The use of biosolids in phytostabilization
of iron ore tailings in Swaziland
EN CELE
23631058
Thesis submitted for the degree Philosophiae Doctor in
Environmental Sciences at the Potchefstroom Campus of the
North-West University
Promoter:
Prof MS Maboeta
Assistant Promoter:
Dr KR Butt
i Preface and acknowledgements
The completion of this thesis is accompanied by the publication of three papers in three journals, which are approved by the Department of Higher Education and Training (DHET) of South Africa. These are the Journal of Environmental Management (impact factor 3.131), International Journal of Environmental Science and Technology (impact factor 2.344), and Environmental Science and Pollution Research (impact factor 2.760). The said papers were authored by Cele and Maboeta (2016) i.e.
(i) A greenhouse trial to investigate the ameliorative properties of biosolids and plants on physicochemical conditions of iron ore tailings: Implications for an iron ore mine site remediation. Journal of environmental Management, 165:167-174 (Annexure A)
(ii) Response of soil enzyme activities to synergistic effects of biosolids and plants in iron ore mine soils. International Journal of Environmental Science and Technology, 13:2117–2126 (Annexure B)
(iii) Amelioration of iron mine soils with biosolids: Effects on plant tissue metal content and earthworms. Environmental Science and Pollution Research, doi: 10.1007/s11356-016-7504-5 (Annexure C).
In addition to publications, results from this study were presented (platform presentations) at two conferences. The first was the 7th Society for Environmental Toxicology and Chemistry (SETAC) Africa conference (Langebaan, Cape Town, South Africa, 5-7 October 2015) (Annexure D). The second was the 4th Annual Land Rehabilitation Society of Southern Africa conference (Kimberley, South Africa, 13-16 September 2016) (Annexure E).
I wish to gratefully acknowledge a number of individuals and organizations in Swaziland and South Africa, without which the planning and carrying out of this study would not have been possible. At the earliest planning stages, Mr. Sam Ntshalintshali (Geological Surveys and Mines) and Mr. Collin Cotterrell (Bulembu) were very instrumental in discussions relating to the history of mining in Swaziland. The Swaziland Water Services Corporation (SWSC) provided financial assistance over a three-year period, as well as access to its biosolids stockpiles at Ezulwini wastewater treatment plant. Salgaocar Swaziland (Pty) Ltd and the Swaziland National Trust Commission (SNTC) were also instrumental in providing access to the study site, Ngwenya iron ore mine. The Unit for Environmental Sciences and Management (UESM) at North-West University provided financial assistance on numerous occasions towards laboratory costs and conference participation. The Agricultural Research Council (ARC) in Potchefstroom provided training and access to its laboratory for soil enzyme analysis. My family was willing to join me in South Africa over the three year period, despite Afrikaans language problems that children had to face at school. Lastly, the dedication and availability of my promoter at all times during the course of this study were instrumental in completing this work.
ii Abstract
Historically, mineral resources (such as tin, gold, iron ore and asbestos) have played an important role in advancing Swaziland economically. Nonetheless, the same mining activities of the past are the cause of a legacy of abandoned mine dumps in a number of places such as Bulembu, Ngwenya and Maloma. On the other hand, the achievement of environmentally sound and economically feasible disposal strategies for biosolids is a major problem in many places, including Swaziland. Currently, there are plans in the country to use biosolids in abandoned mine sites. It is thought that this could improve soil conditions and enhance vegetation re-establishment, and more importantly, serve as a permanent solution to biosolids disposal problems. In order to understand potential problems that this might cause, this study was conducted to investigate the effects of biosolids on iron mine soils with regard to soil conditions, soil enzyme activities, plant growth and metal content, and ecotoxicological effects on earthworms. According to the results obtained, the application of biosolids on iron mine soils followed by planting led to significant increases (p < 0.05) in several parameters related to soil fertility such as Ca2+, PO4–, organic matter, water holding and cation exchange capacities. Soil Cu, Zn, Cd, Hg and Pb were also significantly increased, but remained lower than soil critical concentrations. Significant improvements were also observed in β-glucosidase, alkaline phosphatase and urease soil enzyme activities and plant biomass. Notably, increases in soil metal concentrations (Cu from 17.00–50.13 mg kg–1; Zn from 7.59–96.03 mg kg–1 after plant trials) did not affect enzyme activities. Biosolids-treated mine soils were also favourable to earthworm behaviour (NR>−80 %), biomass and reproduction. There was no immediate threat of metal bioaccumulation in earthworms because amongst the six heavy metals studied, the highest levels were Zn and Pb, which stood at 33.11 and 13.67 mg kg–1 (respectively). Earthworm tissue Ni, Cd and Hg were generally lower than 1 mg kg–1, while Cu ranged from 0.03 – 3.16 mg kg–1. Soil metal concentrations were significantly higher after metal exposure, with Zn and Ni reaching 108.15 and 138.24 mg kg–1. Higher metal uptake by plants was observed, especially Zn, which reached 346 mg kg−1 (in shoots) and 462 mg kg−1 (in roots). Higher bioavailability was observed at 0 – 50 t ha–1 (TBS+P) and at 0 – 75 t ha–1 (TB+P) treatments. From an environmental management point of view, the application of urban biosolids to iron mine soils generally seem favourable, especially when this would immediately be followed by ploughing (to incorporate biosolids into soil) and planting (to avoid aeolian and water erosion). This is particularly an attractive biosolids management strategy considering that wastewater treatment residues are continuously available in large quantities.
Key terms
iii Table of Contents
Preface and acknowledgements ...i
Abstract ... ii
1. INTRODUCTION ... 1
1.1 Origin of the study ... 1
1.2 Mining in Swaziland: A brief economic overview of past mining activities and current environmental problems ... 2
1.2.1 Tin mining ... 2
1.2.2 Gold mining ... 4
1.2.3 Iron ore mining ... 4
1.2.4 Asbestos mining ... 5
1.3 Environmental problems associated with past mining activities ... 7
1.4 Problem statement ... 9
1.5 Broad objective of the study ... 10
1.5.1 Specific objectives of the study ... 10
1.6 Summary ... 10
2. LITERATURE REVIEW ... 12
2.1 Introduction ... 12
2.2 Soils affected by mining operations... 12
2.2.1 Elevated metal content ... 12
2.2.2 Lack of organic matter and plant nutrients, and poor cation exchange capacity ... 13
iv
2.2.4 High salinity ... 17
2.2.5 Lack of soil microorganisms and soil animals ... 19
2.2.6 Lack of soil water ... 19
2.2.7 Poor soil structure ... 20
2.2.8 Poor soil aeration ... 21
2.2.9 High soil compaction, high bulk density and decreased soil porosity ... 22
2.3 Conventional engineering, physicochemical and biological land remediation techniques ... 23
2.4 Low-cost mine soil remediation techniques ... 25
2.4.1 Substrate improvement with biosolids and associated problems ... 25
2.4.3 Plant-based remediation technologies ... 31
2.5 Soil enzymes ... 34
2.5.1 Some types of soil enzymes and their associated functions ... 34
2.5.2 Enzymes and their significance in soil processes ... 35
2.5.3 Mining activities and their adverse impacts on soil enzymes ... 37
2.6 Earthworms: Important ecological guilds ... 38
2.6.1 Epigeic earthworms ... 38
2.6.2 Anecic earthworms ... 39
2.6.3 Endogeic earthworms ... 39
2.7 Earthworms: Their significance in ecotoxicological studies... 39
2.7.1 Earthworms are sensitive to metals ... 39
2.7.2 Earthworms bioaccumulate metals ... 40
v
2.7.4 Heavy metals can cause direct impacts on earthworms ... 42
