Screening of biofuel ash chemical composition and potential environmental risk caused by heavy metals: a six-year...

The growing popularity of biofuels has resulted in the generation of large amounts of biofuel ash, which is rich in nutrients. Biofuel ash is an excellent source of raw material in a circular economy. However, often biofuel ash is contaminated with heavy metals. Therefore, it is important to assess and monitor the chemical composition of biofuel ash to best utilize the potential of it. Study represents six years of data on biofuel ash collected from various boilers across Lithuania. It presents the chemical composition of biofuel ash, an environmental risk assessment, and predictive modelling was performed to estimate the doubling time of background concentrations of heavy metals in soil. The study results showed that concentrations of inorganic pollutants in biofuel ash vary significantly from year to year. When assessing the potential use for biofuel ash in agriculture and forestry, it was found that about 30 % of biofuel ash samples had Cd concentrations exceeding the maximum allowable limits for use in agriculture, more than 50 % - forestry. The Cr concentration exceeded in more than 50 % of tested samples for use in agriculture, about in 20 % - forestry. The estimated cumulative potential ecological risk to the environment is very high, which is determined by the high (Cd) or medium (Hg, Cu) risk of individual metals. Also, after evaluating the 6-year results, it was determined that the use of biofuel ash is limited by Cd, Zn since the background level of these metals in the soil would double most likely after 17 and 27 years.