Influence of pH, competing ions, and salinity on the sorption of strontium and cobalt onto biogenic hydroxyapatite

Experimental analysis and prediction of radionuclide solubility using machine learning models: Effects of organic complexing agents

Radioactive wastes contain organic complexing agents that can form complexes with radionuclides and enhance the solubility of these radionuclides, increasing the mobility of radionuclides over great distances from a radioactive waste repository. In this study, four radionuclides (cobalt, strontium, iodine, and uranium) and three organic complexing agents (ethylenediaminetetraacetic acid, nitrilotriacetic acid, and iso-saccharic acid) were selected, and the solubility of these radionuclides was assessed under realistic environmental conditions such as different pHs (7, 9, 11, and 13), temperatures (10 °C, 20 °C, and 40 °C), and organic complexing agent concentrations (10-5-10-2 M). A total of 720 datasets were generated from solubility batch experiments. Four supervised machine learning models such as the Gaussian process regression (GPR), ensemble-boosted trees, artificial neural networks, and support vector machine were developed for predicting the radionuclide solubility. Each ML model was optimized using Bayesian optimization algorithm. The GPR evolved as a robust model that provided accurate predictions within the underlying solubility patterns by capturing the intricate relationships of the independent parameters of the dataset. At an uncertainty level of 95%, both the experimental results and GPR simulated estimations were closely correlated, confirming the suitability of the GPR model for future explorations.

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.

Hydroxyapatite coatings on cement paste as barriers against radiological contamination

A novel method for precipitating hydroxyapatite (HAp) onto cement paste is investigated for protecting concrete infrastructure from radiological contamination. Legacy nuclear sites contain large volumes of contaminated concrete and are expensive and dangerous to decommission. One solution is to 'design for decommissioning' by confining contaminants to a thin layer. Current layering methods, including paints or films, offer poor durability over plant lifespans. Here, we present a mineral-HAp-coated cement, which innovatively serves as a barrier layer to radioactive contaminants (e.g. Sr, U). HAp is shown to directly mineralise onto a cement paste block in a layer several microns thick via a two-step process: first, applying a silica-based scaffold onto a cement paste block; and second, soaking the resulting block in a PO₄-enriched Ringer's solution. Strontium ingression was tested on coated and uncoated cement paste (~ 40 × 40 × 40mm cement, 450 mL, 1000 mg L^- 1 Sr) for a period of 1-week. While both coated and uncoated samples reduced the solution concentration of Sr by half, Sr was held within the HAp layer of coated cement paste and was not observed within the cement matrix. In the uncoated samples, Sr had penetrated further into the block. Further studies aim to characterise HAp before and after exposure to a range of radioactive contaminants and to develop a method for mechanical layer separation.

Hydroxyapatite Nanopowders for Effective Removal of Strontium Ions from Aqueous Solutions

Drinking water contamination has become a worldwide problem due to the highly negative effects that pollutants can have on human organisms and the environment. Hydroxyapatite (HAp) has the appropriate properties for the immobilization of various pollutants, being considered amongst the most cost-effective materials for water decontamination. The main objective of this study was to use synthesized hydroxyapatite for the elimination of Sr²⁺ ions from contaminated solutions. The hydroxyapatite used in the decontamination process was synthesized in the laboratory using an adapted method. The hydroxyapatite powder (HAp) resulting from the synthesis was analyzed both before and after the elimination of Sr²⁺ ions from contaminated solutions. The efficiency of the HAp nanoparticles in removing Sr²⁺ ions from contaminated solution was determined by batch adsorption experiments. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to study the HAp samples before and after the removal of Sr²⁺ ions. The ability of HAp nanoparticles to eliminate strontium ions from contaminated solutions was established. Moreover, the removal of Sr²⁺ ions from the contaminated aqueous solutions was highlighted by ultrasound measurements. The value of the stability parameter calculated by ultrasonic measurements after the removal of Sr²⁺ ions from the contaminated solution was similar to that of double distilled water whose stability was used as reference. The outcomes of the batch experiments and the parameters obtained from Langmuir and Freundlich models indicated that the HAp nanoparticles had a strong affinity for the elimination of Sr²⁺ ions from polluted solutions. These results emphasized that HAp nanoparticles could be excellent candidates in the development of new technologies for water remediation. More than that, the outcomes of the cytotoxic assays proved that HAp nanoparticles did not induce any noticeable harmful effects against HeLa cells and did not affect their proliferation after 1 day and 7 days of incubation.

