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.

Correction: Emerging investigator series: a holistic approach to multicomponent EXAFS: Sr and Cs complexation in clayey soils.

Correction for 'Emerging investigator series: a holistic approach to multicomponent EXAFS: Sr and Cs complexation in clayey soils' by Pieter Bots et al., Environ. Sci.: Processes Impacts, 2021, DOI: .

Emerging investigator series: a holistic approach to multicomponent EXAFS: Sr and Cs complexation in clayey soils.

Strontium and caesium are fission products of concern at many nuclear legacy sites and Cs is additionally a significant consideration at sites in the aftermath of nuclear accidents and incidents. Such sites require long-term management to minimize the risk of such contaminants to the environment and the public. Understanding the geochemical speciation of Sr and Cs in situ in the soils and groundwater is essential to develop engineered management strategies. Here we developed and utilized a comprehensive approach to fitting the EXAFS of Sr and Cs adsorption to single mineral phases and a composite clayey soil. First, a shell-by-shell fitting strategy enabled us to determine that Sr surface complexes involve the formation of bidentate edge sharing complexes with anatase and illite-smectite, and form at the silicon vacancy sites at the kaolinite basal surfaces. Cs surface complexes form at the silicon vacancy sites at the illite-smectite and kaolinite basal surfaces. Second, using a subsequent holistic approach we determined the predominance of these complexes within a composite clayey soil. Sr was dominated by complexation with illite-smectite (72-76%) and to a lesser extent with kaolinite (25-30%) with negligible complexation with anatase, while Cs complexed roughly equally to both illite-smectite and kaolinite. The presented approach to fitting EXAFS spectra will strengthen predictive modelling on the behaviour of elements of interest. For example, the details on Sr and Cs speciation will enable predictive modelling to characterise their long-term behaviour and the design and validation of evidence-based engineering options for long-term management of nuclear legacy sites.

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

Results of investigations into factors influencing contaminant mobility in a replica trench located adjacent to a legacy radioactive waste site are presented in this study. The trench was filled with nonhazardous iron- and organic matter (OM)-rich components, as well as three contaminant analogues strontium, cesium, and neodymium to examine contaminant behavior. Imposed redox/water-level oscillations, where oxygen-laden rainwater was added to the anoxic trench, resulted in marked biogeochemical changes including the removal of aqueous Fe(II) and circulation of dissolved carbon, along with shifts to microbial communities involved in cycling iron (Gallionella, Sideroxydans) and methane generation (Methylomonas, Methylococcaceae). Contaminant mobility depended upon element speciation and rainfall event intensity. Strontium remained mobile, being readily translocated under hydrological perturbations. Strong ion-exchange reactions and structural incorporation into double-layer clay minerals were likely responsible for greater retention of Cs, which, along with Sr, was unaffected by redox oscillations. Neodymium was initially immobilized within the anoxic trenches, due to either secondary mineral (phosphate) precipitation or via the chemisorption of organic- and carbonate-Nd complexes onto variably charged solid phases. Oxic rainwater intrusions altered Nd mobility via competing effects. Oxidation of Fe(II) led to partial retention of Nd within highly sorbing Fe(III)/OM phases, whereas pH decreases associated with rainwater influxes resulted in a release of adsorbed Nd to solution with both pH and OM presumed to be the key factors controlling Nd attenuation. Collectively, the behavior of simulated contaminants within this replica trench provided unique insights into trench water biogeochemistry and contaminant cycling in a redox oscillatory environment.

Priority issues and key findings from evaluation of disposal records for a legacy radioactive waste site.

This paper reports on a detailed investigation into the disposal procedures and records from the operational period (1960-1968) of the Little Forest Legacy Site (LFLS) in eastern Australia. The aims of the paper are firstly, to highlight the priority issues which are relevant to the radiological assessment of the LFLS, and secondly, to present key lessons that may help to guide future investigations of the records at similar sites. Particular effort was put into assessing the various types of relevant documents and the relationships between them. A specific objective of this work was to evaluate an inventory of the wastes which was reported shortly after the time of disposals. A major finding of the study is that the original actinide inventory for LFLS relied solely on estimates from a limited number of specific records known as 'Scrap Disposal Reports' (SDRs). For example, the estimated amount of plutonium disposed at the LFLS was based on only seven SDR records. Given that there are approximately 50000 buried items, it is possible that other Pu-contaminated items could make a significant additional contribution to the amount of Pu present at the site. For some waste components (e.g. beryllium) the documentation shows that rough estimates of disposal quantities were made, based on the number of disposed Be-contaminated items in each trench. The use of such approximations casts some doubt on the accuracy of the previous inventory of wastes at the site. In addition, the early summaries of radionuclide disposals, which categorized radionuclides into groups according to their radiological hazard, contained significant underestimates of the radionuclide inventory in the most hazardous category (referred to at the time as 'Group I' radionuclides). This was mainly due to the omission of the Pu (which had been recorded on the SDRs) from the Group I inventory, but was also in part because the Group I radionuclide content of disposed sludge drums (from a wastewater treatment plant) was not taken into account for most of the disposal period. Establishing the disposal history and radionuclide inventory at legacy sites is an important pre-requisite to evaluating their radiological impact and developing management options. The detailed investigation of the LFLS records shows the importance of understanding the operational practices of the period and the derivation of the original inventories. These insights should help guide future efforts to better understand disposal histories and inventories at LFLS and elsewhere.

