Breakthrough Of Contaminants Research Articles

Analytical model for two-dimensional contaminant transport in a cut-off wall and aquitard system

The cut-off wall is an effective countermeasure to prevent contaminant transport from a contaminated site. However, the aquitard beneath the cut-off wall may serve as a preferential path, leading to an earlier breakthrough of contaminants in the cut-off wall system. In this study, we present a 2-D analytical model for characterizing the contaminant transport in the cut-off wall and aquitard system. The Green function method and discretization method are used to investigate contaminant migration in a semi-infinite domain. We show that the one-dimensional cut-off wall model may be insufficient to predict contaminant transport when the aquitard hydraulic conductivity (k2) exceeds 5 × 10-10 m/s. The contaminant mass outflow from the aquitard for the case with k2 = 2 × 10-9 m/s may account for 53 % of the total contaminant mass outflow. Increasing k2 from 5 × 10-10 m/s to 5 × 10-9 m/s leads to a 2.9-fold increase of cumulative mass outflow. An approximate fitting formula is proposed to calculate the required minimum thickness of cut-off wall under different type of aquitards. The thicker cut-off wall is required to ensure the cumulative mass outflow will not exceed the allowable value when k2 is relatively high (e.g., 1 × 10-9 m/s).

In silico approaches for the prediction of the breakthrough of organic contaminants in wastewater treatment plants.

The removal efficiency (RE) of organic contaminants in wastewater treatment plants (WWTPs) is a major determinant of the environmental impact of chemicals which are discharged to wastewater. In a recent study, non-target screening analysis was applied to quantify the percentage removal efficiency (RE%) of more than 300 polar contaminants, by analyzing influent and effluent samples from a Swedish WWTP with direct injection UHPLC-Orbitrap-MS/MS. Based on subsets extracted from these data, we developed quantitative structure-property relationships (QSPRs) for the prediction of WWTP breakthrough (BT) to the effluent water. QSPRs were developed by means of multiple linear regression (MLR) and were selected after checking for overfitting and chance relationships by means of bootstrap and randomization procedures. A first model provided good fitting performance, showing that the proposed approach for the development of QSPRs for the prediction of BT is reasonable. By further populating the dataset with similar chemicals using a Tanimoto index approach based on substructure count fingerprints, a second QSPR indicated that the prediction of BT is also applicable to new chemicals sufficiently similar to the training set. Finally, a class-specific QSPR for PEGs and PPGs showed BT prediction trends consistent with known degradation pathways.

Tailoring 3D Printed Micro‐Structured Carbons for Adsorption

AbstractThe manufacture of tailored carbon‐based adsorbent structures with exceptionally low‐pressure drops and improved kinetics using stereolithographic 3D printing is presented. Adsorbent structures are printed from commercial resins with square, circular, and hexagonal cross‐sectional microchannels. These structures can reduce energy use by 50–95% compared to conventional carbon‐packed beds. The activated 3D printed carbon achieves Brunauer–Emmett–Teller surface areas over 1000 m2 g−1 and shows outstanding butane adsorption capacities, over twice the capacity of a commercial carbon and a comparable capacity to phenolic‐based carbons. The structures also show excellent uptakes of cyclohexane, up to 0.62 g g−1 in a saturated feed. The introduction of complex axial geometries including spirals and chevrons enable superior adsorption kinetics and premature breakthrough of contaminants at high gas flow rates. These results demonstrate the success of intelligent manufacturing of low‐pressure drop, high‐capacity micro‐structured adsorbents, allowing for the development of gas separation technologies for applications such as greenhouse gas removal and respiratory protection.

