Phosphate and illite colloid pose a synergistic risk of enhanced uranium transport in groundwater: A challenge for phosphate immobilization remediation of uranium contaminated environmental water

The phosphorus-containing reagents have been proposed to remediate the uranium contaminated sites due to the formation of insoluble uranyl phosphate mineralization products. However, the colloids, including both pseudo and intrinsic uranium colloids, could disturb the environmental fate of uranium due to its nonnegligible mobility. In this work, the transport pattern and micro-mechanism of uranium coupled to phosphate and illite colloid (IC) were investigated by combining column experiments and micro-spectroscopic evidences. Results showed that uranium transport was facilitated in granular media by forming the intrinsic uranyl phosphate colloid (such as Na-autunite) when the pH > 3.5 and CNa+ < 10 mM. Meanwhile, the mobility of uranium depended greatly on the typical water chemistry parameters governing the aggregation and deposit of intrinsic uranium colloids. However, the attachment of phosphate on illite granule increased the repulsive force and enhanced the dispersion stability of IC in the IC-U(VI)-phosphate ternary system. The non-preequilibrium transport and retention profiles, HRTEM-mapping, as well as TRLFS spectra revealed that the IC enhanced uranium mobility by forming the ternary IC-uranyl phosphate hybrid, and acted as the coagulation preventing agent for uranyl phosphate particles. This observed facilitation of uranium transport resulted from the formation of intrinsic uranyl phosphate colloids and IC-uranyl phosphate hybrids should be taken into consideration when evaluating the potential risk of uranium migration and optimizing the in-situ mineralization remediation strategy for uranium contaminated environmental water.

Phosphate (Bio)mineralization Remediation of 90Sr-Contaminated Groundwaters

Historical operations at nuclear mega-facilities such as Hanford, USA, and Sellafield, UK have led to a legacy of radioactivity-contaminated land. Calcium phosphate phases (e.g., hydroxyapatite) can adsorb and/or incorporate radionuclides, including 90Sr. Past work has shown that aqueous injection of Ca-phosphate-generating solutions into the contaminated ground on both laboratory and field scales can reduce the amount of aqueous 90Sr in the systems. Here, two microbially mediated phosphate amendment techniques which precipitated Ca-phosphate, (i) Ca-citrate/Na-phosphate and (ii) glycerol phosphate, were tested in batch experiments alongside an abiotic treatment ((iii) polyphosphate), using stable Sr and site relevant groundwaters and sediments. All three amendments led to enhanced Sr removal from the solution compared to the sediment-only control. The Ca-citrate/Na-phosphate treatment removed 97%, glycerol phosphate 60%, and polyphosphate 55% of the initial Sr. At experimental end points, scanning electron microscopy showed that Sr-containing, Ca-phosphate phases were deposited on sediment grains, and XAS analyses of the sediments amended with Ca-citrate/Na-phosphate and glycerol phosphate confirmed Sr incorporation into Ca-phosphates occurred. Overall, Ca-phosphate-generating treatments have the potential to be applied in a range of nuclear sites and are a key option within the toolkit for 90Sr groundwater remediation.

Phytoremediation of radioactive elements, possibilities and challenges: special focus on agricultural aspects

The radioactive contamination has been reported frequently from agricultural lands and ground water. The main reason behind the radioactive pollution is unprotected mining of radioactive elements, unsafe discard of nuclear industrial waste, military applications, dumping of medically used radioisotopes, globally situated (>400) nuclear power plants and use of phosphate fertilizers in farming. Radionuclides are well known potent carcinogens that may cause the various types of cancers to human and animals due to the long exposure to radioactive contaminated sites. To get rid of from the radioactive pollution there is a need of practically successful and cost effective bioremediation technologies that should able to decontaminate the effected lands and water to benefit the mankind. Microbial and phytoremediation are well studied methods for decreasing or gradually eliminating the radioactive contaminants. In this review, we discussed the different strategies of microbial and phytoremediation of radionuclides and recent advancements, that can play the major role in bioremediation of soil and water.

Phosphorus in the Clouds of Venus: Potential for Bioavailability

Aerosol phase elements such as phosphorus (P), sulfur (S), and metals including iron (Fe) are essential nutrients that could help sustain potential biodiversity in the cloud deck of Venus. While the presence of S and Fe in the venusian cloud deck has been broadly discussed (Zasova et al., 1981; Krasnopolsky, 2012, 2013, 2016, 2017; Markiewicz et al., 2014), less attention has been given to the presence of P in the aerosols and its involvement in the multiphase chemistry of venusian clouds and potential sources of P deposition in the venusian atmosphere. A detailed characterization of phosphorus atmospheric chemistry in the cloud deck of Venus is crucial for understanding its solubility and bioavailability for potential venusian cloud microbiota (Schulze-Makuch et al., 2004; Grinspoon and Bullock, 2007; Limaye et al., 2018). We summarize our current understanding of the presence of P in the clouds of Venus and its role in a hypothetical atmospheric (bio)chemical cycle. The results of the VeGa lander measurements are put into perspective with regard to nutrient limitation for a potential biosphere in venusian clouds. Our work combines the results of the VeGa measurements and focuses on P as an inorganic nutrient component and its potential sources and chemical behavior as part of multiple transformations of atmospheric chemistry. The VeGa data indicate that a plentiful phosphorus layer exists within a layer that reaches into the lower venusian clouds and exceeds minimum P abundances for terrestrial microbial life. Extreme acidification of airborne phases in the atmosphere of Venus may facilitate P solubilization and its bioavailability for a potential ecosystem in venusian clouds. Further sampling and P abundance measurements in the atmosphere of Venus would improve our knowledge of P speciation and facilitate determination of a bioavailable fraction of P detected in venusian clouds. The previous results deserve further experimental and modeling analyses to diminish uncertainties and understand the rates of atmospheric deposition of P and its role in a potential venusian cloud ecosystem.

