Percentage Of Smectite Research Articles

Mineralogy and geochemical characterization of geophagic clays consumed in parts of southern Nigeria

This study seeks to determine health risk associated with consumption of clay in different areas in southern Nigeria. Different clay samples sold for consumption were purchased from Mowe, Ikorodu and Onitsha Markets in southern Nigeria. The clays were subjected to mineralogical and geochemical characterization using X-Ray Diffractometry (XRD), Fourier Transform Infrared (FTIR) and Inductively Coupled Plasma-Atomic EmissionSpectrometry. Potential health risk assessment was calculated using a standardized Hazard Quotient (HQ) and Total Hazard Index (THI). XRD and FTIR results revealed the clay contains varying percentage of quartz, kaolinite, anatase, halite, pyrite, goethite, smectite and palygorskite while geochemical analysis result revealed wide range of elemental concentration (ppm). Mean concentration (ppm) of Cu, Pb, Zn, Cr, Ni and Co from the markets are 5.8 – 8; 17.4 – 29.6; 32.4 – 95.8;111.6 – 103.4; 18.6 – 39.4 and 4 – 16.6 respectively while As was below detection limit in most of the samples. HQ values were used to calculate the THI. THI values for children and adults for Mowe market samples range from 0.9 to 2.1 and 0.5–1.1 respectively, for Ikorodu market samples, THI for children and adult ranges from 1.0- 1.8 and 0.6–1.0 respectively, while THI for children and adults for Onitsha market samples ranges from 1.5 to 2.6 and 0.8–1.84 respectively. In conclusion, children that consume clay from the markets are more exposed to non-carcinogenic risks than adults. It is therefore advisable that the clays should not be consumed for a long period of time to avoid health problems.

Efficient Removal of Basic Fuchsin from Synthetic Medical Wastewater and Competitive Adsorption in the Mixture

The Ziehl-Neelsen stain is a mixture of Basic Fuchsin (BF), phenol (Ph), and methylene blue. It is used to stain the cell walls of Mycobacterium species. In this study, Basic Fuchsin was efficiently removed from synthetic wastewater using natural clay of Gankawa (GC) from Sulaimanyah city, and the effect of the presence of high concentrations of phenol in the adsorption mixture is demonstrated. In addition, X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 gas adsorption analyzer, and field-emission scanning electron microscopy (FESEM) were used to characterize the natural clay. The clay was found to be mostly calcite, with a minor percentage of smectite, and contaminated with low percentages of illite. The adsorption kinetics show a relatively fast equilibration time (60-70 minutes). A second-order pseudokinetic model better fits the experimental kinetic data. The effect of the initial pH of the solution mixture was negligible at the experimental concentration range of the study. Freundlich and Langmuir’s adsorption isotherm models were applied to the equilibrium experimental data using nonlinear regression curve fitting. Both kinetics and isotherm studies point to a chemical adsorption mechanism for the process. For adsorption in the mixture, phenol molecules were found to compete with BF molecules for the active adsorption sites, while a synergetic effect of BF exists on phenol adsorption. As a naturally abundant cheap material, GC shows a superior adsorption capacity toward BF (287.0 mg g-1) over all natural materials and most of the synthetic or modified materials found in the literature.

An Intertropical Convergence Zone shift controlled the terrestrial material supply on the Ninetyeast Ridge

Abstract. Among various climate drivers, direct evidence for the Intertropical Convergence Zone (ITCZ) control of sediment supply on the millennial scale is lacking, and the changes in ITCZ migration demonstrated in paleoclimate records need to be better investigated. Here, we use clay minerals and Sr–Nd isotopes obtained from a gravity core on the Ninetyeast Ridge to track the corresponding source variations and analyze the relationship between terrestrial material supply and climatic changes. On the glacial–interglacial scale, chemical weathering weakened during the North Atlantic cold-climate periods and falling sea level hindered the transport of smectite into the study area due to the exposure of Andaman and Nicobar Islands. However, the influence of the South Asian monsoon on the sediment supply was not obvious on the millennial scale. We suggest that the north–south migration of the ITCZ controlled the rainfall in Myanmar and further directly determined the supply of clay minerals on the millennium scale because the transport of smectite was highly connected with the ITCZ location; thus, the regional shift of the ITCZ induced an abnormal increase in the smectite percentage during the late Last Glacial Maximum (LGM) in our records. The smectite percentage in the studied core is similar to distinct ITCZ records but different in some periods, revealing that regional changes in the ITCZ were significantly obvious, the ITCZ is not a simple north–south displacement, and closer connections occurred between the Northern–Southern Hemisphere in the eastern Indian Ocean during the late LGM.

