Geochemical Transactions最新文献

{"title":"Interaction mechanisms and kinetics of ferrous ion and hexagonal birnessite in aqueous systems","authors":"Tianyu Gao,&nbsp;Yougang Shen,&nbsp;Zhaoheng Jia,&nbsp;Guohong Qiu,&nbsp;Fan Liu,&nbsp;Yashan Zhang,&nbsp;Xionghan Feng,&nbsp;Chongfa Cai","doi":"10.1186/s12932-015-0031-3","DOIUrl":"https://doi.org/10.1186/s12932-015-0031-3","url":null,"abstract":"<p>In soils and sediments, manganese oxides and oxygen usually participate in the oxidation of ferrous ions. There is limited information concerning the interaction process and mechanisms of ferrous ions and manganese oxides. The influence of air (oxygen) on reaction process and kinetics has been seldom studied. Because redox reactions usually occur in open systems, the participation of air needs to be further investigated.</p><p>To simulate this process, hexagonal birnessite was prepared and used to oxidize ferrous ions in anoxic and aerobic aqueous systems. The influence of pH, concentration, temperature, and presence of air (oxygen) on the redox rate was studied. The redox reaction of birnessite and ferrous ions was accompanied by the release of Mn²⁺ and K⁺ ions, a significant decrease in Fe²⁺ concentration, and the formation of mixed lepidocrocite and goethite during the initial stage. Lepidocrocite did not completely transform into goethite under anoxic condition with pH about 5.5 within 30?days. Fe²⁺ exhibited much higher catalytic activity than Mn²⁺ during the transformation from amorphous Fe(III)-hydroxide to lepidocrocite and goethite under anoxic conditions. The release rates of Mn²⁺ were compared to estimate the redox rates of birnessite and Fe²⁺ under different conditions.</p><p>Redox rate was found to be controlled by chemical reaction, and increased with increasing Fe²⁺ concentration, pH, and temperature. The formation of ferric (hydr)oxides precipitate inhibited the further reduction of birnessite. The presence of air accelerated the oxidation of Fe²⁺ to ferric oxides and facilitated the chemical stability of birnessite, which was not completely reduced and dissolved after 18?days. As for the oxidation of aqueous ferrous ions by oxygen in air, low and high pHs facilitated the formation of goethite and lepidocrocite, respectively. The experimental results illustrated the single and combined effects of manganese oxide and air on the transformation of Fe²⁺ to ferric oxides.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-09-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0031-3","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4876261","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The influence of environmental conditions on kinetics of arsenite oxidation by manganese-oxides","authors":"Matthew H. H. Fischel,&nbsp;Jason S. Fischel,&nbsp;Brandon J. Lafferty,&nbsp;Donald L. Sparks","doi":"10.1186/s12932-015-0030-4","DOIUrl":"https://doi.org/10.1186/s12932-015-0030-4","url":null,"abstract":"<p>Manganese-oxides are one of the most important minerals in soil due to their widespread distribution and high reactivity. Despite their invaluable role in cycling many redox sensitive elements, numerous unknowns remain about the reactivity of different manganese-oxide minerals under varying conditions in natural systems. By altering temperature, pH, and concentration of arsenite we were able to determine how manganese-oxide reactivity changes with simulated environmental conditions. The interaction between manganese-oxides and arsenic is particularly important because manganese can oxidize mobile and toxic arsenite into more easily sorbed and less toxic arsenate. This redox reaction is essential in understanding how to address the global issue of arsenic contamination in drinking water.</p><p>The reactivity of manganese-oxides in ascending order is random stacked birnessite, hexagonal birnessite, biogenic manganese-oxide, acid birnessite, and δ-MnO₂. Increasing temperature raised the rate of oxidation. pH had a variable effect on the production of arsenate and mainly impacted the sorption of arsenate on δ-MnO₂, which decreased with increasing pH. Acid birnessite oxidized the most arsenic at alkaline and acidic pHs, with decreased reactivity towards neutral pH. The δ-MnO₂ showed a decline in reactivity with increasing arsenite concentration, while the acid birnessite had greater oxidation capacity under higher concentrations of arsenite. The batch reactions used in this study quantify the impact of environmental variances on different manganese-oxides’ reactivity and provide insight to their roles in governing chemical cycles in the Critical Zone.