Highly Selective Ferric Ion Sorption and Exchange by Crystalline Metal Phosphonates Constructed from Tetraphosphonic Acids

With the motivation of searching for highly selective ferric ion sorbents, two open-framework and microporous
materials, {[Pb7(HEDTP)2(H2O)]·7H2O}n (1) and {[Zn2(H4EDTP)]·2H2O}n (2) [H8EDTP = N,N,N‘,N‘-ethylenediaminetetrakis(methylenephosphonic acid)], have been synthesized and structurally characterized. The structure of
compound 1 results from the seven crystallographically different lead atoms that are bridged by two HEDTP7-
ligands to yield a three-dimensional microporous framework with tunnels along the a and b axes. Compound 2
features a layer architecture built of square waves along the a axis. The layers are connected by hydrogen bonds
between uncoordinated phosphonate oxygen atoms to form a three-dimensional supramolecular network, with one-dimensional tunnels along the a axis. Both compounds 1 and 2 exhibited high ion sorption and exchange capacities
for millimolar concentrations of FeIII. Specifically, when 0.01 g of 1 (or 2) was added to 5 mL of a 1 mM metallic
chloride aqueous solution and the mixture was allowed to stand for 2 days at room temperature, compound 1
adsorbed nearly 100% of FeIII and compound 2 adsorbed 96.8% of FeIII. They were also found to adsorb ferric ions
selectively over other metal ions, such as CaII, CrII, MnII, CuII, ZnII, CdII, etc. Their special ferric ion uptake capacities
may be attributed to the cation exchange, coordination bonding, and electrostatic attraction between ferric ions and
metal phosphonates.