The effect of changes in surface wettability on two-phase saturated flow in horizontal replicas of single natural fractures

By using translucent epoxy replicas of natural single fractures, it is possible to optically measure aperture distribution and directly observe NAPL flow. However, detailed characterization of epoxy reveals that it is not a sufficiently good analogue to natural rock for many two-phase flow studies. The surface properties of epoxy, which is hydrophobic, are quite unlike those of natural rock, which is generally assumed to be hydrophilic. Different surface wettabilities result in dramatically different two-phase flow behavior and residual distributions. In hydrophobic replicas, the NAPL flows in well-developed channels, displacing water and filling all of the pore space. In hydrophilic replicas, the invading NAPL is confined to the largest aperture pathways and flow frequently occurs in pulses, with no limited or no stable channel development, resulting in isolated blobs with limited accessible surface area. The pulsing and channel abandonment behaviors described are significantly different from the piston-flow frequently assumed in current modeling practice. In addition, NAPL never achieved total saturation in hydrophilic models, indicating that significantly more than a monolayer of water was bound to the model surface. Despite typically only 60-80% NAPL saturation, there was generally good agreement between theoretically calculated Young-Laplace aperture invasion boundaries and the observed minimum apertures invaded. The key to determining whether surface wettability is negligible, or not, lies in accurate characterization of the contaminant-geologic media system under study. As long as the triple-point contact angle of the system is low (<20 degrees), the assumption of perfect water wettability is not a bad one.     

The effect of changes in surface wettability on two-phase saturated flow in horizontal replicas of single natural fractures

By using translucent epoxy replicas of natural single fractures, it is possible to optically measure aperture distribution and directly observe NAPL flow. However, detailed characterization of epoxy reveals that it is not a sufficiently good analogue to natural rock for many two-phase flow studies. The surface properties of epoxy, which is hydrophobic, are quite unlike those of natural rock, which is generally assumed to be hydrophilic. Different surface wettabilities result in dramatically different two-phase flow behavior and residual distributions. In hydrophobic replicas, the NAPL flows in well-developed channels, displacing water and filling all of the pore space. In hydrophilic replicas, the invading NAPL is confined to the largest aperture pathways and flow frequently occurs in pulses, with no limited or no stable channel development, resulting in isolated blobs with limited accessible surface area. The pulsing and channel abandonment behaviors described are significantly different from the piston-flow frequently assumed in current modeling practice. In addition, NAPL never achieved total saturation in hydrophilic models, indicating that significantly more than a monolayer of water was bound to the model surface. Despite typically only 60–80% NAPL saturation, there was generally good agreement between theoretically calculated Young–Laplace aperture invasion boundaries and the observed minimum apertures invaded. The key to determining whether surface wettability is negligible, or not, lies in accurate characterization of the contaminant-geologic media system under study. As long as the triple-point contact angle of the system is low (< 20°), the assumption of perfect water wettability is not a bad one.     

Scaling of spontaneous imbibition data with wettability included

Wettability is a dominant parameter governing spontaneous imbibition. However less attention has been paid to the effect of wettability on the scaling of spontaneous imbibition data. Actually few models can include wettability in scaling of spontaneous imbibition data. To this end, a scaling model has been developed for NAPL (oil)-saturated porous media with different wettability based on the fluid flow mechanisms in porous media. Relative permeability, capillary pressure, initial water saturation, and wettability are considered in the scaling model. Theoretically this scaling model is suitable for both cocurrent and countercurrent spontaneous imbibition. The experimental data of countercurrent spontaneous water imbibition at different wettability cannot be scaled using the frequently used scaling model but can be scaled satisfactorily using the scaling model developed in this study. An analytical solution to the relationship between recovery and imbibition time for linear spontaneous imbibition has also been derived in the case in which gravity is ignored. The analytical solution predicts a linear correlation between the recovery by spontaneous water imbibition and the square root of imbibition time, which has been verified against experimental data.     

