Review of some findings of pot and field methods for determining metal Uptake 

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CHAPTER 2 LITERATURE REVIEW

Essential and non- essential heavy metals for plants

Heavy metals are elements with a high relative atomic mass. They occur naturally in the earth’s crust. The term « heavy metal » is used extensively in literature to refer to metals with atomic numbers greater than 20 and is also associated with toxicity or pollution. According to Malan (1999), the term is vague as some authors use it to refer to second and third row transitional metals, others to all transitional metals while many use it to refer to metals not normally found in biological tissue but are harmful. In this study the term heavy metal refers to metals that have atomic numbers greater than 20 and may be harmful to plants and/or animals. These metals include Fe, Zn, Ni, Cr, Cu, As, Hg, Pb and Cd. Pb and Cd have been chosen for investigation in this study because they pose a much higher risk to the human food chain than the rest. They enter the food chain and more easily accumulate to levels that cause health problems to animals and humans.Heavy metals such as Fe, Cu and Zn are essential for plant growth as they participate in oxidation, electron transfer and various enzyme reactions (Polette et al, 1997). Others like Pb and Cd are not known to have any metabolic roles in plants and animals and are therefore non-essential (Johannesson, 2002; Elson and Haas, 2003). In general, essential elements may be defined as metals that are necessary for a plant to complete its life cycle (Welch and Cary, 1987). Non-essential elements are metals with no known role in plant metabolism. Although recent findings indicate that Cd may be essential to certain mushrooms (Johannessson, 2002) the metal is still considered non-essential since its biological functions in plants are still not known. Polette et al (1997) postulated that the mechanisms that allow uptake of nutrients by plants could also facilitate uptake of heavy metals, as the latter are generally indistinguishable from nutrients.

Sources of Pb and Cd

The major sources of heavy metals to the environment are direct deposition from mining and industrial processes, atmospheric deposition from combustion processes and wastewater from mining activities, industrial and domestic processes. The primary production and recycling of Pb (which occurs in over 50 countries in the world) contributes to a total annual production of 6 million tonnes while that of Cd is estimated at 19 000 tonnes per year (Johannesson, 2002). Heavy metals are emitted into the atmosphere as vapour or particulates (dust) or both from combustion processes (power generation, road transport), industrial sources (iron and steel industry, non-ferrous metal industry) and waste incineration (Scottish Executive Environmental and Rural Affairs Department, 2002). From these atmospheric emissions heavy metals are then deposited onto the environment.

 Lead

Pb is a mineral found deep within the earth and mined together with silver deposits (Elson and Haas, 2003). It exists in nature as sulphate (PbSO4), carbonate (PbCO3) and sulphide (PbS), which constitute the principal ore of Pb, known as galena. Impurities in the ore include Ag and gold (Au). Pb ore produces oxides when heated. Lead is a raw material in the manufacture of tetraethyl lead (Pb(C2H5)4), the additive in leaded gasoline. It is used in the production of lead acid storage batteries, pigments and chemicals, solder, other alloys and cables. It therefore becomes part of industrial waste from these industrial activities. WHO (1993) stated that Pb is present in tap water primarily from household plumping systems containing Pb in pipes, solder, fittings or service connections to homes. This makes domestic waste a major source of Pb. The dissolved amount depends on several factors including pH, temperature and water hardness. Wastewater consists of domestic and industrial waste that is treated and may be disposed onto lands, including pasturelands. In the process, treated wastewater may become a major source of Pb on pasturelands. Scottish Executive Environmental and Rural Affairs Department (2002) noted that average human daily Pb intake for adults in the United Kingdom (UK) is estimated at 1.6 µg from air, 20 µg from drinking water and 28 µg from food. Food therefore constitutes a significant proportion of the daily intake of human beings. Subhuti (2001) stated that meat was among the top three main dietary sources of lead. The other two were grasses (mainly grains, such as rice) and common vegetables. The same author noted that the two plants were particularly vulnerable to taking up Pb deposited in the top layers of the soil due to their shallow rooting depths.

Cadmium

Cadmium is present in the earth’s crust at an average of 0.2 mg/kg and usually occurs in association with Zn, Pb and copper sulphide ore bodies. Cadmium is used in the steel and plastics industries and is released to the environment through wastewater (WHO, 1993). The main sources of Cd in the environment are due to:
(1) air emission from Zn, Pb and copper smelters and industries involved in manufacturing alloys, paints, batteries and plastics
(2) wastewater from mining
(3) agricultural use of sludge and fertilisers containing Cd
(4) burning of fossil fuels
(5) deterioration of galvanised materials and Cd-plated containers
Wastewater has been reported as a major source of Cd, although the metal is often not detected in sludges (Lisk, 1972). Doyle (1978) reported Cd accumulations of over 1 mg/kg in the soil, following high rates of application of sludges over a long time. The same author also reported accumulation of 100mg/kg under furrow irrigation with sludge in some extreme cases.
The average daily intake for humans is estimated at 0.15 µg from the air and 1 µg from water,while smoking a 20-cigarette pack can lead to inhalation of around 2-4 µg of Cd (Scottish Executive Environmental and Rural Affairs Department, 2002). Johnston and Jones (1995) noted that plant-based foodstuffs were the largest source of dietary Cd and that the relative contribution of soil Cd content in plants was important but largely unresolved.

