Ion Activity Product Research Articles

Effect of bacteria and salinity on calcium sulfate precipitation

Membrane and thermal desalination are plagued by composite fouling because of inorganic, biological and organic constituents. Interactive effects in composite fouling, though of paramount practical importance, are mostly ignored. This paper presents interactive effects of biological matter, Pseudomonas Fluorescence (PF), on precipitation of calcium sulfate, a sparingly soluble salt and an inorganic foulant. Experiments were conducted in isothermal (25°C) batch tests. The effect of PF and its concentration on CaSO4 precipitation and deposit structure at varying salinities were determined. The Pitzer model, suitable for high salinities, is used in calculation of instantaneous CaSO4's ion activity products. The Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) show that salinity and concentration levels alter the crystal structure but not the crystal phase (gypsum). Under the influence of PF, crystallization of CaSO4 did not occur and sulfur levels significantly increased; it can be surmised that microbial activity of PF reduced the sulfate ions to elemental sulfur and altered precipitation of CaSO4. This does not suggest that biological fouling or presence of bacteria generally inhibits precipitation fouling of calcium sulfate; it underlines that the generally ignored interactive effects are of significant importance and should be considered as corner stone in advancement of fouling research.

Solubility of pyromorphite Pb 5(PO 4) 3Cl–mimetite Pb 5(AsO 4) 3Cl solid solution series

Pyromorphite Pb 5(PO 4) 3Cl and mimetite Pb 5(AsO 4) 3Cl are isostructural minerals with apatite. Due to their high environmental stability, they have gained considerable attention as metals sequestration agents in water treatment and contaminated soil remediation. Pyromorphite and mimetite can form a continuous solid solution series in near-Earth surface environments. Precipitation of the end members and intermediate members of the series is likely to occur in the areas where the cost-effective in situ immobilization reclamation method, based on phosphate amendments, is applied. In contrast to the widely studied thermodynamic parameters of pyromorphite and mimetite, knowledge of the thermodynamics of their solid solutions is sparse. To supplement the data, a number of compounds from the pyromorphite–mimetite series were synthesized at room temperature using a method to simulate the conditions in the near-Earth surface environments. Afterwards, batch dissolution and dissolution–recrystallization experiments of seven synthesized precipitates were conducted at 25 °C, pH = 2 and in a 0.05 M KNO 3 background electrolyte. The experiments were carried out for a period of 6 (dissolution) and 14 (dissolution–recrystallization) months. A plateau in the [Pb] evolution patterns was used to determine equilibrium. All seven dissolutions were congruent, and the ionic activity products (IAP) of the minerals from the pyromorphite–mimetite solid solution series were calculated based on the dissolution reaction: Pb 5 ( PO 4 ) n ( AsO 4 ) 3 - n Cl ↔ 5 Pb 2 + + n PO 4 3 - + ( 3 - n ) AsO 4 3 - + Cl - . The IAPs for pyromorphite and mimetite exhibit a significant difference in values over three orders of magnitude between approximately 10 −79 for pyromorphite and approximately 10 −76 for mimetite. The series appeared to be ideal, and Lippmann and Roozboom diagrams were used for better understanding of its thermodynamics. The results indicated a strong tendency of pyromorphite to partition into the solid phase in the series, which explains some of the naturally observed phenomena. The improvement of the lattice stability of the mimetite due to isostructural phosphate substitutions in anionic sites was observed. The thermodynamic data reported in this study supplement existing databases used in geochemical modeling.

