Alternative model for cationic surfactant adsorption by layer silicates

Migration behavior of Cr(VI) during the transformation of ferrihydrite-Cr(VI) co-precipitates: The interaction between surfactants and co-precipitates

Redistribution of Cr(VI) in ferrihydrite-Cr(VI) co-precipitates (Fh-Cr) was affected by co-precipitates transformation and coexisting substances. These effects were crucial for predicting the migration path of Cr(VI) in ferrihydrite-Cr(VI) co-precipitates. This work investigated the effects of the extensively used surfactants of anionic surfactant sodium dodecylbenzene sulfonate (SDBS) and cationic surfactant cetyltrimethylammonium bromide (CTAB) on the Fh-Cr transformation and redistribution of Cr(VI) for 10 days at different pH values (5.0, 7.5 and 9.0) and concentration of surfactants (0.5, 2.0 and 5.0 mM). The results showed that SDBS hindered the transformation of Fh-Cr to hematite and tended to transform into goethite. SDBS inhibited hematite formation by inhibiting the aggregation of Fh-Cr particles, and it enhanced the dissolution of Fh-Cr to facilitate the formation of goethite. Affected by the inhibition of Fh-Cr transformation, the process of Cr(VI) redistribution was delayed. CTAB did not affect the transformation of Fh-Cr, but allowed more Cr(VI) to enter the interior of iron minerals. When the surfactants were adsorbed on the Fh-Cr, SDBS decreased the adsorption of Cr(VI) by Fh-Cr, while CTAB increased the Cr(VI) adsorption. The findings of this study contribute to understand the effects of surfactants on the transformation of Fh-Cr and the behaviors of Cr(VI) during this process.

Removal of fluoride ions (F-) from aqueous solutions using modified Turkish zeolite with quaternary ammonium

The natural Turkish zeolite has been modified with hexadecyltrimethylammonium bromide (CTAB) for the elimination of fluoride (F^-) from aqueous solutions. The parent natural zeolite (NZ) and modified zeolite (MZ) have been characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), zeta potentials and Brunauer-Emmett-Teller (BET) method. The effect of pH, adsorbent dose, contact time, initial concentration and temperature on adsorption of fluoride ions onto modified zeolite (MZ) has been determined in batch experiments. Fluoride concentration can be reduced to 1.5 mg/L under the optimum condition (pH = 5, adsorbent dose = 20 mg/L, contact time = 60 min and T = 293 K) when initial fluoride concentration of 10 mg/L is employed. The fluoride adsorption on MZ has been described by the Langmuir isotherm and the maximum fluoride adsorption capacity was found as 2.994 mg/g. Kinetics data were best described by the pseudo-second-order model. The thermodynamic studies proved that the adsorption was exothermic and spontaneous.

Removal Properties of Anionic Dye Eosin by Cetyltrimethylammonium Organo-Clays: The Effect of Counter-Ions and Regeneration Studies

The organo-clays (OCs) were prepared by a cation exchange reaction between surfactant (cetyltrimethylammonium, C16TMA) from different counterions (Bromide, Chloride, and Hydroxide). The effect of the counterions was investigated on the physico-chemical properties of the prepared organo-clays. The highest uptake of organic cations (1.60 mmol/g) was achieved using cetyl trimethylammonium bromide solution and the lowest value (0.93 mmol/g) was obtained after modification with cetyl trimethylammonium hydroxide solution starting from the same initial ratio of mmol/g of clay greater than 2.40. The arrangement of C16TMA cations within the interlayer space was assumed to be perpendicular with a tilt angle of 32° to the plane of clay sheets instead of being parallel to the clay surface using C16TMAOH solution at the same ratio. Different techniques were used to characterize these materials. The thermal stability of these organ-clays was investigated using an in-situ X-ray diffraction (XRD) technique. The decomposition of the surfactant moiety occurred at temperatures higher than 215 °C and was accompanied with a shrinkage of the basal spacing value to 1.42 nm. These materials were applied in the removal of an acid dye "eosin." The removed amount of eosin depended on the initial concentrations and the content of surfactants in the organo-clays. The removal of eosin was found to be an endothermic process. The maximum amount of 90 mg/g was achieved. The preheated treatment temperature of two selected OCs did affect the removal properties of eosin. A progressive reduction was observed at temperatures higher than 200 °C. The regeneration of spent OCs was studied and acceptable removal efficiency was maintained after 4 to 6 cycles depending on the used initial concentrations.

