TiO2-degradación de grupos azo

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TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and

mechanistic investigations: A review

Ioannis K Konstantinou , , Triantafyllos A Albanis

Department of Chemistry, Laboratory of Environmental Technology, University of Ioannina,Ioannina 45110, Greece

Received 5 July 2003; revised 24 November 2003; Accepted 24 November 2003. Available

online 7 February 2004.

Abstract

The photocatalytic degradation of azo dyes containing different functionalities has been

reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. Themechanism of the photodegradation depends on the radiation used. Charge injection

mechanism takes place under visible radiation whereas charge separation occurred under UV

light radiation. The process is monitored by following either the decolorization rate and the

formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo

dyes can usually be approximated as pseudo-first-order kinetics for both degradation

mechanisms, according to the LangmuirHinshelwood model. The degradation of dyes depend

on several parameters such as pH, catalyst concentration, substrate concentration and the

presence of electron acceptors such as hydrogen peroxide and ammonium persulphate

besides molecular oxygen. The presence of other substances such as inorganic ions, humic

acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed

degradation of pesticides does not occur instantaneously to form carbon dioxide, but through

the formation of long-lived intermediate species. Thus, the study focuses also on the

determination of the nature of the principal organic intermediates and the evolution of the

mineralization as well as on the degradation pathways followed during the process. Major

identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone,

phenolic compounds and several organic acids. By-products evaluation and toxicity

measurements are the key-actions in order to assess the overall process.

Keywords: Azo dyes; Photocatalytic degradation processes; Operational parameters;

Transformation products

Article Outline

1. Introduction

2. Experimental procedures

2.1. Photocatalytic degradation mechanisms
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2.1.1. Photocatalytic oxidation

2.1.2. Photosensitized oxidation

2.2. Primary substrate disappearance

2.3. Factors influencing the photocatalytic degradation

2.3.1. Effect of initial dye concentration

2.3.2. Effect of TiO2 loading

2.3.3. Effect of pH

2.3.4. Effect of light intensity and irradiation time

2.3.5. Effect of oxidants

2.3.6. Effect of humic acids, natural occurring ions and solvents

2.4. Photocatalytic mineralization of dyes

2.4.1. Analysis of the end products

2.4.2. Nature and evolution of organic intermediates

2.4.2.1. Monoazo dyes

2.4.2.2. Di- and triazo dyes

2.4.2.3. Triazine containing azo dyes

3. Conclusions

References

1. Introduction

Textile dyes and other industrial dyestuffs constitute one of the largest group of organic

compounds that represent an increasing environmental danger. About 120% of the total

world production of dyes is lost during the dyeing process and is released in the textile

effluents [1], [2], [3] and [4]. The release of those colored waste waters in the environment is a

considerable source of non aesthetic pollution and eutrophication and can originate dangerous

byproducts through oxidation, hydrolysis, or other chemical reactions taking place in the

wastewater phase [5], [6], [7] and [8].
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Decolorization of dye effluents has therefore received increasing attention. For the removal of

dye pollutants, traditional physical techniques (adsorption on activated carbon, ultrafiltration,

reverse osmosis, coagulation by chemical agents, ion exchange on synthetic adsorbent resins,

etc.) can generally be used efficiently [9], [10], [11] and [12]. Nevertheless, they are non-

destructive, since they just transfer organic compounds from water to another phase, thus

causing secondary pollution. Consequently, regeneration of the adsorbent materials and post-

treatment of solid-wastes, which are expensive operations, are needed [12] and [13]. Due to

the large degree of aromatics present in dye molecules and the stability of modern dyes,

conventional biological treatment methods are ineffective for decolorization and degradation

[14], [15], [16] and [17]. Furthermore, the majority of dyes is only adsorbed on the sludge and

is not degraded [18]. Chlorination and ozonation are also being used for the removal of certain

dyes but at slower rates as they have often high operating costs and limited effect on carbon

content [12], [19], [20], [21] and [22].

