Chemical Speciation Program Research Articles

Comparison and modelling of extraction methods to assess agronomic effectiveness of fertilizer zinc

AbstractTotal Zn in fertilizer is a poor predictor of Zn availability in granular fertilizers. In this study, we compared different extraction methods using Zn fertilizers for which the agronomic effectiveness had previously been determined. Sixteen fertilizers were extracted by eight different procedures using five extractant solutions: water, 1 M ammonium acetate (pH 7.0), 74 mM EDTA (pH 7.0), 5 mM DTPA + 0.1 M MES (pH 6.2), and 0.12 M bis‐tris (pH 6). Modelling of Zn solubility and speciation was carried out with a chemical speciation program (Visual Minteq). The predicted solubilities were in good agreement with the observations, indicating chemical equilibrium. The three methods using pure water were highly correlated with each other and showed the strongest correlation with agronomic effectiveness (r = 0.81–0.90). The methods using extractants with strong pH buffering or high chelating capacity showed no or weak correlation with the agronomic effectiveness. Thus, water‐only based methods give the best indication of the availability of Zn in granular fertilizers, and are best suited for regulation or labeling of available Zn in fertilizer.

The Effect of Metal Concentration on the Parameters Derived from Complexometric Titrations of Trace Elements in Seawater—A Model Study

In this study we examine the impact of dissolved metal concentrations on the parameters that are commonly determined from complexometric titrations in seawater. We use the non-ideal competitive adsorption (NICA) model within the framework of the chemical speciation program visual MINTEQ with iron as a model metal. We demonstrate that dissolved iron concentrations effect the determined parameters for a heterogeneous binding site distribution with a fixed concentration of dissolved organic carbon. The commonly used terms ‘ligand concentration’ and ‘binding constant’ are therefore dependent on metal concentration, so we adopt the terminology suggested by Town and Filella (Limnology and Oceanography, 45, 1341-1357, 2000) and use the terms ligand quotient and stability quotient here. The systematic increase in the ligand quotient with dissolved iron concentration likely contributes towards the trend of increasing ligand quotient with dissolved iron concentration observed in field studies, and makes it hard to assign an objective meaning to the parameter. We suggest that calculation of the side reaction coefficient, a parameter that describes the probability that any added metal will be complexed, could be less prone to bias and misinterpretation than calculation of conditional stability and ligand quotients. We explore the impact of experimental design on side reaction coefficients by applying different detection windows, and multiwindow and reverse titration approaches. We identify the method that results in the best estimates of side reaction coefficients over a range of iron concentrations between 0.1 and 1.5 nmol L-1. We find that single window titrations can only reliably estimate side reaction coefficients over a limited range of iron concentrations. Multiwindow titrations provided estimates of side reaction coefficients within the 99 % confidence interval of the values calculated directly from the NICA model at all iron concentrations examined here. We recommend that future reports of speciation measurements consider the potential influence of metal concentrations on the determined parameters and future studies focus on developing and applying experimental designs that improve the robustness and rigor of chemical speciation analysis in the marine environment.

Co, Zn and Ag-MOFs evaluation as biocidal materials towards photosynthetic organisms

In the present study, the biocidal activity of three different metal organic frameworks (MOFs) based on Co (Co-SIM1), Zn (Zn-SIM1) and Ag (Ag-TAZ) has been evaluated towards one green alga and two cyanobacteria. These organisms are present in fresh- and seawater and take part in the early stages of the biofouling process. The biocidal activity of these materials was evaluated by measuring chlorophyll a concentration and by inhibition zone testing. After 24h of exposure the three different MOFs caused >50% of chlorophyll a concentration inhibition towards both cyanobacteria, however, although the green alga presented a great sensitivity for Ag-TAZ (reaching 90% of chlorophyll a concentration inhibition), it was much more resistant to the rest of MOFs. Bioavailability of these metals was studied using ICP-MS, the chemical speciation program Visual MINTEQ, and a heavy metal bioreporter bioanalytical tool. We have elucidated that the biocidal activity presented by these MOFs was due to the dissolved metals released from them and more exactly, it depended on the bioavailability presented by these metal ions, which was closely related with the free ion concentration. This article highlights the potential use of different MOFs as biocidal material towards photosynthetic organisms and reveals important differences in the sensitivity between these organisms that should be taken into account in order to increase the biocidal spectrum of these materials.

