Solubility of CaHPO4 · 2H2O in the System Ca(OH)2-H3PO4-H2O at 5, 15, 25, and 37.5 °C

Redefined ion association constants have consequences for calcium phosphate nucleation and biomineralization

Calcium orthophosphates (CaPs), as hydroxyapatite (HAP) in bones and teeth are the most important biomineral for humankind. While clusters in CaP nucleation have long been known, their speciation and mechanistic pathways to HAP remain debated. Evidently, mineral nucleation begins with two ions interacting in solution, fundamentally underlying solute clustering. Here, we explore CaP ion association using potentiometric methods and computer simulations. Our results agree with literature association constants for Ca2+ and H2PO4-, and Ca2+ and HPO42-, but not for Ca2+ and PO43- ions, which previously has been strongly overestimated by two orders of magnitude. Our data suggests that the discrepancy is due to a subtle, premature phase separation that can occur at low ion activity products, especially at higher pH. We provide an important revision of long used literature constants, where association of Ca2+ and PO43- actually becomes negligible below pH 9.0, in contrast to previous values. Instead, [CaHPO4]0 dominates the aqueous CaP speciation between pH ~6-10. Consequently, calcium hydrogen phosphate association is critical in cluster-based precipitation in the near-neutral pH regime, e.g., in biomineralization. The revised thermodynamics reveal significant and thus far unexplored multi-anion association in computer simulations, constituting a kinetic trap that further complicates aqueous calcium phosphate speciation.

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.

Fiber support prevents colloid-facilitated contamination induced by dissolution-precipitation of a calcium phosphate adsorbent

During the adsorptive removal of hazardous metal contaminants, dissolution-precipitation of sparingly soluble adsorbents may result in the formation of toxic colloidal suspensions, triggering secondary pollution. Therefore, we studied the prevention of colloid-facilitated contamination in a model adsorption system of dicalcium phosphate dihydrate (DCPD, CaHPO₄·2H₂O) and Cd²⁺ as an adsorbent and adsorbate. Upon adding pure DCPD powder into a 500 mg L^-1 Cd²⁺ solution of pH ≌ 7.0, aggregates of spheroidal Cd-bearing primary particles, within 0.040-0.95 μm size range, were generated via dissolution-precipitation. The accumulated volume of these submicron particles (10.8%) was greater than that of the submicron particles from the exposure of DCPD to deionized water (4.48%). While the Cd-carrying submicron particles, which are responsible for colloidal recontamination, appeared to form via homogeneous nucleation, their formation was suppressed using polyacrylonitrile fibers (PANFs) as supporting substrates. Thus, heterogeneous nucleation on PANFs formed hexagonal columnar microparticles of a new phase, pentacadmium dihydrogen tetrakis (phosphate) tetrahydrate (Cd₅H₂(PO₄)₄·4H₂O). Together with dissolution-precipitation on the native DCPD, nucleation and growth on the PANFs accelerated the depletion of the dissolved species, reducing the degree of supersaturation along the DCPD-water interface. Although the PANFs decreased the Cd adsorption capacity to 56.7% of that of DCPD, they prevented the formation of small aggregates of Cd-bearing particles. Other sparingly soluble adsorbents can be compounded with PANF to prevent the generation of toxic colloids.

Electron Paramagnetic Resonance Characterization of Sodium- and Carbonate-Containing Hydroxyapatite Cement

Ionizing radiation-induced paramagnetic defects in calcified tissues like tooth enamel are indicators of irradiation dose. Hydroxyapatite (HA), the principal constituent in these materials, incorporates a variety of anions (CO₃^2-, F^-, Cl^-, and SiO₄^4-) and cations (Mn²⁺, Li⁺, Cu²⁺, Fe³⁺, Mg²⁺, and Na⁺) that directly or indirectly contribute to the formation of stable paramagnetic centers upon irradiation. Here, we used an underexploited synthesis method based on the ambient temperature setting reaction of a self-hardening calcium phosphate cement (CPC) to create carbonate-containing hydroxyapatite (CHA) and investigate its paramagnetic properties following γ-irradiation. Powder X-ray diffraction and IR spectroscopic characterization of the hardened CHA samples indicate the formation of pure B-type CHA cement. CHA samples exposed to γ-radiation doses ranging from 1 Gy to 150 kGy exhibited an electron paramagnetic resonance (EPR) signal from an orthorhombic CO₂^•- free radical. At γ-radiation doses from 30 to 150 kGy, a second signal emerged that is assigned to the CO₃^•- free radical. We observed that the formation of this second species is dose-dependent, which provided a means to extend the useful dynamic range of irradiated CHA to doses >30 kGy. These results indicate that CHA synthesized via a CPC cement is a promising substrate for EPR-based dosimetry. Further studies on the CHA cement are underway to determine the suitability of these materials for a range of biological and industrial dosimetry applications.

Involvement of distant octacalcium phosphate scaffolds in enhancing early differentiation of osteocytes during bone regeneration

This study hypothesized that distant octacalcium phosphate (OCP) scaffolds may enhance osteocyte differentiation in newly formed bone matrices. The results obtained were compared with those of Ca-deficient hydroxyapatite (OCP hydrolyzate, referred to as HL hereafter). Granular OCP and HL, 300-500 µm in diameter, were implanted in critical-sized rat calvarial defects for eight weeks and subjected to histology, immunohistochemistry, histomorphometry, and transmission electron microscopy (TEM). Early osteocyte differentiation from an osteoblastic cell line (IDG-SW3) was examined using materials without contacting the surfaces for 10 days. The material properties and the medium composition were analyzed through selected area electron diffraction (SAED) using TEM observation and curve fitting of Fourier transform infrared (FT-IR) spectroscopy. The number of positive cells of an osteocyte earlier differentiation marker podoplanin (PDPN) in bone matrices, along the direction of bone formation, was significantly higher in OCP than that in HL. The ultrastructure around the OCP surfaces observed by TEM showed the infiltration of some cells, including osteocytes adjacent to the OCP surface layers. The OCP structure remained unchanged by SAED analysis. Nanoparticle deposition and hydrolysis on OCP surfaces were detected by TEM and FT-IR, respectively, during early osteocyte differentiation in vitro. The medium saturation degree varied in accord with ionic dissolution, resulting in possible hydroxyapatite formation on OCP but not on HL. These results suggested that OCP stimulates early osteocyte differentiation in the bone matrix from a distance through its metastable chemical properties. STATEMENT OF SIGNIFICANCE: This study demonstrated that octacalcium phosphate (OCP) implanted in critical-sized rat calvaria bone defects is capable of enhancing the early differentiation of osteocytes embedded in newly formed bone matrices, even when the surface OCP is separated from the osteocytes. This prominent bioactive property of OCP was demonstrated by comparing the in vivo and in vitro performances with a control material, Ca-deficient hydroxyapatite (OCP hydrolyzate). The findings were elucidated by histomorphometry, which analyzed the differentiation of osteocytes along the parallel direction of new bone growth by osteoblasts. Therefore, OCP should stimulate osteocyte differentiation through ionic dissolution even in vivo owing to its metastable chemical properties, as previously reported in an in vitro study (Acta Biomater 69:362, 2018).

