pollution problem - Shodhganga

Chapter-1 Introduction and Literature Survey ----------------------------------------------------------------------------------------------------

POLLUTION PROBLEM Pollution is a problem for mankind on this earth. Before 19th century with no much industrial revolution, people lived more in harmony with their immediate environment. As industrialization has spread around the globe, so the problem of pollution has spread with it. When earth's population was much smaller, no one believed pollution would ever cause such a serious problem. It was once popularly believed that the oceans were far too big to pollute. With the population growth, the technology no longer remained simple and man started exploiting nature rather ruthlessly. Today, with over 8 billion people on the planet, it has become apparent that there are limits of creating pollution. Pollution is one of the signs that humans have exceeded its limits. It has attained the stage that the earth’s in-built ecosystem could no longer dilute, decompose and recycle the waste products. Nature’s tolerance capacities and generosity knows no boundaries but man’s carelessness has crossed it, thus degrading environment is the obvious consequence. The splendid plentifulness of nature is a heritage that should never be spoiled. But the unlimited rapacious exploitation of nature by man has disturbed the delicate ecological balance existing between living and non-living components on the planet earth. This undesirable situation created by man has threatened the survival of man himself and other living biota on the earth. It’s about time that we preserve this for our future generation or face catastrophe that cannot be reckoned. Since the beginning of the 20th century there has been a huge growth in the manufacture and use of synthetic chemicals. Environmental pollution and other environmental problems have become important with increase of world’s population and development of industrial applications. There are many possible sources of chemical contaminations. These include wastes from chemicals industries, metal plating operations, textile dying, wood preservatives, leather industries, pesticides run off from agricultural lands and the other industrial applications and productions [1].

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SOURCES OF WATER POLLUTION 

Dumping of industrial wastes, containing heavy metals, harmful chemicals, byproducts, organic toxins and oils, into the nearby source of water is one of the visible causes of water pollution.



Another cause for the contamination of water is the improper disposal of human and animal wastes.



Effluents from factories, refineries, injection wells and sewage treatment plants are dumped into urban water supplies, leading to water pollution.



A number of pollutants, both harmful and poisonous, enter the groundwater systems through rain water.

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The residue of agricultural practices, including fertilizers and pesticides, are some of the major sources of water pollution.

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Untreated pollutants are drained into the nearest water body, such as stream, lake or harbor, causing water pollution.

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Another major source of water pollution comprises of organic farm wastes. When farm land, treated with pesticides and fertilizers, is irrigated, the excess nitrogen and poisons get mixed into the water supply, thereby contaminating it.

Figure 1.1 Sources of water pollution

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Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------All industries use specific chemicals or the other raw materials to produce their final products. If the final production involves long steps reaction which is the total of many reactions then each process can produce hazardous wastes. A waste is considered hazardous if it is reactive, ignitable, corrosive or toxic. About ninety five chemicals have been defined as toxic including phenols on the basis of production volume, exposure and biological effects [2]. The numbers of organic compounds that have been synthesized since the turn of the century now exceeds half a million and 10,000 new compounds are added each year. As a result, many of these compounds are now found in the wastewaters originated from most municipalities and communities. Currently, the release of volatile organic compounds (VOCs), non-volatile or semi-volatile organic compounds and volatile toxic organic compounds (VTOC) found in wastewater is of great concern in the operation of both collection systems and treatment plants. Many industries are located near water or fresh water streams. These industries discharge their untreated effluents into the nearest water reservoirs. Most of the industries discharge highly toxic heavy metals such as chromium, arsenic, lead, mercury etc. along with hazardous organic and inorganic wastes. For example, river Ganges, receives waste from textile, sugar, paper and pulp mills, tanneries, rubber and plastic industries. Most of these pollutants are resistant to breakdown by microorganism (non biodegradable) and chemically polluted water damages the growth of crop and is unsafe for drinking purposes.

HARMFUL EFFECTS OF WATER POLLUTION 

Pollution affects the chemistry of water. The pollutants, including toxic chemicals, can alter the acidity, conductivity and temperature of water.

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As per the records, about 14000 people perish or incur of various infectious diseases due to the consumption of contaminated drinking water.