A hypothesis of calcium stone formation: an interpretation of stone research during the past decades

An interpretation of previous and recent observation on calcium salt crystallization and calcium stone formation provide the basis for formulation of a hypothetical series of events leading to calcium oxalate (CaOx) stone formation in the urinary tract. The various steps comprise a primary precipitation of calcium phosphate (CaP) at high nephron levels, establishment of large intratubular and/or interstitial (sub-epithelial) aggregates of CaP. These crystal masses subsequently might be dissolved during periods with low urine pH. On the denuded surface of subepithelial or intratubularly trapped CaP, release of calcium ions can result in very high ion-activity products of CaOx, particularly during simultaneous periods with peaks of CaOx supersaturation. Crystals of CaOx may result from nucleation in the macromolecular environment surrounding the apatite crystal phase. In the presence of low pH, low citrate and high ion-strength of urine, formation of large CaOx crystal masses can be accomplished by self-aggregation of Tamm-Horsfall mucoprotein. Following dislodgment of the initially fixed CaOx stone embryo, the further development into to clinically relevant stone is accomplished by CaOx crystal growth and CaOx crystal aggregation of the retained stone material. The latter process is modified by a number of inhibitors and promoters present in urine. The retention of the stone is a consequence of anatomical as well as hydrodynamic factors.

Microbial excavation of solid carbonates powered by P-type ATPase-mediated transcellular Ca 2+ transport

Some microbes, among them a few species of cyanobacteria, are able to excavate carbonate minerals, from limestone to biogenic carbonates, including coral reefs, in a bioerosive activity that directly links biological and geological parts of the global carbon cycle. The physiological mechanisms that enable such endolithic cyanobacteria to bore, however, remain unknown. In fact, their boring constitutes a geochemical paradox, in that photoautotrophic metabolism will tend to precipitate carbonates, not dissolve them. We developed a stable microbe/mineral boring system based on a cyanobacterial isolate, strain BC008, with which to study the process of microbial excavation directly in the laboratory. Measurements of boring into calcite under different light regimes, and an analysis of photopigment content and photosynthetic rates along boring filaments, helped us reject mechanisms based on the spatial or temporal separation of alkali versus Acid-generating metabolism (i.e., photosynthesis and respiration). Instead, extracellular Ca(2+) imaging of boring cultures in vivo showed that BC008 was able to take up Ca(2+) at the excavation front, decreasing the local extracellular ion activity product of calcium carbonate enough to promote spontaneous dissolution there. Intracellular Ca(2+) was then transported away along the multicellular cyanobacterial trichomes and excreted at the distal borehole opening into the external medium. Inhibition assays and gene expression analyses indicate that the uptake and transport was driven by P-type Ca(2+)-ATPases. We believe such a chemically simple and biologically sophisticated mechanism for boring to be unparalleled among bacteria.

Extension of a PBPK model for ethylene glycol and glycolic acid to include the competitive formation and clearance of metabolites associated with kidney toxicity in rats and humans

A previously developed PBPK model for ethylene glycol and glycolic acid was extended to include glyoxylic acid, oxalic acid, and the precipitation of calcium oxalate that is associated with kidney toxicity in rats and humans. The development and evaluation of the PBPK model was based upon previously published pharmacokinetic studies coupled with measured blood and tissue partition coefficients and rates of in vitro metabolism of glyoxylic acid to oxalic acid, glycine and other metabolites using primary hepatocytes isolated from male Wistar rats and humans. Precipitation of oxalic acid with calcium in the kidneys was assumed to occur only at concentrations exceeding the thermodynamic solubility product for calcium oxalate. This solubility product can be affected by local concentrations of calcium and other ions that are expressed in the model using an ion activity product estimated from toxicity studies such that calcium oxalate precipitation would be minimal at dietary exposures below the NOAEL for kidney toxicity in the sensitive male Wistar rat. The resulting integrated PBPK predicts that bolus oral or dietary exposures to ethylene glycol would result in typically 1.4–1.6-fold higher peak oxalate levels and 1.6–2-fold higher AUC's for calcium oxalate in kidneys of humans as compared with comparably exposed male Wistar rats over a dose range of 1–1000mg/kg. The converse (male Wistar rats predicted to have greater oxalate levels in the kidneys than humans) was found for inhalation exposures although no accumulation of calcium oxalate is predicted to occur until exposures are well in excess of the theoretical saturated vapor concentration of 200mg/m3. While the current model is capable of such cross-species, dose, and route-of-exposure comparisons, it also highlights several areas of potential research that will improve confidence in such predictions, especially at low doses relevant for most human exposures.

