Distribution Of Macropores Research Articles

Distribution of Macropores in Silica Particles Prepared by Using Multiple Emulsions

The distribution of macropores in silica particles prepared by the hydrolysis and condensation of TEOS in a hexane/water/decyl alcohol (O 1/W/O 2) multiple emulsion was investigated. To stabilize the emulsion structure, hydroxypropyl cellulose (HPC) was added into the O 2 phase and polyethylene glycol (PEG) was added into the water phase. Without HPC, the particles have an irregular shape and hardly have particulate forms. As the concentration of HPC increases, the shape of particles becomes more and more spherical and the size decreases. The size of silica particles was varied from 5 to 1 μm as the concentration of HPC increased from 0.5 to 0.7 wt%. The number and size of the macropores in silica particles were affected by PEG polymer concentration. With the variation in the concentration of PEG, macropores in silica particles were located at the surface of or inside the particles. At high concentrations of PEG, the macropores in particles were located inside the particles, but at low concentrations of PEG the macropores were located at the surfaces of particles. Interestingly, the particles of dimpled surfaces were formed when the molar ratio of water to TEOS ( R w) was 4.0 and the concentrations of PEG and HPC were 2.0 and 0.7 wt% respectively. The surface areas of dimpled silica particles and completely spherical particles, measured by the BET method, were 409 and 433 m 2/g respectively.

Cadmium distribution coefficeints and Cd transport in structured soils

In the case of cadmium transport via soil macropores, the short-term duration of an interaction between the reactive solute in aqueous phase and soil, as well as cadmium precipitation or adsorption on particles < 10–5 m should be taken into account. Two distribution coefficients are proposed for predicting the cadmium transport in a structured soil: the matrix distribution coefficient Kdm, equal to the equilibrium distribution coefficient Kdeq and estimated using the conventional batch technique, and the macropore distribution coefficient KdM, estimated using the modified batch technique. It was found that the conventional approach (using the coefficient Kdeq only) would underestimate a penetration of the part of Cd transported in the macropores about 255-times in the loamy-sand soil in Kalinkovo, 20-times in the loam soil in Macov, and 122-times in the clay soil in Jurová in comparison with the approach proposed in this study.

Phase Separation in Alkylene-Bridged Polysilsesquioxane Sol-Gel Systems

AbstractPolymerization-induced phase separations and concurrent sol-gel transitions in the hydrolysis and polycondensation of alkylene-bridged polysilsesquioxane system resulted in the formation of co-continuous macroporous gels. Gel morphology with co-continuous macropores and micrometer-sized gel skeletons was obtained in the composition region containing much higher amount of water compared with the case with other alkyltrialkoxysilane sol-gel systems. A narrow pore size distribution and high volume fraction of macropores were obtained in the median size range from few micrometers down to 0.1μm.

A model for simulating the influence of a spatial distribution of large circular macropores on surface runoff

This paper reports the development and test, at the scale of 1 m2, of an event‐ based model that aims at simulating the influence of a spatial distribution of large circular macropores on surface runoff. The main originality of this model is that it focuses on the way macropores are supplied with water at the soil surface, by coupling an original model for water interception by individual macropores to a high‐resolution spatialized overland flow model. A three‐step evaluation of the model was carried out, involving (1) an experimental test of the model for water interception by macropores; (2) a sensitivity analysis of the model to time and space discretization; and (3) a comparison between numerical and field results in the case of runoff on a crusted soil surface with a population of large macropores made by termites in the Sahel. The model was found to accurately simulate the effect of a spatial distribution of large macropores on runoff, and it showed that small heterogeneities, like macropores or areas where a crust has been destroyed, which cover a very limited proportion of the soil surface, can have a high impact on runoff.

