Environmental Engineering and Management Journal November/December 2009, Vol. 8, No.6, 1359-1369 http://omicron.ch.tuiasi.ro/EEMJ/

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TEXTILE WASTEWATER TREATMENT BY HOMOGENEOUS

OXIDATION WITH HYDROGEN PEROXIDE

Carmen Zaharia1∗, Daniela Suteu1, Augustin Muresan2, Rodica Muresan2, Alina Popescu3

1”Gheorghe Asachi” Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Department of Environmental Engineering and Management, 71A D. Mangeron Blvd., 700050 Iasi, Romania

2”Gheorghe Asachi” Technical University of Iasi, Faculty of Textiles, Leather and Industrial Management, Department of Chemical Textile Finishing, 53-55 D. Mangeron Blvd, 700050 Iasi, Romania

3National Institute of Research – Development for Textiles and Leather, 16 Lucretiu Patrascanu, 030508 Bucureşti, Romania

Abstract The textile wastewaters have a diverse composition depending both on the used raw materials and applied manufacturing technologies. These wastewaters may contain various pollutants such as organic compounds (e.g. residual dyes), suspended solids, metal ions etc. Most of dyes are synthetic compounds with aromatic molecular structures and non-biodegradable. The oxidative destruction via homogenous oxidation processes with hydrogen peroxide (simple chemical oxidation with H2O2 or advanced oxidation processes (AOPs) as Fenton oxidation, ozonation, photo-oxidation and photo-Fenton oxidation etc.) are attractive alternatives to conventional treatments, easy to be applied and not so expensive. The use of H2O2 in AOPs has the advantage that the decomposition products of organic pollutants are common harmless compounds. Moreover, H2O2 decomposes itself in water and oxygen. This paper is a review of authors’ researches regarding homogenous oxidation with hydrogen peroxide applied for different types of textile dyes in order to perform high textile dye removals considering some relevant factors: pH, agitation regime, temperature, H2O2 concentration, textile dye concentration, oxidation time, ferrous or metallic ions concentration, etc. Key words: AOPs, dyes, homogenous oxidation, hydrogen peroxide, textile wastewaters

∗ Author to whom all correspondence should be addressed: e-mail: e-mail: czah@ch.tuiasi, czaharia2003@yahoo.com; Phone: +40-232-278683 ext. 2175

1. Introduction High levels of environmental contamination

are produced in dyeing and finishing processes of textile units, due to the introduction of large and very diverse quantities of colour, organic and inorganic contaminants, especially non-biodegradable organics, and other hazardous chemicals into the process effluents (i.e. dyes and their derived products, pigments, dye intermediates, auxiliary chemicals and heavy metals etc.). Significant variation in the wastewater characteristics, mainly the pH, colour and wastewater BOD and COD concentrations occur as a result of frequent changes of dyestuff and chemicals used in the dyeing process. A large interval of pH variation is not recommended, since the pH tolerance of conventional biological and chemical treatment systems is very limited (Lin and Chen, 1997). Hence

without proper pH adjustment, normal operation of the treatment processes is essentially impossible. Other important components of the textile wastewater which are difficult to deal with are the strong colour, highly dissolved solid content and high turbidity of effluent.

Pollution of water bodies by dyes is a serious problem in the high industrialized countries. These highly coloured components when discharged with wastewater in the water bodies stop the reoxygenation capacity of the receiving water and cut-off sunlight, thereby upsetting biological activity in aquatic life (Malik and Saha, 2003; Zaharia, 2009). In view of their chemical structures, dyes can be characterised as azo dyes, anthraquinone dyes, heterocyclic dyes etc. Dyes can also be characterised according to their application method into vat dyes, reactive dyes, direct

“Gheorghe Asachi” Technical University of Iasi, Romania

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dyes, acid dyes, basic dyes, disperse dyes, etc. (Malik and Saha, 2003).

Over 700000 tons of approximately 10000 types of dyes and pigments are produced annually worldwide, of which about 20% are assumed to be discharged as industrial effluent during the textile dyeing processes (Azbar et al., 2004). Up to 50% for reactive dyes, 8-20% for disperse dyes and 1% for pigments may be lost directly into effluent resulting in a coloured effluent due to the inefficient dyeing process and the nature of the dyes, that after hydrolyzing it does not react with the fibber (McLullan et al., 2001). The decolourization of the effluent is essential for reuse of dyeing waters or safe discharge into different receivers (i.e. municipal pipe network or different emissaries) even when is released in small concentrations.

Although the traditional decolourization methods such as coagulation-flocculation, sorption on activated carbon, polymeric and mineral sorbents (e.g. peat, fly ash and coal, wood chips, silica gel, corncob, barley etc.), filtration, reverse osmosis, conventional chemical oxidation (i.e. oxidation with hydrogen peroxide, sodium hypochlorite and other chemical agents), ion exchange and biodegradation can remove such organic pollutants from wastewater to certain extent, they are mostly inefficient to degrade stable aromatics such as synthetic dyestuffs because of the complex polyaromatic structure and recalcitrant nature of dyes. Often, the effectiveness of these methods depends on the dye types in wastewaters and, therefore their particular application is restricted (Solozhenko et al., 1995).

