Modeling tetracycline antibiotic sorption to clays

Sorption interactions of three high-use tetracycline antibiotics (oxytetracycline, chlortetracycline, tetracycline) with montmorillonite and kaolinite clays were investigated undervaried pH and ionic strength conditions. Sorption edges were best described with a model that included cation exchange plus surface complexation of zwitterion forms of these compounds. Zwitterion sorption was accompanied by proton uptake, was more favorable on acidic clay, and was relatively insensitive to ionic strength effects. Calcium salts promoted oxytetracycline sorption at alkaline pHs likely by a surface-bridging mechanism. Substituent effects among the compounds in the tetracycline class had only minor effects on sorption edges and isotherms under the same solution pH and ionic strength conditions. At low ionic strength, greater sorption to montmorillonite than kaolinite was observed at all pHs tested, even after normalizing for cation exchange capacity. These results indicate that soil and sediment sorption models for tetracyclines, and other pharmaceuticals with similar chemistry, must account for solution speciation and the presence of other competitor ions in soil or sediment pore waters.     

Diffusion of 22Na and 85Sr in montmorillonite: evidence of interlayer diffusion being the dominant pathway at high compaction

A mechanistic understanding of transport phenomena in compacted clays is essential for the use of such materials as engineered barrier systems for the safe geological disposal of radioactive wastes. The present contribution is a first step in the development of an integrative treatment of the properties of tracer cations in compacted bentonites with respect to diffusion and sorption. The diffusion of 22Na and 85Sr in highly compacted montmorillonite and kaolinite is investigated as a function of the "external salt concentration" (NaClO4), i.e., of the solution in equilibrium with the clay. Consistent results were obtained from through-diffusion experiments and tracer profile analysis. Knowledge of genuine diffusion coefficients of the filter plates turned out to be crucial in cases where the diffusive resistance of the filter plates was similar to that of the clay. Diffusion coefficients formally calculated on the basis of the tracer concentration gradient in the external aqueous phase, and the sorption distribution ratios were found to decrease with increasing external salt concentration in the case of montmorillonite. In a logarithmic representation of these data, a slope of -1 was obtained for the monovalent 22Na, whereas the slope was -2 for the divalent 85Sr. In the case of kaolinite, diffusion coefficients were independent of the external salt concentration. It is postulated that the diffusion of the tracer cation through the interlayer water is the dominant pathway in compacted swelling clays under the experimental conditions tested. Effective diffusion coefficients, based on a tracer concentration gradient in the interlayer water of the clay, were found to be independent of the composition of the external aqueous phase. The latter gradient is assumed to be a function of the external salt concentration, according to a calculated distribution of the tracer cation between free pore water and the interlayer water via cation exchange.     

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 of sulfonamide antimicrobial agents to clay minerals

Adsorption of three sulfonamide antimicrobials to clay minerals was investigated as a function of pH, ionic strength, and type of exchangeable cation. Sulfonamide antimicrobial adsorption exhibited pronounced pH dependence consistent with sorbate speciation and clay properties. Sulfonamide antimicrobials did not intercalate into montmorillonite, and surface charge density influenced sorption by determining adsorption domain size. Adsorption edge data were best fit to a model including terms for the cationic and uncharged species. Adsorption of uncharged sulfamethazine to montmorillonite was relatively insensitive to pH, ionic strength, and type of exchangeable cation, while that to kaolinite was highly sensitive to ionic strength. Adsorption of cationic sulfamethazine to montmorillonite exceeded that of the neutral species by 1-2 orders of magnitude, but was unimportant for kaolinite atthe pH values examined. Cation exchange appeared to contribute to sorption of cationic sulfonamide species to montmorillonite. Anionic sulfamethazine adsorption was negligible. The nature of the sulfonamide R group influenced the degree of adsorption of cationic and neutral species. Our results highlight the importance of considering sulfonamide speciation and clay surface charge density in predicting the transport of these antimicrobials.     