Chemical speciation code CHEMSPEC(C++)and its applications

We used C++ language to write a new chemical speciation program CHEMSPEC (C++), with graphical interfaces. In this paper, the principles and structures of CHEMSPEC (C++) were briefly described. The steps of operating CHEMSPEC (C++) were introduced in details. At the end, several instances were presented to illustrate the features and capacities of CHEMSPEC (C++). CHEMSPEC (C++) is very convenient to understand and operate. The calculation results of CHEMSPEC (C++) are accurate. This new program can be used for calculating the speciations of some radionuclides in defined water-rock systems.

Simulating calcium salt precipitation in the nephron using chemical speciation

Theoretical modeling of urinary crystallization processes affords opportunities to create and investigate scenarios which would be extremely difficult or impossible to achieve in in vivo experiments. Researchers have previously hypothesized that calcium renal stone formation commences in the nephron. In the present study, concentrations of urinary components and pH ranges in different regions of the nephron were estimated from concentrations in blood combined with a knowledge of the renal handling of individual ions. These were used in the chemical speciation program JESS to determine the nature of the solution complexes in the different regions of the nephron and the saturation index (SI) of the stone-forming salts calcium oxalate (CaOx), brushite (Bru), hydroxyapatite (HAP) and octacalcium phosphate (OCP). The effect of independent precipitation of each of the latter on the SI values of other salts was also investigated. HAP was the only salt which was supersaturated throughout the nephron. All of the other salts were supersaturated only in the middle and distal regions of the collecting duct. Supersaturations were pH sensitive. When precipitation of CaOx, Bru and OCP was simulated in the distal part of the collecting duct, little or no effect on the SI values of the other stone forming salts was observed. However, simulation of HAP precipitation caused all other salts to become unsaturated. This suggests that if HAP precipitates, a pure stone comprising this component will ensue while if any of the other salts precipitates, a mixed CaOx/CaP stone will be formed. Application of Ostwald's Rule of Stages predicts that the mixed stone is likely to be CaOx and Bru. Our modelling demonstrates that precipitation of stone-forming salts in the nephron is highly dependent on the delicate nature of the chemical equilibria which prevail and which are themselves highly dependent on pH and component concentrations.

Chemical speciation code CHEMSPEC and its applications

The adsorption and migration behavior of a radionuclide in geological media heavily depends on its chemical forms in a given chemical environment. In order to predict the temporal and spatial distribution of radionuclides around a disposal site when its canister is damaged, it is necessary to develop coupled chemical speciation-solute transport models and relevant software. For that reason, we wrote a new chemical speciation program CHEMSPEC. In this paper, the principles and structure of CHEMSPEC are briefly described, and the strategy and algorithms that were used in this code are interpreted in some detail, such as the measures adopted to prevent divergence in iteratively solving the mass balance equations, the “predictor-corrector” algorithm for calculation of the number and quantities of solid species formed, and the alternate use of “freezing” and “defreezing” oxidation states in handling of co-existent redox and precipitation equilibria. Four examples are given to illustrate CHEMSPEC’s features and capabilities.

GEOCHEM-EZ: a chemical speciation program with greater power and flexibility

GEOCHEM-EZ is a multi-functional chemical speciation program, designed to replace GEOCHEM-PC, which can only be used on DOS consoles. Chemical speciation programs, such as GEOCHEM and GEOCHEM-PC, have been excellent tools for scientists designing appropriate solutions for their experiments. GEOCHEM-PC is widely used in plant nutrition and soil and environmental chemistry research to perform equilibrium speciation computations, allowing the user to estimate solution ion activities and to consider simple complexes and solid phases. As helpful as GEOCHEM-PC has been to scientists, the consensus was that the program was not very user friendly, was difficult to learn and to troubleshoot, and suffered from several functional weaknesses. To enhance the usability and to address the problems found in GEOCHEM-PC, we upgraded the program with a Java graphical interface, added Help files, and improved its power and function, allowing it to run on any computer that supports Windows XP, Vista or Windows 7.

