Controlling the Nonlinear Optical Behavior and Structural Transformation with A-Site Cation in α-AZnPO4 (A = Li, K).

Regulating the crystal structure by A-site cation substitution is one of the effective methods to explore high-performance nonlinear optical (NLO) materials. Herein, two non-centrosymmetric (NCS) compounds, α-MZnPO4 (M = Li, K) with short UV absorption edges 221 and 225 nm, are obtained by performing A-site cation substitution method. It is noteworthy that α-LiZnPO4 (α-LZPO) achieves >10 times second harmonic generation (SHG) response (2.3 × KDP) enhancement compared with that of α-KZnPO4 (α-KZPO) (0.2 × KDP), which is the only case among phosphates with different A-site cations. By structural comparison, it is found that the A-site cations play important roles for anion rearrangements, and further the structure features of the two compounds by designing two suppositional crystal models as well as performing other theoretical calculations are analyzed. The study confirms the feasibility to design promising NLO materials with strengthen SHG response and structural stability in orthophosphate system.

Four New Sb-based Orthophosphates: Cation Regulation to Investigate Diversified Structural Architecture.

In the work, four new Sb-based phosphates, K4 (SbO2 )5 (PO4 )3 , Rb(SbO2 )2 PO4 , Rb3 (SbO2 )3 (PO4 )2 and Cs3 (SbO2 )3 (PO4 )2 (H2 O)1.32 , were successfully synthesized by a high-temperature melt method. Among them, Rb(SbO2 )2 PO4 and Rb3 (SbO2 )3 (PO4 )2 are the first reported examples of Rb-containing alkali metal Sb-based phosphates. They show three-dimensional (3D) frameworks composed of [Sb8 P4 O30 ]∞ layer for K4 (SbO2 )5 (PO4 )3 and [Sb6 P2 O20 ]∞ layer for Rb(SbO2 )2 PO4 , and 2D lamellar structure composed of [Sb3 P2 O10 ]∞ for Rb3 (SbO2 )3 (PO4 )2 and Cs3 (SbO2 )3 (PO4 )2 (H2 O)1.32 . A detailed structural comparison shows that the structure dimensions for them transfer from 1D to complex 3D framework with the increase of (Sb+P)/O ratio, which affects performances of the compounds. Optical property and energy band structure calculations were also carried out based on the density functional theory (DFT). The present study enriches the diversity of Sb-based phosphates and paves the way for further explore their optical properties in the future.

Calcium orthophosphates (CaPO4): Occurrence and properties.

The present overview is intended to point the readers' attention to the important subject of calcium orthophosphates (CaPO4). This type of materials is of the special significance for the human beings because they represent the inorganic part of major normal (bones, teeth and antlers) and pathological (those appearing due to various diseases) calcified tissues of mammals. For example, atherosclerosis results in blood vessel blockage caused by a solid composite of cholesterol with CaPO4, while dental caries (tooth decay) and osteoporosis (a low bone mass with microarchitectural changes) mean a partial decalcification of teeth and bones, respectively, that results in replacement of a less soluble and harder biological apatite by more soluble and softer calcium hydrogenorthophosphates. Due to the compositional similarities to the calcified tissues of mammals, CaPO4 are widely used as biomaterials for bone grafting purposes. In addition, CaPO4 have many other applications. Thus, there is a great significance of CaPO4 for the humankind and, in this paper, an overview on the current knowledge on this subject is provided.

A history of calcium orthophosphates (CaPO4) and their biomedical applications.

The historical development of a scientific knowledge on calcium orthophosphates (CaPO4) from 1770-s till 1950 is described. Many forgotten and poorly known historical facts and approaches have been extracted from old publications and then they have been analyzed, systematized and reconsidered from the modern point of view. The chosen time scale starts with the earliest available studies of 1770-s (to the best of my findings, CaPO4 had been unknown before), passes through the entire 19th century and finishes in 1950, because since then the amount of publications on CaPO4 rapidly increases and the subject becomes too broad. Furthermore, since publications of the second half of the 20th century are easily accessible, the substantial amount of them has been already reviewed by other researchers. The reported historical findings clearly demonstrate that the substantial amount of the scientific facts and experimental approaches has been known for very many decades and, in fact, the considerable quantity of relatively recent investigations on CaPO4 is just either a further development of the earlier studies or a rediscovery of the already forgotten knowledge.