Water structure and aqueous uranyl(VI) adsorption equilibria onto external surfaces of beidellite, montmorillonite, and pyrophyllite: results from molecular simulations

Molecular dynamics simulations were performed to provide a systematic study of aqueous uranyl adsorption onto the external surface of 2:1 dioctahedral clays. Our understanding of this key process is critical in predicting the fate of radioactive contaminants in natural groundwaters. These simulations provide atomistic detail to help explain experimental trends in uranyl adsorption onto natural media containing smectite clays. Aqueous uranyl concentrations ranged from 0.027 to 0.162 M. Sodium ions and carbonate ions (0.027-0.243 M) were also present in the aqueous regions to more faithfully model a stream of uranyl-containing groundwater contacting a mineral system comprised of Na-smectite. No adsorption occurred near the pyrophyllite surface, and there was little difference in uranyl adsorption onto the beidellite and montmorillonite, despite the difference in location of clay layer charge between the two. At low uranyl concentration, the pentaaquouranyl complex dominates in solution and readily adsorbs to the clay basal plane. At higher uranyl (and carbonate) concentrations, the mono(carbonato) complex forms in solution, and uranyl adsorption decreases. Sodium adsorption onto beidellite occurred both as inner- and outer-sphere surface complexes, again with little effect on uranyl adsorption. Uranyl surface complexes consisted primarily of the pentaaquo cation (85%) and to a lesser extent the mono(carbonato) species (15%). Speciation diagrams of the aqueous region indicate that the mono(carbonato)uranyl complex is abundant at high ionic strength. Oligomeric uranyl complexes are observed at high ionic strength, particularly near the pyrophyllite and montmorillonite surfaces. Atomic density profiles of water oxygen and hydrogen atoms are nearly identical near the beidellite and montmorillonite surfaces. Water structure therefore appears to be governed by the presence of adsorbed ions and not by the location of layer charge associated with the substrate. The water oxygen density near the pyrophyllite surface is similar to the other cases, but the hydrogen density profile indicates reduced hydrogen bonding between adsorbed water molecules and the surface.     

Adsorption of the herbicide simazine by montmorillonite modified with natural organic cations

Three organic cations with a natural origin (L-carnitine, L-cystine dimethyl ester, and thiamine) were introduced at different loadings in the interlayer of a low-charge montmorillonite, and the performance of the modified clays as adsorbents of the herbicide simazine was investigated using batch adsorption-desorption experiments. The organic cations were selected on the basis of their natural origin and the presence of diverse functional groups in their structures, which was expected to influence simazine adsorption. Elemental analysis and spectroscopy results demonstrated the presence of the organic cations in the modified montmorillonites and their entrance in the clay mineral interlayers. Batch adsorption results showed that modification with thiamine (K(f) = 96-138), cystine dimethyl ester (K(f) = 400-753), and especially carnitine (K(f) > 10 000) enhanced the adsorption of simazine by montmorillonite (K(f) = 28-47). It appeared that the specific interlayer microenvironment provided by the functional groups of each organic cation was an important factor controlling the adsorption efficiency of the modified clays. For carnitine and cystine dimethyl ester, the increase in simazine adsorption was considerably greater than that observed after montmorillonite modification with "classical" alkylammonium cations, such as phenyltrimethylammonium or hexadecyltrimethylammonium. This illustrated how modification of smectitic clay minerals with natural organic cations containing appropriate functional groups can be a useful strategy to improve the performance of organoclays for the removal of specific organic pollutants from the environment.     

Interactions of DNA with clay minerals and soil colloidal particles and protection against degradation by DNase

Adsorption, desorption, and degradation by nucleases of DNA on four different colloidal fractions from a Brown soil and clay minerals were studied. The adsorption of DNase I and the structures of native DNA, adsorbed and desorbed, were also investigated by Fourier Transform Infrared (FTIR), circular dichroism (CD), and fluorescence spectroscopy, to determine the protection mechanism of DNA molecules by soil colloids and minerals against enzymatic degradation. Kaolinite exhibited the highest adsorption affinity for DNA among the examined soil colloids and clay minerals. In comparison with organomineral complexes (organic clays), DNA was tightly adsorbed by H2O2-treated clays (inorganic clays). FTIR spectra showed that the binding of DNA on kaolinite and inorganic clays changed its conformation from the B-form to the Z-form, whereas montmorillonite and organic clays retained the original B-form of DNA. A structural change from the B- to the C-form in DNA molecules desorbed from kaolinite was observed by CD spectroscopy and confirmed by fluorescence spectroscopy. The presence of soil colloids and minerals provided protection to DNA against degradation by DNase I. The higher level of protection was found with montmorillonite and organic clays compared to kaolinite and inorganic clays. The protection of DNA against nuclease degradation by soil colloids and minerals is apparently not controlled by the adsorption affinity of DNA molecules for the colloids and the conformational change of bound DNA. The higher stability of DNA seemed to be attributed mainly to the presence of organic matter in the system and the adsorption of nucleases on soil colloids and minerals. The information obtained in this study is of fundamental significance for the understanding of the behavior of extracellular DNA in soil environment.     

