Aging and fraction distribution of persistent organic pollutants (POPs) in soil based on sequential extraction

To investigate the dynamics of distribution and availability of persistent organic pollutants (POPs) under aging processes in dry soil, a 130-day indoor microcosm experiment was conducted. Fourteen halogenated POPs, including three organochlorine pesticides (OCPs), five polychlorinated biphenyls (PCBs), and six polybrominated diphenyl ethers (PBDEs) with diverse physico-chemical properties, selected as target pollutants, were spiked to the soil. A four-step sequential extraction including oscillating extraction, ultrasonic extraction, silylation, and alkaline treatment was applied to aged soil to obtain bioaccessible fractions, stable-adsorbed fractions, and two types of non-extractable residues of target POPs. Results indicated that the average volatilization rate of pollutants peaked around the 5th day and significantly decreased thereafter, with minimal volatilization observed after the 30th day. By the 130th day, the fractions of bioaccessible, stable-adsorbed, and non-extractable residue were found to range from 79.5% to 96.3%, 3.0% to 14.2%, and 0.03% to 8.8%, respectively. Changes in the fractions of halogenated POPs were influenced by temperature and the physicochemical properties of the pollutants. Specifically, the content of the bioaccessible fraction increased while that of the stable-adsorbed fraction decreased with rising temperatures. POPs with higher molecular weights exhibited higher proportions of stable-adsorbed fractions and lower proportions of non-extractable residues. The bioaccessible fraction consistently remained the dominant form, indicating sustained higher bioavailability and environmental risk of soil-borne POPs in arid conditions over a longer time period.