Coagulation-flocculation may not remove highly soluble dyes and it may be good for disperse dyes, but it produces a large quantity of sludge (Azbar et al., 2004). Activated carbon sorption is the most commonly applied method for colour removal especially for cationic, mordant, acid dyes and a slightly lesser extent for disperse, direct, vat, pigment and reactive dyes (Azbar et al., 2004; Raghavacharya, 1997). Carbon adsorption of dyes is only successful in some classes of dyes (i.e. ionic type and hydrophobicity) for a given type of carbon in a certain pH range (Hao et al., 2000), and is non-destructive, since it just transfer the pollutant from wastewater to solid matrix that must be regenerated or incinerated (expensive post-treatment operations for solid wastes). The use of other unconventional sorbents (e.g. peat, fly ash and coal, wood chips, silica gel, corncob, barley etc.) has been practiced for textile effluent treatment into developing countries and has the same disadvantages as the other sorbents and generally is considered as low-cost sorbents. The conventional chemical oxidation (i.e. oxidation with hydrogen peroxide – H2O2, sodium hypochlorite – NaClO and other chemical agents) has been widely practiced in the textile industry but the cost of the polishing operation using these chemicals is high (Lin and Chen, 1997).

Ozonation is an efficient technique that has been reported in the scientific literature as a potential

alternative for decolourization purposes (Lin and Chen, 1997) but alone is not effective for disperse dyes (Azbar et al., 2004; Solozhenko et al., 1995). Frequently applied treatments for decolourization of textile effluents consist of integrated processes involving various combinations of physical, chemical and biological processes that are efficient but not cost effective (Azbar et al., 2004).

In addition to their visual effect (aesthetic impact on receiving waters), many synthetic textile dyes are toxic, mutagenic and carcinogenic. In this context, severe physical and chemical processes and operations are required in order to treat and reuse the textile effluents as advanced oxidations, micro filtration, ultra filtration, nano filtration, and other advanced methods (Macoveanu et al., 1997; Suteu et al., 2009). But the most effective decolourization treatments are those which transform the toxic and hazardous pollutants into harmful or easy to treat compounds (Surpateanu and Zaharia, 2004a; Surpateanu and Zaharia, 2004b).

Because of its simple handling, oxidation is the most commonly used chemical decolourization process. Actual progress in the removal of dyes has lead to the development of advanced oxidation processes (AOPs) that commonly include photo-decomposition of hydrogen peroxide (i.e. H2O2/UV), photolysis of ozone (i.e. O3/UV), photo-catalysis (i.e. TiO2/UV etc.), Fenton oxidation (FO) (i.e. Fe2+/H2O2) and photo-Fenton oxidations (photoFO) (i.e. Fe2+/H2O2/UV or M2+/H2O2/UV), radiolytic method and sonochemical method (Joseph et al., 2001; Macoveanu et al., 1997; Neamtu et al., 2004; Neyens and Bayens, 2003; Surpateanu and Zaharia, 2004a; Surpateanu and Zaharia, 2004b; Zaharia, 2006). However, the high electrical energy demand and/or the consumption of chemical reagents are common problems among all AOPs. Specially, the production of photons with artificial light source requires an important energy input (Neamtu et al., 2003).

The simple and advanced oxidation processes with hydrogen peroxide (oxidizing agent) for decomposition of non-biodegradable organic contaminants in textile effluents are attractive alternatives to conventional treatment methods (Andreozzi, 1999; Cisneros et al., 2002; Daneshvar et al., 2003; El-Dein et al., 2003; Kang et al., 2002; Pera-Titus et al., 2004; Zaharia and Suteu, 2008; Zaharia and Surpateanu, 2009). Under suitable operating conditions, the final products can be low molecular weight oxygenated compounds as aliphatic acids, CO2 and H2O (Behnajady et al., 2004) and almost complete mineralization of organics is possible. The oxidation processes with hydrogen peroxide can be explored as wastewater treatment alternatives in two systems: (1) heterogeneous systems based on the use of semiconductors with or without ultraviolet light, such as TiO2, stable modified zeolites with iron (i.e. FeY5, FeY11.5 etc.) (difficulty encountered in the separation of the solid photo catalysts at the end of the process) (Macoveanu et al., 1997; Neamtu et al., 2004; Zaharia et al., 2005;

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Zaharia, 2006) and (2) homogenous systems based on the use of visible or ultraviolet light, soluble catalysts and other chemical activators (e.g. ozone, peroxidase etc.) (Malik and Sanyal, 2003; Neamtu et al., 2003; Surpateanu and Zaharia, 2004a).

This paper is a review of authors’ researches regarding the homogenous oxidation with hydrogen peroxide applied for different types of textile dyes in order to perform high dye removals considering as the main important factors: temperature, pH, textile dye concentration, H2O2 concentration, FeSO4 concentration, oxidation time etc.

2. Oxidation with hydrogen peroxide for textile wastewater treatment

Hydrogen peroxide (H2O2) is a strong oxidant

(i.e. standard potential 1.80 and 0.87 V at pH 0 and 14, respectively) and its application in the treatment of various inorganic and organic pollutants is well established. Numerous applications of H2O2 in the removal of pollutants from wastewaters such as sulphides, hypochlorites, nitrites, cyanides and chlorine are known (Surpateanu et al., 2002; Surpateanu et al., 2003; Zaharia, 2006) and also as a disinfecting agent in the control of undesirable biofilm growth or enhancement of the biodegradation activity (injection of H2O2 into the subsurface) (Neyens and Baeyens, 2003).

