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Why trace elements are important
Fuel Processing Technology 65–66 Ž2000. 21–33 www.elsevier.comrlocaterfuproc
Why trace elements are important Dalway J. Swaine
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CSIRO Energy Technology and Sydney UniÕersity, P.O. Box 136, North Ryde, NSW 1670, Australia Received 7 December 1998; accepted 4 October 1999
Abstract The widespread presence of trace elements is shown by reviewing their roles during the mining, beneficiation and combustion of coal and during metal mining and treatments. Special attention is given to Hg, to the environmentally important mineral, pyrite, to radioactivity, to waste disposal, to reclamation after mining and to health aspects. The 26 trace elements that are considered to be of environmental interest are listed. Important topics for future research are discussed. Although many trace element problems are under control, constant vigilance is a sine qua non and ongoing research is important. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Trace elements; Pyrite; Coal
1. Introduction Trace elements are important because of their association with environmental issues and the health of plants, animals and humans. Consideration must be given to essentiality, non-essentiality and toxicity that depend on concentrations, the form of the element Žspeciation., pH and oxidation–reduction conditions and other factors. In some cases, the difference in concentration between essentiality and unwanted effects, even toxicity, is small. The proper assessment of the environmental status of an element depends critically on proper sampling and analysis with careful attention to minimising contamination, to the use of appropriate reference materials and to the correct choice of analytical methods w1x. There are several sources of trace elements, both natural and anthropogenic. Natural sources include the weathering of rocks, volcanoes, sea spray, thermal springs, lake and river sediments, vegetation and forest fires. Inputs from anthropogenic sources are from )
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0378-3820r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 9 9 . 0 0 0 7 3 - 9
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metal mining and smelting, combustion of coal, oil and wood, industrial operations, waste disposal, agricultural activities, motor tyre-and engine-wear and cremation. It is often difficult to assign the source or sources of trace elements to a particular location and sometimes too much emphasis is given to anthropogenic sources. Volcanic activity is a significant source of several trace elements, for example, Br, Se and Sb w2x. The main source of Zn may be a smelter, a power station, a volcano, fertiliser production, vegetation or motor tyre wear, depending on the nearness of the sampling location to the source. The concept of geochemical cycling, that is, the movement of elements in the various parts of the earth, including the atmosphere, is relevant to a proper understanding of the overall role of trace elements, especially their environmental significance w3x. It must be stressed that the system is dynamic not static and that biogeochemical cycling is now well recognised for the appraisal of trace elements w4x. Based on a personal assessment, 26 trace elements are considered to be of environmental interest ŽTable 1.. The environmental importance of the elements decreases from left to right, and the magnitude of any effects depends on particular situations, keeping in mind that many elements may be essential or hazardous. The notion of toxicity of trace elements per se is unreal and undesirably emotive. The proper status of a trace element has to be established for a particular milieu and set of circumstances. Hence, generalisations should not be made without caveats: for example, with regard to concentrations, the statement of Paracelsus in the 16th century should be heeded, namely, ‘‘All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy.’’ There are wide ranges of concentrations of trace elements in terrestrial materials, waters, air and living matter. Data for igneous rocks, shales, sandstones, limestones, seawater and living matter have been summarised by Bowen w5x. There are extensive compilations of the trace-element contents of soils w6x, of fertilisers w7x and of coals w3x. An overall assessment of trace elements and relevant matters has been given w8x, modes of occurrence of trace elements in coal have been reviewed w3,9x and there is information for inorganic elements in low-rank coals w10x. Table 1 Trace elements considered to be of environmental interest I
II
III
As Cd Cr Hg Pb Se
B Cl F Mn Mo Ni Be Cu P Th U V Zn
Ba Co I Ra Sb Sn Tl
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The importance of trace elements will be discussed for coal during mining, treatment and combustion, for petroleum and for metals during mining and treatment. Special attention will be given to water and air, wastes and the role of pyrite, perhaps the most environmentally important mineral. References will be made to radioactivity and to health aspects. 2. Relevant information on mercury In view of the special interest in Hg, some relevant information will be given. Historically, Hg was widely used in industry, agriculture and medicine, but current concerns about short-term poisoning and about possible long-term ill effects have curtailed its widespread use. At least half of the Hg emitted to the atmosphere comes from anthropogenic activities, including coal combustion, but natural sources, including volcanoes, bedrock, soil, water and vegetation, are also relevant w11x. The volatility of Hg means that recycling occurs readily, thereby enhancing its global atmospheric distribution. This is in keeping with atmospheric Hg being more than 90% in the elemental form w12x. Measurements of the deposition of Hg at a site in the USA, gave values of 43–358, with a mean of 186, ngrm2rweek w13x. Wet deposition predominated over dry deposition. The levels of Hg in fish are an ongoing concern and in some areas, people are advised to limit the consumption of local fish. A thorough study was carried out on several hundred mothers and their children from the Republic of Seychelles, where 85% of the population eat fish daily w14x. Children aged 66 months showed no adverse effects from either prenatal or postnatal exposure to methyl mercury. It was concluded that, ‘‘in the population studied, consumption of a diet high in ocean fish appears to pose no threat to development outcomes through 66 months of age’’ w14x. It must be emphasised that those results are for a particular situation and do not necessarily apply generally. 3. Trace elements in coal 3.1. During mining Trace elements are not expected to cause problems during underground mining, except for minor local effects where some soluble elements may be present in the mine water. However, after mining has ceased, there may be problems from the weathering of coal around the surface of the mine area, especially from the breakdown of pyrite that gives acidic leachates. Such run-off contaminates nearby waterways with an unsightly brown slurry and adds some unwanted trace elements. Nowadays, the abandonment of exhausted coal mines without some containment measures is not permitted in the USA and some other countries. The position with surface mining is different. Here, there may be changes in topography, alterations to the water table and to water quality, aeration of sub-surface material Žsediments and carbonaceous sediments. and changes in the bacterial population of surface material Žsoil.. In many coals, there is the possibility of the oxidation of pyrite ŽFeS 2 ., especially framboidal pyrite, with the consequent lowering of pH. Trace elements, for example, As and Se, could be released from the pyrite and the