Molecular dynamics simulation of secondary sorption behavior of montmorillonite modified by single chain quaternary ammonium cations

Organoclays synthesized from single chain quaternary ammonium cations (QAC) ((CH(3))(3)NR(+)) exhibit different mechanisms for the sorption of nonpolar organic compounds as the length of the carbon chain is increased. The interaction between a nonpolar sorbate and an organoclay intercalated with small QACs has been demonstrated to be surface adsorption, while partitioning is the dominant mechanism in clays intercalated with long chain surfactants. This study presents the results of a molecular dynamics (MD) simulation performed to examine the sorption mechanisms of benzene in the interlayer of three organoclays with chain lengths ranging from 1 to 16 carbons: tetramethylammonium (TMA) clay; decyltrimethylammonium (DTMA) clay; and hexadecyltrimethylammonium (HDTMA) clay. The basis of the overall simulation was a combined force field of ClayFF and CVFF. In the simulations, organic cations were intercalated and benzene molecules were introduced to the interlayer, followed by whole system NPT and NVT time integration. Trajectories of all the species were recorded after the system reached equilibrium and subsequently analyzed. Simulation results confirmed that the arrangement of the surfactants controlled the sorption mechanism of organoclays. Benzene molecules were observed to interact directly with the clay surface in the presence of TMA cations, but tended to interact with the aliphatic chain of the HDTMA cation in the interlayer. The simulation provided insight into the nature of the adsorption/partitioning mechanisms in organoclays, and explained experimental observations of decreased versus increased uptake capacities as a function of increasing total organic carbon (TOC) for TMA clay and HDTMA clay, respectively. The transition of sorption mechanisms was also quantified with simulation of DTMA clay, with a chain length between that of TMA and HDTMA. Furthermore, this study suggested that at the molecular level, the controlling factor for the ultimate sorption capacity is available surface sites in the case of TMA clay, and density of aliphatic chains within the interlayer space for HDTMA clay.     

Sorption of tetracycline and chlortetracycline on K- and Ca-saturated soil clays, humic substances, and clay-humic complexes

Tetracycline (TC) and chlortetracycline (CTC) are used extensively for growth promotion and therapeutic purposes in livestock production. The sorption of TC and CTC on clays, humic substances (HS), and clay-humic complexes (clay-HC) derived from two agricultural soils was quantified using dilute CaCl2 (Ca) and KCI (K) as background solutions. In all systems, the soil components sorbed > 96% of added tetracyclines. Strongest sorption was observed for clays, followed by HS, and then clay-HC. Greater sorption by the Ca systems than the K systems and decreased sorption with increasing pH suggests that cation bridging and cation exchange contribute to sorption. X-ray diffraction analysis showed that TC and CTC were sorbed in the interlayers of smectites and that the presence of HS reduced interlayer sorption of tetracyclines by smectites in clay-HC. The results indicate that tetracyclines are dominantly sorbed on soil clays and that HS in clay-HC either mask sorption sites on clay surfaces or inhibit interlayer diffusion of tetracyclines.     

Investigating the role of mineral-bound humic acid in phenanthrene sorption

Contaminant-soil interaction studies have indicated that physical conformation of organic matter atthe solid-aqueous interface is important in governing hydrophobic organic compound (HOC) sorption. To testthis, organo-clay complexes were constructed by coating montmorillonite and kaolinite with peat humic acid (PHA) in Na+ or Ca2+ dominated solutions with varying pH and ionic strength values. The solution conditions encouraged the dissolved PHA to adopt a "coiled" or "stretched" conformation prior to interacting with the clay mineral surface. Both kaolinite and montmorillonite organo-clay complexes exhibited higher phenanthrene sorption (Koc values) with decreasing pH, indicating that the coiled configuration provided more favorable sorption conditions. Evidence from 1H high-resolution magic angle spinning (HR-MAS) nuclear magnetic resonance (NMR) indicated that polymethylene groups were prevalent at the surface of the organo-clay complexes and may enhance sorptive interactions. Preferential sorption of polymethylene groups on kaolinite and aromatic compounds on montmorillonite may also contribute to the difference in phenanthrene sorption by PHA associated with these two types of clay. This study demonstrates the importance of solution conditions in the sorption of nonionic, hydrophobic organic contaminants and also provides evidence for the indirect role of clay minerals in sorption of contaminants at the soil-water interface.     

Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy

Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono- and bidentate inner-sphere surface complexes attached to the octahedral layer of (010) and (110) edge sites. The maximal site density of inner-sphere sorption sites inferred from molecular simulations agrees well with site capacities of surface complexation sites derived from macroscopic studies and modeling.     

NMR investigation of the behavior of an organothiophosphate pesticide, chlorpyrifos, sorbed on soil components

Phosphorus-31 nuclear magnetic resonance (31P NMR) was used to follow the decomposition of chlorpyrifos (an organothiophosphate pesticide) adsorbed on soil, humic acid, partially hydrated kaolin clay, and partially hydrated montmorillonite clay at high concentration (typically 2-10 wt %). Solid-state 31P NMR (using magic-angle spinning and cross polarization or direct polarization) and liquid-solution 31P NMR of DMSO and acetone extracts indicate that chlorpyrifos is initially physisorbed, appearing by solid-state 31P NMR to exhibit significant motion on the molecular level, which results in almost liquidlike solid-state spectra. Over periods ranging from hours to years, the signals due to unreacted chlorpyrifos sorbed on the clays diminish and are replaced by new 31P NMR peaks resulting from hydrolysis, isomerization, mineralization, and oxidation reactions. The 31P NMR signal characteristics indicate that these decomposition products are much more tightly bound to the clay than is chlorpyrifos. Solid-state 13C and 27AI NMR spectra were less useful for following the decomposition of chlorpyrifos than those obtained by 31P NMR. Solid-state 31P NMR results indicate that a chlorpyrifos loading level of 10% by weight, used in some of the samples to facilitate 31P NMR detection of less-than-dominant decomposition products, exceeds the adsorption capacity of the soil, humic acid, and kaolinite tested, but not Ca2+ -exchanged montmorillonite. This pattern is consistent with intercalation into the montmorillonite, but only surface adsorption on kaolinite.     

Zearalenone removal in synthetic media and aqueous part of canned corn by montmorillonite K10 and pillared montmorillonite K10

The capacities of montmorillonite K10 (K10), aluminum pillared K10 (Al-K10), and iron pillared K10 (Fe-K10) to eliminate zearalenone (ZEN) from synthetic media and the aqueous part of canned corn were studied. Original clay and pillared clays were characterized in terms of X-ray powder diffraction analysis and N(2) adsorption-desorption isotherms. The maximum amounts of adsorption of ZEN by K10, Al-K10, and Fe-K10 at 25°C and pH 7 were 0.202, 1.305, and 1.028 mg/g and 0.264, 0.096, and 0.255 mg/g, calculated from Langmuir and Freundlich isotherms, respectively. The adsorption of ZEN was also studied as a function of adsorbent amount (1 to 30 mg), ZEN concentration (2 to 20 mg/liter), pH of solution (pH 4 to 10), and contact time. Pillared clays could be an excellent alternative for removing ZEN in contaminated food samples and are potentially low-cost adsorbents with a promising future as an alternative to more costly materials.     

Effect of Glycerol on Water Vapor Sorption and Mechanical Properties of Starch/Clay Composite Films

Plasticized starch/clay composite films were prepared by casting aqueous solutions containing oxidized corn starch, different concentrations of glycerol as a plasticizer and 5% clay (sodium montmorillonite, Na⁺‐MMT) on the basis of dry starch. The water‐binding properties of the composite films were evaluated by water vapor sorption isotherms at room temperature and various relative humidities (RHs). Mechanical properties and abrasion resistance were also analyzed for the films with varying glycerol contents at 68% RH and room temperature. Changes in water sorption isotherms suggested that glycerol interacted with both water and starch in a complicated way. A saturation phenomenon of glycerol, depending on RH, was observed based on the isotherms. Above this saturation content, phase separation of the system occurred with the appearance of free glycerol. According to mechanical performance and abrasion resistance, as well as water vapor sorption of the starch blend films, the three‐stage transition was presented to be related to the state of glycerol in the blend system, i.e. adsorption of glycerol onto H‐bonding sites of starch, supersaturation of glycerol as plasticizer and further supersaturation of glycerol. Only above the supersaturation content can glycerol play a plasticizer role in starch‐based composites.     