Development of a thermodynamic approach to assist the control of the precipitation of hydroxyapatites and associated calcium phosphates in open systems

Thermodynamics of the precipitation of calcium phosphates shows the importance of the pH and the order of introduction of the precursor ions on the textural (morphology, surface area) and structural (defects) properties of hydroxyapatites.

Model-driven spatial evaluation of nutrient recovery from livestock leachate for struvite production

Nutrient pollution is one of the major worldwide water quality problems, resulting in environmental and public health issues. Agricultural activities are the main source of nutrient release emissions, and the livestock industry has been proven to be directly related to the presence of high concentrations of phosphorus in the soil, which potentially can reach waterbodies by runoff. To mitigate the phosphorus pollution of aquatic systems, the implementation of nutrient recovery processes allows the capture of phosphorus, preventing its release into the environment. Particularly, the use of struvite precipitation produces a phosphorus-based mineral that is easy to transport, enabling redistribution of phosphorus to deficient locations. However, livestock leachate presents some characteristics that hinder struvite precipitation, preventing extrapolation of the results obtained from wastewater studies to cattle waste. Consideration of these elements is essential to determine the optimal operating conditions for struvite formation, and for predicting the amount of struvite recovered. In this work, a detailed thermodynamic model for precipitates formation from cattle waste is used to develop surrogate models to predict the formation of struvite and calcium precipitates from cattle waste. The variability in the organic waste composition, and how it affects the production of struvite, is captured through a probability framework based on the Monte Carlo method embedded in the model. Consistent with the developed surrogate models, the potential of struvite production to reduce the phosphorus releases from the cattle industry to watersheds in the United States has been assessed. Also, the more vulnerable locations to nutrient pollution were determined using the techno-ecological synergy sustainability metric (TES) by evaluating the spatial distribution and balance of phosphorus from agricultural activities. Although only struvite formation from cattle operations is considered, reductions between 22% and 36% of the total phosphorus releases from the agricultural sector, including manure releases and fertilizer application, can be achieved.

Precipitation and phase transformation of dicalcium phosphate dihydrate in electrolyte solutions of simulated body fluids: Thermodynamic modeling and kinetic studies

The metastable and stable equilibria of a precipitation in the biomimetic system Simulated Body Fluid (SBF)-CaCl₂ -K₂ HPO₄ -KOH-H₂ O were modeled in the pH region 3-7 at a Ca/P molar ratio of 1 using a thermodynamic approach. Saturation indices (SI) of the solid phases were calculated and used to prognose the salt precipitation/dissolution processes. At рН < 4, the solutions are undersaturated (SI < 0) in respect of all solid phases; co-precipitation of dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HA) occurs at рН 4 while at рН > 4 the stable phase is DCPD but the number of other co-precipitated solid phases increases. This result is associated with the increase in HPO₄ ^2- , CaHPO₄ ⁰ , and KНРО₄ ^- species in the studied solution. The phase transformations of five model DCPD-based calcium phosphate precursors in three simulated body fluids differing in their composition, to more stable octacalcium phosphate and hydroxyapatite was thermodynamically prognosed and experimentally confirmed by kinetic studies, as well as by chemical, XRD, SEM, and IR methods.

Technological Challenges of Phosphorus Removal in High-Phosphorus Ores: Sustainability Implications and Possibilities for Greener Ore Processing

With the present rates of iron ore consumption, currently unusable, high-phosphorus iron ore deposits are likely to be the iron ores of the future as higher-grade iron ore reserves are depleted. Consequently, the design and timely development of environmentally-benign processes for the simultaneous beneficiation of high-phosphorus iron ores and phosphorus recovery, currently a technological challenge, might soon become a sustainability challenge. To stimulate interest in this area, phosphorus adsorption and association in iron oxides/hydroxyoxides, and current efforts at its removal, have been reviewed. The important properties of the most relevant crystalline phosphate phases in iron ores are highlighted, and insights provided on plausible routes for the development of sustainable phosphorus recovery solutions from high-phosphorus iron ores. Leveraging literature information from geochemical investigations into phosphorus distribution, speciation, and mobility in various natural systems, key knowledge gaps that are vital for the development of sustainable phosphorus removal/recovery strategies and important factors (white spaces) not yet adequately taken into consideration in current phosphorus removal/recovery solutions are highlighted, and the need for their integration in the development of future phosphorus removal/recovery solutions, as well as their plausible impacts on phosphorus removal/recovery, are put into perspective.

Quartz Crystal Microbalance Assay of Clinical Calcinosis Samples and Their Synthetic Models Differentiates the Efficacy of Chelation-Based Treatments

This paper sets out in vitro protocols for studying the relative effectiveness of chelators used in the dissolution-based treatment of hard calcinosis. Pulverized hard calcinosis samples from human donors or synthetic hydroxyapatite nanoparticles were deposited by electrophoretic deposition on the surface of a quartz crystal microbalance sensor. Over 150 deposits of <20 μg were dissolved over the course of 1 h by aliquots of buffered, aqueous solutions of two calcium chelators, EDTA and citrate, with the surface-limited dissolution kinetics monitored with <1 s time resolution. There was no statistically significant difference in dissolution rate between the four synthetic hydroxyapatite materials in EDTA, but the dissolution rates in citrate were lower for hydroxyapatite produced by acetate or nitrate metathesis. Hard calcinosis and synthetic hydroxyapatites showed statistically identical dissolution behavior, meaning that readily available synthetic mimics can replace the rarer samples of biological origin in the development of calcinosis treatments. EDTA dissolved the hydroxyapatite deposits more than twice as fast as citrate at pH 7.4 and 37 °C, based on a first-order kinetic analysis of the initial frequency response. EDTA chelated 6.5 times more calcium than an equivalent number of moles of citrate. Negative controls using nonchelating N,N,N',N'-tetraethylethylenediamine (TEEDA) showed no dissolution effect. Pharmaceutical dissolution testing of synthetic hydroxyapatite tablets over 6 h showed that EDTA dissolved the tablets four to nine times more quickly than citrate.