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sulfuric acid, formed during the oxidation, may leach trace elements from the coal mineral matter and nearby inorganic material Žrocks.. Some amelioration of these acid conditions occurs if the nearby rocks contain limestone or other carbonates. Under some circumstances, the oxidation of pyrite may produce a slurry of hydrated ferric oxides, surely an undesirable addition to nearby surface waters. However, under these conditions, trace elements may be removed from solution by sorption on clays and iron oxides. Trace elements in the overburden and soil, which will be used in the reclamation of the open-cut after mining, must be taken into account when deciding the nature of the revegetation. Some care should be taken with B and Mn that may affect the growth of some plants under special conditions of pH and drainage. Available Se and Mo will not upset plant growth, but too much of these elements may produce plants that could upset grazing animals. However, unwanted effects for trace elements are rarely found in places previously mined for coal. Successful reclamation depends on a proper agricultural approach, including field trials. It is salutary to recall the statement by M.V. Chadwick that ‘‘disturbance of land is not a disaster it is an opportunity and a challenge.’’ 3.2. During storage Long-term storage of coals after mining and before treatment is not common and hence there should be little weathering. Coal is stockpiled before shipping and at power stations, but this is usually cleaned coal with low contents of pyrite. Weathering is also limited by the compact nature of the stockpile that lessens the access of water and air. 3.3. During beneficiation Various methods are used in the beneficiation of coal to produce cleaned coal. The main aim is to reduce the mineral matter, that is, to lower the ash yield with a consequent lowering of transportation and other costs and to improve combustion. There is also a bonus with the reduction in the levels of various trace elements, notably those associated with sulfide and other minerals. Recent studies in the USA w15,16x have shown that there are marked reductions in the concentrations of various trace elements after beneficiation. Concentrations for 11 trace elements listed in the 1990 Clean Air Act Amendments ŽCAAA. were determined in raw coal and in clean coal using commercial samples from several seams from eastern and mid-western USA and conventional methods of beneficiation w15x. The ash yields of 25–30% in the raw coals were reduced to 6–10% in the clean coals. Significant reductions were found for several trace elements, for example, As, Cr, Mn, Ni and Pb were reduced by 54–75%, Cd, Co and Be by 41–52% and lesser reductions were found for Hg, Sb and Se Ž14–34%.. As expected, there was some connection between the reductions in trace element concentrations and ash yields. Another study was carried out at commercial-scale cleaning plants using conventional methods with heavy-media cyclones and froth flotation with two modified flowsheets w16x. For a coal from the Pratt seam, Alabama, reductions of 39–89% were found for nine trace elements. Further tests were carried out using advanced coal cleaning techniques. Improvements over the conventional processes were found for As,
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Cd, Pb, Ni and Sb. Grinding to 150 and to 75 mm topsize did not give a general improvement in trace element removal. In some coals, even very low concentrations of trace elements may be reduced by beneficiation. For example, three bituminous coals from the Sydney Basin, New South Wales, Australia, had reductions in As from 1–4 ppm to 0.5–1.1 ppm w3x. These coals also had marked reductions in Cd, Hg and Sb, all the concentrations being sub-ppm. Information on the mineral matter in coal is helpful in estimating the likely affects of beneficiation on trace-element reductions. In particular, the mode of occurrence of trace elements is useful, including speciation ascertained by X-ray absorption fine structure spectroscopy w17x. The optimum particle size needs to be determined for particular coals prior to beneficiation. It should be stressed that the cases mentioned above w3,15,16x were for coals that were treated for reductions in ash yields and sulfur contents and not specially for trace-element reductions. Perhaps, attention to other factors would give further reductions in trace-element contents if desired. During beneficiation and the reduction in trace-element levels in the clean coal, there is an associated increase in trace-element contents in the reject material. The disposal of these washery rejects must be undertaken carefully because their increased contents of pyrite are a potential source of acidic solutions that could contaminate local waters with increased concentrations of trace elements. Perhaps this may not be an environmental hazard w16x, but care must be taken to ensure that there is no unwanted contamination from particular washeries. The fate of trace elements during beneficiation has been reviewed w18x. 3.4. During combustion The major use of coal is for the generation of power by combustion on a large scale. This means that most trace elements will be released and redistributed into bottom ash, flyash, fine flyash and the gaseous phase. Up to about 20% of the original mineral matter is found in the bottom ash and up to about 80% in the flyash. The bottom ash remains in the combustion area in the furnace, whereas the flyash is conveyed through the system where most Žmore than 99%. is removed by electrostatic precipitation or fabric filters. However, a small proportion, mostly fine particles of less than about 10 mm in diameter, is emitted to the atmosphere with the stack gases. Very few elements are in the vapour state, probably only variable proportions of the total amount of Hg and the halogens, but even these may be associated with the surfaces of very fine flyash particles. The environmental interest is in the trace elements in the bottom ash and flyash Žremoved by the particle attenuation devices. that are stored in ash disposal ponds, and in the stack emissions. The main concern with the ash disposal ponds is the possibility of the release of leachates that could transport trace elements into nearby underground and surface waters. However, efforts are usually made to prevent or restrict such losses by having the ponds in clay-rich soil and by using clay liners and plastic sheeting. There is a good account of the leaching of trace elements from coal ash, with information on the factors controlling the leaching and on specific elements, As, B, Cd, Cu, Mo, Ni, Se and Zn w19x. The dispersion of trace elements in the stack gases means that varying amounts of trace elements are deposited in the environs of power stations. Measurements of trace
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elements in deposition collected at various locations at different distances and aspects were carried out at Wallerawang power station over a 4-year period w3,20,21x. Wallerawang is about 120 km northwest from Sydney, New South Wales, Australia. The installed capacity is 1240 MW, flyash attenuation is by electrostatic precipitation and the bituminous feed coal is from the Lithgow seam, the ash yield being 21.4 " 4% and the total sulfur being 0.6 " 0.1% S. The method of collection of deposition from the atmosphere was flat fine-mesh envelopes containing 2 g of prepared Sphagnum moss. The envelopes were replaced at 3-monthly intervals and the moss was analysed for up to 39 trace elements using optical emission spectrography, atomic absorption spectrometry and instrumental neutron activation analysis. It was found, as expected, that there was a decrease in the amounts of trace elements deposited with distance from Wallerawang. Variations in amounts at different aspects depended on changes in wind strength and direction. An important finding was the variation in amounts of deposition with time of sampling, the ratio of maximum to minimum values being about 5:1. This stresses the importance of time of sampling at any location in order to obtain significant results. Using the marked difference in Ge contents of local soil and fine flyash emitted to the atmosphere Ža ratio of 1:50., the proportions of flyash and soil in samples of deposition were calculated. This showed for example, that at 1.8 km from the power station, the deposition contained 7–80% with a mean of 40% flyash, while at 5.3 km, there was - 1–5% with a mean of 2.5% flyash. The amounts of trace elements in the depositions were low compared with those in surface soils and mostly less than the amounts from weathering and litter decay w20x. Although this study has established that trace elements from the combustion of a specific coal under the conditions of operation at Wallerawang power station have not been detrimental to the environment, it must be emphasised that these results can at best be only a guide to other situations. Real answers for other places can only be obtained by carrying out similar experiments there. 4. Petroleum There has not been much interest in trace elements in petroleum, perhaps because concentrations are probably mostly low, especially for environmentally sensitive elements, for example, Cd, Hg and Pb. During diagenesis, V and Ni replace some Mg in porphyrin. Other trace elements are gained by the crude oils during migration or in the reservoir. The presence of porphyrin and other organic complexes leads to relatively high concentrations of V Žup to about 1000, with a mean of about 60 ppm V. and Ni Žup to about 150, with a mean of about 20 ppm. in crude oil. These mean values are of the same order as those for most coals w3x. Much lower concentrations of the other trace elements are found in most oils. These levels are not considered to be of environmental concern. The amounts of trace elements emitted during the combustion of oils would be unlikely to be troublesome. A study of rainwater samples at 2, 3 and 4 km from the stack of an oil-fired power station showed values of 0.46, 0.27 and 0.081 ppm V compared with the background 0.065 ppm V w22x. Lower values for Ni, Cu, Zn and Mn indicated that these emissions of trace elements would not be environmentally harmful. It is interesting to recall that deposits from the smokestacks of ships burning certain Venezuelan oils containing 12–15% V2 O5 , were a commercial source of V.
