Sub-ppm Level Research Articles

Achieving Sub-ppm Sensitivity in SO2 Detection with a Chemically Stable Covalent Organic Framework.

We report the inaugural experimental investigation of covalent organic frameworks (COFs) to address the formidable challenge of SO2 detection. Specifically, an imine-functionalized COF (SonoCOF-9) demonstrated a modest and reversible SO2 sorption of 3.5 mmol g-1 at 1 bar and 298 K. At 0.1 bar (and 298 K), the total SO2 uptake reached 0.91 mmol g-1 with excellent reversibility for at least 50 adsorption-desorption cycles. An isosteric enthalpy of adsorption (ΔHads) for SO2 equaled -42.3 kJ mol-1, indicating a relatively strong interaction of SO2 molecules with the COF material. Also, molecular dynamics simulations and Møller-Plesset perturbation theory calculations showed the interaction of SO2 with π density of the rings and lone pairs of the N atoms of SonoCOF-9. The combination of experimental data and theoretical calculations corroborated the potential use of this COF for the selective detection and sensing of SO2 at the sub-ppm level (0.0064 ppm of SO2).

Ultrasensitive Mid-Infrared Optical Gas Sensor Based on Germanium-on-Insulator Photonic Circuits with Limit-of-Detection at Sub-ppm Level

Ultrasensitive Mid-Infrared Optical Gas Sensor Based on Germanium-on-Insulator Photonic Circuits with Limit-of-Detection at Sub-ppm Level

Integrating Metal-Phenolic Networks-Mediated Separation and Machine Learning-Aided Surface-Enhanced Raman Spectroscopy for Accurate Nanoplastics Quantification and Classification.

Increasing accumulation of nanoplastics across ecosystems poses a significant threat to both terrestrial and aquatic life. Surface-enhanced Raman scattering (SERS) is an emerging technique used for nanoplastics detection. However, the identification and classification of nanoplastics using SERS faces challenges regarding sensitivity and accuracy as nanoplastics are sparsely dispersed in the environment. Metal-phenolic networks (MPNs) have the potential to rapidly concentrate and separate various types and sizes of nanoplastics. SERS combined with machine learning may improve prediction accuracy. Herein, we report the integration of MPNs-mediated separation with machine learning-aided SERS methods for the accurate classification and high-precision quantification of nanoplastics, which is tailored to include the complete region of characteristic peaks across diverse nanoplastics in contrast to the traditional manual analysis of SERS spectra on a singular characteristic peak. Our customized machine learning system (e.g., outlier detection, classification, quantification) allows for the identification of detectable nanoplastics (accuracy 81.84%), accurate classification (accuracy > 97%), and sensitive quantification of various types of nanoplastics (polystyrene (PS), poly(methyl methacrylate) (PMMA), polyethylene (PE), and poly(lactic acid) (PLA)) down to ultralow concentrations (0.1 ppm) as well as accurate classification (accuracy > 92%) of nanoplastic mixtures at a subppm level. The effectiveness of this approach is substantiated by its ability to discern between different nanoplastic mixtures and detect nanoplastic samples in natural water systems.

Harmonic errors of a 9.4 T all-REBCO NMR magnet affected by screening current and geometric inconsistency of coated conductors

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Sub-ppm-level detection of nanoplastics using au nanograting and application to disposable plasticware

The detection of sub-ppm levels of nanoplastics using a surface-enhanced Raman scattering sensor was demonstrated. The proposed Au nanograting as surface-enhanced Raman scattering sensor, fabricated using nanoimprint lithography and oblique angle deposition, was designed to contain nanoplastics with enhanced local plasmonic fields. Electromagnetic simulations and Raman measurements show that the plasmonic field enhanced inside the nanograting enables the detection of Raman signal from 50-nm-sized polystyrene nanoparticles in 3-μL-droplets at a concertation of 0.1 ppm. Furthermore, the proposed sensor was applied to detect polystyrene in microwave-heated water in a disposable bowl. The results show that the proposed sensor offers promising potential for detecting nanoplastics released from plasticware.

