ABSTRACT

The presence of heavy metals in the environment constitutes a potential source of both soil and groundwater pollution which is a major environmental concern worldwide. The retention of cadmium and lead by selected Nsukka urban soils obtained from three different sampling locations with a range of soil properties representing the ultisol soil type of tropics was investigated. The effects of contact time, pH, concentration, and temperature on the adsorption process were investigated using the batch technique equilibrated for 24 hours at room temperature. For all soils examined, the study revealed that the adsorption capacities of the soils for cadmium (Cd) and lead (Pb) increased with increase in pH, temperature, contact time, and concentration. The adsorption data were fitted to the Langmuir, Freundlich and Temkins adsorption isotherm. The results indicated that the adsorption isotherm could be satisfactorily described by the Langmuir model. On the basis of the maximum adsorption capacity (q_max), the order of affinity of cadmium and lead for the studied soils was Pb²⁺ > Cd²⁺. The maximum adsorption values for Pb range from 2.06 to 2.54 mg/g while that for Cd range from 1.02 to 1.34 mg/g. Three simplified kinetic models including pseudo-first order, pseudo-second order and Weber and Morris intra-particle diffusion were used to fit the experimental data. The kinetic data of the adsorption process for Pb²⁺ and Cd²⁺ in all soils studied gave better satisfactory fit to pseudo-second order model compared to the Weber and Morris intra-particle diffusion and pseudo-first order, respectively.

 

 

 

TABLE OF CONTENTS

Title Page                                                                                                                                               i

Approval Page                                                                                                                                                  ii

Certification                                                                                                                                         iii

Declaration                                                                                                                                          iv

Dedication                                                                                                                                            v

Acknowledgements                                                                                                                            vi

List of Figures                                                                                                                                     vii

List of Tables                                                                                                                                       x

List of Abbreviations                                                                                                                          xi

Table of Contents                                                                                                                              xii

Abstract                                                                                                                                           xviii

CHAPTER ONE

1.0       INTRODUCTION                                                                                                                  1

• Background of the Study   1
• Behaviour of Metals in Soils   2
• Statement of the Problem   4  4 Aim and Objectives of the Study                                                                                                4

1.4.1    Aim of study                                                                                                                             4

1.4.2    Objectives of Study                                                                                                                  5

• Significance of the Study                                                                                                        5
• Scope of Study                                                                                                                      6

 

CHAPTER TWO

2.0       LITERATURE REVIEW                                                                                                     7

2.1      Adsorption Phenomena                                                                                                             7

2.1.1   Physical adsorption (Physisorption)                                                                                           8

2.1.2   Chemical adsorption (Chemisorption)                                                                                       8

2.2       Sorption in Soils                                                                                                                        9

2.3       Surface Complexation                                                                                                                          9

2.4       Parameters influencing adsorption of heavy metals on soils                                                  12

2.4.1   Soil pH                                                                                                                                     12

2.4.2   Soil organic matter                                                                                                                   12

2.4.3   Metal ion                                                                                                                                  13

2.4.4    Soil type                                                                                                                                  13

2.4.4.1 Oxides in soils                                                                                                                        14

2.4.4.2 Surface functional groups in soils                                                                                           14

2.5        Individual Adsorption Behavior of Cadmium (Cd) and Lead (Pb)                                      15

2.5.1     Cadmium                                                                                                                               15

2.5.2     Lead                                                                                                                                       16

2.6        Adsorption Isotherm                                                                                                              17

2.7        Empirical Models                                                                                                                   18

2.7.1     Linear Adsorption (K_d Approach) Isotherm                                                                          18

2.7.2     Freundlich Isotherm                                                                                                               19

2.7.3     Langmuir Isotherm                                                                                                                20

2.8        Sorption Kinetics                                                                                                                   20

2.8.1     Lagergren Pseudo- first order kinetics model                                                                        21

2.8.2    Pseudo- second order kinetics model                                                                                     21

2.8.3    Intra-particle diffusion  model                                                                                                22

2.9       Review of Some Previous Works Done on Adsorption of Metals by Soils                           23

CHAPTER THREE

3.0       EXPERIMENTALS                                                                                                                        30

3.1       Description of study area                                                                                                        30

3.2       Location of the sample points in the study area                                                                     31

3.2       Collection of Samples                                                                                                             33

3.3       Materials Used                                                                                                                        33

3.3.1    Reagents                                                                                                                                 33

3.3.2    Equipment                                                                                                                               37