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5. Metal mining Although mining matters have been written about since the 4th century B.C., for example, by Aristotle and Theophrastus, the first major treatise was De Re Metallica written by Georgius Agricola in 1556 w23x. This is a thorough, yet scholarly account of the development of mining methods, metallurgical processes, geology, mineralogy and mining law, profusely illustrated by excellent woodcuts. Some adverse effects were noted, for example, ‘‘miners are sometimes killed by the pestilential air which they breathed; sometimes their lungs rot away.’’ Elsewhere he notes that ‘‘if the dust has corrosive qualities, it eats away the lungs . . . hence in the mines of the Carpathian Mountains women are found who have married several husbands, all of whom this terrible consumption has carried off to a premature death.’’ The Roman makers of vermilion Žmercury sulfide. took precautions against breathing its dust. Perhaps other trace elements ŽAs, Cd, Rn. caused health problems during early mining although they were not recognised as such. The sequence of events leading to the production of metals comprises exploration, mining, grinding, separation, extraction and refining. Many ores are sulfides notably those of Cu, Pb, Zn and Hg. There is a close association of Cd with ZnS. Other metals occur as oxides, carbonates, silicates and tellurides, while Ag and Au are found as free metals. The sulfide ores are prone to oxidation after mining and hence there is the possibility of acid formation. In many cases, pyrite is also present. The diversity of modes of occurrence of metals means that there is a variety of methods of mining and treatment and hence different ways by which there could be contamination. There is usually some disturbance of land during exploration, but there is unlikely to be any trace element effect. During underground mining, there may be dispersion of metal-rich dusts and in some cases, increased levels of radon, but good practice dictates proper precautionary measures. In open-cut mining, there is disturbance of land and the formation of fresh surfaces prone to oxidation especially if sulfide ores are being mined. This means that water may be contaminated by traces of the main metal and of minor metals associated with the ore. In placer mining, where large volumes of water are used, trace amounts of metals may be leached and removed in the used water. Good mining practice should cope with these trace-element aspects. In order to separate the valuable metal from the valueless gangue, the mined material is crushed and perhaps, finely ground before a separation process is used. This may be based on differences in specific gravity or wettability of particle surfaces. At this stage, there is the possibility of the disposal of used solutions that may contain some trace metals. Smelting Žthe heating of concentrates in furnaces. is used to prepare Cu, Pb and Zn and these large-scale operations produce emissions containing various trace elements. For example, As, Cd and Se are trace constituents in many ore concentrates fed to smelters. Smelter emissions are difficult to quantify, as they are affected by the composition of the feed, the recycling of dusts, slags and sinter and the collection efficiencies for fumes and dusts. Studies have shown the dispersion of metal-containing dusts around smelters, but good industrial practice is able to control these to ensure minimal contamination. In some places, extractions are carried out to concentrate the valuable metal, thereby producing solutions of unwanted trace metals. The handling of such wastes requires care
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to avoid contaminating local soils and waters. The reworking of wastes is potentially a source of trace metals, some being removed for gain and some being rejected. In general, the mining and treatments of metal ores and concentrates give products with enhanced levels of metals. Hence, any unwanted losses may give higher local levels of contamination than what could be regarded as traces. Modern industrial practices are geared to keeping these losses as low as possible and below relevant guidelines. However, there may be cases where material is spread by winds to give trace levels over large areas. For example, spillage during transport by train or road may not remain in situ, but may be dispersed widely. Prevention of spillages overcomes such incipient problems. 6. Radioactivity As well as the natural radioactive background, there are places where traces of Ra, U and Th give rise to low levels of radon, for example, in some caves. During mining for U, radon is released and mines are exposed to the resulting very low levels. Fatalities occurred amongst miners at Joachimsthal who inhaled radon and radioactive dust. Nowadays, efficient ventilation removes this hazard. The very low levels of radioactivity from coal are not considered to be harmful. It seems reasonable to assume that radioactivity in emissions from coal burning is very low compared with the radioactivity occurring naturally w3x. 7. Pyrite Pyrite is probably the most environmentally interesting mineral, because of its propensity to oxidise during weathering producing sulfuric acid. The raspberry-like texture of framboidal pyrite has discrete, spheroidal aggregates of microcrystallites of about 1 mm in diameter that give ready access to air and water, thereby increasing the rate of oxidation markedly w24x. The initial oxidation yields ferrous iron Žand sulfuric acid. that is further oxidised to ferric iron that can react directly with pyrite to produce ferrous iron and sulfuric acid. The second-stage oxidation to ferric iron is a slow reaction unless it is catalysed by iron-oxidising bacteria, for example, Thiobacillus ferrooxidans. This important finding of the role of the bacterial catalyst is relevant to the oxidation of ferrous iron in natural acidic environments w25x. The bacterium acts best at low pH, less than about 4, so that the sulfuric acid produced favours further rapid oxidation of pyrite. The rate of oxidation of pyrite depends on several factors, namely, the concentration of oxygen, surface saturation with water, pH, temperature, microbiological factors, surface area of pyrite and the presence of other minerals w24x. The association of As with pyrite increases the rate of oxidation of pyrite w17x. The most important factors are air, water and iron-oxidising bacteria. The presence of other minerals, notably carbonate minerals, modifies the acidic conditions by reacting with the sulfuric acid to form sulfates. The presence of pyrite in coal mine and beneficiation wastes and in many metal mine wastes is a major environmental concern. The weathering of pyrite produces acid
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conditions that may leach trace elements and release trace elements associated with pyrite.
8. Waste disposal Major sources of waste needing disposal are mining activities Žcoal and metal., coal beneficiation, smelters, power stations, industrial outfalls, sewage outfalls, water treatment plants and dredge spoils. Trace elements from these sources may reach underground and surface waters where their fate is relevant to the health of plants, fish and humans. The total concentrations of trace elements in water are not a proper guide to their effects. Primarily, it is necessary to know the forms of the elements Žspeciation., possible interactions with other elements and with surfaces of fine particles Žclays, iron oxides, manganese oxides., synergistic effects and the nature of organic matter in water. Knowledge about the speciation of a trace element in water is an essential prerequisite to assessing its biological impact. In some cases, there will be more than one form present, for example, As may be as inorganic As Žoxidation states III and V., methylarsonic acid, dimethylarsinic acid and trimethylarsine oxide-type compounds at low concentrations down to about 0.02 ppb. Hence, the study of speciation is a very demanding task, perhaps one of the most demanding in current analytical chemistry. Sometimes, waste is disposed on land, for example, municipal wastewater and sewage sludge. The main concern about trace elements is the extent of their uptake by plants and by grazing animals. Uptake of Pb into the edible parts of plants is unlikely, but Cd is considered to be the prime element to be monitored. Sewage sludge is sometimes put on land as a fertiliser, but the rates should be carefully controlled to avoid build-up of trace elements and unwanted uptakes by plants. Various aspects of managing trace elements applied to land have been carefully considered and have led to the conclusion that ‘‘It is no light matter to decide to multiply the naturally occurring burdens of heavy metals in soils by a factor of 3 to 5. Yet most soils can carry such burdens without our incurring any more serious penalty than the need for occasionally liming the soil to near neutral point’’ w26x. Another waste, flyash from coal power stations, is potentially a useful addition to land and, in general, the ensuing trace element additions are unlikely to be harmful. In dealing with waste disposal, it is necessary to characterise the waste for its trace element contents and the waste site for its geological nature and hydrogeological features. Such information is helpful in managing the area, in order to lessen runoff of leachates and to plan revegetation.
9. Reclamation after mining Proper concern for the environment demands restitution of mineral areas to an aesthetically satisfying situation where vegetation can be established. Trace elements, many of which are essential to healthy plant growth, have an important role here. In some cases, the waste may contain useful amounts of trace elements; in other cases,
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there may be excessive amounts. Reclamation depends on the nature of the land disturbance, climatic conditions and economic aspects. These are clearly more important than trace-element matters, which however, should be part of the overall plan. An overview deals with matters relevant to surface coal mining, but much is also useful for general reclamation w27x. Attention should be given to the lithology of overburdens, to oxidised and reduced zones and to various chemical and physical properties of topsoils and substitute cover soils, including their contents of several trace elements. Reclamation should be monitored and revegetation should include native species. The most common trace elements that may be troublesome in areas after coal mining are As, B, Mo and Se w27x and probably also Cd, Mn and Zn. ‘‘In general, trace elements in overburden and the like are probably more likely to be useful to plants than detrimental’’ w3x. Agricola w23x apparently appreciated the relevance of reclamation after metal mining by stating ‘‘where woods and glades are cut down, they may be sown with grain . . . so that they repair the losses which the inhabitants suffer.’’
