Movement of phorate under constant and variable surface water flux in initially dry and moist soils

The distribution of phorate was studied in three texturally different soils under constant and variable surface flux of water. Phorate distribution profiles and leaching curves showed that the insecticide was displaced more efficiently and its concentration peak formed at deeper depth with constant than with variable water flux in sandy loam soils irrespective of initial soil water content. However, phorate distribution profiles were found to be independent of the methods of water application in clay soil because of its extremely low permeability.     

砂质多孔介质中土壤颗粒的迁移

通过室内模拟试验,研究了不同土壤悬液浓度和不同多孔介质组成条件下土壤颗粒在砂质多孔介质中的截留和迁出特征,以揭示土壤颗粒在砂质多孔介质中的迁移特征。结果表明,土壤颗粒在砂质多孔介质中迁移时会受到土壤颗粒粒级、浓度和介质组成的影响。截留作用在土壤颗粒通过砂质多孔介质时起主导作用,土壤颗粒截留比例随着土壤颗粒浓度的增大逐渐增加;而随着多孔介质中粗砂比例的增大,其逐渐降低。土壤总颗粒、50μm和1~50μm土壤颗粒迁出量分别与该粒级相应投入量呈幂函数增加趋势,而1μm土壤颗粒的迁出量与其投入量呈线性增加趋势。土壤总颗粒迁出量与介质中粗砂比例之间存在线性增加关系。因此,多孔介质组成、颗粒粒径和浓度在土壤颗粒迁移过程中起着重要作用,在多孔介质中土壤颗粒和污染物迁移研究方面需要充分考虑这些因素。     

Experimental study and numerical simulation of denitrification in saturated porous media

The effect of denitrification on the behavior of nitrate in saturated sandy soil was studied in the laboratory and the results were compared with the temporal and spatial concentration of nitrate in nonreactive (without denitrification) and reactive (with denitrification) cases. A laboratory model was fabricated to study steady one-dimensional flow and to transport nitrate with or without denitrification. Denitrification at various rates has been simulated with varied C∶N ratios and detention time. Retardation constant, dispersivity, and degradation constants have been computed using existing analytical models supporting adsorption and zero and/or first order production or decay. It was found that such analytical models can be used to fit the concentration of nitrate in saturated porous media for a C∶N ratio between 2 and 5.     

Gas diffusivity in soils compared to ideal isotropic porous media

Evidence of anisotropy is reported for advective air and water permeabilities in soils. Thus, anisotropy is likely to exist also for diffusive gas fluxes. Information about direction‐dependent soil gas diffusivity is scarce and most modeling approaches assume isotropy. At hundreds of closely lying positions in a compacted and adjacent undisturbed forest soil, gas diffusivity (D_s/D₀) was measured either in vertical or horizontal direction. The volume‐independent diffusion efficiency (i.e., diffusivity divided by air‐filled porosity) was fitted by a generalized additive model (GAM). Significant regressors were air‐filled porosity (?), soil depth, and the discrete diffusion direction. The model yields in all cases higher vertical diffusion efficiencies. The compaction factor did not yield a significant regressor of its own, i.e., the reduction of diffusivity in the compacted soil was the same as in low‐porosity samples of the undisturbed profile. To elucidate the role of sharing vertically and horizontally orientated pore space and a potential competition between diffusivity in different spatial directions, simple geometric models consisting of 3‐dimensionally crossed pores have been parameterized. These models provided a good explanation of the typical nonlinear D_s/D₀(?) relationship. By simple one‐parameter correction (linear or power function), this mechanistic model could be fitted to the data. The one‐parameter correction of the geometric model could be a straightforward approach to consider direction dependence of measured diffusivities. However, by applying this approach to the observations the anisotropy effect was not clearly evident, which could be attributed to a changing D_s/D₀(?) relationship with depth. As a reason for the preference of the vertical gas diffusion the dominance of vertical stresses and the activity of anecic earthworms are discussed. Direction dependency of gas diffusivity seems to be a basic feature of natural pore systems and has to be considered for modeling gas fluxes in soils. Generally, a preferential vertical diffusion direction reduces horizontal balancing and increases the heterogeneity of gas concentrations in the soil air.     

