Uranium removal from Pyhäsalmi/Finland mine water by batch electrocoagulation and optimization with the response surface methodology

Uranium capture from aqueous solution using palm-waste based activated carbon: sorption kinetics and equilibrium

Carbonaceous materials produced from agricultural waste (palm kernel shell) by pyrolysis can be a proper type of low-cost adsorbent for wide uses in radioactive effluent treatment. In this context, the as-produced bio-char (labeled as PBC) and its sub-driven sulfuric acid and zinc oxide activated carbons (labeled as PBC-SA, and PBC-Zn respectively) were employed as adsorbents for uranium sorption from aqueous solution. Various analytical techniques, including SEM (Scanning Electron Microscopy), EXD (X-ray Diffraction), BET (Brunauer-Emmett-Teller), FTIR (Fourier Transform Infrared Spectroscopy), and Zeta potential, provide insights into the material characteristics. Kinetic and isotherm investigations illuminated that the sorption process using the three sorbents is nicely fitted with Pseudo-second-order-kinetic and Langmuir isotherm models. The picked data display that the equilibrium time was 60 min, and the maximum sorption capacity was 9.89, 16.8, and 21.9 mg/g for PBC, PBC-SA, and PBC-Zn respectively, which reflects the highest affinity for zinc oxide, activated bio-char, among the three adsorbents, for uranium taking out from radioactive wastewater. Sorption thermodynamics declare that the sorption of U(VI) is an exothermic, spontaneous, and feasible process. About 92% of the uranium-loaded PBC-Zn sorbent was eluted using 1.0 M CH3COONa sodium ethanoate solution, and the sorbent demonstrated proper stability for 5 consecutive sorption/desorption cycles.

A review of remediation technologies for uranium-contaminated water

Uranium is a naturally existing radioactive element present in the Earth's crust. It exhibits lithophilic characteristics, indicating its tendency to be located near the surface of the Earth and tightly bound to oxygen. It is ecotoxic, hence the need for its removal from the aqueous environment. This paper focuses on the variety of water treatment processes for the removal of uranium from water and this includes physical (membrane separation, adsorption and electrocoagulation), chemical (ion exchange, photocatalysis and persulfate reduction), and biological (bio-reduction and biosorption) approaches. It was observed that membrane filtration and ion exchange are the most popular and promising processes for this application. Membrane processes have high throughput but with the challenge of high power requirements and fouling. Besides high pH sensitivity, ion exchange does not have any major challenges related to its application. Several other unique observations were derived from this review. Chitosan/Chlorella pyrenoidosa composite adsorbent bearing phosphate ligand, hydroxyapatite aerogel and MXene/graphene oxide composite has shown super-adsorbent performance (>1000 mg/g uptake capacity) for uranium. Ultrafiltration (UF) membranes, reverse osmosis (RO) membranes and electrocoagulation have been observed not to go below 97% uranium removal/conversion efficiency for most cases reported in the literature. Heat persulfate reduction has been explored quite recently and shown to achieve as high as 86% uranium reduction efficiency. We anticipate that future studies would explore hybrid processes (which are any combinations of multiple conventional techniques) to solve various aspects of the process design and performance challenges.

Advances in the management of radioactive wastes and radionuclide contamination in environmental compartments: a review

Several anthropogenic activities produce radioactive materials into the environment. According to reports, exposure to high concentrations of radioactive elements such as potassium (⁴⁰K), uranium (²³⁸U and ²³⁵U), and thorium (²³²Th) poses serious health concerns. The scarcity of reviews addressing the occurrence/sources, distribution, and remedial solutions of radioactive contamination in the ecosystems has fueled data collection for this bibliometric survey. In rivers and potable water, reports show that several parts of Europe and Asia have recorded radionuclide concentrations much higher than the permissible level of 1 Bq/L. According to various investigations, activity concentrations of gamma-emitting radioactive elements discovered in soils are higher than the global average crustal values, especially around mining activities. Adsorption technique is the most prevalent remedial method for decontaminating radiochemically polluted sites. However, there is a need to investigate integrated approaches/combination techniques. Although complete radionuclide decontamination utilizing the various technologies is feasible, future research should focus on cost-effectiveness, waste minimization, sustainability, and rapid radionuclide decontamination. Radioactive materials can be harnessed as fuel for nuclear power generation to meet worldwide energy demand. However, proper infrastructure must be put in place to prevent catastrophic disasters.

