Towards advanced aqueous dye removal processes: A short review on the versatile role of activated carbon

Journal of Environmental Management 102 (2012) 148e164

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Review

Towards advanced aqueous dye removal processes: A short review on the versatile role of activated carbon Gergo Mezohegyi a, Frank P. van der Zee b, Josep Font c, Agustí Fortuny d, Azael Fabregat c, * a

Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, Box 2461, 3001 Leuven, Belgium Biothane Systems Int./VWS, P.O. Box 5068, 2600 GB Delft, The Netherlands c Departament d’Enginyeria Química, ETSEQ, Universitat Rovira i Virgili, Av. Països Catalans 26, 43007 Tarragona, Catalunya, Spain d Departament d’Enginyeria Química, EPSEVG, Universitat Politècnica de Catalunya, Av. Víctor Balaguer s/n, 08800 Vilanova i la Geltrú, Catalunya, Spain b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 July 2011 Received in revised form 25 January 2012 Accepted 18 February 2012 Available online 28 March 2012

During the last decade, several physico-chemical and biological techniques have been developed to remove colour from textile wastewaters. Some of these techniques rely on and many will profit from activated carbon (AC). The role of AC is versatile: (1) it acts as a dye adsorbent, not only in straightforward adsorption processes but also in AC-enhanced coagulation and membrane filtration processes; (2) it generates strong oxidising agents (mostly, hydroxyl (
OH) radicals) in electrochemical dye oxidation; (3) it catalyses
OH production in advanced oxidation processes; (4) it catalyses anaerobic (azo) dye reduction and supports biofilm growth in microbial dye removal. This paper reviews the role of AC in dye decolourisation, evaluates the feasibility of each AC-amended decolourisation technique and discusses perspectives on future research. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Activated carbon Catalysis Catalyst support Decolorization Dye Textile wastewater

1. Introduction Textile processing consumes and discharges large amounts of water. It is estimated that about 40e65 L of textile effluent is generated per kg of cloth produced (Manu and Chaudhari, 2002). One of the problems associated with the release of textile wastewater is the presence of synthetic dyes. These dyes present an environmental concern due to both their high visibility and their recalcitrance. Moreover, many dyes and their degradation products have been associated with toxicity and/or mutagenicity (Weisburger, 2002). Up to day, several physico-chemical and biological methods have been developed to treat dye-containing wastewaters (Slokar and Le Marechal, 1998; Robinson et al., 2001; Forgacs et al., 2004; Dos Santos et al., 2007; Hai et al., 2007). Each technique has its own advantages and drawbacks. The literature indicates that nowadays, those dye removal techniques and investigations are dominant which meet the requirements of stricter environmental regulations (i.e., green(er) and clean(er) technologies). Unfortunately, cleaner dye wastewater treatment methods generally imply higher energy/ * Corresponding author. Tel.: þ34 977 559643; fax: þ34 977 559667. E-mail address: [email protected] (A. Fabregat). 0301-4797/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2012.02.021

operation costs. However, significant reduction of expenses and/or enhancement of dye removal can be achieved e.g., by the combination of different methods in hybrid treatments (Hai et al., 2007). A particular approach for hybrid decolourisation methods is the inclusion of AC and its beneficial features in the physical, chemical or biological process. For a long time, application of AC in textile and dye wastewater treatment was exclusively based on AC as dye adsorbent (Choy et al., 1999; Kannan and Sundaram, 2001; Namasivayam and Kavitha, 2002; Malik, 2003). Indeed, AC is probably the most versatile adsorbent because of its large surface area, polymodal porous structure, high adsorption capacity and variable surface chemical composition (Marsh and Rodríguez-Reinoso, 2006). While AC adsorption of dyes may be discussed as individual treatment, it takes place as an accompanying mechanism in all dye removal methods involving AC either as a porous high-surface support material or as a catalyst. In different catalytic reactions, ACs have been mainly used as supports, but their use as catalysts on their own e especially due to their surface oxygen groups e is growing quickly (Radovic and Rodríguez-Reinoso, 1997; Rodríguez-Reinoso, 1998; Figueiredo and Pereira, 2009), even in dye removal processes. What furthermore makes ACs attractive to facilitate (textile) wastewater treatment is the possibility of tailoring their physical and/or chemical properties in order to optimise their performance.

G. Mezohegyi et al. / Journal of Environmental Management 102 (2012) 148e164

The aim of this review is to summarise the main results of ACamended dye-removal techniques. The diversity of beneficial roles of AC in dye removal will be highlighted, from conventional adsorption to (novel) catalytic decolourisation techniques, with emphasis on the latter ones. The article includes four main sections, i.e., dye removal by single AC adsorption (Section 2), by ACenhanced physico-chemical processes (Section 3), by AC-amended biological degradation (Section 4) and a critical discussion (Section 5) of the topic and the above mentioned treatment methods. 2. AC adsorption of dyes Activated carbons (ACs) are excellent adsorbents of countless pollutants. Their industrial applications involve the adsorptive removal of colour, odour, taste and other undesirable organics and inorganics from drinking water and wastewater. Numerous physicochemical factors affect dye adsorption, including the interaction between the adsorbate and adsorbent, AC surface area and pore structure, AC surface chemistry, effect of other components, characteristics of the dye molecule, AC particle size, pH, temperature, contact time etc. Due to its unique molecular structure, AC has an extremely high affinity for many classes of dyes that is especially determined by the AC characteristics (Table 1). This section does not make an attempt to summarise or compare the hundreds of studies on dye adsorption. Instead, it tries to give a general insight into most of the key issues of dye adsorption that is a requisite for clearly understanding the discussions related to all AC-enhanced processes. 2.1. Dye adsorption isotherms Adsorption isotherms are the basic requirements in the design of adsorption processes. The isotherm indicates how the molecules distribute between the liquid and solid phase when adsorption reaches the equilibrium state. In most studies related to dye adsorption by AC, two well-known isotherms predominate. The Langmuir isotherm assumes monolayer sorption at homogeneous sites of the AC surface without any interaction among the adsorbed molecules possessing equal sorption activation energies. The Langmuir equation can be given as (Eq. (1)):

QE ¼

QM KL CE 1 þ KL CE

(1)

where QE is the dye equilibrium concentration on the AC phase (mg g1 AC ), QM is the maximum amount of dye corresponding to complete monolayer coverage on the carbon surface (mg g1 AC ), CE is the equilibrium concentration in the liquid phase (mg L1) and KL is the Langmuir constant (L mg1). The Freundlich isotherm, on the other hand, presumes heterogeneous surface energies in which the energy term varies as a function of the surface coverage, and can be used for nonideal sorption processes. It is expressed by the following equation (Eq. (2)): 1=n

QE ¼ KF CE

(2)

where KF is the Freundlich constant indicating the dye adsorption capacity (L g1 AC ) and n index shows the adsorption intensity. For example, Acid Yellow 36 adsorption onto ACs prepared from sawdust and rise husk (Malik, 2003) and Methylene Blue adsorption onto various e commercially available and indigenously prepared e activated carbons (Kannan and Sundaram, 2001) fitted well with both isotherms. Some less common isotherms have occasionally been suggested such as the Redlich-Peterson isotherm, incorporating the features of the Langmuir and Freundlich isotherms (Choy et al., 1999; El Qada et al., 2008).

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2.2. Dye adsorption kinetics In general, the mechanism of dye adsorption onto AC involves the following steps: (1) migration of dye from the bulk solution to the AC surface; (2) diffusion of dye through the boundary layer to the AC surface; (3) adsorption of dye at an available active site on the AC surface; and (4) intra-particle diffusion of dye into the interior pores of the AC particle and adsorption (Kannan and Sundaram, 2001). The rate of dye adsorption can be determined by the use of adsorption kinetic models. Among several kinetic approaches (Mall et al., 2005; Kavitha and Namasivayam, 2007), the second-order model was generally found to give very good correlations with the experimental data of dye adsorption onto AC. The model corresponds to the equation (Eq. (3)):

Q ¼

kQE2 t 1 þ kQE t

(3)

where Q represents the dye concentration in the solid phase 1 (mg g1 AC ), QE is the corresponding value at equilibrium (mg gAC ), t is the contact time (min) and k is the adsorption rate constant (gAC mg1 min1). For instance, Kavitha and Namasivayam (2007) tested an activated coir pith carbon prepared from coconut husk to remove Methylene Blue and used different kinetic models to fit the experimental data. The calculated correlations were the closest in case of the second-order model, confirming the chemisorption of the colourant onto the AC particles. The same kinetic model was proposed for the AC adsorption of numerous colourants, such as Congo Red (Namasivayam and Kavitha, 2002), Reactive Red 241 (Órfão et al., 2006), Acid Red 97, Acid Orange 61, Acid Brown 425 (Gómez et al., 2007) or Acid Orange 7 (Mezohegyi et al., 2007). The fast adsorption mechanism by mesoporous AC prepared from coconut coir dust followed a pseudo-second-order kinetics with a significant contribution of intra-particle diffusion in case of Methylene Blue and Remazol Yellow dyes (Macedo et al., 2006). Apart from second-order kinetics, other models for AC adsorption of dyes have occasionally been reported such as the simple firstorder model (Kannan and Sundaram, 2001; Malik, 2003) or the dual resistance modified Matthews-Weber model (Walker and Weatherley, 1999a). 2.3. Effect of pH The solution’s pH plays a major role in the dye adsorption process. Its effect can be described on the basis of the comparison between pH and the point of zero charge (pHPZC) of the adsorbent: AC acts as a positively charged surface in the dye solution for pH < pHPZC and as a negatively charged surface for pH > pHPZC. Consequently, a colourant with cationic characteristics has higher affinity for AC adsorption when pH > pHPZC, whereas a colourant with anionic characteristics has higher affinity when pH < pHPZC (Órfão et al., 2006). E.g., El Qada et al. (2008) showed that the maximum adsorption capacity of AC with a pHPZC value of 6.3 for the basic dye Methylene Blue was nearly doubled by increasing the pH from 4 to 11. The lower adsorption of this dye at acidic pH (
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Table 1 Selected results of dye adsorption studies with different ACs. Dye

AC characteristicsa

Dye adsorption capacityb, mg g1 AC

Reference

Acid Red 114 Polar Blue RAWL Polar Yellow Methylene Blue

com.; d: 500e710

101 101 129 980 143 278 165 343 472 184 87 455 (pH 7) 345 (pH 7) 307 (pH 7) 556 (pH 7) 588 (pH 7) 833 (pH 7) 625 (pH 7) 186 (pH 7) 130 (pH 7) 242 (pH 7) 52 169 222 1111 400 434 546 633 568 714 680 190 157 201 246 223 310 197 244 345 262 289c (pH 5) 219c (pH 5) 309c (pH 5) 282c (pH 5) 272c (pH 5) 52 159 161 189 270 612 904 942 17 318 480 710 432 790

Choy et al. (1999)

Acid Yellow 36 Methylene Blue

Basic Red Basic Yellow Reactive Red 241 (hydrolysed)

Acid Red 97 Acid Orange 61 Acid Brown 425 Remazol Yellow Remazol Red Remazol Black Basic Red 14

