Clay mineral influences on the formation of ferrous layered double hydroxides (LDH) in flooded soil environments
Date
2019
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Publisher
University of Delaware
Abstract
This project involved a series of studies to investigate the interactions between dissolved Fe(II) and soil clay minerals (i.e. phyllosilicates) in redox reducing environments and how this affects the affect the formation and composition of highly Fe(II)-containing layered double hydroxides (LDHs) (e.g. ‘green rusts’). Interactions between Fe(II) and aluminosilicates include adsorption, electron transfer, and dissolution-coprecipitation which can result in Fe(II) hydroxides with a range of Fe(II), Fe(III), Al, Si and Mg contents and surface properties. Ferrous LDHs are potent reductive transformers of trace elements so influences on their formation and character are crucial to understand for natural attenuation processes in fluctuating redox environments. ☐ The ability of natural clay minerals to form Fe(II)-Al LDHs during sorption of Fe(II) was tested using clay isolates high in hydroxy-interlayered minerals (e.g. chlorites and hydroxy-interlayered vermiculites). Clay minerals (aluminosilicates) were isolated from the Ap and Bt horizons of a typic hapludult agricultural soil by removing organic carbon, dithionite-reducible hydroxides and size fractionation below 2µm. The effect of reaction time and near-neutral pHs on Fe precipitation were also tested. Increasing pH in the near-neutral range had a strong effect on Fe(II) uptake with sorption increasing from 99-412 mmol g-1 clay as pH increased from 6.5 to 7.5. Kinetics of Fe(II) uptake at pH 7.5 were described by rapid initial uptake followed by slow continuous removal of Fe(II). Visual observations of a blue-green color indicate GR formation but kinetic citrate bicarbonate (CB) extraction of the Fe(II) reacted clays released labile Fe(II) and Fe(III) with a ratio of 5.48, far exceeding the range of GR (2-3) and suggesting that another highly Fe(II) rich mineral also formed. After reaction with Fe(II), CB extraction of Al and Si were also much higher than compared with unreacted clay which indicates these elements are more labile from the clay is promoted by Fe(II) reaction. In the same Fe(II) reacted clay, XRD showed the dissolution of chlorite (chamosite) which was present in clays before reaction with Fe(II). Fe sorption products after Fe(II) uptake on clay were amorphous to bulk XRD but linear combination fitting of bulk Fe EXAFS with standards identified 30% GR and 15% Fe(II)-Al LDH along with conversion of some Fe from Fe(III) to Fe(II) in the clay structure. Solution pH also affected the product of Fe(II) sorption, with increasing amounts of green rust (GR) being found from pH 6.5, 7.0 to 7.5 and Fe(II)-Al LDH only forming at pH 7.5. ☐ Scanning transmission x-ray microscopy (STXM) at the nano-to-micron scale of clay minerals after Fe(II) reaction identified an Fe-rich particle consistent with ideal GR stoichiometry as well as nanosized, highly ferrous particles. We hypothesize that the nanoparticles are Fe(II)-Al LDHs forming at the surfaces of clay mineral particles. STXM maps also show the mixture of Fe(II), Fe(III) and Al in short-range ordered solid surrounding two plate particles, showing the mixtures of Fe oxidation states at the location of hydroxyl interlayer dissolution. This this is the first evidence of a Fe(II)-Al LDH phase forming in natural soil clay minerals. In this soil, the source of Al reacting with Fe(II) appears to come from delamination of hydroxy-interlayered minerals. Soils containing hydroxy-interlayered minerals may be locations where Fe(II)-Al LDHs can form and contain high amounts of mixed Fe(II) hydroxides during Fe(III) reduction in soil. ☐ As Fe(II)-Fe(III) electron transfer can occur between sorbed Fe(II) and clay minerals containing Fe(III), we tested how changing Fe(II)/Fe(III) ratios between sorbed and structural Fe affect the fate of sorbed Fe(II) by reductive treatments to clay minerals and with Fe(II) isotherms. Initial clay redox status was altered by dithionite reductive treatment of clays which converted 19% of silicate-Fe(III) to silicate-Fe(II). Sorption of Fe(II) to dithionite reduced clay minerals was lower because of less surface oxidation of sorbed Fe(II) by silicate-Fe(III) electron transfer. The electron transfer mechanism was directly observed by using enriched isotopes of 56Fe for Fe(II) sorption isotherm to clay minerals and 57Fe Mossbauer spectroscopy. With this method we measure only the Fe species occurring in the original clay mineral Fe content. Increasing 56Fe(II) concentration with clay minerals