ASSESSMENT OF SOME HEAVY METALS IN SOIL SAMPLES ALONG GUTTERS IN NKPOR, ANAMBRA STATE

 

 

 

 


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TABLE OF CONTENTS

 

Title Page………………………………………………………………….ii Approval Page ……………………………………………........................iii Dedication ………………………………………………………………..iv Acknowledgement ……………………………………………………….v-vi Abstract ……………………………………………………………….vii

 

List of Tables ……………………………………………………………viii

 

List of Figures ………………………………………………………......ix Table of contents ………………………………………………………...x-xi

 

 

CHAPTER ONE

 

1.0                                    General Introduction ………………………………1-2

 

1.1                                    Scope of study ………………………………………2

 

1.2                                    Aims & Objectives of study………….………………3

 

1.3                                    Literature Review ……………………………………4-5

 

1.3.1                            Soil pollution ……………………………………. ……5

 

1.3.2                            Soil Pollution and plant growth……………………….. 5

 

1.3.3                            Soil Organism and biochemistry ………………………5

 

1.3.4                            Soil Acidity …………………………………………..5-6

 

1.3.5                            Heavy Metal Pollution …………………………………6

 

1.3.6                            Heavy Metal Relationship to Living Organisms …….6-7

 

1.3.7                            Reported Cases of Environmental Pollution………….7-8


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1.3.8                            Effects of Cadmium on the Environment………………8

 

1.3.9                            Effect of Antimony on the Environment……………….8-9

 

1.3.10                       Effects of Copper on the Environment…………………9

 

1.3.11                       Effects of Chromium on the Environment…………….9

 

1.3.12                       Effects of Lead on the Environment ……………….9-10

 

1.3.13                       Effects of Mercury on the Environment ……………...10

 

1.3.14                       Effects of Nickel on the Environment ………………..10

 

1.4                                    Solubility of Heavy Metals in soil …………………10-11

 

1.5                                    Phyto Availability of heavy metals in

 

residual-Treated soil..………………………………….11-12

 

1.6                                    Long term environment fate and Bioavailability

 

of Heavy metals in residual treated soil………………12

 

1.7                                      Solute interactions in relation to Bioavailability and

 

remediation of the Environment…………………..12-14

 

1.8                                    A Critical review of the Bioavailability and Impacts

 

of heavy metals in municipal soil waste Composts….14-15

 

1.9                                    Heavy Metals Transport in the soil profiles under

 

the application of sludge and wastewater ……………15-16

 

 

 

 

CHAPTER TWO

 

2.0            Reagents ………………………………………………………..17

 

2.1            Materials ………………………………………………………..17


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2.2            Instrument …………………………………………………….17

 

2.3            Sample collection and preparation …………………………17,19

 

2.4            Sequential Extraction …………………………………………..19 2.4.1 Preparation of reagents ………………………………….......19-20

2.5            Analytical procedures ……………………………………......20-21

 

CHAPTER THREE

 

3.0                                    Result and Discussions …………………………….…22-41

 

3.1                                    Statistical Analysis of the Data …………………...........43

 

3.1.2                            ANOVA (Analysis of Variance) ……………….…….43-44

 

2.1.3                            Correlation of the Fractions. ……………………………..44

 

CHAPTER FOUR

 

4.0            Conclusion and Recommendation ………………………………..45

 

4.1            Conclusion…………………………………………………………45

 

4.2             Recommendations ………………………………………………...45 References ……………………………………………………..46-50


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LIST OF TABLES

 

1. Table 1:               Moisture content of the soil samples ………………..22

 

2 .Table 2:               Total Metal concentration in mg/kg ………………...23

 

3. Table 3:               Total concentration of Chromium

 

In mg/kg of the soil fraction………………………....26

 

4. Table 4:               Total concentration of Nickel in mg/kg

 

of the soil fractions …………………………………29

 

5. Table 5:               Total concentrations of Arsenic in

 

Mg/kg of the soil fraction ……………………………32

 