Removal of cesium ions from clays by cationic surfactant intercalation

We propose a new approach to remediate cesium-contaminated clays based on intercalation of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) into clay interlayers. Intercalation of DTAB was found to occur very rapidly and involved exchanging interlayer cations. The reaction yielded efficient cesium desorption (∼97%), including of a large amount of otherwise non-desorbable cesium ions by cation exchange with ammonium ions. In addition, the intercalation of DTAB afforded an expansion of the interlayers, and an enhanced desorption of Cs by cation exchange with ammonium ions even at low concentrations of DTAB. Finally, the residual intercalated surfactants were easily removed by a decomposition reaction with hydrogen peroxide in the presence of Cu²⁺/Fe²⁺ catalysts.

Chromium immobilization in soil using quaternary ammonium cations modified montmorillonite: Characterization and mechanism

The potential risk of heavy metal in soil can be reduced by stabilization techniques. Stabilizing amendments, montmorillonites modified with tetramethylammonium (TMA) and hexadecyltrimethylammonium (HDTMA) at different loadings were prepared, characterized and assayed for remediation of hexavalent chromium contaminated soil. Characterization results demonstrated that TMA modification resulted in increased surface area and pore volume, whereas HDTMA modification resulted in decreased surface area and pore volume. For HDTMA modified montmorillonites (H-Monts), the arrangement model of surfactant molecules and the structural characteristics strongly depended on the surfactant loading density. Less pronounced packing density dependence was observed for TMA modified montmorillonites (T-Monts). Modified toxicity characteristic leaching procedure (TCLP) was utilized to evaluate the chromium immobilizing capacity of the prepared organo-montmorillonites, and the results revealed that both T-Monts and H-Monts can be reasonably effective in chromium stabilization under different leaching condition. The comparison of chromium leaching behavior manifested by T-Monts and H-Monts suggested that mechanisms for chromium immobilization by organo-montmorillonites amendment proceeded via two adsorption approaches. Each approach involves different functional groups, depending on the leaching pH and surfactant loading concentration. In addition, the effect of HDTMA loading concentration, amendment dosage, initial chromium concentration and incubation time on the immobilizing efficiency of H-Monts was investigated.

Complete Defluorination of Perfluorinated Compounds by Hydrated Electrons Generated from 3-Indole-acetic-acid in Organomodified Montmorillonite

Here we describe a unique process that achieves complete defluorination and decomposition of perfluorinated compounds (PFCs) which comprise one of the most recalcitrant and widely distributed classes of toxic pollutant chemicals found in natural environments. Photogenerated hydrated electrons derived from 3-indole-acetic-acid within an organomodified clay induce the reductive defluorination of co-sorbed PFCs. The process proceeds to completion within a few hours under mild reaction conditions. The organomontmorillonite clay promotes the formation of highly reactive hydrated electrons by stabilizing indole radical cations formed upon photolysis, and prevents their deactivation by reaction with protons or oxygen. In the constrained interlayer regions of the clay, hydrated electrons and co-sorbed PFCs are brought in close proximity thereby increasing the probability of reaction. This novel green chemistry provides the basis for in situ and ex situ technologies to treat one of the most troublesome, recalcitrant and ubiquitous classes of environmental contaminants, i.e., PFCs, utilizing innocuous reagents, naturally occurring materials and mild reaction conditions.

Surfactant adsorption to soil components and soils

Soils are complex and widely varying mixtures of organic matter and inorganic materials; adsorption of surfactants to soils is therefore related to the soil composition. We first discuss the properties of surfactants, including the critical micelle concentration (CMC) and surfactant adsorption on water/air interfaces, the latter gives an impression of surfactant adsorption to a hydrophobic surface and illustrates the importance of the CMC for the adsorption process. Then attention is paid to the most important types of soil particles: humic and fulvic acids, silica, metal oxides and layered aluminosilicates. Information is provided on their structure, surface properties and primary (proton) charge characteristics, which are all important for surfactant binding. Subsequently, the adsorption of different types of surfactants on these individual soil components is discussed in detail, based on mainly experimental results and considering the specific (chemical) and electrostatic interactions, with hydrophobic attraction as an important component of the specific interactions. Adsorption models that can describe the features semi-quantitatively are briefly discussed. In the last part of the paper some trends of surfactant adsorption on soils are briefly discussed together with some complications that may occur and finally the consequences of surfactant adsorption for soil colloidal stability and permeability are considered. When we seek to understand the fate of surfactants in soil and aqueous environments, the hydrophobicity and charge density of the soil or soil particles, must be considered together with the structure, hydrophobicity and charge of the surfactants, because these factors affect the adsorption. The pH and ionic strength are important parameters with respect to the charge density of the particles. As surfactant adsorption influences soil structure and permeability, insight in surfactant adsorption to soil particles is useful for good soil management.

