Abstract

The processes of acid in situ leaching (ISL) uranium (U) mines cause the pollution of groundwater. Phosphate (PO43-) has the potential to immobilize U in groundwater through forming highly insoluble phosphate minerals, but the performance is highly restricted by low pH and high sulfate concentration. In this study, hydrogen peroxide (H2O2) and PO43- were synergistically used for immobilizing U based on the specific properties of groundwater from a decommissioned acid ISL U mine. The removal mechanisms of U and the stability of U on the formed minerals were elucidated by employing X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and kinetic experiments. Our results indicated that the removal of U by simultaneously adding H2O2 and PO43- was significantly higher than the removal of U by individually adding H2O2 or PO43-. The removal of U increased with increasing PO43- concentration from 20 to 200 mg L-1 while decreased with increasing H2O2 concentration from 0.003 to 0.3%. Specifically, the removal efficiency of U from groundwater reached 98% after the application of 0.003% H2O2 and 200 mg L-1 PO43-. Amorphous iron phosphate that preferentially formed at low H2O2 and high PO43- concentrations played a dominant role in U removal, while the formations of schwertmannite and crystalline iron phosphates may be also contributed to the removal of U. This was significantly different from the immobilization mechanism of U through the formation of uranyl phosphate minerals after adding phosphate. The kinetic experimental results suggested that the immobilized U had a good stability. Our research may provide a promising method for in situ remediating U-contaminated groundwater at the decommissioned acid ISL U mines.

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