Low-level Radioactive Wastewater Research Articles

A liquid scintillation analysis method for low-level radioactive wastewater

There is currently general concern over low-level radioactive wastewater from the development of nuclear industry. In this paper, a method based on an ultralow-level liquid scintillation spectrometer for measuring uranium radioactivity in low-level radioactive wastewater is proposed. This method can easily and quickly measure the radioactivity level of uranium in samples and can even distinguish the main isotopes of uranium. The liquid scintillation method directly provides results in units of radioactivity activity concentration, which are more convenient for comparison with relevant national standards to determine whether the emission standards are met. The lowest limit of detection of this method is 0.014 Bq l−1 within 600 min.

Effects of Gamma Irradiation on Organic Membrane Materials

Membranes have been widely used in low-level radioactive wastewater (LLRW) treatment and are under irradiation as a result of radioactive nuclides present in the wastewater, which may cause damage to the membranes and weaken their performances. Irradiation-induced material property changes of several organic membrane matrices and modifiers at different gamma irradiation doses were investigated in this work. The organics and membrane samples were irradiated using a 60Co source at a range of irradiation doses of 0 to 100 kGy. The effects of irradiation on these materials were detected using Fourier transform infrared spectroscopy spectra, ultraviolet spectra, and ion chromatography (used to detect membrane leakage). The results indicated that chain scission and cross linking occurred simultaneously in the membrane matrices, while the modifiers tended to polymerize during the irradiation process. As the irradiation dose increased, the chain scission and polymerization became more significant. The polyamide membrane was observed to be more irradiation tolerant in comparison with the other membranes used in this study. In regard to the modifiers, polyvinyl alcohol and 2,3-epoxypropyl trimethyl ammonium chloride showed significant structural changes at an irradiation dose of 2 kGy and polyetherimide and methyl methacrylate at an irradiation dose of 100 kGy, while chain scission was not detected in the other modifiers at irradiation doses of 2, 10, and 100 kGy, indicating that they remained relatively stable at these irradiation doses. These findings provide useful information for the application of membrane technologies in treating LLRW.

The application of polyethylenimine grafting reverse osmosis membrane in treating boron‐containing low‐level radioactive wastewaters

AbstractBACKGROUNDReverse osmosis (RO), which is widely utilized in wastewater treatment, has been proven to be an effective tool for ion separation with high salts rejection. However, owing to the extremely low concentration of nuclides in boron‐containing low‐level radioactive wastewaters (LRWs), there is a significant reduction in rejection of nuclides by using commercial RO membranes. In this study, in order to obtain higher nuclides rejection under trace‐level condition, novel polyamide RO membranes were fabricated by grafting polyethylenimine (PEI) with different molecular weights on commercial RO membrane surfaces.RESULTSThe properties of PEI‐modified membranes were analyzed, such as surface charge, morphology and hydrophilicity. The results showed that zeta potential at pH 7 increased from −22.89 mV to 20.19 mV, which indicated that surface charge altered from negative to positive. Meanwhile, hydrophilicity of membranes was improved since the contact angle decreased from 65° to 36°. The decontamination factor (DF) of cesium(I) increased from 11.69 to 114.23 and the DF of cobalt(II) increased from 28.62 to 3306.88.CONCLUSIONThe laboratory‐scale experiments with cross‐flow demonstrated the effective rejection of nuclides without obvious membrane flux loss of PEI‐modified membranes. In addition, PEI‐modified membranes offered a stable nuclides removal under high‐level background boron and various pH conditions, which demonstrated a possible application in boron‐containing LRW treatment. © 2019 Society of Chemical Industry

Selective adsorption of Cs+ by MXene (Ti3C2Tx) from model low-level radioactive wastewater

This study explored whether MXene (Ti3C2Tx) could remove radioactive Cs+ from model nuclear wastewater. Various adsorption tests were performed and the physical aspects of the interaction were investigated. We varied the MXene dosage, Cs+ initial concentration, solution pH, solution temperature and exposure time. MXene adsorption exhibited very fast kinetics, based on the fact that equilibrium was achieved within 1 h. MXene exhibited an outstanding adsorption capacity (148 mg g−1) at adsorbent and adsorbate concentrations of 5 and 2 mg L−1, respectively, at neutral pH condition (i.e., pH 7). We explored Cs+ adsorption by MXene in the presence of four different ions (NaCl, KCl, CaCl2 and MgCl2) and three different organic acids (sodium oleate, oxalic acid, and citric acid). The Cs+ removal rate changed in the presence of these components; adsorption of Cs+ by MXene thus involved ion exchange, supported by both Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. We confirmed that MXene was re-usable for at least four cycles. MXene is cost-effective and practical when used to adsorb radionuclides (e.g., Cs+) in nuclear wastewater.

