Redox-Recyclable Extraction and Recovery of Heavy Metal Ions and

Chapter 10

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

Downloaded by UNIV OF MELBOURNE on March 29, 2016 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch010

Steven H. Strauss Department of Chemistry, Colorado State University, Fort Collins, CO 80523

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 . 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 TcO -, ReO -, Cs , Sr , Hg , Cd , Pb , 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 , 10 M TcO -, and 10 M TcO -, and greater than 99% of the extracted radioactivity was recovered in a solid secondary waste. Using MoS , greater than 99% of dissolved Hg was selectively extracted from an aqueous phase containing 1 M H N O and 10 M Hg . The final concentration of aqueous mercury was ≤0.033 μΜ in some cases (6.5 ppb), and greater than 94% of the extracted Hg 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. 2

99

4

137

+

90

2+

2+

2+

2+

+

4

3

-5

99

-10

4

95m

4

2+

2

-3

2+

3

2+

156

©1999 American Chemical Society

Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


157 Solvent extraction and ion-exchange chromatography are mature technologies (1-6). When they are used for the remediation of toxic and/or radioactive aqueous waste streams, however, minimization of the volume of secondary waste (e.g., the strip solution) destined for permanent disposal is still a significant challenge. This is especially problematic where hazardous but dilute waste streams are concerned. In general, the volume of secondary waste per mole of contaminant recovered is inversely proportional to the concentration of contaminant in the primary waste (7). The cost of permanent disposal is direcdy related to the volume of the secondary waste, and, in some solvent extraction and ion-exchange systems, the volume of the strip solution is equal to the volume of the primary waste. Furthermore, in recent years tougher environmental regulations and the high initial cost of new, more effective, and more selective extractants have made the reuse of the extractant an increasingly important issue. For these reasons, the reuse of extractant materials and the minimization of secondary waste volume must become the focus of scientific efforts in chemical separation in the near future. In summary, not only must a modern and effective extractant have (1) a large capacity and (2) a high selectivity for the target pollutant, it must also simultaneously satisfy two other design criteria: (3) it must allow for the recovery of the target species in a minimal volume of secondary waste and (4) it must be reuseable (recyclable). Design Criteria for Redox-Recyclable Extractants We are investigating and developing a relatively unexplored strategy in waste remediation, the use of redox-active transition-metal containing extractants for the separation and recovery of specific pollutant cations or anions (8-14). We have named the strategy, which is represented in Figure 1, Redox-Recyclable Extraction and Recovery (R ER). Our investigations are based on the seminal electrochemicalswitching work of Porter et al. (75), Martin et al. (16), Fabbrizzi et al. (17), 2

spent activator

aqueous waste

Λ

EXTRredox activator

decontaminated * waste EXTR

[EXTRijflON+l

spent deactivator redox and target ion deactivator in small volume Figure 1. The complete, repeatable cycle of extraction-deactivation/recoveryreactivation with a redox-recyclable extractant, EXTR.