</p><p>The reactivity of manganese-oxides investigated was closely linked to each mineral’s crystallinity, surface area, and presence of vacancy sites. δ-MnO₂ and acid birnessite are thought to be synthetic representatives of naturally occurring biogenic manganese-oxides; however, the biogenic manganese-oxide exhibited a lag time in oxidation compared to these two minerals. Reactivity was clearly linked to temperature, which provides important information on how these minerals react in the subsurface environment. The pH affected oxidation rate, which is essential in understanding how manganese-oxides react differently in the environment and their potential role in remediating contaminated areas. Moreover, the contrasting oxidative capacity of seemingly similar manganese-oxides under varying arsenite concentrations reinforces the importance of each manganese-oxide mineral’s unique properties.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0030-4","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4664373","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A synthetic standard for the analysis of carbon isotopes of carbon in silicates, and the observation of a significant water-associated matrix effect","authors":"Christopher H. House","doi":"10.1186/s12932-015-0029-x","DOIUrl":"https://doi.org/10.1186/s12932-015-0029-x","url":null,"abstract":"<p>Due to the biogeochemical fractionation of isotopes, organic material can be heterogeneous at the microscale. Because this heterogentiy preserves in the rock record, the microscale measurement of carbon isotopes is an important frontier of geobiology. Such analyses via secondary ion mass spectrometry (SIMS) have been, however, held back by the lack of an appropriate homogeneous synthetic standard that can be shared between laboratories. Such a standard would need to yield a carbon signal intensity within the same instrument dynamic range as that found for typical rocks, exhibit minimal matrix effects under typical SIMS conditions, and be widely available. In this work, five possible standards were tested with repeated δ¹³C ion microprobe measurements against the PPRG #215-1 Precambrian chert that has been used as a working standard for these types of analyses by several laboratories.</p><p>Results showed that silica powder, pressed, and bonded with Ceramacast 905 produced a useful synthetic standard. The material produced has a secondary ion carbon yield of only about 15× that of the PPRG #215-1 organic-rich chert. Finally, the material, once dried sufficiently, did not demonstrate an observable matrix effect when the carbon isotopes were measured. Another similar material (silica nanopowder, pressed, and bonded with Aremco-Bond 526N) appears to have retained its hydration after a substantial effect to dry it. The isotopes measurements of this more hydrated material showed a significant matrix effect that was diminished by intense pre-sputtering. The results indicate water can affect SIMS carbon isotopic measurements, and an intense beam reduces the effect. A hydrated standard might be useful to monitor the effect.</p><p>A suitable artificial standard for SIMS isotopic measurements of organic material in rocks has been found, and it will allow an acute growth in both the quantity and quality of studies of ancient carbon at the microscale. Also, this work has revealed a novel water-associated matrix effect for carbon isotopes. This newly revealed matrix effect is important because it might have misled previous research. The effect could lead to increased observed heterogeneity of partially hydrated samples and/or produced systematic differences between natural targets and the standards used.