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Pollution is the introduction of a contaminant into the environment. It is created mostly by human actions, but can also be a result of natural disasters. Pollution has a detrimental effect on any living organism in an environment, making it virtually impossible to sustain life when the water is no longer pure and contains pathogens or chemical impurities. All these impurities decrease and lower the quality of the water and can have serious effects on the aquatic life. Water pollution happens in oceans, rivers, lakes, and even in fresh water and water reservoirs.
Pollution is caused by the discharge of large amount of wastewater in to the environment as a result of rapid industrialization and urbanization. Many industries like textile, refineries and chemical, plastic, pharmaceutical and food processing plants produce wastewater characterized by a perceptible content of organics with strong color, high Chemical Oxygen Demand (COD) and with wide variation in pH values.
Hazardous wastes are a continuous problem in today's world, increasing in both quantity and toxicity. Besides inorganic materials, industrial effluents also contain organic pollutants and radioactive chemical toxic materials. Many treatment technologies are in use and have been proposed for the recovery or destruction of these pollutants. These include activated carbon adsorption, solvent extraction for recovery of chemical, electrochemical oxidation for destruction, direct incineration, chemical destruction and even direct immobilization in matrix like cement, polymer, etc. In addition, management of hazardous organic mixed wastes can be done by employing techniques like wet oxidation, photochemical oxidation and electrochemical oxidation. Of these, electrochemical oxidation offers an attractive way of treating solid or liquid organic wastes as it uses electron as a reactant. The electrochemical destruction of hazardous and mixed organic wastes could be carried out by Direct Electrochemical Oxidation (DEO) or by Mediated Electrochemical Oxidation (MEO). A wide range of organic materials can be destroyed by this technique. The process is extensively employed for nuclear industry application, rubber, some plastics, polyurethane, ion exchange resins of various types and hydraulic and lubricating oils, aliphatic and aromatic compounds, chlorinated aliphatic and aromatic compounds (Bersier et al. 1994).
Wastewater treatment serves two main objectives, protecting the environment and conserving the fresh water resources. Many efforts have been made for the biological treatment on wastewater rich in organic compounds. Effluents can be treated by biological methods, flocculation, reverse osmosis, adsorption on activated charcoal, chemical oxidation methods and advanced oxidation process.
An increasing world population with growing industrial demands has led to the situation where the protection of the environment has become a major issue and a crucial factor for the future development of industrial processes, which will have to meet the requirements of sustainable development. Electrochemistry offers promising approaches for the prevention of pollution problems in the process industry. The inherent advantage is its environmental compatibility, due to the fact that the main reagent, the electron, is a 'clean reagent'. The strategies include both the treatment of effluents and waste and the development of new processes or products with less harmful effects, often denoted as process-integrated environmental protection.
Few facts about water pollution are:
Over two-thirds of U.S. estuaries and bays are severely degraded because of nitrogen and phosphorous pollution.
Every year almost 25percent of U.S. beaches are closed at least once because of water pollution.
Over 73 different kinds of pesticides have been found in the groundwater that we eventually use to drink.
1.2 trillion gallons of sewage, storm water and industrial waste are discharged into U.S. waters every year.
40percent of U.S. Rivers are too polluted for aquatic life to survive.
Americans use over 2.2 billion pounds of pesticides every year, which eventually washes into the rivers and lakes (Green Student U, 2010).
1.1 Wastewater characteristics and causes
Wastewater is divided into four types hard-to-treat, highly dispersible, hazardous and toxic, and large volume wastes. Each of these four waste types can be found in all chemical industries and all have their specific characteristics. Treatment is easiest if each waste is considered separately before being combined. Difficult to treat wastes may include dyes, metals, phenols and toxic compounds. This type of waste is resistant to conventional biological treatment and when passed through the treatment system, the treated water sometimes causes toxic effects.
Hydroquinone occurs in the environment as a result of man-made processes as well as in natural products from plants and animals. Due to its physicochemical properties, hydroquinone will be distributed mainly to the water compartment when released into the environment. It degrades both as a result of photochemical and biological processes; consequently, it does not persist in the environment and bioaccumulation was not observed. No data on hydroquinone concentrations in air, soil or water have been found. However, hydroquinone has been measured in mainstream smoke from non-filter cigarettes in amounts varying from 110 to 300μg per cigarette, and also in side stream smoke. Hydroquinone has been found in plant-derived food products (e.g., wheat germ), in brewed coffee, and in tea prepared from the leaves of some berries where the concentration sometimes exceeds 1percent (World Health Organization Geneva, 1996).