Urinary supersaturation as a diagnostic measure in urolithiasis

AIM

To demonstrate that urinary supersaturation per se is not a reliable diagnostic measure of the risk for stone formation.

METHODS

Available physical and chemical data for calcium oxalate monohydrate (COM) and calcium hydrogen phosphate dihydrate (brushite, BRU), and urinary supersaturations with respect to COM and BRU in healthy individuals and stone formers, were obtained from the literature. Classical theory of nucleation was used for calculations.

RESULTS

It was found that the rate of homogeneous nucleation (unaided by substrates) of COM and BRU is nil at all conceivable supersaturations of urine. Consequently spontaneous formation of crystals in urine requires the presence of nucleation substrates for (heteronuclei).

CONCLUSION

Urinary supersaturation with respect to lithiatic compounds is a necessary, but not a sufficient condition for nephrolithiasis. The absence of crystallization inhibitors and the presence of efficient nucleation promoters (heteronuclei) in urine are further necessary conditions of urolithiasis occurrence. Urinary supersaturation per se is not a reliable diagnostic measure of the risk of kidney stone formation.

Cu2+, Co2+ and Cr3+ doping of a calcium phosphate cement influences materials properties and response of human mesenchymal stromal cells

The application of biologically active metal ions to stimulate cellular reactions is a promising strategy to accelerate bone defect healing. Brushite-forming calcium phosphate cements were modified with low doses of Cu²⁺, Co²⁺ and Cr³⁺. The modified cements released the metal ions in vitro in concentrations which were shown to be non-toxic for cells. The release kinetics correlated with the solubility of the respective metal phosphates: 17-45 wt.-% of Co²⁺ and Cu²⁺, but <1 wt.-% of Cr³⁺ were released within 28days. Moreover, metal ion doping led to alterations in the exchange of calcium and phosphate ions with cell culture medium. In case of cements modified with 50mmol Cr³⁺/mol β-tricalcium phosphate (β-TCP), XRD and SEM analyses revealed a significant amount of monetite and a changed morphology of the cement matrix. Cell culture experiments with human mesenchymal stromal cells indicated that the observed cell response is not only influenced by the released metal ions but also by changed cement properties. A positive effect of modifications with 50mmol Cr³⁺ or 10mmol Cu²⁺ per mol β-TCP on cell behaviour was observed in indirect and direct culture. Modification with Co²⁺ resulted in a clear suppression of cell proliferation and osteogenic differentiation. In conclusion, metal ion doping of the cement influences cellular activities in addition to the effect of released metal ions by changing properties of the ceramic matrix.

Reduction of orthophosphates loss in agricultural soil by nano calcium sulfate

Nutrient loss from soil, especially phosphorous (P) from farmlands to natural water bodies via surface runoff or infiltration, have caused significant eutrophication problems. This is because dissolved orthophosphates are usually the limiting nutrient for algal blooms. Currently, available techniques to control eutrophication are surprisingly scarce. Calcium sulfate or gypsum is a common soil amendment and has a strong complexation to orthophosphates. The results showed that calcium sulfate reduced the amount of water extractable P (WEP) through soil incubation tests, suggesting less P loss from farmlands. A greater decrease in WEP occurred with a greater dosage of calcium sulfate. Compared to conventional coarse calcium sulfate, nano calcium sulfate further reduced WEP by providing a much greater specific surface area, higher solubility, better contact with the fertilizer and the soil particles, and superior dispersibility. The enhancement of the nano calcium sulfate for WEP reduction is more apparent for a pellet- than a powdered- fertilizer. At the dosage of Ca/P weight ratio of 2.8, the WEP decreased by 31±5% with the nano calcium sulfate compared to 20±5% decrease with the coarse calcium sulfate when the pellet fertilizer was used. Computation of the chemical equilibrium speciation shows that calcium hydroxyapatite has the lowest solubility. However, other mineral phases such as hydroxydicalcium phosphate, dicalcium phosphate dihydrate, octacalcium phosphate, and tricalcium phosphate might form preceding to calcium hydroxyapatite. Since calcium sulfate is the major product of the flue gas desulfurization (FGD) process, this study demonstrates a potential beneficial reuse and reduction of the solid FGD waste.

Does drinking water influence hospital-admitted sialolithiasis on an epidemiological level in Denmark?

OBJECTIVES

Sialolithiasis, or salivary stones, is not a rare disease of the major salivary glands. However, the aetiology and incidence remain largely unknown. Since sialoliths are comprised mainly of calcium phosphate salts, we hypothesise that drinking water calcium levels and other elements in drinking water could play a role in sialolithiasis. Owing to substantial intermunicipality differences in drinking water composition, Denmark constitutes a unique environment for testing such relations.

DESIGN

An epidemiological study based on patient data extracted from the National Patient Registry and drinking water data from the Geological Survey of Denmark and Greenland retrieved as weighted data on all major drinking water constituents for each of the 3364 waterworks in Denmark. All patient cases with International Statistical Classification of Diseases 10th Revision (ICD-10) codes for sialolithiasis registered between the years 2000 and 2010 were included in the study (n=3014) and related to the drinking water composition on a municipality level (n=98).

PRIMARY AND SECONDARY OUTCOME MEASURES

Multiple regression analysis using iterative search and testing among all demographic and drinking water variables with sialolithiasis incidence as the outcome in search of possible relations among the variables tested.

