Edges Of Clay Particles Research Articles

Chromium(VI) bioreduction by cysteine over clay particle edges: Catalytic mechanisms and influencing factors

Bioremediation is effective for scavenging Cr(VI), one of the eight most toxic chemicals according to the world health organization. This study presents a comprehensive understanding of Cr(VI) bioreduction by cysteine (Cys) over clay minerals that serve as supports, and the central roles of Cys during Cr(VI) bioremediation are verified and unraveled at the molecular level. Impacts of Cys coverage, active sites, and the reducing agent are further discussed. Cr(VI) bioreduction proceeds via competitive mechanisms: Oxidative dehydrogenation (ODH) driven by π-conjugation (C=S) development and SS dimerization (SDM) driven by SS bond formation. The optimal reaction paths for ODH at the Al(III) site, SDM at the Al(III) site, ODH at the Mg(II) site, and SDM at the Mg(II) site have the apparent energy barriers of 124.6, 115.9, 117.3, and 71.9 kJ/mol, respectively. Cr(IV) represents the major product except for SDM at Mg(II) site where Cr(II) dominates. Mechanisms and product distribution are affected by various factors: 1) Increase of Cys coverage triggers Cr(VI) reduction to occur sequentially via Stage 1 of ODH, Stage 2 of ODH, Stage 1 of SDM, and Stage 2 of SDM; 2) Cr(VI) reduction occurs preferentially at the Mg(II) versus Al(III) site, and the conformation transition of CrIV structure promotes reaction and appears for the most preferred paths of the AlIII rather than MgII site; 3) Mechanisms are decided by the Fe(II) content for Fe(II) while by π-conjugation development for alcohols and Cys (ODH): Ethanol is preferred kinetically, while high Cys coverage adopts SDM that lacks for ethanol. Results provide significant new insights for Cr(VI) bioremediation and accelerate catalyst designs for Cr(VI) reduction which conduce to Cr(VI) management and rational utilization of less toxic products.

Molecular-level insights into pH regulation of cation adsorption and exchange at clay particle edges

Clay particle edges are often adsorption and catalytic centers while performances are strongly pH-dependent. Microscopic understanding of adsorption of single (Na+, K+, Cs+, Pb2+) and binary (Na+/Cs+, Na+/K+, K+/Pb2+, Na+/Pb2+) metal ions by montmorillonite particle edges under various pH conditions is addressed by molecular dynamics simulations, and mechanisms regarding how pH regulates adsorption of metal ions, ion-specific effects, impacts of co-ions, thermodynamics and kinetics of ion exchange, and immobilization of heavy metals are unraveled. Under all pH conditions, Na+ and Pb2+ with smaller ionic radii have much higher inner-sphere adsorption densities than K+ and Cs+, and pH elevation generates new adsorption configurations that greatly promote adsorption. The promoting extent is apparently larger for Na+ and Pb2+, while ion-specific sequence (Na+ > K+ > Cs+) remains, which is applicable to single and binary metal ions. Impacts of co-ions onto adsorption rely on their identities and become magnified at higher pH although with consistent trends at all pH conditions; e.g., Na+ always inhibits Pb2+ adsorption while the degree of inhibition is larger due to pH elevation. Whether to promote or suppress adsorption by co-ions can be predicted from relative adsorption preference (Na+ > Pb2+ > K+ > Cs+). Stronger competitive adsorption occurs for binary Na+/Pb2+ than other Aa+/Bb+ ions and for the particle edges than basal and interlayer surfaces. As indicated by selectivity coefficients, montmorillonite particle edges are always Na+-selective, while pH elevation significantly enhances Pb2+ selectivity. Specific adsorption at clay particle edges emerges at lower pH for Na+ than Pb2+ (5.3 vs. 6.9) and increases at higher pH, while the increasing extent is larger for Pb2+, indicating pH elevation favors the immobilization of heavy metals. Results significantly enrich the knowledge about the adsorption/exchange of metal ions and the immobilization of heavy metals by clay minerals.