Hydroquinone is used as a raw material for herbicides, dyestuff and cosmetic products. It is highly toxic to human and aquatic organisms. Cases of intoxication and deaths have been reported after oral ingestion of hydroquinone alone or of photographic developing agents containing hydroquinone. The major signs of poisoning include dark urine, vomiting, abdominal pain, tachycardia, tremors, convulsion and coma.
Irritation has occurred at exposure level of 2.25mg/m3. In a controlled oral study on human volunteers, ingestion of 300-500mg hydroquinone daily for 3-5months did not produce any observable pathological changes in the blood and urine. Its presence in water and soil has become a significant pollution problem, and effective methods for the removal or treatment need to be pursued (World Health Organization Geneva, 1996).
Dermal applications of hydroquinone at concentrations in different bases of less than 3percent caused negligible effects in male volunteers from different human races. However, there are case reports suggesting that skin lightening creams containing 2percent hydroquinone have produced leukoderma, as well as ochronosis. Hydroquinone (1percent aqueous solution or 5percent cream) has caused irritation (erythema or staining). Allergic contact dermatitis due to hydroquinone has been diagnosed (World Health Organization Geneva, 1996).
Combined exposure to airborne concentrations of hydroquinone and quinone causes eye irritation, sensitivity to light, injury of the corneal epithelium, corneal ulcers, and visual disturbances. There have been cases of appreciable loss of vision. Long-term exposure causes staining of the conjunctiva and cornea, and also opacity. Slowly developing inflammation and discolorations of the cornea and conjunctiva have resulted after daily hydroquinone exposure to hydroquinone for at least two years to levels of 0.05-14.4mg/m3 and serious cases have not occurred until five or more years. One report described cases of corneal damage occurring after several years of exposure to hydroquinone. There are no adequate epidemiological data to assess the carcinogenicity of hydroquinone in humans (World Health Organization Geneva, 1996).
Phenol compounds have been widely used as important raw materials such as solvents, chemical cleaning agents, pesticides, insecticides, herbicides and various synthetic compounds. They are typical bio-refractory organic compounds. Most of these compounds are known by their high toxicity levels and their persistence, and thus their recovery or their elimination is required prior to the discharge or the reuse of the waste flow. The ever increasing discharges of these compounds into the environment have become a potential threat to human health. Therefore, these kinds of wastewaters before being discharged into the environment must be treated to meet not only the ever-increasing stringent legislations demands but also the aesthetic standards (Dai et al. 2008).
Aromatic nitro compounds are among the most toxic substances and are commonly used in the manufacture of explosives, pesticides, dyes, plasticizers and pharmaceuticals (Munnecke 1976; Hallas and Alexander 1983; Spain and Gibson 1991; Hanne et al. 1993). These compounds have been detected not only in industrial wastewaters but also in freshwater and marine environments (Lypczynska-Kochany 1992). In particular, para nitrophenol is a toxic derivative of the parathion insecticide and is present in wastewaters from industries such as refineries. Detoxification of water contaminated with nitro aromatic compounds is usually a very difficult process since the presence of a nitro-group confers to the aromatic compound a strong chemical stability and resistant to microbial degradation (Lanouette 1977).
Chlorophenols constitute a significant category of pollutants and cause considerable environmental pollution problems when discharged into surface water sources without proper treatment. Chlorophenols are highly toxic, poorly biodegradable and present carcinogenic and recalcitrant properties. Chlorophenols have been used as wood preservatives, in pesticides, herbicides, insecticides, fungicides, flame retardants, solvents, paint, petrochemical and plastics, glue and in the paper industry. The generalized use of chloroorganic compounds, persistent and highly water-soluble pollutants, and their presence in industrial and urban wastewater, led to accumulation in the environment, and therefore, a serious pollution problem.
Research studies of Kinzell et al. (1979) and Haggblom and Valo (1995) revealed that 2,4-Dichlorophenol and 4-Chlorophenol are widely used in the production of pentachlorophenol, 2,4-Dichlorophenoxyacetic acid and 2,4,5-Trichlorophenoxyacetic acid. In addition, 2, 4-Dichlorophenol has been extensively used as a wood preservative and also as pesticide and as precursor for the synthesis of herbicides (Rape 1980).
