ABSTRACT
This research work assessed the adsorption of Cd2+, Cr6+, Cu2+ and Pb2 in single-metal and multi-metal ions reaction systems onto a standard reference soil (NIST SRM 2709a). The effect of varying adsorbent dosage, contact time and initial concentration of the metal ions on the soil was examined and for most of the metals, the optimum conditions were attained at 20 mg/dm3 initial metal ion concentration, 1.0 g adsorbent dose and 2 hrs contact time. The highest percentage adsorption of Cd2+, Cr6+, Cu2+, and Pb2+obtained from their respective solutions were 92.22%, 56.71%, 99.57% and 97.11%. Equilibrium studies showed that the Freundlich isotherm had a better fit for the selected metals than Langmuir isotherm. The kinetics study revealed that all the selected metal ions followed the pseudo-second order model. The results of the experiments confirmed that adsorption from single-metal solution was more effective than adsorption under multi-metal conditions due to competitive effects. Also, the adsorption of the metals decreased with increase in the number of competing ions. Among the metal ions studied in the multi-metal system, Pb2+ was most preferably removed because of its high affinity to the soil surface. Metal affinity and capacity for the soil surface were found to increase with metal electronegativity Pb >Cu > Cd > Cr in the competitive systems. The results indicated that high removal efficiency could be achieved by the use of natural standard reference soil.
TABLE OF CONTENTS
Cover page
Fly leaf i ii
Title page . ……………………………………………………………………………………………….. iiiiii
Declaration …………………………………………………………………………………………………… iv
Certification . …………………………………………………………………………………………………… v
Dedication …………………………………………………………………………………………………… vi
Acknowledgements ……………………………………………………………………………………………… vii
Abstract …………………………………………………………………………………………………. viii
Table of Contents ………………………………………………………………………………………………….. ix
List of Figures …………………………………………………………………………………………………. xiv
List of Tables ………………………………………………………………………………………………….. xv
List of Appendices ……………………………………………………………………………………………….. xvi
List of Abbreviations …………………………………………………………………………………………… xvii
CHAPTER ONE …………………………………………………………………………………………………… 1
1.0 INTRODUCTION ……………………………………………………………………………………. 1
1.1 The Chemistry of Metal Ions in Soil …………………………………………………………….. 3
1.1.1 Soil surface functional groups ……………………………………………………………………… 4
1.2 Sources of Heavy Metals …………………………………………………………………………….. 5
1.3 Occurrence and Effect of Selected Heavy Metals …………………………………………… 6
1.3.1 Cadmium ………………………………………………………………………………………………….. 6
1.3.2 Copper ……………………………………………………………………………………………………. 7
1.3.3 Lead ……………………………………………………………………………………………………. 7
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1.3.4 Chromium …………………………………………………………………………………………………. 9
1.4 Heavy Metals and Environmental Pollution ………………………………………………… 10
1.4.1 Effect of heavy metal contamination on water ……………………………………………… 10
1.4.2 Effect of heavy metal contamination on plants …………………………………………….. 11
1.5 Statement of Problem ……………………………………………………………………………….. 11
1.6 Justification …………………………………………………………………………………………….. 12
1.7 Aim and Objectives ………………………………………………………………………………….. 12
CHAPTER TWO ………………………………………………………………………………………………… 14
2.0 LITERATURE REVIEW ……………………………………………………………………….. 14
2.1 Adsorption Mechanism of Heavy Metals…………………………………………………….. 14
2.1.1 Ion- Exchange mechanism of heavy metals in soil ……………………………………….. 15
2.2 Factors Influencing Mobility and Adsorption of Heavy Metal Ions in Soil. …….. 15
2.3 Adsorption of Heavy Metals by Different Soils ……………………………………………. 19
2.4 Competitive Adsorption of Metal Ions in Soil ………………………………………….. 2221
2.4.1 Adsorption selectivity sequence of heavy metals in soil ………………………………… 23
2.5 Standard Reference Soil ……………………………………………………………………………. 24
2.6 Applications of Adsorption of Metal Ions from Solution ………………………………. 25
2.7 Principle of Atomic Absorption Spectrophotometry (AAS) …………………………… 25
2.8 Adsorption Isotherms ……………………………………………………………………………….. 26
2.8.1 Langmuir adsorption model ………………………………………………………………………. 26
2.8.2 Freundlich adsorption model ……………………………………………………………………… 27