Reactive Red 241 (hydrolysed)

Acid Blue 113

Methylene Blue

Congo Red

Astrazon Red F3BL

com. (Merck); d: 90 o: bamboo dust; d: 90 o: coconut shell; d: 90 o: groundnut shell; d: 90 o: rice husk; d: 90 o: straw; d: 90 o: mahogany sawdust; d: 74e250; SBET: 516 o: rice husk; d: 74e1190; SBET: 272 com. (Filtrasorb 400); d < 106; SBET: 1216; pHPZC: 7.8 o: BC; d < 106; SBET: 857; pHPZC: 6.3 o: BC; d < 106; SBET: 863; pHPZC: 8.3 com. (Filtrasorb 400); d < 106; SBET: 1216; pHPZC: 7.8 o: BC; d < 106; SBET: 857; pHPZC: 6.3 com. (Filtrasorb 400); d < 106; SBET: 1216; pHPZC: 7.8 o: BC; d < 106; SBET: 857; pHPZC: 6.3 com. (Norit ROX 0.8); d < 50; SBET: 1032; pHPZC: 8.4 com. mod: HNO3; d < 50; SBET: 893; pHPZC: 2.0 com. mod: HNO3/heat under H2; d < 50; SBET: 987; pHPZC: 10.0 com. (J.T. Baker)

com. (Filtrasorb 400); d: 300e500; SBET: 1100; pHPZC: 7.2

com. (Norit GAC 1240 PLUS); d < 50; SBET: 972; pHPZC: 9.7 com. mod: HNO3; d < 50; SBET: 909; pHPZC: 2.7 com. mod: H2O2; d < 50; SBET: 949; pHPZC: 5.4 com. mod: HNO3/heat under H2; d < 50; SBET: 972; pHPZC: 10.8 com. mod: HNO3/heat under N2; d < 50; SBET: 946; pHPZC: 9.9 com. (Norit GAC 1240 PLUS); d < 50; SBET: 972; pHPZC: 9.7 com. mod: HNO3; d < 50; SBET: 909; pHPZC: 2.7 com. mod: H2O2; d < 50; SBET: 949; pHPZC: 5.4 com. mod: HNO3/heat under H2; d < 50; SBET: 972; pHPZC: 10.8 com. mod: HNO3/heat under N2; d < 50; SBET: 946; pHPZC: 9.9 com. (Norit GAC 1240 PLUS); d < 50; SBET: 972; pHPZC: 9.7 com. mod: HNO3; d < 50; SBET: 909; pHPZC: 2.7 com. mod: H2O2; d < 50; SBET: 949; pHPZC: 5.4 com. mod: HNO3/heat under H2; d < 50; SBET: 972; pHPZC: 10.8 com. mod: HNO3/heat under N2; d < 50; SBET: 946; pHPZC: 9.9 com. (BDH, Merck); SBET: 1118; pHPZC: 10.2 com. (F100, Calgon); SBET: 957; pHPZC: 7.8 com. (BPL, Calgon); SBET: 972; pHPZC: 8.6 com. (BPL, Calgon) mod: HCl; SBET: 1015; pHPZC: 6.7 com. (BPL, Calgon) mod: HNO3; SBET: 987; pHPZC: 3.0 o: BC; d: 200e500; SBET: 679; VM/VT: 53.4; pHPZC: 10.2 o: BC; d: 200e500; SBET: 475; VM/VT: 76.8; pHPZC: 11.3 o: BC; d: 200e500; SBET: 331; VM/VT: 82.1; pHPZC: 12.0 o: BC; d: 200e500; SBET: 370; VM/VT: 80.9; pHPZC: 12.2 o: bagasse; d: 250e420; SBET: 446; TA: 750 o: bagasse; d: 250e420; SBET: 545; TA: 780 o: bagasse; d: 250e420; SBET: 593; TA: 810 o: bagasse; d: 250e420; SBET: 607; TA: 840 o: plum kernel; d: 250e420; SBET: 354; TA: 750 o: plum kernel; d: 250e420; SBET: 686; TA: 850 o: plum kernel; d: 250e420; SBET: 825; TA: 875 o: plum kernel; d: 250e420; SBET: 1162; TA: 900 o: corn cob; d: 250e420; SBET: 538; TA: 830 o: corn cob; d: 250e420; SBET: 943; TA: 890

Kannan and Sundaram (2001)

Malik (2003) El Qada et al. (2008)

Órfão et al. (2006)

Gómez et al. (2007)

Al-Degs et al. (2000)

Faria et al. (2004)

Wang et al. (2005b)

Lorenc-Grabowska and Gryglewicz (2007)

Juang et al. (2002)

a Abbreviations: com., commercial; d, AC particle size, mm; o, origin; SBET, BET surface area, m2 g1 AC ; pHPZC, point of zero charge; BC, bituminous coal; mod, modified; VM/VT, mesopore/total pore volume ratio, %; TA activation temperature,  C. b Unless not indicated differently, refers to Langmuir monolayer capacity at natural pH of dye solution. c Adsorption capacities determined by second-order kinetic model.

adsorption capacity (Malik, 2003). Calorimetric studies confirmed that the dyeeAC interaction forces are correlated with the pH of the solution and the increase of pH promoted an endothermic process for the anionic dye Remazol Yellow and an exothermic process for the cationic dye Methylene Blue (Macedo et al., 2006).

2.4. Effect of AC surface chemical characteristics Certain physical and chemical properties of the AC may strongly influence dye adsorption, and one of the most determining factors among them is carbon surface chemistry. Al-Degs et al. (2000) evaluated the adsorption of some anionic reactive dyes on

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a commercial AC and attributed its high adsorption capacity to its net positive surface charge. By modifying the specific properties of AC, it is possible to enhance its effectiveness for contaminant uptakes from aqueous solutions (Yin et al., 2007). Some researches have focused on this issue and one of the key target has become the modification of AC surface chemistry (Figueiredo et al., 1999; Berenguer et al., 2009). Tailoring the carbon surface chemical groups can lead to significant changes in dye adsorption performance. In the works of Pereira et al. (2003), Faria et al. (2004) and Órfão et al. (2006), several ACs were modified by different acid and thermal treatments for the adsorption of different classes of dyes, without inducing any major changes in the AC textural properties. In case of all anionic (reactive, direct and acid) dyes tested, a similar behaviour was observed, following an improvement of the adsorption capacity by increasing the basicity of the AC. On the other hand, for cationic (basic) dyes (Pereira et al., 2003; Faria et al., 2004), the acid oxygen-containing surface groups originating from HNO3-treatment had a positive effect on adsorption, but the thermally treated samples still presented good performances, indicating the existence of two parallel adsorption mechanisms. The first involves electrostatic interactions between the basic dye and the negatively charged carbon surface groups, while the second suggests dispersive interactions between the dye molecule and the graphene layers. Otherwise, the basic AC sample obtained by thermal treatment under H2 flow at 700  C was found to be the best material for adsorption of most of the dyes tested (Pereira et al., 2003; Faria et al., 2004). Some other studies, however, reported dissimilar results referred to basic dye adsorption. For example, a moderate decrease in Methylene Blue (Wang et al., 2005b; Wang and Zhu, 2007) and Crystal Violet (Wang and Zhu, 2007) uptake was observed after treating the AC with HNO3. 2.5. Effect of AC textural characteristics Aside from the surface chemistry, textural properties of AC also play an important role. Most commercially available ACs are predominantly microporous, making them especially suitable for the adsorption of smaller pollutants. The efficiency to adsorb larger molecules is expected to increase with increasing the fraction of mesopores (Macedo et al., 2006). One possible method to enlarge the porosity of ACs is CO2 gasification, e.g., with the help of previously impregnated cobalt, catalysing the formation of mesopores (Pereira et al., 2004). It is evident that if the AC pore size is crucial in dye adsorption, then the molecular size of the colourant is likewise decisive. Tamai et al. (1999) investigated the adsorption of several acid, direct and basic dyes onto both mesoporous and microporous AC fibers in terms of the size of dye molecules and AC pore size. High amounts of sterically small-sized acid and basic dyes were adsorbed on both carbons. Among acid dyes, Acid Blue 74 and Acid Orange 10 with smaller molecular weights did not only perform better adsorption than Acid Blue 9 and Acid Orange 51 with relatively large molecular structures but were also adsorbed in higher amounts on the microporous fiber than on the mesoporous one, indicating the additional importance of AC specific surface area. Large direct dyes adsorbed to a much higher extent on the mesoporous AC than on the microporous AC. Moreover, the extent of direct dye adsorbed on mesoporous AC decreased with increasing size of the dye molecule. Lorenc-Grabowska and Gryglewicz (2007) showed that adsorption of the diazo dye Congo Red increased with both the mesopore volume and the ratio of mesopores to the total pore volume. The so-called ordered mesoporous carbons (OMCs) may be ideal model materials for studying dye adsorption on mesopores due to their large pore volume, high specific surface area, centralised mesopore distribution and tuneable pore diameter. When a microporous carbon and OMCs with varied pore size

151

were simultaneously tested to remove Methylene Blue and Neutral Red from solution, the OMCs showed at least doubled dye adsorption affinities compared to the microporous carbon (Yuan et al., 2007). 2.6. Regeneration of AC Single use of AC for dye adsorption is not cost-effective unless it is prepared from certain solid wastes such as agricultural residues (Kannan and Sundaram, 2001; Juang et al., 2002; Namasivayam and Kavitha, 2002; Dias et al., 2007; Kavitha and Namasivayam, 2007; Demirbas, 2009). AC regeneration is normally not only cheaper than replacement but also more environmentally friendly. Conventional methods for AC recovery have included thermal, biological and solvent regeneration (Cooney et al., 1983; Aktas¸ and Çeçen, 2007). In latter case, the choice of an appropriate solvent which depends on the characteristics of both the AC and the colourant, is essential. E.g., desorption of the basic dye Malachite Green from an AC packed column was nearly complete using acetone (Gupta et al., 1997) and commercial ACs with the adsorbed azo dye Direct Red 79 were effectively regenerated by redundant water at high pressure and temperature (Salvador and Jiménez, 1999). For carbon regeneration, certain surfactants have the advantage of being both efficient under mild operation conditions and relatively harmless to the environment (Purkait et al., 2007). Surfactant enhanced carbon regeneration showed that anionic surfactants performed significantly better than a cationic one during the desorption of anionic dyes Eosin (Purkait et al., 2005) and Congo Red (Purkait et al., 2007). Also some advanced regeneration techniques removing colourants from AC have been reported, such as wet oxidative regeneration (Shende and Mahajani, 2002), microwave irradiation (Quan et al., 2004), electrochemical regeneration (Han et al., 2008) or regeneration with dielectric barrier discharge plasma (Qu et al., 2009). In general, however, these techniques are costly, can result in significant changes in the textural/chemical characteristics of the AC and may cause considerable carbon loss. 3. AC-amended physico-chemical dye removal processes Besides adsorption, physico-chemical dye removal techniques have included coagulation, membrane filtration, electrochemical degradation and advanced oxidation processes (AOPs). Despite that most of these methods have been shown promising and highly efficient, these technologies generally appear to face several limitations since they are financially and often also methodologically demanding. To make them feasible for possible industrial applications, further improvement is still required. As it is presented in the following, physical and chemical dye wastewater treatments can be successfully enhanced by incorporating AC in the operation. 3.1. AC-amended coagulation of dyes Chemical coagulation by the use of inorganic coagulants such as aluminium sulphate, ferrous sulphate or ferric chloride, used to be a feasible way of removing colour from dye wastewater (Chu, 2001; Kim et al., 2004; Golob et al., 2005). Its main advantage is the removal of dye molecules themselves, i.e. without their decomposition to even more harmful and toxic aromatic compounds (Golob et al., 2005). However, the drawback of producing a huge amount of chemical sludge has pushed this method into the background. It is necessary, therefore to develop strategies for improved dye coagulation, such as the reuse of the produced sludge (Chu, 2001), combined electrocoagulation process (Can et al., 2006) or ACamended coagulation (Sanghi and Bhattacharya, 2003). In the