Chlorophenol compounds are resistant to the biological degradation because of the toxic effects of such compounds on microorganisms. Adsorption and ion exchange methods are extensively used to concentrate the chlorophenols on the solid phase, which require further treatment by chemical or biological oxidation for complete mineralization. Chemical oxidation methods are fast, but are expensive and may result in the formation of undesirable by-products. Research studies reveal that biodegradation of chlorophenols by aerobic or anaerobic treatment methods is more specific and relatively inexpensive (Annachatre and Gheewala et al. 1996; Armenante et al. 1999; Atuanya et al. 2000; Bali and Sengul 2002). Usually, a carbohydrate substrate is used as a primary metabolite and the chlorophenols are the co-metabolites in the biodegradation of chlorophenols (Hill et al. 1996; Kim and Hao 1999; Wang and Loh 1999). Palmas et al. (2007) put forth an interesting possibility for the treatment of chlorophenol compounds. He proposed to couple partial electrochemical technologies and biological treatment in order to decrease the toxicity and to increase the biodegradability of the wastewater before biological treatment.
Electrochemical technology has an important role to play as a part of an integrated approach in avoiding pollution, monitoring pollution and increasing process efficiency and also cleaner processing. Electrochemical treatment is one such technique which can either be used as main treatment scheme or as a hybrid technique. Further, it can also be used as a pretreatment scheme for difficult wastewaters for which feasible technologies are not available. Treatment of these wastewaters may require segregation of the culprit wastewater streams at source and pretreatment prior to addition to other wastewaters for treatment and disposal (Walsh 2001).
Chiba (1993) adopted the MEO process by taking 0.5 M Ag(II) as mediator in sulphuric acid medium for the destruction of a number of organic compounds. MEO is an upcoming and one of the most promising technologies extensively used for the destruction of organics since; it is capable of mineralizing the organics totally into carbon dioxide (CO2) and water (H2O) completely, without the emission of any toxic materials like dioxins (Farmer et al.1991; 1992; Steele et al.1992; Chiba et al.1995; Nelson 2001). Direct Electro Oxidation processes have been carried out for a variety of organic compounds. In these processes, the organic compounds are oxidized to CO2 and H2O at the anodic surface. The MEO process is employed for the destruction of various kinds of toxic and refractory organic pollutants (GEF, Report of UNEP, 2004). This process employs an electrochemical cell to generate the oxidizing species and uses the same to destroy the organics at an ambient temperature and at atmospheric pressure (Galla et al. 2000; Turner, 2002). The oxidizing species (mediator ions) are produced at the anode in an acidic medium and are used to destroy the organic compounds into CO2 and H2O. Since the mediated metal ions have a strong potential to oxidize, high temperature is not required for organic oxidation and as a consequence, less volatile and off gases are produced. Several metal oxidizing agents like Ag(II)/Ag(I), Ce(IV)/Ce(III), Co(III)/Co(II), etc., have been tested previously in the MEO process both in pilot and commercial scale systems (Farmer et al. 1992). Bringmann et al. (1998) in their work used Ag(II)-MEO system for the destruction of hydrocarbons and pesticides in sulphuric acid medium. Research studies provide ample evidence that phenol is one of the most common pollutants found in the effluents of many industries such as pharmaceuticals, dyes, synthetic chemical plants, petroleum refineries, pesticides and herbicides treated by several technologies (Lippincot 1990; Comninellis and Pulgarin 1991, 1993; Pifer et al. 1999; Esplugas et al. 2002; Feng and Li 2003). Ince (1999) had found that the Fenton's reagent and the photo-oxidation using UV/H2O2, UV/TiO2 or UV/H2O2/Fe2+ systems were effective in the elimination of organic compounds dissolved or dispersed in aqueous media. Boye et al. (2002) and Oturan (2000) had established that the electro-fenton process was effective in the elimination of synthetic dye mixture from water and the strong odour and ecotoxicological consequences. Raju and Basha (2005) in their study had identified that the mediated metal ions have a strong potential to oxidize and a high temperature was not required for the organic oxidation and therefore, less volatile and off gases were produced.
It may be mentioned here that the advantages of electrochemical approach, compared to several other chemical tools have been well recognized for the recovery of metals in their metallic form from metal ion pollutants because the electrochemical methods are relatively simple and clean (Hwang et al. 1987; Kusakabe et al. 1986; Beauchesne et al. 2005; Chen and Lim 2005). Moreover as the conversion of the response of a chemical reaction or a process into a measurable electronic signal is direct and precise in electrochemical methods (as current or potential), regulation and automation are easier to achieve with them in comparison to the chemical techniques. Since the MEO process minimizes additional treatment of the effluents the process has been identified as one of the best way to reduce Total Organic Content (TOC) for resin effluents using batch recirculation method.
