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Multiple processes affect the fate of the radioactive isotope 129I in the environment. Primary categories of these processes include electron transfer reactions mediated by minerals and microbes, adsorption to sediments, interactions with organic matter, co-precipitation, and volatilization. A description of dominant biogeochemical processes is provided to describe the interrelationship of these processes and the associated iodine chemical species. The majority of the subsurface iodine fate and transport studies in the United States have been conducted at U.S. Department of Energy (DOE) sites where radioisotopes of iodine are present in the environment and stored waste. The DOE Hanford Site and Savannah River Site (SRS) are used to illustrate how the iodine species and dominant processes at a site are controlled by the prevailing site biogeochemical conditions. These sites differ in terms of climate (arid vs. sub-tropical), major geochemical parameters (e.g., pH ~7.5 vs. 4), and mineralogy (carbonate vs. Fe/Al oxide dominated). The iodine speciation and dominant processes at a site also have implications for selection and implementation of suitable remedy approaches for 129I.

Incorporation of iodate into calcite (CaCO3) may be used as an in situ treatment strategy for radioiodine in contaminated soils and groundwater, but the presence of other contaminants may inhibit its efficiency. To this end, the potential for chromate to interfere with iodate incorporation into CaCO3 was investigated as an example of how co-located contaminants may impact in situ remediation efficacy. Here, batch precipitation experiments were periodically sub-sampled over 21 days to determine the kinetic effects of chromate on iodate removal and incorporation into calcite. From these experiments, a decrease in iodate removal from >60 to

Solid-phase interactions and speciation are important to radioiodine transport in groundwater. At the Hanford Site in Southeastern Washington State, iodate (IO3−) is the main aqueous species in dilute radioiodine groundwater plumes. Like other oxyanions, IO3− may be incorporated into and/or adsorbed onto calcite, a common mineral at Hanford, decreasing its mobility in the environment. A series of macroscale batch experiments combined with solid-phase characterization were conducted to identify variables impacting time-dependent aqueous IO3− removal via calcite precipitation and determine the location of IO3− within the calcite crystal structure. Results demonstrated 11.5–97% aqueous IO3− removal during initial rapid calcite precipitation. Incorporation was apparently the main removal mechanism, although later slower precipitation and/or adsorption may have also contributed to IO3− removal. Using a higher concentration of the calcite-forming solutions (i.e., using 1 M vs. 0.1 M concentrations) resulted in an increase in the amount of precipitated calcite and a greater percentage of IO3− removed; however, calcite formed with lower molarity solutions resulted in higher IO3− mass (µg/g) removal. Solubility testing of laboratory-produced calcites showed only small differences in solubility for calcite with and without IO3− incorporated into the structure. Evidence collected from SEM/FIB and TEM/SAED suggested that the IO3− incorporated into calcite was present in regions close to the surface (implying potential easy release upon calcite dissolution).

Technetium-99 (Tc) contamination remains a major environmental problem at legacy nuclear reprocessing sites, including the Hanford Site (Washington State, U.S.A.) where ∼700 Ci of Tc has been relea...

Technetium contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, Washington, USA. At these sites, Tc is present in liquid waste destined for immobilization in a waste form or has been released into the subsurface environment. The high environmental risk associated with Tc is due to its long half-life (214 000 years) and the mobility of the oxidized anionic species Tc(VII)O4-. Under reducing conditions, TcO4- is readily reduced to Tc(IV), which commonly exists as a relatively insoluble and therefore immobile, hydrous Tc-oxide (TcO2·nH2O). The stability of Tc(IV) sequestered as solid phases depends on the solubility of the solid and susceptibility to reoxidation to TcO4-, which in turn depend on the (biogeo)chemical conditions of the environment and/or nuclear waste streams. Unfortunately, the solubility of crystalline TcO2 or amorphous TcO2·H2O is still above the maximum contaminant level (MCL) established by the U.S. EPA (900 pCi/L), and the kinetics of TcO2 oxidative dissolution can be on the order of days to years. In addition to oxygen, sulfur can form complexes that significantly affect the adsorption, solubility, and reoxidation potential of Tc, especially Tc(IV). The principal technetium sulfides are TcS2 and Tc2S7, but much less is known about the mechanisms of formation, stabilization, and reoxidation of Tc-sulfides. A common assumption is that sulfides are less soluble than their oxyhydrous counterparts. Determination of the molecular structure of Tc2S7 in particular has been hampered by the propensity of this phase to precipitate as an amorphous substance. Recent work indicates that the oxidation state of Tc in Tc2S7 is Tc(IV), in apparent contradiction to its nominal stoichiometry. Technetium is relatively immobile in reduced sediments and soils, but in many cases the exact sink for Tc has not been identified. Experiments and modeling have demonstrated that both abiotic and biologic mechanisms can exert strong controls on Tc mobility and that Tc binding or uptake into sulfide phases can occur. These and similar investigations also show that extended exposure to oxidizing conditions results in transformation of sulfide-stabilized Tc(IV) to a Tc(IV)O2-like phase without formation of measurable dissolved TcO4-, suggesting a solid-state transformation in which Tc(IV)-associated sulfide is preferentially oxidized before the Tc(IV) cation. This transformation of Tc(IV)-sulfides to Tc(IV)-oxides may be the main process that limits remobilization of Tc as Tc(VII)O4-. The efficacy of the final waste form to retain Tc also strongly depends on the ability of oxidizing species to enter the waste and convert Tc(IV) to Tc(VII). Many waste form designs are reducing (e.g., cementitious waste forms such as salt stone) and, therefore, attempt to restrict access of oxidizing species such that diffusion is the rate-limiting step in remobilization of Tc.