3.4       Sample Preparation and Analysis                                                                                           39

3.4.1    Preparation of Stock Solution of the Metal Ions                                                                    39

3.4.2    Preparation of 1000 mg/L of Pb(NO₃)₂ Stock Solution                                                         39

3.4.3    Preparation of Diluted Solution of  Pb(NO₃)₂ from Stock Solution                                      39

3.4.4    Preparation of 1000 mg/L of Cd(NO₃)₂ Stock Solution                                                         40

3.4.5    Preparation of Dilute Solution of Cd(NO₃)₂ from Stock Solution                                         40

3.4.6    Preparation of 0.01 M CaCl₂                                                                                                  41

3.4.7     Preparation of 0.05 M NaOH                                                                                                            41

3.4.8     Preparation of 0.10 M NaOH                                                                                                            42

3.4.9     Preparation of 0.10 M KCl                                                                                                    42

3.4.1.0   Preparation of 1 M K₂Cr₂O₇                                                                                                 42

3.4.1.1  Preparation of 1 M FeSO₄                                                                                                     42

3.4.1.2  Preparation of 0.01 M EDTA                                                                                                            43

3.4.1.3  Preparation of 0.05 M HCl                                                                                                    43

3.4.1.4  Preparation of 20 % KOH Solution                                                                                       43

3.4.1.5  Preparation of 1 % Phenolphthalein                                                                                      43

3.4.1.6  Preparation of 1 % Diphenylamine Indicator                                                                        43

3.4.1.7  Preparation of 1 N CH₃COONH₄                                                                                         44

3.4.1.8  Preparation of 0.10 M Potassium Hydrogen Phathalate                                                        44

3.4.1.9  Preparation of 0.10 M Potassium Dihydrogen Phosphate                                                     44

3.4.2.0 Preparation of Ammonium Hydroxide Buffer 10 Solution                                                    44

3.4.2.1 Preparation of pH Buffer 4 Solution                                                                                      44

3.4.2.2  Preparation of pH Buffer 7 Solution                                                                                     45

3.5       Treatment of Soil Samples for Determination of Cadmium and Lead Concentration           45

3.6       Determination of Physico-Chemical Properties of the Soil Samples                                      45

3.6.1    Determination of particle size distribution                                                                             45

3.6.2    pH Determination                                                                                                                   47

3.6.3    Determination of carbon content                                                                                            48

3.6.4    Determination of organic matter                                                                                             48

3.6.5    Determination of cation exchange capacity (CEC)                                                                49

3.6.6    Determination of exchangeable acidity (EA = Al³⁺, H⁺)                                                        49

3.6.6.1 Determination of exchangeable aluminium (Al³⁺)                                                                  50

3.6.6.2 Determination of exchangeable hydrogen ion (H⁺)                                                                51

3.6.7.0 Determination of exchangeable bases (EB = Ca²⁺ and Mg²⁺)                                               51

3.6.7.1 Determination of calcium ion (Ca²⁺)                                                                                       51

3.6.7.2 Determination of magnesium ion (Mg²⁺)                                                                                52

3.6.8    Determination of sodium and potassium ion (Na⁺ and K⁺)                                                    52

3.6.9    Adsorption studies using batch equilibrium technique                                                           52

3.6.9.1 Determination of the effect of pH on adsorption                                                                   53

3.6.9.2 Determination of the effect of contact time on adsorption                                                    54

3.6.9.3 Determination of the effect of initial metal ion concentration on adsorption                                    54

3.6.9.4 Determination of the effect of temperature on adsorption                                                     55

3.7       Adsorption studies data analysis                                                                                            55

CHAPTER FOUR

4.0       RESULTS AND DISCUSSION                                                                                          56

4.1       Results of Concentration of Cadmium and Lead in Soil Samples                                         56  4.2 Results of Physico-Chemical Parameters of Soil Samples                                                           58

4.2.1    Results of effect of pH on adsorption of Pb²⁺ and Cd²⁺                                                        60

4.2.2    Results of effect of temperature on adsorption of Pb²⁺ and Cd²⁺                                          63

4.2.3    Results of effect of initial metal ion concentration on adsorption     of Pb²⁺ and Cd²⁺        66

4.2.4    Results of effect of contact time on adsorption of Pb²⁺ and Cd²⁺                                         69

4.3       Results of adsorption isotherms                                                                                              72

4.3.1    Langmuir isotherm model                                                                                                       72

4.3.2    Freundlich isotherm model                                                                                                     81

