Understanding the Impact of Soil Clay Mineralogy on the Adsorption Behavior of Zinc

Infiltration systems are increasingly used to reduce peak flows and mitigate the impacts of stormwater runoff. Despite the benefits of infiltration systems, there is a risk for associated pollutants, including heavy metals to be introduced to the underlying soil and groundwater. The subsequent movement of metals in the subsurface and their potential to contaminate water resources is uncertain and profoundly depends on the adsorptive behavior of the surrounding soil. We used two soil types, one natural (quartz–kaolinite–muscovite) and one synthetic (quartz and kaolinite only) with different clay mineralogy to test their potential adsorptive capacities through batch systems, with Zn(II) as a representative tracer. Nonlinear isotherms, Freundlich and Langmuir, provided good fits with the experimental sorption data. Kinetic data were well fitted by a pseudo-second-order model, indicating that cation exchange exists between the clay surfaces and Zn(II) in the liquid phase. We found that the natural soil adsorbed far more Zn(II) when compared to the synthetic soil which was attributed to the presence of the muscovite in the natural soil. Comparison of the observed adsorption capacity of the two soils with their predicted adsorption capacities showed that while the adsorption capacities of the single-sized clay minerals are widely reported, these values cannot be linearly extrapolated to estimate the adsorption capacity of a soil that might contain varied fractions of clay. The results suggest that the designers of infiltration systems should first undertake an analysis of clay mineralogy of the subsurface soil to better predict the fate of heavy metals within the surrounding soils.