Changes In Microporosity Research Articles

Facile-Solution-Processed Silicon Nanofibers Formed on Recycled Cotton Nonwovens as Multifunctional Porous Sustainable Materials.

Limited efficiency, lower durability, moisture absorbance, and pest/fungal/bacterial interaction/growth are the major issues relating to porous nonwovens used for acoustic and thermal insulation in buildings. This research investigated porous nonwoven textiles composed of recycled cotton waste (CW) fibers, with a specific emphasis on the above-mentioned problems using the treatment of silicon coating and formation of nanofibers via facile-solution processing. The findings revealed that the use of an economic and eco-friendly superhydrophobic (contact angle higher than 150°) modification of porous nonwovens with silicon nanofibers significantly enhanced their intrinsic characteristics. Notable improvements in their compactness/density and a substantial change in micro porosity were observed after a nanofiber network was formed on the nonwoven material. This optimized sample exhibited a superior performance in terms of stiffness, surpassing the untreated samples by 25-60%. Additionally, an significant enhancement in tear strength was observed, surpassing the untreated samples with an impressive margin of 70-90%. Moreover, the nanofibrous network of silicon fibers on cotton waste (CW) showed significant augmentation in heat resistance ranging from 7% to 24% and remarkable sound absorption capabilities. In terms of sound absorption, the samples exhibited a performance comparable to the commercial standard material and outperformed the untreated samples by 20% to 35%. Enhancing the micro-roughness of fabric via silicon nanofibers induced an efficient resistance to water absorption and led to the development of inherent self-cleaning characteristics. The antibacterial capabilities observed in the optimized sample were due to its superhydrophobic nature. These characteristics suggest that the proposed nano fiber-treated nonwoven fabric is ideal for multifunctional applications, having features like enhanced moisture resistance, pest resistance, thermal insulation, and sound absorption which are essential for wall covers in housing.

Self-regenerative superhydrophobic metallic coatings with enhanced durability

Lotus-inspired superhydrophobic surfaces are highly sought owing to exquisite properties such as de-wetting, self-cleaning, anti-corrosion, anti-fouling. However, their use is limited due to low durability, high processing cost, and scalability issues. In the present work, we used low cost and simple flame spraying for developing self-regenerative superhydrophobic metallic coatings with high durability and corrosion resistance. The aluminum coatings were developed on low-carbon steel at optimized parameters, followed by annealing at different temperatures. The developed coating showed hierarchical structure with nanoscale features superimposed on microscale lamellae. The nano-sponge like morphology obtained through flame spraying after annealing transformed into nano-spheroid with reduced microscale porosity. The fabricated aluminum coatings inherently exhibited superhydrophilicity, which transformed to superhydrophobicity after silanization. The coatings exhibited advancing angles >150° and receding angles >148° with a 5° sliding angle. The change in morphology with annealing showed a significant influence on wettability, adhesion, and electrochemical corrosion. The contact angles decreased slightly with annealing temperature while still in the superhydrophobic regime. The adhesion of the coatings with the water droplet increased from ~2 μN to 7.3 μN with the increase in annealing temperatures while still being less than that of the lotus leaf (18 μN). The electrochemical corrosion resistance of the coatings showed significant improvement after annealing, which is related to the change in microporosity and formation of stable oxides as verified by x-ray photoelectron spectroscopy (XPS). The current density decreased from 0.48 μA/cm2 to 0.06 μA/cm2 due to stable passive layer formation. The coatings showed increased durability against abrasion, and impacting droplets due to improved inter-lamellar cohesion and increased adhesion with the substrate. The present work provide a low cost and facile solution for the development of superhydrophobic coatings for various applications.