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latter study, polyaluminium chloride as coagulant was added to different dye solutions (Direct Orange, Eriochrome Black T and Malachite Green) previously treated with powdered activated carbon (PAC). The role of coagulant was dual; not only to remove the colour but also to flocculate the suspended PAC remaining in the solution after adsorption. Although significant enhancement of dye removal after the addition of a very small dose of coagulant was observed only in case of Eriochrome Black T, the formed sludge settled much faster than PAC alone and could be effectively reused. It is also possible to use AC adsorption indirectly, as a posttreatment of coagulation. The azo dyes Reactive Red 45 and Reactive Green 8 were treated by the two-step, aluminium chloride coagulation/PAC adsorption method resulting in less coagulant consumption and lower sludge volume compared to dye removal by coagulation only (Papic et al., 2004). During the treatment of Reactive Orange 16 and Reactive Black 5, coagulation by aluminium chloride followed by PAC adsorption was found to be more efficient than the reverse adsorption/coagulation method in terms of overall dye removal and chemical requirement (Lee et al., 2006a). 3.2. AC-amended membrane filtration of dyes Pressure-driven membrane processes have been found suitable for the treatment of dye wastewaters from the textile industry (Xu et al., 1999; Marcucci et al., 2001; Chakraborty et al., 2003). Among microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), NF was considered as probably the most adequate membrane process to separate dyes from these effluents (Xu et al., 1999; Chakraborty et al., 2003). Although NF does not reach the retention behaviour of RO, it works under less demanding conditions implying cost reduction (Marcucci et al., 2001). Lately, the use of low-pressure membrane filtration techniques (MF, UF) for dye removal have received greater attention which demand considerably lower operational costs than NF (Lee et al., 2006b) but require intensification or combination with other processes in order to ensure an appropriate permeate quality. One possibility to enhance the MF/UF process for dye removal is their integration with AC dye adsorption. Banat and Al-Bastaki (2004) studied the separation of Methylene Blue from aqueous solutions where the dye feed containing either GAC or PAC was introduced to the UF membrane module. The UF process alone was not able to reject the colourant but 85% of dye removal was achieved in the combined process, with the PAC performing better than GAC. Microfiltration of the reactive dye Ostazin Red HB by a submerged membrane was not effective on its own but the addition of very low doses of PAC to dye wastewater resulted in nearly complete colour removal, although retention of the pollutant decreased with 50% in a few hours of operation (Jirankova et al., 2007). In a hybrid coagulationeadsorptionemembrane process, the submerged microfiltration membrane alone was not sufficient to reject Reactive Black 5 and Reactive Orange 16, and significant membrane fouling occurred (Lee et al., 2006b). However, the filtration performance after alum coagulation and PAC adsorption pretreatments was very high, both concerning dye retention and micro-flocks/carbon particles separation, while only a little decline in permeate flux was observed through the entire hours of experimental run. An efficacious development among combined membrane processes is the membrane bioreactor, nowadays widely used for municipal and industrial wastewater treatment, which can be enhanced by the addition of AC (Park et al., 1999; Hai et al., 2008). During the removal of azo dye Acid Orange II by a submerged MF membrane fungi reactor, PAC addition resulted not only in improved permeate quality but also in stable decolourisation for a period of 1 month, confirming the occurrence of simultaneous adsorption and biodegradation of the dye (Hai et al., 2008).

3.3. AC-amended electrochemical treatment of dyes Electrochemical oxidation uses electrons as the main reagent, requires little or even no chemicals and is a clean and effective wastewater treatment method (Yi et al., 2008). It has been successfully applied for dyeing wastewater treatment, with a wide variety of electrodes (anodes) tested, e.g., iron (Ling and Peng, 1994), conductive diamond (Cañizares et al., 2006), Pt/Ti (Vlyssides et al., 1999) or Pb/PbO2 (Awad and Galwa, 2005). Although the method is quite environmentally friendly, its drawback e apart from electricity costs e is that side-reactions (e.g., electrolysis of water) may compete with the destruction of contaminant (Wang et al., 2005a). Improvement of electrochemical dye degradation can be reached by the use of porous electrodes such as ACs. The AC electrodes reduce the energy requirement for electrolysis and limit side-effects by enhancing the surface area of electrode and regulating the electric current density. Jia et al. (1999) reported efficient treatment of several simulated wastewaters with reactive, acid, direct, cationic and vat dye, as well as with real wastewaters containing vat, reactive or direct dyes or their mixtures. Using activated carbon fiber (ACF) electrodes, nearly all the wastewaters were decolourised for more than 90%, with concomitant COD removal ranging between 40 and 80%. Similarly, Shen et al. (2001) tested the mineralisation of 29 colourants of different classes and confirmed the feasibility of ACF electrolytic process, with colour removals above 85% and TOC removals ranged between 30 and 70%. Depending on the method and electrode configuration, AC may be used as cathode and/or anode. E.g., the anthraquinone dye Alizarin Red S was successfully oxidised using an AC fiber felt as anode with considerable colour (>95%) and COD (>76%) removals within 60 min of electrolysis (Yi et al., 2008; Yi and Chen, 2008). The ACF anode worked much better than a simple carbon fibre anode in terms of colour removal. Probably, this was due to both the high specific surface area of ACF and its capability of promoting the electrogeneration of the strong oxidising agent hydroxyl radical (
OH) from the oxidation of H2O on the anode surface (Yi and Chen, 2008). The results confirmed a three-step mechanism of electrochemical treatment, i.e., (1) dye adsorption onto the ACF surface, (2) electrolytic degradation of the adsorbed colourant and (3) in situ regeneration of the ACF. Larger specific surface area and higher mesopore percentage of the ACF anode lead to more effective electrochemical dye removal (Yi et al., 2008). Electrooxidation of dyes can also be carried out indirectly, based on the electrogeneration of H2O2 from the two-electron reduction of O2 on the cathode. Wang et al. (2005a) studied the mineralization of the azo dye Acid Red 14 by an electro-Fenton reaction based on H2O2 production on an ACF felt cathode. During the Fenton reaction,
OH radicals are generated from H2O2 in the presence of catalytic amounts of ferrous ions (Section 3.4.2). The performance of the ACF cathode was compared with that of the graphite cathode in the absence of Fe2þ. Although similar decolourisation rates were achieved by these two cathode materials, the mineralization ability of ACF felt was much higher than of graphite. This was ascribed to the larger surface area of ACF, as well as to its better ability of peroxide electrogeneration. Due to the existence of fast homogeneous reaction of organics with the great amounts of
OH generated, the electro-Fenton process resulted in better TOC reduction than single H2O2 electrogeneration or simple anodic oxidation. Another variety is the so-called photoelectro-Fenton process, i.e., the UV-enhanced variant of electro-Fenton treatment, by which significantly higher TOC removal can be achieved than with the UVfree electro-Fenton oxidation (Wang et al., 2008). Apart from using ACs as electrodes, electrochemical dye removal can be improved by the so-called three-dimensional electrode

G. Mezohegyi et al. / Journal of Environmental Management 102 (2012) 148e164

system, in which AC granules are used as particle electrodes between the cathode and anode. The removal of Acid Orange 7 in this type of reactor system was found mainly dependent on the oxidation by the active substances (e.g.,
OH) produced both on ACF electrodes and GAC microelectrodes (Xu et al., 2008; Zhao et al., 2010). This method was considered effective as post-treatment for colour and COD removal in a combined ferrous coagulationeelectrooxidation process (Xiong et al., 2001), as well as pre-treatment to a biological process (Zhao et al., 2010). It has to be noticed that AC surface chemistry can be altered by the electrochemical treatment, depending on the applied electrochemical variables (Berenguer et al., 2009). Apart from electrochemical oxidation, GAC as particle cathode could enhance the current efficiency of internal electrolysis during azo dye reduction by zerovalent iron treatment (Liu et al., 2007). 3.4. AC-amended advanced oxidation processes for dye removal Advanced oxidation processes (AOPs) are characterised by a common feature: the capability of exploiting high reactive oxidising agents such as
OH radicals to completely destroy the majority of organic pollutants present in wastewaters (Andreozzi et al., 1999; Gogate and Pandit, 2004). Hydroxyl radicals with their high oxidation potential (E0 ¼ 2.33 V) can be produced by numerous different methods and most of these have been found applicable for treating dye wastewater. Moreover, enhanced dye removal has been reported using AC as catalyst in various processes based on oxidation by
OH (Table 2). Techniques such as electrochemical treatment generating
OH radicals contingently and secondarily, have been discussed in Section 3.3. 3.4.1. Wet oxidation of dyes Catalytic wet oxidation (CWO) is a known process for oxidative treatment of industrial wastewaters. It proceeds under relatively mild conditions (125e220  C, 5e50 bar) in the presence of an adequate catalyst (Garcia et al., 2005). The enhanced solubility of oxygen in aqueous solutions at elevated temperature and pressure provides a strong driving force for the oxidation of contaminants by active oxygen species such as hydroxyl radicals (Levec and Pintar, 2007). Wet oxidation of different toxic contaminants can be catalysed by AC (Santiago et al., 2005; Stüber et al., 2005; Suarez-Ojeda et al., 2005) and CWO of textile dyes using AC catalyst is considered a promising technology as well. Santos et al. (2007) studied the abatement of synthetic dyes Orange G, Brilliant Green and Methylene Blue by means of the CWO method in a fixed-bed reactor packed with AC. At a hydraulic retention time (HRT) of only 15 min, decolourisation was almost complete while TOC removal was moderate. Although the strong oxidation conditions promoted certain changes in AC textural and surface chemical properties, the catalyst kept its activity and stability for at least 200 h of continuous operation at a weight lost of less than 5%. CWO with AC-supported copper catalyst promoted significantly better TOC removal from real dyeing and printing wastewater than the identical treatment without catalyst (Hu et al., 1999). Moreover, moderately better oxidation was achieved by the same catalyst than by either homogeneous (copper nitrate solution) catalyst or aluminiumsupported copper catalyst. A newer class of advanced materials for catalytic applications is nanostructured carbon (Serp et al., 2003) with a broad range of potential applications, e.g., as support for preparing heterogeneous catalysts for liquid-phase reactions. Garcia et al. (2005) tested a platinum catalyst with a multi-walled carbon nanotube (MWNT) support activated by nitric acid oxidation for the catalytic oxidation of textile azo dyes and a real textile effluent. While decolourisation of the dyes by wet oxidation at a temperature of 150  C and 6.9 bar of oxygen partial