Comprehensive understanding of electrochemical treatment systems combined with biological processes for wastewater remediation

The presence of toxic pollutants in wastewater discharge can affect the environment negatively due to presence of the organic and inorganic contaminants. The application of the electrochemical process in wastewater treatment is promising, specifically in treating these harmful pollutants from the aquatic environment. This review focused on recent applications of the electrochemical process for the remediation of such harmful pollutants from aquatic environments. Furthermore, the process conditions that affect the electrochemical process performance are evaluated, and the appropriate treatment processes are suggested according to the presence of organic and inorganic contaminants. Electrocoagulation, electrooxidation, and electro-Fenton applications in wastewater have shown effective performance with high removal rates. The disadvantages of these processes are the formation of toxic intermediate metabolites, high energy consumption, and sludge generation. To overcome such disadvantages combined ecotechnologies can be applied in large-scale wastewater pollutants removal. The combination of electrochemical and biological treatment has gained importance, increased removal performance remarkably, and decreased operational costs. The critical discussion with depth information in this review could be beneficial for wastewater treatment plant operators throughout the world.

An overview of the application of electrocoagulation for mine wastewater treatment

One of the challenges of the twenty-first century is related to the discharge and disposal of mine effluents and wastewater resulting from mine dewatering, precipitation, and surface runoff in mines, especially acidic effluents that contain a variety of toxic and heavy metals and are the main sources of surface and groundwater pollution. Various physical, chemical, and biological methods have been developed and used to treat mine effluents. All proposed methods have their own disadvantages that make their use challenging. One of the new methods used for wastewater treatment is the electrical coagulation process, which has attracted the attention of researchers in recent years due to its advantages such as simplicity, environmental friendliness, and low cost. The present review focused on the applications of electrocoagulation for mine wastewater treatment as well as metals recovery. In addition, the main mechanisms, advantages, and weaknesses of electrocoagulation were reviewed.

Removal of nutrients and other emerging inorganic contaminants from water and wastewater by electrocoagulation process

The continual discharge of emerging inorganic pollutants into natural aquatic systems and their negative effects on the environment have motivated the researchers to explore and develop clean and efficient water treatment strategies. Electrocoagulation (EC) is a rapid and promising pollutant removal approach that does not require any chemical additives or complicated process management. Therefore, inorganic pollutant treatment via the EC process is considered one of the most feasible processes. The potential developments of EC process may make the process a wise choice for water treatment in the future. Thus, the present study mainly focuses on the use of EC technology to remove nutrients and other emerging inorganic pollutants from water medium. The operating factors that influence EC process efficiency are explained. The major advancement of the EC technique as well as field-implemented units are also discussed. Overall, this study mainly focuses on emerging issues, present advancements, and techno-economic considerations in EC process.

A critical review of uranium contamination in groundwater: Treatment and sludge disposal

