Effective Diffusion Path Length Research Articles

Applicability of the microdialysis technique in dry soils: Impact of soil water content depends on perfusion flow rate

Plant nutrient acquisition largely depends on the availability and constant replenishment of nutrients at root surfaces. The main process providing nutrients for root uptake is diffusion which is critically affected by soil physicochemical and biological factors. The effective diffusion path length increases with decreasing soil water content thus limiting diffusive nutrient supply in dry soils. Microdialysis, an increasingly popular sampling tool for monitoring nutrient fluxes in soil in situ, could be used to measure such a limitation provided it operates reliably under dry soil conditions. However, due to its mode of operation, the performance of microdialysis could be impeded in dry soils since a significant amount of perfusate might be lost via leaching to the surrounding soil. Until now, however, no data is available on the applicability of microdialysis in dry soils. In this project, we estimated leaching losses of perfusate to eight soils with different texture maintained at 10, 20, 30, 50 and 80% (w/w) of maximum water-holding capacity in dependence of perfusion flow rate. We found that microdialysis operates reliably even in dry soils when using a high perfusion flow rate (10 μl min−1). At lower perfusion flow rates (1 or 5 μl min−1), however, significant amounts of perfusate were leaching out to soils. Diffusive fluxes of nitrate and ammonium were strongly affected by soil water content, but not fluxes of phosphate. Even at the highest flow rate (where no losses of perfusate where observed) concentrations of both inorganic N forms in dialysates decreased with decreasing soil water content. Overall, we conclude that microdialysis can be applied in dry soils without high risk of perfusate leaching when a high perfusion flow rate (10 μl min−1) is used. Further, our results highlight that microdialysis can be used in much dryer soils than previously suggested (70% MWHC) and allows monitoring the effect of soil water content on nutrient supply for root uptake via diffusion.

Improvement of Gas Barrier Properties for Biodegradable Poly(butylene adipate-co-terephthalate) Nanocomposites with MXene Nanosheets via Biaxial Stretching.

In order to ease the white pollution problem, biodegradable packaging materials are highly demanded. In this work, the biodegradable poly (butylene adipate-co-terephthalate)/MXene (PBAT/Ti3C2TX) composite casting films were fabricated by melt mixing. Then, the obtained PBAT/Ti3C2TX composite casting films were biaxially stretched at different stretching ratios so as to reduce the water vapor permeability rate (WVPR) and oxygen transmission rate (OTR). It was expected that the combination of Ti3C2TX nanosheets and biaxial stretching could improve the water vapor and oxygen barrier performance of PBAT films. The scanning electron microscope (SEM) observation showed that the Ti3C2TX nanosheets had good compatibility with the PBAT matrix. The presence of Ti3C2TX acted as a nucleating agent to promote the crystallinity when the content was lower than 2 wt%. The mechanical tests showed that the incorporation of 1.0 wt% Ti3C2TX improved the tensile stress, elongation at break, and Young’s modulus of the PBAT/Ti3C2TX nanocomposite simultaneously, as compared with those of pure PBAT. The mechanical dynamical tests showed that the presence of Ti3C2TX significantly improved the storage modulus of the PBAT nanocomposite in a glassy state. Compared with pure PBAT, PBAT-1.0 with 1.0 wt% Ti3C2TX exhibited the lowest OTR of 782 cc/m2·day and 10.2 g/m2·day. The enhancement in gas barrier properties can be attributed to the presence of Ti3C2TX nanosheets, which can increase the effective diffusion path length for gases. With the biaxial stretching, the OTR and WVPR of PBAT-1.0 were further reduced to 732 cc/m2·day and 6.5 g/m2·day, respectively. The PBAT composite films with enhanced water vapor and water barrier performance exhibit a potential application in green packaging.