Experimental Investigation on the Effect of Electrically Assisted Rapid Heating in Bulk Metal Forming

Many industries utilize hot metal forming to reduce forming forces and material flow issues. In this study, electrically assisted (EA) rapid heating is used in direct comparison to conventional heating methods. Three materials were investigated: AISI 4340, Al-7075-T6, and Ti-6Al-4V. Samples were heated at an approximate rate of 100 °C/s before applying compressive loads, simulating forging or other bulk deformation processes. Afterwards, samples were prepared for further microstructure evaluation. It was observed that high-rate temperature increase resulted in some developments in the microstructure and phases, as seen in AISI 4330, for example. Additionally, the effect of electricity was noticed to have a significant impact on flow stresses, as observed in many of the compression tests, immediately following the rapid heating process. Experiments and images from optical microscopy show that samples experienced drastic and immediate decrease in flow stress at all temperatures when using the EA rapid heating method versus the conventional method with seemingly no change to the material microstructure. This may be explained by the lack of phase changes at lower temperatures combined with the effect of electricity. Further investigation into the changes in micro-porosity, intermetallic phases, and changes in precipitates are suggested to supplement the current understanding of the drastic stress drop caused by passing electrical currents through metals.

A comparative study of micro-CT and mercury intrusion techniques for predicting permeability and surface area evolution during chemical dissolution

In this paper, we report on experimental and computational studies investigating the evolution of pore structure and permeability of a microporous carbonate rock during chemical dissolution using information provided from X-ray micro-computed tomography (µ-CT) and mercury intrusion porosimetry (MIP) techniques. We consider a chemical dissolution in a core sample by a nonacidic solution wherein a quasi-uniform modification of pore structure occurred. First, we conduct a comparative analysis on the capabilities and limitations of the µ-CT and MIP methods to quantify the evolution of pore size and pore surface area. In particular, we incorporate micropores into the calculations of the pore size-related parameters to highlight the uncertainties that ignoring microporosity causes. In the second part of the paper, we present predictive results on the fractional changes in the permeability of the rock using the Katz-Thompson (KT) and Kozeny-Carman (KC) models. The predictions are based on two characteristic parameters of the pore phase, i.e., the critical pore diameter and the specific pore surface area, and two characteristic macro-scale properties, i.e., porosity and formation factor, all calculated from both the MIP and µ-CT methods. The predicted changes in permeability are compared with those calculated directly on the images using the lattice-Boltzmann (LB) flow simulation on segmented images. The effect of changes in microporosity is also assessed to investigate its role in permeability evolution. The results showed that the KT model could reasonably predict the fractional changes in permeability in terms of the pore structure and macroscale parameters calculated from the MIP and µ-CT methods.

Temporal changes in cortical microporosity during estrogen deficiency associated with perilacunar resorption and osteocyte apoptosis: A pilot study

Osteocytes can actively regulate bone microporosity, through either perilacunar resorption or micropetrosis following apoptosis. Osteocyte apoptosis is more prevalent in estrogen deficiency and changes in the lacunar-canalicular network of osteocytes have been reported. Temporal changes in bone mineralisation and osteocytes cellular strains occur, which might be associated with osteocyte-driven microporosity changes, although time dependant changes in bone microporosity are not yet fully understood. In this pilot study we conducted micro-CT analysis, backscatter electron imaging and histological analysis of femoral cortical bone form an ovariectomized rat model of osteoporosis to investigate whether estrogen deficiency causes temporal changes in lacunar and vascular porosity. We also assessed MMP14 expression, lacunar occupancy and mineral infilling, as indicators of perilacunar resorption and micropetrosis. We report temporal changes in cortical microporosity in estrogen deficiency. Specifically, canalicular and vascular porosity initially increased (4 weeks post-OVX), coinciding with the period of rapid bone loss, whereas in the longer term (14 weeks post-OVX) lacunar and canalicular diameter decreased. Interestingly, these changes coincided with an increased prevalence of empty lacunae and osteocyte lacunae were observed to be more circular with a mineralised border around the lacunar space. In addition we report an increase in MMP14+ osteocytes, which also suggests active matrix degradation by these cells. Together these results provide an insight into the temporal changes in cortical microporosity during estrogen deficiency and suggest the likelihood of occurrence of both perilacunar resorption and osteocyte apoptosis leading to micropetrosis. We propose that microporosity changes arise due to processes driven by distinct populations of osteocytes, which are either actively resorbing their matrix or have undergone apoptosis and are infilling lacunae by micropetrosis.

