Different Types Of Clay Minerals Research Articles

Estimation of Cation Exchange Capacity for Low-Activity Clay Soil Fractions Using Experimental Data from South China

The cation exchange capacity (CEC) of the clay fraction (<2 μm), denoted as CECclay, serves as a crucial indicator for identifying low-activity clay (LAC) soils and is an essential criterion in soil classification. Traditional methods of estimating CECclay, such as dividing the whole-soil CEC (CECsoil) by the clay content, can be problematic due to biases introduced by soil organic matter and different types of clay minerals. To address this issue, we introduced a soil pedotransfer functions (PTFs) approach to predict CECclay from CECsoil using experimental soil data. We conducted a study on 122 pedons in South China, focusing on highly weathered and strongly leached soils. Samples from the B horizon were used, and eight models and PTFs (four machine learning methods, multiple linear regression (MLR) and three PTFs from publication) were evaluated for their predictive performance. Four covariate datasets were combined based on available soil data and environmental variables and various parameters for machine learning techniques including an artificial neural network, a deep belief network, support vector regression and random forest were optimized. The results, based on 10-fold cross-validation, showed that the simple division of CECsoil by clay content led to significant overestimation of CECclay, with a mean error of 14.42 cmol(+) kg−1. MLR produced the most accurate predictions, with an R2 of 0.63–0.71 and root mean squared errors (RMSE) of 3.21–3.64 cmol(+) kg−1. The incorporation of environmental variables improved the accuracy by 2–10%. A linear model was fitted to enhance the current calculation method, resulting in the equation: CECclay = 15.31 + 15.90 × (CECsoil/Clay), with an R2 of 0.41 and RMSE of 4.48 cmol(+) kg−1. Therefore, given limited soil data, the MLR PTFs with explicit equations were recommended for predicting the CECclay of B horizons in humid subtropical regions.

Molecular insights into the binding mechanism of strontium and cesium on phyllosilicates with different expandability

The environmental fate of strontium (Sr) and cesium (Cs), as the critical radioactive fission products, have raised significant concerns regarding radioactive waste disposal and environmental protection. The current study investigated the distinction in the binding configurations of Sr2+ and Cs+ on various 2:1 phyllosilicate (illite, vermiculite, and montmorillonite) by combining batch adsorption, sequential extraction, and spectroscopic analyses. The results show that strontium adsorption is strongly influenced by pH as well as ionic strength, while there is no significant variability in strontium adsorption by different types of clay minerals. EXAFS analysis confirms the outer complexation of strontium on the planar sites of the clay minerals, i.e., Sr2+ is surrounded by ~8.0 O atoms, RSr-O ≈ 2.6 Å, and that process is mainly realized by ion exchange. In contrast, Cs+ adsorption was markedly influenced by the variety of clay minerals but less by pH and ionic strength, the presence of humic acid (HA) inhibited Cs+ adsorption. The inner-sphere complexation formed mainly at the frayed edge sites on illite, and interlayer sites on vermiculite and montmorillonite, was the dominant mechanism for Cs+ adsorption. In addition, the collapse of the interlayer space of vermiculite induced by Cs+ adsorption on the interlayer sites was responsible for the more stable and irreversible immobilization. The findings in present work highlighted the significance of prevailed mineral in governing environmental migration risk of radionuclides, the revealed adsorption mechanism and binding configuration of Sr2+ and Cs+ on typical phyllosilicates would be referable in constructing a reliable migration model of Sr2+ and Cs+ in natural media.

