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  • 1
    Language: English
    In: Water Research, 01 May 2013, Vol.47(7), pp.2572-2582
    Description: Saturated sand-packed column experiments were conducted to investigate the influence of physicochemical factors on the transport and retention of surfactant stabilized silver nanoparticles (AgNPs). The normalized concentration in breakthrough curves (BTCs) of AgNPs increased with a decrease in solution ionic strength (IS), and an increase in water velocity, sand grain size, and input concentration ( ). In contrast to conventional filtration theory, retention profiles (RPs) for AgNPs exhibited uniform, nonmonotonic, or hyperexponential shapes that were sensitive to physicochemical conditions. The experimental BTCs and RPs with uniform or hyperexponential shape were well described using a numerical model that considers time- and depth-dependent retention. The simulated maximum retained concentration on the solid phase ( ) and the retention rate coefficient ( ) increased with IS and as the grain size and/or decreased. The RPs were more hyperexponential in finer textured sand and at lower because of their higher values of . Conversely, RPs were nonmonotonic or uniform at higher and in coarser sand that had lower values of , and tended to exhibit higher peak concentrations in the RPs at lower velocities and at higher solution IS. These observations indicate that uniform and nonmonotonic RPs occurred under conditions when was approaching filled conditions. Nonmonotonic RPs had peak concentrations at greater distances in the presence of excess amounts of surfactant, suggesting that competition between AgNPs and surfactant diminished close to the column inlet. The sensitivity of the nonmonotonic RPs to IS and velocity in coarser textured sand indicates that AgNPs were partially interacting in a secondary minimum. However, elimination of the secondary minimum only produced recovery of a small portion (〈10%) of the retained AgNPs. These results imply that AgNPs were largely irreversibly interacting in a primary minimum associated with microscopic heterogeneity. ► The presence of surfactant affected the shape of the retention profiles (RPs). ► RPs transitioned from hyperexponential, to nonmonotonic, and then to uniform. ► Nanoparticles mainly irreversibly interacted with microscopic heterogeneity.
    Keywords: Stabilized Silver Nanoparticles ; Saturated Porous Media ; Time- and Depth-Dependent Retention ; Surfactant ; Competitive Attachment ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 2
    Language: English
    In: Water Research, 2010, Vol.44(4), pp.1288-1296
    Description: A study was conducted to understand the role of cell concentration and metabolic state in the transport and deposition behaviour of with and without substrate addition. Column experiments using the short-pulse technique (pulse was equivalent to 0.028 pore volume) were performed in quartz sand operating under saturated conditions. For comparison, experiments with microspheres and inactive (killed) bacteria were also conducted. The effluent concentrations, the retained particle concentrations and the cell shape were determined by fluorescent microscopy. For the transport of metabolically-active without substrate addition a bimodal breakthrough curve was observed, which could be explained by the different breakthrough behaviour of the rod-shaped and coccoidal cells of . The 70:30 rod/coccoid ratio in the influent drastically changed during the transport and it was about 20:80 in the effluent and in the quartz sand packing. It was assumed that the active rod-shaped cells were subjected to shrinkage into coccoidal cells. The change from active rod-shaped cells to coccoidal cells could be explained by oxygen deficiency which occurs in column experiments under saturated conditions. Also the substrate addition led to two consecutive breakthrough peaks and to more bacteria being retained in the column. In general, the presence of substrate made the assumed stress effects more pronounced. In comparison to microspheres and inactive (killed) bacteria, the transport of metabolically-active bacteria with and without substrate addition is affected by differences in physiological state between rod-shaped and the formed stress-resistant coccoidal cells of .
