184
uranium in solution was UO 2
2+
when the pH value was below 5, while when the pH
value was between 8.5 and 12, the main species of uranium in solution was
UO 2 (OH) 3
−
. The maximum proportion of UO 2
2+
, (UO 2 ) 3 (OH) 5
+
, and UO 2 (OH) 3
−
in
solutions reached 99.99, 74.73, and 97.2%, respectively, when the pH value was
1.0, 6.5, and 10.5, respectively. However, our pre-experimental results showed that
Azolla-Anabaena could not survive for more than 10 days when the pH value was
below 2.5. Therefore, the pH value for hydroponic experiments cannot be set at 1.0.
The survival duration of Azolla-Anabaena was greatly prolonged when the pH value
was 4.0. At the same time, the main species of uranium in solution was also UO 2
2+
,
and its proportion reached 94%. Thus, UO 2
2+
, (UO 2 ) 3 (OH) 5
+
, and UO 2 (OH) 3
−
were
selected as three different species of uranium for the hydroponic experiments, and
the corresponding pH values in the solutions were 4, 6.5, and 10.5 respectively.
Preparation of UO 2 (CO 3 ) 3
4−
and UO 2 PO 4
−
The proportions of different species of uranium in solutions at different carbonate
and phosphate levels were calculated by the software when the initial uranium concentration was 2 mg L
−1
and the pH value was 7 (Fig. 2). As shown in Fig. 2, the
proportion of different species of uranium in the solution varied with the increase of
carbonate concentration. The main species of uranium in solution was UO 2 (CO 3 ) 3
4−
,
and its proportion reached 70.33%, when the carbonate concentration was
10 mM. The main species of uranium in solution was UO 2 PO 4
−
, when the phosphate
concentration was above 0.2 mM. The maximum proportion of UO 2 PO 4
−
in solution
Fig. 1 Simulated variation of percentages of different species of uranium in deionized water with
pH (the total concentration of uranium is 2 mg L
−1 )
N. Hu et al.