Although many studies have investigated different aspects of an inadvertent release of CO 2 from deep subsurface reservoirs into overlying groundwater aquifers, there are still questions that remain unanswered. This paper includes a short summary of the results obtained mainly from research activities and modeling efforts conducted over the last 5 years at the Pacific Northwest National Laboratory in collaboration with Lawrence Berkeley National Laboratory.

Interactions between mineral surfaces and organic matter are ubiquitous in soils and the environment. Through both physical and chemical mechanisms, organic–mineral assemblages prevent decomposition of soil organic matter by limiting accessibility or reducing efficacy of enzymes and microbes. To understand the mechanisms underlying organic–mineral interactions, researchers have begun to interrogate these systems at micro- and nanometer length scales. Current techniques that maintain a hydrated state and allow researchers to characterize nanometer length scales are limited. Here we chose a model organic–mineral system and performed complementary imaging techniques that allowed direct nanoscale observations in environmentally relevant conditions: cryogenic transmission electron microscopy (cryo-TEM) and in situ liquid cell transmission electron microscopy (TEM). We observed a 3-fold increase in the aggregate size of goethite nanoparticles upon addition of a model organic phosphate ligand and a preference fo...

Systematic flow-through column experiments were conducted using sediments and ground water collected from different subsurface localities at the U.S. Department of Energy's Integrated Field Research Challenge site in Rifle, Colorado. The principal purpose of this study is to gain a better understanding of the interactive effects of groundwater geochemistry, sediment mineralogy, and indigenous bacterial community structures on the efficacy of uranium removal from the groundwater with/without acetate amendment. Overall, we find that the subtle variations in the sediments' mineralogy, redox conditions, as well as contents of metal(loid) co-contaminants showed a pronounced effect on the associated bacterial population and composition, which mainly determines the system's performance with respect to uranium removal. Positive relationship was identified between the abundance of dissimilatory sulfate-reduction genes (i.e., drsA), markers of sulfate-reducing bacteria, and the sediments' propensity to sequester aqueous uranium. In contrast, no obvious connections were observed between the abundance of common iron-reducing bacteria, e.g., Geobacter spp., and the sediments' ability to sequester uranium. In the sediments with low bacterial biomass and the absence of sulfate-reducing conditions, abiotic adsorption onto mineral surfaces such as phyllosilicates likely played a relatively major role in the attenuation of aqueous uranium; however, in these scenarios, acetate amendment induced detectable rebounds in the effluent uranium concentrations. The results of this study suggest that immobilization of uranium can be achieved under predominantly sulfate-reducing conditions, and provide insight into the integrated roles of various biogeochemical components in long-term uranium sequestration.

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.