Compaction Behaviour of Lateritic Soil–Calcium Chloride Mixtures

Laboratory studies on lateritic soil treated with up to 8% calcium chloride (CaCl2) by dry weight of soil was carried out to establish the soil improving potential. Tests carried out include Atterberg limits and linear shrinkage, compaction characteristics (maximum dry density, MDD and optimum moisture content, OMC), strength characteristics (unconfined compressive strength, UCS and California bearing ratio, CBR) and microanalysis. Compaction and strength characteristics test were investigated using three compactive efforts [i.e. British Standard light, BSL (standard Proctor), West African Standard, WAS or ‘intermediate’ and British Standard heavy, BSH (modified Proctor)]. Results obtained show that Atterberg limits decreased with increased calcium chloride content. MDD increased with a corresponding decreased OMC of the soil–CaCl2 mixtures for the three compactive efforts. Peak UCS and CBR values were obtained at 4% CaCl2 content with increasing compactive effort. Microanalysis using Scan Electron Microscope, SEM shows the transformation of surface morphology at the edges of clay particles. Statistical analysis of result shows that CaCl2 content had significant influence on the Atterberg limit parameters and both the variations of CaCl2 content and compactive effort had significant effect on the strength parameters, maximum dry density as well as the optimum moisture content. The R2 values of regression models show that CaCl2, LL, MDD, OMC and CE have considerable influence on the UCS at 7 days curing and CBR values. Peak strength values are below those recommended for sub base and base stabilization, hence CaCl2 is not convenient as a stand-alone stabilizer but can be adequate as a modifier or as admixture in Cement or lime stabilization of lateritic soil.

Heteroagglomeration of zinc oxide nanoparticles with clay mineral modulates the bioavailability and toxicity of nanoparticle in Tetrahymena pyriformis

The extensive use of zinc oxide nanoparticles (ZnO NPs) in cosmetics, sunscreens and healthcare products increases their release in the aquatic environment. The present study explored the possible interaction of ZnO NPs with montmorillonite clay minerals in aqueous conditions. An addition of ZnO NPs on clay suspension significantly (p<0.05) increases the hydrodymic size of clay particles from 1652±90nm to 2158±13nm due to heteroagglomeration. The electrokinetic measurements showed a significant (p<0.05) difference in the electrophoretic mobilities of bare (−1.80±0.03μmcm/Vs) and ZnO NPs-clay association (−1.37±0.03μmcm/Vs) that results to the electrostatic interaction between ZnO NPs and clay particles. The attenuated total reflectance Fourier transform infrared spectroscopy analysis of ZnO NPs-clay association demonstrated the binding of ZnO NPs with the Si-O-Al region on the edges of clay particles. The increase in size of ZnO NPs-clay heteroagglomerates further leads to their sedimentation at 24h. Although, the stability of ZnO NPs in the clay suspension was decreased due to heteroagglomeration, but the bioavailability and toxicity of ZnO NPs-clay heteroagglomerates in Tetrahymena pyriformis was enhanced. These observations provide an evidence on possible mechanisms available in natural environment that can facilitate nanoparticles entry into the organisms present in lower trophic levels of the food web.

산해리도가 다른 무기산에 의한 토양 입자 표면 특성

We conducted this research to observe the changes of surface morphology and composition of clay minerals influenced by various concentrations of fluoric acid. Hydrofluoric acid (HA), a solution of hydrogen fluoride (HF) in water, is a colourless solution that is highly corrosive, capable of dissolving many materials, especially oxides. To do this, we treated several concentrations of HA on the ground soil samples collected from the agricultural experimental station located at Chungnam National University to observe the influence of fluoric acid on the changes of surface structures and elemental composition of clay particles. Generally, microscopic examination showed that the HA can not only attack an edge of clay particles but also start at any point where structural defects and weaknesses predisposed sites to acid. The orderly flake arrangement of clay minerals may reflect certain crystal symmetry elements. The ESEM-EDS results of element composition of clay particles influenced by HA indicated the changes of structures of clay minerals. It is also clear from the formation of etch figures and element composition of clay particles that the product layer at least partially dissolved or disintegrated in the presence of acid. Conclusively, the clay structures can be strongly influenced by concentrations of HA, resulting in changes of physical and chemical properties that can determine the behavior of solute transport as well as mobility of ions in soils.