Analytical solution for one‐dimensional steady‐state contaminant transport through a geomembrane layer (GMBL)/compacted clay layer (CCL)/attenuation layer (AL) composite liner considering consolidation

AbstractA composite liner system consisting of a geomembrane layer (GMBL), a compacted clay layer (CCL) and an attenuation layer (AL) is widely used in the modern landfills. The field and experimental observations highlight the potential detrimental effects on breakthrough of contaminants induced by consolidation of soil liner when a relatively high applied stress is caused by the waste placement. A one dimensional steady‐state model is developed to investigate the behaviour of contaminant transport in GMBL/CCL/AL composite liner system considering consolidation. Effects of the consolidation process on the transport of typical contaminants are investigated by the proposed analytical solution. The analytical results show that the magnitude of volume compressibility coefficient in the AL is important in controlling the overall magnitude of the consolidation induced effects. The process of consolidation shows a larger impact on the absorbable pollutants compared to the non‐absorbable pollutants. This may be due to the fact that the migration of solid particles induced by the consolidation may assist the transport of contaminant absorbed on them. The thickness of the AL shows large impacts on the bottom flux for the case with a high volume compressible coefficient. Increasing the thickness of AL from 1 to 3 m can results in a factor of 2.6 increase of the bottom flux. Increasing the thickness of the AL leads to a larger consolidation advection and a longer consolidation path, which may be the dominant mechanisms for contaminant transport compared to the diffusion process.

Centripetal filtration of groundwater to improve the lifetime of an MgO recycled refractory filter: observations and technical challenges.

In the context of improving permeable reactive barrier (PRB) filters, axial and a centripetal column tests were performed to compare their evolution in terms of chemical and hydraulic performances. For both tests, the MgO reactive media, made of crushed (< 10 mm) spent MgO-C refractory bricks was used to treat water contaminated with Co and Ni by raising the pH and promoting hydroxide precipitation. As opposed to the traditional cylindrical axial configuration, the centripetal column consists of an annulus of reactive media through which the water flows from the outer radius towards the inner radius. Under similar conditions (total reactive mass, porosity), the centripetal column is expected to delay the breakthrough of contaminants because of its higher cross-section and lower flow speeds at the entrance of the media. However, as we found in this study, the design of a granular radial filter poses several technical problems. Indeed, a breakthrough of the contaminants, accompanied by a decline in pH, was observed much sooner in the centripetal (100 pv) than in the axial (375 pv) filter. This lower performance was deemed to be due to a hydraulic shortcut and was supported by the results of a tracer test (average renewal volume much lower (199 ml) than the theoretical one (7530 ml)) as well as the observation of preferential clogging upon dismounting the radial filter. While the design of a filter that induces a purely radial flow still poses a technical challenge, this study contributes to advance the knowledge for centripetal radial filtration of groundwater in PRBs.

Pollution Plume Development in the Primary Aquifer at the Atlantis Historical Solid Waste Disposal Site, South Africa

The monitoring of pollution plumes from municipal landfills is essential in order to control and, where necessary, remediate aquifer contamination. The Atlantis historical landfill was established in 1975 and was unlined as it preceded the promulgation of the Minimum Requirements by the Department of Water and Sanitation. As the underlying, unconfined sandy aquifer serves as a water supply source to the town of Atlantis, regular quarterly hydrochemical monitoring was carried out from 1989 to 1997, at irregular intervals until 2003, and resumed in 2015 when new, deep boreholes were drilled. Groundwater monitoring over nearly three decades provided valuable information on the nature of the chemical reactions that take place in the subsurface and the extent of transport of chemical constituents. Ammonium and organic carbon, which are subject to redox reactions, were lagging compared to chloride and sodium, which are transported advectively. The most recent data indicated the plume consisted mainly of salinity (electrical conductivity (EC) &gt; 200 mS m−1) in the form of sodium, calcium, chloride and bicarbonate ions 350–400 m down-gradient of the landfill, and it is still expanding at a maximum rate of about 25 m a−1, with local deviations from the regional flow pattern. It also became evident that the plume migrated to greater depth as it was transported further from the waste pile. The breakthrough of contaminants being observed at different depths highlights the importance of suitably designed monitoring networks.