153

pressure was very poor in the absence of catalyst, each azo dye was almost completely decolourised in the CWO process catalysed by Pt/MWNT. The treatment of real wastewater under the same conditions showed that the same catalyst could significantly improve both colour and TOC conversions. Furthermore, other nanocarbons such as carbon nanofibers (CNFs), impregnated with copper after their acidic activation, have been reported to improve both TOC and colour removal during CWO from washing textile industrial wastewater (Rodríguez et al., 2008) and from mono,-diand triazo dye solutions (Rodríguez et al., 2009). 3.4.2. Wet peroxide oxidation of dyes A couple of studies have shown that oxidation of different organic compounds by the environmentally friendly H2O2 can be enhanced by AC (Lücking et al., 1998; Huang et al., 2003; Georgi and Kopinke, 2005). Peroxide (E0 ¼ 1.78 V) can be activated on the AC surface involving the formation of free oxidising species such as the hydroxyl radical (Eqs. (4) and (5)):

AC þ H2 O2 /ACþ þ OH þ
OH

(4)

ACþ þ H2 O2 /AC þ HO$2 þ Hþ

(5)

where AC functions as an electron transfer catalyst and ACþ symbolizes the oxidised catalyst state (Kimura and Miyamoto, 1994; Georgi and Kopinke, 2005). High colour removal efficiencies were reported in a combined catalytic peroxide oxidation/ catalytic wet air oxidation system treating Direct Blue 71 and Direct Black 19 (Lin and Lai, 1999). Parameters such as the amount of GAC or the concentration of V2O5 catalyst showed significant effects on decolourisation. The presence of peroxide as additional oxidant was found unnecessary at an air supply of 3 dm3 min1, possibly due to either the high dye removal rates even without peroxide or the (partial) H2O2 decomposition under the applied conditions. A recent study proved that decolourisation of dye solutions by oxidation with H2O2 in the presence of AC is strongly influenced by the carbon surface chemistry (Santos et al., 2009). While the noncatalytic reaction without AC was inefficient in case of most dyes, the combination of AC with H2O2 could significantly enhance the oxidation due to catalytic decomposition of H2O2 into free radicals. The latter findings were explained by the involvement of the free electrons on the graphene basal planes of AC as active centres for the catalytic reaction. Another method for peroxide activation is the Fenton reaction in which ferrous ions as catalysts react with hydrogen peroxide in acidic medium, producing hydroxyl radicals (Eq. (6)):

Fe2þ þ H2 O2 /HO
þ OH þ Fe3þ

(6)

Although the Fenton process has been demonstrated effective in decolourising dye wastewaters (Kuo, 1992; Solozhenko et al., 1995; Shueh et al., 2005), it has the disadvantage of producing effluents with generally higher Fe ion concentrations than the European Union directives allow to release into environment (2 mg L1). By immobilizing Fe ions (or Fe oxides) on an appropriate support, the problem may be overcome. Fan et al. (2006) compared removal efficiencies of the azo dye Acid Black 24 in several treatment processes such as GAC (only adsorption), H2O2 (non-catalytic oxidation), GAC/H2O2 (catalytic oxidation with GAC) and FeGAC/ H2O2 (catalytic oxidation with iron oxide-coated GAC). Among all, FeGAC/H2O2 provided the best dye removal efficiencies, suggesting a synergic effect between AC and iron catalysis. The solution’s pH strongly affected the removal efficiency of the processes tested and treatments with H2O2 were more effective under acidic conditions. In case of a real dye wastewater, decolourisation by GAC/H2O2 and FeGAC/H2O2 methods was about six times higher than by single

oxidation process system characteristicsa

AC catalytic role


OH generation by H2O cont.; A: ACF felt; C: stainless steel sheet; V: 110; HRT: w46.8 oxidation

electrochemical

batch; A: RuO2/Ti; C: ACF felt; V: 500 batch; A: RuO2/Ti; C: ACF felt; Fe2þ (c: 1); V: 500




batch; A: viscose-based ACF felt; C: stainless steel sheet; V: 50 batch; A: stainless steel plate; C: ACF


OH generation by H2O oxidation

electro-Fenton

electrochemical

electrochemical

wet oxidation

batch; A: stainless steel plate; C: ACF; PE: GAC; V: 500 batch; p: 6.9; T: 150; V: 75

OH generation from H2O2 produced on the cathode

SBET: SBET: SBET: SBET: SBET:

893.7; SMIC/SBET: 0.749 1053; SMIC/SBET: 0.720 1374; SMIC/SBET: 0.572 1682; SMIC/SBET: 0.200 1237

SBET: w1000




OH generation from H2O2 SBET: 764.1 produced on the cathode


OH generation from O2-saturated H2O at elevated p and T

ACF (SBET: 764.1) þ GAC (cc: 100; SBET: 910.7) Pt-imp. MWNT; cc: w10.7; SBET: w175

wastewater characteristicsc

SW: Alizarin Red S (c: 200); pH: w6.5

SW: Acid Red 14 (c: 200); pH: 3.0

cont. fixed-bed reactor; p: 16; T: 160; HRT: 15


OH generation from O2-saturated H2O at elevated p and T

GAC (Ind. React FE0160A); d: 800e1000; ccv: w0.32; SBET: 745

wet oxidation

batch; p: 8.7; T: 140; V: 75

OH generation from O2-saturated H2O at elevated p and T

Cu-imp. CNF; d  l: 20e50  50e100; SBET: 188

peroxide oxidation

Fenton

batch; H2O2 (c: 1500); V: 600

batch; H2O2 V: w1000 batch; H2O2 V: w1000 batch; H2O2 V: w1000 batch; H2O2 V: w1000

(c: w1.14); (c: w114); (c: w1.14); (c: w114);


OH generation from peroxide


OH generation from peroxide

e

Yi et al. (2008)

e

Wang et al. (2005a)

XCOL: w1.00 (1.5 h); XTOC: 0.70 (6 h)

XCOL: w1.00 (1.5 h) XTOC: 0.40 (6 h) [C: graphite] XCOL: w0.50 (1 h) [A: carbon fiber]

XCOL > 0.96 (10  1 h) XTOC: 0.460 (10  1 h) Xu et al. XTOC: 0.574 (10  1 h) [C: graphite] (2008) XTOC: 0.413 (10  1 h) [C: stainless steel] XCOL > 0.99 (3 h) e XTOC: 0.719 (3 h) XCOL: 0.995 (2 h) XCOL: 0.0 (2 h) Garcia et al. XTOC: 0.212 (2 h) (2005) XCOL: 0.0 (2 h) XCOL: 1.00 (2 h) XTOC: 0.635 (2 h) XCOL: 0.280 (2 h) XCOL: 0.995 (2 h) XTOC: 0.781 (2 h) XCOL, XTOC: w0.0 Santos et al. XCOL: w1.00 XTOC: w0.40 [glass spheres] (2007) XCOL: w1.00 XTOC: w0.55 XCOL: w1.00 XTOC: w0.55 XCOL: 0.996 (3 h) XCOL: 0.241 (3 h) Rodríguez et al. (2008) XTOC: 0.241 (3 h) XTOC: 0.120 (3 h) XCOL: 0.973 (3 h) XCOL: 1.00 (3 h) XTOC: 0.500 (3 h) XTOC: 0.041 (3 h) XCOL: 0.582 (3 h) XCOL: 1.00 (3 h) XTOC: 0.068 (3 h) XTOC: 0.010 (3 h) XCOL: 1.00 (3 h) XCOL: 0.340 (3 h) XTOC: 0.250 (3 h) XTOC: 0.186 (3 h) k: 4.2  104 Santos et al. k: 6.2  103 (2009)

SW: Acid Orange 7 (c: 300); pH: 3.0 SW: Solophenyl Green BLE (c: 2000) SW: Chromotrop 2R (c: 2000)

SW: Orange G (c: 1000) SW: Brilliant Green (c: 1000)

SW: Acid Orange 7 (c: 1000) SW: Eriochrome Blue Black (c: 1000) SW: Direct Blue 71 (c: 1000)

GAC (Norit GAC 1240 PLUS); d: 100e300; cc: w2.2; SBET: 972; pHPZC: 7.9; CO: 995 GAC mod: HNO3; d: 100e300; cc: w2.2; SBET: 909; pHPZC: 3.0; CO: 2920 GAC mod: HNO3/heat under H2; d: 100e300; cc: w2.2; SBET: 946; pHPZC: 9.8; CO: 575 GAC (F400, Calgon); d: 425e600; cc: 2; SBET: 800e900 GAC (F400, Calgon); d: 425e600; cc: 4; SBET: 800e900 Fe-imp. GAC (w3.7%); d: 425e600; cc: 2 Fe-imp. GAC (w3.7%); d: 425e600; cc: 4

XCOL: w0.63 (l: 507) XCOL: w0.70 (l: 507) XCOL: w0.75 (l: 507) XCOL: w0.82 (l: 507) XTOC: 0.50 (6 h)

SW: Acid Orange 7 (c: 300); pH: 3.0

SW: Reactive Black 5 (c: 1000) peroxide oxidation

without AC

XCOL: w0.98 (1 h) XCOD: 0.765 (1 h)

SW: Methylene Blue (c: 1000)


reference

with AC

SW: Alizarin Red S (c: 700); pH: 7.0

SW: Erionyl Red B (c: 2000) wet oxidation

resultsd

SW: Reactive Red 241 (c: 50); pH: 3.0

Yi and Chen (2008)

k: 5.4  103

k: 8.7  103

SW: Acid Black 24 (c: 120); pH: 2.0 dye wastewater; pH: 1.9

XCOL: 0.74 (3 h)

XCOL: 0.59 (3 h)

XCOL: 0.60 (3 h)

XCOL: 0.12 (3 h)

SW: Acid Black 24 (c: 120); pH: 2.0 dye wastewater; pH: 1.9

XCOL: 0.78 (3 h)

XCOL: 0.59 (3 h)

XCOL: 0.62 (3 h)

XCOL: 0.12 (3 h)

Fan et al. (2006)

G. Mezohegyi et al. / Journal of Environmental Management 102 (2012) 148e164

electrochemical

AC characteristicsb

154

Table 2 Selected results of physico-chemical dye oxidation studies with AC-catalysed decolourisation.

peroxide oxidation Fenton

batch; H2O2 (c: 6); V: 200

ozonation

batch; qG: 285; qO: 2.15; V: 600


OH generation from peroxide




OH generation from O3

o: olive stone; d < 200; cc: 0.2; SBET: 691 Fe-imp. GAC (w7.0%); d < 200; cc: 0.2 GAC (Norit GAC 1240 PLUS); d: 100e300; cc: 0.5; SBET: 972; pHPZC: 9.7

