Brief History of Dyes

Brief history of dyes

The uses of colorants by mankind for painting and dyeing of their surroundings, their skins and their cloths dates back to the dawn of civilization. Until the middle of the 19^th century, all colorants applied were from natural origin. For example, inorganic pigments such as soot, manganese oxide, hematite and ochre have been utilized within living memory; organic natural colorants have also a timeless history of application, especially as textile dye. These dyes are all aromatic compounds, originating from plants but also from insects, fungi and lichens.

Synthetic dye manufacturing started in 1856, when the English chemist W.H. Perkin in an attempt to synthesize quinine, obtained instead a bluish substance with excellent dyeing properties that latter known as aniline purple, Tyrian purple or mauveine. Perkin patented his invention and set up a production line. This concept of research and development was strongly stimulated by Kekule’s discovery of the molecular structure of benzene in 1865. In the beginning of the 20^th century, synthetic dyestuffs had almost completely supplanted natural dyes.

Classification of Dyes

All aromatic compounds absorb electromagnetic energy but only those that absorb light with wavelength in the visible range (~ 350-700 nm) are colored. Dyes contain chromophores, delocalized electron systems with conjugated double bonds, and auxochromes, electron withdrawing or electron donating substituents that cause or intensify the color of the chromophore by altering the overall energy of the electron system. Usual chromophores are –C=C-, -C=N-, -C=O, -N=N-, NO₂ and quinoid rings, usual auxochromes are –NH_3, -COOH, -SO₃H and –OH.

Textile dyes are generally grouped into 14 different classes¹. These are-

(i) Acid dyes: The largest class of dyes in the Color index is referred to as Acid dyes. Acid dyes are anionic compounds that are mainly used for dyeing nitrogen-containing fabrics like wool, polyamide, silk and modified acryl. They bind to the cationic NH₄⁺-ions of those fibres. Most acid dyes are azo (yellow or red) anthraquinone or triarylmethane(blue or green) compounds. The adjective ‘acid’ refers to the pH in acid dye dyebaths rather than to the presence of acid groups (sulphonate, carboxyl) in the molecular structure of these dyes.

(ii) Reactive dyes: Reactive dyes are dyes with reactive groups that form covalent bonds with OH-, NH-, or SH- groups in fibres (cotton, wool, silk, nylon). The reactive group is often a heterocyclic aromatic ring substituted with chloride or fluoride. Another common reactive group is vinyl sulphone . In the Color index, the reactive dyes form the second largest dye class with respect to the amount of active entries.

(iii) Direct dyes: Direct dyes are relatively large molecules with high affinity for cellulose fibres. Van der Waals forces make them bind to the fibre. Direct dyes are mostly azo dyes with more than one azo bond.

(iv) Mordant dyes: Mordant dyes are fixed to fabric by the addition of a mordant, a chemical that combines with the dye and the fibre. Though mordant dyeing is probably one of the oldest ways of dyeing, the use of mordant dyes is gradually decreasing. They are used with wool, leather, silk, paper and modified cellulose fibres. Most mordant dyes are azo, oxazine or triarylmethane compounds.

(v) Sulphur dyes: Sulphur dyes are complex polymeric aromatics with heterocyclic S-containing rings. Dyeing with sulphur dyes involves reduction and oxidation, comparable to vat dyeing. They are mainly used for dyeing cellulose fibres.

(vi) Vat dyes: Vat dyes are water- insoluble dyes that are particularly and widely used for dyeing cellulose fibres. The dyeing method is based on the solubility of vat dyes in their reduced form. Reduced with sodium dithionate, the soluble vat dyes impregnate the fibric. Next, oxidation is applied to bring back the dye in its insoluble form. Almost all vat dyes are anthraquinones or indigoids. ‘Vat’ refers to the vats that were used for the reduction of indigo plants through fermentation.

(vii) Basic dyes: Basic dyes are cationic compounds that are used for dyeing acid-group containing fibres, usually synthetic fibres like modified polyacryl. They bind to the acid group of the fibres. Most basic dyes are diarylmethane, anthraquinone or azo compounds.

(viii) Disperse dyes: Disperse dyes are scarcely soluble dyes that penetrate synthetic fibres (cellulose acetate, polyester, polyamide, acryl, etc.). Dyeing takes place in dyebaths with fine disperse solutions of these dyes.

Among these textile dyes – acid dyes, direct dyes, sulphur dyes, azoic dyes, fibre reactive dyes and disperse dyes are considered as toxic substances.

Environmental concern

Many dyes are visible in water at concentrations as low as 1 mgL^-1. Textile processing wastewaters, typically with dye content in the range 10 – 200 mgL^-1, are therefore usually highly colored and present an aesthetic problem if discharged in open waters. As dyes are designed to be chemically and photolytically stable, they are highly persistent in natural environments. The release of dyes may therefore present an ecotoxic hazard and introduces the potential danger of bioaccumulation that may eventually affect man by transport through the food chain.

Toxicity of dyestaff

Dyestuff toxicity has been investigated in numerous researches . These toxicity (i.e. mortality, genotoxicity, mutagenicity and carcinogenicity) studies diverge from tests with aquatic organisms (fish, algae, bacteria, etc.) to tests with mammals. Furthermore, research has been carried out to effects of dyestuffs and dye containing effluents on the activity of both aerobic and anaerobic bacteria in wastewater treatment systems².

