Water Defect Stabilizes the Bi3+ Lone-Pair Electronic State Leading to an Unusual Aqueous Hydration Structure.

The aqueous hydration structure of the Bi3+ ion is probed using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) simulations of ion-water clusters and condensed-phase solutions. Anomalous features in the EXAFS spectra are found to be associated with a highly asymmetric first-solvent water shell. The aqueous chemistry and structure of the Bi3+ ion are dramatically controlled by the water stabilization of a lone-pair electronic state involving the mixed 6s and 6p orbitals. This leads to a distinct multimodal distribution of water molecules in the first shell that are separated by about 0.2 Å. The lone-pair structure is stabilized by a collective response of multiple waters that are localized near the lone-pair anti-bonding site. The findings indicate that the lone-pair stereochemistry of aqueous Bi3+ ions plays a major role in the binding of water and ligands in aqueous solutions.

Identification and Quantification of Technetium Species in Hanford Waste Tank AN-102.

Technetium-99 (Tc), a high yield fission product generated in nuclear reactors, is one of the most difficult contaminants to address at the U.S. Department of Energy Hanford, Savannah River, and other sites. In strongly alkaline solutions typifying Hanford tank waste, Tc exists as pertechnetate (TcO4-) (oxidation state VII) as well as in reduced forms (oxidation state < VII), collectively known as non-pertechnetate (non-TcO4-) species. Designing strategies for effective Tc management, including separation and immobilization, necessitates understanding the molecular structure of the non-TcO4- species and their identification in actual tank waste samples. Identification of non-TcO4- species would facilitate the development of new treatment technologies effective for dissimilar Tc species. Toward this objective, a spectroscopic library of the Tc(I) [fac-Tc(CO)3]+ and Tc(II, IV, V, VII) compounds was generated and applied to the characterization of the actual Hanford AN-102 tank waste supernatant, which was processed to adjust Na concentration to ∼5.6 M and remove 137Cs by spherical resorcinol-formaldehyde (sRF) ion-exchange resin. Post 137Cs removal, the cesium-loaded sRF column was eluted with 0.45 M HNO3. As-received AN-102, Cs-depleted effluent, and sRF eluate fractions were comprehensively characterized for chemical composition and speciation of Tc using 99Tc nuclear magnetic resonance spectroscopy and X-ray absorption spectroscopy. It was demonstrated for the first time that non-TcO4- Tc present in the AN-102 tank waste is composed of several low-valent Tc species, including the Tc(I) [fac-Tc(CO)3]+ and Tc(IV) compounds. This is the first demonstration of multiple non-TcO4- species co-existing in the Hanford tank waste, highlighting their importance for the waste processing.

Mechanisms of Plutonium Redox Reactions in Nitric Acid Solutions.

Plutonium (Pu) exhibits a complex redox behavior in aqueous solutions. This is due to the ability of the element to adapt a wide range of oxidation states typically from +3 to +6 and the tendency for dynamic interconversion between the oxidation states that primarily depend upon acid concentration and presence of coordinating ligands. This work interrogates the Pu redox behavior in aqueous nitric acid via a combination of voltammetry and in situ vis-NIR spectroelectrochemistry under controlled potentials to map the interconversion between the various Pu oxidation states. The NIR-spectroelectrochemistry studies used to complement the visible spectroscopy bring a new and more complete perspective into the plutonium redox transformations. This allows elucidation of the mechanisms of the involved redox reactions facilitating an in-depth understanding of the relative stability of the Pu oxidation states as a function of redox potentials and nitric acid concentrations. It is observed that oxidation of Pu(III) results in generation of Pu(IV) and Pu(VI) (the latter as PuO22+), bypassing the Pu(V) oxidation state. Further, with increasing acid concentrations, the formation of the Pu(VI) species progressively decreases so that the dynamic equilibrium between the Pu(III) and Pu(IV) oxidation states dominates. These findings have significant implications for developing separation processes for used nuclear fuel reprocessing and treatment.

Evolution of Acid-Dependent Am3+ and Eu3+ Organic Coordination Environment: Effects on the Extraction Efficiency.