Development of SnO2 functionalized In2O3 porous microrods for trace level detection of formaldehyde at room temperature

Room temperature detection is a crucial objective in semiconductor sensor research, given its inherent benefits including reduced energy consumption, extended sensor lifespan, and significant reduction of safety hazards. Herein, novel room temperature sensing materials, SnO2-modified In2O3 porous microrods were synthesized, which exhibited exceptional sensing capability towards formaldehyde at room temperature. Specifically, the 5%-SnO2/In2O3 porous microrods demonstrate an impressive sensing response across varying formaldehyde concentrations, even at sub-ppm levels (0.1 ppm, 5.22), with a theoretical detection limit of 5.47 ppb, thereby enabling the detection of trace amounts of formaldehyde. Moreover, the sensors exhibit superior formaldehyde selectivity, rapid response characteristic, as well as good stability and reproducibility. The exceptional formaldehyde sensing ability of the SnO2/In2O3 sensor primarily arises from the creation of n-n heterojunctions at the SnO2/In2O3 interface, effectively modulating the interfacial potential. Additionally, the presence of SnO2 enhances the amount of surface-adsorbed oxygen species and active sites, thereby boosting sensing response. The porous rod-like structures further facilitate gas adsorption and diffusion, enhancing gas sensing capabilities. This work presents a novel strategy for constructing SnO2-In2O3 heterojunctions to achieve room-temperature formaldehyde detection.

High Sensitivity and High Throughput Magnetic Flow CMOS Cytometers with 2D Oscillator Array and Inter-Sensor Spectrogram Cross-correlation.

In the paper, we present an integrated flow cytometer with a 2D array of magnetic sensors based on dual-frequency oscillators in a 65-nm CMOS process, with the chip packaged with microfluidic controls. The sensor architecture and the presented array signal processing allows uninhibited flow of the sample for high throughput without the need for hydrodynamic focusing to a single sensor. To overcome the challenge of sensitivity and specificity that comes as a trade off with high throughout, we perform two levels of signal processing. First, utilizing the fact that a magnetically tagged cell is expected to excite sequentially an array of sensors in a time-delayed fashion, we perform inter-site cross-correlation of the sensor spectrograms that allows us to suppress the probability of false detection drastically, allowing theoretical sensitivity reaching towards sub-ppM levels that is needed for rare cell or circulating tumor cell detection. In addition, we implement two distinct methods to suppress correlated low frequency drifts of singular sensors-one with an on-chip sensor reference and one that utilizes the frequency dependence of the susceptibility of super-paramagnetic magnetic beads that we deploy as tags. We demonstrate these techniques on a 7×7 sensor array in 65 nm CMOS technology packaged with microfluidics with magnetically tagged dielectric particles and cultu lymphoma cancer cells.

Cobalt-Doped Ceria Sensitizer Effects on Metal Oxide Nanofibers: Heightened Surface Reactivity for High-Performing Chemiresistive Sensors.

Chemiresistive gas sensors based on semiconducting metal oxides typically rely on noble metal catalysts to enhance their sensitivity and selectivity. However, noble metal catalysts have several drawbacks for practical utilization, including their high cost, their propensity for spontaneous agglomeration, and poisoning effects with certain types of gases. As such, in the interest of commercializing the chemiresistive gas sensor technology, we propose an alternative design for a noble-metal-free sensing material through the case study of Co-doped ceria (Co-CeO2) catalysts embedded in a SnO2 matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co-CeO2 catalyst nanoparticles integrated with SnO2 nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co-CeO2@SnO2 NFs exhibited superior gas sensing characteristics toward isoprene (C5H8) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co-CeO2 catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO2 sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C5H8 (compared to pristine SnO2), exceptionally high selectivity, and a low detection limit of 100 ppb. The sensor also exhibited high stability for prolonged response-recovery cycles, attesting to the strong anchoring of Co-CeO2 catalysts in the SnO2 matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co-CeO2, demonstrate strong potential to completely replace noble metal catalysts, thereby advancing the development of the commercially viable chemiresistive gas sensors free from noble metals, capable of detecting target gases at sub-ppm levels.

