Selective Chromium(VI) Trapping by an Acetate-Releasing Coordination Polymer

We report the high-capacity and selective uptake of Cr(VI) from water using the coordination polymer silver bipyridine acetate (SBA, [Ag(4,4′-bipy)][CH3CO2]·3H2O). Cr capture involves the release of acetate, and we have structurally characterized two of the product phases that form: silver bipyridine chromate (SBC, SLUG-56, [Ag(4,4′-bipy)][CrO4]0.5·3.5H2O) and silver bipyridine dichromate (SBDC, SLUG-57, [Ag(4,4′-bipy)][Cr2O7]0.5·H2O). SBA maintains a high Cr uptake capacity over a wide range of pH values (2–10), reaching a maximum of 143 mg Cr/g at pH 4. This Cr uptake capacity is one of the highest among coordination polymers. SBA offers the additional benefits of a one-step, room temperature, aqueous synthesis and its release of a non-toxic anion following Cr(VI) capture, acetate. Furthermore, SBA capture of Cr(VI) remains >97% in the presence of a 50-fold molar excess of sulfate, nitrate, or carbonate. We also investigated the Cr(VI) sequestration abilities of silver 1,2-bis(4-pyridyl)ethane nitrate (SEN, [Ag(4,4′-bpe)][NO3]) and structurally characterized the silver 1,2-bis(4-pyridyl)ethane chromate (SEC, SLUG-58, [Ag(4,4′-bpe)][CrO4]0.5) product. SEN was, however, a less effective Cr(VI) sequestering material than SBA