Changes In Pore Size Distribution Research Articles

Role of amination of a polymeric adsorbent on phenol adsorption from aqueous solution

Adsorption of seven phenols, one aromatic carboxylic acid and one sulfonated aromatic acid from aqueous solution, by a hypercrosslinked polymeric adsorbent (CHA-111) and the derivative animated by dimethylamine was compared. The results of different adsorption isotherms indicated that amino group on the polymeric matrix played a significant role on adsorption of almost all the employed compounds. For most employed compounds adsorption capacities increase to different degree and significantly for those with lower value of pKa. It may be attributed to the enhanced adsorbent–adsorbate interaction for amino group introduced on the polymeric matrix. The empirical Freundlich isotherm equation was employed to interpret the adsorbent–adsorbate interaction. The adsorption enthalpy change indicated the uptake of phenols on MCH-111 to be an enhanced physical adsorption because of the hydrogen-bonding interaction. Adsorption kinetic study of phenols on CHA-111 and MCH-111 was also conducted and amino group on the matrix will reduce the adsorption rate for change of pore size distribution and loss of macroprous volume as well as the hydration effect on the surface partly.

Influence of oxygen-containing functional groups on active carbon adsorption of selected organic compounds

Two commercial activated carbons (AC) were treated with HNO 3 and (NH 4) 2S 2O 8. The changes in surface area and pore size distribution resulting from the oxidising treatments were studied by means of nitrogen adsorption isotherms. The modifications in the surface chemistry were evaluated by means of the point of zero charge (PZC) and the oxygen content. Selective removal of oxygen complexes by heating under N 2 and CO 2 flow at 723 and 1123 K was also studied. The treatment with HNO 3 was found to broaden the microporosity and mesoporosity of the carbons when low concentrations of the oxidising agent were employed. Severe oxidation conditions resulted in an almost total destruction of the texture of the carbons. The point of zero charge values changed systematically with the extent of the oxidation; the more oxidised the carbon, the lower its point of zero charge. Thermal treatment at high temperatures leads to an enhancement of the carbon basicity and a slight decrease in the textural properties of the original carbons.

Measurement and modelling of the development of pore size distribution of wood during pyrolysis

Within the frame adsorbents production from biomass, this study proposes to investigate the change in pore size distribution during the pyrolysis. The mercury intrusion method has been used to measure pore structure and a mathematical model has been developed based on the population balance and the mass balance. This model considers particle shrinkage, different kinds of new pore initiation, and the coalescence of pores. These different kinds of pore size contributions are presented starting from the measured pore size distribution of larch wood.

Stray Field Nuclear Magnetic Resonance of Soil Water

Development, characterization, and preliminary results of a recent technique capable of local measurements of pore-size distribution by a spatially resolved low resolution nuclear magnetic resonance (NMR) technique are described. Potential environmental uses include studying the change in pore-size distribution caused by surface compaction, which influences surface runoff, and obtaining information on the physical state of non-aqueous compounds in porous materials, which should aid the selection of appropriate soil remediation methods. Stray field (STRAFI) imaging is an NMR technique that allows distortion-free imaging of materials with short NMR relaxation times. The sample is placed in the strong axial fringe field gradient of a superconducting NMR magnet. We report on a new, unique, large 5-cm-diameter STRAFI probe, and its use for three preliminary test cases: water in ceramics of known pore size, paraffin wax and oil in sandstone rock, and water in soil at different matric potentials. The imaging is confined to one dimension with a spatial resolution of the order of 100 microm for protons. The optimum position for imaging occurs at 2.62 T and a gradient of 12.1 T/m. Water relaxation decay curves can be measured at any position in the 8-cm-long sample. These curves are decomposed into a series of terms each corresponding to a different pore size. Preliminary results show continuum fits to decay curves for a soil drained to three different matric potentials. Such information will be useful for interpreting water retention curves and will lead to understanding of the behavior of fluids in the vadose zone.