    Keywords: Bacteria Transport ; Colloid Deposition ; Cell Morphology ; Physiological State ; Pseudomonas Fluorescens ; Oxygen Stress ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 3
    Language: English
    In: Water Research, 01 February 2013, Vol.47(2), pp.933-944
    Description: Water-saturated column experiments were conducted to investigate the effect of input concentration ( ) and sand grain size on the transport and retention of low concentrations (1, 0.01, and 0.005 mg L ) of functionalized C-labeled multi-walled carbon nanotubes (MWCNT) under repulsive electrostatic conditions that were unfavorable for attachment. The breakthrough curves (BTCs) for MWCNT typically did not reach a plateau, but had an asymmetric shape that slowly increased during breakthrough. The retention profiles (RPs) were not exponential with distance, but rather exhibited a hyper-exponential shape with greater retention near the column inlet. The collected BTCs and RPs were simulated using a numerical model that accounted for both time- and depth-dependent blocking functions on the retention coefficient. For a given , the depth-dependent retention coefficient and the maximum solid phase concentration of MWCNT were both found to increase with decreasing grain size. These trends reflect greater MWCNT retention rates and a greater number of retention locations in the finer textured sand. The fraction of the injected MWCNT mass that was recovered in the effluent increased and the RPs became less hyper-exponential in shape with higher due to enhanced blocking/filling of retention locations. This concentration dependency of MWCNT transport increased with smaller grain size because of the effect of pore structure and MWCNT shape on MWCNT retention. In particular, MWCNT have a high aspect ratio and we hypothesize that solid phase MWCNT may create a porous network with enhanced ability to retain particles in smaller grain sized sand, especially at higher . Results demonstrate that model simulations of MWCNT transport and fate need to accurately account for observed behavior of both BTCs and RPs. ► Breakthrough curves and retention profiles were measured and numerically modeled. ► We used very low (0.005–1 mg L ) input concentrations of carbon nanotubes (CNTs). ► Breakthrough of CNTs increased with increasing input concentration and grain size. ► Data were simulated well using time- and depth-dependent retention coefficients. ► Model predictions indicate the transport of CNTs to distances greater than 12 cm.
    Keywords: Carbon Nanotubes ; Column Experiments ; Quartz Sand ; Breakthrough Curves ; Retention Profiles ; Transport Modeling ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 4
    Language: English
    In: Water Research, 01 February 2017, Vol.109, pp.358-366
    Description: Saturated soil column experiments were conducted to investigate the transport, retention, and release behavior of a low concentration (1 mg L ) of functionalized C-labeled multi-walled carbon nanotubes (MWCNTs) in a natural soil under various solution chemistries. Breakthrough curves (BTCs) for MWCNTS exhibited greater amounts of retardation and retention with increasing solution ionic strength (IS) or in the presence of Ca in comparison to K , and retention profiles (RPs) for MWCNTs were hyper-exponential in shape. These BTCs and RPs were well described using the advection-dispersion equation with a term for time- and depth-dependent retention. Fitted values of the retention rate coefficient and the maximum retained concentration of MWCNTs were higher with increasing IS and in the presence of Ca in comparison to K . Significant amounts of MWCNT and soil colloid release was observed with a reduction of IS due to expansion of the electrical double layer, especially following cation exchange (when K displaced Ca ) that reduced the zeta potential of MWCNTs and the soil. Analysis of MWCNT concentrations in different soil size fractions revealed that 〉23.6% of the retained MWCNT mass was associated with water-dispersible colloids (WDCs), even though this fraction was only a minor portion of the total soil mass (2.38%). More MWCNTs were retained on the WDC fraction in the presence of Ca than K . These findings indicated that some of the released MWCNTs by IS reduction and cation exchange were associated with the released clay fraction, and suggests the potential for facilitated transport of MWCNT by WDCs.
    Keywords: Multi-Walled Carbon Nanotubes ; Soil ; Breakthrough Curves ; Retention Profiles ; Cation Exchange ; Soil Fractionation ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 5
    Language: English
    In: Water Research, March 1, 2014, Vol.50, p.294(13)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.watres.2013.10.032 Byline: David Vonberg, Jan Vanderborght, Nils Cremer, Thomas Putz, Michael Herbst, Harry Vereecken Abstract: Atrazine was banned in Germany in 1991 due to findings of atrazine concentrations in ground- and drinking waters exceeding threshold values. Monitoring of atrazine concentrations in the groundwater since then provides information about the resilience of the groundwater quality to changing agricultural practices. In this study, we present results of a monitoring campaign of atrazine concentrations in the Zwischenscholle aquifer. This phreatic aquifer is exposed to intensive agricultural land use and susceptible to contaminants due to a shallow water table. In total 60 observation wells (OWs) have been monitored since 1991, of which 15 are sampled monthly today. Descriptive statistics of monitoring data were derived using the "regression on order statistics" (ROS) data censoring approach, estimating values for nondetects. The monitoring data shows that even 20 years after the ban of atrazine, the groundwater concentrations of sampled OWs remain on a level close to the threshold value of 0.1 [mu]g l.sup.