Equilibrium Speciation Research Articles

Biogeochemical controls on mercury methylation in the Allequash Creek wetland.

We measured mercury methylation potentials and a suite of related biogeochemical parameters in sediment cores and porewater from two geochemically distinct sites in the Allequash Creek wetland, northern Wisconsin, USA. We found a high degree of spatial variability in the methylation rate potentials but no significant differences between the two sites. We identified the primary geochemical factors controlling net methylmercury production at this site to be acid-volatile sulfide, dissolved organic carbon, total dissolved iron, and porewater iron(II). Season and demethylation rates also appear to regulate net methylmercury production. Our equilibrium speciation modeling demonstrated that sulfide likely regulated methylation rates by controlling the speciation of inorganic mercury and therefore its bioavailability to methylating bacteria. We found that no individual geochemical parameter could explain a significant amount of the observed variability in mercury methylation rates, but we found significant multivariate relationships, supporting the widely held understanding that net methylmercury production is balance of several simultaneously occurring processes.

Melting phase relations in the systems Mg2SiO4–H2O and MgSiO3–H2O and the formation of hydrous melts in the upper mantle

High-pressure and high-temperature melting experiments were conducted in the systems Mg2SiO4–H2O and MgSiO3–H2O at 6 and 13GPa and between 1150 and 1900°C in order to investigate the effect of H2O on melting relations of forsterite and enstatite. The liquidus curves in both binary systems were constrained and the experimental results were interpreted using a thermodynamic model based on the homogeneous melt speciation equilibrium, H2O+O2−=2OH−, where water in the melt is present as both molecular H2O and OH− groups bonded to silicate polyhedra. The liquidus depression as a function of melt H2O concentration is predicted using a cryoscopic equation with the experimental data being reproduced by adjusting the water speciation equilibrium constant. Application of this model reveals that in hydrous MgSiO3 melts at 6 and 13GPa and in hydrous Mg2SiO4 melts at 6GPa, water mainly dissociates into OH− groups in the melt structure. A temperature dependent equilibrium constant is necessary to reproduce the data, however, implying that molecular H2O becomes more important in the melt with decreasing temperature. The data for hydrous forsterite melting at 13GPa are inconclusive due to uncertainties in the anhydrous melting temperature at these conditions. When applied to results on natural peridotite melt systems at similar conditions, the same model infers the presence mainly of molecular H2O, implying a significant difference in physicochemical behaviour between simple and complex hydrous melt systems. As pressures increase along a typical adiabat towards the base of the upper mantle, both simple and complex melting results imply that a hydrous melt fraction would decrease, given a fixed mantle H2O content. Consequently, the effect of pressure on the depression of melting due to H2O could not cause an increase in the proportion, and hence seismic visibility, of melts towards the base of the upper mantle.

Equilibrium calculations of As, Pb, and Hg speciations during coal combustion in atmospheres 21O2/79N2 and 30O2/70CO2

ABSTRACTThe research of this paper investigated the influence of a coal combustion atmosphere, pressure, and temperature on the equilibrium speciation of As, Pb, and Hg. The input data for calculations in the FactSage were the results of coal from the mine “Bogdanka”. A change in the combustion atmosphere from air to 30%O2/70%CO2 results in an increase in the mole fractions of As, Pb, and Hg species that occur in the gaseous phase, i.e. AsO, AsO2, PbO, PbCl2, HgCl2, and Hg. At temperatures below 900°C, almost all arsenic moves to solid species in the form of salts derived from arsenic acid (V).

Biogeochemical modeling of CO<sub>2</sub> and CH<sub>4</sub> production in anoxic Arctic soil microcosms