Luminescence and advanced mass spectroscopic characterization of sodium zinc orthophosphate phosphor for low-cost light-emitting diodes.

A new rare-earth-free NaZnPO4:Mn(2+) (NZP:Mn) phosphor powder has been developed by our group and investigated meticulously for the first time using secondary ion mass spectroscopy and chemical imaging techniques. The studies confirmed the effective incorporation of Mn(2+) into the host lattice, resulting in an enhancement of photoluminescence intensity. Phase purity has been verified and structure parameters have been determined successfully by Rietveld refinement studies. The NZP:Mn phosphor powder exhibits strong absorption bands in the ultraviolet and visible (300-470 nm) regions with a significant broad yellow-green (~543 nm) emission due to the characteristic spin forbidden d-d transition ((4)T1→(6)A1) of Mn(2+) ions, indicating weak crystal field strength at the zinc-replaced manganese site. The decay constants are a few milliseconds, which is a pre-requisite for applications in many display devices. The results obtained suggest that this new phosphor powder will find many interesting applications in semiconductor physics, as cost-effective light-emitting diodes (LEDs), as solar cells and in photo-physics.

Kinetics of the precipitation reaction between aluminium and contaminant orthophosphate ions.

The removal of phosphorous from wastewater in metal-orthophosphate systems typically occurs by simultaneous adsorption on poorly soluble metal hydroxides and by precipitation reactions between metal ions and orthophosphates in solution. To understand the individual contribution of these mechanisms to the removal of phosphorus, the main aim of this study was to determine the kinetics of consumption of contaminant orthophosphates by the precipitation reaction with aluminium ions in a solution free of insoluble aluminium hydroxide. To define the amount of aluminium and phosphorous compounds to be dissolved in water to have this desired reacting condition at a given pH, the solubilities of KH2PO4(s), Al(OH)3(s) and AlPO4(s) were examined at 25 °C in the pH range ∼2.6 to 7.9. pH-solubility diagrams for these ionic solids were made by solving a system of nonlinear algebraic equations involving dissolution, dissociation and hydrolysis reactions at equilibrium. The kinetics of the reaction between aluminium and orthophosphate ions at a reacting condition free of solids except for the product AlPO4(s) was investigated in a well-stirred batch reactor at pH ∼3.1 and 3.5 at 25 °C. A detailed kinetic model involving ten species, seven reversible reactions of hydrolysis of soluble aluminium and orthophosphate species and one reversible precipitation reaction between aluminium and phosphate ions revealed a rate constant for the latter reaction of 5.968 × 1010 L mol-1 s-1 (p = 0.191). XRD, TGA/DTGA and EDX analyses of the filtered and dried reacting mixture confirmed that the only solid product of the precipitation reaction was hydrated AlPO4(s).

Compositional boundaries for functional dental composites containing calcium orthophosphate particles.

OBJECTIVES: To investigate the interrelationships among handling, degree of conversion (DC), mechanical behavior and Ca2+ release of composites containing dicalcium phosphate dihydrate (DCPD, CaHPO4.2H2O), as a function of total inorganic content and DCPD: glass ratio. METHODS: Twenty-one formulations (1 BisGMA: 1 TEGDMA, in mols) with inorganic fractions ranging from zero to 50 vol% and different DCPD: glass ratios were evaluated for viscosity (parallel plate rheometer, n = 3), DC (near-FTIR spectroscopy, n = 3), fracture toughness/K1C (single-edge notched beam, n = 7-11) and 14-day Ca2+ release (inductively coupled plasma optical emission spectroscopy, n = 3). Data were analyzed by ANOVA/Tukey test (except viscosity, where Kruskal-Wallis/Dunn tests were used, α: 0.05). RESULTS: Viscosity and DC increased with DCPD: glass ratio among composites with the same inorganic content (p < 0.001). At inorganic fractions of 40 vol% and 50 vol%, keeping DCPD content at a maximum of 30 vol% did not compromise K1C. Ca2+ release showed an exponential relationship with DCPD mass fraction in the formulation (R2 = 0.986). After 14 days, a maximum of 3.8% of the Ca2+ mass in the specimen was released. CONCLUSION: Formulations containing 30 vol% DCPD and 10-20 vol% glass represent the best compromise between viscosity, K1C and Ca2+ release. Materials with 40 vol% DCPD should not be disregarded, bearing in mind that Ca2+ release will be maximized at the expense of K1C.

