CHAPTER ONE
1.0 Introduction and literature review
Bentonites are clays rich in smectite whose properties such as crystal structure and size, cation exchange capacity (CEC), hydration and swelling, thixotropy, bonding capacity, impermeability, plasticity and tendency to react with organic compounds make them advantageous for a variety of applications. Smectites are 2:1 type of aluminosilicate having crystal lattice that consists of two dimensional layers where central octahedral sheet of alumina is fused to two external silicate layers. Isomorphic substitution within the layers generates negative charges that are counterbalanced by easily replaceable alkali or alkaline earth cations. These cations are defined as exchangeable cations. Forces holding the stacks together are relatively weak and the intercalation of small molecules between the layers is easy. Smectite can be rendered organophilic by exchanging the exchangeable cations with alkylammonium ions. Quaternary ammonium cations of the general form [(CH3)3NR]+ or [(CH3)2NRR0]+,( where R and R0 are hydrocarbon groups), are usually used in the synthesis of organoclays. Depending on the dimensions of R and R0, organoclays display distinct adsorptive properties and abilities. Clay minerals such as smectite and montmorillonite are abundant in nature and known as swelling clays due to their tendency to swell and hydrate in the presence of water. They are used in a wide range of applications including nanocomposites, catalysts, photochemical reaction reagents and adsorbents. One of the foremost industrial applications is as adsorbents for water purification. In this respect, montmorillonite is the most commonly used clay due to its high cation exchange capacity (CEC), swelling properties, high surface areas, and consequential strong adsorption and abs\um or magnesium ion is octahedrally coordinated to six oxygens or hydroxyls. The isomorphous substitution within the layers (e.g. the replacement of Mg2+ or Zn2+ for Al3+ in the octahedral layer, and Al3+ for Si4+ in the tetrahedral sheets) results in a negatively charged surface. The resultant negatively charged clay surface is counterbalanced by exchangeable cations such as Na+ or Ca2+ in the interlayer space. The hydration of the inorganic cations on the exchangeable sites causes clay mineral surfaces to be hydrophilic in nature and these hydrophilic clays are found to be ineffective adsorbents for the removal of organic compounds. Such ineffectiveness have been overcome through ion exchange of inorganic cations with organic cations such as quaternary ammonium cations (QACs), represented as [(CH3)3NR]+, or [(CH3)2NR2]+, where R is a relatively short hydrocarbon group.
The properties of clay minerals are altered upon the formation of organoclays such as those obtained by the intercalation of cationic surfactant molecules into the interlayer space of MMT through an ion exchange process. This changes the surface properties of organoclays to highly hydrophobic and lipophilic and results in an increase in the interlayer or basal spacing, by promoting the generation of new sorption sites in the interlayer of the clay. It is reported that the adsorption capacity of organoclays is improved over and above untreated clays for the removal of various organic contaminants. Besides, organoclays are more cost effective compared with other adsorbents, such as activated carbon and have been shown to be potentially effective for the uptake of water contaminants in aqueous solution.
In recent years organoclays have attracted great interest because of their academic and industrial importance. Organoclay based nanocomposites exhibit remarkable improvements in properties when compared with virgin polymer or conventional micro and macro composites. These improvements include increased strength and heat resistance, decreased gas permeability and flammability and increased biodegradation of biodegradable polymer. Another important application for organoclays is in adsorption such as in pollution prevention and environmental remediation such as treatment of chemical spills, wastewater treatment and harzadous waste landfills and others. The aforementioned applications strongly depend on the structure and properties of the organoclays.
The study of organoclays is a vital subject in current research since various organoclays are widely used as nanocomposite precursors, adsorbents for organic pollutants, rheological control agents and electric materials. The combination of the hydrophobic nature of the surfactant and the layered structure of the silicate layers leads to unique physicochemical properties. In these applications, the behavior and properties of the organoclays strongly depend on the structure and the molecular environment of the organic molecules within the galleries. Organoclays are synthesized by grafting cationic surfactants such as quaternary ammonium compounds into the interlayer.