Abstract. Soil organic carbon turnover to CO2 and CH4 is sensitive to soil redox potential and pH conditions. However, land surface models do not consider redox and pH in the aqueous phase explicitly, thereby limiting their use for making predictions in anoxic environments. Using recent data from incubations of Arctic soils, we extend the Community Land Model with coupled carbon and nitrogen (CLM-CN) decomposition cascade to include simple organic substrate turnover, fermentation, Fe(III) reduction, and methanogenesis reactions, and assess the efficacy of various temperature and pH response functions. Incorporating the Windermere Humic Aqueous Model (WHAM) enables us to approximately describe the observed pH evolution without additional parameterization. Although Fe(III) reduction is normally assumed to compete with methanogenesis, the model predicts that Fe(III) reduction raises the pH from acidic to neutral, thereby reducing environmental stress to methanogens and accelerating methane production when substrates are not limiting. The equilibrium speciation predicts a substantial increase in CO2 solubility as pH increases, and taking into account CO2 adsorption to surface sites of metal oxides further decreases the predicted headspace gas-phase fraction at low pH. Without adequate representation of these speciation reactions, as well as the impacts of pH, temperature, and pressure, the CO2 production from closed microcosms can be substantially underestimated based on headspace CO2 measurements only. Our results demonstrate the efficacy of geochemical models for simulating soil biogeochemistry and provide predictive understanding and mechanistic representations that can be incorporated into land surface models to improve climate predictions.

Enabling Gibbs energy minimization algorithms to use equilibrium constants of reactions in multiphase equilibrium calculations

The geochemical literature provides numerous thermodynamic databases compiled from equilibrium constants of reactions. These databases are typically used in speciation calculations based on the law of mass action (LMA) approach. Unfortunately, such LMA databases cannot be directly used in equilibrium speciation methods based on the Gibbs energy minimization (GEM) approach because of their lack of standard chemical potentials of species. Therefore, we present in this work a simple conversion approach that calculates apparent standard chemical potentials of species from equilibrium constants of reactions. We assess the consistency and accuracy of the use of apparent standard chemical potentials in GEM algorithms by benchmarking equilibrium speciation calculations using GEM and LMA methods with the same LMA database. In all cases, we use PHREEQC to perform the LMA calculations, and we use its LMA databases to calculate the equilibrium constants of reactions. GEM calculations are performed using a Gibbs energy minimization method of Reaktoro — a unified open-source framework for numerical modeling of chemically reactive systems. By comparing the GEM and LMA results, we show that the use of apparent standard chemical potentials in GEM methods produces consistent and accurate equilibrium speciation results, thus validating our new, practical conversion technique that enables GEM algorithms to take advantage of many existing LMA databases, consequently extending and diversifying their range of applicability.

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Natural variations in 238U/235U of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ∼7.5 and ∼8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ∼8.5, with heavier U isotopes preferentially enriched in the solid phase. 238U/235U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007+0.00002/−0.00003, 1.00005±0.00001, and 1.00003±0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ∼7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)3(aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)3(aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, PCO2, Ca2+, or Mg2+ concentrations. In general, these effects are likely to be small (<0.13‰), but are nevertheless potentially significant because of the small natural range of variation of 238U/235U.

Donnan membrane speciation of Al, Fe, trace metals and REEs in coastal lowland acid sulfate soil-impacted drainage waters

Donnan dialysis has been applied to forty filtered drainage waters collected from five coastal lowland acid sulfate soil (CLASS) catchments across north-eastern NSW, Australia. Despite having average pH values<3.9, 78 and 58% of Al and total Fe, respectively, were present as neutral or negatively-charged species. Complementary isotope dilution experiments with 55Fe and 26Al demonstrated that only soluble (i.e. no colloidal) species were present. Trivalent rare earth elements (REEs) were also mainly present (>70%) as negatively-charged complexes. In contrast, the speciation of the divalent trace metals Co, Mn, Ni and Zn was dominated by positively-charged complexes and was strongly correlated with the alkaline earth metals Ca and Mg. Thermodynamic equilibrium speciation calculations indicated that natural organic matter (NOM) complexes dominated Fe(III) speciation in agreement with that obtained by Donnan dialysis. In the case of Fe(II), however, the free cation was predicted to dominate under thermodynamic equilibrium, whilst our results indicated that Fe(II) was mainly present as neutral or negatively-charged complexes (most likely with sulfate). For all other divalent metals thermodynamic equilibrium speciation calculations agreed well with the Donnan dialysis results. The proportion of Al and REEs predicted to be negatively-charged was also grossly underestimated, relative to the experimental results, highlighting possible inaccuracies in the stability constants developed for these trivalent Me(SO4)2− and/or Me–NOM complexes and difficulties in modeling complex environmental samples. These results will help improve metal mobility and toxicity models developed for CLASS-affected environments, and also demonstrate that Australian CLASS environments can discharge REEs at concentrations an order of magnitude greater than previously reported.