2.7.5 Direct impacts on earthworms can cause negative effects on local biodiversity ... 42
2.8 Earthworms: Their importance in various soil processes ... 44
2.8.1 Earthworms are indicators of soil health and quality ... 44
2.8.2 Earthworm activities contribute to soil formation ... 44
2.8.3 Earthworm activities improve soil structure and fertility ... 44
2.8.4 Earthworm activities result in increased pH, organic matter, plant nutrients and improved plant growth... 45
2.8.5 Earthworm activities may lead to increased metal mobility and bioavailability ... 46
2.9 Summary ... 48
3. A GREENHOUSE TRIAL TO INVESTIGATE THE AMELIORATIVE PROPERTIES OF BIOSOLIDS AND PLANTS ON IRON MINE SOILS: IMPLICATIONS FOR REMEDIATION ... 50
3.1 Introduction ... 50
3.2 Materials and methods ... 51
3.2.1 Location of Ngwenya (study site) ... 51
3.2.2 Soil types ... 51
3.2.3 Climatic conditions ... 53
3.2.4 Vegetation types ... 53
3.2.5 Sample collection and preparation ... 53
3.2.6 Rationale for choice of plant species ... 54
vi
3.2.8 Plant trial design ... 56
3.2.9 Chemical analysis ... 57
3.2.10 Statistical analysis ... 58
3.3 Results ... 59
3.3.1 Pre-trial characterisation of experimental soils ... 59
3.3.2 Influence of biosolids on soil conditions ... 60
3.3.3 Influence of biosolids on plants ... 62
3.3.4 Field soil characterisation subsequent to plant growth trials... 66
3.3.5 Influence of biosolids and plants on soil enzyme activities ... 66
3.4 Discussion of results ... 68
3.4.1 Improvements in soil physicochemical conditions ... 68
3.4.2 Increases in soil metal concentrations ... 69
3.4.3 Improvements in soil enzyme activities ... 71
3.4.4 Plant growth and metal content ... 72
3.5 Summary ... 74
4. AMELIORATION OF IRON MINE SOILS WITH BIOSOLIDS: ECOTOXICOLOGICAL EFFECTS ON EARTHWORMS ... 76
4.1 Introduction ... 76
4.2 Materials and methods ... 78
4.2.1 Soils used in ecotoxicological tests ... 78
4.2.2 Avoidance test with Eisenia andrae ... 78
4.2.3 Mortality, reproduction and bioaccumulation of trace elements in E. andrei ... 79
vii
4.3 Results ... 81
4.3.1 Earthworm behaviour ... 81
4.3.2 Earthworm biomass, and cocoon and juvenile production ... 82
4.3.3 Earthworm tissue metal content ... 83
4.4 Discussion of results ... 85
4.5 Summary ... 88
5. PRACTICAL MATTERS FOR RECYCLING BIOSOLIDS IN DEVELOPING COUNTRIES: MANAGEMENT OF CONCOMITANT PROBLEMS ... 89
5.1 Introduction ... 89
5.2 Methods used in the study ... 90
5.2.1 Investigation of soil physicochemical status, enzyme activities and plant tissue metal content ... 90
5.2.2 Investigation of ecotoxicological effects on earthworms ... 91
5.3 Results, discussion and conclusions ... 91
5.4 Recommendations... 93
5.5 Rationale for recommendations ... 94
5.5.1 Results from the current study were favourable ... 94
5.5.2 Land application of biosolids is practiced in many countries ... 94
5.5.3 Land application is associated with multifaceted benefits ... 95
5.5.4 Biosolids are continuously available in large quantities ... 97
5.5.5 Bioavailability of sludge-bound metals is low ... 98
5.6 Management of biosolids by land application: Important areas to be prioritised ... 99
viii
5.6.2 Trials under greenhouse and field conditions ... 102
5.6.3 Community involvement in biosolids recycling programmes ... 103
5.6.4 Policy formulation ... 105
5.6.5 Implementation and monitoring ... 105
5.7 Remedial measures: Adverse impacts can be managed ... 107
5.7.1 Open air solar drying of sewage sludge ... 107
5.7.2 Composting ... 108
5.7.3 Choice of plant species ... 109
5.7.4 Choice of phytoremediation techniques ... 110
5.7.5 Land use change ... 110
5.8 Summary ... 111
ix List of Tables
Table 1-1: Mineral extraction by type and value in Swaziland (1907-2001)... 7
Table 2-1: Various soil conditions indicated by CEC ... 15
Table 2-2: Soil conditions in different types of mine wastes ... 15
Table 2-3: Functions of mineral elements required as cations from the soil solution ... 16
Table 2-4: Acid description and its effects on the availability of elements in soil ... 17
Table 2-5: Different classes of salinity ... 19
Table 2-6: Conventional soil remediation technologies ... 23
Table 2-7: Maximum permissible concentrations of PTE in soil (United Kingdom) ... 30
Table 2-8: Maximum permissible concentrations of PTE in soil (Scotland) ... 31
Table 2-9: Limit values for PTE in sewage sludge and soils (various countries) ... 31
Table 2-10: Types of phytoremediation ... 34
Table 2-11: Metal levels known to cause negative impacts on earthworms ... 43
Table 3-1: Plant trial designs ... 57
Table 3-2: Pre-trial characterisation of experimental soils ... 60
Table 3-3: Mean (±SD) soil status of the TBS+P treatment after 12 weeks of plant trials ... 61
Table 3-4: Mean (±SD) soil status of the TB+P treatment after 12 weeks of plant trials ... 61
Table 3-5: Mean (±SD) soil metal levels after in TBS+P and TB+P after 12 weeks of plant trials ... 62
Table 3-6: Metal increase/decrease in soil subsequent to plant trials ... 62
Table 3-7: Plants biomass in TBS+P and TB+P after 12 weeks of plant trials ... 63
Table 3-8: BCF and BCFR values of heavy metals in TBS+P and TB+P soils ... 66
x
Table 3-11: Mean (SD) soil enzyme activities after 12 weeks of plant trials ... 67
Table 4-1: Soils used for earthworm trials... 78
Table 4-2: Mean (±SE) biomass (g) of earthworms exposed to tailings subsequent to amendment with biosolids and plants ... 83
Table 4-3: Mean (±SE) number of cocoons and juveniles in TB+P and TBS+P soils over a 28-day earthworm exposure period ... 84
Table4-4: Mean (±SE) total metal concentrations in soils after 28-day earthworm exposure period ... 84
Table 5-1: Biosolids management by land application in selected countries ... 95
Table 5-2: Selected demographic indicators for Swaziland between 1976 and 2007 ... 97
xi List of Figures
Figure 3-1: Location of Ngwenya iron ore mine (within Malolotja Nature Reserve) (SNTC, 2014)... 52 Figure 3-2: Mean (±SD) Cu and Zn concentrations in shoots (a) and roots (b) of plants in
TBS+P soils ... 64 Figure 3-3: Mean (±SD) Cu and Zn concentrations in shoots (a) and roots (b) of plants in
TB+P soils ... 65 Figure 4-1: NR of earthworms in FS+P versus TB+P (a) and FS+P versus TBS+P (b)... 82 Figure 4-2: Mean (±SD) Zn and Pb concentrations in earthworm tissue after 28 days of
exposure to test soils ... 85 Figure 5-1: Sustainable management of biosolids. ... 100 Figure 5-2: Changes of PAHs remaining rate and moisture content of the composting
xii LIST OF ABBREVIATIONS
ANOVA Analysis of variance
ARC Agricultural Research Council
ASTM American Society of Testing and Materials BCFR Bioconcentration coefficient for root
BCF Bioconcentration factor
BMS Bulembu Ministries Swaziland CEC Cation Exchange Capacity CPC Critical Plant Concentrations DHS Demographic Health Survey
DEFRA Department for Environment, Food and Rural Affairs EC Electrical conductivity
FS+P Field soil plus plants
FEC First Environment Consultancy FAO Food and Agricultural Organisation GSMD Geological Survey and Mines Department GDP Gross Domestic Product
ICP–MS Inductively coupled plasma mass spectrometry ISO International Standards Organisation
ISO FDIS International Standards Organisation final draft international standard
K–S Kolmogorov–Smirnov
LOI Loss on ignition
MTL Maximum Tolerable Level
NRC National Research Council
NR Net response
NWU North-West University
OM Organic matter
OECD Organisation for Economic Cooperation and Development OVC Orphaned and Vulnerable Children
PR Percent rate
±SD Plus/minus standard deviation ±SE Plus/minus standard error
PAHs Polycyclic aromatic hydrocarbons PTMs Potentially Toxic Metals
P&T Pump and Teat RGR Relative growth rate
xiii SEPA Scottish Environment Protection Agency SSSSA Soil Science Society of South Africa SEV Soil Vapour Extraction
SMTFCA Songimvelo-Malolotja Trans-Frontier Conservation Area SEA Swaziland Environmental Authority
SIODC Swaziland Iron Ore Development Company SNTC Swaziland National Trust Commission SWSC Swaziland Water Services Corporation
T Tailings
TB+P Tailings, biosolids plus plants TBS+P Tailings, biosolids, soil plus plants T+P Tailings plus plants
