Environmental and human health concerns of Pb and Cd

Get Complete Project Material File(s) Now! »

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.

READ  Measurement and correlations for solid-liquid contacting

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
6.4 Results 
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

GET THE COMPLETE PROJECT

Related Posts