Development of a new ion-exchange/coordinating phosphate ligand for the sorption of U(VI) and trivalent ions from phosphoric acid solutions

A polymer-bound monoprotic phosphoric acid ligand was developed for the sorption of U(VI) from 0.10 to 6.0 M phosphoric acid solutions. The ligands reported are new phosphorylated mono- and triethylene glycol ethyl esters (pEG1M and pEG3M) prepared by combining diethylchlorophosphate and 4-dimethylaminopyridine; they ion exchange through the acid site and coordinate through the phosphoryl oxygen. The binding mechanism is probed by comparing their results to coordinating phosphorylated mono- and triethylene glycol diethyl esters (pEG1 and pEG3) and the diprotic phosphonic acid (DPA) that operates by ion exchange. The affinities for Lu(III), La(III), Fe(III) and Al(III) were also determined. All metal ions were sorbed much more efficiently by the monoprotic ligand relative to the diprotic and coordinating ligands. It is proposed that a decrease in inter-ligand hydrogen bonding within the monoprotic ligand is responsible for the increased metal ion affinities. This is consistent with the FTIR spectra wherein the P=O band appears at 1262 cm−1 for pEG1 and this shifts down to 1229 cm−1 in pEG1M and 1180 cm−1 for DPA. Incorporating ether oxygen into the ligand further enhances metal affinities: pEG3M with three oxygen donors adjacent to the monoprotic site shows the highest affinity for metal ions. The increased affinities compared to pEG1M are due to complexation by the weakly binding ether oxygen donors. High metal ion affinities thus require both ion exchange with acidic protons and coordination with neutral donors. More rapid kinetics evident with pEG3M relative to DPA is further evidence for decreased hydrogen bonding which permits more rapid sorption. The affinity orders of the three acidic polymers are similar: U(VI), Lu(III)>Fe(III) >La(III)>Al(III). Ligand strength is pEG3M>pEG1M>DPA>pEG3>pEG1.