Use of water is critical for human beings on earth. The human activities particularly the industrialization has caused an increase in the amount of pollutants present in the water. This water cannot be utilized unless the pollutants present in it are either completely removed or reduced to a permissible limit. The need for removing these pollutants becomes significant as they cause water pollution and also pose threat to the environment. The industries are under great pressure to reduce the pollutant level discharged in their effluents. In this regard, measures have to be taken first to identify quantitatively the different types of pollutants present in industrial waste water, level of contamination and eventually devising a method for the removal of these pollutants. The contaminants associated with water include suspended solids, biodegradable organics (proteins, fats, and carbohydrates), pathogenic organisms, refractory organics, heavy metals and priority pollutants which include organic compounds that are highly toxic, carcinogenic and mutagenic. Some of the most difficult effluents to treat are those containing ppm quantities of colored, toxic and non biodegradable organics. The discharge limits specified for these organic compounds should be below 1 µg/l (Poste, 2014).
The presence of organic compounds in water not only affects the quality of water but also act as food for microorganisms which results in the consumption of dissolved oxygen. This presents a serious problem as it may lead to the total removal of oxygen from water which is already sparingly soluble in water. This results in a decrease in aerobic activity in water. In addition, presence of these compounds can be harmful to human endocrine system Brook & Wright, 2008.
Amines belong to a class of organic compounds that are often encountered in waste water treatment and are considered as highly toxic, carcinogenic and pose threat to aquatic life and human beings. They incorporate a basic nitrogen atom which has a lone pair of electron and can act as nucleophile which is similar to ammonia .The number of substituent alkyl groups bonded to the Nitrogen atom indicates whether the amine under consideration is a primary, secondary or tertiary. The most important physical property of amines is their solubility. The solubility of tertiary amines is less than the corresponding secondary and primary amines due to lesser hydrogen bonding. The solubility generally decreases with increase in molecular weight. Osman & Djibril, 2005
Amines are present in a wide range in the environment. The strong taste and smell related to certain plants and decomposing organisms are due to the presence of amines. Many aquatic organisms are also responsible for the release of amines in aquatic system. Studies in aquatic systems have highlighted that biological sources are responsible for aliphatic amines such as methyl dimethyl and diethyl amine (MEA), (DMA), (DEA). Many compounds related to amine formation are released by aquatic organisms while they are alive or during decomposition after their death. For example MEA is an important product formed during decarboxylation of amino acids. Seabirds and migratory wild life are also responsible for release of amines into environment. These feaces and urine of these organisms act as a mean of delivering amines and compounds related to amines to aquatic environment. Furthermore, these organisms may act as a source of both nitrate and nitrites which have proved to be important with respect to formation of nitrosamine and nitra amine Snoeyink & Summers (1999) .The process of nitrosation of secondary amines and amines of higher order can lead to the formation of nitrosamine. Nitrosamine are often encountered in discharges from industrial and domestic sources and can be formed during several industrial processes including, manufacture of rubber , tanning of leather, casting of metals and processing of food .Urine also acts as an important source of nitrosamine which is highly carcinogenic and high concentrations of these compounds are found in sewage sludge.. The reported concentration of amines in surface waters can be as low as 10µg/L. The amines that are mostly encountered and usually detected include monoethanol amine, dimethanol amine, ethanol amine diethyl amine and methyl amine respectively MEA, DMA, EA, DEA and MA. The physical properties including solubility data of some common amines have been provided in table 1.
