Reverse flotation separation of muscovite from apatite using a dodecylpyridinium chloride (DPDC) ionic liquid as the collector was studied in this work. The microflotation results depicted that DPDC had a strong collecting for muscovite but had a slight collecting for apatite when using phosphoric acid as a depressant for apatite in a weakly acidic pH value pulp, artificial mixture mineral flotation showed that reverse flotation separation of muscovite from apatite can be effectively achieved in the reagent scheme of phosphoric acid/DPDC, and DPDC had a better separation performance in the muscovite/apatite system than DDA. The adsorption measurements indicated that the adsorption amount of DPDC on the apatite surface was less than that of DPDC on the muscovite surface, and the zeta potential results confirmed that a strong interaction occurred between DPDC and the muscovite surface, while an extremely weak interaction occurred between DPDC and the apatite surface in the presence of phosphoric acid at pH ∼ 5.5. XPS analysis indicated that a hydrogen bond occurred between DPDC and the muscovite surface. Thus, it was inferred that DPDC could be used as an appreciation collector for the reverse flotation of muscovite from apatite.

The research on the interaction of tartrate ions with the surface of hydroskyapatite was presented, including the measurements of the kinetics of tartrate ion adsorption and tartrate ion adsorption as a function of pH. The adsorption of tartrate ions was calculated from the loss of tartrate concentration in the solution as measured by a radioisotope method using C-14 labeled tartaric acid. In order to explain the mechanism of interaction of tartrate ions with hydroxyapatite, supplementary measurements were carried out, i.e., potentiometric measurements of the balance of released/consumed ions in the hydroxyapatite/electrolyte solution system, zeta potential measurements, FTIR spectrophotometric measurements and the hydroxyapatite crystal structure and particle size distribution were characterized. It was found that the adsorption of tartrate ions occurs as a result of the exchange of these ions with hydroxyl, phosphate and carbonate ions. Replacing the ions with the abovementioned tartrate ions leads to the appearance of a negative charge on the surface of the hydroxapatite. On the basis of XRD study and particle size distribution, a decrease in the size of crystallites and the diameter of hydroxyapatite particles in contact with a solution of 0.001 mol/dm³ of tartaric acid was found.

Hydroxyapatite nanoparticles (HAP NPs) are important for medicine, bioengineering, catalysis, and water treatment. However, current understanding of the nanoscale phenomena that confer HAP NPs their many useful properties is limited by a lack of information about the distribution of the atoms within the particles. Atom probe tomography (APT) has the spatial resolution and chemical sensitivity for HAP NP characterization, but difficulties in preparing the required needle-shaped samples make the design of these experiments challenging. Herein, two techniques are developed to encapsulate HAP NPs and prepare them into APT tips. By sputter-coating gold or the atomic layer deposition of alumina for encapsulation, partially fluoridated HAP NPs are successfully characterized by voltage- or laser-pulsing APT, respectively. Analyses reveal that significant tradeoffs exist between encapsulant methods/materials for HAP characterization and that selection of a more robust approach will require additional technique development. This work serves as an essential starting point for advancing knowledge about the nanoscale spatiochemistry of HAP NPs.

Apatite subspecies depend on their halogen and hydroxyl content; chlorapatite, hydroxylapatite and fluorapatite, with additional substitution of other elements within the lattice such as rare earth elements (REE), sodium, strontium and manganese also possible. Rare earth elements are vital to green and emerging technologies, with demand set to outstrip supply. Apatite provides a possible future source of REE. Processing rare earth deposits is often complex, with surface behaviour having a significant effect on the optimization of a process flow sheet. The effect of enrichment of natural apatite and the doping of synthetic apatite on surface behaviour can be determined by measuring the zeta potential and the isoelectric point of the mineral. In this paper, we review zeta potential studies of natural and synthetic apatite to determine the effect of elemental enrichment on surface behaviour. Fifty three studies of natural apatite and forty four studies of synthetic apatite were reviewed. The isoelectric point of apatite varied from pH 1 to pH 8.7, with studies of apatite specified to be >90% pure reducing the variation to pH 3 to pH 6.5. Of the four studies of rare earth enriched apatite found, three had IEP values between pH 3 and pH 4. A study of synthetic apatite showing enrichment of between 1 and 10% by the REE europium does not affect surface behaviour. However, no studies were found that investigated the effect of common REE processing reagents on REE enriched apatite zeta potentials. Therefore, in addition to comparing previous studies we also therefore present new zeta potential measurements of apatite from a REE enriched deposit under water and common flotation collector conditions. The IEP value of this apatite under water conditions was at pH 3.6, shifting to <3.5 under both hydroxamic acid and betacol conditions. When compared to previous studies, the behaviour of REE enriched apatite under collector conditions is similar to non-REE apatite. This result could be important for future processing of apatite enriched with REE, and therefore global apatite and rare earth supply.

