Low Water Flow Rates Research Articles

Use of a High-Pressure Flowmeter for Evaluating Hydraulic Characteristics of Plant Organs and Absorption Functions of Roots

The hydraulic characteristics of plant organs and the absorption functions of roots of corn and tomato plants were analyzed by using a specialized high-pressure flowmeter (HPFM), which was equipped with a root chamber as well as a coupling to connect to the plant organs. The HPFM can detect changes in the hydraulic properties of the respective parts of the plant body. The hydraulic resistances of the leaf and root decreased with an increase in the water flow rate; this dependence was remarkable at low water flow rates. On the other hand, the hydraulic resistance of the stem was only slightly affected by the water flow. However, a positive linear relationship between hydraulic conductance (reciprocal of resistance) and water flow rate was found for the shoot, stem, and root, respectively. Further, the rate of nutrient (NO3−) absorption by the root was evaluated from the water flow (root water absorption) and NO3− concentration in the xylem sap exuded from the root stump. The high-pressure flowmeter with the root chamber can be used for studying water and nutrient transport in plants and their responses to environmental elements.

PVC dust removal from the air by a new dynamical scrubber

Removal of the poly vinyl chloride (PVC) dust particles by a new dynamical scrubber in a PVC manufacturing plant is presented in this work. The effects of inlet air flow rate (containing PVC particles) and water flow rate (the physical absorber of particles) on the PVC dust removal efficiency were investigated. The size of the PVC particles which have been treated in this study was ranging from 0.5 up to 20 μm. The experimental results showed approximately 100% removal efficiency for particles larger than 1 μm and 73% for 0.5 μm particles while employing low water flow rates. The results of these experimental tests indicate a good performance of the scrubber in a short residence time (about 2 s) which is also completely economic for removing fine particles.

Factors Controlling Uranium and Thorium Isotopic Composition of the Streambed Sediments of the River Nile, Egypt

The streambed sediments of the River Nile in Egypt have been analyzed for their long-lived uranium and thorium radionuclides ( 238 U, 234 U, 230 Th) using Alpha Spectrometric Techniques. These sediments are dominated by sand size fractions with average heavy mineral contents of 2.65%. Carbonates and organic materials constitute considerable amounts of the sediments (averages 13.82 and 3.37%, respectively). The average contents of uranium and thorium in the streambed sediments are 2.06 and 6.58 ppm, respectively, which are comparable with the corresponding values of the world rivers. The 230 Th/ 234 U and 230 Th/ 238 U activity ratios of the sediments are greater than unity indicating preferential uranium leaching. During low water flow rate, the streambed sediments probably acquired some uranium from water with high 234 U/ 238 U activity ratios. This results in abnormally high 234 U/ 238 U activity ratios in the sediments. Heavy mineral contents, sorptive uptake of uranium by sediments of relatively high organic contents, geochemical behaviour of radionuclides and flow rates of the River Nile are considered as the most important factors controlling uranium and thorium isotopic composition of the streambed sediments. The sources of the radionuclides are probably the local granitic rocks and phosphate fertilizers used in the cultivated lands adjacent to the River Nile.

Evaporative Cooling of Water in a Rotating Packed Bed (Split Packing)

Rotating packed beds (RPB) have been studied for intensifying mass-transfer processes. This work was undertaken to evaluate the thermal performance of an RPB with split packing for evaporative cooling of water involving simultaneous heat and mass transfer. Higher efficiency was achieved within a smaller contactor volume compared to mechanical draft cooling towers. At all values of the liquid-to-air flow rate ratio studied, efficiency was not significantly affected by rotor speed and decreased with increasing rotor speed beyond a certain limit at low air and water flow rates. A simple mathematical model was used to estimate process parameters from the experimental data based on the Merkel assumption. The hGa/kYaCs ratio varied between 0.85 and 1.18, and the tower characteristic of the RPB was between 1.4 and 4.0.

