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).

Fulvic acid (FA) was extracted and purified from natural soil and the effects of such FA on the bioaccumulation of rare earth elements (REEs, La3+, Gd3+ and y3+) in wheat seedling were investigated. The results indicated that low concentration of FA (<0.4mg C/1 to root, <0.7mg C/1 to tops (stem and leaves)) could increase the bioaccumulation values of REEs in wheat, but when the concentration of FA was high (>0.4mg C/1 to root, >1.5mg C/1 to tops) the bioaccumulation values were decreased. Kinetic experimental results suggested that bioaccumulation values of REEs in roots for 30 days were correlated with the kinetic linear growth equation, and correlation coefficients were higher than 0.861. The kinetic bioaccumulation pattern of REEs in tops was different from that in root. The bioaccumulation values of REEs in wheat root were much higher than in wheat tops. Variations of glutamic oxaloacetic transaminase (GOT) enzyme activities in wheat root and tops were determined. A good correlation existed between the bioaccumulation values of REEs and GOT enzyme activities, and the correlation coefficients were higher than 0.922. GOT is an im-portant parameter influencing the bioavailability of REEs.

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.

In this investigation, the species of spiked Rare Earth Element La, Gd, Y, Ce and Sm in sediment were def'med as watersoluble, acid extractable and organic/sulphide bound species, the correlation relationship of these species to thebioavailability of algae (Chlorella Vulgaris Beijerinck) were established by linear regression. The results indicated thatthere existed good correlation between the concentrations of REEs in algae and the concentrations of three species insediment, respectively. Among three species in the sediment, the water soluble species had the best correlationrelationship to the bioavailability of algae, this suggested that the release of REEs from the sediment were the most important factor affect the bioavailability of REEs to algae.