Dissolved uranium in groundwater at high concentrations is an emerging global threat to human and ecological health due to its radioactivity and chemical toxicity. Uranium can enter groundwater by geochemical reactions, natural deposition from minerals, mining, uranium ore processing, and spent fuel disposal. Although much progress has been made in uranium remediation in recent years, most published reviews on uranium treatment have focused on specific methods, particularly adsorption. This article systematically reviews the major treatment technologies, explains their mechanism and progress of uranium removal, and compares their performance under various environmental conditions. Of all treatment methods, adsorption has received much attention due to its ease of use and adaptability under various conditions. However, salinity and competition from other ions limit its application in actual field conditions. Biosorption and bioremediation are also promising methods due to their low-cost and chemical-free operation. Strong base anion exchange resins are more effective at typical groundwater pH conditions. Advanced oxidation processes like photocatalysis produce less sludge and are effective even at low uranium concentrations. Electrocoagulation shows significantly improved performance when organic ligands are added prior to treatment. The significant advantages of membrane filtration are high removal efficiency and the ability to recover uranium. While each technology has its merits and demerits, no single technology is entirely suitable under all conditions. One major area of concern with all technologies is the need to dispose of liquid and solid waste generated after treatment safely. Future research must focus on developing hybrid and state-of-the-art technologies for effective and sustainable uranium removal from groundwater. Developing holistic management strategies for uranium removal will hinge on understanding its speciation, mechanisms of fate and transport, and socio-economic conditions of the affected areas.

Optimization of electrocoagulation process for treatment of rice grain-based biodigester distillery effluent using surface response methodology approach

Distillery industries are the most water-consuming industries discharging a large amount of wastewater that contain a high organic load. Hence it is first treated in biodigester where significant organics reduces (50–60%) and the outcome of biodigester is commonly known as biodigester effluent (BDE). The present study is an attempt to treat BDE in terms of COD and color removal using a batch electrocoagulation reactor (ECR) where stainless steel (SS) is used as an electrode. To optimize the four independent parameters namely initial pH (pHi: 3.5–9.5), current density (j: 49.5–247 A/m2), electrode gap (g: 1.2–3.2 cm), and reaction time (t: 20–100 min) on the color and COD reduction efficiency, a central composite design (CCD) experiment is applied to evaluate the individual and interactive effects of these parameters. The high coefficients of determination for color (R 2 = 0.9989) and COD (R 2 = 0.9981) were obtained by analysis of variance (ANOVA) between the experimental data and the predicted data using a second-order regression model. At the optimum condition color and COD removal of 81.4 and 91.9%, respectively, were observed. A material balance of SS has also been incorporated.

Uranium isotopes in groundwater in Ho Chi Minh City and related issues: Health risks, environmental effects, and mitigation methods

Groundwater is regularly used for many purposes, such as drinking and agricultural irrigation systems. Still, it contains high levels of radionuclides (e.g., ²³⁸U, ²³²Th, and ²²⁶Ra) that are potentially hazardous to humans and the environment. In this study, activity concentrations of uranium isotopes were analyzed in 15 groundwater samples taken from 15 bored wells in Thu Duc district, Ho Chi Minh City, Vietnam. Environmental effects of the irrigation system with groundwater on agricultural soil in the study area were assessed by models. It was found that the activity concentrations of ²³⁸U and ²³⁴U in groundwater samples were in the ranges of (13.5-268.7) mBq l^-1 and (20.2-438.3) mBq l^-1, respectively. The ratio ²³⁴U/²³⁸U values were ranged from 1.12 to 2, with an average value of 1.44. Based on the model prediction, 25 years irrigation with the groundwater can inject 94.8 Bq both uranium isotopes in 1 kg topsoil. For investigated groundwater samples, the proposed removal method using K₂FeO₄ removed 74.28% and 81.04% for ²³⁴U and ²³⁸U, respectively.

Removal of inorganic pollutants using electrocoagulation technology: A review of emerging applications and mechanisms

Electrocoagulation (ECoag) technique has shown considerable potential as an effective method in separating different types of pollutants (including inorganic pollutants) from various sources of water at a lower cost, and that is environmentally friendly. The EC method's performance depends on several significant parameters, including current density, reactor geometry, pH, operation time, the gap between electrodes, and agitation speed. There are some challenges related to the ECoag technique, for example, energy consumption, and electrode passivation as well as its implementation at a larger scale. This review highlights the recent studies published about ECoag capacity to remove inorganic pollutants (including salts), the emerging reactors, and the effect of reactor geometry designs. In addition, this paper highlights the integration of the ECoag technique with other advanced technologies such as microwave and ultrasonic to achieve higher removal efficiencies. This paper also presents a critical discussion of the major and minor reactions of the electrocoagulation technique with several significant operational parameters, emerging designs of the ECoag cell, operating conditions, and techno-economic analysis. Our review concluded that optimizing the operating parameters significantly enhanced the efficiency of the ECoag technique and reduced overall operating costs. Electrodes geometry has been recommended to minimize the passivation phenomenon, promote the conductivity of the cell, and reduce energy consumption. In this review, several challenges and gaps were identified, and insights for future development were discussed. We recommend that future studies investigate the effect of other emerging parameters like perforated and ball electrodes on the ECoag technique.