Influence of chemical modification of anthracite on the porosity of the resulting activated carbons

Activated carbons in a wide range of porosity (from essentially microporous to essentially mesoporous) have been prepared from anthracite by the combination of a chemical treatment with HClO 4 or Mg(ClO 4) 2 and a physical activation with CO 2 at 850°C. The main effects of the chemical treatment are a modification of anthracite microstructure by insertion and oxidation. The extent of these two reactions was found to be related to the nature of the chemical agents, the treatment time and temperature, and the textural properties of anthracite. As a consequence, the pretreatment of anthracite causes a noticeable reduction of the activation time and a change of the final pore size distribution. The influence of chemical treatment parameters has been analysed by means of XPS, FTIR, TGA, mass spectrometry, elemental analysis and gas adsorption techniques. The optimal conditions for producing activated carbons from chemically modified anthracites were identified. Step by step increasing of the temperature of anthracite treatment by HClO 4 up to 160°C seems to be the best way to obtain a precursor of highly activated carbon with well-balanced micro and meso porosity.

Measurement and estimation of pore shrinkage and pore distribution in a clayey till during soil-water characteristic curve tests

The soil-water characteristic curve (SWCC) of fine-grained soils is usually determined experimentally. In many applications, such as design of mine waste covers and landfill liners, the unsaturated permeability function, k(h), is often derived theoretically from the measured SWCC. Implicit in these derivations is the transformation of the SWCC to a pore-size distribution (PSD), typically assumed to be constant and mono-modal. PSDs of a clayey till compacted at various water contents were measured after compaction, after flexible-wall permeability testing, and during and after SWCC tests. The measurements show that the PSD changes significantly during permeability and SWCC testing. A method is advanced for predicting the observed changes in PSD during SWCC testing. PSDs are determined for soil samples subjected to the highest and lowest suctions applied during the SWCC test. The measured PSDs are transformed to account for pore trapping; the transform assumes that flow occurs through two sets of randomly distributed pores in series. To model pore shrinkage, the pores are idealized as elastic cylinders. PSDs measured after different suction applications in the SWCC tests are compared with predictions of the shrinkage model. The method can also be used to predict the SWCC. Measured and predicted values are compared.Key words: landfill liners, mine waste covers, soil-water characteristic curve, pore-size distribution.

Dissolution Effects on Specific Surface Area, Particle Size, and Porosity of Pentelic Marble

Dissolution of natural stone such as marble is not limited to its surface. The porous structure, known to play an important role in stone decay, is also affected by the conditions of dissolution. In the present work, the changes in pore size distribution of Pentelic marble particles accompanying chemical dissolution in undersaturated solutions and at alkaline pH 8.25 were investigated. The specific surface area and the mesopore distribution of the Pentelic marble tested showed a pronounced decrease to very low values. On the other hand, the sizes of macropores exhibited a tendency to increase with the extent of dissolution due either to dissolution in the interior of the pores or to fusion of small pores into larger. Furthermore, the number of small particles decreased significantly, reaching complete disappearance, depending on the extent of dissolution. At the same time, the relative number of particles of intermediate size increased.

Fouling mechanism of hollow fiber ultrafiltration membrane with pretreatment by coagulation/sedimentation process

The fouling mechanism of the hollow fiber UF membrane with a pretreatment by coagulation/sedimentation process was investigated. The experiments were carried out in three different feed water conditions: river surface water, coagulated water containing micro flocs and clarified water from the coagulation/sedimentation process. The river water contained a lot of suspended particles (high turbidity) and natural organic matter (NOM) such as humic substances. From analysis of the data obtained in the three membrane processes, the mathematical model expressing the change of the filtration resistance is proposed and proved, which includes the pore size distribution of the membrane. The effect of DOC and DOC/E260 on the membrane fouling was also discussed. The analysis of the pore size distribution change with increasing filtration time suggests that the primary mechanism of the fouling is the decrease in the pore number by the high molecular weight humic substances. As a result of experiments and mathematical model analysis, it could be concluded that the coagulation/sedimentation pretreatment process was very effective for expanding ultrafiltration membrane life, because the coagulation/sedimentation process could remove the primary foulants such as high molecular weight humic substances.

