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 (qmax), the order of affinity of cadmium and lead for the studied soils was Pb2+Â > 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+Â and Cd2+Â 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.
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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
1.0Â Â Â Â Â Â INTRODUCTIONÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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
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CHAPTER TWO
2.0Â Â Â Â Â Â LITERATURE REVIEWÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 7
2.1     Adsorption Phenomena                                                                                                           7
2.1.1Â Â Physical adsorption (Physisorption)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 8
2.1.2Â Â Chemical adsorption (Chemisorption)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 8
2.2      Sorption in Soils                                                                                                                      9
2.3      Surface Complexation                                                                                                                        9
2.4       Parameters influencing adsorption of heavy metals on soils                                                12
2.4.1Â Â Soil pHÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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
2.5Â Â Â Â Â Â Â Individual Adsorption Behavior of Cadmium (Cd) and Lead (Pb)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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 (Kd 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
3.0Â Â Â Â Â Â Â EXPERIMENTALSÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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 Â Â Â Preparation of 0.01 M CaCl2Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 41
3.4.7Â Â Â Â Preparation of 0.05 M NaOHÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 41
3.4.8 Â Â Â Â Preparation of 0.10 M NaOHÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 42
3.4.9    Preparation of 0.10 M KCl                                                                                                   42
3.4.1.0Â Â Preparation of 1 M K2Cr2O7Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 42
3.4.1.1 Â Preparation of 1 M FeSO4Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 42
3.4.1.2 Â Preparation of 0.01 M EDTAÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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 Â Preparation of 1 N CH3COONH4Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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
3.6.5Â Â Â Determination of cation exchange capacity (CEC)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 49
3.6.6Â Â Â Determination of exchangeable acidity (EA = Al3+, H+)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 49
3.6.6.1 Determination of exchangeable aluminium (Al3+)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 50
3.6.6.2 Determination of exchangeable hydrogen ion (H+)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 51
3.6.7.0 Determination of exchangeable bases (EB = Ca2+Â and Mg2+)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 51
3.6.7.1 Determination of calcium ion (Ca2+)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 51
3.6.7.2 Determination of magnesium ion (Mg2+)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 52
3.6.8Â Â Â Determination of sodium and potassium ion (Na+Â and K+)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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
4.0Â Â Â Â Â Â RESULTS AND DISCUSSIONÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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
4.2.1Â Â Â Results of effect of pH on adsorption of Pb2+Â and Cd2+Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 60
4.2.2Â Â Â Results of effect of temperature on adsorption of Pb2+Â and Cd2+Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 63
4.2.3   Results of effect of initial metal ion concentration on adsorption    of Pb2+ and Cd2+       66
4.2.4Â Â Â Results of effect of contact time on adsorption of Pb2+Â and Cd2+Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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
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CHAPTER FIVE
5.0Â Â Â Â Â Â Â CONCLUSIONSÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 116
5.1      Conclusions                                                                                                                         116
REFERENCESÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 118
APPENDIXÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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.3Â 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 SO42-, Cl¯, OH¯, PO43-, NO3¯ and CO32-, 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.
- Statement of the Problem
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 researchers22, 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
1.4.1Â Â Â 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.
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1.4.2Â Â Objectives of Study
            The objectives of this research work are to:
- ascertain the physico-chemical properties of soil of the study area.
- 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.
- propose the mechanism of the adsorption process using kinetics models of pseudo-first order, pseudo-second order and Weber and Morris intra-particle diffusion.
- provide baseline data on cadmium and lead adsorption capacity of the soils of the study areas.
1.5Â Â Â Â Â 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.
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1.6Â Â Â Â Â Â 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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