Effect of ions on sorption of tylosin on clay minerals

Tylosin sorption on three ion-exchanged montmorillonites indicated that hydrogen bond interactions were more important than cation exchange in the montmorillonite interlayer.

Comparative study of laterite and bentonite based organoclays: implications of hydrophobic compounds remediation from aqueous solutions

Four cost effective organoclays were synthesized, characterized, and studied for the sorption of hydrophobic compounds (edible oil/grease and hydrocarbon oil) from aqueous solutions. Organoclays were prepared by cation exchange reaction of lattice ions (present onto the surface of laterite and bentonite clay minerals) with two surfactants, hexadecyl trimethyl ammonium chloride (HDTMA-Cl) and tetradecyl trimethyl ammonium bromide (TDTMA-Br). Fourier transform infrared spectroscopy and scanning electron microscopy were used for the characterization of synthesized organoclays. It was found that the amount of surfactant loading and the nature of the surfactant molecules used in the syntheses of organoclay strongly affect the sorption capacity of the clay mineral. Further, it was found that both the laterite and bentonite based organoclays efficiently removed the edible and hydrocarbon oil content from lab prepared emulsions; however, the adsorption capacity of clay mineral was greatly influenced by the nature of hydrophobic compounds as well.

BTEX removal from aqueous solutions by HDTMA-modified Y zeolite

Various technologies have been used for the treatment and remediation of areas contaminated by BTEX (benzene, toluene, ethylbenzene and xylenes), which are organic compounds that are of particular concern due to their toxicity. Potential applications of synthetic zeolites for environmental fieldwork have also been reported worldwide. In this work, a hexadecyltrimethyl ammonium (HDTMA) surfactant-modified synthetic zeolite was investigated for its efficiency in removing BTEX from aqueous solutions. Three surfactant-modified zeolites were synthesized, with amounts of surfactant corresponding to 50%, 100%, and 200% of the total cation-exchange capacity (CEC) of the synthetic zeolite Y. The results of the BTEX adsorption experiments onto both synthetic zeolite and surfactant-modified zeolites (SMZ) showed that the SMZ-100 (zeolite modified with surfactant levels at 100% of CEC) was the most efficient modified zeolite for BTEX removal. Kinetics studies indicated that the multicomponent adsorption equilibrium was reached within 6 h and followed pseudo-second-order kinetics. The Langmuir, Freundlich, Redlich-Peterson and Temkin models were used to evaluate the BTEX adsorption capacity by SMZ-100. The Temkin model was found to be suitable for all BTEX compounds in a multicomponent system. Regeneration cycles of the modified zeolite were also performed, and the results showed that the adsorbent could be used efficiently in as many as four adsorption cycles, except for benzene.

Nature of the interlayer environment in an organoclay optimized for the sequestration of dibenzo-p-dioxin

A Na-smectite clay (Na-SWy-2) was exchanged with various amounts of dimethyldioctadecylammonium bromide (DODA-Br) up to twice the cation exchange capacity (CEC). The organoclay (DODA-SWy-2) with DODA-Br added at 2 × CEC exhibited a maximum 4.2 nm d-spacing and a 31.4% carbon content, which demonstrates DODA(+) intercalation. DODA-SWy-2 was evaluated as an archetype of commercial products used to sequester hydrophobic contaminants, and the nature of the primarily C18 alkylhydrocarbon-chain interlayer environment was emhasized. Shifts in ν(CH) and CH(2) rocking band positions in DODA-SWy-2-complex FTIR-spectra indicate that DODA C18 chains were more ordered as DODA surface coverage was increased. Differential scanning calorimetry analysis indicated a DODA-SWy-2 gel-to-liquid transition temperature much lower than the melting point of crystalline DODA-Br and similar to that of aqueous DODA-Br vesicles. This suggests that the transition was governed by C18 alkyl tail-tail interactions in the clay interlamellar region. Dibenzo-p-dioxin (DD) sorption from water by DODA-SWy-2 was compared to DD sorption by the geosorbents granular activated carbon (GAC), K-exchanged saponite, and a muck soil. The linear K(l) sorption coefficients (log K(l)) from a linear fit of the sorption isotherms were 4.37 for DODA-SWy-2, 5.55 for GAC, 3.19 for muck soil, and 2.46 for K-saponite. The DD-organic-matter-normalized sorption coefficient (K(om)) was ∼2.4 times the octanol-water partition coefficient (K(ow)). This indicates that DD has a higher affinity for the nonpolar interlayer DODA organic phase than for octanol. In contrast, the K(om) for muck soil DD sorption was ~10 times less than K(ow), which reflects the higher polarity of amorphous soil organic matter relative to octanol. Enhanced DD uptake by the DODA-derived lipophilic phase in the organoclay is attributed to the low polarity, "open" C18 alkyl structure due to the physical dimensions of "v-shaped" DODA(+) molecular, and low density of the interlamellar phase (~0.50 g/cm3) density of intercalated DODA(+).