Petrographic characteristics of the Ordovician and Silurian deposits in the Baltic Basin (N Poland) and their relevance for unconventional hydrocarbon accumulations

The Ordovician and Silurian deposits in the Baltic Basin are represented by siltstones, mudstones, limestones, marls, sandstones and, most commonly, claystones with a markedly shaley structure. Claystones that are particularly rich in organic matter are considered as potential reservoirs for shale gas accumulations. Samples from 11 boreholes, including core samples of the Piaśnica, Słuchowo, Kopalino, Sasino, Prabuty, Jantar, Pelplin and locally Pasłęk, Kociewie and Puck formations, were analysed by optical microscopy, cathodoluminescence (CL), scanning electron microscopy (SEM) and energy-dispersive spectrometry and X-ray diffraction analyses (XRD). The Piaśnica, Sasino, Jantar, Pasłęk and Pelplin formations are characterized locally by a dominance of black bituminous claystones with a high content of organic matter, and a generally consistent clay mineral composition. Individual samples only show variations in silt fraction and carbonate content. Very important is the negligible proportion of swelling minerals in the clay fraction; the percentage of smectite in the mixed-layered illite/smectite minerals does not exceed 15%. It is important for the rocks to have adequate [SiO2 >10%; Quartz (Q) + Feldspars (Fs) + Carbonates >40%] brittleness. The content of quartz, feldspars and carbonates (here >40%) suggests that the claystones are susceptible to hydraulic fracturing. The claystones show micropores between detrital grains, between the flakes of clay minerals, within pyrite framboids, and organic matter and secondary micropores within grains.

Contribution of continuously stable sediment input to the formation of the Pearl River delta since the middle Holocene

Pearl River sediments have been reported to have changed greatly in composition due to the impact of human activities on erosion and landscapes in southern China since 2500 years BP. Here, sediments from four cores (PRD-1, PRD-3, PRD-6 and PRD-7) from the Pearl River Delta (PRD) were collected for high-density clay mineral analyses to determine the vertical variations in clay mineral compositions since the middle Holocene. The results showed that the clay mineral compositions of these four cores remained constant. In the PRD, the clay mineral assemblage dominantly consists of illite and kaolinite (averaging more than 85%), with smaller amounts of chlorite (averaging 10%) and scarce smectite (averaging 1%). The stable clay mineral assemblages in the PRD confirm that the clay mineral compositions of the sediments continuously supplied by the Pearl River have not changed. However, the smectite percentage in core PRD-7 increased by 7% downward from a depth of 12.4 m, which was correlated with the weathering of local volcanic materials and sediment transport in the east of the Pearl River under the influence of the Guangdong coastal current in winter. The stable clay mineral compositions in the core sediments indicate that the Pearl River sediments have accumulated naturally in the PRD instead of under the influence of human activities, which would have altered the compositions of the core sediments.

A novel VRo, Tmax, and S indices conversion formulae on data from the fold-and-thrust belt of the Western Outer Carpathians (Poland)

Results are presented on the relationships between the vitrinite reflectance VRo, the Tmax temperatures determined with Rock-Eval pyrolysis, and the smectite (S) indices representing the percentage of smectite in the mixed-layer illite-smectite (I/S) from claystones. Samples locations are recovered from the Polish part of the Outer Carpathian fold-and-thrust belt. Organic indices show thermal maturities of most outcrop samples in the range of 0.47 to 1.49% Ro and 421 to 485 °C Tmax. Inorganic S indices values range from 8 to 60%. Lateral variability of the maturity measured with both organic and inorganic indices reflects mainly differences of maximum tectonic burial in the accretionary prism and subsequent exhumation and denudation. For the Western Outer Carpathians the calculated formulae for Tmax to VRo and S to VRo conversion are VRo = 0.0152 * Tmax - 5.938 (r2 = 0.87) and VRo = 1.1432–0.01295 * S (r2 = 0.65), respectively. Maximum paleotemperatures were calculated for individual samples from S indices by applying the novel formula T = ln (1.1432–0.01295 * S) + 1.68/0.0124. The proposed new conversion formulae might be useful in other regions with a similar geological setting, i.e. for claystone formations dominated by kerogen type III in fold–and-thrust belts, characterized by rapid tectonic burial accompanied by a low geothermal gradient, as well as maturities of the sub-bituminous to the medium volatile bituminous rank (0.5 to 1.50% Ro).