Immobilization of Zn, Cu, and Pb in contaminated soils using phosphate rock and phosphoric acid

Considerable research has been done on P-induced Pb immobilization in Pb-contaminated soils. However, application of P to soils contaminated with multiple heavy metals is limited. The present study examined effectiveness of phosphoric acid (PA) and/or phosphate rock (PR) in immobilizing Pb, Cu, and Zn in two contaminated soils. The effectiveness was evaluated using water extraction, plant uptake, and a simple bioaccessibility extraction test (SBET) mimicking metal uptake in the acidic environment of human stomach. The possible mechanisms for metal immobilization were elucidated using X-ray diffraction, scanning electron microscopy, and chemical speciation program Visual MINTEQ. Compared to the control, all P amendments significantly reduced Pb water solubility, phytoavailability, and bioaccessibility by 72–100%, 15–86%, and 28–92%, respectively. The Pb immobilization was probably attributed to the formation of insoluble Pb phosphate minerals. Phosphorus significantly reduced Cu and Zn water solubility by 31–80% and 40–69%, respectively, presumably due to their sorption on minerals (e.g., calcite and phosphate phases) following CaO addition. However, P had little effect on the Cu and Zn phytoavailability; while the acid extractability of Cu and Zn induced by SBET (pH 2) were even elevated by up to 48% and 40%, respectively, in the H 3PO 4 treatments (PA and PR + PA). Our results indicate that phosphate was effective in reducing Pb availability in terms of water solubility, bioaccessibility, and phytoavailability. Caution should be exercised when H 3PO 4 was amended to the soil co-contaminated with Cu and Zn since the acidic condition of SBET increased Cu and Zn bioaccessibility though their water solubility was reduced.

Influence of EDDS on Metal Speciation in Soil Extracts: Measurement and Mechanistic Multicomponent Modeling

The use of the [S,S]-isomer of EDDS to enhance phytoextraction has been proposed for the remediation of heavy metal contaminated soils. Speciation of metals in soil solution in the presence of EDDS and dissolved organic matter (DOM) received, however, almost no attention, whereas metal speciation plays an important role in relation to uptake of metals by plants. We investigated the influence of EDDS on speciation of dissolved metals in batch extraction experiments using fourfield-contaminated soils with pH varying between 4.7 and 7.2. Free metal concentrations were determined with the Donnan membrane technique, and compared with results obtained with the chemical speciation program ECOSAT and the NICA-Donnan model using a multicomponent approach. Addition of EDDS increased total metal concentrations in our soil extracts by a factor between 1.1 and 32 (Al), 2.1-48 (Cu), 1.1-109 (Fe), 1.1-5.5 (Ni), and 1.3-17 (Zn). In general, Al, Cu, Fe, and Zn had the largest total concentrations in the EDDS-treated extracts, but the contribution of these metals to the sum of total metal concentrations varied significantly between our soils. Free metal concentrations varied between 7.0 and 8.9 (pCd2+), 3.9-9.9 (pCu2+), 6.3-10.2 (pNi2+), and 5.2-7.0 (pZn2+). Addition of EDDS decreased free metal concentrations by a factor between 1.4 and 1.9 (Cd), 3.4-216 (Cu), 1.3-186 (Ni), and 1.3-3.3 (Zn). Model predictions of free metal concentrations were very good, and predicted values were mostly within 1 order of magnitude difference from the measured concentrations. A multicomponent approach had to be used in our model calculations, because competition between Fe and other metals for binding with EDDS was important. This was done by including the solubility of metal oxides in our model calculations. Multicomponent models can be used in chelant-assisted phytoextraction experiments to predict the speciation of dissolved metals and to increase the understanding of metal uptake by plants.