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-09-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0029-x","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4631431","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Mineral precipitation-induced porosity reduction and its effect on transport parameters in diffusion-controlled porous media","authors":"Aurélie Chagneau,&nbsp;Francis Claret,&nbsp;Frieder Enzmann,&nbsp;Michael Kersten,&nbsp;Stephanie Heck,&nbsp;Benoît Madé,&nbsp;Thorsten Schäfer","doi":"10.1186/s12932-015-0027-z","DOIUrl":"https://doi.org/10.1186/s12932-015-0027-z","url":null,"abstract":"<p>In geochemically perturbed systems where porewater and mineral assemblages are unequilibrated the processes of mineral precipitation and dissolution may change important transport properties such as porosity and pore diffusion coefficients. These reactions might alter the sealing capabilities of the rock by complete pore-scale precipitation (cementation) of the system or by opening new migration pathways through mineral dissolution. In actual 1D continuum reactive transport codes the coupling of transport and porosity is generally accomplished through the empirical Archie’s law. There is very little reported data on systems with changing porosity under well controlled conditions to constrain model input parameters. In this study celestite (SrSO₄) was precipitated in the pore space of a compacted sand column under diffusion controlled conditions and the effect on the fluid migration properties was investigated by means of three complementary experimental approaches: (1) tritiated water (HTO) tracer through diffusion, (2) computed micro-tomography (μ-CT) imaging and (3) post-mortem analysis of the precipitate (selective dissolution, SEM/EDX).</p><p>The through-diffusion experiments reached steady state after 15?days, at which point celestite precipitation ceased and the non-reactive HTO flux became constant. The pore space in the precipitation zone remained fully connected using a 6?μm μ-CT spatial resolution with 25?% porosity reduction in the approx. 0.35?mm thick dense precipitation zone. The porosity and transport parameters prior to pore-scale precipitation were in good agreement with a porosity of 0.42?±?0.09 (HTO) and 0.40?±?0.03 (μ-CT), as was the mass of SrSO₄ precipitate estimated by μ-CT at 25?±?5?mg and selective dissolution 21.7?±?0.4?mg, respectively. However, using this data as input parameters the 1D single continuum reactive transport model was not able to accurately reproduce both the celestite precipitation front and the remaining connected porosity. The model assumed there was a direct linkage of porosity to the effective diffusivity using only one cementation value over the whole porosity range of the system investigated.</p><p>The 1D single continuous model either underestimated the remaining connected porosity in the precipitation zone, or overestimated the amount of precipitate. These findings support the need to implement a modified, extended Archie’s law to the reactive transport model and show that pore-scale precipitation transforms a system (following Archie’s simple power law with only micropores present) towards a system similar to clays with micro- and nanoporosity.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-09-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0027-z","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4144466","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms","authors":"Sylvain Grangeon,&nbsp;Alejandro Fernandez-Martinez,&nbsp;Fabienne Warmont,&nbsp;Alexandre Gloter,&nbsp;Nicolas Marty,&nbsp;Agnieszka Poulain,&nbsp;Bruno Lanson","doi":"10.1186/s12932-015-0028-y","DOIUrl":"https://doi.org/10.1186/s12932-015-0028-y","url":null,"abstract":"<p>Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Its layers are built of (MnO₆)^8? octahedra connected through their edges and frequently contain vacancies and? (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements (e.g., Co, Ni, and Zn) and possesses a redox potential that allows for the oxidation of redox-sensitive elements (e.g., As, Cr, Tl). As a result, vernadite acts as a sink for many trace metal elements. In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor. The transformation mechanism is not yet fully understood however and the fate of metals initially contained in vernadite structure during this transformation is still debated. In the present work, the transformation of synthetic vernadite (δ-MnO₂) to synthetic cryptomelane under conditions analogous to those prevailing in soils (dry state, room temperature and ambient pressure, in the dark) and over a time scale of ~10?years was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy.</p><p>Migration of Mn³⁺ from layer to interlayer to release strains and their subsequent sorption above newly formed vacancy in a triple-corner sharing configuration initiate the reaction. Reaction proceeds with preferential growth to form needle-like crystals that subsequently aggregate. Finally, the resulting lath-shaped crystals stack, with <i>n</i>?×?120° (<i>n</i>?=?1 or 2) rotations between crystals. Resulting cryptomelane crystal sizes are ~50–150?nm in the <b>ab</b> plane and ~10–50?nm along <b>c*</b>, that is a tenfold increase compared to fresh samples.</p><p>The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones. This pleads for the relevance of the proposed mechanism to environmental conditions.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-09-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0028-y","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4105564","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Aerobic and anaerobic reduction of birnessite by a novel Dietzia strain","authors":"Huiqin Zhang,&nbsp;Yan Li,&nbsp;Xin Wang,&nbsp;Anhuai Lu,&nbsp;Hongrui Ding,&nbsp;Cuiping Zeng,&nbsp;Xiao Wang,&nbsp;Xiaolei Wu,&nbsp;Yong Nie,&nbsp;Changqiu Wang","doi":"10.1186/s12932-015-0026-0","DOIUrl":"https://doi.org/10.1186/s12932-015-0026-0","url":null,"abstract":"<p>Mn oxides occur in a wide variety of geological settings and exert considerable influences on the components and chemical behaviors of sediments and soils. Microbial reduction of Mn oxides is an important process found in many different environments including marine and freshwater sediments, lakes, anoxic basins, as well as oxic-anoxic transition zone of ocean. Although the pathway of Mn anaerobic reduction by two model bacteria, <i>Geobacter</i> and <i>Shewanella</i>, has been intensively studied, Mn bio-reduction is still the least well-explored process in nature. Particularly, reduction of Mn oxides by other bacteria and in the presence of O₂ has been fewly reported in recent publishes.</p><p>A series of experiments were conducted to understand the capability of <i>Dietzia</i> DQ12-45-1b in bioreduction of birnessite. In anaerobic systems, Mn reduction rate reached as high as 93% within 4?weeks when inoculated with 1.0?×?10¹⁰ cells/mL <i>Dietzia</i> DQ12-45-1b strains. Addition of AQDS enhanced Mn reduction rate from 53 to 91%. The anaerobic reduction of Mn was not coupled by any increase in bacterial protein concentration, and the reduction rate in the stable stage of day 2–14 was found to be in good proportion to the protein concentration. The anaerobic reduction of birnessite released Mn(II) either into the medium or adsorbed on the mineral or bacteria surface and resulted in the dissolution of birnessite as indicated by XRD, SEM and XANES. Under aerobic condition, the reduction rate was only 37% with a cell concentration of 1.0?×?10¹⁰ cells/mL, much lower than that in parallel anaerobic treatment. Bacterial growth under aerobic condition was indicated by time-course increase of protein and pH. In contrast to anaerobic experiments, addition of AQDS decreased Mn reduction rate from 25 to 6%. The reduced Mn(II) combined with carbon dioxide produced by acetate metabolism, as well as an alkaline pH environment given by cell growth, finally resulted in the formation of Mn(II)-bearing carbonate (kutnohorite), which was verified by XRD and XANES results. The system with the highest cell concentration of 1.0?×?10¹⁰ cells/mL gave rise to the most amount of kutnohorite, while concentration of Mn(II) produced with cell concentration of 6.2?×?10⁸ cells/mL was too low to thermodynamically favor the formation of kutnohorite but result in the formation of aragonite instead.