Sanga, Hilda Gerald
(2019)
The use of coal ash from power plants as a soil conditioner.
PhD thesis, University of Nottingham.
Abstract
The disposal of coal ash, produced in large quantities by power plants as a by-product of coal combustion, is a significant environmental concern. Coal ash can be used as an agricultural soil conditioner because of its liming potential and the presence of many essential plant nutrients. However, recommendations for the agricultural use of coal ash should be based on sound knowledge of the coal ash characteristics, particularly the concentrations of potentially toxic elements (PTEs) in the ash. Due to the uptake of PTEs by crop plants it may pose risks to human health following the consumption of food crops.
The aim of this study was to evaluate the potential for the safe application of power station derived coal ash to soil as a beneficial disposal route. The specific objectives included; (i) testing the variability of fly ash obtained from different sources in the UK, Czech Republic and Tanzania, (ii) quantifying the short- and long-term changes in soil characteristics induced by applications of ash, (iii) determining the effects of coal ash on soil enzyme activities, (iv) quantifying the utility of coal ash as a fertiliser by evaluating its effect on growth and yield of wheat and (v) assessing risks of long-term use/multiple applications of coal ash to arable soils.
Coal ash from the Czech Republic, the UK and Tanzania was characterised; the latter two were used in pot experiments to determine their effects on soil enzyme activities, wheat growth and PTE uptake when added to two contrasting soil types (woodland and arable sandy loams). Two incubation experiments were undertaken to quantify short- and long-term effects of the coal ashes on soil characteristics. Calculations were also performed to evaluate the probable risks of increased contamination of soil and plant material as well as human ingestion of PTEs following repeated applications of fly ash to arable soils.
Coal ashes from each source contain varying quantities of essential nutrients and PTEs due to differences in coal ranks and the combustion conditions of the power plants producing each ash. Different batches of ash from the UK and from Tanzania had different characteristics, despite coming from the same industrial source within the respective countries. Application of the first batch of ash collected in the UK (UK1) to woodland soil increased the soil pH, soil respiration and nutritional status during a two-year incubation experiment. Soil amendment with high UK1 ash concentrations (8-16%) contaminated the soil with PTEs through the experiment. In a four-month incubation experiment, the effects of different coal ashes applied to acidic woodland soil varied depending on the characteristics of each individual ash and the amount of ash applied.
In a pot experiment designed to evaluate the effect of coal ash on microbial activities, soil amendment with the UK1 ash increased the pH of woodland and arable soils, while application of the TZ1 ash reduced the pH of both soils. Application of low concentrations (0-4%) of UK1 ash to both soils increased dehydrogenase and urease activities and wheat growth while application of TZ1 ash at high concentrations (8-16%) inhibited the enzyme activities. In pot experiments to evaluate the effects of ash on wheat growth, application of 0-32% of the UK1 ash to woodland and arable soils increased soil pH while application of the TZ1 ash at 0-32% decreased the pH of both soils. Soil amendment with 0-4% of either UK1 or TZ1 ash increased the concentrations and extractability of nutrients and wheat growth and yield, but application of 16-32% of both ashes to both soils contaminated the soils and wheat plants with PTEs. Despite PTE uptake by plants, grain PTE concentrations were within the FAO/WHO ‘safe’ limits for ingestion, except for As and Cd in grains from plants grown in woodland soil amended with the highest concentrations of UK1 and TZ1 ash respectively, which were both present in higher than acceptable concentrations.
Soil and plant concentrations and human consumption of selected PTEs (As, Cd, Cr, Pb and Zn) were calculated following simulated annual applications of TZ1 ash to an arable soil for five consecutive years. This showed that, even when residual contamination over a 25-year period was considered, applications of 2% ash to the soil are unlikely to breach ‘permissible’ standards for soil, wheat grain contamination and human dietary intake of PTEs, which were far below ‘permissible’ limits. It would be possible to apply ash with similar characteristics to TZ1 more frequently or over more than five cropping cycles.
In conclusion, coal ash can be used as an agricultural soil conditioner; however, low concentrations (0-4%) and the strategic agronomic use of ash, specifically targeting problematic soils, are highly recommended for future studies.
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