Comparison of modified montmorillonite adsorbents. Part II: The effects of the type of raw clays and modification conditions on the adsorption performance

This paper builds on the preceding researches to study the effects of the type of clays (montmorillonites K10, KSF) and modifying conditions on the structure and adsorption behavior of resulting clay adsorbents. The raw clays were modified by polymeric Al/Fe species, hexadecyl-trimethylammonium (HDTMA) surfactant and a complex of polymeric Al/Fe-HDTMA. X-ray diffraction spectra was applied to analyze the structure of the raw and modified clays. After modification, the basal spacing of the clays varied, depending on the types of raw clay and modification conditions. Copper and phenol were selected as adsorbates for evaluating the adsorption performance of various clays, which was affected significantly by the types of raw clay and modification conditions. In general the inorganic contaminant (e.g., Cu) tend to be adsorbed by the polymeric Al/Fe modified clay and the organic impurities (e.g., phenol) will be preferably captured by the surfactant modified clay; both due to the specific surface properties resulting from introducing the modifiers. The complex modified clays possessed the ability of adsorbing both inorganic and organic contaminants. In addition, the d 0 0 1 spacing of modified KSF was greater than that of K10; the adsorption performance with modified KSF was thus greater than that with the modified K10. Finally, the ratio of modifiers to the clay (metal:surfactant:clay) has been observed to affect the adsorption performance; the optimal conditions have been defined.     

Improved interpretation of in-diffusion measurements with confined swelling clays

Diffusion is considered the principal transport mechanism of radio-nuclides and other low-molecular-weight pollutants in compacted clays used as barriers at various disposal and storage sites, for example, at projected deep repositories for radioactive waste. Porous filters are routinely used to confine swelling clays in diffusion studies of radio-tracers. The presence of the filter gives rise to considerable mass-transfer limitations at the clay boundary that result in erroneous diffusion parameters. We have solved the problem of in-diffusion with due account for this phenomenon by means of Fourier transforms. By using literature data on the in-diffusion of traces of radioactive cesium in an argillaceous rock (Opalinus clay) and a compacted bentonite (FEBEX bentonite), we have demonstrated that taking into account the mass-transfer limitations considerably improves the quality of the theoretical fit of the time evolution of radio-tracer concentration in the reservoir. Besides that, we have shown that ignoring the mass-transfer limitations leads to a noticeable underestimation of both the effective diffusion coefficient and the specific sorption capacity of the clay.     

有机粘土矿物对水中低浓度菲的吸附性能和机理

分别选用十六烷三甲基溴化铵(HDTMA-Br)、四甲基溴化铵(TMA-Br)、聚乙二醇(PEG)表面活性剂改性天然粘土矿物,研究其对水中低浓度多环芳烃类难降解有机物菲的吸附性能和机理,并讨论了有机粘土的用量对吸附菲的性能的影响,验证了有机粘土矿物吸附菲后的稳定性.3种有机粘土矿物对菲的吸附等温线均呈中凹型,表现为分配系数(Kp)逐渐增大,表明吸附是分配作用和溶剂化效应共同作用的结果.根据Kp及土样有机碳含量(foc)所得的经有机碳归一化的分配系数(Koc)基本为常数,远远高于天然土壤/沉积物的Koc.在相同实验条件下,3种有机粘土矿物中HDTMA改性粘土矿物对菲的吸附性能最强,PEG改性粘土矿物次之,TMA改性粘土矿物最差.     