10. Fine particulates and health The relevance of fine particles ŽPM 10 , PM 2.5 . to health effects has been well reviewed w28x. It is clear that much work is needed ‘‘either to support or refute a causal relationship between low ambient concentrations of particulate matter and observed increased mortality on morbidity risks’’ w28x. These is good evidence for increased concentrations of several trace elements in fine particles compared with those in coarse particles w3x. However, it has not been shown satisfactorily that these trace elements in fine particulates have caused ill health.
11. Health aspects The interest in trace elements is because of their dual role in the health of plants, animals and humans, namely, their essentiality and their potential to be harmful. For essential trace elements, there is an optimal range of concentration for healthy growth of organisms, varying for different plants, animals and humans. Concentrations less than the lower end of this range would give deficiency, while those above the upper end of the range could lead to harmful effects. Excess of B and Mn can damage some plants, while excess of Mo and Se, while not affecting plant growth, can seriously harm animals. A dramatic incidence of a trace element deficiency occurs in some parts of China, where Se is so low in the human diet that there is widespread heart disease, known as Keshan disease, which may affect millions of people. The trend towards removing Pb from petrol and paints is restricting these potential sources of Pb toxicity. The need for constant vigilance is shown by a serious outbreak of ill health in Bangladesh caused by excess of As in drinking water from underground wells. It is important to keep health aspects in proper perspective, avoiding the temptation to stress toxicity of trace elements per se. Although generalisations must be treated only as
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guidelines, it seems most unlikely that trace elements from coal mining and usage should be harmful, provided that due care is taken. For metal mining and processing, each situation needs to be addressed to ensure that environmental and health aspects are not violated.
12. Future work It is clear that new developments in mining and usage, as well as further work on current situations, mean that trace elements must be taken into consideration. The prime reason is to ensure that harmful concentrations do not persist and that essential elements are sufficient. There are various tasks to be carried out, and some of these are outlined below. 1. The trace-element contents of new coals and ores need to be determined. Values for coals should be compared with those for most coals ŽTable 2. to ensure that those with higher values are examined closely before use.
Table 2 Ranges of values for most coals Žas ppm. — based on Ref. w3x Element
Range
As B Ba Be Cd Cl Co Cr Cu F Hg I Mn Mo Ni P Pb Ra Sb Se Sn Th Tl U V Zn
0.5–80 5–400 20–1000 0.1–15 0.1–3 50–2000 0.5–30 0.5–60 0.5–50 20–500 0.02–1 0.5–15 5–300 0.1–10 0.5–50 10–3000 2–80 ;1 in 10 6 0.05–10 0.2–10 1–10 0.5–10 - 0.2–1 0.5–10 2–100 5–300
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2. More information is needed on the modes of occurrence Žspeciation. of trace elements in coals. 3. Experiments should be carried out to ascertain the amounts of trace elements being deposited from the atmosphere in the environs of power stations. 4. The association of trace elements in flyash needs further elucidation. 5. The possible dispersal of trace elements from disposal areas for flyash and for metal-mining wastes should be studied. For clay and plastic liners, information is needed about their effective lifetimes. For clays, the capacity for adsorption of a range of elements should be measured. 6. Experiments are needed to find out the fate of trace elements during the use of coal by fluidised-bed methods, by integrated gas combined cycle and by gasifiers. 7. What is the fate of trace elements during the removal of SO x and NO x? 8. What happens to trace elements deposited from the atmosphere onto soils and plants? The roles of adsorption by clays and iron oxides in soils and possible translocation within plants should be elucidated. 9. The changes in trace element status of wastes after disposal on land needs investigation. 10. The influence of pyrite in metal-mining wastes on the mobilisation of trace elements should be studied. 11. Levels of radioactivity associated with U mining and with resulting wastes should be checked. 12. Can flyash be used as a fertiliser? It is difficult to mine for metals without some side-effects from trace elements, but there is technology to cope with most problems. The use of low-sulfur coals favours low trace element contents and hence less problems during usage. Environmental science needs facts not opinions and therefore more research. Indeed, good science should be a sine qua non for good policy. It is clear that research on various aspects of trace elements, especially environmental and health effects, should be intensified.
References w1x D.J. Swaine, Modern methods in bituminous coal analysis: trace elements, Crit. Rev. Anal. Chem. 15 Ž4. Ž1985. 315–346. w2x E.J. Mroz, W.H. Zoller, Composition of atmospheric particulate matter from the eruption of Heimaey, Iceland, Science 190 Ž1975. 461–464. w3x D.J. Swaine, Trace Elements in Coal, Butterworths, London, 1990, 292 pp. w4x P.A. Trudinger, D.J. Swaine ŽEds.., Biogeochemical Cycling of Mineral Forming Elements, Elsevier, Amsterdam, 1979, 612 pp. w5x Bowen, H.J.M., Environmental Chemistry of the Elements, Academic Press, London, 1979, 333 pp. w6x D.J. Swaine, The Trace-Element Content of Soils, Commonw. Bur. Soil Sci., Tech. Common., No. 48, 1955, 157 pp. w7x D.J. Swaine, The Trace Element Content of Fertilizers, Commonw. Bur. Soils, Tech. Commun., No. 52, 1962, 306 pp. w8x H.J. Gluskoter, N.F. Shimp, R.R. Ruch, Coal analyses, trace elements, and mineral matter, in: M.A. Elliott ŽEd.., Chemistry of Coal Utilisation, Wiley-Interscience, New York, 1981, pp. 369–424.