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.

Dynamic Speciation Modeling to Guide Selection of Complexing Agents for Chemical Bath Deposition: Case Study for ZnS Thin Films

The dynamic behavior of complexing agents often governs the kinetics of chemical bath deposition (CBD), but dynamics are frequently overlooked in favor of a more simplified description of only initial conditions. Here we demonstrate the importance of complexing agent dynamics using a combination of equilibrium speciation modeling and experiments for the case of ZnS thin films grown with three common complexing agents: ethylenediaminetetraacetate (EDTA), nitrilotriacetate (NTA), and citrate, using a reference recipe that ensured fair comparison. Complexing agents control the deposition rate by limiting the availability of free cations. Speciation modeling was used to simulate the free Zn2+ concentration as a function of total Zn2+ for baths with the individual agents. On the basis of the primary stability constants, deposition rate was predicted to decrease abruptly with reaction time for the EDTA bath, whereas baths with NTA or citrate should provide deposition rates that decrease only slightly over time. The predicted deposition profiles of the three baths were observed experimentally using a continuous-flow microreactor for CBD and were supported by the measured total Zn2+ concentration profiles. Understanding and predicting the dynamic behavior of complexing agents by simulation enables strategic design of CBD processes for many material systems.

Measurement and Modeling of U(IV) Adsorption to Metal Oxide Minerals

Chemical or biological reduction of U(VI) produces a variety of poorly soluble U(IV) species. In addition to uraninite (UO2) and biomass-associated noncrystalline U(IV), recent research has found adsorbed U(IV) species on mineral surfaces. To build on these observations, we evaluated equilibrium adsorption of U(IV) to magnetite and rutile as a function of pH and total U(IV) loading. Surface complexation models that could successfully simulate the uptake of U(IV) by accounting for UO2 precipitation and adsorption of U(IV) to both the minerals and the reactor surfaces were developed. Application of the models could determine the conditions under which adsorption as opposed to precipitation would dominate U(IV) uptake with solids. The model-predicted U(IV) surface coverages of the minerals were consistent with a recent spectroscopic study. Such models advance our ability to predict the equilibrium speciation of U(IV) in the subsurface.

Simulating the reactions of CO2 in aqueous monoethanolamine solution by reaction ensemble Monte Carlo using the continuous fractional component method.

Molecular simulations were used to compute the equilibrium concentrations of the different species in CO2/monoethanolamine solutions for different CO2 loadings. Simulations were performed in the Reaction Ensemble using the continuous fractional component Monte Carlo method at temperatures of 293, 333, and 353 K. The resulting computed equilibrium concentrations are in excellent agreement with experimental data. The effect of different reaction pathways was investigated. For a complete understanding of the equilibrium speciation, it is essential to take all elementary reactions into account because considering only the overall reaction of CO2 with MEA is insufficient. The effects of electrostatics and intermolecular van der Waals interactions were also studied, clearly showing that solvation of reactants and products is essential for the reaction. The Reaction Ensemble Monte Carlo using the continuous fractional component method opens the possibility of investigating the effects of the solvent on CO2 chemisorption by eliminating the need to study different reaction pathways and concentrate only on the thermodynamics of the system.

Junction potentials in thermolytic reverse electrodialysis

Reverse electrodialysis (RED) can produce electric energy from waste heat using thermolytic solutions (e.g., NH4HCO3) where waste heat is used to regenerate the high concentration (HC) and low concentration (LC) solutions. The salinity difference between the two solutions in RED is converted into electric potential across an ion exchange membrane (IEM), exploiting the liquid junction potential. Theoretical calculation of the junction potential is cumbersome because the activity coefficients and equilibrium speciation of individual ions are complicated for highly concentrated NH4HCO3 solution. We used a simplification of the Planck–Henderson equation to approximate the junction potential in thermolytic RED systems based on conductivity measurements, and this approximation was consistent with experimentally measured junction potentials. The experimental results also found that NH4HCO3 created greater junction potentials across anion exchange membranes than (NH4)2CO3 solution for a given molar concentration ratio. The junction potential was hardly affected by the magnitude of HC as long as the concentration ratio between HC and LC was maintained. Based on the experimental findings, we recommend that thermolytic RED systems be operated under neutral pH and high concentration ratio conditions (above 1:100 ratio). These findings provide information essential for designing and operating thermolytic RED systems for future study and practical application.