These are the reasons why advanced oxidation processes (AOPs) have been growing during the

last decade since they are able to deal with the problem of dye destruction in aqueous

systems. AOPs were based on the generation of very reactive species such as hydroxy radicals (

OH) that oxidize a broad range of pollutants quickly and non selectively. AOPs such as Fenton

and photo-Fenton catalytic reactions [23], [24], [25], [26] and [27], H2O2/UV processes [28]

and [29] and TiO2 mediated photo-catalysis [11], [30], [31], [32] and [33] have been studied

under a broad range of experimental conditions in order to reduce the color and organic load

of dye containing effluent waste waters.

Among AOPs, heterogeneous photocatalysis using TiO2 as photo-catalyst appears as the most

emerging destructive technology [4], [34], [35], [36], [37], [38] and [39]. The key advantage of

the former is its inherent destructive nature: it does not involve mass transfer; it can be carried

out under ambient conditions (atmospheric oxygen is used as oxidant) and may lead to

complete mineralization of organic carbon into CO2. Moreover, TiO2 photocatalyst is largely

available, inexpensive, non-toxic and show relatively high chemical stability. Finally, TiO2

photocatalytic process is receiving increasing attention because of its low cost when using

sunlight as the source of irradiation. The utilization of combined photocatalysis and solar

technologies may be developed to a useful process for the reduction of water pollution by

dying compounds because of the mild conditions required and their efficiency in the

mineralization [7], [40], [41], [42], [43] and [44].

The application of photocatalytic procedures for remediation of textile wastewater is rather

limited to few investigations [45], [46], [47], [48] and [49]. There are many studies dealing with

the photocatalytic decolorization of specific textile dyes from different chemical categories,

and most of them including a detailed examination of the so-called primary process under

different working conditions [7], [11], [32], [41], [42], [50], [51], [52], [53], [54], [55], [56] and

[57]. On the contrary, little information is available on the reaction mechanisms involved in the

photocatalytic degradation of dyes and on the identification of major transient intermediates

which have been more recently recognized as very important aspects of these processes,

especially in view of their practical applications [6], [34], [35], [44] and [58]. Thus, information

about real mineralization of the dye or decreases in toxicity are scarce and therefore ourattention has been also focused on the reaction types and mechanisms, based on the
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identification of the transformation products. Moreover, the effect of common dyebath

constituents on the photocatalytic treatment efficiency is also discussed in order to examine

the application of the photocatalytic degradation on real wastewater effluents.

Of the dyes available on the market today, approximately 5070% are azo compounds

followed by the anthraquinone group. Azo dyes can be divided into monoazo, diazo, triazoclasses according to the presence of one or more azo bonds ( N N ) and are found in various

categories, i.e. acid, basic, direct, disperse, azoic and pigments [3] and [59]. Some azo dyes and

their dye precursors have been shown to be or are suspected to be human carcinogens as they

form toxic aromatic amines [44], [60] and [61]. Therefore azo dyes are pollutants of high

environmental impact and were selected as the most relevant group of dyes concerning their

degradation using TiO2 assisted photocatalysis.

To our knowledge there is not a review dealing with the photocatalytic degradation of dyes

although that there are some reviews concerning the photocatalytic degradation of other

pollutants such as pesticides [37], [62], [63] and [64]. This review intend to assist workersinvolved in azo dyes photocatalytic treatment using TiO2 by: (a) compiling data on the degree

and on the factors influencing dye photodegradation, and (b) Summarizing and discussing data

on the mineralization degree, the intermediates and reaction mechanisms followed during the

process. The azo dyes were classified in terms of the characteristic structural groups.

2. Experimental procedures

2.1. Photocatalytic degradation mechanisms

2.1.1. Photocatalytic oxidation

The detailed mechanism of the process has been discussed previously in the literature [4], [6],

[58], [65], [66], [67] and [68] and will be only briefly summarized here. It is well established

that conduction band electrons (e) and valence band holes (h+) are generated when aqueous

TiO2 suspension is irradiated with light energy greater than its band gap energy (Eg, 3.2 eV).