Sorption and continuous filtration of heavy metals and radionuclides using novel nano-Farringtonite: Mechanisms delineation using EXAFS

Simultaneous removal of a wide range of toxic heavy metal cations and potential radionuclides from water bodies and their continuous filtration with a single low-cost and eco-friendly material represents several scientific merits. Herein, for the first time, we report the simple and straightforward wet-chemical synthesis of novel nano-Farringtonite (FAR) composed of magnesium and phosphate ions. Further, the potential of known alternate nano-hydroxyapatite (HAP) and novel engineered nano-FAR including their non-stoichiometric variations was evaluated for the removal of mimicking radionuclide (Sr²⁺) and heavy metals (Cd²⁺, and Zn²⁺) from water bodies. Non-stoichiometric FAR (ns-FAR) have shown multifold higher contaminant removal capacities than HAP, i.e., Sr²⁺≈ 85 mg/g vs 49.5 mg/g, Cd²⁺≈ 560 mg/g vs 98.5 mg/g, and Zn²⁺ = 489 mg/g vs 62 mg/g in batch mode. NsFAR showed complete removal of Cd²⁺ and Zn²⁺ with <20% and 0% recovery, respectively in three consecutive sorption-recovery cycles, probing towards permanent incorporation of these ions. Spectroscopic analysis and extended x-ray absorption fine structure (EXAFS) spectroscopy fitting confirmed ion exchange and crystal incorporation as probable removal mechanisms. The high ionic potential of Mg²⁺ allows easy ion exchange with +2 charged metal toxins of varying ionic radius at both Mg1 and Mg2 sites of FAR. nsFAR showed instantaneous separation of these cations in continuous column mode with >2,00,000 L/kg of water filtration capacity (at 1 mg/L), justifying the adsorbent's candidature in water purification applications.

Oil tea shell synthesized biochar adsorptive utilization for the nitrate removal from aqueous media

Various reported methods are devoted to nitrate removal from water over the years. However, recently researchers are focusing on developing the materials that offer bio-based, non-toxic, inexpensive and yet an efficient solution for water treatment. In this study, removal of nitrates from water was carried out using oil tea shells (OTS) as a biosorbent. OTS powder was impregnated with ZnCl₂ and biochar was prepared which was further treated with Cetyltrimethylammonium bromide (CTAB), a cationic surfactant. Both the Langmuir and the Freundlich models were satisfied by the nitrate adsorption of OTS biochar. The adsorption capacity was measured at 15.6 mg/g when the circumstances were at their best. The pseudo-second-order model provided an accurate description of the kinetic data that were collected from batch trials. The adsorption yield goes up when by usage of more adsorbent, but it goes down when adsorption start with a higher concentration of nitrate. The strong basis of analytical equipments were used to characterize the OTS biosorbent. According to the findings of the research, surface-modified OTS biochar is an effective material for the removal of nitrate from aqueous solutions. This means that it has the potential to be utilized in water treatment as an adsorbent that is both inexpensive and kind to the natural environment. Removal of heavy metals and other organic pollutants, both from groundwater and wastewater using OTS biochar seems like a promising and interesting area of study.

Sequestration of heavy metal ions from multi-metal simulated wastewater systems using processed agricultural biomass

Industrial effluents generally contain several metals, so during adsorptive treatment, they may influence the removal of each other. It is essential to explore the effect of co-cations on metal removal in multi-metal solutions. The present study examined the possibility of processed rice husk and saw dust to remove Cr⁶⁺, Ni²⁺, Cu²⁺, Cd²⁺ and Zn²⁺, from the single, binary and multi-component aqueous solutions. A substantial lesser removal of metal ions was observed in the presence of co-ions. This study revealed antagonistic effect on the removal of a particular metal ion, from the industrial effluent, even at optimum process parameters if other metal ions are present in the effluent. Although, a higher concentration of one metal ion than others in effluents increased its removal due to a greater number of ions as compared to other for the biosorption, yet presence of other ions influences the uptake of individual ions. In case of industrial effluents, maximum adsorption was obtained at pH 2.0 for Cr⁶⁺, 5.0 for Ni²⁺ and Cd²⁺ and 6.0 for Zn²⁺ and Cu²⁺. The study confirms the beneficial use of the studied biosorbents in water remediation.

