Interactions of ferrous iron with clay mineral surfaces during sorption and subsequent oxidation

Fe(II)-catalyzed phase transformation of Cd(II)-bearing ferrihydrite-kaolinite associations under anoxic conditions: New insights to role of kaolinite and fate of Cd(II)

Cadmium-bearing ferrihydrite-kaolinite associations (Cd-associations) are commonly found in cadmium-contaminated paddy soils in tropical and subtropical regions. In the presence of anaerobic conditions caused by flooding, the creation of Fe(II) can facilitate the transformation of ferrihydrite into secondary Fe (hydr)oxides, resulting in the redistribution of Cd. However, the role of kaolinite in iron oxides transformation and changes in Cd chemical species have largely not been determined. In this study, Cd-associations were prepared for reaction with Fe(II) under anoxic conditions. The results obtained from powder XRD and EXAFS indicated that the presence of kaolinite association noticeably hastened the transformation of ferrihydrite into crystalline goethite. Specific surface area and electrochemical analyses revealed that smaller particle sizes and higher reactivity of ferrihydrite within Cd-associations collaboratively contribute to the acceleration. Chemical analyses demonstrated a significant negative correlation between ferrihydrite-Fe and aqueous-Cd, and a significant positive correlation between crystalline-Fe and residual-Cd. HRTEM analyses indicated that a portion of the Cd was incorporated into the crystal lattices of lepidocrocite and goethite, with the majority of Cd being sequestered within goethite lattice. These findings provide new insights into the roles of clay minerals in the geochemical cycling of Fe and Cd in paddy soils under anoxic conditions.

The environmental impact of heavy metals in sediments of main valleys in the eastern side of Mosul City, Iraq

Analyzing the geochemical changes in stream sediments can reveal important surface processes on Earth, like weathering, transportation, and cation exchange. The study area is located on the eastern side of Mosul, where valleys named Al-Rashediya, Al-Kharrazi, Al-Khosar, Al-Danffilli, and Al-Shor flow towards the Tigris River. These valleys' sediments contain diverse components like clay minerals, organic matter, iron oxides, carbonates, and heavy metals (H.M.s), either as part of these substances or adsorbed onto them. In this study, 36 sediment samples were gathered from these valleys. They underwent chemical analysis through X-ray fluorescence to ascertain their chemical constituents of major oxides. To understand the distribution of H.M.s in these sediments, correlation coefficient analysis and factor analysis were utilized. The study employed the geoaccumulation index (Igeo) and enrichment factor (E.F.) to evaluate sediment contamination. The results of Igeo ranged from Cr = 0.24 to 1.83, Ni = -0.92 to 0.77, Cu = -2.41 to 0.05, Zn = -1.83 to 0.89, Pb = -1.54 to 0.36, and As = -2.84 to 0.80. These findings suggest that the valley sediments are generally in the range of deficiency to minimal enrichment and moderate enrichment. However, Al-Danffilli Valley shows strong contamination levels for Cu, Zn, and Pb. The E.F. values for Cr = 3.63-12.50, Ni = 1.95-4.19, Cu = 0.69-12.36, Zn = 1.08-16.19, Pb = 1.25-62.16, and As = 0.60-1.79 indicate levels ranging from deficiency to minimal and moderate enrichment. Al-Danffilli Valley, in particular, was identified as ranging from moderate to extremely high enrichment, attributed to its location near industrial areas and its tributaries.