A new model for humic materials and their interactions with hydrophobic organic chemicals in soil-water or sediment-water systems

A generalized model of humic materials in soils and sediments, which is consistent with their observed properties, is presented. This model provides a means of understanding the interaction of hydrophobic pollutants with humic materials. In this model, it is proposed that the humic materials in soils and sediments consist of a number of different oligomers and simple compounds which result from the partial degradation of plant remains. These degradation products are stabilized by incorporation into humic aggregates bound together by weak bonding mechanisms, such as hydrogen bonding, pi bonding, and hydrophobic interactions. The resulting structures are similar to micelles or membranes, in which the interiors of the structures are hydrophobic and the exteriors are hydrophilic. Hydrophobic compounds will partition into the hydrophobic interiors of the humic micelles or “membrane-like” structures.     

Wettability hysteresis and its implications for DNAPL source zone distribution

Subsurface heterogeneity at sites contaminated with nonaqueous phase liquids (NAPLs) reduces the effectiveness of traditional remediation measures. One cause may be the increased proportion of NAPL that is hydraulically isolated due to capillary trapping in heterogeneously-wetted materials. This study examines the wettability of ten materials, ranging from minerals, such as calcite and dolomite, to carbonaceous materials, such shale and coal, in air and water, NAPL and air, and NAPL and water systems. The wettability differed depending on which phase the solid material was initially immersed in: the less crystalline solids, if initially contacted by water were water-wet, but if initially contacted by NAPL were NAPL-wet. This difference, termed here wettability hysteresis, was observed for a suite of halogenated NAPLs and was independent of equilibration time. The degree of wettability hysteresis was greatest in the NAPL and water systems, with the magnitude of the difference increasing with the carbonaceous materials. Since the degree of capillary trapping in subsurface materials is related to wettability, the phenomenon of wettability hysteresis suggests that system history is a factor that may increase the heterogeneity of NAPL source zones.     

Kinetic constraints on theIn-situ remediation of soils contaminated with organic chemicals

Cleanup of contaminated soils to comply with soil quality limits currently receives much interest.In-situ remediation of contaminated soils relies on the ability of the techniques employed to enhance the rate of release of contaminants from the soil-sorbed and nonaqueous phase liquid (NAPL) phases into the aqueous or gaseous phases from which they can be more readily removed and treated. Contaminant concentrations in these “environmentally mobile” forms usually decline over time so that the economic efficiency and the overall success of remediation technologies are subject to the “law of diminishing returns”. In this paper we consider the “state of the art” in our understanding of NAPL dissolution and transport, desorption of soilsorbed contaminants and fluid flow in porous media. The extent to which these processes may constrain the success of bioremediation, pump-and-treat remediation and soil venting in relation to established soil quality limits is addressed. Finally, we suggest directions for future research and comment on legislative considerations.     

Pentachlorophenol sorption by variable-charge soils in methanol-water mixture: pH effect at the low solvent volume fraction

Information on how cosolvents affect sorption of ionizable chemicals by soils with heterogeneous variable-charge mineral surface domains is sparse. In this study, the effect of soil-solution pH in methanol/water solutions on sorption of pentachlorophenol (PCP) by variable-charge soils with a range of hydrophilic (f Hphilic) and hydrophobic (f Hphobic) sorption domains was characterized. PCP sorption by 10 variable-charge soils was measured as a function of apparent pH (pH app) and methanol volume fraction (fc8) decreased log-linearly with increasing fc, but the slope of the relationship was less for anionic PCP. The empirical solvent-sorbent interaction term for anionic PCP (alpha i) was inversely correlated with f Hphilic (r2=0.82), which is consistent with methanol-induced increases in anion exchange. For neutral PCP, the empirical term (alpha n) was positively correlated with f Hphobic (r2=0.84), supporting methanol-induced increases in solution and sorbent hydrophobicity. Sorption of PCP by two soils with varying f Hphilic in the pH app range from 3 to 8 at fc相似文献   