Adsorption of lead onto smectite from aqueous solution

The purpose of this research is to study the effect of a new method of adsorption using membrane filtration to determine the maximum amount of lead adsorbed by clay and investigate the behavior of the clay after adsorption of the said metal. Treatment of wastewater contaminated with heavy metals depends on the characteristics of the effluent, the amount of final discharge, the cost of treatment, and the compatibility of the treatment process. The process of adsorption of heavy metals by clays may be a simple, selective, and economically viable alternative to the conventional physical–chemical treatment. This is justified by the importance of the surface developed by this material, the presence of negative charges on the said surface, the possibility of ion exchange taking place, and its wide availability in nature. The removal of lead from wastewater was studied by using the adsorption technique and using clay as the adsorbent. A method was optimized for adsorption through a membrane approaching natural adsorption. This new method is simple, selective, and the lead adsorption time is about 3 days. The various properties of clay were determined. It was observed that the cation exchange capacity of the clay was 56 meq/100 g of hydrated clay for the raw sample and 82 meq/100 g for the purified sample. The total surface area determined by the methylene blue method was equal to 556 and 783 m²/g for the raw and purified samples, respectively. The adsorption kinetics depends on several parameters. The Pb(II) clay, obeys the Langmuir, Freundlich, and the Elovich adsorption isotherms with high regression coefficients. The use of this adsorbent notably decreases the cost of treatment. It was concluded that clay shows a strong adsorption capacity on Pb(II), the maximum interaction occurring with purified clay treated at high concentration of lead. It is proposed that this adsorption through a membrane be extended for the treatment of effluents containing other metals.     

Consistent interpretation of the results of through-, out-diffusion and tracer profile analysis for trace anion diffusion in compacted montmorillonite

Literature data for anion diffusion in compacted swelling clays contain systematic inconsistencies when the results of through-diffusion tests are compared with those of out-diffusion or tracer profile analysis. In the present work we investigated whether these inconsistencies can be explained by taking into account heterogeneities in the compacted samples; in particular increased porosities at the clay boundaries. Based on the combined results of out-diffusion, tracer profile analysis and the spatial distribution of the electrolyte anion in the clay, we conclude that the inconsistencies can indeed be resolved by taking into account a heterogeneous distribution of the total and the anion-accessible porosity. This, by definition, leads to a position dependence of the effective diffusion coefficient. Neglecting these effects results in a rather subordinate systematic error in the determination of effective diffusion coefficients of anions from through-diffusion tests with clay thicknesses in the centimetre range. However, stronger errors in terms of absolute values and conceptual interpretation may be introduced in out-diffusion tests and profile analyses of the diffused tracer. We recommend that anion diffusion tests should be accompanied by measurements of the total and anion-accessible porosity as a function of position in the direction of diffusion.     

Diffusive transport through compacted Na- and Ca-bentonite

The effect of exchangeable cation — Na⁺ and Ca ²⁺ — on the diffusive transport of I⁻, Sr ²⁺ and ³H (as HTO) in compacted bentonite was examined using a through-diffusion method. Total intrinsic diffusion coefficients, D_i, were determined from the steady-state flux of the diffusants through the clays, and apparent diffusion coefficients, D_a, were obtained from the time lag technique. The clays were compacted to a dry bulk density of 1.3 Mg/m³, and Na-bentonite was saturated with a solution of 100 mol NaCl/m3 and Ca-bentonite with one of 50 mol CaCl₂/m³. The D_i values for all diffusants are 2 to 6 times higher in the Ca- than Na-clay. We attribute this to the larger quasicrystal, or particle, size of Ca- compared to Na-bentonite. Hence, Ca-bentonite has a greater proportion of relatively large pores; this was confirmed by Hg intrusion porosimetry. This means the diffusion pathways in Ca-bentonite are less tortuous than those in Na-bentonite. Moreover, in some cases the effective porosity, or the porosity available for diffusive transport, may be greater in Ca-bentonite. The D_a values are inversely proportional to the distribution coefficients of the diffusants with the clays.     

Sensitivity analysis of radionuclide migration in compacted bentonite: a mechanistic model approach

Mechanistic model calculations for the migration of Cs, Ra, Am and Pb in compacted bentonite have been carried out to evaluate sensitivities with respect to different parameter variations. A surface chemical speciation/electric double layer model is used to calculate: (i) porewater composition and radionuclide speciation in solution and at the bentonite surface, yielding the distribution of mobile and sorbed species and (ii) interaction of diffusing species with negatively charged pore walls to obtain diffusion parameters. The basic scenario considers the interaction of compacted bentonite with a fresh-type groundwater; variations include the presence of bentonite impurities and saline groundwater. It is shown that these scenarios result in significant variations of porewater composition that affect migration via three mechanisms that can partly compensate each other: (1) effects on sorption through radionuclide complexation in solution, and competition of major cations for surface sites; (2) changes in radionuclide solution speciation leading to different diffusing species under different conditions; (3) effects on diffusion through changes in the electric double layer properties of the clay pores as a function of ionic strength.     

