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PubMed2005

Core--shell nanocluster films of hemoglobin and clay nanoparticle: direct electrochemistry and electrocatalysis.

A novel core-shell protein nanocluster film, designated as clay-(Hb/PSS)(n), was fabricated on pyrolytic graphite (PG) electrodes. Positively charged hemoglobin (Hb) at pH 5.5 and negatively charged poly(styrenesulfonate) (PSS) were first assembled layer by layer on surface of clay nanoparticles from their solutions mainly by electrostatic attraction, forming a core-shell nanocluster structure in which clay nanoparticles were the "cores" and (Hb/PSS)(n) multilayers were the "shells". The aqueous dispersion of clay-(Hb/PSS)(n) nanoclusters was then cast on surface of PG electrodes, forming clay-(Hb/PSS)(n) nanocluster films after evaporation of solvent. Hb in clay-(Hb/PSS)(n) films exhibited a pair of well-defined and reversible cyclic voltammetric (CV) peaks at about -0.36 V vs. SCE in pH 7.0 buffers, characteristic of Hb heme Fe(III)/Fe(II) redox couples. Compared with other Hb-containing clay films, clay-(Hb/PSS)(n) films displayed smaller CV peak separation (DeltaE(p)), indicating the better electrochemical reversibility of Hb in these nanocluster films. The partially ordered structure of the films was characterized by X-ray diffraction (XRD) experiments. UV-VIS and reflection absorption infrared (RAIR) spectroscopy suggests that Hb retains its near-native structure in clay-(Hb/PSS)(n) films. Oxygen, hydrogen peroxide, and nitrite were catalytically reduced at clay-(Hb/PSS)(n) film electrodes, showing the potential applicability of the films as the new type of biosensors or bioreactors based on protein direct electrochemistry. The electrochemical and electrocatalytic activity of the films could be tailored by controlling the number of bilayers of the (Hb/PSS)(n) shells on the surface of clay nanoparticle cores.

Aluminum Silicates↗
PubMed2005

Sorption of 2,4,6-trichlorophenol in model humic acid-clay systems.

Humic acids and clays are important soil components that influence the sorption and desorption of organic contaminants; however, it is unclear how humic acids influence the sorption of organic contaminants onto clays and their subsequent desorption. Sorption and desorption of 2,4,6-trichlorophenol (2,4,6-TCP) by and from humic acid-modified K(+)- and Ca(2+)-montmorillonite and -illite were compared with unmodified clays using batch equilibration methods. Commercial humic acid and the humic acid extracted from forest soil were employed in this experiment. The adsorbed amount of 2,4,6-TCP by commercial humic acid was almost twice as large as that adsorbed by the extracted soil humic acid. More 2,4,6-TCP was sorbed onto K(+)- and Ca(2+)-illite than onto K(+)- and Ca(2+)-montmorillonite. K(+) clays were more effective in adsorbing 2,4,6-TCP than Ca(2+) clays. Sorption of 2,4,6-TCP on humic acid-modified Ca(2+)- and K(+)-montmorillonite and -illite increased as compared with unmodified clays. The sorption nonlinearity of 2,4,6-TCP on humic acid-modified Ca(2+)- and K(+)-illite increased remarkably as compared with the unmodified clays. The sorption nonlinearity of 2,4,6-TCP on humic acid-modified Ca(2+)- and K(+)-montmorillonite increased slightly in contrast to unmodified montmorillonites. By comparing sorption and desorption results, we observed hysteresis for all sorbents including humic acids, clays, and humic acid-modified clays. Sorption nonlinearity and hysteresis were dependent on the structure of humic acids. Higher aromaticity of humic acids resulted in greater sorption nonlinearity and desorption hysteresis. In addition, sorption capacity (K(f)') was positively correlated with the humic acid content of the sorbents. These results show that modification of humic acids on clays can not only increase the adsorption ability of clays but also affect the sorption nonlinearity of 2,4,6-TCP, and the desorption hysteresis was probably due to the structural characteristics of humic acids.

Adsorption↗
PubMed2003

Microbial and copper adsorption by smectitic clay--an experimental study.