Highly efficient dual photoredox/copper catalyzed atom transfer radical polymerization achieved through mechanism-driven photocatalyst design

Atom transfer radical polymerization (ATRP) with dual photoredox/copper catalysis combines the advantages of photo-ATRP and photoredox-mediated ATRP, utilizing visible light and ensuring broad monomer scope and solvent compatibility while minimizing side reactions. Despite its popularity, challenges include high photocatalyst (PC) loadings (10 to 1000 ppm), requiring additional purification and increasing costs. In this study, we discover a PC that functions at the sub-ppm level for ATRP through mechanism-driven PC design. Through studying polymerization mechanisms, we find that the efficient polymerizations are driven by PCs whose ground state oxidation potential—responsible for PC regeneration—play a more important role than their excited state reducing power, responsible for initiation. This is verified by screening PCs with varying redox potentials and triplet excited state generation capabilities. Based on these findings, we identify a highly efficient PC, 4DCDP-IPN, featuring moderate excited state reducing power and a maximized ground state oxidation potential. Employing this PC at 50 ppb, we synthesize poly(methyl methacrylate) with high conversion, narrow molecular weight distribution, and high chain-end fidelity. This system exhibits oxygen tolerance and supports large-scale reactions under ambient conditions. Our findings, driven by the systematic PC design, offer meaningful insights for controlled radical polymerizations and metallaphotoredox-mediated syntheses beyond ATRP.

High-Sensitivity Differential Helmholtz Photoacoustic System Combined with the Herriott Multipass Cell and Its Application in Seed Respiration.

A highly sensitive photoacoustic detection system using a differential Helmholtz resonator (DHR) combined with a Herriott multipass cell is presented, and its implementation to sub-ppm level carbon dioxide (CO2) detection is demonstrated. Through the utilization of erbium-doped optical fiber amplifier (EDFA), the laser power was amplified to 150 mW. Within the multipass cell, a total of 22 reflections occurred, contributing to an impressive 33.6 times improvement in the system sensitivity. The normalized noise equivalent absorption coefficient (NNEA) was 8.64 × 10-11 cm-1·W·Hz-1/2 [signal-to-noise ratio, (SNR) = 1] and according to the Allan variance analysis, a minimum detection limit of 500 ppb could be achieved for CO2 at 1204 s, which demonstrates the long-term stability of the system. The system was applied to detect the respiration of rice and upland rice seeds. It is demonstrated that the system can monitor and distinguish the respiration intensity and respiration rate of different seeds in real time.

Freshwater Recovery and Removal of Cesium and Strontium from Radioactive Wastewater by Methane Hydrate Formation.

As human society has advanced, nuclear energy has provided energy security while also offering low carbon emissions and reduced dependence on fossil fuels, whereas nuclear power plants have produced large amounts of radioactive wastewater, which threatens human health and the sustainability of water resources. Here, we demonstrate a hydrate-based desalination (HBD) technology that uses methane as a hydrate former for freshwater recovery and for the removal of radioactive chemicals from wastewater, specifically from Cs- and Sr-containing wastewater. The complete exclusion of radioactive ions from solid methane hydrates was confirmed by a close examination using phase equilibria, spectroscopic investigations, thermal analyses, and theoretical calculations, enabling simultaneous freshwater recovery and the removal of radioactive chemicals from wastewater by the methane hydrate formation process described in this study. More importantly, the proposed HBD technology is applicable to radioactive wastewater containing Cs+ and Sr2+ across a broad concentration range of low percentages to hundreds of parts per million (ppm) and even subppm levels, with high removal efficiency of radioactive chemicals. This study highlights the potential of environmentally sustainable technologies to address the challenges posed by radioactive wastewater generated by nuclear technology, providing new insights for future research and development efforts.

Autonomous execution of highly reactive chemical transformations in the Schlenkputer

We design a modular programmable inert-atmosphere Schlenkputer (Schlenk-line computer) for the synthesis and manipulation of highly reactive compounds, including those that are air and moisture sensitive or pyrophoric. Here, to do this, we constructed a programmable Schlenk line using the Chemputer architecture for the inertization of glassware that can achieve a vacuum line pressure of 1.5 × 10−3 mbar, and integrated a range of automated Schlenk glassware for the handling, storage and isolation of reactive compounds at sub-ppm levels of O2 and H2O. This has enabled automation of a range of common organometallic reaction types for the synthesis of four highly reactive compounds: [Cp2TiIII(MeCN)2]+, CeIII{N(SiMe3)2}3, B(C6F5)3 and {DippNacNacMgI}2, which are variously sensitive to temperature, pressure, water and oxygen. Automated crystallization, filtration and sublimation are demonstrated, along with analysis using inline nuclear magnetic resonance or reaction sampling for ultraviolet–visible spectroscopy. Finally, we demonstrate low-temperature reactivity down to −90 °C as well as safe handling and quenching of alkali metal reagents using dynamic feedback from an in situ temperature probe.

