Sorbent materials for rapid remediation of wash water during radiological event relief

Current and emerging technologies for the remediation of difficult-to-measure radionuclides at nuclear sites

Difficult-to-measure radionuclides (DTMRs), defined by an absence of high energy gamma emissions during decay, are problematic in groundwaters at nuclear sites. DTMRs are common contaminants at many nuclear facilities, with (often) long half-lives and high radiotoxicities within the human body. Effective remediation is, therefore, essential if nuclear site end-state targets are to be met. However, due to a lack of techniques for in situ DTMR detection, technologies designed to remediate these nuclides are underdeveloped and tend to be environmentally invasive. With a growing agenda for sustainable remediation and reduction in nuclear decommissioning costs, there is renewed international focus on the development of less invasive technologies for DTMR clean-up. Here, we review recent developments for remediation of selected problem DTMRs (129I, 99Tc, 90Sr and 3H), with a focus on industrial and site-scale applications. We find that pump and treat (P&T) is the most used technique despite efficacy issues for 129I and 3H. Permeable reactive barriers (PRBs) are a less invasive alternative but have only been demonstrated for removal of 99Tc and 90Sr at scale. Phytoremediation shows promise for site-scale removal of 3H but is unsuitable for 129I and 99Tc due to biotoxicity and bioavailability hazards, respectively. No single technique can remediate all DTMRs of focus. Likewise, there has been no successful site-applied technology with high removal efficiencies for iodine species typically present in groundwaters (iodide/I-, iodate/IO3- and organoiodine). Further work is needed to adapt and improve current techniques to field scales, as well as further research into targeted application of emerging technologies.

Logistics simulation of a remediation effort for a hypothetical radiological contamination scenario

To mitigate the effects following a large-scale nuclear or radiological material release in an urban environment and to expedite recovery, the Integrated Wash-Aid Treatment Emergency Reuse System (IWATERS) was developed. IWATERS consists of three operations: washing contaminated surfaces with an ionic wash solution, collecting, and treating the contaminated wash solution on-site to remove contaminants, and reusing the treated solution throughout operations to preserve the clean water resource. This study develops a framework to simulate the logistics of IWATERS deployment, thereby gaining an understanding of the timeline for decontamination operations. For this purpose, the Analysis of Mobility Platform and GoldSim were leveraged for a hypothetical contamination scenario covering 65,200 m2 of an urban center. The framework reveals that remediation progress is limited by several resources, notably the availability of vermiculite, a reactive clay that is required to treat the contaminated wash solution. This study also presents how the simulation approach can be used to characterize alternatives to reduce the influence of limited resources on operational progress. Overall, this work lays the foundation for evaluating different decontamination methods through detailed logistics simulation, i.e., by refining simulation assumptions and expanding the range of scenarios the simulation can depict.

External Dose to Recovery Teams Following a Wide-area Nuclear or Radiological Release Event

ABSTRACT

The common radionuclide 137Cs is a gamma-ray source term for nuclear reactor accidents, nuclear detonations, and potential radionuclide dispersal devices. For wide-area contamination events, one remediation option integrates water washing activities with on-site treatment of water for its immediate reuse. This remediation option includes washing building and roadways via firehose, collecting the wash water, and passing the contaminated water through chemical filtration beds. The primary objective of this study was to quantify the dose incurred to workers performing a remediation recovery effort for roadways and buildings following a wide-area release event. MicroShield® was employed to calculate the dose to workers at the roadway level and to calculate total dose rates while performing washing activities. This study finds that for a realistic contamination scenario for a wide area of a large urban environment, decontamination crews would be subjected to <220 μSv per person, much less than the 50,000 μSv limit for occupational dose. By extrapolation, one decontamination team of 48 people could continue washing operations on a total of 2.8 km2 before reaching their incurred annual dose limits. Though it is unrealistic to assign one team that entire area, we can conclude external dose will not limit worker deployment given the range of contamination levels adopted in this study.