Sorption of quaternary ammonium compounds in soils: implications to the soil microbial activities

Despite their widespread use in household activities and various industries, information on the toxicity of quaternary ammonium compounds (QACs) to microbial activities in soil is scant. This study investigated the effect of three commonly used QACs namely hexadecyltrimethyl ammonium bromide (HDTMA), octadecyltrimethyl ammonium bromide (ODTMA) and Arquad on dehydrogenase and potential nitrification activities in three different soils. The toxicity of QACs on the dehydrogenase activity and potential nitrification in these soils followed the order: HDTMA>ODTMA>Arquad and Arquad>HDTMA>ODTMA, respectively. HDTMA, ODTMA and Arquad exhibited toxicity to dehydrogenase activity at concentration of 50, 100 and 750 mg kg(-1) soil, respectively, whereas potential nitrification was inhibited by HDTMA and ODTMA even at 50 mg kg(-1) soil. Arquad exhibited toxicity to potential nitrification at comparatively higher concentration of 250 mg kg(-1) soil, with the severity of toxicity very intense at higher concentrations. The nature of QACs and soil properties influenced the toxicity. The toxic effect of QACs on soil microbial activities was more influenced by the relative release of sorbed QACs in soils. This study provides valuable information on the toxicological properties of some widely used QACs on important soil microbial activity parameters. To our knowledge, this is the first report.

Thermal stability of organoclays: effects of duration and atmosphere of isothermal heating on iodide sorption

Heating periods necessary to destroy iodide sorption capacity of the quaternary (alkyl) ammonium and phosphonium modified bentonites were determined using iodide sorption batches. For this purpose, prior to the batches the studied organoclays were isothermally heated in air in the temperature ranges of 110-180 °C and 160-300 °C, respectively. The temperature dependence of the heating periods was found to follow the Arrhenius relationship, which allowed a determination of Arrhenius parameters for the reaction leading to the loss of the iodide sorption capacity of a bentonite modified by CP(+) (cetylpyridinium), BE(+) (benzethonium), CTMA(+) (cetyltrimethylammonium), or TPP(+) (tetraphenylphosphonium) surfactant. At 160 °C, the thermal stability of the iodide sorption capacity of TPP(+)-bentonite is much higher than that of the second most stable CTMA(+)-bentonite (80 days vs 5 days). However, the obtained Arrhenius parameters predict that CTMA(+)-bentonite becomes the most stable one as the heating temperature decreases to 40 °C with iodide sorption still available for ∼12000 years as compared to ∼8000 years for TPP(+)-bentonite. Heating of the organoclays in a N(2)-atmosphere (<70 ppm O(2)) at 160 °C revealed that the strong deficit of molecular oxygen in the contacting atmosphere resulted in a strong increase of their thermal stability. For CTMA(+)-bentonite, this increase is equivalent to the stability increase due to a decrease of the heating temperature by ∼20 °C (from 160 °C to ∼140 °C). Accordingly, the iodide sorption capacity of CTMA(+)-bentonite at a heating temperature of 40 °C is predicted to be retained for ∼350,000 years in the absence of molecular oxygen.