Crystal structure reinvestigation and spectroscopic analysis of tricadmium orthophosphate.

Single crystals of tricadmium orthophosphate, Cd3(PO4)2, have been synthesized successfully by the hydro-thermal route, while its powder form was obtained by a solid-solid process. The corresponding crystal structure was determined using X-ray diffraction data in the monoclinic space group P21/n. The crystal structure consists of Cd2O8 or Cd2O10 dimers linked together by PO4 tetra-hedra through sharing vertices or edges. Scanning electron microscopy (SEM) was used to investigate the morphology and to confirm the chemical composition of the synthesized powder. Infrared analysis corroborates the presence of isolated phosphate tetra-hedrons in the structure. UV-Visible studies showed an absorbance peak at 289 nm and a band gap energy of 3.85 eV, as determined by the Kubelka-Munk model.

Effect of calcium orthophosphate particle size and CaP:glass ratio on optical, mechanical and physicochemical characteristics of experimental composites.

OBJECTIVE: Evaluate light transmittance (%T), color change (ΔE), degree of conversion (DC), bottom-to-top Knoop microhardness (KHN), flexural strength (BFS) and modulus (FM), water sorption/solubility (WS/SL) and calcium release of resin composites containing different dicalcium phosphate dihydrate (DCPD)-to-barium glass ratios (DCPD:BG) and DCPD particle sizes. METHODS: Ten resin-based composites (50 vol% inorganic fraction) were prepared using BG (0.4 µm) and DCPD particles (12 µm, 3 µm or mixture) with DCPD:BG of 1:3, 1:1 or 3:1. A composite without DCPD was used as a control. DC, KHN, %T and ΔE were determined in 2-mm thick specimens. BFS and FM were determined after 24 h. WS/SL was determined after 7 d. Calcium release was determined by coupled plasma optical emission spectroscopy. Data were analyzed by ANOVA/Tukey test (alpha: 0.05). RESULTS: %T was significantly reduced in composites with milled, compared to pristine DCPD (p < 0.001). ΔE > 3.3 were observed with DCPD:BG of 1:1 and 3:1 formulated with milled DCPD (p < 0.001). DC increased at 1:1 and 3:1 DCPD:BG (p < 0.001). All composites presented bottom-to-top KHN of at least 0.8. BFS was not affected by DCPD size but was strongly dependent on DCPD:BG (p < 0.001). Reductions in FM were observed with milled DCPD (p < 0.001). WS/SL increased with DCPD:BG (p < 0.001). At 3DCPD: 1BG, using small DCPD particles led to a 35 % increase in calcium release (p < 0.001). SIGNIFICANCE: A trade-off between strength and Ca2+ release was observed. In spite of its low strength, the formulation containing 3 DCPD: 1 glass and milled DCPD particles is preferred due to its superior Ca2+ release.

Crystal structure of barium dinickel(II) iron(III) tris-[orthophosphate(V)], BaNi2Fe(PO4)3.

The orthophosphate BaNi2Fe(PO4)3 has been synthesized by a solid-state reaction route and characterized by single-crystal X-ray diffraction and energy-dispersive X-ray spectroscopy. The crystal structure comprises (100) sheets made up of [Ni2O10] dimers that are linked to two PO4 tetra-hedra via common edges and vertices and of linear infinite [010] chains of corner-sharing [FeO6] octa-hedra and [PO4] tetra-hedra. The linkage of the sheets and chains into a framework is accomplished through common vertices of PO4 tetra-hedra and [FeO6] octa-hedra. The framework is perforated by channels in which positionally disordered Ba2+ cations are located.