Application of response surface method in the separation of radioactive material: a review

Response Surface Method (RSM) is one of the most popular and powerful tools for experimental design and optimization. This paper first reviewed the research progress of RSM in the separation and recovery of various radioactive materials, and verified the application of RSM in adsorption isotherm analysis and thermodynamic calculation. The main advantage of RSM in radioactive material separation is the reduction in the number of experiments required, resulting in considerably less radioactive material consumption, secondary waste generation, workload and radiation dose, which is valuable for the research of radioactive material separation.

Mitigation of Uranium Mining Impacts—A Review on Groundwater Remediation Technologies

Groundwater contamination is one of the most concerning issues from uranium mining activities. Radionuclides cannot be destroyed or degraded, unlike some organic contaminants (and similar to metals). Besides, sites, where radionuclides may be found, are mainly radioactive and mixed waste disposal areas, and therefore many other contaminants may also be present in groundwater. The state-of-the-art of environmental technology is continually changing, and thus a review on technologies application is of utmost relevance. This work gives an overview of the available remediation technologies for groundwater contaminated with radionuclides resulting mainly from uranium mining. For each technology, a theoretical background is provided; the state of development, limitations, efficiency, and potential adverse effects are also approached. Examples of application and performance monitoring of remediation progress are described, and criteria for the selection of the appropriate remediation technology are given. The most effective remediation technology will always be site-specific as a result of the multitude of geographic and operational factors that influence the effluent quality and impact the technical feasibility of treatment methods. Ion exchange, chemical precipitation, and membrane filtration have been considered by the U.S. Environmental Protection Agency (US EPA) as best demonstrated available technologies for radium and uranium removal. Several factors have been demonstrated to influence the selection of a remediation technology (technological aspects and non-technical factors), but even for the technologies demonstrated or industrial proven, two important challenges remain; the (still) mobile radionuclides and the generation of secondary wastes. Besides, remediation technologies are constantly evolving, but future advancement depends on rigorously monitored, documented efficiency, and results achieved. Therefore, the technologies approached in this paper are by no means exhaustive.

Insight into chitosan/mesoporous silica nanocomposites as eco-friendly adsorbent for enhanced retention of U (VI) and Sr (II) from aqueous solutions and real water

The chitosan chains were integrated with MCM-48 mesoporous silica in an eco-friendly composite (CH/MCM-48) of enhanced adsorption capacity. The prepared CH/MCM-48 composite was applied in systematic retention of U (VI) as well as Sr (II) ions from water as the commonly detected radioactive pollutants. It displayed promising retention capacities of 261.3 mg/g and 328.6 mg/g for U (VI) and Sr (II) considering the equilibrium time interval that was identified after 420 min. The composite showed the kinetic behavior of the Pseudo-First order model and the isotherm properties of the Langmuir assumption. The thermodynamic assessment of the reactions validated the retention of both U (VI) and Sr (II) ions by spontaneous, favorable, and exothermic reactions. Based on the theoretical values of entropy (-5.94 kJ mol^-1 (U (VI)) and -2.93 kJ mol^-1 (Sr (II))), Gibbs free energy (less than 20 kJ mol^-1), and Gaussian energy (5.77 kJ mol^-1 (U (VI)) and 4.56 kJ mol^-1 (Sr (II))) the uptake processes are related to physical adsorption reactions. The CH/MCM-48 composite is of significant recyclability and showed considerable affinities for the studied radioactive ions even in the presence of other metal ions (Cd (II), Pb (II), Zn (II), and Co (II)).