Dissolution Mechanisms and Surface Charge of Clay Mineral Nanoparticles: Insights from Kinetic Monte Carlo Simulations

The widespread use of clay minerals and clays in environmental engineering, industry, medicine, and cosmetics largely stems from their adsorption properties and surface charge, as well as their ability to react with water. The dissolution and growth of minerals as a function of pH are closely related to acid–base reactions at their surface sites and their surface charge. The vivid tapestry of different types of surface sites across different types of clay minerals generates difficulties in experimental studies of structure–property relationships. The aim of this paper is to demonstrate how a mesoscale stochastic kinetic Monte Carlo (kMC) approach altogether with atomistic acid-base models and empirical data can be used for understanding the mechanisms of dissolution and surface charge behavior of clay minerals. The surface charge is modeled based on equilibrium equations for de/protonated site populations, which are defined by the pH and site-specific acidity constants (pKas). Lowered activation energy barriers for these sites in de/protonated states introduce pH-dependent effects into the dissolution kinetics. The V-shaped curve observed in laboratory experiments is reproduced with the new kMC model. A generic rate law for clay mineral dissolution as a function of pH is derived from this study. Thus, the kMC approach can be used as a hypothesis-testing tool for the verification of acid–base models for clay and other minerals and their influence on the kinetics of mineral dissolution and growth.

Preparation and antibacterial activity of new N-salicylideneaniline organomodified Algerian clays

Schiff bases are proved to possess numerous biological activities. Clay minerals on another hand are known for their bio-compatibility and therapeutic properties. The present work aimed to synthetize, characterize and assess the antibacterial activity of two new Clay-Schiff base materials by functionalizing two different types of clay minerals, Montmorillonite (Lalithe) and Palygorskite (Sif Pal) with the N-salicylideneaniline Schiff-base for the formation of the N-Salicylideneaniline modified montmorillonite named HSA- Lalithe and modified palygorskite named HSA- Sif Pal compounds.The new modified materials were characterized using X-ray diffraction (XRD) and Fourier transform Infra-Red (FT-IR). The antibacterial activity was evaluated against gram positive bacteria, Staphylococcus aureus and Staphylococcus aureus (MRSA+) and gram negative bacteria, Escherichia coli and Pseudomonas aeruginosa through the agar dilution method (Muller-Hinton agar, MHA). The results of the minimum inhibitory concentration (MIC) demonstrated moderate antibacterial activity with the four strains of bacteria. HSA-Lalithe shows a better performance against Staphylococcus aureus and Pseudomonas aeruginosa compared to solid HSA- Sif Pal, which could be due to the swelling structure, and or the existence of the HSA Schiff base in the interfoliar space of the Lalithe material.

Diagenetic controls on pore structure variations in fan deltaic sandstones of the Eocene Shahejie Formation, Bohai Bay Basin, China: Implications for reservoir quality and oiliness heterogeneities

The fan deltaic sandstones in the third section of the third member of the Eocene Shahejie Formation (Es33) are important hydrocarbon exploration targets in the northern Nanpu sag, Bohai Bay Basin, China. The Es33 sandstones are characterized by highly variable pore structures. A total of 364 Es33 sandstone samples were measured by a series of petrological and petrophysical analyzes to investigate these variations and their controls on reservoir quality and oiliness heterogeneities. The heterogeneity of Es33 sandstone can be manifested by type I, type II and type III pore structures. Primary intergranular pores and dissolution pores were the main storage spaces for type I sandstones, while dissolution pores and intercrystalline pores were the main storage spaces for type II and type III sandstones, respectively. These results indicate that compaction is the main factor in the loss of primary intergranular porosity, with an average loss of 78%. The grain size controlled the preservation of primary pores in Es33 sandstones with a carbonate cement content of less than 6%. The different types of clay minerals have various effects on the pore structure. Chlorite plays an active role in the preservation of intergranular pores and the improvement of reservoir quality, while mixed layers of illite and smectite greatly reduced the pore size and connectivity and lowered the reservoir quality. The Es33 sandstones have undergone a more open diagenetic system. Dissolution significantly increased the porosity of type I and type II sandstones with more primary intergranular pores, but exerted a minor role in contributing to the permeability. Furthermore, a pore‐throat radius of approximately 0.4 μm is considered to be the threshold for oil accumulation in Es33 fan deltaic sandstones.