H2O2 has also applications in the surface treatment industry involving cleaning, decorating, protecting and etching of metals (Neyens and Baeyens, 2003).

Following enzymatic and non-enzymatic routes, hydrogen peroxide can supply oxygen to microorganisms in biological treatment facilities and in the bioremediation of contaminated sites by dissociation into oxygen and water.

Oxidation by H2O2 alone is effective in homogenous systems for low concentration of inorganic and organic contaminants but is not effective for high concentration of certain non-biodegradable contaminants such as highly chlorinated aromatic compounds and inorganic compounds (e.g. cyanides) because of low rates of reaction at reasonable H2O2 concentrations (stability in pure form) ((Neyens and Baeyens, 2003).

Therefore, into the textile wastewater decolourization, dye decomposition or mineralization hydrogen peroxide need to be activated when is applied.

Transition metal salts (e.g. iron salts, cooper salts etc.), ozone and UV light can activate H2O2 to form hydroxyl radicals which are strong oxidants (i.e. oxidation potential 2.8 V) than H2O2 and ozone. Hydroxyl radicals non-specifically oxidize organic compounds at high reaction rates (i.e. order of 109 L.

mol-1.s-1) (Eqs. 1, 2).

·OH + R-H (Organics) → H2O + R· (mono substituted aromatic radicals) → furher oxidation (1)

R· + OH → P (products) (2)

In general, oxidation processes which are based on the generation of radical intermediates in homogenous systems are named advanced homogenous oxidation processes (AHOP). The H2O2 activation as oxidising agents in homogenous systems for textile dye decomposition can follow the next route.

• Hydrogen peroxide and transitional metal salts

such as iron salts – ferrous salts (Fenton reagent, Fe2+/H2O2) (Eqs. 3, 4).

Fe2+ + H2O2 → Fe3+ + OH· + OH- (chain initiation, k1 ≈ 70 L.mol -1.s-1) (3) OH· + Fe2+ → OH- + Fe3+ (chain termination, k2 ≈ 3.2x108 L.mol -1.s-1) (4)

Ferrous ion (Fe2+) initiates and catalyses the

decomposition of H2O2, resulting generation of hydroxyl radicals in accordance with Eq. 3. Moreover, the newly formed ferric ions may catalyze hydrogen peroxide decomposition into water and oxygen as into Eq. 5. But the mechanism is effective at pH lower than 3.5 when H2O2 and ferrous ions are more stable. At pH values higher than 4.0, ferrous ions easily form ferric ions which have a tendency to produce ferric hydroxo complexes and precipitates that involves coagulation and separation of sludge by sedimentation, filtration or other technique and application of Fenton Sludge Recycling System (FSR process, developed by peroxid-Chemie GmbH) (Solak and Marechal, 1998).

In basic aqueous systems (pH>10), H2O2 is unstable and easily decomposes itself (Kuo, 1992) as into Eqs. 8, 9 following the route described by Eqs. 5-9. Fe3+ + H2O2 → Fe-OOH2+ + H+ (k3 ≈ 0.001-0.01 L.mol-1.s-1) (5) Fe-OOH2+ → HO2·+ Fe2+ (6) Fe2++ HO2·→ Fe3+ + HO2

- (k5 ≈ 1.3x106 L.mol -1.s-1 at pH=3) (7) Fe3++ HO2·→ Fe2+ + O2 + H+ (k6 ≈ 1.2x106 L.mol -1.s-1 at pH=3) (8) OH·+ H2O2 → H2O + HO2· (k7≈ 1.2-4.5x107 L.mol -1.s-1) (9)

H2O2 can act as an initiator (Eq. 1) as well as an OH· scavenger (Eq. 9). The overall Fenton chemistry can be simplified by accounting for water dissociation (Eq. 10).

2Fe2+ + H2O2 + 2H+ → 2Fe3+ + 2H2O (10)

This equation suggests that the presence of an

acidic environment is required in decomposition of H2O2 to produce the maximum amount of hydroxyl

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radicals. The Fenton process (FO) is very suitable for oxidation of textile wastewaters which inhibit biological treatment and are poisonous (Andreozzi et al., 1999; Ince and Tezcanl, 1999; Lin and Chen, 1997; Lin and Peng, 1996; Slokar and Marechal, 1998; Solozhenko et al., 1995). Reactive, direct, metal-complex, pigment, disperse and vat dyes have good decolorization rates except Vat Red (50%) and Disperse Blue (0.5%).

• Hydrogen peroxide and ozone (H2O2/O3)

O3 + H2O2 → OH· + O2 + HO2 (11)

Because of its instability ozone is a very

powerful oxidizing agent (i.e. oxidation potential 2.07 V) for removing hazardous compounds from textile wastewaters (e.g. detergents, chlorinated organic hydrocarbons, phenolic compounds, aromatic hydrocarbons) (Liakou et al., 1997; Slokar and Marechal, 1998). For wastewater treatment ozone has a short half-life in water, it decomposes in about 20 minutes or less if compounds like dyes are present. Its stability is affected by the presence of salts, pH and temperature. Under alkaline conditions ozone decomposes more rapidly than under acidic condition (Gould and Groff, 1987; Slokar and Marechal, 1998). With increasing temperature, ozone solubility decreases. If ozone is used as hydrogen peroxide activator, the rate of decolorization is increased, but additional pollution of wastewater occurs (Azbar et al., 2004; Robinson et al., 2001).