6. Table 6:               Total concentration of Cadmium in

 

Mg/kg of the soil fraction ……………………………33

 

7. Table 7:               Total concentration of Zinc in

 

Mg/kg of the soil fraction ……………………………36

 

8. Table 8:               Total concentration of mercury in

 

Mg/kg of the soil samples ……………………………39

 

9. Table 9:               Percentage Bioavailability of each metal ……………40


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LIST OF FIGURES

 

 

 

 

1. Fig 1:    Total Metal Concentration in Mg/Kg ……………………..24

 

 

 

 

2.Fig 2:         Mean concentration of Cr in Mg/Kg ………………………27

 

 

 

 

3.  Fig 3:    Mean concentration of Ni in Mg/Kg ………………………30

 

 

 

 

4.  Fig 4:    Mean concentration of Cd in Mg/Kg ………………………34

 

 

 

 

5.Fig 5: Mean Concentration of Zn in Mg/Kg ……………………..37

 

 

 

 

6.  Fig 6:    Mean Concentration of Hg in Mg/Kg …………………….40

 

 

 

 

7.  Fig 7:    Percentage bioavailability of each metal …………………41

 

 

 

 

8   Fig 8; Map of Nkpor  Metropolis ……………………………….18


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ABSTRACT

 

Ten representative soil samples were collected along major gutters within Nkpor metropolis. The soil samples were digested using mixtures of HF and aqua regia in ratio1:1. The heavy metals contents of the digested soil samples were determined via Flame Atomic Absorption Spectrophotometer FAAS. Sample A had the highest concentration of heavy metal and the trend was Ni > Cr > Hg > Zn > As ~ Cd, it was followed by sample B with Ni > Cr > Zn > Zn > Cd >g > As, sample C had Ni>Cr >Zn>Hg>Cd>As, sample E had Ni > Zn > Cd > Cr > Hg > As. While sample D had Cr > Ni > Zn > Cd > Hg > As. Also the results of the sequential extraction of each soil sample indicated that Ni had the highest percentage bioavailability (49.61%), Hg (47. 72%), Cr (42. 19%), Zn (39.66%), Cd (35.10%), As (0.00%). The high concentration of Hg, Cr and Cd in most areas of the metropolis indicated gross contamination, which could have resulted from human activities, hence the need to adequately monitor the release of these toxic metals to the environment.


 

CHAPTER ONE

 

INTRODUCTION

 

1.0            GENERAL INTRODUCTION

 

Minerals, metals and metalloids, toxic or essential, are present in soils or sediments in various forms with varying bioavailability, toxicities and mobilities. Heavy metals are natural components of the earth’s crust and the ecosystem with variations in concentration. They cannot be degraded or destroyed and they enter our bodies via food, drinking water and air [1].

 

Unlike organic contaminants, most metals do not undergo microbial or chemical degradation and the total concentration of these metals in soils persist for a long time after their introduction [2].

 

These metals are a cause of environmental pollution (heavy-metal pollution) from a number of sources. For example lead in petrol, industrial effluents and leaching of metal ions from the soil into lakes and rivers by acid rain [3].

 

Living organisms require varying amounts of “heavy metals”. Iron, cobalt, copper, manganese, molybdenum and zinc are required by humans. Excessive levels can be damaging to the organisms. Other heavy metals such as mercury, plutonium and lead are toxic metals that have no beneficial effect on organisms and can cause serious illnesses (3).

 

Therefore, with greater public awareness of the implications of contaminated soils on humans and animal health, there has been increasing interest among scientific communities in the development of technologies to determine the total concentrations of these elements of interest in the soil as well as their chemical forms.