Role of interlayer hydration in lincomycin sorption by smectite clays

Lincomycin, an antibiotic widely administered as a veterinary medicine, is frequently detected in water. Little is known about the soil-water distribution of lincomycin despite the fact that this is a major determinant of its environmental fate and potential for exposure. Cation exchange was found to be the primary mechanism responsible for lincomycin sorption by soil clay minerals. This was evidenced by pH-dependent sorption, and competition with inorganic cations for sorptive sites. As solution pH increased, lincomycin sorption decreased. The extent of reduction was consistent with the decrease in cationic lincomycin species in solution. The presence of Ca2+ in solution diminished lincomycin sorption. Clay interlayer hydration status strongly influenced lincomycin adsorption. Smectites with the charge deficit from isomorphic substitution in tetrahedral layers (i.e., saponite) manifest a less hydrated interlayer environment resulting in greater sorption than that by octahedrally substituted clays (i.e., montmorillonite). Strongly hydrated exchangeable cations resulted in a more hydrated clay interlayer environment reducing sorption in the order of Ca- < K- < Cs-smectite. X-ray diffraction revealed that lincomycin was intercalated in smectite clay interlayers. Sorption capacity was limited by clay surface area rather than by cation exchange capacity. Smectite interlayer hydration was shown to be a major, yet previously unrecognized, factor influencing the cation exchange process of lincomycin on aluminosilicate mineral surfaces.

Cetylpyridinium chloride at the mica-water interface: incomplete monolayer and bilayer structures

Monte Carlo simulations of the interface between the cleaved surface of muscovite mica and aqueous cetylpyridinium chloride (CPCl) solution at ambient conditions are reported. Simulation results reveal that monolayer or bilayer aggregates of CP(+) ions at the muscovite-water interface remain incomplete up to a CP(+) coverage compensating the negative charge of muscovite. It is predicted that at this CP(+) coverage only a partial desorption of K(+) ions occurs and the two aggregates can be distinguished with help of the X-ray reflectivity technique. Formation of inner-sphere and outer-sphere adsorption complexes of CP(+) ions at distances of approximately 3 A and approximately 5 A, respectively, from the surface is observed. Despite an increasing adsorption of CP(+) ions, the structure of the adsorbed water film is largely preserved within approximately 5 A from the surface. A strong decrease of water density beyond this distance and formation of "adsorbed K(+)"-Cl(-) ion pairs result in coadsorption of Cl(-) in an amount equivalent to 1/4 of the negative charge of muscovite as close as approximately 4.3-4.8 A to the surface for the incomplete bilayer aggregate. For the incomplete monolayer aggregate, no segregation between K(+) and CP(+) ions and a displacement of K(+) ions into the adsorption sites approximately 1.6 A from the surface are observed.

Surfactant-enhanced remediation of organic contaminated soil and water

Surfactant based remediation technologies for organic contaminated soil and water (groundwater or surface water) is of increasing importance recently. Surfactants are used to dramatically expedite the process, which in turn, may reduce the treatment time of a site compared to use of water alone. In fact, among the various available remediation technologies for organic contaminated sites, surfactant based process is one of the most innovative technologies. To enhance the application of surfactant based technologies for remediation of organic contaminated sites, it is very important to have a better understanding of the mechanisms involved in this process. This paper will provide an overview of the recent developments in the area of surfactant enhanced soil and groundwater remediation processes, focusing on (i) surfactant adsorption on soil, (ii) micellar solubilization of organic hydrocarbons, (iii) supersolubilization, (iv) density modified displacement, (v) degradation of organic hydrocarbon in presence surfactants, (vi) partitioning of surfactants onto soil and liquid organic phase, (vii) partitioning of contaminants onto soil, and (viii) removal of organics from soil in presence of surfactants. Surfactant adsorption on soil and/or sediment is an important step in this process as it results in surfactant loss reduced the availability of the surfactants for solubilization. At the same time, adsorbed surfactants will retained in the soil matrix, and may create other environmental problem. The biosurfactants are become promising in this application due to their environmentally friendly nature, nontoxic, low adsorption on to soil, and good solubilization efficiency. Effects of different parameters like the effect of electrolyte, pH, soil mineral and organic content, soil composition etc. on surfactant adsorption are discussed here. Micellar solubilization is also an important step for removal of organic contaminants from the soil matrix, especially for low aqueous solubility organic contaminants. Influences of different parameters such as single and mixed surfactant system, hydrophilic and hydrophobic chain length, HLB value, temperature, electrolyte, surfactant type that are very important in micellar solubilization are reviewed here. Microemulsion systems show higher capacity of organic hydrocarbons solubilization than the normal micellar system. In the case of biodegradation of organic hydrocarbons, the rate is very slow due to low water solubility and dissolution rate but the presence of surfactants may increase the bioavailability of hydrophobic compounds by solubilization and hence increases the degradation rate. In some cases the presence of it also reduces the rate. In addition to fundamental studies, some laboratory and field studies on removal of organics from contaminated soil are also reviewed to show the applicability of this technology.