Much effort has been made for the biological treatment of wastewater rich in organic compounds. The microorganisms such as Pseudomonas sp, (Chitra et al. 1995) Phaneuochaere sp (Perez et al. 1997) have been used for the degradation of organic compounds. Effluents can be treated by biological methods, flocculation, and reverse osmosis, adsorption on activated charcoal, chemical oxidation methods and advanced oxidation processes (Slokar and LeMarechal 1998). Flocculation, reverse osmosis and adsorption methods transfer the pollutants from one media to other, thus causing secondary pollution (Tanaka et al. 2000; Suksaroj et al. 2005) and chemical oxidation methods are not cost effective (Baban et al. 2003). The chemical coagulation may induce secondary pollution caused by added chemical substances. These disadvantages encouraged many studies on the use of electro coagulation for the treatment of several industrial effluents containing refractory organic contaminants (Adhoum et al. 2004).
1.2 Refractory organic materials
These organics tend to resist congenital methods of wastewater treatment. Typical examples include surfactants, phenols, and agricultural pesticides. A waste stream is termed recalcitrant if it is resistant to biological treatment, because the chemical compounds present are chemically and metabolically inert and are toxic to the microorganisms. Toxicity in particular, can be a major problem as it can effectively shut down a biological treatment plant.
1.3 Electrochemistry and environment
Grimm et al. (1998) in his article has stated that electrochemistry as a branch of physical chemistry plays an important role in most areas of science and technology. The electrochemical technique offers promising approaches for the prevention of pollution in the process industry. The inherent advantage is its environmental compatibility, due to the fact that it uses a clean reagent, i.e. the electron. The strategies include both the treatment of effluents and waste and the development of new processes or products with less harmful effects, often denoted as process-integrated environmental protection (Tennakoon et al. 1996). Electrochemical technologies have gained importance during the past two decades. At present, electrochemical technologies have reached such a state that they are not only comparable with other technologies in terms of cost, but sometimes they are more efficient and compact. The development, design and application of electrochemical technologies in water and wastewater treatment have been focused on particularly in some technologies such as electrodeposition, electrocoagulation, electroflocculation, and electrooxidation (Rajeshwar et al.1994).
1.4 Advantages and disadvantages of electrochemical treatment
Bockris and Drazic (1972) have discussed the advantages and disadvantages of electrochemical treatment.
The main media used is electron, which is harmless, and usually there is no need for adding extra chemicals which may introduce additional pollutants.
No generation of secondary pollutants and the process is eco-friendly.
Easy automation of processing, as current and voltage are the only variables.
Low temperature requirements than that of non electrochemical processes (e.g. Thermal incineration)
Capability of continuous operation with constant efficiency in process with complete automation for easy monitoring of process parameters.
Applicability for various processes like direct or indirect oxidation, reduction, phase separation, concentration, or dilution which deal with pollutants that are in the gaseous, liquid and solid states which can be treated from millilitres to millions of litre.
Whenever possible, recovery of valuable metals can be aimed at.
The synergic effect of low temperature and the presence of inhibitory substances can lead to the block of nitrification of industrial wastewater even at 15-16oC.
In this context, electrochemical process being independent of temperature range as experienced for tannery wastewater can be an interesting alternative to traditional biological treatment.
The 'sacrificial electrodes' are dissolved into wastewater streams as a result of oxidation, and need to be regularly replaced.
The use of electricity may be expensive in many places.
An impermeable oxide film may be formed on the cathode leading to loss of efficiency of this unit.
High conductivity of the wastewater suspension is required. Gelatinous hydroxide may tend to solubilize in some cases.
1.5 Electrochemical methods
The application of electrochemistry for the protection of the environment has already been the topic of several books and reviews (Bockris and Drazic 1972; Pletcher and Walsh 1990; Brito and Sequeira 1994; Rajeshwar et al. 1994; Tatapudi and Fenton 1995; Rajeshwar and Ibanez 1997; Simonsson 1997). Besides the process oriented benefits, electrochemistry is also playing a key role in sensor technology. Electro analytical techniques for monitoring and trace level detection of pollutants in air, water and soil as well as of micro-organisms are needed for process automation. An interesting view on the role of electro catalysis for electrochemistry and environment was given by De Pauli and Trasatti (1995).