The acute toxicity of dyestuffs is generally low. The most acutely toxic dyes for algae are – cationic – basic dyes. The chance of human mortality due to acute dyestuff toxicity is probably very low. However, acute sensitization reactions by humans to dyestuff often occur. Especially some disperse dyestuffs have been found to cause allergic reactions, eczema or contract dermatitis.

Chronic effects of dyestuffs, especially of azo dyes, have been studied for several decades. Researchers were traditionally mostly focused on the effects of food colorants, usually azo compounds. Furthermore, also the effects of occupational exposure to dyestuffs of human workers in dye manufacturing and dye utilizing industries have received attention. Azo dyes in purified form are seldom directly mutagenic or carcinogenic, except for some azo dyes with free amino groups. However, reduction of azo dyes, i.e. cleavage of the dye’s azo  linkage(s), leads to formation of aromatic amines and several aromatic amines are known mutagens and carcinogens. In mammals, metabolic activation (= reduction) of azo dyes is mainly due to bacterial activity in the anaerobic parts of the lower gastrointestinal tract. Various other organs, especially the liver and the kidneys, can, however, also reduce azo dyes.

After azo dye reduction in the intestinal tract, the released aromatic amines are absorbed by the intestine and excreted in the urine .The acute toxic hazard of aromatic amines is carcinogenesis, especially bladder cancer. The carcinogenicity mechanism probably includes the formation of acyloxy amines through N-hydroxylation and N-acetylation  of the aromatic amines followed by O-acylation. These acyloxy amines can be converted to nitremium and carbonium ions that bind to DNA and RNA, which includes mutations and tumor formation.

The mutagenic activity of aromatic amines is strongly related to the molecular structure. In 1975 and in 1982, the International Agency for Research on Cancer (IARC)summarized the literature on suspected azo dyes, mainly amino-substituted azo dyes , fat-soluble azo dyes and benzidine azo dyes, but also a few sulphonated azo dyes. Most of the dyes on the IRAC list were taken out of prosuction.

Generally stated genotoxicity is associated with all aromatic amines with benzidine moieties, as well as with some aromatic amines with toluene, aniline and naphthalene moieties. The toxicity of aromatic amines depends strongly on the spatial structure of the molecules or in other words the location of the amine group(s). For instance, whereas there is strong evidence that 2- naphthylamine is a carcinogen, 1- naphthalamine is much less toxic. The toxicity of aromatic amines depends furthermore on the nature and the location of the substituents. As an example, the substitution with nitro methyl or methoxy groups or halogen atoms may increase the toxicity, whereas substitution of carboxyl or sulphonate groups generally lowers the toxicity.

As most soluble commercial azo dyestuffs contain one or more sulphonate groups, insight in the potential danger of sulphonated aromatic amines is particularly important. In an extensive review of literature data on genotoxicity and carcinogenicity of sulphonated aromatic amines, it was concluded that sulphonated aromatic amines, in contrast to some of their unsulphonated analogues have generally no or very low genotoxic and tumorigenic potential.

 The state of Bangladeshi water

The increasing urbanization and industrialization of Bangladesh have negative implications for water quality. The pollution from industrial effluents in some water bodies and rivers has reached at an alarming level. The long-term effects of this contamination by organic and inorganic substances, many of them toxic, are severe.

In Bangladesh, industrial units are mostly located along the banks of the rivers, which provide transportation for incoming raw materials and outgoing finished products. Unfortunately as a consequence, industrial units drain effluents directly into the rivers without any consideration of the environment. Textile industry is one of the most problematic industries for the water sector. A complex mixture of hazardous chemicals, both organic and inorganic is discharged into the water bodies from these industries, usually without treatment.

 The Textile Industries

The textile industries use vegetable fibres such as cotton, animal fibres such as wool and silk and a wide range of synthetic material such as nylon, polyester and acrylics. The amount of production of natural fibres is approximately equal to the amount of production of synthetic materials. The stages of textile production are fibre production; fibre processing and spinning; yarn preparation; fabrics production; bleaching; dyeing and printing and finishing. Each stages produces waste that requires proper management.

This cycle of textile production involves what are termed “wet processes”, which emits volatile organic compounds ( VOCs). The wastewater is typically alkaline and contains solids, oils and potentially toxic organics, such as phenols from dyeing and halogenated organics from bleaching. Dye wastewaters are frequently highly colored and may contain heavy metals such as copper and chromium.

As the diagram demonstrates, this historical trend means that established production facilities become more environmentally aware. Over time, they move towards the most efficient mode of treating pollution: prevention

Pollution Prevention

The most successful way to combat environmental pollution is to prevent it in the first instance. Pollution prevention in the textile industry should focus on reducing water use and on more efficient use of process chemicals. Changes to textile production processes that affect type and volume of effluent include the following:

• Matching process variables (e.g. dye) to the type and weight of fabric. This can reduce waste by 10-20 percent.
• Matching batches to minimize waste at the end of cycle.
• Avoiding non-degradable or less degradable washing and scouring chemicals.
• Using pad batch dyeing. This dyes the fabric at full width. The fabric is passed through containing the dye and between two heavy rollers, which force the dye into the cloth and squeeze the excess dye. This saves up to 80 percent of energy requirements and 90 percent of water consumption. It also reduces dye and salt usage.
• Using fewer toxic dye carriers and finishing agents. Avoid carriers containing chlorine, such as chlorinated aromatics.
• Reusing dye solution from the dye bath.
• Recovering and reusing process chemicals.
• Controlling the quantity and temperature of the wastewater.

