Immobilization of heavy metals in soils by the application of bauxite residues: pot experiments under field conditions

In previous greenhouse experiments red mud, a residue of the alumina industry, was identified as effective amendment for in situ fixation of heavy metals. In the present study, we further evaluated the efficiency and potential drawbacks of red mud in an outdoor pot experiment. Application of 5% (w/w) red mud (RM) should reveal possible drawbacks of red mud due to indigenous pollutants such as As, Cr, and V. Three soils from arable land in Lower Austria named Untertiefenbach (U) (Eutric Cambisol), Weyersdorf (W) (Dystric Cambisol), and Reisenberg (R) (Calcic Chernozem) were spiked with Cd, Zn, Cu, Ni, and V at two concentration levels in 1987, two soils originate from long-term industrially polluted sites, located in Carinthia (Arnoldstein-Rendzic Leptosol; Zn, Cd, and Pb) and Tyrol (Brixlegg-Dystric Fluvisol; Cu, Zn). Zea mays was cultivated in pots for three months in outdoor conditions. Extraction with 1 M NH4 NO3 was used to assess the influence of RM on the labile metals. Lability of Cd, Zn, Ni, and Pb was reduced upon RM treatment on a sandy soil up to 91%, 94%, 71%, and 83% of the control, respectively. Metal accumulation in shoots was reduced for Cd and Zn up to 54% and for Ni up to 75% (soil W), but not for Pb (soil A). Addition of RM (5% w/w) increased the total As, Cr, and V concentrations in soils by 5, 20, and 50 mg kg(-1), respectively. Whereas the lability of Cr was not affected, 1 M NH4 NO3-extractable As and V exceeded the trigger value for water quality according to PrueB (1994). Lability of Cu increased upon RM application, especially on the Cu polluted industrial soil (B), while Cu toxicity appeared to be reduced as indicated by the higher corn biomass production. Red mud holds promise as soil amendment in terms of reduction Cd, Zn, and Ni bioavailability. However, at additions as high as 5% (w/w) large As, Cr, and V concentrations of this material may limit its application.