Valorization of Hemp Stalk Waste Through Thermochemical Conversion for Energy and Electrical Applications

The presented research aimed at finding new ways to value hemp by-products (stalks) from the cannabidiol industry through thermochemical conversion. Chemical and elemental composition of hemp biomass was investigated by successive chemical extractions and Scanning Electron Microscopy along with Energy-dispersive X-ray Spectroscopy. Proximate and elemental analyses completed the chemical characterization of the hemp biomass and its biochar. Thermogravimetric analysis of the hemp biomass allowed to understand its kinetic of decomposition during thermal conversion. The carbon structure and porosity of the biochar were assessed by Raman spectroscopy and CO2 gas adsorption. Properties of interest were the energy production measured through calorific values, and the electrical conductivity. Two ways of valorisation of the hemp biomass were clearly identified, depending mainly on the chosen pyrolysis temperature. Hemp biochar carbonized at 400–600°C were classified as lignocellulosic materials with a good potential for solid biofuel applications. Specifically, the resulting carbonized biochar presented low moisture content (below 2.50%) favourable for high fuel quality, low volatile matter (27.1–10.4%) likely to show lower particle matter emissions, limited ash content (6.8–9.8%) resulting in low risk of fouling issues during the combustion, high carbon content (73.8–86.8%) suggesting strong energy density, associated with high higher heating values (28.45–30.95 MJ kg−1). Hemp biochar carbonized at 800–1000 °C displayed interesting electrical conductivity, opening opportunities for its use in electrical purposes. The electrical conductivity was related to the evolution of the biochar microstructure (development of graphite-like structure and changes in microporosity) in regard with the thermochemical conversion process parameters.Graphical abstract

Granite microporosity changes due to fracturing and alteration: secondary mineral phases as proxies for porosity and permeability estimation

Abstract. Several alteration facies of fractured Lipnice granite are studied in detail on borehole samples by means of mercury intrusion porosimetry, polarized and fluorescent light microscopy, and microprobe chemical analyses. The goal is to describe the granite void space geometry in the vicinity of fractures with alteration halos and to link specific geometries with simply detectable parameters to facilitate quick estimation of porosity and permeability based on, for example, drill cuttings. The core of the study is the results of porosity and throat size distribution analyses on 21 specimens representing unique combinations of fracture-related structures within six different alteration facies basically differing in secondary phyllosilicate chemistry and porosity structure. Based on a simple model to calculate permeability from the measured porosities and throat size distributions, the difference in permeability between the fresh granite and the most fractured and altered granite is 5 orders of magnitude. Our observations suggest that the porosity, the size of connections and the proportion of crack porosity increase with fracture density, while precipitation of iron-rich infills as well as of fine-grained secondary phyllosilicates acts in the opposite way. Different styles and intensities of such end-member agents shape the final void space geometry and imply various combinations of storage, transport and retardation capacity for specific structures. This study also shows the possibility to use standard mercury intrusion porosimetry with advanced experimental settings and data treatment to distinguish important differences in void space geometry within a span of a few percent of porosity.