Induced polarization of volcanic rocks. 3. Imaging clay cap properties in geothermal fields

Smectite-rich clay caps form permeability seals in geothermal systems. The presence of smec-tite is also responsible for a strong surface (interfacial) electrical conductivity and polarization due to their electrical double layer properties. We developed new complex conductivity models using both differential effective medium (DEM) and volume averaging theories accounting for both conduction and polarization of these high cation exchange capacity (CEC) materials. These models predict that the chargeability is also a non-linear function of the pore water conductivity reaching a constant value at pore water conductivity far above the so-called iso-conductivity point. The iso-conductivity point is characterized by the equality between the conductivity of the rock and the conductivity of the pore water. We apply the DEM conductivity model (which requires only two textural parameters) to smectite-rich volcanic and sedimentary rocks using data sets from the literature. When smectite is present in the volcanic rocks, the CEC of the rock is dominated by the CEC of smectite. The grain conductivity and the normalized chargeability are related to each other by a dimensionless number R = 0.10 (independent of temperature and saturation) and both are controlled by the excess of charge per unit pore volume Q V , which can be determined from the CEC and porosity. Our petro-physical model is also able to predict the permeability of the rock as well from the CEC and the porosity. It is applied to a 3-D data set at Krafla volcano (Iceland). The porosity, the CEC, the percentage of smectite, and the permeability of the clay-cap are imaged by 3-D induced polarization tomography. Electrical conductivity tomography alone does not allow separation of the contribution of the bulk pore space from the interfacial properties related to alteration and therefore should be used with caution.

Clay mineral compositions in surface sediments of the Ganges-Brahmaputra-Meghna river system of Bengal Basin, Bangladesh

Clay minerals are significant indicators that can be used to identify sources and transport patterns of both fluvial and marine sediments. The Ganges, Brahmaputra, and Surma-Meghna (SM) rivers are the most important rivers of the Bengal Basin (BB), loading a large amount of sediments from the Himalayas into the Bay of Bengal. Clay minerals of 132 surface sediment samples from the Ganges-Brahmaputra-Meghna (GBM) river system have been analyzed with X-ray diffraction (XRD) to show the variation of clay mineral assemblages in the sediments from upstream to downstream for the major rivers of the BB and to verify the validity of clay minerals as an important index for sediment provenance in the Bay of Bengal after these sediments are discharged into the neighboring seas. The results show that the clay mineral assemblage in the GBM river system consists dominantly of illite (average 66%), with smaller amounts of smectite, kaolinite, and chlorite (8%, 8%, and 17%, respectively). Smectite is diagnostic in the Ganges River sediments, with a higher content (23%) than that in the Brahmaputra (2%) and the SM River (1%); however, chlorite (21%) is high in the SM River sediments and the Brahmaputra River sediments have a high illite percentage (77%). Downstream, the smectite percentages in the Ganges River increase significantly, but the kaolinite percentages in sediments of the SM River increase slightly. Due to the continuous mixture of first the Brahmaputra River and then the SM River discharge, the clay mineral assemblages of this river system's sediments change greatly, e.g., smectite percentages decrease from 23% in the Ganges River to 8% in the GBM River via the stretch with 12% in the Ganges-Brahmaputra confluence river.