Characteristic clinical and biochemical profile of recurrent calcium-oxalate nephrolithiasis in patients with metabolic syndrome

Metabolic syndrome is a risk factor for nephrolithiasis. This study was performed to evaluate the clinical and biochemical profile of calcium-oxalate nephrolithiasis in stone formers with metabolic syndrome. A total of 526 recurrent stone formers, 184 of them with metabolic syndrome, and 214 controls were examined on a free diet and after a sodium-restricted diet (sodium intake < 100 mmol/24 h). On free diet, stone formers with metabolic syndrome showed higher sodium excretion [mean (95% confidence interval), 196 (176-218) vs 160 (150-168) mmol/24 h; P < 0.01] and lower citrate excretion [2.23 (1.99-2.58) vs 2.84 (2.51-3.17) mmol/24 h; P < 0.01] compared to controls, whereas stone formers without metabolic syndrome showed higher calcium and oxalate excretion [5.43 (5.01-5.82) vs 3.58 (2.84-4.19) and 0.34 (0.32-0.36) vs 0.26 (0.20-0.31)m mmol/24 h for calcium and oxalate, respectively; P < 0.01] and lower citrate excretion [2.18 (1.98-2.38) vs 2.84 (2.51-3.17) mmol/24 h; P < 0.01] compared to controls. The ion activity product of urinary calcium-oxalate salts was similar between stone formers with and without metabolic syndrome [1.41 (1.31-1.59) vs 1.40 (1.35-1.45); P > 0.05]. After the test diet, this index was lower in diet-compliant stone formers with metabolic syndrome compared to diet-compliant stone formers without metabolic syndrome [1.15 (1.10-1.21) vs 1.39 (1.31-1.45); P < 0.01]. The biochemical profiles and responses to the sodium-restricted diet were significantly different between stone formers with metabolic syndrome and those without. Dietary habits play a central role in the pathogenesis of nephrolithiasis in stone formers with metabolic syndrome.

Speciation and supersaturation model in papermaking streams

Environmental responsibility of papermaking industry leads to intensive reuse of water streams. As well known in chemical engineering, this closure of loops causes the accumulation of soluble species, including calcium ions and inorganic carbon. The result is increased scaling, mainly by calcium carbonate. Due to complex behaviour, predicting scaling in papermaking streams is not straightforward and process water disposal should be kept at a high level in order to prevent scaling. This is contradictory with the objective of preserving the environment. Calcium carbonate precipitation is considered as complex since: - speciation calculation must take into account several complexes and protonated species ( CaC O 3 0 , CaHC O 3 + , CaOH + , HC O 3 − , H 2 C O 3 ) in addition to Ca 2+ and C O 3 2 − ; - several polymorphs can precipitate; - the nucleation requires a rather high supersaturation to occur in homogeneous way. We combined a mass balance approach using the CTP proprietary software PS2000 (Ruiz et al., 2003), specially developed for the simulation of actual papermaking plants and the USGS software PHREEQC (Parkhurst and Appelo, 1999), devoted to the speciation in complex aqueous solutions, to predict the local value of the Ionic Activity Product (I.A.P.) in the nodes of paper plants streams. These data were analysed using the results and methodology by Elfil and Roques (2004), comparing the I.A.P. with the solubility products of the three relevant polymorphs (anhydrous calcium carbonate, monohydrate calcium carbonate, calcite) to predict the risk of scaling. A campaign of analysis on an industrial site permitted to benchmark the calculation.