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The use of sequential extraction techniques to fractionate metals in soils and evaluate their potential effects has become very useful and well recognized (4)

 

1.1            SCOPE OF STUDY

 

This research covers the analysis of six heavy metals in each of the five selected drainage pathways or gutters in Nkpor. The heavy metals are mercury (Hg)

 

Nickel; (Ni), Zinc (Zn), Arsenic (As), Cadmium (Cd), Mercury (Hg)

 

Soil samples were collected from each of the drainage systems as follows:

 

Five soil samples each along Nkpor/Enugu Old Road, Nkpor/Obosi Road (site for Geolis Cables Industry), Limca Road Nkpor, New Market Road Nkpor and Nkpor/Enugu Express Road.

 

The samples will be collected at a distance of 10 meters each to avoid bias. A total of 25 soil samples will be analysed to determine the total concentration of each metal in each sample.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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1.2            AIMS AND OBJECTIVES OF STUDY

 

Heavy metal content of soil sample in some selected drainage systems in Nkpor will be investigated. As earlier on stated, heavy metals are dangerous because they tend to bioaccumulate i.e increase in their concentration over time. Also heavy metals can enter water supply by industrial and consumer waste or even from acidic rain and release heavy metal into streams, lakes, rivers and ground water.

 

Hence, the study will help in assessing the potential environmental impacts of these metals by determination of their concentrations in soils as well as the chemical form of these metals in the soil.

 

OBJECTIVES OF THIS STUDY

 

The objectives of this study are:

 

(a)              To determine the total concentration of some selected heavy metals in gutter.

 

(b)             To use sequential extraction technique to fractionate metals in soil samples.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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1.3            LITERATURE REVIEW

 

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 metallic 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 [5]

 

Pollution is the release of substances which alter the environment and make it unfavourable to man, animals and plants [6]. Water, air and soil stand the risk of being polluted.

 

1.3:1             SOIL POLLUTION

 

Soil pollution is the introduction of undesirable materials such as solids, liquids or even gases into the soil, which interfere with its sustenance of plant or animal life [7]. Soil particles may hold chemicals and nutrients making them available for plant roots and keeping them from moving into lakes or streams or entering the ground water [8]. Metals from soils come from various sources. They may have been present in the geologic rock, or they may occur as atmospheric additions of copper, mercury, lead and zinc [9]. Metals also may have been deposited by past industrial activities, such as; battery productions, brass and steel manufacturing industries, mining and many different processes involving Nickel, Cadmium, Copper and Lead. Lead is especially evident in the soil near roadways because of automobile emissions. [10]. Again as lead paints and some soldered pipes used in houses wear and deteriorate they add lead to nearby soils [11]. Other on-going sources of metals and organic waste materials in the soil are landfills [12] and dump sites that are poorly maintained or unregulated


 

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[13]. Land fill materials eventually decompose and form a highly variable type of unseen soil. Heavy metals remain in topsoil layer which is a result of chemical reaction between heavy metals and organic matter [14].

 

1.3.2                            SOIL POLLUTION AND PLANT GROWTH

 

Metal contamination on a site maybe evidenced by plant growth [15], animal behaviour [16] or paint flasks containing lead from older building [17]. Many plants simply cannot grow well where the level of certain metals is high. Other plants grow well in contaminated soil but fail to set seed or do not grow as well as expected [15]. Absence of plant growth in a locality is a warning sign that a site may be severely contaminated. Metals may be present at a site but with no risk for gardening or recreation, depending on the soil properties, drainage and vegetation at the site.

 

1.3:3             SOIL ORGANISMS AND BIOCHEMISTRY

 

Soil is made up of mineral particles and organic matter [12] as well as the decomposed remains of living things [18]. Bacterial fungi and other micro organisms are largely responsible for breaking down dead plants and animals in the soil. Small organisms (microbes) have negatively charged sites where soil nutrients and metals can bond to form soil aggregates and compounds [19]. Earthworms and larger animals eat and digest organic materials and minerals; transform them into soil aggregates and deposit them as waste.