TF Translocation Factor
UK United Kingdom
USA EPA United States Environment Protection Agency US$ United States dollars
USA United States of America VOCs Volatile Organic Compounds WWTP Wastewater treatment plant WHC Water Holding Capacity
Page 1 1. INTRODUCTION
1.1 Origin of the study
The origin of this study is the Kingdom of Swaziland; the smallest country in southern Africa, with a total land area of 17 364 km2, and a population of just over 1.2 million. It is landlocked and bordered by South Africa to the north, west and south, and Mozambique to the east (Central Statistics Office & Macro International, 2008; Masilela et al., 2006; Food and Agricultural Organization & World Food Programme, 2008; Whiteside & Whalley, 2007). From an industrial point of view, mining has, in the past, played an important role in Swaziland’s economic development. Regrettably, lack of proper mine closure and soil remediation practices have left a legacy of abandoned mine sites in a number of places. Inadequate legislation on environmental management is partly to be blamed for the current state of affairs with regards to mined areas. For instance, in section 21 of the Mining Act (No. 5) of 1958 (Swaziland Environmental Authority (SEA) & United Nations Environmental Programme 2005), there is a heading for “land restoration”, but there is neither description of nor specific instructions regarding land restoration. Other than requiring miners to provide an environmental bond to the Commissioner of Mines, there is still no description of land restoration in the Mines and Minerals Act of 2011. Poor environmental legislation on mining activities in the country are perhaps some of the reasons why there are still dumps of mine tailings from iron ore, asbestos, coal and diamond mining. Some of these dumps are found in close proximity to human settlements and water sources, while others are within wildlife sanctuaries. While mined areas are in a degraded state, Swaziland has large stockpiles of biosolids for which there are currently no disposal plans (SWSC, personal communication, June 18, 2013).
Biosolids are defined as the nutrient-rich organic material resulting from the treatment of domestic sewage in a treatment facility, which may be recycled and applied as fertilizers to improve and maintain productivity of soils and stimulate plant growth (Chen et al., 2010). In Swaziland, conventional processes are used in treating wastewater. These are primary treatment, secondary treatment and tertiary treatment, as described by Berg et al. (2011), Cunningham and Cunningham (2012), and Manahan (2001). Subsequent to these three stages, sewage sludge is subjected to open air drying in sand beds. According to Girovich (1996), biosolids produced in this way are Class B products with respect to the pathogen and vector attraction reduction requirements and as such they can be land applied. In many countries, biosolids have been utilized to restore mine soils (Boyer & Wratten, 2010; Borden & Black, 2011). In fact, the use of biosolids for soil remediation and agricultural purposes are the two principal methods for biosolids disposal (Fytili & Zabanitou, 2008; Bai et al., 2010). Taking the above into consideration, land affected by mining operations and its possible remediation
Page 2
through the use of biosolids and plant establishment were the key areas of investigation in this study. Bearing in mind that mining has played an important economic role in Swaziland, as noted above, this chapter seeks to provide an economic overview of past mining activities and resultant environmental problems. In view of these problems, the objectives of this study are also described in this chapter.
1.2 Mining in Swaziland: A brief economic overview of past mining activities and current environmental problems
In many countries, mining is one of the most important industrial activities that contribute significantly to economic advancement and general wellbeing of their populations. Mineral discovery and exploitation have been among the major factors responsible for economic emergence of Africa (Scott, 1950) and particularly the mainstays for over a century in southern African economies (McCulloch, 2003).
Developing countries consider mining as the engine of development to promote technological and economic growth (Aswathanarayana, 2003). Currently, the mining sector accounts for a major percentage of the gross domestic product (GDP) of many countries (Chaturvedi et al., 2014; Wolff et al., 2011; Yan et al., 2013; Zhao et al., 2013). Botswana for example, with a population of a little over one million, earns almost US$3 billion from the mining sector, principally diamonds. This equates to approximately US$ 3000 per capita per annum, which is more than 20 times the GDP per capita of neighbouring Mozambique (Aswathanarayana, 2003). In fact, the significance of the mining industry to global economic development has led to the intensification of mining activities in some parts of the world. The main reasons for growth in this sector include the rapid growth and affluence of human populations in the last decades, demand for feedstock indispensable for the supply of goods that are necessary to modern life (Rodrigues et al., 2013), technological advances (Johnston, 2002), and increased capacity of extracting natural resources around the globe (Monjezi et al., 2009).
As a developing country, Swaziland has also relied on mineral resources in the past. A careful investigation of the country’s history in mining shows that there were important contributions to the economy from mineral resources like tin, gold, iron ore and asbestos, as discussed below.
1.2.1 Tin mining
The tin mining industry is known to have existed for 70 years in Swaziland (Crush, 1988); however, the official tin period may be regarded as extending from 1913 to 1938, because it was during this era whereby the annual production of tin by value far exceeded the output of
Page 3
gold, and the aggregate production was five times more valuable (Scott, 1950). Some of the most important areas of stream tin were discovered towards the close of the 19thcentury (Doveton, 1937); however, it was only in 1902 that these deposits were exploited (Doveton, 1937; Crush 1988). The best years of the tin industry in Swaziland seem to have been during the First and Second World War years. During the First World War, for instance, the high prices of over £300.00/ton (£18,750.00 in 2016, http://inflation.stephenmorley.org/, accessed in October 20, 2016) stimulated the production of several thousand tons of dressed tin with a market value of over £200, 000.00 (£12,500,000.00 in 2016). The greatest value obtained for the mineral in any one year was £76,870.00 (£4,804,375.00 in 2016) during the last year (1918-9) of the Great War, (Doveton, 1937). Thereafter, the industry went into slow decline discouraged by the 1920s crash in tin prices, high costs, and the gradual exhaustion of payable reserves. However, between 1934 and 1946, prices picked up and tin to the value of £119 000 was raised (£7,009,100.00 in 2016) (Crush, 1988).
Tin was not smelted in Swaziland, though a smelting plant was erected. For some reason, the climate was found to be unsuitable, and so, smelting was abandoned. The ore was shipped to Lourenço Marques (now Maputo) in Mozambique (Doveton, 1937; Crush 1988) and on to Malaya (now Malaysia) where it was smelted with the ores of that country (Doveton, 1937). During its 70-year existence, the tin mining industry experienced rapid and dramatic changes in fortune. The quantity of tin dressed and the revenue derived from its export fluctuated sharply from year to year. Amongst the many challenges, there were four factors that constantly undermined the industry, and these were; high transportation costs, the cost of feeding the labour force, the labour-intensive nature of the production process, and worker resistance to the demands of management (Crush, 1988). The sale of tin concentrates virtually ceased in 1985 (McLoughlin & Mehra, 1988), bringing the tin era, which began in the 1890s to an end (Crush, 1988).