Evaluating the mobility of polymer-stabilised zero-valent iron nanoparticles and their potential to co-transport contaminants in intact soil cores

The use of zero-valent iron nanoparticles (nZVI) has been advocated for the remediation of both soils and groundwater. A key parameter affecting nZVI remediation efficacy is the mobility of the particles as this influences the reaction zone where remediation can occur. However, by engineering nZVI particles with increased stability and mobility we may also inadvertently facilitate nZVI-mediated contaminant transport away from the zone of treatment. Previous nZVI mobility studies have often been limited to model systems as the presence of background Fe makes detection and tracking of nZVI in real systems difficult. We overcame this problem by synthesising Fe-59 radiolabelled nZVI. This enabled us to detect and quantify the leaching of nZVI-derived Fe-59 in intact soil cores, including a soil contaminated by Chromated-Copper-Arsenate. Mobility of a commercially available nZVI was also tested. The results showed limited mobility of both nanomaterials; <1% of the injected mass was eluted from the columns and most of the radiolabelled nZVI remained in the surface soil layers (the primary treatment zone in this contaminated soil). Nevertheless, the observed breakthrough of contaminants and nZVI occurred simultaneously, indicating that although the quantity transported was low in this case, nZVI does have the potential to co-transport contaminants. These results show that direct injection of nZVI into the surface layers of contaminated soils may be a viable remediation option for soils such as this one, in which the mobility of nZVI below the injection/remediation zone was very limited. This Fe-59 experimental approach can be further extended to test nZVI transport in a wider range of contaminated soil types and textures and using different application methods and rates. The resulting database could then be used to develop and validate modelling of nZVI-facilitated contaminant transport on an individual soil basis suitable for site specific risk assessment prior to nZVI remediation.

Effect of pH and Pressure on Uranium Removal from Drinking Water Using NF/RO Membranes.

Groundwater is becoming an increasingly important drinking water source. However, the use of groundwater for potable purposes can lead to chronic human exposure to geogenic contaminants, for example, uranium. Nanofiltration (NF) and reverse osmosis (RO) processes are used for drinking water purification, and it is important to understand how contaminants interact with membranes since accumulation of contaminants to the membrane surface can lead to fouling, performance decline and possible breakthrough of contaminants. During the current study laboratory experiments were conducted using NF (TFC-SR2) and RO (BW30) membranes to establish the behavior of uranium across pH (3-10) and pressure (5-15 bar) ranges. The results showed that important determinants of uranium-membrane sorption interactions were (i) the uranium speciation (uranium species valence and size in relation to membrane surface charge and pore size) and (ii) concentration polarization, depending on the pH values. The results show that it is important to monitor sorption of uranium to membranes, which is controlled by pH and concentration polarization, and, if necessary, adjust those parameters controlling uranium sorption.

Study on Vertical Air Curtains for Multi-deck Viewer Refrigerated Display

Air curtains are devices used in refrigerated display cabinets and the entrance of air conditioned spaces is aimed to reduce the breakthrough of heat, moisture and contaminants between the conditioned environment and the surrounding ambient, consequently reducing energy costs. The main objective of the present work is to study the parameter affecting the performance of air curtains used in viewer refrigerators using one and two air curtains at different ambient conditions, in order to reduce the air infiltration and consequently the energy consumption. The results of one and two air curtains are compared, where two air curtains are more effective than one air curtain

Enhancement of adsorptive chemical filters via titania photocatalysts to remove vapor-phase toluene and isopropanol

Adsorption-type chemical filtration is essential in many indoor purification applications. However, these chemical filters are expensive and have a limited filter life after breakthrough of contaminants. With toluene and isopropanol (IPA) as model contaminants, this study investigates the breakthrough behaviors of adsorptive chemical filters and TiO 2-enhanced chemical filters with various configurations—a chemical filter containing TiO 2-preloaded activated carbon (Scenario A), a chemical filter preceded by a TiO 2-coated non-woven sheet (Scenario B), and a chemical filter pressed against a TiO 2-coated non-woven sheet (Scenario C). The photocatalytic performance of the TiO 2-coated non-woven sheet was evaluated using several key operating parameters such as air moisture (relative humidity), TiO 2 loading density, light intensity, and challenge concentrations of model contaminants. The generation and accumulation of oxidative intermediate products from photocatalysis of toluene and IPA were also examined. Additionally, the Yoon–Nelson breakthrough model was experimentally validated and applied to generate adsorption breakthrough curves of several hypothetical challenge concentrations. The breakthrough studies indicate that the life of the filter in Scenario C was markedly longer than those of filters in scenarios A and B, even when photocatalysts were de-activated as a result of benzaldehyde accumulation from toluene decomposition. By pressing the TiO 2-coated non-woven sheet against the front face of the chemical filter, TiO 2 particles in the non-woven sheet and activated carbon granules in the chemical filter comprised a quasi-homogeneous system, yielding synergistic effects that enhance the operating lifespan of a chemical filter via photocatalysis.