Formation of Caries-Like Lesions in vitro on the Root Surfaces of Human Teeth in Solutions Simulating Plaque Fluid

Lesion formation on root surfaces of human posterior teeth was studied in acetate/lactate buffers with a background electrolyte composition based on plaque fluid analyses. Lesion depth after 28 days at 37°C was measured in relation to: the presence or absence of cementum; the concentration of undissociated buffer; the presence or absence of magnesium ions at plaque fluid concentration. Each factor was evaluated at several values of –log(ion activity product for hydroxyapatite): pI_HA. Solutions were formulated to minimize variation in pH, which varied by ≤0.03 for a given comparison (individual pI_HA) and by 0.42–0.82 over the range of pI_HA within experiments. Lesions on surfaces from which cementum had been ground were significantly deeper than on intact surfaces, but this is considered to be due to subsurface mechanical damage and not to a solubility difference. Neither the concentration of undissociated buffer nor the presence of magnesium ions significantly affected lesion depth. Lesion depth was strongly influenced by the correlated variations in pI_HA and pH. At pI_HA 54 and 55, only extremely shallow lesions formed. From pI_HA 56, lesion depth increased with increasing pI_HA. The results confirm that the solubility of the mineral of root tissues is higher than that of hydroxyapatite, but indicate that it is probably lower than suggested by Hoppenbrouwers et al. [Arch Oral Biol 1987;32:319–322]. For calcium concentrations of 3–12 mM, the critical pH for root tissue mineral was calculated as 5.22–5.66 assuming solubility equivalent to pI_HA 54 and 5.08–5.51 assuming pI_HA 55.

A minimally destructive technique for removing the smear layer from dentine surfaces

Objectives To develop a minimally destructive technique for removing the smear layer produced by cutting and polishing specimens of dentine prepared for use in experimental studies, e.g. on occlusion of dentinal tubules by oral health products. The aim was to avoid the damage caused by conventional techniques utilising short exposures to solutions with very low pH. Methods Two acetate buffers, pH 5.5, containing different concentrations of calcium and phosphate, with −log(ion activity product with respect to hydroxyapatite) (p I HA) of 55 or 56, were tested on slices of dentine using scanning electron microscopy (SEM). Results A solution which, from previous work, was slightly undersaturated with respect to dentine mineral, with a p I HA of 56, was found to remove smear layers produced by cutting and/or polishing after 15 min. However, to reliably remove debris occluding the tubules an exposure time of 2 h, followed by brief ultrasonication, was necessary. After 2 h treatment with this buffer, only a small amount of demineralization of the surface was detectable by SEM, while calcium and phosphorus were detectable by X-ray dispersive spectroscopy. Conclusion It is possible to remove smear layers, and to open dentinal tubules, by a reasonably short exposure to an acidic buffer which is undersaturated with respect to dentine mineral.

Predicting the occurrence of iron chlorosis in grapevine with tests based on soil iron forms

&lt;p style="text-align: justify;"&gt;&lt;strong&gt;Aims&lt;/strong&gt;: Iron deficiency symptoms (leaf chlorosis and depressed growth) are common in grapevine growing on calcareous soils. The objective of this study was to investigate the relationships between these symptoms and the properties of Spanish vineyard soils.&lt;/p&gt;&lt;p style="text-align: justify;"&gt;&lt;strong&gt;Methods and results&lt;/strong&gt;: Soils from thirty vineyards with leaf chlorosis were used to pot '110 Richter' rootstock for three seasons. Mean leaf chlorophyll concentration, as measured via SPAD, was positively correlated with the soil content in poorly crystalline Fe oxides, as estimated by extraction with various reagents, and negatively correlated with the contents in calcium carbonate equivalent and active lime. Iron deficiency affected plant growth but none of the measured growth variables was correlated with soil properties. The relationships between SPAD and acid ammonium oxalate-, citrate/ascorbateand unbuffered hydroxylamine-extractable Fe conformed to the linear-plateau model, from which clear-cut critical levels (345, 425 and 8 mg kg&lt;sup&gt;-1&lt;/sup&gt;, respectively) could be established. This was not possible with tests based on the alkalinity properties (e.g. the calcite ion activity product), the IPC (« Indice du pouvoir chlorosant »), and the DTPA-extractable Fe test, which generally exhibited a poor predictive value.&lt;/p&gt;&lt;p style="text-align: justify;"&gt;&lt;strong&gt;Conclusion&lt;/strong&gt;: Tests based on reagents capable of extracting, in part or quantitatively, the most reactive forms of soil Fe were useful to estimate the risk of Fe chlorosis in grapevine. Acid ammonium oxalate, citrate/ascorbate and unbuffered hydroxylamine were the best extractants for this purpose.&lt;/p&gt;&lt;p style="text-align: justify;"&gt;&lt;strong&gt;Significance and impact of study&lt;/strong&gt;: This study has shown the limited usefulness of tests based on the contents and reactivity of the soil carbonate to predict the occurrence of Fe chlorosis in grapevine; tests capable of estimating the contents of the labile soil Fe forms constitute the best alternative.&lt;/p&gt;