158

Echegoyen, Gokel et al. (18), Shinkai et al. (79), and Beer et al. (20), but with an added emphasis on recovering the target pollutant in a minimal volume of secondary waste. These groups, as well as others, have shown that the binding of ions can be enhanced by electrochemically switching (or redox switching) an extractant molecule. However, when one considers practical factors such as duty-cycletime,extractant stability under harsh conditions, extractant effectiveness over many cycles, extractant cost, and secondary-waste volume, much work remains to be done before useful R ER schemes can be developed and reduced to practice. The key features of R ER are as follows. The neutral, deactivated extractant (EXTR) has little or no affinity for an ionic pollutant. When it is activated by either one-electron oxidation or reduction, it becomes cationic or anionic, respectively (EXTR*), and therefore develops an electrostatic affinity for an ion of opposite charge. If an EXTR salt dissolved in a water-immiscible phase is shaken with an aqueous phase containing an ionic pollutant, the pollutant may migrate to the extractant phase (i.e., the pollutant ion will undergo exchange with the original counterion of EXTR*). The degree of ion-exchange (pollutant migration) will depend on factors that normally control ion migration between phases, such as relative hydration free energies and shape selectivity. When the cationic or anionic extractant is subsequentiy deactivated by either reduction or oxidation, respectively, it becomes neutral again (EXTR) and loses the electrostatic affinity it formerly had for the ionic pollutant. Depending on the choice of redox deactivator, the ionic pollutant can be recovered in a relatively small volume of secondary waste. The design criteria for molecular redox-recyclable extractants are as follows. They should be transition-metal complexes that are very stable as neutral complexes and as one-electron oxidized or reduced cations or anions, respectively. Ideal complexes will be kinetically inert to substitution in both redox states. This criterion immediately suggests the use of polydentate ligands. The complexes should not contain acid- or base-labile functional groups. They should have redox potentials that allow the use of simple, inexpensive oxidants or reductants. They should undergo rapid one-electron oxidation or reduction, but should not undergo over-oxidation or over-reduction in the presence of an excess of oxidant or reductant. The complexes should be relatively nontoxic (e.g., iron complexes would be preferable to chromium complexes). In addition, the complexes should be relatively inexpensive (e.g., iron complexes would be preferable to ruthenium complexes). Finally, the complexes must have negligible water solubility in both working oxidation states, whether die process they will be used for is solvent extraction or stationary-phase physisorbed ion exchange. The design criteria for solid-state redox-recyclable extractants are similar. First, the solid-state layered or channeled extractants must be stable in contact with a wide variety of aqueous phases in both active (reduced) and inactive (oxidized) forms. Second, the solid-state extractants must have suitable electrochemical potentials in the presence of simple intercalant ions such as L i or Na , so that they can be shuttled between their active and inactive oxidation states using relatively mild and inexpensive reductants and oxidants. Third, solid-state extractant activation and deactivation redox reactions must be sufficiently rapid to allow for reasonably short duty times for complete extraction-deactivation/recovery-reactivation cycles. Fourth, the solid-state extractants must not dissolve in prolonged contact with the aqueous phase to be treated. 2


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+


159 They must not undergo any other form of decomposition, including irreversible over reduction or over-oxidation, during many cycles of extraction, deactivation/recovery, and reactivation. Radiolytic stability is also important for nuclear waste treatment. Finally, practical solid-state extractants should have low toxicity, low cost, and should be relatively easy to prepare and handle.


HEP. À Molecular Redox-Recyclable Extractant for Solvent Extraction and Ion-Exchange Chromatography Liquid-Liquid Extraction with HEP. Our most complete study of redoxrecyclable extraction and recovery to date involves Tc0 "~ and its non-radioactive surrogate Re0 ~ (8,9,12,13). (Work in our laboratory confirmed earlier reports that the solvent extraction behavior of Tc0 ~ and Re0 ~ are similar enough that Re0 ~ extraction experiments can be used to test the potential effectiveness of Tc0 ~ extractants (21).) The key component in our studies is the stable, lipophilic complex HEP, a tetraalkylated ferrocene (HEP = l,l,3,3-tetrakis(2-methyl-2-hexyl)ferrocene). 4

4

4

4

4

4

,

HEP

,

=

The cycle of extraction-deactivation/recovery-reactivation is given by the following three simplified chemical reactions (simplified in that HEP+TeO^org) is really a mix ture of ca. ΚΗ-ΙΟ" M HEP N0 -(org) and only ca. 10" M HEP Tc0 "(org) and that the solid secondary waste contains Fe(N03)3*xH 0 as well as Fe(Tc0 )3> (8,13): 3

+

5

+

3

4

2

4

+

HEP N0 -(org) + Tc0 -(aq waste) -> HEP+TcO^rg) + N0 ~-(aq waste) 3

4

HEPncO -(0r£) 4

3

+ Fe(.s) -> HEP(org) + Fe(Tc04)3(*) + TcO 0s) 2

+

3+

EEP(org) + Cd^(aq) + N 0 " ( ^ ) -> ΗΕΡ Ν0 ->Γ£) + Ce (o#) 3

3

Note that the middle reaction (deactivation/recovery) is not balanced. Some data for the extraction step are listed in Table I (for these experimental results, Tc0 ~ = T c 0 ~ and Tc0 ~). The figures of merit for these extractions are D(Tc0 ") and D(Re0 "), which were calculated using the following equation: 99

