A Strategy for Separating and Recovering Aqueous Ions:  Redox-Recyclable Ion-Exchange Materials Containing a Physisorbed, Redox-Active, Organometallic Complex

A series of anion-exchange materials were prepared by adsorption of the dark-green organometallic salt HEP+NO3 - or HEP+ReO4 - dissolved in organic solvents onto three different silica gels (HEP = 1,1‘,3,3‘-tetrakis(2-methyl-2-hexyl)ferrocene). Adsorption isotherms showed that the amount of HEP+ salt adsorbed depended on the choice of counteranion, solvent, surface area, and pore size diameter of the silica gel. After drying the HEP+NO3 -/SiO2 and HEP+ReO4 -/SiO2 solid materials, the organometallic salts did not desorb into the aqueous phase when the solids were treated with aqueous solutions containing NaNO3 and/or HNO3. The HEP+NO3 -/SiO2 materials functioned as redox-recyclable ion exchangers. Treatment of the materials with aqueous waste simulants containing KReO4, NaNO3, and HNO3 resulted in NO3 -/ReO4 - ion exchange as follows: HEP+NO3 -/SiO2(s) + ReO4 -(aq) ⇌ HEP+ReO4 -/SiO2(s) + NO3 -(aq). The distribution coefficient for one of the new materials was 100 mL/g (440 mL/mmol of HEP+) for an aqueous waste simulant containing ReO4 - and 1.0 M HNO3. This can be compared with 290 mL/g (87 mL/mmol of cationic sites) for Reillex-HPQ, a commercial non-redox-recyclable ion-exchange resin which has been studied for ReO4 - and TcO4 - extraction. The higher distribution coefficient per millimole of cationic sites suggests that HEP+NO3 -/SiO2 is more selective for ReO4 - than Reillex-HPQ under these conditions. The recovery of adsorbed ReO4 - was accomplished by treating the exchanged materials with aqueous ferrocyanide, which caused the reduction of adsorbed HEP+ to adsorbed HEP and concomitant release of the adsorbed counterions ReO4 - and NO3 -. Reactivation of HEP/SiO2 to HEP+NO3 -/SiO2 was accomplished with aqueous ferric nitrate. Five complete extraction−deactivation/(ReO4 - recovery)−reactivation cycles (duty cycle time 94 min) consistently showed a slow decrease in distribution coefficient (∼20% over five cycles). Nevertheless, the data indicate that redox-recyclable anion exchange is a viable concept and that redox-recyclable ion-exchange materials with improved stability should be considered as viable alternatives to traditional anion-exchange resins in the future.