Page 4 1.2.2 Gold mining
Gold occurrences were discovered over a hundred years ago in north-western Swaziland between Motshane and Horo (Geological Survey & Mines Department (GSMD), 2006), by Tom McLachlan and Walter Cater (Doveton, 1937). Gold mining dominated mineral production in Swaziland from 1880, when the Forbes Reef mine began operating until 1913, after which tin mining assumed greater importance (Scott, 1950; GSMD, 2006). The main gold mines were Pigg’s Peak, Forbes Main Reef, Wyldesdale, Horo, Daisey, Gobolondo and Devil’s Reef (GSMD, 2006; Scott, 1950; Doveton, 1937). Forbes Reef accounted for the largest gold output than any mine in Swaziland prior to 1912, while the Pigg’s Peak mine accounted for the largest gold output of any mine in Swaziland (Scott, 1950).
The gold mining sector in Swaziland did not develop significantly due to a number of reasons, and some of these included the inaccessibility of the gold region (Doveton, 1937; Scott, 1950). As a result, mining was sporadic, occurring between 1882 and 1914 and between 1920 and 1967 (GSMD, 2006). Bridle paths, often the only routes through the mountains, were impassable even to ox-wagons, the principal means of transportation until as late as 1930. Only the Forbes Reef and Pigg’s Peak mines came to be served with better roads, which is perhaps the reason why these two mines accounted for the greatest output compared to the rest of the mines in the region (Scott, 1950). The mountainous nature of the country between the various mines also hampered any efforts aimed at assessing gold in the area (Doveton, 1937).
1.2.3 Iron ore mining
The Ngwenya iron ore mine (hereafter referred to as “Ngwenya”) is reputed to be the second oldest iron ore mine in the world. Excavations carried out between 1964 and 1966 led to firm conclusions that iron ore was already being mined some 28 000 years ago at Ngwenya (Dart & Beaumont, 1969). Following the above findings, commercial production of iron ore started in 1964 (Waïtzenegger et al., 1970; Sneesby, 1968), when about 66000 short tons were mined with a value of £319,000.00 (£6,029,100.00 in 2016), and reached over 2 million short tons in 1967 (Sneesby, 1968), rising to 1.7 million tons in later years. Commercial production came after the signing of a £40 million (£756,000,000.00 in 2016) contract between the Swaziland Iron Ore Development Company (SIODC) Ltd and Japanese iron and steel companies, where the former was to supply about 12-15 million tons of iron ore to the latter, over a 10-year period (1964-1974).
Ngwenya was operated by SIODC, which was controlled jointly by Anglo-American Corporation of South Africa and Guest and Keen & Nettlefold Ltd (Waïtzeneggeret al., 1970).The opening of Ngwenya was particularly facilitated by the construction of a new railway line through Swaziland linking it (Swaziland) with the Portuguese territory of Mozambique. This
Page 5
enabled easy transportation of iron ore from Ngwenya to Lourenço Marques in Mozambique and on to Japan (Sneesby, 1968; Waïtzenegger et al., 1970). The contract between the SIODC and the Japanese iron and steel companies was fulfilled in the second part of 1978 (GSMD, 2006) and in 1980 Ngwenya was closed (McLoughlin & Mehra, 1988).
In accordance with the Swaziland Government’s commitment to re-establish the mining sector in the country, in June 2011, Salgaocar Swaziland (Pty) Ltd was successfully granted a mining lease by INgwenyama (King Mswati the III) on behalf of the Swazi nation, to re-open Ngwenya. Salgaocar’s objective is primarily to re-process the old dumps to recover some of the remaining iron ore. Anglo-American Corporation left about 32.1 million metric tonnes of iron dumps consisting of overburden and materials with low grade iron around the mine pit (Swaziland Environmental Authority (SEA), 2011). A mineral recovery of 46% is expected from the dumps and total production of 10-15 million tonnes over a period of seven (7) years. The area leased includes the dumps, the tailings dam and the slimes dam totalling 401.9 ha (GSMD, 2006).
1.2.4 Asbestos mining
Asbestos is perhaps one of the mineral resources that were extracted at a commercial scale for a much longer period of time compared to the other minerals discussed above. The Havelock asbestos deposit was discovered in 1928 (Scott, 1950; Baton, 1982), but lack of transportation facilities held up production for nearly ten years. Motor transport was unable to reach the site until 1932 when a track was built, connecting the mine with Pigg’s Peak. In that year, operations began on a small scale, but were soon abandoned. Mining started again in 1937 (Scott, 1950) and in 1939 the mine was officially opened as the Havelock asbestos mine (hereafter referred to as “Havelock”) (Baton, 1982; McCulloch, 2005; McDermott et al., 1982). Within ten years, chrysotile asbestos accounted for 70% of the value of Swazi exports (McCulloch, 2005). In the years that followed, Havelock became a very important economic area in Swaziland. Output rose from 4591 short tons in 1939 to a maximum of 32,660 short tons in 1944 (Scott, 1950). In 1966, 36,142 short tons were produced with a total value of nearly £2,500,000.00 (£43,750,000.00 in 2016) (Sneesby, 1968). Towards 1980, production had risen to an average of 30 000 tons of asbestos per year (Baton, 1982).
In order to facilitate exportation of asbestos over the rugged terrain of the Barberton mountain land, a 20km aerial ropeway was constructed in 1939 from Havelock to the nearest railhead at Barberton, South Africa (Baton, 1982; Waïtzenegger et al., 1970; Sneesby, 1968; Scott, 1950; McDermott et al., 1982) by Messrs Turner & Newall (Doveton, 1937). The bulk of asbestos was subsequently transported to Lourenço Marques in Mozambique via South African
Page 6
Railways, for shipment to the United Kingdom, which was the major market (Waïtzenegger et al., 1970).
For most of its life the mine was owned and operated by New Amianthus Mines, under the control of the British asbestos company, Messrs Turner & Newall (Waïtzenegger et al., 1970). Havelock was, for some decades, the largest single employer in Swaziland and a major source of public revenue. The mine also played an important role in the nation’s labour history. By 1960, Havelock was one of the world’s major fibre producers. Output peaked in 1976 at 42,000 tons with most fibre being exported to Europe, Africa and the Far East (McCulloch, 2005).The work force at Havelock consisted of about 2000 Africans and 100 Europeans who formed a community that was the third largest centre of population in Swaziland. Most of the Africans were from Malawi, Mozambique, and South Africa, as well as mainly Swazis (McDermott et al., 1982).
Messrs Turner & Newall experienced heavy losses from the middle to the late 1980s, and were bankrupt in 1991 (McLoughlin & Mehra, 1988; Sam Ntshalintshali1, personal communication, September 11, 2013). After liquidation in the same year, the company was bought by HVL Asbestos Swaziland, who changed the name from Havelock to Bulembu mine. According to Colin Cotterrell2
(personal communication, May 24, 2011 and November 4, 2012) when HVL took over, their main focus was on re-processing of the coarse tailings. Initially, the milling process at Havelock, under Messrs Turner & Newall was quite crude, and so, a lot of dumps contained large quantities of chrysotile asbestos, so much that even Messrs Turner & Newall had started re-processing the discarded material in their last years (McCulloch, 2005). In 2000 output had fallen to 11,000 tons, and the mine was closed in 2001 (McCulloch, 2005). At this time, about 40 ha of land was in a derelict state (O'Dell & Claassen, 2009).
In 2006, the small village of Bulembu (a Swazi name for Havelock) was bought by a non-profit-making organization called Bulembu Ministries Swaziland (BMS). The vision of BMS is to restore Bulembu to a self-sustaining community. Already, BMS has initiated about nine (9) income generating projects. Apart from income generation, the road to recovery for Bulembu also comprises of a number of vibrant community–oriented programmes, such as care for orphaned and vulnerable children (commonly referred to as OVCs in Swaziland), schools, clinics, etc. (Colin Cotterrell, personal communication, May 24, 2011 and November 4, 2012). The production and income generated from the mining activities described above are summarised in Table 1-1.