 

1.3:4             SOIL ACIDITY

 

An acid is a substance that has positive charge and usually yields hydrogen ions when dissolved in water. The pH scale (1-14) is a common measure of soil reaction [12]. The lower the number, the greater the acidity. The midpoint of the pH scale is neutral


 

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(7.0) which is a good level for the growth of most plants. Changes in soil reaction, as measured by pH, have significant effects on metals in soils. Metal toxicity to plants and animals increases in strongly acid soils with low pH (3.5) [20]. Metals in these soils are released from negative sites back into soil solution. At a higher pH (8.5), the metals often are sequestered in the soil [21]. The term ‘sequestered’ indicates that the positively charged metal ions are bound tightly to negatively charged sites in the soil [22]. These sites may be on clays, mineral compounds, or organic matter, including the surfaces of some microorganisms.

 

1.3:5             HEAVY METAL POLLUTION

 

These metals are a cause of environmental pollution from a number of sources including lead in petrol, industrial effluents and leaching of metal ions from the soil into lakes and rivers by acid rain [3].

 

Heavy metals occur naturally in the ecosystem with large variations in concentration. Today, anthropogenic sources of heavy metals ie pollution, have been introduced into the ecosystem. Wastes derived fuels are especially prone to contain heavy metals so there should be a central concern in a consideration of their use.

 

Heavy metal pollution can arise from many sources but most commonly arises from the purification of metals e.g. the smelting of copper and the preparation of nuclear fuels. Electroplating is the primary source of chromium and cadmium. Through precipitation of their compounds or by ion exchange into soils and mud.

 

1.3:6             HEAVY METAL RELATIONSHIP TO LIVING ORGANISMS

 

Some metals are necessary for humans in minute amounts (cobalt, copper, chromium, Nickel etc). Some of them are dangerous to health or to the environment even at low


 

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concentration (mercury, cadmium, arsenic, lead, chromium etc). Some are harmful in other ways e.g. Arsenic may pollute catalysts, whilst others are carcinogenic or toxic thereby affecting, among others, the central nervous system (mercury, lead, Arsenic), the kidneys or liver (Mercury, lead cadmium, copper) or skin, bones or teeth ( Nickel, cadmium, copper, chromium) [23]. Certain elements are normally toxic to some organisms while others may be beneficial e.g. Vanadium, tungsten and cadmium [24]. In medical circle, heavy metals are loosely defined [24] and include all toxic metals irrespective of their atomic weights. Heavy metal poisoning can occur when excessive amounts of Iron, Manganese, Aluminum or Beryllium (the fourth lightest element) or such as semi metal as Arsenic are present in the environment. Early farmers in the New Zealand suffered from bush sickness which was later discovered to be a deficiency in cobalt [ 24 ]

 

1.3:7  REPORTED CASES OF ENVIRONMENTAL POLLUTION

 

In 1932, sewage containing mercury was released by Chisso’s Chemical works into Minamata Bay in Japan. The mercury accumulated in sea creatures, leading eventually to mercury poisoning in the population.

 

Again, Denis L. Feron and others used a secret discharge pipe to deliberately pump hundreds of tons of heavy metal wastes into the Mississippi River from 1986 to 1996. Feron is now an international fugitive and one of the Environmental Protection Agency’s most wanted man because of the environmental hazards caused by his pollution of the River.


 

 

 

 

 

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In addition to that, the first incidents of mercury poisoning appeared in the population of Minimata bay in Japan, in 1952. This was caused by consumption of fish polluted with mercury which brought about over 500 fatalities. [25].

 

In 1986, in Sandoz, water used to extinguish a major fire carried C. 3ot fungicide containing mercury into the upper Rhine. Fishes were killed over a stretch of 100km. The shock drove many Federal Environmental Agency (FEA) projects forward.

 

Also, toxic chemicals in water from a burst dam belonging to a mine contaminated the Coto De Danana nature reserve in Southern Spain, C.5 in Southern Spain, C.5 million M-of mind containing sulphur, lead, copper, zinc and cadmim flowed down the Rio Guardimar. Experts estimated that Europe’s largest bird sanctuary as well as Spain’s Agriculture and fisheries suffered permanent damage from this pollution.[25]

 

1.2.8                            EFFECTS OF CADMIUM ON THE ENVIRONMENT.

 

Cadmium derives its toxicological properties from its chemical similarity to zinc an essential micro element for plants, animals and humans (25).

