Determining the Level of Heavy Metal in Orange (Citrus sinensis) within AngloGold Ashanti, Obuasi Mine

Bachelor Thesis 2012 60 Pages

Environmental Sciences










1.1 Research Background
1.2 The Problem Statement
1.3 Justification
1.4 The Research Objectives
1.5 The Structure of the Thesis

2.1 Introduction
2.2.1 Taxonomic Classification and Physiology of Orange
2.2.2 Nutritional Benefits of Orange
2.2.3 Definition of Heavy Metals
2.2.4 Bio-accumulation of Heavy Metals
2.2.5 Heavy Metal Tolerance
2.2.6 Heavy Metal Remediation
2.2.7 Factors Affecting Metal Availability
2.2.8 Plant Uptake and Transport of Metals
2.2.9 Uptake Mechanisms of Plants
2.2.10 Transport within the Plant
2.2.11 Xylem Transport
2.2.12 Phloem Transport
2.2.13 Sources and Uses of Copper
2.2.14 Lead Poisoning
2.2.15 Natural Sources of Lead
2.2.16 Mining Operations as a Source of Lead
2.2.17 Transport and Bioaccumulation of Lead
2.2.18 Chemistry of Arsenic
2.2.19 Arsenic in the Environment
2.2.20 Health Effects of Arsenic
2.2.21 Environmental Effects of Arsenic
2.2.22 Chemistry of Zinc
2.2.23 Occurrence of Zinc
2.2.24 Environmental Impacts of Zinc
2.2.25 Tailings Composition and Storage
2.2.26 Phytoremediation

3.1 Introduction
3.2 Study Area
3.2.1 Location
3.2.2 Climate
3.3 Materials
3.4 Experimental Design
3.5 Methods
3.5.1 Acid Digestion
3.6 Limitations of the Study

4.1 Introduction
4.2 Results for Heavy Metal Concentration
4.2.1 Concentration of Zinc in Orange Samples
4.2.2 Concentration of Arsenic in Orange Samples
4.2.3 Concentration of Copper in Orange Samples
4.2.4 Concentration of Lead in Orange Samples

5.1 Introduction
5.2 Concentration of Zinc
5.3 Concentration of Arsenic
5.2 Concentration of Copper
5.3 Concentration of Lead

6.1 Introduction
6.3 Contribution to Knowledge
6.4 Recommendations
6.5 Issues for Future Research




Appendix I Concentration of heavy metals in orange samples from tailings site

Appendix II. Concentration of heavy metals in orange samples from control site

Appendix III. Two-sample t-test for Zinc

Appendix IV. Two- sample t-test for Arsenic

Appendix V. Two- sample t-test for Copper

Appendix VI Two- sample t-test for Lead

Appendix VII Concentration of heavy metal in orange samples


Figure 3.1 Map of study area

Figure 4.1 Mean concentration of Zinc

Figure 4.2 Mean concentration of Arsenic

Figure 4.3 Mean concentration of Copper

Figure 4.4 Mean concentration of Lead


This work is dedicated to the Almighty God, for His grace, mercies and love throughout my academic journey. I dedicate it as well to my parents, Mr S. Y Boadu Duah and Mrs Margaret Kissi Boadu and finally to my siblings and niece Gloria Boadu, Gladys Boadu, Rose Boadu and Francisca Irene Boadu.


The levels of four different heavy metals Arsenic (As), Zinc (Zn), Lead (Pb) and Copper (Cu) were determined in orange samples (Citrus sinensis) cultivated near the Sansu tailings dam of AngloGold Ashanti Obuasi mine. The concentrations of the four metals in the orange samples were analysed from twenty (20) orange samples. Ten samples were collected randomly from the tailings site and extra ten (10) samples were purchased as control from Akrokeri (outside the Obuasi Municipality). Atomic absorption spectrometer was used to determine the concentrations of these metals in the fruits. The average concentrations of the heavy metals (As, Zn, Cu, and Pb) in the orange samples from the tailings site were 4.81mg/kg, 1.52 mg/kg, 1.04 mg/kg, and 0.74 mg/kg respectively. The average concentrations of the heavy metals (As, Zn, Cu, and Pb) in the orange samples from the control were 0.43 mg/kg, 0.25 mg/kg, 0.32 mg/kg, and 0.15 mg/kg respectively. The WHO gives the maximum permissible level of As, Zn, Pb, and Cu as 0.5mg/kg, 0.4mg/kg, 0.4mg/kg, and 0.3mg/kg respectively. Based on these levels it was concluded that the orange grown at the tailings site is a health hazard for human consumption.


