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Aquatic Geochemistry (v.19, #1)
Using Mg Isotopes to Trace Cyanobacterially Mediated Magnesium Carbonate Precipitation in Alkaline Lakes by Liudmila S. Shirokova; Vasileios Mavromatis; Irina A. Bundeleva; Oleg S. Pokrovsky; Pascale Bénézeth; Emmanuelle Gérard; Christopher R. Pearce; Eric H. Oelkers (pp. 1-24).
This study assesses the potential use of Mg isotopes to trace Mg carbonate precipitation in natural waters. Salda Lake (SW Turkey) was chosen for this study because it is one of the few modern environments where hydrous Mg carbonates are the dominant precipitating minerals. Stromatolites, consisting mainly of hydromagnesite, are abundant in this lake. The Mg isotope composition of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments were measured. Because Salda Lake is located in a closed basin, mass balance requires that the Mg isotopic offset between Lake Salda water and precipitated hydromagnesite be comparable to the corresponding offset between Salda Lake and its water inputs. This is consistent with observations; a δ26Mg offset of 0.8–1.4 ‰ is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a δ26Mg 0.9–1.1 ‰ more negative than its corresponding fluid phase. This isotopic offset also matches closely that measured in the laboratory during both biotic and abiotic hydrous Mg carbonate precipitation by cyanobacteria (Mavromatis, V., Pearce, C., Shirokova, L. S., Bundeleva, I. A., Pokrovsky, O. S., Benezeth, P. and Oelkers, E.H.: Magnesium isotope fractionation during inorganic and cyanobacteria-induced hydrous magnesium carbonate precipitation, Geochim. Cosmochim. Acta, 2012a. 76, 161–174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg5(CO3)4(OH)2·5(H2O)) and hydromagnesite (Mg5(CO3)4(OH)2·4H2O) with morphological features similar to those of natural samples. Concurrent abiotic control experiments did not exhibit carbonate precipitation demonstrating the critical role of cyanobacteria in the precipitation process.
Keywords: Magnesium isotope fractionation; Hydromagnesite precipitation; Alkaline lakes
Effect of Microbial Siderophore DFOB on Pb, Zn, and Cd Sorption Onto Zeolite by Lotfallah Karimzadeh; Sreejesh Nair; Broder J. Merkel (pp. 25-37).
Recent studies suggest that siderophores form stable complexes with divalent metals and affect their mobility. In this work, effects of trihydroxamate microbial siderophores and desferrioxamine-B (DFOB) on Pb(II), Zn(II), and Cd(II) sorption by two kinds of synthesized zeolites (13X and Na–Y) as a function of pH were investigated. Results showed that 13X zeolite has a higher sorption affinity for studied metals than Na–Y. DFOB strongly affected metal sorption on both zeolites. Under slightly acidic to neutral condition, DFOB increased the metal sorption on zeolites due to the sorption of positively charged heavy metal–DFOB complexes. Whereas by increasing pH (>7), the mobilizing effect of DFOB was observed for Pb, Zn, and Cd. DFOB drastically decreased (80 %) Zn sorption in alkaline condition. As a result, siderophores can weaken the treatment efficiency of zeolites and increase the bioavailability of metals in soils. Surface complexation modeling revealed that the effects of DFOB on metal sorption by 13X and Na–Y zeolites can be explained by the differences in their surface charge. In general, the result shows the influence of DFOB on metal sorption by zeolites over the pH range 4–9 and decreasing in the sequence Zn > Pb > Cd.
Keywords: Microbial siderophore DFOB; Sorption; Zeolite; Toxic metals; Pb; Zn; Cd
A New Analytic Integration of the Rate Equation for Batch Dissolution of Salts in the Presence of Common Ion by Victor W. Truesdale; Cristiana Sebu (pp. 39-56).
