Diffusive Transport of VOCs through LLDPE and Two Coextruded Geomembranes

The diffusive properties of two coextruded geomembranes, one with a polyamide inner core and the other with an ethylene vinyl-alcohol (EVOH) inner core, and a standard 0.53-mm (20-mil) linear low-density polyethylene (LLDPE) geomembrane were examined. Diffusion and sorption laboratory tests were performed to estimate the parameters controlling diffusive migration, including the partitioning, diffusion, and permeation coefficients of the geomembrane in both the aqueous and vapor phases. Results indicate a significant reduction in mass flux through the coextruded geomembranes compared to conventional LLDPE. The EVOH coextruded geomembrane had the lowest permeation coefficients (Pg) with a range of (2–6)×10?12?m2?s?1 for diffusion from the aqueous phase. These values for EVOH are upper bounds and the actual values may be lower than as stated. The polyamide (nylon) coextruded geomembrane had higher values than for EVOH, with a Pg range of (0.7–2.2)×10?11?m2?s?1 from the aqueous phase. The highest permeation coefficients were for the standard 20-mil LLDPE, which ranged from (0.6–1.1)×10?10?m2?s?1. Thus the permeation coefficient for LLDPE was about one order of magnitude greater than for the nylon coextruded and at least two orders of magnitudes higher than for the EVOH coextruded geomembrane. Both coextruded geomembranes showed decreased Pg values and therefore improved diffusive resistance to volatile organic compounds over traditional 0.56-mm PVC geomembranes. The EVOH geomembrane showed a 5–12-fold decrease in Pg in comparison to a 2.0-mm high density polyethylene geomembrane.     

Estimation of Diffusion Coefficients and Solubilities for Organic Solvents Permeation through High-Density Polyethylene Geomembrane

Experiments on the permeation of several chlorinated and aromatic hydrocarbons through high-density polyethylene (HDPE) geomembranes were conducted using the ASTM F-739 standard test method. The diffusion coefficients were estimated by a one-dimensional diffusion equation based on Fick’s second law, and the solubilities of the solvents in HDPE were determined by the steady state permeation rates. The one-dimensional transient model was able to simulate the permeation concentrations and implied that equilibrium partition between organic solvent and HDPE geomembrane was not achieved during the initial permeation. The solubilities of organic solvents in the HDPE geomembranes obtained by immersion tests or weight gain methods of permeation experiments were not an appropriate boundary condition for the model simulation of permeation. It was found that the diffusion coefficients and solubilities of organic solvents correlated well with their molecular weights and dipole moment, respectively. The present work provides information on the extent of organic compounds permeations through HDPE geomembranes as applied in hazardous waste landfills.     

Estimation of Mass Transport Parameters of Organic Compounds through High Density Polyethylene Geomembranes Using a Modifield Double-Compartment Apparatus

A modified double-compartment apparatus (MDCA) is used to estimate mass transport parameters of organic compounds through high density polyethylene (HDPE) geomembranes and to investigate the effects of aging and external tension of HDPE geomembranes on the mass transport of organic compounds. A developed one-dimensional partition–diffusion mass transport model successfully explains the mass transport of the organic compounds through the HDPE geomembranes in a dilute aqueous solution–geomembrane system. Similar to batch immersion tests, the HDPE–water partition coefficient (KHDPE–W) values of organic compounds are found to have close relationships with the octanol–water partition coefficient and the aqueous solubility; furthermore, the diffusion coefficient (D) values decrease with the increase of their molecular diameter. For HDPE geomembranes served in the landfill liner for 5 years and stretched by 8% of their initial length, KHDPE–W values for organic compounds increase by 5–58%, D values for organic compounds increase by 10–86%, and breakthrough times are faster, indicating more amounts of organic compounds may break through the HDPE geomembrane in fields than expected. The mass transport parameters from MDCA tests could be used with those from batch immersion tests interchangeably after mass loss and immobilization of organic compounds in MDCA tests are considered.     

Effects of Thickness on the Aging of HDPE Geomembranes

The results of an accelerated aging test program to evaluate the effect of thickness on the depletion of antioxidants from high-density polyethylene (HDPE) geomembranes and subsequent degradation of the physical properties are reported. Three commercially available HDPE geomembranes having nominal thicknesses of 1.5, 2.0, and 2.5 mm were examined. The geomembranes were immersed in a synthetic leachate at 85, 70, 55, and 22°C and tested for oxidative induction time, crystallinity, melt index (MI), tensile properties, and stress-crack resistance. The antioxidant depletion rate for the 1.5 mm geomembrane was faster than for the 2.0 and 2.5 mm geomembranes. Antioxidant depletion time was predicted at representative landfill temperatures of 20–60°C using Arrhenius modeling and was found to increase with geomembrane thickness for the three geomembranes examined. Based on the results of crystallinity, MI, and stress-crack resistance, the degradation of the geomembrane was slowest for the thickest geomembrane. These results suggest that a thicker geomembrane may have a longer service life (other things being equal).     