A new technique for preparing ceramics for catalyst support exhibiting high porosity and high heat resistance

A new technique for preparing magnesia ceramics of high porosity and high temperature resistance has been developed. Spray freeze drying of magnesium sulfate aqueous solution produced fine salt particles having open pores due to sublimation of ice crystals. The particles were calcined to porous magnesium oxide and formed a green body. Highly porous magnesia was produced by firing the green body. The porous magnesia exhibited a bimodal pore size distribution of macro-pores of micron order and meso-pores smaller than 100nm. Porosity was 87–90%. After addition of an aluminum additive with an amount 3–5mol%, the magnesia exhibited high heat resistance; surface area was greater than 20m2g−1 after 20h exposure in a 1573K oven. Thus, the porous magnesia is expected to be very suitable for combustion catalyst support used in a high temperature environment.

Fractures and Biopores in Weichselian Clayey Till Aquitards at Flakkebjerg, Denmark

The distribution of fractures and biopores in a clayey and sandy till aquitard was investigated in two excavations, on southern Zealand, Denmark. A large number of fracture data were measured and quantitative fracture properties calculated. The formation of fractures can be related to the depositional history of the tills during two glacier-advances/retreats and subsequent climatic changes during the Quaternary. The fractures including faults are separated into four systems: 1) Large vertical/subvertical fractures related to the oldest glacier-advance or interstadial climatic influence. 2) Horizontal -subhorizontal fractures related to subglacial drag/shear during a glacial advance from northeast and in an initial stage of a younger glacier-advance from southeast. 3) Normal faults related to subglacial loading during a late stage of the youngest glacier-advance and, 4) Minor desiccation and freeze-thaw fractures related to post-glacial climatic influence. A conceptual macropore distribution model was developed that consists of three zones with different macropore characteristics.

Denitrification in pasture and cropped soil clods as affected by pore space structure

To assess the influence of the pore space structure and organic matter on denitrification, a comparative study was performed on clods in a soil under cropped and pasture managements. For each management, the potential denitrification rate was estimated. Denitrification under oxic conditions was also measured on 100 clods, which were saturated with KNO 3 solution (4 g l −1). Size and density fractions of the soil were separated, and the C and N contents of their particulate organic matter were determined. Clod porosities were measured and the distributions of distances of any point within the clod to the nearest air-filled pore were estimated on 20 thin sections for each soil management. Potential denitrification rates were similar (105×10 −11 and 98×10 −11 mol N 2O kg −1 dry soil s −1 for pasture and cropped soil, respectively). The mean denitrification rate under oxic conditions was only equal to 0.14% of the potential denitrification rate for pasture, whereas it was about 2.1% for cropped soil. The total and soluble organic C content was significantly higher in pasture than in cropped soil clods. The quantity or the quality of organic matter fractions did not explain the difference in denitrification activities. Even if macroporosity represented a small fraction of the total porosity in both soils, the differences in macropore distribution induced by soil management practices led to significantly different maximal distances between any clod point and the nearest air-filled pore (8 and 14 mm for pasture and cropped soils, respectively). Consequently, we demonstrated that the pore space structure appears to be the major factor explaining the difference in mean denitrification rates between pasture and cropped soil clods, while the distribution of particulate organic matter is suspected to be involved in the differences in denitrifying activity distribution between the clods of the two soils.

Pd/SiO2-Cogelled Aerogel Catalysts and Impregnated Aerogel and Xerogel Catalysts: Synthesis and Characterization

Pd/SiO 2 aerogel catalysts have been synthesized by cogelation of tetraethoxysilane with complex Pd 2+ [NH 2 -CH 2 -CH 2 -NH-(CH 2 ) 3 -Si(OCH 3 ) 3 ] 2 containing a hydrolysable silyl group. Pd catalysts have also been prepared by impregnation of silica aerogels and xerogels with solutions of this complex or Pd 2+ (NH 3 ) 4 followed by drying under hypercritical conditions or under vacuum. It appears that, in cogelled samples, the Pd 2+ complex acts as a nucleation agent in the formation of silica particles. The resulting catalysts are then composed of completely accessible palladium crystallites with a diameter of about 2.3 nm located inside silica particles with a monodisperse microporous distribution centered on a hydraulic radius of 0.43 nm. The continuous meso- and macropore distribution is located in voids between particles and between aggregates constituted of those particles. In impregnated samples, it appears that Pd particles are located outside SiO 2 particles. The relation between the texture of cogelled and impregnated catalysts and their behavior toward sintering has been studied in detail. In cogelled samples, it has been shown that Pd crystallites cannot migrate because they are trapped in the pores of SiO 2 particles. These catalysts are sinterproof during hypercritical drying. When Pd crystallites are not trapped inside SiO 2 particles, hypercritical drying leads to an extensive sintering of metal particles. During calcination of impregnated catalysts dried under vacuum, sintering occurs to a various extent depending on the support texture. The Pd crystallites size decreases when the pore size distribution is shifted toward the small sizes.

