As the operation conditions may vary greatly among seepage waters from different U mines, the effects of different operation conditions on the performance of the PEC method were examined. We first evaluated the PEC uranium extraction performance at pH from 4.2 to 9.0. The results show that similar U extraction rate in the first 60 min was achieved for all the tested pH values, while lower rate was observed for higher pH at 60-120 min (Figure ). One plausible explanation would be the changes in the speciation of aqueous U(VI) with the increasing pH (Figure ). At acidic pH (4.2), U(VI) was predominantly present as uranyl (UO2 2+ ) ions, and the complexation of UO2 2+ with OH -or CO3 2-ions increased with the increasing pH. In the case of higher pH, as the U extraction proceeded, the residual aqueous U concentration decreased, so the proportion of UO2 2+ -OH -and UO2 2+ -CO3 2-complexes increased with the extraction time. CV curves of the Ti cathode in the synthetic U-containing water at various pH show that these UO2 2+ -OH -and UO2 2+ -CO3 2-complexes had more negative reduction potentials than UO2 2+ ions (Figure ), implying that the reduction of these hydroxyl and carbonate complexes predominant at higher pH was less thermodynamically favorable.