Purpose: Wood ash, characterized by high content of certain nutrients and charcoal, can be applied to soils as a means of managing this waste product improving the soil quality. The associated environmental risk must be assessed. The objective of this study was to characterize the bottom and fly ash collected from 15 biomass power plants in Spain by determining the benzene, toluene, ethylbenzene, xylene and styrene (BTEX+S), PAHs and aliphatic hydrocarbon contents of both types of ash. Biochar was also used for comparison purposes.
Methods: Gas chromatography-mass spectrometric methods were used for the identification and determination of both BTEX+S and aliphatic hydrocarbon contents in bottom and fly ashes, as well as biochar. High performance liquid chromatography with fluorescence detection was used for PAHs measurements. Multivariate correlation analysis was used to determine the relationship between sample characteristics and pollutants identified by partial least squares regression analysis.
Results and discussion: In general, the degree to which organic matter in the sample is burned increases with T50 or the "50% burn off" temperature (possibly due to the addition of fuel), and the BTEX+S also tended to increase. However, as the Q/MO (the heat of combustion divided by organic matter mass) increased, the combustion decreased or proceeded with less oxygen, which appears to be related to an increased presence of PAHs. The results confirm that the amounts of organic pollutants (PAHs and BTEX+S, together with total aliphatic hydrocarbons) in the wood ash do not exceed limits established for different soil or industrial uses.
Conclusions: Both types of ash, together with biochar, may therefore be suitable for application to soil as a fertilizer and an organic amendment, taking into account the target organic pollutants.
Keywords: BTEX; PAHs; Soil impact; Styrene; Total aliphatic hydrocarbons; Wood ash and biochar.
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