GAC mod: HNO3; d: 100e300; cc: 0.5; SBET: 909; pHPZC: 2.7

ozonation




OH generation from O3

GAC; d: w5000; cc: 50; SBET: 893 ozonation

batch; qG: 150; qO: 7.5; V: 700




OH generation from O3

GAC (Norit GAC 1240 PLUS); d: 100e300; cc: 0.5; SBET: 909

GAC/cerium oxide composite (CeeO: w45%); d: 100e300; cc: 0.5; SBET: 583

photochemical

photocatalytic

photocatalytic

batch; UV, H2O2 (c: 12, without GAC; c: 9, with GAC); V: w5000 batch; UV, TiO2 (cPC: 1); V: 800

batch; UV, TiO2 (cPC: 1); V: 800

XCOL: 0.036 (20 h)

Ramirez et al. (2007)

k: 0.412 XTOC: w0.35 (1.5 h) k: 0.326 XTOC: w0.50 (1.5 h) k: 0.470 XTOC: w0.15 (1.5 h) k: 0.412 k: 0.326 k: 0.470 k: 0.412 k: 0.326 k: 0.470 XCOL: w0.94 (1 h) XCOD: w0.29 (1 h)

Faria et al. (2005)

XCOL: 0.98 (4 h) SW: Acid Blue 113 (c: 100)

k: 0.470 XTOC: w0.59 (1.5 h) SW: H. Reactive Red 241 (c: 100) k: 0.330 XTOC: w0.63 (1.5 h) SW: Basic Red 14 (c: 100) k: 0.532 XTOC: w0.54 (1.5 h) SW: Acid Blue 113 (c: 100) k: 0.462 SW: H. Reactive Red 241 (c: 100) k: 0.325 SW: Basic Red 14 (c: 100) k: 0.456 SW: Acid Blue 113 (c: 100) k: 0.413 SW: H. Reactive Red 241 (c: 100) k: 0.337 SW: Basic Red 14 (c: 100) k: 0.520 TW; COD: 484 XCOL: w0.95 (1 h) XCOD: w0.40 (1 h) XCOL: w0.98 (1 h) XCOD: w0.77 (1 h) SW: Acid Blue 113 (c: 50); XTOC: w0.87 (2 h) pH: 5.8 SW: Reactive Yellow 3 (c: 50); XTOC: w0.81 (2 h) pH: 5.8 SW: Reactive Blue 5 (c: 50); XTOC: w0.79 (2 h) pH: 5.6 TW (bio-treated); TOC: 150; XTOC: w0.40 (2 h) pH: 9.3 SW: Acid Blue 113 (c: 50); XTOC: w0.98 (2 h) pH: 5.8 SW: Reactive Yellow 3 (c: 50); XTOC: w0.97 (2 h) pH: 5.8 SW: Reactive Blue 5 (c: 50); XTOC: w1.00 (2 h) pH: 5.6 TW (bio-treated); TOC: 150; XTOC: w0.35 (2 h) pH: 9.3 SW: H. Everzol Black GSP k: 0.29 (20 min, l: 596) (c: 36); pH: 7 kTOC: 0.019 (45 min) SW: Solophenyl Green BLE k: 0.0475 (c: 40e50 ADA) k: 0.0382


OH generation from peroxide

GAC (Prolab); d: 840e1680; cc: 8

sensitization of photocatalytic
OH generation

PAC (Norit C GRAN); cc: 0.2; SBET: 1400 PAC (Norit GAC 1240 PLUS); cc: 0.2; SBET: 1000 PAC (Norit ROX 0.8); cc: 0.2; SBET: 1100 PAC (ROX 0.8) mod: H2O2; cc: 0.2; SBET: 908 PAC (ROX 0.8) mod: HNO3; cc: 0.2; SBET: 893 PAC (ROX 0.8) mod: HNO3/heat under H2; cc: 0.2; SBET: 987 SW: Solophenyl Green BLE GAC (Norit C GRAN); (c: 41e50 ADA) d: 100e300; cc: 0.2; SBET: 1400

sensitization of photocatalytic
OH generation

XCOL: 0.98 (20 h)

XCOL: w0.97 (5 min) XTOC: w0.87 (2 h) XCOL: w0.88 (5 min) XTOC: w0.73 (2 h) XCOL: w0.98 (5 min) XTOC: w0.64 (2 h) XTOC: w0.27 (2 h)

Lin and Lai (2000)

Faria et al. (2009a)

XCOL: w0.97 (5 min) XTOC: w0.87 (2 h) XCOL: w0.88 (5 min) XTOC: w0.73 (2 h) XCOL: w0.98 (5 min) XTOC: w0.64 (2 h) XTOC: w0.27 (2 h) k: 0.20 (20 min,

l: 596) kTOC: 0.011 (45 min) k: 0.0243

Ince et al. (2002) Da Silva and Faria (2003)

k: 0.0457

G. Mezohegyi et al. / Journal of Environmental Management 102 (2012) 148e164

batch; qG: 4000; qO: 0.5; V: 2000

GAC mod: HNO3/heat under H2; d: 100e300; cc: 0.5; SBET: 972; pHPZC: 10.8 GAC; d: w5000; cc: 5; SBET: 893

SW: Orange II (c: 35); pH: 3.0

k: 0.0435 k: 0.0245 k: 0.0389 k: 0.048

SW: Erionyl Red B (c: 41e50 ADA)

k: 0.063

SW: Chromotrope 2R (c: 41e50 ADA)

k: 0.034

min)

Silva et al. (2006)

min) min) [UV] min) min) [UV] min) (continued on next page)

155

k: 0.042 (9 [UV] k: 0.024 (9 [UV/TiO2] k: 0.030 (9 k: 0.059 (9 [UV/TiO2] k: 0.024 (9 k: 0.023 (9 [UV/TiO2]

XCOL: 0.90 (3 h) [EPC] XCOL: 0.78 (3 h) [PC] SW: Acid Orange II (c: 200)

k: 0.143 SW: Acid Fuchsine (c: 120)

sensitization of photocatalytic OH generation


batch; UV; A: TiO2/ACF (5 wt. %, 12 cm2); V: w86 electrophotocatalytic

TiO2-imp. ACF; SBET: 1237

k: 0.355 SW: Methyl Orange (c: 120) TiO2-imp. ACF; SBET: 555.1; TC: 800

k: 0.251 SW: Acid Fuchsine (c: 120)

a Abbreviations: cont., continuous; A, anode; C, cathode; V, volume of wastewater treated, mL; HRT, hydraulic residence time, min; c, concentration, mM; PE, particle electrode; p, oxygen partial pressure, bar; T, temperature, C; qG, ozoneegas mixture flow rate, cm3 min1; qO, ozone mass flow rate, mg min1; UV, ultraviolet irradiation; cPC photocatalyst concentration, g L1. b Abbreviations: SBET, BET surface area, m2 g1; SMIC/SBET, microporous surface area/BET surface area ratio; cc, carbon concentration in the liquid phase, g L1 imp., impregnated; MWNT, multi-walled carbon nanotube; d, AC particle size, mm; ccv, carbon concentration in the fixed-bed volume, g mL1; CNF, carbon nanofiber; d  l, diameter  length, nm  nm; pHPZC, point of zero charge; CO, amount of CO-emitting groups on AC surface, mmol g1; mod, modified; o, origin; TC, calcination temperature,  C. c Abbreviations: SW, simulated wastewater; c, inlet concentration, mg L1; TW, textile wastewater; H, hydrolysed; COD, chemical oxygen demand, mg L1; TOC, total organic carbon, mg L1; ADA, after dark adsorption. d Abbreviations: XCOL, colour removal; l, wavelength of colour analysis, nm; XTOC, total organic carbon removal; XCOD, chemical oxygen demand removal; k, first-order decolourisation rate constant, min1; kTOC, first-order TOC removal rate constant, min1.; EPC, electro-photocatalysis; PC, photocatalysis.



Shi et al. (2008) without AC

k: 0.253 (1st cycle, 30 min) [UV/TiO2] k: 0.164 (1st cycle, 30 min) [UV/TiO2] k: 0.253 (1st cycle, 30 min) [UV/TiO2] k: 0.164 (1st cycle, 30 min) [UV/TiO2] XCOL: 0.24 (3 h) [A: TiO2/Ti] k: 0.379

with AC

SW: Methyl Orange (c: 120) TiO2-imp. ACF; SBET: 434.9; TC: 600

sensitization of photocatalytic
OH generation batch; UV, TiO2 (cPC: 2); V: w250 photocatalytic

resultsd wastewater characteristicsc AC characteristicsb AC catalytic role oxidation process system characteristicsa

Table 2 (continued )

Hou et al. (2009)

G. Mezohegyi et al. / Journal of Environmental Management 102 (2012) 148e164

reference

156

H2O2 oxidation. In a heterogeneous Fenton-like reaction for azo dye Orange II decolourisation, the catalytic activity of iron-impregnated AC catalyst as peroxide activator surpassed that of AC support itself (Ramirez et al., 2007). However, iron loss from the support was found a limiting factor. This problem may be diminished either by preparing other types of carbon supports such as carbon aerogels in which iron is within the aerogel structure (Ramirez et al., 2007) or, alternatively, by tailoring the AC to meet the demands of the catalytic reaction considered and using it as a catalyst on its own (Santos et al., 2009). 3.4.3. Ozonation of dyes Ozone is a more powerful oxidising agent (E0 ¼ 2.07 V) than other well-known oxidants such as H2O2. It is capable of oxidising a vast range of organic pollutants in industrial wastewaters, such as wastewaters from the textile industry (Lin and Lin, 1993). Single ozonation, however, rarely leads to total mineralisation. The oxidation can be strongly enhanced in terms of ozone consumption efficiency, operation time and mineralisation rate by homogeneous or heterogeneous catalysis (Legube and Leitner, 1999). Apart from the most common catalysts such as metal oxides or supported metal oxides (Legube and Leitner, 1999; Kasprzyk-Hordern et al., 2003), AC was suggested as an attractive alternative, accelerating the transformation of O3 into
OH radicals (Jans and Hoigné, 1998). In general, catalytic ozonation with AC is strongly influenced by its textural and surface chemical features and favoured by ACs with large surface areas and high basicity (Faria et al., 2005, 2006; Sánchez-Polo et al., 2005). It has to be noted that even in the liquid phase, ozone may oxidise the AC surface to a certain extent, resulting in both the introduction of oxygenated electronwithdrawing groups and the decrease of catalytic activity (Faria et al., 2006). Most of the studies suggesting AC catalysis during the ozonation of a dye wastewater showed that ozone alone is strong enough to decolourise the colourants and that AC plays merely a role in COD/ TOC removal. E.g., Lin and Lai (2000) investigated the ozone oxidation of a textile wastewater in a fluidised or fixed GAC bed reactor. While the amount of GAC had a relatively little effect on wastewater colour removal, COD reduction was considerably enhanced by adding more GAC into the reactor. Non-catalytic ozonation of Acid Blue 113, Reactive Red 241 and Basic Red 14 quickly decolourised all the dye solutions but it did not result in satisfactory TOC removal (Faria et al., 2005). However, AC-amended ozonation not only increased decolourisation but also mineralisation of the organic matter, the latter showing a close relationship with the basicity of the AC sample. Gül (2007) ozonised the azo dyes Reactive Red 194 and reactive Yellow 145 in aqueous solutions and reached double TOC removal efficiencies in the presence of GAC. The authors showed that although the formation of hydroxyl radicals took place on the GAC surface, the main oxidative reactions proceeded in the bulk of the solution. The positive contribution of AC to dye removal efficiencies during ozonation was furthermore evidenced by the results of a study to the removal of Bomaplex Red CR-L using the so-called Taguchi method. The study investigated the effects of HCO 3 ions, temperature, ozoneeair flow rate, dye concentration, AC amount, H2O2 concentration, pH and treatment time on decolourisation (Oguz et al., 2006). The parameter affecting the dye removal in the highest extent was the amount of AC. The HCO 3 ions have a negative effect on decolourisation in the catalytic ozonation process, probably due to their scavenging effect on the
OH radicals (Kasprzyk-Hordern et al., 2003; Oguz et al., 2006; Oguz and Keskinler, 2008). Under continuous operation of AC-enhanced ozonation, the reactor configuration (e.g., distribution of gas bubbles) can affect colour and TOC removal as well as ozone requirement (Soares et al., 2007).

