The Study About Partition Behaviors And Mechanisms Of Typical Perfluoroalkyl Substances In Sediment

Perfluoroalkyl substances (PFASs) are widely used in numerous industrial and commercial applications due to their unique physicochemical properties, such as thermal and acid resistance, high surface activity, both hydro-and lipophobicity. As a result of the high demand and wide application, PFASs have been detected in water, sediment, soil, wildlife and humans all over the world. In May2009, perfluorooctane sulfonate (PFOS), its salts and perfluorooctane sulfonyl fluoride were added to Annex B of the Stockholm Convention on persistent organic pollutants by the Fourth Conference of Parties.Due to the low volatility and high solubility of PFASs, aquatic environment is an important sink for PFASs. Some studies suggested the only environmental sinks for PFASs were sediment burial and transport to the deep oceans. Although there are many studies about PFASs in water, most of them reported the pollution level of PFASs in water, sediment and aquatic biota. There are a few studies about sorption of PFASs on sediment, while most of them focused on PFOS, little has been done to other PFASs. Systematic studies about sorption and desorption behaviors of PFASs on sediment are obviously insufficient. It remains unclear how PFASs are distributed in sediment and the distribution mechanism.This study provides particle-scale understanding of perfluoroalkyl substances (PFASs) distribution in two sediments with different compositions and properties. Three perfluoroalkane sulfonates (PFSAs) and six perfluoroalkyl carboxylates (PFCAs) were selected as target compounds. Wet sieving was performed to separate each sediment into eight fractions according to particle size and density. The partitioning and desorption behaviors of PFASs in each fraction of the two sediments were compared. The study aimed to illustrate the impacts of the properties of PFASs and the compositions and structure of sediments on sorption. Even though lower density carbonaceous fractions contributed only17.8%and22.3%of the total sediment mass, they displayed stronger affinity to PFASs with much higher PFAS concentrations than in the heavy fractions. Hydrophobic interaction predominated the partition of longer chain PFASs while electrostatic interaction could affect the sorption of short chain PFASs in sediment fractions. Both the length of the perfluorocarbon chain and the functionality of the head group affected the distribution and desorption of the anionic PFAS surfactants in the sediment fractions. The individual PFAS concentrations increased with increasing perfluorocarbon chain length, and the concentrations of PFSAs in the same sediment fraction were higher than PFCAs with the same chain length. Desorption experiments indicated desorption became difficult as the chain length increased, and PFSAs were harder to be desorbed than the corresponding PFCAs. LogKoc was in the range of1.70-3.80for C5-C12PFCAs and1.75-2.97for C4-C8PFSAs. Short-chain PFASs displayed a higher potential for aqueous long-range transport, while sediment could act as a sink for long-chain PFASs. The results suggest that the bioavailability of PFASs in sediment decreases with increasing the carbon chain length.To better understand the sorption mechanisms of PFSAs on soils/sediments, different fractions of humic substances (HSs), including two humic acids (HAs), and humin (HM) were sequentially extracted from a single peat soil. The sorption of the two typical PFSAs, including perfluorohexane sulfonate (PFHxS) and PFOS, on the HS fractions were investigated. Batch experiments including sorption kinetics, sorption and desorption isotherms were conducted. The effect of solution pH on their sorption and the possible sorption mechanisms were investigated. The sorption kinetics and isotherms showed the adsorption of PFOS on the HSs was much higher than PFHxS. For the same PFSA compound, the sorption on HSs followed the order of HM>HA2>HA1. These suggest that hydrophobic interaction plays a key role in the sorption of PFSAs on HSs. Due to the higher water solubility, the PFSA sorption capacities on the HSs are significantly weaker than the typical hydrophobic organic compounds (HOCs), such as benzo[a]phrene, phenanthrene and lindane. The sorption capacities of PFSAs on HSs were significantly related to their aliphaticity, but negatively correlated to aromatic carbons, indicating the importance of aliphatic groups in the sorption of PFSAs. As compared to PFOS, PFHxS displayed distinct desorption hysteresis, probably due to irreversible pore deformation after sorption of PFHxS. Solution pH displayed significant effect on their sorption on the HSs, and the sorption of the two PFSAs on HSs decreased with an increase in pH in the solution. Under normal environmental pH conditions (pH5-8), the sorption of PFSAs on HSs is mainly contributed by hydrophobic interaction. This is ascribed to the electrostatic interaction and hydrogen bonding at lower pH. Hydrophobic interaction might also be stronger at lower pH due to the aggregation of HSs.To further illustrate the interaction of PFASs with organic matters in soil/sediment, the influence of contact duration on the chemical extractability and degradation of PFASs in three peat soils and a farmland soil were investigated under antibacterial and non-antibacterial regimes. Three PFSAs and eight PFCAs were selected as target compounds. Earthworms were exposed to artificially PFASs contaminated soils to investigate the bioaccumulation and bioavailability of PFASs in soil. The results indicated that most of PFASs were extractable in the farmland soil while were sequestrated in the peat soils, suggesting that the sequestration of PFASs in soils was caused by soil organic matters. The accumulation of long carbon chain PFCAs and PFSAs in earthworms was obvious, and PFSAs showed stronger bioaccumulation potential than the corresponding PFCAs. The bioavailability of the PFASs in soils significantly decreased along with the sequestration and aging. Long chain PFCAs were more strongly influenced by aging than the short ones. The extractability of PFASs from the soil without NaN3was much lower than the soil with NaN3, indicating that the presence of microbes promoted the formation of a larger nonextractable residual fraction. Part of PFASs might be degraded by the microbes. Further studies are deemed to understand the funtions of the microbes.