Hot water flushing for immiscible displacement of a viscous NAPL

Thermal remediation techniques, such as hot water flooding, are emerging technologies that have been proposed for the removal of nonaqueous phase liquids (NAPLs) from the subsurface. In this study a combined laboratory and modeling investigation was conducted to determine if hot water flooding techniques would improve NAPL mass removal compared to ambient temperature water flushing. Two experiments were conducted in a bench scale two-dimensional sandbox (55 cmx45 cmx1.3 cm) and NAPL saturations were quantified using a light transmission apparatus. In these immiscible displacement experiments the aqueous phase, at 22 degrees C and 50 degrees C, displaced a zone with initial NAPL saturations on the order of 85%. The interfacial tension and viscosity of the selected light NAPL, Voltesso 35, are strongly temperature-dependent. Experimental results suggest that hot water flooding reduced the size of the high NAPL saturation zone, in comparison to the cold water flood, and yielded greater NAPL mass recovery (75% NAPL removal vs. 64%). Hot water flooding did not, however, result in lower residual NAPL saturations. A numerical simulator was modified to include simultaneous flow of water and organic phases, energy transport, temperature and pressure. Model predictions of mass removal and NAPL saturation profiles compared well with observed behavior. A sensitivity analysis indicates that the utility of hot water flooding improves with the increasing temperature dependence of NAPL hydraulic properties.     

浅析环境监测QC/QA过程的采样环节

对环境监测布点采样中的QC/QA以及由采样引起的误差来源做了简要分析,并对监测采样过程中QC/QA的操作提出了一些设想。     

False-negative prenatal exclusion of Wiskott-Aldrich syndrome by measurement of fetal platelet count and size

The study of the fetal platelet count and size can, according to the literature, be used for the prenatal diagnosis of the Wiskott-Aldrich syndrome (WAS). So far, no affected fetuses have been identified by this method. All pregnancies in which this method had been applied to resulted, as correctly predicted, in the birth of normal children. Here we report on a familial case of WAS where the haematological parameters failed to reveal the affected second child. Hence we assume that the platelet count and size of platelets remain normal in fetuses with WAS to the gestational age of 22 weeks and cannot be used for prenatal diagnosis.     

Enhanced stability and dechlorination activity of pre-synthesis stabilized nanoscale FePd particles

Nanoscale zero-valent iron (NZVI) particles are promising materials for the in-situ remediation of a wide variety of source zone contaminants. This study presents the results of a systematic investigation of the stability of bimetallic FePd nanoparticle suspensions in water and their capability to degrade trichloroethylene (TCE) synthesized in the presence of various stabilizers (i.e., carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP), and guar gum). Results indicate a dramatic improvement in FePd suspension stability when the stabilizer is present in the matrix during the nanoparticle synthesis step. Stability enhancement is controlled by iron nanoparticle/stabilizer electrostatic and steric interactions, which are a function of the molecular structure of the stabilizer. Stabilization mechanisms differed for each stabilizer with CMC and guar gum exhibiting the best nanoparticle suspension stability improvement. Results suggest that the complexation of iron precursors with the stabilizer, during synthesis, plays a key role in nZVI stability improvement. In case of guar gum, gelation during synthesis significantly increased suspension viscosity, enhancing suspension stability. The capability of these materials to degrade TCE was also investigated. Results demonstrated that when stabilizers were present in the matrix dechlorination rates increased significantly. FePd nanoparticles in CMC had the highest observed rate constant; however the highest surface area-normalized rate constant was obtained from FePd stabilized in PVP360K. Results from this study can be used to aid in the selection of appropriate iron nanoparticle stabilizers. Stabilizer selection should be assessed on a case by case basis as no stabilizer will meet the needs of all in-situ remediation applications.     

Selectivity of Nano Zerovalent Iron in In Situ Chemical Reduction: Challenges and Improvements

Nano zerovalent iron (nZVI) is a promising remediation technology utilizing in situ chemical reduction (ISCR) to clean up contaminated groundwater at hazardous waste sites. The small particle size and large surface area of nZVI result in high reactivity and rapid destruction of contaminants. Over the past 20 years, a great deal of research has advanced the nZVI technology from bench‐scale tests to field‐scale applications. However, to date, the overall number of well‐characterized nZVI field deployments is still small compared to other alternative remedies that are more widely applied. Apart from the relatively high material cost of nZVI and questions regarding possible nanotoxicological side effects, one of the major obstacles to the widespread utilization of nZVI in the field is its short persistence in the environment due to natural reductant demand (NRD). The NRD for nZVI is predominantly due to reduction of water, but other reactions with naturally present oxidants (e.g., oxygen) occur, resulting in situ conditions that are reducing (high in ferrous iron phases and H₂) but with little or no Fe(0). This article reviews the main biogeochemical processes that determine the selectivity and longevity of nZVI, summarizes data from prior (laboratory and field) studies on the longevity of various common types of nZVI, and describes modifications of nZVI that could improve its selectivity and longevity for full‐scale applications of ISCR. © 2016 Wiley Periodicals, Inc.     

Prediction of two-phase capillary pressure-saturation relationships in fractional wettability systems

Capillary pressure/saturation data are often difficult and time consuming to measure, particularly for non-water-wetting porous media. Few capillary pressure/saturation predictive models, however, have been developed or verified for the range of wettability conditions that may be encountered in the natural subsurface. This work presents a new two-phase capillary pressure/saturation model for application to the prediction of primary drainage and imbibition relations in fractional wettability media. This new model is based upon an extension of Leverett scaling theory. Analysis of a series of DNAPL/water experiments, conducted for a number of water/intermediate and water/organic fractional wettability systems, reveals that previous models fail to predict observed behavior. The new Leverett–Cassie model, however, is demonstrated to provide good representations of these data, as well as those from two earlier fractional wettability studies. The Leverett–Cassie model holds promise for field application, based upon its foundation in fundamental scaling principles, its requirement for relatively few and physically based input parameters, and its applicability to a broad range of wetting conditions.     

Infiltration of PCE in a system containing spatial wettability variations

A two-dimensional infiltration experiment was conducted to investigate and quantify the effect of spatial wettability variations on DNAPL migration and entrapment in saturated sands. Experimental observations of tetrachloroethylene (PCE) infiltration showed that organic-wet sand lenses acted as very effective capillary barriers, retaining PCE and inhibiting its downward migration. A multiphase numerical simulator was used to model this sand box experiment. The simulator incorporates wettability-modified van Genuchten and Brooks-Corey capillary pressure/saturation relationships as well as Burdine and Mualem relative permeability relationships. PCE mass distributions, estimated by image analysis of digital photographs taken during the infiltration event, were compared to simulation results. Although both relative permeability models were qualitatively able to predict the PCE retention in the organic-wet layers, simulations with the Mualem model failed to capture the observed rate of PCE migration. A traditional multiphase simulator, incorporating water-wet capillary retention relations, failed to predict both PCE pathways and retention behavior. This study illustrates the potential influence of subsurface wettability variations on DNAPL migration and entrapment and supports the use of modified capillary relations in conjunction with the Burdine model in multiphase flow simulators.