Genesis and Effects of Particles Produced during In Situ Chemical Oxidation Using Permanganate

A research effort was undertaken to investigate the genesis of particles produced during in situ chemical oxidation (ISCO) of trichloroethene (TCE) with permanganate (MnO4?) and to explore the effects of those particles on system permeability and metal mobility. The experimental approach included characterization of soil and groundwater samples from an ISCO field site, batch experiments with a replicated 25 factorial design, and flow-through column experiments. Analyses of intact soil cores from an ISCO field site revealed that MnO2 solids were present in the subsurface near an injection well for NaMnO4 but at low levels (2.3–2.5 mg/g dry wt media) calculated to fill <1% v/v of the aquifer porosity. Batch tests revealed that the mass of filterable solids (>0.45 μm) produced during chemical oxidation with MnO4? was increased at higher TCE concentrations (54 versus 7 mg/L) and in the presence of ambient silt/clay-sized particles in the groundwater (750 versus 7.5 mg/L). Under otherwise comparable conditions, increasing the MnO4? dose markedly increases the oxidant consumption and also increases the solids production. The oxidant form (NaMnO4 versus KMnO4) or reaction time (15 versus 300 min) had little effect on oxidant consumption or filterable solids production. During MnO4? oxidation of higher levels of TCE in a groundwater with ambient silt/clay particles present, there can be substantial increases in filterable solids generated, which are <1 μm in size and consist of MnO2, commingled with other mineral matter. Conceivably, low volumetric fillings of these solids could cause permeability loss. Flow-through column experiments revealed that permeability loss was possible during ISCO but only under conditions with very high MnO2 solids production. On the positive side, the MnO2 solids produced can increase the sorption potential for metals such as cadmium and can represent a mode of immobilization. This research demonstrated that ISCO with permanganate has the potential to yield system permeability loss under some conditions as well as to affect metal mobility. The magnitude of these effects is related to the subsurface conditions, target organic chemical mass, and permanganate dose and delivery method. The production of solids during ISCO needs to be carefully considered during process design and operation to avoid solids-related performance problems while exploiting potential benefits.     

Diffusive Transport of Permanganate during In Situ Oxidation

The transport of permanganate in low permeability media (LPM) and its ability to degrade trichloroethylene (TCE) in situ were studied through diffusive transport experiments with intact soil cores. A transport cell was developed to measure the effective diffusion coefficient (Deff) of a Br? tracer through intact cores of silty clay LPM obtained from a field site and enable calculation of the apparent tortuosity (τa) of the medium. Then, 5000 mg?L?1 of KMnO4 was added to the cell and diffusive transport and soil matrix interactions were observed. After three months, the soil cores were dissected for morphologic examination and characterization of matrix ions, total organic carbon, MnO4?, and manganese oxides (MnO2). The experiment was then repeated after 2 μL of pure phase TCE were delivered into the center of each of two intact cores. Permanganate transport was observed for one month and then an extraction of the entire soil core was made to determine the extent of TCE degradation. This research demonstrated that permanganate can migrate by diffusion and yield reactive zones that can be predicted based on the properties of the LPM and the oxidant source. Under the experimental conditions examined, permanganate had little effect on the LPM’s pore structure or continuity, and appreciable soil organic matter remained even after 40–60 days of exposure to the oxidant. MnO2 solids, an oxidation by-product, were observed in the LPM, but not at levels sufficient to cause pore filling or alter the apparent matrix tortuosity, even when TCE was present. During diffusive transport of permanganate, TCE in the silty clay LPM was degraded by 97%.     

Factors Affecting Effectiveness and Efficiency of DNAPL Destruction Using Potassium Permanganate and Catalyzed Hydrogen Peroxide