The photogenerated electrons could reduce the dye or react with electron acceptors such as

O2 adsorbed on the Ti(III)-surface or dissolved in water, reducing it to superoxide radical anion

O2 . The photogenerated holes can oxidize the organic molecule to form R+, or react with

OH or H2O oxidizing them into OH radicals. Together with other highly oxidant species

(peroxide radicals) they are reported to be responsible for the heterogeneous TiO2

photodecomposition of organic substrates as dyes. According to this, the relevant reactions at

the semiconductor surface causing the degradation of dyes can be expressed as follows:

(1)

(2)

(3)
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(4)

(5)

(6)

(7)

(8)

The resulting OH radical, being a very strong oxidizing agent (standard redox potential +2.8 V)

can oxidize most of azo dyes to the mineral end-products. Substrates not reactive toward

hydroxyl radicals are degraded employing TiO2 photocatalysis with rates of decay highly

influenced by the semiconductor valence band edge position [69]. The role of reductive

pathways (Eq. (8)) in heterogeneous photocatalysis has been envisaged also in the degradation

of several dyes but in a minor extent than oxidation [58] and [70].

2.1.2. Photosensitized oxidation

The mechanism of photosensitized oxidation (called also photo-assisted degradation) by visible

radiation (>420 nm) is different from the pathway implicated under UV light radiation. In the

former case the mechanism suggests that excitation of the adsorbed dye takes place by visible

light to appropriate singlet or triplet states, subsequently followed by electron injection from

the excited dye molecule onto the conduction band of the TiO2 particles, whereas the dye is

converted to the cationic dye radicals (Dye +) that undergoes degradation to yield products as

follows [32], [34], [50], [51], [67], [68], [70], [71] and [72]:

(9)

(10)

(11)

(12)
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The cationic dye radicals readily reacts with hydroxyl ions undergoing oxidation via reactions

13 and 14 or interacts effectively with O2 , HO2 or HO species to generate intermediates

that ultimately lead to CO2 ((15), (16), (17), (18) and (19)).

(13)

(14)

(15)

(16)

(17)

(18)

(19)

In experiments that are carried out using sunlight or simulated sunlight (laboratory

experiments) it is suggested that both photooxidation or photosensitizing mechanism occurred

during the irradiation and both TiO2 and the light source are necessary for the reaction to

occur. In the photocatalytic oxidation, TiO2 has to be irradiated and excited in a near-UV

energy to induce charge separation. On the other hand, dyes rather TiO2 are excited by visible

light followed by electron injection onto TiO2 conduction band, which leads to photosensitizedoxidation. It is difficult to conclude whether the photocatalytic oxidation is superior to the

photosensitizing oxidation mechanism, but the photosensitizing mechanism will help to

improve the overall efficiency and make the photobleaching of dyes using solar light more

feasible [51].

2.2. Primary substrate disappearance

Several experimental results indicated that the destruction rates of photocatalytic oxidation of

various dyes over illuminated TiO2 fitted the LangmuirHinshelwood (LH) kinetics model [4],

[9], [65], [73], [74] and [75]:
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(20)

where r is the oxidation rate of the reactant (mg/l min), C the concentration of the reactant

(mg/l), t the illumination time, k the reaction rate constant (mg/l min), and K is the adsorption

coefficient of the reactant (l/mg).

When the chemical concentration Co is a millimolar solution (Co small) the equation can be

simplified to an apparent first-order equation [4], [9] and [37]:

(21)

A plot of ln Co/C versus time represents a straight line, the slope of which upon linear

regression equals the apparent first-order rate constant kapp.

Generally first-order kinetics are appropriate for the entire concentration range up to few ppm

and several studies were reasonably well fitted by this kinetic model [8], [38], [43], [44], [51],

[54], [55], [58], [60] and [76]. The LH model was established to describe the dependence of

the observed reaction rate on the initial solute concentrations.

It has been agreed, with minor doubts that the expression for the rate of photomineralization

of organic substrates such dyes with irradiated TiO2 follows the LangmuirHinshelwood (LH)

law for the four possible situations; (a) the reaction takes place between two adsorbed

substances, (b) the reaction occurs between a radical in solution and an adsorbed substrate

molecule, (c) the reaction takes place between a radical linked to the surface and a substrate

molecule in solution, and (d) the reaction occurs with the both of species being in solution. In

all cases, the expression for the rate equation is similar to that derived from the LH model,

which has been useful in modeling the process, although it is not possible to find out whether

the process takes place on the surface in the solution or at the interface [6].