Interactions of the dye, C.I. direct orange 34 with natural clay

Clay minerals are efficient adsorbent for dyes due to their colloidal properties. Tunisian raw clay collected from mine of Tamra composed of two species of clay minerals (kaolinite and halloysite) has shown efficiency for removal of azo dye “Direct orange 34” (DO34) from aqueous solutions. For this purpose, the adsorption of DO34 was studied in batch mode under various parameters such as: pH, contact time, adsorbent dose and temperature. The results showed that acid pH is favorable for the adsorption of dye and physisorption on broken edges of clay particles seemed to play a major role in the adsorption process. It was found that the rate of adsorption decreases with increasing temperature and the process is exothermic. The adsorption kinetics followed the pseudo-second-order equation. The intraparticle diffusion and film diffusion are the rate limiting steps. The raw halloysite-rich clay sample (HC) plays a major role in anionic dye adsorption due to mainly to its acidic surface sites.

Mechanochemical treatment of vermiculite in vibration milling and its effect on lead (II) adsorption ability

Vermiculite was subjected to mechanochemical treatment using a vibration mill. The treated samples were characterized by XRD, FE-SEM, N2-adsorption, FT-IR and 29Si-MAS NMR. The mechanochemical treatment led to a reduction in the particle size and a significant increase in the surface area. Surface hydroxyl (silanol, magnesiol and aluminol) groups, located on the clay particle edges and amorphous silica phase, were also generated. The mechanochemical treatment of vermiculite gave a dramatic improvement in the lead (II) adsorption capacity.

Efficient anionic dye adsorption on natural untreated clay: Kinetic study and thermodynamic parameters

This research involved the efficient adsorption of an anionic dye (Reactive Red 120) by natural untreated clay, a low-cost material abundant in highly weathered soils. The adsorption kinetics was investigated using the parameters such as contact time, stirring speed, initial dye concentration, initial pH, ionic strength, and solution temperature. According to these experiments the maximum removal is observed after 80 to 100min. The results showed that acid pH is favorable for the adsorption of dye and physisorption on broken edges of clay particles seemed to play a major role in the adsorption process. It was found that the rate of adsorption decreases with increasing temperature and the process is exothermic. The adsorption kinetics followed the pseudo-second-order equation for the dye investigated in this work. Furthermore the thermodynamic activation parameters such as the enthalpy and entropy were determined. Compared to adsorption experiments on other clays, the results show that the Fouchana natural clay is an efficient adsorbent for Reactive Red 120 dye.

Thermodynamic Concepts in Modeling Sorption at the Mineral-Water Interface

Sorption phenomena on mineral-water interfaces (MWI) are known to control the retention of many trace elements and organic substances of environmental concern, as well as the kinetics of mineral dissolution/precipitation (Sposito 1984; Davis and Kent 1990; Dzombak and Morel 1990; Goldberg 1992; Stumm 1992; Morel and Hering 1993; Langmuir 1997; Zachara and Westall 1999; Benjamin 2002b; Ganor et al. 2009; and references therein). However, there is still no agreement about a uniform thermodynamic concept which can account for the relevant physicochemical interactions at MWI and can be implemented in popular geochemical modeling codes. This is a major obstacle for compiling a unified (ad)sorption thermodynamic database, also marred by disagreements in defining standard and reference states for (ad)sorbed species. In turn, confusion stems from the multiplicity of thermodynamic concepts used in the description of sorption phenomena on MWIs. The goal of this contribution is to summarize these concepts in a hope to illuminate ways along which they can eventually be made mutually consistent. A minor element (M) can be retained on mineral-water interfaces by a number of sorption mechanisms which also depend on time. On a kinetic basis, this can be summarized in the so-called sorption continuum . At short reaction times, aqueous complexation competes for M with ion exchange on permanently-charged surfaces (such as basal planes or interlayer of clay minerals), as well as the outer-sphere surface complexation on (hydr)oxides. These fast processes are (almost) reversible. At longer reaction times, specific ( multi-site ) adsorption on clay particle edges or (hydr)oxides takes place; this kind of binding into inner-sphere surface species often displays an energetic heterogeneity at low total M concentrations in the system and becomes progressively irreversible with time. At still longer reaction times, the incorporation of (ad)sorbed species deeper into …