 

In humans, long-term exposure is associated with renal disfunction. High exposure can lead to lung disease and has been linked to lung cancer. The average daily intake

 

for humans is estimated as 0.15 mg from air and 1 ΅g from water. Smoking of 1

 

packet of 20 cigarettes can lead to the inhalation of around 2.4 ΅g of cadmium. [25]

 

1.3:9             EFFECT OF ANTIMONY ON THE ENVIRONMENT.

 

Antimony is a metal used in the compound antimony trioxide, a flame retardant. Exposure to high levels of antimony for short period of time causes nausea, vomiting and diarrhea. There is little information on the effects of long-term exposure, but it is


 

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a suspected human carcinogen. Most antimony compounds do not bioaccumulate in aquatic life. [25].

1.3:10           EFFECTS OF COPPER ON THE ENVIRONMENT

 

Copper is an essential substance to human life, but in high doses it can cause anaemia, liver and kidney damage. It can also lead to stomach and intestinal irritation. People with Wilson’s disease are at greater risk from over exposure to copper. Copper normally occurs in drinking water from copper pipes and also from additives designed to control alga growth.

 

1.3.11                       EFFECTS OF CHROMIUM ON THE ENVIRONMENT

 

Chromium is used in metal alloys and pigments for paints, cement, paper and rubber. Low-level exposure can irritate the skin and cause ulceration. Long-term exposure can cause kidney and liver damage and damage to circulatory and nerve tissues. It often accumulates in aquatic environments thereby adding to the danger of eating fish that may have been exposed to high levels of chromium.

 

1.3.12                       EFFECTS OF LEAD ON THE ENVIRONMENT

 

Lead is a trace metal that is present naturally in soils and water in trace amounts (26). In humans, exposure to lead can result in a wide range of biological effects depending on the level and duration of exposure. Various effects occur over a broad range of doses, with the developing foetus and infant being more sensitive than adult. High levels of exposure may result in toxic biochemical effects on humans, which in turn cause problems in the synthesis of hemoglobin, effects on the kidneys, gastro intestinal tract, joints and reproductive system and acute or chronic damage to the nervous system. [25].


 

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Average daily lead intake for adult in UK is estimated at 1.6 ΅gfrom air, 20 ΅gfrom drinking water and 28΅g from foodkoftheir. Altho lead intake from food, in specific populations, other sources may be more important,

such as water in areas with lead piping and air near point of source emissions, soil, dust, paint flakes in old houses of contaminated land. [25]

 

1.3.13                       EFFECTS OF MERCURY ON THE ENVIRONMENT

 

Mercury is a toxic substance which has no known function in human biochemistry and physiology and does not occur naturally in living organisms. Inorganic mercury poisoning is associated with tumor and/or minor psychological changes together with spontaneous abortion and congenital malformation.

 

Monomethyl mercury causes damage to the brain and the central nervous system, it also gives rise to abortion, congenital malformation and development changes in young children. [25]

 

1.3.14                       EFFECTS OF NICKEL ON THE ENVIRONMENT

 

Small amounts of nickel are needed by the human body to produce red blood cells. In great amounts, it can become mildly toxic. Long term exposure can cause decreased body weight, heart and liver damage but short-term over exposure to Nickel is not known to cause any problems. nickel can accumulate in aquatic life, but its presence is not magnified along food chain. [25]

 

1.4                                  SOLUBILITY OF HEAVY METALS IN SOIL.

 

Solubility of heavy metals is directly related to sorption capacity of residuals and soil. Soil pH and iron oxides are very important factors controlling metal solubility in these systems. Sorption is an important chemical process that regulates partitioning of


 

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heavy metals between solution and solid phases in soils. Recent studies on this have shown that heavy metal solubility and availability in land applied residuals is governed by fundamental chemical reactions between metal constituents and soil and residual components [27].

