ABSTRACT
A field trial to investigate the phyto-extraction of Cd, Co, Cu, Ni, Pb and Zn by Vetivera zizanioide (Jema), Ipomoea asarifolia (duman kada) Helianthus annus (sunflower), Ricinus communis L. (Castor oil plant) and Cymbopogon citratus (Lemon grass) planted on contaminated soils at Dakace, Gaskiya and Hanwa in Zaria, Nigeria was carried out between July to September, 2013. The effect of application of ethylenediamine-tetraacetic acid (EDTA) on the phyto-extraction of the studied metals by the test plants was also investigated. The harvested plants and the soils collected from the experimental and control sites were analyzed using atomic absorption spectrometry to determine the concentrations of these elements in different parts of each plant species and the soil treated and untreated with EDTA. The physicochemical parameters of the soil such as pH, moisture content, particle size, cation exchange capacity (CEC), organic matter (OM), chloride, phosphates, nitrates and sulphates were determined. The soils was subjected to sequential extraction to ascertain the bioavailability of the metals in the soils. Statistical analysis on the data obtained was carried out using One- way ANOVA. The results indicated that the soils at the experimental and control sites were sandy loam in texture, slightly acidic for the experimental soils and slightly alkaline for the control soil. The %OM and CEC for the experimental soils were higher than the soils of the Nigerian savanna. The concentrations of the studied metals in the soils from experimental sites were higher than the corresponding values from the control site, and higher than the recommended limits given by the Joint Food and Agricultural Organization and the World Health Organization (FAO/WHO). The Nemero Pollution Index showed that soils at the experimental sites were slightly to moderately polluted with Cd, Co, Ni and Zn and very heavily polluted with Pb. The
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percentage bioavailability of metals at the experimental sites were: At Dakace Cd – 22.7%, Co – 29.7%, Cu – 80.7%, Ni – 53.2%, Pb – 24.3%, Zn – 64.3%; at Gaskiya, Cd – 22.4%, Co – 30.3%, Cu – 56.7%, Ni – 43.0%, Pb – 22.3%, Zn – 47.6% and at Hanwa, Cd – 21.4%, Co – 34.8%, Cu – 41.2%, Ni -42.5%, Pb – 18.7%, Zn – 63.4%. The Bioaccumulation factor (BF) and Transfer factor (TF) for the metals varied from plant to plant. The percentage removal of the studied metals from the EDTA treated soils and non EDTA treated soils were: At Dakace site, without EDTA application (natural) were: Cd- 52.5%, Co – 0.6%, Cu – 44.3%, Ni – 88.4%, Pb – 13% and Zn – 28%; with EDTA application: Cd – 70.3%, Co – 1.2%, Cu – 70.5%, Ni – 89.9%, Pb – 38.3% and Zn – 36%; At Gaskiya site without EDTA application (natural): Cd – 45.7%, Co – 15.7%, Cu – 24.5%, Ni – 69.7%, Pb – 21.2% and Zn – 17.1%; with EDTA application: Cd -62%, Co – 19.2%, Cu – 60.4%, Ni – 90.5%, Pb – 27.9% and Zn – 29.1%. At Hanwa site without EDTA application (natural): Cd – 36.7%, Co – 1.2%, Cu – 28.4%, Ni – 20.5%, Pb – 17.4% and Zn – 17.2%; with EDTA application: Cd – 59.7%, Co – 2.3%, Cu – 32.1%, Ni – 35.6%, Pb – 42.1% and Zn – 19.9%. The bioaccumulation and transfer factors for the studied metals by the test plant species are indications that these plant species have the potentials for phyto-remediation under field conditions even on soils that were untreated with EDTA. The phyto-extraction potentials of the test plant species were in the order of Cymbopogon citratus (Lemon grass) > Vetivera zizanioide (Jema) > Helianthus annus (sunflower) > Ricinus communis L. (Castor oil plant) > Ipomoea asarifolia (duman kada).