Cavity assisted high-resolution THz spectrometer

The analysis of gases by THz radiation offers a high degree of discrimination due to the narrow linewidths that are observed at low pressure. The sensitivity of existing high-resolution instruments is limited by the availability and performance of critical system components. This study uses two key components with physical structures at the wavelength scale to realise a high finesse THz cavity. The cavity is characterised and incorporated into a spectrometer. Sensitivity limits of the instrument are experimentally demonstrated for trace and pure gases. Both CEAS (Cavity Enhanced Absorption Spectroscopy) and CRDS (Cavity Ring-Down Spectroscopy) configurations are shown to give sub-ppm detection levels. The cavity has also been used to measure the atmospheric losses.

A highly sensitive and reproducible fluorescence sensor for continuously measuring hydrogen peroxide at the sub-ppm level.

A highly sensitive fluorescence sensor for monitoring low concentrations of hydrogen peroxide was designed. The sensor employs the commercially available palladium or platinum metal on activated charcoal as catalysts to decompose hydrogen peroxide into water and molecular oxygen. The produced oxygen concentration can be measured in real time using an oxygen-sensitive layer doped with photostable oxygen probes. The sensor exhibits high sensitivity that is able to measure hydrogen peroxide concentration down to 20 ppb and can measure hydrogen peroxide concentration in the range of 0.1-100 ppm and 0.02-100 ppm, respectively. The response is fully reversible and the typical response time is less than one minute, which makes it suitable to continuously measure hydrogen peroxide over a long duration. Due to the excellent batch-to-batch consistency of palladium or platinum metal on activated charcoal, the sensor can be massively produced with good reproducibility and affordable price, which holds great potential for constructing sensors for industrial and practical applications.

Thermocatalytic oxidation of a binary mixture of formaldehyde and toluene at ambient levels by a titanium dioxide supported platinum catalyst

The thermocatalytic oxidative potential of various supported noble metal catalysts (SNMCs) is well-known for hazardous volatile organic compounds (VOCs), e.g., formaldehyde (FA) and toluene. However, little is known about SNMC performance against ambient VOC pollution with low concentration (subppm levels) relative to industrial effuluents with high concentrations (several hundred ppm). Here, the thermocatalytic oxidation performance of a titanium dioxide (TiO2)-supported platinum catalyst (Pt/TiO2) has been evaluated for a low-concentration binary mixture of FA and toluene at low temperatures and in the dark. A sample of TiO2 containing 1 wt% Pt with thermal reduction pre-treatment under hydrogen achieved 100 % conversion of FA (500 ppb) and toluene (100 ppb) at 130 °C and a gas hourly velocity of 59,701 h−1. Its catalytic activity was lowered by either a decrease in catalyst mass or an increase in VOC concentration, relative humidity, or flow rate. In situ diffuse reflectance infrared Fourier transform spectroscopy, density functional theory simulations, and molecular oxygen (O2) temperature–programmed desorption experiments were used to identify possible VOC oxidation pathways, reaction mechanisms, and associated surface phenomena. The present work is expected to offer insights into the utility of metal oxide-supported Pt catalysts for the low-temperature oxidative removal of gaseous VOCs in the dark, primarily for indoor air quality management.

The Role of Zn Ions in the Structural, Surface, and Gas-Sensing Properties of SnO2:Zn Nanocrystals Synthesized via a Microwave-Assisted Route.