인공강우 시 나지교란사면 토사유출에 미치는 다짐처리의 영향

토양의 다짐은 강우 시 지표유출 및 토사유출에 큰 영향을 끼친다. 다짐은 체적밀도 증가, 전단강도 증가, 공극률 변형, 투수계수 등과 같은 토양특성 변화를 야기하기 때문이다. 본 연구는 인공강우 실험을 활용하여 개발지의 사면조건과 유사한 나지교란사면에서 표면의 다짐처리가 지표유출 및 토사유출에 미치는 영향을 파악하였다. 표면처리(다짐, 비다짐), 강우강도(68.5mm/hr, 95.6mm/hr), 사면경사(<TEX>$5^{\circ}$</TEX>, <TEX>$12.5^{\circ}$</TEX>, <TEX>$20^{\circ}$</TEX>)의 각 조건별 3회 반복하여 총 36회의 강우모의에 따른 지표유출 및 토사유출을 측정하였다. 연구결과, 다짐처리 후 토양의 체적밀도 및 전단강도는 유의적으로 증가하였다. 그러나 이러한 물리적 특성의 변화가 지표유출에 미치는 영향은 강우강도와 사면경사에 따라 다르게 반응하였다. 평균 토사유출량은 강우강도와 사면경사가 증가함에 따라 유의적으로 증가하였다. 또한, 토사유출량은 강우강도와 사면경사 별 다짐처리 유 무에 따라 다른 반응을 보였다. 완경사(<TEX>$5^{\circ}$</TEX>)에서는 다짐처리에서 더 많은 토사가 유출되었으나, 급경사(<TEX>$20^{\circ}$</TEX>)에서의 다짐처리는 토사유출을 감소시키는 역할을 한 것으로 나타났다. 나지교란사면에서는 토사유출에 대한 다짐효과의 천이구간이 존재하는 것으로 파악되며, 본 연구의토양조건 및 강우조건에서 천이구간은 완경사와 급경사 사이로서 사면경사 <TEX>$10{\sim}15^{\circ}$</TEX> 범위에 존재하는 것으로 판단된다. Surface compaction significantly impacts runoff and soil erosion under rainfall since it leads to changes of soil physical characteristics such as increase of bulk density and shear stress, change of microporosity, and decrease of hydraulic conductivity. This study addressed surface compaction effects on runoff and soil loss from bare and disturbed soils that are commonly distributed on construction sites. Thirty-six rainfall simulations from three replicates of each involving rainfall intensities (68.5 mm/hr, 95.6 mm/hr) and plot gradients (<TEX>$5^{\circ}$</TEX>, <TEX>$12.5^{\circ}$</TEX>, <TEX>$20^{\circ}$</TEX>) were conducted to measure runoff and soil loss for two different soil surface treatments (compacted surface, non-compacted surface). Compacted surface increased significantly soil bulk density and soil strength. However, the effect of surface treatments on runoff changed with rainfall intensity and plot gradient. Rainfall intensity and plot gradient had a positive effect on mean soil loss. In addition, the effect of surface treatments on soil loss responded differently with rainfall intensity and plot gradient. Compacted surfaces increased soil loss at gentle slope (<TEX>$5^{\circ}$</TEX>) while they decreased soil loss at steep slope (<TEX>$20^{\circ}$</TEX>). These results indicate that there exists transitional slope range (<TEX>$10{\sim}15^{\circ}$</TEX>) between gentle and steep slope by surface compaction effects on soil loss under disturbed bare soils and simulated rainfalls.

Microbial diversity affects self-organization of the soil–microbe system with consequences for function

Soils are complex ecosystems and the pore-scale physical structure regulates key processes that support terrestrial life. These include maintaining an appropriate mixture of air and water in soil, nutrient cycling and carbon sequestration. There is evidence that this structure is not random, although the organizing mechanism is not known. Using X-ray microtomography and controlled microcosms, we provide evidence that organization of pore-scale structure arises spontaneously out of the interaction between microbial activity, particle aggregation and resource flows in soil. A simple computational model shows that these interactions give rise to self-organization involving both physical particles and microbes that gives soil unique material properties. The consequence of self-organization for the functioning of soil is determined using lattice Boltzmann simulation of fluid flow through the observed structures, and predicts that the resultant micro-structural changes can significantly increase hydraulic conductivity. Manipulation of the diversity of the microbial community reveals a link between the measured change in micro-porosity and the ratio of fungal to bacterial biomass. We suggest that this behaviour may play an important role in the way that soil responds to management and climatic change, but that this capacity for self-organization has limits.