4.3.3    Temkins isotherm model                                                                                                         88

4.4       Results of adsorption  kinetics                                                                                                95

4.4.1    Pseudo-first order kinetics                                                                                                      95

4.4.2    Pseudo-second  order kinetics                                                                                                          102

4.4.3    Weber and morris intra-particle diffusion kinetics                                                               109

 

CHAPTER FIVE

5.0       CONCLUSIONS                                                                                                                116

5.1       Conclusions                                                                                                                           116

REFERENCES                                                                                                                                118

APPENDIX                                                                                                                                      126

 

 

CHAPTER ONE

• INTRODUCTION
• Background of the Study

Soil is one of the most important components of all terrestrial ecosystems and is of essential importance and it plays a pivotal role in the sustenance of all life forms on the planet. Some of the roles played by soil range from the simplest of functions like providing anchorage and nutrients for the growth of crops, trees and grassland, and regulating water supplies, to more complex functions such as helping in maintaining a clean environment via degradation and transfer of biomass and being a source and sink for atmospheric gases¹. The importance of soils to all life forms cannot be overemphasize.

Soil is a very complex heterogeneous mixture, which consists of solid phases (the soil matrix) containing minerals and organic matter and fluid phase (the soil water and the soil air), which react with each other and ions entering the soil system².

Due to growing industrialization and urbanization, heavy metals are increasingly introduced into the environment mainly soil, via a variety of sources. These sources include industrial processes, application of sewage sludges, fertilizers, pesticides, and fungicidal sprays applied to plants, atmospheric deposition, municipal effluent, and disposal of electronic waste.³ Unfortunately, some of these heavy metals can be taken up by crops, thereby entering the food chain. Hence, soils provide a potential pathway with which heavy metals may become bioavailable to humans.

Soils in urban environment have direct influence on public health, this is because they receive higher than normal loads of contaminants from anthropogenic activities, mostly in industrial areas. Most of the heavy metals are bound to particles in sediments, but only a small quantity becomes dissolved in the water and it can spread widely in the food chain⁴.

The term heavy metal refers to any metallic element that has a relatively high density, toxic or poisonous at low concentrations, that are stable and cannot be degraded or eliminated⁵. Heavy metals have been classified into essential and non-essential metals. The essential metals are needed by living organisms in trace quantities for optimum performance of life processes. They include Ni, Fe, Zn, Co, Mo, e.t.c.⁶. Insufficient supply of these essential metals in an organism, leads to problems associated with growth and ability to complete its life cycle, while sufficient supply results in optimum conditions   and excess supply results in toxic effects and possibly death⁷.

The non-essential elements include As, Ag, Cd, Hg, and Pb and the ability of various organisms to accommodate these non-essential metals are limited⁸. They may be tolerated at very low concentrations, some are toxic even if their concentration is very low, and their toxicity increases with accumulation in water and soils⁹.

Heavy metal ions are the most toxic inorganic pollutants which are present in soils and can be of natural or anthropogenic origin¹⁰. Heavy metals may be found in soils¹¹, ground water¹², sediments, plants¹³ and even in dust¹⁴. They cause many health problems, some of which include cancer, renal damage, Wilson’s disease(neurological or psychiatric symptoms of liver disease, compounded with heavy metal deposits), lung damage, dermatitis, nausea, chronic asthma, headache, dizziness, rapid respiration, e.t.c.^{12, 15}.

• Behaviour of Metals in Soils

The soil is the primary recipient either by design or accident of  diverse waste products and chemicals used in industry¹⁶. Natural source of trace metal elements in soils is parent materials of the soil. Anthropogenic sources, including industrial emissions and effluents, bio-solids, fertilizers, and pesticides can also contribute to the increase in the amount of heavy metals in soils¹⁷. Heavy metals are spread to the ground in several ways, such as spillage or corrosion of products that contain heavy metals. The heavy metals that are spread to the ground by airborne particles remain mainly in the ground where they are deposited, because heavy metals have strong affinity for the soil matrix¹⁷. In the ground, heavy metals are distributed as either soluble in the soil solution, sorbed to the soil matrix or precipitated as solids¹⁷. While in the soil solution, heavy metal can exist as a free ion, as well as soluble complexes with different organic and inorganic ligands¹⁷. Common inorganic ligands in the soil are SO₄^2-, Cl^¯, OH^¯, PO₄^3-, NO₃^¯ and CO₃^2-, and common organic ligands are low molecular weight aliphatic, aromatics, amino acids and soluble constituents of fulvic acids such as phenol¹⁷.