RESULTS

The nationwide incidence of hospital-admitted sialolithiasis was 5.5 cases per 100,000 citizens per year in Denmark. Strong relations were found between the incidence of sialolithiasis and the drinking water concentration of calcium, magnesium and hydrogen carbonate, however, in separate models (p<0.001). Analyses also confirmed correlations between drinking water calcium and magnesium and their concentration in saliva whereas this was not the case for hydrogen carbonate.

CONCLUSIONS

Differences in drinking water calcium and magnesium may play a role in the incidence of sialolithiasis. These findings are of interest because many countries have started large-scale desalination programmes of drinking water.

New Approaches to Enhanced Remineralization of Tooth Enamel

Dental caries is a highly prevalent diet-related disease and is a major public health problem. A goal of modern dentistry is to manage non-cavitated caries lesions non-invasively through remineralization in an attempt to prevent disease progression and improve aesthetics, strength, and function. Remineralization is defined as the process whereby calcium and phosphate ions are supplied from a source external to the tooth to promote ion deposition into crystal voids in demineralized enamel, to produce net mineral gain. Recently, a range of novel calcium-phosphate-based remineralization delivery systems has been developed for clinical application. These delivery systems include crystalline, unstabilized amorphous, or stabilized amorphous formulations of calcium phosphate. These systems are reviewed, and the technology with the most scientific evidence to support its clinical use is the remineralizing system utilizing casein phosphopeptides to stabilize and deliver bioavailable calcium, phosphate, and fluoride ions. The recent clinical evidence for this technology is presented and the mechanism of action discussed. Biomimetic approaches to stabilization of bioavailable calcium, phosphate, and fluoride ions and the localization of these ions to non-cavitated caries lesions for controlled remineralization show promise for the non-invasive management of dental caries.

Molecular mechanisms of crystallization impacting calcium phosphate cements

The biomineral calcium hydrogen phosphate dihydrate (CaHPO(4).2H(2)O), known as brushite, is a malleable material that both grows and dissolves faster than most other calcium minerals, including other calcium phosphate phases, calcium carbonates and calcium oxalates. Within the body, this ready formation and dissolution can play a role in certain diseases, such as kidney stone and plaque formation. However, these same properties, along with brushite's excellent biocompatibility, can be used to great benefit in making resorbable biomedical cements. To optimize cements, additives are commonly used to control crystallization kinetics and phase transformation. This paper describes the use of in situ scanning probe microscopy to investigate the role of several solution parameters and additives in brushite atomic step motion. Surprisingly, this work demonstrates that the activation barrier for phosphate (rather than calcium) incorporation limits growth kinetics and that additives such as magnesium, citrate and bisphosphonates each influence step motion in distinctly different ways. Our findings provide details of how, and where, molecules inhibit or accelerate kinetics. These insights have the potential to aid in designing molecules to target specific steps and to guide synergistic combinations of additives.

Effect of addition of citric acid and casein phosphopeptide-amorphous calcium phosphate to a sugar-free chewing gum on enamel remineralization in situ

Casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) has been shown to remineralize enamel subsurface lesions in situ. The aim of this study was to investigate the effects of CPP-ACP in a fruit-flavoured sugar-free chewing gum containing citric acid on enamel remineralization, and acid resistance of the remineralized enamel, using an in situ remineralization model. The study utilized a double-blind, randomized, crossover design with three treatments: (i) sugar-free gum (2 pellets) containing 20 mg citric acid and 18.8 mg CPP-ACP, (ii) sugar-free gum containing 20 mg citric acid alone, (iii) sugar-free gum not containing CPP-ACP or citric acid. Ten subjects were instructed to wear removable palatal appliances, with 4 half-slab insets of human enamel containing demineralized subsurface lesions and to chew gum (2 pellets) for 20 min 4 times per day for 14 days. At the completion of each treatment the enamel half-slabs were removed and half of the remineralized lesion treated with demineralization buffer for 16 h in vitro. The enamel slabs (remineralized, acid-challenged and control) were then embedded, sectioned and subjected to microradiography to determine the level of remineralization. Chewing with gum containing citric acid and CPP-ACP resulted in significantly higher remineralization (13.0 +/- 2.2%) than chewing with either gum containing no CPP-ACP or citric acid (9.4 +/- 1.2%) or gum containing citric acid alone (2.6 +/- 1.3%). The acid challenge of the remineralized lesions showed that the level of mineral after acid challenge was significantly greater for the lesions exposed to the gum containing CPP-ACP.

Surface texture and composition of titanium brushed with toothpaste slurries of different pHs

OBJECTIVES

This in vitro study characterized the surface texture and composition of titanium brushed with toothpaste slurries of different pHs, and thereby elucidated mechanochemical interactions between the metal and abrasive material in dentifrice.

METHODS

Two fluoride-free toothpastes, which contained crystalline CaHPO(4).2H(2)O and amorphous SiO(2) particles as abrasive, were mixed with acidic buffers to provide slurries of pH 6.8 and 4.8. Specimens were cast from CP Ti, mirror-polished, and then toothbrushed at 120strokes/min for 350,400 strokes under a load of 2.45N. Specimen surfaces were characterized by means of SPM and EPMA. The obtained data were compared with the already reported results of water-diluted alkaline slurries. SPM data of each paste were analyzed using one-way ANOVA, followed by post hoc Tukey test.

RESULTS

Irrespective of toothpaste, neutral slurries, as with alkaline slurries, yielded a chemically altered surface with rough texture, whereas acidic slurries formed a chemically clean surface with relatively smooth texture. Mechanochemical polishing effect might be mainly responsible for the cleanness and smoothness.

SIGNIFICANCE

Acidic slurry-induced smooth surface may minimize plaque formation. However, the augmentation of released titanium ions may be adverse to the human body. For evaluation of toothpaste abrasion effects on titanium, paste slurry pH should be taken into account.