It is likely that sorption of the dye is an important parameter in determining photocatalytic

degradation rates. All isotherms showed L-shape curves according to the classification of Giles

et al. [77] that means there is no strong competition between the water and the dye molecules

to occupy the TiO2 surface sites. The adsorption isotherms fit well to Langmuirian type

implying a monolayer adsorption model [4], [73], [75] and [76].

The color removal of the dye solution was determined usually with the absorbance value at

the maximum of the absorption spectrum for every dye by monitoring UV-Vis spectrum in

200800 nm zone using a spectrophotometer [41], [51], [72] and [78]. Alternatively, the

disappearance of dye was monitored by high performance liquid chromatography equipped

with a UV diode array detector [42], [58], [79] and [80] or MS detector [3].

2.3. Factors influencing the photocatalytic degradation
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2.3.1. Effect of initial dye concentration

It is important both from a mechanistic and from an application point of view to study the

dependence of the photocatalytic reaction rate on the substrate concentration. It is generally

noted that the degradation rate increases with the increase in dye concentration to a certain

level and a further increase in dye concentration leads to decrease the degradation rate of thedye [8] and [42]. The rate of degradation relates to the probability of OH radicals formation

on the catalyst surface and to the probability of OH radicals reacting with dye molecules. As

the initial concentrations of the dye increase the probability of reaction between dye

molecules and oxidizing species also increases, leading to an enhancement in the

decolorization rate. On the contrary, the degradation efficiency of the dye decreases as the

dye concentration increases further. The presumed reason is that at high dye concentrations

the generation of OH radicals on the surface of catalyst is reduced since the active sites are

covered by dye ions. Another possible cause for such results is the UV-screening effect of the

dye itself. At a high dye concentration, a significant amount of UV may be absorbed by the dye

molecules rather than the TiO2 particles and that reduces the efficiency of the catalytic

reaction because the concentrations of OH and O2 decrease [9], [52], [53], [68], [74], [81]

and [82].

The major portion of degradation occurs in the region near to the irradiated side (termed as

reaction zone) where the irradiation intensity is much higher than in the other side [83]. Thus

at higher dye concentration, degradation decreases at sufficiently long distances from the light

source or the reaction zone due to the retardation in the penetration of light. Hence, it is

concluded that as initial concentration of the dye increases, the requirement of catalyst

surface needed for the degradation also increases [7].

2.3.2. Effect of TiO2 loading

Whether in static, slurry or dynamic flow reactors the initial reaction rates were found to be

directly proportional to catalyst concentration indicating the heterogeneous regime. However,

it was observed that above a certain level of concentration the reaction rate even decreases

and becomes independent of the catalyst concentration. Most of studies reported enhanced

degradation rates for catalyst loading up to 400500 mg/l [8], [42], [52], [53], [55], [84] and

[85]. Only a slight enhancement or decrease was observed when TiO2 concentration further

increased up to 2000 mg/l. This can be rationalized in terms of availability of active sites on

TiO2 surface and the light penetration of photoactivating light into the suspension. Theavailability of active sites increases with the suspension of catalyst loading, but the light

penetration, and hence, the photoactivated volume of the suspension shrinks. Moreover, the

decrease in the percentage of degradation at higher catalyst loading may be due to

deactivation of activated molecules by collision with ground state molecules [7].

Agglomeration and sedimentation of the TiO2 particles were observed elsewhere when 2000

mg/l of TiO2 was added to the dye solution [52]. In such a condition, part of the catalyst

surface probably became unavailable for photon absorption and dye adsorption, thus bringing

little stimulation to the catalytic reaction. On the contrary, continuous increase of the

photocatalytic degradation rate of Reactive Black 5 was found up to 3500 mg/l TiO2[74]. The

crucial concentration depends on the geometry, the working conditions of the photoreactor
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and the type of UV-lamp (power, wavelength). The optimum amount of TiO2 has to be added

in order to avoid unnecessary excess catalyst and also to ensure total absorption of light

photons for efficient photomineralization. This optimum loading of photocatalyst is found to

be dependent on the initial solute concentration [62].

