Sorption and genotoxicity of sediment-associated pentachlorophenol and pyrene influenced by crop residue ash

Purpose The bioavailability and potential toxicity of sediment-associated organic compounds are affected by sorption processes. The particulate matter (ash) from field burning of crop residues is one of the primary sources for environmental black carbon, which is regarded as supersorbent for organic contaminants and may reduce their ecotoxicity. This study aimed to investigate the influence of ash on sediment-associated organic pollutants' potential toxicity. Materials and methods Pentachlorophenol (PCP) and pyrene (PYE) were chosen as the target organic pollutants, earthworms (Eisenia fetida) were used to evaluate the genotoxicity of contaminated sediments in the exposure experiments, and the genotoxicity (expressed as DNA damage) was quantified by comet assay. Sediments amended with different levels of ash (1%, 3%, 5%, and 10%) were selected as the sorbent to investigate the influence of ash on the sorption and genotoxicity of sediment-associated organic compounds. In order to study the toxic effect of ash, the ash-amended sediments without pollutants addition were adopted as exposure systems in the comet assay. Results and discussion The sorption capacity for PCP and PYE increased with ash content in the sediment. The high adsorptivity of ash was attributed to the black carbon contained in the ash, which is in line with many previous studies. When spiked at a concentration of 200 μg/kg, both PCP and PYE exerted significant genotoxic effect on earthworms. The genotoxicity of sediment-associated pollutants decreased as the ash content in sediment increased. The DNA damage in the 5% ash-amended exposure showed no significant difference with that in the control exposure, probably due to the strong sorption capactiy of ash, which decreased bioavailability as well as toxicity of sediment-associated pollutants. However, the 10% ash treatment had a higher DNA lesion than that in the 5% char-added exposure. This may have been caused by the genotoxic compounds and high alkalinity contained in the ash. Conclusions Addition of ash increased sorption and inhibited the genotoxicity of sediment-associated organic toxicants. When added at a high rate (e.g., 10%), ash could also exert genotoxic effect. As field burning is a common land-cleaning method in many agricultural areas, the ecological influence of ash should be given much attention in the future study. More research should be conducted to better understand the mechanism of toxic effect of ash.