Miscible Displacement of Zinc in Soil Columns: Linear and Nonlinear Modeling

Miscible displacement column experiments were performed to investigate the mobility of Zn in two different soils, Windsor (mixed, mesic Typic Udipsamment) and Webster (fine-loamy, mixed, superactive, mesic Typic Endoaquoll). In the Webster soil, strong Zn retention was observed due to a higher percentage of clay, organic matter, amorphous Fe and Al, and the presence of carbonates compared with Windsor soil. Observed Zn breakthrough curves (BTCs) were asymmetric and exhibited extensive retardation and slow desorption. The Webster soil exhibited limited Zn mobility, with only 14% recovery of Zn in the effluent solution, whereas considerable mobility was exhibited by Windsor, with 59% recovery. The Zn distribution with depth in the soil column showed that, for Windsor, Zn was retained within the surface layer and decreased with depth, whereas for Webster, a concentration maximum was observed below the soil surface. The presence of P in the Zn pulse solution increased the amount of Zn sorbed for the Windsor soil, whereas the opposite was observed for the Webster soil. Measured BTCs for P indicated higher P sorption for Windsor, with P recovery of 67%, than for Webster, with recovery of 85%. Simulations using a linear model with a first-order irreversible reaction provided inadequate predictions of Zn BTCs. Therefore, the linear model is not recommended. Improved Zn predictions of the BTC were obtained when a multireaction and transport model (MRTM) was used. Nevertheless, the MRTM model, which accounted for nonlinear kinetic reversible and irreversible retention reactions, provided inadequate Zn BTC predictions for the Webster soil. There is a need for an improved MRTM model that accounts for the various retention mechanisms governing adsorption-desorption reactions during transport in soils.