2.3.3. Effect of pH

The interpretation of pH effects on the efficiency of dye photodegradation process is a very

difficult task because of its multiple roles. First, is related to the ionization state of the surface

according to the following reactions,

(22)

(23)

as well as to that of reactant dyes and products such as acids and amines. pH changes can thus

influence the adsorption of dye molecules onto the TiO2 surfaces, an important step for the

photocatalytic oxidation to take place [86]. Bahnemann et al. [87] have already reviewed that

acid-base properties of the metal oxide surfaces can have considerable implications upon their

photocatalytic activity. The point of zero charge (pzc) of the TiO2 (Degussa P25) is at pH 6.8

[88]. Thus, the TiO2 surface is positively charged in acidic media (pH6.8).

Second, hydroxyl radicals can be formed by the reaction between hydroxide ions and positive

holes. The positive holes are considered as the major oxidation species at low pH whereas

hydroxyl radicals are considered as the predominant species at neutral or high pH levels [78]

and [89]. It was stated that in alkaline solution OH are easier to be generated by oxidizing

more hydroxide ions available on TiO2 surface, thus the efficiency of the process is logically

enhanced [54], [55], [65], [90] and [91]. Similar results are reported in the photocatalysed

degradation of acidic azo dyes and triazine containing azo dyes [7], [9], [52], [74], [92] and [93],

although it should be noted that in alkaline solution there is a Coulombic repulsion between

the negative charged surface of photocatalyst and the hydroxide anions. This fact could

prevent the formation of OH and thus decrease the photoxidation. Very high pHs have beenfound favorable even when anionic azo dyes should hamper adsorption on the negatively

charged surface [81]. At low pH, reduction by electrons in conduction band may play a very

important role in the degradation of dyes due to the reductive cleavage of azo bonds.

Third, the TiO2 particles tend to agglomerate under acidic condition and the surface area

available for dye adsorption and photon absorption would be reduced [86]. Hence, pH plays an

important role both in the characteristics of textile waters and in the reaction mechanisms that

can contribute to dye degradation, namely, hydroxyl radical attack, direct oxidation by the

positive hole and direct reduction by the electron in the conducting band.
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The degradation rate of some azo dyes increases with decrease in pH as reported elsewhere

[42], [67], [94] and [95]. At pH6.8 as dye molecules are negatively charged in alkaline media, their adsorption is also

expected to be affected by an increase in the density of TiO groups on the semiconductor

surface. Thus, due to Coulombic repulsion the dyes are scarcely adsorbed [44], [65] and [76].

For the above reasons the photocatalytic activity of anionic dyes (mainly sulphonated dyes)

reached a maximum in acidic conditions followed by a decrease in the pH range 711 [42],

[53], [58], [68], [75] and [76]. Moreover, the higher degradation rate at acid pH is seen also for

Vis/TiO2 experiments due to the efficient electron-transfer process due to strong surface

complex bond formation. This effect is less marked in neutral/basic pH solutions [67].

On the contrary, different optimal pHs (67) have been observed for the photocatalytic

degradation of other azo dyes, and a decrease of degradation in both acidic and alkaline pH

was reported [82] and [96]. The inhibitory effect seems to be more pronounced in the alkaline

range (pH=1113). At high pH values the hydroxyl radicals are rapidly scavenged and they do

not have the opportunity to react with dyes [97]. An additional explanation for the pH effects

can be related with changes in the specification of the dye. That is, protonation or

deprotonation of the dye can change its adsorption characteristics and redox activity [7].

Since the influence of the pH is dependent on dye type and on properties of TiO2 surface his

effect on the photocatalytic efficiency must be accurately checked before any application.

2.3.4. Effect of light intensity and irradiation time

Ollis et al. [98] reviewed the studies reported for the effect of light intensity on the kinetics ofthe photocatalysis process and stated that (i) at low light intensities (020 mW/cm2), the rate

would increase linearly with increasing light intensity (first order), (ii) at intermediate light

intensities beyond a certain value (approximately 25 mW/cm2) [62], the rate would depend on

the square root of the light intensity (half order), and (iii) at high light intensities the rate is

independent of light intensity. This is likely because at low light intensity reactions involving

electronhole formation are predominant and electronhole recombination is negligible.

