Geochemical Transactions最新文献

{"title":"Oxidation of arsenite to arsenate on birnessite in the presence of light","authors":"Samantha L. Shumlas,&nbsp;Soujanya Singireddy,&nbsp;Akila C. Thenuwara,&nbsp;Nuwan H. Attanayake,&nbsp;Richard J. Reeder,&nbsp;Daniel R. Strongin","doi":"10.1186/s12932-016-0037-5","DOIUrl":"https://doi.org/10.1186/s12932-016-0037-5","url":null,"abstract":"<p>The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (k_obs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04?h^?1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-10-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0037-5","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4277928","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":"Sources of salinity and arsenic in groundwater in southwest Bangladesh","authors":"John C. Ayers,&nbsp;Steven Goodbred,&nbsp;Gregory George,&nbsp;David Fry,&nbsp;Laura Benneyworth,&nbsp;George Hornberger,&nbsp;Kushal Roy,&nbsp;Md. Rezaul Karim,&nbsp;Farjana Akter","doi":"10.1186/s12932-016-0036-6","DOIUrl":"https://doi.org/10.1186/s12932-016-0036-6","url":null,"abstract":"<p>High salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012–2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy.</p><p>Drill cuttings, exposures in pits and regional studies reveal a?&gt;5?m thick surface mud cap overlying a?~30?m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60?m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of?~2?cm/year through the early Holocene, with local subsidence or compaction rates of 1–3?mm/year. Most tubewells are screened at depths of 15–52?m in sediments deposited 8000–9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na–Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11–57?mg/L and As 2–258?ug/L, and low sulfur (S) 2–33?mg/L.</p><p>Groundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO₄) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured ¹⁴C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46?% for As and 100?% for salinity.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0036-6","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4460155","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 study on the dissolution rates of K-Cr(VI)-jarosites: kinetic analysis and implications","authors":"Iván A. Reyes,&nbsp;Ister Mireles,&nbsp;Francisco Patiño,&nbsp;Thangarasu Pandiyan,&nbsp;Mizraim U. Flores,&nbsp;Elia G. Palacios,&nbsp;Emmanuel J. Gutiérrez,&nbsp;Martín Reyes","doi":"10.1186/s12932-016-0035-7","DOIUrl":"https://doi.org/10.1186/s12932-016-0035-7","url":null,"abstract":"&lt;p&gt;The presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others. Understanding the dissolution reactions of jarosite-type compounds is notably important for an environmental assessment (for water and soil), since some of these elements could either return to the environment or work as temporary deposits of these species, thus would reduce their immediate environmental impact.&lt;/p&gt;&lt;p&gt;This work reports the effects of temperature, pH, particle diameter and Cr(VI) content on the initial dissolution rates of K-Cr(VI)-jarosites (KFe&lt;sub&gt;3&lt;/sub&gt;[(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2???X&lt;/sub&gt;(CrO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;X&lt;/sub&gt;](OH)&lt;sub&gt;6&lt;/sub&gt;). Temperature (T) was the variable with the strongest effect, followed by pH in acid/alkaline medium (H&lt;sub&gt;3&lt;/sub&gt;O&lt;sup&gt;+&lt;/sup&gt;/OH&lt;sup&gt;?&lt;/sup&gt;). It was found that the substitution of CrO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;2?&lt;/sup&gt;in &lt;i&gt;Y&lt;/i&gt;-site and the substitution of H&lt;sub&gt;3&lt;/sub&gt;O&lt;sup&gt;+&lt;/sup&gt; in &lt;i&gt;M&lt;/i&gt;-site do not modify the dissolution rates. The model that describes the dissolution process is the unreacted core kinetic model, with the chemical reaction on the unreacted core surface. The dissolution in acid medium was congruent, while in alkaline media was incongruent. In both reaction media, there is a release of K&lt;sup&gt;+&lt;/sup&gt;, SO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;2?&lt;/sup&gt; and CrO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;2?