Part IV—sorption of hydrophobic organic contaminants

Abstract

Behavior of hydrophobic organic contaminants (HOCs) in the environment has attracted research interest for more than three decades. It has been clearly concluded that humic substances (HSs), which are the main content of soil/sediment organic matter (SOM) and dissolved organic matter (DOM), controls the sorption of HOCs in soils/sediments. In order to predict the movement of HOCs in the environment, many studies have been conducted to relate HOCs sorption characteristics with HS chemical properties. However, no consensus has been reached on precisely what HS chemical properties regulate HOC sorption, indicating that other HS properties (besides chemical properties) may also control HOC-HS interactions. Increasing amounts of research reveal that SOM physical properties can affect the accessibility of HOCs to SOM sorption sites and thus are of great importance for altering HOC-SOM interactions. Therefore, different from the past reviews on HOCs sorption in soils/sediments, this current one emphasizes physical conformation of HSs for both solid and dissolved forms. SOM chemical properties such as aromatic content, aliphatic content, polarity, and molecular weight have all been reported to affect HOC sorption. No general model has been proposed to predict SOM sorption characteristics from any individual chemical properties. Physical conformations of both solid SOM and DOM are of great importance for altering HOC-SOM interactions. The terms of glassy and rubbery domains have been used to describe physical conformations of solid SOM, and efficiency of chemical oxidation and glassy-rubbery transition temperature are indirect methods to describe SOM rigidity. Various techniques and parameters have been employed to study DOM conformation, such as microscopic images, pyrene-probing hydrophobic region, surface tension, and zeta potential. However, DOM nonideal sorption properties are not properly regarded. HOC-DOM interactions are investigated using solubility enhancement, gas-phase partitioning, fluorescence quenching, and dialysis equilibration methods. The limitations of all the methods are discussed in this review. Relatively, a dialysis equilibration experiment is a better design to study the true HOC-DOM interactions. Physical conformation of SOM are of the same importance as, if not more important than, SOM chemical properties for HOC sorption. Although increasing amounts of research focus on SOM physical conformation regarding HOC sorption, proper mathematical description of its physical conformation and the relationship between SOM physical conformation and its sorption properties are still unclear. Quantitative characterization of SOM conformation regarding its sorption properties with HOCs is a topic worth of further research. The HOC-DOM interactions could not be adequately addressed because of the inappropriate research approach; thus, a reevaluation of HOC-DOM interactions is also required.