Microbial physiology-based model of ethanol metabolism in subsurface sediments

Dense non-aqueous phase liquids (DNAPLs) present in the subsurface may contain surface active compounds that impact DNAPL migration and distribution. While a number of studies have revealed the role surface active compounds play in altering the wettability of quartz sand, few have considered the implications for other minerals common to contaminated sites. This study extends understanding of DNAPL/surfactant wettability to iron oxide surfaces. Specifically, quartz and iron oxide-coated sands in a tetrachloroethene (PCE)/water system containing the organic base (an organic molecule that acts as a base) dodecylamine (DDA) were compared at a variety of scales. Wettability of the minerals' surfaces, and the impact of wettability on capillary resistance to DNAPL entry, were assessed as a function of pH through: (i) advancing and receding contact angles, (ii) primary drainage capillary pressure-saturation experiments, and (iii) small, two-dimensional, flow cell experiments. The work revealed that, at neutral pH and under identical boundary capillary pressures, DNAPL invaded quartz sand but not iron oxide-coated sand; however, at low pH, DNAPL invaded both sands equally. These differences were demonstrated to be due to wettability alterations associated with the strength of attractive forces between DDA and the mineral surface, dictated by the isolectric point of the minerals and system pH. Observed differences in DNAPL invasion behavior were consistent with measured intrinsic contact angles and P(c)-S relationships, the latter requiring scaling by the operative contact angle inside the porous medium for a meaningful comparison. This study suggests that the distribution of minerals (and, more specifically, their isoelectric points), as well as the aqueous phase pH at a given site, may have a significant impact on the DNAPL source zone architecture.     

Evaluation of PPCPs removal in a combined anaerobic digester-constructed wetland pilot plant treating urban wastewater

The removal efficiency of 16 pharmaceuticals and personal care products (PPCPs) from urban wastewater (dissolved and particulate phases) was evaluated for the first time in a hybrid pilot plant consisting of an upflow anaerobic sludge blanket reactor followed by two sequentially connected horizontal flow constructed wetlands: a surface flow wetland (SF CW) and a subsurface flow wetland (SSF CW). Whereas the PPCP removal associated with the dissolved phase exhibited a seasonal pattern, the fraction associated with the suspended solids showed less seasonality. In the dissolved phase, the overall removal efficiency in summer ranged from 70% to 85% for salicylic acid (SAL), methyl dihydrojasmonate, caffeine (CAF), ketoprofen and triclosan, whereas in winter it declined for most of the PPCPs to between 30% and 50%, except for CAF and SAL (>80%) and carbamazepine and butylated hydroxyl toluene (11-18%). In the suspended solids, the removal exceeded 80% for most of the target PPCPs. The efficiency of the different treatment steps was also compound-dependent, but the SF CW generally exhibited the highest removal efficiency for most of the contaminants analyzed. The characterization of the organic matter retained in the wetland gravel beds revealed the occurrence of hydrophobic contaminants such as phthalate esters and fragrances at moderate concentrations (i.e., up to 3.5 μg kg⁻¹), which declined strongly over the course of the different treatment steps. In the SF CW, the net mass accumulation rates of tonalide and galaxolide were 4 and 23 g y⁻¹ respectively, whereas in the SSF CW they were 0.3 and 1.8 g y⁻¹ respectively.     

亲水性塑料弹性填料生物膜法处理模拟废水的研究

通过比较普通塑料弹性填料与亲水性改进后的塑料弹性填料在污水生化处理中的应用效果发现 :亲水性改进的塑料弹性填料在污水的厌氧生化处理中可提高污水处理效率 15 %— 30 % ,在好氧生化处理中可提高污水处理效率5 %— 10 % ,且亲水性改进大大提高了塑料弹性填料的挂膜速度。     

Nontargeted evaluation of the fate of steroids during wastewater treatment by comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry

Emerging organic contaminants in wastewater are usually analyzed by targeted approaches, and especially estrogens have been the focus of environmental research due to their high hormonal activity. The selection of specific target compounds means, however, that most of the sample components, including transformation products and potential new contaminants, are neglected. In this study, the fate of steroidal compounds in wastewater treatment processes was evaluated by a nontargeted approach based on comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry. The potential of the nontargeted approach to generate comprehensive information about sample constituents was demonstrated with use of statistical tools. Transformation pathways of the tentatively identified compounds with steroidal four-ring structure were proposed. The purification efficiency of the wastewater treatment plants was studied, and the distribution of the compounds of interest in the suspended solids, effluent water, and sludge was measured. The results showed that, owing to strong adsorption of hydrophobic compounds onto the solid matter, the steroids were mostly bound to the suspended solids of the effluent water and the sewage sludge at the end of the treatment process. The most abundant steroid class was androstanes in the aqueous phase and cholestanes in the solid phase. 17β-estradiol was the most abundant estrogen in the aqueous phase, but it was only detected in the influent samples indicating efficient removal during the treatment process. In the sludge samples, however, high concentrations of an oxidation product of 17β-estradiol, estrone, were measured.     

