Assessment of the Competitive Adsorption of Some Heavy Metal Ions onto Soil.
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
Fly leaf ii
Title page iii
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
INTRODUCTION 1
- The Chemistry of Metal Ions in Soil 3
- Soil surface functional groups 4
- Sources of Heavy Metals 5
- Occurrence and Effect of Selected Heavy Metals 6
- Lead 7
- Chromium 9
- Heavy Metals and Environmental Pollution 10
- Effect of heavy metal contamination on water 10
- Effect of heavy metal contamination on plants 11
- Statement of Problem 11
- Justification 12
- Aim and Objectives 12
CHAPTER TWO
LITERATURE REVIEW 14
- Adsorption Mechanism of Heavy Metals 14
- Ion- Exchange mechanism of heavy metals in soil 15
- Factors Influencing Mobility and Adsorption of Heavy Metal Ions in Soil 15
- Adsorption of Heavy Metals by Different Soils 19
- Competitive Adsorption of Metal Ions in Soil 21
- Adsorption selectivity sequence of heavy metals in soil 23
- Standard Reference Soil 24
- Applications of Adsorption of Metal Ions from Solution 25
- Principle of Atomic Absorption Spectrophotometry (AAS) 25
- Adsorption Isotherms 26
- Langmuir adsorption model 26
- Freundlich adsorption model 27
- Adsorption Kinetics 28
- Pseudo – first Order model 28
- Pseudo – second Order model 28
CHAPTER THREE
MATERIALS AND METHODS 30
- Apparatus and Equipment 30
- Collection of Soil Sample 30
3..2.1 Preparation of soil sample 30
- Determination of Soil pH 31
- Preparation of Stock Solutions 31
- Cadmium solution 31
- Copper solution 31
- Lead solution 31
- Chromium solution 32
- Preparation of binary solutions 32
- Preparation of ternary solutions 32
- Preparation of quaternary solutions 32
- Standard Working Solution of Metal Ions 33
- Adsorption Experiments 33
- Effect of initial concentrations on the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 34
- Effect of adsorbent dose on the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 34
- Effect of contact time on the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 35
- Modeling Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 35
- Adsorption Kinetics of Cd2+, Cr6+, Cu2+ and Pb2+ 36
- Competitive Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 36
- Adsorption from binary systems 36
- Adsorption from ternary systems 37
- Adsorption from quaternary systems 37
- Statistical Analysis 37
CHAPTER FOUR
RESULTS 39
- Scanning Electron Microscope (SEM) Analysis 39
- Adsorption of the Selected Metal Ions 41
- Effect of initial metal ion concentration on the adsorption of selected metal ions 41
- Effect of adsorbent dose on the adsorption of selected metal ions 41
- Effect of contact time on the adsorption of selected metal ions 41
- Adsorption Modeling 45
- Adsorption Kinetics 55
- Competitive Adsorption of Selected Metal Ions 65
- Comparison between the adsorption of selected metal ions in single and mixed systems using statistical analysis 65
CHAPTER FIVE
- DISCUSSION 68
- Scanning Electron Microscope (SEM) Analysis 68
- Soil pH 68
- Effect of Initial Metal ion Concentration on the Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 69
- Effect of Adsorbent Dose on the Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 70
- Effect of Contact Time on the Adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ 71
- The Langmuir and Freundlich Isotherms Modeling for Metals Uptake 72
- Kinetics Modeling of Cd2+, Cr6+, Cu2+ and Pb2+ Adsorption 74
- Competitive Adsorption of Heavy Metal Ions 76
- Comparison of the adsorption of Cd2+, Cr6+, Cu2+ and Pb2+ in the single and mixed systems 81
CHAPTER SIX
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 82
- Summary 82
- Conclusion 83
- Recommendation for Further Studies 84
REFERENCES 86
APPENDICES 95
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).
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