A tissue engineered 3D printed calcium alkali phosphate bioceramic bone graft enables vascularization and regeneration of critical-size discontinuity bony defects in vivo.

Introduction: Recently, efforts towards the development of patient-specific 3D printed scaffolds for bone tissue engineering from bioactive ceramics have continuously intensified. For reconstruction of segmental defects after subtotal mandibulectomy a suitable tissue engineered bioceramic bone graft needs to be endowed with homogenously distributed osteoblasts in order to mimic the advantageous features of vascularized autologous fibula grafts, which represent the standard of care, contain osteogenic cells and are transplanted with the respective blood vessel. Consequently, inducing vascularization early on is pivotal for bone tissue engineering. The current study explored an advanced bone tissue engineering approach combining an advanced 3D printing technique for bioactive resorbable ceramic scaffolds with a perfusion cell culture technique for pre-colonization with mesenchymal stem cells, and with an intrinsic angiogenesis technique for regenerating critical size, segmental discontinuity defects in vivo applying a rat model. To this end, the effect of differing Si-CAOP (silica containing calcium alkali orthophosphate) scaffold microarchitecture arising from 3D powder bed printing (RP) or the Schwarzwalder Somers (SSM) replica fabrication technique on vascularization and bone regeneration was analyzed in vivo. In 80 rats 6-mm segmental discontinuity defects were created in the left femur. Methods: Embryonic mesenchymal stem cells were cultured on RP and SSM scaffolds for 7d under perfusion to create Si-CAOP grafts with terminally differentiated osteoblasts and mineralizing bone matrix. These scaffolds were implanted into the segmental defects in combination with an arteriovenous bundle (AVB). Native scaffolds without cells or AVB served as controls. After 3 and 6 months, femurs were processed for angio-µCT or hard tissue histology, histomorphometric and immunohistochemical analysis of angiogenic and osteogenic marker expression. Results: At 3 and 6 months, defects reconstructed with RP scaffolds, cells and AVB displayed a statistically significant higher bone area fraction, blood vessel volume%, blood vessel surface/volume, blood vessel thickness, density and linear density than defects treated with the other scaffold configurations. Discussion: Taken together, this study demonstrated that the AVB technique is well suited for inducing adequate vascularization of the tissue engineered scaffold graft in segmental defects after 3 and 6 months, and that our tissue engineering approach employing 3D powder bed printed scaffolds facilitated segmental defect repair.

Developing Porous Ortho- and Pyrophosphate-Containing Glass Microspheres; Structural and Cytocompatibility Characterisation.

Phosphate-based glasses (PBGs) are promising materials for bone repair and regeneration as they can be formulated to be compositionally similar to the inorganic components of bone. Alterations to the PBG formulation can be used to tailor their degradation rates and subsequent release of biotherapeutic ions to induce cellular responses, such as osteogenesis. In this work, novel invert-PBGs in the series xP2O5·(56 - x)CaO·24MgO·20Na2O (mol%), where x is 40, 35, 32.5 and 30 were formulated to contain pyro (Q1) and orthophosphate (Q0) species. These PBGs were processed into highly porous microspheres (PMS) via flame spheroidisation, with ~68% to 75% porosity levels. Compositional and structural analysis using EDX and 31P-MAS NMR revealed that significant depolymerisation occurred with reducing phosphate content which increased further when PBGs were processed into PMS. A decrease from 50% to 0% in Q2 species and an increase from 6% to 35% in Q0 species was observed for the PMS when the phosphate content decreased from 40 to 30 mol%. Ion release studies also revealed up to a four-fold decrease in cations and an eight-fold decrease in phosphate anions released with decreasing phosphate content. In vitro bioactivity studies revealed that the orthophosphate-rich PMS had favourable bioactivity responses after 28 days of immersion in simulated body fluid (SBF). Indirect and direct cell culture studies confirmed that the PMS were cytocompatible and supported cell growth and proliferation over 7 days of culture. The P30 PMS with ~65% pyro and ~35% ortho phosphate content revealed the most favourable properties and is suggested to be highly suitable for bone repair and regeneration, especially for orthobiologic applications owing to their highly porous morphology.