Effect of draw solution concentration and operating conditions on forward osmosis and pressure retarded osmosis performance in a spiral wound module

Forward osmosis (FO) and pressure retarded osmosis (PRO) are concentration-driven membrane processes. While they can be potentially used in water, wastewater, and energy applications, these processes suffer from the concentration polarization inside the porous membrane support resulting in severe flux decrease, a phenomenon known as internal concentration polarization (ICP). Researchers have investigated the effect of ICP both in theoretical and experimental studies. The current study extends the existing ICP model to include the effect of draw solution dilution by membrane permeate flow in a spiral wound FO module (SWFO). FO and PRO experiments were performed using a Hydrowell ® SWFO under both submerged and cross-flow conditions. The effect of draw solution concentration, draw solution flow rate, feed water flow rate, and membrane orientation on FO and PRO water flux performance was systematically investigated. Permeate flow increased with greater draw solution concentration in both FO and PRO modes. ICP was found to drastically limit the available membrane flux in the concentration-driven membrane processes, and its adverse effect was more severe at greater draw solution concentration. Membrane flux was also affected by the dilution of draw solution when the permeate flow rate was comparable or greater than the draw solution flow rate. The submerged FO configuration performed nearly as good as the cross-flow configuration with feed water circulating outside of the membrane envelope (shorter flow path). In this case, the feed water flow rate only had limited effect on membrane flux likely due to its low mass transfer resistance. In contrary, the membrane flux can be adversely affected at low feed water flow rate when it was circulated inside of the membrane envelope (longer flow path).

Relationships between water flow rate and geophysical measurements in an alluvial aquifer

Our study focuses on the potential usefulness of surface geophysical data to constrain the water content within an alluvial aquifer. On a study area where two wells have been drilled, we have performed several geophysical measurements, including ground penetrating radar, DC resistivity prospecting, seismic refraction survey and magnetic resonance soundings. From these data, we estimated several parameters, namely, the water height in the deposits, the effective porosity, the water content, the permeability, and the transmissivity of alluvial deposits. These physical parameters allow us to characterize the alluvial deposits in order to constrain the estimation of the potential water flow. The lithology and water flow rate known from the wells enabled us to compare geophysical results obtained in a high water flow rate zone to those in a low water flow rate zone. Correlation has been found between the water flow rate observed in both wells and the geophysical data obtained in the vicinity of these wells.

Study on Design of Air-water Two-phase Flow Centrifugal Pump Based on Similarity Law

A conventional centrifugal pump causes a drastic deterioration of air-water two-phase flow performances even at an air-water two-phase flow condition of inlet void fraction less than 10% in the range of relatively low water flow rate. Then we have developed a two-phase flow centrifugal pump which consists of a tandem arrangement of double rotating cascades and blades of outer cascade have higher outlet angle more than <TEX>$90^{\circ}$</TEX>. In design of the two-phase flow pump for various sized and operating conditions, similarity relations of geometric dimensions to hydraulic performances is very useful. The similarity relations of rotational speed, impeller diameter and blade height are investigated for the developed impeller in the present paper. As the results, the similarity law of rotational speed and impeller diameter is clarified experimentally even in two-phase flow condition. In addition, influences of blade height on air-water two-phase flow performances indicate a little difference from the similarity relations.

The impact of water–rock interaction and vegetation on calcium isotope fractionation in soil- and stream waters of a small, forested catchment (the Strengbach case)

This study aims to constrain the factors controlling the calcium isotopic compositions in surface waters, especially the respective role of vegetation and water–rock interactions on Ca isotope fractionation in a continental forested ecosystem. The approach is to follow changes in space and time of the isotopic composition and concentration of Ca along its pathway through the hydro-geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls, percolating soil solutions and springs. The study is focused on the Strengbach catchment, a small forested watershed located in the northeast of France in the Vosges mountains. The δ 44/40Ca values of springs, brooks and stream waters from the catchment are comparable to those of continental rivers and fluctuate between 0.17 and 0.87‰. Soil solutions, however, are significantly depleted in lighter isotopes (δ 44/40Ca: 1.00–1.47‰), whereas vegetation is strongly enriched (δ 44/40Ca: −0.48‰ to +0.19‰). These results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions, with depletion in “light” calcium in the soil solutions from deeper parts of the soil compartments due to preferential 40Ca uptake by the plants rootsystem. However, mass balance calculations require the contribution of an additional Ca flux into the soil solutions most probably associated with water–rock interactions. The stream waters are marked by a seasonal variation of their δ 44/40Ca, with low δ 44/40Ca in winter and high δ 44/40Ca in spring, summer and autumn. For some springs, nourishing the streamlet, a decrease of the δ 44/40Ca value is observed when the discharge of the spring increases, with, in addition, a clear covariation between the δ 44/40Ca and corresponding H 4SiO 4 concentrations: high δ 44/40Ca values and low H 4SiO 4 concentrations at high discharge; low δ 44/40Ca values and high H 4SiO 4 concentrations at low discharge. These data imply that during dry periods and low water flow rate the source waters carry a Ca isotopic signature from alteration of soil minerals, whereas during wet periods and high flow rates admixture of significant quantities of 40Ca depleted waters (vegetation induced signal) from uppermost soil horizons controls the isotopic composition of the source waters. This study clearly emphasizes the potential of Ca isotopes as tracers of biogeochemical processes at the water–rock–vegetation interface in a small forested catchment.