Besides prevention there are also various physical, chemical and biological pretreatment, main treatment and post treatment techniques can be employed to remove color or degrade the pollution from the dye containing wastewaters.

Physicochemical techniques include membrane filtration, coagulation / flocculation, precipitation, flotation, adsorption, ion exchange, ion pair extraction, ultrasonic mineralization, electrolysis, advance oxidation (chlorination, bleaching, ozonation, Fenton oxidation and photo catalytic oxidation) and chemical reduction.

On the other hand biological techniques include bacterial and fungal biosorption and biodegradation in aerobic, anaerobic or combined anaerobic/aerobic treatment processes.

Several factors determine the technical and economic feasibility of each single dye removal technique:

• Dye type
• Wastewater composition
• Dose and cost of required chemicals.
• Operation cost (energy and material)
• Environmental fate and handling costs of generated waste products.In general each technique has its limitations. The use of one individual process may often not be sufficient to achieve complete decolorisation or complete degradation. Dye removal strategies most often therefore consists of a combination of different techniques.

The most important dye removal techniques are-

• Membrane filtration,
• Coagulation/flocculation,
• Sorption and ion exchange,
• Electrolysis,
• Advance oxidation process and
• Biological techniques

Among the above techniques we will briefly discussed the ‘Advance oxidation processes’ which include oxidation by Fenton’s Reagent.

Advance oxidation processes

Advance oxidation can be defined as oxidation by compounds with an oxidation potential (E₀) higher than that of oxygen (1.23 V), i.e. hydrogen peroxide (E₀ = 1.78 ), ozone (E₀ = 2.07 V) and the hydroxyl radical (E₀ = 2.28 V). Hydrogen peroxide alone is, however, usually not powerful enough. Advanced oxidation process (AOPs) are therefore mostly based on the generation of highly reactive radical species (especially the hydroxyl radical HO^.) that can react with a wide range of compounds, also with compounds that are otherwise difficult to degrade, e.g. dimolecules. The four AOPs that have been most widely studied are ozonation, UV/H₂O₂, Fenton’s reagent (Fe²⁺/H₂O₂) and UV/TiO₂.

In the ozonation process, hydroxyl radicals are formed when O₃ decomposes in water:

H₂O + O₃                 HO₃⁺ + OH^–                 2OH₂        + 2O₃        OH^. + 2 O₂

Though ozone itself is a strong oxidant, hydroxyl radicals are even more reactive. Decomposition of ozone requires high pH (>10). Ozone treatment of organic molecules proceeds therefore faster in alkaline solutions than at neutral or acidic pH where ozone is the main oxidant.

Ozone rapidly decolourises water-soluble dyes but non-soluble dyes (vat dyes and disperse dyes) react much slower. Textile-processing wastewater furthermore usually contains many refractory constituents other than dyes (e.g. surfactants) that will react with ozone, thereby increasing the ozone demand. It is advised, therefore, to pre-treat the wastewater before ozonation is applied. For example, in Leek, England, ozonation is used as the final stage (after biological treatment and filtration) for treating textile-processing wastewater at full-scale. This concept is, however, not logical as ozonation seldom leads to complete oxidation. Instead, ozone converts the organic compounds into smaller (usually biodegradable) molecules like dicarboxylic acids and aldehydes. The reduction of COD is therefore low, while some of the ozonation products (especially the aldehydes) are highly toxic. It is better, therefore, to treat the effluent of the ozonation stage, logically by using inexpensive biological methods.

Fenton oxidation is based on the generation of  hydroxyl radicals from Fenton’s reagent (Fe²⁺/H₂O₂) when ferrous iron is oxidized by hydrogen peroxide:

 Fe²⁺ + H₂O₂                    Fe (OH)²⁺ +OH^.                                         [1.2]

Also higher oxidized iron species like [Fe(OH)₂(H₂O)₅]²⁺ may be formed and it may even be possible that these species are the main oxidants in Fenton oxidation processes. In addition, re-reduction of ferric iron (redox cycling) can take place, thereby enabling iron to act as a catalyst in the generation of radicals:

Fe³⁺ + H₂O₂                         Fe²⁺ + HO₂^. +H⁺                                     [1.3]

However, reaction (1.3) proceeds much slower than reaction (1.2), unless at very high temperatures or when the reaction is catalysed by UV-light. In the latter case, both Fenton’s reagent and Fenton-like reagent (Fe³⁺/H₂O₂) can be used. Another enhanced Fenton-like process uses H₂O₂ in combination with iron powder. The oxidation reaction here is the conventional dark Fenton’s process but adsorption of dyes to the iron powder increases its effectiveness.

Fenton or Fenton-like oxidation can decolourise a wide range of dyes.  In comparison to ozonation, the process is relatively cheep and results generally in a larger COD reduction, although post-treatment (by for instance activated sludge) may still be required. A drawback for application of Fenton or Fenton-like oxidation for the treatment of  -the usually highly alkaline- textile-processing wastewaters is that the process requires low pH (2 – 5). At higher pH, large volumes of waste sludge are generated by the precipitation of ferric iron salts and the process loses effectiveness as H₂O₂ is catalytically decomposed to oxygen. Fenton or Fenton-like oxidation will furthermore be negatively affected by the presence of radical scavengers and strong chelating agents in the wastewater

Photocatalytic oxidation process (UV/H₂O₂, UV/TiO₂; UV/Fenton’s reagent; UV/O₃ and other) are all based on the formation of free radicals due to UV irradiation. Typically, as UV light does not penetrate sufficiently in highly coloured waste streams, application of photocatalytic processes is limited to the post-treatment stage.