Table 1:- Physical Properties of Some Common Amines Bansal & Goyal, 2005
Name Structure Boiling point(0C) Water solubility (g/100ml) at 25 0C
Primary Amines
Methyl amine CH3-NH2 -6 Very soluble
Ethyl amine CH3-CH2-NH2 17 Very soluble
Propoyl amine CH3-CH2-CH2-NH2 49 Very soluble
Cyclo hexyl amine C6H11-NH2 134 Slightly soluble
Benzyl amine C6H5-CH2-NH2 185 Slightly soluble
Aniline C6H5-NH2 184 3.7
Secondary Amines
Dimethyl Amine (CH3)2-NH 7 Very soluble
Diethyl Amine (CH3-CH2)2NH 56 Very soluble
Dipropyl Amine (CH3-CH2-CH2)2 NH 110 Very soluble
N-methyl Aniline C6H5-NH-CH3 196 Slightly Soluble
Diphenyl Amine (C6H5)2-NH 302 Insoluble
Tertiary Amines
Trimethyl Amine (CH3)3-N 2.9 Very soluble
Triethyl Amine (CH3-CH2)3N 90 14
Tripropyl Amine (CH3-CH2-CH2)3N 156 Slightly soluble
N,N-dimethyl Aniline C6H5-NH-(CH3)2 194 Slightly soluble
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The different types of amines present in the various water streams from different industrial sources are given in the table-2:
Table 2 :-Concentration of Amines in Different Industrial Wastes Snoeyink & Summers, (1999)
Waste water Sample Compounds Concentrations(µg/L)
Pharmaceutical waste water 5 Chloro 2-methyl aniline 200
Petrochemical industry Methyl amine, Dimethyl amine , Diethyl amine
30,70,30
water from rubber additives Aniline 6200
print works waste water 3,4 dicholoranaline 88.2
Many amines possess an unpleasant smell and are dangerous for health, cause irritation of human skin, eye, respiratory tract and can adversely affect the mucous membranes. The amines like EDA, DEA, DETA, TETA are responsible for allergic dermatoses, Hepatic, renal and cardiac injuries in laboratory animals if subjected to high concentration of amine compounds. The requirement for removal of amines from waste water and to devise a method for their removal can be justified by their toxicity.
The removal processes for amines can be classified in many ways but broadly they can be classified as Biological, Physical and Chemical. The processes available for removal of amines are as follows:
1. Biological: Aerobic and anaerobic
2. Physical : coagulation and flocculation, biofilteration and ion exchange
3. Chemical: Advanced oxidation process and adsorption Matilainen, Veino & Tuhkenen (2006).
Aerobic process means a process carried out by using air (oxygen); while anaerobic process is the one carried out without air (oxygen). These two processes are strongly dependant on two factors primarily the type of microorganisms or bacteria involved in the degradation process of organic compounds in waste water stream and the type of bioreactor employed for carrying out these processes. The biological methods include the activated sludge process, membrane bioreactor process (MBR) and moving bed bioreactors (MBBR).These biological methods have several disadvantages due to which their preference for removal of organics on large scale is low. These disadvantages include requirement of large area of land, production of undesirable odors, low treatment efficiency, requirement of large area for land filling of sludge, extensive use of mechanical devices, high capital cost and requirement of a technically skilled manpower for operations and maintenance.
Chemical processes include coagulation ,flocculation which may be used in combination with flotation and filtration processes, precipitation- flocculation with Fe(II)/Ca(OH)2 , electro kinetic coagulation, conventional oxidation methods by the use of an oxidizing agent like ozone , irradiation and electro-flotation. These chemical techniques have proved to be quite effective in the removal of organic compounds from the wastewater streams but are found to be less cost effective in most cases due to the use of different chemicals and therefore are not employed frequently. In addition to the above mentioned disadvantage the disposal problem is often encountered because of the accumulation of undesired concentrated sludge. These methods are not considered economical owing to high demand of energy, greater use of the chemical reagents and high cost involved.
A number of different physical methods are also available for removal of amines such as filtration processes employing membranes such as nano-filtration, Reverse osmosis, electro dialysis. However these processes also possess several disadvantages which make these processes uneconomical for treatment of various organics. The disadvantages shown by these methods include the need of high pressure for use of membranes, fouling of membrane, and low lifetime of membrane, cost of replacement of membrane, high capital cost and low capability for treating large volumes of water.