Adsorption of Sr(II) from water by mercerized bacterial cellulose membrane modified with EDTA

The treatment of low-level radioactive wastewater is a critical and considerable challenge. Bacterial cellulose membrane (BCM) modified with ethylenediaminetetraacetic acid (EDTA) using (3-aminopropyl) triethoxysilane (APTES) as a crosslinker were used to remove Sr2+ in this work. SEM, XPS, and FTIR were used to characterize the morphology, structure, chemical shift, and functional groups of the as-prepared adsorbent. The synthesized BCM@APTES-EDTA presented a three-layer structure of membrane-net-membrane with nano-sized fibers (<100 nm). The adsorption of Sr2+ onto BCM@APTES-EDTA was investigated as a function of contact time and initial concentration of Sr2+. Results showed that the adsorption of Sr2+ followed the pseudo second-order kinetic model (R2 = 0.999), and fitted well with the Langmuir isotherm model (R2 = 0.996). The maximum adsorption capacity was calculated to be 44.86 mg g-1, which was comparable to other adsorbents. Additionally, the mechanism of Sr2+ adsorbed by the as-prepared adsorbent was studied through FTIR and XPS analysis, which indicated that the tertiary amines and carboxylate from grafted EDTA participated in the adsorption of Sr2+.

Removal of strontium from simulated low-level radioactive wastewater by nanofiltration.

The performance of different nanofiltration (NF) membranes for the treatment of strontium-containing radioactive wastewater was investigated. The effects of the initial strontium concentration, solution pH and complexation phenomena on strontium removal were described. For all the three membranes, the strontium rejection increased with decreasing initial strontium concentration. Meanwhile, the strontium rejection was minimum at the membrane isoelectric point (pH 5) primarily due to decreased co-ion electrostatic repulsion. In the presence of a complexing agent (polyacrylic acid or ethylenediamine tetraacetic acid disodium salt), the strontium rejection was generally higher than those obtained without a complexing agent for NF 270 and XN 45. Membrane cleaning experiments were also conducted to recover the performance of the membranes, which exhibited degradation during long-time filtration. The performance of the membranes after cleaning was close to that of the virgin membranes, especially for XN 45, whose recovery percentage was nearly 100%.

The influence of salts on the reverse osmosis performance treating simulated boron‐containing low level radioactive wastewater

AbstractBACKGROUNDWith the rapid development of nuclear energy in China, a broad spectrum of low level radioactive wastewater (LLRW) has been generated and must be purified prior to discharging to the environment. Reverse osmosis (RO) has been proved an effective technology in the treatment of high boron‐containing LLRW in nuclear power plants (NPPs). However, there is always a large amount of salts in the LLRW as background compounds. It is important to understand the influences of salts on the rejection of nuclides and boron.RESULTSA lab‐scale cross‐flow membrane filtration unit was used. With the increase of ion concentration, the permeate flux decreased by 15% and 25% for BW30XFR and LE, respectively. Nuclides rejection increased by 2–8% and boron rejection increased by 5–10% when raising salts concentration to 2.5 mmol L–1. Divalent ions such as Mg2+ and SO42− had a stronger impact than monovalent ions such as Na+ and Cl− on nuclides and boron rejection.CONCLUSIONIncreasing ion strength promoted the rejection of trace nuclides and boron, but at the cost of membrane flux loss. The results of dealing with simulated wastewater would give us information to better understand how to treat real radioactive wastewater. © 2018 Society of Chemical Industry

Poly(vinyl pyrrolidone) modified poly(vinylidene fluoride) ultrafiltration membrane via a two-step surface grafting for radioactive wastewater treatment

Abstract Poly(vinylidene fluoride) (PVDF) ultrafiltration (UF) membranes were modified with poly(vinyl pyrrolidone) (PVP) through a two-step surface grafting method in order to enhance the antifouling properties. PVP of small molecular was used to modify the membrane pores and cross-linked PVP modified the membrane surface. The pore modification enhanced the membrane flux from 130.0 L m−2 h−1 to 170.7 L m−2 h−1, and the surface modification increased the membrane hydrophilicity (i.e., the contact angle decreases from 93° to 83°). During the fouling tests with bovine serum albumin (BSA) solution and laboratory simulation low-level radioactive wastewater (LLRW), the modified membrane had a much slower flux reduction compared with the raw membrane. After water wash, the flux recovery rates of the BSA fouled membranes were 98% for the modified membrane and 46% for the raw membrane, respectively. Additionally, the modified membrane had the same rejection on nuclide and surfactant as the raw membrane. Based on these results, the two-step modified membrane is suitable to be used in LLRW treatment with advantages of less frequent membrane cleaning and longer membrane lifetime.