 

Iron, aluminum and manganese oxide soil minerals are important sinks for heavy metals in soil and residual amended soils. Heavy metal cations sorb to soil organic matter (som) and other forms of humified natural organic matter (NOM). The type of sorption by NOM affects the environmental fate of heavy metals. Heavy metal cations form sparingly soluble phosphates, carbonates, sulphides and hydroxides. Sorption and many metal precipitation processes are highly pH dependent. The pH of the soil-residual system is often the most important chemical property governing sorption and precipitation of heavy metals. [27]

 

1.5 PHYTO AVAILABILITY OF HEAVY METALS IN RESIDUAL –

 

TREATED SOILS.

 

Application of residuals to soil affects phytoavailability by introducing heavy metal into the soil and / or redistributing heavy metal into different chemical pools that vary in phytoavailability (28). Application of biosolids increases heavy metal solubility and availability in soil. Increases in availability are a function of metal type and metal loading. Transmission of heavy metal through the food chain is affected by the soil-plant barrier (29). The barrier limits transmission of metal through the food chain either by soil chemical processes that limit solubility (e.g soil barrier) or by plant senescence (showing sign of old age) from phytotoxicity (e.g. plant barrier). The soil plant barrier limits transmission of many heavy metals through the soil-crop-animal


 

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food chain, except aluminum, zinc, molybdenum, and selenium. Cadmium, which has lower affinity for metal-sobbing phases (e.g. oxides, NOM), has the greatest potential for transmission through the food chain in levels that present risk to consumers (30).

 

1.6            LONG TERM ENVIRONMENTAL FATE AND BIOAVAILABILITY OF

 

HEAVY METALS IN RESIDUAL TREATED SOILS.

 

Heavy metals do not degrade in soil and many are considered persistent bioacumulative toxins (PBTs). The risk to human and ecosystem health from land application of PBTs in residuals depends on solubility and bioavailabilty of these contaminants in the residual – treated soil. Uncertainties exist in the effect of decomposition of soil organic matter complexes that bind metal and uncertainties of the effect of slower long term reactions between metals adsorbed to inorganic oxide surfaces in soils on metal solubility and bioavailability. Recent research findings show that heavy metal is adsorbed to oxide phases of Biosolids (31). Heavy metals sequestered to oxide surfaces will likely remain sequestered longer than metal complexed by biosolids NOM. However, the stability of metal sequestered by oxide depends on the metal and the mineral oxide surface. Long term mineral crystallization reactions may “eject” metals from the solid phase into solution. The long-term stability of sequestered metal bounded to metal oxide surfaces remains uncertain.

 

1.7            SOLUTE INTERACTIONS IN SOILS IN RELATION TO BIOAVAILABILITY AND REMEDIATION OF THE ENVIRONMENT.

For diffuse distribution of metals (e.g). Fertilizer-derived cadmium input in pasture soils), remediation options generally include amelioration of soils to minimize the metal bioavailability.


 

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The bioavailability of a chemical in the soil environment has been defined as the fraction of the total contaminant in the interstitial pore water (i.e., soil solution) and soil particle that is available to the receptor organism [32]. Bioavailability can be minimized through chemical and biological immobilization of metals using a range of inorganic compounds such as lime and phosphate (p) compounds (e.g. apatite rocks), and organic compound such as “exceptional quality” biosolids [33]. Reducing metal availability and maximizing plant growth through inactivation may also prove to be an

effective method of in situ soil remediation on industrial, urban, smelting and mining sites.

The more localized metal contamination found in urban environments (e. g chromium contamination in timber treatment plants) is remedied by metal mobilization process that include bioremediation (including phyto remediation) and chemical washing.

 

Bioavailability of metals in soils can be examined using chemical extraction and bioassay tests. Chemical extraction test includes single extraction and sequential fractionation [34]. Bioassay involves plants, animals and microorganism [35].