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The concentration of bacteria and viruses in polluted water causes increase in solids suspended in the water body, which in turn, leads to health problems.

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Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------

Marine life becomes deteriorated due to water pollution. Lethal killing of fish and aquatic plants in rivers, oceans and seas is an after-effect of water contamination only.

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Diseases affecting the heart, poor circulation of blood, the nervous system, and ailments like skin lesion, cholera and diarrhea are often linked to the harmful effects of water pollution.

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Carcinogenic pollutants found in polluted water might cause cancer.

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Alteration in the chromosomal makeup of the future generation is foreseen, as a result of water pollution.

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Discharges from power stations to nearest water body reduce the availability of oxygen.

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The flora and fauna of rivers and oceans is adversely affected by water pollution.

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Polluted municipal water supplies are found to pose a threat to the health of people using them.

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A number of waterborne diseases are produced by the pathogens present in contaminated water, affecting humans and animals alike. The wastewaters from the industries contain hydrocarbons which may

present in many forms such as chlorinated hydrocarbons, halogenated hydrocarbons, organophosphates and non volatile or semi volatile aromatic hydrocarbons. Phenol, as an aromatic semi volatile hydrocarbon is present in wastewaters of most industries such as high temperature coal conversion, petroleum refining, resin and plastic, leather and textile manufacturing [3], oil refineries, chemical plants, coke ovens, aircraft maintenance, foundry operations, paper-processing plants, paint manufacturing, rubber reclamation plants, nitrogen works, and fiberglass manufacturing in different ranges from 1ppm to 7000 ppm (Table 1.1) [4]. Phenolic constituents stand at eleventh rank out of the 126 chemicals which have been pointed as priority pollutants according to United States Environmental Protection Agency (EPA) [5]. Phenolic derivatives belong to a group of common environmental contaminants and basic structural unit for variety of synthetic ---------------------------------------------------------------------------------------------------~4~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------organic compounds. Phenols and substituted phenols are significant contaminants in medical, food and environmental matrices. Their presence even at low concentrations can be an obstacle to the use of water. Phenols cause unpleasant taste and odour of drinking water and can exert negative effects on different biological processes. Phenolic compounds are a class of polluting chemicals, highly soluble in water, easily absorbed by animals and humans through the skin and mucous membranes. Their toxicity affects directly a great variety of organs and tissues, primarily lungs, liver, kidneys and genitourinary system. Human consumption of phenol-contaminated water can cause severe pain leading to damage of the capillaries ultimately causing death. Table 1.1 Levels of phenolic compounds reported in industrial wastewaters Industrial Source Petroleum refineries

Concentration of phenolic compounds (ppm) 40 - 185

Petrochemical

200 - 1220

Textile

100 - 150

Leather

4.4 - 5.5

Coke ovens (without dephenolization)

600 - 3900

Coal conversion

1700 - 7000

Ferrous industry

5.6 - 9.1

Rubber industry

3 - 10

Pulp and paper industry

22

Wood preserving industry

50 - 953

Phenolic resin production

1600

Phenolic resin Fiberglass manufacturing Paint manufacturing

1270 - 1345 40 - 2564 1.1

Phenol containing water, when chlorinated during disinfection of water also results in the formation of chlorophenols. The majority releases of chlorophenols to the environment are caused by spills and leachate from treated wood. They are also breakdown products of agricultural pesticides. Chlorophenols are also emitted ---------------------------------------------------------------------------------------------------~5~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------during treated wood combustion. Significant amounts of chlorophenol are produced during the chlorine bleaching process in pulp and paper-mills, incineration and water chlorination. The releases from manufacturing industries are significant with accidental spillage being negligible. There are believed to be no significant natural sources of them. Most of these compounds are recognized as toxic carcinogens. Nitroaromatic compounds are used in the synthesis of dyes, explosives, pesticides and drugs. In particular, nitrophenols are involved in much of this chemistry. For example, 4-Nitrophenol (4-NP/PNP) is the final product of the enzyme catalyzed, biochemical reaction of acetyl cholinesterase and paraoxon. Nitrophenol isomers have even served as model systems to explore the role of silver nanoparticles in the conversion of aromatic nitro-compounds to aromatic amines, which has industrial and environmental significance. In the US, eleven phenols are listed as priority pollutants by the EPA including 2-Nitrophenol (2-NP) and 4-NP [6]. Industrial sources of phenolic contaminants such as pulp and paper, resin manufacturing, gas and coke manufacturing, explosives, tanning, textile, plastics, wood preserving chemicals rubber, pharmaceutical, oil refineries, coal gasification sites and petrochemical units, etc. [7-10] generate large quantity of phenols residue. Besides that the phenolic derivatives are widely used as intermediates in the synthesis of plastics, colours, pesticides and insecticides, etc. Degradation of these substances means the appearance of phenol and its derivatives in the environment [11]. Henceforth, the determination of phenolic compounds is of great importance due to their toxicity and persistency in the environment [12]. The estimation and detoxification of phenol from the wastewaters is, therefore of great importance.

