The content of macroelements, heavy metals and their fractions in ash after biomass combustion

The aim of the study was to assess the content of macroelements and heavy metals and their fractions in ash after biomass combustion in a combined heat and power plant. Ash samples from biomass combustion in the combined heat and power plant in Bialystok were collected quarterly in 2021. Single-stage extraction with aqua regia was performed to determine the total content of the following elements in the samples: P, K, Na, Ca, Mg, S, Zn, Cu, Cr, Ni, Pb, Cd. The analysis was made using the ICP-AES method. Metal fractions were extracted using extractants according to the Tessier method. The content of fractions was determined using the ICP-AES method. Then, the share of individual metal fractions in their total content was calculated. The total content of the determined elements varied depending on the sampling date, which resulted from the type of biomass sent for combustion. The average amount of macroelements in the tested ash in the period of the study was: phosphorus 1.3 %, potassium 3.4 %, sodium 0.13 %, calcium 12.1 %, magnesium 1.6 %, and sulfur 0.4 %. The of average amount of metals were: zinc 118 mg/kg, copper 34 mg/kg, chromium 52 mg/kg, nickel 53 mg/kg, lead 55 mg/kg, and cadmium 6 mg/kg. The distribution of metals in individual fractions varied to some extent depending on the sampling date and was characteristic for each element. Zinc, chromium, nickel, and lead were most abundant in fraction IV, i.e. they were associated with manganese, iron, and aluminum oxides. Copper was present mainly in the residual fraction, and cadmium in the soluble fraction, in the fraction associated with iron, manganese and aluminum oxides and in the residual fraction. The content of trace elements did not exceed the legal limits for fertilizers and waste used for fertilization. Due to the distribution of the metals determined in the individual fractions, the use of ash for fertilization is safe. The tested ash is a source of macroelements for plants, and due to the significant content of calcium and magnesium, it can affect the regulation of pH in acidified soils.