1 Commissioner of Mines, Swaziland
Page 7
Table 1-1: Mineral extraction by type and value in Swaziland (1907-2001)
Years Cassiterite Tin Gold
Volume (Tons)
Value (£) Value in 2016 (£) Volume (Tons) Value (£) Value in 2016 (£) 1907-10 1507 119,941.00 12,833,687.00 21059 88,579.00 9,477,953.00 1911-20 5054 572,889.00 26,639,338.50 73408 314,060.00 14,603,790.00 1921-30 2133 369,999.00 21,200,942.70 3932 23,643.00 1,354,743.90 1931-34 384 77,393.00 4,999,587.80 8,064.00 520,934.40
Asbestos Iron ore
1939 4591 1944 32660 1966 32142 £2.5 M 43,750,000.00 1754.0 £10.3 M 180,250,000.00 1967 42200 £5.9 M 99,120,000.00 1921.9 £11.3 M 189,840,000.00 1968 42900 £6.0 M 98,400,000.00 2260.2 £11.8 M 193,520000.00 1976 42000 1980 30000 2001 11000
(Sources: Doveton, 1937; Waïtzenegger et al., 1970; Scott, 1950; Sneesby, 1968; Baton, 1982; McCulloch, 2005).
1.3 Environmental problems associated with past mining activities
While mining activities are economically important to any country’s developmental agenda, as illustrated in the above account, they are often destructive to the natural environment (Berg et al., 2011; Stuben et al., 2001; Tordoff et al., 2000; Pietrzykowski et al., 2014), especially in the absence of proper mine closure and rehabilitation programmes. Since the advent of mining in Swaziland, there is no record of rehabilitation of land affected by mining operations (Sam Ntshalintshali, personal communication, September 11, 2013), and as a result, former mine sites are currently wastelands that are located within wildlife sanctuaries and in close proximity to human dwellings and water sources.
According to O'Dell & Claassen (2009), land affected by asbestos mining and the accumulation of dumps at Havelock is about 40 ha. At Ngwenya, the total area affected by iron ore extraction is about 401.9 ha (GSMD, 2013), while at Maloma coal mine (hereafter referred to as “Maloma”), 29 ha of land was said have already been disturbed by coal mining by the end of 2013 (First Environmental Consultancy (FEC), 2014). Mineral extraction and associated activities in these areas have resulted in barren landscapes because by their nature, mining activities affect (remove and/or bury) top soil, which leads to loss of important biological features that support vegetation. These features include water-holding capacity, cation exchange capacity, organic matter, plant essential nutrients, plant seed, mycorrhizal propagules, and associated microbial communities. Loss of such features leads to drastically disturbed, unproductive landscapes, which are erosive and fail to naturally revegetate, even after a
Page 8
prolonged period of time (O'Dell & Claassen, 2009). Most importantly, the susceptibility of spoils and tailings to wind and water erosion contributes to their infertility (Miller & Spoolman, 2012).
Asbestos tailings are also a major concern with regard to public health because there are still a lot of people that live within the Bulembu village. Concerns stem from the fact that asbestos fibres can be blown by air and be deposited in water, on vegetation, on people’s houses, etc. McCulloch (2005) writes that “on the day I visited Havelock in July 2002, it was windy and fibre was being blown over the entire settlement, including the primary school”. The proximity of occupied houses to the asbestos tailings, and thus possibility of inhalation mesothelioma (cancer of the pulmonary membrane and peritoneum) present major concerns because it is well accepted that asbestos fibres are the cause of virtually all cases of human malignant mesothelioma. It is also known that all asbestos types, including chrysotile and amphiboles, have been shown in epidemiological and toxicological studies to be fully capable of inducing mesothelioma (LaDou, 2010; Suzuki & Yuen, 2002). Other risks include asbestosis and bronchial carcinoma (Joshi & Gupta, 2005; Turci et al., 2009, Swartjes & Tromp, 2008; Harrison et al., 1999). More frighteningly, there is no available data to support a threshold limit for exposure to asbestos below which there is no risk of malignant mesothelioma (Suzuki & Yuen, 2002), and thus there is no room for relaxation of public health controls on asbestos (Joshi & Gupta, 2006).
Areas degraded by mineral extraction are also easily accessed by domestic and protected wild animals. Both Havelock and Ngwenya are located within the 700 km² Songimvelo-Malolotja Trans-Frontier Conservation Area (S-MTFCA); a wildlife sanctuary located on the South Africa-Swaziland border between Barberton (in South Africa) and Pigg's Peak (in Swaziland). The core of the TFCA is formed by the 49 000 ha Songimvelo Nature Reserve (in South Africa) and 18 000 ha Malolotja Nature Reserve (in Swaziland) which share a common border. SNTC (2004),
In the south of the country, Maloma is also controversial because the mine was established on a 477-ha private farm. Prior to the establishment of the mine, the farm was used for grazing, at 2 large animal units/ha. Presently (2014-2016), cattle graze freely within the mine site, perhaps due to the availability of fresh water on some dams, which over the years have served as livestock watering holes because generally the area is dry (FEC, 2014). The controversy surrounding access of animals to mine sites is that mine dumps and land affected by mining operations are often associated with elevated metal content (McCabe & Otte, 2000; Dafana et al., 2010; Tordoff et al., 2000; Chaturvedi et al., 2014). During grazing, animals sometimes consume soil particles (Smith et al., 2010; Adriano et al., 2004; Roggeman et al., 2013). In fact, the main pathway of transfer of potentially toxic heavy metals (PTMs) from pasture to the grazing animals has been accepted as the ingestion of soil (directly from the soil
Page 9
surface, plant roots, snout licking or dust) (Hillman et al., 2003). This could represent a special exposure pathway of animals to Pb, Cd and other hazardous heavy metals (Adriano et al., 2004), to an extent that the consumption of meat-based products might be affected. For instance, Cd accumulation in the offal of grazing animals in New Zealand and Australia once made it unsuitable for human consumption and affected access of meat products to overseas markets (Bolan et al., 2014). Animals could also be exposed through consumption of metal-contaminated plants (Kabata-Pendias, 2011; Roggeman et al., 2013). Plants can accumulate trace heavy metals in their tissues due to their great ability to adapt to variable chemical properties of the environment. Thus they are intermediate reservoirs through which trace elements from soils move to man and animals (Kabata-Pendias, 2011). The accumulation of rain water in surface pits presents another problem because according to Roggeman et al. (2013), animals can also be exposed to PTMs via drinking water.
As far as hydrological aspects are concerned, the presence of iron ore tailings within Malolotja is a cause for concern. The area consists of a number of perennial streams and rivers. There are several upland vleis that retain water throughout the year, the most important of which is the Malolotja vlei. The reserve includes virtually the entire catchment area for the Malolotja, Mgwayiza and Mhlangamphepha rivers. The Nkomati River also cuts through the reserve, in a west–easterly direction (SNTC, 2014). The western side of the Ngwenya Mountain is a source of the tributary to Mlondolozi stream, which flows towards the border with South Africa. The Motshane River originates on the north-eastern side of Ngwenya Mountain and flows towards the Hawane Dam, which supplies water to Mbabane (Swaziland’s capital city). There are also subsurface streams that emerge further down slope, and these are the Ngwenya and Ndlotane streams (SEA, 2011). Further, there are a number of small streams within and outside Havelock. The Luhhumaneni River, for instance, which originates in South Africa, borders the asbestos tailings on the east side, flowing in a north–southerly direction. Several other water courses also appear below the asbestos dumps on the south side, and join together to form the Mzilaneni River (Colin Cotterrell, personal communication, May 24, 2011 and November 4, 2012).