Following a recent suggestion of a new rate equation specifically for the batch dissolution of salts in solutions containing a common ion, this paper describes an analytic solution to its integration. The equation has been tested by dissolving 250 μm gypsum- rock particles in water (26.7 g l−1) containing various mixtures of sodium and calcium chlorides, all at an ionic strength of 0.060 M. The model fitted the experimental curves very well and showed that the dissolution slowed slightly overall when the initial calcium concentration was increased from 0 to 0.020 M. The dissolution curves were also modelled as a simple exponential, whence the fit was comparable to that with the new equation, with the exponential rate constant varying between 0.025 and 0.019 (±0.0004) for 0 and 0.020 M initial calcium concentration, respectively. Conventional Electrolyte theory from thermodynamics is used to show that the new equation is an inevitable consequence of modelling the net rate of dissolution in terms of a back reaction that is first order with respect to the dissolved substance, as per the recently described Shrinking Object model. Moreover, it is shown how the simple exponential model (which is a well-used plot in dissolution kinetics) provides the linear end-member to an infinite number of curvilinear plots of rate of dissolution versus reaction progress developed by the new model—it is the special case where common ion is absent. The results are now judged good enough to identify a generic batch dissolution rate equation for all salts dissolving without significant complication from either contaminants or their own gaseous species, as in calcium carbonate dissolution.
Keywords: Dissolution; Dissolution kinetics; Batch dissolution; Shrinking Object model; Gypsum; Rate equation; Common ion effect; Electrolyte theory; Analytic integral
Speciation and Mobility of Selected Trace Metals (As, Cu, Mn, Pb and Zn) in Sediment with Depth in Cam River-Mouth, Haiphong, Vietnam by Huu Hieu Ho; Rudy Swennen; Valérie Cappuyns; Elvira Vassilieva; Tom Van Gerven; Tan Van Tran (pp. 57-75).
The speciation and mobility of some selected trace metals (As, Cu, Mn, Pb and Zn) in sediments with depth was investigated in the Cam River-mouth (Vietnam) by collecting sediment cores and analysing porewater and sediment composition, complemented with single (ammonium-EDTA) and sequential (BCR 3-step) extractions and mineralogical analysis (XRD). All trace metals show overall decreasing trends with depth in porewater as a result of anthropogenic input in upper sediment layers. High porewater concentrations of As, Mn and Pb in oxic and suboxic sediment layers may result in groundwater pollution. Sediment-bound Pb and Mn dominate in the reducible and the acid-soluble fraction, respectively, while Cu and Zn distribute rather evenly between four extracted fractions. The porewater metal speciation, as predicted by a geochemical model Visual MINTEQ version 3.0, indicates that the toxicity of Cu, Mn, Pb and Zn (presented by the proportions of free metal ions) decreases with depth, while the toxicity of As increases when As(III) becomes more abundant. The dissolved concentrations of trace metals are not only controlled by the precipitation/dissolution of discrete hydroxide/oxide, carbonate and phosphate minerals, but also by sorption processes on major sorbents (i.e. As on goethite, and Mn and Zn on calcite and dolomite). Sulphide minerals do not show any control even in the anoxic zone most likely because of the low concentration of sulphur.
Keywords: Trace metal; Speciation; Mobility; Porewater; Sediment; Solubility-controlling minerals
Monitoring and Modeling of Metal Concentration Distributions in Anoxic Basins: Aitoliko Lagoon, Greece by Areti Gianni; Miltiadis Zamparas; Ioannis T. Papadas; George Kehayias; Yiannis Deligiannakis; Ierotheos Zacharias (pp. 77-95).
The balance between physicochemical processes, influencing vertical and temporal distributions of metal compounds in one relatively isolated anoxic environment, constitutes the objective of the present work. Ion activity product (IAP) was calculated for manganese and iron sulfides, in order to define the metal sulfide forms that control Fe and Mn solubility in the bottom waters of anoxic lagoons. Iron solubility depended on amorphous FeS formation, while manganese sulfides were a minor component in a solid solution lowering its solid-phase activity. A theoretical physicochemical model was developed for the iron speciation, based on experimental pH and redox potential data. A very good match was achieved for the measured and the theoretical total dissolved iron, at all depths. The dominance of oxidant iron species Fe(OH) 3 − in the surface waters and their sequence by FeSH+ and FeSaq in the deeper layers brings out the influence of physicochemical parameters (dissolved oxygen, sulfide, pH and Eh) in vertical distribution of dissolved metal species, in anoxic/hypoxic basins. Based on these findings, we can conclude that the distribution of manganese and iron is of special interest, not only because these are the indicators of redox conditions but also for the role of their oxidized/reduced forms in the formation of the biogeochemical structure of redox zone.
Keywords: Anoxia; Iron; Manganese; Hydrogen sulfide; Aitoliko lagoon