Natural and anthropogenic radionuclides in water and wastewater: Sources, treatments and recoveries

Water-energy nexus in the context of changing climate amplifies the importance of comprehending the transport, fate and recovery of radioisotope. While countries have been more interested for zero/low greenhouse gas emission technologies, energy production from nuclear power plant (NPP) can be a prominent solution. Moreover, radioisotopes are also used for other benefits such as in medical science, industrial activities and many more. These radionuclides are blended accidently or intentionally with water or wastewater because of inefficacious management of the nuclear waste; and therefore, it is an imperative task to manage nuclear waste so that the harmful consequences of the waste on environment, ecology and human health can be dispelled. Due to generation of significant amount of waste throughout its utilization, a noticeable number of physical, chemical and biological processes has been introduced as remediation processes although mechanisms of optimum removal process are still under investigation. Removal mechanisms and influencing factors for radionuclide removal are elucidated in this review so that, further, operation and process development can be promoted. Again, resource recovery, opportunities and challenges are also discussed for elevating the removal rates and minimizing the knowledge gaps existing in development and applications of novel decontamination processes.

Removal of U(VI) from nuclear mining effluent by porous hydroxyapatite: Evaluation on characteristics, mechanisms and performance

The effluents from nuclear mining processes contain relatively high content of radionuclides (such as uranium), which may seriously threaten the environment and human health. Herein, a novel adsorbent, porous hydroxyapatite, was prepared and proven highly efficient for removal of uranyl ions (U(VI)) given its high U(VI) uptake capacity of 111.4 mg/g, fast adsorption kinetics, and the potential stabilization of adsorbed U(VI). A nearly complete removal of U(VI) was achieved by porous HAP under optimized conditions. Langmuir model could well describe the adsorption equilibrium. The data fit well with pseudo-second-order kinetic model, suggesting that U(VI) adsorption is primarily attributed to chemisorption with porous HAP. Intraparticle diffusion analysis showed that the intraparticle diffusion is the rate-limiting step for U(VI) adsorption by porous HAP. After removal by porous HAP, the adsorbed U(VI) ions were incorporated into tetragonal autunite, which has a low solubility (log Ksp: -48.36). Our findings demonstrate that the porous HAP can effectively remediate uranium contamination and holds great promise for environmental applications.

Magnetically modified hydroxyapatite nanoparticles for the removal of uranium (VI): Preparation, characterization and adsorption optimization

Recently, magnetically modified nanomaterials have gained a great interest in the field of wastewater remediation. In this regard, the present work introduces a facile microwave-assisted pathway for the preparation of magnetically modified hydroxyapatite nanoparticles (MNHA) and evaluates its adsorption capability towards the removal of uranium (VI) ions from wastewaters. The prepared magnetic nanocomposite went through a full characterization procedure using different techniques, such as transmission electron microscope (TEM), X-ray diffraction (XRD), FT-IR, Brunauer-Emmett-Teller (BET) surface area measurements and magnetization curve. Involvement of the prepared MNHA in the remediation of wastewater containing U(VI) ions was investigated and the factors that influence the adsorption capacity were considered and optimized. The adsorption's optimum pH was found to be 5.0 and equilibrium was attended after 120 min. A maximum adsorption capacity of 310 mg/g was achieved after 120 min at 25 °C. The experimental data were well explained by Langmuir adsorption isotherm model. Kinetically, the adsorption process follows the pseudo-second order model. Thermodynamically, it is endothermic, irreversible and spontaneous adsorption process. Removal of U(VI) ions was found to take place via complex formation between the phosphate groups on the adsorbent and uranyl ions. The recovery of U(VI) ions from MNHA beads and the reusability of the spent beads were also explored. It was concluded that the prepared MNHA nanocomposite is simple, fast, ecofriendly adsorbent for the removal of U(VI) ions from water with excellent adsorption capacity.