HEALTH EFFECTS OF STUDIED PHENOLS PHENOL Phenol has acute and chronic effects on human health. Inhalation and dermal exposure to phenol is highly irritating to skin, eyes and mucous. These inverse effects also known as acute (less than 14 days-exposure) effects of phenol. ---------------------------------------------------------------------------------------------------~6~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------The other acute health effects are headache, dizziness, fatigue, fainting, weakness, nausea, vomiting and lack of appetite at high levels. Effects from chronic exposure (longer than 365 days) include irritation of the gastrointestinal tract. Phenol also can change blood pressure and can cause liver and kidney damage. Nervous system is affected negatively for long time exposures (EPA, 2002). EPA has classified phenol as a Group D, not classifiable as to human carcinogenicity. Animal studies have not shown tumors resulting from oral exposure to phenol, while dermal studies have reported that phenol applied to the skin may be a tumor promotor and/or a weak skin carcinogen in mice.

ortho-CHLOROPHENOL Inhalation of o-Chlorophenol (OCP) can cause cough, shortness of breath, sore throat, abdominal pain, convulsion, drowsiness and weakness. Skin and eye absorption may cause redness, pain and blurred vision. Chlorophenol spills have resulted in fish kills. Exposure to large quantities of chlorophenol impairs algal primary production and reproduction. Biodegradation in soils is likely to be reasonably rapid (days-weeks) and it binds moderately to soil/sediment particles, however for significant spills to land, leaching to groundwater may be possible. Bioaccumulation of chlorophenols appears to be moderate.

para-NITROPHENOL Exposure of para-Nitrophenol (PNP) irritates eyes, skin and respiratory tract and may cause the inflammation of those parts. It has a delayed interaction with blood

and

forms met-haemoglobin which

is

responsible

for

the

met-

hemoglobinemia, potentially causing cyanosis, confusion and unconsciousness. When ingested, it causes abdominal pain and vomiting. Prolonged contact with skin may cause allergic response.

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USES OF STUDIED PHENOLS PHENOL Phenol is used in making plywood, construction, automotive and appliance industry as a raw chemical and in the production of nylon, epoxy resins. In addition, it becomes a disinfectant, slime-killing agent and an additive in medicines. Production of biphenol A is another usage area of phenol [4].

ortho-CHLOROPHENOL It is used as disinfectant agent and pesticide. This particular compound has fewer applications but is an intermediate in the polychlorination of phenol.

para-NITROPHENOL It is used for the synthesis of drugs as an intermediate eg. paracetamol. It is used as the precursor for the preparation of phenetidine and acetophenetidine indicators and raw materials for fungicides. In peptide synthesis, carboxylate ester derivatives of para-nitrophenol may serve as activated components for construction of amide moieties. Most of the chemicals present in wastewater are generally toxic and cause adverse effects on aquatic ecosystem and human life. As a result of development of better analytical systems and better health monitoring technologies, the acceptable minimum concentration of these chemicals is progressively decreasing. As such, stringent regulations have been introduced by most countries with respect to presence of these chemicals in water and which binds industries and other bodies to minimize the concentrations appreciably before the wastewater is discharged into natural water bodies containing good quality of water. In view of importance of pollution control technologies require substantial financial input and many times, their use is restricted because of cost factors overriding the importance of pollution control. It has therefore, been the efforts of many workers to develop cost effective technologies for the wastewater treatments. Thus, the search for safe, convenient and cost effective treatment of wastewater is still going on. ---------------------------------------------------------------------------------------------------~8~