</p><p>\u0000 <i>Dietzia</i> DQ12-45-1b was able to anaerobically and aerobically reduce birnessite. The rate and extent of Mn(IV) reduction depend on cell concentration, addition of AQDS or not, and presence of O₂ or not. Meanwhile, Mn(IV) bioreduction extent and suspension conditions determined the insoluble mineral products.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-08-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0026-0","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4331323","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Decolorization of RhB dye by manganese oxides: effect of crystal type and solution pH","authors":"Hao-Jie Cui,&nbsp;Hai-Zheng Huang,&nbsp;Baoling Yuan,&nbsp;Ming-Lai Fu","doi":"10.1186/s12932-015-0024-2","DOIUrl":"https://doi.org/10.1186/s12932-015-0024-2","url":null,"abstract":"<p>Organic dye pollution in water has become a major source of environmental pollution. Mn(III/IV) oxides \u0000have attracted a great deal of attention to remove organic dye pollutants due to their unique structures and physicochemical properties. Numerous studies have reported the removal of dye by various Mn(III/IV) oxides through catalytic degradation and adsorption. The crystalline structures of manganese oxides and solution pH may exert substantial impact on the removal of dyes. However, few studies have focused on the oxidative degradation of RhB dye using Mn(III/IV) oxides with different crystal structures during a spontaneous reaction. In the present study, three manganese oxides with different crystal type (α-MnO₂, β-MnO₂, and δ-MnO₂) were prepared by refluxing process to decolorize RhB dye in various pH solutions.</p><p>The results showed that the decolorization efficiencies of RhB for the three manganese oxides all increase with decrease solution pH. α-MnO₂ exhibited highest activity and could efficiently degrade RhB at pH 2–6. The degradation of RhB by β-MnO₂ and δ-MnO₂ could be observed at pH 2–3, and only little adsorption RhB on manganese oxides could be found at pH 4–6. The UPLC/MS analysis suggests that the decolorization of RhB by manganese oxides consists of three main stages: (1) cleavage of the ethyl groups from RhB molecular to form Rh; (2) further destruction of –COOH and –CNH₂ from Rh to form the small molecular substances; (3) mineralization of the small molecular substances into CO₂, H₂O, NO₃\u0000 ^? and NH₄\u0000 ⁺.</p><p>Overall, these results indicate that α-MnO₂ may be envisaged as efficient oxidants for the treatment of organic dye-containing wastewater under acid conditions.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0024-2","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5342803","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Characterization, dissolution and solubility of synthetic cadmium hydroxylapatite [Cd5(PO4)3OH] at 25–45°C","authors":"Yinian Zhu,&nbsp;Zongqiang Zhu,&nbsp;Xin Zhao,&nbsp;Yanpeng Liang,&nbsp;Liuqin Dai,&nbsp;Yanhua Huang","doi":"10.1186/s12932-015-0025-1","DOIUrl":"https://doi.org/10.1186/s12932-015-0025-1","url":null,"abstract":"&lt;p&gt;The substitution of Ca&lt;sup&gt;2+&lt;/sup&gt; in Ca-hydroxylapatite by toxic Cd&lt;sup&gt;2+&lt;/sup&gt; can cause the forming of Cd-hydroxylapatite and is a significant issue in a great variety of research areas, which hence needs an understanding of the essential physicochemical characteristics. Unfortunately, the solubility product and thermodynamic data for Cd-hydroxylapatite in water under a variety of conditions now are lacking. Little information has been reported by previous researchers. Additionally, the dissolution mechanism of Cd-hydroxylapatite has never been studied.&lt;/p&gt;&lt;p&gt;Dissolution of the synthetic cadmium hydroxylapatite [Cd-HAP, Cd&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH] in HNO&lt;sub&gt;3&lt;/sub&gt; solution (pH?=?2), ultrapure water (pH?=?5.6) and NaOH solution (pH?