Dendritic anion hosts: Perchlorate uptake by G5-NH₂ poly(propyleneimine) dendrimer in water and model electrolyte solutions

Perchlorate (ClO₄^-) has emerged as a major groundwater and surface water contaminant in the United States. Ion exchange (IX) is the most widely used technology for treating water containing lower concentrations of perchlorate (<100 ppb). However, a major drawback of IX is the need for frequent regeneration or disposal of the perchlorate-laden resins. As a first step toward the development of high-capacity, selective and recyclable dendritic ligands for the recovery of perchlorate from aqueous solutions by dendrimer filtration, we tested the hypothesis that dendrimers with hydrophobic cavities and positively charged internal groups should selectively bind ClO₄^- over more hydrophilic anions such as Cl ^-, NO₃^-, SO₄^2-, and HCO₃^-. We measured the uptake of ClO₄ ^- by the fifth generation (G5-NH₂) poly(propyleneimine) (PPI) dendrimer with a diaminobutane core and terminal NH₂ groups in deonized water and model electrolyte solutions as a function of (i) anion-dendrimer loading, (ii) solution pH, (iii) background electrolyte concentration, and (iv) reaction time. The ClO₄^- binding capacity of this dendrimer is comparable to those of perchlorate-selective IX resins. However, its ClO₄^- binding kinetics is faster and reaches equilibrium in ∼1 h. Note also that only a high pH (∼9.0) aqueous solution is needed to release more than 90% of the bound ClO ₄^- anions by deprotonation of the dendrimer tertiary amine groups. The overall results of this study suggest that dendritic macromolecules such as the G5-PPI NH₂ dendrimer provide ideal building blocks for the development of high-capacity, selective and recyclable ligands for the recovery of anions such as perchlorate from aqueous solutions by dendrimer enhanced filtration.