2.9 Adsorption Kinetics ………………………………………………………………………………….. 28
2.9.1 Pseudo – first Order model ………………………………………………………………………… 28
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2.9.2 Pseudo – second Order model …………………………………………………………………….. 28
CHAPTER THREE…………………………………………………………………………………………….. 30
3.0 MATERIALS AND METHODS……………………………………………………………… 30
3.1 Apparatus and Equipment …………………………………………………………………………. 30
3.1.1 Reagents …………………………………………………………………………………………………. 30
3.2 Collection of Soil Sample …………………………………………………………………………. 30
3..2.1 Preparation of soil sample …………………………………………………………………………. 30
3.3 Determination of Soil pH ………………………………………………………………………….. 31
3.4 Preparation of Stock Solutions …………………………………………………………………… 31
3.4.1 Cadmium solution ……………………………………………………………………………………. 31
3.4.2 Copper solution ……………………………………………………………………………………….. 31
3.4.3 Lead solution …………………………………………………………………………………………… 31
3.4.4 Chromium solution …………………………………………………………………………………… 32
3.4.5 Preparation of binary solutions ………………………………………………………………….. 32
3.4.6 Preparation of ternary solutions …………………………………………………………………. 32
3.4.7 Preparation of quaternary solutions …………………………………………………………….. 32
3.5 Standard Working Solution of Metal Ions …………………………………………………… 33
3.6 Adsorption Experiments ……………………………………………………………………………. 33
3.6.1 Effect of initial concentrations on the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ … 34
3.6.2 Effect of adsorbent dose on the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ …………. 34
3.6.3 Effect of contact time on the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ …………….. 35
3.7 Modeling Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ ………………………………………. 35
3.8 Adsorption Kinetics of Cd2+, Cr6+, Cu2+ and Pb2+ …………………………………………. 36
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3.9 Competitive Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ …………………………………… 36
3.9.1 Adsorption from binary systems ………………………………………………………………… 36
3.9.2 Adsorption from ternary systems ……………………………………………………………….. 37
3.9.3 Adsorption from quaternary systems ………………………………………………………….. 37
3.10 Statistical Analysis …………………………………………………………………………………… 37
CHAPTER FOUR ………………………………………………………………………………………………. 39
4.0 RESULTS ………………………………………………………………………………………………. 39
4.1 Scanning Electron Microscope (SEM) Analysis …………………………………………… 39
4.2 Adsorption of the Selected Metal Ions ………………………………………………………… 41
4.2.1 Effect of initial metal ion concentration on the adsorption of selected metal ions 41
4.2.2 Effect of adsorbent dose on the adsorption of selected metal ions ………………….. 41
4.2.3 Effect of contact time on the adsorption of selected metal ions ………………………. 41
4.3 Adsorption Modeling ……………………………………………………………………………….. 45
4.4 Adsorption Kinetics ………………………………………………………………………………….. 55
4.5 Competitive Adsorption of Selected Metal Ions …………………………………………… 65
4.5.1 Comparison between the adsorption of selected metal ions in single and mixed systems using statistical analysis ………………………………………………………………… 65
CHAPTER FIVE ………………………………………………………………………………………………… 68
5.0 DISCUSSION ………………………………………………………………………………………… 68
5.1 Scanning Electron Microscope (SEM) Analysis …………………………………………… 68
5.2 Soil pH ………………………………………………………………………………………………….. 68
5.3 Effect of Initial Metal ion Concentration on the Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ ……………………………………………………………………………………………………………………………………….. 69
5.4 Effect of Adsorbent Dose on the Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ ………. 70
CHAPTER ONE
1.0 INTRODUCTION
Soil is one of the key elements in the ecosystems. It provides the nutrient-bearing environment for plant life and it is important for degradation of biomass. Soil is a very complex heterogeneous medium, which consists of solid phases (the soil matrix) containing minerals and organic matter and fluid phases (soil water and the soil air); which interact with each other and ions entering the soil system (Brady and Weil, 2002). The ability of soils to adsorb metal ions from aqueous solution is of special interest and has consequences for both agricultural issues (such as soil fertility) and environmental questions such as remediation of polluted soils and waste deposition (Bradl, 2004).