Theoretical investigation of the effects of consolidation on contaminant transport through clay barriers

AbstractConsolidation of clayey contaminant barriers such as landfill liners has been postulated as a cause of early breakthrough of contaminants. In this paper we theoretically investigate this proposition. For this purpose a sophisticated one‐dimensional, large‐deformation model of coupled mechanical consolidation and solute transport is employed. This new model is a generalization of existing coupled consolidation and solute transport models described in the literature. It takes into account both non‐linearities in geometry as well as constitutive relations. The latter relate the compressibility, hydraulic conductivity and coefficient of effective diffusivity to the deformation of the soil. The model is applied to a case study of a clay liner and geomembrane system. Results obtained from numerical solution of the model equations are compared with those from various simplified models, including a ‘diffusion only’ (i.e. a rigid soil) model traditionally used in contaminant barrier design. For barriers incorporating low compressibility soils (as for many well compacted clays), there is little difference between contaminant transit (i.e. breakthrough) times predicted by the two models. However, for contaminant barriers incorporating more compressible soils, consolidation is shown to significantly accelerate transport. These results indicate the potential importance of accounting for the effects of soil consolidation and highlight the limitations of existing models when modelling solute transport through composite barriers utilizing soft soils. Based on these limited results, we suggest a possible way of taking into account soil consolidation using simplified models. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Natural Attenuation Processes during In Situ Capping

Chlorinated solvents are common groundwater contaminants that threaten surface water quality and benthic health when present in groundwater seeps. Aquatic sediments can act as natural biobarriers to detoxify chlorinated solvent plumes via reductive dechlorination. In situ sediment capping, a remedial technique in which clean material is placed at the sediment-water interface, may alter sedimentary natural attenuation processes. This research explores the potential of Anacostia River sediment to naturally attenuate chlorinated solvents under simulated capping conditions. Results of microcosm studies demonstrated that intrinsic dechlorination of dissolved-phase PCE to ethene was possible, with electron donor availability controlling microbial activity. A diverse microbial community was present in the sediment, including multiple Dehalococcoides strains indicated by the amplification of the reductive dehalogenases tceA, vcrA, and bvcA. An upflow column simulating a capped sediment bed subject to PCE-contaminated groundwater seepage lost dechlorination activity with time and only achieved complete dechlorination when microorganisms present in the sediment were provided electron donor. Increases in effluent chloroethene concentrations during the period of biostimulation were attributed to biologically enhanced desorption and the formation of less sorptive dechlorination products. These findings suggest that in situ caps should be designed to account for reductions in natural biobarrier reactivity and for the potential breakthrough of groundwater contaminants.

Role of membranes and activated carbon in the removal of endocrine disruptors and pharmaceuticals

Endocrine disrupting chemicals and pharmaceuticals represent two classes of emerging contaminants that are ubiquitously present in municipal wastewater effluents. Some of these contaminants have been shown to impact aquatic organisms at trace concentrations (i.e., ng/L). Moreover, the public has expressed human health concerns regarding the presence of emerging contaminants in water reuse projects. The primary objective of this investigation was to determine the efficacy of various membranes and activated carbons for the removal of endocrine disruptors, pharmaceuticals, and personal care products. A suite of structurally diverse target compounds was selected for evaluation based largely upon occurrence and molecular structure. Several membrane types and applications were evaluated at pilot- and/or full-scale, including: microfiltration, ultrafiltration, nanofiltration, reverse osmosis, electrodialysis reversal, membrane bioreactors, and combinations of membranes in series. Granular activated carbon was evaluated at bench-scale using rapid small scale column tests and at two full-scale utilities. Microfiltration and ultrafiltration were found to reject very few target compounds; however, some loss of steroidal type compounds was observed. Nanofiltration and reverse osmosis were capable of significant rejection of nearly all target compounds, though compounds were detectable at trace levels in permeates. Granular activated carbon was highly effective at removing all target chemicals. However, break-through curves clearly demonstrated that compounds with greater hydrophilicity breach activated carbon faster than hydrophobic compounds. In full-scale applications, the impact of regeneration was observed as activated carbon filters that received regular regeneration had minimal breakthrough of organic contaminants, while non-regenerated filters displayed no removal of target compounds. Findings confirm that membrane and carbon processes are capable of greatly reducing the concentrations of emerging contaminants; however, several compounds are detectable in membrane permeate and carbon effluent.