Antioxidant Depletion from a High Density Polyethylene Geomembrane under Simulated Landfill Conditions

Accelerated aging tests to evaluate the depletion of antioxidants from a high density polyethylene geomembrane are described. The effects of temperature, high pressure, and continuous leachate circulation on the aging of geomembranes in composite liner systems are examined. The antioxidant depletion rates (0.05, 0.19, and 0.41?month?1 at 55, 70, and 85°C, respectively) obtained for the simulated landfill liner at 250 kPa vertical pressure are consistently lower than that obtained from traditional leachate immersion tests on the same geomembrane (0.12, 0.39, and 1.1?month?1 at 55, 70, and 85°C). This difference leads to a substantial increase in antioxidant depletion times at a typical landfill liner temperature (35°C) with 40 years predicted based on the data from the landfill liner simulators tests, compared to 15 years predicted for the same geomembrane based on leachate immersion tests. In these tests, the crystallinity and tensile yield strain of the geomembrane increased in the early stages of aging and then remained relatively constant over the testing period. There was no significant change in other geomembrane properties within the testing period.     

Permeation of Organics through Linear Low Density Polyethylene Geomembranes under Mechanical Deformation

The effect of mechanical deformation on the permeation of methylene chloride (MEC), trichloroethylene (TCE) as well as mixtures thereof through linear low-density polyethylene (LLDPE) geomembranes was studied using a new experimental technique. A total of fifty experimental conditions involving five different deformations, five different concentrations and two types of LLDPE geomembranes were evaluated following a full factorial experimental design. Mixtures of TCE and MEC (0.66, 0.50, and 0.34 volumetric fraction) were tested through geomembranes elongated uniaxially (10% and 20%) as well as in the biaxial mode (10%×10%?and?20%×20%). The breakthrough times for the permeation of TCE and MEC decreased with elongation for both types of geomembranes. A reduction in breakthrough times between 38 and 45% was observed for the permeation of MEC and TCE though geomembranes elongated 20%×20% in the biaxial mode compared to the nonelongated specimens. Evidence of stress-enhanced transport was also observed as the steady-state permeation rates of MEC increased between 200 and 300% where geomembranes were elongated 20%×20% in the biaxial mode compared to the nonelongated samples. Enhanced transport of MEC was also noted during the permeation of the MEC–TCE mixtures.     

Volatile Organic Compound (VOC) Transport through Compacted Clay

Movement of volatile organic compounds (VOCs) through compacted clay liners was investigated using laboratory-scale column and tank tests. Hydraulic conductivity of the compacted clay was not significantly impacted by the introduction of VOCs in concentrations up to 20 mg∕L. Soil-water partition coefficients of the seven VOCs tested had a strong logarithmic relationship with the octanol-water partition coefficient. Partition coefficients from batch tests were in good agreement with those measured directly on soil samples at the termination of the column∕tank tests. The VOCs were degraded in the clay, with estimated half-lives ranging from 2 to 116 days. Mechanical dispersion was not significant in the range of the hydraulic conductivities of the test specimens (i.e., <10?7 cm∕s). Effective molecular diffusion coefficients were mostly in 10?6 cm2∕s and generally decreased with increasing aqueous solubility. Mass transport parameters of VOCs in clay liners can be estimated from laboratory batch tests and properly prepared small-scale column tests. However, accounting for degradation of VOCs and minimizing the number of transport parameters that are simultaneously estimated from a single response-time record are important considerations for accurate determination of transport parameters.     

Transport of Organic Contaminants in Geomembranes under Stress

The transport properties of aqueous solutions of benzene, dichloromethane, and trichloroethylene through extended polyvinyl chloride and high-density polyethylene geomembranes are investigated. It is found that extension enhances the permeation rates of the penetrants through polyvinyl chloride geomembranes, and the opposite effect is found in the case of high-density polyethylene. This difference in response is attributed to the type of structural change, which occurred as a result of the extension. The diffusivities of a mixture of the three organic solvents through the geomembranes are also determined.     

