PAH desorption from sediments with different contents of organic carbon from wastewater receiving rivers.

BACKGROUND AND AIM

Many rivers have to receive treated or untreated wastewater as the main water sources in the world, especially in the countries facing with water shortage. For instance, the Haihe and Huaihe River Basin, which are among areas facing crises for water resources in China, receive 33,400 million tons of wastewater per year. As the sediment has large capacity for sorbing hydrophobic organic pollutants such as polycyclic aromatic hydrocarbons (PAHs), it can act as a natural repository for the contaminants. This means pollutants can release into water phase again under some conditions, such as resuspension, sediment dredging, etc. The objective of this research was to study the trends of PAH release from sediments in rivers receiving much wastewater, such as Yongding New River (YD), Northsewer (NS), and Southsewer (SS) from Haihe River Basin. These rivers received most of the wastewater from Tianjin, China and merge into Bohai Bay finally.

METHODS

Sediments (namely YD, NS, and SS) were collected from Yongding New River, Northsewer, and Southsewer, respectively. The physical and chemical properties of the sediments, including particle size distribution, total organic carbon (TOC) and black carbon (BC) contents, elemental compositions, organic functional groups analyzed from Fourier transformed infrared (FTIR), and nuclear magnetic resonance (NMR) spectra, were characterized. PAH desorption from the sediments was measured with the Tenax shaken slurry desorption method.

RESULTS

The NS sediment had a greatly higher TOC and BC contents, while lower BC-to-TOC ratio than YD and SS sediments. NMR and FTIR analysis showed that aliphatic carbons were more abundant in NS sediment than SS sediment. The desorption experiments showed that PAHs desorbed more rapidly from YD and NS sediments than from SS sediment. PAH fraction desorbed after each interval during the experiment except 0.8 day was significantly correlated with PAH properties among all the three sediments.

DISCUSSION

The higher BC-to-TOC ratio in SS sediment corresponded to slower desorption rate and less desorption extent of PAHs from SS sediment among the three sediments. This probably could be explained by the extremely strong sorption ability of BC. It also proved that the sequestration of PAHs in sediment was not simply the result of higher organic carbon content.

CONCLUSIONS

Considering the physical and chemical characterization of the sediments and PAH desorption properties comprehensively, BC-to-TOC ratio was a more important factor controlling the behavior of PAHs in sediments than absolute TOC or BC content.