Adsorption and removal of strontium in aqueous solution by synthetic hydroxyapatite

Enhanced sorption of strontium radionuclides onto a modified molybdenum titanate composite

A study was conducted to investigate the sorption of 85Sr from aqueous solutions using a fabricated magnesium molybdenum titanate (MgMoTi) composite. The MgMoTi composites were synthesized through the co-precipitation technique and characterized using different analytical tools, including FT-IR, XRD, SEM, and EDX. The sorption studies focused on 85Sr and examined factors such as shaking time, pH, ionic strength, temperature, initial ion concentration, and saturation capacity. The results obtained from the study indicated that, under optimum sorption conditions, the saturation capacity for 85Sr onto S-4 and S-5 was determined to be 23.31 and 37.72 mg g-1, respectively. The sorption of 85Sr exhibited dependence on pH and ionic strength. The kinetics of the sorption process followed the pseudo-2nd-order model, while the thermodynamics revealed an endothermic and spontaneous nature. Desorption studies revealed that 0.1 M HCl was the most effective eluent for the complete recovery of 85Sr. Furthermore, the recycling results demonstrated the excellent recyclability of MgMoTi, suggesting its potential application as a sorbent for the removal of 85Sr from aqueous solutions. Overall, the study highlights MgMoTi as a promising composite with practical utility in the sorption of 85Sr from aqueous solutions.

Sorption behavior of strontium and europium ions from aqueous solutions using fabricated inorganic sorbent based on talc

Sorption of Sr(II) and Eu(III) from aqueous solutions was studied using tin molybdate talc sorbent synthesized by the precipitation technique. The synthesized sorbent was characterized using different analytical tools, such as; FT-IR, SEM, XRD, XRF, TGA, and DTA. The sorption studies applied to Sr(II) and Eu(III) include the effects of shaking time, pH, concentrations, and saturation capacity. The sorption of Sr(II) and Eu(III) depends on pH, reaction kinetics obey the pseudo-2nd-order model, and the Langmuir model is better suited for the sorption isotherm. The thermodynamic parameters reflect an endothermic and spontaneous sorption process. Desorption studies showed that 0.1 M HCl was the best desorbing agent for the complete recovery of Sr(II) (96.8%) and Eu(III) (92.9%). Finally, the obtained data illustrates that the synthesized sorbent can be applied and used as an efficient sorbent for the sorption of Sr(II) and Eu(III) from aqueous solutions and can be used as a promising sorbent to remove Sr(II) and Eu(III).

Efficient uptake of 85Sr and 60Co using fabricated inorganic sorbent for reducing radiation doses of simulated low-level waste

The present study investigated the sorption behavior of 85Sr and 60Co radionuclides from aqueous solutions onto tin molybdate (SnMo) sorbent. SnMo has been synthesized using the precipitation method and was characterized using four analytical techniques including FT-IR, XRD, SEM, and XRF. The sorption studies applied on 85Sr and 60Co include the effect of shaking time, pH, concentration, and saturation capacity. The experimental data revealed that the sorption process was carried out after equilibrium time (180 min). The saturation capacity for 85Sr and 60Co is measured to be 58.1 and 52.2 mg g-1, respectively. The sorption behavior of studied radionuclides is dependent on pH values. Sorption kinetic better fit with the pseudo-second-order model. Furthermore, the sorption isotherm is better represented by the model proposed by Langmuir. The results of the desorption investigations indicated that the most effective eluents for achieving full recovery of investigated radionuclides were identified. Finally, the recycling results demonstrate the suitability of SnMo for affected sorbing of 85Sr and 60Co from aqueous solutions. All the obtained data clarify that the SnMo sorbent is an effective means of removing 85Sr and 60Co from liquid waste.

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.

Antibacterial peptides-loaded bioactive materials for the treatment of bone infection

Bacterial bone infection in open fractures is an urgent problem to solve in orthopedics. Antimicrobial peptides (AMPs), as a part of innate immune defense, have good biocompatibility. Their antibacterial mechanism and therapeutic application against bacteria have been widely studied. Compared with traditional antibiotics, AMPs do not easily cause bacterial resistance and can be a reliable substitute for antibiotics in the future. Therefore, various physical and chemical strategies have been developed for the combined application of AMPs and bioactive materials to infected sites, which are conducive to maintaining the local stability of AMPs, reducing many complications, and facilitating bone infection resolution. This review explored the molecular structure, function, and direct and indirect antibacterial mechanisms of AMPs, introduced two important AMPs (LL-37 and β-defensins) in bone tissues, and reviewed advanced AMP loading strategies and different bioactive materials. Finally, the latest progress and future development of AMPs-loaded bioactive materials for the promotion of bone infection repair were discussed. This study provided a theoretical basis and application strategy for the treatment of bone infection with AMP-loaded bioactive materials.

