Influence of pH on sediment-associated sorption reactions of benzidine

Benzidine has been marked as a priority chemical on the National Priorities List by the U.S. Environmental Protection Agency because of its carcinogenic nature. Benzidine sorbs to the sediment matrix after entering water-sediment ecosystems and undergoes at least three different fate processes, including cation exchange, hydrophobic partitioning, and covalent binding. Sediment samples taken from Lake Macatawa (MI, USA) were used after drying and grinding treatments in this study. Sorption experiments were conducted in the buffered deionized water-sediment slurries with a pH range of approximately 3 to 7. Experimental results indicated that low pH conditions (e.g., pH 3.2) favored sorption of benzidine onto sediment, where a large proportion of benzidine species protonated and sorbed predominantly through the fast cation exchange process. Sorption kinetics data suggested that reactions between protons and carbonate components residing in the sediment matrices led to a shift of sorption mechanisms from cation exchange to hydrophobic partitioning, covalent binding, or both when the slurry pH increased from 3 to 7. A sorption mechanism-based model is presented to describe benzidine sorption behavior in the sediment-water systems at different pH values. This model comprises three components mathematically: the linear hydrophobic partitioning, Langmuir-type covalent binding, and quadratic cation exchange. On the basis of nonlinear regression, this model fits the experimental data well. The organic carbon-normalized distribution coefficient value calculated from this model (1,914 L/kg at pH 6.9), and the available covalent binding sites in the sediment matrices were 27 to 52 mmol/kg organic carbon in the pH range of 5.0 to 6.9. The predicted model parameters are in good agreement with the reported literature values. By this model, the individual contribution from each sorption mechanism can be quantified with a wide pH range (e.g., from pH 3 to 7). This model strategy could provide an alternative way to predict the complex sorption processes of aromatic amines containing one or two amino groups in the aqueous-sediment environment.