Decolorization by H2O2/O3 combination was applied for direct, metal-complex or blue disperse dyes (Szpyrkowicz et al., 2001). There are some problems with decolourization of acid and red disperse dyes as well as with mixtures of direct, metal-complex, disperse and reactive dye (blue dyes even more than red dyes). The homogenous oxidation with hydrogen peroxide is not effective as preliminary treatment, but can be used as a secondary treatment before the biological treatment in order to reduce the biologically organic and inorganic inhibitors or as a tertiary treatment following an activated sludge process.

• Hydrogen peroxide and UV light (H2O2/UV)

H2O2 + hυ → 2OH· (12)

Into photo oxidation with hydrogen peroxide,

the increasing of H2O2 concentrations deal with two problems: (1) more hydroxyl radicals are available to attack the aromatic rings and the rate of organic oxidation decreases, and (2) above 50 mg/L H2O2, hydroxyl radicals efficiently reacts with H2O2 and produces HO2· radicals (Eq. 13).

The H2O· radicals are less reactive than OH· and the increasing of hydroperoxide radicals generates negligible contribution in dye destruction (Galindo et al., 2001).

H2O2 + OH· → HO2· + H2O (13) All the above mentioned environmental

problems (e.g. sludge formation and regeneration, increased pollution of wastewater caused by ozone) can be avoided by oxidation with hydrogen peroxide activated with UV light (Colonna et al., 1999; Surpateanu and Zaharia, 2004b).

The only chemical used in the treatment is H2O2 which is no problematic due to its final decomposition into oxygen. The most commonly factors influencing H2O2/UV treatment are H2O2 concentration, intensity of UV irradiation, pH, dye structure and dyebath composition. The most effective decolourization is performed at neutral pH, at higher UV radiation intensity (1600 W rather than 800 W), with an optimal H2O2 concentration that depends of dye type and structure (e.g. 5-100 mg/L H2O2) and with a dyebath that does not contain oxidizing agents having an oxidizing potential higher than of hydrogen peroxide (Slokar and Marechal, 1998; Surpateanu and Zaharia, 2004a; Zaharia, 2006). The acid dyes are the easiest to decompose and the decolorization efficiency decreases with the increasing number of azo groups.

The yellow and green reactive dyes need longer decolourization times, while other reactive dyes as direct, metal-complex and disperse dyes are decomposed quickly (Pittroff and Gregor, 1992; Slokar and Marechal, 1998). For pigments, H2O2/UV treatment is not suitable, because they form a film-like coating which is difficult to remove.

• Hydrogen peroxide, UV light and transitional

metal salts such as iron salts – ferrous salts (photo-Fenton reagent, Fe2+/H2O2/UV)

The dye oxidation rate with Fenton reagent

(Eq. 1) is strongly accelerated by irradiation with UV/VIS light. The main advantages resulted from UV/VIS irradiation at wavelength higher than 300 nm is that, in these conditions, the photolysis of Fe3+

complexes (i.e. Fe(III)-hydroxy complexes) allows Fe2+ regeneration. Photolysis of these complexes can yield HO· which is believed to be formed as a result of electron transfer from ligand (OH-) to Fe(III) in the excited states (Eq. 14) (Joseph et al., 2001).

Fe(OH)2+ + hυ → [Fe(OH)2+]*→ Fe2+ + HO· (14)

Under light irradiation, Fe3+ is continuously

reduced to Fe2+ (Eq. 15) (Kang et al, 1999; Surpateanu and Zaharia, 2001a; Surpateanu and Zaharia, 2001b; Surpateanu and Zaharia, 2001c; Neamtu et al., 2003; Zaharia, 2006).

Fe3+ + H2O + hυ → Fe2+ + HO· + H+ (15)

The application of this process in wastewater

treatment, as photo-Fenton process, is still limited since a strict pH control is imposed.

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Also, it can be formed sludge that includes the contaminants from wastewater with related disposal and reuse problems. All azo dyes were efficient removed from textile wastewaters using this process and good removals of COD, BOD and TOC were performed.

• H2O2 and peroxidase

For decolorization purposes, peroxidase can

also be used as hydrogen peroxide activator. The effectiveness of treatment depends on the peroxidase used, its concentration, pH and on the temperature of the aqueous medium (Solakar and Marechal, 1998). Three types of peroxidases were commonly studied as peroxide activator in the decolourization of acid dye: horseradish (HRP), soybean (SPO) and arthromyces ramosus (ARP).

By measuring the absorbance, it was found that the rate constant was the greatest using ARP (Morita et al., 1996).

The decolorization rate increased with increasing peroxidase concentration and temperature being higher than pH 9.5 (Slokar et al., 1997).

3. Experimental 3.1. Materials

Some homogenous oxidation studies were done with hydrogen peroxide applied for some indigene and imported textile dyes used as commercial salts that are characterized in Table 1.

The main characteristics of the studied homogeneous oxidation with hydrogen peroxide for these textile dyes are summarized into Table 2.