All glory and honour to Almighty God for seeing me through this work successfully. I would not have been able to get this far without His grace and mercies.

My special gratitude goes to Mr. Simon Abugre, for his supervision, guidance and unflinching encouragement and support throughout the research work. I cannot find enough words to express my gratitude to my family Mr. and Mrs. Boadu Duah, Gloria Boadu, Gladys Boadu and Rose Boadu, may God bless you richly.

I gratefully acknowledge the staff of the Environmental Department of AngloGold Ashanti Ghana especially Mr. Peter Yeboah, Mr. Edmund Addai Cudjoe, Prince-Peter Kponyo, Theophilus Bruce, for their help during my trace metal analysis. It is a pleasure to pay tribute to my friends, Dela Setsoafia, Edwin Danso, Pastor Anthony Baidoo and all NUPS-G KNUST members. This list cannot be complete without extending my gratitude to Selasi Cyril Tayviah and Mr. Daniel Mawutor, may the blessings of God be your portion.

Finally, I would like to thank all who in diverse ways helped me to complete this work. God bless you all.


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1.1 Research Background

Heavy metals are members of a loosely defined subset of elements that exhibit metallic properties (Duffus, 2002). Its contamination issue in human dietary has elicited significant responses and worldwide concerns pivotally entailing fruits consumption. Some heavy metals like zinc and copper are a necessary part of good nutrition. According to Sharma et al. (2007), plants growing in a polluted environment can accumulate heavy metals at high concentrations, causing a serious risk to human health.

The presence of rich mineral ore in Ghana has led to the proliferation of mining companies in Ghana with its accompanying environmental problems. The long history of mining at Obuasi has generated huge environmental legacy issues in the area. Perhaps, the most significant of the environmental challenges is that of heavy elements contamination. (Amonoo-Neizer et al., 1995).

During mining, enormous quantities of fine rock particles, in sizes ranging from sand-sized down to as low as a few microns together with other heavy metals are produced. These fine-grained wastes are known as "tailings". By far, the larger proportion of ore mined in most industry sectors ultimately becomes tailings that must be disposed of. In the gold industry, only a few hundredths of an ounce of gold may be produced for every ton of dry tailings generated (USEPA, 1994). Tailings need to be properly managed because they are a major source of the release of many heavy metals into the environment (Ahmad and Carboo, 2000). Due to the toxic level of the tailings, AngloGold Ashanti, Obuasi mine ensures that they are confined in a safe place to prevent them from getting into the environment. However, these tailings in their liquid form are capable of seeping into the soil.

This study will determine the issue of gold mines tailings dams as a potential source of heavy metal contamination in oranges planted on adjoining soils at the AGA tailings dam. The main sources of heavy metals to plants are the air and soil from which metals are taken up by the root or foliage. The uptake of metal concentration by roots depends on speciation of metal and soil characteristics and type of plant species.

Widespread interest in heavy metal contamination in plant systems has emerged only over the last three decades and several research articles reported concentrations of a number of heavy metals in local crops and other plants as a consequence of anthropogenic emissions (Bernard et al, 2000). The consequence of heavy metals in food such as vegetables and fruits has been a considerable interest because of their toxicity effect which is important in human beings (Asaolu, 2010).