Corrosion Behavior of 13Cr Casing in Cement Synthetic Pore Solution

A test procedure was designed and employed for exposure and degradation testing of well cements in a simulated downhole environment of offshore deep drilling. A Class H cement sample was crushed to coarse grains and was exposed to 5 % mass NaCl(aq) solution in contact with CO2(g) at 100 ˚C. The partial pressure of CO2 was 10 MPa. After the 200-hour exposure, the liquid phase from the testing, or the ‘original pore solution’, was diluted and extracted for qualitative and quantitative analyses of the key ionic species. Significant variation was observed between samples drawn in situ and ex situ from the original pore solution. Ca2+(aq), Na+(aq), and some K+(aq) were determined to be the primary cationic species. Cl-(aq) and SO4 2-(aq) were found to be major anionic species, though the concentration of Cl-(aq) was lower than that originally in the NaCl(aq) solution. The compounds used to achieve the equilibrium speciation corresponding to the “original pore solution” were modeled using commercial software OLI Analyzer Studio 9.0, and were used to develop the cement synthetic pore solution (CSPS) recipe. The CSPS was then used for corrosion characterization of high chromium well casing steel, type 13Cr, grade L-80. The steel samples were exposed to CSPS at 100 ˚C and 10 MPa pressure in contact with CO2(g) with a Pt mesh counter electrode and a custom made Ag/AgCl electrode. The corrosion behavior of the steel was investigated using a set of electrochemical methods consisting of the open circuit potential, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) at 3-hour intervals to detect steady state conditions. The CO2-rich CSPS was stirred during these experiments, which lasted 60 hours. At the end of 60-hour exposure, the stirring was stopped and the same set of electrochemical tests was performed without stirring to detect any effect of convection or diffusion. The corrosion potential in the measurements was similar, between -0.4 and -0.5 V vs. standard hydrogen electrode. The average corrosion rate of the steel samples was on the order of 0.1 mm/year. This was considered high for an environment in which steels expected to be passivated. Polarization resistance obtained from EIS data was similar to that determined through LPR. No significant change in the corrosion behavior was observed between stirred and unstirred conditions, and the EIS data did not suggest that corrosion process was controlled by diffusion. The high corrosion rate was believed to be due to neutralization of the calcium hydroxide by the dissolved CO2, which then moved the corrosion potential of the steel from the passive region to the active region. This would provide a highly corrosive environment when combined with the high chloride content and high temperature of the CSPS. While it was noted that the condition studied here represented a worst-case scenario for degradation of the cement material, the magnitude of the corrosion rate shows that care must be taken to prevent damage to the cement coating, as the resulting environment could result in severe corrosion and premature failure of critical steel components like a well casing.

Electrochemical Codeposition of Nickel and Tin from Gluconate Solutions

Ni-Sn alloys show excellent properties having considerable technological interest. Commercial codeposition of nickel with tin is usually carried out from acidic fluoride-chloride electrolytes, but these are intended to replace by less aggressive baths. In this work codeposition of Ni-Sn alloys from acidic solutions containing sodium gluconate as a cheap and nontoxic complexing agent was carried out. The electrodeposition process was studied using various electrochemical techniques. Cathodic polarization curves have shown that tin (II) ions were reduced under limiting current. Chronoamperometric studies indicated nucleation of the alloys according to the instantaneous mechanism. Equilibrium speciation of the baths was also calculated. Chemical and phase composition as well as morphology of the coatings were investigated. Corrosion resistance of the alloys was also studied.