Table 1. Characteristics of the textile dyes used in homogeneous

Name / Abbreviation Formula MW, g/mol

λmax, nm

Dye concen-tration, mg/L

Acid Red G (ColR) / RG unitary acid dye C32H20O8N4S2Na2 698.64 535 50, 250 Vopsider Brown DMG 218 (ColR) /

VB DMG Direct dyes mixture, predominant dye

C39H25O6N8SNa2 820 565 100

Vopsider Brown DNRL 101 (ColR)/ VB DNRL

unitary direct fast dye C36H21O9N6SNa3 782.63 542 100

Methylene Blue (Basic Blue 9) / MB unitary direct dye C16H18N3SCl 319.86 660 100 Procion Gelb H-E4R (BASF) / PGb unitary reactive dye C50H34Cl2N14O3S10 1702.45 406 100

Table 2. The main features of the studied homogenous oxidation with hydrogen peroxide

Oxidation type Characteristics

Simple oxidation with

(H2O2)

Oxidation equipment: a 300 mL glass reactor equipped with magnetic stirrer with no, intermittent or continuous stirring. Operational conditions: different values of pH (1-13), temperature (t=17°-20°C), addition of different volumes of 30% H2O2 (0.1-12.5 mL) Experimental studies: decolourization kinetics in order to follow the removal progress of the studied dye (Surpateanu and Zaharia, 2001a; 2001b)

Fenton oxidation

(Fe2+/H2O2)

Oxidation equipment: a 300 mL glass reactor equipped with magnetic stirrer with no, intermittent or continuous stirring. Operational conditions: different values of pH (1-13), temperature (t=17°-20°C), addition of different volumes of 30% H2O2 (0.1-2.5 mL), of FeSO4 solution of 17.67 mmol/L Fe(II) (0.1-10 mL) Experimental studies: decolourization kinetics in order to follow the removal progress of the studied dye (Surpateanu and Zaharia, 2001a; 2001b, 2008; Zaharia, 2009)

Oxidation with H2O2 and

ozone (H2O2/O3)

Oxidation equipment: a laboratory ozonizer (ozone production) adapted for different tension variation between electrodes (1000-9000 V) and operation time (10-360 min), the air through-put being kept constantly (5-40 l/h); two bubbling vessels in series: (1) for analysed dye solution and (2) for the control of residual ozone (15 mL sodium thiosulphate 0.01N, 5 mL potassium iodine 10% and 55 mL phosphate buffer of pH=7) Operational conditions: air through-put of 20 l/h, ozonization time of 15 min, applied tension of 3148 V (production of 2.67 mg O3/L water), different values of pH (2-12) Experimental studies: decolourization kinetics in order to follow the removal progress of the studied dye (Surpateanu and Zaharia, 2003)

Photooxidation with H2O2 (H2O2/hν)

Oxidation equipment: photooxidation reactor - a tubular reactor of 0.30 dm3 capacity and 0.45 m in height, having a casing for keeping a constant temperature. The UV light source is a lamp with mercury vapours of medium pressure having 0.12 m in length, 0.020 m in diameter (emission of radiation 200-380 nm). Operational conditions: different values of pH (1-13), addition of different volumes of 30% H2O2 (0.1-2.5 mL) Experimental studies: photo oxidation kinetics in order to follow the removal progress of the studied dye (Surpateanu and Zaharia, 2001a; 2001b; 2004a; 2004b; Zaharia et al., 2004)

Photo Fenton oxidation

(Fe2+/H2O2/ UV )

Oxidation equipment: photo oxidation reactor - a tubular reactor of 0.30 dm3 capacity and 0.45 m in height, having a casing for keeping a constant temperature. The UV light source is a lamp with mercury vapours of medium pressure having 0.12 m in length, 0.020 m in diameter (emission of radiation 200-380 nm). Operational conditions: different values of pH (1-13), addition of different volumes of 30% H2O2 (0.1-2.5 mL), of FeSO4 solution of 34 mmol/L Fe(II) (0.1-5 mL) Experimental studies: photo Fenton kinetics in order to follow the removal progress of the studied dye (Surpateanu and Zaharia, 2001a; 2001b; 2004a; 2004b; Zaharia et al., 2004)

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The homogenous oxidation has proven a

promising and attractive treatment method for the effective decolourization and decomposition of textile dyes.Among these processes, advantages of the Fenton oxidation, photo-oxidation and photo-Fenton oxidation are numerous including high efficiency, simplicity in destroying the contaminants leaving no or small quantity of residue, stability to treat a wide range of substances, non-necessity of supplementary construction (adaptation of the existing ones at the operational conditions or equipment). 3.2. Homogenous oxidation studies

Homogenous oxidation experiments were

performed in batch conditions, by addition of oxidizing agent in 50, 100 or 250 mL aqueous dye solutions of known initial concentration, into an oxidation reactor placed in a thermostated bath at desired temperature. After temperature stabilization at selected value and the correction of pH, different volumes of 30% H2O2 solution were added to achieve the selected H2O2 concentrations, these being the basic conditions for the experimental tests. Depending on the homogenous oxidation type, other activators or catalysts were added (i.e. ferrous ions or other M(II) metallic ions or ozone). The initial solution pH was adjusted by adding dilute HCl or NaOH solutions and directly measured with a HACH ONE LABORATORY pH-Meter. After an adequate time (i.e. usually a time interval of 5-10 minutes) aqueous samples were prelevated and the amount of dye or colour were measured by spectrophotometric method at specific wavelength, using a DRELL DR/2000 spectrophotometer, Hach Company. Hydrogen peroxide was removed from the samples by increasing pH value to basic interval (pH 9-10). The decolourization efficiency was evaluated by percentage of dye removal or decolourization degree:

( )0 0R% C C 100/C= − ⋅ , where: C0 and C are the initial and the final concentration of dye in aqueous solution (mg/L).

