project 99-CTS-3-UR), Alexandria, Virginia, for funding this research. Authors. Jennie Perey Saxe received her Ph.D. from the Department of Civil and Environmental Engineering at University of Delaware. Brian L. Lubenow received his M.C.E. from the University of Delaware in 2002 and is currently with Camp Dresser Water Environment Research, Volume 78, Number 1
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& McKee, Inc. Pei C. Chiu is an associate professor, C. P. Huang is the distinguished professor, and Daniel K. Cha is an associate professor in the Department of Civil and Environmental Engineering at University of Delaware. Correspondence should be addressed to Daniel K. Cha, Department of Civil and Environmental Engineering, 301 DuPont Hall, University of Delaware, Newark, DE 19716; e-mail:
[email protected]. Submitted for publication April 1, 2003; revised manuscript submitted August 10, 2004; accepted for publication September 1, 2004. The deadline to submit Discussions of this paper is April 15, 2006. References Agrawal, A.; Tratnyek, P. G. (1996) Reduction of Nitro Aromatic Compounds by Zero-Valent Iron Metal. Environ. Sci. Technol., 30, 153–160. American Public Health Association; American Water Works Association; Water Pollution Control Federation (1992) Standard Methods for the Examination of Water and Wastewater, 17th ed.; Washington, D.C. Arslan, I.; Balcioglu, I. A. (1999) Degradation of Commercial Reactive Dyestuffs by Heterogenous and Homogenous Advanced Oxidation Processes: A Comparative Study. Dyes Pigments, 43, 95–108. Cao, J.; Wei, L.; Huang, Q.; Wang, L.; Han, S. (1999) Reducing Degradation of Azo Dye by Zero-Valent Iron in Aqueous Solution. Chemosphere, 38, 565–571. Chun, H.; Yizhong, W. (1999) Decolorization and Biodegradability of Photocatalytic Treated Azo Dyes and Wool Textile Wastewater. Chemosphere, 39, 2107–2115. Chung, K.-T.; Stevens, S. E. (1993) Degradation of Azo Dyes by Environmental Microorganisms and Helminths. Environ. Toxicol. Chem., 12, 2121–2132. Coughlin, M. F.; Kinkle, B. K.; Bishop, P. L. (2003) High Performance Degradation of Azo Dye Acid Orange 7 and Sulfanilic Acid in a Laboratory Scale Reactor after Seeding with Cultured Bacterial Strains. Water Res., 37, 2757–2763. Devlin, J. F.; Klausen, J.; Schwarzenbach, R. P. (1998) Kinetics of Nitroaromatic Reduction on Granular Iron in Recirculating Batch Experiments. Environ. Sci. Technol., 32, 1941–1947. Doha´nyos, M.; Madeˇra, V.; Sedla´cˇek, M. (1978) Removal of Organic Dyes by Activated Sludge. Prog. Water Technol. 10, 559–575.
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Gheewala, S. H.; Annachhatre, A. P. (1997) Biodegradation of Aniline. Water Sci. Technol., 36 (10), 53–63. Hitz, H. R.; Huber, W.; Reed, R. H. (1978) The Adsorption of Dyes on Activated Sludge (Publication Sponsored by ETAD). J. Soc. Dyers Colourists, 94, 71–76. Johnson, T. L.; Scherer, M. M.; Tratnyek, P. G. (1996) Kinetics of Halogenated Organic Compound Degradation by Iron Metal. Environ. Sci. Technol., 30, 2634–2640. Kudlich, M.; Hetheridge, M. J.; Knackmuss, H.-J.; Stolz, A. (1999) Autoxidation Reactions of Different Aromatic o-Aminohydroxynaphthalenes that are Formed during the Anaerobic Reduction of Sulfonated Azo Dyes. Environ. Sci. Technol., 33, 896–901. Liakou, S.; Pavlou, S.; Lyberatos, G. (1997) Ozonation of Azo Dyes. Water Sci. Technol., 35, 279–286. Nam, S.; Tratnyek, P. G. (2000) Reduction of Azo Dyes with Zero-Valent Iron. Water Res., 34, 1837–1845. O’Neill, F. J.; Bromley-Challenor, K. C. A.; Greenwood, R. J.; Knapp, J. S. (2000) Bacterial Growth on Aniline: Implications for the Biotreatment of Industrial Wastewater. Water Res., 34, 4397–4409. Orth, R.; Dauda, T.; McKenzie, D. E. (1998) Reductive Dechlorination of DNAPL Trichloroethylene by Zero-Valent Iron. Practice Period. Hazard. Toxic Radioactive Waste Manage., 2, 123–128. Perey, J. R.; Chiu, P. C.; Huang, C.-P.; Cha, D. K. (2002) Zero-Valent Iron Pretreatment for Enhancing Biodegradability of Azo Dyes. Water Environ. Res., 74, 221–225. Reardon, E. J. (1995) Anaerobic Corrosion of Granular Iron: Measurement and Interpretation of Hydrogen Evolution Rates. Environ. Sci. Technol., 29, 2936–2945. Shaul, G. M.; Holdsworth, T. J.; Dempsey, C. R.; Dostal, K. A. (1991) Fate of Water Soluble Azo Dyes in the Activated Sludge Process. Chemosphere, 22, 107–119. Singh, J.; Comfort, S. D.; Shea, P. J. (1998) Remediating RDXContaminated Water and Soil Using Zero-Valent Iron. J. Environ. Qual., 27, 1240–1245. Tan, N. C. G.; Borger, A.; Slenders, P.; Svitelskaya, A.; Lettinga, G.; Field, J. A. (2000) Degradation of Azo Dye Mordant Yellow 10 in a Sequential Anaerobic and Bioaugmented Aerobic Bioreactor. Water Sci. Technol., 42 (5) 337–344. Weber, E. J. (1996) Iron-Mediated Reductive Transformations: Investigation of Reaction Mechanism. Environ. Sci. Technol., 30, 716–719. Zissi, U.; Lyberatos, G. (1996) Azo-Dye Biodegradation Under Anoxic Conditions. Water Sci. Technol., 34, 495–500.
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