Heavy metals are group of elements between copper and lead on the Periodic Table, having atomic weights between 63.55 and 200.59, high density, precisely greater than 5 gcm-3 and specific gravities greater than 4.0 (Hawkes, 1997). Heavy metals in the soil include metals of significant biological toxicity, such as mercury, cadmium, lead and chromium (Ademoroti, 1996).
Environmental pollution with toxic metals is a global phenomenon and the potential effects of metallic contaminants on human health and the environment is on the increase. Therefore, research on the fundamental, applied and health aspects of heavy metals in the environment is also increasing (Yang and Sun, 2009). The coastal regions are some of the most sensitive environments and yet they are subject to growing human pressures because of increasing urbanization, industrial development and recreational activities. Human activities affect the
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natural geological and biological redistribution of heavy metals by altering the chemical forms of the metals released to the environment. The main anthropogenic sources of heavy metal contamination are mining, disposal of untreated and partially treated effluents containing heavy metals, as well as metal chelates from different industries and indiscriminate use of fertilizer and pesticides containing heavy metals (Hatje et al., 1998).
Heavy metals are non-biodegradable and can accumulate in the human body system, causing damage to the nervous system and other internal organs (Lee et al., 2007). Ni and Cu are tumor promoting factors, whose carcinogenic effect has attracted global concerns (Baryla et al., 2011). Workers who are in close contact with the nickel powder are more likely to suffer from respiratory cancer, and the content of Ni in the environment is positively correlated with nasopharyngeal carcinoma (Chen, 2011).
Aquatic organisms may be adversely affected by heavy metals, slightly elevated metal levels in natural waters may cause the following sublethal effects in aquatic organisms: histological or morphological change in tissues, changes in physiology such as suppression of growth and development, poor swimming performance, changes in circulation, changes in biochemistry such as enzyme activity and blood chemistry, changes in behaviour and reproduction (Cheung et al., 2001).
Conventional methods for removing heavy metals from aqueous solutions include chemical precipitation, ion exchange, adsorption and membrane filtration technologies (Gode and Pelalivan, 2006). In most soil environment, sorption is the dominating speciation process and
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thus the largest fraction of heavy metal in a soil is associated with the solid phase of that soil (Basta and Tabatai, 1992).
Adsorption is known as one of the most economical, effective and widely used methods for the removal of heavy metals from aqueous environments. The great advantage of adsorption over others is the low generation of residues, easy metal recovery and the possibility for the reuse of the adsorbent. Several low cost adsorbents such as agricultural/industrial wastes and natural/synthetic soil minerals have been used as effective adsorbents for the removal of heavy metals, organics and radionuclides in water treatment because of their strong ion-exchange and complex formation abilities with the heavy metals (Srivastava et al., 2005; Bekkouche et al., 2012;). Adsorption of metal ions from aqueous solution onto soil, clay minerals and clays has been a subject of interest in chemistry as well as in other research areas. It is considered that the adsorption of heavy metal ions onto soil occurs as a result of ion exchange, surface complexation, hydrophobic and electrostatic interaction (Jung et al., 1998). This is because the organic components of soils having carboxyl, phenol or amine groups may take part in heavy metal ion retention by complexation (Jung et al., 1998).
1.1 The Chemistry of Metal Ions in Soil
The chemistry of metal ions in soils may be affected by three main factors: specific adsorption to various solid phases, precipitation of sparingly soluble or highly stable compounds and formation of relatively stable complexes or chelates with soil components (Mouni et al., 2009). Soil is known to be either positively or negatively charged. Soil with a net positive charge is said to have an anion exchange capacity while a soil is said to have a cation exchange capacity
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when the soil particles have a net negative charge and they attract and retain cations (Mouni et al., 2009). Soil cations are divided into two categories which are; (i) the base cations which include ammonium, calcium, magnesium, potassium and sodium ions (ii) the acid cations which are aluminum and hydrogen ions. The pH of soil is one of the most important properties involved in understanding how rapidly reactions occur in soil and mostly the pH of soils usually range from 4 to 10 (Brady and Weil, 2002). Soil chemistry may also depend on the presence of organic matter (OM) containing many hydrogen and carbon compounds. The arrangement and formation of these compounds influence soil’s ability to handle spilt chemicals and other pollutants (Brady and Weil, 2002).