G. Mezohegyi et al. / Journal of Environmental Management 102 (2012) 148e164

Further improvement of dye mineralisation can be achieved when catalytic ozonation is combined with an advanced catalyst, e.g. AC in combination with a metal or metal oxide. Faria et al. (2009a) studied the catalytic ozonation of dyes Acid Blue 113, Reactive Yellow 3, Reactive Blue 5 and textile effluents by AC, cerium oxide and ceria-AC composite as catalyst materials. Whereas the extent of colour removal was more or less equal for all conditions tested, the extent of dye mineralisation was higher in the presence of catalysts. With dye solutions, the metal oxide performed better than AC alone but the highest mineralisation efficiencies were obtained with the ceria-AC composite (Fig. 1). In contrast, with real textile wastewater, O3/AC provided better TOC reduction than ozonation with either cerium oxide or composite catalyst. This behaviour was attributed to the scavenging effect of bicarbonate and carbonate ions in the effluent (Kasprzyk-Hordern et al., 2003; Oguz et al., 2006; Oguz and Keskinler, 2008) that was less significant in case of single AC catalysis due to oxidation reactions occurring at the AC surface. Another study of these authors (Faria et al., 2009b) examined the effect of AC impregnation by manganese, cobalt or cerium on the removal of diazo dye Acid Blue 113 by catalytic ozonation. As it could be expected, the catalytic treatments resulted in an increase, particularly of dye mineralisation, compared to ozonation in the absence of catalysts. Although the best results were obtained using the cerium-containing catalyst, the catalytic effect observed with the supported metal oxides was mainly attributed to the AC support. From the studies above it can be concluded that AC may work efficiently as catalyst on its own for catalytic ozonation of real (textile) wastewaters. 3.4.4. Photochemical and photocatalytic removal of dyes In recent years, UV-based photochemical processes for the removal of hazardous organic compounds have received growing attention because they are efficient, environmentally friendly and cheap. The role of UV in advanced oxidation processes is the formation of hydroxyl radicals when used in combination with a strong oxidising agent such as hydrogen peroxide (Eq. (7)):

H2 O2 þ hv/ 2
OH

(7)

The UV/H2O2 treatment was suitable for dye removal from wastewater (Shu et al., 1994; Ince and Gönenç, 1997). During the UV/H2O2 photochemical treatment of the textile reactive dye Eversol Black-GSP, both decolourisation and TOC reduction was

Fig. 1. TOC removal during non-catalytic and catalytic ozonation of anthraquinone dye Reactive Blue 5 (c0: 300 mg/L). [Reprinted from Faria et al. (2009a), Copyright (2009), with permission from Elsevier].

157

enhanced by GAC addition (Ince et al., 2002). Although this increment was ascribed solely to GAC adsorption, it is probably also due to the AC-catalysed peroxide decomposition producing additional
OH radicals (Section 3.4.2). Heterogeneous photocatalysis has received increasing attention as it is carried out under ambient conditions, does not require expensive oxidants or additives and the catalysts are generally inexpensive, chemically stable and non-toxic (Herrmann, 2005). The process is initiated by UV irradiation of the photocatalyst with the formation of electron/hole pairs (e/hþ), which can migrate rapidly to the surface and initiate redox reactions with suitable substrates. Oxygen over the catalyst acts as electron acceptor to form superoxide radicals while adsorbed OH groups and H2O molecules are available as electron donors to yield the hydroxyl radical (Da Silva and Faria, 2003):

CATALYST þ hv/e þ hþ

(8)

CATALYSTðe Þ þ ðO2 Þads /CATALYST þ O, 2

(9)

CATALYSTðhþ Þ þ ðH2 OÞads /CATALYST þ Hþ þ
OH CATALYSTðhþ Þ þ OH


ads

/CATALYST þ
OH

(10) (11)

The degradation of colourants by semiconductor-assisted photocatalytic processes has been thoroughly investigated (Gonçalves et al., 1999; Tanaka et al., 2000; Lachheb et al., 2002; Konstantinou and Albanis, 2004). While most of these studies already demonstrated efficient colour and TOC removal in the absence of AC, other studies have shown an additional impact of AC. For instance, Da Silva and Faria (2003) investigated photochemical and photocatalytic degradation of the triazo dye Solophenyl Green BLE in aqueous solution by UV irradiation. Photocatalysis was carried out by using both commercial titanium dioxide and mixtures of TiO2 with different ACs suspended in the solution. Although non-catalysed UV treatment performed the best at lower dye concentrations, photocatalytic decolourisation was more efficient at higher dye concentrations. The presence of AC increased the photoefficiency of TiO2 catalyst over the entire range of dye concentrations studied, and best performances took place in case of ACs with some electron availability (ACs with basic or slightly acid surfaces). This supports the hypothesis that AC acts as a photosensitiser, triggering the photocatalytic formation of
OH radicals by injecting an electron in the conduction band of TiO2. During photooxidation of the azo dyes Solophenyl Green BLE, Erionyl Red B and Chromotrope 2R, the powerful decolourisation efficiency of AC-amended photocatalytic treatment by suspended TiO2 (and GAC) was furthermore confirmed by the mineralisation degrees and the initial quantum yields (Silva et al., 2006). Although a direct relationship between structural characteristics of azo dyes and their photodegradation could not be established, it would be interesting to verify whether there exists a correlation between the dyes’ electrochemical properties and their decolourisation rates. The problem of possible catalyst leaching and their recovery from suspended solutions urged researchers to investigate photocatalyst immobilisation. Wang et al. (2007) prepared nanocrystalline TiO2/AC composite catalysts by a modified sol-gel method for the photocatalytic degradation of the monoazo dye Chromotrope 2R. The composite exhibited higher activities than TiO2 alone and photocatalytic decolourisation was more efficient than photolytic degradation. Increased dye removal efficiencies by photocatalysis were also reported with TiO2-impregnated AC (Subramani et al., 2007), TiO2 immobilized on ACF (Shi et al., 2008) and even with

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both TiO2 and AC being immobilized on a support material such as silicone rubber film (Gao and Liu, 2005) or silica beads (Follansbee et al., 2008). Apart from the popular titanium dioxide, zinc oxide has been found a suitable alternative for photocatalytic decolourisation processes. ZnO indicated the synergism with AC in the form of ZnO/AC composites which showed promising results under solar radiation (Byrappa et al., 2006; Sobana and Swaminathan, 2007). By doping another semiconductor on the surface of a photocatalyst, it is possible to further improve its photocatalytic efficiency. This is due to the increased charge separation and the extended photoresponding range. Sun et al. (2009) showed that photocatalytic decolourisation rate of the azo dye Congo Red was faster with (WO3eTiO2)/AC catalyst than with TiO2/AC. Another way to improve AC-amended photocatalysis is altering the carbon material itself. Both (activated) carbon nanotubes with unique electronic properties (Yu et al., 2005) and ordered mesoporous carbon with periodic pore structure (Park et al., 2008) have been reported to facilitate the photocatalytic activity of TiO2 in dye degradation more efficiently than AC. Recently, some combined photocatalytic dye treatment systems presented powerful decolourisation potentials such as photo-Fenton reaction with Fe-ACF/TiO2 composite catalyst (Zhang and Oh, 2009), microwave-enhanced photodegradation using TiO2/AC composite catalyst (He et al., 2009) or electrophotocatalytic oxidation using TiO2 immobilized on the ACF anode (Hou et al., 2009). However, the possibly significant additional costs of operation were not evaluated. 3.4.5. Miscellaneous dye removal processes Lately, some individual AC-enhanced oxidation treatments for dye degradation have been published which could not be clearly classified into any of the previously discussed categories. This section includes these methods such as oxidation by non-thermal plasma or by microwave irradiation. In a study by Zhang et al. (2007a), a gas-liquid series electrical discharge reactor was applied for the degradation of Methyl Orange in the presence of different GACs. The pulse discharge alone ensured fast decolourisation. The non-thermal plasma could generate various reactive agents such as radicals (
O,
OH) or molecular species (H2O2, O3). However, the removal of colour and COD could be further increased by the combination of pulse discharge and GAC. The role of GAC was to promote the decomposition of molecular oxidising agents into hydroxyl radicals. The study of Zhang et al. (2007b) was based on the fact that AC, aside from being an excellent adsorbent, can strongly absorb microwave energy, resulting in fast chemical oxidation of pollutants on the AC surface. The results indicated that decolourisation of the Congo Red dye solution proceeded much faster by PAC-enhanced microwave irradiation than by AC adsorption or microwave treatment solely. In addition, the combined system provided very efficient TOC removals, probably due to the only minor formation of intermediate products. However, the catalytic activity of PAC decreased with its cyclic reuse because of its simultaneous oxidation with the colourant. Microwave irradiation may also be applied for dye degradation indirectly. Yang et al. (2009) investigated the azo dye Acid Orange 7 degradation in an oxidation process using free 0 sulphate radicals (SO$ 4 ) (E ¼ 2.6 V) produced from the decomposition of persulphate anions (S2O2e 8 ) by microwave activation, both in the absence and in the presence of AC. Although the persulphate anion is a strong oxidising agent itself (E0 ¼ 2.01 V), it reacts only very slowly at ambient temperatures. In the control experiments, no colour removal was observed by microwave irradiation or persulphate solely. In contrast, their simultaneous application led to rapid and complete decolourisation. The oxidation could be improved by addition of AC, mainly due to its catalytic role under microwave treatment (Zhang et al., 2007b).