Treated wastewater as source of Pb and Cd

Treated waste material from sewage treatment plants is disposed on land as effluent or liquid sludge or dried sludge. Research has noted that most chemical pollutants are held by the organic fraction of treated sewage, that is the sludge and not the effluent. Primary sludge constitutes particulate organic material and secondary sludge consists mostly of microorganisms. However, WHO (1989) reported that conventional treatment processes, such as the activated sludge and the bio-filtration systems have little effect on removing chemical contaminants from wastewater. This suggests that chemical contaminants may also be present in treated effluent. Junkins et al (1983) explains that during wastewater treatment, soluble (dissolved) and insoluble suspended materials are adsorbed into microorganism cells where they are broken down (digested). Digestion includes synthesis (reproduction of more cells) and oxidation (formation of carbon dioxide (CO2), water (H2O) and energy. Junkins et al (1983) described the process of activated sludge using the following equations:
As the microorganisms die, they break open, making nutrients and heavy metals available to other microorganisms. Liquid digested sludge differs from air-dried sludge in that during anaerobic digestion, much of the organic nitrogen present in the sludge is mineralised to ammonia, thereby bringing the ammonium ion into solution. When added to the soil, ammonium ions may volatilise, or become adsorbed onto clay minerals or organic matter, absorbed by plants or nitrified (Doyle 1978). The mineralisation process also releases metals, including Pb and Cd into solution, allowing for their adsorption onto clay minerals, hydroxides or uptake by plants. The rate of decomposition of digested sludge was found to depend on soil moisture and texture and most decomposition took place within one month of addition of sludge to soils(Miller, 1974). This therefore suggests that liquid sludge has a higher proportion of readily available metals in solution than dried sludge. As the soil dries after addition of liquid sludge, decomposition decreases thereby reducing metal availability. Joffe (1955) attributed the decrease in mineralisation upon drying of the soil to the retardation of microbial activity. King and Morris (1972) reported decreases in soil pH and increase in cations available for plant uptake in a sandy clay loam due to the application of liquid sludge to land. The same authors also noted that large applications of sludge to soils have also been reported to create anaerobic soil conditions that increase mineralisation of organic matter present in the sludge as well as lower soil pH.Birley and Lock (2001) noted that nearly all Cd ions applied through irrigation water are found in the topsoil due to strong sorption. However it has been observed that after filling all available attachment sites, the soil particles gradually decrease the sorption rate (Christensen 1989a). Murray (2003) noted that metal behaviour in sewage sludge amended soils and plant uptake is difficult to generalise because it strongly depends on nature of metal, sludge, soil properties and crop.