Interlayer expansion and mechanisms of anion sorption of Na-montmorillonite modified by cetylpyridinium chloride: a Monte Carlo study

To study the change of interlayer structure of a Wyoming-type Na-montmorillonite as a result of the replacement of interlayer Na+ ions by cetylpyridinium (CP+) ions, a series of NPT Monte Carlo simulations of the clay mineral with different contents of CP+, Na+, Cl- ions and water in its interlayer space is carried out. In agreement with conclusions from experimental studies, the simulations show that the CP+ ions form monomolecular, bimolecular, and pseudotrimolecular layers with increasing interlayer contents. Calculated potential energies reveal that clay-organic interactions are stronger than organic-organic interactions in CP+-modified montmorillonite, which is in conformity with observations of earlier thermogravimetric experiments. The simulation results indicate that the pseudotrimolecular arrangement of CP+ ions is a prerequisite for the experimentally observed interlayer sorption of inorganic anions. Furthermore, in the interlayer space with a pseudotrimolecular layer, chloride ions favor the formation of pairs with inorganic rather than organic cations. On the basis of these findings and available experimental data, we propose that the interlayer sorption of inorganic anions from the pore space of an organically modified montmorillonite may occur not only in pairs with organic cations, as suggested earlier, but also in pairs with inorganic cations, which represents a so-far unconsidered and maybe more important mechanism of anion sorption on clay minerals.

High resolution electron microscopy structural studies of organo-clay nanocomposites

Engineering of clay nanocomposite materials by modification of their surfaces can enable the control of retention, transport, and persistence of toxic chemicals in the geosystem. The properties and interactions of clay nanocomposites have been widely studied, but little information exists on their microstructure at a range of scale extending down to atomic dimensions. The pairing of Na-montmorillonite clay with organic cations as well as with the herbicide fluridone, chosen as a model for an organic pollutant, was studied. Three organic cations were selected: hexadecyltrimethylammonium, benzyltrimethylammonium, and benzyltriethylammonium at 0%, 60%, and 100% of cation exchange capacity (CEC) loadings. A detailed microstructural analysis of the organo-clay nanocomposites and of the fluridone nanocomposites was undertaken by high-resolution transmission electron microscopy (HRTEM) and X-ray energy-dispersive spectroscopy (EDS). Morphological observations and chemical analyses were performed simultaneously on the same sample. The combined HRTEM and EDS measurements strongly suggest (a) heterogeneous local intercalation of the organic cations manifested by a range in the measured d001 spacing, implying random expansion of the clay layered structure with increased loading of the organic cations; (b) intercalation within the external layers, which is thoroughly influenced by local defect microstructure and/or edge availability of the montmorillonite nanoparticles as well as by the molecular structure of the intercalating organic cation. Additional intercalation of fluridone molecules did not affect the structure (d001 spacing) of the organo-clay nanocomposites.

Sorption of hydrophobic organic compounds onto organoclays

The behavior and fate of nonionic hydrophobic organic compounds (HOCs) in the environment are mainly controlled by their interactions with various components of soils and sediments. Due to their large surface area and abundance in many soils, smectites may greatly influence the fate and transport of the contaminants in the environment. In our experiments, HOC sorption by hexadecyltrimethylammonium (HDTMA)-modified smectite linearly increased with the amount of HDTMA added to the clay. However, tetramethylammonium (TMA)- and dodecyltrimethylammonium (DTMA)-modified smectites showed not only inferiority in their sorption of HOC compared with the HDTMA-smectite, but also a partially decreased HOC sorption at specific surfactant loading levels. This means that the sorption of organoclays for organic contaminants was significantly influenced by the amount and size of the surfactants added on the clay. In addition, it seems that the interlayer structure (e.g., pore size) formed at each surfactant loading level plays an important role to adsorb HOC in different amount.