1.2 The Problem Statement

There is a global outreach to ensure that all arable lands are put to good use to ensure availability of food to prevent any future food insecurity. Increase in world population has led to a corresponding increase in demand for food. Farming is a primary occupation for many in tropical Africa and the main source of income as well.

According to a research conducted by the Third World Network on the impact of gold mining on poor people in Obuasi from the 29th of May to 4th June 2006, the cultivation of fruit and vegetables including ‘Obuasi oranges’ on polluted land poses a risk to peoples’ health and prevents farmers from selling their produce in local markets. The research also showed serious poisoning of local crops in areas of historic gold mining activity.

Most research works on the soil and crops in Obuasi have attributed the issue of contamination on cyanide with little emphasis on heavy metals which are also potential sources of contamination. It has become expedient to find out the level of heavy metal contamination in the orange fruits so that the necessary remediation measure can be taken.

1.3 Justification

Certain plants can accumulate heavy metals in their tissues. Uptake of heavy metals increases generally in plants that are grown in areas with increased soil contamination with heavy metals. Therefore, many people could be at risk of adverse health effects from consuming common crops cultivated in contaminated soil (Le Coultre, 2001).

Orange is one of the commonest fruits that can be found in the Obuasi municipality. It is the preferred choice in fruit plantations because of its high economic value. Orange plays an important role in the diet of humans. As a major source of vitamin C, it plays an important role for the proper functioning of our immune system.

Buszewski (2000), revealed that the high concentration of heavy metals in soils is reflected by higher concentrations of metals in plants, and consequently in animal and human bodies. This work will complement the work done by other researchers on the level of heavy metals in food crops and also give an idea on the level of pollution. It will also provide data that will be necessary in determining the need for remediation.

Publicity regarding the high level of heavy metals in Obuasi has created apprehension and fear in the public as to the presence of heavy metal residues in their daily food. Hence the need for his research

1.4 The Research Objectives

The project seeks to find out if the cultivation of orange on adjoining soils of the tailings dam affects the level of heavy metals in the orange.

The specific objectives of this study are:

- To determine the level of heavy metals (As, Cu, Pb, and Zn) in oranges planted on adjoining soil of the tailings dam of AngloGold Ashanti, Obuasi mine.
- To compare the level of the heavy metals to the highest permissible level of the World Health Organization.

1.5 The Structure of the Thesis

Chapter 1 which is deals with the introduction of the study presents the focus of the study. In this chapter the problem that the study seeks to address and its justification are stated. Chapter 2 expounds the supporting literature of the study. It provides basis for which discussions are extracted. The study area and the materials and methods by which the data for the study were collected are contained in chapter 3. The results are presented in chapter 4 which is then followed by discussion in chapter 5. The conclusion of the study and the necessary recommendations are made in chapter 6.



2.1 Introduction

In the preceding chapter, the reasons that prompted this research have been disclosed. Having done that it is imperative to discuss the necessary literatures that support various aspects of the work. Thus this chapter examines some of the crucial issues concerning heavy metal contamination. This chapter reveals the taxonomic classification of orange, sources of heavy metals and presents literary and related platform upon which the results will be discussed.

2.2.1 Taxonomic Classification and Physiology of Orange

Orange specifically, the sweet orange is taxonomically known as Citrus ​sinensis. It is the most commonly grown tree fruit in the world. The orange belongs to kingdom Plantae and family rutaceae. The orange is a hybrid of ancient cultivated origin, possibly between pomelo (Citrus maxima) and mandarin (Citrus reticulata). It is an evergreen flowering tree generally growing to 9–10 m in height (although very old specimens have reached 15 m). The leaves are arranged alternately, are ovate in shape with crenulated margins and are 4–10 cm long. The orange fruit is a hesperidium, a type of berry.

Like all citrus fruits, the orange is acidic: pH levels have been reported by reliable sources as low as 2.9 and as high as 4.0 Orange trees are widely cultivated in tropical and subtropical climates for the delicious sweet fruit, which is peeled or cut (to avoid the bitter rind) and eaten whole, or processed to extract orange juice, and also for the fragrant peel.