Nano-Hydroxyapatite Derived from Biogenic and Bioinspired Calcium Carbonates: Synthesis and In Vitro Bioactivity.

Biogenic calcium carbonates naturally contain ions that can be beneficial for bone regeneration and therefore are attractive resources for the production of bioactive calcium phosphates. In the present work, cuttlefish bones, mussel shells, chicken eggshells and bioinspired amorphous calcium carbonate were used to synthesize hydroxyapatite nano-powders which were consolidated into cylindrical pellets by uniaxial pressing and sintering 800-1100 °C. Mineralogical, structural and chemical composition were studied by SEM, XRD, inductively coupled plasma/optical emission spectroscopy (ICP/OES). The results show that the phase composition of the sintered materials depends on the Ca/P molar ratio and on the specific CaCO3 source, very likely associated with the presence of some doping elements like Mg2+ in eggshell and Sr2+ in cuttlebone. Different CaCO3 sources also resulted in variable densification and sintering temperature. Preliminary in vitro tests were carried out (by the LDH assay) and they did not reveal any cytotoxic effects, while good cell adhesion and proliferation was observed at day 1, 3 and 5 after seeding through confocal microscopy. Among the different tested materials, those derived from eggshells and sintered at 900 °C promoted the best cell adhesion pattern, while those from cuttlebone and amorphous calcium carbonate showed round-shaped cells and poorer cell-to-cell interconnection.

Effect of calcium orthophosphate: Reinforcing glass ratio and prolonged water storage on flexural properties of remineralizing composites.

OBJECTIVES: To compare the effects of replacing reinforcing barium glass particles by DCPD (dicalcium phosphate dihydrate), as opposed to simply reducing glass filler content, on composite flexural properties and degree of conversion (DC). On a second set of experiments, composites with different "DCPD: glass" ratios were exposed to prolonged water immersion to verify if the presence of DCPD particles increased hydrolytic degradation. METHODS: Two series of composites were prepared: 1) composites with total inorganic content of 50 vol% and "DCPD: glass" ratios ranging from zero (glass only) to 1.0 (DCPD only), in 0.25 increments, and 2) composites containing only silanized glass (from zero to 50 vol%). Disk-shaped specimens were fractured under biaxial flexural loading after 24 h in water. Another set of specimens of composites with different "DCPD: glass" ratios was stored in water for 24 h, 30, 60, 90 and 120 days and tested in flexure. DC was determined using FTIR spectroscopy. Data were analyzed using Kruskal-Wallis/Dunn test (flexural properties) or ANOVA/Tukey test (DC, alpha: 0.05). RESULTS: For glass-only composites, reducing inorganic content caused a linear decrease in strength. The presence of DCPD did not affect composite strength up until a "DCPD: glass" ratio of 0.5. On the other hand, materials with 0.75 and 1.0 DCPD showed significantly lower strength than the glass-only composite with 12.5 vol% filler and the unfilled resin, respectively (p < 0.001). Except for the 0.25 DCPD composite, the presence of DCPD did not contribute to increase flexural modulus. After water storage, composites containing DCPD showed higher percent reductions in properties than the control, but only in a few cases the effect was statistically significant (strength: 0.5 DCPD, modulus: 0.25 and 1.0 DCPD). DC was only marginally affected by DCPD fraction. SIGNIFICANCE: For composites with "DCPD: glass" of 0.25 and 0.5, reductions in strength were related to the lower glass content, and not due to the presence of DCPD. Flexural modulus was primarily defined by glass content. Overall, composites containing DCPD particles presented higher reductions in properties after water storage, but it remained within limits reported for commercial materials.

Probing structural and photophysical features of Eu3+ activated NaCdPO4 orthophosphate phosphor.