Impact of Chlorinated Water Exposure on Contaminant Transport and Surface and Bulk Properties of High-Density Polyethylene and Cross-Linked Polyethylene Potable Water Pipes

The aim of this work was to determine if the aging of polyethylene (HDPE, PEX-A and PEX-B) water pipes by exposure to chlorinated water altered polar and nonpolar contaminant diffusivity and solubility by analyzing new, laboratory-aged, and exhumed water-distribution system polyethylene (PE) pipes. After 141?days of aging in pH 6.5 water with 45??mg/L free chlorine, the surface chemistry and bulk properties of PEX-A pipe were unaffected. Carbonyl bonds (σ = 1,713??cm-1) were detected on the surfaces of HDPE and PEX-B pipe, and these oxygenated surfaces became more hydrophilic, resulting in statistically significant increases in diffusion rates. All 10 contaminant and four pipe material combinations had diffusivity increases on average of 50% for polar contaminants and 5% for nonpolar contaminants. Contaminant solubility was slightly increased for aged PEX-A and slightly decreased for PEX-B pipes. Toluene and trichloromethane diffusivity and solubility values for 7- to 25-year-old buried water utility pipes were similar to values for new and laboratory-aged HDPE-based materials. Because chlorinated water exposure alters how polar contaminants interact with aged PE pipes, results of this work should be considered in future health risk assessments, water quality modeling, pipe performance, and service-life considerations.     

Depletion of Antioxidants from a HDPE Geomembrane in a Composite Liner

The results of two series of accelerated aging tests are reported. Both series of tests were conducted at temperatures of 85, 70, 55, and 26°C over a period of about 3?years. In the simulated liner series, the top of the geomembrane was covered with a geotextile (protection) layer that was exposed to simulated municipal solid waste (MSW) landfill leachate while the bottom of the geomembrane was in contact with a hydrated geosynthetic clay liner. In the immersion series, the geomembrane was immersed in the simulated MSW leachate, and hence, both sides were exposed to leachate. The results from oxidative induction time tests indicate that the antioxidant depletion is about 2.2–4.8 times faster for the leachate immersed geomembrane than for geomembrane in a composite liner. The higher rates are attributed to the higher extraction of antioxidants from two sides of the geomembrane immersed in leachate. The measured antioxidant depletion rates are extrapolated to a range of temperatures (0–60°C) using Arrhenius modeling. At a liner temperature of 35°C, the calculated time for the depletion of antioxidants is about 40?years for a geomembrane in a composite liner compared to 10?years if it is simply immersed in leachate. These tests suggest that to obtain realistic estimates of geomembrane service life one needs data from tests that simulate the expected field conditions and that prediction based on immersion tests may underestimate the service life.     

Interactions between PVC Geomembranes and Compacted Clays

The interactions between plastic soils and polyvinyl chloride (PVC) geomembranes were studied using a direct shear device under as-compacted conditions. The PVC geomembranes had smooth or textured surfaces, and the soils were of plasticity index (PI) ranging from 35 to 100%. The peak and residual failure envelopes were expressed using Coulomb failure criteria. The adhesion and angle of friction increased for PIs up to 70% and subsequently recorded a decrease. The adhesion is larger for the peak strength compared to the residual strength, but it was the reverse for the angle of friction. The efficiency in terms of adhesion appeared more relevant than that of the angle of friction in expressing the interactions between geomembrane and cohesive soils. The smooth and textured geomembranes showed little difference in results at the residual state.     

Flow of Gasoline through Composite Liners

A composite liner composed of a soil/clay liner and a flexible membrane is widely used for waste containment facilities. In this research, an organically modified clay (organoclay BB-40) liner and a high-density polyethylene (HDPE) membrane were studied for preventing the leakage and migration of gasoline from underground storage tanks into the surrounding environment. The equivalent hydraulic conductivity of intact HDPE to gasoline was determined using a specially built system, and the conventional hydraulic conductivity testing method was employed to determine the hydraulic conductivity of compacted organoclays and the permeation rate of gasoline through composite liners. The equivalent hydraulic conductivity of intact HDPE to gasoline was about 10–13 cm/s, and the hydraulic conductivity of the organoclay liner was approximately 6.0×10?9?cm/s, which is nearly 4 orders of magnitude lower than that obtained for unmodified clay. These results show that both organoclay and HDPE are effective in reducing the release of gasoline by advective flow, especially the intact HDPE. The flow of gasoline through the composite liners under the worst condition, was of the same magnitude as that through a single organoclay liner, independent of the flow shape. It can be anticipated that under good contact conditions, the defective HDPE would still be beneficial in reducing the permeation of gasoline due to the decrease of the wetted area of the underlying layer exposed to gasoline leakage.     