Infiltration of Microporous Activated Charcoal by Pyrolysis of CH4 and Its Effect on Enhancement of Resistance against Oxidation.

Pores within activated charcoal (A.C.) were infiltrated by pyrolitic carbon derived from thermal decomposition of CH4. The infiltrated activated charcoal showed an increase in resistance against oxidation with CO2. To elucidate the mechanism of this effect, the pore size distribution of macropores, mesopores and micropores including submicropores were characterized for the original A.C., infiltrated A.C., oxidized A.C. and A.C. oxidized after infiltration, respectively. It was found that the infiltration decreased the volume and the surface area of pores, particularly the surface area of micropores. When oxidized, the carbon which deposited in macropores is almost burned but that in meso- and micropores is only partly burned; The ratio of increase in surface area of the infiltrated sample is smallest for the micropores. From these experimental results, it is deducible that the deposited carbon in micropores plays the major role for blocking CO2 intruding, thereby the resistance against oxidation is enhanced.

Anion Transport in a Piedmont Ultisol: II. Local-Scale Parameters

Field-scale transport parameters obtained from tile drain data in two large conventionally tilled plots were reported in the first paper in this series. Our objectives in this paper were to determine the relationship between local-scale transport parameters and macropore distributions measured with columns in one of these plots and to compare local-scale transport parameters with the field-scale parameters. Forty undisturbed soil columns (15 cm in diameter, 33-64 cm in length) were taken from Plot 1 described in the first paper. Transport parameters were determined from a Br breakthrough curve (BTC) with CXTFIT. Macropore distributions were determined from computed tomography (CT) scanning and by sectioning the columns after adding methylene blue dye. Local-scale saturated hydraulic conductivity (K s ), pore water velocity (V), and dispersivity (λ) were highly variable (coefficient of variation of 77, 74, and 41%, respectively) and lognormally distributed. The K s was positively correlated with the number of 5- to 10-mm-diameter CT scan macropores at a depth of 35 to 40 cm near the top of the Bt horizon. Dye patterns indicated that preferential flow began at about this depth. This may be the zone where flow becomes restricted and highly dependent on macropores. Mean local-scale λ (6.6 cm) was similar to the estimated field-scale values (3.4 and 5.3 cm) for the unsaturated zone to a depth of 20 cm. This implied that variation among local-scale V in the field experiment was much less than that observed in the columns, probably because the surface was not ponded in the field. A deterministic approach with mean values of local-scale parameters adequately described the estimated field-scale BTC.

Properties of Porous Carbons Prepared from the Wood of Japanese Cypress

In order to obtain basic data for manufacture of porous carbon materials from various woody wastes, porous carbons were prepared from wood of the Japanese cypress by carbonizing at 900°C and the effect of holding time at 900°C on the properties of the carbons obtained were examined.A slight amount of air entered the vessel for carbonization during the heat treatment at 900°C and an activation reaction occurred and weight loss of the carbon was observed. The weight loss corresponded with a formation of micropores in the carbon. The pore-size distribution curve in macropore and mesopore ranges of the carbons was similar to that of a commercial activated carbon. An increase of holding time at 900°C caused a development of mesopores. In macropore range, an increase of holding time at 900°C caused a lowering of the height of peak at 500nm radius and an increase of the height of peak at 1500nm radius. The change of macropore distribution was not appreciably followed by weight change. Porous carbon with similar adsorptive capacity as commercial activated carbon could be prepared by prolonged heating at 900°C.