Microstructural changes caused by carbonation of cement mortar

The change of specific surface area and pore size distribution due to carbonation of an ordinary Portland cement mortar is investigated. The adsorption of water vapor on noncarbonated and well-carbonated cement mortar is measured in order to evaluate the difference in specific surface area for the two samples using the BET theory. From the measured desorption the pore size distribution is calculated using the Kelvin formula. A sorption balance is used to measure the sorption characteristics for the two studied sample qualities. In this method dry and saturated air are mixed in desired proportion in a closed system. One of the benefits of the method is that the samples not are exposed to carbon dioxide during testing, i.e., undesired effects caused by carbonation on the sorption can be eliminated. The specific surface area for a noncarbonated sample was calculated, using the measured adsorption data, to be 8% higher than for the well-carbonated sample. The difference in pore size distributions was more marked than the difference in specific surface area for the two samples. The well-carbonated mortar had about twice as much volume attributed to small pores as the noncarbonated cement mortar.

Weathering and decay of granitic rocks: its relation to their pore network

A study is presented on the pore size distribution in granites, both fresh and artifically aged by freezing/thawing cycles (−20/110°C), in the diameter range 0–25 nm, by nitrogen adsorption at −196°C. The pore size distribution found permits a plausible explanation for the behaviour shown by these materials during degradation processes associated to fluid flows through their pores (thermoclasty and gelifraction). If low porous granites are used, artificial ageing has only a minor, limited effect on their pore network; however, changes in pore size distribution (diameter < 10 nm) are observed in samples where a pore network already exists in the unaltered samples. Changes in porosity observed upon artificial ageing are similar to those observed in historical buildings after natural decay.

Residual compressive strength and microstructure of high performance concrete after exposure to high temperature

Experimental programs were carried out to study compressive strength and microstructure of high performance concrete (HPC) subjected to high temperature compared with normal strength concrete (NSC). After the concrete specimens were exposed to a peak temperature of 800°C, the compressive strength was tested. Changes of porosity and pore size distribution of the concrete were measured by using mercury intrusion porosimetry (MIP). Test results show that high performance concrete had higher residual strength although the strength of high performance concrete degenerated much more than the normal strength concrete after high temperature exposed. Variations in pore structure of high performance concrete after high temperature indicated the degradation of the mechanical properties. A model by optimizing the parameters in Ryshkewitch model was developed to predict the relationship between porosity and compressive strength of the high performance concrete.

In situ measurement of light scattering in porous ceramics during sintering

Measuring the pore size distribution in ceramics via preparation of polished sections and image analysis is a tedious task and other measuring methods for that purpose should be investigated. The measurement of light transmission gives a sensitive tool for the detection of changes in pore size distribution. Light transmission measurements are performed in situ during sintering of alumina samples at temperatures up to 1923 K. They are compared to shrinkage measurements by dilatometry. The temperature dependence of light transmission and the influence of absorption in alumina is discussed. Very pure alumina samples show no influence of absorption on the transmission at high temperatures. In less pure samples, however, increasing absorption with increasing temperature may strongly influence the transmission. The scattering and absorption coefficients could be calculated separately by numerical methods.

Methane combustion on sol-gel Rh/ZrO2-SiO2 catalysts

Rh/ZrO2–SiO2 catalysts prepared by impregnation of mixed ZrO2–SiO2 oxides obtained by using the sol–gel method were characterised and used in the combustion of CH4. It was found that the specific area and porosity depend strongly on the pH of gelation. Two different Zr precursors were used: Zr(acac)4 and Zr alkoxide. When the former precursor is used, no significant changes in the surface area are detected, however, important changes in pore size distribution are observed. In those samples prepared from Zr alkoxide, both the surface area and the porosity change significantly. No important changes in rhodium dispersion were observed by TEM, in agreement with XPS results. On the contrary hydrogen chemisorption results indicate that the hydrogen uptake of the sulfated samples drops drastically compared with those in which sulfate ions are absent. This behaviour has been attributed to a poisoning of rhodium particles by sulfide species produced by reduction of sulfate ions, as was revealed by TPR. The catalytic activity in the methane combustion is affected by the presence of chloride ions, which induce an inhibitory effect on the combustion. Additionally, the presence of micropores also decreases the activity due to diffusional limitations. © 1999 Society of Chemical Industry