Apparent solubility distributions of hydroxyapatite and enamel apatite

Samples of human dental enamel and hydroxyapatite were equilibrated at 5 mg/40 ml for 9 days at 37 degrees C with acetate buffers adjusted to a range of saturations with respect to hydroxyapatite. Sigmoidal apparent solubility distributions, in which the fraction dissolved was plotted against--log(ion activity product for hydroxyapatite) (pIHA), were constructed. About 10% of the hydroxyapatite and 14% of the enamel was very soluble, dissolving even at pIHA 55. The apparent solubility distributions for both solids were invariant with pH (4.5, 5.0, 5.5), within experimental error, showing that solubility was controlled by a phase with the stoichiometry of hydroxyapatite, probably in the form of a surface layer or complex on the crystals, in agreement with other studies on carbonate-apatites and bone mineral. The pIHA at 50% dissolution was employed as an average value. The pIHA (50%) values for pooled data (58.76 for enamel and 60.17 for hydroxyapatite) were lower than the respective pIHA previously measured by conventional equilibration techniques. However, the average pIHA measured for enamel was higher than that obtained by the same technique in another study, possibly because of differences in specimen preparation and equilibration time. The possible implications of the findings for understanding the process of dental caries are discussed.

Phosphorus retention in calcareous soils and the effect of organic matter on its mobility

A survey of the interactions between phosphorus (P) species and the components of calcareous soils shows that both surface reactions and precipitation take place, especially in the presence of calcite and limestone. The principal products of these reactions are dicalcium phosphate and octacalcium phosphate, which may interconvert after formation. The role of calcium carbonate in P retention by calcareous soils is, however, significant only at relatively high P concentrations - non-carbonate clays play a more important part at lower concentrations. In the presence of iron oxide particles, occlusion of P frequently occurs in these bodies, especially with forms of the element that are pedogenic in origin. Progressive mineralization and immobilization, often biological in nature, are generally observed when P is added as a fertilizer. Manure serves both as a source of subsurface P and an effective mobilizing agent. Blockage of P sorption sites by organic acids, as well as complexation of exchangeable Al and Fe in the soil, are potential causes of this mobilization. Swine and chicken manure are especially rich P sources, largely due the practice of adding the element to the feed of nonruminants. Humic materials, both native and added, appear to increase recovery of Olsen P. In the presence of metal cations, strong complexes between inorganic P and humates are formed. The influence of humic soil amendments on P mobility warrants further investigation.

Theoretical analysis of calcium phosphate precipitation in simulated body fluid

The driving force and nucleation rate of calcium phosphate (Ca-P) precipitation in simulated body fluid (SBF) were analyzed based on the classical crystallization theory. SBF supersaturation with respect to hydroxyapatite (HA), octacalcium phosphate (OCP) and dicalcium phosphate (DCPD) was carefully calculated, considering all the association/dissociation reactions of related ion groups in SBF. The nucleation rates of Ca-P were calculated based on a kinetics model of heterogeneous nucleation. The analysis indicates that the nucleation rate of OCP is substantially higher than that of HA, while HA is most thermodynamically stable in SBF. The difference in nucleation rates between HA and OCP reduces with increasing pH in SBF. The HA nucleation rate is comparable with that of OCP when the pH value approaches 10. DCPD precipitation is thermodynamically impossible in normal SBF, unless calcium and phosphate ion concentrations of SBF increase. In such case, DCPD precipitation is the most likely because of its highest nucleation rates among Ca-P phases. We examined the influences of different SBF recipes, interfacial energies, contact angle and molecular volumes, and found that the parameter variations do not have significant impacts on analysis results. The effects of carbonate incorporation and calcium deficiency in HA were also estimated with available data. Generally, such apatite precipitations are more kinetically favorable than HA.

Physicochemical Characterization of Casein Phosphopeptide-Amorphous Calcium Phosphate Nanocomplexes

Milk caseins stabilize calcium and phosphate ions and make them available to the neonate. Tryptic digestion of the caseins yields phosphopeptides from their polar N-terminal regions that contain clusters of phosphorylated seryl residues. These phosphoseryl clusters have been hypothesized to be responsible for the interaction between the caseins and calcium phosphate that lead to the formation of casein micelles. The casein phosphopeptides stabilize calcium and phosphate ions through the formation of complexes. The calcium phosphate in these complexes is biologically available for intestinal absorption and remineralization of subsurface lesions in tooth enamel. We have studied the structure of the complexes formed by the casein phosphopeptides with calcium phosphate using a range of physicochemical techniques including x-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and equilibrium binding analyses. The amorphous nature of the calcium phosphate phase was confirmed by two independent methods: x-ray powder diffraction and selected area diffraction. In solution, the ion activity product of a basic amorphous calcium phosphate phase was the only ion product that was a function of bound phosphate independent of pH, consistent with basic amorphous calcium phosphate being the phase stabilized by the casein phosphopeptides. Detailed investigations of calcium and calcium phosphate binding using a library of synthetic homologues and analogues of the casein phosphopeptides have revealed that although the fully phosphorylated seryl-cluster motif is pivotal for the interaction with calcium and phosphate, other factors are also important. In particular, calcium binding and calcium phosphate stabilization by the peptides was influenced by peptide net charge, length, and sequence.

Saturation of human saliva with respect to calcium salts

It may be assumed that free ionic concentrations of calcium and phosphate in resting saliva tend to equilibrate with those in plaque fluid, and that salivary data can therefore be used to illustrate chemical conditions in both saliva and plaque. In the present study, salivary data collected from the literature or obtained in our laboratory were used to calculate degrees of super- and undersaturation with respect to apatites, brushite, beta-tricalcium phosphate, octacalcium phosphate, calcium carbonate and calcium fluoride in the pH range from 3 to 9. Concentrations of calcium, phosphate, fluoride, carbonate, and background ion strength of resting parotid saliva, resting submandibular saliva, and resting and stimulated whole saliva were entered into a computer program, and curves illustrating saturation in the pH range 3-9 constructed. It was found that oral fluids are supersaturated with respect to apatites above pH 5.3 and with respect to octacalcium phosphate and beta-tricalcium phosphate above pH 6. Parotid saliva was undersaturated with respect to brushite whilst submandibular saliva was supersaturated with respect to that salt in the pH range 6-8. Stimulated whole saliva with 25 mmol/l carbonate became supersaturated with respect to calcium carbonate only above pH 7.3, which may explain the absence of this salt in the human oral cavity. To maintain the saturation of oral fluids with respect to calcium fluoride, i.e. to ensure its survival in the mouth required 6 ppm fluoride in the aqueous phase. Therefore, this salt, the outcome of topical fluoride therapy, will inevitably dissolve in the oral fluids.

