Effect of organic carbon and mineral surface on the pyrene sorption and distribution in Yangtze River sediments

Abstract

The effect of organic carbon (OC) and mineral surface on the sorption of polycyclic aromatic hydrocarbon (PAH) pyrene molecule to four Yangtze River sediments was investigated by sorption batch techniques using fluorescence spectroscopy. Pyrene sorption to the mineral fraction was estimated with model sorbent illite, the main clay mineral in Yangtze sediment. The Freundlich model fitted sorption to illite and to sediments was normalized to the specific surface area (SSA). Comparison of the SSA-normalized sorption capacities of illite and sediments suggests a negligible contribution of the pyrene sorption to the mineral fraction. In addition, composite models, such as the linear Langmuir model (LLM) and the linear Polanyi-Dubinin-Manes model (LPDMM) were applied for fitting the sorption of pyrene to the pristine sediments. The application of composite models allows assessing the partition of pyrene into amorphous organic carbon (AOC) and the adsorption in the porous structure of black carbon (BC). The modelling results indicate that the pyrene adsorption to the minor BC components (<0.2%) is more effective than the partition to AOC (0.5-1.3%). Besides the pristine sediments, sediments preheated at 375 degrees C were also studied, in which the AOC fraction was removed during the preheating treatment. The modelling results with LPDMM and Polanyi-Dubinin-Manes model (PDMM) indicate a similar adsorption capacity of BC in pristine and preheated sediments, respectively. The low AOC concentrations in sediments do not diminish the BC micropore filling with pyrene. Simulation of pyrene distribution in the investigated Yangtze River sediments support the importance of the BC fraction in the PAH immobilization under environmental conditions.