When UV is used in combination with hydrogen peroxide, hydroxyl radicals are formed according to the following (simplified) reaction:

H₂O₂ + hu                          2HO^.

Drawbacks of the UV/H₂O₂ process are the relatively high costs and the occasional lack of effectiveness. Faster, cheaper and more effective photocatalytic processes receive therefore increasing attention, especially those based on catalysis by solid semiconductor materials, mostly TiO₂ particles. When this material is irradiated with photons of less than 385 nm, the band gap energy is exceeded and an electron is promoted from the valance band to the conduction band. The resultant electron-hole pair has a lifetime in the space-charge region that enables its participation in chemical reactions. In general, oxygen is used to scavenge the conduction band electron to produce a superoxide anion radical (O₂^.-), while adsorbed water molecules are oxidized to hydroxyl radicals:

O₂ + H₂O       hv/TiO₂       O₂^.- + HO^.

With TiO₂ catalysed UV treatment, a wide range of dyes can be oxidized. The dyes are generally not only decolourised but also highly mineralised.

Fenton’s Reagent

A versatile oxidant is highly reactive hydroxyl radicals (^.OH) generated from iron-catalysed oxygen transfer reaction of hydrogen peroxide which has wide applications in environmental remediation of many organic pollutants.

Introduction

Many metals have special oxygen transfer properties, which improve the utility of hydrogen peroxide. By far the most common of these is iron which, when used in the prescribed manner, results in the generation of highly reactive hydroxyl radicals (^.OH). The reactivity of this system was first observed in 1894 by its inventor H.J.M. Fenton, but its utility was not recognized until the 1930’s once the mechanism was identified.

What is Fenton’s reagent

Fenton’s reagent is the result between hydrogen peroxide (H₂O₂) and ferrous iron (Fe²⁺), producing the hydroxyl radical (^.OH). The hydroxyl radical is a strong oxidant capable of oxidizing various organic compounds.

Today Fenton’s reagent is used to treat a variety of industrial wastes containing a range of toxic organic compounds such as phenols, formaldehyde, BTEX and complex wastes derived from dyestuffs, pesticides, wood preservatives, plastics additives and rubber chemicals. The process may be applied to wastewater, sludges, or contaminated solids. The Fenten’s  reagent has been found to be effective in the-

• Destruction of organic pollutants
• Reduction of toxicity
• Improvement biodegradability
• Removal of BOD/COD
• Removal of order and color

 General chemistry of Fenton’s reagent

The procedure requires: (i) Adjusting the wastewater to pH 3-5, (ii) Adjusting the iron catalyst (as a solution of FeSO₄); and (iii) Adding slowly the H₂O₂. If the pH is too high, the iron precipitates as Fe(OH)_3­ and catalytically decomposed the H₂O₂ to oxygen.

Oxidation by Fenton’s reagent – mechanism

The reaction mechanism with Fenton’s chemistry, i.e. oxidation reaction initiated by Fenton’s Reagent, are still not fully understood, despite the intensive research for more than 60 years. It is generally believed that the active species in Fenton’s chemistry is the free OH radical, which is produced by the iron catalyzed dissociation of hydrogen peroxide

               Fe²⁺  +  H₂O₂                        →                         Fe³⁺  +  OH^–   +  ^. OH

               Fe²⁺   +  H₂O₂                →                 FeO²⁺  +  H₂O

Reaction rates with Fenton’s Reagent are generally limited by the rate of ^.OH generation (i.e. concentration of the iron catalyst). Fenton’s Reagent is most effective as a pretreatment tool, where COD’s are > 500 mg/L .This is due to the loss in selectivity as pollutant level decreases:

                ^.OH  +  H₂O₂         →            ^.O₂H  +  H₂O

                 ^.OH   +   Fe³⁺          →            Fe²⁺   +  OH^–

In addition to free radical scavengers, the process is inhibited by (iron) chelants such as phosphates, EDTA, formaldehyde, and citric/oxalic acids. Because of the sensitivity of Fenton’s Reagent to different wastewaters, it is recommended that the reaction always be characterized through laboratory treatability tests before proceeding to plant scale.

Hydroxyl radical reactivity

The hydroxyl radical is one of the most reactive chemical species known, second only to elemental fluorine in its reactivity.

The chemical reactions of the hydroxyl radical in water are of four types:

                ^.OH  +  C₆H₆             →              (OH)C₆H₆

where the hydroxyl radical adds to an unsaturated compound, aliphatic or aromatic, to from a free radical product (cyclohexadienyl radical shown above).

Hydrogen Abstraction :

                ^.OH  +  CH₃OH              →              ^.CH₂OH  +H₂O

                where an organic free radical and water are formed.

Electron Transfer

^.OH  +  [Fe(CN)₆]^4-                 →              [Fe(CN)₆]^3-  +  OH^-+

where ions of a higher valance state are formed, or an atom or free radical if a mononegative ion is oxidized.

Reaction Interaction

                 ^.OH  +  ^.OH                →            H₂O₂

where the hydroxyl radical reacts with another hydroxyl radical, or with an unlike radical, to combine or to disproportionate to form a stable product.

In applying Fenton’s Reagent for industrial waste treatment, the conditions of the reactions are adjusted so that first two mechanisms (hydrogen abstraction and oxygen addition) predominate.

