Evidence of Physical and Chemical Nonequilibrium in Lead and Cadmium Transport and Sorption in Acid Soils

Abstract

Environmental issues regarding soil trace metals often center on mobility and bioavailability. Transport of metals through the soil system could be affected by physical or chemical nonequilibrium processes. In this study, the physiochemical factors involved in Pb and Cd transport and sorption in soils were examined to determine the fate of metal pollutants. A series of metal miscible displacement experiments were executed following a full factorial statistical design including fi ve sources of variability: soil (S1 and S2), metal (Pb and Cd), metal initial concentration (C0), water fl ux rate (Jw), and depth (0–4 and 4–8 cm). Temporal moment analysis of the measured breakthrough curves (BTCs) revealed that Cd transport is characterized by larger mass recovery values and lower estimated values of the transport timing—mean travel time (μ) and vertical solute spreading (σ2)—than Pb. This indicates the high affi nity and retention of Pb in the soil and the high mobility of Cd. On the other hand, the sorption parameters for both Pb and Cd were estimated from a sequential chemical extraction of the metal retained in the soil column. The effect of the sources of variability on both transport and sorption parameters were determined. The functional relationship between sorption and transport parameters was also estimated through a linear regression and canonical correlation analysis. Physical and chemical nonequilibrium in transport processes were demonstrated by BTC tailing, changing BTC characteristics (μ and σ2) with Jw and C0, and semi-irreversible sorption identifi ed by sequential extraction.