Post-substitution of magnesium at CaI of nano-hydroxyapatite surface for highly efficient and selective removal of radioactive 90Sr from groundwater

We have modified the ion-exchange affinity of nano-Hydroxyapatite (Ca₅(PO₄)₃OH, HAP) surface for the rapid and selective adsorption of ⁹⁰Sr from groundwater. The modification was achieved by the post-substitution of cations (Na⁺, Mg²⁺, Cu²⁺, Ba²⁺, Fe³⁺, and Al³⁺) for parent Ca²⁺ within surface structure of HAP. The diffraction patterns of modified HAP showed a slight shift of the (002) peak between 25° and 27° 2θ depending the ionic radius of the substituted cation. Magnesium substituted HAP, Mg-HAP, exhibited the highest removal efficiency (>95%) for 10 ppm of Sr²⁺, which is attributable to the higher ion-exchange affinity of substituted Mg²⁺ than parent Ca²⁺ toward Sr²⁺. The results of various analyses revealed that Mg substitution dominantly occurred at the Ca^I site of HAP, which enabled the Mg-HAP to adsorb Sr²⁺ at both of Ca^I and Ca^II sites whereas bare HAP could adsorb Sr²⁺ mainly at Ca^II site. Adsorption isotherms and the kinetics of Mg-HAP for Sr²⁺ were evaluated using a bi-Langmuir isotherm and a pseudo-second-order kinetic model, which demonstrated the Mg-HAP exhibited the highest adsorption capacity (64.69 mg/g) and fastest adsorption kinetics (0.161-1.714 g/(mg·min)) than previously modified HAPs. In the presence of competing cations at circumneutral pHs, the enhanced performance of the Mg-HAP led to a greater than 97% reduction of ⁹⁰Sr (initial radioactivity = 9500 Bq/L) within 1 h. The distribution coefficient of Mg-HAP was 1.3-6.6 × 10³ mL/g while that of bare HAP was 1.2-6.6 × 10² mL/g. The findings in the present study highlight that the ion-exchange affinity of Ca^I and Ca^II sites on HAP surface plays a key-role in ⁹⁰Sr uptake. The proposed modification method can simply increase the affinity of HAP surface, therefore, this work can further improve the deployment of an in situ remediation technology for ⁹⁰Sr contaminated groundwater, i.e., Mg-HAP-based permeable reactive barrier.

Characterization of bio-adsorptive removal performance of strontium through ureolysis-mediated bio-mineralization

The adsorptive removal performance of strontium (Sr) through bio-mineralization metabolism under various parameters was evaluated in this study. The primary mechanism of bio-mineralization used in this study was the urea hydrolysis process through bacterial enzymatic catalysis. Bacillus sp, which was isolated from river sediment, was used as a ureolytic bacteria. Various environmental conditions were set as different initial concentrations of Sr (10, 50, 100, 200, and 500 mg/L), and various ratios of Mg/Ca (4, 2, 1, 0.5, and 0.25). The concentrations of Sr²⁺, Ca²⁺, and Mg²⁺ in the solution of the batch experiment were measured to identify the bio-mineralization performance and the removal rate of Sr. In addition, the main Sr removal mechanism of ureolytic bacteria was identified. As a result, for Sr removal of bacteria, the bio-mineralization mechanism was more predominant than the adsorption of Sr. The rapid growth and high nucleation site production were observed when the initial concentration of Sr²⁺ increased and the Mg/Ca ratio was lowered, resulting in high biomineralization performance and Sr removal rate. The main phases of carbonate minerals formed in the presence of Sr, Ca, and Mg were SrCO₃ and SrCa(CO₃)₂. Mg²⁺ could retard the bacterial growth and participate in the formation of carbonate minerals, when a large amount of Mg²⁺ was present. Furthermore, the desorption rate of Sr²⁺ from bacterial pastes containing the carbonate minerals increased as the concentration of HCl increased, although the carbonate minerals were in a stable state.