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.

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.

Removal of potentially toxic elements from contaminated soil and water using bone char compared to plant- and bone-derived biochars: A review

Conversion of hazardous waste materials to value-added products is of great interest from both agro-environmental and economic points of view. Bone char (BC) has been used for the removal of potentially toxic elements (PTEs) from contaminated water, however, its potential BC for the immobilization of PTEs in contaminated water and soil compared to bone (BBC)- and plant (PBC)-derived biochars has not been reviewed yet. This review presents an elaboration for the potentials of BC for the remediation of PTEs-contaminated water and soil in comparison with PBC and BBC. This work critically reviews the preparation and characterization of BC, BBC, and PBC and their PTEs removal efficiency from water and soils. The mechanisms of PTE removal by BC, BBC, and PBC are also discussed in relation to their physicochemical characteristics. The review demonstrates the key opportunities for using bone waste as feedstock for producing BC and BBC as promising low-cost and effective materials for the remediation of PTEs-contaminated water and soils and also elucidates the possible combinations of BC and BBC aiming to effectively immobilize PTEs in water and soils.

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.

Prediction of batch sorption of barium and strontium from saline water

Celestite and barite formation results in contamination of barium and strontium ions hinder oilfield water purification. Conversion of bio-waste sorbent products deals with a viable, sustainable and clean remediation approach for removing contaminants. Biochar sorbent produced from rice straw was used to remove barium and strontium ions of saline water from petroleum industries. The removal efficiency depends on biochar amount, pH, contact time, temperature, and Ba/Sr concentration ratio. The interactions and effects of these parameters with removal efficiency are multifaceted and nonlinear. We used an artificial neural network (ANN) model to explore the correlation between process variables and sorption responses. The ANN model is more accurate than that of existing kinetic and isotherm equations in assessing barium and strontium removal with adj. R² values of 0.994 and 0.991, respectively. We developed a standalone user interface to estimate the barium and strontium removal as a function of sorption process parameters. Sensitivity analysis and quantitative estimation were carried out to study individual process variables' impact on removal efficiency.

Preparation of Porous Hydroxyapatite Using Cetyl Trimethyl Ammonium Bromide as Surfactant for the Removal of Lead Ions from Aquatic Solutions

In the present study, a new low-cost bioceramic nanocomposite based on porous hydroxyapatite (HAp) and cetyl trimethyl ammonium bromide (CTAB) as surfactant was successfully obtained by a simple chemical co-precipitation. The composition and structure of the HAp-CTAB were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) spectrometer, and N2 adsorption/desorption analysis. The capacity of HAp-CTAB nanocomposites to remove the lead ions from aqueous solutions was studied by adsorption batch experiments and proved by Langmuir and Freundlich models. The Pb2+ removal efficiency of HAp-CTAB biocomposite was also confirmed by non-destructive ultrasound studies. The cytotoxicity assays showed that the HAp-CTAB nanocomposites did not induce any significant morphological changes of HeLa cells after 24 h of incubation or other toxic effects. Taken together, our results suggests that the obtained porous HAp-CTAB powder could be used for the decontamination of water polluted with heavy metals, such as Pb2+.

Dual-functional micro-adsorbents: Application for simultaneous adsorption of cesium and strontium

In current work, Prussian blue (PB)- and hydroxyapatite (HAp)-embedded micro-adsorbents (PB-HAp-MAs) were rationally fabricated through an easy and flexible custom-made micronozzle system as a novel bifunctional adsorbent. The adsorption performance of the as-prepared samples was conducted based on the removal of cesium (Cs⁺) and strontium (Sr²⁺) ions. Adsorption behaviors of the PB-HAp-MAs were also evaluated as function extrusion dimensions and adsorbate concentration. The adsorption isotherm was well fitted by the Langmuir model with adsorption capacities of 24.688 mg g^-1 and 29.254 mg g^-1 for Cs⁺ and Sr²⁺, respectively. Specially, the enhanced adsorption activity can be synergistically attributed to the porous nature of the developed alginate backbone with a high surface area of encapsulated functional nanoparticles, thus leading to rapid saturation within 1 min. In addition, the as-synthesized PB-HAp-MAs were successfully separated from the aqueous solution within 10 s by applying a magnetic field. We expect that our findings will provide valuable guidelines towards developing highly efficient adsorbents for environmental remediation.