BACKGROUND Natural nanomaterials have always been abundant during Earth’s formation and throughout its evolution over the past 4.54 billion years. Incidental nanomaterials, which arise as a by-product from human activity, have become unintentionally abundant since the beginning of the Industrial Revolution. Nanomaterials can also be engineered to have unusual, tunable properties that can be used to improve products in applications from human health to electronics, and in energy, water, and food production. Engineered nanomaterials are very much a recent phenomenon, not yet a century old, and are just a small mass fraction of the natural and incidental varieties. As with natural and incidental nanomaterials, engineered nanomaterials can have both positive and negative consequences in our environment. Despite the ubiquity of nanomaterials on Earth, only in the past 20 years or so have their impacts on the Earth system been studied intensively. This is mostly due to a much better understanding of the distinct behavior of materials at the nanoscale and to multiple advances in analytic techniques. This progress continues to expand rapidly as it becomes clear that nanomaterials are relevant from molecular to planetary dimensions and that they operate from the shortest to the longest time scales over the entire Earth system. ADVANCES Nanomaterials can be defined as any organic, inorganic, or organometallic material that present chemical, physical, and/or electrical properties that change as a function of the size and shape of the material. This behavior is most often observed in the size range between 1 nm up to a few to several tens of nanometers in at least one dimension. These materials have very high proportions of surface atoms relative to interior ones. Also, they are often subject to property variation as a function of size owing to quantum confinement effects. Nanomaterial growth, dissolution or evaporation, surface reactivity, and aggregation states play key roles in their lifetime, behaviors, and local interactions in both natural and engineered environments, often with global consequences. It is now possible to recognize and identify critical roles played by nanomaterials in vital Earth system components, including direct human impact. For example, nanomaterial surfaces may have been responsible for promoting the self-assembly of protocells in the origin of life and in the early evolution of bacterial cell walls. Also, weathering reactions on the continents produce various bioavailable iron (oxy)hydroxide natural and incidental nanomaterials, which are transported to the oceans via riverine and atmospheric pathways and which influence ocean surface primary productivity and thus the global carbon cycle. A third example involves nanomaterials in the atmosphere that travel locally, regionally, and globally. When inhaled, the smallest nanoparticles can pass through the alveolar membranes of the lungs and directly enter the bloodstream. From there, they enter vital organs, including the brain, with possible deleterious consequences. OUTLOOK Earth system nanoscience requires a convergent approach that combines physical, biological, and social sciences, as well as engineering and economic disciplines. This convergence will drive developments for all types of intelligent and anticipatory conceptual models assisted by new analytical techniques and computational simulations. Ultimately, scientists must learn how to recognize key roles of natural, incidental, and engineered nanomaterials in the complex Earth system, so that this understanding can be included in models of Earth processes and Earth history, as well as in ethical considerations regarding their positive and negative effects on present and predicted future environmental and human health issues.

This study of vadose zone and aquifer sediments beneath a former dichromate spill site showed that the persistence of CrVI in the sediments and the large differences in released mass and rate was caused by the dissolution of multiple CrVI surface phases. Vadose zone sediments contained numerous 1 to 10 μm high solubility calcium chromate crystals, with lesser amounts of unidentified phases indicated by Ba/Cr association in weathered pyroxenes and Ca/Cr association in weathered Ca-rich plagioclase. Most (>90%) of the CrVI mass in these vadose zone sediments was readily leached in laboratory columns at high concentrations (up to 187 mg/L CrVI) likely from the highly soluble calcium chromate. Additional CrVI associated with other CrVI surface phases was additionally slowly released over 100 s of hours. The source of Ca and Ba for the CrVI precipitates may be from mineral dissolution associated with the historical surface spills of CrVI as an acidic dichromate solution. In contrast, aquifer sediments contained significantly less CrVI, which was slowly released over 100 s of hours. Small-sized CrVI-containing precipitates (

Co-precipitation of iodate and chromate with calcium carbonate (CaCO3) minerals is a potential approach that can significantly decrease their mobility in the subsurface. There is a need to identify the impact of pH and elements of natural abundance such as Si on co-precipitation process to understand potential for natural attenuation or targeted removal via injection of CaCO3 forming solutions in the subsurface. Batch experiments were combined with solid phase characterization studies to examine the potential for co-precipitation/incorporation of iodate and chromate into CaCO3 minerals. These experiments considered uptake during and after precipitation of CaCO3 phases and measured the release of contaminants during dissolution. Experiments conducted at variable pH (6.5, 8.0 and 9.0) and Si concentrations (0 mM, 0.5 mM and 20 mM) showed that the incorporation of iodate was pH dependent and was favored at lower pH and higher Si concentration within 7 days. The presence of chromate had little effect on the removal of iodate except at pH 6.5 where iodate uptake increased 6–10% due to the formation of dietzeite, Ca2H2O(IO3)2(CrO4). More aqueous iodate was removed in all experiments compared to chromate except some of the investigated conditions (Si 20 mM, pH 9) where the removal of CrO42− was found similar to IO3−. The release of iodate and chromate from dissolving CaCO3 was also pH and Si concentration dependent. The I fraction released from CaCO3 doped with IO3− was 1.5–2.3%, which reached a steady state after 24h; however, 2.6–4.6% of I was released from CaCO3 doped with IO3− and CrO42− for all tested conditions. More chromate (up to 36.5 ± 4.6%) than iodate (3.2 ± 0.07%) was released during dissolution at pH 6.5 and 20 mM Si. Although less uptake of Cr occurred, it may have led to increased release of I when both were co-precipitated simultaneously.