TABLE OF CONTENTS
Title page i Declaration ii Certification iii Acknowledgement iv Abstract v Table of Contents vii List of Tables xiv List of Figures xv List of Plates xxii List of Appendices xxiii Abbreviations xxviii CHAPTER ONE
1.0 INTRODUCTION 1
1.1 Statement of Problem 3
1.2 Justification 3
1.3 Aim of the Research 3
1.4 Objectives 4
CHAPTER TWO
2.0 LITERATURE REVIEW 5
2.1 Municipal Solid Waste Management 6
2.2 Heavy Metals 7
2.2.1 Heavy metal toxicity 7
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2.2.2 Heavy metal bioavailability 8
2.2.3 Cadmium 10
2.2.4 Copper 12
2.2.5 Nickel 14
2.2.6 Cobalt 16
2.2.7 Lead 17
2.2.8 Zinc 19
2.3 Phytoremediation 22
2.3.1 Advantages of phytoremediation compared to conventional remediation 23
2.3.2 Disadvantages of phytoremediation compared to conventional remediation 23
2.3.3 Phytovolatilization 24
2.3.4 Rhizofiltration 24
2.3.5 Phytostabilization 25
2.3.6 Phytodegradation 25
2.3.7 Phytoaccumulation/extraction 25
2.3.8 Hyperaccumulators 26
2.3.9 Advantages and limitations of phytoremediation 27
2.3.10 Chelate-assisted phytoextraction 28
2.4 Plants for Phytoextraction 32
2.4.1 Vetivera zizanioide 32
2.4.2 Ipomoea asarifolia 34
2.4.3 Cymbopogon citratus 34
2.4.4 Helianthus annus 36
2.4.5 Ricinus cummunis 37
2.5 Soil Parameters 38
2.5.1 Cation exchange capacity 38
2.5.2 Soil pH 39
2.5.3 Particle size 40
2.5.4 Organic matter 41
2.5.5 Soil moisture content 42
2.5.6 Nitrates 43
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2.5.7 Chlorides 44
2.5.8 Phosphorus 44
2.6 Temperature and Climate 45
CHAPTER THREE 3.0 MATERIALS AND METHODS 47 3.1 Study Areas 47 3.2 Experimental Design 47 3.3 Sample Collection and Treatment 56 3.4 Preparation of EDTA Solution 56 3.5 Sequential Extraction 57 3.6 Determination of Soil pH 59 3.7 Determination of Soil Moisture Content 59 3.8 Determination of Particles Size 60 3.9 Determination of Cation Exchange Capacity 60 3.10 Determination of Chloride 62 3.11 Determination of Phosphate 63 3.12 Determination of Sulphate 65 3.13 Determination of Nitrate 66 3.14 Determination of Organic Matter 68 3.15 Determination of Cd, Co, Cu, Ni, Pb and Zn 69 3.16 Preparation of Calibration Curve 70
3.17 Pollution Index 71
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3.18 Quality Assurance Validation 72 3.18.1 Preparation of multi-element standard solution 72 3.18.2 Spiking experiment 73 3.19 Statistical Analysis 74 CHAPTER FOUR 4.0 RESULTS 75
4.1 Results of the Physicochemical Parameters of Soils 75
4.2 Quality Assurance Validation 75
4.3 Metal Levels in Soils 75
4.4 Concentration of Metals in Different Fractions of Soils 76
4.5 Bioavailability of Metals in Soils 89
4.6 Metal Contents in Different Parts of Plants without EDTA
Application at Dakace 89 4.6.1 Metal concentration in different parts of Vetivera zizanioide 89
4.6.2 Metal concentration in different parts of Cymbopogon citratus 90
4.6.3 Metal concentration in different parts of Helianthus annus 90
4.6.4 Metal concentration in different parts of Ipomoea asarifolia 91
4.6.5 Metal concentration in different parts of Ricinus communis 91
4.7 Bioaccumulation and Transfer Factors for the Metals in Different
Plant Species at Dakace 92
4.8 Metal Contents in Different Parts of Plants without EDTA
Application at Gaskiya 92 4.8.1 Metal concentration in different parts of Vetivera zizanioide 92
4.8.2 Metal concentration in different parts of Cymbopogon citratus 109
4.8.3 Metal concentration in different parts of Helianthus annus 109
4.8.4 Metal concentration in different parts of Ipomoea asarifolia 110
4.8.5 Metal concentration in different parts of Ricinus communis 110
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4.9 Bioaccumulation and Transfer Factors for the Metals in Different Plant Species at Gaskiya 111 4.10 Metal Contents in Different Parts of Plants without EDTA Application at Hanwa 128
4.10.1 Metal concentration in different parts of Vetivera zizanioide 128
4.10.2 Metal concentration in different parts of Cymbopogon citratus 129
4.10.3 Metal concentration in different parts of Helianthus annus 129
4.10.4 Metal concentration in different parts of Ipomoea asarifolia 130