TECHNOLOGIES

AVAILABLE

FOR

THE

REMOVAL

OF

PHENOLIC COMPOUNDS Wastewaters are typically classified as industrial wastewater and municipal wastewater. Industrial wastewater with characteristics compatible with municipal wastewater is often found to be discharged to the municipal sewers. From industries to industries the characteristics of industrial wastewaters vary greatly and consequently the treatment process for industrial wastewater also varies. There are several methods reported for the removal of pollutants from the effluents/wastewater. These technologies can be divided into three categories: physical, chemical and biological. All of them have advantages and drawbacks. Because of the high cost and disposal problems, many of these conventional methods for treating phenol bearing wastewater have not been widely applied at large scale in the industries. Water treatment process selection is tedious assignment involving the consideration of many factors which include available space for the treatment, facilities, reliability of process equipment, waste disposal constraints, desired finished water quality and capital and operating cost, including chemical cost. Wastewater to be treated must be characterized fully, particularly with a thorough chemical analysis of possible waste constituents and their chemical and metabolic products. Current treatment technologies are available to remove phenols from wastewaters. Both physicochemical and biological treatment techniques are successful in full scale industrial use and high efficiencies of phenol removal can be obtained. Phenolic wastes also contain other contaminants which require additional special treatment procedures. Some of them involve physico-chemical processes, such as coagulation [13, 14], reverse osmosis by membrane filtration [15, 16], electrochemical oxidation [17], catalytic oxidation [18, 19], ion exchange [20], biological methods [21, 22], enzyme treatment [23], solvent extraction [24], adsorption [3, 8, 9, 25, 26], pervaporation [27], advanced oxidation processes [28, 29], disinfection by ozone [30, 31], activated sludge [32], photo catalysis [33] etc. For example, in case of wastewaters from petroleum industry, organic pollutants are removed by biological treatment or chemical oxidation methods [4]. In addition ---------------------------------------------------------------------------------------------------~9~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------to biological treatment and chemical oxidation methods, another method for phenol removal is also used such as adsorption on to granular activated carbon [34, 35]. Choice of a suitable and effective treatment technique depends on economic factors and special wastewater characteristics. Among these, a particular treatment may not be effective sometimes in removing all pollutants and in such cases; a number of processes may be incorporated in conjunction, so that all type of pollutants can be tackled [35]. However, these methods have their own shortcomings and limitations. For example, the method based on chemical/ biological oxidation, ion exchange and solvent extraction have shown low efficiency for the removal of trace levels of pollutants [37]. Further, coagulation requires pH control and causes the problem of sludge disposal, whereas ozonation while removing colour effectively does not minimize chemical oxygen demand (COD) and also comprise high operational cost. Most of methods suffer from some drawbacks, such as high capital and operational cost, regeneration cost and problem of residual disposal. Among the various technologies available for water pollution control listed above, the ‘sorption’ process is considered to be the most effective and proven technology having wide potential applications in both water and wastewater treatment [38]. The commonly used treatment methods are discussed in the following paragraphs.

CHEMICAL METHODS Chemical methods include coagulation or flocculation combined with flotation and filtration, precipitation–flocculation with Fe (II)/Ca(OH)2, electro flotation, electro kinetic coagulation, conventional oxidation methods by oxidizing agents (ozone), irradiation or electrochemical processes [17-19]. These chemical techniques are often expensive, and although the phenols are removed, accumulation of concentrated sludge creates a disposal problem. There is also the possibility that a secondary pollution problem will arise because of excessive chemical use. Chemical oxidation by ozone and chlorine has been reported effective for some toxic organics including phenol. It is possible to reach 48 % removal efficiency for phenol at pH 7 and initial phenol concentration of 1000mg/L ---------------------------------------------------------------------------------------------------~ 10 ~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------using ozone as an oxidant. Several factors influence the effectiveness of the oxidation process, such as reactivity of the ozone itself with the target compound, the rate of reactivity, the ozone demand to achieve a desired degree of treatment, the extent of incidental stripping associated with ozone dispersion and other treatment variables such as pH and temperatures [28-31]. For example, the ozone treatment of phenol proceeds approximately twice faster at pH 11 than at pH 7 [4]. Recently, other emerging techniques, known as advanced oxidation processes [2729], which are based on the generation of very powerful oxidizing agents such as hydroxyl radicals, have been applied with success for pollutant degradation. Although these methods are efficient for the treatment of waters contaminated with pollutants, they are very costly and commercially unattractive. The high electrical energy demand and the consumption of chemical reagents are common problems.