Influence of ion exchange resin on the performance of continuous electrodeionization (CEDI) treating low-level radioactive wastewater

ABSTRACTWith the fast development of nuclear power, low-level radioactive wastewater (LLRW) treatment will meet more severe requirements. Continuous electrodeionization (CEDI) is a novel technology to treat LLRW with advantages of lower level effluent and smaller waste production. In this study, the influences of ion exchange resins (IERs) in the concentrate and dilute compartments on the performance of CEDI were investigated. The three stacks filled with anion exchange resins in the dilute compartments were more efficient for boron removal than metal ions, while the three stacks filled with cation exchange resins were more efficient for the removal of metal ions than boron. The stack filled with a type of mixed IERs was good at removing both metal ions and boron. The correlation analysis showed that no significant correlations were found between ion exchange capacities or ion exchange kinetics with the decontamination factors of metal ions. The removal ratios of boron were positively correlated with the ion exchange capacities, but were not correlated with the ion exchange kinetics. The results can guide the CEDI stack design by filling different IERs for different purposes.

Effect of ion exchange membrane on the removal efficiency of continuous electrodeionization (CEDI) during low level radioactive wastewater treatment

The treatment of low level radioactive wastewater (LLRW) has gained more attention with the development of nuclear energy. Continuous electrodeionization (CEDI) process is a promising technology to remove trace nuclides and minimize radioactive wastes. In this study, seven CEDI stacks filled with different ion exchange membranes (IEM) were compared for their performance during the LLRW treatment. The removal ratios of boron ranged from 5% to 16%, and the decontamination factors (DF) of nuclides ranged from 15 to 960. The performance of the CEDI stacks varied from each other. Some stacks had high removal efficiencies on both boron and nuclides, while some had high DF values of nuclides but low removal ratio of boron. Correlation analysis showed that ion exchange capacity of IEMs had no significant correlation with the efficiency of CEDI stacks. The diffusion dialysis coefficient had a negative effect on the removal of nuclides, but had no significant effects on the removal of boron. The transport of strong electrolyte nuclides and weak electrolyte boric acid was quite different, which helps to guide the CEDI stack design for different purposes, such as the separation of boron and nuclides or removal them simultaneously.

Influence of low dose gamma-ray irradiation on the performance and degradation of PVDF ultrafiltration membrane

Poly vinylidene fluoride (PVDF) UF membranes in water were exposed to gamma rays irradiation from 0 to 100kGy using a cobalt source. The absorbed dose provoked the rupture of C-F bonds, giving 0.02–0.38mg of free F- ion per gram membrane. Increasing the absorbed dose led to a more hydrophilic membrane surface, more fissures in the membrane, and a lower of water permeability. The pure water flux of a membrane irradiated at 100kGy was reduced by 44% compared to the pristine membrane. During filtration of low level radioactive wastewater that contained surfactants, the flux decreased at a slower rate for the irradiated membrane than for the pristine membrane. The retention of most ions remained stable. SEM images of the fouled membranes show a decrease in salt crystal contents on the fouling layer with increasing absorbed dose. Lower levels of salt deposits may contribute to a decrease in Ag(I) retention. Overall, upon exposure to low levels of irradiation, the membrane pure water flux was reduced, and membrane fouling was alleviated. These factors should be considered in membrane system design.

Removal of Cs + , Sr 2+ , and Co 2+ Ions from the Mixture of Organics and Suspended Solids Aqueous Solutions by Zeolites

Abstract Serving as an excellent adsorbent and inorganic ion exchanger in the water purification field, zeolite 4A has in this work presented a strong capability for purifying radioactive waste, such as Sr 2+ , Cs + , and Co 2+ in water. During the processes of decontamination and decommissioning of suspended solids and organics in low-level radioactive wastewater, the purification performance of zeolite 4A has been studied. Under ambient temperature and neutral condition, zeolite 4A absorbed simulated radionuclides such as Sr 2+ , Cs + , and Co 2+ with an absorption rate of almost 90%. Additionally, in alkaline condition, the adsorption percentage even approached 98.7%. After conducting research on suspended solids and organics of zeolite 4A for the treatment of radionuclides, it was found that the suspended clay was conducive to absorption, whereas the absorption of organics in solution was determined by the species of radionuclides and organics. Therefore, zeolite 4A has considerable potential in the treatment of radioactive wastewater.