4. AC-amended biological dye removal processes Microbial decolourisation and degradation of dyes is one of the oldest but probably the most cost-effective among all methods for colour removal from wastewaters. Moreover, it is an environmentally friendly process that does not require hazardous or aggressive chemicals. Some excellent reviews have been published reporting the variety of aspects in microbial dye degradation (Banat et al., 1996; Stolz, 2001; Pearce et al., 2003). As it is well-documented in the literature, the addition of AC into a biological system treating dye or textile wastewaters presents a positive contribution to pollutant removal (Table 3). These studies may be grouped according to the main role of AC considered: whether the process enhancement is rather due to higher biomass activity (biofilm on AC surface) and dye adsorption or to AC catalysis. 4.1. AC-supported microbial degradation of dyes Several studies have demonstrated a positive impact of AC on biological dye wastewater treatment. The predominant conclusion was that the synergy is due to the simultaneous occurrence of welldeveloped biomass on the AC support, carbon adsorption and biodegradation. Most traditional biological systems decolourising dyes with the assistance of AC operated with aerobic activated sludge (Shaul et al., 1983; Specchia and Gianetto, 1984). Dye degradation is strongly dependent on the biomass concentration and overload of the microorganisms in the reactor can inhibit biological activity near to the AC surface, resulting in colour removal decrease (Márquez and Costa, 1996; Mezohegyi et al., 2008). From a technological point of view, GAC is a better choice than PAC because it can more easily be retained in bioreactors. Some sequential batch reactor (SBR) treatments showed that removal efficiencies of disperse (Sirianuntapiboon and Srisornsak, 2007) and direct dyes (Sirianuntapiboon et al., 2007; Sirianuntapiboon and Sansak, 2008) increased through the addition of GAC into the SBR system while GAC acted mainly as a media for biofilm and not as a dye adsorbent. Although it is not practical to work with pure cultures since mixed ones are effective for dye biodegradation and mineralisation as well (Pearce et al., 2003), a couple of studies evidenced that the use of AC in combination with a specific microorganism in the reactor outperformed the conventional biotreatment of dyes (Walker and Weatherley, 1999b; Zhang and Yu, 2000). Commercial dyes are not uniformly susceptible to microbial attack in aerobic treatment because of their unique and stable chemical structures. Nevertheless, azo dyes e representing the largest class of dyes e can be decolourised by the reduction of the azo bond(s) in anaerobic bioreactors (Delée et al., 1998; Van der Zee et al., 2001a) and some can be mineralised completely by a sequential anaerobiceaerobic treatment (Van der Zee and Villaverde, 2005). So far, using AC-enhanced biomass for azo dye removal under anaerobic conditions has not been so widespread. E.g., Kuai et al. (1998) carried out a sequential anaerobic/aerobic process of a textile wastewater containing soluble acid and metalcomplex azo colourants and used GAC in the UASB (upflow anaerobic sludge blanket) reactor in order to protect the top-layer granular sludge from toxicants. The combined GAC-amended UASB/aerobic activated sludge treatment showed a stable performance with a COD and colour removal of 98 and 95%, respectively. Since AC plays a role in obtaining high concentrations of active microorganisms in the bioreactor, the logical way is to operate with high GAC apparent volume/reactor volume ratios. Effective biodecolourisation of the azo dye Acid Orange 7 has been reported under reductive environment in some packed-bed-like reactor configurations with immobilised mixed cultures on GAC (Ong et al.,

Table 3 Selected results of AC-amended biological dye removal studies. reactor systema

organisms used

AC characteristicsb

wastewater characteristicsc

resultsd

reference

with AC

without AC

continuous mixed and aerated reactor; HRT: 3.75; BC: 2000 aerobic sequencing batch reactor; HRT: 72 (SW) and 120 (TW)

activated sludge

PAC (Panreac PR); d: 74e88; cc: 0.5

SW: Acid Orange 7 (c: 20); peptone (c: 160); COD: 250

XCOL: 0.97 XCOD: w0.75

n.m.

Márquez and Costa (1996)

bio-sludge

GAC (CGC-11, C. Gigan Co.); SBET: 1050e1150; cc: 1

bio-sludge

GAC (CGC-11, C. Gigan Co.); SBET: 1050e1150; cc: 1

aerobic stirred tank reactor; VL: 3

Pseudomonas putida

GAC (Filtrasorb 400); SBET: 1050e1200; cc: w1.6

XCOL: 0.942 XCOD: 0.974 XCOL: 0.845 XCOD: 0.973 XCOL: 0.930 XCOD: 0.922 XCOL: 0.810 XCOD: 0.983 XCOL: 0.848 XCOD: 0.972 XCOL: 0.571 XCOD: 0.636 XCOL: 0.760 XCOD: 0.862 kAV (TB4R): 8.297 (6 h) kAV (TR2B): 6.090 (6 h) kAV (TO3G): 12.58 (6 h)

XCOL: 0.881 XCOD: 0.962 XCOL: 0.762 XCOD: 0.959 XCOL: 0.877 XCOD: 0.908 XCOL: 0.756 XCOD: 0.967 XCOL: 0.799 XCOD: 0.959 XCOL: 0.511 XCOD: 0.609 XCOL: 0.702 XCOD: 0.843 kAV (TB4R): 0.918 (6 h) kAV (TR2B): 0.240 (6 h) kAV (TO3G): 0.558 (6 h)

Sirianuntapiboon and Srisornsak (2007)

aerobic sequencing batch reactor; HRT: 72 (SW) and 180 (TW)

aerobic fluidized-bed reactor; HRT: 20 upflow anaerobic sludge blanket reactor; HRT: 5e5.5

Trametes versicolor

PAC; d: 37e150; cc: w50

XCOL: 0.55 (after 7 d)

n.m.

Zhang and Yu (2000)

activated granular sludge

GAC (Norit SA-4); cc: 10

SW: Disperse Red 60 (c: 80); glucose (c: 1875); COD: 2008 SW: Disperse Blue 60 (c: 80); glucose (c: 1875); COD: 2008 TW: mixed disperse dyes; COD: 1517 SW: Direct Red 23 (c: 40); glucose (c: 1875); COD: 2000 SW: Direct Blue 201 (c: 40); glucose (c: 1875); COD: 2000 TW: mixed direct dyes; COD: 1615 TW: mixed direct dyes; glucose (c: 890); COD: 2438 SW: mixture of Tectilon Blue 4R-01, Tectilon Red 2B and Tectilon Orange 3G (c: 3  33); CM1 nutrient broth (5%) SW: Acid Violet 7 (c: 100); glucose (c: 5000) SW: hydrolysed Reactive Red 2 (c: 42); VFA mixture (COD: 1500)

XCOL: w0.90 (after 130 d)

XCOL: w0.35 (after 130 d)

Van der Zee et al. (2003)

upflow anaerobic stirred packed-bed reactor; VPB: w2

mixed bacterial consortium

n.m.

Mezohegyi et al. (2009)

n.m.

Mezohegyi et al. (2010)

k: 10.2

Pereira et al. (2010)

anaerobic batch reactor; BC: 1000

mixed bacterial consortium

granular biomass

GAC (Norit ROX 0.8); d: 300e700; ccv: w0.5; SBET: 1055; pHPZC: 8.3; CO: 911 GAC mod: gas. (BO: 28); d: 300e700; ccv: w0.4; SBET: 1097; CO: 1136 GAC mod: gas. (BO: 50); d: 300e700; ccv: w0.3; SBET: 1336; CO: 1448 GAC mod: HNO3; d: 300e700; ccv: w0.5; SBET: 1004; pHPZC: 4.5; CO: 2041 GAC mod: HNO3/heat under N2; d: 300e700; ccv: w0.5; SBET: 977; pHPZC: 8.6; CO: 106 GAC (Norit ROX 0.8); d < 315; cc: 0.1; SBET: 1032; pHPZC: 8.4; CO: 814 GAC mod: HNO3/heat under H2; d < 315; cc: 0.1; SBET: 987; pHPZC: 10.8; CO: 590

SW: Orange II (c: 100); sodium acetate (c: 200) SW: Acid Red 88 (c: 100); sodium acetate (c: 200) SW: hydrolysed Reactive Black 5 (c: 100); sodium acetate (c: 200) SW: hydrolysed Reactive Black 5 (c: 100); sodium acetate (c: 200)

XCOL: w0.43 (after 112 d) k1: 2.46

Walker and Weatherley (1999b)

k1: 2.63 k1: 3.51 k1: 2.83

k1: 5.70 k1: 9.05

G. Mezohegyi et al. / Journal of Environmental Management 102 (2012) 148e164

upflow anaerobic stirred packed-bed reactor; VPB: w2e3

GAC (Norit SA-4); cc: 0.4 GAC (Merck); d: 300e700; ccv: w0.5

Sirianuntapiboon et al. (2007)

k1: 4.85 k1: 5.21

SW: Mordant Yellow 10 (c: 110); VFA mixture (c: 2000)

k: 10.2 k: 19.4

Abbreviations: HRT, hydraulic residence time, h; BC, biomass concentration, mg L1; SW, simulated wastewater; TW, textile wastewater; VL, volume of treated dye solution, L; VPB apparent volume of the packed-bed, mL. Abbreviations: d, AC particle size, mm; cc, carbon concentration in the liquid phase, g L1; SBET, BET surface area, m2 g1; ccv, carbon concentration in the packed-bed volume, g mL1; pHPZC, point of zero charge; CO, amount of CO-emitting groups on AC surface, mmol g1; mod, modified; gas., gasification; BO, AC burn-off, %. c Abbreviations: c, inlet concentration, mg L1; COD, chemical oxygen demand, mg L1; VFA, volatile fatty acid. 1 d Abbreviations: XCOL, colour removal; XCOD, COD removal; n.m., not measured; kAV, average reaction rate constant, mg h1; k1, kinetic constant of the model suggested by Mezohegyi et al. (2008), mmol g1 k, firstAC min order decolourisation rate constant, day1. a

b

159

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2005, 2008a,b). Moreover, GAC inoculated with single species of Bacillus OY1-2 was able to reduce the azo dye Red B in a packed reactor under anoxic conditions (Li et al., 2004). 4.2. AC-catalysed bioreduction of azo dyes Anaerobic azo dye reduction proceeds usually rather slowly. Bacterial azo reduction is generally considered as a non-specific reduction process that can be helped by redox mediators, shuttling electrons from bacteria to the azo dyes (Keck et al., 1997). Therefore, the use of such agents is a logical strategy for accelerating anaerobic decolourisation. Quinone-like compounds have been reported to catalyse the decolourisation of azo dyes (Van der Zee et al., 2001b; Dos Santos et al., 2004; Van der Zee and Cervantes, 2009). However, continuous dosing of the redox mediator results additional process costs. This problem can be avoided by immobilizing the electron mediator in the bioreactor. For this purpose, AC was considered as it can be retained in the reactor for prolonged time and it generally contains carbonyl/quinone sites on its surface. The first study considering AC as catalytic redox mediator in an advanced reduction process (ARP) was reported by Van der Zee et al. (2003) treating the hydrolysed azo dye Reactive Red 2 in a lab-scale UASB reactor and using volatile fatty acids as electron donors. Mixing of AC with granular sludge greatly improved both the dye removal and the formation of its reduction product aniline, giving evidence of AC’s catalytic behaviour. Moreover, batch experiments with azo dye Acid Orange 7 proved that chemical reduction of the dye was accelerated by AC addition. These results suggest that AC accepts electrons from microbial oxidation of organic acids and transfers the reducing equivalents to the azo dye, thereby accelerating the colour removal. Recently, some studies examined the different aspects of AC-catalysed ARPs of azo dyes. Mezohegyi et al. (2007) investigated the anaerobic reduction of Acid Orange 7 in different reactor systems, in the presence of solid electron mediators. Batch experiments with graphite, incorporated with the sludge, showed that electron-mediating capability of the solid particles contributed to higher decolourisation rates. The use of AC in a continuous upflow packed-bed bioreactor (UPBR) resulted in rather fast dye conversion, up to 99% in 2 min of space time (w5.4 min of HRT) (Fig. 2). The continuous reactor with AC was found more effective than the identical reactor working with