Coordination of trivalent lanthanide and actinide metal ions by lipophilic diglycolamides and phosphonic acids has been proposed for their separation through extraction from aqueous nitric acid solutions. However, the nature of M3+ coordination complexes in these combined solvent systems is not well understood, resulting in low predictability of their behavior. This work demonstrates that a combination of N,N,N',N'-tetrakis(2-ethylhexyl)diglycolamide (T2EHDGA) and weakly acidic 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) in n-dodecane exhibits a complicated extraction mechanism for Eu3+ and Am3+, which continuously evolves as a function of the aqueous phase acidity. At low aqueous phase nitric acid concentrations, M3+ ions are primarily extracted via exchange of the phosphonic acid proton and coordination with HEH[EHP]. At high aqueous phase nitric acid concentrations, HEH[EHP] remains protonated, and M3+ ions are transported to the organic phase by the coextraction of nitrate anions from the aqueous phase, thus forming complex species with T2EHDGA. At moderate acid regimes, both ligands participate in the coordination of M3+ ions and show a synergistic relationship resulting in considerable enhancement of M3+ transport into the combined solvent system over the simple sum of the individual extractants. The observed synergism is caused by differences in organic phase M3+ speciation and has a significant impact on the performance of the organic solvent. Distribution studies with Eu3+ indicate that nominally two or three T2EHDGA ligands participate in metal extraction in the presence of phosphonic acid, while nominally three diglycolamide ligands participate in the presence or absence of phosphonic acid. While synergistic behavior has been observed in many solvent-extraction processes, this system demonstrates a clear correlation between the continuously changing organic speciation of M3+ and its transport into the organic solvent. This paper reports the spectroscopic characterization of the organic phase M3+ species by IR, X-ray absorption, and visible spectroscopies. Spectroscopic evidence indicates a mixed-ligand complex, i.e., a ternary complex at the moderate acid regime, where the greatest degree of synergism is observed. Differences in synergistic extraction of Am3+ and Eu3+ at the low acid regime were observed, indicating their dissimilar extraction behavior.

Spectroscopic Characterization of Aqua [ fac-Tc(CO)3]+ Complexes at High Ionic Strength.

Understanding fundamental Tc chemistry is important to both the remediation of nuclear waste and the reprocessing of nuclear fuel; however, current knowledge of the electronic structure and spectral signatures of low-valent Tc compounds significantly lags behind the remainder of the d-block elements. In particular, identification and treatment of Tc speciation in legacy nuclear waste is challenging due to the lack of reference data especially for Tc compounds in the less common oxidation states (I-VI). In an effort to establish a spectroscopic library corresponding to the relevant conditions of extremely high ionic strength typical for the legacy nuclear waste, compounds with the general formula of [ fac-Tc(CO)3(OH2)3- n(OH) n]1- n (where n = 0-3) were examined by a range of spectroscopic techniques including 99Tc/13C NMR, IR, XPS, and XAS. In the series of monomeric aqua species, stepwise hydrolysis results in the increase of the Tc metal center electron density and corresponding progressive decrease of the Tc-C bond distances, Tc electron binding energies, and carbonyl stretching frequencies in the order [ fac-Tc(CO)3(OH2)3]+ > [ fac-Tc(CO)3(OH2)2(OH)] > [ fac-Tc(CO)3(OH2)(OH)2]-. These results correlate with established trends of the 99Tc upfield chemical shift and carbonyl 13C downfield chemical shift. The lone exception is [ fac-Tc(CO)3(OH)]4 which exhibits a comparatively low electron density at the metal center attributed to the µ3-bridging nature of the -OH ligands causing less σ-donation and no π-donation. This work also reports the first observations of these compounds by XPS and [ fac-Tc(CO)3Cl3]2- by XAS. The unique and distinguishable spectral features of the aqua [ fac-Tc(CO)3]+ complexes lay the foundation for their identification in the complex aqueous matrixes.

Theoretical Modeling of (99)Tc NMR Chemical Shifts.

