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During TEM-EDS (transmission electron microscopy coupled with an X-ray energy dispersive spectrometer) analysis of Dixie clay artificially contaminated with Pb(II), we observed that Pb(II) was preferentially adsorbed and precipitated on the surface of TiO2. To deepen the understanding of the mechanism and importance of this phenomenon, batch sorption experiments, XANES (X-ray absorption near edge spectroscopy) analysis, and sequential extraction analysis were performed. The TiO2 in Dixie clay was found to be anatase, and anatase showed a higher Pb(II) sorption propensity than rutile, α-FeOOH, and one of two MnO2 investigated in this study. Our experimental results indicated that the Pb precipitates preferentially formed on the surface of anatase was Pb(II) hydroxide or Pb(II) oxide. Additionally, sequential extraction analysis showed that at least 32% and 42% of Pb(II) was sorbed onto anatase in the Dixie clay contaminated with a Pb content of 736 mg Pb/kg and 1,958 mg Pb/kg, respectively. These results demonstrated that in addition to Fe and Mn oxides that are well-known metal oxides that serve as sinks for Pb(II) in the soil environment, TiO2 is also a metal oxide that controls the behavior and fate of Pb(II) in soils.

This study was performed to enhance the understanding of the Cd(II) leaching mechanism from contaminated kaolinite with a focus on the role of anatase impurities. Experimental data obtained from titration and batch Cd(II) adsorption experiments were analyzed using a surface complexation model coupled with ion exchange reaction. Anatase showed significantly higher Cd(II) adsorption capability than those of kaolinite and goethite (α-FeOOH), with inner-sphere complex formation as the dominant adsorption mechanism. Under acidic and near-neutral conditions, kaolinite absorbs Cd(II) mainly via outer-sphere complex formation, while both outer-sphere and inner-sphere complex formation are important at higher pH. Binding constants obtained from modeling analyses were then used to predict the leaching extent of Cd(II) from contaminated kaolinite with different contents of anatase impurity at pH 4.0–7.0. The experimental data and model predictions consistently showed that the leaching of Cd(II) decreased with increasing anatase content and this trend was more significant under acidic conditions. These results showed that the content of anatase impurity is a factor controlling the leaching extent of Cd(II) from kaolinite when exposed to acid rain.