Characterisation and Pre-concentration of Chromite Values from Plant Tailings Using Floatex Density Separator

Classification is a method of separation of fines from coarse particles and also lighter particles from heavier particles. The conventional classifiers, such as, hydrocyclone or mechanical classifiers, decreases the efficiency of the grinding and concentration circuits due to their imperfect separation. In the process of improving the efficiency of classification, a device that has been gaining popularity in recent years is the teeter-bed or hindered-bed separator such as Floatex density separator. Generally for processing chromite ores, different types of gravity methods are employed after crushing, grinding followed by classification. The Tata Steel Chrome Ore Beneficiation (COB) plant is generating 50 tph of tailings assaying 17% Cr2O3. A critical review on practice of the plant operating personnel is concerned in the grade-recovery characteristics of unit operations. But separation insight and influence of different operating and process parameters are essential to understand and control the process. The objective of the present investigation was to study the effect of the important operating variables on floatex density separator and preconcentration of COB plant tailings for the further beneficiation process and found that significant removal of iron bearing mineral such as goethite and silica is possible using FDS in a single stage operation. The maximum of 83% recovery of chromite is possible with 22 to 23% Cr2O3 content and thus obtained FDS underflow is suitable for flotation circuit. A low teeter water flow rate with a high bed pressure removes iron bearing mineral like goethite efficiently in an FDS.

On-tap passive enrichment, a new way to investigate off-flavor episodes in drinking water

Because taste and odor events in drinking water are often fleeting and unpredictable phenomena, an innovative enrichment sampler has been developed to trap off-flavor compounds directly at the consumer's tap. The ARISTOT (Advanced Relevant Investigation Sampler for Taste & Odor at Tap) consists of a tap adapter in which seven polydimethylsiloxane (PDMS) coated stir bars are placed, allowing the stir bar sorptive extraction (SBSE) of organic compounds during each tap opening. In order to study the efficiency of ARISTOT, a private network pilot unit has been constructed in our laboratory, equipped with four faucets in parallel, solenoid valves for an automation of the system and a mixing chamber to spike drinking water with odorous compounds in order to have homogenously smelling water at each tap. After enrichment, the stir bars are taken out, in-line thermo-desorbed and analyzed by gas chromatography coupled with a mass spectrometer. The results showed the high sensitivity of ARISTOT, which was able to quickly monitor odorous compounds at the sub ng/L level. A “multishot” method was developed to analyze chemicals concentrated on the seven stir bars in only one chromatographic run, thereby increasing the sensitivity of the system. Higher enrichment factors were obtained under low water flow rates or by using longer stir bars and/or stir bars with a higher PDMS film thickness. No significant loss of extracted compounds was reported for flow rates between 2 and 4 L/min. This allowed us to spike the stir bars with an internal standard prior to sampling in order to monitor the analytical variations. It was also observed that hot water increases the loss of enriched solutes but the quantification can be corrected by internal standard addition.

Experimental investigation on the slip between oil and water in horizontal pipes

This work is devoted to study of the slip phenomenon between phases in water–oil two-phase flow in horizontal pipes. The emphasis is placed on the effects of input fluids flow rates, pipe diameter and viscosities of oil phase on the slip. Experiments were conducted to measure the holdup in two horizontal pipes with 0.05 m diameter and 0.025 m diameter, respectively, using two different viscosities of white oil and tap water as liquid phases. Results showed that the ratios of in situ oil to water velocity at the pipe of small diameter are higher than those at the pipe of big diameter when having same input flow rates. At low input water flow rate, there is a large deviation on the holdup between two flow systems with different oil viscosities and the deviation becomes gradually smaller with further increased input water flow rate.