Technetium-99 (Tc) displays a rich chemistry due to its wide range of accessible oxidation states (from -I to +VII) and ability to form coordination compounds. Determination of Tc speciation in complex mixtures is a major challenge, and (99)Tc nuclear magnetic resonance (NMR) spectroscopy is widely used to probe chemical environments of Tc in odd oxidation states. However, interpretation of (99)Tc NMR data is hindered by the lack of reference compounds. Density functional theory (DFT) calculations can help to fill this gap, but to date few computational studies have focused on (99)Tc NMR of compounds and complexes. This work evaluates the effectiveness of both pure generalized gradient approximation and their corresponding hybrid functionals, both with and without the inclusion of scalar relativistic effects, to model the (99)Tc NMR spectra of Tc(I) carbonyl compounds. With the exception of BLYP, which performed exceptionally well overall, hybrid functionals with inclusion of scalar relativistic effects are found to be necessary to accurately calculate (99)Tc NMR spectra. The computational method developed was used to tentatively assign an experimentally observed (99)Tc NMR peak at -1204 ppm to fac-Tc(CO)3(OH)3(2-). This study examines the effectiveness of DFT computations for interpretation of the (99)Tc NMR spectra of Tc(I) coordination compounds in high salt alkaline solutions.

Development of Online Spectroscopic pH Monitoring for Nuclear Fuel Reprocessing Plants: Weak Acid Schemes.

In nuclear fuel reprocessing, separating trivalent minor actinides and lanthanide fission products is extremely challenging and often necessitates tight pH control in TALSPEAK (Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes) separations. In TALSPEAK and similar advanced processes, aqueous pH is one of the most important factors governing the partitioning of lanthanides and actinides between an aqueous phase containing a polyaminopolycarboxylate complexing agent and a weak carboxylic acid buffer and an organic phase containing an acidic organophosphorus extractant. Real-time pH monitoring would significantly increase confidence in the separation performance. Our research is focused on developing a general method for online determination of the pH of aqueous solutions through chemometric analysis of Raman spectra. Spectroscopic process-monitoring capabilities, incorporated in a counter-current centrifugal contactor bank, provide a pathway for online, real-time measurement of solution pH. The spectroscopic techniques are process-friendly and can be easily configured for online applications, whereas classic potentiometric pH measurements require frequent calibration/maintenance and have poor long-term stability in aggressive chemical and radiation environments. Raman spectroscopy discriminates between the protonated and deprotonated forms of the carboxylic acid buffer, and the chemometric processing of the Raman spectral data with PLS (partial least-squares) regression provides a means to quantify their respective abundances and therefore determine the solution pH. Interpretive quantitative models have been developed and validated under a range of chemical composition and pH conditions using a lactic acid/lactate buffer system. The developed model was applied to new spectra obtained from online spectral measurements during a solvent extraction experiment using a counter-current centrifugal contactor bank. The model predicted the pH of this validation data set within 11% for pH > 2, thus demonstrating that this technique could provide the capability of monitoring pH online in applications such as nuclear fuel reprocessing.

Highly Selective Colorimetric and Luminescence Response of a Square-Planar Platinum(II) Terpyridyl Complex to Aqueous TcO4(-).

Molecular recognition of an aqueous pertechnetate (TcO4(-)) anion is fundamentally challenging partly due to the charge-diffuse nature of this anion, which hampers design of new technologies for its separation and detection. To address this gap, simple salts of transition metal complexes that undergo a distinct spectroscopic change upon exposure to aqueous anions were explored. The Pt(II) complex [Pt(tpy)Br]SbF6 (tpy = 2,2';6',2″-terpyridine) undergoes a dramatic color change and intense luminescence response upon TcO4(-) uptake due to concomitant enhancement of Pt···Pt interactions. The spectroscopic response was highly selective and quantitative for aqueous TcO4(-) among other competing anions. Complementary Raman spectroscopy and microscopy techniques, structural determination, and theoretical methods were employed to elucidate the mechanism of this response at the molecular level.

Water O-H stretching Raman signature for strong acid monitoring via multivariate analysis.

A distinct need exists for real time information on an acid concentration of industrial aqueous streams. Acid strength affects efficiency and selectivity of many separation processes, including nuclear fuel reprocessing. Despite the seeming simplicity of the problem, no practical solution has been offered yet, particularly for the large-scale schemes involving toxic streams such as highly radioactive nuclear wastes. The classic potentiometric technique is not amiable for online measurements due to the requirements of frequent calibration/maintenance and poor long-term stability in aggressive chemical and radiation environments. Therefore, an alternative analytical method is needed. In this work, the potential of using Raman spectroscopic measurements for online monitoring of strong acid concentration in solutions relevant to dissolved used nuclear fuel was investigated. The Raman water signature was monitored for solution systems containing nitric and hydrochloric acids and their sodium salts of systematically varied composition, ionic strength, and temperature. The trivalent neodymium ion simulated the presence of multivalent f metals. The gaussian deconvolution analysis was used to interpret observed effects of the solution nature on the Raman water O-H stretching spectrum. The generated Raman spectroscopic database was used to develop predictive multivariate regression models for the quantification of the acid and other solution components, as well as selected physicochemical properties. This method was validated using independent experiments conducted in a flow solvent extraction system.

