Assessing degradation rates of chlorinated ethylenes in column experiments with commercial iron materials used in permeable reactive barriers

Multiple column experiments were performed using two commercial iron materials to evaluate the necessity and usefulness of preliminary investigations in permeable reactive barrier (PRB) design for chlorinated organics. Experiments were performed with contaminated groundwater and involved fresh iron granules or altered iron material excavated from PRBs. The determination of first-order rate coefficients by global nonlinear least-squares fittings indicated a variability in rate coefficients on 1 or 2 orders of magnitude. Geometric mean values of surface area normalized rate coefficients (in 10(-5) L m(-2) h(-1)) for fresh gray cast iron and iron sponge, respectively, are: tetrachloroethene (4.5, 2.6), trichloroethene (8.1, 3.3), cis-1,2-dichloroethene (3.1, 2.9), trans-1,2-dichloroethene (9.5, 5.3), 1,1-dichloroethene (4.0, 4.4), and vinyl chloride (1.6, 6.1). The increasing rate coefficients with decreasing grade of chlorination, which characterize degradation at iron sponge are linearly related to diffusion coefficients in water, suggesting diffusion limitation in the degradation process for this particular material, possibly due to a high inner surface. The variability in rate coefficients seems to be too high to use mean rate coefficients from published studies in the design procedure of PRBs, and variabilities cannot be related to groundwater characteristics, waterflow through the reactive cells, or secondary corrosion reactions.     

Most-probable-number determination of salmonella levels in naturally contaminated raw almonds using two sample preparation methods

Pathogens occurring in particulate foods may be unevenly distributed, which may impact interpretation of most-probable-number (MPN) values. The MPN analysis of Salmonella in naturally contaminated raw almonds was conducted using two sample preparation methods. Raw almond kernels (3,698 samples) and inshell almonds (455 samples) were collected from almond processors throughout California during the 2006 and 2007 harvests, and 100-g samples were enriched for Salmonella. The prevalence of Salmonella on kernels and inshell almonds was 1.6 and 0.9%, respectively, in 2006, and 0.83 and 2.2%, respectively, in 2007. Almond kernel samples from 2006 were further enriched for Salmonella, and levels of the organism were determined for positive samples by three-tube MPN analysis (25 g, 2.5 g, 0.25 g). Almonds were either divided into subsamples prior to blending and enrichment (method A), or samples were blended in enrichment broth prior to preparation of subsamples (method B). Salmonella was not isolated (<1.2 MPN/100 g) upon retesting of 19 of 31 (method A) or 23 of 29 (method B) positive samples. When detected, levels were 1.4 to 15.5 MPN/100 g (average 2.3 MPN/100 g) or 1.4 to 18.3 MPN/100 g (average 2.1 MPN/100 g) using methods A or B, respectively. A total of 23 different Salmonella serovars were identified from the original almond samples. Salmonella Muenchen was the most frequently isolated serovar (15%) from the 53 Salmonella-positive samples, followed by Newport (12%), Enteritidis (10%), and Typhimurium (8%). No correlation was found between presence of Salmonella and E. coli levels, aerobic plate counts, or counts of yeasts or molds.     

On properties of predictors derived with a two-step bootstrap model averaging approach—A simulation study in the linear regression model

In many applications of model selection there is a large number of explanatory variables and thus a large set of candidate models. Selecting one single model for further inference ignores model selection uncertainty. Often several models fit the data equally well. However, these models may differ in terms of the variables included and might lead to different predictions. To account for model selection uncertainty, model averaging procedures have been proposed. Recently, an extended two-step bootstrap model averaging approach has been proposed. The first step of this approach is a screening step. It aims to eliminate variables with negligible effect on the outcome. In the second step the remaining variables are considered in bootstrap model averaging. A large simulation study is performed to compare the MSE and coverage rate of models derived with bootstrap model averaging, the full model, backward elimination using Akaike and Bayes information criterion and the model with the highest selection probability in bootstrap samples. In a data example, these approaches are also compared with Bayesian model averaging. Finally, some recommendations for the development of predictive models are given.     

单壁纳米管与对三联苯的交互作用及其束散现象

研究了单壁纳米管(SWCNT)与对三联苯(p-Terphenyl)分子的交互作用和束散现象.SWCNTs分别由电弧放电法和一氧化碳高压分解法(Hipco法)制备.比较了不同制备工艺及纯化处理后SWCNTs与p-Terphenyl交互作用及其束散程度.采用分光和显微镜等技术,探讨了SWCNTs及其与p-Terphenyl交互作用.应用X光能量消散(EDAX)技术,给出了纯化处理前后SWCNTs试样的元素分析.利用萤光分析和原子显微镜技术评估了SWCNTs交互作用及束散程度.研究显示:SWCNTs束散程度及其和p-Terphenyl交互作用的强弱与SWCNTs纯化程度有关.     