CHAPTER PAGE
EXECUTIVE SUMMARY 
THESIS CONTRIBUTION TO KNOWLEDGE 
ACKNOWLEDGEMENT 
1.0 INTRODUCTION 
1.1 Environmental and human health concerns of Pb and Cd 
1.2 Metal pollution from wastewater 
1.3 Paucity of data on accumulation of Pb and Cd in star grass 
1.4 Challenges in modelling plant metal uptake from soils 
1.4.1 Soil metal concentrations and sampling depth
1.4.2 Differences in uptake characteristics of plants
1.4.3 Influence of uptake by other metals
1.5 Objectives of study 
1.6 Scope of study 
1.7 Organisation of thesis 
2.0 LITERATURE REVIEW 
2.1 Essential and non-essential heavy metals for plants
2.2 Sources of Pb and Cd 
2.2.1 Lead
2.2.2 Cadmium
2.3 Treated wastewater as source of Pb and Cd 
2.4 Chemistry of Pb and Cd 
2.4.1 Lead
2.4.2 Cadmium
2.5 Metal contamination and toxicity
2.5.1 Lead
2.5.2 Cadmium
2.6 Bio-availability of heavy metals 
2.7 Lead and cadmium health hazards to humans 
2.8 Plants as soil cleaners and pathway of Pb and Cd to food chain 
2.9 Treated sewage as source of Pb and Cd hazard to grazing animals via plants 
2.10 Potential of grasses to accumulate Pb and Cd 
2.11 Cynodon nlemfuensis 
2.12 Reliability of standard permissible toxic metal guidelines 
2.13 Reliability of guidelines of loading rates for wastewater on soils 
2.14 On land sewage disposal methods 
2.15 Influence of plant and other chemical species on metal uptake 
2.16 Models for heavy metal content prediction 
2.16.1 Mass balance approach
2.16.2 Use of soil-plant system models for metal prediction
2.17 Metal uptake in sewage amended soils 
2.18 Review of methods of measuring bio-available metal concentrations 
2.19 Review of some findings of pot and field methods for determining metal Uptake 
2.20 Review of sewage treatment systems in Zimbabwe
2.21 Problem statement and hypotheses 
2.21.1 Problem statement
2.21.2 Potential benefits of study
2.21.3 Hypotheses
3.0 METHODOLOGY
3.1 Introduction 
3.2 Background of study area 
3.2.1 Location of study area
3.2.2 Sources of pollutants for study area
3.2.3 Treatment plants
3.3 Study design 
3.3.1 Baseline assessment of Pb and Cd levels in study area
3.3.2 Greenhouse Pb and Cd uptake by star grass under treated sewage application
3.3.3 Field assessment of Pb and Cd uptake
3.3.4 Data analysis
4.0 BASELINE ASSESSMENT OF LEAD AND CADMIUM LEVELS IN STUDY AREA 
4.1 Introduction 
4.2 Objectives 
4.3 Detailed methods and materials 
4.3.1 Analysis of past records on levels Pb and Cd in treated sewage
4.3.2 Baseline assessment of chemical characteristics of study area
4.4 Results
4.4.1 Analysis of past records on levels of Pb and Cd in treated sewage
4.4.2 Chemical characteristics of study area
4.5 Discussion 
4.5.1 Analysis of past records on levels of Pb and Cd in treated sewage
4.5.2 Pb and Cd accumulation in soils and grasses
4.5.3 Implications of findings
5.0 ASSESSMENT OF LEAD AND CADMIUM UPTAKE BY Cynodon nlemfuensis UNDER REPEATED APPLICATION OF TREATED WATER 
5.1 Introduction 
5.2 Objectives 
5.3 Detailed methods and materials 
5.3.1 Experimental set-up
5.3.2 Grass establishment
5.3.3 Soil treatment and irrigation application
5.3.4 Soil sampling and testing
5.3.5 Grass sampling and testing
5.3.6 Sewage effluent and sludge collection and testing
5.3.7 Data analysis
5.4 Results 
5.4.1 Bio-available Pb and Cd content of soils
5.4.2 Extraction capacity of star grass
5.4.3 Grass metal content response to bio-available soil metal content in single treatments
5.4.4 Yield response to Pb and Cd content of grass in single treatments
5.4.5 Interactions of Pb and Cd in mixed treatments
5.4.6 Correlations of Pb and Cd in grass
5.4.7 Yield response to combined Pb and Cd
5.4.8 Yield, grass and soil metal content models and critical limits of Pb and Cd
5.4.9 Pb and Cd levels in effluent and sludge mixture
5.5 Discussion 
5.5.1 Extraction capacity of star grass
5.5.2 Grass yield response to Pb and Cd
5.5.3 Metal uptake models and critical metal limits
5.5.4 Implications of findings
6.0 FIELD ASSSESSMENT OF LEAD AND CADMIUM UPTAKE BY Cynodon nlemfuensis UNDER REPEATED APPLICATION OF TREATED WASTEWATER 
6.1 Introduction 
6.2 Objectives 
6.3 Detailed methods and materials
6.3.1 Estimated irrigation requirements of star grass
6.3.2 Experimental set-up
6.3.3 Preparation of field plots
6.3.4 Irrigation of grass
6.3.5 Soil sampling and testing
6.3.6 Grass sampling and testing
6.3.7 Sewage effluent and sludge sampling and testing
6.3.8 Data analysis
6.4 Results 
6.4.1 Soil pH, cation exchange capacity and clay content
6.4.2 Bio-available Pb and Cd content of soils and grass
6.4.3 Soil bio-available Pb and Cd response to treatment
6.4.4 Grass Pb and Cd content response to treatment
6.4.5 Correlations between bio-available and grass Pb and Cd contents for each grass crop
6.4.6 Correlation between average bio-available Pb and Cd in soils and average Pb and Cd contents in grass
6.4.7 Rate of metal application from treated sewage
6.5 Discussion
7.0 GENERAL DISCUSSION 
7.1 Long-term Pb and Cd accumulation in soils and bio-available levels 
7.2 Capacity of star grass to absorb Pb and Cd 
7.3 Yield responses to increasing bio-available Pb and Cd 
7.4 Yield-metal uptake models for Pb and Cd and toxic limits in grass
7.5 Soil bio-available-grass metal uptake models and critical metal limits 
7.6 Co-presence of Pb and Cd 
7.7 Appropriate Pb and Cd levels in effluent and digested sludge 
8.0 CONCLUSIONS AND RECOMMENDATIONS
8.1 Main conclusions 
8.2 Recommendations 

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MODELLING LEAD AND CADMIUM UPTAKE BY STAR GRASS UNDER IRRIGATION WITH TREATED WASTEWATER