Organoclays have become an important part of the treatment train to remove creosote and PNAH (poly nuclear aromatic hydro-carbon) from contaminated groundwater at old wood treating facilities and MGP (manufactured gas plant) sites. Organoclays consist of bentonite that is modified with quaternary amines. Bentonite is a volcanic rock whose main constituent is the clay mineral montmorillonite. This gives the bentonite an ion exchange capacity of 70-90 meq/gram. By exchanging the nitrogen end of a quaternary amine onto the surface of the clay platelets, by cation exchange (Exchanging the sodium or calcium ion on the surface for the nitrogen which is positively charged), the bentonite now becomes organically modified and thus organophilic, which also means hydrophobic (Lagaly, 1984). The clay is arranged in a layered structure, platelets stacked on top of each other. When these platelets are placed into water, the amine chains are activated and stand up like dry hair causing pillaring of the platelets, and allowing the end of the amine chains to stand or dangle into the water, reacting with organics that pass by (Mortland et al, 1986). The chains will then dissolve or partition into large organic compounds such as sparingly soluble chlorinated hydrocarbons. Oil is the most prominent of these (Smith et al, 1990). These same compounds, on the other hand, will blind the pores of activated carbon.
The layered expandable clay minerals (e.g., smectite, hydrotalcite) always possess charges on their layer sheet, and these charges will be compensated by counter inorganic ions. Because of the strong hydration of these inorganic ions, the interlayer spaces of the clay minerals are hydrophilic in nature. As a result, the natural clay minerals show rather weak affinity to most of the hydrophobic organic compounds (HOCs), and they are seldom used as sorbents for HOCs (Yariv and Cross, 2001).
Under suitable conditions, the inorganic ions on clay minerals can be replaced by organic ions, and then the interlayer spaces become hydrophobic (Lee et al., 1999; Wang et al., 2004; Volzone et al., 2006; Frost et al., 2008). As a result, the sorption capacity of the modified clay minerals (i.e., organoclays) towards HOC’s can be significantly improved, and the organoclays have found applications in a wide range of organic pollution control fields (Volzone et al., 2006; Frost et al., 2008; Laha et al., 2009). Organoclays can be efficient sorbent for removal of organic pollutants from water (Zhou et al., 2007a,b; Huang et al., 2007; Zhu and Zhu, 2007; Lin and Juang, 2009) and air (Zhu and Su, 2002; Tian et al., 2004; Volzone et al., 2006; Park et al., 2008), and they are also used as landfill liner and sorptive barrier to prevent down gradient pollution of groundwater and aquifer from organic pollutants (Lo, 2001; Lo and Yang, 2001; Brixie and Boyd, 1994). Sorptive mechanisms of organoclays towards HOC’s have been extensively studied in the past decades, and various methods for improving their sorption capacities are proposed accordingly (Sheng et al., 1996; Shen, 2004; Zhu et al., 2007, 2008a,b). For the organoclays modified with organic cations containing long alkyl chains, it is believed that HOCs molecules are incorporated into the alkyl chain formed organic phases (Sheng et al., 1996; Shen, 2004; Zhu et al., 2007). Accordingly, it is suggested that regulating the arrangement model of the alkyl chains of the ions can optimize the sorption capacity of the organoclays (Zhu et al., 2007, 2008a). If the organoclays are synthesized with small organic cations, the HOCs molecules are believed to be primarily adsorbed on the hydrophobic siloxane surface of the organoclays (Shen, 2004; Bartelt-Hunt et al., 2003). Increasing the exposed siloxane surface can improve the sorption capacity of this class of organoclays (Shen, 2004; Zhu et al., 2008b). To reduce the pollution control costs, researchers have also developed some novel processes for the application of organoclays in pollution control (Shen, 2002; Ma and Zhu, 2007; Scurtu et al., 2008). Among these processes, the one-step treatment process is of particular interesting (Shen, 2002; Ma and Zhu, 2007). In this process, the synthesis of organoclays and the application of organoclays in pollution control are combined in one step. As thus, the waste water treatment processes are simplified and the pollution control costs can be greatly reduced (Shen, 2002;Ma and Zhu, 2007).
1.1. What is clay?
Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried or fired. Clay deposits are mostly composed of clay minerals (phylloosilicate minerals) and variable amounts of water trapped in the mineral structure by polar attraction. Organic materials which do not impart plasticity may also be a part of clay deposits.