&lt;/sup&gt; from the KFe&lt;sub&gt;3&lt;/sub&gt;[(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2???X&lt;/sub&gt;(CrO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;X&lt;/sub&gt;](OH)&lt;sub&gt;6&lt;/sub&gt; structure, although the latter is rapidly absorbed by the solid residues of Fe(OH)&lt;sub&gt;3&lt;/sub&gt; in alkaline medium dissolutions. The dissolution of KFe&lt;sub&gt;3&lt;/sub&gt;[(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2???X&lt;/sub&gt;(CrO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;X&lt;/sub&gt;](OH)&lt;sub&gt;6&lt;/sub&gt; exhibited good stability in a wide range of pH and T conditions corresponding to the calculated parameters of reaction order &lt;i&gt;n&lt;/i&gt;, activation energy &lt;i&gt;E&lt;/i&gt;\u0000 &lt;sub&gt;\u0000 &lt;i&gt;A&lt;/i&gt;\u0000 &lt;/sub&gt; and dissolution rate constants for each kinetic stages of induction and progressive conversion.&lt;/p&gt;&lt;p&gt;The kinetic analysis related to the reaction orders and calculated activation energies confirmed that extreme pH and T conditions are necessary to obtain considerably high dissolution rates. Extreme pH conditions (acidic or alkaline) cause the preferential release of K&lt;sup&gt;+&lt;/sup&gt;, SO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;2?&lt;/sup&gt; and CrO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;2?&lt;/sup&gt; from the KFe&lt;sub&gt;3&lt;/sub&gt;[(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2???X&lt;/sub&gt;(CrO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;X&lt;/sub&gt;](OH)&lt;sub&gt;6&lt;/sub&gt; structure, although CrO&lt;sub&gt;4&lt;/sub&gt;\u0000 &lt;sup&gt;2?&lt;/sup&gt; is quickly adsorbed by Fe(OH)&lt;sub&gt;3&lt;/sub&gt; solid residues. The precipitation of phases such as KFe&lt;sub&gt;3&lt;/sub&gt;[(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2???X&lt;/sub&gt;(CrO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;X&lt;/sub&gt;]","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0035-7","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4545268","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 the hydroxypyromorphite–hydroxyapatite solid solution [(PbxCa1−x)5(PO4)3OH] at 25 °C and pH 2–9","authors":"Yinian Zhu,&nbsp;Bin Huang,&nbsp;Zongqiang Zhu,&nbsp;Huili Liu,&nbsp;Yanhua Huang,&nbsp;Xin Zhao,&nbsp;Meina Liang","doi":"10.1186/s12932-016-0034-8","DOIUrl":"https://doi.org/10.1186/s12932-016-0034-8","url":null,"abstract":"&lt;p&gt;The interaction between Ca-HAP and Pb&lt;sup&gt;2+&lt;/sup&gt; solution can result in the formation of a hydroxyapatite–hydroxypyromorphite solid solution [(Pb&lt;sub&gt;x&lt;/sub&gt;Ca&lt;sub&gt;1?x&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;(OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil. Therefore, it’s necessary to know the physicochemical properties of (Pb&lt;sub&gt;x&lt;/sub&gt;Ca&lt;sub&gt;1?x&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;(OH), predominantly its thermodynamic solubility and stability in aqueous solution. Nevertheless, no experiment on the dissolution and related thermodynamic data has been reported.&lt;/p&gt;&lt;p&gt;Dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb&lt;sub&gt;x&lt;/sub&gt;Ca&lt;sub&gt;1?x&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;(OH)] in aqueous solution at 25?°C was experimentally studied. The aqueous concentrations were greatly affected by the Pb/(Pb?+?Ca) molar ratios (X&lt;sub&gt;Pb&lt;/sub&gt;) of the solids. For the solids with high X&lt;sub&gt;Pb&lt;/sub&gt; [(Pb&lt;sub&gt;0.89&lt;/sub&gt;Ca&lt;sub&gt;0.11&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH], the aqueous Pb&lt;sup&gt;2+&lt;/sup&gt; concentrations increased rapidly with time and reached a peak value after 240–720?h dissolution, and then decreased gradually and reached a stable state after 5040?h dissolution. For the solids with low X&lt;sub&gt;Pb&lt;/sub&gt; (0.00–0.80), the aqueous Pb&lt;sup&gt;2+&lt;/sup&gt; concentrations increased quickly with time and reached a peak value after 1–12?h dissolution, and then decreased gradually and attained a stable state after 720–2160?h dissolution.&lt;/p&gt;&lt;p&gt;The dissolution process of the solids with high X&lt;sub&gt;Pb&lt;/sub&gt; (0.89–1.00) was different from that of the solids with low X&lt;sub&gt;Pb&lt;/sub&gt; (0.00–0.80). The average &lt;i&gt;K&lt;/i&gt;\u0000 &lt;sub&gt;sp&lt;/sub&gt; values were estimated to be 10&lt;sup&gt;?80.77±0.20&lt;/sup&gt; (10&lt;sup&gt;?80.57&lt;/sup&gt;–10&lt;sup&gt;?80.96&lt;/sup&gt;) for hydroxypyromorphite [Pb&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH] and 10&lt;sup&gt;?58.38±0.07&lt;/sup&gt; (10&lt;sup&gt;?58.31&lt;/sup&gt;–10&lt;sup&gt;?58.46&lt;/sup&gt;) for calcium hydroxyapatite [Ca&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH]. The Gibbs free energies of formation (Δ&lt;i&gt;G&lt;/i&gt;\u0000 &lt;span&gt;\u0000 &lt;sup&gt;&lt;i&gt;o&lt;/i&gt;&lt;/sup&gt;&lt;sub&gt;\u0000 &lt;i&gt;f&lt;/i&gt;\u0000 &lt;/sub&gt;\u0000 \u0000 &lt;/span&gt;) were determined to be ?3796.71 and ?6314.63?kJ/mol, respectively. The solubility decreased with the increasing Pb/(Pb?