Fig. 2. Azo dye Acid Orange 7 (cIN: 100 mg/L) conversion in continuous UPBR using different support materials in the bioreactor: (A) activated carbon, (:) graphite, and (-) alumina [Reprinted with permission from Mezohegyi et al. (2007). Copyright (2007) American Chemical Society].

graphite, proving the importance of AC’s beneficial surface properties in biological azo reduction. Compared to the UPBR, improvement in Acid Orange 7 conversion was achieved by using a noveltype upflow stirred packed-bed bioreactor (USPBR) containing GAC (Mezohegyi et al., 2008). The USPBR provided more reproducible data to make kinetic modelling of azo dye bioreduction possible. The electrochemical characteristics of azo dyes were a key factor in ARPs: the bioreducibility of azo colourants could be predicted by their reduction potential values in the catalytic USPBR system, independently of the azo dye type, complexity and adsorption affinity (Mezohegyi et al., 2009). As for the AC characteristics, biological removal of azo dyes Orange II and Reactive Black 5 in USPBRs was significantly affected by the AC textural properties and the reduction rate constants were proportional to the AC surface area (Mezohegyi et al., 2010). Variation of the AC surface chemistry seemed to have less effect on dye conversion rates, even though the hypothesis of AC catalysis by the carbonyl/quinonic sites in the ARP was confirmed. González-Gutiérrez et al. (2009) proposed pathways for the anaerobic degradation of azo dye Reactive Red 272 in a fixedbed bioreactor containing GAC and, similarly, considered AC besides extracellular enzymes or coenzymes to transfer electrons by means of its quinonic groups. Nevertheless, also other mechanisms may influence catalytic decolourisation, particularly in the absence of significant densities of surface oxygen-containing groups, whereas not the quinonic groups but rather the delocalised p-electrons are involved in the reduction (Mezohegyi et al., 2010). The latter theory was furthermore confirmed by Pereira et al. (2010) who observed that the biological decolourisation rate of the azo dye Mordant Yellow 10 proceeded twice as fast with thermally treated AC than with unmodified AC in the bioreactor. 5. Discussion Considering the numerous different methods in which AC has been reported to facilitate colour removal as well as colourant mineralisation, the versatile and beneficial roles of this material are obvious. A couple of important and related issues, however, suffer lack of information in the literature such as the efficacy of AC-aided hybrid treatment operating with real (textile) wastewaters and comparison of these enhanced techniques from economic point of view. Textile effluents are characterised by extreme fluctuations in many parameters (e.g., chemical and biological oxygen demand, pH, salinity) and may contain diverse pollutants such as size agents and starch (from desizing), oils, fats, waxes and surfactants (from scouring), NaOH (from mercerizing), dyes, metals and sulphide (from dyeing) (Dos Santos et al., 2007). Despite the complexity of these wastewaters, most studies from the literature deal with treating a ‘simulated effluent’ containing only one or more dyes. Only a few researchers (e.g. Is¸ık and Sponza (2008)) have investigated dye removal with rather realistic simulated wastewater, albeit without involving AC. Research with real textile wastewaters is more widely published than with simulated effluents, although their accessibility is now limited due to the textile industry’s dense geographic situation in the eastern world. Only a limited number of publications discuss results from AC-amended hybrid textile wastewater treatment technologies (Lin and Lai, 2000; Sirianuntapiboon and Srisornsak, 2007; Sirianuntapiboon et al., 2007; Faria et al., 2009a). All of these studies confirmed the synergetic effect of AC. Nevertheless, textile auxiliaries can have a negative effect on AC performance, i.e., on final colour and COD removal due to e.g., competitive AC adsorption or scavenging effects (Oguz et al., 2006; Faria et al., 2009a). The investment, operation and maintenance costs of dye removal are undeniably decisive factors for full-scale application.

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To the authors’ knowledge, minimal number of studies have been reported on the systematic cost comparison of different textile wastewater techniques among either industrial applications or labscale operations. Hai et al. (2007) in their critical review made a cost evaluation for different hybrid decolourisation processes and concluded that AOPs are generally more costly than biological processes, although comparison is rather difficult due to the numerous factors (e.g., feed condition, reactor configuration, effluent quality) which also have an impact on total expenses. Some reports have been occasionally published about cost analysis of an individual dye treatment (Bayramoglu et al., 2004; El-Dein et al., 2006). Although no data are currently available for the economic assessment of AC-amended colourant removal techniques, their total cost may be evaluated by adding the value of AC (including its eventual replacements) to the single process expenses. According to the promising results (some comparative data are shown in Table 2 and Table 3), these AC-enhanced treatments in general anticipate cheaper dye removal solutions in up-scaled systems. Each dye decolourisation method has its own advantages and drawbacks (Robinson et al., 2001). When they are combined with AC, some other features can be further discussed. The use of AC for (single) adsorption of dyes from textile wastewaters falls far from economically attractive consideration unless it is produced from e.g., solid wastes (Dias et al., 2007; Demirbas, 2009). Moreover, it generates secondary waste that has to be treated, and in case of real wastewaters, competitive adsorption of other pollutants can decrease dye removal rates. According to the research trends in AC dye adsorption, it slowly goes out of fashion and lower-cost adsorbents with similar adsorption properties are preferred to investigate (Gupta and Suhas, 2009). The role of AC in enhanced dye coagulation is not more than additional adsorption of the colourant. This hybrid method has not become popular for textile wastewater treatment probably due to both the vast use of coagulants and production of secondary waste sludge, making the method one of the least environmentally friendly among all. Studies for membrane filtration of dye effluents are of permanent interest, especially because of the progressive development of membrane materials and their more economic availability. Although textile wastewater decolourisation by AC-aided membrane filtration does not anticipate its introduction at full-scale operations, the intensive application of membrane bioreactors (MBRs) for treating these effluents can be expected in the future. MBR research for both municipal and industrial wastewaters have become very intensive nowadays (Meng et al., 2009) and their enhancement by AC seems in particular promising for dye removal (Hai et al., 2008, 2011). The complex role of AC in the improved MBR process (Hai et al., 2011) included several beneficial components such as being a dye adsorbent, biofilm support, catalyst (anaerobic azo dye reduction) and control agent for membrane fouling. Electrochemical oxidation of dyes is a green method, although the (electrical) energy usage can result in very high expenses in case of high-rate wastewaters. AC can both reduce the costs and increase process efficiency by the electrogeneration of strong oxidising agent(s) either as electrode or particle electrode. Most AOPs are environmentally suitable methods and very effective for both colour and TOC removal from textile wastewaters. The inclusion of AC in these oxidation processes enhances removal rates due to its catalytic
OH generation or sensitization of (photocatalytic)
OH production (Table 2). However, most of these techniques have not been applied at industrial scale, probably due to the lack of data referred to economic persuasion. In addition, some of these processes may seem methodologically too demanding for up-scaling. The lowestcost solutions among all textile wastewater cleaning methods certainly are the biological treatments. These make up one of the most investigated dye removal techniques and are the most widely

161

adapted processes for full-scale textile effluent treatment thanks to their relatively simple, cost-efficient and environmentally applicable features. As for their most significant drawback, compared e.g. to AOPs, biological dye removal usually proceeds rather slowly. This has been proved by many bioreactor studies achieving nearly complete decolourisation but only at very low removal rates or at inadequately high HRTs. However, dye removal can be speeded up by incorporating AC in the bioreactor (Table 3) due to both biofilm formation on the AC surface and contingent AC catalysis (Section 4.2). 6. Conclusions Activated carbon can play an important role in both physicochemical and biological dye decolourisation. Although AC has in general a high dye adsorption affinity, adsorption is by far not the only mechanism that contributes to higher dye removal rates in ACamended treatment technologies. Certain features of AC always play a part in the combined process, irrespective of the applied method, such as its strong propensity to adsorb dyes and its (normally) high-surface area; others contribute to colour removal specifically, depending on the treatment method, such as AC’s ability to conduct electrons, to support impregnated elements/ bacteria on its surface or to catalyse dye degradation on its own. By tailoring AC textural and surface chemical properties, dye wastewater remediation can be optimised for the applied decolourisation technique. The advantageous roles of AC or innovative (activated) carbon materials in advanced MBRs, advanced oxidation processes and advanced (azo) dye bioreduction methods make these treatments or their combinations especially attractive choices/opportunities for economically and environmentally improving textile/ dye wastewater technologies. However, more research with real textile effluents and total cost evaluation are required for making the full-scale implementations possible. Acknowledgements This work has been partly funded by the Spanish Ministry of Science and Innovation (CTM2008-03338) and the Catalan government (2007ITT-00008). Dr. Gergo Mezohegyi is also indebted to Rovira i Virgili University for providing a fellowship. References Aktas¸, Ö., Çeçen, F., 2007. Bioregeneration of activated carbon: a review. Int. Biodeter. Biodegr 59 (4), 257e272. Al-Degs, Y., Khraisheh, M.A.M., Allen, S.J., Ahmad, M.N., 2000. Effect of carbon surface chemistry on the removal of reactive dyes from textile effluent. Water Res. 34 (3), 927e935. Andreozzi, R., Caprio, V., Insola, A., Marotta, R., 1999. Advanced oxidation processes (AOP) for water purification and recovery. Catal. Today 53 (1), 51e59. Awad, H.S., Galwa, N.A., 2005. Electrochemical degradation of Acid Blue and Basic Brown dyes on Pb/PbO2 electrode in the presence of different conductive electrolyte and effect of various operating factors. Chemosphere 61 (9), 1327e1335. Banat, F., Al-Bastaki, N., 2004. Treating dye wastewater by an integrated process of adsorption using activated carbon and ultrafiltration. Desalination 170 (1), 69e75. Banat, I.M., Nigam, P., Singh, D., Marchant, R., 1996. Microbial decolorization of textile-dye-containing effluents: a review. Bioresour. Technol. 58 (3), 217e227. Bayramoglu, M., Kobya, M., Can, O.T., Sozbir, M., 2004. Operating cost analysis of electrocoagulation of textile dye wastewater. Sep. Purif. Technol. 37 (2), 117e125. Berenguer, R., Marco-Lozar, J.P., Quijada, C., Cazorla-Amorós, D., Morallón, E., 2009. Effect of electrochemical treatments on the surface chemistry of activated carbon. Carbon 47 (4), 1018e1027. Byrappa, K., Subramani, A.K., Ananda, S., Rai, K.M.L., Sunitha, M.H., Basavalingu, B., Soga, K., 2006. Impregnation of ZnO onto activated carbon under hydrothermal conditions and its photocatalytic properties. J. Mater. Sci. 41 (5), 1355e1362. Can, O.T., Kobya, M., Demirbas, E., Bayramoglu, M., 2006. Treatment of the textile wastewater by combined electrocoagulation. Chemosphere 62 (2), 181e187.

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