Equilibrium State of PAHs in Bottom Sediment-Water-Suspended Sediment System of a Large River Considering Freely Dissolved Concentrations

Abstract

In natural waters, the equilibrium state of hydrophobic organic compounds among bottom sediment (BS), suspended sediment (SPS), and water is fundamental to infer their transfer flux and aqueous bioavailability. However, this type of information remains scarce and fragmented. This study systematically evaluated the equilibrium state of polycyclic aromatic hydrocarbons (PAHs) in the Yangtze River. Total and freely dissolved concentrations of the 16 priority PAHs in pore water and overlying water (including surface and near-bottom) of the Yangtze middle reaches were investigated, as were the concentrations of attached PAHs in SPS and BS. Results showed that concentrations of total/freely dissolved PAHs, dissolved organic carbon (DOC), and SPS in surface water were not statistically different from those in near-bottom water, and the DOC-water distribution coefficients of PAHs in pore water were not statistically different from overlying water. However, significant disequilibrium was found at the sediment-water interface; concentrations of total/freely dissolved PAHs in pore water were 1 to 2 orders of magnitude higher than those in overlying water. This study offers a complete analysis of the potential disequilibrium of PAHs in BS-water-SPS system of large rivers and suggests that distribution of hydrophobic organic compounds between BS and overlying water is essential in controlling their equilibrium state in the BS-water-SPS system of natural waters.

Suspended Particulate Matter—A Source or Sink for Chemical Mixtures of Organic Micropollutants in a Small River under Baseflow Conditions?

“…In contrast, a similar composition but a higher concentration in SPM than in the sediment was observed for PFCs in Tokyo Bay, Japan, and Pearl River, China . The preferential sorption on SPM was also reported for estrogens in the Yangtze Estuary for PAHs in the Yangtze River and for PAHs, organochlorine pesticides (OCPs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs) in Lake Chaohu, China . Even though the number of detected neutral and hydrophobic chemicals and charged or hydrophilic chemicals was similar in both bulk SPM and sediment extracts (Figure B), neutral and hydrophobic chemicals contributed more to the sediment-associated pollution (>96%) than SPM-associated pollution (<40%) (Figure S7C,E).…”

“…Once introduced into a river, organic micropollutants are distributed among different river compartments, especially water, suspended particulate matter (SPM), and sediments. SPM provides a link between water and the sediment and represents an effective vector for the transport and accumulation of contaminants. , After being sorbed to SPM, the organic chemicals may undergo dynamic and advective processes like transport along the river, sedimentation, and remobilization over time. A wide variety of factors, such as the physicochemical properties of chemicals, the characteristics of the hydrologic system, and the external inputs, determine the distribution of contaminants between phases. − This distribution also governs the exposure pathways of chemicals, including the bioavailability to the local organisms and their communities. , Therefore, considering the partitioning processes of pollutants as well as their bioavailability would be beneficial for understanding the big picture of the risk caused by organic chemicals in aquatic systems.…”

“…Further studies have demonstrated that the freely dissolved concentration of chemicals could also be deduced with passive equilibrium sampling (PES) using polydimethylsiloxane (PDMS) as the sampling phase . The comparison of dissolved concentrations obtained from different methods also allows the evaluation of the role of DOC and the identification of diffusive flux directions between SPM and the water phase …”

“…SPM represents the most mobile fraction of sediments. The contaminants sorbed to SPM can deposit and form the bed sediment, which, in turn, can be resuspended as a source of SPM with the change of hydrodynamic conditions . Under dry weather conditions, the influence of sediment remobilization caused by agitation and the associated chemical inputs can be disregarded.…”

Effect of water-sediment regulation of the Xiaolangdi reservoir on the concentrations, characteristics, and fluxes of suspended sediment and organic carbon in the Yellow River

“…Therefore, for the sediment in the watercourse, effect of their resuspension/deposition on the DOC concentrations is insignificant. In addition, although the DOC level in pore water of the sediment might be higher than that in overlying water (Lai et al, 2015), the effect of DOC releasing from the pore water on the DOC concentrations in overlying water during sediment resuspension might be insignificant due to the small amount of pore water during water regulation.…”