In 2008, 68.5 million tons of oranges were grown worldwide, primarily in Brazil and the US states California and Florida. Oranges probably originated in Southeast Asia and were cultivated in China by 2500 BC. The fruit of Citrus sinensis is called sweet orange to distinguish it from Citrus aurantium, the bitter orange (Bailey et. al 1976).

This study uses sweet orange (Citrus sinensis) because of its nutritional benefits to man and its popularity in fruit plantation.

2.2.2 Nutritional Benefits of Orange

Orange has a high concentration of vitamin C and is also a source of dietary fibre, vitamins B1 and A, potassium, calcium and calories. In recent research studies, the healing properties of oranges have been associated with a wide variety of phytonutrient compounds. Vitamin C is the primary antioxidant in the body, disarming free radicals and preventing damage in the aqueous environment both inside and outside the cells. Inside cells, a potential result of free radical damage to DNA is cancer. This is why a good intake of vitamin C is associated with a reduced risk of colon cancer (Fulker, 2001).

2.2.3 Definition of Heavy Metals

A heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed some based on density, some on atomic number or atomic weight, and some on chemical properties or toxicity (Morel &Lane, 1998). Its contamination issue in human dietary has elicited significant responses and worldwide concerns pivotally entailing fruits consumption. The contamination of fruits with heavy metals possess a critical threat to the society and the environment as regards to increasing concerns of food safety issues potential health risks and detrimental effects upon soil ecosystem (McLaughlin et al., 2000). It is this health concern that has necessitated this research since food safety is one of the greatest threats to food security.

Heavy metals occur naturally in the ecosystem with large variations in concentration. In modern times, anthropogenic sources of heavy metals have been introduced to the ecosystem. This is mainly due to high industrialization and mechanised approach to farming. Some examples of heavy metals are; Aluminium (Al), Antimony (Sb), Arsenic (As), Barium (Ba), Cadmium (Cd), Cobalt (Co), Chromium (Cr), Copper (Cu), Iron (Fe), Nickel (Ni), Lead (Pb), Manganese (Mn), Molybdenum (Mo), Rubidium (Rb), Scandium (Sc), Selenium (Se), Strontium (Sr), Tin (Sn), Titanium (Ti), Tungsten (W), Vanadium (V), Zinc (Zn).

2.2.4 Bio-accumulation of Heavy Metals

Heavy metals are dangerous because they tend to bio- accumulate . Bioaccumulation means an increase in the concentration of a chemical in a biological organism over time, compared to the chemical's concentration in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down (metabolized) or excreted. It is due to this property of plants

2.2.5 Heavy Metal Tolerance

Excessive levels of heavy metals in agricultural land constitute an increasingly serious threat not only for intact plant growth and yield, but also for environment and human health (Gratão et al., 2005). Some heavy metals are toxic to plants even at very low concentrations, while others may accumulate in plant tissues up to a certain level without visible symptoms or yield reduction. Non-nutrient heavy metals such as cadmium, arsenic, lead, and mercury, which are harmful for both plants and humans, are introduced to agricultural ecosystems from various sources, including industry, reclaimed wastewater, and soil amendments originating from various sources. Although the problem of heavy metal contamination in fruit vegetables is currently not widespread, some recent reports are worrying (Verkleij et al., 1990). Moreover, Edelstein and Ben-Hur (2007) studied the effects of grafting on heavy metal and trace mineral concentrations in the fruit under field conditions, using melon plants, ungrafted and grafted onto the commercial Cucurbita rootstock and irrigated using marginal quality water. The concentrations of B, Zn, Sr, Mn, Cu, Ti, Cr, Ni, and Cd were lower in fruit from grafted than from ungrafted plants. The lower heavy metal and trace element concentrations in fruits were ascribed mainly to differences in characteristics of the root systems between the two plant types. This work will however not focus on the grafted and ungrafted means of cultivation.

