Biogeochemical Mobility of Contaminants from a Replica Radioactive Waste Trench in Response to Rainfall-Induced Redox Oscillations

Transformation of Natural Organic Matter in Simulated Abiotic Redox Dynamic Environments: Impact on Fe Cycling

Redox fluctuations within redox dynamic environments influence the redox state of natural organic matter (NOM) and its interaction with redox-active elements, such as iron. In this work, we investigate the changes in the molecular composition of NOM during redox fluctuations as well as the impact of these changes on the Fe-NOM interaction employing Suwannee River Dissolved Organic Matter (SRDOM) as a representative NOM. Characterization of SRDOM using X-ray photoelectron spectroscopy and Fourier transform infrared spectrometry showed that irreversible changes occurred following electrochemical reduction and reoxidation of SRDOM in air. Changes in the redox state of SRDOM impacted its interaction with iron with higher rates of Fe(III) reduction in the presence of reduced and reoxidized SRDOM than in the presence of the original SRDOM. The increased rate of Fe(III) reduction in the presence of reduced SRDOM was due to the formation of reduced organic moieties on SRDOM reduction. The Fe(II) oxidation rate also increased in the presence of reduced SRDOM due to the formation of redox-active moieties that were capable of oxidizing Fe(II). Overall, our study provides useful insights into the changes in SRDOM that may occur in redox dynamic environments and the associated impact of these changes on Fe transformations.

Potential amendments of coal fly ash-derived zeolite to beryllium contaminated soil at a legacy waste disposal site

Management of Be contamination using industrial solid waste or solid waste-derived amendments is not well understood. This study investigated the potential of Australian coal fly ash (CFA), derived synthesized zeolite (SynZ) and chitosan-modified zeolite (ModZ), for Be immobilization at the Little Forest Legacy Waste Site (LFLS), a low-level radioactive waste disposal site near Sydney, Australia. In laboratory simulation experiments, the SynZ and ModZ were separately applied as an amendment to both naturally contaminated soil and simulated contaminated (spiked) soil. Different techniques, including pore water (PW), batch desorption, and microbial activities were assessed to provide insight into immobilization mechanisms. Results revealed that amendment of 2% ModZ in soils, substantially decreased Be concentrations in PW (PWBe) ranging from 13.3% to 99.5% across all concentrations of Be. In contrast, PWBe increased while using SynZ, which could be attributed to the increased solubility of different organic-inorganic elements in PW. Moreover, batch desorption using Milli-Q water, simulated acid rainwater [H2SO4/HNO3 = 60/40, (v/v), and 0.11 M acetic acid solution also revealed similar patterns of Be immobilization as found in PWBe analysis. Soil amendments boosted microbial biomass carbon, and phosphorous (MBC,P), along with basal respiration (BRCO2). This indicates increased microbial activities, which are linked with environmental eco-friendliness. This effect was substantially noticed in ModZ-amended soils, exhibiting up to 22 times higher in BRCO2 values compared to unamended soil. Additionally, reduced PWBe was correlated with soluble organic-inorganic elements, desorbed Be in the batch study, and soil MBc. The differences in behavior between SynZ and ModZ underline the importance of carefully studying the various potential amendment materials and the need to evaluate their performance before application in field situations. This study highlights ModZ's effectiveness in eco-friendly Be immobilization, underlining the role of organic functional groups in zeolite architecture, a key factor in controlling Be in soils.

Synthesised and modified zeolite for effective management of beryllium contaminants in aqueous media under different conditions