Biopolymer coated clay particles for the adsorption of tungsten from water

Biopolymer coated clay (i.e. chitosan coated montmorillonite clay called biosorbent) particles were synthesized. The effects of tungsten concentration in feed water (20–500 ppm) and water pH (4, 5.5 and 6.4) on the zeta potential of adsorbent particles, tungsten removal, and adsorption equilibrium were studied using biosorbent and natural clay. Coating clay particles with chitosan shifts the net surface charge of clay from negative to positive and the point of zero charge (PZC) of clay from 2.8 and 5.8. The net surface charge of biosorbent particles decreases with an increase in the tungsten concentration of feed water because the positively charged sites are consumed by the adsorption of tungsten anions. Biosorbent was found to be much more effective than natural clay for the removal of tungsten. The tungsten removal efficiencies of both biosorbent and natural clay decrease with an increase in the pH level and an increase in the tungsten concentration of the feed water. Adsorption equilibrium studies show that tungsten removal is the highest at pH 4. Adsorption equilibrium data were modeled with the Langmuir, Temkin, and Freundlich isotherms and the model parameters were obtained. Adsorption oftungsten species on both biosorbent and natural clay seemed to obey Langmuir isotherm within the range of concentrations and pH investigated. The maximum tungsten adsorption capacities of chitosan, biosorbent and natural clay were found to be 632 mg tungsten/g of chitosan, 23.9 mg tungsten/g of biosorbent, and 5.45 mg tungsten/g of natural clay at pH 4, respectively. The tungsten species adsorption on biosorbent was found to be governed by the ionic attraction between the protonated surface groups on chitosan and the negatively charged tungsten species. Whereas, the tungsten species adsorption on natural clay is governed by the positively charged clay particle edges formed by broken bonds of Al-O and Si-O.

Flocculation characteristics of organo-modified clay particles in poly(L-lactide) / montmorillonite hybrid systems

Abstract The stacking characteristics of organo-modified montmorillonite particles in poly(L-lactide) / clay hybrids have been investigated through FT-IR measurements and transmission electron microscopy. The clay particles tend to flocculate with hydrogen bonding among the hydroxyl groups of the surfactant, those located at the edge of clay particles, and/or those existing at the ends of polylactide chains.

Organic matter in small mesopores in sediments and soils

The three-way correlation among organic matter concentrations, specific surface area and small mesopores observed for many soils and sediments led to the hypothesis that enclosure within the pores might explain the apparent protection of organic matter by minerals. We test this hypothesis by examining whether the bulk of organic matter resides within small mesopores. Pore volumes as a function of pore width were measured before and after organic matter removal, and the volume differences ascribed to organic matter filling of pores. Minor changes in small mesopore size distributions upon treatments such as centrifugation and muffling indicate the robustness of the mineral matrices that form these pores. We developed an additional method to assess organic matter densities using high-resolution pycnometry, and used these densities to convert pore volumes to organic matter contents. Although smaller mesopores are shown to have sufficient volumes to contain significant fractions of the total organic matter, only small fractions of total organic matter were found to reside in them. These results are consistent with preferential association between organic matter and aluminous clay particle edges, rather than the largely siliceous clay faces that contribute most surface area and form pore walls. While simple enclosure within smaller mesopores cannot, therefore, explain protection, network effects working at larger size scales may account for exclusion of digestive agents and hence organic matter protection.

