Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO 2 2+) for over half a century and have exhibited exceptional extractability and selectivity toward the uranyl ion due to the presence of the phosphoryl group (OP). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO 2 2+ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO 2 2+ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σand π-type interactions with U 5f and 6d orbitals in the UO 2 Cl 2 (TRPO) 2 complexes. Although both the axial (O yl ) and equatorial (O eq ) oxygen atoms in the UO 2 Cl 2 (TRPO) 2 complexes contribute to the Xray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by O eq and is assigned to the transition from O eq 1s orbitals to the unoccupied molecular orbitals of 1b 1u + 1b 2u + 1b 3u symmetries resulting from the σand π-type mixing between U 5f and O eq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of O eq 2p character in these orbitals for the four UO 2 Cl 2 (TRPO) 2 complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U−TRPO bonding interactions in the UO 2 Cl 2 (TRPO) 2 complexes. The covalent bonding interactions between TRPO and UO 2 2+ , especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.

The characteristics of high-solids anaerobic digestion (AD) of sewage sludge were investigated by comparison with conventional low-solids processes. A series of batch experiments were conducted under mesophilic condition and the initial solid contents were controlled at four levels of 1.79%, 4.47%, 10.28% and 15.67%. During these experiments, biogas production, organic degradation and intermediate products were monitored. The results verified that high-solids batch AD of sewage sludge was feasible. Compared with the low-solids AD with solid contents of 1.79% or 4.47%, the high-solids processes decreased the specific biogas yield per gram of sludge volatile solids slightly, achieved the same organic degradation rate of about 40% within extended degradation time, but increased the volumetric biogas production rate and the treatment capability of digesters significantly. The blocked mass and energy transfer, the low substrate to inoculum rate and the excessive cumulative free ammonia were the main factors impacting the performance of high-solids batch AD.

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