Co-transport of biogenic nano-hydroxyapatite and Pb(II) in saturated sand columns: Controlling factors and stochastic modeling

Biogenic nano-hydroxyapatite (bio-nHAP) has recently gained great interest in many domains, especially in the remediation of heavy metal-contaminated soil, due to its high reactivity, low cost, and eco-friendly nature. The co-transport and reaction of bio-nHAP with Pb(II) in saturated porous media, however, are not well understood. This work investigated the effects of ionic strength (IS), ionic composition (IC), dissolved organic matter (DOM), and flow velocity on transport-reaction dynamics of Pb(II) and bio-nHAP by combining column breakthrough experiments and model simulations. Results showed that the mobility of Pb(II) was significantly enhanced with increasing IS/IC but less affected by flow velocity during the transport-reaction process of bio-nHAP and Pb(II) in the saturated sand column; while the transport of bio-nHAP was restricted by increasing IS/IC but facilitated by increasing velocity. IC, IS, and velocity only slightly affected the reaction kinetics between Pb(II) and bio-nHAP, likely due to the fast reaction rate between Pb(II) and bio-nHAP and precipitation of pyromorphite. The transport dynamics of bio-nHAP and Pb(II) were significantly changed by DOM, and this effect depended strongly on the type of DOM with different molecular weights. Breakthrough curves of Pb(II) and bio-nHAP exhibited apparent "anomalous", sub-diffusive transport behaviors, which could be well quantified by a novel tempered fractional derivative bimolecular reaction equation (T-FBRE). Our findings highlighted the accurate simulation of the co-transport and reaction of bio-nHAP with Pb(II) using T-FBRE and had a great benefit for risk assessment and remediation strategy development for Pb(II) contaminated soil.

Novel Hydroxyapatite Beads for the Adsorption of Radionuclides from Decommissioned Nuclear Power Plant Sites

Although a powdered form of hydroxyapatite (p-HdA) has been studied for the adsorption of heavy metals that contaminate the restoration sites of decommissioned nuclear power plants, most of the studies are limited in the laboratory due to the head loss and post-separation in practical applications. Herein, we fabricated a porous bead form of HdA (b-HdA) as a novel adsorbent for removing radionuclides from aqueous environments via a facile synthesis by mixing the p-HdA precursor and polyvinyl butyral (PVB) as a binder and added a sintering process for the final production of a porous structure. The spherical b-HdA with an approximate diameter of 2.0 mm was successfully fabricated. The effectiveness of the b-HdA at removing Co(II) was investigated via the adsorption equilibrium at various experimental temperatures. The b-HdA exhibited the adsorption capacity for Co(II) ions with a maximum of 7.73 and 11.35 mg/g at 293 K and 313 K, respectively. The experimental kinetic data were well described using a pseudo-second-order kinetic model, and the adsorption mechanisms of Co(II) onto the b-HdA were revealed to be a chemisorption process with intraparticle diffusion being the rate-limiting step. In addition, the competitive adsorption onto the b-HdA with the order of U(VI) > Co(II) > Ni(II) > Sr(II) > Cs(I) was also observed in the multi-radionuclides system. Considering the advantages of the size, applicability to the continuous-flow column, and the easy separation from treated water, the b-HdA can be an excellent absorbent with high potential for practical applications for removing radionuclides.

Efficient separation of strontium radionuclides from high-salinity wastewater by zeolite 4A synthesized from Bayer process liquids

The efficient, selective, and economical sorbents for the removal of Sr radionuclides are largely needed for the decontamination of effluents with high salinity. In this study, the removal of Sr was investigated using the zeolite produced from the Bayer process liquids. Based on the XRD, SEM/EDS analysis, the product was pure and highly crystalline zeolite 4A (Z4A). Removal of Sr was fast (5 min for 100% removal at 8.80 mg/L), with high maximum sorption capacity (252.5 mg/L), and independent on the initial pH in the range 3.5-9.0. Specific sorption of Sr by protonated groups on the Z4A surface was operating in addition to ion-exchange with Na ions. The selectivity of Z4A decreased in the order Sr > Ca > K > Mg > Na. 84% of Sr was separated from seawater within 5 min, at the Z4A dose of 5 g/L, while efficiency increased to 99% using the dose of 20 g/L. Desorption of radioisotope ⁸⁹Sr from seawater/Z4A solid residue was very low in deionized water (0.1-0.2%) and groundwater (0.7%) during 60 days of leaching. Z4A is a cost-effective, selective, and high-capacity medium for Sr removal, which provides high stability of retained radionuclides.