This paper describes laboratory studies conducted to evaluate the impact of varying environmental conditions (dense nonaqueous phase liquid (DNAPL) type and mass, and properties of the subsurface porous media) and design features (oxidant type and load) on the effectiveness and efficiency of in situ chemical oxidation (ISCO) for destruction of DNAPL contaminants. Porous media in 160?mL zero-headspace reactors were employed to examine the destruction of trichloroethylene and perchloroethylene by the oxidants potassium permanganate and catalyzed hydrogen peroxide. Measures of oxidation effectiveness and efficiency include (1) media demand (mg-oxidant/kg-porous media), (2) oxidant demand (mol-oxidant/mol-DNAPL), (3) reaction rate constants for oxidant and DNAPL depletion (min?1), (4) the percent (%) DNAPL destroyed, and (5) the relative treatment efficiency, i.e., the rate of oxidant depletion versus rate of DNAPL destruction. While an obvious goal of ISCO for DNAPL treatment is high effectiveness (i.e., extensive contaminant destruction), it is also important to focus on oxidation efficiency, or to what extent the oxidant is utilized for contaminant destruction instead of competing side reactions, for improved cost effectiveness and/or treatment times. Results indicate that DNAPL contaminants can be treated both effectively and efficiently under many environmental and design conditions. In some cases, DNAPL treatment was more effective and efficient than dissolved/sorbed phase treatment. In these experiments, permanganate was a more effective oxidant, however catalyzed hydrogen peroxide treated contaminants more efficiently (e.g., less oxidant required per mass contaminant treated). Results also indicate that oxidation treatment goals can be dictated by environmental conditions, and that specific treatment goals can dictate remediation design parameters (e.g., faster contaminant destruction was realized in catalyzed hydrogen peroxide systems, whereas greater contaminant destruction occurred in permanganate systems).     

Chemistry of Modified Fenton’s Reagent (Catalyzed H2O2 Propagations–CHP) for In Situ Soil and Groundwater Remediation

The use of modified Fenton’s reagent, or catalyzed H2O2 propagations (CHP), has become increasingly popular for the in situ and ex situ treatment of surface soils and the in situ remediation of the subsurface. The process is based on the catalyzed decomposition of hydrogen peroxide by soluble iron, iron chelates, or iron minerals to generate the strong oxidant hydroxyl radical as well as other reactive oxygen species. Some of these species function as reductants and nucleophiles and may be responsible for the enhanced treatment of sorbed and nonaqueous phase liquid (NAPL) contaminants that is sometimes observed in the field. This paper serves as a review of the process chemistry of CHP; the goal is to provide researchers and practitioners with fundamental concepts that will aid in applying the CHP process to soil and groundwater contamination. Although the importance of well placement and the method of reagent injection must be considered in CHP remediation, understanding and promoting the most effective process chemistry is essential to successful soil and groundwater remediation.     

Applying a One-Dimensional Mass Transport Model Using Groundwater Concentration Data

One-dimensional analytical mass transport models are familiar to environmental professionals because they are typically used as learning devices in undergraduate groundwater courses. The application of the models requires relatively certain knowledge of contaminant release to the saturated zone. However, release data are typically not reliably known at sites with uncontrolled contaminant releases. A mass balance approach has been developed to calculate contaminant release parameters based on site-specific groundwater concentration data. Standard numerical calibration and sensitivity analysis techniques were modified for use with the one-dimensional spreadsheet model. A groundwater concentration dataset from a Superfund site was used to evaluate three schemes for calculating the model initial concentration. The site application demonstrates how the spreadsheet model could be used for preliminary remediation system comparisons including restoration time estimating. The use of the spreadsheet model may reduce the effort associated with subsequent numerical modeling typically required for remedial design. The spreadsheet application highlights the importance of collecting physical data with groundwater concentration data.     

Field Monitoring of a Permeable Reactive Barrier for Removal of Chlorinated Organics

The application of zero-valent iron (Fe0) in the funnel-and-gate permeable reactive barrier (PRB) installed at the Vapokon site, Denmark, was conducted in 1999 to remediate the groundwater contaminated by chlorinated aliphatic hydrocarbons (CAHs). Over the past 4?years, except in September 2002 and January 2003, about 92.4–97.5% CAH removal could be achieved with the PRB. Although there was a continuous decrease in total alkalinity (90.3%), calcium (81.7%), and sulfate (69.2%) ions in the groundwater crossing the PRB, probably caused by mineral precipitation and resulting in 0.88% porosity loss per year, no noticeable deterioration of the barrier’s performance was observed between March 2000, and August 2003. Instead, climatic variation in the barrier’s performance on CAH dechlorination was examined. The dechlorination rates in the cold season (January 2003 and March 2000) were generally smaller than those in the hot season (August 2003, September 2000, and September 2001). Besides, 1,2-dichloroethane and dichloromethane, which were proven to be not treatable by Fe0, could also be removed with the PRB, thereby suggesting enhancement from Fe0 adsorption or microbial degradation.     