Enhanced rejection of cations by low-level cationic surfactant during ultrafiltration of low-level radioactive wastewater

Abstract The influence of cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB) on the rejection of metal salts was investigated during ultrafiltration of low-level radioactive wastewater (LLRW). In the presence of CTAB below the critical micelle concentration (CMC), the rejection of nuclides significantly increased from 24%–33% to 50% for Cs(I) and above 90% for Sr(II), Co(II), and Ag(I). These increased rejections are attributed to CTAB fouling layer, which led to a more hydrophilicity and more positively-charged membrane surface. The nuclide content on the fouling layer increased with increasing CTAB concentration, and 55–80% of Co(II), Sr(II), and Ag(I) were adsorbed on the membrane at 400 mg/L CTAB. The high deposition of Ag(I) is partially due to the formation of AgBr. Based on these results, low-levels of cationic surfactant can significantly enhance the UF rejections of cations, but membrane fouling and nuclide deposition should not be ignored.

Effect of gamma-ray irradiation at low doses on the performance of PES ultrafiltration membrane

The influence of gamma irradiation on the performance of polyether sulfone (PES) ultrafiltration (UF) membrane was investigated at low absorbed doses (0–75kGy) using a cobalt source. The performance of the UF membranes was tested using low level radioactive wastewater (LLRW) containing three types of surfactants (anionic, cationic and nonionic surfactants). The physical and chemical properties of membrane surface were analyzed, and relationships between these properties and separation performance and fouling characteristics were determined. At 10–75kGy irradiation, there were no significant changes observed in the membrane surface roughness or polymer functional groups, however the contact angle decreased sharply from 92° to ca. 70° at irradiation levels as low as 10kGy. When membranes were exposed to the surfactant-containing LLRW, the flux decreased more sharply for higher dosed irradiated membranes, while flux in virgin membranes increased during the filtration processes. The study highlights that fouling properties of membrane may be changed due to the changes of surface hydrophilicity at low dose irradiation, while other surface properties and retentions remain stable. Therefore, a membrane fouling test with real or simulated wastewater is recommended to fully evaluate the membrane irradiation resistance.

Filtering of Low-Level Radioactive Wastewater by Means of Vacuum Membrane Distillation

The rapid development of nuclear power plants (NPPs) in China has caused increasing attention to be paid to the treatment of low-level radioactive wastewaters (LLRWs). One possibility is the application of vacuum membrane distillation (VMD). In this study, a commercial hydrophobic microporous polypropylene membrane was investigated with respect to nuclide decontamination and permeate flux performance in the VMD process. The results demonstrate that vacuum pressure has the most obvious influence on permeate flux, followed by feed temperature and feed velocity. Despite the influence of operational parameters, effective nuclide filtering can be achieved with average decontamination factor (DF) values consistently higher than 1700. The salt concentration in the feed solution decreases the permeate flux and nuclide filtering. However, the VMD process still offers high average DF values of 6000 for Cs(I), 3700 for Sr(II), and 8300 for Co(II), even when the feed salt concentration reaches 80 g L−1. After operation at a high salt concentration, there is no obvious variation in the chemical composition on the membrane surface based on the attenuated total reflectance–Fourier transform infrared spectra. A brief comparison shows that the process integrating reverse osmosis and VMD is a promising method for treating LLRWs and minimizing radioactive waste in NPPs.

Removal of strontium(II) and cobalt(II) from acidic solution by manganese antimonate

Radioactive strontium and cobalt are two major contaminants in the low-level radioactive wastewaters (LLRWs). In this study, a manganese-antimony composite oxide (MnSb) were prepared to study the sorption behavior and the removal of Sr(II) and Co(II) from acidic wastewaters. The as-prepared MnSb was identified and characterized by X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), Transmission Electron Microscope (TEM), zeta potential, and surface analysis, and its sorption performance was tested by batch experiments. Our results suggest that the MnSb oxides, with their microcrystal rutile structures and high specific surface areas, are promising candidate for the removal of Co (KdCo=2.3×106mL/g) and Sr (KdSr=8.3×105mL/g) in aqueous solutions at pH>2. Co-existing ions, like Na+, Mg2+ and Ca2+, affect the adsorption of Co and Sr to a limited degree. Adsorption of Co and Sr onto MnSb is fast and increases with elevating operating temperature. The equilibrium isotherms of Sr and Co could be best fitted by the Freundlich model with KF values up to 1.98 and 0.54mmol/g for Co and Sr at 323K, respectively. Co ions were found to be more competitive than Sr ions onto MnSb with a selectivity ratio of S(Co/Sr)=1.8. The kinetic performance of Co and Sr could be described well by the pseudo-second order equation, and film diffusion was the rate-controlling step under our experimental conditions.