4. Results and discussion

Previous experimental studies released that the

main factors influencing the sorption equilibrium can be classified in two categories: (1) process variables such as pH, agitation regime, oxidant dose, initial dye concentration, operational time and (2) variables depending on homogenous oxidation type or oxidizing agent such as hydrogen peroxide or hydrogen peroxide with different activators (M2+, O3 etc.). 4.1. pH and agitation influence on dye decomposition

The homogenous oxidation with hydrogen

peroxide of the studied dyes is strongly dependent on the solution pH. The dye oxidation with simple

hydrogen peroxide is no efficient (i.e. only maximum 19% dye removal after 90 minutes oxidation). Fenton and photo-Fenton oxidation applied for the studied dyes is efficient in slowly acidic medium (i.e. maximum dye removal after 30 minutes between 26.91-91.64%). The photo-oxidation with H2O2 of these studied dyes requires slowly acidic or medium condition for good decolourization efficiency (Surpateanu and Zaharia, 2001a; Surpateanu and Zaharia, 2001b; Surpateanu and Zaharia, 2004b; Zaharia et al, 2004) (Fig. 1a). It is clearly observed into Fig. 1a that RG dye removal by photo-oxidation is no dependent of pH value and has the highest efficiency comparison with the other dyes removals using Fenton oxidation. In Fig. 1b is presented the influence of pH on VB DNRL dye removal for different types of homogenous oxidation with H2O2. It is clearly evidence that the dye type and structure influences the efficiency of dye removal. Positive influence on all dye removals (decolourization) has also the continuous stirring or combination of only 10 minutes stirring at low rate (50 rpm) followed by interruption of agitation during the rest period of oxidation (economy of energy and costs) (Fig. 1c,d). 4.2. H2O2, initial dye concentration and oxidation time influence on dye decomposition

The previous studies conclude that the

concentration of hydrogen peroxide play an important role in dye decolourization. In the homogenous AOPs with hydrogen peroxide, the oxidation rate increases with increasing of H2O2 concentration but only to a certain dose (Surpateanu and Zaharia, 2004a; Surpateanu and Zaharia, 2004b; Zaharia and Suteu, 2008; Zaharia, 2009). These studies indicated that the high decolourization rate of these studied dyes increases proportional in the first 30 minutes, followed by a slower variation. The fastest Fenton oxidation occurs with 17.18 – 88.23 mM H2O2 depending on dye type and pH (a point of inflexion indicate depletion of H2O2, formation of compounds resistant to the oxidative processes or presence of intermediate products competing for the produced HO· radicals) (Surpateanu and Zaharia, 2004a; Zaharia and Suteu, 2008; Zaharia, 2009). In the case of VB DNRL dye photo oxidation, it was reported an increase of 48.6% and 46.07% dye removal after 10 minutes for 8.82 mM H2O2 and, respectively, 88.23 mM H2O2 in comparison with a dye removal of 16.49% in absence of H2O2 using photo oxidative processes (Surpateanu and Zaharia, 2001b; Surpateanu and Zaharia, 2001c; Surpateanu and Zaharia, 2004a). For VB DNRL dye photo-Fenton oxidation, it was reported an increase of 49.2% and 68.8% dye removal after 10 minutes for 8.82 mM H2O2 and, respectively, 88.23 mM H2O2 (pH=5.5; 8.8x10-2 mM Fe(II)). The experimental results performed for PGb dye Fenton oxidation into acidic and neutral medium (pH=4.5, respectively pH=6.5 with 0.72 mM Fe(II)) are presented into Fig. 2a,b

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indicating a maximum admissible H2O2 concentration of 0.22 M H2O2 for 88.86% dye removal after 10 minutes (95.21% dye removal after 30 minutes).

Some previous studies concluded that the decolourization efficiency by photo oxidation and Fenton oxidative processes decreases with the increase of initial dye concentration (Surpateanu and Zaharia, 2004a; Surpateanu and Zaharia, 2004b). The experiments performed with these dyes at different initial concentration (50-200 mg/L dye) indicated that a rapid decolourization occurs after first period of 20-30 minutes of intense oxidation because the reaction follows first or second order kinetics with respect to the dye concentration in second stage reaction. All Fenton oxidative processes and photo oxidation lead to high decolourization efficiency in a relatively short time (i.e. 30 minutes) whereas mineralization require longer reaction times (i.e. > 120 minutes). For PGb dye, an increase of dye removal from 87.4% at 5 minutes of oxidation time to 93.23% after 15 minutes occurs when the photo-Fenton process was used. 4.3. Ferrous and other metallic ions influence on dye decomposition

Fenton and photo Fenton processes use ferrous

ions as homogenous catalyst or activator of H2O2 activity, generally as sulfate, in a large concentration

range (2-100 mg/L Fe2+) but at high doses of ferrous ions a decrease in decolourization efficiency and COD removal was observed. When the initial ferrous ions concentration was 8.8x10-2- 17.86x10-2 mM the decolourization efficiency remains almost the same after 30 minutes of reaction.