Decontamination of urban surfaces contaminated with radioactive materials and consequent onsite recycling of the waste water

Enhancing rapid remediation strategies is paramount for recovery after a large-scale nuclear contamination event in an urban environment. Some current strategies recommend use of readily available equipment, materials, and facilities to expedite recovery. For example, applying pressurized water to contaminated surfaces may effectively remove radioactive contamination. In this study, a commercial power washer removes soluble forms of ¹⁵²Eu³⁺, ⁸⁵Sr²⁺, and ¹³⁷Cs⁺ contamination from common porous building materials, and computer simulations characterize the recycling of the resultant contaminated wash water. Pressure washing the porous building materials under spray conditions typical with do-it-yourself units improved decontamination factors (DFs) for ¹⁵²Eu compared to low-pressure application of tap water (majority of two-tailed t-test p-values < 0.1), but pressure did not improve DFs for ¹³⁷Cs or ⁸⁵Sr. For both pressurized and low-pressure applications, adding potassium ions (K⁺) to promote ion exchange reactions produced significantly higher DFs for tested radionuclides on asphalt, brick, and concrete. The resultant contaminated wash water can be processed through self-prepared chemical filtration beds of clay and sand. Modeled in a prior study, the beds yielded linear trends (R² > 0.98) in sensitivity analyses between most bed configuration variables and bed performance variables, permitting flexible ad-hoc bed design. The experimental and simulation results led to estimates of the remediation rate and waste generated after cleaning 250 m² of cesium-contaminated concrete from the combined deployment of a power washer and two different mobile treatment beds. The first treatment bed was designed to reduce treatment time and processed 1900 L of wash solution in 70 min using 880 kg of clay/sand infill material. Designed to reduce the solid waste generated, the second bed processed the same solution volume in 1040 min (17 h) using 170 kg of clay/sand infill material. The results of this analysis warrant further investigation of power washing with recycled salt solution as an effective rapid decontamination method with manageable waste.

Penetration of fission product ions into complex solids and the effect of ionic wash methods

During washing of radiologically impacted building surfaces, penetration of radionuclide ions into complex solids associated with these surfaces may occur. This study investigates the penetration of 137Cs, 85Sr, and 152Eu solutions into numerous common building materials and radionuclide behavior when these materials were exposed to a static bath or low-pressure flow of tap water, 0.1 M potassium chloride (KCl), and 0.5 M KCl. The decontamination efficacy and the depth profile for residual contamination were measured to determine the conditions under which applying a wash solution has benefit compared to physically removing the surface material. On asphalt, 70-80% of the radionuclides were found to be within 0.02 mm of the surface. Concrete is more porous than asphalt, and 80% of the radionuclides were within 0.2 mm of the surface for 137Cs and 152Eu and 50-80% for 85Sr. Water effectively removed all contaminants from hard nonporous surfaces. Finally, this paper illustrates that a wash penalty factor concept-defined as ratio of the depth at which 50% of the radioactivity is found in the washed sample divided by the depth at which 50% of radioactivity is found in the control-can serve as a way to quantify whether the wash method increases the depth at which contamination penetrates into the material and thus the material becomes more difficult to decontaminate.

Hydrothermal Desorption of Cs with Oxalic Acid from Hydrobiotite and Wastewater Treatment by Chemical Precipitation

A hydrobiotite (HBT) clay contains more cesium (Cs)-specific adsorption sites than illitic clay, and the capacity of frayed edge sites can increase as the weathering of micaceous minerals proceeds. Thus, Cs can be selectively adsorbed to HBT clay. In this study, we investigated the removal efficiency of non-radioactive (133Cs) and radioactive (137Cs) Cs from HBT, using oxalic acid. We found the minimum optimal concentration of 0.15 M oxalic acid removed more than 90% of Cs. Subsequently, cations and Cs ions were removed using Ca(OH)2 and sodium tetraphenylborate (NaTPB) to treat the washing wastewater generated at the optimum concentration of the desorbent (0.15 M oxalic acid). In order to remove cations and heavy metal ions in the waste solution, Ca(OH)2 was treated at a mass ratio of 0.025 g/mL and pH 9–10 to derive optimal conditions. As a final step, to remove Cs, NaTPB was treated with a mass ratio of 2 mg/mL and reduced to below 0.1 mg/L Cs to find the optimal dose. The novelty of this study is that the amount of radioactive waste can be drastically reduced by removing the non-radioactive cations and heavy metals separately in the first step and removing the remaining radioactive Cs in the second step.