Radionuclide distributions and migration pathways at a legacy trench disposal site.

This paper examines the distributions of several anthropogenic radionuclides (239+240Pu, 241Am, 137Cs, 90Sr, 60Co and 3H) at a legacy trench disposal site in eastern Australia. We compare the results to previously published data for Pu and tritium at the site. Plutonium has previously been shown to reach the surface by a bath-tubbing mechanism, following filling of the former trenches with water during intense rainfall events. This has led to some movement of Pu away from the trenched area, and we also provide evidence of elevated Pu concentrations in shallow subsurface layers above the trenched area. The distribution of 241Am is similar to Pu, and this is attributed to the similar chemistry of these actinides and the likely in-situ generation of 241Am from its parent 241Pu. Concentrations of 137Cs are mostly low in surface soils immediately above the trenches. However, similar to the actinides, there is evidence of elevated 137Cs and 90Sr concentrations in shallow subsurface layers above the trenched area. While the subsurface radionuclide peaks suggest a mechanism of subsurface transport, their interpretation is complicated by the presence of soil layers added following disposals and during the subsequent years. The distribution of 90Sr and 137Cs at the ground surface shows some elevated levels immediately above the trenches which were filled during the final 24 months of disposal operations. This is in agreement with disposal records, which indicate that greater amounts of fission products were disposed in this period. The surface distribution of 239+240Pu is also consistent with the disposal documents. Although there is extensive evidence of a mobile tritium plume in groundwater, migration of the other radionuclides by this pathway is limited. The data highlight the importance of taking into account multiple pathways for the mobilisation of key radioactive contaminants at legacy waste trench sites.

Measurement of tributyl phosphate (TBP) in groundwater at a legacy radioactive waste site and its possible role in contaminant mobilisation.

At many legacy radioactive waste sites, organic compounds have been co-disposed, which may be a factor in mobilisation of radionuclides at these sites. Tri-butyl phosphate (TBP) is a component of waste streams from the nuclear fuel cycle, where it has been used in separating actinides during processing of nuclear fuels. Analyses of ground waters from the Little Forest Legacy Site (LFLS) in eastern Australia were undertaken using solid-phase extraction (SPE) followed by gas chromatographic mass spectrometry (GCMS). The results indicate the presence of TBP several decades after waste disposal, with TBP only being detected in the immediate vicinity of the main disposal area. TBP is generally considered to degrade in the environment relatively rapidly. Therefore, it is likely that its presence is due to relatively recent releases of TBP, possibly stemming from leakage due to container degradation. The ongoing presence and solubility of TBP has the potential to provide a mechanism for nuclide mobilisation, with implications for long term management of LFLS and similar legacy waste sites.

The Sorption Processes of U(VI) onto SiO2 in the Presence of Phosphate: from Binary Surface Species to Precipitation.

The ternary system containing aqueous U(VI), aqueous phosphate and solid SiO2 was comprehensively investigated using a batch sorption technique, in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy, time-resolved luminescence spectroscopy (TRLS), and surface complexation modeling (SCM). The batch sorption studies on silica gel (10 g/L) in the pH range 2.5 to 5 showed no significant increase in U(VI) uptake in the presence of phosphate at equimolar concentration of 20 µM, but significant increase in U(VI) uptake was observed for higher phosphate concentrations. In situ infrared and luminescence spectroscopic studies evidence the formation of two binary U(VI) surface species in the absence of phosphate, whereas after prolonged sorption in the presence of phosphate, the formation of a surface precipitate, most likely an autunite-like phase, is strongly suggested. From SCM, excellent fitting results were obtained exclusively considering two binary uranyl surface species and the formation of a solid uranyl phosphate phase. Ternary surface complexes were not needed to explain the data. The results of this study indicate that the sorption of U(VI) on SiO2 in the presence of inorganic phosphate initially involves binary surface-sorption species and evolves toward surface precipitation.

Inhibition of uranium(VI) sorption on titanium dioxide by surface iron(III) species in ferric oxide/titanium dioxide systems.