Calcite and gypsum solubility products in water‐saturated salt‐affected soil samples at 25°C and at least up to 14 dS m−1

Calcite and gypsum are salts of major ions characterized by poor solubility compared with other salts that may precipitate in soils. Knowledge of calcite and gypsum solubility products in water‐saturated soil samples substantially contributes to a better assessment of processes involved in soil salinity. The new SALSOLCHEMIS code for chemical equilibrium assessment was parameterized with published analytical data for aqueous synthetic calcite and gypsum‐saturated solutions. Once parameterized, SALSOLCHEMIS was applied to calculations of the ionic activity products of calcium carbonate and calcium sulphate in 133 water‐saturated soil samples from an irrigated salt‐affected agricultural area in a semi‐arid Mediterranean climate. During parameterization, sufficiently constant values for the ionic activity products of calcium carbonate and calcium sulphate were obtained only when the following were used in SALSOLCHEMIS: (i) the equations of Sposito &amp; Traina for the free ion activity coefficient calculation, (ii) the assumption of the non‐existence of the Ca (HCO 3)+ and CaCO3o ion pairs and (iii) a paradigm of total ion activity coefficients. The value of 4.62 can be assumed to be a reliable gypsum solubility product (pKs) in simple aqueous and soil solutions, while a value of 8.43 can only be assumed as a reliable calcite solubility product (pKs) in simple aqueous solutions. The saturated pastes and saturation extracts were found to be calcite over‐saturated, with the former significantly being less so (p IAP = 8.29) than the latter (p IAP = 8.22). The calcite over‐saturation of saturated pastes increased with the soil organic matter content. Nevertheless, the inhibition of calcite precipitation is caused by the soluble organic matter from a dissolved organic carbon threshold value that lies between 7 and 12 mm. The hypothesis of thermodynamic equilibrium is more adequate for the saturated pastes than for the saturation extracts.

Initial stages of weathering and soil formation in the Morteratsch proglacial area (Upper Engadine, Switzerland)