High pressure decontamination of building materials during radiological incident recovery

The release of radiological material from a nuclear incident has the potential to cause extensive radiological contamination requiring rapid decontamination. A promising method for rapid remediation is the use of pressure washers to decontaminate building and street surfaces. Pressure washers utilize both physical removal through surface ablation and chemical removal through desorption of bonded radionuclides. To understand the extent that each removal mechanism is present, overall removals, depth profiles, and wash water were analyzed from the pressure washing of various surfaces contaminated with cesium, strontium, and europium. Removals were dependent on surface type with over 80% of the radionuclides removed from concrete, 50–80% from asphalt, and only 20–25% from brick. Generally, the closer the radionuclide was to the surface of the material, the higher the removal, with europium being removed most readily followed by cesium then strontium, though some exceptions were evident. Comparing these removals and depth profiles of radionuclides in non-decontaminated coupons revealed that cesium and europium are mostly removed through surface ablation. Strontium, on the other hand, is desorbed from the surface, especially from brick and asphalt surfaces. Correspondingly, cesium and europium were attached to the particulates that were likely removed with the pressurized water. Strontium was primarily dissolved in the wash water, supporting the observation that the radionuclide is desorbed from each surface. Finally, the faster the surfaces were brought through the high pressure spray, the lower the removals, arising from decreases in both the physical and desorption mechanisms. Pressure washers were concluded to be a promising decontamination method during radiological incident relief. However, the surface and radionuclide identity must be considered when developing proper procedures.

Silicate coating to prevent leaching from radiolabeled surrogate far-field fallout in aqueous environments

Recent characterization of radioactive particles indicate that a large percentage of the radioactivity observed during the Fukushima Daiichi nuclear meltdown was insoluble ¹³⁷Cs bound within silica microparticles. Therefore, much of the decontamination research performed prior to the Fukushima incident that used either soluble radionuclides deposited onto wet surfaces or large (∼100 μm) particles characteristic of nuclear weapons fallout do not accurately represent the characteristics of potential contamination. Thus, the common practice of extrapolating radioactive decontamination methods generically to all radioactive release events is, at best, suspect. In response, a method to produce chemically-inert, radiolabeled silica particles was developed. Binding ¹⁵²Eu within a sodium silicate coating required proper temperature control and ethanol was beneficial as a volatile dispersant to limit residues. In the end, a step-wise method, which first deposited ¹⁵²Eu or ²⁴¹Am as a nitrate salt, decomposed the salt to a sesquioxide, and finally coated the surface with sodium silicate led to dispersed particles of the desired 2 or 0.5 μm diameters. Dynamic light scattering and scanning election microscopy confirmed the particle size was unchanged. Leaching studies into several common decontaminants were performed to ensure particle inertness. Our approach allows for substitution of other radionuclides making it a robust, simple, and novel method to produce inert particle surrogates for a release event that allows direct comparison of decontamination techniques and contaminant fate studies, greatly aiding the development of response and recovery plans.

In Situ Cs and H Exchange into Gaidonnayite and Proposed Mechanisms of Ion Diffusion

The microporous mineral gaidonnayite Na₂ZrSi₃O₉·2H₂O was studied to better understand its ion-exchange mechanisms, specifically for Cs⁺ and H⁺ ions. In situ Raman spectroscopy, in situ X-ray diffraction (XRD), simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), and in situ X-ray fluorescence were used to determine the exchange processes involved. The Raman spectra contain strong peaks that can be attributed to the vibrational modes for the 3MR symmetric stretch at 500 cm^-1, Si-O-Zr-O chain stretches at 938 cm^-1, and Si-O stretching in the 1000-1100 cm^-1 range. The most prominent Raman shift during ion exchange is found near the 520 cm^-1 peak, which corresponds to distortions of the 3MR substructure of gaidonnayite. In all instances of this study, the 3MR exhibited the highest amount of distortion during ion exchange, and the evolution of this distortion is compared to unit-cell changes as measured from XRD data and elemental changes via XRF. The correlations between the Raman, XRD, and XRF data show rapid deformation of the 3MR during the onset of H⁺ ion exchange in the Na form of gaidonnayite. Even when unit-cell volume changes were small (<3 ų) as in the cases for Cs⁺ into Na-gaidonnayite and Cs⁺ into H-gaidonnayite, significant changes in the ≈520 cm^-1 peak were measured. By comparing XRD data and Raman data, and verifying the cation uptake by XRF, we were able to identify and confirm conformational changes and distortions in the crystal structure before, during, and after Cs⁺ and H⁺ exchange. Cs exchange occurred the fastest and with the greatest capacity when starting in the H-form at room temperature, and at elevated temperatures when starting in the Na-form.