Clays are distinguished from other fine-grained soils by various differences in composition. Silts, which are fine grained soils which do not include clay minerals tend to have large particle sizes than clays but there is some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include both silts and clays. The distinction between silts and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2µm (clays being finer than silts), sedimentologists often use 4-5µm, and colloid chemists use 1um. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, ISO 14688 grades; clay particles as being smaller than 0.063mm and silts one larger.
There are three or four main groups of clays; kaolinite, montmorillonite-smecite, illite and chlorite. Chlorites are not always considered a clay, sometimes being classified as a separate group within the phyllosilicates. There are approximately thirty different types of “pure” clays in these categories but most “natural” clays are mixtures of these different types along with other weathered minerals( Lagaly,1984.).
1.1.1 Clay minerals
Clay minerals likely are the most utilized minerals not just as the soils that grow plants for foods and garment, but a great range of applications, including oil absorbants, iron casting, animal feeds, pottery, china, pharmaceuticals, drilling fluids, waste water treatment, food preparation, paint e.t.c.
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays form flat hexagonal sheets similar to the micas. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone, and siltstone and in fine grained metamorphic slate and phyllite. Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so may require special analytical techniques for their identification/study. These include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, and SEM-EDS or automated mineralogy solutions. These methods can be augmented by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.
Clay minerals can be classified as 1:1 or 2:1, this originates from the fact that they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in the structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite and montmorillonite.
Clay minerals include the following groups:
Mixed layer clay variations exist for most of the above groups. Ordering is described as random or regular ordering, and is further described by the term reichweite, which is German for range or reach.
1.1.2. Structure of clay minerals
Like all phyllosilicates, clay minerals are characterized by two-dimensional sheets of corner sharing SiO4 tetrahedral and/or AlO4 octahedral. The sheet units have the chemical composition (Al,Si)3O4. Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedral forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedral "point" in the same direction; i.e. all of the unshared vertices are on the same side of the sheet. In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminum or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedral. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as 1:1 clay. The alternative, known as 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet. Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrugated or twisted; causing di-trigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite. Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.
Fig 1. Structure of montmorillonite ( Papke,Keith1970)
1.1.3 Cation exchange capacity of clay
CEC may be defined as the quality of exchangeable cations expressed in milliequivalent per 100g of ignited weight of clay (Newman 1987).
The cation exchange capacity of clay is a very important tool in the preparation of Organoclay. The cation exchange capacity (CEC) of a clay is a measure of the quantity of negatively charged sites on clay surfaces that can retain positively charged ions (cations) such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+), by electrostatic forces. Cations retained electrostatically are easily exchangeable with cations in the clay solution so a clay with a higher CEC has a greater capacity to maintain adequate quantities of Ca2+, Mg2+ and K+ than a clay with a low CEC. A clay with a higher CEC may not necessarily be more fertile because a clay’s CEC can also be occupied by acid cations such as hydrogen (H+) and aluminum (Al3+). However, when combined with other measures of clay fertility, CEC is a good indicator of clay quality and productivity. Clay CEC is normally expressed in one of two numerically equivalent sets of units: meq/100 g (milliequivalents of charge per 100 g of dry clay) or cmolc/kg (centimoles of charge per kilogram of dry clay). Because of the differing methods to estimate CEC, it is important to know the intended use of the data. For clay classification purposes, a clay’s CEC is often measured at a standard pH value. Examples are the ammonium acetate method of Schollenberger and Dreibelbis (1930) which is buffered at pH 7, and the barium chloride-triethanolamine method of Mehlich (1938) which is buffered at pH 8.2 (Rhoades,1982.) .
1.1.4 Quaternary ammonium cations
Quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure NR4+, R being an alkyl group or an aryl group. Unlike the ammonium ion (NH4+) and the primary, secondary, or tertiary ammonium cations, the quaternary ammonium cations are permanently charged, independent of the pH of their solution. Quaternary ammonium salts or quaternary ammonium compounds (called quaternary amines in oilfield parlance) are salts of quaternary ammonium cations with an anion. (Sheng et al., 1996; Shen, 2004).
