Guest Editorial: Environmental aspects of trace elements in coal
It is clear that coal will be used increasingly as a major source of power for at least the next decade, and hence more attention will be paid to environmental and health aspects. Much effort is expended on the effects of sulphur and nitrogen oxides (acid rain) and on carbon dioxide (greenhouse effect), but trace elements in coal should also be considered during mining, usage and reclamation after mining. In popular literature it is common to stress the toxicity of trace elements, but this should be avoided because it is usually irrelevant for trace elements in most coals. Scientists know that most common trace elements may be biologically essential or hazardous. "Essential elements m a y show d e f i c i e n c y or t o x i c i t y d e p e n d i n g on concentration and availability, the form of the element being important, that is, whether it is present as an ion or as a simple or complex inorganic or organic compound" (Swaine, 1990). This dual role was known to Paracelsus in the sixteenth century when he stated that "... all substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy". During mining, changes in oxidation potential and pH may lead to an increase in solubility of some elements which may cause increases in concentration of these elements in underground (mine) waters, and possibly surface waters. An important factor is the oxidation of pyrite (FeS2) to give very acidic solutions which may leach trace elements from the associated coals and sediments. However, it cannot be assumed that these elements will travel far and be hazardous, because there may be attenuation by clays and by carbonate minerals. Many effects during surface mining must be taken into account for the rehabilitation of the area after mining. For example, boron and manganese can affect plant growth, while
selenium and molybdenum may upset grazing animals. Similar conditions should be applied to residues from beneficiation and other wastes. The widespread use of coal power production means that trace elements are redistributed into bottom ash and flyash which are usually removed to ash-disposal ponds where leaching may release some elements to nearby areas. In modern power stations, more than 99% of flyash is removed by electrostatic precipitators or fabric filters, the remaining proportion, mostly fine particles, being emitted into the atmosphere with the stack gases. Ultimately this atmospheric material is deposited by dry and wet deposition, mostly within about 20 km of the source. The few studies made around power stations have shown that the amounts of trace elements deposited are very small compared with the amounts already present in most soils. In general, the assessment of trace elements in coal in relation to environmental and health effects is not simple. Each coal needs to be considered in relation to a particular situation, thereby ensuring that untoward effects, which could have been prevented, are not found out too late. Much information on trace elements in coal is available and should be used in assessing new coals for possible environmental effects. "It seems that trace elements cannot be regarded as a hazard during coal mining and usage, assuming that due care is taken to ensure proper operational conditions" (Swaine, 1990). D.J. Swaine
Reference Swaine, D.J. 1990. Trace elements in Coal, 296 pp. Butterworths, London.
It is clear that coal will be used increasingly as a major source of power for at least the next decade, and hence more attention will be paid to environmental and health aspects. Much effort is expended on the effects of sulphur and nitrogen oxides (acid rain) and on carbon dioxide (greenhouse effect), but trace elements in coal should also be considered during mining, usage and reclamation after mining. In popular literature it is common to stress the toxicity of trace elements, but this should be avoided because it is usually irrelevant for trace elements in most coals. Scientists know that most common trace elements may be biologically essential or hazardous. "Essential elements m a y show d e f i c i e n c y or t o x i c i t y d e p e n d i n g on concentration and availability, the form of the element being important, that is, whether it is present as an ion or as a simple or complex inorganic or organic compound" (Swaine, 1990). This dual role was known to Paracelsus in the sixteenth century when he stated that "... all substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy". During mining, changes in oxidation potential and pH may lead to an increase in solubility of some elements which may cause increases in concentration of these elements in underground (mine) waters, and possibly surface waters. An important factor is the oxidation of pyrite (FeS2) to give very acidic solutions which may leach trace elements from the associated coals and sediments. However, it cannot be assumed that these elements will travel far and be hazardous, because there may be attenuation by clays and by carbonate minerals. Many effects during surface mining must be taken into account for the rehabilitation of the area after mining. For example, boron and manganese can affect plant growth, while
selenium and molybdenum may upset grazing animals. Similar conditions should be applied to residues from beneficiation and other wastes. The widespread use of coal power production means that trace elements are redistributed into bottom ash and flyash which are usually removed to ash-disposal ponds where leaching may release some elements to nearby areas. In modern power stations, more than 99% of flyash is removed by electrostatic precipitators or fabric filters, the remaining proportion, mostly fine particles, being emitted into the atmosphere with the stack gases. Ultimately this atmospheric material is deposited by dry and wet deposition, mostly within about 20 km of the source. The few studies made around power stations have shown that the amounts of trace elements deposited are very small compared with the amounts already present in most soils. In general, the assessment of trace elements in coal in relation to environmental and health effects is not simple. Each coal needs to be considered in relation to a particular situation, thereby ensuring that untoward effects, which could have been prevented, are not found out too late. Much information on trace elements in coal is available and should be used in assessing new coals for possible environmental effects. "It seems that trace elements cannot be regarded as a hazard during coal mining and usage, assuming that due care is taken to ensure proper operational conditions" (Swaine, 1990). D.J. Swaine
Reference Swaine, D.J. 1990. Trace elements in Coal, 296 pp. Butterworths, London.