Effect in vitro acidification on plaque fluid composition with and without a NaF or a controlled-release fluoride rinse

Plaque fluid ion concentration changes, especially fluoride, in response to the pH decrease associated with a cariogenic episode are important components of the caries process. A "controlled-release" (CR) fluoride rinse, based on the controlled release of fluoride in the presence of calcium, has been shown to form large fluoride reservoirs in resting plaque. In this study, the in vitro acid-induced release of fluoride, and other ions, was examined in 48-hour-fasted plaque fluid from subjects (n = 11) who received no rinse, or who used a 228-ppm CR or NaF fluoride rinse 1 hr before being sampled. After collection, the plaque was centrifuged to yield plaque fluid, acidified (0.1 microL of 0.5 mol/L HCl per milligram plaque), and then re-centrifuged before a second sample was obtained. Although previous studies indicated a higher plaque fluid fluoride after the new rinse relative to NaF, no statistically significant difference was observed here. Average fluoride release after acidification (average pH, 5.2) was statistically greater following the use of the CR rinse (153 micromol/L) compared with the NaF rinse (17 micromol/L). No fluoride release was seen in the no-rinse samples. The pH, free calcium, phosphate, acetate, propionate, and buffer capacity were not affected by the different amounts of fluoride deposited in the plaque. However, following acid addition, an increase in free calcium and phosphate was observed, which was also independent of the rinse. The large release of fluoride following acidification suggests that the new rinse may provide an improved cariostatic effect.

Measurement of Calcium Activity in Oral Fluids by Ion Selective Electrode: Method Evaluation and Simplified Calculation of Ion Activity Products

The activity of calcium in plaque fluid is needed to calculate the saturation level of that fluid relative to the tooth mineral. One method to determine the calcium activity in very small plaque fluid samples is by micro ion-selective electrode (ISE). Two commercially available calcium ionophores, a neutral-carrier and a charged-carrier, were evaluated in micro ISEs and compared to a commercially available macro ISE using saliva as a model for plaque fluid. The neutral-carrier containing ISEs gave results consistent with those of the macro ISE. Calcium activity measurements made with micro ISEs that contained the neutral ion-carrier of whole plaque samples and plaque fluid samples obtained by centrifugation of whole plaque showed that the activities did not change due to centrifugation. Estimates of the saturation with respect to hydroxyapatite were made from these measurements. A simplified calculation method is presented to estimate the ion activity product (IAP) of the calcium-phosphate minerals. The method is based on the relative abundance of some of the possible calcium-binding species and a fixed ionic strength for plaque fluid. Calculations show that within a normal pH range for plaque fluid (5.0 to 7.5) the differences in the IAP calculations for hydroxyapatite using the simplified method are less than those estimated from propagation of uncertainty calculations.

Use of Dicalcium Phosphate Dihydrate as a Probe in an Approach for Accurate Calculations of Solution Equilibria in Buffered Calcium Phosphate Systems

Both synthetic and biological carbonated apatites have been shown to possess metastable equilibrium solubility (MES) distributions. Investigation of their MES behavior requires accurate knowledge of the activities of the relevant ions over a range of solution compositions. This in turn requires that the activity coefficients for these ions and the stability constants for the various solution complexes be accurately known. Since the solubility of dicalcium phosphate dihydrate (DCPD) can be easily determined, it can be used as a probe to study the appropriateness of proposed sets of stability constants and activity coefficients. In the present study, the solubility of DCPD was determined in acetate buffer solutions at pHs ranging from 4.5 to 6.5 and this allowed for the determination and evaluation of the stability constants of the NaHPO(-)(4), CaHPO(0)(4), CaH(2)PO(+)(4), and CaAc(+) complexes. The activity coefficients were calculated by a modification of the extended Debye-Hückel method and also by Bockris and Reddy's method, in which the activity of water and the hydration numbers of ions are also considered. The solution conditions were controlled to simulate the dissolution media used for the determination of MES distributions of apatites in our laboratory. Both methods of activity coefficient determination gave very consistent DCPD solubility product (K(DCPD)) values, and values for the stability constants of the relevant complexes determined using the two methods were close to each other. This approach of determining the stability constants under a set of conditions similar to those in apatite dissolution studies offers an effective set of parameters which, though not necessarily always exactly correct in absolute terms, are internally consistent and should allow for the quantitative characterization of the MES behavior of apatites. Copyright 1999 Academic Press.

Edgard C. Moreno, PhD: a tribute to his major contributions to oral biology research

The seminal contributions of Dr. Edgard C. Moreno to physical chemical aspects of oral biology research are reviewed, with personal insights provided by the author. Dr. Moreno has made major contributions to our understanding of fundamental processes which occur within the oral cavity and which control the formation of mineralized tissues. These contributions include extensive research in the areas of: the chemistry of synthetic calcium phosphates and biominerals, the interaction of salivary proteins and other molecules of biological interest with calcium phosphate surfaces, the mechanism of dental caries formation and the effects of fluoride on this process, the mechanism of dental calculus formation and prevention, and the mechanism of dental enamel formation.

Remineralization of enamel subsurface lesions by casein phosphopeptide-stabilized calcium phosphate solutions

Casein phosphopeptides (CPP) stabilize amorphous calcium phosphate (ACP), localize ACP in dental plaque, and are anticariogenic in animal and in situ human caries model. In this vitro study, CPP-stabilized calcium phosphate solutions were shown to remineralize subsurface lesions in human third-molar enamel. Solutions were used to examine the effect of CPP-calcium phosphate concentration on remineralization. Other solutions were used to examine the effect of increasing pH, which decreased the concentrations of free calcium and phosphate ions and increased the level of CPP-bound ACP. Although most of the remineralizing solutions were supersaturated with respect to the amorphous and crystalline calcium phosphate phases, the solutions were stabilized by the CPP such that spontaneous precipitation of calcium phosphate did not occur. After a ten-day remineralization period, enamel lesions were sectioned, subjected to microradiography, and the mineral content determined by microdensitometry. All solutions deposited mineral into the bodies of the lesions, with the 1.0% CPP-calcium phosphate (pH 7.0) solution replacing 63.9 +/- 20.1% of mineral lost at an averaged rate of 3.9 +/- 0.8 x 10(-8) mol hydroxyapatite/m2/s. The remineralizing capacity was greater for the solutions with the higher levels of CPP-stabilized free calcium and phosphate ions. Remineralization was not significantly correlated with either the CPP-bound ACP of the degrees of saturation for hydroxyapatite, octacalcium phosphate, or ACP. However, remineralization was significantly correlated with the degree of saturation for dicalcium phosphate dihydrate (CaHPO4.2H2O), but his was attributed to the significant correlation of remineralization with the activity gradients from the solution into the lesion of some calcium phosphate ions and ion pairs, in particular the neutral ion pair CaHPO4(0). The CPP, by stabilizing calcium phosphate in solution, maintain high-concentration gradients of calcium and phosphate ions and ion pairs into the subsurface lesion and thus effect high rates of enamel remineralization.