Adsorption isotherms of homologous alkyldimethylbenzylammonium bromides on sodium montmorillonite

The adsorption of homologous alkyldimethylbenzylammonium bromides, [C(6)H(5)CH(2)N(CH(3))(2)R]Br, on sodium montmorillonite from aqueous NaCl solutions at room temperature has been studied. R stands for the methyl-, butyl-, hexyl-, octyl-, decyl-, and dodecyl-group, and the corresponding ammonium cations will be denoted as C1+, C4+, C6+, C8+, C10+, and C12+, respectively. C1+, the reference cation, attains the plateau region of adsorption at a level close to the cation exchange capacity (CEC) of the clay. The chain-length dependence on adsorptivity of the homologous cations exhibits an unexpected peculiarity. In the case of short-chain homologues of C1+ their adsorption onto sodium montmorillonite decreases in the order C1+>C4+>C6+. This behavior is due, presumably, to the growing steric hindrances at the surface of clay, which occur because of the limited area available for the bulky organic cations at the exchange sites. These limitations appear to be out-balanced in the case of higher homologues for which the increasingly growing hydrophobic effects lead to the expected sequence of adsorptivity of the cations, i.e., C1+<C8+<C10+<C12+. The extent of adsorption of the long-chain homologous cations at the plateau region exceeds the CEC value and indicates the commencement of formation of a bilayer or admicelle. As expected, the adsorption data for the short-chain homologues fit fairly well to the Langmuir isotherm, whereas the data for the C10+ and C12+ cations show an increasing departure from linearity of the corresponding plots. Results of X-ray analysis of organo-clays fully loaded with C1+, C4+, and C12+ cations suggest that in the case of short-chain homologues, the ammonium cations lay flatly and interact relatively strongly with the montmorillonite packet surface, whereas the long-chain homologue forms an interdigitated system of coiled hydrocarbon chains.

Simultaneous sorption of lead and chlorobenzene by organobentonite

Clays or organoclays have been used as a barrier to prevent the transport of hazardous contaminants in landfills. However, clays are known to effectively sorb mostly inorganic contaminants, while organoclays are mainly used for organic contaminants. Since the organoclays are basically clay particles modified with cationic surfactants, there might exist an optimal coverage of cationic surfactant on the clay particles to sorb both inorganic and organic contaminants. In order to determine the optimal mass of cationic surfactants on the bentonites, sodium bentonites were treated with various ratios of hexadecyltrimethylammonium (HDTMA) to bentonites. Chlorobenzene and lead were selected as representative contaminants. When either chlorobenzene or lead exists as a single contaminant, chlorobenzene sorption increased with increasing HDTMA to bentonite ratios, and lead sorption decreased with increasing HDTMA to bentonite ratios. Sorption of chlorobenzene was a function of HDTMA coverage on the bentonites, while lead sorption was much more influenced by the initial lead concentration rather than the mass of HDTMA added to the bentonites.

The sedimentation capabilities of hexadecyltrimethylammonium-modified montmorillonites

Natural montmorillonite was modified with a quaternary ammonium compound, hexadecyltrimethylammonium (HDTMA). The sedimentation capabilities of unmodified and modified montmorillonites were then investigated. The sedimentation velocity of modified montmorillonites increased if the amounts of adsorbed HDTMA were from 0.3 to 1.0 times the cation exchange capacitity (CEC). It also emerged that the sedimentation capability of modified montmorillonites was improved and that the variously CEC-modified montmorillonites had similar sedimentation capabilities after they had sorbed organic matter from oily wastewater. Thus, modified montmorillonites (especially 0.5 CEC treatment) had good sedimentation capabilities for sorbing organic substance and can act as carriers in wastewater biotreatment.

Spectroscopic study of nitroaromatic-smectite sorption mechanisms

Sorption mechanisms of 1,3- and 1,4-dinitrobenzene, 1,3,5-trinitrobenzene (TNB), dinitro-o-creasol, and 6-sec-butyl-2,4-dinitrophenol (DINOSEB) on smectite were investigated using FTIR spectroscopy and HPLC methods. A quantitative method was developed that established a direct link between the HPLC and the FTIR data. Freundlich sorption values ranged from 47 (L g(-1)) for 1,3,5-TNB to 3.7 for DINOSEB and showed that the extent of nitroaromatic compounds (NAC) sorption was strongly dependent on the number and position of the nitro substituents as well as other substituents and steric effects. The amount of 1,3,5-TNB sorbed to smectite was strongly influenced by the nature of the exchangeable cation. Furthermore, the exchangeable cation significantly influenced the positions and relative intensities of the vibrational modes of the -NO2 groups. The strongest perturbations were observed for cations with lower enthalpies of hydration (e.g., K+) and included a red shift of the v(asym)(NO) band, a concomitant blue shift of the v(sym)(NO) band. These changes were accompanied by a 2-fold increase in the relative intensity of the v(asym)(NO) band relative to the intensity of the v(sym) (NO) band. Molecular quantum mechanics calculations were used to rationalize frequency shifts in terms of nitroaromatic interactions with interlayer cations. Results indicate that the sorption of NACs to smectite surfaces is controlled largely by the hydration characteristics of the exchangeable cation, which regulates both cation-nitroaromatic complexation and swelling of the smectite.