Biosorption of heavy metals by Fucus spiralis

The sorption uptake of cadmium, nickel, zinc, copper and lead by marine brown alga Fucus spiralis was investigated in bimetallic, trimetallic and multimetallic solutions. The experimental data fitted very well to Langmuir model. In bimetallic systems, the affinity of biomass for lead and copper increased and the sorption uptake of these metals was not affected by increasing concentrations of cadmium, nickel or zinc. However, in solutions with both metals there was a significant mutual decrease of their sorption levels at high concentrations of the other metal. The sorption uptake of cadmium, nickel and copper was investigated in trimetallic aqueous systems. Based on the kinetic parameter b, the affinity of F. spiralis for copper was considerably higher than for cadmium or nickel: b Cd = 6.39, b Ni = 1.82 and b Cu = 17.89. In all tests, the maximum sorption uptake remained practically constant around 1 mmol/g, indicating that the number of active sites on the biomass was limited. Tests with four and five metals showed that copper was preferentially adsorbed. The differences between the experimental sorption data and those given by the chemical speciation program PHREEQCI were negligible. In general, the software used provided satisfactory estimated data for each metal and hence can be a useful tool to predict or simulate the real process.

Geochemical assessment of metal transport in glacial till during electrokinetic remediation

This paper presents the chemical speciation and retention behavior of chromium (Cr), nickel (Ni), and cadmium (Cd) prior to and after the electrokinetic remediation in glacial till soil. The speciation of the metals was predicted using the chemical speciation program MINEQL(+). The simulations were performed for single-contaminant with only Cr(VI) or Ni, and multi-contaminants consisting of: (1) Cr(VI), Ni, and Cd; (2) Cr(III), Ni and Cd; (3) Cr(VI), Cr(III), Ni and Cd; (4) Cr(VI), Ni, and Cd with reducing agents; and (5) Cr(III), Ni, and Cd with oxidizing agent (Mn). The results showed that the speciation and distribution of cationic metals [Ni, Cd, and Cr(III)] in glacial till soil remain unaffected or slightly affected during electrokinetics. This is attributed to the high pH buffering capacity of the glacial till, leading the metals to precipitate in the soil prior to and after electrokinetics. This study showed that during electrokinetics, Cr(VI) existed as anionic complex and migrated towards the anode and the migration is maximum in case of a single-contaminant system. The study also showed that near the anode in the absence of any reducing and oxidizing agent, Cr(VI) mostly adsorbed, and some of Cr(VI) reduced to Cr(III) and migrated towards the cathode and finally precipitated due to high pH conditions. Ni and Cd remain adsorbed or precipitated due to the high pH conditions throughout the soil. Among the reducing agents, the sulfide had significant effect on the migration of metals compared to ferrous ions. While in the presence of oxidizing agent (Mn), no noticeable Cr(VI) was found in the soil sample indicating the reduction of Cr(VI) to Cr(III) and the predominance of reducing conditions due to the presence of naturally occurring iron in the glacial till soil. Overall, this study provides a reasonable explanation of the speciation and distribution of chromium, nickel and cadmium during the electrokinetic remediation of glacial till soil.

Effect of citric acid on aluminum hydrolytic speciation

The mechanisms of the influence of organics on Al transformation were not fully understood. This study investigated the effect of citric acid on Al speciation in partially neutralized aluminum solution. The partially neutralized solution was prepared with 20 mmol L −1 AlCl 3 without citrate (citrate/Al molar ratio of 0, control) or with citrate (citrate to Al molar ratios between 0.1 and 3.0) at pH between 3.0 and 7.0. The nature of aluminum hydrolytic products as influenced by citrate complexation was investigated by turbidity measurement, ferron kinetic color development, peak line width in 27Al nuclear magnetic resonance (NMR) deconvolution demodule quantitative methods, and the MinteqA2 chemical speciation program. Sulfate precipitates from Al solution as influenced by citrate were examined by X-ray diffraction (XRD) analysis and atomic force microscopy (AFM). The turbidity of the Al solution increased with increasing pH values. Increases in citrate/Al molar ratio from 0 to 0.1 decreased dramatically the turbidity due to citrate complexation. The 27Al NMR peak at 6 ppm of the Al solution at a citrate/Al molar ratio of 0.5 shifted to 8 and 10 ppm in the solutions at citrate/Al molar ratio of 1.0 and 3.0, respectively. Comparison of 27Al NMR data and the data obtained from the MinteqA2 chemical speciation program, indicate that the Al-citrate complexes as revealed by 27Al NMR data are largely Al(citrate), AlH(citrate) +, and Al ( citrate ) 2 3 ‐ complexes (99–112%) in the pH range of 4.5–6.5. The non-detected Al fractions by the MinteqA2 program account for 82–99% of the non-detected Al fractions by NMR quantitation in the same pH range. The AFM of sulfate precipitates from solutions with low citrate/Al molar ratios (i.e., 0.01, pH 4.5, aged 40 days) shows that Al 13 sulfate precipitates were ellipse-shaped. These ellipse-shaped precipitates were aggregated when solution pH increased from 4.5 to 7.0 (aged 40 days), indicating the fast hydrolytic rate of Al at high pH. The sulfate precipitates from solution with a high citrate/Al molar ratio (i.e., 0.05, pH 4.5, aged 40 days) also shows aggregate of particles, and XRD non-crystallized precipitates the hampering effect of citrate on Al precipitates.