1.4 Problem statement
Taking the above into consideration, it is clear that there are a number of environmental problems linked with past mining activities. While the country’s abandoned mine sites are in a derelict state, the disposal of biosolids in Swaziland is also a problem to which no solution (an environmental management plan) has yet been identified (SWSC, personal communication, June 18, 2013). In fact, the handling of biosolids and the achievement of environmentally sound and economically feasible disposal strategies are currently major issues in wastewater
Page 10
treatment, not only in Swaziland but in many places around the world (Deepesh et al., 2014; Fytili & Zabanitou, 2008; Contin et al., 2012). Therefore, in order to identify a permanent solution to biosolids management, the SWSC recently (2013) proposed the spreading of biosolids in mine sites. It was thought that this might serve as a permanent biosolids disposal strategy for Swaziland, while assisting in remediating soils degraded by mining operations. However, there are a number of problems associated with these plans. Amongst many pollutants, biosolids often contain elevated metal content which might affect the quality and health of receiving soils (Fytili & Zabanitou, 2008). Also, since some of the mine sites are located within wildlife sanctuaries, resultant metal accumulation in plant tissue might expose plant-dependant animals to metals.
1.5 Broad objective of the study
The overall objective of this study was to investigate the effects of utilizing biosolids in phytostabilization of iron ore mine soil in Swaziland.
1.5.1 Specific objectives of the study
In order to achieve the broad objective of the study, it was necessary to investigate various related aspects. Therefore, the specific objectives of the study were:
(i) To investigate the influence of biosolids on iron mine soils with regard to soil physicochemical conditions, belowground bioindicators (soil enzymes) and plant growth and tissue metal content.
(ii) To investigate ecotoxicological effects on earthworms subsequent to biosolids application and planting, with regard to earthworm behaviour, mortality, reproduction and bioaccumulation of trace elements.
(iii) To propose recommendations regarding the use of biosolids in remediating iron mine soils in Swaziland.
1.5.2 Summary
Mineral extraction has, in the past, played an important economic role in Swaziland. However, the management of land affected by mining has largely been neglected, and this has resulted in a legacy of abandoned mine sites. Chapter one aimed at presenting an overview of such mining activities and the environmental problems that ensued. Based on these problems, chapter 1 further outlined the main objectives for carrying out this study.
While a number of problems were described in section 1.3, including the proximity of asbestos tailings to human dwellings, access of animals to mine-disturbed areas, etc, the focus
Page 11
of this study was on the effects of biosolids on iron mine soil. As demonstrated in the literature review section (chapter 2), the most dangerous pollutants in biosolids are heavy metals. These are indestructible in the environment, they are linked with animal and human health effects, they affect soil processes, etc. Accordingly, in Chapters 3 and 4, the effects of heavy metals on soil physicochemical processes, soil enzymes, plant growth and earthworms were investigated, and outcomes from these investigations led to the formulation of recommendations presented in Chapter 5.
While the effects of disease vectors in biosolids, effects of other pollutants in biosolids (e.g., pharmaceuticals, detergents, etc – as listed in chapter 2), aesthetic effects of biosolids, the effects of mine wastes on human health (such as airborne asbestos), etc, are all important, they were not investigated in the current study because they were not part of the main objectives.
Page 12 2. LITERATURE REVIEW
2.1 Introduction
Ever since the Industrial Revolution, which began in the mid-18th century, global socio-economic development has depended heavily on the mining industry for provision of mineral resources (Yan et al., 2013). This is because the industry plays a very important role in the generation of wealth and contributes a major percentage of gross national products (Wolff et al., 2011). However, although mining can bring much economic prosperity, large areas of industrial dereliction often result once mining has ceased (Stuben et al., 2001; Tordoff et al., 2000). This dereliction is a result of a combination of activities which, amongst others, include land clearance, transportation activities and generation of vast amounts of mine wastes on top of existing vegetation (Yan et al., 2013). According to Dafana et al. (2010), mine wastes represent the highest proportion of waste produced by industrial activity, with billions of tonnes being produced annually.
In Swaziland, the extraction of mineral resources over a long period of time, without land restorative programmes, has resulted in a legacy of abandoned mine dumps in many places. As stated in Chapter 1, abandoned mine sites in Swaziland are presently (2014-2016) being considered as favourable places where land application of biosolids might be practiced in order to remediate soils affected by years of mineral extraction. In line with these plans, the broad and specific objectives of this study were formulated. The set objectives were important criteria in determining related literature. Specific objective 1 touches on biosolids, mine soils, below-ground indicators and plants. Therefore, thorough review of problems affecting mine soils, influence of biosolids in soil, belowground indicators and their significance in mine soils, and plant establishment in mine are some of the areas that need to be studied. A review of literature on earthworms and their significance in soil is also necessary because earthworms are the subject of specific objective 2.
2.2 Soils affected by mining operations
According to O'Dell and Claassen (2009), mining activities can result in drastically disturbed and unproductive landscapes, which are erosive and incapable of natural vegetation re-establishment, even after a prolonged period of time. The specific problems that cause infertility in mine soils are described below.
2.2.1 Elevated metal content
One of the negative impacts of mining activities on the environment is the accumulation of spoils and tailings (Miller & Spoolman, 2013; Juwarkar & Jambhulkar, 2008; Hudson et al., 1999;
Page 13
Mendez & Maier, 2008). Tailings are characterized by elevated concentrations of heavy metals (McCabe & Otte, 2000; Dafana et al., 2010; Tordoff et al., 2000; Chaturvedi et al., 2014; Bhuiyan et al., 2010), which include Al, Hg, pyrite (FeS2) (Monjezi et al., 2009), As, Cd, Cu, Mn, Pb, Zn (1–50 g/kg) (Chaturvedi et al., 2012; Mendez & Maier, 2008), Ni and Cr. These heavy metals mostly derive from the oxidation of some easily weathered and metal-rich minerals, particularly sulfides (FeS2) (Vega et al., 2004), sphalerite (ZnS) and chalcopyrite (CuFeS2) (Li et al., 2013). Although physical factors may severely limit vegetation establishment, the chemical properties of metalliferous wastes are considered as the ones that play a key role in inhibiting plant establishment. Heavy metals inhibit root growth, which leads to increased susceptibility to droughts (Tordoff et al., 2000). Heavy metals diminish the absorption of water and nutrients by plants, the breathing of roots, and metosis in the meristematic root regions (Vega et al., 2004). Further discussions on heavy metals can be found in section 2.4.1.1 and Chapter 5.
2.2.2 Lack of organic matter and plant nutrients, and poor cation exchange capacity Tailings usually contain no organic matter (Lange et al., 2012; Norland & Veith, 1995; Yan et al., 2013; Chaturvedi et al., 2014; Mendez & Maier, 2008). When available in soil, organic matter leads to positive changes in physical and chemical soil properties, such as water holding and sorption capacities, nutrient content and availability, soil bulk density (Zhao et al., 2013), capacity, cation exchange capacity, pH buffering, chelation and thus bioavailability of trace elements to plants (Khan, 2002). Organic matter is also an energy source for soil biota, which drives decomposition and mineralisation of plant residues, thereby releasing nutrients (Zhao et al., 2013). Further, high organic matter content enhances the retention of metals, reducing their availability for uptake by plants (Carvalho et al., 2013). Thus, the scarcity of organic matter on tailings and land affected by mining often leads to generally poor physicochemical conditions. Apart from organic matter, mine soils are also generally poor in plant nutrients. During various mining processes, nutrients are leached out due to accelerated erosion rates, so the productive soil profile is totally devastated and hence the spoil dumps do not have supportive and reproductive properties to anchor the plants (Juwarkar & Jambhulkar, 2008). Some of the important nutrients to plants, which are leached out during mining, include N, P (Tordoff et al., 2000; Juwarkar & Jambhulkar, 2008; Borgegard & Rydin, 1989), K, Ca (Carvalho et al., 2013), C, Mg and S (Pietrzykowski et al., 2014).