Soft sedimentary rocks and their boreability characteristics in mechanised tunnelling projects

AbstractSoft sedimentary rocks are frequently found in many mountainous regions of the world like Zagros Mountains located in the south and west of Iran, where most of Iran's water conveyance projects have been constructed or are under construction. These rocks usually contain considerable amounts of different types of clay minerals. The clay mineral‐rich rocks cause many problems in the deep and long mechanised tunnelling projects. In particular, the boreability of these rocks is very different from other rocks, and due to their plastic behaviour and the lack of complete formation of chips during the cutting process by disc cutters, the prediction of cutterhead penetration in these rocks, with existing models, is often inaccurate. In this paper, a new model for estimating the disc cutter penetration in these rocks is introduced, which is based on data obtained from the main tunnelling projects completed in sedimentary rocks of Zagros Mountains. Then, the validity of the developed model is checked using actual data obtained from a tunnel recently constructed in the area. The new model can offer a more accurate estimate of machine performance in similar rocks.

Influence of Different Types of Clay Minerals on the Shape and Form of Silica-Carbonates (Biomorphs) of Ca(II), Ba(II), and Sr(II)

The array of atmospheric and chemical conditions that prevailed during the Precambrian era favored the origin of life in our planet. The minerals and rocks that existed in that era catalyzed the synthesis of the first biomolecules. It has been shown that mineral clays possess the best-suited mineral surface to study the polymerization of protobiomolecules because they catalyze the binding of monomers to form peptides and nucleic acids and foster the binding between them. Inorganic structures exist formed by silica-carbonates of calcium, barium, or strontium, called biomorphs, which emulate the morphological structure of diverse organisms. Some of these morphologies resemble several of the microfossils found in the Precambrian cherts. Notwithstanding, it has not been documented yet whether biomorphs are a consequence of the dissolution of clays in alkaline environments (rich in alkaline earth metals) during the initial stages of the Earth, which would favor the hypothesis that the biomorphs or cherts existed since that time. To know whether biomorphs could have been embedded within minerals or in diverse types of clays and contributed to the synthesis of protobiomolecules, we aimed at synthesizing biomorphs in the presence of different clays of natural origin in two synthesis conditions that resemble both the atmosphere of the Precambrian era and that of the current time, regarding the CO2 concentration. Our results revealed that in the condition emulating the Precambrian, polymorphs of calcite, aragonite, and vaterite were formed. The barium and strontium biomorphs adopted the morphology of radiolarians, diatoms, and foraminifers in this same condition, indicating that they could be the first structure of these organisms. Results also revealed that the clays present in the Precambrian together with the prevailing environmental conditions played a preponderant role as catalyzers in the structural morphology, chemical composition, and crystalline habit adopted by the first organisms. Biomorphs could have been the first inorganic structures with a structural order and characteristic shape (preserving biomolecules in their interior when they were formed) that were synthetized in the early era of our planet, but they have not been dated as a fossil registry.

The Effects of Hydrochloric Acid Pretreatment on Different Types of Clay Minerals

Clay minerals are common in geological samples and are useful paleoclimate and sediment provenance proxies. Acid pretreatment is the most common method for the separation and purification of clay minerals. Given that hydrochloric acid (HCl) can dissolve chlorite and distinguish it from kaolinite, the HCl digestion method is used to simplify the routine method of clay mineral analysis. However, there have been few studies of the effects of acid digestion on different clay minerals in the context of extracting paleoclimate indicators. In this study, we used illite, chlorite, kaolinite, and two types of smectite to assess the effects of pretreatment with different HCl concentrations at variable temperatures. Our results show that chlorite is the most soluble clay mineral in HCl and can be effectively dissolved in HCl with concentrations of >1 N. The variable crystal structure of smectite affects its solubility in HCl. Ca-rich smectite, which has more cation substitution of octahedral Al, can be dissolved with HCl. However, Na-rich smectite, which has less cation substitution for octahedral Al, is hardly dissolved in HCl of any concentration or at any temperature. Illite can be partly dissolved in HCl, and the threshold beyond which dissolution occurs is 5 N HCl at 70 °C. Kaolinite is relatively difficult to dissolve in HCl. Given that the HCl digestion method uses the peak intensity of the bulk sample X-ray diffraction (XRD) analysis, whereas the routine method uses the peak area of clay particles, we compared the results of clay mineral quantification and the paleoclimate proxy obtained using the two methods for synthetically prepared mixed and natural clay samples. The results obtained with the HCl digestion method are less accurate than those obtained with the routine method because of the dissolution of illite and smectite in HCl. Therefore, the HCl pretreatment method is not suitable for clay mineral analysis in paleoclimate studies. The present results provide reference data for future studies that employ the acid dissolution pretreatment of clay mineral samples to acquire and quantify paleoclimate proxies.