Pedo-hydrological and sediment responses to simulated rainfall on soils of the Konya uplands (Turkey)

Microplot experiments using a portable rainfall simulator were carried out in April 1992 on 15 Turkish soils with textures ranging from clay loam to loamy clay and slopes from 3° to 12°. In order to determine the effects of a change in land use on infiltration and erosion, pairs of sites with soils developed on identical parent material were chosen. Several physical and chemical properties of the soil were determined and related by regression analysis to various soil loss and infiltration variables. The regression equations showed that plant cover, and therefore the type of land use, was the most important factor. Plant cover influenced various important soil characteristics which determine the infiltration behaviour of the soil, namely humus, root content and macropore distribution. The slope, physical, textural, and chemical properties of the soils and aggregate stability were only of secondary importance. The shear vane test was not well-suited for measuring the surface properties of these soils, which were initially cohesionless and consisted of loose aggregates. Erosion damage mapping showed high rill erosion rates after snow melting and spring rainfalls on tilled and seedbed sites, up and down hill culture, slopes with gradients > 8 degrees, and on northwest exposed slopes. Results of the microplot tests and erosion damage mapping were in general qualitative agreement. On the catchment scale, the variations of the mesoclimate due to exposure and altitude became important.

Assessment of in situ permeability with emphasis on packer testing in Kuwait

This paper discusses in situ permeability testing performed at two sites in bore holes drilled in an extensive hydrogeological exploration program in Kuwait. The testing program, comprising packer, falling and rising head tests, was carried out in calcareous and variedly textured cemented sand formed during post-depositional diagenetic processes with the colloquial name ‘Gatch’. The program was undertaken to assist in the design and construction of a pilot drainage scheme aimed at lowering of the groundwater table in Kuwait City and suburbs. The results indicated that ‘Gatch’ is an almost impermeable soil. However, the hydraulic conductivity of the subsoils at the investigated sites is significantly influenced by the distribution and volume of macropores. These permeable zones act as preferential pathways for the movement of water bypassing the fine matrix and, thereby, improve markedly the overall drainage characteristics of the sites. Various in situ test methods, including packer, compare results satisfactorily, to give the sites a variable hydraulic conductivity rating that straddles between very low and medium. The packer device, by faring well against other routine in situ tests, shows promises for use as a tool in the assessment of drainage parameters of cemented soils. Laboratory tests, performed in parallel, proved inadequate to characterize the permeability properties of the subsoils in Kuwait.

Monodisperse polymer beads as packing material for high‐performance liquid chromotography: Effect of divinylbenzene content on the porous and chromatographic properties of poly(styrene‐co‐divinylbenzene) beads prepared in presence of linear polystyrene as a porogen

AbstractThe effect of concentration of divinylbenzene on pore size distribution and surface areas of micropores, mesopores, and macropores in uniformly sized porous poly(styrene‐co‐divinylbenzene) beads prepared in the presence of linear polystyrene as a component of the porogenic mixture has been studied. While the total specific surface area was clearly determined by the content of divinylbenzene, the sum of pore volumes for mesopores and macropores as well as their size distribution does not change within a broad range of DVB concentrations. Consequently, the size exclusion chromatography calibration curves are almost identical for all the beads prepared with different percentages of crosslinking monomer. However, the more crosslinked beads have better mechanical and hydrodynamic properties. © 1994 John Wiley & Sons, Inc.