For growth and survival, plants need a range of elements, which are usually categorised into macronutrients and micronutrients. In various literature sources, there seems to be slight differences on what constitute macronutrients and micronutrients. For instance, according to Wild (1995), macronutrients are N, P, K, Ca, Mg and S, while micronutrients are Fe, Mn, Cu, Zn, B, Mo, Cl, Ni. The third category is what Wild (1995) classifies as beneficial elements, and these
Page 14
are Co, Na and Si. High concentrations of Ca and N are essential for plant growth. The largest pool of N, about 95%, is held organic matter, which is strongly related to soil fertility (Tordoff et al., 2000; Carvalho et al., 2013; Bradshaw, 2002; Haygarth et al., 2013). Amongst key macronutrients, N and K are usually taken up by plants in the largest amounts (Wild, 1995; Bradshaw, 2002), while N and P are the most important macronutrients that sustain soil, ecological and human life on earth (Haygarth et al., 2013). In order to ensure satisfactory plant growth, soils should contain N and P in the range of 500-3000 mg kg–1 and 200-1500 mg kg–1, respectively (Ma et al., 2003). N is taken up as ammonium (NH4+) or nitrate (NO3-) (Wild, 1995; Dickinson, 2002; Haygarth et al., 2013). Microorganisms and fauna play an important role in making organic N available to plants through mineralisation, resulting in the release of ammonium, and via microbial immobilization that reduces the availability of N to plants. NH4+, which is produced during mineralization processes, is converted to NO3- by nitrification and involves the oxidation of NH4+ to nitrite (NO2-) and then to NO3-, which as a highly mobile ion, forms the basis upon which the transfer of N to the atmosphere or water can potentially happen (Haygarth et al., 2013).Other important nutrients are taken up as follows; P as H2PO4- and HPO42-, S as SO42- and the heavy metals as cations, e.g., Ca2+, Mg2+, K+, etc (Wild, 1995).
The cation exchange capacity (CEC) of a soil is defined as the sum of positive (+) charges of the adsorbed cations that a soil can adsorb at a specific pH. Each adsorbed K+ contributes one + charge, and each adsorbed Ca2+ contributes 2 + charges to the CEC. The CEC is the sum of the + charges of all of the adsorbed cations (Forth, 1990; Pulford, 2007; White & Greenwood, 2013). It influences the extent to which soils can meet plant demands for cations such as K+ and Mg2+ without frequent applications of these macronutrients (White & Greenwood, 2013). In other words, cation exchange in soil is the mechanism by which K, Ca, Mg, and essential trace-level heavy metals are made available to plants (Manahan, 2001). According to Jones (2012), CEC within 1-10 and 11-50 meq/100g ranges is indicative of the aspects described in Table 2-1.
Although soils affected by mining operations often exhibit similar problems, it appears that nutritional issues vary considerably between different types of mined soils (Dickinson, 2002), as outlined in Table 2-2. Colliery spoil, for instance, is totally deficient in N and almost equally deficient in plant available P. Many of the problems encountered in revegetating colliery spoil are concerned with extreme acidity, which is generated by oxidation of iron pyrites (FeS2). Colliery spoil contains significant quantities of pyrite, and these oxidise spontaneously when in contact with the atmosphere. In many cases, the rate of acid production and quantity of acid produced exceeds the buffering capacity of the spoil material, and the pH of the spoil falls to extreme low levels, making aluminium and manganese available in toxic amounts to plants (Ye
Page 15
et al., 2002). The main functions of some of the nutrients discussed above are shown in Table 2-3.
Table 2-1: Various soil conditions indicated by CEC
CEC within 1-10meq/100g* CEC within 11-50meq/100g
High sand content High clay content
Low organic matter content High organic matter content
Less aglime required to change the soil pH More aglime required to change the soil pH Low capacity to hold plant nutrient elements, loss
by leaching from the soil profile
High capacity to hold plant nutrient elements within the soil profile
Physical ramifications associated with high sand content
Physical ramifications associated with high clay content
Low water holding capacity High water-holding capacity (Source: Jones, 2012)
Table 2-2: Soil conditions in different types of mine wastes Mine waste type Factors associated with nutrients
Overburdens General lack of major nutrients, especially N.
Colliery spoil Commonly, pyrite (FeS2) oxidation creates acidity and toxicity problems; low N and P; sometimes high salinity.
Metalliferous mine waste Extremely high concentrations of mined heavy metals and companion heavy metals (e.g. Pb, Zn, Cu, Cd, Ni); inadequate soil structure and major nutrient deficiency; extremes of pH often influence availability of heavy metals to plants.
(Source: Dickinson, 2002)
2.2.3 Acidic pH
Tailings usually exhibit acidic pH (Lange et al., 2012; Chaturvedi et al., 2014; Mendez & Maier, 2008), although some tailings may be alkaline (Mendez & Maier, 2008). Acidity creates chemical and biological conditions in soil which are harmful to many plants, although there are exceptions (Wild, 1995). Acidity determines the availability of heavy metals to plants (Tordoff et al., 2000), and soils with either low or high pH are adverse for plant growth due to its effect on nutrient availability. It is known that a lower pH (measure of acidity or alkalinity) and redox potential (measure of the aeration status) enhance the mobility and higher plant uptake of most heavy metals (Forsberg & Ledin, 2006; Carvalho et al., 2013) such as Pb, Zn, Cu, as well as accessory heavy metals such as Mn and Al (Tordoff et al., 2000). This is because when soils become acidic their capacity to adsorb cations is reduced so that nutrient cations, especially Ca2+ and Mg2+, pass into solution and are leached in drainage water. As the pH decreases to about 5.5 and below, the concentrations of aluminium ions; Al(OH)2+, AlOH2+ and Al3+begin to increase in soil solution and become dominant in that order as the pH falls, and they displace other cations from exchange sites.
Page 16
Table 2-3: Functions of mineral elements required as cations from the soil solution Element and form
required Role in plants Phosphorus (P)H2PO4 -and HPO4 2-,
Respiration and photosynthesis in green leaves; microbial turnover and decomposition of litter; seed and root formation; crop quality and strength; accumulation and release of energy during cellular metabolism; constituent of many proteins, coenzymes, nucleic acids, and metabolic substrates.
Potassium (K), K+ Enzyme activation; cellular osmoticum; counter-cation for anion accumulation and electrogenic transport; photosynthesis; carbohydrate; translocation; protein synthesis, etc.
Calcium (Ca), Ca2+ Membrane and cell-wall structure; counter-cation for anion accumulation in vacuoles; cystosolic signalling.
Magnesium (Mg), Mg2+
Constituent of chlorophyll; photosynthetic charge separation; enzyme cofactor; nucleic acid stabilization.
Iron (Fe), Fe2+, Fe3+ chelates
Photosynthesis; mitochondrial respiration; C and N metabolism; production and scavenging of reactive oxygen species; regulation of transcription and translation; hormone biosynthesis.
Manganese (Mn), Mn2+, Mn chelates
Photosystem II; enzyme activation in photosynthesis, C and N metabolism, RNA polymerase; CO2 assimilation; production of lignin, flavonoids, fatty acids, growth hormone, N metabolism.
Copper (Cu), Cu+, Cu2+, Cu chelates
Photosynthesis; mitochondrial respiration; C and N metabolism; protection against oxidative stress; protein metabolism; lignification of cell walls; N fixation. Zinc (Zn), Zn2+, Zn
chelates
Structural stability of proteins; regulation of transcription and translation; oxidoreductases and hydrolytic enzymes; protection of cells from damage. Nickel (Ni), Ni2+, Ni
chelates
Constituent of urease; healthy embryo and seedling vigour; plant disease resistance.
Sodium (Na), Na+ Osmotic replacement of K; counter-cation for anion accumulation and electrogenic transport; C4 and CAM metabolism.
Aluminium (Al), Herbivore defence; prevention of Fe toxicity. Cobalt (Co), Co2+ Nitrogen fixation
Nitrogen (N),NH4+, NO3
- Constituent of all proteins, chlorophyll, and in coenzymes and nucleic acids. Sulphur (S), SO4
2
- Important constituent of plant proteins.
Boron (B) Sugar translocation and carbohydrate metabolism.