A prominent radionuclide of concern in nuclear wastes, 129I, is present in low-activity wastes (LAW) at the Hanford site. Several Ag-containing materials were tested as immobilization agents, or “getters”, for I (as iodide, I−) removal from deionized (DI) water and a liquid LAW simulant: Ag impregnated activate carbon (Ag–C), Ag exchanged zeolite (Ag–Z), and argentite. In anoxic batch experiments with DI water, the Ag–C and argentite were most effective, with maximum Kd values of 6.2 × 105 mL/g for the Ag–C and 3.7 × 105 mL/g for the argentite after 15 days. Surface area and Ag content were found to influence the performance of the getters in DI water. In the anoxic batch experiments with LAW simulant, Ag–Z vastly outperformed the other getters with Kd values of 2.2 × 104 mL/g at 2 h, which held steady until 15 days, compared with 1.8 × 103 mL/g reached at 15 days by the argentite. All getters were stable over long periods of time (i.e. 40 days) in DI water, while the Ag–Z and argentite were also stable in the LAW simulant. Ag–Z was found to have consistent I removal upon crushing to a smaller particle size and in the presence of O2, making it a strong candidate for the treatment of LAW containing I.

Mitigation of hazardous and radioactive waste can be improved through conversion of existing waste to a more chemically stable and physically robust waste form. One option for waste conversion is the fluidized bed steam reforming (FBSR) process. The resulting FBSR granular material was encapsulated in a geopolymer matrix referred to here as Geo-7. This provides mechanical strength for ease in transport and disposal. However, it is necessary to understand the phase assemblage evolution as a result of geopolymer encapsulation. In this study, we examine the mineral assemblages formed during the synthesis of the multiphase ceramic waste form. The FBSR granular samples were created from waste simulant that was chemically adjusted to resemble Hanford tank waste. Another set of samples was created using Savannah River Site Tank 50 waste simulant in order to mimic a blend of waste collected from 68 Hanford tank. Waste form performance tests were conducted using the product consistency test (PCT), the Toxicity Characteristic Leaching Procedure (TCLP), and the single-pass flow-through (SPFT) test. X-ray diffraction analyses revealed the structure of a previously unreported NAS phase and indicate that monolith creation may lead to a reduction in crystallinity as compared to the primary FBSR granular product.

Sodalites have been proposed as a possible host of certain radioactive species, specifically 99 Tc and 129 I, which may be encapsulated into the cage structure of the mineral. To demonstrate the ability of this framework silicate mineral to encapsulate and immobilize 99 Tc and 129 I, single-pass flow-through (SPFT) tests were conducted on a sodalite-bearing multi-phase ceramic waste form produced through a steam reforming process. Two samples made using a steam reformer samples were produced using non-radioactive I and Re (as a surrogate for Tc), while a third sample was produced using actual radioactive tank waste containing Tc and added Re. One of the non-radioactive samples was produced with an engineering-scale steam reformer while the other non-radioactive sample and the radioactive sample were produced using a bench-scale steam reformer. For all three steam reformer products, the similar steady-state dilute-solution release rates for Re, I, and Tc at pH (25 °C) = 9 and 40 °C were measured. However, it was found that the Re, I, and Tc releases were equal or up to 4.5x higher compared to the release rates of the network-forming elements, Na, Al, and Si. The similar releases of Re and Tc in the SPFT test, and the similar time-dependent shapes of the release curves for samples containing I, suggest that Re, Tc, and I partition to the sodalite minerals during the steam reforming process.

Multiphase, reactive transport modeling was used to identify the mechanisms controlling trace metal release under elevated CO2 conditions from a well-characterized carbonate aquifer. Modeling was conducted for both batch and column experiments. The column experiments resulted in higher trace metal concentrations because the rock to water ratio was higher. A kinetic desorption model fits the overall trends in release for seven trace metals observed in batch and column experiments exposing Edwards Aquifer material to elevated concentrations of CO2. Observed and predicted trace metal concentrations are compared to groundwater concentrations from this aquifer to determine the potential for leaking CO2 to adversely impact drinking water quality. Finally, a three-dimensional multiphase flow and reactive-transport simulation of CO2 leakage from an abandoned wellbore into a generalized model of the shallow, unconfined portion of the aquifer is used to determine potential impacts on groundwater quality. As a measure of adverse impacts on groundwater quality, both the EPA’s regulatory limits and the maximum trace metal concentration observed in the aquifer were used as threshold values. Results of the field-scale simulations indicate that CO2 leakage into a carbonate aquifer is likely to cause decreases in pH and increases in TDS beyond observed ranges in the aquifer and beyond regulatory limits. However, trace metal concentrations are not predicted to exceed either the observed maximums or the regulatory limits.