Heavy metal contamination is a general term given to describe a condition in which high levels of toxic metals are introduced into the environment. Common examples are copper, lead, cadmium and arsenic. This contamination can be very real, detrimental to health and deadly.

2.2.6 Heavy Metal Remediation

The overall objective of any soil remediation approach is to create a final solution that is protective of human health and the environment. Remediation is generally subject to an array of regulatory requirements and can also be based on assessments of human health and ecological risks where no legislated standards exist or where standards are advisory. The regulatory authorities will normally accept remediation strategies that center on reducing metal bioavailability only if reduced bioavailability is equated with reduced risk, and if the bioavailability reductions are demonstrated to be long term. For heavy metal-contaminated soils, the physical and chemical form of the heavy metal contaminant in soil strongly influences the selection of the appropriate remediation treatment approach. Information about the physical characteristics of the site and the type and level of contamination at the site must be obtained to enable accurate assessment of site contamination and remedial alternatives (Martin et al., 1983).

2.2.7 Factors Affecting Metal Availability

Plants cannot usually access the total pool of a metal present in the growth substrate. Instead, that fraction of the metal which plants can absorb is known as the available or bioavailable fraction. Metals present in a soil can be divided into a number of fractions including; the soluble metal in the soil solution, metal-precipitates, metal sobbed to clays, hydrous oxides and organic matter, and metals within the matrix of soil minerals. These different fractions are all in dynamic equilibrium with each other (Norvell, 1972). However, while the soluble metal in the soil solution is directly available for plant uptake other soil metal pools are less available (Davis and Leckie, 1978). For example, change in the concentration of metal in the matrix of soil minerals is slow relative to exchange and desorption reactions between clays, hydrous oxides, organic matter and the soil solution (Whitehead, 2000).

Metals within the soil solution are the only soil fraction directly available for plant uptake (Fageria et al., 1991). Hence, factors which affect the concentration and speciation of metals in the soil solution will affect the bioavailability of metals to plants. Soil factors which have an effect on metal bioavailability include the total metal present in the soil, pH, clay and hydrous oxide content, organic matter and redox conditions.

2.2.8 Plant Uptake and Transport of Metals

Plants have developed a range of mechanisms to obtain metals from the soil solution and transport these metals within the plant. Much of the research, and understanding, of these mechanisms has been at sufficiency and deficiency levels of metals. However, from an understanding of the mechanisms operating at deficiency and sufficiency levels of metals, supplemented with what is understood at excess metal supply, an understanding can be gained of the processes affecting metal uptake and transport by plants.

2.2.9 Uptake Mechanisms of Plants

Uptake of metals into plant roots is a complex process involving transfer of metals from the soil solution to the root surface and inside the root cells. Understanding of uptake processes is hampered by the complex nature of the rhizosphere which is in continual dynamic change interacted upon by plant roots, the soil solution composing it and microorganisms living within the rhizosphere (Laurie and Manthey, 1994).

Metal uptake by plants is regulated by the electrochemical potential gradient for each metal ion that exists across the plasma membrane of root cells (Welch, 1995). Kochian (1991), elucidated that most plants have a plasma membrane potential between 0.120 and 0.180 mV, hence a large electrical gradient exists that powers metal uptake. As well, the metal ions in the cytoplasm are maintained at low activity to prevent harmful redox reactions which can result from the presence of free ionic forms of these reactive metals (Laurie and Manthey, 1994; Welch, 1995). These two factors combine to create a large passive gradient for metal uptake. Hence, as opposed to macronutrients, there is little need for the plant to utilise thermodynamically active processes for the uptake of metal ions (Kochian, 1993; Welch, 1995).