Earthworm Macropores and Preferential Transport in a Long‐Term Manure Applied Typic Hapludalf

AbstractDeep burrowing earthworm species have been found to be present in soils with a history of manure application. This study was designed to quantify the effects of long‐term application of liquid dairy manure and inorganic fertilizer on the distribution of earthworm macropores and in turn on the preferential transport of water and tracer through a typical soil of the karst area of the upper mid‐western USA. Large (≈ 30 cm diam. by 90 cm long) undisturbed soil columns were taken from plots where liquid dairy manure or inorganic fertilizers had been applied continuously for 8 yr. The number and size distribution of macropores in soil columns were nearly the same for both inorganic and manure treatments, however, visible surface macropores were continuous to much deeper depth in soil columns taken from manure than from the inorganic fertilizer plot. Identification of the earthworms a year later showed the presence of Apporectodea tuberculata, A. trapezoides, and Lumbricus rubellus, subsurface burrowers, as well as, L. terristris, a deeper burrowing species in the manure applied plot. Apporectodea tuberculata was the only species present in the inorganic fertilizer plot. Number of macropores and macroporosity varied with soil depth. The maximum macroporosity was <2.5% and it occurred at 2‐cm depth. The predominant macropore sizes were between 1‐ and 2‐mm radii for both treatments. During breakthrough experiments, Cl− appeared earlier in soil columns taken from the manure plot thereby indicating a greater continuity of macropores in the manure compared with the inorganic fertilizer treatment. Thearly appearance of Cl− in the manure treatment, however, was much slower than one would expect based on the number of macropores and their continuity estimated from the serial sectioning. This suggests that intrusive serial sectioning and image analysis techniques probably overestimate the continuity of macropores possibly due to vacuuming of the earthworm casts and other debris that plugs the macropore channels. Based on macropore size distribution with depth and related breakthrough curves, it is likely that most existing models of water and contaminant transport that simulate macropore flow, will not accurately predict the transport of water and contaminant because of their assumption that surface visible macropores are continuous to deeper soil depths. Data from this study showed that macropore size distribution could be described by a normal or log‐normal distribution function. These functions in combination with information on continuity and tortuosity of macropores may be sufficient, when used in some current macropore models, to adequately describe the conducting efficiency of macropores in soils.

A Laboratory Analysis of the Effect of Macropores on Solute Transport

AbstractMacropores play an important role in many soils in relation to ground‐water contamination by providing preferential pathways from the root zone to the water table. Surface‐applied agrochemicals may hereby be carried quickly to the shallow ground water. The reduced retention combined with a small contact area between the flowing water and the soil implies that little removal from physical, chemical, and microbiological processes may take place, thus increasing the risk of ground‐water contamination.A laboratory procedure for evaluating the effect of macropores in a given soil is proposed and tested. The procedure involves the following sequence of experiments on two undisturbed soil monoliths: (1) measurements of the distribution of outflow, (2) measurements of breakthrough curves, (3) dye application for visual observation of macropores; and (4) horizontal slicing of the monoliths to measure macropore distribution and continuity of macropores. The experimental procedure is based on well‐documented techniques that combined will provide evidence on the significance of macropores in relation to ground‐water contamination and guidance for selecting the appropriate model for simulating flow and transport at the field in question.The laboratory procedure was tested on two large, undisturbed soil monoliths (30 cm in diameter) removed from the unsaturated zone at a clayey moraine agricultural field in Denmark. This soil was known to have many macropores. The investigations documented the strong influence of these structures on flow and transport suggesting that a traditional flow and transport model would be inadequate for simulating the processes occurring in the field.

An Improved Method for Quantifying Soil Macroporosity

Quantitative information on macroporosity is needed to predict water flow and solute transport in field soils. A method was developed for determining the number, shape, and size distribution of soil macropores. Horizontal serial sections sawed from paraffin-impregnated soil cores were photographed under ultraviolet (UV) light. Anthracene, mixed with the paraffin, fluoresces a bright bluish white under UV light and provides a sharp contrast between the soil matrix and the paraffin-filled pore space. Section photographs were converted to 256 level, grey-scale digital images using a flat-bed scanner. Image processing was used to classify each pixel in a digital image as pore space or soil matrix, to group the pore space pixels into pores, and to measure the area and perimeter of each pore. The method was able to measure pores with an equivalent radius ≥85 µm. Macroporosity in soil cores sampled form a tillage path and from an adjacent, undisturbed (notill) region was quantified. Tillage sections contained, on the average, 9.4 macropores/cm2 with an equivalent macroporosity of 8%, while no-till sections contained 0.8 macropores/cm2 with an equivalent macroporosity of 0.3%. Computed intrinsic permeabilities for tillage sections that included macropore information were significantly larger than values computed using micropore information alone, suggesting that macropores must be included in permeability calculations when the number of macropores is large. The developed method should be useful for quantifying macroporosity in nonskeletal soils.