Role of bentonite colloids on europium and plutonium migration in a granite fracture

Abstract To establish when the presence of colloids can favour contaminant migration is a critical task for the performance assessment of a high level radioactive waste repository. In this study, the effects of the presence of bentonite colloids on the migration of highly sorbing elements, such as Eu and Pu, in a granite environment were investigated. Special efforts were made to quantify the filtration of bentonite colloids in this medium and to determine the experimental uncertainties that could bias the interpretation of the results. The migration of bentonite colloids in a granite fracture was studied in the laboratory by dynamic column experiments under low water flow rates (4–11 mL/h). The breakthrough curves of bentonite colloids always showed a peak in a similar position to conservative tracers, but the colloids recovery critically depended on their initial concentration and on the water flow rate. In the presence of colloids, the breakthrough curves of Eu and Pu always showed a peak in the same position as conservative tracers, thus indicating the migration of these radionuclides to be mostly colloid-driven. The recovery of Pu adsorbed onto the colloids was approximately that expected when the experimentally determined bentonite colloid filtration in the column was accounting for but the Eu recovery was always significantly lower.

Landfill gas upgrading with countercurrent water wash

A pilot-scale countercurrent absorption process for upgrading landfill gas to produce vehicle fuel was studied using absorption and desorption units and water as absorbent. The height-to-diameter ratio of the absorption column used was 3:1 instead of the more conventionally used 20:1 ratio, and a higher pressure was used along with a lower water flow rate. The effects of pressure (10-30 bar) and water and gas flow-ratios on the upgrading process were studied. Methane content in the product gas increased to near or above 90% with both gas flows (50 and 100 l/min) used at over 20 bar pressure with 10 l/min water flow and at 30 bar pressure with 5l/min water flow. Carbon dioxide content with these upgrading parameters ranged from 3.2% to 4.8%. The remaining fraction of the product gas was nitrogen (from 6% to 7%), while hydrogen sulphide was removed to below the detection limit with all of the upgrading parameters used. The methane content of exhaust gas increased with increasing pressure. In conclusion, the pilot-scale gas upgrading process studied here appears to be able to produce gas with high energy content (>90% methane), apparently suitable as vehicle fuel, from landfill gas.

Effects of feeding, stocking density and water-flow rate on fecundity, spawning frequency and egg quality of Nile tilapia, Oreochromis niloticus (L.)

This study investigated the effects of two levels of feeding (1 or 4% BW day−1), two levels of stocking densities (3 or 10 female m−2) and two levels of water-flow rates (0.06±0.00 or 0.35±0.04 L s−1) on the fecundity, relative fecundity, spawning frequency and egg quality of Nile tilapia, Oreochromis niloticus (L.) with the goal of improving hatchery seed production. Males and females, 125 days of age, were stocked in recirculating concrete tanks. Eggs were collected every four days directly from the mouths of incubating females. Fecundity (eggs spawn−1), body weight gain (g day−1) and egg diameter (mm) and weight (mg), percent protein and lipid, fertilization and hatchability were measured over 120 days. Higher eggs kg female−1 day−1 (312±36) was observed in treatments with lower levels of stocking density (3 female m−2) and water-flow rate (0.06±0.00 L s−1) under both feeding rates. The rate was 2 to 4 fold higher than the others. Overall, high feeding level (4% BW day−1) increased growth (g day−1) by 35% and eggs spawn−1 by 18% and did not affect eggs kg female−1 day−1, spawn female−1 and eggs m−2 day−1. Higher stocking density (10 female m−2) lowered eggs spawn−1 by 19%, eggs kg female−1 day−1 by 52% and spawn female−1 by 40%. Higher water-flow rate (0.35±0.04 L s−1) increased growth (g day−1) by 33%, and lowered eggs kg female−1 day−1 by 51%. The variables also did not affect percent egg fertilization, hatchability, crude lipid and protein or egg diameter (mm). However, egg weight (mg) was higher in groups fed a lower ration (1% BW day−1) with higher stocking density (10 female m−2) and higher water-flow rates (0.35±0.04 L s−1). Feeding levels interacted with water-flow rates in improving growth (g day−1) while stocking density interacted with water-flow rates in lowering eggs kg female−1 day−1 and spawn female−1. The eggs kg female−1 day−1 obtained in this study from the best treatments is comparable with the highest egg production rates reported by others for O. niloticus. This suggests that lower feeding level (1% BW day−1), lower stocking density (3 female m−2) and lower water-flow rate (0.06±0.00 L s−1) could be adopted as a management strategy to improve current tilapia hatchery seed production, although, optimum water flow-related stocking density needs further investigation.