1.8 Review of treatment of textile dyes, organic chlorocompounds and some other organic substances by Fenton’s reagent

The term “degradation” is usually used to refer the complete oxidation mineralization, which is the conversion of organic compounds to CO_2, H₂O, NO_3, SO₄^2- or other oxides, halides, phosphates etc.

Fenton’s process has been widely studied in the past two decades for the mineralization of organic hazardous and toxic components in aqueous medium.

Bae  et al³ have investigated the evaluation of predominant reaction mechanisms for the Fenton process in textile dyeing wastewater treatment. They quantitatively evaluated the predominant reactions in a large-scale Fenton process that treated dyeing wastewater and suggested an economical and effective treatment process. They have gone through plant analysis and found that a great part of the COD was removed by ferric coagulation. The comparative evaluation of Fenton’s oxidation and ferric coagulation revealed that ferric coagulation was the predominant mechanism to remove COD and color. In Fenton oxidation, the removal efficiencies of SCOD and color were 67.7% and 84.7%, respectively.

Kang et al⁴ have studied about the result of pre-oxidation and coagulation of textile wastewater by the Fenton process. These results evaluate the Fenton process involving oxidation and coagulation, for the removal of color and chemical oxygen demand (COD) from synthetic textile wastewater containing polyvinyl alcohol and a reactive dyestuff, R94H. The experimental variables studied include dosages of iron salts and hydrogen peroxide, oxidation time, mixing speed and organic content. The result shows that color was removed mainly by Fenton oxidation. The color removal reached a maximum of 90% at a reaction time of five minutes under low dosages of H₂O₂ and Fe²⁺. In contrast, the COD was removed primarily by Fenton coagulation, rather than by Fenton-oxidation.

 Perkowski⁵ has investigated the treatment of textile dyeing wastewater by hydrogen peroxide and ferrous ions. The results of investigations on the applicability of Fenton’s reagent in the treatment of textile dyeing wastewater were discussed. The optimum conditions and efficiency of the method were determined, taking as an example of three types of wastewater produced while dyeing cotton, polyacrylonitrile and polyester. The effect of the type and dose of coagulant was investigated. Two types of iron salts were used: sulphate (FeSO₄x7H₂O) and chloride (FeCl₂x4H₂O); to adjust the pH of the wastewater, a 1% solution of calcium oxide (CaO) was used. The process of pollutant decomposition which took place in the influence of hydrogen peroxide alone at different concentrations was investigated. It was found that the tested dyeing wastewater revealed high susceptibility to treatment using a combined action of ferrous salts and hydrogen peroxide. The main parameters of wastewater, i.e. the color threshold number, chemical oxygen demand and anionic surfactants, were reduced by dozens of percent.

Besides textile dyes various organic Fenton’s reagent has also degraded various organic chlorocompounds. For example, p-chlorophenol has been degraded by Fenton’s process ⁶. For this study batch experiments were carried out to investigate the effects of pH, Fe²⁺ and H₂O₂ levels, p-chlorophenol concentration and chloride level. The decomposition proceeded rapidly only within a limited pH of 2-4. It was a typical decomposition that an initial fast  decomposition rate was significantly reduced after a few minutes. The fast initial rate was first order with respect to p-chlorophenol and its rate constant was proportional to the initial level of Fe²⁺ and H₂O₂. The occurrence of the slow rate of decomposition was primarily attributed to the Fe²⁺ depletion caused by Fe-organic complex formation. The effect of chloride was pH dependent and could be significant as its concentration was increased. These observations strongly suggest that the reactions accounting for the conflicting effects of organic intermediates and products and heteroatoms should properly be included in the future models.

The study of degradation of 2,4 dinitrophenol (DNP)⁷, mainly concentrations of Fe²⁺, H₂O₂ and their effects on the removal of DNP have been studied.

The stoichiometry of Fenton’s reagent in the oxidation of dichloroethylene (DCE), trichloroethane (TCE), tetrachloroethane (Tet.CE) and dichloroethane (DCEA) has been studied by Tang ⁸. The theoretical optimal ratio between H₂O₂ and Fe²⁺ was II. However the optimal pH was 3.5. The amount of H₂O₂ required for a specific percentage of removal of organic compounds to be oxidized. The amount of H₂O₂ required to achieve a certain percent removal follows the order of TCE>Tet.CE>DCE>>DCEA.

Heterogeneous decomposition of COD components in ash filtrate samples by Fenton’s reagent, TiO₂ and granite porphyry powders has been studied by Moto and his co-workers ⁹. The oxidation of COD species by treatment with heterogeneous suspensions of Fenton’s reagent, TiO₂ and granite porphyry along with H₂SO₄ prior to heavy metal removal improves the subsequent ferric treatment efficiency.

Destruction of TNT by Fenton’s process has been studied ¹⁰. Treating an aqueous TNT solution with Fenton’s reagent on the dark resulted 100% destruction of TNT within 24 hours. This coincided with 40% mineralisation. Subsequent exposure to light resulted in >90% mineralisation. The generation of 2,4,6, trinitro benzoic acid and 2,4,6, trinitrobenzene within 15 min following Fenton oxidation of TNT were also observed. This indicated that initial TNT destruction likely occurred by Me group oxidation and decarboxylation. Subsequently transformation involves nitro moiety removal with ring hydroxylation and cleavage as evidenced by the stoichiometric recovery of TNT-N and NO₃^– and production of oxalic acid. Upon exposure to light, Fe(II) was formed and the oxalate produced from ¹⁴C-TNT oxidation disappeared. This coincided with a decrease in solution ¹⁴C activity. An azo dye (procian red) has been degraded by advance oxidation (Fenton process and UV light) process. At pH~3 and with H₂O₂/Fe(II) 79% of the dye has been degraded in 20 minutes.