An optimal ratio of Fe2+/H2O2 of 0.02 (1:20) for Fenton oxidation and of 0.66 or less (1:15) for photo Fenton oxidative process was proposed corresponding to literature data for other organic dyes (Surpateanu and Zaharia, 2004a; Neamtu et al., 2003; Malik and Saha, 2003). For VB DMG dye an increase of decolourization efficiency was performed using 3.58x10-3 mM Fe(II) from 13.57% into Fenton process to 86.08% into photo Fenton processes after 60 minutes of oxidation in the same operational conditions (Surpateanu and Zaharia, 2001b, Surpateanu and Zaharia, 2004a).

Optimal decolourization efficiency of RG dye (i.e. 99.23% after 10 minutes of UV irradiation) was performed into photo Fenton process for 6.52x10-2 mM Fe(II) and 18.314 mM H2O2. Other results of metallic ions influence that agrees the above mentioned general observations are presented into Fig. 3 for Fenton oxidation (88.23 mM H2O2) of PGb dye with Fe(II) ions (Fig. 3a) or in comparison with Cu(II) ions (i.e. 0.35 mM M2+) (Fig. 3b).

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12

pH

Dye

rem

oval

, %

VB DMG

VB DNRL

MB

RG

PG H-E4R

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

0 2 4 6 8 1 0 1 2

pH

VB D

NR

L dy

e re

mov

al, %

H2O 2F e(II)/H2O 2UV /H2O 2UV /H2O 2/F e(II)

(a) (b)

0

20

40

60

80

100

120

0 20 40 60 80

time, min

Dye

rem

oval

, %

aeration

H2O2+10 min stirring/after no

H2O2+continuous stirring

stirring

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140

Oxidation tim e, m in

PGb

dye

rem

oval

, %

no stirring

10 min stirring/no stirring

continuous stirring

(c) (d)

Fig. 1. The influence of some operational factors of the studied dye oxidation with H2O2 at room temperature:

a) influence of pH on the dye Fenton oxidation (VB DMG, VB DNRL, MB) (i.e. 88.23 mM H2O2, 8.8x10-2 mM Fe(II)) and photo oxidation (RG) after 30 minutes; b) influence of pH on VB DNRL dye homogenous oxidations (i.e. 70.58 mM H2O2, 8.8x10-2 mM Fe(II)) after 30 minutes;

c) influence of agitation regime (no, intermittent or continuous stirring at 50 rpm) at Fenton oxidation of VB DNRL (i.e. 88.23 mM H2O2 and 8.8x10-2 mM Fe2+; pH=5.5; t= 17°C); d) influence of agitation (no, intermittent or continuous stirring at 50 rpm) at Fenton oxidation of PGb (i.e.

88.23 mM H2O2 and 8.8x10-2 mM Fe2+; pH=5.5; t= 17°C)

Zaharia et al./Environmental Engineering and Management Journal 8 (2009), 6, 1359-1369

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A comparison between different metallic

activators of H2O2 as homogenous catalysts (e.g. Fe2+, Cu2+, Ni2+, Co2+, Mn2+) into almost neutral medium (pH=6.5, 18.3 mM H2O2, 0.0652 mM M2+) apllied for RG dye photo oxidation and photo Fenton process is shown into Fig. 3c and presented in detail into previous studies (Surpateanu and Zaharia, 2004a; Surpateanu and Zaharia, 2004b). The decolourization efficiency by Fenton process applied for all studied dyes is shown into Fig. 3d into the same operational

condition (i.e. room temperature, continuous stirring, pH= 4.5, 18.32 mM H2O2, 0.0882 mM Fe2+). 4.4. Ozone influence on dye oxidation with hydrogen peroxide

This researches were performed on aqueous

PGb dye solutions (50 mg/L dye) into a laboratory bubbling vessel were it was continuous introduced ozone generated into the laboratory ozonizer (2.67 mg O3/L water).

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140

Oxidation time, min

PGb

dye

rem

oval

, % 0.044 M H2O20.132 M H2O20.176 M H2O20.22 M H2O2

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140

Oxidation time, min

PGb

dye

rem

oval

, %

0.044 M H2O20.088 M H2O20.132 M H2O20.176 M H2O2

(a) (b)

Fig. 2. The influence of some operational factors of the studied dye oxidation with H2O2 at room temperature:

a) influence of initial H2O2 concentration on PGb dye Fenton oxidation in acidic condition (pH=4.5 and 0.72 mM Fe(II)); b) influence of initial H2O2 concentration on PGb dye Fenton oxidation in neutral condition (pH=6.5 and 0.72 mM Fe(II))

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140

Oxidation time, min

PGb

dye

rem

oval

, %

0.072 mM Fe(II)0.144 mM Fe(II)0.216 mM Fe(II)

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140

Oxidation tim e, m in

PGb

dye

rem

oval

, %

Fe(II)

Cu(II)

(a) (b)

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35

Oxidation tim e, m in

RG

dye

rem

oval

, %

Fe(II)Cu(II)Ni(II)Co(II)Mn(II)

