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SOIL CHEMISTRY

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

^a Instituto de Ciencias Agrarias, Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, Serrano 115 dpdo, 28006, Madrid, Spain
^b Lawrence Livermore National Lab., 7000 East Ave., L-627, Livermore, CA 94550
^c Now at: Geosyntec Consutants, Oakland, CA 94612
^d Centro Técnico de Informática, Consejo Superior de Investigaciones Científicas, Pinar 19, 28006 Madrid, Spain

^* Corresponding author (fernando.garrido{at}ccma.csic.es).

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 five sources of variability: soil (S1 and S2), metal (Pb and Cd), metal initial concentration (C₀), water flux rate (J_w), 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 (²)—than Pb. This indicates the high affinity 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 ²) with J_w and C₀, and semi-irreversible sorption identified by sequential extraction.

Abbreviations: BTC, breakthrough curve • ICP–AES, inductively coupled plasma–atomic emission spectrometry