Evaluating Solid Sorbents for Recycling Wash Waters Containing Strontium and Calcium

A system for rapid reduction of radioactive contamination and recycle of contaminated waters is called the Integrated Wash-Aid, Treatment, and Emergency Reuse System (IWATERS). First developed for cesium contaminations, IWATERS prescribes the use of salt and surfactant additives to decontaminate radionuclides from urban surfaces. The water is collected and recycled after passing through reactive filtration beds containing selective sorbents. To adapt the IWATERS for strontium contaminations, potential additives to enhance its decontamination from urban surfaces are identified. One possible additive is calcium (Ca2+). However, its concentration can have a very strong detrimental effect on the ability of selective sorbents to remove strontium from spent wash water. We recognized that studies on off-the-shelf sorbents that include Ca2+ concentrations at relevant levels (greater than millimolar) are absent in the literature. To understand better the effect of Ca2+, we completed a literature review, batch tests, and surface complexation modeling to reveal few sorbent options. Only silico-titanate sorbents exhibited high K d values in the presence of Ca2+, but have significant drawbacks in cost and availability. Given the state of the art, it is imperative that alternatives to alkaline earth ions in the IWATERS be identified to permit in situ recycle of the wash waters.

Decontamination of radioactive wastewater: State of the art and challenges forward

Radioactive substances have been widely used in many industrial sectors, e.g. nuclear power station, biomedical engineering, etc. With increasing applications of nuclear technology, more and more radioactive wastewater is being generated via different channels, which indeed is posing an emerging challenge and threat to the environment and human health. Given such a situation, this review attempts to offer a holistic view with regard to the state of the art of technology for decontamination of radioactive wastewater as well as shed lights on the challenges forward. Different from reclamation of other types of wastewaters, the most challenging issue in decontamination of radioactive wastewater is the effective stabilization and solidification of soluble radioactive nuclides present in wastewater, which are critical for final disposal. Moreover, the potential risk of human exposure to wastewater radiation needs to be carefully assessed, and this issue should also be taken into consideration in the selection, design and operation of the radioactive wastewater treatment process. These clearly differentiate the treatment principle of radioactive wastewater from those of traditional industrial and municipal wastewaters. Lastly, the challenges from the perspectives of technology development, environmental and human health impacts and possible solutions forward are also elucidated.

Phosphonate and carboxylic acid co-functionalized MoS2 sheets for efficient sorption of uranium and europium: Multiple groups for broad-spectrum adsorption

It is significant to develop novel materials and techniques for efficient removal of radionuclides from radioactive wastes due to the radioactive and chemical toxicity. In this paper, we report a strategy for broad-spectrum adsorption of radionuclides by multiple groups-decorated adsorbents. Specifically, the adsorbents were prepared by grafting diethyl-(4-vinylbenzyl) phosphonate and maleic anhydride copolymers onto molybdenum disulfide sheets for the sorption of uranium(VI) and europium(III). The sorption efficiencies exhibited a dependency on pH, contact time and initial concentrations. The sorption reached the equilibrium within 60 min and followed a pseudo-second-order kinetic model. The maximum sorption capacities of the sorbents were 448.4 mg/g and 171.2 mg/g at pH 4.0 and 298.15 K for uranium(VI) and europium(III), respectively. The sorbent possessed a high efficiency of 98% in five sorption-desorption cycles without damage in chemical structures. XPS spectra showed that the sorption of uranium(VI) and europium(III) on the sorbents were originated from the interaction between multiple groups (such as sulfur, COOH, PO and PO) and uranium/europium. This work demonstrates that the adsorbent can be utilized as a promising material for the separation of broad-spectrum radionuclides from an aqueous solution.