However, at increased light intensity electronhole pair separation competes with

recombination, thereby causing lower effect on the reaction rate. In the studies reviewed

here, the enhancement of the rate of decolorization as the light intensity increased was also

observed [7], [42], [52], [74] and [75].

It is evident that the percentage of decolorization and photodegradation increases with

increase in irradiation time. The reaction rate decreases with irradiation time since it follows

apparent first-order kinetics and additionally a competition for degradation may occur

between the reactant and the intermediate products. The slow kinetics of dyes degradation

after certain time limit is due to: (a) the difficulty in converting the N-atoms of dye into

oxidized nitrogen compounds [99], (b) the slow reaction of short chain aliphatics with OH

radicals [100], and (c) the short life-time of photocatalyst because of active sites deactivation

by strong by-products deposition (carbon etc.).

2.3.5. Effect of oxidants
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It was observed that H2O2 and S2O82 addition was beneficial for the photoxidation of the

dyes of different chemical groups included azo dyes [6], [8], [43] and [52]. The reactive radical

intermediates ( SO4 and OH) formed from these oxidants by reactions with the

photogenerated electrons can exert a dual function: as strong oxidant themselves and as

electron scavengers, thus inhibiting the electronhole recombination at the semiconductor

surface [101] according to the following equations:

(24)

(25)

(26)

(27)

(28)

Moreover, the solution phase may at times be oxygen starved, because of either oxygen

consumption or slow oxygen mass transfer. Peroxide addition thereby increases the rate

towards what it would have been an adequate oxygen supply. The presence of S2O82

positively influences the mineralization rate, despite the decreasing of pH as the oxidant

properties of the system probably prevail on the effect of pH reduction. On the contrary, as far

as the substrate is concerned, the faster degradation rate can be due to both the decrease of

the pH and the oxidant action of S2O82[43].

However, H2O2 can also become a scavenger of valence band holes and OH, when present at

high concentration, [68], [102], [103] and [104]:

(29)

(30)

(31)
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As both hVB+ and OH are strong oxidants for dyes, the photocatalytic oxidation will be

inhibited when H2O2 level gets too high. Furthermore, H2O2 can be adsorbed onto TiO2

particles to modify their surfaces and subsequently decrease its catalytic activity.

Since the influence of the above additives, in particular H2O2, has been in some cases

controversial and it appeared dependent on the substrate type and on various experimentalparameters [105] the usefulness of which must be accurately checked before their application

[43].

2.3.6. Effect of humic acids, natural occurring ions and solvents

The occurrence of dissolved inorganic ions is rather common in dye-containing industrial

wastewater. Often, wastewater contains a mixture of pollutants, organic solvents as well as

dissolved organic matter and humic substances, if mixed with other waste streams. These

substances may compete for the active sites on the TiO2 surface or deactivate the

photocatalyst and, subsequently, decrease the degradation rate of the target dyes.Alternatively, they may act as light screens, thus reducing the photon receiving efficiency.

The Vis/TiO2 photocatalytic degradation of different classes of dyes is reported to be retarded

by many commonly used industrial solvents and acids, as well as by many naturally abundant

mineral species and dissolved organic matter [99]. The retardation by humic substances may

be by the combination effects of light attenuation, competition for active sites and surface

deactivation [106], [107] and [108]. Finally, various solvents such as acetonitrile and ethanol

were found to have a significant retardation effect on the photobleaching of dyes even at low

concentrations [68] and [106] as it is also stated for phenols and aromatic products [109]. Of

the anionic species studied (HCl, NaCl, NaNO3, HNO3, H3PO4 and NaHCO3), HCl exhibited thestrongest inhibition effect followed by H3PO4[68] and [106]. Inhibition effects of anions can be

explained as the reaction of positive holes and hydroxyl radical with anions, that behaved as

h+ and OH scavengers ((32), (33), (34), (35), (36) and (37)) resulting prolonged color removal.

Probably the adsorbed anions compete with dye for the photo-oxidizing species on the surface

and preventing the photocatalytic degradation of the dyes [87], [93] and [110]. Formation of

inorganic radical anions (e.g. Cl , NO3 ) under these circumstances is possible to occur [111].