Nutrient losses by surface run-off following the application of organic manures to arable land. 1. Nitrogen

Research was conducted on nitrogen (N) surface run-off losses following organic manure applications to land, utilising a purpose-built facility on a sloping site in Herefordshire under arable tillage. Different rates and timing of cattle slurry, farmyard manure and inorganic N and phosphorus (P) fertiliser were compared, over a 4-year period (1993-97). P losses from the same studies are reported in a separate paper. The application of cattle slurries to the silty clay loam soil increased the loss of solids and NH4(+)-N in surface water flow compared to control plots receiving inorganic fertiliser only, or no treatment, but had little effect on NO3(-)-N losses by this route. Results were consistent with other observations that rainfall events immediately after manure applications are particularly likely to be associated with nutrient run-off losses. Losses via subsurface flow (30 cm interflow) were consistently much lower than via surface water movement and were generally unaffected by treatment. Increasing slurry application rate and, in particular, slurry solids loading, increased solids and NH4(-)-N losses via surface run-off. The threshold, above which the risk of losses via surface run-off appeared to be greatly increased, was ca. 2.5-3.0 t/ha slurry solids, which approximates to the 50 m3/ha limit suggested for slurry within UK 'good agricultural practice'. Sealing of the soil surface by slurry solids appears to be a possible mechanism by which polluting surface run-off may occur following slurry application on susceptible soils. Total losses of NH4(+)-N and NO3(-)-N during the 4-year monitoring period were insignificant in agronomic terms, but average soluble N concentrations (NH4(+)-N + NO3(-)-N) in run-off, ranging from ca. 2.0 mg/l, up to 14.0 mg/l for the higher rate slurry treatments. Peak concentrations of NH4(+)-N > 30 mg/l, are such as to be of concern in sensitive catchments, in terms of the potential for contribution to accelerated eutrophication and adverse effects on freshwater biota.     

Predicting surfactant modified soil/water distribution coefficients using micellar HPLC

Soil water/distribution coefficients (Kd) have been measured for the partitioning of naphthalene, phenanthrene and pyrene between aqueous surfactant solutions and a clean soil. The surfactants used are ABA block copolymers constructed from ethylene oxide (the monomer used to synthesise the hydrophilic A blocks) and propylene oxide (used for the manufacture of the hydrophobic B block). Three of these surfactants comprising the same size propylene oxide block but different ethylene oxide/propylene oxide ratios were investigated. Increasing amounts of surfactant in the system result in a progressive decrease in the Kd values signifying an increasing tendency for the hydrophobic solutes to be dispersed in aqueous solution due to the action of the surfactant. More significantly for equal surfactant doses the most hydrophobic surfactant possessing the lowest ethylene oxide/propylene oxide ratio reduces Kd by the greatest amount whereas the most hydrophilic surfactant reduces Kd the least. Finally micellar HPLC using the above surfactants and hydrophobic solutes was undertaken. Interpolated capacity factors evaluated for particular surfactant doses correlated well with Kd values calculated for the same surfactant doses. The relationship between Kd and capacity was found to be log-linear and the correlation line could be fitted to the data obtained for all three surfactants. It is therefore concluded that micellar HPLC may be used for preliminary evaluations of the effectiveness of particular surfactants proposed for contaminated soils restoration schemes.     