Simulating the changes in carbon structure during the burn-off process

Using a simple energetic criterion, we modelled the process of activation of ‘soft’ activated carbons. Eighteen carbon samples, differing in degree of graphitisation, and obtained using Molecular Dynamics annealing of an amorphous carbon precursor were studied. For all samples, the geometric pore size distribution was calculated using the method proposed by Bhattacharya and Gubbins. Adsorption isotherms for Ar at 87K were simulated and analysed using different approaches widely applied in adsorption science (αs, DA, APD, ND, BET). It is shown that our approach leads to similar changes in microporosity (with the rise in carbon burn-off) to those observed in real experiments. Moreover, the conclusions about the reality of popular methods of carbon porosity characterisation are given.

A study of porosity of synthetic polymer nanoparticles using PALS

Positron annihilation lifetime spectroscopy (PALS) has been used to study the free volume in dry synthetic polymer nanoparticles of various sizes. A series of poly(styrene/divinyl benzene) particles with diameters in the range of 100 to 500 nm were synthesized and then carefully chemically treated using the sulfonation process, to increase their porosity. The particles were characterised by Scanning Electron Microscopy (SEM), light scattering and PALS. Light scattering gave larger size for the treated particles, reflecting the hydration effect and therefore the increase in porosity. PALS spectra of untreated and treated particles gave four and three life-time components, respectively. Analysis by PAScual version 1.3.0 program indicated there was a reduction in the intensity and the type of the micropores in the treated particles. The data suggest PALS is a sensitive tool for detecting changes in microporosity in particles. The conflicting results obtained for light scattering compared to PALS for chemically treated particles is difficult to resolve and suggests sample preparation of polymeric materials for PALS is the critical factor.

Soil degradation by land use change in an agropastoral area in Sardinia (Italy)

In recent decades the clearing of Mediterranean maquis along with the creation of new pastures has been a major factor of land degradation in Sardinia (Italy). This was due to an inadequate implementation of agricultural policies. Consequently, tillage and water erosion intensified over a wide area. The present work assesses the impacts of land use change on soil properties in a representative area of central-eastern Sardinia. Paired forest and pasture sampling sites were selected in relation to present land use, land suitability, and land use history. Different soil properties were considered: physical (sand, silt, clay, soil thickness, bulk density, and penetration resistance), chemical (pH, OC, N, C/N, Ca, Mg, Na, K, CEC, and BS), biological (BQI), and micromorphological (microporosity and microstructure). The comparison of forest and pasture soils showed a significant soil loss (in terms of soil thickness, −22%), and a clear decrease in organic carbon storage (−64% on average). An increase in bulk density (+ 44%) and a change in microporosity and its vertical distribution were also observed, respectively by field measurements and micromorphological quantification on digital images.

Regeneration of activated carbons saturated with benzene or toluene using an oxygen-containing atmosphere

Regeneration of activated carbons (ACs) after their use for benzene and toluene removal at low concentration was studied in an oxygen-containing atmosphere, analysing the effect of porosity of the adsorbent. Attention was paid to the effect of regeneration cycles on the adsorption capacity and surface chemistry of the AC. Results show that ACs with large mesopore volumes, together with wide micropore size distributions, are desired to achieve complete regeneration at low temperatures (250–300 °C). The effect of successive regeneration cycles under an oxygen-containing atmosphere indicates that 250 °C is suitable for regeneration of most benzene-exhausted adsorbents, whereas for toluene the process requires higher temperatures (300–350 °C). The experiments show that in the case of benzene, regeneration efficiencies remain essentially constant with the number of cycles and the achieved regeneration efficiencies are close to 100%. The slight differences in regeneration efficiencies can be explained considering changes in microporosity and surface chemistry of the AC with the cycles. For regeneration of toluene-saturated AC, the regeneration percentage only remains constant for one AC, whereas some decrease is observed for the rest. This decrease can be explained, in general, by a reduction in the narrow micropore volume with cycling.