4.10.4 Metal concentration in different parts of Ricinus communis 141
4.11 Bioaccumulation and Transfer Factors for the Metals in Different
Plant Species at Hanwa 141
4.12 Metal Contents in Different Fractions of Soils with EDTA Application
at the Study Areas and Control 149
4.13 Metal Contents in Different Parts of Plant with EDTA Application
at Dakace 151 4.13.1 Concentrations of metals in different parts of Vetivera zizanioide 151 4.13.2 Concentrations of metals in different parts of Cymbopogon citratus 152 4.13.3 Concentrations of metals in different parts of Helianthus annus 157
4.13.4 Concentrations of metals in different parts of Ipomoea asarifolia 157
4.13.5 Concentrations of metals in different parts of Ricinus communis 158 4.14 Bioaccumulation and Transfer Factors for the Metals in Different Plant Species at Dakace Soil Treated with EDTA 169
4.15 Metal Contents in Different Parts of Plant with EDTA Application
at Gaskiya 176
4.15.1 Metal concentration in different parts of Vetivera zizanioide 176
4.15.2 Metal concentration in different parts of Cymbopogon citratus 179
4.15.3 Metal concentration in different parts of Helianthus annus 179
4.15.4 Metal concentration in different parts of Ipomoea asarifolia 180
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4.15.5 Metal concentration in different parts of Ricinus communis 181
4.16 Bioaccumulation and Transfer Factors for the Metals in Different Plant Species at Gaskiya Soil Treated with EDTA 181
4.17 Metal Contents in Different Parts of Plant with EDTA Application
at Hanwa 190
4.17.1 Metal concentration in different parts of Vetivera zizanioide 190
4.17.2 Metal concentration in different parts of Cymbopogon citratus 197
4.17.3 Metal concentration in different parts of Helianthus annus 198
4.17.4 Metal concentration in different parts of Ipomoea asarifolia 198 4.17.5 Metal concentration in different parts of Ricinus communis 199 4.18 Bioaccumulation and Transfer Factors for the Metals in Different Plant Species at Hanwa Soil Treated with EDTA 200 4.19 Metal Concentrations in the Non- EDTA Treated Soils after Harvest 217
4.20 Metal Concentrations in the EDTA Treated Soils after Harvest 217
4.21 Percentage Metal Removal after Natural and EDTA-assisted Phytoextraction 219 4.22 Pollution Indexes for Metal after Natural and EDTA-assisted Phytoextraction 220 CHAPTER FIVE
5.0 DISCUSSION 229
5.1 Physicochemical Parameters of the Soils 229
5.2 Metal Levels in Soils before Planting 230
5.3 Metals Bioavailability 232
5.4 Bioaccumulation and Transfer Factors 235
5.4.1 Vetivera zizanioide plant 235 5.4.2 Cymbopogon citratus plant 237
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5.4.3 Helianthus annus plant 238 5.4.4 Ipomoea asarifolia plant 239 5.4.5 Ricinus communis plant 240 5.5 Effect of EDTA Application on the Concentrations of Metals in Different Fractions of the Soils 243 5.6 Effect of EDTA Application on Phytoextraction of Metals 246 5.6.1 EDTA-assisted phytoextraction by Vetivera zizanioide plant 246 5.6.2 EDTA-assisted phytoextraction by Cymbopogon citratus plant 247 5.6.3 EDTA-assisted phytoextraction by Helianthus annus plant 248 5.6.4 EDTA-assisted phytoextraction by Ipomoea asarifolia plant 249 5.6.5 EDTA-assisted phytoextraction by Ricinus communis plant 250 5.7 Efficiency of the Phytoextraction 253 CHAPTER SIX 6.0 SUMMARY, CONCLUSION AND RECOMMENDATIONS 255 REFERENCES 258 APPENDICES 280
CHAPTER ONE
1.0 INTRODUCTION The global problem concerning contamination of the environment as a consequence of human activities is on the increase. Contamination has resulted from industrial activities, such as mining and smelting of metalliferous ores, electroplating, gas exhaust, energy and fuel production, fertilizer and pesticide application, and generation of municipal wastes (Kabata Pendias and Pendias, 1989). These have resulted in environmental build up of waste products of which heavy metals are of particular concern (Appel and Ma, 2002). Excessive metal concentration in the soil pose significant hazard to human, animal and plant health, and to the ecosystem in general (Nascimento and Xing, 2006).