Measurement of the Oxygen Mass Transfer Through the Air-Water Interface (5 pp)

Goal, Scope and Background Gas mass transfer through the liquid-gas interface has enormous importance in various natural and industrial processes. Surfactants or insoluble compounds adsorbed onto an interface will inhibit the gas mass transfer through the liquid-gas surface. This study presents a technique for measuring the oxygen mass transfer through the air-water interface. Experimental data obtained with the measuring device were incorporated into a novel mathematical model, which allowed one to calculate diffusion conduction of liquid surface layer and oxygen mass transfer coefficient in the liquid surface layer. Methods A special measurement cell was constructed. The most important part of the measurement cell is a chamber containing the electrochemical oxygen sensor inside it. Gas exchange between the volume of the chamber and the external environment takes place only through the investigated surface layer. Investigated liquid was deoxygenated, which triggers the oxygen mass transfer from the chamber through the liquid-air interface into the liquid phase. The decrease of oxygen concentration in the cell during time was measured. By using this data it is possible to calculate diffusional parameters of the water surface layer.Results Diffusion conduction of oxygen through the air-water surface layer of selected wastewaters was measured. The diffusion conduction of different wastewaters was about 3 to 6 times less than in the unpolluted water surface. It was observed that the dilution of wastewater does not have a significant impact on the oxygen diffusion conduction through the wastewater surface layer. This fact can be explained with the presence of the compounds with high surface activity in the wastewater. Surfactants achieved a maximum adsorption and, accordingly, the maximum decrease of oxygen permeability already at a very low concentration of surfactants in the solution. Oxygen mass transfer coefficient of the surface layer of the water is found to be.Conclusion A simple technique for measuring oxygen diffusion parameters through the air-water solution surface has been developed. Derived equations enable the calculation of diffusion parameters of the surface layer at current conditions. These values of the parameters permit one to compare the resistances of the gas–liquid interface to oxygen mass transfer in the case of adsorption of different substances on the surface layer. Recommendation and Outlook This simple technique may be used for a determination of oxygen permeability of different water-solution surface layers. It enables one to measure the resistance to the oxygen permeability of all inflowing wastewater surface layers in the wastewater treatment plant, and to initiate a preliminary cleaning of this wastewater if required. Similarly, we can measure oxygen permeability of natural waterbodies. Especially in the case of pollution, it is important to know to what extent the oxygen permeability of the water surface layer has been decreased. Based on the tehnique presented in this research, fieldwork equipment will be developed.     

Adsorption of picloram on clays nontronite,illite and kaolinite: equilibrium and herbicide-clays surface complexes

The picloram (PCM) adsorption on nontronite, illite and kaolinite was studied at pH 3, 5 and 7. The adsorption isotherms had well-fitted to Langmuir and Freundlich models equations. The interactions of PCM with the clay mineral surfaces exhibited an anionic profile adsorption, with a decrease in adsorption when the pH increases. The PCM adsorption capacity increases in the following order: kaolinite?<?illite?<?nontronite. The X-ray diffraction (XRD) analysis of PCM-clay samples revealed that the picloram molecule does not enter into the clays basal space. The interaction of PCM with clays surface sites through nitrogen of the pyridine ring was confirmed by X-ray photoelectron spectroscopy (XPS). Due to the anionic form of PCM, the adsorption onto the external and edges surface sites of the clay minerals was proposed.     