New archive of another significant potential sediment source in the South China Sea

Sediment provenance is an important part of the sediment source-to-sink transport process from land to sea. However, we sometimes cannot explain some contradictory problems well when assessing sediment sources in certain areas. The northern South China Sea (SCS) is currently considered to be dominantly supplied with sediments from the Pearl River, Taiwan and Luzon. For smectite (one type of clay mineral), Luzon is normally thought to be a significant source when considering only the high smectite percentages in Luzon-derived sediments and neglecting their limited sediment discharge into the sea and longer transport distance of its sediments from this island. Recently, clay mineral compositions of the Pearl River originated sediments were reported to change from rich in smectite from ~2500 years ago due to the anthropogonic activities. By contrast, the wide area in the east of the Pearl River (EPR) is similarly characterized by sediments with high smectite percentages and a shorter transport distance than those from Luzon. However, a previous study showed that surface sediment samples with high smectite percentages were occasionally distributed in bays. Here, we present a sediment archive of 16 gravity cores from neighboring bays/harbors along the northern SCS coast. Unlike surface sediments with low smectite percentages, core sediments from the coastal bays/harbors of the EPR show high smectite percentages, indicating that the EPR besides the Pearl River and/or Luzon might be as another potential sediment source for the northern SCS. In South China, widely distributed igneous rocks/volcanic materials are prodigious sources for sediments in the northern SCS slope. In future work, weathering of local materials must be considered when differentiating various sediment sources.

The impact of water saturation on the elastic anisotropy dispersion in the Wellington shale at seismic frequencies

The anisotropic behaviour of shales is commonly associated with the properties of a transversely isotropic medium, which are determined by five elastic constants such as five independent components of the compliance or stiffness matrix. In this study, we utilize the laboratory low-frequency technique based on stress-strain relationship to measure the dispersion of five independent stiffness tensor components and Thomsen’s anisotropy parameters of shale samples saturated with water at four different values of humidity in the range from 12% to 97.5% (12, 44, 72 and 97.5%). We have investigated three shale samples from the Wellington formation cored along the horizontal, vertical and 45°-inclination directions with respect to the bedding plane at seismic frequencies between 0.1 Hz and 100 Hz.The obtained experimental data show an increased softening of the samples, which manifests itself in reduction of the transversely isotropic Young’s moduli and Thomsen’s parameters of elastic anisotropy e and y, no noticeable changes in parameter S were found. We also observed large reductions in normal and shear stiffness tensor components with saturation. When the samples were saturated at a relative humidity of 97.5 %, the softening at the higher frequencies was partly compensated by the modulus dispersion.We presume that the weakening of the elastic moduli and components of the stiffness tensor is caused by the significant percentage of water-swellable smectite in the Wellington shale.

Provenance of Sediments Filling a Paleo-Channel that Formed on the Western Yellow Sea Continental Shelf During the Last Glacial Period

Several buried paleo-channels are located on the continental shelf of the western Yellow Sea. Research on the paleochannels is significant for both theoretical studies and practical applications. In this paper, we analyse and discuss the mineralogy of sediments in a core (SYS-0803) recovered from a buried paleo-channel on the continental shelf of the western Yellow Sea. The aim is to determine the provenance of sediments that fill the paleo-channel. The heavy mineral assemblage of sediments in the core consists of schistose minerals, common hornblende, epidote, and ilmenite. The light mineral assemblage consists of plagioclase, quartz, lithic fragments, and K-feldspar. Analysis of the compositional maturity of the sand fraction revealed a quartz/feldspar ratio of < 1. A relatively high percentage of smectite is recorded throughout the entire paleo-channel fill, with the greatest percentage in the middle to lower parts. The detrital mineral assemblage and clay mineral content indicate that the paleo-channel sediments were sourced mainly from the Huanghe River during the last glacial period.

Assessment of the effect of mineralogy on the geotechnical parameters of clayey soils: A case study for the Orta County, Çankırı, Turkey

Abstract The purpose of this study is to investigate and relate the geotechnical parameters with the mineralogical properties of the Miocene and Plio-Quaternary (Pliocene to Quaternary) lacustrine and fluvial sediments, particularly clayey soils in the Orta County, Cankiri. The study area is located about 110 km northeast of Ankara which is the capital city of the Republic of Turkey and Orta is a small county of Cankiri. The study area is a structural depression in the northern part of the Orta plain which is filled by fault-controlled continental sedimentation. In order to correlate the geotechnical and mineralogical characteristics and to establish a relation between these properties, comprehensive geotechnical laboratory studies have been performed. For correlation purposes, disturbed and undisturbed soil samples were collected from twenty-two locations of the study area. On these samples, mineralogical (i.e., methylene blue absorption, specific surface area determination, X-ray powder diffraction (XRD) and scanning electron microscope (SEM-EDAX)) analysis and geotechnical (i.e., sieve analysis, hydrometer, Atterberg (consistency) limits, oedometer and swelling) tests were implemented. By the aid of the statistical analysis, cross-correlation of the soil properties was established by the regression analyses performed. In particular, the plasticity index was correlated with cation exchange capacity, specific surface area, clay content, percent of fines, smectite and smectite/kaolinite ratio (SKR). There was a direct relationship between PI and smectite percentage as well as between PI and SKR. Additionally, the liquid limit was correlated with the clay content and the effect of the specific surface area on the swelling characteristics was studied. The effects of the mineralogical properties of the Orta clays together with the geological history on geotechnical parameters were investigated by considering the relationships mentioned. The results of the study revealed that the mineralogical characteristics had a significant effect on the geotechnical behavior of clayey soils of the Orta County which was demonstrated by regression analyses. Finally it was identified that the geotechnical properties of clayey soils can be approximated and more reliably determined if the mineralogical character and composition of clay is accurately investigated and well developed.