Modeling Uncoupled Solute Transport in Natural Organic Matter Size Fractionation by Ultrafiltration

Physicochemical separation of organic macrosolutes and colloidal particles is routinely required during the analysis of natural, waste, and process waters derived from aquatic and terrestrial environmental samples. This study was conducted to demonstrate the utility of a two-parameter nonlinear permeation coefficient model (PCM) in describing the uncoupled transport of solutes in dilute heterogeneous solutions subjected to batch ultrafiltration (UF). The PCM was used in conjunction with natural organic matter (NOM) permeate data for a natural water and six hydrophobic and hydrophilic subfractions to determine permeation coefficients p and NOM concentrations Cr0 with apparent molecular weight less than membrane specific cutoff values of moderately hydrophilic YC/YM series Amicon? UF membranes. Experimentally measured permeation coefficients p determined for the whole water were found to correlate well with composite permeation coefficients p? calculated using a mass-fraction weighted average of individual NOM subfraction permeation coefficient values. Correlation of experimentally measured and calculated permeation coefficient values (p and p?) indicated that the PCM can adequately describe uncoupled transport of chemically distinct solute fractions during batch UF of heterogeneous dilute solutions.     

Characterizing Polyurethane Foam as a Sink for or Source of Volatile Organic Compounds in Indoor Air

Polyurethane foam (PUF) is widely used in indoor consumer products. Despite strong potential interactions with volatile organic compounds (VOCs), the effect of PUF on indoor concentrations of VOCs has not been examined. This study determines the behavior of PUF as a potential sink for or source of VOCs in indoor air. A flexible polyether-type foam and eight aromatic VOCs ranging in molecular weight from naphthalene to benzene were studied. Rapid determinations of PUF–air partition coefficient (K) and PUF–phase diffusion coefficient (D) were achieved using a dynamic microbalance procedure. A diffusion model was applied to interpret the experimental data. The PUF sample was assumed to conform to semi-infinite cylindrical geometry with solid-phase diffusion being the rate limiting step. The results indicate that sorption of VOCs by PUF is fully reversible. For the VOCs studied, K can be correlated with vapor pressure and D with molecular free surface area. Humidity appears to reduce the extent of sorption and slow the sorption kinetics. These findings should facilitate the prediction of the source/sink behavior of PUF and the related impact on VOC concentrations in the indoor environment.     

Kinetics of permeation of nucleotide through oil/water interface in the interaction of nucleotide with octadecylamine and dodecyl guanidine

The transports of tritiated ATP, ADP and AMP from the aqueous to scintillator phase with and without octadecylamine (or dodecyl guanidine) have been studied by the layered scintillation method and a theory suitable for an explanation of the results has been presented. (1) Transport processes were all expressed by the first order kinetics. (2) For the simple partitioning of ATP, the reciprocal of the rate constant of the backward permeation was linear with respect to the square of the partition coefficient. (3) For the transport of nucleotide with chemical reaction, the reciprocal of the rate constant of the backward permeation was linear against the overall partition coefficient of nucleotide. (4) A theory was presented on the basis of a general diffusion equation by assuming the two-film model with potential energy near the interface. (5) The theory could explain the dependences of the permeation rates on the partition coefficients. (6) From the finding that the ratio of the apparent diffusion coefficient in aqueous to scintillator phase was much smaller than unity, the occurrence of an energy barrier at interface was suggested. For the simple partitioning of ATP, the energy barrier was not significant.     