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w9x R.B. Finkelman, Modes of occurrence of environmentally sensitive trace elements in coal, in: D.J. Swaine, F. Goodarzi ŽEds.., Environmental Aspects of Trace Elements in Coal, Kluwer Academic Publishing, Netherlands, 1995, pp. 24–50. w10x S.A. Benson, S.K. Falcone, F.R. Karner, Elemental distribution and association with inorganic and organic components in two North Dakota lignites, Am. Chem. Soc. Div. Fuel Chem. Prepr. 29 Ž4. Ž1984. 36–47. w11x W.F. Fitzgerald, Is mercury increasing in the atmosphere? The need for an atmospheric mercury network, Water, Air and Soil Pollution 80 Ž1995. 245–254. w12x A. Carpi, S.E. Lindberg, Application of a Teflon dynamic flux chamber for quantifying soil mercury flux: tests and results over background soil, Atmos. Environ. 32 Ž95. Ž1998. 873–882. w13x S. Vermette, S. Lindberg, N. Bloom, Field tests for a regional mercury deposition network — sampling design and preliminary test results, Atmos. Environ. 29 Ž11. Ž1995. 1247–1251. w14x P.W. Davidson, G.J. Myers, C. Cox, C. Axtell, C. Shamlaye, J. Sloanereeves, E. Cernichiari, L. Needham, A. Choi, Y.N. Wang, M. Bertin, T.W. Clarkson, Effects of prenatal and postnatal methyl mercury exposure from fish consumption on neurondevelopment — outcomes at 66 months of age in the Seychelles child development study, J. Am. Med. Assoc. 280 Ž8. Ž1998. 701–707. w15x M.S. DeVito, L.W. Rosendale, V.B. Conrad, Comparison of trace element contents of raw and clean commercial coals, Fuel Process. Technol. 39 Ž1994. 87–106. w16x D.J. Akers, The redistribution of trace elements during the beneficiation of coal, in: D.J. Swaine, F. Goodarzi, ŽEds.., Environmental Aspects of Trace Elements in Coal, Kluwer Academic Publishing, Netherlands, 1995, pp. 93–110. w17x F.E. Huggins, G.P. Huffman, Mode of occurrence of trace elements in coal from XAFS spectroscopy, Int. J. Coal Geol. 32 Ž1996. 31–53. w18x D.J. Swaine, Trace elements during the mining and beneficiation of coal, Coal Preparation 19 Ž3–6. Ž1998. 177–193. w19x D.R. Jones, The leaching of major and trace elements from coal ash, in: D.J. Swaine, F. Goodarzi ŽEds.., Environmental Aspects of Trace Elements in Coal, Kluwer Academic Publishing, Netherlands, 1995, pp. 221–262. w20x D.J. Swaine, Trace elements in coal and their dispersal during combustion, Fuel Process. Technol. 39 Ž1994. 121–137. w21x W.C. Godbeer, D.J., Swaine, The deposition of trace elements in the environs of a power station, in: D.J. Swaine, F. Goodarzi ŽEds.., Environmental Aspects of Trace Elements in Coal, Kluwer Academic Publishing, Netherlands, 1995, pp. 178–203. w22x A.J. Stamm, P. Caplan, R.A. Hinrichs, Rain chemistry under the plume from an oil-fired power plant — a case study, Atmos. Environ. 18 Ž4. Ž1984. 817–823. w23x A. Georgius, De Re Metallica, Seen in translation by H.C. Hoover, L.H. Hoover ŽEds.., Dover Publications, New York, 1556, 638 pp. w24x D.J. Swaine, The rapid weathering of a siltstone, J. Proc. R. Soc. N. S. W. 119 Ž1986. 83–88. w25x P.C. Singer, W. Stumm, Acid mine drainage: the rate limiting step, Science 167 Ž1970. 1121–1124. w26x G.W. Leeper, Managing the Heavy Metals on the Land, Marcel Dekker, New York, 1978, 121 pp. w27x L.P. Gough, R.C. Severson, Mine-land reclamation: the fate of trace elements in arid and semi-arid areas, in: D.J. Swaine, F. Goodarzi ŽEds.., Environmental Aspects of Trace Elements in Coal, Kluwer Academic Publishing, Netherlands, 1995, pp. 275–307. w28x I.M. Smith, L.L. Sloss, PM 10 rPM 2.5 — emissions and effects, IEA Coal Research, CCCr08, 1998, 75 pp.
Why trace elements are important Dalway J. Swaine
)
CSIRO Energy Technology and Sydney UniÕersity, P.O. Box 136, North Ryde, NSW 1670, Australia Received 7 December 1998; accepted 4 October 1999
Abstract The widespread presence of trace elements is shown by reviewing their roles during the mining, beneficiation and combustion of coal and during metal mining and treatments. Special attention is given to Hg, to the environmentally important mineral, pyrite, to radioactivity, to waste disposal, to reclamation after mining and to health aspects. The 26 trace elements that are considered to be of environmental interest are listed. Important topics for future research are discussed. Although many trace element problems are under control, constant vigilance is a sine qua non and ongoing research is important. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Trace elements; Pyrite; Coal
1. Introduction Trace elements are important because of their association with environmental issues and the health of plants, animals and humans. Consideration must be given to essentiality, non-essentiality and toxicity that depend on concentrations, the form of the element Žspeciation., pH and oxidation–reduction conditions and other factors. In some cases, the difference in concentration between essentiality and unwanted effects, even toxicity, is small. The proper assessment of the environmental status of an element depends critically on proper sampling and analysis with careful attention to minimising contamination, to the use of appropriate reference materials and to the correct choice of analytical methods w1x. There are several sources of trace elements, both natural and anthropogenic. Natural sources include the weathering of rocks, volcanoes, sea spray, thermal springs, lake and river sediments, vegetation and forest fires. Inputs from anthropogenic sources are from )
Tel.: q61-2-94873130; fax: q61-2-9490-8909
0378-3820r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 9 9 . 0 0 0 7 3 - 9
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metal mining and smelting, combustion of coal, oil and wood, industrial operations, waste disposal, agricultural activities, motor tyre-and engine-wear and cremation. It is often difficult to assign the source or sources of trace elements to a particular location and sometimes too much emphasis is given to anthropogenic sources. Volcanic activity is a significant source of several trace elements, for example, Br, Se and Sb w2x. The main source of Zn may be a smelter, a power station, a volcano, fertiliser production, vegetation or motor tyre wear, depending on the nearness of the sampling location to the source. The concept of geochemical cycling, that is, the movement of elements in the various parts of the earth, including the atmosphere, is relevant to a proper understanding of the overall role of trace elements, especially their environmental significance w3x. It must be stressed that the system is dynamic not static and that biogeochemical cycling is now well recognised for the appraisal of trace elements w4x. Based on a personal assessment, 26 trace elements are considered to be of environmental interest ŽTable 1.. The environmental importance of the elements decreases from left to right, and the magnitude of any effects depends on particular situations, keeping in mind that many elements may be essential or hazardous. The notion of toxicity of trace elements per se is unreal and undesirably emotive. The proper status of a trace element has to be established for a particular milieu and set of circumstances. Hence, generalisations should not be made without caveats: for example, with regard to concentrations, the statement of Paracelsus in the 16th century should be heeded, namely, ‘‘All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy.’’ There are wide ranges of concentrations of trace elements in terrestrial materials, waters, air and living matter. Data for igneous rocks, shales, sandstones, limestones, seawater and living matter have been summarised by Bowen w5x. There are extensive compilations of the trace-element contents of soils w6x, of fertilisers w7x and of coals w3x. An overall assessment of trace elements and relevant matters has been given w8x, modes of occurrence of trace elements in coal have been reviewed w3,9x and there is information for inorganic elements in low-rank coals w10x. Table 1 Trace elements considered to be of environmental interest I
II
III
As Cd Cr Hg Pb Se
B Cl F Mn Mo Ni Be Cu P Th U V Zn
Ba Co I Ra Sb Sn Tl
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The importance of trace elements will be discussed for coal during mining, treatment and combustion, for petroleum and for metals during mining and treatment. Special attention will be given to water and air, wastes and the role of pyrite, perhaps the most environmentally important mineral. References will be made to radioactivity and to health aspects. 2. Relevant information on mercury In view of the special interest in Hg, some relevant information will be given. Historically, Hg was widely used in industry, agriculture and medicine, but current concerns about short-term poisoning and about possible long-term ill effects have curtailed its widespread use. At least half of the Hg emitted to the atmosphere comes from anthropogenic activities, including coal combustion, but natural sources, including volcanoes, bedrock, soil, water and vegetation, are also relevant w11x. The volatility of Hg means that recycling occurs readily, thereby enhancing its global atmospheric distribution. This is in keeping with atmospheric Hg being more than 90% in the elemental form w12x. Measurements of the deposition of Hg at a site in the USA, gave values of 43–358, with a mean of 186, ngrm2rweek w13x. Wet deposition predominated over dry deposition. The levels of Hg in fish are an ongoing concern and in some areas, people are advised to limit the consumption of local fish. A thorough study was carried out on several hundred mothers and their children from the Republic of Seychelles, where 85% of the population eat fish daily w14x. Children aged 66 months showed no adverse effects from either prenatal or postnatal exposure to methyl mercury. It was concluded that, ‘‘in the population studied, consumption of a diet high in ocean fish appears to pose no threat to development outcomes through 66 months of age’’ w14x. It must be emphasised that those results are for a particular situation and do not necessarily apply generally. 3. Trace elements in coal 3.1. During mining Trace elements are not expected to cause problems during underground mining, except for minor local effects where some soluble elements may be present in the mine water. However, after mining has ceased, there may be problems from the weathering of coal around the surface of the mine area, especially from the breakdown of pyrite that gives acidic leachates. Such run-off contaminates nearby waterways with an unsightly brown slurry and adds some unwanted trace elements. Nowadays, the abandonment of exhausted coal mines without some containment measures is not permitted in the USA and some other countries. The position with surface mining is different. Here, there may be changes in topography, alterations to the water table and to water quality, aeration of sub-surface material Žsediments and carbonaceous sediments. and changes in the bacterial population of surface material Žsoil.. In many coals, there is the possibility of the oxidation of pyrite ŽFeS 2 ., especially framboidal pyrite, with the consequent lowering of pH. Trace elements, for example, As and Se, could be released from the pyrite and the