Investigations in Alpine soils indicate that mineral weathering is much faster in ‘young’ soils (< 1000 yr) than in ‘old’ soils (∼ 10,000 yr). However, little is known about the initial stages of weathering and soil formation, i.e. during the first decades of soil genesis. In this study we investigated rock-forming minerals weathering at very early stages of soil formation. Due to the continuous retreat of the Morteratsch glacier (Upper Engadine, Swiss Alps), the proglacial area offers a full time sequence from 0 to 150 yr old surfaces. A low slope and the absence of glacier which might have interrupted soil formation processes, contributed to the choice of the Morteratsch proglacial valley for this case study. The area is well documented regarding vegetation and soils. The tectonic unit is the Bernina-crystalline, which is mainly constituted of granitoid rocks. Consequently, the glacial till has an acidic character. Mineralogical measurements were carried out on the soil fraction < 2 mm using XRD and DRIFT for qualitative and quantitative phase analysis. In addition, chemical analyses of the stream water from the main channel, tributaries and of rainwater were performed with a special focus on Ca/Sr and Sr isotope ratios ( 87Sr/ 86Sr). Furthermore, the accumulation of organic matter within the time sequence and physical soil properties were measured. Decreasing grain size with time shows active physical weathering processes. Soil organic matter has been accumulated during 150 yr at very high rates. Special emphasis has been given to chemical weathering and to the formation and transformation mechanisms of minerals. Of special interest were biotite, chlorite, epidote, plagioclase and calcite. Biotite has been continuously transformed into illite-like components. Within 150 yr, the concentration of epidote significantly decreased. The high Ca/Sr as well as 87Sr/ 86Sr ratios in the stream and spring waters confirmed that Ca bearing minerals are weathering and transforming at very high rates in the proglacial area. Also in cryic, ice-free environments, chemical weathering rates are high leading to the formation and transformation of minerals. Disseminated calcite in granitoid rocks, not confined to sedimentary carbonate rocks, also plays a role in subglacial environments. It is, however, not known for how long such an influence is significant and measurable. The high Ca/Na and Ca/Sr ratio in the stream and tributary waters showed that calcite contributes to the supply of soluble Ca, although the ion activity product calculations clearly demonstrated that the waters were undersaturated with respect to this mineral.

Nucleation and Polymorphism of Calcium Carbonate by a Vapor Diffusion Sitting Drop Crystallization Technique

The nucleation and polymorphism of calcium carbonate have been studied using a microdevice named the crystallization mushroom. This setup allows carrying out precipitation experiments reproducibly by the vapor diffusion sitting drop technique. Within the range of concentrations investigated (from 10 to 500 mmol/L CaCl2 and from 1 to 25 mmol/L NH4HCO3), the dominant polymorph to appear first in the drops was calcite, or mixtures of calcite and vaterite followed by aragonite. Additionally, amorphous calcium carbonate (ACC) was not observed. The order of appearance of the polymorphs in the droplets is explained by intrinsic features of the crystallization mushroom, that is, the slow increase in the ionic activity product caused by slow diffusion of NH3 and CO2 gases, which favors the least soluble phase calcite to crystallize before other more soluble polymorphs. The appearance of calcite as the first nucleating dominant polymorph in the drops allowed us to calculate its surface free energy from induction ti...

Phosphorus availability in boreal forest soils: A geochemical and nutrient uptake modeling approach

Changes in soil surface charge and phosphorus (P) sorption capacity of soils may affect the availability of P in soils. We investigated how changes in soil properties in the Bear Brook Watershed in Maine (BBWM), which is a long-term paired-watershed experimental acidification study, may affect P availability using the Cushman–Barber plant nutrient uptake simulation model. Both NH 4Cl- and water-extractable P and Al in mineral soils were significantly related to soil organic matter content. The results from this study suggest that P solubility in the BBWM soils is controlled through the formation of amorphous aluminum phosphate (Al(OH) 2H 2PO 4) with a log ion activity product (IAP) between − 28.6 and − 28.8. Surface charge titration and batch P adsorption experiments were conducted on mineral soils from BBWM. The surface charge titration curve was fit to a three-site Langmuir model with the Type 1 site having an average OH − binding strength log K 1 of 8.7 and 12% of the surface charge, Type 2 site with log K 2 of 6.0 and 23% of the surface charge, and the Type 3 site with a log K 3 of 4.2 and 65% of the surface charge. The Cushman–Barber simulation model predicted a 15% and 31% increase in P availability in the acid-treated watershed soils for the softwood and hardwood stands, respectively. This was primarily due to increased water-soluble P content in these soils, suggesting that soil acidification leads to increased P availability to plants. The results from this study clearly demonstrate the importance of the Al–P–organic matter interaction in determining the level of soluble P in soils, a key determinant of P bioavailability to plants.