Sorption of Acridine Orange on Non-Swelling and Swelling Clay Minerals

Acridine orange (AO) is a cationic fluorescent dye commonly used in DNA analyses. Extensive studies were conducted for its metachromasy under different solution concentrations and different amounts of AO sorbed on a solid surface. Meanwhile, for the safe disposal of wastewater, AO removal from water using different materials was also evaluated extensively. Clay minerals, due to their large specific surface area, high cation exchange capacity, and vast reserves, have been evaluated as potential sorbents for the removal of a variety of different types of contaminants, including color dyes. In this study, the sorption of AO on different types of clay minerals was contrasted. The sorption of co-presenting Zn2+ was much less than the sorption of AO, suggesting that clay minerals have higher affinities for AO in comparison to inorganic Zn2+. The desorption of exchangeable cations was linearly related to AO sorption, and the amounts of AO sorbed were close to the CEC values of the minerals, confirming that cation exchange is the dominating mechanism for AO sorption. Molecular dynamics simulation results showed that, under low and high AO loading levels, the sorbed AO formed monolayers and bilayers on the mineral surfaces of non-swelling clay minerals, except halloysite, as well as in the interlayer of swelling clay minerals, due to its relatively large dimer constant in solution. Overall, clay minerals are good candidates for the removal of cationic dyes from solution even in the presence of competing inorganic cations.

An integrated chemo- bio- mineral technology for agricultural drainage water remediation

ABSTRACT During the last decades, technologies for agricultural drainage water remediation had been widely explored to oppose the arising intimidation of their pollutants on health, biotas and farming sustainability. The present study targeted to augment the remediation competence of inorganic pollutants and faecal coliforms from agricultural drainage water using different types of clay minerals. Kaolinite, Bentonite, Ball clay and Aswan clay were used either solely or in amalgamation with some biological premeditative amendments (cultures of Phosphate dissolving Bacteria (PDB) and/or Acidithiobacillus sp.] for drainage water remediation of samples delivered from Belbies and Rahawy agricultural drains. Nineteen trails and four groups were implemented for each drain together with a control sample. Water samples were analysed before and after clay minerals’ trails application for detection of Cd(II), Cu(II), Ni(II), Zn(II) and As(V) by Atomic Spectroscopy. Total faecal bacteria presence was also quantified through direct culturing on MacConkey agar plates. The obtained results confirmed a significant decrease in the concentrations of Cd, Cu, Ni, Zn and As pollutants as well as a decrease in the faecal coliform intensities under all trailed treatments compared to control samples. Worthy to mention that 4th group and especially T18 (Bentonite + Acidithiobacillus+ phosphate dissolving bacteria) was the most efficient in minimising the hazards of PTEs and the existence of faecal microbes.

Quantitative Interpretation of Water Sensitivity Based on Well Log Data: A Case of a Conglomerate Reservoir in the Karamay Oil Field

Abstract In the process of oil reservoir development, clay minerals in the pores of a reservoir swell because of water absorption, blocking the pore channels, and decreasing the reservoir permeability. This phenomenon is referred to as reservoir water sensitivity. To develop and protect oil and gas fields more scientifically and effectively and save development cost, we examine the water sensitivity of the low-permeability conglomerate reservoir of the Lower Karamay Formation in the 530-well area of District 8 of the Karamay oil field. Based on multiple linear regression and neural network methods, the quantitative calculation of different types of clay minerals was performed by comprehensively using natural gamma ray, neutron, density, sonic transit time, and X-ray diffraction (XRD) analysis data. The water sensitivity experiment was performed in the study area, and the logging interpretation method of reservoir water sensitivity was established by considering the clay mineral content as the medium; moreover, the reservoir water sensitivity degree was quantitatively explained.