+?Ca) molar ratios (X&lt;sub&gt;Pb&lt;/sub&gt;) of (Pb&lt;sub&gt;x&lt;/sub&gt;Ca&lt;sub&gt;1?x&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;(OH). For the dissolution at 25?°C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (Pb&lt;sub&gt;x&lt;/sub&gt;Ca&lt;sub&gt;1?x&lt;/sub&gt;)&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;(OH) dissolved stoichiometrically at the early stage of dissolution and moved gradually up to the Lippmann &lt;i&gt;solutus&lt;/i&gt; curve and the saturation curve for Pb&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OH, and then the data points moved","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0034-8","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4264130","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":"Effects of common groundwater ions on chromate removal by magnetite: importance of chromate adsorption","authors":"Amanda H. Meena,&nbsp;Yuji Arai","doi":"10.1186/s12932-016-0033-9","DOIUrl":"https://doi.org/10.1186/s12932-016-0033-9","url":null,"abstract":"<p>Reductive precipitation of hexavalent chromium (Cr(VI)) with magnetite is a well-known Cr(VI) remediation method to improve water quality. The rapid (&lt;a few hr) reduction of soluble Cr(VI) to insoluble Cr(III) species by Fe(II) in magnetite has been the primary focus of the Cr(VI) removal process in the past. However, the contribution of simultaneous Cr(VI) adsorption processes in aged magnetite has been largely ignored, leaving uncertainties in evaluating the application of in situ Cr remediation technologies for aqueous systems. In this study, effects of common groundwater ions (i.e., nitrate and sulfate) on Cr(VI) sorption to magnetite were investigated using batch geochemical experiments in conjunction with X-ray absorption spectroscopy.</p><p>In both nitrate and sulfate electrolytes, batch sorption experiments showed that Cr(VI) sorption decreases with increasing pH from 4 to 8. In this pH range, Cr(VI) sorption decreased with increasing ionic strength of sulfate from 0.01 to 0.1?M whereas nitrate concentrations did not alter the Cr(VI) sorption behavior. This indicates the background electrolyte specific Cr(VI) sorption process in magnetite. Under the same ionic strength, Cr(VI) removal in sulfate containing solutions was greater than that in nitrate solutions. This is because the oxidation of Fe(II) by nitrate is more thermodynamically favorable than by sulfate, leaving less reduction capacity of magnetite to reduce Cr(VI) in the nitrate media. X-ray absorption spectroscopy analysis supports the macroscopic evidence that more than 75?% of total Cr on the magnetite surfaces was adsorbed Cr(VI) species after 48?h.</p><p>This experimental geochemical study showed that the adsorption process of Cr(VI) anions was as important as the reductive precipitation of Cr(III) in describing the removal of Cr(VI) by magnetite, and these interfacial adsorption processes could be impacted by common groundwater ions like sulfate and nitrate. The results of this study highlight new information about the large quantity of adsorbed Cr(VI) surface complexes at the magnetite-water interface. It has implications for predicting the long-term stability of Cr at the magnetite-water interface.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-04-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0033-9","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5104883","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":"Erratum to: Zn (II) and Cu (II) adsorption and retention onto iron oxyhydroxide nanoparticles: effects of particle aggregation and salinity","authors":"Rebecca B. Chesne,&nbsp;Christopher S. Kim","doi":"10.1186/s12932-015-0032-2","DOIUrl":"https://doi.org/10.1186/s12932-015-0032-2","url":null,"abstract":"","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"16 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2015-11-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-015-0032-2","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4764344","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":"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}