Alendronate Reduces the Excretion of Risk Factors for Calcium Phosphate Stone Formation in Postmenopausal Women with Osteoporosis

Objective: Osteoporosis is associated with the pathogenesis and risk of urolithiasis, which is higher among postmenopausal women (as opposed to premenopausal). Bisphosphonates potently inhibit bone resorption, and are used in the management of bone disease. We investigated the ability of a bisphosphonate to prevent calcium stone formation. Methods: We studied 12 postmenopausal women (63.8 ± 7.3 years) who were not receiving osteoporosis therapy, and had stones comprised of calcium phosphate (CaP; n = 3), calcium oxalate (CaOx; n = 3) and CaP + CaOx (n = 6). We measured bone mineral density (BMD), serum and urinary values in 24-hour urine specimens before and 3 months after the oral administration of 5 mg/day of alendronate (ALN). The indexes of the ionic activity product of calcium oxalate, AP(CaOx), and of calcium phosphate, AP(CaP), were estimated using the Tiselius method. Results: ALN significantly reduced the AP(CaP) index (1.53 ± 1.37 to 0.89 ± 0.81, p <0.05). Urinary calcium, oxalate, phosphate and the AP(CaOx) index did not significantly change. BMD improved in 11 of the 12 patients. Urinary stones did not develop in any of the patients during the course of the study. Conclusion: The results suggested that ALN not only improves BMD and osteoporosis, but also reduces the risk of calcium phosphate stone formation in postmenopausal women.

Comparison of Phosphate Materials for Immobilizing Cadmium in Soil

A study was conducted to compare the effects of phosphate (P) materials in reducing cadmium extractability. Seven P materials (commercial P fertilizers--fused phosphate (FP), 'fused and superphosphate' [FSP], and rock phosphate [RP]; P chemicals--Ca[H(2)PO(4)](2).H(2)O, [NH(4)](2)HPO(4), KH(2)PO(4), and K(2)HPO(4)) were selected for the test. The selected P source was mixed with Cd-contaminated soil at the rate of 0, 200, 400, 800, and 1,600 mg P kg(-1) under controlled moisture conditions at 70% of water holding capacity, then incubated for 8 weeks. FP, Ca(H(2)PO(4))(2) H(2)O, KH(2)PO(4), and K(2)HPO(4) significantly decreased NH(4)OAc-extractable Cd (plant-available form) concentrations with increasing application rates. Compared to other phosphate materials used, K(2)HPO(4) was found to be the most effective in reducing the plant-available Cd concentration in soil, mainly due to the negative charge increase caused by soil pH and phosphate adsorption. Contrary to the general information, FSP and (NH(4))(2)HPO(4) increased Cd extractability at low levels of P application (<400 mg kg(-1)), and thereafter Cd extractability decreased significantly with increasing application rate. RP scarcely had an effect on reducing Cd extractability. Ion activity products of CdHPO(4), Cd(OH)(2), and CdCO(3) analyzed by the MINTEQ program were significantly increased by K(2)HPO(4) addition, but the effect of Cd-P compound formation on reducing Cd extractability was negligible. Conclusively, the P-induced alleviation of Cd extractability can be attributed primarily to Cd immobilization due to the increase in soil pH and negative charge rather than Cd-P precipitation, and therefore, alkaline P materials such as K(2)HPO(4) are effective for immobilizing soil Cd.

Feldspar dissolution kinetics and Gibbs free energy dependence in a CO 2-enriched groundwater system, Green River, Utah