Stability of binary complexes of Pb(II), Cd(II) and Hg(II) with maleic acid in TX100-water mixtures

Binary complexes of maleic acid with toxic metal ions such as Pb(II), Cd(II) and Hg(II) have been studied in 0.0-2.5% v/v tritonX-100 (TX100) - water media at 303 K at an ionic strength of 0.16 M. The active forms of the ligand are LH 2 , LH - and L 2- . The derived ‘best fit’ chemical speciation models are based on crystallographic R-factors, χ 2 and Skewness and Kurtosis factors. The predominant species formed are of the type ML 2 , ML 2 H and ML 3 . The trend in variation of complex stability constants with change in the mole fraction of the medium is explained on the basis of prevailing electrostatic and non-electrostatic forces. The species distribution as a function of pH at different compositions of TX100-water mixtures and plausible speciation equilibria are presented and discussed. KEY WORDS : Maleic acid, TritonX-100, Toxic metal ions, Mole fraction, Binary complexes Bull. Chem. Soc. Ethiop. 2014 , 28(3), 383-391. DOI: http://dx.doi.org/10.4314/bcse.v28i3.7

Influence of pH, Ionic Strength, and Multidentate Ligand on the Interaction of CdIIwith Biochars

Pyrolysis temperature-dependent changes in pH, ionic strength, and metal ion-coordinating dissolved organic carbon (DOC) control the sorption of CdII and other transition metals on biochar. Sorption of CdII on 300–700 °C pecan shell biochars was strongly pH- and ionic strength-dependent. Equilibrium speciation calculation suggested that CdOH+ and other hydrolysis products were the “reactive” CdII species engaging in the surface interaction with biochar. Sorption of CdII on 700 °C (lowest O:C) biochar progressively increased from pH 3 to 7 and was not affected by the citrate concentration. In contrast, low concentration of citrate ([CdII]:[citrate] molar ratios of 4–24) dramatically enhanced CdII sorption on lower temperature (300–500 °C) biochars; higher citrate concentrations diminished CdII sorption. Citrate-enhanced CdII sorption was accompanied by (i) a several-fold increase in DOC and (ii) pH buffering near 7 at equilibrium. Carboxyl ligands form an unusually strong hydrogen bond with the surface fun...

The 2015 IBS MacArthur &amp; Wilson Award

Announcements ISSN 1948-6596 The 2015 IBS MacArthur & Wilson Award The International Biogeography Society Board is pleased to announce that the MacArthur & Wilson Award will be presented to Dr. Daniel L. Rabosky at the society’s biennial meeting in Bayreuth, Ger- many, happening from 8 th to 12 th of January 2015. This award, which is sponsored by Frontiers of Bio- geography, was created to recognize an out- standing early-career scientist that has made in- novative, substantive, broad and important contri- butions to the discipline of biogeography and is named in honor of Robert H. MacArthur and Ed- ward O. Wilson. Daniel Rabosky received his PhD from Cor- nell University in 2009, completed a post-doctoral position at the University of California Berkeley, and is currently an Assistant Professor at the Uni- versity of Michigan. Interested in evolutionary diversification, ranging from the role of ecological interactions in the speciation process to factors that determine the fates of clades over epochal timescales, Daniel was nominated by IBS members for his work involving both comparative analyses of diversification as well as the development of analytical tools for hypothesis testing. His work is published in the highest-profile scientific journals, such as Systematic Biology, PNAS, Trends in Ecol- ogy and Evolution, Nature, and Science. A fuller account of his work will be featured in a future edition of Frontiers, following the presentation of this award at the upcoming biennial conference. The following four papers were submitted by his nominators to represent the broad range of his outstanding contributions to biogeography: Rabosky, D.L. (2009) Ecological limits on clade diversification in higher taxa. American Naturalist, 173, 662-674. Rabosky, D.L. (2009) Diversity dynamics of marine planktonic diatoms across the Cenozoic. Nature, 457, 183-186. Rabosky, D.L. (2010) Equilibrium speciation dynamics in a model adaptive radiation of island lizards. Proceed- ings of the National Academy of Sciences of the United States of America, 51, 22178-22183. Rabosky, D.L. (2013) Diversity-dependence, ecological speci- ation, and the role of competition in macroevolution. Annual Review of Evolution, Ecology, and Systemat- ics, 44, 481-502. The IBS Board notes with thanks the work of the MacArthur & Wilson Award Committee: Miguel B. Araujo, Christy M. McCain, Robert E. Ricklefs, Brett R. Riddle (chair) Brett R. Riddle School of Life Sciences, University of Nevada, Las Ve- gas, USA Chair of the 2015 IBS MacArthur & Wilson Award Com- mittee To post jobs, advertisements, or other announcements please contact the Frontiers of Biogeography editoral office (frontiersofbiogeography@googlemail.com). Advertisements are $50 per quarter page for professional or institutional positions. Announcements are free to members for vacancies in their lab. All advertisements and announcements also appear at no additional cost on the IBS facebook pa- ge (in abbreviated form) and on the IBS Blog (in long, or longer form). frontiers of biogeography 6.2 2014 — © 2014 the authors; journal compilation © 2014 The International Biogeography Society