A series of Eu3+ doped NaCd1-xPO4: x Eu3+ (0 ≤ x ≤ 0.1) phosphors were synthesized and a systematic investigation of structural, morphological, optical and photophysical properties were carried out. Structural characteristics were analyzed both qualitatively as well as quantitatively to establish the phase purity, supported by FTIR and Raman spectroscopy methods. The valance state of the dopant, as well as surface elemental compositions were investigated by XPS technique. The optical characteristics were analyzed by employing UV-Visible DRS, which shows that the host bandgap is getting broadened upon doping as a result of the Burstein-Moss (BM) effect. Eu3+ doped phosphor exhibits emissions rich in red (617 nm) under suitable excitations at 393 nm while the optimal doping concentration was found to be around x = 0.06. The effective non-radiative transfer of energy from host to activator is governed by the dominant dipole-dipole transitions. The hypersensitive transition line 5D0 â†’ 7F2 is observed owing to the induced electric dipole (ED) transition of Eu3+ centered at 617 nm. Local site symmetry has been analyzed to ascertain the environment around Eu3+ and presence of any non-equivalent cation sites. The concentration quenching effect of phosphors was explained on the basis of Dexter's theory and charge compensation mechanism. The Commission Internationale de l'Eclairage (CIE 1931) chromaticity coordinates for the prepared phosphors were estimated and found to lie in the red region of color space. The photoluminescence decay time was measured for the most intense emission line 5D0 â†’ 7F2 at 617 nm under 393 nm excitation and the results indicate that Eu3+ activated NaCdPO4 can be a suitable red phosphor for white LED using UV-LED chip owing to it's near UV-excitation characteristics.

Microwave-Assisted Fabrication of Mesoporous Silica-Calcium Phosphate Composites for Dental Application.

Herein, the microwave-assisted wet precipitation method was used to obtain materials consisting of mesoporous silica (SBA-15) and calcium orthophosphates (CaP). Composites were prepared through immersion of mesoporous silica in different calcification coating solutions and then exposed to microwave radiation. The composites were characterized in terms of molecular structure, crystallinity, morphology, chemical composition, and mineralization potential by Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDX). The application of microwave irradiation resulted in the formation of different types of calcium orthophosphates such as calcium deficient hydroxyapatite (CDHA), octacalcium phosphate (OCP), and amorphous calcium phosphate (ACP) on the SBA-15 surface, depending on the type of coating solution. The composites for which the progressive formation of hydroxyapatite during incubation in simulated body fluid was observed were further used in the production of final pharmaceutical forms: membranes, granules, and pellets. All of the obtained pharmaceutical forms preserved mineralization properties.

Mechanochemical synthesis of hydroxyapatite using cuttlefish bone and chicken eggshell as calcium precursors.

This work explores the possibility of synthesising hydroxyapatite via mechanochemical route using biogenic calcium carbonate sources, namely calcite in chicken eggshell and aragonite in cuttlebone. The calcium source and orthophosphoric acid in ratio complying with Ca/P = 1.67 were submitted to high-energy ball milling with transferred energy values in the 1.6 to 123.0 kWh/g range, in the presence of 6.4 wt% water. Results show that increasing transferred energy results in CaCO3 → DCPD → HA reaction sequence when the used calcium source is cuttlebone, and in CaCO3 → DCPD → DCPA → HA when eggshell is used. The produced calcium orthophosphates exist in delimited energy transfer ranges; HA forms monophasic regions at the highest transferred energy ranges tested. 52.5 kWh/g is the minimum energy value for hydroxyapatite formation starting from eggshell, while only 6.2 kWh/g is required starting from cuttlebone. Calcium orthophosphate dimensionality depends on the supplied energy and on the starting CaCO3 polymorph, and includes nanospheres and nanoplates, and more complex flower-like geometries built from individual nanoparticles. Milling maps were built to systematise the effect of initial CaCO3 polymorph and transferred milling energy on the conditions for hydroxyapatite mechanochemical formation. Obtained results demonstrate the potential of chicken eggshell and cuttlefish bone as natural precursors to produce hydroxyapatite, and the ability of high-energy milling as the corresponding processing route. This indicates an opportunity window for the development of reliable, scalable, fast and cost-effective HA production method.