Heavy metals are conventionally defined as elements with metallic properties (ductility, conductivity, stability as cations, ligand specificity) and with atomic numbers greater than 20 ( Lasat, 2002). The most common heavy metal contaminants are: Cd, Cr, Cu, Hg, Pb, and Zn. Metals are natural components in soil. Due to the potential toxicity and high persistence of metals, soils polluted with these elements constitute environmental problem that requires effective and affordable solution. A number of techniques have been developed to remove metals from contaminated soils such as: solidification and stabilization; soil flushing/washing; electrokinetics; chemical reduction/oxidation; low temperature thermal desorption; incineration; vitrification; pneumatic fracturing; excavation/retrieval and landfill. These physico-chemical technologies used for soil remediation render the land useless as a medium for plant growth, as they remove all biological activities. These methods of disposal solely shift the contamination problem
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elsewhere along with the hazards associated with transportation of contaminated soil and migration of contaminants from landfill into an adjacent environment.
Due to the high cost of these conventional technologies, the use of plants to clean up contaminated environments known as phytoremediation has emerged (Hosh and Ingh, 2005). Phytoremediation takes the advantage of the unique and selective uptake capabilities of plant root systems, together with the translocation, bioaccumulation, and contaminant degradation abilities of the entire plant body. The method is comprised of phytoextraction, phytostabilization, phytovolatilization, phytodegradation and phytofiltration. However, only phytoextraction which can effectively remove contaminants from contaminated soils is the most promising for commercial application (Sun et al., 2011). Successful phytoextraction depends not only on metal concentration in shoots but also on high biomass production. Selection of plant species is based on high tolerance and accumulation rate for several metals, adaptation to local climates, high biomass, depth root structure, growth rate, ease of planting and maintenance, and ability to take up large quantities of water through the roots (Zhao et al.,2001). If metal availability in the soil is not adequate for plant uptake, chelates may be used to liberate them into the soil solution (Huang and Cunningham, 1996; Huang et al., 1997; Lasat et al., 1998). The dumping of industrial and municipal solid wastes especially scrap-metals into the environment has contributed greatly to the increase in levels of heavy metals in soil and vegetations grown in dumpsites and finally find their way through the food chain into man. The soil and plants on these dumpsites will constitute a serious threat to the health of people living around such areas. Dakace, Gaskiya (both in Zaria L.G.A) and Hanwa (in Sabon Gari L.G.A) are among the locations where such scrap-
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metal dumpsites can be found. Farming activities take place and different types of crops are grown on the soil around the dumpsites.
1.1 Statement of the Problem
Toxic metal pollution of soils is a major environmental problem, and most conventional remediation approaches do not provide acceptable solutions.
1.2 Justification
Phytoextraction has been accepted widely both in developed and developing nations for its potential to clean up the polluted and contaminated sites (Sun et al., 2011). This technology is currently gaining considerable importance due to its potential for application to real world ecosystems and, however, it is still in its testing stages. It is cheaper and does not degrade the physical or chemical health of the soil. It seems to be the most promising technique and has received increasing attention from researchers as a technology to clean up metal polluted soils. The majority of phytoextraction studies have focused on pot experiments and laboratory hydroponic studies. Very few studies have attempted to evaluate the potential of natural hyperaccumulators or high biomass crops for phytoextraction under field conditions. 1.3 Aim of the Research The aim of this research work is to make use of hyper-accumulators and high biomass plants to extract potentially toxic metals from polluted soils under field conditions with a view to either clean up or reduce the level of metal pollution in the soil to tolerable levels.
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1.4 Objectives The aim of the research work will be achieved through the following objectives:
(i) To determine the concentrations of Cd, Co, Cu, Ni, Pb and Zn in the soils before planting and after harvest, and compare with the given standards set by relevant regulatory bodies such as Joint Food and Agricultural Organization and World Health Organisation (FAO/WHO) with a view of assessing levels of pollution.
(ii) To carry out sequential extraction of metals from the soil to determine the bioavailable metals fractions.
(iii)To compare the uptake and distribution of heavy metals within different plant species.
(iv) To determine the actual effectiveness of a plant for cleaning up of metal polluted soils, by calculating the bioaccumulation factor (BF) and the transfer factor (TF).
(v) To investigate the effect of application of chelating agent (EDTA) on the metal levels in different soil fractions and their absorption by the plants.
(vi) To determine the concentrations of the metals in the soils after the application of the EDTA to ascertain the efficiency of the phyto-extraction.
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