Adsorption technique is the most extensively used technique for removing certain classes of organic pollutants from water, especially those that cannot be removed effectively by the use of conventional biological wastewater treatment techniques. The various advantages in using this technique involve the use of relatively inexpensive adsorbents, high adsorption capacity of adsorbent lower installation and operational costs and lesser requirement of land. Use of Activated carbon as an adsorbent is found to be quite effective in removing various organics from waste water. The major attraction in using activated carbon are its structural characteristics and its porous structure that give it a greater and improved surface area available for adsorption , furthermore its chemical nature can be easily modified by chemical treatment in order to increase its adsorption capacity McKay, (1996).
Activated carbon has two types, Granulated Activated carbon(GAC) and Powdered activated carbon(PAC) .In case of waste water treatment granulated activated carbon(GAC) is generally used , however powdered form of activated carbon can also be employed under specific conditions . GAC is a porous and offers large surface area which is needed for efficient adsorption. GAC can remove total organic carbon in a water treatment plant with range of 1-16mg/l and removal efficiency ranging in between 10%-90% Bayer, (2005). The use of GAC as adsorbent is depicted by the following table 3.
Table 3: Organic compound amenable to Adsorption by GAC Osman & Djibril, 2005.
CLASS EXAMPLES
Aromatics Benzene, toluene and xylene
Poly nuclear aromatics Naphthalene and biphenyl
Chlorinated Aromatics Chlorobenzene, PCBS, DDT
Phenolics Phenol, cresol
Aromatic and Aliphatic Amines Aniline, toluene diammine
Surfactants Alkyl benzene sulphonates
Soluble Organic Dyes Methylene blue and textile dyes
Chlorinated Solvents Carbon tetra chloride, perchlorethylene
Aliphatic and Aromatic Acids Benzoic acid
Pesticides atrazine, simazine, aldicarb, alachlor
Although the use of GAC is quite helpful in removing a wide variety of organic compounds but quite often GAC filters get exhausted with extensive remove organic matters and their adsorption capacity decreases with time due to saturation of activated carbon Moreover , its adsorption efficiency rapidly decreases when used for water streams with high level of contamination of organic compounds are treated, as activated carbon exhibits a non specific nature in removal of contaminants from waste water streams. After the carbon gets exhausted due to adsorption of various organic compounds , it can be either incinerated, disposed of in a land fill, which is normally governed by the fact the GAC used is small in quantity. An alternate method is the thermal regeneration of spent GAC and this option is practiced and widely as it is more cost effective as compared to disposal or incineration. Muhammad, Roberts & Brown (2011). However, thermal regeneration requires a large amount of energy and often requires transportation to regeneration facilities which may be quite far away from the sites at which the GAC is being used this may result in each regeneration cycle leading to about 10% loss of GAC . Another problem that may arise during the reactivation is that the regeneration process may lead to enlargement of the pores in the activated carbon due to heating effects and this can adversely affect its capacity to absorb smaller organic molecules Lai & Sheish (1996).The other regeneration techniques include biological and steam regeneration, however due to economical reasons they are not preferred. An ideal regeneration process is one which gives the highest possible rate of desorption of adsorbed compound. It should also exhibit least capacity to get eroded and moreover it should show little or no alteration of adsorbent should be easily accessible and ecologically safe to use. Moreover it should show easy separation of desired compounds from desorbate and should lead to invariable qualitative composition of the desorbed components during desorption process. The electrochemical regeneration exhibits most of the above mentioned ideal regeneration process characteristics and enjoys the following features control of selectivity and reaction rate through electrode potential and makes possible reactions at ambient condition. It can reduce the number of steps and makes use of cheap starting material .