Dissolution of powdered human enamel suspended in acid solutions at a high solid/solution ratio under a 5% CO2 atmosphere at 20 degrees C

The aim was to examine the nature of enamel dissolution in aqueous suspensions with a high solid/solution ratio and in a CO2-rich atmosphere. Before experimentation, a water-saturated mixture of 95% N2-5% CO2 was passed through the acid solutions for 24 hr. Samples of 2 g of powdered enamel were suspended in 7 ml of either 5 or 10 mmol/l HClO4, with or without 2 parts/10(6) fluoride and kept gently agitated for 24 hr in the above atmosphere. The same enamel samples were repeatedly exposed to fresh acid for 26 runs. All experiments were duplicated. The aqueous phase was analysed after 20 min and 24 hr for calcium, phosphate, fluoride, chloride, sodium and magnesium. It was found that after 20 min the fluoride was invariably taken up in the enamel and the solution was supersaturated with respect to hydroxyapatite with pH ranging 6.7-5.6. During the following 24 hr pH increased further, the supersaturation remained unchanged and the concentrations of calcium and phosphate in solution decreased. In contrast, sodium, magnesium and chloride were released from enamel during the entire period. In the later runs, the supersaturation with respect to hydroxyapatite was only modest and the decrease of calcium and phosphate concentrations limited, as were the release of sodium, magnesium and chloride. It is concluded that despite a CO2-rich atmosphere, calcium, phosphate and carbonate were released from enamel and quickly established a supersaturation with respect to hydroxyapatite with a secondary reprecipitation of mineral. It indicates that within the dental caries lesion in vivo, lesion fluid cannot exist undersaturated with respect to enamel apatite.

Variations in solution chemistry during calcium-deficient and stoichiometric hydroxyapatite formation from CaHPO4.2H2O and Ca4(PO4)2O

This study explores the mechanistic paths taken when calcium-deficient hydroxyapatite, CDHAp (Ca/P = 1.50), and stoichiometric hydroxyapatite, SHAp (Ca/P = 1.67), form by reaction between particulate calcium phosphate salts. The acidic reactant was CaHPO4.2H2O (DCPD) and the basic reactant was Ca4(PO4)2O (TetCP). Variations in pH, calcium and phosphate concentrations, and the solids present during apatite formation, were determined as functions of reaction temperature (25.0 degrees, 37.4 degrees, and 50.0 degrees C) and time. It was found that the dissolution of TetCP was rate limiting for both hydroxyapatite (HAp) compositions at all three temperatures. However, the retrograde solubility and incongruent dissolution of DCPD became increasingly important in influencing the kinetics as the reaction temperature was increased. An amorphous intermediate phase was observed regardless of the HAp stoichiometry. The solutions from which the SHAp formed approached equilibrium at much shorter reaction times (1-2 days) than those from which the CDHAp formed. The latter continued to display changes in pH and in calcium and phosphate concentrations for 6 months. CDHAp was shown to be a thermodynamically stable phase. The dissolution of CDHAp is incongruent, showing a Ca/P molar ratio in solution less than 0.5.

A computer program for correlating dental plaque pH values, cH+, plaque titration, critical pH, resting pH and the solubility of enamel apatite

A computer program was written in Visual Basic (Microsoft) to calculate (a) the area between a plaque pH curve (as seen after a sucrose rinse) and either a resting pH (around pH 7) or a critical pH value (around 5.5) above at least parts of the pH curve; (b) the solubility of apatite at the pH values in plaque; (c) the area between the plaque pH solubility curve and the apatite solubility at the resting pH/critical pH; (d) the area between plaque cH+ curve and the cH+ value at resting pH/ critical pH; and (e) the area between a plaque pH curve and a cut-off pH value below the curve, e.g. pH 3. It was found that because both the cH+ and the solubility of apatite increased logarithmically with a pH drop, the two latter area functions (d, e) were basically different from those based directly on pH curves. Thus, pH changes around the resting pH value had little effect on areas calculated from concentrations of H+ and solubility. In contrast, a small pH change around pH 4 had a strong impact on both demineralization potential and areas based on cH+. Also, because of the logarithmic nature, demineralization potentials were generally large in comparison to remineralization potentials, a point that has hitherto received little attention.

Anticariogenicity of calcium phosphate complexes of tryptic casein phosphopeptides in the rat

Casein phosphopeptides (CPP) stabilize calcium phosphate through the formation of casein-phosphopeptide amorphous calcium-phosphate complexes (CPP-CP). The ability of CPP-CP to reduce caries activity was investigated by use of specific-pathogen-free rats inoculated with Streptococcus sobrinus. The animals consumed a defined cariogenic diet free of dairy products. Solutions (100 microL) of the CPP-CP (0.1, 0.2, 0.5, 1.0% w/v) were applied to the animals' molar teeth twice daily. Other groups of animals received solutions containing 500 ppm F, the non-phosphorylated peptides of a casein tryptic digest (0.5% w/v), or the calcium-phosphate complex of a synthetic octapeptide, Ac-Glu-Ser(P)-Ile-Ser(P)-Ser(P)-Ser(P)-Glu-Glu-NHMe, corresponding to the common sequence in the CPP. The CPP-CP significantly reduced caries activity in a dose-response fashion, with 1.0% CPP-CP producing 55% and 46% reductions in smooth surface and fissure caries activity, respectively, being similar to that of 500 ppm F. The anticariogenic effects of CPP-CP and fluoride were additive, since animals receiving 0.5% CPP-CP plus 500 ppm F had significantly lower caries activity than those animals receiving either CPP-CP or fluoride alone. The tryptic digest of casein with the phosphopeptides selectively removed showed no anticariogenic activity. The synthetic octapeptide-calcium phosphate complex significantly reduced caries activity, confirming that this calcium-phosphate-stabilizing portion of the casein phospho-peptides is associated with anticariogenicity. The CPP-CP did not significantly affect the level of S. sobrinus in fissure plaque.(ABSTRACT TRUNCATED AT 250 WORDS)