Although semiconducting metal oxide (SMOx) nanoparticles (NPs) have attracted attention as sensing materials, the methodologies available to synthesize them with desirable properties are quite limited and/or often require relatively high energy consumption. Thus, we report herein the processing of Zn-doped SnO2 NPs via a microwave-assisted nonaqueous route at a relatively low temperature (160 °C) and with a short treatment time (20 min). In addition, the effects of adding Zn in the structural, electronic, and gas-sensing properties of SnO2 NPs were investigated. X-ray diffraction and high-resolution transmission electron microscopy analyses revealed the single-phase of rutile SnO2, with an average crystal size of 7 nm. X-ray absorption near edge spectroscopy measurements revealed the homogenous incorporation of Zn ions into the SnO2 network. Gas sensing tests showed that Zn-doped SnO2 NPs were highly sensitive to sub-ppm levels of NO2 gas at 150 °C, with good recovery and stability even under ambient moisture. We observed an increase in the response of the Zn-doped sample of up to 100 times compared to the pristine one. This enhancement in the gas-sensing performance was linked to the Zn ions that provided more surface oxygen defects acting as active sites for the NO2 adsorption on the sensing material.

(Invited) Silicon Quantum Dot Single-Electron Pumps for the Closure of the Quantum Metrology Triangle

Si quantum dot (QD) devices fabricated by MOSFET technology are promising for future and ultimate quantum electronics such as quantum electrical standards and quantum computers. We have developed Si tunable-barrier QD devices with a quasi-gate-all-around (GAA) nanowire [1], which are designed to be used as basic elements such as single-electron (SE) transfer devices and qubits; the precise manipulation of charges and their states are realized in the QDs. The device is especially suitable for high-speed tunable-barrier SE pumps driven at GHz frequencies [2,3] which can generate an electric current via a single QD precisely equal to the elementary charge times the driving frequency and thereby are promising for quantum current standards [4].To achieve a high accuracy using tunable-barrier SE pumps [5-7], proper designs of device structures are required to secure robust charge quantization. The potential shape of the QD and the entrance tunnel barrier [8] are critical parameters to dictate the pump accuracy because they determine the charging energy and energy filtering effect during SE capture into the QD. Furthermore, deep understanding of electron dynamics such as mechanism crossover of the dynamical charge quantization process [9] and quantum nonadiabatic excitation [10]/electron-phonon relaxation inside the QD are important to get optimal accuracy and extend the frequency breakdown of SE pumps. We have recently developed a device simulator of Si GAA-nanowire SE pumps [11], which is a useful tool to find optimal design and control parameters in all the above-mentioned respects.One of the long-desired targets of SE pumps is to close the quantum metrology triangle (QMT) [12] by checking the consistency among three quantum electrical standards: the SE current standard, the quantum Hall resistance standard, and Josephson voltage standard (JVS). The Japanese QMT project [13] has aimed to conduct the QMT experiment with sub-ppm uncertainty in a single dilution refrigerator by placing all the quantum electric standards inside it. Our key approaches are to use quantum Hall array resistance (QHAR) [14] and parallelized Si SE pumps, which provide higher resistance and current respectively, making the generated voltage more easily compared to JVS. In the project we have succeeded in developing a current-reversal technique with electromechanical relays to cancel out the drift and low frequency noise from various sources in the measurement setup, and applying it to high accuracy measurement of current delivered by a Si SE pump [15]. We have also recently performed a comparison measurement of two Si SE pump operated in parallel and thereby have confirmed a sub-ppm level accuracy [16]. All these progresses will lead to a more precise closure of QMT at deep sub-ppm level in the new future. Si MOSFET technology continues to expand its versatility into quantum electronics technology. This work was supported by JSPS KAKENHI Grant Number JP18H05258. A. Fujiwara et al., Appl. Phys. Lett. 84, 1323 (2004); Appl. Phys. Lett. 88 053121 (2006).A. Fujiwara et al., Appl. Phys. Lett. 92, 042102 (2008)G. Yamahata et al., Appl. Phys. Lett. 106, 023112 (2015).N.-H. Kaneko et al., Meas. Sci. Technology 27 032001 (2016).B. Kaestner and V. Kashcheyevs, Rep. Prog. Phys. 78, 103901 (2015).G. Yamahata et al., Appl. Phys. Lett. 109, 013101 (2016).S. P. Giblin et al., Metrologia 56, 044004 (2019).N. Johnson et al., Appl. Phys. Lett. 115, 162103 (2019).G. Yamahata et al., Phys. Rev. B 103, 245306 (2021).G. Yamahata et al., Nat. Nanotechnol. 14, 1019 (2019).A. Fujiwara et al., in the Conference on Precision Electromagnetic Measurements (CPEM 2022).F. Piquemal and G. Genevès, Metrologia 37, 207 (2000).https://www.jsps.go.jp/file/storage/grants/j-grantsinaid/12_kiban/ichiran_30/e-data/h_30_eng_18h05258.pdf; http://www.brl.ntt.co.jp/people/afuji/kakenS/ (only in Japanese).T. Oe et al., IEEE Trans. Instrum. Meas. 66, 1475 (2017).S. Nakamura et al., IEEE Trans. Instrum. Meas, under review.S. Nakamura et al., in preparation.