Transient Response of Flow-Direction-Switching Vapor-Phase Biofilters

Transient loading of vapor-phase biofilters may result in exceedence of the local reaction or mass transfer capacity of the inlet region. In such cases, higher concentrations of contaminants are carried deeper into the bed where there is less active biomass and, in some cases, breakthrough of contaminants may occur. Previous studies have demonstrated that periodic reversal of the flow direction results in improved transient-loading response. However, quantitative information on the extent of the benefit is lacking. Step function increases in toluene concentration were applied to unidirectional-flow and flow-direction-switching laboratory reactors operated in parallel. Contaminant concentration was monitored at several points along the packed beds. Relative to unidirectional mode of operation, periodic flow reversal produced a more uniform distribution of microbial reaction capacity along the length of the packed bed. Directional switching at a 12-h interval did not result in a loss of activity or removal capacity. Mass-removal rates under transient-loading conditions were similar in the first-half of both biofilters but, in the second-half of the units, significant removals were observed only in the flow-direction-switching biofilter. As a result, maximum mass-removal rates under transient-loading conditions were approximately twice as great for the flow-direction-switching biofilter relative to the conventional unidirectional-flow biofilter receiving similar mass loading.

An in situ toxicity identification evaluation method part I: Laboratory validation

Identification of individual chemical groups is critical in evaluating sediment quality and fractionating these groups of chemicals in a mixture is important to determine the primary chemicals causing toxicity. The in situ toxicity identification evaluation (iTIE) is a novel method that was developed to fractionate chemicals in contaminated sediments and waters and assess toxicity organisms. The study objectives were to verify that the iTIE can help identify contaminant chemical classes and improve the toxicity assessment process; and compare the iTIE and U.S. Environmental Protection Agency's (U.S. EPA) toxicity identification evaluation (TIE) methods. The iTIE exposure chamber is powered by a portable air pump that suctions pore water, via a Venturi system, through selective sorption materials. After passing through the sorptive materials, the pore water passes into an exposure chamber containing Daphnia magna. The chemical sorption materials included Ambersorb 563 for nonpolar organic chemical adsorption, Chelex for metals chelation, and multiple zeolite types for ammonia adsorption. The laboratory studies were performed using water and sediments spiked with ammonia, cadmium, and fluoranthene. The laboratory validation of the iTIE approach showed that different classes of compounds readily could be separated via the resin treatments, resulting in significant differences in concentrations and thus exposures to in situ exposed organisms. Ammonia, cadmium, and fluoranthene were significantly removed by zeolite, Chelex, and Ambersorb, respectively. Although there was some cross-adsorption to the other nontarget resins, it was limited and allowed for treatment differences to be detected. Survival in the treatment resins exposed to the target compounds was as high as control survivals. A 24-h exposure period appeared optimal, allowing for replacement of initial culture water with pore waters, while longer exposures occasionally allowed for breakthrough of contaminants. The iTIE was more sensitive than the U.S. EPA TIE method, in that it detected toxicity more readily due to the greater loss of contaminant concentrations in the TIE manipulation process.