多孔介质干燥过程传热传质研究

该文根据非平衡态热力学和相平衡理论，建立了多孔介质对流干燥内部传热传质过程的二维数学模型，该模型充分考虑了传热与传质之间的相互耦合，用控制容积积分法采用全隐格式对该模型进行分析求解，并与玉米干燥实验数据进行对比。结果表明，表面对流换热系数、干燥介质温度和湿度的变化对热、湿迁移过程均有较大影响，而表面对流传质系数对湿迁移过程影响不大，温度和湿度梯度的耦合产生“局部增湿”现象。     

Pore network and water retention characteristics of volcanic porous media

Aggregate media are often characterized by multi‐porous systems, which have structural and water retention characteristics that depend on the complex interaction between intra‐ and inter‐aggregate pores. Here we investigate the structure and water retention dynamics of rigid aggregate volcanic materials. In particular, we focus on commercially used pumices, lapilli and zeolites. The aim was to estimate the air and water content through complex dual‐porous systems, and thus to evaluate their suitability for vegetation growth. Both inter‐ and intra‐aggregate characteristics were determined by means of mercury intrusion porosimetry, X‐ray microtomography and water retention curves. The wilting point was determined with pressure plates, a dew point hygrometer and the sunflower method to assess their reliability at small matric potentials. Results indicate that aggregate porous media were bimodal and their heterogeneous pore network affected the water retention dynamics because (i) the large inter‐aggregate pores allowed a rapid drainage near saturation and (ii) the intra‐aggregate porosity held water available for root uptake and plant growth. In contrast, volcanic powders were less affected by the inter‐ and intra‐aggregate dual‐porosity. The use of a dew point hygrometer instead of pressure plates for determining small matric potentials is also suggested because pressure plates might over‐estimate the water content because of poor plate and soil conductance. However, the reference potential at wilting point should be set at values greater than ?1471.5 kPa (?784.8 kPa) to consider the interaction between plant roots and porous media with small hydraulic conductivity. Results from this work indicate that aggregate multi‐porous media allow the simultaneous supply of oxygen and available water for plants, although the heterogeneous nature of the pore network involves uncertainties regarding water balance and root–matrix interactions.     

氧化石墨烯和五价砷在改性多孔介质中的共迁移特征

砷是农田土壤重金属污染的主要元素之一,在砷污染农田土壤的修复过程中往往忽视纳米颗粒能够使结合态的砷重新释放,导致有效态砷浓度升高,探究土壤中黏土矿物对氧化石墨烯(Graphene Oxide,GO)和五价砷(As(V))在多孔介质中迁移行为的影响,对进一步完善农田土壤砷修复理论以及提高农作物产量、保护人体健康具有重要意义。该研究利用蒙脱石和高岭石改性石英砂,通过砂柱迁移试验系统地研究了GO、As(V)和GO-As(V)在填加0%、10%、30%和50%的蒙脱石和高岭石改性石英砂柱中的迁移行为。研究结果表明,随着高岭石和蒙脱石改性石英砂填加比例的增加,GO和As(V)的迁移能力均呈降低趋势,且GO和As(V)在不同条件下的迁移曲线均存在显著差异(P<0.05);GO在50%高岭石和蒙脱石改性石英砂柱中的回收率相对于石英砂柱分别下降了14%和17%,As(V)分别下降了15%和12%;在共迁移试验中,GO和As(V)在石英砂柱中回收率分别上升至99%和100%。分析表明,As(V)在蒙脱石改性石英砂柱中的迁移能力大于高岭石改性石英砂,而GO与之相反;当GO和与As(V)共迁移时,二者...     

Modeling of throughfall chemistry and indirect measurement of dry deposition

Precipitation over a forested watershed is altered by interaction with plant surfaces which act as a filter for airborne gases and particles. This results in a major transfer to the forest floor of materials captured, washed, and leached from the forest canopy. Sequential samples of wetfall and sequential samples of throughfall under deciduous and coniferous trees were collected and chemically analyzed for major anions and cations. A simple washoff, mixing model based on leaf area index was used to simulate throughfall chemistry and to decouple foliar exudation from dry deposition. Model results gave excellent predictions of the measured sequential throughfall using estimated values of dry deposition. The model can also be used to calculate dry deposition, if the sequential throughfall data and wetfall data are used as input variables.     