Tailoring mesoscopically structured H-ZSM5 zeolites for toluene methylation

Mesoscopically structured zeolites based on H-ZSM5 were designed and synthesized as highly active and shape selective catalysts for methylation of toluene by tuning diffusion and acid site concentration of the catalysts. This was achieved by combining desilication, subsequent dealumination and chemical deposition of a mesoporous SiO2 overlayer of several nanometer thickness. The decreasing effective diffusion length in zeolite crystals achieved by desilication and dealumination increased the turnover rate of toluene by favoring activation of methanol and facilitating desorption of the produced xylenes, albeit with some loss in p-xylene selectivity. The presence of the SiO2 overlayer increased the p-xylene selectivity by enhancing the tortuosity of the zeolite, randomly blocking pore openings at the surface, and increasing the effective diffusion path length. The final material combines the higher catalyst utilization with enhanced selectivity leading to rates comparable to the parent zeolite, but at significantly higher selectivity.

Evaluation of MD process performance: Effect of backing structures and membrane properties under different operating conditions

The paper presents investigations on the process performance of different membranes with and without backing structure in direct contact membrane distillation. Influences of backing structures and their orientation are identified. An integrated membrane and backing model was developed. Laminates with different backing designs were examined over a wide range of operating conditions and compared to results for the same membranes without backing. Experimental results and model predictions for flux and thermal efficiency are compared. The model predictions agree well with the experimental results. An influence on the effective area for diffusion, an increase in effective diffusion path length in the membrane and the formation of a complex network of thermal resistances are considered to be the main effects, leading to a significant reduction in process performance due to backing structures. The backing was identified to be one of the key components for further improvement in MD applications. An integrated assessment of membrane properties in combination with backing structures was carried out by analysing membranes with different thicknesses and different pore sizes. The experimental observations show good agreement with theoretical considerations. A combined analysis of specific flux and thermal efficiency leads to comprehensive understanding of the process and its potential for optimisation.

Chemical enhancer induced changes in the mechanisms of transdermal delivery of zinc oxide nanoparticles

The overlapping wavelength of photoluminescence (PL) of zinc oxide nanoparticles (ZnO NPs) and autofluorescence (AF) from the stratum corneum (SC) has for a long time held back researchers from investigating the chemically enhanced penetration pathways of ZnO NPs into the SC lipids. However, the non-linear polarization effect of second harmonic generation (SHG) may be used for ZnO NPs to be distinguished from the AF of the SC. This study combined the SHG of ZnO NPs and the AF of the SC to image the transdermal delivery of ZnO NPs under the chemical enhancer conditions of oleic acid (OA), ethanol (EtOH) and oleic acid–ethanol (OA–EtOH). In addition to qualitative imaging, the microtransport properties of ZnO NPs were quantified to give the enhancements of the vehicle-to-skin partition coefficient ( K), the SHG intensity gradient ( G) and the effective diffusion path length ( L). The results showed that OA, EtOH and OA–EtOH were all capable of enhancing the transdermal delivery of ZnO NPs by increasing the intercellular lipid fluidity or extracting lipids from the SC.

First‐principles, structure‐based transdermal transport model to evaluate lipid partition and diffusion coefficients of hydrophobic permeants solely from stratum corneum permeation experiments

To account for the effect of branched, parallel transport pathways in the intercellular domain of the stratum corneum (SC) on the passive transdermal transport of hydrophobic permeants, we have developed, from first-principles, a new theoretical model-the Two-Tortuosity Model. This new model requires two tortuosity factors to account for: (1) the effective diffusion path length, and (2) the total volume of the branched, parallel transport pathways present in the SC intercellular domain, both of which may be evaluated from known values of the SC structure. After validating the Two-Tortuosity model with simulated SC diffusion experiments in FEMLAB (a finite element software package), the vehicle-bilayer partition coefficient, K(b), and the lipid bilayer diffusion coefficient, D(b), in untreated human SC were evaluated using this new model for two hydrophobic permeants, naphthol (K(b) = 225 +/- 42, D(b) = 1.7 x 10(-7) +/- 0.3 x 10(-7) cm(2)/s) and testosterone (K(b) = 92 +/- 29, D(b) = 1.9 x 10(-8) +/- 0.5 x 10(-8) cm(2)/s). The results presented in this paper demonstrate that this new method to evaluate K(b) and D(b) is comparable to, and simpler than, previous methods, in which SC permeation experiments were combined with octanol-water partition experiments, or with SC solute release experiments, to evaluate K(b) and D(b).

