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Evaluation of Fluoride-Induced Metal Mobilization in Soil Columns

Soil Physics Div., Univ. of Bayreuth, D-95440 Bayreuth, Germany;
Inst. of Soil Science and Soil Geography, Univ. of Bayreuth, D-95440 Bayreuth, Germany;
Soil Science and Conservation Research Inst., Mládenícka 36, SK-97405 Banská Bystrica, Slovakia.

^* Corresponding author (wolfgang.wilcke{at}uni-bayreuth.de).

ABSTRACT

Fluoride in soil solution may cause substantially increased leaching of dissolved organic carbon (DOC), Al, and heavy metals from soils. Our objective was to examine whether increased metal solubility is attributable to formation of fluoro- or organo-complexes. The A horizon of a Slovak soil contaminated with heavy metals and F by Al smelter emissions was used to conduct two column experiments under saturated and steady-state water flow conditions. The columns were leached with 5 mM Na₂SO₄, 1 mM NaCl, and 9 mM NaF solutions at pH 5.5 and an electrical conductivity of 1000 µS cm^–1. Samples were collected in 70-min intervals for approximately 7 d. Two experiments were performed. In the first experiment (single F pulse), a fluoride (F) pulse was applied for 36 pore volumes (PVs), while in the second experiment (dual F pulse), two consecutive F pulses (each for 2.8 PVs) were applied. For both experiments, Cl breakthrough curves (BTCs) revealed a convection-dominated transport regime. The breakthrough of Cd, Cr, Fe, Ni, and Pb paralleled DOC breakthrough in both experiments indicating metal leaching as organo-complex. In the dual F pulse experiment, peak separation of the DOC and the complexed F (F_bound) breakthrough was achieved. The highest F_bound concentration occurred 3.8 PVs later than that of DOC. The Al BTC paralleled that of Fb_bound but showed tailing with a turning point simultaneous to the highest DOC concentration. Thus, the increase in Al solubility was mainly caused by fluoro-complexation and only to a lesser extent by organo-complexation.