SOLVENT EXTRACTION Solvent extraction method can be predominantly applied for the separation of organic materials from wastewaters. Solvent extraction is also called liquid extraction and liquid-liquid extraction. Solvent extraction occurs when a waste constituent in the wastewater is selectively removed when it is contacted with an organic solvent, because it has more solubility in the solvent than it is in the wastewater [24]. In this process, the solvent and the waste stream are mixed to allow mass transfer of the contaminant from the waste to the solvent. The solvent, immiscible in water, is then allowed to separate from the water by gravity. The solvent solution containing extracted contaminants is called the extract. The extracted waste stream with the contaminants removed is called the raffinate. If the extract is sufficiently enriched, it may be possible to recover the useful materials. For the recovery of the solvent and reusable organic chemicals from organic materials, distillation is often employed. The solvent extraction process has found wide application in the ore processing, food processing and the petroleum industry [39]. Large quantity of solvent is required in this method to remove the contaminants and recovery of the solvent involves tedious processes. Solvent extraction methods are economically and environmentally unfavoured at large ---------------------------------------------------------------------------------------------------~ 11 ~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------scale.

BIOLOGICAL TREATMENT PROCESS Biological remediation methods are often cheaper and more environmentally friendly than their physical or chemical counterparts (i.e., incineration, ozonation). Biological treatment is often the most economical alternative when compared with other physical and chemical processes. Biodegradation methods such as fungal decolonization, microbial degradation, adsorption by (living or dead) microbial biomass and bioremediation systems are commonly applied to the treatment of industrial effluents because many microorganisms such as bacteria, yeasts, algae and fungi are able to accumulate and degrade different pollutants. Unfortunately, they are also less versatile as microbial activity is more easily affected by process parameters such as the effluent toxicity. Hence, a more cost-efficient alternative consists in combining physical and chemical processes with biological methods [40]. Phenolic compounds, especially chlorinated ones, are similar to herbicides and pesticides in structure and they are difficult to remove by biological treatment processes because of their resistance of biodegradation [3]. However, Phenol can be removed from wastewater by different treatment method including biochemical ways [41]. Biological treatment involves the action of living microorganisms. The various microorganisms utilize the waste material as food and convert it into simpler substances by natural metabolic process. Organic waste from the petroleum industry can be treated biologically. In addition to the traditional biological treatment systems (activated sludge and trickling filter processes), a treatment method called land farming and land treatment may be used. The waste is carefully applied to and mixed with surface soil, microorganisms and nutrients may also be added to the mixture, as needed. The toxic organic material is degraded biologically, whereas inorganic materials are adsorbed in the soil [42]. Phenol concentrations up to 500 mg/L are generally considered suitable for biological treatment techniques [4]. Certain organic hazardous wastes can be treated in slurry from in an open lagoon or in a closed vessel called a bioreactor. A bioreactor has ---------------------------------------------------------------------------------------------------~ 12 ~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------fine bubble diffusers to provide oxygen and mixing device to keep the slurry solids in suspension [42]. However, their application is often restricted because of technical constraints. Biological treatment requires a large land area and is constrained by sensitivity toward diurnal variation as well as toxicity of some chemicals, and less flexibility in design and operation. Biological treatment is incapable of obtaining satisfactory phenol removal with current conventional biodegradation processes. Moreover, although many organic molecules are degraded, many others are recalcitrant due to their complex chemical structure and synthetic organic origin.

