Kinetics of lead release from soils at historic mining and smelting sites, determined by a modified electro-ultrafiltration

Within a pilot study, after pedological and mineralogical characterization, various kinetic models were tested to fit lead (Pb)-mobilization kinetics from soils at historic mining and smelting sites. Pb mobilization was obtained by modified electro-ultrafiltration (EUF) after addition of diethylenetriaminepentaacetic acid (DTPA) at variable conditions of extraction. 10 fractions were sequentially produced, under mild conditions at 20°C/200 V (to simulate an initial release) for fractions 1-5, and subsequently harder conditions at 80°C/400 V (to simulate a long-term release) for fractions 6-10. The special samples treated within this work yielded higher extraction rates within the first runs. Closest fits in terms of the coefficient of determination (R2) were obtained from the 2nd order polynomial model y = a + bt + ct2, and in terms of re-calculated results by the parabolic equation y = a + b √t. The fitted constants obtained by the modified EUF method correlated better with soil pH than with organic carbon and clay contents. From this, it remains open, whether the dissolution of the Pb-minerals in the electric field or concentration resp. diffusion of the DTPA is rate-determining.

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