Biokinetic modeling of ureolysis in Sporosarcina pasteurii and its integration into a numerical chemodynamic biocalcification model

A dynamic model for Sporosarcina pasteurii-mediated ureolytic biocalcification process has been proposed, which incorporates microbial growth-dependent ureolysis, equilibrium speciation of dissolved inorganic carbon (DIC), dissolved inorganic nitrogen (DIN), different sodium and calcium containing species, kinetics of gas/solution exchange (for CO2, NH3), and a dynamic charge balance. The kinetics of microbial growth and ureolysis (including parameters) were selected from a set of relevant representative models using designed batch ureolysis experiments and were incorporated into the dynamic model. The gas/solution exchange rate constants for CO2 and NH3 were assessed from independent sets of abiotic experiments while regressing their measured DIC-pH profiles or DIN-pH profiles, respectively. Subsequently, residual parameters associated with CaCO3 precipitation were estimated by calibrating one or more measured profiles (DIN, urea, Ca2+, and pH) from the ureolytic-biocalcification experiment with the dynamic model. Bootstrap confidence intervals and sensitivity analysis of these model parameters are presented.

Metal mobilization from soils by phytosiderophores - experiment and equilibrium modeling.

AimsTo test if multi–surface models can provide a soil-specific prediction of metal mobilization by phytosiderophores (PS) based on the characteristics of individual soils.MethodsMechanistic multi-surface chemical equilibrium modeling was applied for obtaining soil-specific predictions of metal and PS speciation upon interaction of the PS 2’-deoxymugineic acid (DMA) with 6 soils differing in availability of Fe and other metals. Results from multi-surface modeling were compared with empirical data from soil interaction experiments.ResultsFor soils in which equilibrium was reached during the interaction experiment, multi-surface models could well predict PS equilibrium speciation. However, in uncontaminated calcareous soils, equilibrium was not reached within a week, and experimental and modeled DMA speciation differed considerably. In soils with circum-neutral pH, on which Fe deficiency is likely to occur, no substantial Fe mobilization by DMA was predicted. However, in all but the contaminated soils, Fe mobilization by DMA was observed experimentally. Cu and Ni were the quantitatively most important metals competing with Fe for complexation and mobilization by DMA.ConclusionThermodynamics are unable to explain the role of PS as Fe carrier in calcareous soils, and the kinetic aspects of metal mobilization by PS need to be closer examined in order to understand the mechanisms underlying strategy II Fe acquisition.Electronic supplementary materialThe online version of this article (doi:10.1007/s11104-014-2128-3) contains supplementary material, which is available to authorized users.

The influence of sulfate ions on the electrodeposition of Ni–Sn alloys from acidic chloride–gluconate baths

Codeposition of nickel and tin from acidic gluconate baths of various compositions was investigated. It was found that cathodic process was seriously affected by sulfate ions resulting in lower nickel percentages in the deposits, changes in the phase composition as well as improved surface morphology of the alloys. Partial cathodic polarization curves have shown that tin ions were reduced under limiting current. Chronoamperometric studies indicated nucleation of the alloys according to the instantaneous mechanism. Anodic stripping analysis confirmed phase composition of the alloys – NiSn and Ni3Sn4 were formed in the chloride bath, while NiSn phase was produced in the presence of sulfate anions. It was accordant with thermodynamic predictions. Equilibrium speciation of the baths was also calculated.