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 + NO 3 - or HEP + ReO 4 - 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 + NO 3 - /SiO 2 and HEP + ReO 4 - /SiO 2 solid materials, the organometallic salts did not desorb into the aqueous phase when the solids were treated with aqueous solutions containing NaNO 3 and/or HNO 3 . The HEP + NO 3 - /SiO 2 materials functioned as redox-recyclable ion exchangers. Treatment of the materials with aqueous waste simulants containing KReO 4, NaNO 3, and HNO 3 resulted in NO 3 - /ReO 4 - ion exchange as follows: HEP + NO 3 - /SiO 2 (s) + ReO 4 - (aq) ⇌ HEP + ReO 4 - /SiO 2 (s) + NO 3 - (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 ReO 4 - and 1.0 M HNO 3 . 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 ReO 4 - and TcO 4 - extraction. The higher distribution coefficient per millimole of cationic sites suggests that HEP + NO 3 - /SiO 2 is more selective for ReO 4 - than Reillex-HPQ under these conditions. The recovery of adsorbed ReO 4 - 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 ReO 4 - and NO 3 - . Reactivation of HEP/SiO 2 to HEP + NO 3 - /SiO 2 was accomplished with aqueous ferric nitrate. Five complete extraction−deactivation/(ReO 4 - 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.