Removal of heavy metals and radionuclides from water using nanomaterials: current scenario and future prospects

There is an increase in concern about the hazardous effects of radioactivity due to the presence of undesirable radioactive substances in our vicinity. Nuclear accidents such as Chernobyl (1986) and Fukushima (2011) have further raised concerns towards such incidents which have led to contamination of water bodies. Conventional methods of water purification are less efficient in decontamination of radioisotopes. They are usually neither cost-effective nor environmentally friendly. However, nanotechnology can play a vital role in providing practical solutions to this problem. Nano-engineered materials like metal oxides, metallic organic frameworks, and nanoparticle-impregnated membranes have proven to be highly efficient in treating contaminated water. Their unique characteristics such as high adsorption capacity, large specific surface area, high tensile strength, and excellent biocompatibility properties make them useful in the field of water purification. This review explores the present status and future prospects of nanomaterials as the next-generation water purification systems that can play an important role in the removal of heavy metals and radioactive contaminants from aqueous solutions.

Brushite-infused polyacrylonitrile nanofiber adsorbent for strontium removal from water

The Fukushima Daiichi nuclear disaster and the decommissioning of over a hundred nuclear reactors worldwide led to the increase in the demand for efficient water treatment technologies to remove radionuclides, such as ⁹⁰Sr. Brushite or dicalcium phosphate dihydrate (DCPD) is a potential adsorbent to remove strontium from water. In this study, composite poly(acrylonitrile) (PAN) nanofiber (NF) adsorbents with DCPD (PAN/DCPD) were prepared, characterized, and investigated for strontium adsorption in water. Material characterization revealed mechanically suitable, hydrophilic, and macroporous composite NF adsorbents with average fiber diameters of <500 nm. As-prepared DCPD powder exhibited a superior strontium uptake capacity of 81.7 mg g^-1 at pH ≅ 10 of aqueous Sr²⁺ solution over its biogenic and synthetic predecessor, hydroxyapatite. Increased DCPD loading resulted in higher adsorption. Maximum Sr²⁺ uptake of PAN/DCPD NF with 70 wt% DCPD loading (PAN/70DCPD NF) was 146 mg g^-1 considering the Sips isotherm model. Kinetic studies revealed that Sr²⁺ removal by PAN/DCPD NF was a chemisorption process which involved ion exchange and surface complexation. PAN/70DCPD NF as a dead-end membrane filter exhibited superior removal efficiency over pure PAN NF. The overall results of this study revealed the potential application of PAN/DCPD NF adsorbent for ⁹⁰Sr removal from water.

Enhancement of Ion Pairing of Sr(II) and Ba(II) Salts by a Tritopic Ion-Pair Receptor in Solution

Tritopic ion-pair receptors can bind bivalent salts in solution; yet, these salts have a tendency to form ion-pairs even in the absence of receptors. The extent to which such receptors can enhance ion pairing has however remained elusive. Here, we study ion pairing of M2+ (Ba2+ , Sr2+ ) and X- (I- , ClO4- ) in acetonitrile with and without a dichlorooxacalix[2]arene[2]triazine-related receptor containing a pentaethylene-glycol moiety. We find marked ion association already in receptor-free solutions. When present, most of the MX+ ion-pairs are bound to the receptor and the overall degree of ion association is enhanced due to coordinative, hydrogen-bonding, and anion-π interactions. The receptor shows higher selectivity for iodides but also stabilizes perchlorates, despite the latter are often considered as weakly coordinating anions. Our results show that ion-pair binding is strongly correlated to ion pairing in these solutions, thereby highlighting the importance of taking ion association in organic solvents into account.

Synthesis of Metal-Organic Framework ZnO x -MOF@MnO2 Composites for Selective Removal of Strontium Ions from Aqueous Solutions

A Zn(II)-based metal-organic framework (MOF) compound and MnO2 were used to prepare ZnO x -MOF@MnO2 composites for selective Sr2+ removal in aqueous solutions. The ZnO x -MOF@MnO2 composites were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, and Brunauer-Emmett-Teller surface area analysis. The functional groups, morphologies, thermal stabilities, and specific surface areas of the composites were suitable for Sr2+ adsorption. A maximum adsorption capacity of 147.094 mg g-1 was observed in batch adsorption experiments, and the sorption isotherms were fit well by the Freundlich model of multilayer adsorption. Adsorption reached equilibrium rapidly in kinetic experiments and followed the pseudo-second-order kinetic model. The adsorption capacity of the ZnO x -MOF@MnO2 composite with the highest MnO2 content was high over a wide pH range, and the composite was highly selective toward Sr2+ in solutions containing coexisting competing ions. Also, it has a good reusability for removing Sr2+.