Degradation of selected pesticide active ingradients and commercial formulation in water by the photo assisted Fenton reaction has been studied by M.Becker¹¹. Destruction of pesticide active ingradients (AI) and compound formulations in acidic aqueous solution with the catalytic photo assisted Fenton process. The temperature rise and UV photolysis increase the rate of mineralisation of the organic compounds. In some cases, intermediate products such as formate, acetate and oxalate appeared in the early stage of degradation. The inert ingradients present in the medium had no affect for the compound Furadan.

A novel advance oxidation system using combined UV/H₂O₂ has recently been developed ¹². Deys like MG, methyle orange, methylene blue, orange(II), rhodammene, p-nitrodimythylaninene have been degraded by this process. Similar to Fenton process hydroxyl radical has been identified as the main degrading agent.

Plan of the present work

The untreated dyes in effluents from dyeing factories and leather industries are a group of hazardous chemicals as well as important sources of water pollution in Bangladesh. The hazards are rendered more acute by their widespread presence. Dyes undergo chemical as well as biological changes in the aquatic system, consume dissolved oxygen and thus disturb the aquatic eco-system. Consequently survival of fishes and other aquatic wild lives become difficult. In presence of these organic compounds water becomes unusable for practical purposes like irrigation. It is, therefore, necessary to treat the water containing dyes and other organic compounds before discharging into water streams.

In the present work degradation of a textile dye (RED 8B) mediated by Fenton’s reagent has been studied. It was considered worthwhile to examine how this dye is degraded in the homogeneous medium in presence of Fenton reagent.

The mineralisation of dyes and other organic compounds by Fenton process in the homogeneous medium has been a research of increasing importance. The dyes are degraded in the dark but the rate of degradation and the extent of degradation can be enhanced by the light. In the present study we have used, Fe(II) + H₂O₂, as a source of  radicals (^.OH) which ultimately degraded the dye.

CHAPTER     2

EXPERIMENTAL

Chemicals and reagents used

The following chemicals and reagents were used without any further treatment,

1)                  Ferrous sulphate (BDH)

2)                  Hydrogen peroxide (BDH)

3)                  Red 8B (textile dye)

4)                  Buffer solution

5)         De-ionized water

 Equiepments  used

Equipments used for the analytical purposes were:

1)                  Double- beam 140 UV-visible spectrometer (Shimadzu)

2)                  Electronic balance

3)                  pH meter

4)                  Visible light source

5)                  UV light source

 Preparation of solutions

 Stock solution of Red 8B

The dye selected for this study was Red 8B which is a commercial dye extensively used in textile industries in Bangladesh.  0.028 g dye was taken to prepare 250.0 mL, 100 ppm dye solution using de-ionized water.

 Preparation of 2000 ppm H₂O₂ (30%) solution

0.45 cm³ of H₂O₂ was taken to prepare 250.0 mL, 2000 ppm H₂O₂ (30%) solution.

 Preparation of 2000 ppm FeSO₄ solution

0.515 g FeSO₄.7H₂O was taken to prepare 250.0 mL, 2000 ppm FeSO₄ solution.

 Experimental procedure

Determination of the absorption spectrum of Red 8B in the aqueous solution

The absorption spectrum of Red 8B in the aqueous solution has a distinguishable peak at 545 nm (Fig. 1). From this spectrum, the peak at 545 nm has been chosen as its characteristic peak. Accordingly this peak has been taken as the working wavelength for the experimental measurement of degradation of Red 8B.

 Degradation of Red 8B by Fenton’s reagent

The degradation of Red 8B by Fenton’s reagent was carried out both in the dark and in the presence of light. For this purpose 50.0 mL reaction mixture was prepared by taking requisite amount of Red 8B solution, FeSO₄ solution and H₂O₂ solution of known concentration. Deionized water was used to make the volume. The reaction was started by the addition of H₂O₂ solution. The absorbance of the solution was measured at 545 nm at different time intervals. For the photo-Fenton process the reaction mixture under illumination was withdrawn from the reactor at various intervals and its absorbance was also recorded at 545 nm. The procedure for the measurement of degradation was repeated under different conditions. The conditions were

a)      Variation of H₂O₂ concentration

b)      Variation of FeSO₄ concentration

 Determination of the (%) degradation

 Let us consider that, A₀ and A_t are the initial absorbance and absorbance at any time t respectively. Then the % degraded was calculated from the following relationship:

                        % Degraded =

Absorbance of the dye solution at five different concentrations

Reference                                :           water

l_max  of dye solution               :           545 nm

Temperature                           :            29^.0⁰C

Table 2.1.1: Absorbance of the dye solutions of different concentrations

 Calibration curve

A calibration curve based on the experimental data presented in Table 2.1.1 is shown in Fig. 2.1 The Absorbance vs. Concentration plot shows that the results follow Beer-Lambert Law within the range of concentrations considered. The molar extinction co-efficient (Î) was found to be 0.0216 (UNIT??). The unknown concentrations of the dye were evaluated from this calibration curve.

Degradation of the dye solution in the absence of FeSO₄ and H₂O₂ solution

Dye                 :           Red 8B; 50 ppm

Reference        :           Water

l_max                  :            545 nm

Table 2.1.2: Data obtained from the measurement of absorbance of dye solution at different times in the absence of Fenton’s reagent

 Conclusion: Red 8B alone does not undergo any degradation in the absence of FeSO₄ and H₂O₂ solution.