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70

Oxidation time, min

Dye

rem

oval

, %

VB DMGVB DNRLMBPGbRG

(c) (d)

Fig. 3. The influence of some operational factors of the studied dye oxidation with H2O2 at room temperature: a) influence of ferrous ions concentration on PGb dye Fenton oxidation (pH=4.5); b) influence of metallic ions concentration on PGb dye Fenton oxidation (pH=4.5); c) influence of metallic ions concentration on RG dye photo oxidation (pH=6.5, 18.3 mM

H2O2); d) comparison of Fenton oxidation efficiencies for the studies organic dyes

Using of homogenous oxidation with hydrogen peroxide for textile wastewater treatment

1367

0

10

20

30

40

50

60

70

0 5 10 15

pH

Dye

rem

oval

, % ozone

ozone/H2O2

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 1 2 3 4 5 6 7 8 9

p H

Res

idua

l ozo

ne, m

g

ozone

ozone/H2O2

(a) (b)

Fig.4. The influence of ozone on the PGb dye homogenous oxidation:

a) influence of pH on decolourization efficiency after 15 minutes of ozonation; b) variation of residual ozone quantity into aqueous samples vs. pH for two systems (O3; O3/H2O2)

The dye decolourization with only ozone and

ozone/H2O2 system (7.14 M H2O2 corresponding to 0.5 mL H2O2 30%/dye solution sample) was studied into the bubbling vessel with 75 mL dye solution. The highest decolourization efficiency (60.12-63.41%) after 15 minutes of ozonation at pH=5.0 were performed for ozone/H2O2 system comparison with only ozone system (35.21-42.32%) (Fig. 4).

These data are in accordance with the literature data or a little bit lower (Siminiceanu, 2003) mainly because the tested laboratory ozonizer is not a commercial one (it was self-produced by a researcher team over more than 25 years ago) (Antonescu et al., 1986; Surpateanu and Zaharia, 2003) but its performance is still satisfactory, being used especially for didactical activities.

4.5. The irradiation nature and temperature influence on dye oxidation with hydrogen peroxide

Photocatalytic decolourization of dyes can be enhanced by action of solar light in the case of Fenton reagent or under UV radiation for photo-oxidation and photo-Fenton oxidation. The Fenton decolourization is faster than the experiment in the dark (more than 3.9 times) (Neamtu et al., 2003; Siminiceanu, 2003) because of directly dye photolysis or/and photoreaction involving H2O2 or metallic ions. These mentioned studies were performed with a source of UV radiation based on a medium pressure mercury lamp (UV radiation 200-380 nm) (L=0.12 m, φ=0.020 m). The nature of UV lamp is very important for high decolourization efficiency. On the market exists numerous types of commercial UV lamps (i.e. different low or medium pressure UV lamps). Generally, the decolourization rate increases with increase of UV intensity as the amount of UV radiation absorbed by H2O2 was increased. However, UV power increasing appears to be favourable for colour removal (Surpateanu and Zaharia, 2004a).

Our experiments were initiated at constant room temperature. The scientific literature mention that for Fenton process the increase of temperature

has no important effect on colour and COD removal between 20°-40°C (i.e. at 20°C the decolourization rate is about 4.5 h in comparison with 1 h at 40°C) but negatively affects the COD removal if temperature increases up to 60°C (Lin and Chen, 1997; Talinli and Anderson, 1992).

5. Conclusions

Discharging of textile wastewaters in river streams without an efficient treatment can cause undesired environmental problems. Therefore, a constant concern to find and apply the most efficient treatments in order to reduce both colour and organic content is required.

There are a large number of methods for dyes removal, but some of them are not efficient because they can only transfer, more or less effective, the pollutants from one phase to another, leaving the final environmental problem unsolved. Therefore, it is more indicated to transform the toxic pollutants such as textile dyes into harmful or easy-to-treat compounds using advanced homogenous oxidative processes with H2O2 (AHOP).

The studied organic dyes used into the textile industry by homogenous oxidative processes are: Vopsider Brown DMG 218, Vopsider Brown DNRL 101, Acid Red G, Methylene Blue, Procion Gelb H-E4R, hazardous chemicals because of its complex aromatic structure and type.

The use of hydrogen peroxide has as major attribute the fact that can be considered a friendly oxidant because by his decomposition water and oxygen are generated.

The dye oxidation rate with H2O2 was increased by use of different activators such as UV radiation, metallic ions (especially ferrous ions as sulphate), ozone etc.

The principal studied factors with great influence on dye homogenous oxidation with H2O2 are: pH, agitation regime, H2O2 concentration, initial dye concentration, ferrous or other metallic ions concentration, oxidation time, temperature, etc.

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Modifying these principal factors, the oxidation processes are certified as attractive technique for decolourization of textile effluents, with similar or better results given by conventional treatments.

It is difficult to apply the same homogenous oxidation conditions for all the studied dyes in order to get high decolourization efficiency. This study clearly indicates that for each dye and studied oxidation technique exist specific optimal operational conditions for a complete or adequate dye removal (> 70%).

To increase the treatment efficiency when is used an oxidation process, a great interest will have the use of combined treatment steps (biological, physical and chemical steps). One of the main criteria of textile effluent treatment that must be followed is the operating cost.

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