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sulfuric acid, formed during the oxidation, may leach trace elements from the coal mineral matter and nearby inorganic material Žrocks.. Some amelioration of these acid conditions occurs if the nearby rocks contain limestone or other carbonates. Under some circumstances, the oxidation of pyrite may produce a slurry of hydrated ferric oxides, surely an undesirable addition to nearby surface waters. However, under these conditions, trace elements may be removed from solution by sorption on clays and iron oxides. Trace elements in the overburden and soil, which will be used in the reclamation of the open-cut after mining, must be taken into account when deciding the nature of the revegetation. Some care should be taken with B and Mn that may affect the growth of some plants under special conditions of pH and drainage. Available Se and Mo will not upset plant growth, but too much of these elements may produce plants that could upset grazing animals. However, unwanted effects for trace elements are rarely found in places previously mined for coal. Successful reclamation depends on a proper agricultural approach, including field trials. It is salutary to recall the statement by M.V. Chadwick that ‘‘disturbance of land is not a disaster it is an opportunity and a challenge.’’ 3.2. During storage Long-term storage of coals after mining and before treatment is not common and hence there should be little weathering. Coal is stockpiled before shipping and at power stations, but this is usually cleaned coal with low contents of pyrite. Weathering is also limited by the compact nature of the stockpile that lessens the access of water and air. 3.3. During beneficiation Various methods are used in the beneficiation of coal to produce cleaned coal. The main aim is to reduce the mineral matter, that is, to lower the ash yield with a consequent lowering of transportation and other costs and to improve combustion. There is also a bonus with the reduction in the levels of various trace elements, notably those associated with sulfide and other minerals. Recent studies in the USA w15,16x have shown that there are marked reductions in the concentrations of various trace elements after beneficiation. Concentrations for 11 trace elements listed in the 1990 Clean Air Act Amendments ŽCAAA. were determined in raw coal and in clean coal using commercial samples from several seams from eastern and mid-western USA and conventional methods of beneficiation w15x. The ash yields of 25–30% in the raw coals were reduced to 6–10% in the clean coals. Significant reductions were found for several trace elements, for example, As, Cr, Mn, Ni and Pb were reduced by 54–75%, Cd, Co and Be by 41–52% and lesser reductions were found for Hg, Sb and Se Ž14–34%.. As expected, there was some connection between the reductions in trace element concentrations and ash yields. Another study was carried out at commercial-scale cleaning plants using conventional methods with heavy-media cyclones and froth flotation with two modified flowsheets w16x. For a coal from the Pratt seam, Alabama, reductions of 39–89% were found for nine trace elements. Further tests were carried out using advanced coal cleaning techniques. Improvements over the conventional processes were found for As,
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Cd, Pb, Ni and Sb. Grinding to 150 and to 75 mm topsize did not give a general improvement in trace element removal. In some coals, even very low concentrations of trace elements may be reduced by beneficiation. For example, three bituminous coals from the Sydney Basin, New South Wales, Australia, had reductions in As from 1–4 ppm to 0.5–1.1 ppm w3x. These coals also had marked reductions in Cd, Hg and Sb, all the concentrations being sub-ppm. Information on the mineral matter in coal is helpful in estimating the likely affects of beneficiation on trace-element reductions. In particular, the mode of occurrence of trace elements is useful, including speciation ascertained by X-ray absorption fine structure spectroscopy w17x. The optimum particle size needs to be determined for particular coals prior to beneficiation. It should be stressed that the cases mentioned above w3,15,16x were for coals that were treated for reductions in ash yields and sulfur contents and not specially for trace-element reductions. Perhaps, attention to other factors would give further reductions in trace-element contents if desired. During beneficiation and the reduction in trace-element levels in the clean coal, there is an associated increase in trace-element contents in the reject material. The disposal of these washery rejects must be undertaken carefully because their increased contents of pyrite are a potential source of acidic solutions that could contaminate local waters with increased concentrations of trace elements. Perhaps this may not be an environmental hazard w16x, but care must be taken to ensure that there is no unwanted contamination from particular washeries. The fate of trace elements during beneficiation has been reviewed w18x. 3.4. During combustion The major use of coal is for the generation of power by combustion on a large scale. This means that most trace elements will be released and redistributed into bottom ash, flyash, fine flyash and the gaseous phase. Up to about 20% of the original mineral matter is found in the bottom ash and up to about 80% in the flyash. The bottom ash remains in the combustion area in the furnace, whereas the flyash is conveyed through the system where most Žmore than 99%. is removed by electrostatic precipitation or fabric filters. However, a small proportion, mostly fine particles of less than about 10 mm in diameter, is emitted to the atmosphere with the stack gases. Very few elements are in the vapour state, probably only variable proportions of the total amount of Hg and the halogens, but even these may be associated with the surfaces of very fine flyash particles. The environmental interest is in the trace elements in the bottom ash and flyash Žremoved by the particle attenuation devices. that are stored in ash disposal ponds, and in the stack emissions. The main concern with the ash disposal ponds is the possibility of the release of leachates that could transport trace elements into nearby underground and surface waters. However, efforts are usually made to prevent or restrict such losses by having the ponds in clay-rich soil and by using clay liners and plastic sheeting. There is a good account of the leaching of trace elements from coal ash, with information on the factors controlling the leaching and on specific elements, As, B, Cd, Cu, Mo, Ni, Se and Zn w19x. The dispersion of trace elements in the stack gases means that varying amounts of trace elements are deposited in the environs of power stations. Measurements of trace
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elements in deposition collected at various locations at different distances and aspects were carried out at Wallerawang power station over a 4-year period w3,20,21x. Wallerawang is about 120 km northwest from Sydney, New South Wales, Australia. The installed capacity is 1240 MW, flyash attenuation is by electrostatic precipitation and the bituminous feed coal is from the Lithgow seam, the ash yield being 21.4 " 4% and the total sulfur being 0.6 " 0.1% S. The method of collection of deposition from the atmosphere was flat fine-mesh envelopes containing 2 g of prepared Sphagnum moss. The envelopes were replaced at 3-monthly intervals and the moss was analysed for up to 39 trace elements using optical emission spectrography, atomic absorption spectrometry and instrumental neutron activation analysis. It was found, as expected, that there was a decrease in the amounts of trace elements deposited with distance from Wallerawang. Variations in amounts at different aspects depended on changes in wind strength and direction. An important finding was the variation in amounts of deposition with time of sampling, the ratio of maximum to minimum values being about 5:1. This stresses the importance of time of sampling at any location in order to obtain significant results. Using the marked difference in Ge contents of local soil and fine flyash emitted to the atmosphere Ža ratio of 1:50., the proportions of flyash and soil in samples of deposition were calculated. This showed for example, that at 1.8 km from the power station, the deposition contained 7–80% with a mean of 40% flyash, while at 5.3 km, there was - 1–5% with a mean of 2.5% flyash. The amounts of trace elements in the depositions were low compared with those in surface soils and mostly less than the amounts from weathering and litter decay w20x. Although this study has established that trace elements from the combustion of a specific coal under the conditions of operation at Wallerawang power station have not been detrimental to the environment, it must be emphasised that these results can at best be only a guide to other situations. Real answers for other places can only be obtained by carrying out similar experiments there. 4. Petroleum There has not been much interest in trace elements in petroleum, perhaps because concentrations are probably mostly low, especially for environmentally sensitive elements, for example, Cd, Hg and Pb. During diagenesis, V and Ni replace some Mg in porphyrin. Other trace elements are gained by the crude oils during migration or in the reservoir. The presence of porphyrin and other organic complexes leads to relatively high concentrations of V Žup to about 1000, with a mean of about 60 ppm V. and Ni Žup to about 150, with a mean of about 20 ppm. in crude oil. These mean values are of the same order as those for most coals w3x. Much lower concentrations of the other trace elements are found in most oils. These levels are not considered to be of environmental concern. The amounts of trace elements emitted during the combustion of oils would be unlikely to be troublesome. A study of rainwater samples at 2, 3 and 4 km from the stack of an oil-fired power station showed values of 0.46, 0.27 and 0.081 ppm V compared with the background 0.065 ppm V w22x. Lower values for Ni, Cu, Zn and Mn indicated that these emissions of trace elements would not be environmentally harmful. It is interesting to recall that deposits from the smokestacks of ships burning certain Venezuelan oils containing 12–15% V2 O5 , were a commercial source of V.