PHYSICAL METHODS Different physical methods are also widely used, such as membranefiltration processes [13-16] (nanofiltration, reverse osmosis, electrodialysis, etc.) and adsorption techniques. The major disadvantage of the membrane processes is that they have a limited lifetime before membrane fouling occurs and the cost of periodic replacement must thus be included in any analysis of their economic viability. In accordance with the very abundant literature data, liquid-phase sorption is one of the most popular methods for the removal of pollutants from wastewater since proper design of the adsorption process will produce a high quality treated effluent. This process provides an attractive alternative for the treatment of contaminated waters, especially if the sorbent is inexpensive and does not require an additional pre-treatment step before its application. Sorption is a well known equilibrium separation process and an effective method for water decontamination applications. Adsorption has been found to be superior compared to other techniques for water reuse in terms of initial cost, flexibility and simplicity of design, ease of operation and insensitivity to toxic pollutants. Also, adsorption does not result in the formation of harmful substances.

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ADSORPTION ONTO GRANULAR ACTIVATED CARBON The most common method for the removal of dissolved organic material is sorption on activated carbon [43], a product that is produced from a variety of carbonaceous materials, including wood, pulp-mill char, wheat, rice husk, peat, lignite etc. Effectiveness of these materials comes from its tremendous surface area. The carbon is produced by charring the raw material anaerobically below 600°C followed by an activation step consisting of partial oxidation. Carbon dioxide may be employed as an oxidizing agent at 600-700°C, or the carbon may be oxidized by water at 800-900°C [34, 35]. These processes develop porosity, increase the surface area and leave the carbon atoms in arrangements that have affinities for organic compounds. Activated carbon might be in two general types: granulated activated carbon, consisting of particles 0.1-1mm in diameter and powdered activated carbon, in which most of the particles are 50-100µm in diameter. For water treatment, currently granular carbon is most widely used. It may be employed in a fixed bed, through which water flows downward. Accumulation of particulate matter requires periodic backwashing. Economics require regeneration of the carbon, which is accomplished by heating it to 950°C in a steam air atmosphere. This process oxidizes adsorbed organics and regenerates the carbon surface, with an approximately 10% loss of carbon [1]. Activated carbons are the most widely used adsorbents due to their excellent adsorption abilities for organic pollutants. The high adsorption capacities of activated carbons are usually related to their highsurface-area, pore volume, and porosity.

ADSORPTION PROCESS The term adsorption was proposed by Bios-Reymond but introduced into the literature by Kayser [44]. Ever since then, the adsorption process has been widely used for the removal of solutes from solutions and harmful gases from atmosphere. Adsorption process is efficient for the removal of organic matter from waste effluents. Adsorption is the physical and/or chemical process in which a substance is accumulated at an interface between two phases. For the purposes of water or ---------------------------------------------------------------------------------------------------~ 14 ~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------wastewater treatment, adsorption from solution occurs when impurities in the water accumulate at a solid-liquid interface. The substance which is being removed from the liquid phase to the interface is called as sorbate and solid phase in the process is known to be sorbent. The use of term ‘sorption’ instead of adsorption became common in 19th century, for the surface activities. Sorption is defined as being the attraction of an aqueous species to the surface of a solid. Sorption is a rapid phenomenon of passive sequestration separation of sorbate from an aqueous/gaseous phase onto a solid phase. Sorption occurs between two phases in transporting pollutants from one phase to another. It is considered to be a complex phenomenon and depends mostly on the surface chemistry or nature of the sorbent, sorbate and the system conditions in between the two phases. Sorption processes offer the most economical and effective treatment method for removal of pollutants. The process is often carried out in a batch mode, by adding sorbent to a vessel containing contaminated water, stirring the mixture for a sufficient time, then letting the sorbent settle and drawing off the cleansed water. At the surface of the most solids, there are unbalanced forces of attraction which are responsible for sorption. In cases where the sorption is due to weak Van der Waals forces, it is called physical sorption which is reversible in nature with low enthalpy values. On the other hand, in many systems there may be a chemical bonding between sorbate and sorbent molecule. Such type of sorption is chemisorption. As a result of chemical bonding, the sorption is irreversible in nature and has high enthalpy of sorption. Sorption phenomenon is operative in most natural physical, biological and chemical systems. Sorption operations employing solids such as activated carbon and synthetic resins are used widely in industrial applications and for purification of waters and wastewaters. Dissolved species may participate directly in air-water exchange while sorbed species may settle with solids. Figure 1.2 illustrates a brief sorption process for a general aromatic organic matter [45].