To better understand the role of CO2-utilizing processes in determining geochemical outcomes in CO2-impacted, reduced subsurface environments; we studied hydrogenotrophic methanogenesis (a key process across a range of subsurface systems) and associated changes in CO2–CH4, pH–Eh and metal(loid) dynamics in CO2-enriched pyrite-, arsenopyrite- and galena-containing batch reactors. Hydrogenotrophic methanogenesis proceeded via two first-order, microbially-mediated steps. The first involved microbially-enhanced gas-to-solution transfer of CO2 with a rate constant of 0.95 ± 0.20 d−1 (compared to 0.17 ± 0.34 d−1 in non-methanogenic reactors). The second step involved the pseudo-first order reduction of HCO3−-to-CH4 with a rate constant of 0.46 ± 0.10 d−1. Bicarbonate reduction accounted for 76% of the CO2–C removed from the reactors’ headspace and triggered a decrease in Eh (−0.215 to −0.332 V) and an increase in pH (6.93 to 8.03). No significant effect on dissolved Pb (or its release from galena) was observed but dissolved Fe decreased and dissolved As increased during methanogenesis. Changes in dissolved Fe and As were proportional to methanogenesis-induced pH–Eh shifts and attributable to an initial CO2-induced release of As and Fe from arsenopyrite (and Fe from pyrite) with subsequent methanogenesis-induced increase in pH and electron activity triggering the precipitation of Fe, as amorphous FeCO3·6H2O. In addition to the role of methanogenesis in enhancing, (1) aqueous dissolution of CO2, (2) CO2 to carbonate mineralization and (3) immobilization of some metals, our findings suggested that due to its occurrence after CO2 dissolution and before carbonate precipitation, extensive HCO3−-to-CH4 reduction would lower solubility- and mineral-trapping of CO2. Such considerations are central to assessments of how CO2-utilizing processes may alter long-term outcomes at geologic storage sites.

A series of batch and column experiments combined with solid phase characterization studies was conducted to evaluate the impacts of the potential leakage of carbon dioxide (CO2) from deep subsurface storage reservoirs to overlying potable carbonate aquifers. The main objective was to gain an understanding on CO2 gas-induced changes in aquifer pH and mobilization of major, minor, and trace elements from dissolving minerals in rocks representative of an unconfined, oxidizing carbonate aquifer within the continental US. Samples from the unconfined portion of the Edwards limestone aquifer in Texas were exposed to a CO2 gas stream or were leached with a CO2-saturated influent solution simulating different leaking scenarios [i.e., sudden, fast, and short-lived release of CO2 (batch experiments) and gradual release (column experiments)]. The results from the batch and column experiments confirmed that exposure to excess CO2 gas caused significant decrease in pH (about two pH units); the release of major chemical elements into the contacting aqueous phase (such as Ca, Mg, Ba, Sr, Si, Na, and K); the mobilization and possible rapid immobilization of minor elements (such as Al and Mn), which are able to form highly reactive secondary phases; and sustained but low-concentration releases of some trace elements (such as Mo, Cs, Sn) in some samples. Spikes of low concentrations of other trace elements (such as As, Cd, Pb, Cu, Zn, Se, etc.), were observed sporadically during these experiments. The results help in developing a systematic understanding of how CO2 leakage is likely to influence pertinent geochemical processes (such as dissolution/precipitation and sorption/desorption) in the aquifer sediments and will support site selection, risk assessment, policy-making, and public education efforts associated with geologic CO2 sequestration.

A practical approach for in situ hydrous ferric oxide (HFO) precipitation was developed for iodine immobilization under field-scale conditions at the Hanford Site. A series of 1D meter-long bench-top column experiments packed with Hanford sediments was conducted with a single acidic ferric solution (0.1 M, pH = 1.5) injection. Because carbonate and clay minerals are widely present in sediments, self-pH buffering of the injected acidic ferric solution occurred due to mineral dissolution, leading to HFO precipitation under a neutral condition. Up to ~12 mg/g Fe as HFO successfully precipitated and evenly distributed in the column sediments, and the remobilization of the neoformed HFO precipitates was limited (≤ ~3.16 wt% after over 100 pore volumes (pv) of flushing). The transport of iodate (IO3−) in the HFO-amended sediments was strongly retarded through both adsorption and co-precipitation processes. However, reversible adsorption of iodine on HFO was observed, which might limit its application to slow-moving groundwater systems.