The interaction between Ca-HAP and Pb(2+) solution can result in the formation of a hydroxyapatite-hydroxypyromorphite solid solution [(PbxCa1-x)5(PO4)3(OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil. Therefore, it's necessary to know the physicochemical properties of (PbxCa1-x)5(PO4)3(OH), predominantly its thermodynamic solubility and stability in aqueous solution. Nevertheless, no experiment on the dissolution and related thermodynamic data has been reported.

Dissolution of the hydroxypyromorphite-hydroxyapatite solid solution [(PbxCa1-x)5(PO4)3(OH)] in aqueous solution at 25 °C was experimentally studied. The aqueous concentrations were greatly affected by the Pb/(Pb + Ca) molar ratios (XPb) of the solids. For the solids with high XPb [(Pb0.89Ca0.11)5(PO4)3OH], the aqueous Pb(2+) concentrations increased rapidly with time and reached a peak value after 240-720 h dissolution, and then decreased gradually and reached a stable state after 5040 h dissolution. For the solids with low XPb (0.00-0.80), the aqueous Pb(2+) concentrations increased quickly with time and reached a peak value after 1-12 h dissolution, and then decreased gradually and attained a stable state after 720-2160 h dissolution.

The dissolution process of the solids with high XPb (0.89-1.00) was different from that of the solids with low XPb (0.00-0.80). The average K sp values were estimated to be 10(-80.77±0.20) (10(-80.57)-10(-80.96)) for hydroxypyromorphite [Pb5(PO4)3OH] and 10(-58.38±0.07) (10(-58.31)-10(-58.46)) for calcium hydroxyapatite [Ca5(PO4)3OH]. The Gibbs free energies of formation (ΔG f (o) ) were determined to be -3796.71 and -6314.63 kJ/mol, respectively. The solubility decreased with the increasing Pb/(Pb + Ca) molar ratios (XPb) of (PbxCa1‒x)5(PO4)3(OH). For the dissolution at 25 °C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (PbxCa1-x)5(PO4)3(OH) dissolved stoichiometrically at the early stage of dissolution and moved gradually up to the Lippmann solutus curve and the saturation curve for Pb5(PO4)3OH, and then the data points moved along the Lippmann solutus curve from right to left. The Pb-rich (PbxCa1-x)5(PO4)3(OH) was in equilibrium with the Ca-rich aqueous solution. Graphical abstractLippmann diagrams for dissolution of the hydroxypyromorphite-hydroxyapatite solid solution [(PbxCa1-x)5(PO4)3OH] at 25 ˚C and an initial pH of 2.00.

The substitution of Ca(2+) in Ca-hydroxylapatite by toxic Cd(2+) can cause the forming of Cd-hydroxylapatite and is a significant issue in a great variety of research areas, which hence needs an understanding of the essential physicochemical characteristics. Unfortunately, the solubility product and thermodynamic data for Cd-hydroxylapatite in water under a variety of conditions now are lacking. Little information has been reported by previous researchers. Additionally, the dissolution mechanism of Cd-hydroxylapatite has never been studied.