Clay Addition to Sandy Soil—Influence of Clay Type and Size on Nutrient Availability in Sandy Soils Amended with Residues Differing in C/N ratio

Addition of clay-rich subsoil to sandy soil results in heterogeneous soil with clay peds (2-mm) or finely ground (< 2 mm) clay soil (FG), which may affect the nutrient availability. The aim of this study was to assess the effect of clay soil particle size (FG or peds) and properties on nutrient availability and organic C binding in sandy soil after addition of residues with low (young kikuyu grass, KG) or high (faba bean, FB) C/N ratio. Two clay soils with high and low smectite percentage, clay and exchangeable Fe and Al were added to a sandy soil at a rate of 20% (weight/weight) either as FG or peds. Over 45 d, available N and P as well as microbial biomass N and P concentrations and cumulative respiration were greater in soils with residues of KG than FB. For soils with KG residues, clay addition increased available N and initial microbial biomass C and N concentrations, but decreased cumulative respiration and P availability compared to sandy soil without clay. Differences in measured parameters between clay type and size were inconsistent and varied with time except the increase in total organic C in the < 53 μm fraction during the experiment, which was greater for soils with FG than with peds. We concluded that the high exchangeable Fe and Al concentrations in the low-smectite clay soil can compensate a lower clay concentration and proportion of smectite with respect to binding of organic matter and nutrients.

Mineralogical characteristics of upper Jurassic Mikulov Marls, the Czech Republic, in relation to their thermal maturity

The Upper Jurassic Marls of Mikulov present a formation that is considered to be the most promising strata to produce hydrocarbons in the Vienna basin. The marls are composed of dark pelagic marlstones that frequently contain layers of limestone with thickness reaching several hundreds of meters. Twenty-seven core samples from selected wells located in the south-eastern portion of the Czech Republic representing depths ranging from 2300 to 4500m were analyzed by x-ray diffraction to assess bulk mineralogy and the progress of smectite illitization.Bulk mineralogy of the Mikulov Marls comprises carbonates (mean value=54.4 mass%), clay minerals (26.6 mass%), quartz (15.0 mass%), and feldspar (1.6 mean%). In the decreasing order, the clay mineral fraction is composed of illite/mica, kaolinite, illite-smectite, and chlorite. The amount of smectite in illite-smectite decreases with depth from 70% to 28%. There is a change from random to ordered interstratification at the depth of 3300m. The transition from short-range ordering (R1) to long-range ordering (R3) occurs at depths greater than 4,500m.There was a good correspondence between thermal maturity parameters: the percentage of smectite in illite-smectite structures and vitrinite reflectance as a parameter of organic matter. The increase of the metamorphic grade was compared in respect to the geothermal gradient with adjacent basins.

Experimental study of the electrokinetic behaviour of kaolinite–smectite mixtures

The evolution of the behaviour of kaolinite–smectite mixtures was studied using mechanical and electrokinetic tests. Oedometric tests showed that the compression index of the mixtures increases with increasing smectite percentage and that the curves feature a double slope in the [log σv,e] (where σv is the vertical mechanical stress and e is the void ratio) coordinate system when the percentage of smectite is strictly higher than 25%. Electrokinetic tests show that, of smectite the electrical conductivity and electro-osmotic flow tend towards that of the smectite. Measurements performed after the electrokinetic tests showed that the pH and conductivity are constant when the amount of smectite is lower than 25%. For higher smectite content, acidification of the medium is not totally obtained and the electrical conductivity is higher near the anode because of the slow diffusion of H+ ions in the structure. The tests also highlight that the electro-osmotic permeability is affected by the hydraulic permeability, although the variation in electro-osmotic permeability remains small compared with that of hydraulic permeability.