Electrical Ion exchange
Combined electrical treatments
Figure 1.1: Electrochemical methods
Using electricity to treat wastewater was first proposed in England in 1889. The application of electrolysis in mineral beneficiation was patented by Elmore in 1904 (Elmore 1905). Electrocoagulation with aluminum and iron electrodes was patented in the United States in 1909. The electrocoagulation of drinking water was first applied on a large scale in the United States in 1946 (Stuart 1946; Bonilla 1947). Because of the relatively large capital investment and the expensive electricity supply, electrochemical of water or wastewater technologies did not find wide application worldwide initially. However, in the United States and the former USSR, extensive research during the following half century has accumulated abundant amount of knowledge. With the ever increasing standard of drinking water supply and the stringent environmental regulations regarding the wastewater discharge, electrochemical technologies have regained their importance worldwide during the past two decades. There are companies supplying facilities for metal recoveries, for treating drinking water or process water, treating various wastewaters resulting from tannery, electroplating, dairy, textile processing, oil and oil in water emulsion, and so on. Nowadays, electrochemical technologies have reached such a state that they are not only comparable with other technologies in terms of cost but are also more efficient and more compact (Yu and Ji 1993; Goodridge and Scott, 1995; Scott 1995; Rajeshwar and Ibanez 1997).
A toxic industrial effluent and a recalcitrant organic compound leads to severe environmental problems. Traditional treatment methods like biological, chemical and physical treatments are ineffective and thus a number of alternatives have been studied such as super critical water oxidation, photo chemical oxidation etc. Electrochemical degradation is one of the alternatives for the degradation of these compounds, and it is suitable for low volume application and environmental compatibility.
Biological treatment of chlorophenols attracts more attention than physical and chemical methods, because a variety of microorganisms such as Pseudomonas pickettii, Alcalilgenes eutrophus, Desulfomonile tiedjei, Phanerochaete chrysosporium since they utilize chlorophenols as the sole carbon or energy source (Mohn and Kennedy 1992; Fava et al. 1995; Hill et al. 1996; Perez et al. 1997).
However, conventional biological treatments often fail to achieve high efficiency in removing chlorophenols from wastewater due to the toxicity or inhibition of chlorophenols to microorganisms. In addition, chlorophenol degrading populations in bio-treatment plants are likely small and often account for a small fraction of the total organics in influent (Makinen et al. 1993). Here, the objective of the pre-treatment is to decrease the toxicity and to increase the biodegradability of the wastewater before biological treatment.
1.6 Electrochemical treatment
The electrochemical method is based on the well-known basic principles of electrochemical reactions such as anodic oxidation, cathodic reduction, deposition, ionic transport through a membrane etc. Depending on the nature of the pollutant, with due considerations for the toxicity of the reactants and products, an appropriate choice of the methods has to be made. Rajeshwar and Ibanez (1997) in their article have declared that it may be even necessary to use more than one method for getting effective treatment of effluent.
In electrochemical treatment, the wastewater is treated by applying electric current through electrodes in a reactor. The electrodes generate positive and negative ions which combine to form metal hydroxide flocs. These metal hydroxide flocs combine with pollutant particles and settle down. Application of electric current also generates oxidizing groups like HOCl which destroy or oxidize organic pollutants. Along with ion generation, gas bubbles are also generated from the cathode in the form of hydrogen gas. These gas bubbles stick to the pollutant particles and float them to the surface of water. This technology is adopted for the removal of metals, colloidal solids and particles, and soluble inorganic pollutants from aqueous media by introducing highly charged polymeric metal hydroxide species.
1.7 Electrochemical degradation
The advantages of using electrochemical techniques (electrooxidation and electrocoagulation) are: environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability to automation, and cost effectiveness (Rajeshwar et al. 1994). The use of electrochemical technology has been widely studied as a method for the removal of organic substances (Juttner et al. 2000; Chen 2004). Vlyssides et al. (2004) identified that good removal rates were observed and they suggested that the electrochemical method could be used as a pre-treatment step in pesticide waste disposal.
The dimensionally stable anodes are promising materials for many electro-organic applications and have been classified as 'active' or 'non-active', depending on its chemical nature (Simond et al. 1997; Malpass and Motheo 2001). Active electrodes mediate the oxidation of organic species via the formation of higher oxidation states of the metal (MOx+1). Whenever a higher oxidation state can be reached by the metal oxide (e.g., RuO2 or IrO2) it leads to selective oxidation that makes the non-biocompatible organic effluent to biocompatible organic compounds. Non-active electrodes present no higher oxidation state available and the organic species is directly oxidized by an adsorbed hydroxyl radical, generally resulting in incomplete combustion of the organic molecule (e.g., SnO2 or PbO2).