4

4

95m

4

4

[M0 -] 4

D(M0 ") 4

[M0 ~]

forg

=

4

-

ijaq

4

[M0 -] 4

f>aq

= [M0 "] 4

[M0 -]

f>aq

4

f)aq

1-

Note that the greater selectivity of HEP for Tc0 ~ and Re0 ~ relative to N0 ~ is the result of a natural bias based on the free energies of hydration of these anions (this has been discussed at length in an important review by Moyer and Bonneson (22)). In other words, this is an example of Hofmeister separation (23-26). Our new extractant HEP N0 ~ is more selective than Aliquat-336 N0 ~ for Re0 " relative to N0 ~ (14). 4

+

4

3

+

3

3

4


3

160 +

Table I. Extraction of Tc0 ~ and Re0 ~ Using H E P N 0 D(Re0 ") D(Tc0 ~) organic aqueous waste simulant phase 230(10) 430(20) toluene lMNaOH/1.5MNaN03 270(10) 120(10) lMNaOH/1.5MNaN03 0-C6H4CI2 lMNaOH/1.5MNaN0 2-nonanone 470(20) 60(10) 270(10) 190(10) toluene 1 M HNO3 210(10) 230(10) IMHNO3 2-nonanone 190(10) 180(10) IMHNO3 4

4

3

4

4

3

a

2

- 5

99

The aq. waste simulant contained either 10~ M KReC>4 or - Î O M L i T c 0 plus -10" M L i T c 0 . The organic phase contained 0.1 M [HEP][N0 ]. Estimated standard deviations are given in parentheses. D(M0 ~~) values are for a single contact of equal volumes of the two phases.


4

10

9 5 m

4

3

4

+

For the most part, however, the fact that HEP gives D(Tc0 ~) values well over 100 is not a significant breakthrough except to show that HEP is a lipophilic cation that is reasonably stable in the presence of aqueous 1 M NaOH and 1 M HNO3 and can be used for " T c 0 ~ extraction from relevant nuclear waste simulants. The real advantage of using HEP N03~ for T c 0 ~ remediation is that technetium-99 can be recovered as a solid of minimal volume. The recovery step consists of deactivating HEP*" by one-electron reduction to neutral HEP. Several bulk metals can serve as the reducing agent, including iron, which is inexpensive and non toxic. The plots in Figure 2 show the time course of reduction by metallic iron of H E P to HEP in three organic solvents with the concomitant formation and precipitation of Fe(Tc0 ) -3H 0 and Tc0 (s) (8,13). Note that H E P N 0 " and HEP Tc0 ~ dissolved in toluene were converted to HEP and Tc-containing solids in only 10-20 minutes. 4

+

4

+

9 9

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4

3

2

2

3

+

4

contact time with Fe powder (min.) Figure 2. Time course of reactions of iron powder with ~0.01 Μ [ΗΕΡ ][Νθ3 ], -ΙΟ" M [HEP ][ Tc0 -], and - Î O " M [HEP ][ Tc0 -] dissolved in watersaturated toluene, 2-nonanone, or 1,2-dichlorobenzene. The γ activity of T c in the organic phase and/or the solid phase was monitored. +