Characterization of Gorleben groundwater colloids by atomic force microscopy.

Groundwater colloids from the Gorleben site (Lower Saxony, Germany) are characterized in the presence of Eu(III) by tapping-mode atomic force microscopy (AFM) with phase contrast imaging. Using a liquid cell the method allows investigations of samples being in contact with aqueous solution. This ensures that complex structures are kept in their native hydrated state. Different types of colloids and aggregates are found by AFM, e.g., spherical particles, fibrous structures, and structures which appear to be hollow. A partial coating of the edges of clay particles with humic colloids can be assumed from phase contrast images. Therefore, aquatic colloids and their aggregates found in Gorleben groundwater can be characterized as a complex mixture of components, which may influence the migration of groundwater contaminants in different processes.

Lime pile treatment of black cotton soils

This paper examines the efficiency of 75 mm diameter lime piles in altering the physico-chemical and engineering properties of a compacted expansive soil. Calcium and hydroxyl ions from the lime pile migrated into the surrounding soil mass and increased the soil pH and pore salinity. The increase in soil pH from lime migration could not increase the soil pH to levels (⩾ 12) that were conducive for pozzolanic reactions. The increase in soil pH, however, increased the exchangeable calcium ion content of the soil by apparently imparting additional negative charge to the clay particle edges. Lime modification reactions due to increased pore salinity and exchangeable calcium ions suppressed the plasticity index and swell potential, and slightly increased the unconfined compressive strength of the compacted expansive soil. Dans cet exposé, nous examinons la capacité des piles de chaux de 75 mm de diamètre ont de à changer les propriétés physico-chimiques et industrielles d'un sol expansif compacté. Les ions de calcium et d'hydroxyle présents dans la pile de chaux migrant dans la masse du sol avoisinant environnant, augmentant son pH et la salinité des pores. L'augmentation du pH du sol causée par la migration de chaux ne peut atteindre des niveauzx (⩾ 12) qui favorisent favorisant des réactions pouzzolani-ques. Cependant, l'augmentation du pH du sol augmente la teneur de celui-ci en ions de calcium échangeables en apportant apparemment une charge négative supplémentaire sur les bords des particules d'argile. Les réactions de modification de la chaux dues à une plus grande salinité de pores et aux ions de calcium échangeables suppriment l'indice de plasticité et le potential de gonflement et augmentent légèrement la résistance à la compression non confinée du sol expansif compact.

The effect of clay structure on peptide bond formation catalysis

The catalytic efficiency in peptide bond formation of various clays was tested by reactions of glycine (gly), diglycine (gly 2), glycine+alanine (ala), gly 2+ala. The main effects of clay structure and composition resulted as following: (1) Clay composition influences the activation of reactant molecules at clay particle edges. (2) Acidity (basicity) of the clay surface can change on the state of the reactant. (3) The clay structure is related to suspension stability and thus accessibility of clay catalytic sites. Mg-rich trioctahedral clays hectorite (smectite) and talc are the most efficient catalysts. Oligomerization of gly and gly 2 proceeds on all clays, whereas oligopeptides including ala units are formed with much lower yields and only on the most efficient catalysts. Besides dipeptides and other linear peptides, also cyclic anhydride (diketopiperazine) is formed with relatively high yields from diglycine. Cyclic anhydrides can directly act in the formation of the linear oligopeptides by ring opening molecular rearrangement and addition of another amino acid (oligopeptide).

The effect of reaction conditions on montmorillonite-catalysed peptide formation

Oligomerization of glycine (gly) and diglycine (gly2) on montmorillonite was performed as cyclic, drying-wetting process at temperatures below 100°C, under varying reaction conditions. The influence of substrate/clay ratio, temperature and pH was found to be different for amino acid (AA) dimerization, cyclic anhydride (CA) formation and peptide chain elongation. High temperatures and neutral pH favour CA formation over diglycine production. An AA/catalyst ratio of 0.2 mmol/g leads to optimal yields. This supports the assumption that amino acid dimerization and CA formation take place at the edges of clay particles. Peptide chain elongation, starting from gly2, produces higher yields at higher temperatures and neutral pH.