The mechanism of electrochemical oxidation of wastewater is a complex phenomenon involving coupling of electron transfer reaction with a dissociate chemisorption step. Basically, two different processes occur at the anode or cathode. In anodes having high electro-catalytic activity, oxidation occurs at the electrode surface (direct electrolysis). On the other hand on metal oxide electrode, oxidation occurs via surface mediator on the anodic surface continuously (indirect electrolysis). In direct electrolysis, the rate of oxidation depends on electrode activity, diffusion rate of pollutants and current density. A generalized scheme of the electrochemical conversion of organics on noble oxide coated catalytic anode (MOx) is shown in Figure 1.2. In the first step, H2O is discharged at the anode to produce adsorbed hydroxyl radicals according to the reaction.
MOx +H2O → MOx(•OH) + H+ +e− ----------------------- (1)
Figure 1.2: Scheme of the electrochemical conversion/ combustion of organics on noble oxide coated catalytic anode
In the second step, the adsorbed hydroxyl radicals may interact with the oxygen already present in the oxide anode with possible transition of oxygen from the adsorbed hydroxyl radical to the oxide forming the higher oxide MOx+1.
MOx(•OH) → MOx+1 +H+ +e− --------------------(2)
At the anode surface, the active oxygen can be present in two states. Either as physisorbed hydroxyl radicals (•OH) and as chemisorbed (oxygen in the lattice, MOx+1). In the absence of oxidizable organics, the active oxygen produces dioxygen according to the following reactions:
MOx (•OH) → MOx + O2 + H+ + e− ---------- (3)
MOx+1 → MOx + O2 ------------------------------- (4)
When NaCl is used as a supporting electrolyte, Cl ion may anodically react with MOx(•OH) to form adsorbed OCl radicals according to the following reaction:
MOx (•OH) + Cl−→ MOx (•OCl) + H+ +2e− --------- (5)
Further, in the presence of Cl ion, the adsorbed hypochlorite radicals may interact with the oxygen already present in the oxide anode with possible transition of oxygen from the adsorbed hypochlorite radical to the oxide forming the higher oxide MOx+1 according to the following reaction and also MOx (•OCl) simultaneously react with chloride ion to generate active oxygen (dioxygen) and chlorine according to the following reactions:
MOx (•OCl) + Cl−→ MOx+1 +Cl2 +e− ----------- (6)
MOx (•OCl) + Cl− → MOx + O2 + Cl2 + e− ----------- (7)
In the presence of oxidizable organics, the physisorbed active oxygen (•OH) could cause predominantly the complete combustion of organics and chemisorbed will participate in the formation of selective oxidation products according to the following reactions (Simond et al. 1997; Malpass and Motheo 2001).
R + MOx (•OH) → ROO + H+ + e− + MOx ------------- (8)
R + MOx +1→ RO + MOx --------------- (9)
The physisorbed route of oxidation is the preferable way for waste treatment. Since the organic hydrogen peroxides formed are relatively unstable, decomposition of such intermediates leads to molecular breakdown and formation of subsequent intermediates with lower carbon numbers. These sequential reactions continue until the formation of carbon dioxide and water (Bindu et al. 2000; Raghu and Basha 2007). In this case, the diffusion rate of organics on the anode area controls the combustion rate (Buso et al. 2000; Panizza et al. 2001). On the other hand, temperature, pH and diffusion rate of generated oxidants determine the rate of oxidation in indirect electrolysis. In the same way, indirect electrochemical oxidation mechanism has been proposed for metal oxide with chloride as supporting electrolyte for wastewater treatment (Comninellis and Pulgarin 1993; Miwa et al. 2006; Malpass et al. 2007). In indirect electrooxidation, chloride salts of sodium are added to the wastewater for better conductivity and generation of hypochlorite ions. The reactions of anodic oxidation of chloride ions to form chlorine in bulk of solution is given as
Cl2 + H2O H+ + Cl− + HOCl ------------------ (10)
HOCl H+ + OCl− ------------------------------- (11)
Organic + OCl− CO2 + H2O + Cl− + P ----- (12)
Since organic compounds of the effluent are electrochemically inactive, the primary reaction occurs at the anodes is chloride ion oxidation (Equations 6 and 7) with the liberation of Cl2, which is a robust oxidizing agent. As regards to the reactions in the bulk, gaseous Cl2 dissolves in aqueous solution due to ionization as indicated in equation 10. The rate reaction is less in acidic solution due to OH− instability and considerably more in basic solution due to ready formation of OCl− (pKa 7.44) ion in equation 11 implying that the basic or neutral pH conditions are more favorable for conducting reactions involving chlorine. The direct electrooxidation rate of organic pollutants depends on the catalytic activity of the anode, on the diffusion rate of the organic compounds in the active points of anode and applied current density. The indirect electrooxidation rate of organic pollutants depends on the diffusion rate of the oxidants into the solution, flow rate of the effluent, temperature and the pH.
