Transport of Cadmium and Phosphate in Soils

Abstract

This study quantified and modeled cadmium (Cd) transport in the presence to phosphate (P) in three different soils, Mahan (acidic, kaolinitic), Webster (neutral, mixed mineralogy), and Windsor (acidic, sandy). Two sets of miscible displacement experiments were carried out: (1) sequential pulses of 200 mg L−1 Cd as Cd(NO3)2 then 200 mg L−1 P as KH2PO4, both in 0.01 M KNO3 background; and (2) two consecutive pulses of mixed 200 mg L−1 Cd and P in background solution. Isotherms for Cd and P (1-d equilibration) were also determined. Breakthrough curves (BTCs) of Cd applied without P were generally consistent with the (Freundlich) isotherms and soil properties, Cd retention decreasing: Webster, 72% > Mahan, 61% > Windsor, 37%. Retention of Cd increased when applied with P in Webster and Windsor but decreased in Mahan. Phosphate BTCs indicated high mobility (≥92% recovery) and little effect of Cd, thus limited Cd-P precipitation. Residual distribution of Cd with depth also showed no clear evidence for precipitation. Cadmium BTCs could not be described solely by either linear or nonlinear equilibrium isotherms. Rather, the data were best described by nonlinear, reversible kinetic sorption with an irreversible phase. However, P sorption during transport was well described without irreversible sorption, thus indirect evidence of limited Cd-P precipitation. Although transport of Cd applied without P was well described (r2 = 0.95, Webster and Windsor), description of BTCs was more qualitative when applied with P, accurately locating effluent peak but approximating the sorption and desorption sides.