Arvia Technology Ltd., developed a process known as the Arvia Process , which offers an alternative to removal of organic pollutants from water streams using GAC. An electrochemical method which makes use of a unique electrically conductive type of activated carbon called Nyex a graphite intercalation compound by oxidizing organic pollutants in water. The GIC is produced by treating graphite flakes in the presence of sulphuric acid using chemically or electrochemically oxidizing conditions. The Nyex adsorbs the pollutants and they are then gets electrochemically oxidized. The Arvia – technology developed at university of Manchester, U.K and has been commercialized as Arvia Technology. The Arvia Technology shows combined effects of both adsorption and advanced oxidation using GIC. GIC is a electrically conducting and non porous derivative of carbon . The non porous structure of GIC results in the achievement of ` its full adsorptive capacity within a short time and its electrical conductivity enables it to become a part of the electrode in an electrochemical cell. As a result, adsorption process by using GIC is quick and leads to fast electrochemical regeneration and results in 100% regeneration efficiency of the adsorbent. The basic principle underlying this technology is that the GIC bed is placed in a tank containing electrodes made of graphite and a separator is installed in the form of a microporus polyethylene membrane (Deramic-350). In front of the cathode a membrane is installed and the gap between them is filled with sodium chloride solution (0.3%) by making use of pump this is done to keep the cell voltage as low as possible. GIC act as the anode during regeneration process. This process can be carried out in both batch or in a continuous operation. Muhammad et al. (2011)
Typical operation follows the following treatment steps:
1. Adsorption – This is achieved by mixing the GIC and effluent with the injection of fluidizing air at the bottom of the reactor. Vigorous and extensive mixing and the non-porous nature of the GIC results in quick adsorption (typically 15 – 30 minutes).
2. Sedimentation- once the fluidized air is switched off the dense Nyex particles settle down rapidly under the influence of gravity to form a bed (typically in 5 – 10 minutes). This bed is formed within a compartment containing an anode and a micro-porous membrane. The cathode is placed behind the separator with an electrolyte solution whose purpose is to provide conductivity between the separator and the electrode.
3. Electrochemical Regeneration– A direct electric current is passed through the GIC adsorbent bed which serves as the anode. The high electrical conductivity of Nyex means only low voltage is needed (3.5-5V) depending on the current density and hence the electricity consumption is low. At the cathode electrochemical reduction leads to the production of hydrogen and hydroxide ions from reduction of water. The concentration of hydrogen is kept below10% of its explosive limits to ensure its safe operation. The hydroxide ions are neutralized using hydrochloric acid to control the PH of the catholyte solution. Regeneration typically takes 3-45 min depending on the organic loading. The regeneration process is capable of recovering 100% of the adsorptive capacity with a negligible loss of adsorbent. After regeneration the treated water is pumped out of the tank and GIC is ready for the next water treatment cycle.
The process of adsorption with electrochemical regeneration using graphite based adsorbents is capable of reducing operating costs Owing to two factors. (i) Since the mass transfer process is involved only during the adsorption stage, therefore the mass transfer limitation is reduced in electrochemical processes involving low concentrations of organics and (ii) the relatively higher electrical conductivity of graphitic adsorbents results in low cell voltages. Two mechanisms of electrochemical regeneration of activated carbon has been recognized in literature, namely, the direct destruction of adsorbed contaminants onto the surface of GAC or desorption of adsorbed contaminants and their destruction in solution. In an ideal regeneration process, the adsorbed contaminants are completely converted into carbon dioxide and water and thereby leaving no objectionable components into the solution.
The key advantages of the process include targeting waste water streams with low concentration of toxic, hazardous organics, the technology there by offering a complete waste water solution and destroying of organics, disruption of microorganisms and leads to residual chlorination. In comparison to other waste water treatment technologies it requires no substantial chemicals, no secondary waste is generated , the energy consumption is dependent on the concentration of organics and the adsorbent can be easily regenerated. It consists of a mobile, skid mounted unit that can be easily fitted into an existing plant or stream Brown et al 2012.