Zwitterionic dye rhodamine B (RhB) uptake on different types of clay minerals

Increased use of color dyes in textile industries resulted in great needs of treating dye-containing wastewater. As such, researches and developments for effective removal of dyes from wastewater have been conducted extensively and good progresses have already been made. However, there are still needs to understand the mechanisms of interactions between dyes and solids and to evaluate factors controlling such interactions. In this study, the interactions between different types of clay minerals and rhodamine B (RhB), a zwitterionic dye, were examined under different physic-chemical conditions. The solution pH, ionic strengths, and temperature had minimal influence of RhB uptake on these clay minerals. Uptake of RhB was limited to the external surfaces for the non-swelling clay minerals kaolinite and palygorskite. In contrast, both external and interlayer spaces are responsible for the uptake of RhB on swelling clay mineral montmorillonite. The RhB molecules intercalated into the interlayer of montmorillonite may adopt a monolayer or a bilayer configuration under low and high uptake levels. Under neutral pH condition, the RhB molecules exist as zwitterionic species in solution. The extensive uptake of RhB by the clay minerals, up to their cation exchange capacity (CEC) values, was attributed to electrostatic interactions between cationic end of the amine and the negatively charged clay minerals surfaces. The significant RhB uptake suggested that clay minerals are also good sorbents for the removal of zwitterionic dyes.

Removal of rhodamine 6G with different types of clay minerals

With an increased use of color dye in textile industries and elevated fabrics output, more scientific studies and technology developments are needed to effectively treat wastewater containing dyes. However, better understanding of the interactions between dyes and suspended solids is a necessity to advance such developments. In this study the interactions between rhodamine 6G (R6G), a cationic dye, and different types of clays minerals, commonly found in the wastewater sludge, were elucidated. The uptake of R6G on the clay minerals was attributed to cation exchange on the external surfaces of non-swelling clays, and at both external and interlayer spaces for swelling clays. In the interlayer of montmorillonite, the R6G molecules form monolayer and bilayer configurations under low and high uptake levels. The significant amounts of R6G uptake indicate that clay minerals are good sorbents for the removal of cationic dyes from water. And the R6G could be readily removed in wastewater treatment by adding small amount of clays and flocculated the clays out.

Ultrasonic velocity and mechanical anisotropy of synthetic shale with different types of clay minerals

Clay minerals are the most abundant materials in shale. Their presence significantly influences the elastic behavior of reservoir rocks as a function of mineral type, volume, and distribution, and their orientation controls the shale’s intrinsic anisotropic behaviors. Thus, knowing the elastic properties of shale with different types of clay minerals is imperative for fully understanding the seismic properties of the reservoir. However, it is extremely difficult to measure the elastic properties of natural shale by means of a single variable (in this case, the type of clay), due to the influences of multiple factors, including water, total organic carbon content, complex mineral composition, and so on. Thus, we use quartz, clay (kaolinite, illite, and smectite), carbonate, and kerogen extract as the primary materials to construct synthetic shale with different types of clay. Ultrasonic experiments were conducted to study the anisotropy of velocity and mechanical properties (Young’s modulus and Poisson’s ratio) in dry synthetic shale samples as a function of applied axial stress. The results show that the velocity of samples increases with applied pressure and the rate of velocity increase is higher at low pressures. Similarly, the dynamic Young’s modulus and Poisson’s ratio increase with applied pressure; [Formula: see text] is always larger than [Formula: see text], but [Formula: see text] may be larger or smaller than [Formula: see text]. Furthermore, velocity anisotropy and mechanical anisotropy decrease with the increase of stress and are sensitive to stress and lithology. The closure of large aspect-ratio pores (and/or microcracks) seems to be a dominant mechanism controlling the change of anisotropy. Finally, the changes in mechanical anisotropy under applied stress are larger compared with the changes in velocity anisotropy, indicating that mechanical properties are more sensitive to the changes in rock property.