Effect of ozone on the structure and physicochemical properties of ultradisperse diamond and shungite nanocarbon elements

A comparative study of the effect of ozonization on the physicochemical properties of naturally and synthetically generated nanocarbon materials, shungite carbon (ShC) and ultra-disperse diamond (UDD) after special treatment in the form of monodisperse single nano-diamond particulates (mdsn-D), that have a similar two-level structural pattern (hierarchical system structure) and curved graphene shells or their fragments contribute to the formation of the structure and surface properties of both ShC and mdsn-D particles, was conducted. The ozonization kinetics of ShC and mdsn-D showed their high catalytic activity during ozone decomposition. Upon ozonization, the graphene shell is removed selectively from the surface of the diamond core of mdsn-D, as shown by Fourier transform-infrared (FT-IR) spectroscopy and derivatography data. A distinctive characteristic of ShC is a substantial change in structural parameters upon ozonization: structural porosity increases and the size of coherent scatter domains decreases in the direction perpendicular to graphene layers [as shown by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) data], which agrees with adsorption experiments that showed an increase in ultramicroporosity and mesoporosity upon ozonization of ShC. The molecular probe technique was used to monitor changes in microporosity of ShC and mdsn-D.

CO 2 capture from syngas via cyclic carbonation/calcination for a naturally occurring limestone: Modelling and bench-scale testing

The intrinsic rate constants of the CaO – CO 2 reaction, in the presence of syngas, were studied using a grain model for a naturally occurring calcium oxide-based sorbent using a thermogravimetric analyzer. Over temperatures ranging from 580 to 700 °C, it was observed that the presence of CO and H 2 (with steam) during carbonation caused a significant increase in the initial rate of carbonation, which has been attributed to the CaO surface sites catalyzing the water–gas shift reaction, increasing the local CO 2 concentration. The water–gas shift reaction was assumed to be responsible for the increase in activation energy from 29.7 to 60.3 kJ/mol for limestone based on the formation of intermediate complexes. Changes in microporosity due to particle sintering during calcination have been credited with the rapid initial decrease in cyclic CaO maximum conversion for limestone particles, whereas the presence of steam during carbonation has been shown to improve the long-term maximum conversion in comparison to previous studies without steam present.

The influence of SiMn slag on chemical resistance of blended cement pastes

During the last decades, industrial by-products and wastes recycling as complementary building materials is focusing a large number of researching studies, most of them dedicated to the cement and concrete production, studying technical, economical and environmental aspects. This paper presents an original contribution to this tendency, which consists on the possibility of recycling a SiMn slag from ferroalloys industry as a complementary construction material and its influence on the resistance of cement paste in different aggressive solutions. Different parameters, such as waste nature, additions, mechanical resistance, different aggressive solutions or morphological and microporosity changes were investigated. The results have showed that blended cement pastes elaborated with SiMn slag (5% and 15% additions) after 56 days of curing have a good resistance in some aggressive solutions, no losing weight and presenting an excellent resistance index compared to the reference matrix. Aggressive solutions presence do not alter elaborated pastes microporosity not minding if they are prepared with and without SiMn slag additions in the working conditions studied in this study.

Change in Microporosity of Granitic Building Stones upon Consolidation Treatments

The microporosity of granite stones has been studied in both the untreated condition as well as after being treated with a consolidation treatment using two commercial products. A decrease in the specific surface area and pore size distribution (especially micropores) has been found in samples treated with an organic silicate consolidant. Treatment with a consolidant containing polysiloxane (a water repellent agent) leads to variable results, depending on the precise pore structure of the untreated stone. The use of consolidation treatments may restrict flow of fluids (e.g., water, saline solutions, etc.) through the pores of the stones, which may decrease degradation.

