Although coal combustion residues present an increasing waste disposal problem in Australia, there is little documentation of either the characteristics of fly ash from Australian power stations or the bioavailabillty of elements in the ash. This study investigated the agronomic properties of these ashes and focussed on their B content and release characteristics.
The first part of this study involved the characterisation of the agronomic properties of fly ashes from 13 Australian power stations. All were dominated by both amorphous and crystalline aluminosilicates and quartz, and these were associated with minor amounts of Fe oxides, lesser amounts of Ca, Mg, Na, K, Ti and P oxides, and variable levels of incompletely combusted C. The ashes consisted of particles predominantly in the silt (0.02 - 0.002 mm) plus fine sand (0.2 - 0.02 mm) fraction (67 - 98%), and electron microscopy revealed that ash matrices consisted of glassy spherical particles and less regularly shaped spongy particles. The available water capacity was high and varied from 27 to 105%, with 11 of the 13 samples having values >40%.
Twelve of the 13 ashes were alkaline to strongly alkaline (pH range 8.0 - 12.8). Soluble salt levels were related to the coal source, and electrical conductivity of the saturation extracts (ECse) varied from 0.63 to 7.0 mS/cm for 11 of the samples; two ashes produced from brown coal had ECse values of 46.0 and 55.0 mS/cm.
The ability of ashes to supply nutrients was assessed on samples leached with water to reduce the soluble salts to equilibrium levels. The ashes contained negligible amounts of N, but their P status was variable. The amounts of NH40Ac-extractabIe Ca and Mg were generally high in most samples and were considered more than adequate for most plant species; however, NH40Ac-extractable K values were generally low. Adequate levels of sulfate-S existed in 12 of the 13 samples, and levels of DTPA-extractable Cu, Zn, Mn and Fe were high when compared with published critical values. Most of the unleached samples contained potentially toxic levels of B.
Following identification of B as one of the elements in fly ash likely to have a high bioavailability, laboratory studies were undertaken to investigate the characteristics of B dissolution from selected fly ashes, and glasshouse trials were conducted to examine the potential for B toxicity to plants grown in ashamended media. Short term (≤ 32 h equilibration) dissolution studies at various ash:solution ratios did not provide any evidence that sparingly soluble compounds were controlling the solubility of B in the ashes studied. However, long term (up to 112 days) moist storage of ashes, after an initial 'leaching' phase to decrease soluble B, demonstrated the capacity of all except one ash to regenerate high B levels. The amount of soluble B regenerated increased with increasing time of moist storage. It is hypothesised that diffusion of B from and through the solid phase of the fly ash may explain this regeneration phenomenon. Decreasing the pH of aqueous suspensions of fly ash resulted in marked increases in the dissolution of B, and there was circumstantial evidence that the B dissolved was associated with Ca. Although the ashes (washed to remove readily soluble B) did not exhibit any significant capacity to adsorb added B, particle size/water-extractable B relationships and surface analysis (secondary ion mass spectroscopy) studies on freshly precipitated ashes provided clear evidence for a surface predominance of B.
French bean (Phaseolus vulgaris cv. Redland Pioneer) and Rhodes grass (Chloris gayana cv. Pioneer) were grown in a series of glasshouse experiments to investigate the potential for B phytotoxicity in selected ashes and to relate plant B uptake to various chemical availability indicies. The ashes used were either freshly precipitated, leached or adjusted to pH 6.5 and subsequently leached. In the first experiment, the yield and B status of plants grown on five fly ashes mixed (5 and 10% by weight) with an acid-washed sand were measured and, with the exception of one ash, yield differences among ash sources and among ash treatments were attributed to differences in the degree of B toxicity. In a second experiment, a fly ash with properties representative of most Australian ashes was mixed (0, 15, 30, 70 and 100% by weight) with a sandy loam, and the yield and mineral composition of plants grown on these mixtures determined. Although the available water capacity of the soil was substantially increased by fly ash addition, incorporating large proportions of untreated fly ash resulted in poor plant growth primarily due to B toxicity. In both experiments, leaching the ash reduced the potential for B toxicity, whereas adjustment of the pH to 6.5 and subsequent leaching of the fly ash resulted in plants with normal levels of B.
There were marked differences in both the tissue levels of B and the extent of B toxicity symptoms between the two plant species grown on the mixtures of ash and sand or ash and sandy loam soil. Rhodes grass appeared to be able to tolerate higher B contents in the growing medium by taking up much less of the element than French bean. The results indicate that phytotoxicity of B could be a major problem In establishing vegetation on ash dams and in the agronomic utilization of unweathered fly ashes in Australia.
In a third experiment, eight fly ashes were either untreated or separately batch leached to various extents to give a range of extractable B levels. The freshly precipitated ashes and leached ashes were separately mixed (10% by weight) with acid-washed sand and French bean grown on the various mixes. When growth and B concentration in plant tops were plotted against various measures of extractable B (e.g., hot water, 1 M NH4OAc, 0.05M mannitol), the relationships were not as good as those obtained when plant parameters were plotted against solution B concentration (B concentration in a 10 kPa matric suction moisture content extract of the growth medium). Above about 3 ma B/L in the solution extract, the growth of French bean was reduced.