Dissolution and Vertical Transport of Uranium from Stable Mineral Forms by Plants as Influenced by the Co-occurrence of Uranium with Phosphorus

Plants could mobilize (dissolution followed by vertical transport) uranium (U) from mineral forms that are otherwise stable. However, the variability of this plant-mediated mobilization of U as a function of the presence of various essential plant nutrients contained in these minerals remains unknown. A series of column experiments were conducted using Andropogon virginicus to quantify the vertical transport of U from stable mineral forms as influenced by the chemical and physical coexistence of U with the essential nutrient, phosphorus (P). The presence of plants significantly increased the vertical migration of U only when U was precipitated with P (UO₂HPO₄·4H₂O; chernikovite) but not from UO₂ (uraninite) that lacks any essential plant nutrient. The U dissolution was further increased when chernikovite co-occurred with a sparingly available form of P (FePO₄) under P-limited growing conditions. Similarly, A. virginicus accumulated the highest amount of U from chernikovite (0.05 mg/g) in the presence of FePO₄ compared to that of uraninite (no-P) and chernikovite supplemented with KH₂PO_4. These results signify an increased plant-mediated dissolution, uptake, and leaching of radioactive contaminants in soils that are nutrient deficient, a key factor that should be considered in management at legacy contamination sites.

Treatment of Cd2+ and Cu2+ Ions Using Modified Apatite Ore

Apatite ore from Lao Cai (Vietnam) has large reserves and low prices. Its main component is fluorapatite. The purification and modification of apatite ore can produce a material that can be used as an absorbent for heavy metals with high efficiency. The molecular structure, phase component, specific surface area, element component, and morphology of modified apatite ore from Lao Cai province, Vietnam, were characterized by IR, XRD, BET, EDX, and SEM methods. The IR and XRD results show that the modified process transformed apatite ore from fluorapatite to nanohydroxyapatite. The specific surface area of modified apatite ore (100.79 m2/g) is much higher than the original ore (3.97 m2/g). The modified apatite ore was used to adsorb Cd2+ and Cu2+ ions in water. The effect of adsorbent mass, pH, contact time, and initial concentration of Cd2+ and Cu2+ on the adsorption efficiency and capacity was investigated. Besides, the isotherm adsorption model was determined using Freundlich and Langmuir theories.

Effective removal of uranium via phosphate addition for the treatment of uranium laden process effluents

We have investigated the suitability of phosphate addition, in the form MH₂PO₄ (M = Na⁺, K⁺ or NH₄⁺), for the selective removal of uranium from a complex waste effluent. The effluent in question is generated as part of a treatment strategy for a spent uranium catalyst, used in the production of acrylonitrile (Sohio process), which has been in temporary storage in Korea since 2004. Both pH (3.0-11.0) and phosphate dosages (0.25-10 mM) have been screened to identify the optimal conditions of 6.25 and 1 mM, respectively, for an initial uranium concentration of 0.16 mM. Precipitation kinetics have been investigated revealing the rapid removal of uranium from solution, with 30 min found to be optimal. The effluent was effectively decontaminated via Meta-ankoleite (K(UO₂)(PO₄)·3H₂O) formation to uranium levels below the Korean release limit of 1 ppm for uranium-bearing liquid wastes, with KH₂PO₄ addition being chosen for the real process. Final decontamination factors of the order of ≥ 8000 were readily achieved. Aluminium, calcium and iron containing coagulants were screened for the clean-up of the remaining supernatant, post-uranium removal, ensuring the final effluent meets the relevant release criteria (pH, total suspended solids, total phosphate and turbidity) for general, non-radioactive, effluents. A process scheme is presented and discussed for adaptation to similar uranium containing effluents.