Soil organic matter (SOM) dynamics are central to soil biogeochemistry and fertility. The retention of SOM is governed initially by interactions with minerals, which mediate the sorption of chemically diverse organic matter (OM) molecules via distinct surface areas and chemical functional group availabilities. Unifying principles of mineral-OM interactions remain elusive because of the multi-layered nature of biochemical-mineral interactions that contribute to soil aggregate formation and the heterogeneous nature of soils among ecosystems. This study sought to understand how soil mineralogy as well as nitrogen (N) enrichment regulate OM composition in grassland soils. Using a multi-site grassland experiment, we demonstrate that the composition of mineral-associated OM depended on the clay content and specific mineral composition in soils across the sites. With increasing abundance of ferrihydrite (Fh) across six different grassland locations, OM in the hydrophobic zone became more enriched in lipid- and protein-like compounds, whereas the kinetic zone OM became more enriched in lignin-like molecules. These relationships suggest that the persistence of various classes of OM in soils may depend on soil iron mineralogy and provide experimental evidence to support conceptual models of zonal mineral-OM associations. Experimental N addition disrupted the accumulation of protein-like molecules in the hydrophobic zone and the positive correlation of lignin-like molecules in the kinetic zone with Fh content, compared to unfertilized soils. These data suggest that mineralogy and clay content together influence the chemical composition not only of mineral-associated OM, but also of soluble compounds within the soil matrix. If these relationships are prevalent over larger spatial and temporal scales, they provide a foundation for understanding SOM cycling and persistence under a variety of environmental contexts.

Extensive vadose zone metals and organic contamination remains at many former industrial and defense manufacturing sites, and effective remedial solutions are needed to slow or prevent its migration to groundwater. In this study, the application of gaseous substrates to stimulate microbial respiratory reduction of comingled radioisotopes and nitrate under unsaturated conditions was examined for possible application at the Hanford Site, a former nuclear production facility in southeastern WA, USA. First, screening studies were performed to qualitatively measure the sediment respiratory response to 14 gaseous or volatile organic substrates at two moisture contents, 4% and 8%. Volatile substrates produced the strongest respiratory response, among them were butyrate, pentane, butyl acetate. Ethane and butane were the most effective gaseous substrates but only at 8% water content. Hanford sediment from two waste sites with distinctive chemistries were wetted to 7% moisture content, packed into columns, and treated with ethane or butane. After 4 weeks, columns were then leached to quantify retardation in the mobility of aqueous contaminant concentrations compared to no gas control columns. Treatment with both gases resulted in >80% removal of Cr from the aqueous phase. However, NO3 concentration and a waste sites exposure history to NO3 had a major effect on U and Tc reduction. Incomplete nitrate reduction outcompeted U and Tc in waste site sediments having limited prior exposure to NO3. Conversely, waste site sediments co-contaminated with NO3 were able to achieve highly reduced conditions resulting in complete denitrification of NO3, and delayed leaching of U and Tc. This implied effective reduction of both contaminants to less mobile species. This study demonstrates that unsaturated vadose sediments at Hanford waste sites have the capacity for a sustained respiratory response to gaseous substrate injection, which could potentially be deployed as part of an overall strategy to reduce the flux of long-lived radionuclides to groundwater at Hanford and other legacy waste sites.

Iodate (IO3–) incorporation in calcite (CaCO3) is a potential sequestration pathway for environmental remediation of radioiodine-contaminated sites (e.g., Hanford Site, WA), but the incorporation mechanisms have not been fully elucidated. Ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure spectroscopy (EXAFS) were combined to determine the local coordination environment of iodate in calcite, the associated charge compensation schemes (CCS), and any tendency for surface segregation. IO3– substituted for CO32– and charge compensation was achieved by substitution of Ca2+ by Na+ or H+. CCS that minimized the I–Na/H distance or placed IO3– at the surface were predicted by density functional theory to be energetically favored, with the exception of HIO3, which was found to be metastable relative to the formation of HCO3–. Iodine K-edge EXAFS spectra were calculated from AIMD trajectories and used to fit the experimental spectrum. The best-fit combination consisted of a ...