Hydrogel chitosan sorbent application for nutrient removal from soilless plant cultivation wastewater.

In this study, we determined the effectiveness of removal of nutrients (nitrates and orthophosphates) from greenhouse wastewaters (GW) using non-cross-linked chitosan (CHs) and chitosan cross-linked with epichlorohydrin (CHs-ECH) in the form of hydrogel beads. GW used in the study had the following parameters: N-NO3 621.1 mg/L, P-PO4 60.8 mg/L, SO42- 605.0 mg/L, Cl- 0.9 mg/L, Ca2+ 545.0 mg/L, Mg2+ 178.0 mg/L, K+ 482.0 mg/L, hardness 113° dH, and pH 6.2. The scope of the study included determination of the effect of pH on wastewater composition and effectiveness of nutrient sorption, analyses of nutrient sorption kinetics, and analyses of the effect of sorbent dose on percentage removal of nutrients from GW. CHs-ECH was able to sorb 79.4% of P-PO4 and 76.7% of N-NO3 from GW, whereas CHs to remove 92.8% of P-PO4 and 53.2% of N-NO3.

Meta-analysis of non-reactive phosphorus in water, wastewater, and sludge, and strategies to convert it for enhanced phosphorus removal and recovery.

Current and future trends indicate that mining of natural phosphorus (P) reserves is occurring faster than natural geologic replenishment. This mobilization has not only led to P supply concerns, but has also polluted many of the world's freshwater bodies and oceans. Recovery and reuse of this nuisance P offers a long-term solution simultaneously addressing mineral P accessibility and P-based pollution. Available physical, chemical, and biological P removal/recovery processes can achieve low total P (TP) concentrations (≤100 µg/L) and some processes can also recover P for direct reuse as fertilizers (e.g., struvite). However, as shown by our meta-analysis of over 20,000 data points on P quantity and P form, the P in water matrices is not always present in the reactive P (RP) form that is most amenable to recovery for direct reuse. Thus, strategies for removing and recovering other P fractions in water/wastewater are essential to provide environmental protection via P removal and also advance the circular P economy via P recovery. Specifically, conversion of non-reactive P (NRP) to the more readily removable/recoverable RP form may offer a feasible approach; however, extremely limited data on such applications currently exist. This review investigates the role of NRP in various water matrices; identifies NRP conversion mechanisms; and evaluates biological, physical, thermal, and chemical processes with potential to enhance P removal and recovery by converting the NRP to RP. This information provides critical insights into future research needs and technology advancements to enhance P removal and recovery.

The use of cross-linked chitosan beads for nutrients (nitrate and orthophosphate) removal from a mixture of P-PO4, N-NO2 and N-NO3.

A hydrogel chitosan sorbent ionically cross-linked with sodium citrate and covalently cross-linked with epichlorohydrin was used to remove nutrients from an equimolar mixture of P-PO4, N-NO2 and N-NO3. The scope of the study included, among other things, determination of the influence of pH on nutrient sorption effectiveness, nutrient sorption kinetics as well as determination of the maximum sorption capacity of cross-linked chitosan sorbents regarding P-PO4 (H2PO4-, HPO42-), N-NO2 (HNO2, NO2-), and N-NO3 (NO3-). The effect of the type of the cross-linking agent on the affinity of the modified chitosan to each nutrient was studied as well. The kinetics of nutrient sorption on the tested chitosan sorbents was best described with the pseudo-second order model. The model of intramolecular diffusion showed that P-PO4, N-NO2 and N-NO3 sorption on cross-linked hydrogel chitosan beads proceeded in two phases. The best sorbent of nutrients turned out to be chitosan cross-linked covalently with epichlorohydrin; with P-PO4, N-NO2 and N-NO3 sorption capacity reaching: 1.23, 0.94 and 0.76mmol/g, respectively (total of 2.92mmol/g). For comparison, the sorption capacity of chitosan cross-linked ionically with sodium citrate was: 0.43, 0.39 and 0.39mmol/g for P-PO4, N-NO2 and N-NO3, respectively (total of 1.21mmol/g).