Differential Protein Mobility of the γ-Aminobutyric Acid, Type A, Receptor α and β Subunit Channel-lining Segments

The gamma-aminobutyric acid, type A (GABAA), receptor ion channel is lined by the second membrane-spanning (M2) segments from each of five homologous subunits that assemble to form the receptor. Gating presumably involves movement of the M2 segments. We assayed protein mobility near the M2 segment extracellular ends by measuring the ability of engineered cysteines to form disulfide bonds and high affinity Zn(2+)-binding sites. Disulfide bonds formed in alpha1beta1E270Cgamma2 but not in alpha1N275Cbeta1gamma2 or alpha1beta1gamma2K285C. Diazepam potentiation and Zn2+ inhibition demonstrated that expressed receptors contained a gamma subunit. Therefore, the disulfide bond in alpha1beta1E270Cgamma2 formed between non-adjacent subunits. In the homologous acetylcholine receptor 4-A resolution structure, the distance between alpha carbon atoms of 20' aligned positions in non-adjacent subunits is approximately 19 A. Because disulfide trapping involves covalent bond formation, it indicates the extent of movement but does not provide an indication of the energetics of protein deformation. Pairs of cysteines can form high affinity Zn(2+)-binding sites whose affinity depends on the energetics of forming a bidentate-binding site. The Zn2+ inhibition IC50 for alpha1beta1E270Cgamma2 was 34 nm. In contrast, it was greater than 100 microM in alpha1N275Cbeta1gamma2 and alpha1beta1gamma2K285C receptors. The high Zn2+ affinity in alpha1beta1E270Cgamma2 implies that this region in the beta subunit has a high protein mobility with a low energy barrier to translational motions that bring the positions into close proximity. The differential mobility of the extracellular ends of the beta and alpha M2 segments may have important implications for GABA-induced conformational changes during channel gating.

Speciation and solubility of Cu, Ni and Pb in contaminated soils

SummaryA long‐term incubation experiment was established to assess the solid ⇅ solution equilibria of (Cu2+), (Pb2+) and (Ni2+) in soil pore water. The experiment comprised 23 soils spiked with 135, 300 and 75 mg kg−1 of Cu, Pb and Ni, respectively, added as nitrate salts in solution. Samples of soil pore water were extracted several times during the incubation period of 818 days and concentrations of Cu, Pb, Ni, dissolved organic carbon and major cations and anions were measured. Similar measurements were carried out on a smaller selection of historically contaminated soils to extend the range of data and assess compatibility of the measurements with the incubated soils. The chemical speciation program ‘WHAM VI’ was used to speciate metal ions in solution. A pH‐dependent Freundlich equation was used to describe free ion activities, p(M2+), for Cu, Pb and Ni using total adsorbed metal, soil pH, soil organic carbon content and ionic strength as determining variables. For all three metals the greatest improvements in model fit were found when metal ions were assumed to be adsorbed exclusively on soil organic carbon rather than on the ‘whole soil’. Further improvements in the description of p(Cu2+) and p(Ni2+) were found when the ionic strength of the soil pore water was included within the model formulation. Residual standard deviations (–log10(M2+) units) for the best‐fit models were 0.36, 0.53 and 0.29 for p(Cu2+), p(Pb2+) and p(Ni2+), respectively. The effects of progressive fixation of Pb, Cu and Ni on model parameterization, during the course of the experiment, were found to be small. Independent datasets from both published and unpublished sources were used to compare experimental protocols and validate the model for the determination of (M2+) in soil pore water.