Metal Removal from Nickel-Containing Effluents Using Mineral-Organic Hybrid Adsorbent

Nickel is one of the most dangerous environmental pollutants and its removal from wastewater is an important task. The capacity of a mineral–organic hybrid adsorbent, consisting of Shewanella xiamenensis biofilm and zeolite (clinoptilolite of the Chola deposit), to remove metal ions from nickel-containing batch systems under different experimental conditions was tested. The obtained biosorbent was characterized using neutron activation, SEM, and FTIR techniques. It was established that maximum removal of cations, up to 100%, was achieved at pH 6.0. Several mathematical models were applied to describe the equilibrium and kinetics data. The maximum adsorption capacity of the hybrid biosorbent, calculated using the Langmuir model, varied from 3.6 to 3.9 mg/g. Negative Gibbs energy values and positive ∆H° values indicate the spontaneous and endothermic character of the biosorption process. The effects of several parameters (pH and biosorbent dosage) on Ni(II) removal from real effluent, containing nickel with a concentration of 125 mg/L, were investigated. The optimal pH for Ni(II) removal was 5.0–6.0 and an increase of sorbent dosage from 0.5 to 2.0 led to an increase in Ni(II) removal from 17% to 27%. At two times effluent dilution, maximum Ni(II) removal of 26% was attained at pH 6.0 and sorbent dosage of 1.0 g. A 12-fold effluent dilution resulted in the removal of 72% of Ni(II) at the same pH and sorbent dosage values. The obtained hybrid biosorbent can be used for Ni(II) removal from industrial effluents with low Ni(II) concentrations.

Review on Selection and Experiment Method of Commonly Studied Simulated Radionuclides in Researches of Nuclear Waste Solidification

Although many types of simulated radionuclides have been widely used as a substitute for actual nuclear waste in the studies of nuclear waste solidification, the understanding of the applicability and validity of simulated radionuclides is still insufficient. In particular, the selection and use of simulated radionuclides, which can play a decisive role in the accuracy of the experimental results, still lack unified or integrated references. This paper provides a critical review on the selection, experimental methods, and applicability of the most commonly studied simulated radionuclides, followed by a careful discussion and recommendation of simulated radionuclides suitable for different solidified bodies. The main factors (e.g., temperature, pH, and atmosphere) affecting the choice of simulated radionuclides were analyzed in detail. This work helps to integrate the selection and use of simulated radionuclides, and it will be beneficial for improving the effectiveness of nuclide solidification research.

Synthesis, Characterization and Mechanism Study of Green Aragonite Crystals from Waste Biomaterials as Calcium Supplement

In present work, environmentally benign green aragonite crystals were synthesized from waste chicken eggshells and bivalve seashells through a simple and low-cost wet carbonation method. This method involves a constant stirring of calcium oxide slurry and magnesium chloride suspension in aqueous solution with constraint carbon dioxide injection at 80 °C. The physicochemical properties of the synthesized aragonite were further compared with the aragonite synthesized from commercial calcium oxide. The morphological analysis, such as acicular shape and optimum aspect ratio (~21), were confirmed by scanning electron microscopy. The average crystal size (10–30 µm) and specific surface area (2–18 m2 g−1) were determined by particle size and Brunauer–Emmett–Teller analysis, respectively. Moreover, a schematic crystal growth mechanism was proposed to demonstrate the genesis and progression of aragonite crystal. Green aragonite can bridge the void for numerous applications and holds the potential for the commercial-scale synthesis with eggshells and bivalve seashells as low-cost precursors.