Degradation of the dye solution in presence of H₂O₂ solution

Dye                 :           Red 8B; 50 ppm

H₂O₂(30%)      :           2000 ppm

pH                         :           3.0

l_max                       :          545 nm

Π                    :           0.0216

Table 2.1.3: Data obtained from the Measurement of absorbances of the dye solution as a function of time in presence of 2000 ppm H₂O₂ solution.

 % decomposed =

Conclusions: Red 8B has been degraded 20% in the presence of H₂O₂ solution, and in this particular experiment no FeSO₄ solution was used.

 Degradation of the dye solution in presence of FeSO₄ solution

Dye                 :           Red 8B; 50 ppm

FeSO₄                  :           100 ppm

pH                         :           3.0

l_max                      :           545 nm

Π                    :           0.0216

Table 2.1.4: Data obtained from the Measurement of absorbances of the dye solution as a function of time in presence of 100 ppm FeSO₄ solution.

 % decomposed =

Conclusions: Red 8B has been degraded up to 22% in the presence of FeSO₄ solution and in this particular experiment no H₂O₂ solution was used

Degradation of the dye solution in presence of FeSO₄ (100 ppm ) and H₂O₂ (900 ppm) solution (Fenton’s reagent).

 Dye                 :           Red 8B; 50 ppm

FeSO₄                  :           100 ppm

H₂O₂(30%)      :           900 ppm

pH                         :           3.0

l_max                       :          545 nm

Π                    :           0.0216

Table 2.1.5: Data obtained from the Measurement of absorbances of the dye solution as a function of time in presence of 100 ppm FeSO₄ and 900 ppm H₂O₂ solutions

 % decomposed =

Conclusions: Degradation was found upto 20% in presence of both FeSO₄ and H₂O₂ solutions

Degradation of the dye solution in presence of FeSO₄ (200 ppm)and H₂O₂ (900 ppm) solution (Fenton’s reagent).

Dye                 :           Red 8B; 50 ppm

FeSO₄                  :           200 ppm

H₂O₂(30%)      :           900 ppm

pH                         :           3.0

l_max                      :           545 nm

Π                    :           0.0216

Table2.1.6: Data obtained from the Measurement of absorbances of the dye solution as a function of time in presence of 200 ppm FeSO₄ and 900 ppm H₂O₂ solutions

 % decomposed =

Conclusions: At a fixed dye and H₂O₂ concentration, change in FeSO₄ concentration did not affect the degradation process.

 Degradation of the dye solution in presence of FeSO₄ (100 ppm)  and H₂O₂ (1800 ppm) solution (Fenton’s reagent).

Dye                 :           Red 8B; 50 ppm

FeSO₄                  :           100 ppm

H₂O₂(30%)      :           1800 ppm

pH                         :           3.02

l_max                       :           545 nm

Π                    :           0.0216

Table2.1.7: Data obtained from the Measurement of absorbances of the dye solution as a function of different time in presence of 100 ppm FeSO₄ and 1800 ppm H₂O₂ solutions

 % decomposed =

Conclusions: At a fixed dye concentration and at a fixed FeSO₄ concentration, an increase of H₂O₂ concentration increased the rate of degradation.

Degradation of the dye solution in presence of FeSO₄ (100 ppm)and H₂O₂ (3600 ppm) solution (Fenton’s reagent)

DYE                :           Red 8B; 50 ppm

FeSO₄                  :          100 ppm

H₂O₂(30%)      :          3600 ppm

pH                   :          3.02

l_max                       :          545 nm

Π                    :           0.0216

Table2.1.8: Data obtained from the Measurement of absorbance of the dye solution at different times in presence of 100 ppm FeSO₄ and 3600 ppm H₂O₂ solutions.

 % decomposed =

Conclusions: Again further increase in H₂O₂ concentration significantly increased the rate of degradation

Degradation of the dye solution in presence of FeSO₄ (100 ppm), H₂O₂ (3600 ppm) solution (Fenton’s reagent) and UV light.

DYE                :           Red 8B; 50 ppm

FeSO₄                  :           100 ppm

H₂O₂ (30%)     :           3600 ppm

pH                   :           3.02

l_max                       :           545 nm

Π                    :           0.0216

Table2.1.9: Data obtained from the Measurement of absorbance of the dye solution at different times in presence of 100 ppm FeSO₄, 3600 ppm H₂O₂ and UV light

 Conclusions: In presence of UV light the degradation was up to 69%

2.7.7 Degradation of the dye solution in presence of FeSO₄ (100 ppm), H₂O₂ (5400 ppm) solution (Fenton’s reagent) and UV light

DYE                :           Red 8B; 50 ppm

FeSO₄                  :           100 ppm

H₂O₂(30%)      :           5400 ppm

P^H                    :           3.02

l_max                       :           545 nm

Π                    :           0.0216

Table2.1.9.1: Data obtained from the Measurement of absorbance of the dye solution at different times in presence of 100 ppm FeSO₄, 5400 ppm H₂O₂ and UV light

Conclusions: The rate of degradation was further increased with the increase of the H₂O₂ concentration under UV radiation.

