Redox-Recyclable Extraction and Recovery of Heavy Metal Ions and Radionuclides from Aqueous Media

Abstract

Scientists involved in the separation of ionic pollutants such as radionuclides or toxic heavy metal ions from water have designed extractants with high selectivities and large capacities. Although there is still room for improvement in these parameters, there is a more urgent need to develop processes that allow the target pollutants to be recovered in a minimal volume of secondary waste and that allow the extractants to be reused (recycled). We have studied redox-active transition-metal -containing extractants which undergo reversible electron-transfer activation and deactivation as the target ions are extracted and recovered. The "redox-recyclable" extractants investigated so far include molecular organometallic complexes such as substituted ferrocenes and layered metal chalcogenides such as MoS 2 . The molecular complexes can be dissolved in water-immiscible organic solvents for solvent-extraction processes or can be immobilized on inert supports for ion-exchange chromatography. The bulk metal chalcogenides themselves function as redox-recyclable ion-exchange materials. Several extractants have been tested for the repeatable extraction and recovery of aqueous 99 TcO 4 -, ReO 4 -, 137 Cs + , 90 Sr 2+ , Hg 2+ , Cd 2+ , Pb 2+ , and Ag + . Using a substituted ferrocene, greater than 99% of the radioactivity was extracted from an aqueous phase containing 1 M NaOH, 1.5 M NaNO 3 , 10 -5 M 99 TcO 4 -, and 10 -10 M 95m TcO 4 -, and greater than 99% of the extracted radioactivity was recovered in a solid secondary waste. Using MoS 2 , greater than 99% of dissolved Hg 2+ was selectively extracted from an aqueous phase containing 1 M HNO3 and 10 -3 M Hg 2+ . The final concentration of aqueous mercury was ≤ 0.033 μΜ in some cases (6.5 ppb), and greater than 94% of the extracted Hg 2+ was recovered as elemental mercury after stripping. In both cases, the volume of secondary waste containing the target pollutant was a small fraction of the volume of the primary aqueous waste, and in both cases the stripped (deactivated) extractants were reusable. 156

XAFS Studies of Soft-Heavy-Metal-Ion-Intercalated MxMoS2 (M = Hg2+, Ag+) Solids

“…Many of these materials have potential applications as heterogeneous catalysis, − superconductors, , ionic or electronic conductors, , or reversible battery electrodes. − We are currently investigating a new application of these materials in another area: waste remediation. We have reported that Li 1.3 MoS 2 is an effective, selective, and redox-recyclable extractant for the removal of Hg(II) from aqueous solution. − Therefore, for our purposes, we are very interested in the mechanism of ion removal.…”

ATR–FTIR detection of ≤ 25 μg/L Aqueous Cyanide, Perchlorate, and PFOS

“…The related compound HEP+NO 3 − [HEP + = 1,1′,3,3′tetrakis(2‐methyl‐2‐hexyl)ferrocenium cation] has been used to selectively extract and recover the related tetrahedral monoanions 99 TcO 4 − and ReO 4 − from aqueous solutions containing up to 100,000 times as much nitrate ion (Chambliss et al, 1999a; Chambliss et al, 1999b; Strauss, 1999; Chambliss et al, 1998; Clark et al, 1996). The ion‐exchange process can readily be seen in Figure 3, which displays IR spectra recorded after 5 and 10 min when the coated probe was immersed in 15 μg/L aqueous ClO 4 − .…”

“…The compounds DEC + NO 3 − and HEP + NO 3 − are examples of redox‐recyclable extractants, which are being developed in the authors' laboratory for the efficient removal and recovery from water of weakly hydrated aqueous ions, even in the presence of 10 5 times as much nitrate ion in the aqueous phase (Chambliss et al, 1999a; Chambliss et al, 1999b; Clark et al, 1999, Strauss, 1999; Chambliss et al, 1998; Clark et al, 1996; Clapsaddle et al, unpubl.). The basis of redox‐recyclable extraction and recovery (R 2 ER) is that the neutral extractant, in this case a hydrophobic ferrocene [Fe(Cp′) 2 ], is the deactivated low‐affinity form of the extractant.…”

“…Only a few coatings that can concentrate ions have been reported, and none are known to detect aqueous ions at concentrations below 1,000 μg/L (Barja et al, 1999; Degenhardt & McQuillan, 1999; Dobson & McQuillan, 1999; Dobson & McQuillan, 1997; Hug, 1997; Persson et al, 1996; Yang & Saavedra, 1995; Ahn & Franses, 1994). In this initial report, the authors demonstrate that CN − , ClO 4 − , and C 8 F 17 SO 3 − can be detected in water at concentrations equal to 25 μg/L using a commercially available silicon ATR‐FTIR immersion probe that has been coated with thin films of suitable organometallic ion‐exchange compounds (Chambliss et al, 1999a; Chambliss et al, 1999b; Clark et al, 1999; Strauss, 1999; Chambliss et al, 1998; Clark et al, 1996; Clapsaddle et al, unpubl. ).…”