Accounting for uncertainty in pedotransfer functions in vulnerability assessments of pesticide leaching to groundwater

A simulation tool for site-specific vulnerability assessments of pesticide leaching to groundwater was developed, based on the pesticide fate and transport model MACRO, parameterized using pedotransfer functions and reasonable worst-case parameter values. The effects of uncertainty in the pedotransfer functions on simulation results were examined for 48 combinations of soils, pesticides and application timings, by sampling pedotransfer function regression errors and propagating them through the simulation model in a Monte Carlo analysis. An uncertainty factor, f(u), was derived, defined as the ratio between the concentration simulated with no errors, c(sim), and the 80th percentile concentration for the scenario. The pedotransfer function errors caused a large variation in simulation results, with f(u) ranging from 1.14 to 1440, with a median of 2.8. A non-linear relationship was found between f(u) and c(sim), which can be used to account for parameter uncertainty by correcting the simulated concentration, c(sim), to an estimated 80th percentile value. For fine-textured soils, the predictions were most sensitive to errors in the pedotransfer functions for two parameters regulating macropore flow (the saturated matrix hydraulic conductivity, K(b), and the effective diffusion pathlength, d) and two water retention function parameters (van Genuchten's N and alpha parameters). For coarse-textured soils, the model was also sensitive to errors in the exponent in the degradation water response function and the dispersivity, in addition to K(b), but showed little sensitivity to d. To reduce uncertainty in model predictions, improved pedotransfer functions for K(b), d, N and alpha would therefore be most useful.

Evaluation of MACRO model by short-term water and solute transport simulation in Maury silt loam soil

Modeling preferential flow has been a concern of many academic fields in the past 30 years all over the world and helps to prevent groundwater contamination. A dual-porosity model, MACRO, was evaluated for short-term (less than 2 days) simulation of water flow and non-reactive solute (chloride) transport through the profile of six plots in well-structured Maury silt loam soil. Water flow in micropores is calculated by the Richards’ equation while simple gravity flow is assumed in the macropores. Solute transport in the micropores is calculated by the convection-dispersion equation (CDE) while the dispersion and diffusion in the CDE is neglected for the solute transport in the macropores. The applied water and chloride reached the bottom of the profile during the 2 and 1-hour(s) application periods in studies 2 and 3, respectively. There is a strong indication of macropore flow in this soil. Based on the statistical criteria, the model accurately simulated water flow and solute transport with depth and time in all plots. The mean values of three statistical parameters (coefficient of residual mass, model efficiency, and correlation coefficient) for water and chloride transport were –0.0014, 0.791, 0.903 and 0.0333, 0.923, 0.956, respectively. Preliminary studies showed that the model could not simulate flow and transport well enough with the one-domain flow concept. In the two-domain flow, effective diffusion path-length, boundary hydraulic conductivity, and boundary soil water pressure were the three most important parameters that control flow and transport between the two domains. The effective diffusion path-length represented the structural development with depth in the Maury silt loam soil.

