Wood ash is biomass obtained by the combustion of wood products (wood chips, logs, etc.). Burning wood products produces about 6-10% ash from the total weight of wood. Wood ash contains a high amount of macro- and micro-elements. However, several studies show that wood ash also contains rare earth elements. Wood ash is produced in large quantities through the combustion of wood for energy and heat production. For example, factories (for energy production) and boiler houses (for heat production) use wood products as fuel.
Nowadays, several applications have been found for the use of wood ash. In several European Union countries, wood ash is used in road construction and asphalting works. However, the most common application of wood ash is as a fertilizer for agricultural and forest soil. The high content of macro- and micro- elements in the wood ash ensures plants with the necessary nutrients and reduces soil acidity. The use of wood ash can create biocycle, increase the amount of renewable energy, reduce the number of landfill sites and CO2 emissions.
Wood ash dived into two main types – bottom ash and fly ash. Bottom ash is a coarsest and granulated ash fraction with particle size 0.4-4.7 mm that contains impurities (stones, sands, etc.) and most of the wood ash mass consists of silicon (~57%) and calcium (~19%). Fly ash is the finest fraction with particle size 12-250 μm which contains inorganic compounds and a small amount of unburned carbon.
The aim of this research was to evaluate rare earth element content in the bottom and fly wood ash. Both types of wood ash samples were collected from boiler houses from different parts of Latvia in 2014. Wood ash samples were extracted in 5 mL of concentrated HNO3 by using a shaker (5 hours, 190 rpm). The resulting solution was diluted with deionized water and filtered (Whatman 541) to get rid of undissolved particles. The rare earth element content was determined by ICP - MS (Agilent 8900 ICP-QQQ) analysis method. The pH and electrical conductivity were also determined in the extract in the water.
Obtained results show that wood ash is a highly basic material (pH > 12). The electrical conductivity of bottom ash is higher than fly ash. The content of rare earth elements shows the diversity of the contents between the bottom and fly wood ash.