Matrimid Aerogels by Temperature‐Controlled,Solution‐Based Crosslinking

This paper reports on a temperature‐controlled, solution‐based method to prepare diamine crosslinked Matrimid aerogels. Addition of a diamine to a preheated polymer solution resulted in a well‐dispersed solution, allowing formation of a homogeneous gel upon cooling. The gels (studied by FTIR and AFM) were dried by solvent extraction with supercritical CO₂. The resulting aerogels showed surface areas of approximately 150 m² · g^?1 and porosities of 0.66–0.69 mL · g^?1 with polymer domains and pore sizes of tens of nanometers. A room temperature‐prepared, inhomogeneous aerogel gave approximately 250 m² · g^?1 and 0.31 mL · g^?1 with meso‐ and micropores. SEM images of the aerogels show similar surface features as AFM images of the Matrimid solvent gels.

     

A Regulator Based “Semi-Targeted” Approach to Activate Silent Biosynthetic Gene Clusters

By culturing microorganisms under standard laboratory conditions, most biosynthetic gene clusters (BGCs) are not expressed, and thus, the products are not produced. To explore this biosynthetic potential, we developed a novel “semi-targeted” approach focusing on activating “silent” BGCs by concurrently introducing a group of regulator genes into streptomycetes of the Tübingen strain collection. We constructed integrative plasmids containing two classes of regulatory genes under the control of the constitutive promoter ermE*p (cluster situated regulators (CSR) and Streptomyces antibiotic regulatory proteins (SARPs)). These plasmids were introduced into Streptomyces sp. TÜ17, Streptomyces sp. TÜ10 and Streptomyces sp. TÜ102. Introduction of the CSRs-plasmid into strain S. sp. TÜ17 activated the production of mayamycin A. By using the individual regulator genes, we proved that Aur1P, was responsible for the activation. In strain S. sp. TÜ102, the introduction of the SARP-plasmid triggered the production of a chartreusin-like compound. Insertion of the CSRs-plasmid into strain S. sp. TÜ10 resulted in activating the warkmycin-BGC. In both recombinants, activation of the BGCs was only possible through the simultaneous expression of aur1PR3 and griR in S. sp. TÜ102 and aur1P and pntR in of S. sp. TÜ10.     

Dissecting the performance of YouTube video streaming in mobile networks

Video streaming applications constitute a significant portion of the Internet traffic today, with mobile accounting for more than half of the online video views. The high share of video in the current Internet traffic mix has prompted many studies that examine video streaming through measurements. However, streaming performance depends on many different factors at different layers of the TCP/IP stack. For example, browser selection at the application layer or the choice of protocol in transport layer can have significant impact on the video performance. Furthermore, video performance heavily depends on the underlying network conditions (eg, network and link layers). For mobile networks, the conditions vary significantly, since each operator has a different deployment strategy and configuration. In this paper, we focus on YouTube and carry out a comprehensive study investigating the influence of different factors on streaming performance. Leveraging the Measuring Mobile Broadband Networks in Europe (MONROE) test bed that enables experimentation with 13 different network configurations in four countries, we collect more than 1800 measurement samples in operational mobile networks. With this campaign, our goal is to quantify the impact of parameters from different layers on YouTube's streaming quality of experience (QoE). More specifically, we analyze the role of the browser (eg, Firefox and Chrome), the impact of transport protocol (eg, TCP or QUIC), the influence of network bandwidth, and signal coverage on streaming QoE. Our analysis reveals that all these parameters need to be taken into account jointly for network management practices, in order to ensure a high end‐user experience.     

Virtual Support Communities and Psychological Well‐Being: The Role of Optimistic and Pessimistic Social Comparison Strategies

Do the beneficial or detrimental effects of CMC activity depend on the specific social comparison strategy individuals use? The present study aimed to answer this question by examining social comparison strategies, different measures of online activity within the community, and psychological well‐being of users of online breast cancer support communities. Results showed that the relationship between online activity (i.e., length of visits and frequency of posts) and psychological well‐being (i.e., breast cancer related concerns and depression) was determined by users' pessimistic social comparison strategy; downward identification influenced especially highly active users. Findings suggest that active CMC users should be careful not to become entrapped by negative social comparison processes.