Relating model parameters to basic soil properties

Relating model parameters to basic soil characteristics can help to differentiate and classify soils based on their flow and transport characteristics and ultimately helps to develop a sound management tool to protect groundwater from industrial and agricultural contaminants. In this study, the model parameters (effective diffusion path-length or aggregate half-width, boundary soil water pressure, boundary hydraulic conductivity, saturated hydraulic conductivity, tortuosity in macropores, dispersivity, mixing depth) obtained from simulation of water flow and solute transport for 3 soils (Maury, Cecil, Lakeland) with contrasting properties were related to see whether these derived parameters can be related to variation in fundamental soil properties such as texture and structure and thus the flow and transport characteristics of the soils. The boundary is a division point in which the soil porosity is divided into macropores and micropores. The ANOVA test showed that the parameter values of effective diffusion path-length and tortuosity in macropores for 3 soils were not different from each other, but the parameter values of saturated and boundary hydraulic conductivities including the texture (clay content) were statistically different. Moreover, the means of boundary soil water pressure, dispersivity, and mixing depth for 3 soils were significantly different. These results suggest that relating model parameters to basic soil properties in order to differentiate and classify soils based on their flow and transport characteristics is promising and needs further study.

Simulation of water and solute transport with MACRO model in Cecil loamy sand soil

Modelling preferential flow increases understanding of flow and transport processes in the unsaturated (vadose) zone and, hence, helps prevention of groundwater contamination. A dual-porosity model, MACRO, was evaluated for long-term drainage flow and short-term chloride-tagged water flow simulations in well-structured Cecil loamy sand soil. Water flow in micropores is calculated by the Richards’ equation, and simple gravity flow is assumed in the macropores. Solute transport in the micropores is calculated by the convection–dispersion equation (CDE), and the dispersion and diffusion in the CDE is neglected for the solute transport in the macropores. Based on the statistical criteria, the model accurately simulated drainage flow with depth and time. The average values of 3 statistical parameters (coefficient of residual mass, model efficiency, correlation coefficient) for drainage flow of different plots and times were 0.0057, 0.972, and 0.987, respectively. Similarly, the average values of the 3 statistical parameters in the same order for water flow and chloride transport of 4 plots were 0.097, 0.628, and 0.915, and 0.167, 0.938, and 0.982, respectively. The model simulated long-term drainage flow better than the short-term applied chloride-tagged water. The percentage recovery of measured water and chloride 2 h after the application ceased was 83 and 63 in the 1.05-m-deep profile of plot 1. This was a strong indication of preferential flow in this soil. Two-domain flow concept was required for acceptable simulation of this type of flow. In the 2-domain flow, boundary tension, boundary hydraulic conductivity, and effective diffusion path-length were the 3 most important parameters controlling flow and transport between the 2 domains. The effective diffusion path-length represented the structural development with depth in Cecil loamy sand soil. The relationships between the variability in flow and transport characteristics and fundamental soil properties and, hence, the associated key model parameters suggest that pedotransfer functions can be developed for the estimation of dual-porosity model parameters that control preferential flow.

Effects of triaxial stressing on creep cavitation of grain boundaries

We investigate the influence of triaxial stressing on the growth of cavities on grain boundaries by the combined processes of plastic creep flow and grain boundary diffusion. The coupling arises from local accommodation of matter on the grain boundary near the cavity tip due to plastic creep deformability of the grains. This has the effect of shortening the diffusion path length on the grain boundary and increasing the cavity volumetric growth rate. An increase in triaxiality is found to accelerate the matter flux flowing from the cavity surfaces onto the grain boundaries and thus increase the cavity volumetric growth rate. This occurs at attainable levels of triaxiality. However, the enhancement in the triaxial stress state does not affect the effective diffusion path length very significantly. A simple formula for the volumetric growth rate of the cavity is suggested and it is found to give a good approximation to the numerically computed results.

Gaseous Diffusion Evaluation in Forest Humus

AbstractPenman's ratio of direct length to effective diffusion path length, L/Le, is not directly predictable from the air content of forest humus, probably because of anisotropic pore orientation. The diffusion coefficient of a gas in the unsaturated porous medium can be estimated from temperature, air pressure, porosity, water content, and an empirically determined value of L/Le. A steady state CO2 diffusion method is proposed for measuring L/Le and gaseous diffusion coefficient in either loose or structurally intact forest humus samples.