-1 without any considerable decrease. The spatial distribution of atrazine concentrations is highly heterogeneous with OWs exhibiting permanently concentrations above the regulatory threshold on the one hand and OWs were concentrations are mostly below the limit of quantification (LOQ) on the other hand. A deethylatrazine-to-atrazine ratio (DAR) was used to distinguish between diffuse - and point-source contamination, with a global mean value of 0.84 indicating mainly diffuse contamination. Principle Component Analysis (PCA) of the monitoring dataset demonstrated relationships between the metabolite desisopropylatrazine, which was found to be exclusively associated with the parent compound simazine but not with atrazine, and between deethylatrazine, atrazine, nitrate, and the specific electrical conductivity. These parameters indicate agricultural impacts on groundwater quality. The findings presented in this study point at the difficulty to estimate mean concentrations of contamination for entire aquifers and to evaluate groundwater quality based on average parameters. However, analytical data of monthly sampled single observation wells provide adequate information to characterize local contamination and evolutionary trends of pollutant concentration. Author Affiliation: (a) Agrosphere Institute (IBG-3), Forschungszentrum Julich GmbH, 52425 Julich, Germany (b) Erftverband, Am Erftverband 6, 50126 Bergheim, Germany Article History: Received 7 May 2013; Revised 6 September 2013; Accepted 14 October 2013
    Keywords: Aquifers -- Analysis ; Aquifers -- Electric Properties ; Metabolites -- Analysis ; Metabolites -- Electric Properties ; Atrazine -- Analysis ; Atrazine -- Electric Properties ; Electrical Conductivity -- Analysis ; Electrical Conductivity -- Electric Properties ; Groundwater -- Analysis ; Groundwater -- Electric Properties
    ISSN: 0043-1354
    Source: Cengage Learning, Inc.
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  • 6
    Language: English
    In: Water research, 2010, Vol.44, pp.1288-1296
    Description: In the special issue: Transport and Fate of Colloids and Microbes in Granular Aqueous Environments / Edited by David Dixon, Nathalie Tufenkji and Monica B. Emelko. Includes references ; p. 1288-1296.
    Keywords: Pseudomonas Fluorescens ; Colloids ; Oxygen ; Microbial Contamination ; Porous Media ; Metabolism ; Drinking Water ; Saturated Conditions
    ISSN: 0043-1354
    Source: AGRIS (Food and Agriculture Organization of the United Nations)
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  • 7
    Language: English
    In: Water Research, 01 March 2014, Vol.50, pp.294-306
    Description: Atrazine was banned in Germany in 1991 due to findings of atrazine concentrations in ground- and drinking waters exceeding threshold values. Monitoring of atrazine concentrations in the groundwater since then provides information about the resilience of the groundwater quality to changing agricultural practices. In this study, we present results of a monitoring campaign of atrazine concentrations in the Zwischenscholle aquifer. This phreatic aquifer is exposed to intensive agricultural land use and susceptible to contaminants due to a shallow water table. In total 60 observation wells (OWs) have been monitored since 1991, of which 15 are sampled monthly today. Descriptive statistics of monitoring data were derived using the “regression on order statistics” (ROS) data censoring approach, estimating values for nondetects. The monitoring data shows that even 20 years after the ban of atrazine, the groundwater concentrations of sampled OWs remain on a level close to the threshold value of 0.1 μg l without any considerable decrease. The spatial distribution of atrazine concentrations is highly heterogeneous with OWs exhibiting permanently concentrations above the regulatory threshold on the one hand and OWs were concentrations are mostly below the limit of quantification (LOQ) on the other hand. A deethylatrazine-to-atrazine ratio (DAR) was used to distinguish between diffuse – and point-source contamination, with a global mean value of 0.84 indicating mainly diffuse contamination. Principle Component Analysis (PCA) of the monitoring dataset demonstrated relationships between the metabolite desisopropylatrazine, which was found to be exclusively associated with the parent compound simazine but not with atrazine, and between deethylatrazine, atrazine, nitrate, and the specific electrical conductivity. These parameters indicate agricultural impacts on groundwater quality. The findings presented in this study point at the difficulty to estimate mean concentrations of contamination for entire aquifers and to evaluate groundwater quality based on average parameters. However, analytical data of monthly sampled single observation wells provide adequate information to characterize local contamination and evolutionary trends of pollutant concentration.
    Keywords: Atrazine ; Groundwater Monitoring ; DAR ; Nondetects ; Data Censoring ; Principle Component Analysis ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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