Irreversible sorption has been widely observed for hydrophobic organic contaminants in natural sediments. It is a complicated process and impacts sediment quality and contaminated sediment management. In this research, experiments were conducted to further understand the nature of irreversible sorption and the mechanism(s) controlling this process.
The unique characteristics of the irreversible compartment were delineated with the sorption and desorption of five chlorinated benzenes in four natural sediments. It was observed that each chemical-sediment combination exhibited a fixed amount of maximum irreversible sorption capacity, qirrmax . However, the organic carbon based partition coefficient associated with the irreversible compartment was essentially constant for different chemical-sediment combinations--- Kirroc=105.42+/-0.17 . The desorption behaviors were modeled with an irreversible sorption model, in which a Langmurian-type expression (representing the irreversible fraction) was added to the linear model. For the sorption and desorption of different compounds in the same sediment qirrmax seems to be related to both Kow and solubility. For sorption and desorption of the same compound in different sediments qirrmax depends mainly on sediment organic carbon content (foc), but may also be affected by the origin of the sediments. The release from the irreversible compartment was not affected by a number of factors, such as extended desorption time, competitive sorption, caustic treatment of sediments, and external mechanic forces. Many observations in this research suggest that sorption and desorption of hydrophobic organic contaminants in natural sediments are not controlled by the same mechanism, and irreversible sorption may be partly due to the association of organic sorbate molecules to some highly insoluble and hydrophobic materials in sediments.
Irreversible sorption greatly affects the fate and transport of contaminants in surface and subsurface environments---it causes contaminants to persist in soils and sediments, and it also reduces the efficiency of pump and treat and bioremediation. Nevertheless, the same process could also greatly reduce the ecological and human-health risks of contaminated sediments. Thus, it is necessary to establish risk-based sediment quality standards, which could be of enormous economical impact. Also, to quantitatively address the impact of irreversible sorption on sediment quality, a one-dimensional solute-transport model was developed. This simple model could provide valuable support for environmental decision making.