Time and pH-dependent uptake of benzidine by XAD-4 hydrophobic resin

[Display omitted] The environmental persistence of hydrophobic organic contaminants (HOCs) has incited a number of studies to both describe and predict fate and transport of these chemicals in the environment. A known human carcinogen, benzidine, has been used in the textile industry for the production of benzidine-based dyes. Its solubility was found to decrease with increase in pH. When released to the environment, benzidine can follow a number of sorption pathways. In this study, sorption of benzidine molecules on hydrophobic Amberlite XAD-4 resin was investigated. Experiments were carried out in DI water and in sodium citrate/citric acid buffered systems (pH = 3.0, 3.85, 5.85, and 6.5) in order to bracket the two pKa values of benzidine and ensure varying degrees of benzidine protonation in XAD-4 resin/water media. The sorption experiments were conducted from six hours to 56 days duration. Nonlinear uptake was observed for the selected resin under all conditions investigated and fitted with dual-mode model (DMM). Hole filling was found to be the predominant sorption mechanism, while partitioning appeared to play a minor role in the uptake of benzidine by XAD-4 resin. This indicated that benzidine uptake was not a diffusion-limited process: benzidine molecules had no problem navigating the mesoporous structure of the resin. Only when the hole capacity has been reached does the partitioning process become important. Therefore, the long-term objective of this research is to create a time-dependent model for sorption of aromatic amines in sediment and soil media.