CHAPTER    3

RESULTS AND DISCUSSION

Degradation of dyes by Fenton’s process

^. OH radical has been generated by the interaction with Fe (III) and Fe (II) solutions with hydrogen peroxide ¹⁰. Hydroxyl radicals oxidize the dyes to produce carbon dioxide, water etc. According to Fenton, Fe (II) in solution will undergoes the following reaction

 Fe (II) +H₂O₂  +  H⁺                       Fe (III)  +  ^.OH  +H₂O                (1)

Fe (II) and Fe (III) are ferrous and ferric species in aqueous solution respectively. These could be Fe²⁺ (aq) and Fe³⁺  (aq) or any other ferrous and ferric species capable of reaction with H₂O₂ (aq).

If Fe (III) is initially present following reactions have been suggested for the formation of Fe (II)

Fe (III) + H₂O₂                                      Fe (II) + ^.OOH +H⁺             (2)

Fe (III) +^{. .}OOH                                 Fe (II) +O₂+ H⁺                          (3)

However the reaction (2), which initiates the generation of Fe (III) is three order magnitude slower than the reaction (1). In the presence of light, which excites the dyes, there is an alternative route of generation of Fe (II)

Dye  +  hn                                  Dye^*                         (4)

Dye^* + Fe (III)                                 Dye⁺ + Fe (II)

Finally,

Dye  +  ^.OH                                 products

 Effect of iron concentration

In the absence there is no evidence of hydroxyl radical formation when, for example, H₂O₂ is added to a phenolic wastewater (i. e., no reduction in the level of phenol occurs). As the concentration of iron is increased, phenol removal accelerates until a point is reached where further addition of iron becomes inefficient. This feature (an optimul dose range for iron catalyst) is a characteristic of Fenton’s reagent, although the definition of the range varies between wastewaters.

 Effect of iron type (ferrous or ferric)

For most applications, it does not matter whether Fe²⁺ or Fe³⁺ salts are used to catalyze the reaction – the catalytic cycle begins quickly if H₂O₂ and organic material are in abundance. However if low doses of Fenton’s reagent are being used (e.g., < 10-25 mg/L H₂O₂), some research suggests ferrous iron may be preferred. Neither does it matter whether a chloride or sulphate salts of the iron is used, although with the former, chlorine may be generated at high rates of application.

It is also possible to recycle the iron following the reaction. This can be done by raising the pH, separating the iron floc, and re-acidifying the iron sludge.

 Effect of Hydrogen peroxide

Experiments showed that Red 8B was not degraded simply by iron catalyst or hydrogen peroxide or by light. The presence of H₂O₂ was essential, as H₂O₂ helped generating ^.OH radicals for the degradation of the dye to be initiated. The effect of the concentration of H₂O₂ on the degradation of Red 8B are shown in Fig.   And fig.   Respectively. The higher the concentration of H₂O₂ tends to minimize the induction period of decomposition of dyes. If the concentration of H₂O₂ is sufficientlyincreased, the induction period of decomposition of dyes seem to be eliminated.

Effect of pH

The optimal pH occurs between pH 3 and pH 6.The drop in efficiency on the basic side is attributed to the transition of iron from a hydrated ferrous ion to a colloidal ferric species. In the latter form, iron catalytically decomposes the H₂O₂ into oxygen and water, without forming hydroxyl radicals.

Effect of reaction time

The time needed to complete a Fenton reaction will depend on many variables discussed above, most notably catalyst dose and wastewater strength. For simple phenol oxidation, typical reaction times are 30-60 minutes. For more complex or more concentrated wastes, the reaction may take several hours. In such cases performing the reaction in steps (adding both iron and H₂O₂) may be more effective (and safer) than increasing the initial charges.

Determining the completion of the reaction may prove troublesome. The presence of residual H₂O₂ will interfere with many wastewater analyses. Residual H₂O₂  may be removed by raising the pH to e.g., pH 7-10. Often, observing color changes can used to access the reaction progression. Wastewaters will typically darken upon H₂O₂ addition and clear up as the reaction reaches completion.

Effect of temperature

The rate of reaction with Fenton’s reagent increases with increasing temperature, with the effect more pronounced at temperatures < 20 ^o C. However as temperature increases above 40-50 ^o C, the efficiency of H₂O₂ utilization declines. This is due to the accelerated decomposition of H₂O₂ into oxygen and water. As a practical matter, most commercial applications of Fenton’s reagent occur at temperatures between 20-40 ^o C. And in this particular work temperature was always within a range of 29-31 ^o C ( or room temperature). Moderating the temperature is important not only for economic reasons, but for safety reasons as well.

 Table: 3.1 The effects of variables on the degradation of Red 8B by Fenton and photo Fenton processes

Conclusion

Spectrophotometric method has been used to analyze the textile dye Red 8B in aqueous solutions. The dye has absorption maxima in the visible range (545 nm). Dilute solution of the dye obeys Beer-Lambert law.

The spectrum of Red 8B remains unaffected throughout our study. The dye undergoes degradation in the dark by Fenton process. As the rate of degradation is pH dependent, and the optimum pHrange is 3-5, so the pH   of the dye solution was kept » 3 throughout the investigation. Higher (%) degradation is obtained as time goes on but after a certain period the change is very negligible. The experiment was also carried out in presence of UV radiation. And it has been found that even UV radiation alone cannot degrade the aqueous solution of the dye, but in presence of Fenton’s reagent. UV radiation can make the degradation rate even faster. The whole work was carried out at room temperatures.

CHAPTER    4

REFERENCES

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Van Der Zee. F. P. 2002. Anaerobic azo dye reduction. Doctoral Thesis, Wageningen University. Wageningen, The Netherlands, 142 pages

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