Simultaneous and synergistic effect of heavy metal adsorption on the enhanced photocatalytic performance of a visible-light-driven RS-TONR/TNT composite

A hydrothermally synthesized rhodium/antimony co-doped TiO₂ nanorod and titanate nanotube (RS-TONR/TNT) composite was prepared for removal of heavy metals and organic pollutants from water under visible light irradiation. The composite provides the dual function of simultaneous adsorption of heavy metal ions and enhanced degradation of dissolved organic compounds. Acid treatment transformed titanate nanotubes to irregular tubular structures distributed homogeneously over untransformed RS/TONRs. Synergistic removal and degradation was studied with various heavy metals, Orange (II) dye, and Bisphenol A. The adsorption capacity of the composite for heavy metal ions was Pb(II) > Cd(II) > Cu(II) > Zn(II). The adsorbed metals enhanced photocatalytic degradation of the organic pollutants, but Cu was most effective, with degradation exceeding 70% for the dye and 80% for Bisphenol A after 5 h of treatment. Photocatalytic activity was enhanced more by adsorption than photodeposition of Cu ions. A decrease in XRD rutile peak intensity with adsorbed metal indicates a change in crystallinity which may enhance photocatalytic activity. Thick and bulging nanostructures in FE-SEM images signify ion adsorption within titanate pores. BET analysis indicated titanate nanotubes with adsorbed metal are mesoporous but their tubular structure persists. XPS showed more active Cu 2p_3/2 states under light, supporting an active role of Cu⁺ in photocatalytic ROS generation. Detection of ROS and Cu species using methanol, EDTA, pCBA, and benzoic acid probes provided strong evidence for degradation via a charge transfer mechanism. Findings demonstrate the potential of the RS-TONR/TNT composite for simultaneous removal of heavy metals and degradation of organic pollutants.

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.

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.

Studies on adsorption of oxytetracycline from aqueous solutions onto hydroxyapatite

The presence of antibiotics in the water and wastewater has raised problems due to potential impacts on the environment and consequently their removal is of great importance. For this reason, this article aims to perform a study on the possibility of oxytetracycline (OTC) adsorption from aqueous medium by using the hydroxyapatite (HA) nanopowders as adsorbent materials. The hydroxyapatite nanopowders were synthesized by wet precipitation method by using orthophosphoric acid and calcium hydroxide as raw materials and investigated by XRD, SEM-EDX, FTIR and BET methods. The uncalcined and calcined hydroxyapatite samples have hexagonal crystal structure with crystal sizes smaller than 100nm and a specific surface area of 316m²/g and 139m²/g, respectively. The adsorption behavior of oxytetracycline, a zwitterionic antibiotic, on nanohydroxyapatite was investigated as a function of pH, contact time, adsorbent dosage and drug concentration by means of batch adsorption experiments. High oxytetracycline removal rates of about 97.58% and 89.95% for the uncalcined and calcined nanohydroxyapatites, respectively, were obtained at pH8 and ambient temperature. The adsorption process of oxytetracycline onto nanohydroxyapatite samples was found to follow a pseudo-second order and intraparticle diffusion kinetic models. The maximum adsorption capacities of 291.32mg/g and 278.27mg/g for uncalcined and calcined nanohydroxyapatite samples, respectively, have been found. The adsorption mechanism of OTC on the hydroxyapatite surface at pH8 can be established via surface complexation. The obtained results are indicative of good hydroxyapatite adsorption ability towards oxytetracycline drug.

Recent developments in nanostructured inorganic materials for sorption of cesium and strontium: Synthesis and shaping, sorption capacity, mechanisms, and selectivity-A review

Liquid wastes containing non-ferrous heavy metal ions and some radionuclides, ¹³⁷Cs and ⁹⁰Sr in particular, represent one of the most dangerous sources of environmental contamination. The remediation of wastewater containing such pollutants continue to be among the biggest challenges of Sustainable Development and Environmental Safety. Sorption-based technologies have proven their efficiency also in reducing the radionuclide content in aqueous streams to low-level residual activity, with the concomitant decrease in the amount of ultimate solid waste generated. Although sorption of cesium and strontium by resins, clays, and zeolites has been investigated intensively and even used in real applications, there is still considerable scope for improvement in terms of retention capacity and selectivity. Recent progress in design and preparation of nanostructured inorganic materials has attracted growing interest based on the potential for improving the retention performance when coupling such functionalities as ion exchange capacity, structural flexibility that may result in steric retention effects, as well as the propensity to interact specifically with the target metal cations. Titanate, vanadate, and tungsten based materials, manganese oxides, hexacyanoferrates, metal sulfides, ammonium molybdophosphates, or hydroxyapatite, characterized by various structures and morphologies, are reviewed with the emphasis being put on synthesis and shaping of such materials, their structure in relationship with the capacity and selectivity of trapping cesium and strontium from either single or multi-component aqueous solutions, as well as the possible retention mechanism. The potential candidates for remediation uses are selected with regard to their sorption capacity and distribution coefficient towards target cations, and also the pH window for an optimum cation capture.

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.