The effectiveness of bone char in the defluoridation of water in relation to its crystallinity, carbon content and dissolution pattern

The procedure for charring bone has been found to influence the nature of its mineral phase, and also affects the defluoridation capacity of the char. The aim of this study was to examine the efficiency of defluoridation by char produced from various parts of bones and charred for various lengths of time at various temperatures, relating the preparation of the char to its X-ray diffraction pattern, its content of pyrophosphate and its capacity for defluoridation. Bone was charred for 1 and 4 h at 400 degrees C and 30 min, 4 h and 48 h at 550 degrees C. Batches of the chars were suspended in amounts of 0.25 g in 100 ml of distilled water containing 0.53 mmol/l fluoride for up to 6 days under gentle agitation. At intervals, the pH and concentrations of fluoride, calcium and phosphate in the water were determined and the degree of saturation with respect to the calcium phosphate salts calculated. The charring procedure reduced the organic content of the bone from the 44-26% in intact bone to 3.4% in bone charred at 400 degrees C and to almost zero when charred at 550 degrees C for 48 h. When charred at 400 degrees C for up to 4 h the X-ray diffraction pattern of the bone showed a poorly crystallized apatite similar to that of untreated bone. Heating for 48 h or more at 550 degrees C led to considerably sharper apatite reflections, indicative of a well-crystallized salt.(ABSTRACT TRUNCATED AT 250 WORDS)

Physicochemical properties of calcific deposits isolated from porcine bioprosthetic heart valves removed from patients following 2-13 years function

The purpose of this study was to characterize the physicochemical properties of calcific deposits that cause the failure of tissue-derived heart valve bioprostheses. This was done in an effort to understand the mechanism of pathologic biomineralization in the cardiovascular system and potentially prevent deterioration of bioprostheses. Calcific deposits taken from 10 failed bioprosthetic valves that had been implanted in patients for 2-13 years were characterized by chemical analysis, x-ray diffraction, FTIR spectroscopy, scanning electron microscopy, polarized light microscopy, and solubility measurements. The combined results identified the biomineral as an apatitic calcium phosphate salt with substantial incorporation of sodium, magnesium and carbonate. The average Ca/PO4 ratio for this "young" pathologic biomineral was approximately 1.3, considerably lower than approximately 1.7 found in mature atherosclerotic plaque biomineral and mature skeletal biomineral, both of which approximate hydroxyapatite in composition. Deproteinated calcific deposits from bioprostheses had thermodynamic solubilities comparable to those of both atherosclerotic plaque, typical pathologic biomineral and hydrolyzed octacalcium phosphate (OCP, Ca4H(PO4)3 x 2.5 H2O), a proposed precursor phase to biomineral apatite. This later finding, together with chemical composition and structural details of the bioprosthetic deposits themselves, supports a mechanism of cardiovascular calcification in which OCP plays a crucial role in the formation of the final apatitic phase. This suggests an approach toward prevention of bioprosthetic tissue calcification through control of the formation of the kinetically favored OCP precursor and/or its transformation into bioapatite.

Effects of low fluoride concentrations on formation of caries-like lesions in human enamel in a sequential-transfer bacterial system

Human enamel surfaces were exposed to sequential batch cultures of Streptococcus mutans NCTC 10832 in a sucrose-rich medium containing 0-5 mg/l added fluoride (F). In 10-day experiments, subsurface lesion formation was partly inhibited by 1 mg/l F and completely by 2 and 5 mg/l F, but small lesions formed in 2 mg/l F in 21-day experiments. Analysis of the spent media, together with analogous, bacteria-free experiments, suggested that lesion inhibition involved two main effects. First, inhibition of bacterial acid production reduced the pH fall, resulting in reduced undersaturation with respect to hydroxyapatite and consequently reduced rate of demineralization. Secondly, interaction of F with enamel mineral resulted in a small increase in reprecipitation during periods of supersaturation and a much larger reduction in demineralization during periods of undersaturation. It is concluded that, at low F concentrations, inhibition of bacterial acid production is a major factor in lesion inhibition, which may contribute significantly to caries prevention in vivo where plaque fluid F levels are raised by frequent topical applications.

Defluoridation of water at high pH with use of brushite, calcium hydroxide, and bone char

The aim of this study was to improve the efficiency of the bone-char method of water defluoridation by pre-treating the water with brushite and calcium hydroxide. Various amounts of brushite, calcium hydroxide, and bone char were suspended batchwise in 100 mL of distilled water containing 0.53 mmol/L fluoride for 24 h under gentle agitation. At suitable intervals, pH and the concentrations of fluoride, calcium, and phosphate in the water were determined and, when possible, the degrees of saturation with respect to brushite, hydroxyapatite, and fluorapatite calculated. Bone char used alone took up fluoride slowly and inefficiently. The addition of brushite and calcium hydroxide resulted in high concentrations of calcium and phosphate, making the solutions highly supersaturated with respect to fluorapatite, and led to a 20-fold increase in fluoride removal from the water. The combined use of all three salts left low concentrations of phosphate in solution and optimized the fluoride uptake capacity. Repeated use of the same bone char for 18 consecutive runs demonstrated that uptake of fluoride by the bone char was improved by repeated use, provided that brushite and calcium hydroxide were added. Therefore, addition of the two salts to the water may prolong the life of the bone char indefinitely, ensure the removal of fluoride, and thus avoid the problem of determining when the bone char is exhausted. In conclusion, we show that the bone-char defluoridation technique can be improved by addition of brushite and calcium hydroxide to the water. The problem of high terminal pH remains, however, and further work is required to improve potability.