The objective of this study was to quantify copper-, bacteria- and bacteriophage-binding capacities of natural clay with the aim of predicting the adsorption of heavy metals, human pathogenic bacteria and viruses by a clayey landfill liner. X-ray diffraction analysis of six natural clays showed that the dominant phase in all deposits consists of smectites together with illite, kaolin and, sometimes, palygorskite and sepiolite. The specific surface areas of different clay substrates were very high ranging from 293 to 351 m2 g(-1), and indicating a high proportion of phyllosilicates, consisting especially of smectites. The physico-chemical identification of separated smectites showed a high potential adsorbent character indicative of a large industrial use. The Kb12 smectite substrate chosen arbitrarily among six separated substrates, appeared as an excellent copper adsorbent. Copper was adsorbed to clay in a proportion ranging from 94.6 to 96.0% with an average of 95.1% and its adsorption occurred rapidly in less than 30 min. Organic contents of the clay substrate, evaluated as 17% of dry mass, may contribute and enhance copper adsorption. Different elution protocols using distilled water, 2 and/or 5% nitric acid revealed that while nitric acid resulted in the removal of more than 59% of the metal at the lower concentration and its complete depletion with a further elution at the higher concentration, distilled water alone was unable to remove more than 1% of adsorbed copper. This finding suggested that copper ions form high-energy bonds with layer-silicate surfaces. Interestingly, the use of a regenerated substrate as copper adsorbent subsequent to abundant washings of the used substrate consecutively, with 0.1 N HNO3 and distilled water, reduced copper adsorption by approximately 14%, suggesting a slight disturbance of clay initial structure. Batch adsorption experiments with phage T7 and raw clay Kb12 showed that the tested clay substrate appeared as a relatively moderate phage adsorbent since the quantity of adsorbed phage averaged 98.2 +/- 0.88% (2 log10 retention) as measured by infectivity for Escherichia coli ATCC 11303. As shown by two types of separating procedures, natural sedimentation and a low speed centrifugation, bacteriophage particles were bound essentially to fine and not to relatively coarse particles of the clay suspension. The retention capacity of purified clay Kb12 appeared low, with average values lower than 60 and 50%, for Pseudomonas aeruginosa ATCC 15442 and Bacillus cereus ATCC 1135, respectively. A significant increase of retention, in the order of 30%, was found for both bacteria when the mixture clay-bacteria was incubated at laboratory temperature for 6 hours.

Adsorption↗
PubMed2003

[Kinetics and mechanism of removing Microcystis aeruginosa using clay flocculation].

Twenty-six natural clays were studied for their kinetics of flocculating and removing algal cells of Microcystis aeruginosa. According to the 8 h equilibrium removal efficiencies and removal rates at a clay-loading of 0.7 g.L-1, all the 26 clays were classified into three categories. Type-I clay, which includes talc, ferric oxide, sepiolite, ferroferric oxide, and kaolinite, has an equilibrium removal efficiency greater than 90%, a t50 (time needed to remove 50% of the algae) of less than 30 min, and a t80 (time needed to remove 80% of the algae) of less than 2.5 h. Type-II clay, which includes argillanceous rocks, attapulgite, rectorite, illite, and argil, etc., has an equilibrium removal efficiency of 50%-80%, a t50 of less than 2.5 h, and a t80 of more than 5 h. Type-III clay consists of 14 minerals, including laterite, zeolite, mica, clinoptilolite, pumice, tripoli, feldspar and quartz, etc. with the removal efficiency less than 50%, and t50 > > 8 h. When the clay loading was decreased to 0.1-0.2 g.L-1, the 8 h equilibrium removal efficiencies for 25 clays declined to below 60%, except for sepiolite, a Type-I clay, which maintained around 90%. After the sepiolite was modified with Fe3+ to increase its surface charge (Zeta potential from -24.0 mV to +0.43 mV at pH 7.4), the initial removal rate was increased remarkably although its 8 h equilibrium removal efficiency was not improved substantially. As a comparison, the 8 h equilibrium removal efficiency of PAC was no greater than 40% at loadings of 0.02-0.2 g.L-1. Following the analysis of the flocculation mechanism it was concluded that the effect of bridging and netting may play a key role in the clay-algae flocculation processes, which may be important for selecting and modifying clays to improve significantly the removal efficiency.