Evaluation of direct and alternating current on nitrate removal using a continuous electrocoagulation process: Economical and environmental approaches through RSM

This study aims to investigate the effects of alternating current (AC) and direct current (DC) for nitrate removal and its operating costs by using a continuous electrocoagulation (CEC) process. For this purpose, two series of 31 experiments, which were designed by response surface method (RSM), were carried out in both cases of the AC and the DC modes. In each series, the effect of selected parameters, namely, initial nitrate concentration, inlet flow rate, current density and initial pH along with their interactions on the nitrate removal efficiency as well as its operating costs, as responses, were investigated separately. According to the analysis of variance (ANOVA), there is a reasonable agreement between achieving results and the experimental data for both responses. The nitrate removal in the AC mode was slightly more efficient than that of the DC mode. In addition, the average operating costs of the DC mode, including the energy and the electrode consumption for the CEC process were achieved 54 US$/(kg nitrate removed); whereas this amount was calculated 29 US$/(kg nitrate removed) for the AC mode. Therefore, the average of the operating costs was improved more than 40% using the AC mode, which was mainly related to reduction of aluminum electrode consumption.

Treatment of oil sands produced water using combined electrocoagulation and chemical coagulation techniques

Hybrid electrocoagulation-chemical coagulation (EC-CC) process has attracted a growing attention for the removal of various types of wastewaters contaminants. In this paper, the feasibility of EC-CC technique as an alternative to conventional chemical processes for the treatment of steam assisted gravity drainage (SAGD) produced water has been systematically studied. Eight parameters, namely electrode material, cell configuration, pH and temperature of the solution, chemical coagulant dosage, intensity of the electrical current, mixing rate, and treatment time were studied. To explore the synergistic effect of the design parameters, the experimental trials were arranged using Taguchi method. Analysis of variance (ANOVA) was performed to evaluate the effect of each design parameter on the organic matter removal from the SAGD produced water. It was found that all parameters except the electrode arrangement had a significant effect on the removal efficiency of the EC-CC process. Among these parameters, the chemical coagulant and the treatment time had the most significant contribution to the efficiency by 40% and 26%, respectively. The optimum condition for the highest TOC removal efficiency (39.8%) was obtained by applying 0.34 A to Al electrode in a bipolar (BP) configuration when the pH, temperature, coagulant concentration, mixing rate, and reaction time were set to 8, 60 °C, 200 mg/L, 700 rpm, and 90 min, respectively. Moreover, a second-order polynomial regression model was proposed to predict the removal efficiency in terms of design parameters. An excellent agreement between the model predictions and experimental data was obtained with the adjusted R² of about 99%.

Optimization of Synthesis Conditions of Carbon Nanotubes via Ultrasonic-Assisted Floating Catalyst Deposition Using Response Surface Methodology

The growing use of carbon nanotubes (CNTs) in a plethora of applications has provided to us a motivation to investigate CNT synthesis by new methods. In this study, ultrasonic-assisted chemical vapor deposition (CVD) method was employed to synthesize CNTs. The difficulty of controlling the size of clusters and achieving uniform distribution—the major problem in previous methods—was solved by using ultrasonic bath and dissolving ferrocene in xylene outside the reactor. The operating conditions were optimized using a rotatable central composite design (CCD), which helped optimize the operating conditions of the method. Response surface methodology (RSM) was used to analyze these experiments. Using statistical software was very effective, considering that it decreased the number of experiments needed to achieve the optimum conditions. Synthesis of CNTs was studied as a function of three independent parameters viz. hydrogen flow rate (120⁻280 cm³/min), catalyst concentration (2⁻6 wt %), and synthesis temperature (800⁻1200 °C). Optimum conditions for the synthesis of CNTs were found to be 3.78 wt %, 184 cm³/min, and 976 °C for catalyst concentration, hydrogen flow rate, and synthesis temperature, respectively. Under these conditions, Raman spectrum indicates high values of (I_G/I_D), which means high-quality CNTs.