Hydraulic-conductivity reduction, reaction-front propagation, and preferential flow within a model reactive barrier

Recent work has demonstrated the utility of reactive barriers for transformation or immobilization of contaminants in ground water. However, reaction-induced changes in hydraulic conductivity may compromise the reactive barrier by diverting flow or focusing breakthrough of contaminants. For an acidic, ferric solution flowing through calcareous sand in column experiments, we investigated how hydraulic-conductivity reduction, reaction-front propagation, and fingering depend upon the reactive solid volume, reactive surface area, and pore-water velocity. Hydraulic-conductivity reductions are greater with larger initial pore-water velocity, calcite surface area, or calcite volume. Reductions in hydraulic conductivity (up to four orders of magnitude) result primarily from CO 2 (g) exsolution rather than from ferric oxyhydroxide precipitation and may be at least partly reversed as bubbles migrate. Although fingering is both driven and repressed by pore-scale hydraulic changes, the velocity of the reaction front, width of the primary reaction zone, and maximum length of the largest finger appear to be insensitive to macroscopic changes in hydraulic conductivity at a constant flow rate. Reaction-front velocity increases as the ratio of initial calcite volume to pore-water velocity decreases, whereas zonal width and maximum finger length appear to increase as the Damköhler number decreases for a given calcite grain-size distribution. These results offer guidelines for improving the efficiency of reactive barriers when the reaction rate equals or exceeds the rate of mass transfer.

Transport of reactive colloids and contaminants in groundwater: effect of nonlinear kinetic interactions

Transport of reactive colloids in groundwater may enhance the transport of contaminants in groundwater. Often, the interpretation of results of transport experiments is not a simple task as both reactions of colloids with the solid matrix and reactions of contaminants with the solid matrix and mobile and immobile colloids may be time dependent and nonlinear. Further colloid transport properties may differ from solute transport properties. In this paper, a one-dimensional model for coupled colloid and contaminant transport in a porous medium (COLTRAP) is presented together with simulation results. Calculated breakthrough curves (BTC's) during contamination and decontamination show systematically the effect of nonlinear and kinetic interactions on contaminant transport in the presence of reactive colloids, and the effect of colloid transport properties that differ from solute transport properties. It is shown that in case of linear kinetic reactions, the rate of exchange of mobile and immobile colloids have a large impact on the shape of BTC's even if the solid matrix is saturated with respect to colloids. BTC's during the contamination and decontamination phase have identical shapes in this case. Moreover, the slow reactions of contaminants and colloids may lead to unretarded breakthrough of contaminants. Independent of reaction rates, nonlinear reactions lead to BTC's that are steeper during contamination than in the linear case. A characteristic aspect of nonlinear sorption is that shapes of BTC's differ during the contamination and decontamination phase. It has been observed that shapes of some of the simulated adsorption and desorption curves are similar as shapes found in experiments reported in literature. This stresses the importance of incorporating both kinetics and nonlinearity in models for coupled colloid and contaminant transport and the capability of COLTRAP to interpret experimental results. Finally, to figure out whether nonlinear processes play a role, it is very important to consider both contamination and decontamination in transport experiments.

Transport of Low‐Concentration Contaminants in Saturated Earthen Barriers

Laboratory tests indicate that a one-dimensional advective-dispersive transport model for reactive solutes provides reasonable estimates of the breakthrough of low-concentration organic contaminants in saturated fine-grained earthen barriers. Based upon this transport model, a design/analysis methodology and respective guidelines are developed for earthen barriers used in waste disposal applications. The limitations and restrictions of the model are discussed and fundamental variables affecting the transport are parametrically analyzed. Transport of low-concentration trichloroethylene at a selected study site was selected for illustrating the presented procedure.

Diffusive contaminant transport in natural clay: a field example and implications for clay-lined waste disposal sites

Vertical core samples were obtained from an impervious, unweathered, water-saturated clay deposit beneath a 5-year-old hazardous waste landfill at a site in southwestern Ontario. Sections of the cores were analyzed for chloride and volatile organic compounds. Waste-derived chloride was detected in the clay to a maximum depth of 83 cm below the bottom of the landfill. The most mobile organic compounds were found only to a depth of /approximately/ 15 cm. The downward transport of these chemical species into the clay was the result of simple Fickian diffusion. This study has implications for low-permeability clay liners used at waste disposal sites. For liners of typical thickness (/approximately/ 1 m), simple diffusion can cause breakthrough of mobile contaminants in approximately 5 years; the diffusive flux of contaminants out of such liners can be large.