(32)

(33)

(34)

(35)
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(36)

(37)

Although the reactivity of these radicals may be considered, they are not as reactive as h+ and

OH [112] and thus, the observed retardation effect is still thought to be the strong adsorption

of the anions on the TiO2 surface [110].

The effect of several types of metal ions (Cu2+, Zn2+, Fe3+, Al3+ and Cd2+) on the

photodegradation of non-azo dyes in TiO2 aqueous dispersions under visible light illumination,

has been investigated by Chen et al. [113]. They have concluded that Cu2+ and Fe3+ ions have

a strong suppressing effect on the photodegradation of all three dyes examined, by altering

the interfacial electron-transfer pathway under visible light irradiation. They noted that the

addition of Cu2+ and Fe3+ decreases the reduction of O2 by the conduction electrons,

subsequently blocks the formation of reactive oxygen species (O2 / OOH, OH) and hence

suppresses the photodegradation of dyes under visible irradiation. However, other metal ions

such as Zn2+, Cd2+ and Al3+ affect the photoreaction only slightly through an alteration of the

adsorption of dyes.

On the basis of hydroxyl radical formation through photocatalytic reactions of Fe3+ ions and

the products of their hydrolysis in aqueous solutions [114] is assumed that, the presence of

Fe3+ in the reaction environment, together with TiO2, should increase the rate of the

photocatalytic processes. An increased degradation rate was observed in the photocatalytic

degradation of azo dye acid red 1 in TiO2 suspensions containing Fe(III) aquo ions (105 to

104 M)[115]. This beneficial behavior was attributed to the increased amount of dye

adsorbed on the iron(III)-modified TiO2 surface and this was further confirmed by the fact that

iron species such as Fe2+ not adsorbed on the semiconductor had no kinetic effects. The

beneficial effect of Fe3+ ions was also found on the photocatalytic degradation of rhodamine B

in aqueous TiO2 suspensions [33].

Baran et al. [116] studied the photocatalytic degradation of several anionic and cationic azo

dyes in the presence of TiO2 and FeCl3. They have found that Fe3+ ions have a catalytic

influence on the decolorization of the studied anionic dyes but an inhibiting influence on the

decolorization of the cationic dyes. In conclusion, the role of Fe3+ ions on the photocatalytic

degradation of several dyes shows a controversial behavior depending on the physico-

chemical properties of dyes. Thus, in the presence of these ions the specific azo dye

degradation should be considered in order to determine the treatment efficiency.

The photocatalytic decolorization of the triazine azo dye MX-5B was reported to increase

slightly in the presence of 1 M of Cu2+ and Ni2+ at pH=2.4 [112]. Their reduced forms could

trap holes and that explains the decrease of the e/h+ recombination rate and a higher

production of OH. Excess of Cu2+ and Ni2+ led to short-circuiting reactions, which created a

cyclic process without generating active OH and retarded the reaction. However, at pH 10.8

the photodegradation of MX-5B was completely inhibited by the trace quantities of Cu2+ and
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Ni2+. The deposition of NiO2 on the surface of TiO2 was found to deactivate the photocatalyst

[112].

An understanding of the retardation effects not only aids in assessing the feasibility of using

photocatalytic oxidations to treat wastewater, but also allows a thoughtful photocatalytic

oxidation design.

2.4. Photocatalytic mineralization of dyes

2.4.1. Analysis of the end products

In order to assess the degree of mineralization reached during the photocatalytic treatment

the formation of CO2 and inorganic ions [6], [32], [34], [44], [58], [91] and [117], is generally

determined. However, in the presence of real wastewaters the monitoring of inorganic ions

and CO2 gives only a global estimation on the well functioning of the treatment, but does not

provide information on the real decay of the contaminant. In such cases the determination of

total organic carbon (TOC) and/or the measurement of the chemical oxygen demand (COD) or

the biological oxygen demand (BOD) of the irradiated solution is generally used for monitoring

the mineralization of the dye [7], [32], [42], [52], [58], [76], [80] and [118]. In general, at low

reactant levels or for compounds which do not form important intermediates, complete

mineralization and reactant disappearance proceed with similar half lives, but at higher

reactant levels where important intermediates occur, mineralization is slower than the

degradation of the parent compound. Until now, total mineralization has been observed for

the photacatalytic degradation of most of the azo dyes even at longer irradiation periods [42],

[44], [58], [74], [76] and [119]. Only in the case of triazine containing dyes, the mineralization

was not complete due to the high stability of triazine nucleus and the stable cyanuric acid wasformed, as in the case of s-triazine herbicides [120], which fortunately is not toxic [41], [52],

[80] and [121].