A novel sorbent based on Ti-Ca-Mg phosphates: synthesis, characterization, and sorption properties

This work focuses on the synthesis procedure of a new sorbent based on a TiCaMg phosphate. The synthesis strategy includes stepwise interaction between solid precursors and phosphorus-containing agents. The solid precursors were ammonium titanyl sulfate and calcined dolomite, which were used as titanium, calcium, and magnesium sources. The effect of the nature and concentration of phosphoric agent on the sorbent composition and properties has been investigated using elemental analysis, TG, XRD, IR spectroscopy, BET, and SEM techniques. The novel sorbent has been demonstrated to be a composite material consisting of the following components: TiO(OH)H₂PO₄·H₂O, Ti(HPO₄)₂·H₂O, CaHPO₄·2H₂O, MgНPO₄·3H₂O, and NH₄MgPO₄·6H₂O. The ratio between these phases in the composite depends on synthesis conditions. The optimal conditions, ensuring full conversion of Ti, Ca, and Mg containing in the initial precursors into the final product, have been found. The sorption properties of the obtained composite sorbent towards Co²⁺, Cs⁺, and Sr²⁺ cations and their radionuclide analogues have been studied. The obtained data has indicated that the purification effect was based on both precipitation and ion exchange mechanism. The combined action of the individual components of the composite sorbent ensures its high sorption capacity towards different cations in a wide pH range. The new sorbent shows high sorption ability towards radionuclides in multicomponent liquid radioactive waste (LRW) systems, and the distribution coefficient of the studied radionuclides was found to be 10⁵ mL g^-1. The presence of different types of functional groups in the composite sorbent allows realizing the one-step purification process of LRW that, in turn, simplifies the sorption system design.

Study of 223Ra uptake mechanism on hydroxyapatite and titanium dioxide nanoparticles as a function of pH

The mechanism of 223Ra uptake on hydroxyapatite and titanium dioxide nanoparticles was studied as a function of pH. Both materials are widely used in food industry and medicine. They offer properties suitable for labelling with medicinal radionuclides, particularly for targeted radionuclide therapy. The selected isotope, 223Ra, is an alpha emitter widely used in targeted alpha particle therapy due to high-dose delivery in very small tissue volume, nevertheless the results are applicable for any radium isotope including 226Ra. The study was performed in the pH range 4.5 to 12 for hydroxyapatite nanoparticles and 2 to 12 for titanium dioxide nanoparticles in Britton-Robinson buffer solution. Both nanomaterials at pH 6 and higher showed that over 95% of the radium has been sorbed. According to the applied chemical equilibrium model, the most important species playing a role in sorption on the edge-sites were RaCO3, RaPO4 -, RaHPO4 and Ra(Ac-)2, and Ra2+ and RaH2PO4 + on layer-sites. All experiments were conducted under free air conditions and no negative impact of CO2 was found. The surface complexation model was found suitable for describing radium uptake by the studied hydroxyapatite and titanium dioxide nanomaterials.

Immobilization of radioactive corrosion products by cold sintering of pure hydroxyapatite

An efficient method for the consolidation of cobalt (Co(II)) adsorbed calcium hydroxyapatite was investigated to develop a simplified route for decontamination of the coolant system of nuclear power plants and direct immobilization of as-spent adsorbent. Calcium hydroxyapatite nano-powder synthesized by a wet precipitation method was used as an adsorbent and 94% Co(II) surrogate removal from simulated water was measured. The as-spent adsorbent was sintered at 200 °C, a temperature significantly lower than conventional sintering temperatures (900-1300 °C) for hydroxyapatite, under a uniaxial pressure of 500 MPa for 10 min. The relative density after the cold sintering was >97% and sintered samples displayed good compressive strength (175 MPa). The normalized leaching rate of the Co(II) was measured as per ASTM-C1285 standard and found to be 2.5 × 10^-5 g/m²/day. ANSI/ANS-16.1 test procedure was used to analyze the leachability of the sintered matrices and the measured leaching index value was 6.5. Thus, the use of pure calcium hydroxyapatite nano-powder as adsorbent and its cold sintering offers a mean by which radioactive waste form can be processed in an environment friendly manner.