On the development of a three-dimensional transient thermal model to predict ingot cooling behavior during the start-up phase of the direct chill-casting process for an AA5182 aluminum alloy ingot

The control of the heat transfer during the start-up phase of the direct-chill (DC) casting process for aluminum sheet ingots is critical from the standpoint of defect formation. Process control is difficult because of the various inter-related phenomena occurring during the cast start-up. First, the transport of heat to the mold is altered as the ingot base deforms and the sides are pulled inward during the start-up phase. Second, the range of temperatures and water flow conditions occurring on the ingot surface as it emerges from the mold results in the full range of boiling-water heat-transfer conditions—e.g., film boiling, transition boiling, nucleate boiling, and convection—making the rate of transport highly variable. For example, points on the ingot surface below the point of water impingement can experience film boiling, resulting in the water being ejected from the surface, causing a dramatic decrease in heat transfer below the point of ejection. Finally, the water flowing down the ingot sides may enter the gap formed between the ingot base and the bottom block due to butt curl. This process alters the heat transfer from the base of the ingot and, in turn, affects the surface temperature on the ingot faces, due to the transport of heat within the ingot in the vertical direction. A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the DC casting process. The model, based on the commercial finite-element package ABAQUS, includes primary cooling via the mold, secondary cooling via the chill water, and ingot-base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of ingot-surface temperature, water flow rate, impingement-point temperature, and position relative to the point of water impingement. In addition, a secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat-extraction rates). The algorithm used to describe ingot-base cooling includes both the drop in contact heat transfer due to gap formation between the ingot base and bottom block (arising from butt curl) as well as the increase in heat transfer due to water incursion within the gap. The model has been validated against temperature measurements obtained from two 711×1680 mm AA5182 ingots, cast under different start-up conditions (nontypical “cold” practice and nontypical “hot” practice). Temperature measurements were taken at various locations on the ingot rolling and narrow faces, ingot base, and top surface of the bottom block. Ingot-based deflection data were also obtained for the two test conditions. Comparison of the model predictions with the data collected from the cast/embedded thermocouples indicates that the model accounts for the processes of water ejection and water incursion and is capable of describing the flow of heat in the early stages of the casting process satisfactorily.

Decolorization of Rhodamine B in Water by Pulsed High-Voltage Gas Discharge

The red coloring in water colored by an organic dye, rhodamine B, was removed in as fast as 10 s by pulsed gas discharge. Water was atomized by air and flowed as a mist down the central part of a reactor and as a water film down the inner wall. Radicals, such as ozone, OH and O, which were produced by radial barrier discharge, decolorized rhodamine B. Color was completely removed at a low water flow rate and high total input energy, but the removal efficiency was highest at a high water flow rate and low total input energy. By using oxygen, the color removal and removal efficiency were improved due to higher ozone concentration produced than in the case of using air. The efficiency was improved in proportion to the initial concentration of rhodamine B. The highest removal efficiency of 8.2 µg/J was obtained at 22 mg/L and a water flow rate of 150 mL/min. The results of the discharge performed in Ar suggest that the radicals produced from water considerably contributed to the decolorization. By changing the reactor radius, we clarified that the water flowing down the reactor wall as a water film was decolorized as well as the atomized water in the reactor.

The effect of water ejection and water incursion on the evolution of thermal field during the start-up phase of the direct chill casting process

A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the direct chill casting process. The model, based on the commercial finite element package ABAQUS, includes primary cooling to the mould, secondary cooling to water and ingot base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of surface temperature, water flow rate and position relative to the point of water impingement. In addition, the secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat extraction rates). The algorithm used to describe ingot base cooling, includes the drop in contact heat transfer due to base deformation (butt curl) and also the increase in heat transfer due to water incursion between the ingot base and the bottom block. The model has been verified against temperature measurements obtained from two 711×1680 mm AA5182 ingots, cast under different conditions (non-typical “cold” practice and non-typical “hot” practice). Ingot base deflection data has also been obtained for the two test castings. Comparison of the model predictions with the data collected from the embedded thermocouples indicates that a 2-D longitudinal model is capable of describing the flow of heat in the early stages of the casting process in a region close to the centre of the rolling face. A sensitivity analysis completed with the model has clearly identified the link between ingot base cooling and secondary water-cooling heat transfer during the start-up phase.

Diffusion-Limited Mobilization and Transport of Natural Colloids in Macroporous Soil

This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time-dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time-dependent and possibly diffusion-limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.

Diffusion‐Limited Mobilization and Transport of Natural Colloids in Macroporous Soil

This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time‐dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time‐dependent and possibly diffusion‐limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.

Diffusion-Limited Mobilization and Transport of Natural Colloids in Macroporous Soil

This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time-dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time-dependent and possibly diffusion-limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.