The occurrence of pesticides and their residues in char produced by the combustion of wood pellets in domestic boilers

•Char produced by combustion of bark pellets in automatic boilers contains pesticides.•Char from pellets contains 11 kinds of herbicides, insecticides and fungicides.•The enrichment factor for insecticide carbaryl for char from pellets reaches 2.44.•Decomposition of pesticides adsorbed on elemental carbon requires temperature 750 °C.•Pesticides bound in char have molecular weight in the range from 201 to 721. Pesticides were observed in the analysis of char from the combustion of bark pellets in modern boilers with automatic loading. Altogether, eleven pesticides were identified: fungicides (fuberidazole, bitertanol, epoxiconazole, chlozolinate + 3 metabolites), herbicides (bifenox + 1 metabolite), and insecticides (carbaryl, carbophenothion, methyl carbophenothion, bromophos, azadirachtin, deltamethrin). Metabolites or fragments of pesticides include chemical compounds that contain benzimidazole, isoxazole, and phthalimide. Analyses of char samples determined that some pesticides do not decompose or only decompose partially at high temperatures. The enrichment factor for pesticides contained in char from bark pellets varied from 0.07 to 2.44 (carbaryl) and from 0.6 to 0.67 (carbaryl) for the char from bark briquettes. During the combustion process, some pesticides (such as carbaryl) were adsorbed onto elemental carbon, which made their thermal decomposition possible only at temperatures higher than 750 °C.