生物过滤塔处理实验室废气

研究了生物过滤塔处理实验室排放的模拟混合废气,考察了反应器对苯、甲苯、二甲苯、乙醇、丙酮、乙酸乙酯和甲烷等废气的去除效果。运行结果表明,在设备稳定运行期间,进气中总挥发性有机物(TVOCs)的浓度为124～380 mg/m3,而出气浓度在10～40 mg/m3,去除效率保持在85%以上。实验室废气中的多种污染物在生物过滤塔中去除机理不同,亲水性污染物的去除效率高于疏水性污染物。通过系统关停后重启,污染物的去除效果在第2天就能恢复,这为生物过滤塔处理实验室废气过程的停运检修或者系统闲置提供了可行性。     

Sorption of isoxaflutole diketonitrile degradate (DKN) and dicamba in unsaturated soil

When analyzing the sorption characteristics of weakly sorbing or labile pesticides, batch methods tend to yield a high margin of error attributable to errors in concentration measurement and to degradation, respectively. This study employs a recently developed unsaturated transient flow method to determine the sorption of isoxaflutole's herbicidally active diketonitrile degradate (DKN) and dicamba. A 20-cm acrylic column was packed with soils with varied texture that had been uniformly treated with 14C-labeled chemical.The antecedent solution herbicide in equilibrium with sorbed phase herbicide was displaced by herbicide-free water, which was infiltrated into the column. Sorption coefficients, Kd, were obtained from a plot of total herbicide concentration in the soil versus water content in the region where the antecedent solution accumulated. DKN Kd values were approximately 2-3 times (average Kd = 0.71 L kg-1) greater using the unsaturated transient flow method as compared to the batch equilibration method in clay loam (Kd = 0.33 L kg-1), but similar for the two methods in sand (0.12 vs 0.09 L kg-1) soils. Dicamba Kd values were 3 times greater using the unsaturated transient flow method as compared to the batch equilibration method in the clay loam soil (0.38 vs 0.13 L kg-1), however, the Kd values were the same for the two methods in the sand (approximately 0.06 L kg-1). This demonstrates that to determine sorption coefficients for labile hydrophilic pesticides, an unsaturated transient flow method may be a suitable alternative to the batch method. In fact, it may be better in cases where transport models have overpredicted herbicide leaching when batch sorption coefficients have been used.     

Volatilisation of o-xylene from sandy soil

The diffusive release of o-xylene from two soils with different contents of organic carbon (1.1 % and 0.11 % TOC) and with two different water contents (app. 5 % w/w and 15 % w/w was studied in the laboratory. The soils were spiked with o-xylene in the laboratory. The fluxes were measured over a period of 24 hours. The measured fluxes were compared to predictions by two models. Model I, which is an analytical model, assumed instant local equilibrium between soil air, water and solids. The distribution coefficients were measured for the two soils, and Henry's constant and the diffusion coefficient in air were taken from the literature. This model overestimated the flux for o-xylene for all the tested combinations. The ratios between estimated and observed fluxes at 1 h were between 1.7 and 7.3. Model II assumed that the mass transfer of o-xylene between the solids and the water phase was kinetically controlled and was solved numerically. However, the predictions by the more advanced model were not significantly better than the prediction by the simple analytical model. The results indicate that prediction of o-xylene volatilisation from unsaturated soil is associated with substantial uncertainty.     

Size and XAD fractionations of trihalomethane precursors from soils

Soil organic matter is an important source of allochthonous dissolved organic matter inputs to the Sacramento-San Joaquin Delta waterways, which is a drinking water source for 22 million people in California, USA. Knowledge of trihalomethane (THM) formation potential of soil-derived organic carbon is important for developing effective strategies for organic carbon removal in drinking water treatment. In this study, soil organic carbon was extracted with electrolytes (deionized H2O and Na- or Ca-based electrolytes) of electrical conductivity bracketing those found in Delta leaching and runoff conditions. The extracts were physically and chemically separated into different fractions: colloidal organic carbon (0.45-0.1 microm), fine colloidal organic carbon (0.1-0.025 microm), and dissolved organic carbon (DOC) (<0.025 microm); hydrophobic acid (HPOA), transphilic acid, and hydrophilic acid. Two representative Delta soils, Rindge Muck (a peat soil) and Scribner Clay Loam (a mineral soil) were examined. Results showed that less than 2% of soil organic carbon was electrolyte-extractable and heterogeneous organic fractions with distinct THM reactivity existed. Regardless of soil and electrolytes, DOC and HPOA fractions were dominant in terms of total concentration and THMFP. The amounts of extractable organic carbon and THMFP were dependent on the cation and to a lesser extent on electrical conductivity of electrolytes. Along with our previous study on temperature and moisture effects on DOC production, we propose a conceptual model to describe the impacts of agricultural practices on DOC production in the Delta. DOC is mainly produced in the surface peat soils during the summer and is immobilized by accumulated salt in the soils. DOC is leached from soils to drainage ditches and finally to the Delta channels during winter salt leaching practices.     