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Figure 1.2 Illustration of sorbed species behave differently from dissolved molecules of the same substance Physical sorption (physisorption) is relatively non-specific and is due to the operation of weak forces between molecules. In this process, the sorbed molecule is not affixed to a particular site on the solid surface; it is free to move over the surface. The physical interactions among molecules, based on electrostatic forces, include dipole-dipole interactions, dispersion interactions and hydrogen bonding. When there is a net separation of positive and negative charges within a molecule, it is said to have a dipole moment. Molecules such as H2O and N2 have permanent dipoles because of the configuration of atoms and electrons within them. Hydrogen bonding is a special case of dipole-dipole interaction and hydrogen atom in a molecule has a partial positive charge. Positively charged hydrogen atom attracts an atom on another molecule which has a partial negative charge. When two neutral molecules which have no permanent dipoles approach each other, a weak polarization is induced because of interactions between the molecules, known as the dispersion interaction [46]. Figure 1.3 illustrates the main interactions and forces during physical sorption processes [45].

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Figure 1.3 Illustration of the various molecular interactions arising from uneven electron distributions

In water treatment, sorption of an organic sorbate from polar solvent (water) onto a nonpolar sorbent (carboneous material) has an often interest. In general, attraction between sorbate and polar solvent is weaker for sorbates of a less polar nature; a nonpolar sorbate is less stabilized by dipole-dipole or hydrogen bonding to water. Nonpolar compounds are sorbed more strongly to nonpolar sorbents. This is known as hydrophobic bonding. Hydrophobic compounds sorb on to carbon more strongly. Longer hydrocarbon chain is more nonpolar, so, degree of this type of sorption increases with increasing molecular length [46]. Additionally, branched chains are usually more sorbable than straight chains, an increasing length of the chain decreases solubility. An increasing solubility of the solute in the liquid decreases its sorbability. For example, a hydroxyl group generally reduces sorption efficiency. Carboxyl groups have variable effects according to the host molecule. Double bonds affect sorbability of organic compounds depending on the carboxyl groups. The other effective factor on sorption is molecular size [47]. Aromatic and substituted aromatic compounds ---------------------------------------------------------------------------------------------------~ 17 ~

Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------are more sorbable than aliphatic hydrocarbons [4]. Figure 1.4 illustrates the sorption of an aromatic compound on to a polar surface.

Figure 1.4 Illustration of the aromatic hydrocarbon sorption on a polar inorganic surface

Chemical sorption (chemisorption) is also based on electrostatic forces, but much stronger forces act a major role on this process [48]. In chemisorption, the attraction between sorbent and sorbate is a covalent or electrostatic chemical bond between atoms, with shorter bond length and higher bond energy [46]. The enthalpy of chemisorption is very much greater than that for physisorption and typical values are in the region of 200 kJ/mol, whereas this value for physisorption is about 20 kJ/mol. Except in the special cases, chemisorption must be exothermic. A spontaneous process requires a negative free energy (ΔG) value. Because, the translational freedom of the sorbate is reduced when it is sorbed, entropy (ΔS) is negative. Therefore, in order for ΔG to be negative, ΔH must be negative and the process exothermic. If the enthalpy values less negative than -25 kJ/mol, system is physisorption and if the values more negative than -40 kJ/mol it is signified as chemisorption [49].

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Chapter-1 Introduction and Literature Survey ---------------------------------------------------------------------------------------------------Table 1.2 The bond energies of various mechanisms for the sorption Interaction between sorbent and sorbate

Enthalpy (kJ/mol)

Electrostatic chemical bonding Dispersion

interactions

and

hydrogen

- ΔH

+ ΔH

> 40

> 200

8 -40

chemisorption physisorption

bonding Dipole-dipole interaction