Aluminum Silicates↗
PubMed2006

[Mechanisms of removing red tide organisms by organo-clays].

We tested the influence of the preparation conditions of the quaternary ammonium compounds (QACs) modified clays on their capacities to remove red tide organisms, then discussed the mechanisms of the organo-clays removing red tide organisms. Hexadecyltrimethylammonium (HDTMA) improved the capacity of clays to flocculate red tide algae, and the HDTMA in metastable state enhanced the toxicity of the clay complexes to algae. The capacities of the organo-clays correlated with the toxicity and the adsorbed amount of the QACs used in clays modification, but as the incubation time was prolonged the stability of the organo-clays was improved and the algal removal efficiencies of the clay complexes decreased. When the adsorbed HDTMA was arranged in different clays in which the spatial resistance was different, there was more HDTMA in metastable state in the three-layer montmorillonite. Because of the homo-ion effect the bivalent or trivalent metal ions induced more HDTMA in metastable state and the corresponding organo-clays had high capacities to remove red tide organisms. When the reaction temperature was 60 degrees C the adsorbed HDTMA was easily arranged on cation exchange sites, if the temperature rose or fell the metastable HDTMA would increase so that the capacity of the clays was improved.

Adsorption↗
PubMed2005

A surface-mediated origin of the RNA world: biogenic activities of clay-adsorbed RNA molecules.

The involvement of clay surfaces in the origin of the first genetic molecules on Earth has long been suggested. However, the formation of these polymers was not sufficient by itself to initiate the evolutionary process leading to the appearance of life. These macromolecules had to persist in primeval habitats so that their biological potentiality could be expressed. In this study, we assess the possibility of development of the RNA world on a clay substrate by investigating the capacity of different RNA molecules adsorbed/bound on the clay minerals montmorillonite (M) and kaolinite (K) to persist in the presence of a degrading agent (RNase-A), to interact specifically with complementary RNA strands, and to transmit the information contained in their nucleotide sequences. The RNase-A degradation of clay-adsorbed 23S rRNA from Escherichia coli was significantly slower (75-80%) than that observed for free rRNA, and the complete digestion of nucleic acid in the presence of clay was obtained in 2 vs. 1 h. Clay-adsorbed Poly[A] homopolymer was able to recognize the complementary Poly[U] homopolymer present in the surrounding water solution and to establish a specific interaction (association) with it, possibly leading to the formation of double strands. Reverse transcription and amplification (RT-PCR) amplification of free and clay-adsorbed 16S indicated that the presence of clay particles partially reduced the efficiency and processivity of reverse transcriptase but did not inhibit its activity, demonstrating that clay-adsorbed RNA is still available for enzymatic replication. These findings indicate that primordial genetic molecules adsorbed on clay minerals would have been protected against degrading agents present in the environment and would have been in the right conditions to undergo evolutionary processes.

Aluminum Silicates↗
PubMed2006

The use of fortified soil-clay as on-site system for domestic wastewater purification.

The quest for simple, low-cost and high-performance decentralized wastewater treatment system for domestic application in developing nations necessitated this study. Clay samples collected from different deposits in Nigeria were characterized by studying the mineralogical and geochemical composition using X-ray diffraction (XRD) and atomic absorption spectroscopy (AAS), respectively. Three major clay minerals of kaolinite, illite and smectite were identified. The geochemical studies showed the abundance of SiO2, Al2O3 and H2O+ in each of the clay samples. Performance efficiency studies were conducted to determine the best combination ratio of pebbles/soil-clay. Soil-clay fortified by pebbles in combination ratios of 1:3 (i.e. pebbles:soil-clay = 1:3 (w/w) showed the optimum water purification, while the combination 3:1 gave the least. The flow rate studies showed that the wastewater had a longer residence time in non-fortified soil-clay than in fortified soil-clay. Two modes of treatment methods were employed-single and double column treatment methods (SCT and DCT). The two methods gave effluents of good quality characteristics, but those from the DCT were of better quality. The quality of effluents also varies from one clay type to another. The quality of effluents from media containing smectite clay mineral was better than those from other columns. Repeated usage of the fortified clay column showed a decrease of pH, TS and DO, and an increase of COD when monitored over a period of 10 days.