The effective management of beryllium (Be) in solution is not well established. In this study, zeolite was synthesised from coal fly ash (CFA) and further modified to enhance Be sorption. Results indicated zeolite NaP1 was effectively synthesised, and cross-linked chitosan was grafted in/on the zeolite structure during modification. The Brunauer, Emmett, and Teller (BET) surface area substantially increased from 1.05 m2/g in CFA to 94.0 m2/g in the synthesised zeolite (SZ). Furthermore, the modified zeolite (MZ) showed improved functionality as a reactive site for Be sorption. A comparative sorption study revealed inferior sorption (11.3 %) and higher desorption (56.1 %) of Be using CFA than the sorption using SZ (93.0 % sorption, 2.9 % desorption) and MZ (93.0 % sorption, 1.5 % desorption). Consequently, SZ and MZ exhibited higher sorption efficacy than commercial zeolite (57.4 %) and other commercial sorbents. At an experimental pH of 5.5 [relevant to the pH of Little Forest Legacy Waste Site (LFLS) soil, a representative site for potential Be contamination], MZ showed higher sorption than SZ. The higher sorption in MZ resulted from its elevated ligand complexation [with nitrogen (N), phosphorous (P), and oxygen (O)] and some ion exchange (with Na+, -NH3+, and H+ ions) mechanisms. Moreover, increased sorption (up to 99 %) was observed using colloidal soil solution (CSS) collected from LFLS soil to simulate field conditions after extensive rainfall. Different environmental factors (e.g. pH, temperature, time, CSS, concentrations of sorbate, and sorbent) regulated Be sorption. The sorption mechanism was best described by the Langmuir model, and the pseudo-second-order kinetic model (R2 = 0.999). Moreover, the sorption reaction was spontaneous (ΔG = -Ve), enthalpically, and entropically influenced. Desorption hysteresis (ndesorption/nsorption < 1) suggested irreversible sorption, and the chemisorption mechanism of Be was confirmed by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis.

Microbial community dynamics and cycling of plutonium and iron in a seasonally stratified and radiologically contaminated pond

Plutonium (Pu) cycling and mobility in the environment can be impacted by the iron cycle and microbial community dynamics. We investigated the spatial and temporal changes of the microbiome in an iron (Fe)-rich, plutonium-contaminated, monomictic reservoir (Pond B, Savannah River Site, South Carolina, USA). The microbial community composition varied with depth during seasonal thermal stratification and was strongly correlated with redox. During stratification, Fe(II) oxidizers (e.g., Ferrovum, Rhodoferax, Chlorobium) were most abundant in the hypoxic/anoxic zones, while Fe(III) reducers (e.g., Geothrix, Geobacter) dominated the deep, anoxic zone. Sulfate reducers and methanogens were present in the anoxic layer, likely contributing to iron and plutonium cycling. Multinomial regression of predicted functions/pathways identified metabolisms highly associated with stratification (within the top 5%), including iron reduction, methanogenesis, C1 compound utilization, fermentation, and aromatic compound degradation. Two sediment cores collected at the Inlet and Outlet of the pond were dominated by putative fermenters and organic matter (OM) degraders. Overall, microbiome analyses revealed the potential for three microbial impacts on the plutonium and iron biogeochemical cycles: (1) plutonium bioaccumulation throughout the water column, (2) Pu-Fe-OM-aggregate formation by Fe(II) oxidizers under microaerophilic/aerobic conditions, and (3) Pu-Fe-OM-aggregate or sediment reductive dissolution and organic matter degradation in the deep, anoxic waters.

Contaminant release, mixing and microbial fluctuations initiated by infiltrating water within a replica field-scale legacy radioactive waste trench

Numerous legacy near-surface radioactive waste sites dating from the mid 20th century have yet to be remediated and present a global contamination concern. Typically, there is insufficient understanding of contaminant release and redistribution, with invasive investigations often impractical due to the risk of disturbing the often significantly radiotoxic contaminants. Consequently, a replica waste trench (~5.4 m³), constructed adjacent to a legacy radioactive waste site (Little Forest Legacy Site, LFLS), was used to assist our understanding of the release and mixing processes of neodymium (Nd) - a chemical analogue for plutonium(III) and americium(III), two significant radionuclides in many contaminated environments. In order to clarify the behaviour of contaminants released from buried objects such as waste containers, a steel drum, representative of the hundreds of buried drums within the LFLS, was placed within the trench. Dissolved neodymium nitrate was introduced as a point-source contaminant to the base of the trench, outside the steel drum. Hydrologic conditions were manipulated to simulate natural rainfall intensities with dissolved lithium bromide added as a tracer. Neodymium was primarily retained both at its point of release at the bottom of the trench (>97 %) as well as at a steel container corrosion point, simulated through the emplacement of steel wool. However, over the 8-month field experiment, advective mixing initiated by surface water intrusions rapidly redistributed a small proportion of Nd to shallower waters (~1.5-1.7 %), as well as throughout the buried steel drum. Suspended particulate forms of Nd (>0.2 μm) were measured at all depths in the suboxic trench and were persistent across the entire study. Analyses of the microbial communities showed that their relative abundances and metabolic functions were strongly influenced by the prevailing geochemical conditions as a result of fluctuating water depths associated with rainfall events. The site representing steel corrosion exhibited divergent biogeochemical results with anomalous changes (sharp decrease) observed in both dissolved contaminant concentration as well as microbial diversity and functionality. This research demonstrates that experimental trenches provide a safe and unique method for simulating the behaviour of subsurface radioactive contaminants with results demonstrating the initial retention, partial shallow water redistribution, and stability of particulate form(s) of this radioactive analogue. These results have relevance for appropriate management and remediation strategies for the adjacent legacy site as well as for similar sites across the globe.