=?9) was experimentally studied at 25, 35 and 45°C. Characterization by XRD, FT-IR and FE-SEM proved that Cd-HAP solids showed no recognizable change during dissolution. For the Cd-HAP dissolution in aqueous acidic media at initial pH 2 and 25°C, the solution cadmium and phosphate concentrations increased rapidly and reached the peak values after 20–30?days and 10?days reaction, respectively. Thereafter, the Cd-HAP dissolution rate decreased slowly, whereas the solution Cd/P molar ratio increased constantly from 1.65–1.69 to 6.61–6.76. The mean &lt;i&gt;K&lt;/i&gt;\u0000 &lt;sub&gt;sp&lt;/sub&gt; values for Cd&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH were determined to be 10&lt;sup&gt;?64.62&lt;/sup&gt; (10&lt;sup&gt;?64.53&lt;/sup&gt;–10&lt;sup&gt;?64.71&lt;/sup&gt;) at 25°C, 10&lt;sup&gt;?65.58&lt;/sup&gt; (10&lt;sup&gt;?65.31&lt;/sup&gt;–10&lt;sup&gt;?65.80&lt;/sup&gt;) at 35°C and 10&lt;sup&gt;?66.57&lt;/sup&gt; (10&lt;sup&gt;?66.24&lt;/sup&gt;–10&lt;sup&gt;?66.90&lt;/sup&gt;) at 45°C. Based on the obtained solubility data from the dissolution at initial pH 2 and 25°C, the Gibbs free energy of Cd&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH forming &lt;span&gt;(Delta G_{f}^{o})&lt;/span&gt; was determined to be ?3,970.47?kJ/mol (?3,969.92 to ?3,970.96?kJ/mol). Thermodynamic parameters, Δ&lt;i&gt;G&lt;/i&gt;\u0000 &lt;sup&gt;0&lt;/sup&gt;, Δ&lt;i&gt;H&lt;/i&gt;\u0000 &lt;sup&gt;0&lt;/sup&gt;, Δ&lt;i&gt;S&lt;/i&gt;\u0000 &lt;sup&gt;0&lt;/sup&gt;, and &lt;span&gt;(Delta C_{text{p}}^{ 0})&lt;/span&gt; for the dissolution process of Cd-HAP in aqueous acidic media at initial pH 2 and 25°C were calculated 368,710.12?J/K?mol, ?158,809.54?J/mol, ?1,770.20 and ?869.53?J/K?mol, respectively.&lt;/p&gt;&lt;p&gt;Based on the experimental results of the present work and some previous researches, the cadmium hydroxylapatite (Cd-HAP) dissolution in aqueous media is considered to have the following coincident processes: the stoichiometric dissolution coupled with protonation and complexation reactions, the non-stoichiometric dissolution with Cd&lt;sup&gt;2+&lt;/sup&gt; release and PO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;3?&lt;/sup&gt; sorption and the sorption of Cd&lt;sup&gt;2+&lt;/sup&gt; and PO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;3?&lt;/sup&gt; species from solution backwards onto Cd-HAP surface. The obtained solubility products (&lt;i&gt;K&lt;/i&gt;\u0000 &lt;sub&gt;sp&lt;/sub&gt;","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0025-1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4719724","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Formation of todorokite from “c-disordered” H+-birnessites: the roles of average manganese oxidation state and interlayer cations","authors":"Huaiyan Zhao,&nbsp;Xinran Liang,&nbsp;Hui Yin,&nbsp;Fan Liu,&nbsp;Wenfeng Tan,&nbsp;Guohong Qiu,&nbsp;Xionghan Feng","doi":"10.1186/s12932-015-0023-3","DOIUrl":"https://doi.org/10.1186/s12932-015-0023-3","url":null,"abstract":"<p>Todorokite, a 3?×?3 tectomanganate, is one of three main manganese oxide minerals in marine nodules and can be used as an active MnO₆ octahedral molecular sieve. The formation of todorokite is closely associated with the poorly crystalline phyllomanganates in nature. However, the effect of the preparative parameters on the transformation of “<i>c</i>-disordered” H⁺-birnessites, analogue to natural phyllomanganates, into todorokite has not yet been explored.</p><p>Synthesis of “<i>c</i>-disordered” H⁺-birnessites with different average manganese oxidation states (AOS) was performed by controlling the MnO₄\u0000 ^?/Mn²⁺ ratio in low-concentrated NaOH or KOH media. Further transformation to todorokite, using “<i>c</i>-disordered” H⁺-birnessites pre-exchanged with Na⁺ or K⁺ or not before exchange with Mg²⁺, was conducted under reflux conditions to investigate the effects of Mn AOS and interlayer cations. The results show that all of these “<i>c</i>-disordered” H⁺-birnessites exhibit hexagonal layer symmetry and can be transformed into todorokite to different extents. “<i>c</i>-disordered” H⁺-birnessite without pre-exchange treatment contains lower levels of Na/K and is preferably transformed into ramsdellite with a smaller 1?