Removal of nuclides and boron from highly saline radioactive wastewater by direct contact membrane distillation

The volume of low level radioactive wastewaters (LLRWs) produced from nuclear power plants (NPPs) should be concentrated to as small as possible before the final solidification. The direct contact membrane distillation (DCMD) method was used to treat highly saline LLRWs with commercial polypropylene (PP) membranes. The effect of different salt concentration on the rejections of nuclides (Co(II), Sr(II), Cs(I)) and boron (B) in DCMD was investigated in both short- and long-term concentration tests. Due to the hydrophobic properties of PP membrane, DCMD processes could give a high rejection for nuclides with DF above 105, which was weakly influenced by the concentrations of feed salt and B in LLRWs. B concentrations in the permeate remained below 2.0mgL−1, with a rejection factor (RF) greater than 99.97%, even when feed B concentration reached 5000mgL−1 or feed salt was maintained at 300gL−1. Scaling of the DCMD system by CaSO4 resulted in a significant reduction in permeate flux. The addition of boric acid and high concentrations of NaNO3 minimized scaling by CaSO4 on the membrane surface. Scaling could result in an increase in permeate B concentrations due to the co-precipitation of B and CaSO4 on the membrane surface.

Rejection of nuclides and silicon from boron-containing radioactive waste water using reverse osmosis

Abstract Reverse osmosis has been successfully utilized for waste separation across various fields. In this work, RO membrane was investigated for the separation of weakly electrolytic silica and boron, which was meaningful for potential boron reuse and waste reduction during treatment of low level radioactive waste water. The influences of temperature, pressure, and pH were tested. As the recovery rate rose to 80%, the flux decreased and the permeate concentration increased slightly. A higher temperature of 55 °C led to a higher silica and boron separation efficiency without a large loss in nuclides rejection. Under these conditions the recovery of boron is 46.58%, and the decontamination factors (DF) of silicon, Cs, Sr, and Co are 29.3, 4.54, 58.88, and 70.41, respectively. Assuming reasonable system design, the recovered water containing a high concentration of boron and a low concentration of silica is acceptable for reuse.

EDTA fouling in dead-end ultrafiltration of low level radioactive wastewater

EDTA is widely used as a detergent, and finally enters into wastewater. The influence of EDTA on ultrafiltration of low level radioactive wastewater (LLRW) was investigated under different operation conditions. As the main organic pollutant, EDTA led to unrecoverable membrane fouling and the normalized flux decreased from 100% to 85% depending on its concentration. The clogging caused by EDTA increased the surface roughness of the membrane, leading to the flux reduction. Both nuclide rejections and depositions on the membrane surfaces were enhanced with EDTA addition, due to the strong complexation of the nuclides with EDTA. However, Ag deposition on the membrane decreased slightly in the presence of EDTA, which may be caused by the stronger attraction of Ag to the unmodified membrane than that to the EDTA-modified one. Transmembrane pressure (TMP) and molecular weight cut off (MWCO) of membranes had negligible effects on membrane fouling, while the nuclide rejections by membrane and the depositions of nuclides on membrane both decreased significantly when the TMP increased to 0.2MPa and MWCO increased from 5kDa to 30kDa. Based on these results, it clearly showed that EDTA even at a low concentration had strong effects on the performance of UF treating LLRW. Therefore, it is suggested for industrial application that pretreatments to reduce EDTA or alkaline/ultrasonic washing involved in UF process were necessary to reduce the nuclide depositions on the membrane surfaces and irradiation dose of membrane surface.

Competitive adsorption of strontium and cobalt onto tin antimonate

Radioactive strontium (90Sr) and cobalt (60Co) are two major contaminants in low-level radioactive wastewaters from nuclear power plants. In this study, a tin antimonate sorbent (SnSb) used to simultaneously remove Sr and Co was prepared via a new and safe method. The as-prepared SnSb consisted of a composite structure of both pyrochlore and rutile microcrystal dispersed homogeneously at the nanometer scale. The sorbent had a good affinity to both Sr(II) ions and Co(II) ions across a wide pH range of 2–12 or in the presence of various non-radioactive ions. The adsorption competition performance of Sr(II) and Co(II) was investigated, with the adsorption isotherms following the extended Freundlich multicomponent isotherm and Sheindorf–Rebuhn–Sheintuch equation. Although Co(II) was more readily adsorbed on the sorbent, Sr(II) clearly inhibited Co adsorption. The kinetic performance of Sr(II) and Co(II) was described well by a pseudo-second order equation. When compared to the single-component adsorption, the adsorption rate of Sr(II) with Co(II) in the binary-component system decreased due to a competitive adsorption process.