1.1.1 Soil surface functional groups
Soil surface functional groups consist of the chemically reactive molecular units bound onto the structure of a soil solid phase at its periphery, such that the reactive components of these units are in contact with the solution phase (Bradl, 2004). Soil surfaces display a variety of hydroxyl groups having different reactivity. Alumina surfaces, for example, possess terminal –OH groups which are more likely to accept additional protons in acidic solutions compared to a bridging –OH groups. Goethite (α-FeOOH) possesses four types of surface hydroxyls whose reactivity depends on the coordination environment of the oxygen atom in the Fe– OH group (Bradl, 2004). Alumosilicates (such as clay minerals, micas, zeolites, and most Mn oxides) display both aluminol (≡Al–OH) and silanol (≡Si–OH) edge-surface groups. The deprotonated aluminol group (i.e ≡Al–O−) binds metals in the form of more stable surface complexes (McBride, 1994).
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In a natural environment, adsorption by carboxylic groups is more important than adsorption by phenolic groups due to the wide difference between their acidity constants (Wood, 2005). Soil colloidal particles provide large interfaces and specific surface areas which play an important role in regulating the concentrations of many trace elements and heavy metals in natural soils and water systems. Weathering may also produce interlayer hydroxypolymers, interstratification, external-surface organic and inorganic coatings on smectite, as well as organic and iron oxide coatings on kaolinite (Bradl, 2004).
1.2 Sources of Heavy Metals
Metals are natural constituents of rocks, soils, sediments and water. They are released into the environment through volcanism, weathering of rocks and largely by human activities (Qin et al., 2008). Heavy metals may enter into an aquatic environment through acid rain, industrial or consumer waste deposition (Stumm and Morgan, 2012). Heavy metals occurring in most rivers are carried by suspended particles and only a fraction is transported in soluble form (Horowithz, 1991). However, heavy metals tend to accumulate more in sediments than in water and aquatic organisms (Lee et al., 2003). In soils, heavy metals can be present in various chemical forms and generally exhibit different physical and chemical behaviour in terms of chemical interaction, mobility, biological availability and potential toxicity (Li et al., 2000).
Heavy metals are persistent in all parts of the environment because they cannot be degraded or destroyed. Living organisms require trace amounts of some heavy metals such as vanadium, chromium, manganese, iron and nickel but excessive levels can be detrimental to the organisms.
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Their accumulation over time in the bodies of mammals can cause serious illnesses (Chao et al., 2014).
1.3 Occurrence and Effects of Selected Heavy Metals
1.3.1 Cadmium
Cadmium occurs in a few minerals and in small quantities in other ores, especially zinc ores, from which it is produced as a by – product. Some lead ores also contain small quantities of cadmium and if it is present in sufficient quantity, it is recovered by a cycle of operations similar to that used by zinc smelters (Chronopoulos et al., 1997). The concentrations of cadmium in agricultural soils depend upon the amounts present in the parent rocks from which the soil is formed, the amounts added in the form of fertilizers and soil amendments, the amounts deposited onto soils from the atmosphere and the amounts removed by harvested crops and by leaching (Volesky and Holan, 1995). On the average, sedimentary rocks contain greater concentrations of cadmium than either igneous or metamorphic rocks and therefore, recent soils derived from sedimentary rock should contain greater concentrations of cadmium than those derived from igneous or metamorphic rocks (Volesky and Holan, 1995).
In acute cadmium poisoning by ingestion, irritation of the gastrointestinal tract is the major symptom manifesting as nausea, vomiting, diarrhoea and abdominal cramps. Chronic exposure to cadmium by inhalation can damage the nervous system, kidney and liver. Other exposures to cadmium compounds may lead to skin lesions, lung tumour, decrease in heart contraction and damaged blood vessel in humans (Jenne et al., 2011).