Reduction Pathway-Dependent Formation of Reactive Fe(II) Sites in Clay Minerals

Structural Fe in clay minerals is an important, potentially renewable source of electron equivalents for contaminant reduction, yet our knowledge of how clay mineral Fe reduction pathways and Fe reduction extent affect clay mineral Fe(II) reactivity is limited. Here, we used a nitroaromatic compound (NAC) as a reactive probe molecule to assess the reactivity of chemically reduced (dithionite) and Fe(II)-reduced nontronite across a range of reduction extents. We observed biphasic transformation kinetics for all nontronite reduction extents of ≥5% Fe(II)/Fe(total) regardless of the reduction pathway, indicating that two Fe(II) sites of different reactivities form in nontronite at environmentally relevant reduction extents. At even lower reduction extents, Fe(II)-reduced nontronite completely reduced the NAC whereas dithionite-reduced nontronite could not. Our ⁵⁷Fe Mössbauer spectroscopy, ultraviolet-visible spectroscopy, and kinetic modeling results suggest that the highly reactive Fe(II) entities likely comprise di/trioctahedral Fe(II) domains in the nontronite structure regardless of the reduction mechanism. However, the second Fe(II) species, of lower reactivity, varies and for Fe(II)-reacted NAu-1 likely comprises Fe(II) associated with an Fe-bearing precipitate formed during electron transfer from aqueous to nontronite Fe. Both our observation of biphasic reduction kinetics and the nonlinear relationship of rate constant and clay mineral reduction potential E_H have major implications for contaminant fate and remediation.

Freshwater suspended particulate matter-Key components and processes in floc formation and dynamics

Freshwater suspended particulate matter (SPM) plays an important role in many biogeochemical cycles and serves multiple ecosystem functions. Most SPM is present as complex floc-like aggregate structures composed of various minerals and organic matter from the molecular to the organism level. Flocs provide habitat for microbes and feed for larger organisms. They constitute microbial bioreactors, with prominent roles in carbon and inorganic nutrient cycles, and transport nutrients as well as pollutants, affecting sediments, inundation zones, and the ocean. Composition, structure, size, and concentration of SPM flocs are subject to high spatiotemporal variability. Floc formation processes and compositional or morphological dynamics can be established around three functional components: phyllosilicates, iron oxides/(oxy)hydroxides (FeOx), and microbial extracellular polymeric substances (EPS). These components and their interactions increase heterogeneity in surface properties, enhancing flocculation. Phyllosilicates exhibit intrinsic heterogeneities in surface charge and hydrophobicity. They are preferential substrates for precipitation or attachment of reactive FeOx. FeOx form patchy coatings on minerals, especially on phyllosilicates, which increase surface charge heterogeneities. Both, phyllosilicates and FeOx strongly adsorb natural organic matter (NOM), preferentially certain EPS. EPS comprise various substances with heterogeneous properties that make them a sticky mixture, enhancing flocculation. Microbial metabolism, and thus EPS release, is supported by the high adsorption capacity and favorable nutrient composition of phyllosilicates, and FeOx supply essential Fe.

Soil parameters affecting the levels of potentially harmful metals in Thessaly area, Greece: a robust quadratic regression approach of soil pollution prediction

The behavior and possible contamination risk due to the presence of potentially harmful metals (PHM) were studied based on 2250 soil samples that were collected in a 5-year period (2013-2017) from the plain of Thessaly (prefectures of Karditsa, Trikala, and Larissa). The vertical distribution of metals was also investigated from sample profiles at three depths 0-30, 30-60, and 60-90cm. The soils of the sampling belong to four taxonomy soil orders that are dominant in the studied area (Alfisols, Inceptisols, Endisols, and Vertisols). In a novel approach, robust quadratic regression analysis on multiple variables was used to define prediction models of the concentrations of two metals: Fe which is an essential metal and the toxic Cd. Linear and quadratic regression formulae were estimated based on the iteratively reweighted least squares robust regression approach in an effort to eliminate the impact of the outliers. These formulae define how several soil properties affect the distribution of the considered metals in each soil order. The evaluation of the estimated regression equations based on the R² metric indicates that they constitute a useful, reliable, and valuable tool for managing, describing, and predicting the pollution in the studied area.

Ferrihydrite enrichment in the rhizosphere of unsaturated soil improves nutrient retention while limiting arsenic and uranium plant uptake