 

A number of amendments are used either to mobilize or immobilize heavy metals into soil solution, which is subsequently removed using higher plants. In contrast, in case of the immobilization technique, the metal concerned is removed from soil solution either through adsorption, complexation and precipitation reactions, thereby rendering the metals unavailable for plant uptake and leading to groundwater.

 

Since one of the primary objectives of remediating contaminated sites is to reduce the

 

bioavailability of metals, in-situ immobilization using soil amendments that are low in heavy metal content may offer a promising option. However, a major inherent


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problem associated with immobilization technique is that although the heavy metals become less bioavailable, their total concentrations in soils remain unchanged. The immobilized heavy metal may become bioavailable with time through natural weathering process or through breakdown of high molecular weight organic metal complexes.

 

1.8     A CRITICAL REVIEW OF THE BIOAVAILABILITY AND IMPACTS

 

OF HEAVY METALS IN MUNICIPAL SOIL WASTE COMPOSTS.

 

The concentration, behaviour and significance of heavy metals in composted waste materials is important from two potentially conflicting aspects of environmental legislation in terms of: (a) defining end-of-water criteria and increasing recycling of composted residuals on land and (b) protecting soil quality by preventing contamination. All types of municipal soil waste (m s w) compost contain more heavy metals than the background concentrations present in soil and will increase their contents in amended soil. [36] Total concentrations of heavy metals in such segregated and green waste compost are typically below UK PAS100 limits and mechanical segregated material can also comply with the metal limits in UK PAS100 , although this is likely to be more challenging [36]. Zinc and lead are the elements presents in the largest amounts in MSW-compost. Lead is the most limiting element to use of mechanical segregated compost in domestic gardens, but concentrations are typically below risk based thresholds that protect human health.

 

There is general consensus in the scientific literature that aerobic composting processes increase the complexation of heavy metals in organic waste residuals, and that metals are strongly bound to the compost matrix and organic matter, limiting their


 

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solubility and potential bioavailability in soil. Lead is the most strongly bound element and nickel the weakest, with zinc, copper and Cadmium showing intermediate sorption characteristics. The strong metal sorption properties of compost produced from m s w or sewage sludge have important benefits for the remediation of metals contaminated industrial and urban soils.

 

The availability of metals in soil depends on the nature of the chemical association between a metal with the organic residual and soil matrix, the pH value of the soil, the concentration of the element in the compost and the soil, and the ability of the plant to regulate the uptake of a particular element.

 

However, there is good experimental evidence demonstrating the reduced bioavailability and crop uptake of metal from composted biosolids compared to other types of sewage sludge. [36] The total metal concentration in compost is important in controlling crop uptake of labile elements, like zinc and copper, which increases with increasing total content of these elements in the compost.

 

1.9.1                     HEAVY METALS TRANSPORT IN THE SOIL PROFILES UNDER THE APPLICATION OF SLUDGE AND WASTEWATER.

 

The results of packed-column studies may be overly optimistic in predicting soil immobilization of metals, by pass flow via preferential flow paths in field soils may allow significant metal transport to ground water (37). The lack of significant metal deposition in subsoil may not be reliable evidence for immobility of sludge application metals (38).

 

Alloway and Jackson (1991) cited several studies reporting some downward metal translocation in soil, noting a potential correlation with climate (39). Soluble and


 

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colloidal organics have been shown experimentally to moblize metals (40). Land application of sludge or compost can have both beneficial and harmful aspects. Its organic matter content, which constitutes approximately 50% of the solid fraction, may improve soil physical properties. Nitrogen and phosphorous, ranging from 2 to 8% and 1 to 4% respectively, in sewage sludge and sludge compost are nutrients essential for growth of crops. Municipal sludge, however, often contain undesirable chemicals which may be toxic to plants and/or eventually toxic to animals and human that consume edible parts of such plants (41).

 

It was concluded that the use of wastewater and sludge application in agricultural lands, enriched soils with heavy metals to concentrations that may pose, potential environmental and health risks in the long term.

 

 

 

 

 

 

 

 

 

 

 

 

 

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