2.2.10 Transport within the Plant Once within a plant the two major transport mechanisms for metals are via the xylem and phloem. The effects of metals on the rate of movement and composition of the xylem and phloem sap may impact on plant response to metal toxicity. Translocation effects include the relative proportions of metals in roots versus shoots, potential sites of toxic action of metals and the translocation of other nutrients within the plant

2.2.11 Xylem Transport

Transport of metal ions within the xylem is essentially driven by mass upward flow of water created by the transpiration stream (Kochian, 1991; Welch, 1995). Water transpiration rate has a large effect on macronutrient translocation rate, however at low supply, processes such as xylem loading and unloading and transfer between xylem and phloem have been shown to be more important for the rate of nutrient supply (Welch, 1995). There is little to suggest that the case would be different for metals, and hence, under conditions of excess metal it is likely that the rate of transpiration would dominate metal movement in the xylem sap.

In addition, the composition, pH and redox potential of the xylem sap would affect the types and amounts and therefore movements of metal species in the xylem sap (Welch, 1995). Both xylem, pH and redox potential are important for regulating the solubility and speciation of metal within the xylem (Welch, 1995), hence affecting the concentration that can be transported throughout the plant. No comprehensive research exists on the effects of excess metal on xylem pH, redox potential or ionic strength; however, the little which does exist suggests plants are able to maintain these characteristics (White et al., 1979).

2.2.12 Phloem Transport

Transport of metals within the phloem is thought to occur via the positive hydrostatic pressure gradient developed from the loading of sucrose into the phloem from mature actively photosynthesizing leaves and unloading of sucrose into the sink tissues such as rapidly growing tissues, apical root zones and reproductive organs (Hocking, 1980). As in the xylem, the pH, redox potential, ionic strength and organic constituents of the phloem sap will determine the loading, transport and unloading of metals in the phloem (Welch, 1995). However, unlike xylem cells, phloem cells are alive and metabolically active. Hence, metabolic reactions within the phloem have the potential to make the phloem sap more responsive to changes in the internal plant environment than the xylem sap (Welch, 1995).

Typically phloem sap has of a pH of 8 or greater (Hocking, 1980; Kochian, 1991), is more reducing and has a higher solute concentration (Hocking, 1980) and ionic strength (Welch, 1995) than xylem sap. Hence, the activity and speciation within the phloem sap is likely to be considerably different to the xylem sap. Sugars have been found to compose from 14 to >24% of the phloem sap (Hocking, 1980). No comprehensive research has studied the effect of excess metals on phloem composition and so it is not known whether plants are able to maintain a stable pH, redox state and ionic strength under excess metal supply.

2.2.13 Sources and Uses of Copper

The major sources of release are mining operations, agriculture, and municipal and industrial solid waste. Mining and milling contribute the most waste. Copper is released to water as a result of natural weathering of soil and discharges from industries and sewage treatment plants. Industrial releases are only a fraction of the total environmental releases of copper and copper compounds. Other sources of copper release into the environment originate from domestic waste water, combustion processes, wood production, phosphate fertilizer production, and natural sources (e.g., windblown dust, volcanoes, decaying vegetation, forest fires, sea spray, etc.), (Adalsteinsson, 1994). Most copper is used for electrical equipment (60%); construction, such as roofing and plumbing (20%); industrial machinery, such as heat exchangers (15%) and alloys (5%). The main long established copper alloys are bronze, brass (a copper-zinc alloy), copper-tin-zinc, which was strong enough to make guns and cannons, and was known as gun metal, copper and nickel, known as cupronickel, which was the preferred metal for low-denomination coins. Copper does not break down in the environment and because of that it can accumulate in plants and animals when it is found in soils. On copper-rich soils only a limited number of plants have a chance of survival. That is why there is not much plant diversity near copper-disposing factories. Due to the effects upon plants, copper is a serious threat to the productions of farmlands. Copper can seriously influence the proceedings of certain farmlands, depending upon the acidity of the soil and the presence of organic matter. Despite of this, copper-containing manures are still applied. Copper can interrupt the activity in soils, as it negatively influences the activity of microorganisms and earthworms. The decomposition of organic matter may seriously slow down because of this.



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determining citrus anglogold ashanti obuasi




Title: Determining the Level of Heavy Metal in Orange (Citrus sinensis) within AngloGold Ashanti, Obuasi Mine