Improvement of nutrient use efficiency and limiting trace elements such as arsenic and uranium bioavailability is critical for sustainable agriculture and food safety. Arsenic and uranium possess different properties and mobility in soils, which complicates the effort to reduce their uptake by plants. Here, we postulate that unsaturated soil amended with ferrihydrite nanominerals leads to improved nutrient retention and helps reduce uptake of these geogenic contaminants. Unsaturated soil is primarily oxic and can provide a stable environment for ferrihydrite nanominerals. To demonstrate the utility of ferrihydrite soil amendment, maize was grown in an unsaturated agricultural soil that is known to contain geogenic arsenic and uranium. The soil was maintained at a gravimetric moisture content of 15.1 ± 2.5%, typical of periodically irrigated soils of the US Corn Belt. Synthetic 2-line ferrihydrite was used in low doses as a soil amendment at three levels (0.00% w/w (control), 0.05% w/w and 0.10% w/w). Further, the irrigation water was fortified (~50 μg L^-1 each) with elevated arsenic and uranium levels. Plant dry biomass at maturity was ~13.5% higher than that grown in soil not receiving ferrihydrite, indicating positive impact of ferrihydrite on plant growth. Arsenic and uranium concentrations in maize crops (root, shoot and grain combined) were ~ 20% lower in amended soils than that in control soils. Our findings suggest that the addition of low doses of iron nanomineral soil amendment can positively influence rhizosphere geochemical processes, enhancing nutrient plant availability and reduce trace contaminants plant uptake in sprinkler irrigated agroecosystem, which is 55% of total irrigated area in the United States.

Remediation of Toxic Heavy Metal Contaminated Soil by Combining a Washing Ejector Based on Hydrodynamic Cavitation and Soil Washing Process

Based on the features of hydrodynamic cavitation, in this study, we developed a washing ejector that utilizes a high-pressure water jet. The cavitating flow was utilized to remove fine particles from contaminated soil. The volume of the contaminants and total metal concentration could be correlated to the fine-particle distribution in the contaminated soil. These particles can combine with a variety of pollutants. In this study, physical separation and soil washing as a two-step soil remediation strategy were performed to remediate contaminated soils from the smelter. A washing ejector was employed for physical separation, whereas phosphoric acid was used as the washing agent. The particles containing toxic heavy metals were composed of metal phase encapsulated in phyllosilicates, and metal phase weakly bound to phyllosilicate surfaces. The washing ejector involves the removal of fine particles bound to coarse particles and the dispersion of soil aggregates. From these results we determined that physical separation using a washing ejector was effective for the treatment of contaminated soil. Phosphoric acid (H3PO4) was effective in extracting arsenic from contaminated soil in which arsenic was associated with amorphous iron oxides. Thus, the obtained results can provide useful information and technical support for field soil washing for the remediation of soil contaminated by toxic heavy metals through emissions from the mining and ore processing industries.

Antibacterial Mechanisms of Reduced Iron-Containing Smectite-Illite Clay Minerals

Reduced nontronite has been demonstrated to be antibacterial through the production of hydroxyl radical (^•OH) from the oxidation of structural Fe(II). Herein, we investigated the antibacterial activity of more common smectite-illite (S-I) clays toward Escherichia coli cells, including montmorillonite SWy-3, illite IMt-2, 50-50 S-I rectorite RAr-1, 30-70 S-I ISCz-1, and nontronite NAu-2. Under an oxic condition, reduced clays (with a prefix r before mineral names) produced reactive oxygen species (ROS), and the antibacterial activity followed the order of rRAr-1 > rSWy-3 ≥ rNAu-2 ≫ rIMt-2 ≥ rISCz-1. The strongest antibacterial activity of rRAr-1 was contributed by a combination of ^•OH and Fe(IV) generated from structural Fe(II)/adsorbed Fe²⁺ and soluble Fe²⁺, respectively. Higher levels of lipid and protein oxidation, intracellular ROS accumulation, and membrane disruption were consistent with this antibacterial mechanism of rRAr-1. The antibacterial activity of other S-I clays depended on layer expandability, which determined the reactivity of structural Fe(II) and the production of ^•OH, with the expandable smectite being the most antibacterial and nonexpandable illite the least. Our results provide new insights into the antibacterial mechanisms of clay minerals.