resulted in a proportional decrease in silicate-Fe(III) and near equal rise in structural Fe(II) combined with an increase in short-range ordered Fe(III) hydroxides. This suggests that Fe(II) sorption reduces silicate-Fe(III) by electron transfer and also promotes chemical weathering and secondary Fe precipitation. The ratio of Fe(II) sorbed to effective electron transfer was less efficient (<1:1) with increasing quantity of Fe(II) indicating that Fe(II) is sorbing to an intermediate Fe(III)hydroxide covering the clay surface. Fe EXAFS shell fitting and LCF with standards of Fe(II) reacted clays identified increasing proportion of green rust and the more reduced Fe(II)-Al LDH was only observed at the highest sorption level of Fe(II). In clay minerals and after dithionite reduction, low concentrations of Fe(II) fully oxidized to Fe(III)hydroxides but at the highest uptake it produced a mixture of green rust and Fe(II)-Al LDH. The results of highly ferrous Fe-Al LDH indicates that initial clay mineral redox status can have a cascading effect on the oxidation state of surface precipitates with a more reduced clay and greater Fe(II) sorption creating a more reduced Fe(II) LDH at its surface. STXM images of the clay mineral indicate a high degree of heterogeneity in total Fe content and Fe oxidation state across particles. This indicates that each particle may have coexisting and vastly different capacities for electron transfer and Fe(II) surface oxidation during uptake. ☐ In the fourth chapter, we explored silicon (Si) coprecipitation with green rusts (GR) and how Si affected GR crystal formation and air-oxidation products. Green rusts (GR) are meta-stable and highly-reactive mixed-valent iron hydroxides that form in Fe-reducing soil and water environments. Green rusts play an important role in natural attenuation because they sorb and reductively transform many problematic trace elements (chromium, mercury, uranium, selenium), dechlorinate organic contaminants and denitrify nitrates and nitrite. Silicon (Si) is ubiquitous in environmental waters and above certain concentrations can decrease GR plate size and prevent dissolution-reprecipitation during oxidation. If reprecipitation is inhibited, trace elements could be less well sequestered because they are unable to incorporate into Fe(III) hydroxides. Our study seeks to answer how Si coprecipitation during Fe hydrolysis affects the formation of two major types of GR (sulfate and carbonate) and their oxidative transformation mechanisms after air oxidation. ☐ GR sulfate (GR(SO4)) and carbonate (GR(CO3)) both were coprecipitated by base titration to Fe and Si salts in increasing Si/Fe molar ratios (Si absent, 0.01, 0.1, 0.5) using a modified Ruby et al. 2006 method. Synthesis was monitored by pH measurements for pH response curves. Formed minerals and end products of rapid air-oxidation were assessed by traditional and synchrotron-source XRD, SEM, and N2-BET SA measurements. At every concentration of Si (Si/Fe 0-0.5), GRs were the only mineral identified by XRD for GR(SO4) and GR(CO3), respectively. SEM images showed a wide range of plate widths that only decreased size significantly at the highest Si content (0.5 Si/Fe molar ratio). The decrease in plate area was matched by an increase in BET surface area (m2g-1) 12.2(0.89) = 11.3(0.90) = 12.8(1.08) < 76.6(5.75) for Si/Fe molar ratios (Si absent, 0.01, 0.1, 0.5), respectively. GR plate thickness was not changed by Si content indicating that interlayer silication did not occur. Air oxidation caused GR(SO4) to transform by dissolution-oxidation precipitation to lepidocrocite and goethite but increasing Si content trended with shorter range ordered ferric hydroxides. Air oxidation caused GR(CO3) to transform to a mixture of magnetite and goethite but Si inhibited reprecipitation and above 0.1 Si/Fe MR, completely prevented it. Defining these conditions will help better predict how natural GRs interact during reductive transformation of contaminants and Fe cycling in Fe-reducing environments. ☐ The implication of these results are that parent material can influence the quantity and character of Fe(II) precipitation products by simultaneous adsorption, electron transfer and dissolution-precipitation. These interactions are a function of the soil properties and parent material mineralogy and should be considered as variables affecting Fe cycling in reducing geochemical environments. This provides more detail on Fe cycling and mineral transformations that occurs in dynamic, reducing geochemical environments.
Description
Keywords
Electron transfer, Green rust, Iron redox, Layered double hydroxide, Phyllosilicates, X-ray absorption spectroscopy