Role of beryllium in the environment: Insights from specific sorption and precipitation studies under different conditions

In this study, we examined factors influencing the environmental behaviour of Be, specifically considering soils collected from a legacy radioactive waste disposal site near Sydney (Australia). The precipitation study showed the formation of Be(OH)₂ (amorphous) from ICP standard solution, but a mixture of Be(OH)₂ (alpha), Be(OH)₂ (beta) and ternary Na/S-Be (ΙΙ)-OH(s) solid phase were formed from BeSO₄ solutions. The precipitation of Be started at relatively lower pH at higher concentrations than at the lower Be concentration as indicated by both laboratory data and simulation. Across the pH range, the Be sorption curve was divided into three phases, these being pH 3-6, pH 6-10, and pH > 10, within which sorption of Be with soil was 9-97%, 90-97%, and 66-90%, respectively. Beryllium solubility was limited at pH > 7, but a sorption study with soil showed chemisorption under both acidic and alkaline pH (pH 5.5 and 8) conditions, which was confirmed by FTIR and XPS analysis. At pH 5.5 (specifically relevant to the study site), sorption of Be was 72-95%, in which 77% and 46% Be was respectively sorbed by separated fulvic and humic acid fractions. The irreversible chemisorption mechanism was controlled by SOM at higher pH, and by metal oxyhydroxides at lower pH. Both organic and inorganic components synergistically influence the specific chemisorption of Be at the intermediate pH 5.5 of field soil.

Genomic Insights Into the Archaea Inhabiting an Australian Radioactive Legacy Site

During the 1960s, small quantities of radioactive materials were co-disposed with chemical waste at the Little Forest Legacy Site (LFLS, Sydney, Australia). The microbial function and population dynamics in a waste trench during a rainfall event have been previously investigated revealing a broad abundance of candidate and potentially undescribed taxa in this iron-rich, radionuclide-contaminated environment. Applying genome-based metagenomic methods, we recovered 37 refined archaeal MAGs, mainly from undescribed DPANN Archaea lineages without standing in nomenclature and 'Candidatus Methanoperedenaceae' (ANME-2D). Within the undescribed DPANN, the newly proposed orders 'Ca. Gugararchaeales', 'Ca. Burarchaeales' and 'Ca. Anstonellales', constitute distinct lineages with a more comprehensive central metabolism and anabolic capabilities within the 'Ca. Micrarchaeota' phylum compared to most other DPANN. The analysis of new and extant 'Ca. Methanoperedens spp.' MAGs suggests metal ions as the ancestral electron acceptors during the anaerobic oxidation of methane while the respiration of nitrate/nitrite via molybdopterin oxidoreductases would have been a secondary acquisition. The presence of genes for the biosynthesis of polyhydroxyalkanoates in most 'Ca. Methanoperedens' also appears to be a widespread characteristic of the genus for carbon accumulation. This work expands our knowledge about the roles of the Archaea at the LFLS, especially, DPANN Archaea and 'Ca. Methanoperedens', while exploring their diversity, uniqueness, potential role in elemental cycling, and evolutionary history.