Intercalation compounds between nicotine and a high surface area Saponite

The retention of nicotine by a high surface area saponite has been studied using two methods: (1) in repose at room temperature, and (2) heating at 70 °C under reflux. Different contact times were considered for each method. The intercalation compounds obtained were studied by chemical analysis, X-ray diffraction, infrared spectroscopy and thermal analysis. Nicotine is retained both in the interlayer region and at the edge of clay particles. The amount of nicotine retained in the interlayer region has been calculated from thermal analysis and CHN measurements.

Some electrokinetic behaviour of tin tailing slimes

Tin tailing slimes from three ex-mining ponds located in different parts of Malaysia and with different clay compositions were studied. Pengkalan slime contained about 8% of montmorillonite clay; Serendah (SH) slime had a high content of amorphous materials (24%) while the main constituents of all samples were kaolinite and illite with a small amount of organic materials and microorganisms as exemplified by Malim Nawar slime. The surface nature of the clay minerals in these slimes was characterised using the particle microelectrophoresis technique. The electrokinetic behaviour of the slime basically followed that of kaolinite but differences due to compositional variations were evident. This is especially so for the SH slime where the amorphous materials present completely modified the surface property of the kaolinite. The magnitude of the electrophoretic mobility depended strongly upon the exchangeable cations associated with the clay particles and pore fluid chemistry of the slime samples. The evaluation of the isoelectric point of the edges of clay particles by the electrokinetic method is discussed. The importance of understanding the charged surfaces of the clay minerals in the slimes in relation to the design of an efficient dewatering programme is briefly discussed.

Electrophoretic Mobility of Illite and Micaceous Soil Clays

AbstractExamination of the colloid chemistry of clays may provide an increased understanding of the structural stability of soil clays under field conditions. Electrophoretic mobility (EM) gives specific information about the net charge of a particle at the shear plane. An evaluation was made of the effect of the important water quality variables, electrolyte concentration, sodium adsorption ratio (SAR), and pH on the EM of Silver Hill illite and three micaceous soil clays. Increases in electrolyte concentrationg enerally increased the mobility of the colloidal particles, contrary to double‐layer prediction. The EM behavior is thus not explained by the commonly used double‐layer theory of infinite plates; however, results can be explained if this double‐layer theory is extended to consider charge spillover into clay particle edges and changes in variable edge charge. The EM increased faster when the SAR increased from 5 to 20 (mmol L−1)0.5 than at higher values of this parameter. These results are in agreement with tactoid formation and demixing theory previously developed by others for smectites. The EM for micaceous soil clays and specimen Silver Hill illite was more sensitive to changes in pH as the electrolyte concentration increased. These results are consistent with observations on variable‐charge minerals where variable charge increases with increasing ionic strength. Removal of organic matter increased the EM in the pH 4 to 10 range studied.

Boron Adsorption on California Soils

AbstractBoron adsorption on 15 arid zone soil samples was investigated both as a function of equilibrium B concentration (0–100 g m−3) and as a function of solution pH (5.5–11.5). Boron adsorption as a function of equilibrium B concentration could be described by the Freundlich adsorption isotherm over the entire concentration range studied and by the Langmuir adsorption isotherm over a limited concentration range. Boron adsorption increased from pH 5.5 to 8.5, exhibited a peak in the pH range 8.5 to 10, and decreased from pH 10 to 11.5. The adsorption maximum was highly correlated with both specific surface area and clay content. It was assumed that B adsorbs by ligand exchange with hydroxyl groups at the edges of clay particles and Al and Fe oxide minerals. The constant capacitance model was able to describe B adsorption on the soil samples. While the fit was generally good, the chemical significance of one of the surface complexation constants is uncertain for a few soil samples. An average set of surface complexation constants could be used to predict B adsorption on most of the soil samples studied.