Synthesis
Quaternary ammonium compounds are prepared by alkylation of tertiary amines, in a process called quaternization. Typically one of the alkyl groups on the amine is larger than the others. A typical synthesis is for benzalkonium chloride from a long-chain alkyldimethylamine and benzyl chloride:
CH3(CH2)nN(CH3)2 + ClCH2C6H5 → [CH3(CH2)nN(CH3)2CH2C6H5]+Cl- ........................ (1)
Applications
Quaternary ammonium salts are used as disinfectants, surfactants, fabric softeners, and as antistatic agents (e.g. in shampoos). In liquid fabric softeners, the chloride salts are often used. In dryer anticling strips, the sulfate salts are often used. Spermicidal jellies also contain quaternary ammonium salts (Huang et al., 2007; Zhu and Zhu, 2007; Lin and Juang, 2009).
1.2 Adsorption
Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where matter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic.
Fig 2. Schematic demonstration of difference between adsorption and absorption (Ahmedna, 2000)
If we have to remove soluble material from the solution phase, but the material is neither volatile nor biodegradable, we often employ adsorption processes.
1.2.1 Thermodynamics of surface adsorption
In solutions certain particles tend to concentrate at the surface. These particles are those that have low affinity for the water (solvent). These are hydrophobic molecules. Because they have low affinity for the solvent the can get to the surface easily since they have low bond energy in the bulk phase. The water system prefers to have these molecules at the surface because the placement at the surface requires less energy than a water molecule -- hydrophobic molecules decrease surface energy (surface tension) relative to a pure water system. On the other hand if a particle has a high affinity for the solvent phase (hydrophilic) it will tend to remain in the bulk solution because of its strong bond with water. In fact, this bonding makes the water bonding stronger and, therefore, there is a larger energy required to get water molecules to the surface-- therefore, hydrophilic molecules increase surface tension, e.g. salts such as NaCl.
As particles concentrate at surface there becomes a "surface excess". Surface excess is calculated thus …………………………… (2)
Where "Volume" is the volume of the solution from which the adsorption is occurring onto the surface with total surface area = "surface area".
Surface excess is defined as the mass adsorbed per surface area. A more fundamental definition is given by the Gibbs relationship. ………………………………. (3)
where: µ1 = the molar free energy of solute i. Ci is the bulk concentration of this solute. The Gibb’s expression simply uses G as a proportionality constant to relate the change in solute molar free energy to surface tension (g) during adsorption. The underlying principle here is that for the adsorption process changes in the sum of all solute free energy must be accounted for in changes in the surface tension during the adsorption process.
1.2.2 Types of adsorption:
1)Lack of solvent-solute interactions (hydrophobicity –surfactants)
2)Specific solid-solute interaction
Generally some combination of physical and chemical adsorption is responsible for activated carbon adsorption in water and wastewater.surface. This is relatively weak, reversible, adsorption capable of multilayer adsorption.
1.2.3 Adsorption equilibria
If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by isotherms.
qe = mass of material adsorbed (at equilibrium) per mass of adsorbent.
Ce = equilibrium concentration in solution when amount adsorbed equals qe.
qe/Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption, etc. Some general isotherms are shown in the figure below
Fig 3. General adsorption isotherms (Archana 2007)
1.2.4 Isotherm models
The figures below show that there are four common models for isotherms. 
Fig 4. Adsorption isotherm models (Hassler, 1963)
Langmuir Isotherm:
This model assumes monolayer coverage and constant binding energy between surface and adsorbate. The model is: …………....………………………. (4)
Qoa represents the maximum adsorption capacity (monolayer coverage) (g solute/g adsorbent).
Ce has units of mg/L. K has units of L/mg
BET (Brunauer, Emmett and Teller) isotherm:
This is a more general, multi-layer model. It assumes that a Langmuir isotherm applies to each layer and that no transmigration occurs between layers. It also assumes that there is equal energy of adsorption for each layer except for the first layer
………………………………….. (5)
CS =saturation (solubility limit) concentration of the solute. (mg/liter)
KB = a parameter related to the binding intensity for all layers.
Note: when Ce << CS and KB >> 1 and K = KB/Cs BET isotherm approaches Langmuir isotherm.