Dissolution of the synthetic cadmium hydroxylapatite [Cd-HAP, Cd5(PO4)3OH] in HNO3 solution (pH = 2), ultrapure water (pH = 5.6) and NaOH solution (pH = 9) was experimentally studied at 25, 35 and 45°C. Characterization by XRD, FT-IR and FE-SEM proved that Cd-HAP solids showed no recognizable change during dissolution. For the Cd-HAP dissolution in aqueous acidic media at initial pH 2 and 25°C, the solution cadmium and phosphate concentrations increased rapidly and reached the peak values after 20-30 days and 10 days reaction, respectively. Thereafter, the Cd-HAP dissolution rate decreased slowly, whereas the solution Cd/P molar ratio increased constantly from 1.65-1.69 to 6.61-6.76. The mean K sp values for Cd5(PO4)3OH were determined to be 10(-64.62) (10(-64.53)-10(-64.71)) at 25°C, 10(-65.58) (10(-65.31)-10(-65.80)) at 35°C and 10(-66.57) (10(-66.24)-10(-66.90)) at 45°C. Based on the obtained solubility data from the dissolution at initial pH 2 and 25°C, the Gibbs free energy of Cd5(PO4)3OH forming [Formula: see text] was determined to be -3,970.47 kJ/mol (-3,969.92 to -3,970.96 kJ/mol). Thermodynamic parameters, ΔG (0), ΔH (0), ΔS (0), and [Formula: see text] for the dissolution process of Cd-HAP in aqueous acidic media at initial pH 2 and 25°C were calculated 368,710.12 J/K mol, -158,809.54 J/mol, -1,770.20 and -869.53 J/K mol, respectively.

Based on the experimental results of the present work and some previous researches, the cadmium hydroxylapatite (Cd-HAP) dissolution in aqueous media is considered to have the following coincident processes: the stoichiometric dissolution coupled with protonation and complexation reactions, the non-stoichiometric dissolution with Cd(2+) release and PO4 (3-) sorption and the sorption of Cd(2+) and PO4 (3-) species from solution backwards onto Cd-HAP surface. The obtained solubility products (K sp) 10(-64.62) (10(-64.53)-10(-64.71)) for Cd-HAP was approximately 7.62-5.62 log units lower than 10(-57)-10(-59) for calcium hydroxylapatite (Ca-HAP).Graphical abstractDissolution of cadmium hydroxylapatite [Cd5(PO4)3OH].

Hydroxyapatite chromatography is shown to be an excellent method for chromatographically purifying monoclonal antibodies (Mab). Mab contained in eluates from Protein A columns was partially purified on ceramic hydroxyapatite (CHT™) Type I, 40 μm ceramic hydroxyapatite using two scouting methods which provide milligram amounts of Mab typical at laboratory scale. The result from one of the scouting methods was optimized to obtain a high concentration of purified Mab with acceptable clearance of cell culture impurities. Several techniques (linear phosphate screening, linear alkaline salt screening, and two alkaline salt step gradients) are described for obtaining high concentrations of purified Mab in a lab-scale CHT chromatography column.

The specific adsorption of citric acid ions at hydroxyapatite interface was investigated by the means of radioisotope method (¹⁴C) as a function of citric acid ions concentration, NaCl concentration and pH. Application of the hydroxyapatite has become wide in the biomaterial field as the Ca₁₀(OH)₂(PO₄)₆ possess biocompatibility with human hard tissue. Hydroxyapatite was synthesized using three different methods. The physical properties of the resulting powder were characterized by DTA/TG, XRD, AFM and SEM microscopy. Physicochemical qualities characterizing the electrical double layer of the hydroxyapatite/NaCl solution interface were determined. The zeta potential and the adsorption of citric acid molecule were studied as a function of pH. The point of zero charge and the isoelectric point of samples were determined. Electrical double layer parameters of hydroxyapatite/NaCl interface are influenced by a synthesis method. The points pH_pzc and pH_IEP for sample 1 are pH_pzc 7.5 and pH_IEP 3; for sample 2 pH_pzc 7.05 and pH_IEP 3, for smaple 3 pH_pzc 6.7 and pH_IEP 3. Temperature has weak influence both on pure substance and with citric acid adsorbed, as derivatographic analysis has shown, and characterization of hydroxyapatite structure may be carried out by this thermal analysis. Two phenomena are responsible for citric acid adsorption: phosphate group's replacement at hydroxyapatite surface by citric ions parallel to intraspherical complexes formation.

