Exceptional Perrhenate/Pertechnetate Uptake and Subsequent Immobilization by a Low-Dimensional Cationic Coordination Polymer: Overcoming the Hofmeister Bias Selectivity

We
report one of the most efficient scavenger materials, a cationic
crystalline coordination polymer SBN for trapping ReO4–, a surrogate for 99TcO4–, as an anionic radioactive contaminant of great concern.
The uptake capacity for ReO4– reaches
786 mg/g, a value noticeably higher than the state of art anion-exchange
resins and other inorganic or hybrid anion sorbents. Once being captured,
ReO4– is greatly immobilized, as almost
no ReO4– can be eluted using large excess
of nitrate, carbonate, and phosphate anions. The processes are featured
by a complete and irreversible single-crystal to single-crystal structural
transformation from SBN to the ReO4–-incorporated
phase (SBR). The coordination environments of NO3– and ReO4– probed by single-crystal
structures clearly unravel the underlying mechanism, where each ReO4– in SBR binds to multiple Ag+ sites forming strong Ag–O–Re bonds, and to 4,4′-bipyridine
through a dense hydrogen bond network. These structural insights lead
to a significant difference in solubility product constants between
SBN and SBR, which is further confirmed by first principle calculations
showing a large binding energy difference of 35.61 kcal/mol. To the
best of our knowledge, SBR is the least soluble perrhenate/pertechnetate
salt reported, which may be considered as a potential waste form for
direct immobilization of TcO4–.