Freundlich Isotherm:
For the special case of heterogeneous surface energies (particularly good for mixed wastes) in which the energy term, “KF”, varies as a function of surface coverage we use the Freundlich model …………………………………………………..(6)
n and KF are system specific constants.
1.2.5 Determination of appropriate model
To determine which model to use to describe the adsorption for a particular adsorbent/adsorbate isotherms experiments are usually run. Data from these isotherm experiments are then analyzed using the following methods that are based on linearization of the models
For the Langmuir model linearization gives:
…………………………………………………… (7)
A plot of Ce/qe versus Ce should give a straight line with intercept : and slope:
Or:
Here a plot of 1/qe versus 1/Ce should give a straight line with intercept 1/Qao and slope
For the Freundlich isotherm use the log-log version :
…………………....…………………………..... (8)
A log-log plot should yield an intercept of log KF and a slope of 1/n
For the BET isotherm we can arrange the isotherm equation to get
……………………………………… (9)
Intercept = Slope =
1.2.6 Factors which affect adsorption
1.3 Copper
Copper is a transition metal that is stable in its metallic state and forms monovalent (cuprous) and divalent (cupric) cations. Common copper compounds include the following: Copper(II) acetate monohydrate [Cu(C2H3O2)2·H2O] , Copper(II) chloride [CuCl2], Copper(II) nitrate trihydrate [Cu(NO3)2·3H2O], Copper(II) oxide [CuO], Copper(II) sulfate pentahydrate [CuSO4·5H2O].
1.3.1 Properties of copper
Copper is classified as a noble metal (as are silver and gold) and can be found in nature in the elemental form. Its chemical and physical properties include high thermal conductivity, high electrical conductivity, malleability, low corrosion, alloying ability, and aesthetically pleasing appearance. These properties make it one of the most important metals (U.S. Department of Health & Human Services 1990)
Table 1 Physico-chemical properties of copper (U.S. Department of Health & Human Services 1990).
|
PROPERTY |
COPPER |
|
Atomic/molecular weight |
63.546 |
|
Color |
Reddish |
|
Physical state |
Solid |
|
Melting point oC |
1083.4 |
|
Boiling point oC |
2567 |
|
Density g/cm3 |
8.92 |
|
Solubility Water (g/100ml) Organic solvents |
Insoluble (CuSO4 14.3 @ 0OC) (CuSO4 soluble in methanol, slightly soluble in ethanol) |
|
Vapour pressure, mmHg |
10(1870oC) |
Dissolved copper can sometimes impart a light blue or blue-green colour and an unpleasant metallic, bitter taste to drinking-water. The concentration at which 50% of 61 volunteers could detect the taste of copper (i.e., taste threshold) as the sulphate or chloride salt in tap or demineralized water ranged from 2.4 to 2.6 mg/litre (Zacarias et al., 2001). The taste threshold increased in the presence of other solutes (Olivares & Uauy, 1996a; Zacarias et al., 2001). Blue to green staining of porcelain sinks and plumbing fixtures occurs from copper dissolved in tap water.
1.3.2 Sources of copper to the environment
Copper and its compounds are naturally present in the earth's crust. Natural discharges to air and water may therefore be significant. Most of the copper released to the environment is released to soil and least is released to air.
1.3.2.1 Releases to Air
On a global scale, natural and anthropogenic copper emissions to air are similar in magnitude. Principal natural sources are wind-borne soil particles and volcanoes. The main anthropogenic source is emissions from the primary non-ferrous metal industry (Pacyna et al. 1995). In the United States, copper emissions to air are estimated to be only 0.4% of copper released to the environment with windblown dust as the primary natural source (U.S. Department of Health and Human Services 1990).
1.3.2.2. Releases to Water
Both natural and anthropogenic sources contribute copper to water: natural weathering of soil, atmospheric deposition, and discharges from industry and wastewater treatment plants. A 1976 evaluation in the United States determined that 2.4% of the identified copper releases to the environment enters waterways. The major source is from land runoff through natural weathering (68%; U.S. Department of Health and Human Services 1990). Copper sulfate use represented 13% of the releases to water; urban runoff contributed 2% (Perwack et al. 1980; U.S. Department of Health and Human Services 1990).