Sorption and cosorption of organic contaminant on surfactant-modified soils

Three kinds of soils were modified with the cationic surfactants, hexadecyltrimethylammonium (HDTMA) bromide and tetramethylammonium (TMA) bromide to increase their sorptive capabilities. Sorption of chlorobenzene in simulated groundwater by these soils was investigated. HDTMA-modified soil has a higher ability to sorb chlorobenzene from simulated groundwater than unmodified soil. TMA-modified soil did not show the superiority. HDTMA thus can be used to modify soil to improve its sorption capability. Cosorption of chlorobenzene in simulated groundwater in the absence or presence of nitrobenzene and dichloromethane on HDTMA-modified soil was also investigated. Nitrobenzene facilitated sorption of chlorobenzene on all HDTMA-modified soil. Dichloromethane did not influence the sorption of chlorobenzene by HDTMA-modified soil. The results suggest that HDTMA-modified soil is a highly effective sorbent for chlorobenzene and multiple organic compounds did not impede the uptake of chlorobenzene.

Influence of modified soils on the removal of diesel fuel oil from water and the growth of oil degradation micro-organism

Three soils were modified with two kinds of cationic surfactants in order to increase their sorptive capabilities for organic contaminants. Sorption of diesel fuel oil in water by these modified soils had been investigated. Modified soils can effectively sorb diesel fuel oil from water. The sorption capability of modified soils is: HDTMA-black soil > HDTMA-yellow brown soil > HDTMA-red soil > TMA-black soil > TMA-yellow brown soil > TMA-red soil. Sorption of diesel fuel oil by natural soils and HDTMA modified soils is via partition, the sorption isotherms can be expressed by Henry equation, and logK(SOM) is 2.42-2.80, logK(HDTMA) is 3.37-3.60. Sorption isotherms of TMA modified soils can be expressed by Langmuir equation, the saturation sorption capacities are 1150 (TMA-black soil), 750 (TMA-yellow-brown soil), 171 mg/kg (TMA-red soil), respectively. A diesel fuel oil degradation micro-organism (Pseudomonas sp.) was isolated in the lab. To test the influence of the modified soils on the micro-organism, various growth curves of Pseudomonas in different conditions were drawn. Pseudomonas can grow very well with natural soils and TMA modified soils. The acclimation period of Pseudomonas is reduced. As to HDTMA modified soils, HDTMA loading amount is very important. When HDTMA loading amount is no higher than 0.5 CEC, the micro-organism can grow very well after a long acclimation period.

Thermodynamics of Cationic Surfactant Sorption onto Natural Clinoptilolite

Sorption enthalpies of hexadecyltrimethylammonium bromide (HDTMA) as monomers and micelles and tetraethylammonium bromide (TEA) were used with surfactant, counterion, and co-ion sorption isotherms to infer the conformation, sorption mechanism, and relative stability of the sorbed surfactants on natural clinoptilolite. The average value of the sorption enthalpy was -10.38 kJ/mol for monomers, -11.98 kJ/mol for micelles, and +3.03 kJ/mol for TEA. Sorption of monomers produced a lower sorption plateau than equivalent micelle sorption (maxima 145 mmol/kg, 225 mmol/kg). Analysis of the sorption data demonstrated a change in the sorption mechanism at the external cation exchange capacity (ECEC) of clinoptilolite. Sorption data from below and above the ECEC were fit to a simple polynomial model and the Gibbs free energy of sorption (DeltaG0m) and sorption entropies were calculated. Resultant values of DeltaG0m were -9.27 and -14.38 kJ/mol for HDTMA monomers and micelles, respectively, for sorption below the ECEC, and -16.11 and -23.10 kJ/mol, respectively, for sorption above the ECEC. The value for TEA was -1.04 kJ/mol, indicating weaker sorption than for HDTMA. Monomer sorption to clinoptilolite exceeded the ECEC, even when the solution concentration was below the critical micelle concentration. Hydrophobic (tail-tail) components of DeltaG0m were the driving force for sorption of HDTMA, both below and above the ECEC. A significant kinetic effect was observed in the sorption isotherms with a period of rapid sorption followed by slow equilibration requiring 7 days to achieve steady state for HDTMA; TEA equilibration occurred within 24 h. Copyright 1998 Academic Press.