Aluminum Silicates↗
PubMed2003

An analytical model for the interpretation of pulse injection experiments performed for testing the spatial variability of clay formations.

This paper presents an analytical model to describe pulse injection experiments. This model solves the advection-diffusion equation while taking into account back diffusion from the clay core to the inlet and from the outlet to the clay core. In most analytical models, back diffusion is neglected. For sufficiently high Péclet numbers, this is a good approximation. However, in experiments where the Péclet number is low, back diffusion is important and must be taken into account. An additional advantage of the present model is that both concentration and flux are conserved at the inlet and at the outlet of the clay core. This model is used to fit pulse injection experiments with iodide and tritiated water (HTO) in clay cores. The (new) model is required for fitting the experimental results since in clay layers advection is very slow leading to a low Péclet number. The experiments are performed on clay cores taken from different depths from the Boom Clay and the Ypres Clay layer under the site of the nuclear power plant of Doel (Belgium). The quality of all fits is excellent and the obtained parameter values are coherent. For HTO, the fitted value for the diffusion accessible porosity is consistent with measurements of the water content in Ypres Clay cores. In both types of clays, the apparent diffusion coefficient at zero flow is between 10(-10) and 2 x 10(-10) m(2)/s for iodide and between 2 x 10(-10) and 3 x 10(-10) m(2)/s for HTO. The dispersion length is in the order of 10(-3) m. The average value for the diffusion accessible porosity is between 0.35 and 0.4 for HTO and between 0.2 and 0.25 for iodide.

Aluminum Silicates↗
PubMed2003

Sorption of paraquat on clay components in Taiwan's oxisol.

The sorption of herbicides in soils is mainly influenced by clay components. The objectives of this study were to evaluate the contribution of clay components on paraquat sorption. The surface soils (0-20 cm) of a Laopi pedon (Fine, mixed, Hyperthermic Typic Hapludox) were separated clays into whole (< 2.0 microm), coarse (0.2-2.0 microm), and fine (< 0.2 microm) fractions with the treatments of removals of organic matter (OM) and free Fe (Fed) oxides. Results indicated that sorption isotherm of paraquat was fitted by the nonlinear Freundlich equation with R2 values ranged in 0.79-0.96, respectively. The shape of paraquat adsorption isotherm on the fine fraction was H-type, but their shapes on the whole and coarse fractions were L-types. The fine clay fractions gave higher contribution on paraquat sorption than the coarse clay fractions identified by their K(f) values. Organic matter associated with fine clay fraction had high CEC contributing to relatively high affinity for paraquat. The DCB treatment created high-affinity sites for paraquat on the fine clay, but had little effect on paraquat sorption for the coarse clay. Chemisorption is the major mechanism for retention of paraquat on clay components, not ion exchange. However, the silicate clay had the highest affinity for paraquat and free Fe compound had the lowest.

Adsorption↗
PubMedDate not supplied

Factors affecting the ratio of cation exchange capacity to clay content in lignite overburden.

Unusually high cation exchange capacity (CEC) values relative to clay content are frequently reported for lignite overburden and minesoils. The CEC to percent clay ratio is commonly greater than one and would require that the average charge of the clay fraction be greater than 100 cmol(c) kg(-1). A comparison of methods for particle-size distribution suggests that the major reason lignite overburden samples have CEC to percent clay ratios greater than one is incomplete dispersion of aggregates of clay minerals or shale fragments. Preliminary investigations revealed the presence of shale fragments, smectite, and partially weathered mica in the silt fraction. Methods commonly used in soil textural analysis underestimated clay content by approximately 24%. The silt fraction may, therefore, provide a "hidden" source of CEC. Another important factor influencing the CEC to percent clay ratio was the presence of organic materials (lignite) in the samples. Lignite may make a significant contribution to CEC in overburden materials. In a study designed to estimate the pH-dependent charge of both the mineral and organic fractions, the CEC of overburden organic constituents was determined to be approximately 158 cmol(c) kg(-1) at pH 8.2. The high CEC to percent clay ratio in lignite overburden and minesoils may be resolved by adjusting methods for clay determination to optimize dispersion and by accounting for CEC due to organic materials. An alternative approach is to use existing methodology and use correction factors to account for incomplete dispersion of clay minerals and the charge contributions of organic materials.