Development of an equation to relate electrical conductivity to soil and water salinity in a Mediterranean agricultural environment

A linear equation to relate total ionic concentration to electrical conductivity (EC) in natural aqueous systems from low to medium level salt content was derived from the theory of EC in aqueous solutions. Some of the empirical equations used in soil science to characterise the salinity of soil and water based on the EC can be reduced to that equation. Also, it can be seen that aqueous systems with different relative ionic compositions have different values for the regression coefficient that relates EC to total ionic concentration. The equation was validated on the basis of the Faraday constant with some soil saturation extracts and irrigation waters sampled in different places along the Valencian Community (eastern Spain, western Mediterranean basin), and with data from the literature. The validation was performed using the chemical speciation program SOILSOLN. The results of the validation improved when activities were considered instead of concentrations, and in more dilute aqueous systems. Finally, the validation showed that the formation of ion pairs in the aqueous systems analysed was negligible due to the relatively low concentrations of sulfate, magnesium, and calcium.

Predicting the activity of Cd 2+ and Zn 2+ in soil pore water from the radio-labile metal fraction

Cadmium and zinc were added at 3 and 300 mg kg −1, respectively, to 23 soils and incubated at 16°C and 80% field capacity for 818 d. Following addition of metal, changes in the radio-labile concentrations of both elements were examined on seven separate sampling occasions over 818 d. At each sample time, soil pore water was extracted using Rhizon soil solution samplers, and concentrations of Cd, Zn, dissolved organic carbon, and major cations and anions were determined. The chemical speciation program WHAM 6 was used to determine free metal ion activity, (M 2+). Similar measurements were made on a set of historically contaminated soils from old mining areas, sewage sludge disposal facilities, and industrial sources. The two data sets were combined to give a range of values for p(Cd 2+) and p(Zn 2+) that covered 5 and 4 log 10 units, respectively. A pH-dependent Freundlich model was used to predict Zn 2+ and Cd 2+ ion activity in soil pore water. Total and radio-labile metal ion concentration in the solid phase was assumed to be adsorbed on the “whole soil,” humus, or free iron oxides to provide alternative model formats. The most successful models assumed that solubility was controlled by adsorption on soil humus. Inclusion of ionic strength as a model variable provided small improvements in model fit. Considering competition with Ca 2+ and between Zn 2+ and Cd 2+ produced no apparent improvement in model fit. Surprisingly, there was little difference between the use of total and labile adsorbed metal as a model determinant. However, this may have been due to a strong correlation between metal lability and pH in the data set used. Values of residual standard deviation for the parameterized models using labile metal adsorbed on humus were 0.26 and 0.28 for prediction of p(Cd 2+) and p(Zn 2+), respectively. Solubility control by pure Zn and Cd minerals was not indicated from saturation indices. However there may have been fixation of metals to non-radio-labile forms in CaCO 3 and Ca-phosphate compounds in the soils in the higher pH range. Independent validation of the Cd model was carried out using an unpublished data set that included measurements of isotopically exchangeable Cd. There was good agreement with the parameterized model.

Copper speciation and accumulation in the gill microenvironment of carp ( Cyprinus carpio) in the presence of kaolin particles.

Carp were exposed to copper adsorbed on kaolin particles at various concentrations (0-2.4 mg/L), and net accumulation due to elevated adsorbed copper in the surrounding water was observed. Copper speciation in the water and fish gill microenvironment was modeled in the presence and absence of kaolin using a chemical speciation program (MINTEQA2). The adsorption affinity constants of kaolin for copper at various pH values used in the speciation calculations were experimentally determined, and the quantitative relationship between the affinity constant and pH was modeled. Copper accumulations in fish gills exposed to kaolin-adsorbed copper was then observed. The results indicate that desorption occurred in the fish gill microenvironment due to both mucus competition of copper and slight increase in water pH. Furthermore, the available copper species increased as a result of desorption, causing a net accumulation of Cu by the gills.