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9 5 m


161 The following analysis shows the extent of the secondary-waste volume reduction that may be possible. We will make the following assumptions: (1) the concentration of TeC>4~ in an aqueous waste stream of concern is 5 χ 10~ M; (2) the recovered solid is primarily Fe(N0 ) -9H 0 (density = 1.68 g cm" ); (3) HEP N0 ~ dissolved in toluene is the extractant; (4) 100 L of aqueous waste is treated. Therefore: for one extraction-deactivation-reactivation cycle with 0.1 Μ HEP N0 ~, Dtotal(Tc04~) = 430 (8), the mole ratio of N0 -/Tc0 - in the precipitate ([NO^/TcCVls) = 2,000, and the volume of the solid precipitate (V ) = 0.79 L; for two R ER cycles with 0.01 M H E P N 0 - , D (Tc0 -) ~ 2,100 (46 χ 46) (13), [N0 -/Tc0 -] = 400, and V = 0.16 L; for four R%R cycles with 0.001 Μ HEP N0 ", D (Te0 ~) ~ 256 (4 χ 4 χ 4x4) (13), [N0 -/Tc0 -] = 80, and V = 0.032 L, a volume reduction factor of 3,000. Although the aqueous waste simulant we used for these experiments contained only 1.0 M NaOH and 1.5 NaN0 , this analysis is relevant because prior experiments with Aliquat-336 N0 " have shown that D(Tc04~) is not affected by more than a factor of two by the addition of 0.86 M NaN0 , 0.49 M NaAl(OH) ,0.39 M Na C0 , 0.11 M Na P0 , 0.093 M NaCl, 0.073 M Na (citrate), 0.031 M Na S0 , and 0.002 M Ca(N0 ) to a waste simulant containing 1.0 M NaOH and 1.5 M NaN0 (21). 99

5

3

3

3

+

2

3

+

3

3

4

2

s

+

3

tot2a

4

3

4

s

s

+


3

3

4

s

total

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s

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Ion-Exchange Chromatography with Physisorbed H E P N 0 " . We have also prepared R ER ion-exchange "resins" from HEP N0 ~ physisorbed onto different materials ranging from silica gels to organic polymers (9,12). A complete R ER cycle for HEP N0 " loaded onto 100 Â pore size S i 0 (surface area = 300 m g" ) is shown in Figure 3. In this case, K (Re0 ~) from 1 Μ H N 0 containing 0.32 mM KRe0 was found to be approximately 100 mL g" (9), which can be compared with 289 mL g for Reillex-HPQ (27). Although it seems as if K for our new material is lower than for Reillex-HPQ, we have been limited so far by how much HEP N0 ~ can be loaded onto the resins. When Kd(Re04~) is expressed in mL per mmol cation exchange group, the values are 450 and 87 for HEP N0 "/Si0 and Reillex-HPQ, respectively, demonstrating that the redox-recyclable ion-exchange material H E P N 0 ~ / S i 0 is at least four times more selective than Reillex-HPQ for extraction of Re04~ from 3

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1

2

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- 1

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+

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+

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ΗΕΡ+Ν0 ~

HEP [/on S i 0

+

HEP Re0 "

3

$|/onSi02

4

î / ο η SiQ

-N0 -(aq) 3

ΗΕΡ+ΝΚν 2

2

m +Re(>4 (aq)

2

j/on S i û

2

3+

+Fe (a#), 4

+Fe(CN) -(og) 6

-Re0 -(a#),

+NQ -(ag) 3

2+

-Fe (a?)

4

3

-Fe(CN) -(a ) 6

€

τ direction I of flow

Figure 3. R ER cycle for Re0 ~ extraction and recovery using HEP N0 "/Si0 . 2

+

4


3

2

162 1 M HNO3. Furthermore, our Re04~-loaded material can be recycled by passing a 25 mM aqueous solution of FeiCNJg " through the column, which reduces physisorbed HEP to physisorbed HEP and strips all of the bound ReC>4~ in only 10 minutes (9). We found that decreased by about 5% per complete cycle over five cycles (9). The decrease in K could be due to desorption of physisorbed extractant and/or decomposition of the physisorbed exctractant. Improving the hydrophobicity and stability of R ER extractants over many cycles are important goals of our ongoing research. 4