Usually COD or TOC values decrease with increase in irradiation time whereas the amount of

NH4+ and NO3 ions increase with increase in irradiation time. However, the formation of Cl

and SO42 increases initially and subsequently remains unchanged. COD or TOC curves have

an exponential or sigmoidal shape. The sigma-shaped curves indicating that is related to the

formation of relative tolerant by-products [44], [52] and [118]. This pattern means that during

the first steps of the process where the solution is still colored there is only a small decrease of

the parameter measured (TOC or COD or BOD) due to the fact that dye molecules aredecomposed to lower molecular weight compounds and the resulting intermediates still

contribute to the COD of the solution. After the decolorization of the solution the COD

decreases sharply (the linear segment of the S shaped curve) reaching a plateau that

corresponds to the oxidation of most stable compounds indicating that almost complete

mineralization of intermediates has occurred.

For chlorinated dye molecules, Cl ions are easily released in the solution and are the first of

the ions appearing during the photocatalytic degradation [42], [43] and [80]. This could be

interesting in a process, where photocatalysis would be associated with a biological treatment

which is generally not efficient for chlorinated compounds. Nitrogen is mineralized into NH4+,NO3 and N2. The proportion depends mainly on the initial oxidation degree of nitrogen, the
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926337303005411#ref_BIB112


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substrate structure and on irradiation time [122], [123] and [124]. By comparing the initial

rates, NH4+ appears as the primary product with respect to NO3 in the case of amine

compounds. The nitrogen atoms in the amino-groups of the dyes can lead to NH4+ ions by

successive attacks by hydrogen species

(38)

(39)

The total amount of nitrogen-containing ions present in the solution at the end of the

experiments is usually lower than that expected from stoichiometry indicating that N-

containing species remain adsorbed in the photocatalyst surface or most probably, that

significant quantities of N2 and/or NH3 have been produced and transferred to the gas-phase.[42], [44] and [76]. The formation of N2 in azo dyes can be accounted for by the same

processes responsible for NH4+ formation:

(40)

(41)

When nitrogen is present in the 3 state as in amino groups or in pyrazoline ring, it

spontaneously evolves as NH4+ cations with the same oxidation degree, before being

subsequently and slowly oxidized into nitrate [58]. In the azo bonds each nitrogen atom is in its

+1 oxidation degree. This oxidation degree favors the evolution of gaseous dinitrogen by the

two step reduction process expressed previously. N2 evolution constitutes the ideal case for a

decontamination reaction involving totally innocuous nitrogen-containing final product.

The dyes containing sulfur atoms are mineralized into sulfate ions [43] and [80]. In all the

studies the formation of SO42 was always observed and in most cases its stoichiometric

formation was found in the final steps of the photoreaction when organic intermediates stillwere present [43], [52] and [80]. The reported initial slopes are positive indicating that SO42

ions are initial products, directly resulting from the initial attack on the sulfonyl group. Release

of sulphate ions upon dye degradation was a little slower than decolorization but much faster

than TOC loss. Non-stoichiometric formation of sulphate ions is usually explained by a strong

adsorption on the photocatalyst surface [44], [125] and [126]. This strong adsorption could

partially inhibit the reaction rate which, however, remains acceptable [111] and [127].

Generally, it is found that nitrate anions have little effect on the kinetics of reaction whereas

sulfate, chloride and phosphate ions, especially at concentrations of greater than 103 mol

dm3, can reduce the rate by 2070% due to the competitive adsorption at the photoactivated

reaction sites [111]. The release of SO42 can be accounted by an initial attack by a photo-induced OH radical:
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