Resource manipulation in uranium and arsenic attenuation by Lemna gibba L. (duckweed) in tailing water of a former uranium mine

Influences of phosphorus and nitrogen on uranium and arsenic accumulation in Lemna gibba L. were investigated in the laboratory hydroponic cultures and in the field pot experiments. The initial uranium and arsenic concentrations in solutions for the hydroponic cultures were 1000 μ g l^?1 each, while in situ trials used tailing water containing 198.7 ± 20.0 μ g U l^?1 and 75.0 ± 0.4 μ g As l^?1 at a former uranium mine in eastern Germany. A test of three PO₄ ^3? concentrations (0.01, 13.6 and 40.0 mg l^?1) in the hydroponic cultures, highest uranium accumulated in L. gibba under the culture with highest PO₄ ^3?. Significant differences in uranium accumulation were between 0.01 mg l^?1 and 13.6 mg l^?1 PO₄ ^3? cultures only (ANOVA p = 0.05). In the field, addition of 40.0 mg l^?1 PO₄ ^3? increased the bioaccumulation of uranium significantly. Contrary, high PO₄ ^3? concentrations suppressed the bioaccumulation of arsenic in both the laboratory and the field. The bioaccumulation of both uranium and arsenic increased slightly with the increase of NH₄ ⁺ concentration. However, high NH₄ ⁺ concentrations reduced the yield in the control experiments. The concentration of uranium rose temporarily to 856.0 ± 294.0 μ g l^?1, while the concentration of arsenic sunk slightly and temporarily immediately after amending the tailing waters with 40 mg l^?1 PO₄ ^3?. The speciation of uranium in the tailing water was modelled with geochemical code PhreeqC, which predicted that uranyl carbonate species dominated before addition of phosphates, but after increasing the PO₄ ^3? concentrations, uranyl phosphates species became dominant. Addition of NH₄ ⁺ to the tailing water had negligible influence on free available uranium and arsenic concentrations. Thus, manipulations to enhance uranium and arsenic attenuation by L. gibba has limitation when the amendments interact with other elements including the contaminants in the milieu, and when the target contaminants have antagonistic behaviour in the tailing water.