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5. Metal mining Although mining matters have been written about since the 4th century B.C., for example, by Aristotle and Theophrastus, the first major treatise was De Re Metallica written by Georgius Agricola in 1556 w23x. This is a thorough, yet scholarly account of the development of mining methods, metallurgical processes, geology, mineralogy and mining law, profusely illustrated by excellent woodcuts. Some adverse effects were noted, for example, ‘‘miners are sometimes killed by the pestilential air which they breathed; sometimes their lungs rot away.’’ Elsewhere he notes that ‘‘if the dust has corrosive qualities, it eats away the lungs . . . hence in the mines of the Carpathian Mountains women are found who have married several husbands, all of whom this terrible consumption has carried off to a premature death.’’ The Roman makers of vermilion Žmercury sulfide. took precautions against breathing its dust. Perhaps other trace elements ŽAs, Cd, Rn. caused health problems during early mining although they were not recognised as such. The sequence of events leading to the production of metals comprises exploration, mining, grinding, separation, extraction and refining. Many ores are sulfides notably those of Cu, Pb, Zn and Hg. There is a close association of Cd with ZnS. Other metals occur as oxides, carbonates, silicates and tellurides, while Ag and Au are found as free metals. The sulfide ores are prone to oxidation after mining and hence there is the possibility of acid formation. In many cases, pyrite is also present. The diversity of modes of occurrence of metals means that there is a variety of methods of mining and treatment and hence different ways by which there could be contamination. There is usually some disturbance of land during exploration, but there is unlikely to be any trace element effect. During underground mining, there may be dispersion of metal-rich dusts and in some cases, increased levels of radon, but good practice dictates proper precautionary measures. In open-cut mining, there is disturbance of land and the formation of fresh surfaces prone to oxidation especially if sulfide ores are being mined. This means that water may be contaminated by traces of the main metal and of minor metals associated with the ore. In placer mining, where large volumes of water are used, trace amounts of metals may be leached and removed in the used water. Good mining practice should cope with these trace-element aspects. In order to separate the valuable metal from the valueless gangue, the mined material is crushed and perhaps, finely ground before a separation process is used. This may be based on differences in specific gravity or wettability of particle surfaces. At this stage, there is the possibility of the disposal of used solutions that may contain some trace metals. Smelting Žthe heating of concentrates in furnaces. is used to prepare Cu, Pb and Zn and these large-scale operations produce emissions containing various trace elements. For example, As, Cd and Se are trace constituents in many ore concentrates fed to smelters. Smelter emissions are difficult to quantify, as they are affected by the composition of the feed, the recycling of dusts, slags and sinter and the collection efficiencies for fumes and dusts. Studies have shown the dispersion of metal-containing dusts around smelters, but good industrial practice is able to control these to ensure minimal contamination. In some places, extractions are carried out to concentrate the valuable metal, thereby producing solutions of unwanted trace metals. The handling of such wastes requires care
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to avoid contaminating local soils and waters. The reworking of wastes is potentially a source of trace metals, some being removed for gain and some being rejected. In general, the mining and treatments of metal ores and concentrates give products with enhanced levels of metals. Hence, any unwanted losses may give higher local levels of contamination than what could be regarded as traces. Modern industrial practices are geared to keeping these losses as low as possible and below relevant guidelines. However, there may be cases where material is spread by winds to give trace levels over large areas. For example, spillage during transport by train or road may not remain in situ, but may be dispersed widely. Prevention of spillages overcomes such incipient problems. 6. Radioactivity As well as the natural radioactive background, there are places where traces of Ra, U and Th give rise to low levels of radon, for example, in some caves. During mining for U, radon is released and mines are exposed to the resulting very low levels. Fatalities occurred amongst miners at Joachimsthal who inhaled radon and radioactive dust. Nowadays, efficient ventilation removes this hazard. The very low levels of radioactivity from coal are not considered to be harmful. It seems reasonable to assume that radioactivity in emissions from coal burning is very low compared with the radioactivity occurring naturally w3x. 7. Pyrite Pyrite is probably the most environmentally interesting mineral, because of its propensity to oxidise during weathering producing sulfuric acid. The raspberry-like texture of framboidal pyrite has discrete, spheroidal aggregates of microcrystallites of about 1 mm in diameter that give ready access to air and water, thereby increasing the rate of oxidation markedly w24x. The initial oxidation yields ferrous iron Žand sulfuric acid. that is further oxidised to ferric iron that can react directly with pyrite to produce ferrous iron and sulfuric acid. The second-stage oxidation to ferric iron is a slow reaction unless it is catalysed by iron-oxidising bacteria, for example, Thiobacillus ferrooxidans. This important finding of the role of the bacterial catalyst is relevant to the oxidation of ferrous iron in natural acidic environments w25x. The bacterium acts best at low pH, less than about 4, so that the sulfuric acid produced favours further rapid oxidation of pyrite. The rate of oxidation of pyrite depends on several factors, namely, the concentration of oxygen, surface saturation with water, pH, temperature, microbiological factors, surface area of pyrite and the presence of other minerals w24x. The association of As with pyrite increases the rate of oxidation of pyrite w17x. The most important factors are air, water and iron-oxidising bacteria. The presence of other minerals, notably carbonate minerals, modifies the acidic conditions by reacting with the sulfuric acid to form sulfates. The presence of pyrite in coal mine and beneficiation wastes and in many metal mine wastes is a major environmental concern. The weathering of pyrite produces acid
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conditions that may leach trace elements and release trace elements associated with pyrite.