Retention of Iodide and Chloride by Formation of a Green Rust Solid Solution GR-Cl1-xIx: A Multiscale Approach

The uptake of iodide and chloride during the synthesis of green rust (GR), the Fe endmember of the layered double hydroxide (LDH) group, was investigated. GR compounds were prepared by aerial oxidation of Fe(OH)₂ in suspension, considering various I/Cl ratios at constant ionic strength. Only GR compounds formed in all experiments, and the associated I/Cl ratio increased with that of the starting suspension. No preferential uptake of any halide could be detected, and all compounds had comparable morphology. Furthermore, the height of the interlayer gallery increased with the I/Cl ratio from ∼7.7 Å for the chloride endmember to ∼8.3 Å for the iodide endmember, and the observed linear increase was attributed to increasing interlayer iodide content. In all compounds, Fe K-edge X-ray absorption spectroscopy evidenced the presence of sixfold coordinated iron with a Fe²⁺/Fe³⁺ ratio of 3, homogeneously distributed within flattened octahedral sites, with six Fe as next-nearest neighbors. The Fe short-range environment was not affected by the interlayer composition, and no halide from the interlayer could be detected. Furthermore, iodide and chloride anions are located in a water-like environment, being loosely bound by weak electrostatic interactions to the octahedral sheet likely above ferric iron. Results consistently hint at the formation of a solid solution between chloride and iodide GR endmembers, certainly facilitated by the crystallization of both compounds in the same space group. This study provides further insights into the ability of LDH to simultaneously accommodate several anionic species of various sizes. The formation of such LDH compounds in a deep geological repository for nuclear waste thus represents a possible retention barrier to the migration to the far field of anionic species like ³⁶Cl^- and ¹²⁹I^- mobilized from the waste matrix. The extent of retention in disposal sites will depend, among others, on the availability of GR and on the concentration of competing anions.

Vertical redox zones of Fe-S-As coupled mineralogy in the sediments of Hetao Basin - Constraints for groundwater As contamination

The formation of iron-sulfur-arsenic (Fe-S-As) minerals during biogeochemical processes in As contaminated aquifers remains poorly understood despite their importance to understanding As release and transport in such systems. In this study, X-ray absorption and Mössbauer spectroscopies complemented by electron microscopy, and chemical extractions were used to examine vertical changes of As, Fe and S speciation for the example of sediments in the Hetao Basin. Reduction of Fe(III), As(V) and SO₄^2- species were shown to co-occur in the aquifers. Iron oxides were observed to be predominantly goethite and hematite (36 - 12%) and appeared to decrease in abundance with depth. Furthermore, reduced As (including arsenite and As sulfides) and sulfur species (including S(-II), S(-I) and S⁰) increased from 16% to 76% and from 13% to 44%, respectively. Iron oxides were the major As carrier in the sediments, and the lower groundwater As concentration consists with less desorbable and reducible As in the sediments. The formation of As-Fe sulfides (e.g., As containing pyrite and greigite) induced by redox heterogeneities likely contribute to localized lower groundwater As concentrations. These results help to further elucidate the complex relationship between biogeochemical processes and minerals formation in As contaminated aquifers.

The Effect of Aeration on Mn(II) Sorbed to Clay Minerals and Its Impact on Cd Retention

Manganese is a redox-sensitive element in soils and sediments that plays an important role in the retention of trace elements. Under anoxic conditions, clay minerals were shown to increase Cd retention by favoring the precipitation of Mn(II) phases. In this study, we investigated the influence of aeration on anoxically formed Mn solid phases and its impact on Cd retention in the presence of two clay minerals with low Fe contents, a natural kaolinite (KGa-1b) and a synthetic montmorillonite (Syn-1). Ca-saturated KGa-1b and Syn-1 were pre-equilibrated with Mn²⁺ and Cd²⁺ under anoxic conditions for 1 or 30 days and subsequently exposed to air for 1 or 30 days. The analysis with synchrotron X-ray absorption spectroscopy (XAS) revealed that extended anoxic pre-equilibration (30 days) partially prevented the oxidation of sorbed Mn(II) (MnSiO₃ and Mn(II)Al-LDH). Extended exposure to ambient air and short anoxic pre-equilibration favored the formation of feitknechtite (β-MnOOH) and birnessite (δ-MnO₂). Aeration resulted in a decrease of pH and a net release of Cd²⁺ into the solution, indicating that Cd re-sorption by Mn(III/IV)-phases was insufficient to compensate for the release of Cd²⁺ due to dissolution of Mn(II)-containing phases and the decrease in pH. Our results demonstrate the significance of clay minerals in the (trans)formation of Mn-containing phases and their impact on trace metal retention in environments undergoing fluctuating redox conditions.