This is a process involving many chemical and physical phenomena in which the coagulating ions are produced in situ and it involves three successive stages: formation of coagulants by electrolytic oxidation of the sacrificial electrode, destabilization of the contaminants, particulate suspension (and breaking of emulsions) and aggregation of the destabilized phases to form flocs. Electrocoagulation has been successfully employed in industrial effluents for removal of organic contaminants, oil and greases. In this process, a potential is applied to the metal anodes, typically fabricated from either iron or aluminum, which causes two separate reactions.
The electrocoagulation process is based on the continuous in situ production of a coagulant in the contaminated water. It had been shown that electrocoagulation is able to eliminate a variety of pollutants from wastewaters; for example metal and arsenic (Ratna Kumar et al. 2004; Gao et al. 2005; Hunsom et al. 2005), clay minerals (Matteson et al. 1995; Holt et al. 2004), oil (Xu and Zhu 2004), chemical oxygen demand (Murugananthan et al. 2004; Xu and Zhu 2004) and color and organic substances (Jiang et al. 2002). Electro coagulation is an effective technique for the treatment of effluent of various origins (Mameri et al. 1998). Compared with the traditional flocculation and coagulation methods electrocoagulation has the advantage of removing even the smallest charged particles because of the electric field that sets them in motion. The main features of electrocoagulation are simple construction and easy operation, brief reactive retention period, less equipment for adding chemicals and decreased amount of sludge formation (Mollah et al. 2001). Therefore electrocoagulation has been successfully used to treat water containing food and protein wastes, synthetic detergents, effluent mine wastes and heavy metal containing solutions (Lin and Peng 1996; Tsai et al. 1997; Lin et al. 1998). Hence, electrocoagulation has been attracting with special interest in all applications of electrochemical treatment for the purification of wastewater.
1.10 Types of biological processes for wastewater treatment
The principle biological processes used for the wastewater treatment can be divided into two main categories: suspended growth and attached growth (or biofilm) processes.
1.10.1 Suspended growth processes
In suspended growth processes, the microorganisms responsible for treatment are maintained in liquid suspension by appropriate mixing methods. Many suspended growth processes used in industrial wastewater treatment are operated with a positive dissolved oxygen concentration (aerobic), but applications exist where suspended growth anaerobic (no oxygen present) reactors are used, such as for high organic concentration in industrial wastewater and organic sludge.
1.10.2 Attached growth processes
In an attached growth processes, the microorganisms responsible for the conversion of organic matter or nutrients are attached to an inert packing material. The organic material and nutrients are removed from wastewater flowing past the attached growth also known as biofilm. The packing materials used in attached growth processes include rocks, gravel, slag, sand, redwood, and a wide range of plastic and other synthetic materials. Attached growth processes can also be operated as aerobic or anaerobic processes. The packing can be submerged completely in liquid or not submerged, with air or gas space above the biofilm liquid layer. The most common aerobic attached growth process is trickling filter in which wastewater is distributed over the top area of a vessel containing non-submerged packing material.
1.11 Process according to operational conditions
1.11.1 Aerobic process
The process that essentially requires the presence of molecular oxygen for metabolic activity of microorganisms is called an aerobic process. The process can be designed to supply required oxygen either naturally (as in trickling filter, aerobic stabilization ponds) or by artificial/mechanical means (as in activated sludge processes, aerated lagoons) in the reactor. The process normally fails in the absence of oxygen. Aerobic biological treatment basically involves stabilization of organic content of wastewater by the mixed population of microorganisms. In the simplest term, during the stabilization of organic content, biodegradable organic matter is oxidized or synthesized by microorganisms in aerobic conditions to produce new cells, inert solids and other simple end products.
1.11.2 Stoichiometry of aerobic biological oxidation
In aerobic oxidation, the conversion of organic matter is carried out by mixed bacterial cultures in general accordance with the stoichiometry shown below:
Oxidation and synthesis:
CONHS (organic matter) + O2 + nutrients ƒ CO2 + NH3 + C5H7NO2 + other end products ---------- (1)
C5H7NO2 + 5O2ƒ CO2 + NH3 + 2H2O + energy ----------- (2)
In equation (1), CONHS is used to represent the organic matter in wastewater, which serves as the electron donor while the oxygen serves as the electron acceptor. Although the endogenous respiration equation (2) is shown as resulting in relatively simple end products and energy, stable organic end products are also formed (Tchobanoglous and Burtun 1991).
