The use of coal ash from power plants as a soil conditioner
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Date
2015
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Publisher
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 fertilizer 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 characterized; 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 agronomical use of ash, specifically
targeting problematic soils, are highly recommended for future studies.
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Keywords
Coal ash, Soil conditioner