The persistence of heavy metals in the soil and its reduction in mobility involves the phenomena of sorption, desorption, precipitation, complexation, oxidation/reduction and dissolution. Although these phenomenons can occur simultaneously, adsorption mechanisms are known to be determinant in the control of metal availability and solubility in the soil¹⁸. Adsorption phenomenon in soils controls the concentration of metal ions and complexes in the soil solution and thus can exert a major influence on translocation of the metal ions in plant¹⁹.

The term ‘sorption’ is often broadly used to encompass all processes that involve either the retention or release of contaminants in soil. Sorption process is one of the most important chemical processes that occurs in soils, it governs the rate and quantity of nutrient uptake by plants, transport of heavy metals, radionuclides, pesticides, organic pollutants and other materials within soil systems. Sorption processes are complex and are influenced by soil properties such as texture, bulk density, pH, organic matter, cation exchange capacity, and the type and amount of the different clay minerals present in any particular soils²⁰.

Adsorption of heavy metals in soil involves two mechanisms. One is selective adsorption, in which surface complexes are formed; while the other is non-selective adsorption, in which the metallic cations act as counter-ions in the diffuse layer. The importance of each of these mechanisms depends on the metal and the nature of soil²¹.

Contamination of the environment most especially soils, by heavy metals, through increasing anthropogenic activities, has been on the rise in recent years and this has been of considerable concern due to environmental hazards, and various health problems it poses such as cancer, gene mutation, neurological disorder, e.t.c. Moreover, since soils offer a possible sink for pollutants, there is a need to investigate how pollutants are sorbed by soil and the fate of the pollutants in the environment. Furthermore, the presence of heavy metals in Nsukka urban soil has been well documented by researchers^{22, 23}.However, the adsorption capacity of the soil for heavy metals has not been reported. Also, reports on how Nigerian soils function as sorbent for heavy metals are limited in literature. In addition, IUPAC observed that the majority of investigations of fate of pollutants in the environment have been conducted in temperate soils, predominately in Europe and North America, with limited report on tropic soils most especially Africa²⁴. However, approximately one-half of the earth’s population and roughly one-third of its land mass are found in the tropics²⁴. Hence, this has necessitated and underscores the need for this study.

• Aims and Objectives of the Study

1.4.1    Aims of Study

The main aim of the present work is to study the retention capacity of cadmium (Cd) and lead (Pb) by selected Nsukka urban soils as well as to investigate the effect of some parameters on the adsorption of Cd and Pb by the soils using the batch technique.

 

 

 

 

 

1.4.2   Objectives of Study

            The objectives of this research work are to:

1. ascertain the physico-chemical properties of soil of the study area.
2. determine cadmium and lead adsorption capacity of Nsukka urban soils by investigating

the effects of pH, concentration, temperature and contact time on the adsorption process.

iii.  evaluate the adsorption isotherm of the adsorption process using the empirical models;   Langmuir, Freundlich and Temkin isotherm.

1. propose the mechanism of the adsorption process using kinetics models of pseudo-first order, pseudo-second order and Weber and Morris intra-particle diffusion.
2. provide baseline data on cadmium and lead adsorption capacity of the soils of the study areas.

1.5      Significance of the Study

This study is of a great significance to the researcher as the knowledge created in relation to the study will be of immense benefit towards the understanding of how cadmium and lead are sorbed by the soil and the bioavailability of these metal ions in soil of the study areas. Additionally, the knowledge created regarding the study can give a clue on cadmium and lead translocation in plants grown on the soils.

Also, the outcome of this study can be of help towards contributing to the design of effective remediation strategies and providing possible alternatives for a preventive environmental control that is needed by environmentalist via formulation of guidelines.

Furthermore, the outcome of the study can serve as additional database to the researchers in onward adsorption studies of heavy metals by soils, most especially tropical soils.

 

 

1.6       Scope of Study

The research focuses on and is limited to cadmium and lead adsorption capacity of Nsukka urban soils using batch equilibrium techniques. The investigation was done by varying operational variables such as pH, initial concentration, contact time, and temperature to determine the effects of the variables on the adsorption process. The adsorption capacity of the soils for the metal ions is described using empirical models; Langmuir, Freundlich and Temkins adsorption isotherm model. The mechanism of the reaction is described using the pseudo-first order, pseudo-second order and Weber and Morris intra-particle diffusion kinetics model.

 

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