Methane combustion on sol–gel Rh/ZrO2–SiO2 catalysts

Rh/ZrO2–SiO2 catalysts prepared by impregnation of mixed ZrO2–SiO2 oxides obtained by using the sol–gel method were characterised and used in the combustion of CH4. It was found that the specific area and porosity depend strongly on the pH of gelation. Two different Zr precursors were used: Zr(acac)4 and Zr alkoxide. When the former precursor is used, no significant changes in the surface area are detected, however, important changes in pore size distribution are observed. In those samples prepared from Zr alkoxide, both the surface area and the porosity change significantly. No important changes in rhodium dispersion were observed by TEM, in agreement with XPS results. On the contrary hydrogen chemisorption results indicate that the hydrogen uptake of the sulfated samples drops drastically compared with those in which sulfate ions are absent. This behaviour has been attributed to a poisoning of rhodium particles by sulfide species produced by reduction of sulfate ions, as was revealed by TPR. The catalytic activity in the methane combustion is affected by the presence of chloride ions, which induce an inhibitory effect on the combustion. Additionally, the presence of micropores also decreases the activity due to diffusional limitations. © 1999 Society of Chemical Industry

Microwave plasma polymerization of acrylic acid on poly(ethylene terephthalate) track-etched membranes

Remote microwave plasma polymerization of acrylic acid for the formation of layers containing carboxylic acid groups on poly(ethylene terephthalate) track-etched membranes is investigated. The influence of deposition parameters — microwave power, pulsed or continuous wave and pressure — on film formation is examined. The content of carboxylic acid groups is determined by fluorescence labeling with thionine acetate and X-ray photoelectron spectroscopy, the change of pore-size distribution by porometry and water permeation experiments. The pores were covered regularly, and the pore size could be adjusted. Layers deposited at 200 W and 90 Pa contained a concentration of carboxylic acid groups of 0.9 mmol cm−3.

99/02183 Simulation of change of pore size distribution in coal char during combustion: Zhang, J. et al. Ranliao Huaxue Xuebao, 1998, 26, (4), 362–366. (In Chinese)

99/02183 Simulation of change of pore size distribution in coal char during combustion: Zhang, J. et al. Ranliao Huaxue Xuebao, 1998, 26, (4), 362–366. (In Chinese)

Wastewater Effects on Montmorillonite Suspensions and Hydraulic Properties of Sandy Soils

Recycled wastewater effluent is an important source of irrigation water in arid and semiarid regions. In these regions, however, irrigation water quality is one of the main factors limiting plant growth. Wastewater effluents generally contain high concentrations of suspended and dissolved solids, both organic and inorganic. Inorganic dissolved solids are only minimally removed from the effluent during conventional sewage treatment (secondary or tertiary). As a result, most of the salts added during domestic and industrial usage remain in the irrigation water and may eventually reach the soil. A number of researchers have reported reduced hydraulic conductivity for soils to which treated wastewater has been applied. In this research, the influence of dissolved organic matter (DOM) contained in reclaimed wastewater effluents (mainly humic substances) on the flocculation of montmorillonite and on the hydraulic properties of soils was studied. Flocculation values (FVs) for Na-montmorillonite increased with increasing concentrations of DOM at all pH levels analyzed (5.5, 7.0, and 8.5). Maximum FV levels were exhibited for Na-montmorillonite at the highest DOM concentrations. The effect of DOM on FV can be explained by the mechanisms of edge-charge reversal and mutual flocculation. The hydraulic conductivity (HC) of a sandy soil was determined in the laboratory by leaching columns with an electrolyte solution chemically similar to that of the wastewater effluent (but without DOM). In columns treated with wastewater effluent, the HC exhibited a sharp decrease to only 20% of its initial value. The adverse effect of DOM on HC was evident for this soil despite a relatively low exchangeable sodium percentage (ESP). The reduction in HC is likely to be the result of decreased soil pore-size, which reflects two processes: (i) retention of part of the DOM during water percolation; and (ii) a change in pore-size distribution due to swelling and dispersion of clay particles. The latter may result from a higher percentage of adsorbed sodium combined with the presence of humic substances from the wastewater effluent.