With proper environmental conditions, seed source, and acclimation time, a wide range of toxic and recalcitrant organic compounds have been found to serve as growth substrates for heterotrophic bacteria. Such compounds include phenol, benzene, toluene, polyaromatic hydrocarbons, pesticides, alcohols, ketones, methylene chloride, vinyl chloride, and chlorinated phenols. However, many chlorinated organic compounds cannot be attacked readily by aerobic heterotrophic bacteria and thus do not serve as growth substrates. Fortunately, a number of chlorinated organic compounds are degradable by cometabolic degradation. It should be noted that organic compounds that are saturated fully with chlorine are degraded only by anaerobic dechlorination.Cometabolic degradation is possible by bacteria that produce nonspecific mono-oxygenase or dioxygenase enzymes. These enzymes mediate a reaction with oxygen and hydrogen.
1.11.3 Anaerobic process
The process that operates in the absence of molecular oxygen in the reactor for the growth of microbes and normally fails in the presence of excessive oxygen is called anaerobic process, e.g. anaerobic sludge digester, and anaerobic up-flow filters. For the treatment of high strength industrial wastewater, anaerobic treatment has shown to provide a very cost-effective alternative to aerobic processes with saving in energy, nutrient, and reactor volume. Because the quality is not as good as that obtained with aerobic treatment, anaerobic treatment is commonly used as a pretreatment step process to an aerobic. Three basic steps are involved in the overall anaerobic oxidation of a waste: (1) hydrolysis, (2) fermentation (also known as acidogenesis), and (3) methanogenesis. Many toxic organic compounds are degraded under anaerobic conditions, with the compound serving as a growth substrate with fermentation and ultimate methane production. Typical examples include non-halogenated aromatic and aliphatic compounds such as phenol, toluene, alcohols and ketones. However, most chlorinated organic compounds are not attacked easily under anaerobic conditions and do not serve as growth substrate. Fortuitously, many of these compounds serve as electron acceptors in anaerobic oxidation reduction reaction and hydrogen produced in fermentation reactions provides the main electron donor. Hydrogen replaces chlorine in the molecule, and such reactions have generally been referred to as anaerobic dehalogenation or anaerobic dechlorination.
1.11.4 Anoxic process
This is a biological process in which microbes convert the nitrate and nitrogen of wastewater into nitrogen gas in the absence of oxygen. It is also known as denitrification process. Biological denitrification involves the biological oxidation of many organic substrates in wastewater treatment using nitrate or nitrite as the electron acceptor instead of oxygen. In the absence of dissolved oxygen or under limited dissolved oxygen concentrations, the nitrate reductase enzyme in the electron transport respiratory chain is induced, and helps to transfer hydrogen and electrons to nitrate as a terminal electron acceptor. The nitrate reduction reactions involve the following steps from nitrate to nitrite, to nitric oxide, to nitrous oxide, and to nitrogen gas. (Tchobanoglous and Burtun 1991)
NO3- ƒ NO2- ƒ NO ƒ N2O ƒ N2 ------------ (3)
1.12 Objectives of the present study
The main objective of the thesis is to investigate electrochemical approaches for the degradation of selected refractory organics. In this study, the electrochemical parameters are mainly considered for the treatment of Resin, Hydroquinone, 2,4-Dichlorophenol and para nitrophenol.
To study the effect of TOC reduction by electrochemical oxidation on synthetic resin effluent using RuO2/Ti by batch and batch recirculation process.
To study the effect of COD reduction by electrocoagulation on Hydroquinone removal from water using flow electrolyser in both Monopolar and Bipolar configurations in a batch recirculation mode of operation.
To study the effect of COD reduction by combined electrochemical oxidation and biological processes on wastewater containing 2,4-Dichlorophenol optimization using response surface methodology.
To study the effect of COD reduction by combined electrochemical oxidation and biological processes on wastewater containing para nitrophenol optimization using response surface methodology
To study optimum COD reduction on Hydroquinone, 2,4-Dichlorophenol and para nitrophenol by varying various operational parameters like current density, supporting electrolyte concentration, flow rate, volume of effluent, time of electrolysis, and concentration of effluent using response surface methodology.