Comment on “Holocene evolution in weathering and erosion patterns in the Pearl River delta” by Hu et al.

A recent paper [Hu et al., 2013] reports Holocene evolution in weathering and erosion patterns in the Pearl River delta. The paper concludes the modern Pearl River is a poor proxy for understanding the typical erosional flux from southern China to the South China Sea (SCS), principally based on low smectite percentage in the modern river sediments but high smectite percentage in two cores through much of the Holocene, which may lead to misinterpretation of depositional environment in the deep-sea basin from the SCS. In this comment, we first point out that core locations determine sedimentary records in the Pearl River delta. Second, we show that high smectite percentage at both cores during the Holocene was correlated with weathering of neighboring islands or even transported by coastal currents from nearby bays, instead of sediment supply changes from South China via the Pearl River. We thus conclude that sediment flux of the Pearl River remained relatively stable during the Holocene and correlated parameters derived from the river sediments could be utilized to effectively trace sediment sources in deepwater environments from the northern SCS.

An Experimental Design for the Optimization of Brick Manufacturing Procedure: Application on the Late Miocene Clays, Southern Tunisia

Clay deposits of the Segui formation (Jebel Stah of the Gafsa basin, Southwestern Tunisia) were used as natural material for brick manufacturing. Factors influencing the preparation procedure were optimized by an experimental design to find out the most propitious conditions for the preparation of bricks of high quality. The collected clay samples were characterized by various techniques, including X-ray diffraction (XRD), Fourier transform infrared (FTIR) and pycnometry. The optimum conditions for the preparation of high quality materials were investigated by studying the effects of clay minerals percentage, sintering time and temperature. Then various tests including sintering shrinkage, water absorption and compressive strength of the newly prepared brick samples were evaluated to determine the optimum preparation conditions. Results showed that the optimized conditions were: 80% illite, 830°C sintering temperature and 2 h sintering time. The same results showed that brick shrinkage depended mainly on water absorption, illite and smectite percentages. These results suggested that clay samples from Jebel Stah are promising natural materials that could be used to prepare bricks with better mechanical properties.

Clay mineralogy and geochemistry investigations in the host rocks of the Chelungpu fault, Taiwan: Implication for faulting mechanism

The Chelungpu fault, with northward propagating ruptures, was ruptured as a result of the Mw 7.6 earthquake which struck Central Taiwan on 21st September 1999. To understand the consequences of clays in fault gouges, we examined the clay mineralogy and major element geochemistry of the “host rocks” of the Chelungpu fault from four outcrops, shallow boreholes at Fengyuan (455.3m in depth), Nantou (211.9m in depth), and a deep borehole of the Taiwan Chelungpu fault Drilling Project (2003m in depth). Results revealed different degrees of chemical weathering, the intensity, as documented by differences in the smectite content, could be further understood through the relative clay percentage of smectite, illite chemistry index, and illite crystallinity. These mineralogical proxies combined with the chemical index of alteration (CIA) and the intensity of chemical weathering indicate that the degree of chemical weathering is a function of depth, i.e., the most severe on the surface and the mildest in the TCDP samples. The mineralogical and geochemical data obtained in this study also suggest that chemical weathering, rather than leaching, seems to be the main driving force for the phase changes of clays. The amount of smectite produced by chemical weathering varies with depth, and it suggests that the previous idea of weak-fault caused by the presence of smectite in the fault zone will be risky without considering weathering processes. The observations of clay mineralogy and major element geochemistry in this study indicate that the presence of smectite in the outcrops may not play a significant role during faulting, and suggest that fault-weakening as a result of the presence of smectite cannot be applied to the Chelungpu fault.