Determination of gas phase adsorption isotherms--a simple constant volume method

Single and ternary solute gas phase adsorption isotherms were conducted in this study to evaluate the effectiveness of a simple constant volume method, which was utilized by using Tedlar gas sampling bags as a constant volume batch reactor. For this purpose, gas phase adsorption of toluene, methyl ethyl ketone (MEK), and methyl isobutyl ketone (MIBK) on two types of activated carbons, BPL-bituminous base and OVC--coconut base, were investigated. For the single solute adsorption, the experimental adsorption data were found to be well correlated with Freundlich and Myers adsorption equations. The pore size distribution of adsorbents was found to affect their adsorption capacities; its effect was dependant on the solute concentration. The ternary adsorption experimental isotherms were accurately predicted by using the well-known model, i.e., ideal adsorbed solution theory (IAST).     

Contributions of advective and diffusive oxygen transport through multilayer composite caps over mine waste

The relative contributions of four mechanisms of oxygen transport in multilayer composite (MLC) caps placed over oxygen-consuming mine waste were evaluated using numerical and analytical methods. MLC caps are defined here as caps consisting of earthen and geosynthetic (polymeric) components where a composite barrier layer consisting of a geomembrane (1-2 mm thick polymeric sheet) overlying a clay layer is the primary barrier to transport. The transport mechanisms that were considered are gas-phase advective transport, gas-phase diffusive transport, liquid-phase advective transport via infiltrating precipitation and liquid-phase diffusive transport. A numerical model was developed to simulate gas-phase advective-diffusive transport of oxygen through a multilayer cap containing seven layers. This model was also used to simulate oxygen diffusion in the liquid phase. An approximate analytical method was used to compute the advective flux of oxygen in the liquid phase. The numerical model was verified for limiting cases using an analytical solution. Comparisons were also made between model predictions and field data for earthen caps reported by others. Results of the analysis show that the dominant mechanism for oxygen transport through MLC caps is gas-phase diffusion. For the cases that were considered, the gas-phase diffusive flux typically comprises at least 99% of the total oxygen flux. Thus, designers of MLC caps should focus on design elements and features that will limit diffusion of gas-phase oxygen.     

Preparation of a porous clay heterostructure and study of its adsorption capacity of phenol and chlorinated phenols from aqueous solutions.

A porous clay heterostructure (PCH) from a Mexican clay was prepared and characterized, and its aqueous phenol and dichlorophenols (DCPs) adsorption capacities were studied using a batch equilibrium technique. The PCH displayed a surface area of 305.5 m2/g, 37.2 A average porous diameter, and a basal space of 23.2 A. The adsorption capacity shown by the PCH for both phenol and DCPs from water (14.5 mg/g for phenol; 48.7 mg/g for 3,4-DCP; and 45.5 mg/g for 2,5-DCP) suggests that the PCH has both hydrophobic and hydrophilic characteristics, as a result of the presence of silanol and siloxane groups formed during the pillaring and calcination of the PCH. The values of maximal adsorption capacity for dichlorophenols were higher than those reported for aluminum pillared clays and some inorgano-organo clays and comparable with some ionic exchange resins.     

Photostabilization of the herbicide bensulfuron-methyl by using organoclays

Photostable formulations of the herbicide bensulfuron-methyl [BSM, 2-(4,6-dimethoxypyrimidin-2-carbamoylsulfamoyl)-o-toluic acid methyl ester] were achieved by adsorbing it on clays or on clays pre-adsorbed with the organic cation malachite green (MG). Fourier-transform infra-red (FTIR) spectra showed the existence of strong interactions between the pre-adsorbed MG and the herbicide. The photostabilization of BSM obtained with clay-MG was mainly due to pacification of clay surface by MG and a deactivation mechanism via energy transfer between the two organic molecules adsorbed on the surface of the clay.     

Multiphase flow and transport in fractured clay/sand sequences.