A compact portable photoacoustic sensor for sub-ppm level SO2 detection with a DUV-LED and a non-resonant photoacoustic cell

A photoacoustic module based on a novel 273 nm deep-ultraviolet light-emitting diode (DUV-LED) was developed for sulfur dioxide (SO2) detection. The DUV-LED with an emission angle of 60° emitted a power of 10 mW. A high-sensitivity non-resonant photoacoustic cell with a gas chamber volume of only 0.39 mL was introduced to reduce the thermal noise generated by the contact of scattered DUV light with the photoacoustic cell. Assembly of the DUV-LED with the non-resonant photoacoustic cell resulted in a robust and portable photoacoustic module without any moving parts. The minimum detection limit of 725 ppb was achieved with 1s integration time at room temperature and atmospheric pressure. Furthermore, the photoacoustic module performance was evaluated in terms of DUV modulation frequency, linearity and the long-term stability.

Reliable NO2 sensing and alert system based on Pd-PdO thin film with sub ppm level detection at room temperature

Human and environmental health is greatly affected due to toxic gas emissions in the environment from power generation, transportation and other industries. For real time monitoring of polluting gases, gas sensor should be reliable, robust and stable in all environmental conditions. We report, nanostructured PdO thin film based portable and low voltage (< 5 V) operating NO2 sensor for sensing sub-ppm level of NO2, with limit of detection (LOD) 80 ppb at room-temperature. To optimize the sensor response of PdO thin film prepared by cost effective self-assembly technique films were calcined at 200 °C, 300 °C and 400 °C. Effect of calcination temperatures on film’s surface morphology, composition and energy band gap have been studied using various techniques. The charge carrier concentration, charge carrier transport mechanism, sensitivity, selectivity, repeatability, reproducibility, reliability, linearity etc. have been studied. PdO thin films calcined at 400 °C are most sensitive with approx. 60% and 100% response for 500 ppb and 1 ppm NO2, respectively at room-temperature. After stable response upto 6 months and more was obtained for these, a portable, pocket size electronic device of size (8 × 5 cm) was fabricated and tested successfully, coupling with the sensor film to prepare a wearable sensor.

CO2 competes with radioactive chemicals for freshwater recovery: Hydrate-based desalination

Here, we introduce CO2 hydrate-based desalination (CHBD) technology for freshwater recovery from radioactive wastewater, for water particularly containing Cs and Sr. The hydrate equilibrium curves of CO2 hydrates shift towards lower temperature and higher pressure regions as the concentrations of CsCl and SrCl2 increase. X-ray diffraction and Raman spectroscopy measurements found that neither CsCl nor SrCl2 can affect the structure of CO2 hydrates. The high-pressure micro-differential scanning calorimetric results demonstrate that CO2 hydrates in the presence of CsCl and SrCl2 started to dissociate at lower temperatures due to the enrichment of CsCl and SrCl2 in the remaining solutions. The formation kinetics results indicate that increases in the concentrations of the radioactive chemicals lead to a decrease in the initial reaction rate and sub-cooling temperature. Solid-state nuclear magnetic resonance spectroscopy was utilized to confirm the exclusion of radioactive isotopes from solid gas hydrates. Importantly, the CHBD technology proposed in this study is applicable to radioactive wastewater containing Cs+ and Sr2+ across broad concentration ranges, spanning from a percent to hundreds of parts per million (ppm), and even sub-ppm levels, with comparable recovery efficiency. This study presents new insights into the potential of environmentally sustainable technologies to overcome the challenges posed by radioactive wastewater.