The potential of ryegrass (Lolium perenne L.) to clean up multi-contaminated soils from labile and phytoavailable potentially toxic elements to contribute into a circular economy

Aided phytoremediation was studied for 48 weeks with the aim of reducing extractable and phytoavailable toxic elements and producing potential marketable biomass. In this sense, biomass of ryegrass was produced under greenhouse on two contaminated garden soils that have been amended with two successive additions of phosphates. After the first addition of phosphates, seeds of ryegrass were sown and shoots were harvested twice. A second seedling was performed after carefully mixing the roots from the first production (used as compost), soils and phosphates. Forty-eight weeks after starting the experiments, the concentrations of Cd, Pb, Zn, Cu, Fe, and Mn extracted using the rhizosphere-based method were generally lower than those measured before the addition of phosphates and cultivation (except for Pb and Fe in the most contaminated soil). The concentrations of metals in the shoots of ryegrass from the second production were lower than those from the first (except for Al). The best results were obtained with phosphates and were the most relevant in the lowest contaminated soil, demonstrating that the available metal concentrations have to be taken into account in the management of contaminated soils. In view of the concentration of metals defined as carcinogens, mutagens, and reprotoxics (e.g., Cd, Pb) and those capable to be transformed into Lewis acids (e.g., Zn, Fe), the utilization of ryegrass in the revegetation of contaminated soils and in risk management may be a new production of marketable biomass. The development of phytomanagement in combination with this type of biomass coincided with the view that contaminated soils can still represent a valuable resource that should be used sustainably.

Phosphorus Stress-Induced Changes in Plant Root Exudation Could Potentially Facilitate Uranium Mobilization from Stable Mineral Forms

Apparent deficiency of soil mineral nutrients often triggers specific physio-morphological changes in plants, and some of these changes could also inadvertently increase the ability of plants to mobilize radionuclides from stable mineral forms. This work, through a series of sand-culture, hydroponics, and batch-equilibration experiments, investigated the differential ability of root exudates of Andropogon virginicus grown under conditions with variable phosphorus (P) availability (KH₂PO₄, FePO₄, Ca₃(PO₄)₂, and no P) to solubilize uranium (U) from the uranyl phosphate mineral Chernikovite. The mineral form of P, and hence the bioavailability of P, affected the overall composition of the root exudates. The lower bioavailable forms of P (FePO₄ and Ca₃(PO₄)₂), but not the complete absence of P, resulted in a higher abundance of root metabolites with chelating capacity at 72 hrs after treatment application. In treatments with lower P-bioavailability, the physiological amino acid concentration inside of the roots increased, whereas the concentration of organic acids in the roots decreased due to the active exudation. In batch dissolution experiments, the organic acids, but not amino acids, increase the dissolution U from Chernikovite. The root exudate matrix of plants exposed to low available forms of P induced a >60% increase in U dissolution from Chernikovite due to 5-16 times greater abundance of organic acids in these treatments. However, this was ca. 70% of the theoretical dissolution achievable by this exudate matrix. These results highlight the potential of using active management of soil P as an effective tool to alter the plant-mediated mobilization of U in contaminated soil.

Ionic Strength Differentially Affects the Bioavailability of Neutral and Negatively Charged Inorganic Hg Complexes

Mercury (Hg) bioavailability to bacteria in marine systems is the first step toward its bioamplification in food webs. These systems exhibit high salinity and ionic strength that will both alter Hg speciation and properties of the bacteria cell walls. The role of Hg speciation on Hg bioavailability in marine systems has not been teased apart from that of ionic strength on cell wall properties, however. We developed and optimized a whole-cell Hg bioreporter capable of functioning under aerobic and anaerobic conditions and exhibiting no physiological limitations of signal production to changes in ionic strength. We show that ionic strength controls the bioavailability of Hg species, regardless of their charge, possibly by altering properties of the bacterial cell wall. The unexpected anaerobic bioavailability of negatively charged halocomplexes may help explain Hg methylation in marine systems such as the oxygen-deficient zone in the oceanic water column, sea ice or polar snow.

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

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

Bio-rescue of marine environments: On the track of microbially-based metal/metalloid remediation

The recent awareness of the huge relevance of marine resources and ecological services is driving regulatory demands for their protection from overwhelming contaminants, such as metals/metalloids. These contaminants enter and accumulate in different marine niches, hence deeply compromising their quality and integrity. Bioremediation has been flourishing to counteract metal/metalloid impacts, since it provides cost-effective and sustainable options by relying on ecology-based technologies. The potential of marine microbes for metal/metalloid bioremediation is the core of many studies, due to their high plasticity to overcome successive environmental hurdles. However, any thorough review on the advances of metal/metalloid bioremediation in marine environments was so far unveiled. This review is designed to (i) outline the characteristics and potential of marine microbes for metal/metalloid bioremediation, (ii) describe the underlying pathways of resistance and detoxification, as well as useful methodologies for their characterization, (iii) identify major bottlenecks on metal/metalloid bioremediation with marine microbes, (iv) present alternative strategies based on microbial consortia and engineered microbes for enhanced bioremediation, and (v) propose key research avenues to keep pace with a changing society, science and economy in a sustainable manner.

Ex-Situ Remediation Technologies for Environmental Pollutants: A Critical Perspective

Pollution and the global health impacts from toxic environmental pollutants are presently of great concern. At present, more than 100 million people are at risk from exposure to a plethora of toxic organic and inorganic pollutants. This review is an exploration of the ex-situ technologies for cleaning-up the contaminated soil, groundwater and air emissions, highlighting their principles, advantages, deficiencies and the knowledge gaps. Challenges and strategies for removing different types of contaminants, mainly heavy metals and priority organic pollutants, are also described.