Hydraulic conductivity of a soil irrigated with treated sewage effluent

Effluent disposal on soil solves the immediate problem of contamination caused by direct discharge into rivers; nevertheless, other problems may appear if no agricultural management is applied. Among the current concerns about effluent reuse in agriculture, the most critical one regards the charge of soil physical properties, particularly of the hydraulic conductivity. This paper aims to determine the hydraulic conductivity in an Oxisol area irrigated with treated domestic sewage effluent (Effluent treatment) and with treated groundwater (Water treatment), during 24 months, by comparing the results with those of an area under natural conditions (Control treatment). The unsaturated soil hydraulic conductivity K was measured in the experimental area using the instantaneous profile method from 12.5 to the 187.5 cm deep (every 25 cm). The determination of the saturated hydraulic conductivity K sat was carried out in laboratory using the falling head method. The results showed that the soil hydraulic conductivity changed in both areas, i.e., the Water and Effluent treatments, when compared to the Control area. Irrigation with treated water and with effluent showed similar changes in the K(θ) function, due to a high Na content in both irrigation waters. The irrigated areas had a significant decrease of K(θ) at high water contents compared to the Control treatment; a significant decrease of K sat was also observed in the irrigated areas. K(θ) increased in the irrigated soil at low water contents, suggesting soil structure and macro- and microporosity changes. Changes in the hydraulic conductivity were more pronounced in subsurface soil layers (to about the 1-m depth), which could be a consequence of the concentration of sodium in the leaching fraction and its subsequent increasingly damaging effect in the soil subsurface.

Citrate impairs the micropore diffusion of phosphate into pure and C-coated goethite

Anions of polycarboxylic low-molecular-weight organic acids (LMWOA) compete with phosphate for sorption sites of hydrous Fe and Al oxides. To test whether the sorption of LMWOA anions decreases the accessibility of micropores (<2 nm) of goethite (α-FeOOH) for phosphate, we studied the kinetics of citrate-induced changes in microporosity and the phosphate sorption kinetics of synthetic goethite in the presence and absence of citrate in batch systems for 3 weeks (500 μM of each ion, pH 5). We also used C-coated goethite obtained after sorption of dissolved organic matter in order to simulate organic coatings in the soil. We analyzed our samples with N 2 adsorption and electrophoretic mobility measurements. Citrate clogged the micropores of both adsorbents by up to 13% within 1 h of contact. The micropore volume decreased with increasing concentration and residence time of citrate. In the absence of citrate, phosphate diffused into micropores of the pure and C-coated goethite. The C coating (5.6 μmol C m −2) did not impair the intraparticle diffusion of phosphate. In the presence of citrate, the diffusion of phosphate into the micropores of both adsorbents was strongly impaired. We attribute this to the micropore clogging and the ligand-induced dissolution of goethite by citrate. While the diffusion limitation of phosphate by citrate was stronger when citrate was added before phosphate to pure goethite, the order of addition of both ions to C-coated goethite had only a minor effect on the intraparticle diffusion of phosphate. Micropore clogging and dissolution of microporous hydrous Fe and Al oxides may be regarded as potential strategies of plants to cope with phosphate deficiency in addition to ligand-exchange.

Thermal aging of silica gels in water

The classical synthesis route for highly porous silica aerogels comprises supercritical drying (SCD) to prevent the collapse of the delicate gel backbone upon the extraction of the liquid phase. Although the capillary forces approach zero at supercritical conditions, shrinkage on the order of 20–30vol.% is still observed by different authors. Heat treatment of silica gels in water has been shown previously to increase the mechanical stability of the aerogel backbone. The detailed investigation of the effects that thermal aging in water has on the structural and mechanical characteristics of the silica aerogel and its precursor demonstrates that the change in microporosity and surface groups is a key factor for drying shrinkage upon supercritical extraction.