Electrochemistry and Spectroelectrochemistry of europium(III) chloride in 3LiCl-2KCl from 643 to 1123 K.

The electrochemical and spectroelectrochemical behavior of europium(III) chloride in a molten salt eutectic, 3LiCl-2KCl, over a temperature range of 643-1123 K using differential pulse voltammetry, cyclic voltammetry, potential step chronoabsorptometry, and thin-layer spectroelectrochemistry is reported. The electrochemical reaction was determined to be the one-electron reduction of Eu(3+) to Eu(2+) at all temperatures. The redox potential of Eu(3+/2+) shifts to more positive potentials, and the diffusion coefficient for Eu(3+) increases as temperature increases. The results for the number of electrons transferred, redox potential, and diffusion coefficient are in good agreement between the electrochemical and spectroelectrochemical techniques. This research extends our ability to develop a spectroelectrochemical sensor for lanthanides and actinides into molten salt media.

Reductive removal of pertechnetate and chromate by zero valent iron under variable ionic strength conditions.

Radioactive technetium-99 (Tc) present in waste streams and subsurface plumes at legacy nuclear reprocessing sites worldwide poses potential risks to human health and environment. This research comparatively evaluated efficiency of zero-valent iron (ZVI) toward reductive removal of Tc(VII) in presence of Cr(VI) from NaCl and Na2SO4 electrolyte solutions under ambient atmospheric conditions. In both electrolytes, anticorrosive Cr(VI) suppressed oxidation of ZVI at elevated concentrations resulting in the delay of initiation of Tc(VII) reduction to Tc(IV). In the absence of Cr(VI), no delay was observed in the analogous systems. At low ionic strength (IS), retarded ZVI oxidation inhibited Tc(VII) reduction. Higher IS favored reduction of both Tc(VII) and Cr(VI), which followed second-order reaction rates in both electrolytes attributed to the more efficient iron oxidation as evident from solids characterization studies. Magnetite was the primary iron oxide phase, and its higher fraction in the SO42- solutions facilitated reductive removal of Tc(VII) and Cr(VI). In the Cl- matrix, Cr(VI) promoted further oxidation of magnetite as well as formation of chromite diminishing overall reductive capacity of this system and resulting in less effective removal of Tc(VII) compared to the SO42- solutions.

Accumulation mechanisms for contaminants on weak-base hybrid ion exchange resins.

Mechanism of hexavalent chromium removal (Cr(VI) as CrO42-) by the weak-base ion exchange (IX) resin ResinTech® SIR-700-HP (SIR-700) from simulated groundwater is assessed in the presence of radioactive contaminants iodine-129 (as IO3-), uranium (U as uranyl UO22+), and technetium-99 (as TcO4-), and common environmental anions sulfate (SO42-) and chloride (Cl-). Batch tests using the acid sulfate form of SIR-700 demonstrated Cr(VI) and U(VI) removal exceeded 97%, except in the presence of high SO42- concentrations (536 mg/L) where Cr(VI) and U(VI) removal decreased to ≥ 80%. However, Cr(VI) removal notably improved with co-mingled U(VI) that complexes with SO42- at the protonated amine sites. These U-SO42- complexes are integral to U(VI) removal, as confirmed by the decrease in U(VI) removal (<40%) when the acid chloride form of SIR-700 was used instead. Solid phase characterization revealed that CrO42- is removed by IX with SO42- complexes and/or reduced to amorphous Cr(III)(OH)3 at secondary alcohol sites. Tc(VII)O4- and I(V)O3- also undergo chemical reduction, following a similar removal mechanism. Oxyanion removal preference is determined by the anion reduction potential (CrO42->TcO4->IO3-), geometry, and charge density. For these reasons, 39% and 69% of TcO4- and 17% and 39% of IO3- are removed in the presence and absence of Cr(VI), respectively.