Characterization of clays from the Corumbataí formation used as raw material for ceramic industry in the Santa Gertrudes district, São Paulo, Brazil

The Ceramic District of Santa Gertrudes (CDSG) is the largest producer of ceramic floor and wall tiles in Brazil. Tile manufacturers in the CDSG use the single raw material, clayey rocks from the Corumbataí formation. Mineralogical, chemical, thermal and microstructural characterization of the clayey rocks from the Corumbataí formation have been performed in order to better understand effect of the studied properties on dry route tilemaking process. The results revealed different compositional features among the studied clayey raw materials. The bulk samples were dominated by clay minerals associated with quartz, feldspars and minor amount of hematite. All samples were complex mixtures where different types of clay minerals (illite, smectite, illite–smectite, kaolinite–smectite, chlorite, sometimes kaolinite) coexist in variable amount. The amount of clay minerals increased with a higher degree of weathering of studied samples. Different technological behavior was expected for studied raw materials due to their heterogeneous nature, in terms of mineral composition and weathering degree. To improve technological behavior of studied materials during tilemaking process an upgrade of the currently used raw materials preparation is recommended (raw materials maturation, more aggressive milling of raw materials, addition of plastic component and homogeneous mixing of plastic and non-plastic components).

Geo-environmental assessment of Al-Wala dam sediments, Jordan

Surface sediments of Al-Wala Reservoir central Jordan, were evaluated to characterize trace element distribution and sources related to various physicochemical variables. Metallic micronutrient distribution is determined by grain size analysis, parent material in the catchment, and anthropogenic activities. The suitability of reservoir sediment as a source of natural additives for replacing eroded topsoil was assessed by comparing the physiochemical characteristics of bottom sediments to normal worldwide soils. Textural analysis of bottom sediments is mainly composed of very fine-grained facies (clayey-silty facies), characterized by high content of different types of clay minerals and organic matters, medium-high cation exchange capacity, and optimal pH ranges for availability of most micronutrients. Among the metallic micronutrients measured: Fe, Zn, Cd, Cu and Mn, total abundance of Zn and Cd slightly exceeded the recommended safe levels for normal soils, but these concentrations my are decline when the sediments are exposed to air. Comparing our results with natural soils, it can be concluded that clayey-silty facies sediments can be considered as a good source of non-manufactured additives, whereas coarse-grained (sandy and granule facies) sediments as a good agricultural soil.

Spectral stratigraphy and clay minerals analysis in parts of Hellas Planitia, Mars

Abstract. Absorption features that occur in reflectance spectra are a sensitive indicator of mineralogy and chemical composition for a wide variety of materials. The investigation of the mineralogy and chemical composition of surfaces give information about the origin and evolution of planetary bodies. On Mars, the processes of formation of different types of clay minerals result from different types of wet conditions viz. hydrothermalism, subsurface/groundwater weathering, surface alteration etc. The image analyzed in the present study was frt000947f- 164-trr3 (−27.87N–65.06E). Through the spectral stratigraphic characterization along a crater wall, eight (8) different layers were identified considering the spectral variability and their position. In Hellas Planitia, the alteration minerals identified by CRISM based on distinctive absorptions from 0.4 to3.9 μm include Al-rich smectite, montmorillonite, phyllosilicate mineral at 2.2 μm and 2.35 μm, including strong absorption feature noticed at 1.9 μm. We conclude that the layers exposed in the crater wall help characterize the compositional stratigraphy for confirming the presence of hydrated minerals in this region as an outcome of geohydrological weathering process.

Thermal magnetic behaviour of Al-substituted haematite mixed with clay minerals and its geological significance