Surface protolytic property characterization of hydroxyapatite and titanium dioxide nanoparticles

We provide characterization data of hydroxyapatite (nHAp) and titanium dioxide (nTiO2) nanoparticles as potential materials for ion sorption, e.g. in targeted therapy, barrier materials for waste repositories or photovoltaics. The study is focused on the determination of the values of protonation and ion exchange constants and site densities (∑SOH, ∑X; [mol kg-1]) of nTiO2 and nHAp for further Ra kinetics and sorption experiments. These data are very important for further investigation of the materials, which can be used e.g. as drug delivery systems or in engineered barriers of deep geological repositories. The characterization was based on the evaluation of the dependence of titrating agent consumption on pH. Titration results were evaluated on the basis of several model combinations, however the combination of the Chemical Equilibrium Model (CEM) and Ion Exchange Model (IExM) fits best to the experimental titration curves. However, the differences between the two sorbents were relatively large. Due to stability in a broad pH range and available surface sites, nTiO2 seems to have a wide application range. The applicability of nHAp is not so wide because of its dissolution under pH 5. Both sorbents are virtually able to sorb cationic species on deprotonated edge and layer sites with different capacities, which can be important for sorption and decontaminating applications.

A facile synthesis of hydroxyapatite for effective removal strontium ion

Hydroxyapatite (HA) with perforated porous structure was successfully synthesized using shell powder as the raw material by double interfacial diffusion method. The structure of obtained products was examined by X-ray diffraction, Fourier transform infrared spectrograph, field-emission scanning electron microscopy, transmission electron microscopy, particle size, thermogravimetry and nitrogen adsorption-desorption analysis etc. Results indicate that the perforated porous structure is composed of nanosheets and has high specific surface area (up to 188.5 m² g^-1). Thus, investigation of adsorbing Sr²⁺ in solution was further examined by discussing factors such as initial pH, ion strength, adsorbent dosage, contact time, initial Sr²⁺ concentration and temperature. The kinetics and equilibrium adsorption data followed the nonlinear pseudo-second-order kinetic and Liu isotherm models. The maximum removal (%) was up to 98.94% at 313.15 K, and the adsorption process of Sr²⁺ was endothermic, feasible, and spontaneous in nature as studied via thermodynamic analysis (ΔG° < 0, ΔH° > 0, and ΔS° > 0). A possible adsorption mechanism was proposed. Meanwhile, leaching and desorption experiments was used to evaluate recycling capacity. All the outcomes effectively reveal that the synthesized HA shows great potential in removing Sr²⁺ from nuclear effluents.

Strontium in the environment: Review about reactions of plants towards stable and radioactive strontium isotopes

Radiostrontium is released to the environment from routine and accidental discharge and acts on living organisms either from external sources or after absorption. When incorporated by plants, it enters the food chain and causes primary threat to human health and the environment. Understanding the mechanisms of plants for strontium uptake and retention is therefore essential for decision making concerning agriculture: are uptake rates low enough so that plants can serve as food? Or is radiostrontium accumulated so that plants should not be eaten but could be probably used for extracting strontium from water and soil in hot spots of pollution? The review presents a summary of studies about the origin of stable and radioactive strontium in the environment and effects coming from both internal and external exposure of plants. Mobility and availability of strontium to plant roots in soil are controlled by external factors such as chemical composition of the soil and pH, temperature and agricultural soil cultivation as well as soil biological networks built by microbial communities. Plant surfaces may receive input of strontium from deposition induced by atmospheric pollution or by acquisition from water through the whole immersed surface. Cells have entry mechanisms for strontium such as plasma membrane transporters for calcium and potassium. Part of absorbed strontium can be lost via processes discussed in this review. We give examples on strontium transfer factors for 149 plants to estimate plant absorption capacity for strontium from soil, water and air. Uptake efficiency of terrestrial and aquatic plants is deciding about their remediation potential to either remove radiostrontium by accumulation and rhizofiltration or to retain it in roots or aerial parts. Data of strontium content in soils after fallout and edible plants from long-term monitoring support the evaluation of the potential hazards posed by strontium input to the food chain.