Separating and stabilizing phosphate from high-level radioactive waste: process development and spectroscopic monitoring.

Removing phosphate from alkaline high-level waste sludges at the Department of Energy's Hanford Site in Washington State is necessary to increase the waste loading in the borosilicate glass waste form that will be used to immobilize the highly radioactive fraction of these wastes. We are developing a process which first leaches phosphate from the high-level waste solids with aqueous sodium hydroxide, and then isolates the phosphate by precipitation with calcium oxide. Tests with actual tank waste confirmed that this process is an effective method of phosphate removal from the sludge and offers an additional option for managing the phosphorus in the Hanford tank waste solids. The presence of vibrationally active species, such as nitrate and phosphate ions, in the tank waste processing streams makes the phosphate removal process an ideal candidate for monitoring by Raman or infrared spectroscopic means. As a proof-of-principle demonstration, Raman and Fourier transform infrared (FTIR) spectra were acquired for all phases during a test of the process with actual tank waste. Quantitative determination of phosphate, nitrate, and sulfate in the liquid phases was achieved by Raman spectroscopy, demonstrating the applicability of Raman spectroscopy for the monitoring of these species in the tank waste process streams.

Removable coatings: Thermal stability and decontamination of steel surfaces from 241Am.

This study presents a comparative analysis of several commercial removable materials for radioactive decontamination of steel surfaces using 241Am as representative radionuclide. The selection criteria of removable coatings for this study included a history of application, commercial availability, easy handling conditions and different composition and formulation. Carbon steel and stainless steel coupons were utilized as common industrial materials, and the experimental series were expanded to include the rusting treatment of these surfaces as it is common for decommissioned nuclear facilities. Radionuclide 241Am was deposited on the coupon surfaces and used to evaluate decontamination efficiency of the removable coatings, which were pre-screened for the ease of application and removal from the surface. Selected coatings were characterized with Fourier-transform infrared spectroscopy and thermogravimetric analysis, decontamination efficiencies for different types of steel surfaces, and potential enhancement of the removal efficiencies of the select removable coatings via amendment with EDTA. Across all the coatings, decontamination efficiencies for stainless steel (both pristine and with oxidizing treatment) were higher than for pristine carbon steel, which in turn were higher than for rusted carbon steel. Amendment with EDTA improved removal efficiency of a removable coating. CC Strip coating exhibited easy handling and high decontamination efficiency, (up to 97% when EDTA-amended), but its drying time was the longest, and thermal analysis indicated higher release of energy during thermal decomposition compared to the other coatings. Hydrogel-based DeconGel coating, even though not the easiest in handling among the rest of materials, exhibited high decontamination efficiency, efficient drying at the ambient temperature leading to the loss of about 80 wt% due to solvent evaporation, and extremely low heat released during thermal decomposition; therefore, it is considered a preferable choice for the considered factors.

Time dependent zero valent iron oxidation and the reductive removal of pertechnetate at variable pH.

Elemental iron Fe0 is a promising reductant for removal of radioactive technetium-99 (Tc) from complex aqueous waste streams that contain sulfate, halides, and other inorganic anions generated during processing of legacy radioactive waste. The impact of sulfate on the kinetics of oxidation and reduction capacity of Fe0 in the presence of Tc has not been examined. We investigated the oxidative transformation of Fe0 and reductive removal of TcO4- in 0.1 M Na2SO4 as a function of initial pH (i.e., pHi 4, 7, and 10) under aerobic conditions up to 30 days. Tc reduction was the fastest at pHi 7 and slowest at pHi 10 (Tc reduction rate pHi 7 > 4 > 10). Aqueous fraction of Tc was measured at 0.4% at pHi 7 within 6 h, whereas ≥ 97% of Tc was removed from solutions at pHi of 4 and 10 within 24 h. Solid phase characterization showed that magnetite was the only oxidized crystalline phase for the first 6 h regardless of initial pH. Lepidocrocite was the most abundant oxidized product for pHi 10 after 5 days, but was not observed at pH of 4 or 7.

Simultaneous immobilization of aqueous co-contaminants using a bismuth layered material.

