Removal of CrO₄^2–, a Nonradioactive Surrogate of ⁹⁹TcO₄^–, Using LDH–Mo₃S₁₃ Nanosheets

Abstract

Removal of chromate (CrO4 2–) and pertechnetate (TcO4 –) from the Hanford Low Activity Waste (LAW) is beneficial as it impacts the cost, life cycle, operational complexity of the Waste Treatment and Immobilization Plant (WTP), and integrity of vitrified glass for nuclear waste disposal. Here, we report the application of [MoIV 3S13]2– intercalated layer double hydroxides (LDH–Mo3S13) for the removal of CrO4 2– as a surrogate for TcO4 –, from ppm to ppb levels from water and a simulated LAW off-gas condensate of Hanford’s WTP. LDH–Mo3S13 removes CrO4 2– from the LAW condensate stream, having a pH of 7.5, from ppm (∼9.086 × 104 ppb of Cr6+) to below 1 ppb levels with distribution constant (K d) values of up to ∼107 mL/g. Analysis of postadsorbed solids indicates that CrO4 2– removal mainly proceeds by reduction of Cr6+ to Cr3+. This study sets the first example of a metal sulfide intercalated LDH for the removal of CrO4 2–, as relevant to TcO4 –, from the simulated off-gas condensate streams of Hanford’s LAW melter which contains highly concentrated competitive anions, namely F–, Cl–, CO3 2–, NO3 –, BO3 3–, NO2 –, SO4 2–, and B4O7 2–. LDH–Mo3S13’s remarkable removal efficiency makes it a promising sorbent to remediate CrO4 2–/TcO4 – from surface water and an off-gas condensate of nuclear waste.

Unveiling the Potential of [Sn2S6]4– Functionalized Layered Double Hydroxides for the Sorption of ReO4– as a Surrogate for 99TcO4–

“…23 Our investigations demonstrated that the removal of CrO 4 2− , a redox surrogate of TcO 4 − , occurs mainly through reductive precipitation where cooperative oxidation of S 2 2− and Mo 4+ infers the reductive precipitation of soluble Cr 6+ to insoluble Cr 3+ . 16 This intriguing finding requires further investigation of diverse metal sulfide anion intercalated LDHs and analysis of their sorption mechanisms under different experimental conditions.…”

“…LDH can be defined by the general formula [M II (1– x ) M III x (OH) 2 ] x + [(A n – ) x / n · m H 2 O] x − , where M II and M III correspond to divalent and trivalent cations and A n – is an anion. − In LDH, the positively charged metal hydroxide layers are counterbalanced with exchangeable anions that inhabit the interlayer spacings. , Because of such distinct structural features, LDHs adopt diverse sorption mechanisms for TcO 4 – and its oxoanionic surrogates, such as adsorption on the outer surface via M 2+ /M 3+ –O–H···O 4 –Tc interactions, ion-exchange-driven trapping in between the positively charged host LDH layers, and reconstruction of the LDH structures . Furthermore, metal sulfides/polysulfides are known to remove TcO 4 – from the solution by reductive precipitation (Tc 7+ (sol) → Tc 4+ (s) ) and surface sorption through S···TcO 4 covalent interactions. − These features, along with the LDH’s intrinsic sorption properties toward TcO 4 – , appealed to us to design and fabricate an LDH with metal sulfide anions to produce LDH-Mo 3 S 13 . It should be noted that structurally Mo 3 S 13 2– is presented as [Mo IV 3 S­(S 2 ) 6 ] 2– , where the trinuclear cluster of Mo 4+ ions are surrounded by six disulfide (S 2 2– ) groups and an epical S 2– anion .…”

“…In recent years, metal–organic frameworks (MOFs), specifically cationic MOFs, have appealed to attention because of their efficient uptake of TcO 4 – from defense legacy nuclear waste, but their complex synthetic procedures, scalability, reusability, and high cost hinder their practical application. − On the other hand, layered double hydroxides (LDHs), a hydrotalcite type clay, are attractive because of their low cost, scalability, and efficient sorption properties for TcO 4 – . LDH can be defined by the general formula [M II (1– x ) M III x (OH) 2 ] x + [(A n – ) x / n · m H 2 O] x − , where M II and M III correspond to divalent and trivalent cations and A n – is an anion. − In LDH, the positively charged metal hydroxide layers are counterbalanced with exchangeable anions that inhabit the interlayer spacings. , Because of such distinct structural features, LDHs adopt diverse sorption mechanisms for TcO 4 – and its oxoanionic surrogates, such as adsorption on the outer surface via M 2+ /M 3+ –O–H···O 4 –Tc interactions, ion-exchange-driven trapping in between the positively charged host LDH layers, and reconstruction of the LDH structures . Furthermore, metal sulfides/polysulfides are known to remove TcO 4 – from the solution by reductive precipitation (Tc 7+ (sol) → Tc 4+ (s) ) and surface sorption through S···TcO 4 covalent interactions. − These features, along with the LDH’s intrinsic sorption properties toward TcO 4 – , appealed to us to design and fabricate an LDH with metal sulfide anions to produce LDH-Mo 3 S 13 .…”

“…For the LDH-Mo 3 S 13 , chromate separation is predominantly accomplished by the reduction of soluble Cr(VI) to insoluble Cr(III) species. 16 Hence, such a distinction between the LDH-Mo 3 S 13 and LDH-Sn…”

Efficient and Superstable Mineralization of Toxic Cd2+ Ions through Defect Engineering in Layered Double Hydroxide Nanosheets

“…As a clay-like material, layered double hydroxides (LDHs) have been extensively researched in recent years for heavy metal removal. − LDHs typically have the general formula [(M 2+ ) 1– x (M 3+ ) x (OH) 2 ] x + (A m – ) x / m · n H 2 O] and a layered structure consisting of M 2+ and M 3+ cations octahedrally coordinated by oxygen in positively charged sheets, and anions (A m – ) and water in the interlayer region between the sheets. The LDH structure can accommodate a wide range of divalent and trivalent cations; hence through isomorphous substitution processes, LDHs can offer excellent removal performance for certain heavy metal ions (like Cd 2+ , Ni 2+ , and Cu 2+ ions) leading to superstable mineralization structures (SSMS). − Recently, we reported that CaAl-LDH could be used as a mineralizer to selectively remove Cd 2+ ions from water with a capacity of 592 mg/g via isomorphous substitution to form a SSMS CdAl-LDH .…”