At the Hanford Site in southeastern Washington, discharge of radionuclide laden liquid wastes resulted in vadose zone contamination, providing a continuous source of these contaminants to groundwater. The presence of multiple contaminants (i.e., 99Tc and 129I) increases the complexity of finding viable remediation technologies to sequester contaminants in situ and protect groundwater. Although previous studies have shown the efficiency of zero valent iron (ZVI) and sulfur modified iron (SMI) in reducing mobile Tc(VII) to immobile Tc(IV) and iodate incorporation into calcite, the coupled effects from simultaneously using these remedial technologies have not been previously studied. In this first-of-a-kind laboratory study, we used reductants (ZVI or SMI) and calcite-forming solutions to simultaneously remove aqueous Tc(VII) and iodate via reduction and incorporation, respectively. The results confirmed that Tc(VII) was rapidly removed from the aqueous phase via reduction to Tc(IV). Most of the aqueous iodate was transformed to iodide faster than incorporation into calcite occurred, and therefore the I remained in the aqueous phase. These results suggested that this remedial pathway is not efficient in immobilizing iodate when reductants are present. Other experiments suggested that iodate removal via calcite precipitation should occur prior to adding reductants for Tc(VII) removal. When microbes were included in the tests, there was no negative impact on the microbial population but changes in the makeup of the microbial community were observed. These microbial community changes may have an impact on remediation efforts in the long-term that could not be seen in a short-term study. The results underscore the importance of identifying interactions between natural attenuation pathways and remediation technologies that only target individual contaminants.

Long residence times of soil organic matter have been attributed to reactive mineral surface sites that sorb organic species and cause inaccessibility due to physical isolation and chemical stabilization at the organic–mineral interface. Instrumentation for probing this interface is limited. As a result, much of the micron- and molecular-scale knowledge about organic–mineral interactions remains largely qualitative. Here we report the use of force spectroscopy to directly measure the binding between organic ligands with known chemical functionalities and soil minerals in aqueous environments. By systematically studying the role of organic functional group chemistry with model minerals, we demonstrate that chemistry of both the organic ligand and mineral contribute to values of binding free energy and that changes in pH and ionic strength produce significant differences in binding energies. These direct measurements of molecular binding provide mechanistic insights into organo–mineral interactions, which could potentially inform land-carbon models that explicitly include mineral-bound C pools., Most molecular scale knowledge on soil organo–mineral interactions remains qualitative due to instrument limitations. Here, the authors use force spectroscopy to directly measure free binding energy between organic ligands and minerals and find that both chemistry and environmental conditions affect binding.

This paper provides an overview of the existing data and knowledge presented in recent literature about the potential leaking of CO2 from the deep subsurface storage reservoirs and the effects on groundwater quality. The objectives are to: 1) present data and discuss potential risks associated with the groundwater quality degradation due to CO2 gas and brine exposure; 2) identify the set of geochemical data required to develop models to assess and predict aquifer responses to CO2 and brine leakage; and 3) present a summary of the findings and reveal future trends in this important and expanding research area. The discussion is focused around aquifer responses to CO2 gas and brine exposure and the degree of impact; major hydrogeological and geochemical processes and site-specific properties known to control aquifer quality under CO2 exposure conditions; contributions from the deep reservoirs (plume characteristics and composition); and the possibility of establishment of a new network of reactions and processes affecting or controlling the overall mobility of major, minor, and trace elements and the fate of the elements released from sediments or transported with brine. This paper also includes a discussion on the development of conceptual and reduced order models (ROMs) to describe and predict aquifer responses and whether or not the release of metals following exposure to CO2 is harmful, which are an essential tool for CO2 sequestration related risk assessment. Future research needs in this area are also included at the end of the paper.

One of the key challenges for radioactive waste management is the efficient capture and immobilization of radioiodine, because of its radiotoxicity, high mobility in the environment, and long half-life (t1/2 = 1.57 × 107 years). Silver-functionalized silica aerogel (AgAero) represents a strong candidate for safe sequestration of radioiodine from various nuclear waste streams and subsurface environments. Batch sorption experiments up to 10 days long were carried out in oxic and anoxic conditions in both deionized water (DIW) and various Hanford Site Waste Treatment Plant (WTP) off-gas condensate simulants containing from 5 to 10 ppm of iodide (I−) or iodate (IO3−). Also tested was the selectivity of AgAero towards I− in the presence of other halide anions. AgAero exhibited fast and complete removal of I− from DIW, slower but complete removal of I− from WTP off-gas simulants, preferred removal of I− over Br− and Cl−, and it demonstrated ability to remove IO3− through reduction to I−.