Preparation of a porous clay heterostructure and study of its adsorption capacity of phenol and chlorinated phenols from aqueous solutions.

A porous clay heterostructure (PCH) from a Mexican clay was prepared and characterized, and its aqueous phenol and dichlorophenols (DCPs) adsorption capacities were studied using a batch equilibrium technique. The PCH displayed a surface area of 305.5 m2/g, 37.2 A average porous diameter, and a basal space of 23.2 A. The adsorption capacity shown by the PCH for both phenol and DCPs from water (14.5 mg/g for phenol; 48.7 mg/g for 3,4-DCP; and 45.5 mg/g for 2,5-DCP) suggests that the PCH has both hydrophobic and hydrophilic characteristics, as a result of the presence of silanol and siloxane groups formed during the pillaring and calcination of the PCH. The values of maximal adsorption capacity for dichlorophenols were higher than those reported for aluminum pillared clays and some inorgano-organo clays and comparable with some ionic exchange resins.     

Entrapment and dissolution of DNAPLs in heterogeneous porous media

Two-dimensional multiphase flow and transport simulators were refined and used to numerically investigate the entrapment and dissolution behavior of tetrachloroethylene (PCE) in heterogeneous porous media containing spatial variations in wettability. Measured hydraulic properties, residual saturations, and dissolution parameters were employed in these simulations. Entrapment was quantified using experimentally verified hydraulic property and residual saturation models that account for hysteresis and wettability variations. The nonequilibrium dissolution of PCE was modeled using independent estimates of the film mass transfer coefficient and interfacial area for entrapped and continuous (PCE pools or films) saturations. Flow simulations demonstrate that the spatial distribution of PCE is highly dependent on subsurface wettability characteristics that create differences in PCE retention mechanisms and the presence of subsurface capillary barriers. For a given soil texture, the maximum and minimum PCE infiltration depth was obtained when the sand had intermediate (an organic-wet mass fraction of 25%) and strong (water- or organic-wet) wettability conditions, respectively. In heterogeneous systems, subsurface wettability variations were also found to enhance or diminish the performance of soil texture-induced capillary barriers. The dissolution behavior of PCE was found to depend on the soil wettability and the spatial PCE distribution. Shorter dissolution times tended to occur when PCE was distributed over large regions due to an increased access of flowing water to the PCE. In heterogeneous systems, capillary barriers that produced high PCE saturations tended to exhibit longer dissolution times.     

Removal of humic acid using TiO2 photocatalytic process--fractionation and molecular weight characterisation studies

The photocatalytic removal of humic acid (HA) using TiO₂ under UVA irradiation was examined by monitoring changes in the UV₂₅₄ absorbance, dissolved organic carbon (DOC) concentration, apparent molecular weight distribution, and trihalomethane formation potentials (THMFPs) over treatment time. A resin fractionation technique in which the samples were fractionated into four components: very hydrophobic acids (VHA), slightly hydrophobic acids, hydrophilic charged (CHA) and hydrophilic neutral (NEU) was also employed to elucidate the changes in the chemical nature of the HA components during treatment. The UVA/TiO₂ process was found to be effective in removing more than 80% DOC and 90% UV₂₅₄ absorbance. The THMFPs of samples were decreased to below 20 μg l⁻¹ after treatments, which demonstrate the potential to meet increasingly stringent regulatory level of trihalomethanes in water. Resin fractionation analysis showed that the VHA fraction was decreased considerably as a result of photocatalytic treatments, forming CHA intermediates which were further degraded with increased irradiation time. The NEU fraction, which comprised of non-UV-absorbing low molecular weight compounds, was found to be the most persistent component.