1.3.2.3. Releases to Soil
On a global scale, the two principal sources of copper to soils are disposal of ash residues from coal combustion and municipal and industrial wastes on land. Mine tailing and slags and wastes from smelters can also be a major source of copper to soils: 657-1577 103 t/year (Nriagu and Pacyna 1988)
1.3.3 Environmental concentrations of copper
1.3.3.1 Soil
The crustal average concentration of copper has been estimated at 55 mg/kg but varies with the type of rock (10 mg/kg for granite and 100 mg/kg for basalt). Copper ranks 25th in abundance among the elements present in the earth's crust. Copper concentrations in soils range between 2 and 100 mg/kg dry weigth with a mean value of 20 mg/kg (Bowen 1966, Demayo and Taylor 1981).
1.3.3.2. Sediment
Mean copper concentrations in freshwater sediments ranged from 12 to 57 mg/kg with individual values as high as 4000 mg/kg (Spear and Pierce 1979). Copper concentrations in pristine environments are generally less than 50 mg/kg dry weigth, while concentrations in polluted environments can be several thousand mg/kg (Harrison and Bishop 1989). The highest copper concentrations in sediments were found in reservoirs and sites with high levels of organic matter (Anderson et al. 1994).
1.3.3.3. Freshwater
Copper is widely distributed in water since it is a naturally occurring element. Copper levels in river waters range from 0.6 to 400 :g/L, with a median of 10 :g/L. Dissolved copper levels in uncontaminated freshwaters usually range from 0.5 to 1.0 :g/L, increasing to > 2 :g/L in urban areas (Moore and Ramamoorthy 1984).
1.3.4. Environmental fates of copper
The fate of elemental copper in water is complex and influenced by pH, dissolved oxygen and the presence of oxidizing agents and chelating compounds or ions (US EPA, 1995). Surface oxidation of copper produces copper(I) oxide or hydroxide. In most instances, copper(I) ion is subsequently oxidized to copper(II) ion. However, copper(I) ammonium and copper(I) chloride complexes, when they form, are stable in aqueous solution.
In pure water, the copper(II) ion is the more common oxidation state (US EPA, 1995) and will form complexes with hydroxide and carbonate ions. The formation of insoluble malachite [Cu2(OH)2CO3] is a major factor in controlling the level of free copper(II) ion in aqueous solution. Copper(II) ion is the major species in water up to pH 6; at pH 6–9.3, aqueous CuCO3 is prevalent; and at pH 9.3–10.7, the aqueous [Cu(CO3)2]2- ion predominates (Stumm & Morgan, 1996). Dissolved copper ions are removed from solution by sorption to clays, minerals and organic solids or by precipitation. Copper strongly adsorbs to clay materials in a Ph dependent fashion, and adsorption is increased by the presence of particulate organic materials (Barceloux, 1999; Landner & Lindestrom, 1999). Copper discharged to wastewater is concentrated in sludge during treatment. Various studies of leaching from sludge indicate that the copper is not mobile (ATSDR, 2002). Free copper ions are chelated by humic acids and polyvalent organic anions (Landner & Lindestrom,1999). Atmospheric copper is removed by gravitational settling, dry disposition, rain and snow.
1.3.5. Effects of copper on humans
Copper is an essential trace nutrient that is required in small amounts (5-20 micrograms per gram (µg/g)) by humans, other mammals, fish and shellfish for carbohydrate metabolism and the functioning of more than 30 enzymes. It is also needed for the formation of haemoglobin and haemocyanin, the oxygen-transporting pigments in the blood of vertebrates and shellfish respectively. However, copper concentrations that exceed 20 micrograms per gram (µg/g) can be toxic, as explained by Heike Bradl (2005) and Wright and Welbourn (2002).
1.3.5.1. Acute exposure
The acute lethal dose for adults lies between 4 and 400 mg of copper(II) ion per kg of body weight, based on data from accidental ingestion and suicide cases (Chuttani et al., 1965; Jantsch et al., 1984–1985; Agarwal et al., 1993). Individuals ingesting large doses of copper present with gastrointestinal bleeding, haematuria, intravascular haemolysis, methaemoglobinaemia, hepatocellular toxicity, acute renal failure and oliguria (Agarwal et al., 1993).
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