The hydroxyapatite (HA) formation on the DNA molecules in SBF was examined. After immersion for four weeks in SBF at 36.5 °C, the HA crystallites of ~1-14 μm in diameter grew on the surface of DNA molecules. Various morphologies were successfully observed through scanning electron microscopy analysis. The Ca/P mol ratio (1.1-1.5) in HA was estimated by energy dispersive X-ray analysis. Original peaks of both of DNA and HA were characterized by Fourier transform infrared spectroscopy. The molecular orbital computer simulation has been used to probe the interaction of DNA with two charge-balancing ions, i.e., CaOH(+) and CaH2PO4(+). The adsorption enthalpy of the two ions on ds-DNA and/or ss-DNA having large negative value (~ -60 kcal/mol per charge-balancing ion) was the evidence for the interface in mineralization of HA in SBF.

Non-collagenous proteins (NCPs) inhibit hydroxyapatite (HA; Ca(5)(PO(4))(3)OH) formation in living organisms by binding to nascent nuclei of HA and preventing their further growth. Polar and charged amino acids (AAs) are highly expressed in NCPs, and the negatively charged ones, such as glutamic acid (Glu) and phosphoserine (P-Ser) seem to be mainly responsible for the inhibitory effect of NCPs. Despite the recognized importance of these AAs on the behaviour of NCPs, their specific effect on HA crystallization is still unclear, and controversial results have been reported concerning the efficacy of HA inhibition of positively versus negatively charged AAs. We focused on a positively charged (arginine, Arg) and a negatively charged (Glu) AA, and their combination in the same solution. We studied their inhibitory effect on HA nucleation and growth at physiological temperature and pH and we determined the mechanism by which they can affect HA crystallization. Our results showed a strong inhibitory effect of Arg on HA nucleation; however, Glu was more effective in inhibiting HA crystal growth during the growth stage. The combination of Glu and Arg was less effective in controlling HA nucleation, but it inhibited HA crystal growth. We attributed these differences to the stability of complexes formed between AAs and calcium and phosphate ions at the nucleation stage, and in bonding strength of AAs to HA crystal faces during the growth stage. The AAs also influenced the morphology of synthesized HA. Presence of either Arg or Glu resulted in the formation of spherulites consisting of preferentially oriented nanoplatelets orientation. This was attributed to kinetic factors favoring growth front nucleation (GFN) mechanism.

Eight dissolution models of calcium apatites (both fluorapatite and hydroxyapatite) in acids were drawn from the published literature, analyzed and discussed. Major limitations and drawbacks of the models were conversed in details. The models were shown to deal with different aspects of apatite dissolution phenomenon and none of them was able to describe the dissolution process in general. Therefore, an attempt to combine the findings obtained by different researchers was performed which resulted in creation of the general description of apatite dissolution in acids. For this purpose, eight dissolution models were assumed to complement each other and provide the correct description of the specific aspects of apatite dissolution. The general description considers all possible dissolution stages involved and points out to some missing and unclear phenomena to be experimentally studied and verified in future. This creates a new methodological approach to investigate reaction mechanisms based on sets of affine data, obtained by various research groups under dissimilar experimental conditions.

This paper explores the feasibility of converting waste Rohu fish (Labeo rohita) scale into a high-performance, reusable, low-cost heterogeneous catalyst for synthesis of biodiesel from soybean oil. The thermo-gravimetric analysis (TGA) and X-ray diffraction (XRD) analysis revealed that a significant portion of the main component of fish scale i.e. HAP (hydroxyapatite) could be transformed into β-tri-calcium phosphate when calcined above 900°C for 2 h. Scanning Electron Microscopy (SEM) morphology studies of the calcined scale depicted a fibrous layer of porous structure; while a BET surface area of 39 m(2)/g was measured. Response surface methodology (RSM) was employed to determine the optimal parametric conditions viz. methanol/oil molar ratio, 6.27:1, calcination temperature, 997.42°C and catalyst concentration, 1.01 wt.% of oil corresponding to a maximum FAME yield of 97.73%. Reusability results confirmed that the prepared catalyst could be reemployed up to six times, procreating a potentially applicable avenue in biodiesel synthesis.