Microbial reduction of structural iron in interstratified illite‐smectite minerals by a sulfate‐reducing bacterium

Clay minerals are ubiquitous in soils, sediments, and sedimentary rocks and could coexist with sulfate-reducing bacteria (SRB) in anoxic environments, however, the interactions of clay minerals and SRB are not well understood. The objective of this study was to understand the reduction rate and capacity of structural Fe(III) in dioctahedral clay minerals by a mesophilic SRB, Desulfovibrio vulgaris and the potential role in catalyzing smectite illitization. Bioreduction experiments were performed in batch systems, where four different clay minerals (nontronite NAu-2, mixed-layer illite-smectite RAr-1 and ISCz-1, and illite IMt-1) were exposed to D.vulgaris in a non-growth medium with and without anthraquinone-2,6-disulfonate (AQDS) and sulfate. Our results demonstrated that D.vulgaris was able to reduce structural Fe(III) in these clay minerals, and AQDS enhanced the reduction rate and extent. In the presence of AQDS, sulfate had little effect on Fe(III) bioreduction. In the absence of AQDS, sulfate increased the reduction rate and capacity, suggesting that sulfide produced during sulfate reduction reacted with the phyllosilicate Fe(III). The extent of bioreduction of structural Fe(III) in the clay minerals was positively correlated with the percentage of smectite and mineral surface area of these minerals. X-ray diffraction, and scanning and transmission electron microscopy results confirmed formation of illite after bioreduction. These data collectively showed that D.vulgaris could promote smectite illitization through reduction of structural Fe(III) in clay minerals.

Bioreduction of Fe-bearing clay minerals and their reactivity toward pertechnetate (Tc-99)

99Technetium ( 99Tc) is a fission product of uranium-235 and plutonium-239 and poses a high environmental hazard due to its long half-life ( t 1/2 = 2.13 × 10 5 y), abundance in nuclear wastes, and environmental mobility under oxidizing conditions [i.e., Tc(VII)]. Under reducing conditions, Tc(VII) can be reduced to insoluble Tc(IV). Ferrous iron, either in aqueous form (Fe 2+) or in mineral form [Fe(II)], has been used to reduce Tc(VII) to Tc(IV). However, the reactivity of Fe(II) from clay minerals, other than nontronite, toward immobilization of Tc(VII) and its role in retention of reduced Tc(IV) has not been investigated. In this study the reactivity of a suite of clay minerals toward Tc(VII) reduction and immobilization was evaluated. The clay minerals chosen for this study included five members in the smectite–illite (S–I) series, (montmorillonite, nontronite, rectorite, mixed layered I–S, and illite), chlorite, and palygorskite. Surface Fe-oxides were removed from these minerals with a modified dithionite–citrate–bicarbonate (DCB) procedure. The total structural Fe content of these clay minerals, after surface Fe-oxide removal, ranged from 0.7% to 30.4% by weight, and the structural Fe(III)/Fe(total) ratio ranged from 45% to 98%. X-ray diffraction (XRD) and Mössbauer spectroscopy results showed that after Fe oxide removal the clay minerals were free of Fe-oxides. Scanning electron microscopy (SEM) revealed that little dissolution occurred during the DCB treatment. Bioreduction experiments were performed in bicarbonate buffer (pH-7) with structural Fe(III) in the clay minerals as the sole electron acceptor, lactate as the sole electron donor, and Shewanella putrefaciens CN32 cells as a mediator. In select tubes, anthraquinone-2,6-disulfate (AQDS) was added as electron shuttle to facilitate electron transfer. In the S–I series, smectite (montmorillonite) was the most reducible (18% and 41% without and with AQDS, respectively) and illite the least (1% for both without and with AQDS). The extent and initial rate of bioreduction were positively correlated with the percent smectite in the S–I series (i.e., layer expandability). Fe(II) in the bioreduced clay minerals subsequently was used to reduce Tc(VII) to Tc(IV) in PIPES buffer. Similar to the trend of bioreduction, in the S–I series, reduced NAu-2 showed the highest reactivity toward Tc(VII), and reduced illite exhibited the least. The initial rate of Tc(VII) reduction, after normalization to clay and Fe(II) concentrations, was positively correlated with the percent smectite in the S–I series. Fe(II) in chlorite and palygorskite was also reactive toward Tc(VII) reduction. These data demonstrate that crystal chemical parameters (layer expandability, Fe and Fe(II) contents, and surface area, etc.) play important roles in controlling the extent and rate of bioreduction and the reactivity toward Tc(VII) reduction. Reduced Tc(IV) resides within clay mineral matrix, and this association could minimize any potential of reoxidation over long term.