Remediation of TCE-Contaminated Groundwater in a Sandy Aquifer Using Pulsed Air Sparging: Laboratory and Numerical Studies

The objective of this study was to investigate, through laboratory and numerical investigations, the effectiveness of a pulsed air sparging system for remediation of groundwater contaminated with trichloroethylene (TCE) in a sandy aquifer. In laboratory experiments, air was pulsed into TCE source zone on a daily basis in order to remediate TCE-contaminated groundwater. Most dissolved TCE was removed at the end of experiments although its concentrations fluctuated due to the air pulsing. The measured gaseous phase TCE concentration increased whereas the aqueous phase TCE concentration decreased during air sparging pulses. Experimental data were assessed by using a numerical code STOMP (subsurface transport over multiphases) with some modification based on the TCE dissolution kinetics. The unmeasured residual TCE mass was predicted through numerical simulations. Results show that aqueous concentrations for TCE are still much higher than the maximum contaminant level in spite of successful removal of 95% of residual TCE. It may imply that it would be more appropriate to apply air sparging combined with other remediation technologies such as bioremediation for remediation of TCE-contaminated groundwater.     

Impact of Reaction Conditions on MnO2 Genesis during Permanganate Oxidation

To enhance the understanding of the behavior and effects of the precipitation of MnO2 particles in the subsurface generated during in situ chemical oxidation (ISCO) using permanganate, laboratory batch experiments were completed to examine the influence that varied reaction matrix conditions have on the generation and properties of manganese oxides. The conditions examined include organic material type and concentration, permanganate concentration, pH, and the presence of calcium (as a representative divalent cation) in solution. Experimental studies included: (1) spectrophotometric examination of permanganate depletion and manganese oxides generation over time during reactions with trichloroethene; (2) scanning electron microscopy analyses of manganese particle morphology; (3) particle size distribution (filtration) characterization studies; and (4) optical particle sizing and numeration studies. Bench-scale, batch experiments were conducted to focus on fundamental chemical properties affecting particle development under varied potential environmental conditions. The amount of manganese oxides particles that develop, grow, and potentially settle as a result of permanganate ISCO of organic contaminants is a function of the particle size and concentration, the time allowed for particle development, and the impact of matrix conditions on the ability of particles to agglomerate.     

Effects of Seepage Velocity and Temperature on the Dechlorination of Chlorinated Aliphatic Hydrocarbons

The influence of seepage velocity and groundwater temperature on the dechlorination rates of trichloroethylene (TCE) and tetrachloroethylene (PCE) by zero-valent iron (Fe0) were investigated by running laboratory column tests at seepage velocities ranging from 31 to 1,884?m/year at temperatures of 10 and 23°C. By increasing the seepage velocity from 31 to 1,884?m/year at 10°C, there were approximately seven- and nine-fold increases in the normalized dechlorination rate constants (SA) of TCE and PCE, respectively. Similarly, a four-fold increase in the SA of TCE and PCE was also observed at 23°C when increasing the seepage velocity from 103 to 1,183?m/year. Raising the groundwater temperature from 10 to 23°C at a given seepage velocity resulted in 2.7 and 1.1 times increases in the TCE SA and PCE SA, respectively. With the application of the Arrhenius equation, activation energies of 70.3?kJ/mol for TCE and 38.6?kJ/mol for PCE dechlorination were determined, indicating domination of the electron transfer process over the mass transfer as a major rate-limiting step of the dechlorination reactions by Fe0.     

Mass Transfer of Ozone in a Novel In Situ Ozone Generator and Reactor

A porous tubular reactor that also served as an electrode for ozone generation was studied in this research to determine the effects of in situ ozone generation on mass transfer and reaction rates. Experimental data over a range of gas flow rates and ozone generation rates gave KLa values in the range 0.77–1.14?min?1. These values are more than double the values typically reported for bubble columns, and about 30% higher than that for packed beds. The specific power requirement for the laboratory-scale in situ reactor is an order of magnitude lower than that for bubble columns and stirred tank reactors that are used for ozone dissolution. A compartments-in-series fluid flow model was developed to describe the reactor system, and this model provides a good comparison to the experimental data for dissolved ozone and off-gas concentrations in the reactor. Sensitivity analyses indicate that the dissolved and off-gas ozone profiles are most sensitive to the gas–liquid partition coefficient and the overall mass transfer coefficient.