Displacement imaging in porous media using the line scan NMR technique

Displacement imaging is a recent, powerful NMR method with which distributions of displacements can be acquired of e.g. fluids within a porous medium. Both motion parallel and perpendicular to the flow direction may be observed within a time window of a few milliseconds to several seconds. By combining displacement imaging with the line scan technique, one-dimensionally resolved measurements with a high temporal resolution ( < 1 min) of the spatial dependency of motion can be obtained. Here we present displacement images of flow through two simple model systems for soil: an unconsolidated glass bead water system and a sintered glass bead filter. It is demonstrated that the combination of displacement imaging and spatial resolution along a line is important to access both bulk displacement and local displacements in relation to the local porosity.     

A technique for the measurement of gaseous diffusion in porous media

An apparatus composed of two well-mixed vessels connected by a tube was used to measure the effective diffusivity of a gas (sulphur hexafluoride, SF₆) in porous beds of sand, steel wool and glass beads. The general solution to Fick's second law for the apparatus was employed in the analysis. The ratio of the measured effective diffusivity of SF₆ to SF₆-air diffusivity was found to equal approximately the void fraction of the porous bed raised to the 1.43 power. This relationship agreed well with previously collected data and with the theoretical models of Millington (1959) and Marshall (1959). The effective diffusivity of wet porous materials was also measured and, at higher moisture contents, found to deviate strongly from that measured in dry materials. The effective diffusivity of SF₆ in sand with 20 per cent moisture (volume basis) was found to be about one-fourth as large as the value expected from the dry-bed experiments. No significant deviation was found for sand of 10 per cent moisture content or lower.     

Solute transport in aggregated porous media: Comparing model independent and dependent parameter estimation

In many studies on solute transport soil column experiments arc used to obtain the transport characteristics for Convection-Dispersion-Models. Early breakthrough of the solute pulse has been attributed to a non-equilibrium in phase exchange. It is a standard procedure to determine several model parameter values from such breakthrough curves (BTC). This investigation is focused on the physical significance of simultaneously fitted parameter values used in the convection and diffusion-controlled mass transfer model (mobile — immobile phase concept). Saturated column experiments were conducted with solid phases consisting of porous and solid glass beads. One set of model parameter values was obtained from the breakthrough curves by simultaneous optimization and a second set was determined by independent measurements of individual parameter values. Both sets of parameter values described the BTCs equally well but deviated substantially from each other. These discrepancies were analysed in terms of local parameter sensitivities.     

Simulation of gravity fingering in porous media using a modified invasion percolation model

We introduce a modified invasion percolation (MIP) model for the immiscible displacement of a nonwetting fluid by a wetting fluid within a porous network. The model includes the influence of gravity and is applicable in the quasi-static limit of infinitesimal flow rate where viscous forces are negligible with respect to gravity and capillary forces. The incorporation of gravity alone creates complicated, pore-scale gravity fingers. To properly model wetting fluid invasion where macroscopic gravity fingers form, we incorporate a pore-scale geometric capillary smoothing function we refer to as facilitation.Facilitation models the physics of wetting fluid invasion of pores by modifying the capillary pressure required to fill a pore based on the number of adjacent necks connecting the pore to the invading wetting fluid. The wetting fluid invasion facilitation process creates compact clusters and macroscopic fronts in horizontal simulations and in combination with gravity, creates macroscopic, gravity fingers that are in qualitative agreement with physical experiments. The MIP model yields much different imbibition front structures than standard invasion percolation. For MIP, capillary fingering, capillary facilitation, and gravity fingering compete to determine the wetted network structure as a function of pore-size distribution.     