+

d

2


Other Molecular Redox-Recyclable Extractants for Solvent Extraction Fe(Tp')2« We have investigated substituted bis(hydridotris(pyrazolylborate)) iron complexes, abbreviated Fe(Tp')2, as potential R ER extractants for anions such as Re04" and " T C O 4 - (14). In one series of experiments, the two aqueous waste simulants contained 0.010 M KRe0 and either 3 M HNO3 or 3 M NaN0 . The two organic phases examined contained 0.05 M [Fe(Tp)2][N03] or 0.05 M Aliquat336 N0 ~ dissolved in dichloromethane. The D(Re0 ") values for [Fe(Tp) ][N0 ] were 0.89(1) for 3 M H N 0 and 7.8(1) for 3 M NaN0 . The corresponding values for Aliquat-336 N03~ were 0.77(1) and 4.4(1), respectively. Preliminary results indicated that greater than 88% of the extracted Re04~ could be recovered in a solid precipitate from the [Fe(Tp)2][Re04]-containing dichloromethane phase using iron powder as the deactivating reagent (14). 2

4

3

+

3

4

3

2

3

3

+

Fe(Cp)(Dc*) Fe(Cp)(Dc'). We have also synthesized organometallic extractants that cycle between their uncharged and anionic forms and can extract ^Sr "** and C s from a nuclear waste simulant containing 1 M NaOH and 1.5 M NaM>3 (11). These are complexes of iron containing a cyclopentadienyl ligand (Cp) and a substituted dicarbollide ligand (Dc'). When a 0.05 M toluene solution of the salt [Na][Fe(Cp)(Dc')] was used, D(Sr ) and D(Cs ) were 2 and 23, respectively (R = C12H25) (11)- These values are higher than for the recently developed and very promising extractant sodium bis(dicarbollide)cobalt(l-) (28). These results are especially important because Fe(Cp)(Dc')~ complexes, like HEP and Fe(Tp')2 , are redox-recyclable, whereas Co(Dc)2~ is not redox-recyclable (the Ej/2 value for the Fe(Cp)(Dc') ~ couple is 0.17 V vs. SCE). We are currently developing complete R ER cycles using Fe(Cp)(Dc')~ extractants to recover ^Sr " " and C s in a minimal volume of secondary waste. 2

2+

1 3 7

+

+

+

0/1

2

+

2

1

1 3 7

+


163 M0S2.

A Redox-Recyclable Material for Heavy-Metal Ion-Exchange

2

2 +

R E R of Aqueous H g Using M0S2. We have found that lithium-intercalated metal chalcogenides such as L i M o S 2 * L i W S 2 will rapidly undergo cationexchange with H g in aqueous solution (0.25 < χ < 1.9) (10). The capacity of Li MoS2 ^B 580 mg mercury per gram of extractant. Most importantly, greater than 94% of the ion-exchanged mercury in Hg MoS2 was efficiently recovered as metallic mercury in a cold trap when Hg MoS2 heated under vacuum at 425°C. Thus, M0S2 and WS2 represent redox-recyclable extractants for aqueous H g . Note that metallic mercury, which represents the secondary waste recovered, is only 0.015% of the volume of the primary waste simulant, 10 mM aqueous H g . Furthermore, the molybdenum-containing material recovered after the heat-induced recovery step was essentially M0S2, which was reactivated (recycled) to Li MoS2 with «-BuLi in hexanes for subsequent R ER cycles. Extraction and recovery data for three sucessive cycles with the same sample of M0S2 are listed in Table II, which also includes a scheme for the complete cycle (10). anc

x

x

2 +

w a s a s

x

n

n

a s

y

w

a

s

y

2+


2+

x

2

Table II. Use of a Sample of Li MoS2 for Three Consecutive Mercury Extraction-Deactivation/Recovery-Reactivation Cycles x

a

2+

Hg (^) Li MoS x

2

0.1MHNO (o#)

Hg MoS y

heat, vacuum 1 » Hg MoS -Hg°

2

z

3

2

w-BuLi cycle

b

: value

c

y value

d

: value

/2-BuLi titration

Li/Mo ratio from digestion

digestion

filtrate

1.4 1.7 2.0

1.3 1.6 1.8

0.35 0.39 0.50

0.32 0.50 0.50

0.02

Redox-Recyclable Extraction and Recovery of Heavy Metal Ions and

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