Changes in Concrete Pore Size Distribution Due to Electrochemical Chloride Migration Trials

Concrete structures suffering reinforcement corrosion are starting to be rehabilitated by means of electrochemical techniques, such as cathodic protection, chloride extraction, and realkalization that, in some cases, involve the application of relatively large electrical potentials or currents. The generalized movement of the ions dissolved in the aqueous phase of pores leads researchers to think that microstructure of concrete can be altered. In this study, results are presented on the changes in the pore size distribution after penetration of chlorides driven by an electrical field (migration) in comparison with natural diffusion penetration. Results indicate that the application of an electrochemical treatment to a structure induces an increase in total porosity, because of the solution of precipitated phases initially located in the smallest capillary pores that, after some time, evolve to be found in the range of 5 micro m to 0.05 micro m.

Interaction between clay microstructure, decomposition of plant residues and humification

SummaryWe have studied microstructural changes in montmorillonite and kaolinite, which were amended with organic debris (leaves of chestnut or beech), microbial inoculum and nutrients and subjected, for 30 days at 25°C, to alternate drying and wetting (D–W) or kept continuously moist at water‐holding capacity (WHC). The objective was to evalute the interactions between the decomposition of plant residues and clay microstructural organization in different pore–water regimes. The microstructure was studied by Hg porosimetry, N adsorption at 77 K, and scanning electron microscopy. Decomposition was assessed by measuring residual C and N, the amount of humified material, the relative contents of humic (HA) and fulvic (FA) acids, and their molecular weight distribution. The structural organization at the end of the experiments was different for the two clays, and new classes of pores were found as the result of the microbial metabolism. The decomposition was also significantly affected by clay type. Decomposition of the organic matter (OM) was rapid in the montmorillonite. The humified material occured mainly as low molecular weight FAs. By contrast, transformation was slow in kaolinite, leaving much residual C, and more of the humified material consisted of HAs. Alternate D–W had little effect on microstructure and decomposition. Samples that were kept moist and amended with chestnut leaves produced the greatest amount of humified substances. The dynamics of the decomposition was significantly affected by the nature of the leaves as indicated by the differences in residual C, humified material and C/N ratio. By contrast, changes in pore size distribution (PSD) were litle influenced by the type of organic debris.

Effect of some physical parameters on the swelling properties of cross-linked amylose matrices

The influence of compression force (CF) on initial tablet porosity and water uptake kinetics in cross-linked amylose (CLA) matrices was determined by mercury intrusion and gravimetry, respectively. Major changes in tablet porosity and pore size distribution occurred in the CF range of 1–5 T with little or no effect on the gelation properties of CLA matrices. Compression properties of CLA tablets were also characterized by electron scanning microscopy. The presence of macrovoids within micro- or non-porous regions was observed. This may reflect a certain fragmentation taking place in addition to plastic deformation during compression. The effect of pH, ionic strength and osmolarity of the medium on the uptake properties of CLA tablets was also evaluated. Although the swelling profiles of CLA matrices were found to be independent of pH (1.4–8) and of medium osmolarity (0–0.1 M sucrose solutions), a significant drop in the swelling capacity of tablets was observed with increased NaCl concentration. This decrease—not related to ionic interactions—may be due to changes in the gelation properties of the polymer.