8. Waste disposal Major sources of waste needing disposal are mining activities Žcoal and metal., coal beneficiation, smelters, power stations, industrial outfalls, sewage outfalls, water treatment plants and dredge spoils. Trace elements from these sources may reach underground and surface waters where their fate is relevant to the health of plants, fish and humans. The total concentrations of trace elements in water are not a proper guide to their effects. Primarily, it is necessary to know the forms of the elements Žspeciation., possible interactions with other elements and with surfaces of fine particles Žclays, iron oxides, manganese oxides., synergistic effects and the nature of organic matter in water. Knowledge about the speciation of a trace element in water is an essential prerequisite to assessing its biological impact. In some cases, there will be more than one form present, for example, As may be as inorganic As Žoxidation states III and V., methylarsonic acid, dimethylarsinic acid and trimethylarsine oxide-type compounds at low concentrations down to about 0.02 ppb. Hence, the study of speciation is a very demanding task, perhaps one of the most demanding in current analytical chemistry. Sometimes, waste is disposed on land, for example, municipal wastewater and sewage sludge. The main concern about trace elements is the extent of their uptake by plants and by grazing animals. Uptake of Pb into the edible parts of plants is unlikely, but Cd is considered to be the prime element to be monitored. Sewage sludge is sometimes put on land as a fertiliser, but the rates should be carefully controlled to avoid build-up of trace elements and unwanted uptakes by plants. Various aspects of managing trace elements applied to land have been carefully considered and have led to the conclusion that ‘‘It is no light matter to decide to multiply the naturally occurring burdens of heavy metals in soils by a factor of 3 to 5. Yet most soils can carry such burdens without our incurring any more serious penalty than the need for occasionally liming the soil to near neutral point’’ w26x. Another waste, flyash from coal power stations, is potentially a useful addition to land and, in general, the ensuing trace element additions are unlikely to be harmful. In dealing with waste disposal, it is necessary to characterise the waste for its trace element contents and the waste site for its geological nature and hydrogeological features. Such information is helpful in managing the area, in order to lessen runoff of leachates and to plan revegetation.
9. Reclamation after mining Proper concern for the environment demands restitution of mineral areas to an aesthetically satisfying situation where vegetation can be established. Trace elements, many of which are essential to healthy plant growth, have an important role here. In some cases, the waste may contain useful amounts of trace elements; in other cases,
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there may be excessive amounts. Reclamation depends on the nature of the land disturbance, climatic conditions and economic aspects. These are clearly more important than trace-element matters, which however, should be part of the overall plan. An overview deals with matters relevant to surface coal mining, but much is also useful for general reclamation w27x. Attention should be given to the lithology of overburdens, to oxidised and reduced zones and to various chemical and physical properties of topsoils and substitute cover soils, including their contents of several trace elements. Reclamation should be monitored and revegetation should include native species. The most common trace elements that may be troublesome in areas after coal mining are As, B, Mo and Se w27x and probably also Cd, Mn and Zn. ‘‘In general, trace elements in overburden and the like are probably more likely to be useful to plants than detrimental’’ w3x. Agricola w23x apparently appreciated the relevance of reclamation after metal mining by stating ‘‘where woods and glades are cut down, they may be sown with grain . . . so that they repair the losses which the inhabitants suffer.’’
10. Fine particulates and health The relevance of fine particles ŽPM 10 , PM 2.5 . to health effects has been well reviewed w28x. It is clear that much work is needed ‘‘either to support or refute a causal relationship between low ambient concentrations of particulate matter and observed increased mortality on morbidity risks’’ w28x. These is good evidence for increased concentrations of several trace elements in fine particles compared with those in coarse particles w3x. However, it has not been shown satisfactorily that these trace elements in fine particulates have caused ill health.
11. Health aspects The interest in trace elements is because of their dual role in the health of plants, animals and humans, namely, their essentiality and their potential to be harmful. For essential trace elements, there is an optimal range of concentration for healthy growth of organisms, varying for different plants, animals and humans. Concentrations less than the lower end of this range would give deficiency, while those above the upper end of the range could lead to harmful effects. Excess of B and Mn can damage some plants, while excess of Mo and Se, while not affecting plant growth, can seriously harm animals. A dramatic incidence of a trace element deficiency occurs in some parts of China, where Se is so low in the human diet that there is widespread heart disease, known as Keshan disease, which may affect millions of people. The trend towards removing Pb from petrol and paints is restricting these potential sources of Pb toxicity. The need for constant vigilance is shown by a serious outbreak of ill health in Bangladesh caused by excess of As in drinking water from underground wells. It is important to keep health aspects in proper perspective, avoiding the temptation to stress toxicity of trace elements per se. Although generalisations must be treated only as
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guidelines, it seems most unlikely that trace elements from coal mining and usage should be harmful, provided that due care is taken. For metal mining and processing, each situation needs to be addressed to ensure that environmental and health aspects are not violated.
12. Future work It is clear that new developments in mining and usage, as well as further work on current situations, mean that trace elements must be taken into consideration. The prime reason is to ensure that harmful concentrations do not persist and that essential elements are sufficient. There are various tasks to be carried out, and some of these are outlined below. 1. The trace-element contents of new coals and ores need to be determined. Values for coals should be compared with those for most coals ŽTable 2. to ensure that those with higher values are examined closely before use.
Table 2 Ranges of values for most coals Žas ppm. — based on Ref. w3x Element
Range
As B Ba Be Cd Cl Co Cr Cu F Hg I Mn Mo Ni P Pb Ra Sb Se Sn Th Tl U V Zn
0.5–80 5–400 20–1000 0.1–15 0.1–3 50–2000 0.5–30 0.5–60 0.5–50 20–500 0.02–1 0.5–15 5–300 0.1–10 0.5–50 10–3000 2–80 ;1 in 10 6 0.05–10 0.2–10 1–10 0.5–10 - 0.2–1 0.5–10 2–100 5–300
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D.J. Swainer Fuel Processing Technology 65–66 (2000) 21–33
2. More information is needed on the modes of occurrence Žspeciation. of trace elements in coals. 3. Experiments should be carried out to ascertain the amounts of trace elements being deposited from the atmosphere in the environs of power stations. 4. The association of trace elements in flyash needs further elucidation. 5. The possible dispersal of trace elements from disposal areas for flyash and for metal-mining wastes should be studied. For clay and plastic liners, information is needed about their effective lifetimes. For clays, the capacity for adsorption of a range of elements should be measured. 6. Experiments are needed to find out the fate of trace elements during the use of coal by fluidised-bed methods, by integrated gas combined cycle and by gasifiers. 7. What is the fate of trace elements during the removal of SO x and NO x? 8. What happens to trace elements deposited from the atmosphere onto soils and plants? The roles of adsorption by clays and iron oxides in soils and possible translocation within plants should be elucidated. 9. The changes in trace element status of wastes after disposal on land needs investigation. 10. The influence of pyrite in metal-mining wastes on the mobilisation of trace elements should be studied. 11. Levels of radioactivity associated with U mining and with resulting wastes should be checked. 12. Can flyash be used as a fertiliser? It is difficult to mine for metals without some side-effects from trace elements, but there is technology to cope with most problems. The use of low-sulfur coals favours low trace element contents and hence less problems during usage. Environmental science needs facts not opinions and therefore more research. Indeed, good science should be a sine qua non for good policy. It is clear that research on various aspects of trace elements, especially environmental and health effects, should be intensified.
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