Uranium (U(VI)) sorption in systems containing titanium dioxide (TiO(2)) and various Fe(III)-oxide phases was investigated in the acidic pH range (pH 2.5-6). Studies were conducted with physical mixtures of TiO(2) and ferrihydrite, TiO(2) with coprecipitated ferrihydrite, and with systems where Fe(III) was mostly present as crystalline Fe(III) oxides. The presence of ferrihydrite resulted in decreased U(VI) sorption relative to the pure TiO(2) systems, particularly below pH 4, an unexpected result given that the presence of another sorbent would be expected to increase U(VI) uptake. In mixtures of TiO(2) and crystalline Fe(III) oxide phases, U(VI) sorption was higher than for the analogous mixtures of TiO(2) with ferrihydrite, and was similar to U(VI) sorption on TiO(2) alone. X-ray absorption spectroscopy of the TiO(2) system with freshly precipitated Fe(III) oxides indicated the presence of an Fe(III) surface phase that inhibits U(VI) sorption-a reaction whereby Fe(III) precipitates as lepidocrocite and/or ferrihydrite effectively blocking surface sorption sites on the underlying TiO(2). Competition between dissolved Fe(III) and U(VI) for sorption sites may also contribute to the observed decrease in U(VI) sorption. The present study demonstrates the complexity of sorption in mixed systems, where the oxide phases do not necessarily behave in an additive manner, and has implications for U(VI) mobility in natural and impacted environments where Fe(III) (oxyhydr)oxides are usually assumed to increase the retention of U(VI).

Electroacoustic isoelectric point determinations of bauxite refinery residues: different neutralization techniques and minor mineral effects.

Bauxite refinery residue (BRR) is a highly caustic, iron hydroxide-rich byproduct from alumina production. Some chemical treatments of BRR reduce soluble alkalinity and lower residue pH (to values <10) and generate a modified BRR (MBRR). MBRR has excellent acid neutralizing (ANC) and trace-metal adsorption capacities, making it particularly useful in environmental remediation. However, soluble ANC makes standard acid-base isoelectric point (IEP) determination difficult. Consequently, the IEP of a BRR and five MBRR derivatives (sulfuric acid-, carbon dioxide-, seawater-, a hybrid neutralization, i.e, partial CO(2) neutralization followed by seawater, and an activated-seawater-neutralized MBRR) were determined using electroacoustic techniques. Residues showed three significantly different groups of IEPs (p < 0.05) based around the neutralization used. Where the primary mineral assemblage is effectively unchanged, the IEPs were not significantly different from BRR (pH 6.6-6.9). However, neutralizations generating neoformational minerals (alkalinity precipitation) significantly increased the IEP to pH 8.1, whereas activation (a removal of some primary mineralogy) significantly lowered the IEP to pH 6.2. Moreover, surface charging curves show that surfaces remain in the ±30 mV surface charge instability range, which provides an explanation as to why MBRRs remove trace metals and oxyanions over a broad pH range, often simultaneously. Importantly, this work shows that minor mineral components in complex mineral systems may have a disproportionate effect on the observable bulk IEP. Furthermore, this work shows the appropriateness of electroacoustic techniques in investigating samples with significant soluble mineral components (e.g., ANC).

Uranium sorption on various forms of titanium dioxide--influence of surface area, surface charge, and impurities.

Titanium dioxide (TiO(2)) has often served as a model substrate for experimental sorption studies of environmental contaminants. However, various forms of Ti-oxide have been used, and the different sorption properties of these materials have not been thoroughly studied. We investigated uranium sorption on some thoroughly characterized TiO(2) surfaces with particular attention to the influence of surface area, surface charge, and impurities. The sorption of U(VI) differed significantly between samples. Aggressive pretreatment of one material to remove impurities significantly altered the isoelectric point, determined by an electroacoustic method, but did not significantly impact U sorption. Differences in sorption properties between the various TiO(2) materials were related to the crystallographic form, morphology, surface area, and grain size, rather than to surface impurities or surface charge. In-situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopic studies showed that the spectra of the surface species of the TiO(2) samples are not significantly different, suggesting the formation of similar surface complexes. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption.

Assessment of surface area normalisation for interpreting distribution coefficients (K(d)) for uranium sorption.

Adsorption of radionuclides on soils and sediments is commonly quantified by distribution coefficients (K(d) values). This paper examines the relationship between K(d) values for uranium(VI) adsorption and the specific surface area (SSA) of geologic materials. We then investigate the potential applicability of normalising uranium (U) K(d) measurements using the SSA, to produce 'K(a) values' as a generic expression of the affinity of U for the surface. The data for U provide a reasonably coherent set of K(a) values on various solid phases, both with and without ligands. The K(a) representation provides a way of harmonising datasets obtained for materials having different specific surface areas, and accounting for the effects of ligands in different systems. In addition, this representation may assist in developing U sorption models for complex materials. However, a significant limitation of the K(a) concept is that sorption of radionuclides at trace levels can be dominated by interactions with specific surface sites, whose abundances are not reflected by the SSA. Therefore, calculated K(a) values should be interpreted cautiously.