A numerical model (Queen's University Multi-Phase Flow Simulator, QUMPFS) was used to assess the rate of trichloroethylene (TCE) dense, non-aqueous phase liquid (DNAPL) migration through fractured clay, with special attention focused on the influence of interbedded sand lenses. The presence of these sand lenses was found to increase the time required for the non-wetting phase to migrate through the full 30 m vertical extent of the clay sequence from a few days to several years. Applied vertical hydraulic gradients were found to be moderately influential in systems consisting solely of fractured clays, yet one of the dominant factors controlling speed of vertical migration when sand lenses were present. Larger displacement pressure of the sands relative to that of the fractures leads to slower DNAPL migration rates, due to the delays that occur during build-up of capillary pressures. Dissolution of DNAPL and subsequent matrix diffusion of the aqueous phase has little effect on the rate of DNAPL migration through systems consisting of fractured clay only, yet slows the rate of migration in systems containing sand lenses. In all cases examined, the rate of DNAPL loading to the lower aquifer far exceeded the rate of aqueous phase mass loading. It was also found that DNAPL reaches the lower aquifer at approximately the same time as the aqueous phase plumes even for systems experiencing downward groundwater flow due to the attenuation of the aqueous phase through matrix diffusion.     

Solute transport properties of compacted Ca-bentonite used in FEBEX project

The present Spanish concept of a deep geological high level waste repository includes an engineered clay barrier around the canister. The clay presents a very high sorption capability for radionuclides and a very small hydraulic conductivity, so that the migration process of solutes is limited by sorption and diffusion processes. Therefore, diffusion and distribution coefficients in compacted bentonite (i.e. in "realistic" liquid to solid ratio conditions) are the main parameters that have to be obtained in order to characterise solute transport that could be produced after the canister breakdown. Through-Diffusion (TD) and In-Diffusion (ID) experiments with HTO, Sr, Cs and Se were carried out using compacted FEBEX bentonite, which is the reference material for the Spanish concept of radioactive waste disposal. Experiments were interpreted by means of available analytical solutions that allow the estimation of diffusion coefficients and, in some cases, distribution coefficients. Analytical solutions are simple to use, but rely on hypotheses that do not hold in all the experiments. These experiments were interpreted also using an automatic parameter estimation code that overcomes the limitations of analytical solutions. Numerical interpretation allows the simultaneous estimation of porosity, diffusion and distribution coefficients, accounts for the role of porous sinters and time-varying boundary concentrations, and can use different types of raw concentration data.     

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.     

Modelling the adsorption of mercury onto natural and aluminium pillared clays

Introduction

The removal of heavy metals by natural adsorbent has become one of the most attractive solutions for environmental remediation. Natural clay collected from the Late Cretaceous Aleg formation, Tunisia was used as a natural adsorbent for the removal of Hg(II) in aqueous system.

Methods

Physicochemical characterization of the adsorbent was carried out with the aid of various techniques, including chemical analysis, X-ray diffraction, Fourier transform infrared and scanning electron micrograph. Batch sorption technique was selected as an appropriate technique in the current study. Method parameters, including pH, temperature, initial metal concentration and contact time, were varied in order to quantitatively evaluate their effects on Hg(II) adsorption onto the original and pillared clay samples. Adsorption kinetic was studied by fitting the experimental results to the pseudo-first-order and pseudo-second-order kinetic models. The adsorption data were also simulated with Langmuir, Freundlich and Temkin isotherms.

Results

Results showed that the natural clay samples are mainly composed of silica, alumina, iron, calcium and magnesium oxides. The sorbents are mainly mesoporous materials with specific surface area of <250 m² g^?1. From the adsorption of Hg(II) studies, experimental data demonstrated a high degree of fitness to the pseudo-second-order kinetics with an equilibration time of 240 min. The equilibrium data showed the best model fit to Langmuir model with the maximum adsorption capacities of 9.70 and 49.75 mg g^?1 for the original and aluminium pillared clays, respectively. The maximum adsorption of Hg(II) on the aluminium pillared clay was observed to occur at pH 3.2. The calculated thermodynamic parameters (?G°, ?H° and ?S°) showed an exothermic adsorption process. The entropy values varied between 60.77 and 117.59 J?mol^?1 K^?1, and those of enthalpy ranged from 16.31 to 30.77 kJ mol^?1. The equilibrium parameter (R _L) indicated that the adsorption of Hg(II) on Tunisian smectitic clays was favourable under the experimental conditions of this study.

