The bioavailability of rare earth elements (REEs) in soils was evaluated, based on the combination of chemicalfractionation and multiple regression analysis. REEs in soils were partitioned by a sequential extraction procedure intowater soluble (F(ws)), exchangeable (F(ec)), bound to carbonates (F(cb)), bound to organic matter (F(om)), bound toFe

Fulvic acids (FAs) were extracted by alkali extraction from different soil samples in China, then purified using resins and characterized by Fourier transform infrared spectroscopy. The complexing ability of FAs was investigated by measuring the stability constants of rare earth elements (REEs) (La3+, Ce3+, Sm3+, Gd3+, Y3+) with FAs by the ion exchange technique. The results indicated that maximum binding ability forY3+ (4.41～4.44) was higher than other REEs (La3+, Ce3+, Sm3+, Gd3+) (0.72～1.03). There were two types of binding sites in the functional groups of fulvic acids. The complexing reaction followed two steps. The stability constants (K1 and K2) of REEs with Fas were calculated from experimental data by division of Scatchard plots into two straight-line segments. Y3+ (logK1=5.72±log K2=4.83±0.01) also has higher stability constants than the other four REEs (log K1=4.37±0.16, log K2=3.62±0.28).

Physiological and biochemical perturbations in the liver ofCarassius auratus were investigated in vivo following 40 days ofexposure to ytterbium solutions of di4erent concentration.Glutamate8pyruvate transaminase (GPT) activity in gold5shliver was stimulated at 0.05mg/L Yb3+ and inhibited at higherYb3+ concentrations. Activity of the antioxidant enzymesuperoxide dismutase (SOD) was stimulated at Yb3+ higher than 0.05mg/L, and catalase (CAT) activity was strongly inhibitedafter 40 days of exposure. Detoxifying enzymes glutathioneS-transferase (GST) and glutathione peroxidase (GSH-Px) werestimulated at 0.05mg/L and inhibited at 0.1mg/L after 40 daysof exposure. Among the parameters determined, CATin gold5shliver was most sensitive to Yb3+, indicating that CATmightbe considered a potential tool in the biomonitoring of exposureto Yb3+ in an aquatic ecosystem.

Equilibrium release experiments were conducted under three different pH values of 3.5, 5.5 and 7.5 as well as three redox potentials of 400, 0 and -100Mv to investigate the influence of redox potential and pH value on the La, Ce, Gd and Y release of from the simulated-REEs-accumulation (SRA) soil. Oxygen and nitrogen were allowed to flow into soil suspension to adjust redox potential to a preset value, and 1mol/1 HC1 or lmol/1NaOH solutions were added into the soil suspension to keep pH at a preset value. Results indicated that La, Ce, Gd and Y release increased gradually with the decrease of pH value or Eh, and the influence of redox potential on Ce was more remarkable than on La, Gd and Y. At the same time. It was observed that La, Ce, Gd and Y releases were positively correlated with the release of Fe and Mn, indicating that La, Ce, Gd and Y releases might originate from dissolution of Fe-Mn oxyhydroxides under reduction and low pH conditions. Moreover, it was found that alteration of pH value and redox potential might affect the change of La, Ce, Gd and Y species in the soil. The contents of La, Ce, Gd and Y in exchangeable fraction and Fe-Mn oxide fraction in the solid phase from soil suspension separation decreased with the decline of pH value and redox potential. Multiple stepwise regression analysis showed that exchangeable fraction and Fe-Mn oxide fraction pre-dominately contributed to the La, Ce, Gd and Y release. Low pH value and redox potential were more favorable to La, Ce, Gd and Y releases following the change of their species. The La, Ce, Gd and Y contents in exchangeable fraction and Fe-Mn oxide fraction are the main contributors to their release.