Molybdenum (Mo) Nitrogen fixation; functioning of enzymes involved in redox reactions. Chlorine (Cl) Activates system for production of O2 in photosynthesis
(Sources: Haygarth et al., 2013; Forth, 1990; Wild, 1995; White & Greenwood, 2013; Alloway, 2013)
Acidic soils therefore usually have low contents of calcium and magnesium, and the supply to plants may be deficient (Wild, 1995). Also, this can have a negative effect on germination and
Page 17
seedling development (Carvalho et al., 2013). On the other hand, the solubility of most heavy metals decreases as pH increases (Verdugo et al., 2011). A pH of 5.0 or higher is suitable for nitrogen fixation (Maiti & Saxena, 1998), and generally, the maintenance of a circumneutral pH reduces the risk of metal toxicity in plants and animals (Forsberg & Ledin, 2006). That is why even in agricultural practices, pH is maintained at 6.0-6.5 (Wild, 1995). With regard to soil types, the optimum pH range for mineral soils is 5.5-6.5, organic soils is 5.4-6.0 and organic soilless medium is 5.2-5.8 (Jones, 2012). The description of acidity and pH ranges at which some of the heavy metals are mobile and available to plants are summarised in Table 2-4.
The genesis of extreme acidity in mine waste is related to sulphur or sulphide minerals, such as FeS2. When exposed to the atmosphere or rainfall, during or after mining, these can be oxidised forming large amounts of sulphuric acid (H2SO4). This results in pronounced pH decrease, and acid mine drainage that can leach heavy metals from tailings, constituting a threat to surrounding ecosystems (Bhuiyan et al., 2010; Novo et al., 2013; Tordoff et al., 2000; Bian et al., 2010).
Table 2-4: Acid description and its effects on the availability of elements in soil
Soil reactivity Effects of pH on the relative availability of chemical elements in soil
Acid Description pH range pH range Highest mobility and availability Extremely acid <4.5 Low pH (˂5.5) Al, Fe, Mn, Zn, Cu, Cd, Pb Very strong 4.5 – 5.5 Intermediate pH NO3, PO4, K, Mg, S, B, Cu Medium acid 5.6 – 6.0 High pH (˃7.0) Ca, Mo, As, Se
Slightly acid 6.1 – 6.5 Neutral 6.6 – 7.3 Mildly alkaline 7.4 – 7.8 Medium alkaline 7.9 – 8.4 Strongly alkaline 8.5 – 9.0 Very alkaline >9.1
(Sources: Dickinson, 2002; Johns, 2012)
2.2.4 High salinity
Salinity has been reported to be one of the major concerns in mine wastes, especially spoils. Salinity refers to the content of soluble salts in soils, and common chemical components include sodium, chloride, calcium, magnesium, potassium and sulphate. There are two types of salt-affected soils, namely sodic soils and saline soils. The main differences between these two are related to the nature of anions and the pH of the soil (Rasool et al., 2013). In saline soils, the dominant soluble salts are NaCl and Na2SO4 and sometimes there are appreciable quantities of chloride (Cl−) and sulphate (SO42–) of calcium (Ca2+) and magnesium (Mg2+). A sodic soil has excessive amount of Na+ associated with the negatively charged clay particles in soils.
Page 18
Otherwise, the term “salt-affected” refers to both saline and sodic soils (Hasanuzzaman et al., 2013).
Excellent reviews on the impacts of salinity can be found Hasanuzzaman et al. (2013), Rasool et al. (2013) and Djanaguiraman and Prasad (2013). Briefly, salinity can causes problems on crop yield, soil biota, growth and survival of microorganisms and soil animals (Owojori et al., 2008). On plants, the adverse effects of salt stress are expressed on whole plant level, and appear during all developmental stages including germination, seedling and vegetative stages (Chandna et al., 2013). Specifically, salinity can significantly affect enzyme activities, nutrition balance, membrane functions and metabolic processes (Orcutt & Nilsen, 2000). It is well known that high salt levels can limit plant growth and thus can be a major constraint for natural biological colonization and phytoremediation (Mingorance et al., 2013; Li et al., 2013; Maiti & Saxena, 1998). It has been reported that salinity detrimentally influences microbiological processes due to the restricted availability of water or the influence on cellular physiology and metabolic processes. It has been shown that salinity over a critical level reduces enzyme and soil microbial biomass. Furthermore, soil salinity may often increase the microbial metabolic quotient (qCO2), a specific indicator, used to determine the effects of environmental stresses on soil microbial activity (Ghollarata & Raiesi, 2007). Moreover, salt movement through surface and groundwater and seepage may salinize the surrounding receiving environments and cause a degradation of ecosystems (Li et al., 2013). In natural soils, conductivity ranges from 0.2 – 0.8dS m–1, which is optimum for plant growth (Maiti & Saxena, 1998). According to Meeinkuirt et al. (2012), 0.2 dS m–1 is the standard level of salinity, while soils with electrical conductivity values greater than 4 dSm-1 are considered saline (Hodson & Donner, 2013). The different classes of salinity are shown in Table 2-5.
With regard to the genesis of salinity in spoil, there are a number of factors that are involved. Many waste materials, particularly those produced by mining for heavy metals and also some colliery spoils, contain sulphides, such as pyrite, and also carbonates particularly of calcium and magnesium. When the pyrite oxidises, it can give calcium and magnesium sulphates. Neither of these is at all toxic at normal concentrations. These are soluble and, in arid climates, may accumulate and cause extreme salinity in the surface layers of the waste as the soil moisture evaporates (Ye et al., 2002; Li et al., 2013; Mendez & Maier, 2008). Salinity of tailings may therefore result from interaction of products of pyrite weathering with native carbonates, concentration of naturally occurring salts due to recycling of water, additions made to tailings to adjust effluent pH and excessive evaporation from the surface (Ye et al., 2002).
Page 19 Table 2-5: Different classes of salinity
Salinity class ECe range (dS/m)
Non-saline 0-2 Low salinity 2-4 Moderate salinity 4-8 High salinity 8-16 Severe salinity 16-32 Extreme salinity >32
(Source: Rasool et al., 2013)
2.2.5 Lack of soil microorganisms and soil animals
Lack of soil microorganisms is another significant problem in mine waste (Mendez & Maier, 2008; Tordoff et al., 2000; Norland & Veith, 1995). Soil microorganisms are essential for key ecosystem processes such as decomposition and mineralization. The quantity and activity of soil microorganisms also sensitively reflect soil conditions and degree of development. The development of active microbial populations has a major impact on the activation of the vital processes of element cycling, which lead to the transformation of organic compounds and conversion of inorganic forms of N and P into plant available forms and, in turn, to enhance soil development (Zhao et al., 2013). Soil microorganisms decompose plant residues and form soil organic matter. They also physically bind soil particles, and are fundamental to soil structure as well as soil formation (Khan, 2002). Key members of the soil mesofauna have long been linked with maintenance and improvement of soil structure. This is particularly true of mesofauna such as earthworms and dung beetles. The effects of these can be very considerable, and some soils and their associated organic matter inputs (animal and plant detritus) are heavily processed by these animals. Along with plant roots, earthworms and other burrowing animals of sufficient size move soil particles to create and then stabilize (with mucilaginous stabilizing agents and faecal materials) cylindrical section pores, which are good for soil structure (Standing & Killham, 2013).
2.2.6 Lack of soil water
Mining destroys the soil’s ability to retain water, which affects the overall productivity of soil. Soil water is a key property for plant growth and development (Mouazen et al., 2014). Water is required for photosynthesis, for tissue rigidity (turgidity), to produce carbohydrates (Verhoef & Egea, 2013), for microbial activity and also affects gas exchange and many soil chemical reactions (Khan, 2002; Asgarzadeh et al., 2010). Some of the most important aspects of soil-water relations include soil-water holding capacity.
The supply of water to plants is determined by the water storage capacity of the soil or WHC, its ability to be replenished from surface applied water, its internal drainage, and the depth and distribution of the root system (Bell, 2002). WHC is defined as the total amount of