Environmental contextNanominerals are more reactive than bulk minerals, a property that strongly influences the fate of nutrients and contaminants in soils and plants. This review discusses applications of Fe- and Mn-nano-oxides in agricultural systems and their potential to be used as fertiliser and contaminant adsorbents, while addressing potential phytotoxicity. We discuss areas where significant advances are needed, and provide a framework for future work. AbstractRising population growth and increase global food demand have made meeting the demands of food production and security a major challenge worldwide. Nanotechnology is starting to become a viable remediation strategy of interest in farming. Ultimately, it may be used as a sustainability tool in agricultural systems. In these roles, it could be used to increase the efficiency of techniques such as food monitoring, pathogen control, water treatment and targeted delivery of agrochemicals. In addition to these uses, nanoparticles, particularly nano-metal-oxides (NMOs), have been engineered to act as contaminant scavengers and could be applied to a wide range of systems. Numerous studies have investigated the scavenging ability of NMOs, but few have investigated them in this role in the context of agricultural and food systems. Within these systems, however, research has demonstrated the potential of NMOs to increase crop health and yield but few have studied using NMOs as sources of key micronutrients, such as Fe and Mn. In this review, we address previous research that has used Fe- and Mn-NMOs in agricultural systems, particularly the worldwide crop production of the four major staple foods – rice, wheat, maize and soybeans – highlighting their application as fertilisers and sorbents. Fe- and Mn-NMOs are strong candidates for immobilisation of agricultural contaminants in soils and, because they are naturally ubiquitous, they have the potential to be a cost-effective and sustainable technology compared with other remediation strategies.

At the U.S. Department of Energy Hanford Site, Tc-99 is a component of low-activity waste (LAW) fractions of the nuclear tank waste and removal of Tc from LAW streams would greatly benefit the site remediation process. In this study, we investigated the removal of Tc(VII), as pertechnetate, from deionized water (DIW) and a LAW simulant through batch sorption testing and solid phase characterization using tin (II) apatite (Sn-A) and SnCl2. Sn-A showed higher levels of Tc removal from both DIW and LAW simulant. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/XEDS) and X-ray absorption spectroscopy (XAS) of reacted Sn-A in DIW showed that TcO4- is reduced to Tc(IV) on the Sn-A surface. The performance of Sn-A in the LAW simulant was lowered due to a combined effect of the high alkalinity, which lead to an increased dissolution of Sn from the Sn-A, and a preference for the reduction of Cr(VI).

Large scale deployment of CO2 geological sequestration requires the assessment of the risks. One of the potential risks is the impact of CO2 leakage on shallow groundwater overlying the sequestration site.The understanding of the key chemical processes and parameters are critical for building numerical models for risk assessment. Model interpretation of laboratory and field tests is an effective way to enhance such understanding. As part of this investigation, column experiments in which the CO2 saturated synthetic groundwater flowed through a column packed with materials from the High Plains aquifer, were conducted. Changes in concentrations of several constituents in the column effluent and pH were determined. In this paper, a reactive transport model was developed to describe and interpret the observed concentration changes, attempting to shed light on the chemical reactions and mechanisms and key parameters that control the changes in effluent chemistry. The reactive transport model described fairly well the changes in pH and the concentration changes of Ca, Mg, Ba, Sr, Cs, As and Pb. Calcite dissolution and Ca-driven cation exchange reactions were the major drivers for the concentration changes of Ca, Ba, Sr, and Cs. The pH-driven adsorption/desorption reactions led to a concentration increase of As and Pb. The volume fraction and reactive surface area of calcite, CEC and sorption capacity were key parameters in controlling the magnitude of concentration increase. Model results also showed that Ba, which is an important chemical element released into the aqueous phase during these experiments, may be incorporated into the calcite crystal structure and the dissolution of Ba-bearing calcite could be an alternative pathway to explain the increase in aqueous Ba concentration when sediments are exposed to the CO2 saturated leaching groundwater.

This review paper attempts to summarize the progress made in research efforts conducted over the past years to study the surface chemical properties of the tropical and subtropical soils, usually called variable charge soils, and the way they response to different management. The paper is composed of an introductory section that provides a brief discussion on the surface chemical properties of these soils, and five other review sections. The focus of the following sections is on the evolution of surface chemical properties during the development of the variable charge properties (second section), interactions between oppositely charged particles and the resulting effects on the soil properties and especially on soil acidity (third section), the surface effects of low molecular weight organic acids sorbed to mineral surfaces and the chemical behavior of aluminum (fourth section), and the crop-straw-derived biochar-induced changes of the surface chemical properties of these soils (fifth section). A discussion on the effect of climate change variables on the properties of the variable charge soils is included at the end of this review paper (sixth section).