Clay minerals and Al-substituted haematite (Al-hm) usually coexist in soils and sediments. However, effects of clay minerals on Al-hm during thermal magnetic measurements in argon environment have not been well studied. In order to quantify such effects, a series of Al-hm samples were synthesized, and were then mixed with clay minerals (illite, chlorite, kaolinite and Ca-montmorillonite). The temperature dependence of magnetic susceptibility curves in an argon environment showed that Al-substituted magnetite was produced during the thermal treatment via the reduction of Al-hm by the clay mineral, which leads to a significant magnetic enhancement of the thermal products. In addition, the reductive capacity varies among different types of clay minerals, that is, illite > chlorite > kaolinite > Ca-montmorillonite. Furthermore, the iron content in the clay minerals and Al content of Al-hm are two predominant factors controlling the reduced haematite content. The iron is released from the clay minerals and provides the reducing agent, while Al decreases the crystallinity of haematite and thus facilitates the chemical reaction. Therefore, the thermal magnetic measurements can be used to quantify the Al content of Al-hm in natural samples. Our study provides significant information for palaeomagnetism and environmental magnetism studies, such as thermal magnetic analysis and palaeomagnetic intensity reconstruction using ancient pottery and kilns.

Evaluation of the Adsorption of Mono- and Polytungstates onto Different Types of Clay Minerals and Pahokee Peat

Detailed knowledge about the fate and transport of tungsten in soils is critical to understanding and effectively addressing tungsten behavior in the environment. Recent studies have shown that tungsten anions may polymerize (depending upon concentration, pH, and aquatic geochemistry) in aquatic and soil systems. However, to date, of all soluble tungstate species only monotungstates have been scrutinized to a fair extent in adsorption studies. There is a lack of information evaluating adsorption mechanisms of mono- and polytungstates onto clay minerals. The objective of this work is to investigate the adsorption behavior of monotungstates (sodium tungstate, Na2WO4) and polytungstates (sodium metatungstate, 3Na2WO4·9WO3) onto different types of clay minerals (montmorillonite, kaolinite, illite) and an organic adsorbent (Pahokee peat). Batch equilibrium experiments as a function of concentration (adsorption isotherms) and pH (adsorption envelopes) were performed to provide information about mono- and polytungstate adsorption onto clays and Pahokee peat. Adsorption equilibrium data for mono- and polytungstates onto different types of clay minerals and Pahokee peat were modeled with Freundlich and Langmuir isotherms. The adsorption affinity of clays and Pahokee peat for monotungstates follows the order: Pahokee peat>kaolinite>montmorillonite>illite; for polytungstates, the order is as follows: kaolinite>Pahokee peat>montmorillonite>illite. Results of this study suggest that the charges of the clay mineral surface, tungsten species, and solution pH are the main factors controlling tungsten adsorption. Moreover, polymeric tungsten species (i.e., metatungstate) appear to be more mobile in the environment than monomeric tungstate.

Tracing TiO2 photocatalytic degradation of humic acid in the presence of clay particles by excitation–emission matrix (EEM) fluorescence spectra

Excitation–emission matrix (EEM) fluorescence spectral features were evaluated for the elucidation of the photocatalytic degradation of sole humic acid (HA) and HA in the presence of clay minerals. Montmorillonite (Mt) or kaolinite (Kt) was selected to represent two different types of clay minerals. EEM fluorescence signatures displayed irradiation time dependent transformation of humic-like fluorophores to fulvic-like fluorophores in accordance with the photocatalytic mineralization of HA. The role of Ca2+ ions on the photocatalytic degradation of HA could also be visualized through EEM under the specified conditions. Upon photocatalytic treatment, the role of Mt was more pronounced with respect to the effects observed in the presence of Kt in comparison to the results achieved for sole HA. Under prolonged irradiation conditions (tirr = 180 min) almost complete removal of fluorophoric groups was detected in relation to a substantial degree of mineralization (>75% removal of dissolved organic carbon, DOC). As a correlative approach, fluorescence-derived index defined as fluorescence intensity (FI) represented by the ratio of the emission intensity at λemis = 450 nm to that at λemis = 500 nm, following excitation at λexc = 370 nm was successfully employed. FI values were correlated to specified UV parameters (UV254 and UV280) revealing an inverse relationship. Moreover, through normalization of the specified UV parameters to DOC contents, the derived specific UV parameters (SUVA254 and SUVA280) could also be successfully correlated to FI. The obtained results indicated that besides photocatalytic mineralization efficiencies, UV–vis and fluorescence spectroscopic properties could also be employed to gain further information on the humic structural changes attained through non-selective oxidation mechanism in comparison to the data obtained under non-oxidative conditions.