×?2 tunnel structure rather than todorokite. Na⁺ pre-exchange, i.e. to form Na-H-birnessite, greatly enhances transformation into todorokite, whereas K⁺ pre-exchange, i.e. to form K-H-birnessite, inhibits the transformation. This is because the interlayer K⁺ of birnessite cannot be completely exchanged with Mg²⁺, which restrains the formation of tunnel “walls” with 1?nm in length. When the Mn AOS values of Na-H-birnessite increase from 3.58 to 3.74, the rate and extent of the transformation sharply decrease, indicating that a key process is Mn(III) species migration from layer into interlayer to form the tunnel structure during todorokite formation.</p><p>Structural Mn(III), together with the content and type of interlayer metal ions, plays a crucial role in the transformation of “<i>c</i>-disordered” H⁺-birnessites with hexagonal symmetry into todorokite. This provides further explanation for the common occurrence of todorokite in the hydrothermal ocean environment, where is usually enriched in large metal ions such as Mg, Ca, Ni, Co and etc. These results have significant implications for exploring the origin and formation process of todorokite in various geochemical settings and promoting the practical application of todorokite in many fields.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0023-3","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4607412","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Annual sulfur cycle in a warm monomictic lake with sub-millimolar sulfate concentrations","authors":"Nadav Knossow,&nbsp;Barak Blonder,&nbsp;Werner Eckert,&nbsp;Alexandra V Turchyn,&nbsp;Gilad Antler,&nbsp;Alexey Kamyshny Jr.","doi":"10.1186/s12932-015-0021-5","DOIUrl":"https://doi.org/10.1186/s12932-015-0021-5","url":null,"abstract":"<p>We studied the annual variability of the concentration and isotopic composition of main sulfur species and sulfide oxidation intermediates in the water column of monomictic fresh-water Lake Kinneret. Sulfate concentrations in the lake are &lt;1?mM and similar to concentrations that are proposed to have existed in the Paleoproterozoic ocean. The main goal of this research was to explore biogeochemical constrains of sulfur cycling in the modern low-sulfate fresh-water lake and to identify which processes may be responsible for the isotopic composition of sulfur species in the Precambrian sedimentary rocks.</p><p>At the deepest point of the lake, the sulfate inventory decreases by more than 20% between March and December due to microbial sulfate reduction leading to the buildup of hydrogen sulfide. During the initial stages of stratification, sulfur isotope fractionation between sulfate and hydrogen sulfide is low (11.6?‰) and sulfur oxyanions (e.g. thiosulfate and sulfite) are the main products of the incomplete oxidation of hydrogen sulfide. During the stratification and at the beginning of the lake mixing (July–December), the inventory of hydrogen sulfide as well as of sulfide oxidation intermediates in the water column increases and is accompanied by an increase in sulfur isotope fractionation to 30?±?4?‰ in October. During the period of erosion of the chemocline, zero-valent sulfur prevails over sulfur oxyanions. In the terminal period of the mixing of the water column (January), the concentration of hydrogen sulfide decreases, the inventory of sulfide oxidation intermediates increases, and sulfur isotope fractionation decreases to 20?±?2?‰.</p><p>Sulfide oxidation intermediates are present in the water column of Lake Kinneret at all stages of stratification with significant increase during the mixing of the water column. Hydrogen sulfide inventory in the water column increases from March to December, and sharply decreases during the lake mixis in January. Sulfur isotope fractionation between sulfate and hydrogen sulfide as well as concentrations of sulfide oxidation intermediates can be explained either by microbial sulfate reduction alone or by microbial sulfate reduction combined with microbial disproportionation of sulfide oxidation intermediates. Our study of sulfur cycle in Lake Kinneret may be useful for understanding the range of biogeochemical processes in low sulfate oceans over Earth history.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-07-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0021-5","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4085558","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}