Reactions between CO 2-charged brines and reservoir minerals might either enhance the long-term storage of CO 2 in geological reservoirs or facilitate leakage by corroding cap rocks and fault seals. Modelling the progress of such reactions is frustrated by uncertainties in the absolute mineral surface reaction rates and the significance of other rate limiting steps in natural systems. Here we use the chemical evolution of groundwater from the Jurassic Navajo Sandstone, part of a leaking natural accumulation of CO 2 at Green River, Utah, in the Colorado Plateau, USA, to place constraints on the rates and potential controlling mechanisms of the mineral–fluid reactions, under elevated CO 2 pressures, in a natural system. The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. The mineral reactions are close to stoichiometric with plagioclase and K-feldspar dissolution largely balanced by precipitation of clay minerals and carbonate. Mineral modes, in conjunction with published surface area measurements and flow rates estimated from hydraulic head measurements, are then used to quantify the kinetics of feldspar dissolution. Maximum estimated dissolution rates for plagioclase and K-feldspar are 2 × 10 − 14 and 4 × 10 − 16 mol m − 2 s − 1 , respectively. Fluid ion-activity products are close to equilibrium (e.g. Δ G r for plagioclase between − 2 and − 10 kJ/mol) and lie in the region in which mineral surface reaction rates show a strong dependence on Δ G r. Local variation in Δ G r is attributed to the injection and disassociation of CO 2 which initially depresses silicate mineral saturation in the fluid, promoting feldspar dissolution. With progressive flow through the aquifer feldspar hydrolysis reactions consume H + and liberate solutes to solution which increase mineral saturation in the fluid and rates slow as a consequence. The measured plagioclase dissolution rates at low Δ G r of 2 × 10 − 14 mol m − 2 s − 1 would be compatible with far-from-equilibrium rates of ~ 1 × 10 − 13 mol m − 2 s − 1 as observed in some experimental studies. This suggests that the discrepancy between field and laboratory reaction rates may in part be explained by the differences in the thermodynamic state of natural and experimental fluids, with field-scale reactions occurring close to equilibrium whereas most laboratory experiments are run far-from-equilibrium.

X-ray Absorption Near Edge Structure Study of Lead Sorption on Phosphate-Treated Kaolinite

Presorbed phosphate significantly increases Pb sorption on the phyllosilicate kaolinite in the pH range from 4to 8. The sorbed Pb-to-P molar ratios over this pH range stray little from the molar ratio found in the mineral pyromorphite, suggesting sorbed phosphate reacts with soluble Pb to form a surface precipitate similar to pyromorphite. X-ray absorption near edge structure (XANES) studies at the Pb L3-edge support this interpretation. In particular, the fine structure of first-derivative Pb L3-edge XANES spectra of Pb species sorbed to phosphate-treated kaolinite samples covering a pH range extending from 4 to 10 match the fine structure of the spectrum of pyromorphite. Although the productis structurally and compositionally similarto pyromorphite, the ion activity product in the pH range 4-6 was undersaturated with respect to the solubility product of pyromorphite.

Crystalline behaviors of hydroxyapatite in the neutralized reaction with different citrate additions

Ca(OH) 2 and H 3PO 4 were used to synthesize nano-sized hydroxyapatite. In the process of acid–base reaction, citric acid or sodium citrate was added to influence the crystallinity of hydroxyapatite. Formation of Ca-citrate complex resulted in variations of ionic concentrations and ionic activity products for solutions with different citrate additions. The degree of supersaturate was found higher for solutions with less citric acid or more sodium citrate addition. Consequently, hydroxyapatite particles with smaller size were gained. These crystalline behaviors were investigated and explained by mechanism of crystal nucleation and growth in solutions.

Bone-like apatite layer formation on the new resin-modified glass-ionomer cement

In this study, the apatite-forming ability of the new resin-modified glass-ionomer cement was evaluated by soaking the cement in the simulated body fluid. The Fourier Transform Infrared (FTIR) spectrometer and X-Ray Diffraction (XRD) patterns of the soaked cement pointed to the creation of poorly crystalline carbonated apatite. It was found that the releasing of calcium ions from the soaked cement will dominate the undesirable effect of polyacrylic acid on apatite formation. Consequently, the ionic activity products (IAPs) of the apatite in the surrounding medium increased which accelerated apatite nucleation induced by the presence of the Si-OH and COOH groups. Accordingly, the apatite nuclei started to form via primary heterogeneous nucleation and continued by secondary nucleation. Therefore, nucleation and growth occurs as in the layer-by-layer mode so that finite numbers of monolayers are produced. Subsequent formation of film occurs by formation of discrete nuclei (layer-plus-island or SK growth).