Aluminum Silicates↗
PubMed1979

The effect of clays on the oligomerization of HCN.

The reaction of 0.1 M HCN and dilute solutions of diaminomaleonitrile (DAMN) at pH 8--9 and 25 degrees C in the presence of suspensions of montmorillonite (bentonite) clays were investigated. Montmorillonite clays inhibit the oligomerization of aqueous solutions of HCN. Yields of colored oligomers, ura, and DAMN, are all diminished by clays, but the rate of loss of cyanide is not significantly decreased. The inhibition of oligomer formation is due to the clay-catalyzed decomposition of DAMN. The absence of strong binding of DAMN to clays was suggested by our failure to detect DAMN when a clay that had been incubated with DAMN was washed with spermidine (6 x 10(-3) g/liter). It was established that DAMN does not simply bind to the clays by the observation that the bulk of the radioactivity was recovered from the supernatant in the reaction of 14C-DAMN with montmorillonite. The clay-catalyzed decomposition of DAMN was observed when montmorillonite from two different sources was used and with a variety of homoinic montmorillonites and bentonites. A modification of the established procedure for using the cyanide electrode for cyanide analyses was used to follow the release of HCN from DAMN. This new method can be used in both the acidic and basic pH range and it does not result in the destruction of DAMN by the reagents used for the analysis. Quantitative analyses of the reaction solution from the clay-catalyzed decomposition of DAMN revealed the formation of 1--2 equivalents of HCN per mole of DAMN. The possible significance of these clay-catalyzed reactions in chemical evolution is discussed.

Bentonite↗
PubMed2004

Impact of clay minerals on sulfate-reducing activity in aquifers.

Previous studies have shown that sulfate-reduction activity occurs in a heterogeneous manner throughout the terrestrial subsurface. Low-activity regions are often observed in the presence of clay minerals. Here we report that clays inhibit sulfate reduction activity in sediments and in a pure culture of Desulfovibrio vulgaris. Clay minerals including bentonite and kaolinite inhibited sulfate reduction by 70-90% in sediments. Intact clays and clay colloids or soluble components, capable of passing through a 0.2-microm filter, were also inhibitory to sulfate-reducing bacteria. Other adsorbent materials, including anion or cation exchangers and a zeolite, did not inhibit sulfate reduction in sediments, suggesting that the effect of clays was not due to their cation-exchange capacity. We observed a strong correlation between the Al2O3 content of clays and their relative ability to inhibit sulfate reduction in sediments (r2 = 0.82). This suggested that inhibition might be a direct effect of Al3+ (aq) on the bacteria. We then tested pure aluminum oxide (Al2O3) and showed it to act in a similar manner to clay. As dissolved aluminum is known to be toxic to a variety of organisms at low concentrations, our results suggest that the effects of clay on sulfate-reducing bacteria may be directly due to aluminum. Thus, our experiments provide an explanation for the lack of sulfate-reduction activity in clay-rich regions and presents a mechanism for the effect.

Aluminum Oxide↗
PubMed2004

Geochemical modulation of pesticide sorption on smectite clay.