A novel system for reducing leaching from formulations of anionic herbicides: clay-liposomes

A new approach was developed for reducing leaching of herbicides and contamination of groundwater. Liposome-clay formulations of the anionic herbicides sulfometuron and sulfosulfuron were designed for slow release by incorporating the herbicide in positively charged vesicles of didodecyldimethylammonium (DDAB), which were adsorbed on the negatively charged clay, montmorillonite. Freeze fracture electron microscopy demonstrated the existence of DDAB vesicles and aggregated structures on external clay surfaces. X-ray diffraction results for DDAB with montmorillonite imply the existence of DDAB bilayers with an oblique orientation to the basal plane within the clay interlayer space at adsorbed amounts beyond the cation exchange capacity of the clay. Adding DDAB with sulfometuron or sulfosulfuron to montmorillonite yielded 95% or 83% adsorption of the herbicide at optimal ratios. Liposome-clay formulations exhibited slow release of the herbicides in water. Analytical measurements in soil columns demonstrated 2-10-fold reduction in leaching of the herbicides from liposome-clay formulations in comparison with commercial formulations. Percents of root growth inhibition of a test plant in the upper soil depths were severalfold higher for the liposome-clay formulations than for the commercial ones. Consequently, liposome-clay formulations of anionic herbicides can solve environmental and economical problems by reducing their leaching.     

Reaction-based model describing competitive sorption and transport of Cd, Zn, and Ni in an acidic soil

Predicting the mobility of heavy metals in soils requires models that accurately describe metal adsorption in the presence of competing cations. They should also be easily adjustable to specific soil materials and applicable in reactive transport codes. In this study, Cd adsorption to an acidic soil material was investigated over a wide concentration range (10(-8) to 10(-2) M CdCl2) in the presence of different background electrolytes (10(-4) to 10(-2) M CaCl2 or MgCl2 or 0.05 to 0.5 M NaCl). The adsorption experiments were conducted at pH values between 4.6 and 6.5 A reaction-based sorption model was developed using a combination of nonspecific cation exchange reactions and competitive sorption reactions to sites with high affinity for heavy metals. This combined cation exchange/specific sorption (CESS) model accurately described the entire Cd sorption data set. Coupled to a solute transport code, the model accurately predicted Cd breakthrough curves obtained in column transport experiments. The model was further extended to describe competitive sorption and transport of Cd, Zn, and Ni. At pH 4.6, both Zn and Ni exhibited similar sorption and transport behavior as observed for Cd. In all transport experiments conducted under acidic conditions, heavy metal adsorption was shown to be reversible and kinetic effects were negligible within time periods ranging from hours up to four weeks.     

Solid- and solution-phase organics dictate copper distribution and speciation in multicomponent systems containing ferrihydrite, organic matter, and montmorillonite

Copper retention by ferrihydrite, leaf compost, and montmorillonite was studied over 8 months in systems that emulate a natural soil where different solid phases compete for Cu through a common solution in a compartmentalized batch reactor. Copper speciation in solution (total dissolved, DPASV-labile, and free) and exchangeable and total Cu in individual solid phases were determined. Organic carbon in solution (DOC) and that retained by the mineral phases were also determined. Cu sorption reached steady-state after 4 months and accounted for 80% of the Cu initially added to the system (0.15 mg L(-1)). The remaining 20% stayed in solution as nonlabile (82.8%), labile (17%), and free (0.2%) Cu species. Copper sorption followed the order organic matter > silicate clays > iron oxides. Within each solid phase, exchangeable Cu was < or = 10% of the total Cu sorbed. DOC reached steady state (22 mg L(-1)) after 4 months and seemed to control Cu solubility and sorption behavior by the formation of soluble Cu-DOC complexes and by sorbing onto the mineral phases. DOC sorption onto ferrihydrite prevented Cu retention by this solid phase. Using a multicomponent system and 8 months equilibrations, we were able to capture some of the more important aspects of the complexity of soil environments bytaking into account diffusion processes and competition among solid- and solution-phase soil constituents in the retention of a metal cation.     