Conclusion

The clay of the Aleg formation, Tunisia was found to be an efficient adsorbent for Hg(II) removal in aqueous systems.     

镉溶液污染黏土微观结构演化规律

镉溶液在压实黏土中迁移会引起黏土微观结构的变化。通过一系列扫描电子显微镜与低温氮吸附实验,观察了镉溶液污染黏土的表面微观形貌与孔隙结构的演化规律。扫描电子显微镜观察结果显示,受镉溶液污染严重的土柱上表面孔隙明显,土颗粒之间主要以点-点排列方式为主;而受镉溶液污染较弱的下表面,土颗粒之间主要以片-片状排列为主。低温氮吸附实验结果表明,Z=0 cm、Z=10 cm、Z=20 cm、Z=30 cm处,污染黏土吸附量分别为682.65、631.72、583.25和523.36 cm3/g。随土柱中土层深度的增大,黏土颗粒间微孔增多,二次孔减少,孔隙体积与比表面积减小。镉溶液污染黏土孔隙主要集中在10 nm以下。     

Coupled production and transport of selenium vapor in unsaturated soil: evaluation by experiments and numerical simulation

Volatilization of selenium (Se) from soil to the atmosphere involves several sequential chemical reactions that form volatile Se species, followed by transport of the gaseous Se through the soil. This paper describes a numerical model that simulates the chemical and physical processes governing the production and transport of Se vapor in unsaturated soil. The model couples the four Se species involved in the production of Se vapor through chemical reactions, and allows each to migrate through the soil by advection, liquid or vapor diffusion depending on its affinity for the dissolved or vapor phase. The coupled transformations and transport of the four Se species, i.e., selenate, selenite, elemental and organic Se, and Se vapor, were calculated based on the Crank-Nicolson finite difference method. The model was used to analyze fluxes of Se vapor measured from a soil amended with inorganic Se in the form of selenate and covered with unamended clean soil of various thicknesses. Evolution of Se vapor from the soil was very fast, with measurable amounts of Se detected within 24 h. The peak of Se volatilization, detected at the 6th day, reached 3.31 Se microgram/day for the uncovered soil, but was reduced to near the detection limit (0.05 microgram/day) in the presence of a 8- or 16-cm clean soil cover. With two reaction rate coefficients fitted to the data, the model described Se volatilization very well. The estimated rate coefficient of Se methylation was unexpectedly high, with a value of 0.167/day. The net volatilization of Se, however, was severely inhibited by the fast demethylation, i.e., the reverse reaction which converted volatile Se species back into nonvolatile forms. As a result, Se vapor only penetrated a few centimeters in the soil. The demethylation rate coefficient, assessed by independent transport experiments using dimethyl selenide, was estimated as 186.8/day, corresponding to a half-life of only 5.3 min for Se vapor. Results of this study indicated that rapid demethylation of Se vapor during its diffusive transport through a soil is probably an important limiting factor in the volatilization of Se under natural conditions.     

酸活化凹凸棒黏土对印染废水中亚甲基蓝的吸附性能

通过稀硫酸活化凹凸棒黏土,再与高黏土混合,制备颗粒黏土吸附剂。通过X射线衍射（XRD）、场发射扫描电镜（FE-SEM）、N2吸附-脱附等温线和傅里叶红外光谱（FT-IR）对吸附剂粉末进行表征测试。结果表明：凹凸棒黏土由水硅锰钙石（Kittatinnyite）、坡缕石（Palygorskite）和石英（Quratz）3种成分组成;黏土表面呈杂乱堆积的纤维状和针棒状,比表面积高达126.43 m2·g-1;动力学吸附数据符合准二级动力学模型和离子内扩散模型,热力学吸附数据符合Langmuir吸附模型,在323 K时,最大吸附量达到153.85 mg·g-1;颗粒吸附剂对印染废水中亚甲基蓝具有较好的吸附性能,是一种具有发展前景的新型吸附材料。