Pesticide adsorption by soil clays can be dramatically influenced by the exchangeable cations present. Among the common exchangeable base cations in soils (Ca2+, Mg2+, K+, and Na+), K+-saturated clays frequently demonstrate the strongest affinity for pesticides. In the presence of multiple exchangeable cations in the system, we hypothesize that the magnitude of pesticide sorption to soil minerals is proportional to the fraction of clay interlayers saturated with K+ ions. To test this hypothesis, we measured sorption of three pesticides with different polarities (dichlobenil, monuron, and biphenyl) by homoionic K- and Ca-smectite (SWy-2) in KCl/CaCl2 aqueous solutions. The presence of different amounts of KCl and CaCl2 resulted in varying populations of K+ and Ca2+ on the clay exchange sites. The sorption of dichlobenil and, to a lesser extent monuron, increased with the fraction of K+ on clay mineral exchange sites. Ca- and K-SWy-2 displayed the same sorption capacities for nonpolar biphenyl. X-ray diffraction patterns indicated that at lower fractions of K+-saturation, exchangeable K+ ions were randomly distributed in clay interlayers and did not enhance pesticide sorption. At higher populations of K+ (vs Ca2+), demixing occurred causing some clay interlayers, regions, or tactoids to become fully saturated by K+, manifesting greatly enhanced pesticide sorption. The forward and reverse cation exchange reactions influenced not only K+ and Ca2+ populations on clays but also the nanostructures of clay quasicrystals in aqueous solution which plays an important, if not dominant, role in controlling the extent of pesticide sorption. Modulating the cation type and composition on clay mineral surfaces through cation exchange processes provides an environmental-safe protocol to manipulate the mobility and availability of polar pesticides, which could have applications for pesticide formulation and in environmental remediation.

Adsorption↗
PubMed2001

Investigation of organophilic montmorillonite clay inclusion in zearalenone-contaminated diets using the mouse uterine weight bioassay.

Previous studies with low-pH montmorillonite (LPHM) clay exchanged with alkylammonium compounds showed that these organo clays were quite effective in sorbing the estrogenic mycotoxin zearalenone (ZEN) from aqueous solution. The potential toxicity of these types of clays, in particular hexadecyltrimethylammonium (HDTMA) LPHM, led to the investigation of the sorption efficacy of clay exchanged with a less toxic primary amine analog, hexadecylamine (HDA). Isothermal analysis studies showed that HDA LPHM was able to bind ZEN, but less effectively than HDTMA LPHM as evidenced by a significantly lower Freundlich K (63,900 vs. 845). The in vivo effectiveness of these two clays to bind ZEN was tested utilizing the mouse uterine weight bioassay. At a dietary inclusion level of 0.25%, the clays did not have a negative impact on overall animal health as measured by final body weight; however, they did not protect the animals from the estrogenic effects induced by 35 mg ZEN/kg in the feed (i.e., the uterine weights were not reduced in comparison to ZEN alone). In fact, the HDTMA LPHM group showed an increase in uterine weight that was more than the ZEN treatment group. When the animals were fed 0.5% clay, both exchanged clays (i.e., HDTMA LPHM and HDA LPHM) resulted in decreased body weight gain. The uterine weights of ZEN-fed animals (either alone or in combination with clays) were not significantly different from each other. In contrast, the uterine:body weight ratio showed a dramatic increase in the groups fed exchanged clay + ZEN compared to ZEN alone. These results suggest that alkylamine groups may assist the transport or uptake of ZEN and result in an enhanced toxicity from contaminated feed. The findings from this study clearly demonstrate the need for careful testing of all mycotoxin-binding agents before their inclusion in the diet.

Adsorption↗
PubMed1995

Insecticidal Activity of the Toxins from Bacillus thuringiensis subspecies kurstaki and tenebrionis Adsorbed and Bound on Pure and Soil Clays.

The release of transgenic plants and microorganisms expressing truncated genes from various subspecies of Bacillus thuringiensis that encode active insecticidal toxins rather than inactive protoxins could result in the accumulation of these active proteins in soil, especially when bound on clays and other soil particles. Toxins from B. thuringiensis subsp. kurstaki and B. thuringiensis subsp. tenebrionis, either free or adsorbed at equilibrium or bound on pure clay minerals (montmorillonite or kaolinite) or on the clay size fraction of soil, were toxic to larvae of the tobacco hornworm (Manduca sexta) and the Colorado potato beetle (Leptinotarsa decemlineata), respectively. The 50% lethal concentrations (LC(inf50)) of free toxins from B. thuringiensis subsp. kurstaki were higher than those of both bound and adsorbed complexes of these toxins with clays, indicating that adsorption and binding of these toxins on clays increase their toxicity in diet bioassays. The LC(inf50) of the toxin from B. thuringiensis subsp. tenebrionis that was either free or adsorbed on montmorillonite were comparable, whereas the toxin bound on this clay had higher LC(inf50) and the toxin bound on kaolinite had lower LC(inf50) than when adsorbed on this clay. Results obtained with the clay size fraction separated from unamended soil or soil amended with montmorillonite or kaolinite were similar to those obtained with the respective pure clay minerals. Therefore, insecticidal activity of these toxins is retained and sometimes enhanced by adsorption and binding on clays.