Coupling the use of computer chemical speciation models and culture techniques in laboratory investigations of trace metal toxicity

The bioavailability and toxicity of a dissolved metal are closely linked to the metal’s chemical speciation in solution. A variety of inorganic and organic ligands are often used in laboratory toxicity tests to control the concentration of labile trace metal in solution. Computerised chemical speciation models based on thermodynamic principles can be used to estimate metal speciation under such experimental conditions. However, these models are sensitive to the quality of their thermodynamic databases. Detailed protocols for the incorporation of reliable equilibrium formation constants into widely available computer chemical speciation programs (e.g., MINEQL+ and MINTEQ) are provided. The examples demonstrate both the benefits and the potential pitfalls involved in the use of chemical speciation models. The application of chemical speciation modelling to metal toxicity studies is discussed and guidelines are proposed for its proper use. Both defined media and chemical speciation programs have co-existed for two decades but the combined use of these techniques has been reserved for those possessing in-depth knowledge of both chemistry and biology. The techniques presented should enable an investigator with basic biological, chemical and computing skills to design an aqueous medium and incorporate correct thermodynamic constants into a computer chemical speciation program, starting from a standardised database, thereby providing a sound framework for critically assessing the biological response of a particular test organism to a given metal.

Solubility and Ion Activity Products of Calcium Phosphate Minerals

AbstractMost phosphate mineral solubility data in use were measured more than 20 yr ago when instrumentation and methods were generally less sophisticated. The objective of this study was to measure calcium phosphate mineral solubilities using modern analytical equipment and techniques. Natural and synthetic Ca phosphate minerals (<50 µm) were equilibrated in water and dilute HCl for 6 mo with continuous agitation. X‐ray diffraction (XRD) was used to identify compositions before and after equilibration. Filtered (<0.2 µm) solution aliquots were analyzed for pH and chemistry. Chemical compositions were determined using inductively‐coupled plasma spectroscopy (ICP). Aqueous carbonates were determined from alkalinity titrations after phosphate alkalinity corrections. The chemical speciation model “Soilchem” was used to calculate free ion concentrations and ionic strengths. Ion activity products (IAPs) were determined from free ion concentrations and single ion activity coefficients. Experimental log IAPs of monetite (CaHPO4) were consistent (−6.60) with published values for brushite (CaHPO4·2H2O) suggesting that the IAPs of these minerals are identical. No change in mineralogy during equilibration was revealed by XRD. Experimental log IAPs of synthetic whitlockite (−30.74, β‐Ca3(PO4)2) indicated somewhat lower solubility than published values. Log IAPs of synthetic hydroxyapatite (−56.02, Ca5(PO4)3OH) were within the range of published values. Natural samples of whitlockite and fluorapatite did not attain equilibrium due to extraneous siderite (FeCO3) and silica phases within the samples. A long equilibration time, modern analytical equipment, and a chemical speciation program can yield more reliable solubility data for phosphates and other minerals.

The chemical behavior of Be, Al, Fe, Ca and Mg during AMS target preparation from terrestrial silicates modeled with chemical speciation calculations

Using the chemical speciation program MINEQL +, we have modeled the chemical speciation of Be and Al during the various steps whereby they are extracted from terrestrial quartz samples for surface exposure dating. The distribution coefficients we used we culled from the literature from reviewed sources (i.e. making our own database) rather than using the constants included in the MINEQL + package. We set-up a formalism whereby we could also apply the program to model the exchange of cations on an exchange resin. With the model we successfully calculated the species distribution of Be and Al in solution in the presence of Cl and F over the whole pH range. Next we investigated the effect of Fe, Ca and Mg on the outcome of the precipitation as well as on the column separation. The presence of such interfering cations is frequently encountered when relatively large quartz mineral separates must be dissolved (i.e. on the order of 50 g). Several points are thus revealed as important during sample preparation: because the presence of F will cause Ca to be precipitated as flourite together with Be- and Al-hydroxide, it is very important that all the SiF4 be fumed off right after total digestion. Once in the sample, Ca cannot be effectively separated from Al using cation exchange. Second, the pH of the first precipitation step is important, values that are too high or too low can cause loss of some of the Be or Al, the latter being more sensitive in this respect. High pH can also cause the inclusion of Mg as brucite. Further, excess cations can cause Be to be eluted off the column earlier than expected. By modeling the chemical speciation of Be and Al, our calculations allow the prediction of the fate and location of Be and Al during sample preparation, as well as revealing the reasons behind such behavior.