Evaluation of fluoride-induced metal mobilization in soil columns

Fluoride in soil solution may cause substantially increased leaching of dis solved organic carbon (DOC), Al, and heavy metals from soils. Our objective was to examine whether increased metal solubility is attributable to forma tion of fluoro- or organo-complexes. The A horizon of a: Slovak soil contam inated with heavy metals and F by Al smelter emissions was used to conduct two column experiments under saturated and steady-state water flow conditio ns. The columns were leached with 5 mM Na2SO4, 1 mM NaCl, and 9 mM NaF solu tions at pH 5.5 and an electrical conductivity of 1000 mu S 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) pu lse 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. Fo r both experiments, Cl breakthrough curves (BTCs) revealed a convection-dom inated transport regime. The breakthrough of Cd, Cr, Fe, Ni, and Pb paralle led DOC breakthrough in both experiments indicating metal leaching as organ o-complex. In the dual F pulse experiment, peak separation of the DOC and t he complexed F (F-bound) breakthrough was achieved. The highest F-bound con centration occurred 3.8 PVs later than that of DOG. The Al BTC paralleled t hat of F-bound but showed tailing with a turning point simultaneous to the highest DOC concentration. Thus, the increase in Al solubility was mainly c aused by fluoro-complexation and only to a lesser extent by organo-complexa tion.