Journal Article↗
PubMed1980

Clays in prebiological chemistry.

In this review an attempt is made to highlight the structures and properties of clay that may contribute to a better understanding of the role of clays in chemical evolution. The adsorption of organic molecules on clays has been demonstrated, as has the synthesis of bioorganic monomers in the presence of clays. For instance, amino acids (glycine, aspartic acid, threonine, alanine and others) as well as purines and pyrimidines, have been obtained from CO and NH3 in the presence of clays at relatively high temperatures (250-325 degrees C). Carbohydrates are also easily derived from formaldehyde at relatively low temperatures (approximately equal to 80 degrees C). The oligomerization of biochemical monomers, mediated by clays has also been shown to result in the formation of polymer molecules basic to life. For instance the condensation of amino acyl adenylates at room temperature in the presence of montmorillonite is known to yield polypeptides in discrete ranges of molecular weights with degrees of polymerization up to 56. Clays have also been found to affect the condensation of mononucleotides to oligonucleotides. Although the role of clays in the origin or metabolic pathways has not been demonstrated, it is possible that clays may have played a cooperative role with catalytic peptides in an intermediate stage of prebiological chemistry preceding the emergence of life on this planet.

Adsorption↗
PubMed2000

Toxicity of cadmium-contaminated clay to the zebrafish Danio rerio.

In this study, the toxicity of cadmium-contaminated clay to the zebrafish Danio rerio was evaluated. Kaolin clay was spiked with CdCl(2) (1.28 mg Cd/g clay) and adult zebrafish were exposed to 0, 500, 1,000, and 2,000 mg/L Cd-contaminated clay in a continuous recirculation system. Cumulative mortality was evaluated as a function of exposure time, and LT50 values were calculated. Results of acute toxicity tests showed that an exposure to 1,000 mg/L Cd-contaminated clay resulted in LT50 values of 12 and 92 h (n = 2 experiments) and in a LT50 value of 22 h in both experiments with 2,000 mg/L Cd-contaminated clay. Positive control experiments with corresponding measured dissolved Cd concentrations were performed to evaluate by comparison the toxicity of the clay-bound Cd. These control experiments gave LT50 values higher than 144 h for both conditions. Moreover, no toxicity (LT50 > 144 h) of 2,000 mg/L uncontaminated clay was observed. This study showed that cadmium present on clay particles can be bioavailable and exert a toxic effect to the zebrafish D. rerio.

Aluminum Silicates↗
PubMed2002

Effects of surface coatings on electrochemical properties and contaminant sorption of clay minerals.

Surface charges play a major role in determining the interactions of contaminants with soils. The most important sources of soil charges are clay mineral colloids, whose electrochemical properties are usually modified by metal-oxides and organic matter in natural environments. In this study, effects of coatings of organic matter and Fe- and Al-oxides on a series of electrochemical properties and heavy metal sorption of three clay minerals (kaolinite, montmorillonite and illite) predominant in natural soils were investigated using batch techniques. The results indicate that the coatings increased the specific surface area of the clay minerals, except for the Al-oxide coated montmorillonite and organic matter coated 2:1 clay minerals. The sesquioxide coatings increased amount of positive charges but decreased negative charges. This causes great reduction of the negative potential on the clay surfaces, shift of the zero point of charge to a higher pH, and promotion of fluoride sorption due to presence of more OH- and OH2 on the oxide surfaces than on the clay surfaces. In contrast, the organic coating significantly increased the negativity of surface charges, and thus the zero point of charge and zeta-potential of the clays dropped down. The organic coating also induced a reduction of fluoride sorption on the clays. With respect to the sorption of lead and cadmium, the sesquioxide coatings produced insignificant effects. The experiments of lead/cadmium competitive sorption show that on both the oxide-coated surface and the original clay surface there exist different types of sites, each of which preferentially binds with a heavy metal.

Adsorption↗