The effect of calcite supernatant, calcium, and carbonate ions on the hydroxyapatite (HA) zeta potential without and in the presence of sodium oleate (1x10(-4) mol L(-1)) was examined within the pH range from 4 to 12. The interpretation of results was based on the HA surface and oleate solution chemistry, and on some floatability tests. HA, with different positive and negative surface sites formed depending on its solubility and pH, had a negative zeta potential over the whole pH range. This mineral is not naturally floatable (flotation recovery, 5%<R<18%). The oleate ions (Ol(-)), present in a very low concentration in an acidic medium (pH from 4.8 to 6), chemisorb individually on HA surface centers [triple bond]Ca(+), [triple bond]HPO(4)Ca(+), and [triple bond]OH(2)(+), increasing the negative zeta potential of the mineral. Within the pH range from 7 to 9, the dominant oleate species Ol(-) ion and ion-molecule complex, H(Ol)(2)(-), adsorbed on HA by head groups toward the solid and associated due to chain-chain interaction in hemimicelles, made the HA surface with zeta potential about -22/-23 mV, and more floatable (R=80-100%) than in 4<pH<7 (R=15-35%) or in pH>9.3. The HA surface is less negatively charged in calcite supernatant than in water from pH 6.6 to 9.2 due to the adsorption on HA negative surface active centers ([triple bond]HPO(4)(-) and [triple bond]PO(4)(2-)) of the Ca(2+), and CaOH(+) ions (present in the calcite supernatant), producing more surface sites [triple bond]HPO(4)Ca(+), [triple bond]PO(4)Ca, [triple bond]HPO(4)CaOH, and [triple bond]PO(4)(-) CaOH, and new centers [triple bond]HPO(4)CaHCO(3) and [triple bond]PO(4)(-) CaHCO(3). In the presence of 1x10(-3) mol L(-1) CaCl(2), the HA sample has positive zeta potential, the same as calcite from the same deposit, up to IEP at pH 11.25. Carbonate ions (1x10(-3) mol L(-1) Na(2)CO(3)) do not affect the HA zeta potential. However, a possible process can be the ion-exchange reaction between bicarbonate (or carbonate) and some anion from the surface sites formed on HA. The obtained values of the HA zeta potential with the collector (1x10(-4) mol L(-1) Na-oleate) added into hydroxyapatite/calcite supernatant suspensions corroborate the weak chemisorption of Ol(-) and H(Ol)(2)(-). The likely processes in this system also are the ion-exchange reactions on [triple bond]HPO(4)CaOH and [triple bond]PO(4)(-) CaOH, [triple bond]HPO(4)CaHCO(3) and [triple bond]PO(4)(-) CaHCO(3) between oleate ion and surface hydroxyl and bicarbonate ions, surface and bulk precipitations of calcium oleate, Ca(Ol)(2), and the surface and bulk precipitations of Ca[H(Ol)(2)(-)](2) over the pH range from 7 to 9. Calcite supernatant does not influence natural floatability of the mineral. However, calcite supernatant depresses the hydroxyapatite flotation in the presence of 1x10(-4) mol L(-1) Na-oleate (pH 9, R approximately 50%), a likely result of the weak chemisorption due to the steric effect of heterogeneous HA surface formed in calcite supernatant, Ca(Ol)(2) and Ca[H(Ol)(2)(-)](2) surface and bulk precipitations.

Hydroxyapatite is a very interesting material given that it is the main component in tooth enamel and because of its uses in bone implant applications. Therefore, not only the characterization of its surface is of high relevance but also designing reliable methods to study the interfacial properties of films adsorbed onto it. In this paper we apply the colloidal probe atomic force microscopy method to investigate the surface properties of commercially available hydroxyapatite surfaces (both microscopic particles and macroscopic discs) in terms of interfacial and frictional forces. In this way, we find that hydroxyapatite surfaces at physiological relevant conditions are slightly negatively charged. The surfaces were then exposed to human whole saliva, and the surface properties were re-evaluated. A thick film was formed that was very resistant to mechanical stress. The frictional measurements demonstrated that the film was indeed highly lubricating, supporting the argument that this system may prove to be a relevant model for evaluating dental and implant systems.