3-D-electron microscopy configuration of TMOS wet silica gels prepared by the quick-freeze, deep-etching-rotary-replication technique

Silica gel provides a useful medium for crystal growth; solution growth is confined to pores left free by the polymer during its development. All growth steps depend on the gel structure, which is not completely known for crystal growth conditions. Therefore, a three-dimensional (3-D) visualization has been performed for two TMOS aqueous gels, which are rather fragile: the quick-freeze, deep-etching, rotary-replication method has been applied for sample preparation. An original surface labeling technique has been used for surface recognition. The results concern the distribution of macropores that are responsible for crystal nucleation; micropores whose total volume is larger have not been visualized due to the limits of the method. These results are discussed in comparison with previous data performed by light scattering.

Macropore Characterization for Two Tillage Systems Using Resin‐Impregnation Technique

AbstractThis study used resin impregnation and image analysis to characterize macroporosity of a Nicollet loam soil (fine‐loamy, mixed, mesic Aquic Hapludoll) under no‐tillage and conventional tillage. Soil samples (7.5 by 5 by 5 cm) were taken at each 5‐cm depth interval to determine macroporosity after conducting solute‐leaching experiments on undisturbed soil columns. These soil samples were impregnated with either polyester or epoxy resin. Impregnated soil blocks were sectioned in the middle and ground to a smooth finish. Photographic slides were made of the horizontal faces and an automatic image analyzer was used to calculate the percentage of area occupied by macropores, and total perimeter, number, and size‐frequency distribution of macropores. Percentage of area, perimeter, and number of macropores were not statistically different for the two treatments. Macroporosity data obtained from the samples did not support the observations made in the solute‐leaching studies on saturated soil columns (i.e., a greater degree of preferential flow in no‐tillage columns) because the air‐dried samples and the saturated columns had different porosity characteristics and because small, two‐dimensional images were unable to sample any less frequently occurring larger pores. Another source of discrepancy between the results of the two studies may be macropore continuity. Two‐dimensional analysis of porosity images does not provide a measure of pore continuity, which can be a decisive factor in solute transport through soil columns under different tillage systems. Results from the resin‐impregnation study were not consistent with the results of a related field study. Inconsistency between the macroporosity data of these two studies were attributed to: the difference in minimum cut‐off diameter of macropores in the two studies, the difference in sample size, and the difference in moisture status of the field and lab samples.

Measurement and Characterization of Macropores by using AUTOCAD and Automatic Image Analysis

AbstractMacropores (with equivalent diameters greater than 1600 µm) were quantified under no‐till and conventional tillage practices by using AUTOCAD and automatic image analysis. To quantify macropores, undisturbed soil blocks (50 by 50 cm) were impregnated with plaster of paris slurry. After the plaster of paris set, soil layers were taken out in 5‐cm increments to a total depth of 60 cm. At each soil plane pictures were taken by a 35‐mm camera and macropores were traced on transparent acetate sheets. The pictures and transparent sheets were analyzed by using an automated image analyzer and AUTOCAD, respectively, for total number, perimeter, area, and size frequency distribution of macropores. The AUTOCAD method proved to be simpler and better than the image analysis technique. Larger and continuous cracks accounted for greater macropore area in conventional tillage sites than in no‐till sites. On the other hand, more worm holes and root holes accounted for a greater number and larger perimeters of macropores in no‐till sites in comparison to conventional tillage sites.