Environmentally friendly slow release formulations of alachlor based on clay-phosphatidylcholine

A new clay-liposome complex was developed for reducing leaching of herbicides and contamination of groundwater. The liposomes were composed of the neutral and Environmental Protection Agency approved phospholipid phosphatidylcholine (PC). Adsorption of PC liposomes on the clay mineral montmorillonite could exceed the cation exchange capacity of the clay, and was well simulated by the Langmuir equation. X-ray diffraction results for 6 mM PC and 1.6 g/L clay (3 day incubation) yielded a basal spacing of 7.49 nm, which was interpreted as the formation of a supported planar bilayer on montmorillonite platelets. Fluorescence methods demonstrated structural changes which reflected adsorption of PC followed by loss of vesicle integrity as measured by the penetration of dithionite into the internal monolayer of fluorescently labeled liposomes, resulting in a decrease in fluorescence intensity to 18% of initial after 4 h. Energy transfer was demonstrated after 1 h from labeled liposomes to montmorillonite labeled by an acceptor. The neutral herbicide alachlor adsorbed on the liposome-clay complex, yielding a formulation of up to 40% active ingredient, and 1.6-fold reduction in herbicide release in comparison to the commercial formulation. Hence, the PC-montmorillonite complex can form a basis for environmentally friendly formulations of herbicides, which would yield reduced leaching.     

Sorption of phenanthrene by reference smectites

Fate and behavior of nonionic hydrophobic organic compounds (HOCs) in the environment is 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 HOCs in the environment. We used phenanthrene as a probe to explore the potential of reference smectites to sorb HOCs from aqueous solution. Batch experiments were used to construct phenanthrene sorption isotherms, and possible sorption mechanisms were inferred from the shape of the isotherms. Our results demonstrate that smectites can retain large amounts of phenanthrene from water. Phenanthrene sorption capacities of the reference smectites investigated in this study were comparable to those of soil clays containing a considerable amount of organic matter. Hectorite exhibited the highest sorption affinity and capacity followed by Panther Creek montmorillonite. The lack of correlation between Freundlich sorption constants (K'f) and indices of charge or hydrophobicity suggests that sorption of phenanthrene by smectites is primarily a physical phenomenon. Capillary condensation into a network of nanoor micropores created by quasicrystals is likely to be a dominant mechanism of phenanthrene retention by smectites.     

载锆蒙脱石对蔗糖溶液中没食子酸的吸附特性分析

该研究以氧氯化锆和蒙脱石为原料,通过共沉淀法制备载锆蒙脱石。通过扫描电镜（SEM）、傅里叶红外光谱（FT-IR）、X射线衍射（XRD）、X射线光电子能谱（XPS）和比表面积分析（BET）研究改性和吸附对载锆蒙脱石表观结构和表面基团的影响,并考察了载锆蒙脱石对蔗糖溶液的吸附特性。表征结果显示,载锆蒙脱石相较于蒙脱石表面更加粗糙,比表面积达到了204.47 m2/g,是改性前的3.39倍;表面有纳米小颗粒堆积,FT-IR和XPS结果证明氧化锆成功负载到蒙脱石表面,载锆蒙脱石等电点为7.39。吸附试验表明载锆蒙脱石吸附量相较于蒙脱石吸附量提高了102.54%,载锆蒙脱石对蔗糖溶液中没食子酸吸附的最佳pH为7.0,时间为300 min时达到平衡。吸附过程可以准二级吸附动力学模型和Langmuir等温吸附模型准确描述,饱和吸附量达到129.87 mg/g;热力学研究表明吸附过程为吸热过程,能自发进行。经过5次再生后,载锆蒙脱石可以保持初次吸附量的80%以上。吸附过程主要通过静电引力和氧化锆的配位离子交换吸附没食子酸。综上所述,载锆蒙脱石对蔗糖溶液中没食子酸具有良好的吸附性能,是一种有前景的糖用吸附剂。