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1.3.2 Copper
The major portion of copper produced in the world is used by electrical industries, most of which is combined with other metals to form alloy. Important series of alloys in which copper is the chief constituent are brass (copper and zinc) and bronze (copper and tin) (Morse, 1994). Copper resists the action of the atmosphere and sea water on its exposure for a long period of time. This may result in the formation of thin green protective coatings known as patina which is a mixture of hydroxocarbonate, hydrosulphate and small amount of other compounds. Copper is a moderately noble metal, unaffected by oxidizing or complexing dilute acids in the absence of air; it will however, dissolve readily in HNO3 and in H2SO4 in the presence of oxygen (Amin and Khaled, 2010)
Copper is among the heavy metals that are essential to life but could be toxic at elevated levels. It is toxic at high concentration in water and is known to cause brain damage in mammals. Elevated levels of this metal have been found to be toxic (Hukabee et al., 2011). Toxicity of copper in plants through chemical-treated agricultural soil has also been reported (Babula and Adam, 2008). Deposition of metals from industries, dumpsites and power plants that burn fuels have also contributed to the elevated level of copper in the environment (Baryla et al., 2011).
1.3.3 Lead
Lead is present in several minerals but all are of minor significance except the sulphide, PbS (galena), which is the major source of lead production throughout the world. Lead is also found in anglesite (PbSO4) and cerussite (PbCO3). The elimination of lead from insecticides and paint pigments and the use of respirators and other protective devices in areas of exposure have
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reduced lead poisoning (Sanborn et al, 2002). The recognition that the use of tetraethyl lead [Pb(C2H5)4] as an antiknock additive in gasoline was polluting the air and water led to the compound’s elimination as a gasoline constituent in the 1980s (Taylor, 1995). The major causes of lead poisoning in humans are: drinking of water carried through lead pipes and ingestion of materials that has lead-based paints. Children who chew painted toys and furnishings or eat paint peelings from walls are most likely to be affected (Sahu et al., 2008). Industries in which workers encounter lead-containing solids, dusts or fumes include: the petroleum, mining, smelting, printing, cutlery, plumbing, glass and ammunitions, gas fittings, ceramics and storage-battery manufacturing industries. Other possible sources of lead poisoning include: the agricultural use of insecticides containing lead compounds to spray fruits and vegetables which may eventually affect the workers and the consumers. In the mid-20th century, constant exposure to exhaust fumes of motor vehicles powered by fuel containing tetraethyl lead became a significant cause of lead poisoning, especially in children (Kersten et al., 2003).
Symptoms of lead poisoning may develop gradually or appear suddenly after chronic exposure (Jenne, et al., 2011). The poisoning affects the entire body especially the nervous system, the gastrointestinal tract and the blood forming tissues. The victim usually becomes palled, moody irritable and may complain of a metallic taste. Digestion is deranged, the appetite fails and there may be severe abdominal pain, with spasms of the abdominal muscles and constipation. A black line (lead line) may appear at the base of the gums. Other symptoms such as; anaemia headache, dizziness, confusion and visual disturbances may also be observed (Sahu et al., 2008). Peripheral nerve involvement results in a paralysis (lead palsy) that generally first affects the fingers, hands and wrists (wrist drop) (Goyer, 1996). The most serious effects are seen in
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children under the age of six, in whose brain and nervous system development is still occurring. In these children, even a small amount of lead can result in permanent damage and loss of function of the affected area of the brain. Complications may occur, such as learning disabilities, slowed growth, blindness, deafness, and, in extreme cases, convulsions and coma ending in death. Brain injury may also occur in adults after massive exposure (Sahu et al., 2008).
1.3.4 Chromium
Chromium is a naturally occurring element present in the earth’s crust, with oxidation states (or valence states) ranging from chromium (II) to chromium (VI) (Jacob and Testa, 2005). Chromium enters into various environmental matrices (air, water, and soil) from a wide variety of natural and anthropogenic sources with the largest release coming from industrial establishments. Industries with the largest contribution to chromium release include metal processing, tannery facilities, chromate production, stainless steel welding, and ferrochrome and chrome pigment production (Jacobs and Testa, 2005). The increase in the environmental concentrations of chromium has been linked to air and wastewater release of chromium, mainly from metallurgical, refractory, and chemical industries (Jacobs and Testa, 2005). Chromium is widely used in numerous industrial processes and as a result, it is a contaminant of many environmental systems (Cohen et al., 1993). Commercially chromium compounds are used in industrial welding, chrome plating, dyes and pigments, leather tanning and wood preservation. Chromium is also used as anticorrosive in cooking systems and boilers (Cohen et al., 1993). Chromium enhances insulin activity in the body and thus it is essential to the body (Jacobs and Testa, 2005).