Removal of Sr2+ using high-surface-area hydroxyapatite synthesized by non-additive in-situ precipitation

Owing to their high-risk factor, many attempts have been made to remove radionuclides from water. Sr²⁺ ions are the target of removal by synthesized hydroxyapatite in this research. A facile method for synthesizing high-surface-area hydroxyapatite by in-situ precipitation using excess diammonium phosphate solution and without any additive was developed. The highest surface area achieved using this method was 177.00 m²/g, and the synthesized hydroxyapatite was also mesoporous. The effects of different pH, temperatures, and ion concentrations during synthesis on the properties of the hydroxyapatite were assessed, and it was found that a low temperature and high pH were optimal for synthesizing high-surface-area hydroxyapatite. The maximum strontium removal capacity of 28.51 mg/g was achieved when the pH-7.5 solution was used. This performance is competitive in comparison with previously developed synthesized materials. Synthesized hydroxyapatite could effectively remove radioactive strontium from an aqueous solution for nuclear waste management.

Synthesis and applications of biogenic nanomaterials in drinking and wastewater treatment

The continuous increase in water pollution by various organic & inorganic contaminants has become a major issue of concern worldwide. Furthermore, the anthropogenic activities for the manufacturing of various products have boosted this problem manifold. To overcome this serious issue, nanotechnology has initiated to explore various proficient strategies to treat waste water in a more precise and accurate way with the support of various nanomaterials. In recent times, nanosized materials have proved their applicability to provide clean and affordable water treatment technologies. The exclusive features such as high surface area and mechanical properties, greater chemical reactivity, lower cost and energy, efficient regeneration for reuse allow the nanomaterials perfect for water remediation. But the conventional routes of synthesis of nanomaterials encompass the involvement of hazardous and volatile chemicals; therefore the use of nanomaterials further creates the secondary pollution. This issue has intrigued the scientists to develop biogenic pathways and procedures which are environmentally safer and inexpensive. It has led to the new trends that involve developing bio-inspired nano-scale adsorbents and catalysts for the removal and degradation of a wide range of water pollutants. Carbohydrates, proteins, polymers, flavonoids, alkaloids and several antioxidants obtained from plants, bacteria, fungi, and algae have proven their effectiveness as capping and stabilizing agents during manufacture of nanomaterials. Application of biogenic nanomaterials for waste water treatment is relatively newer but rapidly escalating area of research. In the present review, promises and challenges for the synthesis of various biogenic nanomaterials and their potential applications in waste water treatment and/or water purification have been discussed.

Investigation of MnO2 nanoparticles-anchored 3D-graphene foam composites (3DGF-MnO2) as an adsorbent for strontium using the central composite design (CCD) method

Strontium-90 is one of the dangerous fission products generated during electricity production in nuclear reactors and the separation of this radionuclide from contaminated water is an important step in safeguarding human health and minimizing the impact on the environment.

Atomic-level design of CoOH+–hydroxyapatite@C catalysts for superfast degradation of organics via peroxymonosulfate activation

In situ formation of CoOH⁺–hydroxyapatite@C via ion exchange between Ca and Co realises the simultaneous adsorption of Co²⁺ and catalytic peroxymonosulfate oxidation for superfast oxidative degradation of organic contaminants.

Calcium-deficient Hydroxyapatite as a Potential Sorbent for Strontium

A calcium (Ca)-deficient hydroxyapatite was investigated for its potential to remove Sr²⁺ from environmentally relevant water. We conducted sorption tests on solutions containing magnesium ion (Mg²⁺) and calcium ion (Ca²⁺) as competing cations at a strontium ion (Sr²⁺) concentration of 0.05 mmol/L. The Ca-deficient hydroxyapatite maintained a high Sr²⁺ sorption ratio of above 80% in the presence of Mg²⁺ and Ca²⁺ at the concentrations between 0.1 and 1.0 mmol/L, whereas the stoichiometric hydroxyapatite showed a lower ratio even in the presence of small amounts of Mg²⁺ and Ca²⁺ (72% for Mg²⁺ and 51% for Ca²⁺ at 0.1 mmol/L). For solutions with various Sr²⁺ concentrations between 0.01 and 10 mmol/L, Ca-deficient hydroxyapatite exhibited a higher Sr²⁺ sorption ratio than stoichiometric hydroxyapatite. The bonding states of Sr²⁺ on the Ca-deficient hydroxyapatite were evaluated by extended X-ray absorption fine structure measurements. The results indicated that there are specific sorption sites in Ca-deficient hydroxyapatite where Sr²⁺ is stably and preferentially immobilized.