The remediation of co-located contaminants in the vadose zone can be challenging due to accessibility and responses of different contaminants to remedial actions. At the Hanford Site (WA, USA), multiple radionuclides and other hazardous contaminants are present in the vadose zone and groundwater, including iodine-129 (I), technetium-99 (Tc), uranium-238 (U), chromium (Cr), and nitrate (NO3-). We evaluated a layered Bi oxyhydroxide material for its potential to remove individual and co-located contaminants with a series of batch experiments that investigated a range of plume conditions, followed by solid phase characterization of the reacted bismuth material. The results demonstrated successful removal of four contaminants (>98% removal of I, Tc, U, and Cr from the aqueous phase after 30 days) when tested individually. When contaminants were combined, a slight decrease in Tc removal occurred (-6%p). The addition of sediment decreased the removal for Tc and I, but U and Cr removal was unaffected. The results of these batch tests demonstrated that the bismuth based oxy-hydroxide material is a promising material for sequestering multiple contaminants in situ.

Spontaneous redox continuum reveals sequestered technetium clusters and retarded mineral transformation of iron.

The sequestration of metal ions into the crystal structure of minerals is common in nature. To date, the incorporation of technetium(IV) into iron minerals has been studied predominantly for systems under carefully controlled anaerobic conditions. Mechanisms of the transformation of iron phases leading to incorporation of technetium(IV) under aerobic conditions remain poorly understood. Here we investigate granular metallic iron for reductive sequestration of technetium(VII) at elevated concentrations under ambient conditions. We report the retarded transformation of ferrihydrite to magnetite in the presence of technetium. We observe that quantitative reduction of pertechnetate with a fraction of technetium(IV) structurally incorporated into non-stoichiometric magnetite benefits from concomitant zero valent iron oxidative transformation. An in-depth profile of iron oxide reveals clusters of the incorporated technetium(IV), which account for 32% of the total retained technetium estimated via X-ray absorption and X-ray photoelectron spectroscopies. This corresponds to 1.86 wt.% technetium in magnetite, providing the experimental evidence to theoretical postulations on thermodynamically stable technetium(IV) being incorporated into magnetite under spontaneous aerobic redox conditions.

Author Correction: Closing the Nuclear Fuel Cycle with a Simplified Minor Actinide Lanthanide Separation Process (ALSEP) and Additive Manufacturing.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Technetium immobilization by materials through sorption and redox-driven processes: A literature review.

The objective of this review is to evaluate materials for use as a barrier or other deployed technology to treat technetium-99 (Tc) in the subsurface. To achieve this, Tc interactions with different materials are considered within the context of remediation strategies. Several naturally occurring materials are considered for Tc immobilization, including iron oxides and low solubility sulfide phases. Synthetic materials are also considered, and include tin-based materials, sorbents (resins, activated carbon, modified clays), layered double hydroxides, metal organic frameworks, cationic polymeric networks and aerogels. All of the materials were evaluated for their potential in-situ and ex-situ performance with respect to long-term Tc uptake and immobilization, environmental impacts and deployability. Other factors such as the technology maturity, cost and availability were also considered. Given the difficulty of evaluating materials under different experimental conditions (e.g., solution chemistry, redox conditions, solution to solid ratio, Tc concentration etc.), a subset of these materials will be selected, on the basis of this review, for subsequent standardized batch loading tests.

Iodine immobilization by materials through sorption and redox-driven processes: A literature review.

Radioiodine-129 (129I) in the subsurface is mobile and limited information is available on treatment technologies. Scientific literature was reviewed to compile information on materials that could potentially be used to immobilize 129I through sorption and redox-driven processes, with an emphasis on ex-situ processes. Candidate materials to immobilize 129I include iron minerals, sulfur-based materials, silver-based materials, bismuth-based materials, ion exchange resins, activated carbon, modified clays, and tailored materials (metal organic frameworks (MOFS), layered double hydroxides (LDHs) and aerogels). Where available, compiled information includes material performance in terms of (i) capacity for 129I uptake; (ii) long-term performance (i.e., solubility of a precipitated phase); (iii) technology maturity; (iv) cost; (v) available quantity; (vi) environmental impact; (vii) ability to emplace the technology for in situ use at the field-scale; and (viii) ex situ treatment (for media extracted from the subsurface or secondary waste streams). Because it can be difficult to compare materials due to differences in experimental conditions applied in the literature, materials will be selected for subsequent standardized batch loading tests.