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However, Chromium (VI) is one of the pronounced toxins; it easily permeates biological membrane due to its oxidizing potential. Occupational and environmental exposure to Cr(VI)-containing compounds is known to cause multiorgan toxicity such as renal damage, allergy, asthma, and cancer of the respiratory tract in humans (Bae et al., 2001).
1.4 Heavy Metals and Environmental Pollution
1.4.1 Effect of heavy metal contamination of soil on water
Heavy metal pollution of surface and underground water sources results in considerable soil pollution and pollution increases when mined ores containing toxic metals are dumped on the ground surface for manual dressing (Duruibe et al., 2007).
Irrigation water may transport dissolved heavy metals to agricultural fields thereby leading to its accumulation in plant roots and possibly throughout the plant (Jaishankar et al., 2014). Consequently, Animals that graze on such contaminated plants or drink from the polluted waters also accumulate such metals in their tissues, and milk (if lactating). Drinking and swimming in contaminated water causes skin rashes and health problems like cancer, reproductive problems, typhoid fever and stomach sickness in humans. Industrial chemicals and agricultural pesticides containing heavy metals that end up in aquatic environments can accumulate in marine lives. Fishes are easily poisoned with metals that are also later consumed by humans (Jaishankar et al., 2014).
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1.4.2 Effect of heavy metal contamination of soil on the plants
Plants experience oxidative stress upon exposure to heavy metals that leads to cellular damage and disturbance of cellular ionic homeostasis (Yadav, 2010). High concentration of heavy metals in the soil affects the growth of plants and may lead to the death of the plant if not controlled. Research found that the growth of cabbage and bean seedling under cadmium concentration of 30 μ mol/L was inhibited, decreasing the root length, the plant height and leaf area (Qin et al., 2008). Cadmium may interfere with crop photosynthesis and protein synthesis, and cause membrane damage (Chao et al., 2014). Heavy metals in urban or agricultural soils may either go into the body directly through ingestion, contact with the skin or absorbed and accumulated by crops. Ingesting heavy metals through the soil – crop system is a major way of damaging human health (Aeliona et al., 2008).
1.5 Statement of Problem
Heavy metals are non-biodegradable and can accumulate in the human body system, causing damage to the nervous system and other internal organs. Heavy metals are adsorbed either by initial fast reactions (in seconds, minutes or hours) or by slow adsorption reactions and are, thereafter, redistributed into different chemical forms with varying bioavailability, mobility, and toxicity. It is therefore expected that when several heavy metals exist in the water or wastewater stream, some will be more difficult to be removed than others and a competitive environment will be created. Several studies have been carried out to develop more effective and selective adsorbent materials, which are abundant in nature and require minimal processing in order to reduce cost. Many authors have also investigated the adsorption of metals on different soil minerals under different experimental conditions. However, the application of a standard
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reference soil for the simultaneous removal of multi-component heavy metal from water and wastewater systems has not been assessed.
1.6 Justification
In order to arrive at an accurate remedial alternative for heavy metal contaminated water, a thorough knowledge of the chemistry of their dissolution, transport, adsorption and selectivity is necessary. Assessing the adsorption capacity of Cd2+, Cr6+, Cu2+ and Pb2+ on a standard reference soil will aid in modeling a removal scheme for water contaminated with these heavy metals. This study will as well be useful in designing control strategies for the amendments and fixation of heavy metals in water in order to achieve better groundwater protection. Also, the knowledge derived from the competitive assessment of Cd2+, Cr6+, Cu2+ and Pb2+ will be significant in the design of remediation methods for polluted farmland soil especially when there is multi-elemental interaction between these metals on the soil.
1.7 Aim and Objectives
The aim of this research was to study the competitive adsorption of Cd, Cu, Cr and Pb ions onto a standard reference soil. This aim was achieved through the following objectives:
(i) optimization of the initial metal ion concentrations, adsorbent dosages and contact times for the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ from aqueous solution onto soil
(ii) modeling the adsorption characteristics of Cd2+, Cr6+, Cu2+ and Pb2+ using Langmuir and Freundlich adsorption isotherms.
(iii) determination of the appropriate kinetic models that best describe the adsorption process.
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(iv) investigation of the competitive adsorption of the selected metal ions in the binary, ternary and quaternary systems.
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