Diffusion constraints and neuron-glia interaction during aging

Changes in brain extracellular space (ECS) volume, composition, and geometry are a consequence of neuronal activity, of glial K+, pH, and amino acid homeostasis, and of changes in glial cell morphology, proliferation, and function. They occur as a result of repetitive neuronal activity, seizures, anoxia, injury, inflammation, and many other pathological states in the CNS, and may significantly affect signal transmission in the CNS. Activity-related or CNS damage-related cellular swelling is compensated for by ECS volume shrinkage and, as a consequence, by a decrease in the apparent diffusion coefficients (ADCs) of neuroactive substances diffusing in the ECS. Changes in cellular morphology, such as occur during aging, could also result in changes of ECS volume and geometry. We provide evidence for limited diffusion in rat cortex, corpus callosum, and hippocampus in the aging brain that correlates with changes in glial volume and the extracellular matrix. In all structures, the mean ECS volume fraction alpha (alpha = ECS volume/total tissue volume) and nonspecific uptake k' are significantly lower in aged rats (26-32 months old) than in young adult brain. Compared to young adult brain, in the aged brain we found an increase in GFAP staining and hypertrophied astrocytes with thicker processes which, in the hippocampus, lost their radial organization. The tortuosity (lambda = square root of D/ADC) was lower in the cortex and CA3 region. Immunohistochemical staining for fibronectin and chondroitin sulfate proteoglycans revealed a substantial decrease that could account for a decrease in diffusion barriers. Diffusion parameters alpha, lambda, and k' in the aging brain after cardiac arrest changed substantially faster than in the young adult brain, although the final values were not significantly different. This suggests that the smaller extracellular space during aging results in a greater susceptibility of the aging brain to anoxia/ischemia, apparently due to a faster extracellular acidosis and accumulation of K+ and toxic substances, for example, glutamate. We conclude that during aging the movement of substances is more hindered in the narrower clefts. This is partly compensated for by a decrease in the diffusion barriers that may be formed by macromolecules of the extracellular matrix. Diffusion parameters can affect the efficacy of synaptic as well as extrasynaptic transmission by a greater accumulation of substances, because they diffuse away from a source more slowly, or induce damage to nerve cells if these substances reach toxic concentrations. Diffusion parameters are also of importance in the "crosstalk" between synapses, which has been hypothesized to be of importance during LTP and LTD. We can, therefore, assume that the observed changes in ECS diffusion parameters during aging can contribute to functional deficits and memory loss.     

Contaminant Diffusion, Solubility, and Material Property Differences between HDPE and PEX Potable Water Pipes

The objectives of this work were to identify differences between the composition, bulk properties, contaminant diffusivity and solubility for new high-density polyethylene (HDPE) pipe and crosslinked polyethylene (PEX) pipe, as well as determine which contaminant and polymer properties are useful for predicting contaminant fate in water pipe. Variations in PE pipe density (0.9371–0.9547?g/cm3), crystallinity (69–72%), crosslinking (60 and 76%), and oxidative induction time (33 to >295?min.) were detected. While numerically these differences seem minor, results show that slight material differences have a notable effect on contaminant diffusivity and solubility. Nonpolar contaminant diffusivity and solubility were best predicted by bulk density. Polar contaminants were more soluble and diffused faster through PEX than HDPE pipes because PEX pipes contained a greater amount of oxygen. For all materials, dipole moment and Log Kow were good predictors of contaminant fate and molecular volume was only useful for predicting diffusivity and solubility values for haloalkane and nonpolar aromatic contaminants.     

某矿山尾矿库渗流电模拟试验研究

某矿山尾矿库三期扩建设计方案中,堆积高程为734 m时洪水运行的最小干滩长度不能满足要求。为了确定此堆积高程坝体逸出点位置及浸润线情况,进行了三组电模拟试验,即设计方案、管式排渗方案和铺设土工膜方案。试验结果表明,管式排渗和铺设土工膜均可有效地降低坝体浸润线,有利于尾矿库坝体的稳定。     

Comparison of Solute Transport in Three Composite Liners

Three composite landfill liners were compared in this study based on leakage rate, mass flux, and sorptive capacity. One composite liner consisted of a geomembrane and a geosynthetic clay liner (GCL). The other two had a geomembrane and a thicker soil barrier (61 or 122 cm). The analyses employed one- and three-dimensional numerical models that were developed for analyzing contaminant transport through defects in the geomembrane component of composite liners and diffusion of volatile organic compounds through intact composite liners (i.e., composite liners without holes in the geomembrane). Cadmium was used to represent inorganic leachate constituents and toluene was used to represent organic leachate constituents. The composite liner, having a GCL had the lowest leakage rate of the three composite liners. For cadmium, the mass flow rate and sorptive capacity for the three composite liners varied within an order of magnitude. However, for toluene, the mass flux from the GCL composite liner was two to three orders of magnitude greater than that through composite liners having a thicker soil liner. Additionally, for leachate having similar concentrations of cadmium and toluene, the mass flux of toluene can be as much as seven orders of magnitude greater than that for cadmium. For toluene, the sorptive capacity of thicker liners was an order of magnitude greater than that for the GCL composite liner. Similar behavior is expected for other inorganic and organic solutes.     

垃圾填埋场渗沥液调节池的浮盖设计与施工

在调节池设计上采用浮盖技术，并用高密度聚乙烯（HDPE）土工防渗膜作为浮盖材料，结合工程经验对填埋场渗沥液调节池大面积池面加盖的设计与施工进行阐述。