Measurement of hydraulic characteristics of porous media used to grow plants in microgravity

Understanding the effect of gravity on hydraulic properties of plant growth medium is essential for growing plants in space. The suitability of existing models to simulate hydraulic properties of porous medium is uncertain due to limited understanding of fundamental mechanisms controlling water and air transport in microgravity. The objective of this research was to characterize saturated and unsaturated hydraulic conductivity (K) of two particle-size distributions of baked ceramic aggregate using direct measurement techniques compatible with microgravity. Steady state (Method A) and instantaneous profile measurement (Method B) methods for K were used in a single experimental unit with horizontal flow through thin sections of porous medium providing an earth-based analog to microgravity. Comparison between methods was conducted using a crossover experimental design compatible with limited resources of space flight. Satiated (natural saturation) K ranged from 0.09 to 0.12 cm s-1 and 0.5 to >1 cm s-1 for 0.25- to 1- and 1- to 2-mm media, respectively. The K at the interaggregate/intraaggregate transition was approximately 10(-4) cm s-1 for both particle-size distributions. Significant differences in log(10)K due to method and porous medium were less than one order of magnitude and were attributed to variability in air entrapment. The van Genuchten/Mualem parametric models provided an adequate prediction of K of the interaggregate pore space, using residual water content for that pore space. The instantaneous profile method covers the range of water contents relevant to plant growth using fewer resources than Method A, all advantages for space flight where mass, volume, and astronaut time are limited.     

Effect of humic acid on the sedimentation and transport of nanoparticles silica in water-saturated porous media

Journal of Soils and Sediments - Nano silicon particles (nSiO2) is one of the most widely used industrial engineered nanomaterials (ENMs). The extensive applications of nSiO2 may pose potential...     

生物材料冻干过程分形多孔介质传热传质模拟

为了研究生物材料冻干过程中的超常传热传质机理,建立了考虑已干层分形特点的冻干模型,水蒸气和惰性气体在已干层中的连续方程采用了分形多孔介质中的扩散方程,扩散系数随已干层厚度的增加呈指数下降。该理论模型可确定已干层的分形特点对生物材料冻干过程升华界面的移动和干燥时间的影响,预测生物材料冻干过程中物料内部温度和结合水浓度的分布。以螺旋藻为对象,利用分形模型借助Matlab和Fluent软件模拟了螺旋藻的冻干过程,模拟结果与试验结果吻合较好。     

Observation of flow and transport processes in artificial porous media via magnetic resonance imaging in three dimensions

In two series of laboratory experiments solute transport in saturated artificial porous media was observed using magnetic resonance imaging. The objective was to get qualitative and quantitative information on transport in three dimensions including heterogeneity effects and density driven convection phenomena. Characteristic finger growth rates were evaluated and compared to calculation results. The experiments show that magnetic resonance imaging measurements can be used successfully to clarify the role of heterogeneities in transport and to check the results of numerical calculations of density driven flow.     

Magnetic resonance imaging methods to reveal the real-time distribution of nickel in porous media

Direct and non‐destructive measurement of the sorption, diffusion and mobility of ions and molecules in porous media has applications in industry and environmental science. We used magnetic resonance imaging (MRI) to visualize the dynamic distribution of paramagnetic nickel (Ni⁺²) ions in porous media. Various MRI sequences were tested to image Ni²⁺ at small concentrations. Noisy gradient echo images had poor contrast between samples containing various Ni²⁺ concentrations. Turbo spin echo and spoiled gradient echo images showed a linear relation between Ni²⁺ concentrations and signal intensity over a wide range of concentrations. Spoiled gradient echo images resolved Ni²⁺ concentrations (down to 30 mg litre^?1) better than turbo spin echo images. However, for smaller concentrations, uncertainty in intensity values increased. A T₁ measurement, obtained using an inversion recovery sequence, showed a linear correlation between T₁ and Ni²⁺ concentration down to 1.5 mg litre^?1. In a glass bead medium with an ion exchange resin as an Ni sink, the real‐time development of the Ni²⁺ depletion zone around the resin, as Ni was sorbed into the resin, was imaged by T₁ mapping. A spatial resolution of 0.58 mm and a temporal resolution of less than a minute were achieved. The two‐dimensional Ni²⁺ gradient that was determined from MRI agreed well with geochemical modelling results. The results of this study showed that MRI, in particular T₁ mapping, can quantify microscale behaviour of paramagnetic species in porous media. However, ferromagnetic components that naturally occur in most soils can easily disturb the MRI signal.