1. SERDP SYMPOSIUM 2008 POSTER NO. 138
Evaluation of the concept of effective reactivity zone for optimum placement of nano-scale zero-valent iron for treatment of aquifers contaminated with DNAPLs
Fritjof Fagerlund3, Menka Mittal1, Tissa H. Illangasekare Sidika Turkbey Cihan1, Tanapon Phenrat2, Hye-Jin Kim2 and Greg Lowry2
1 Center for Experimental Study of Subsurface Environmental Processes (CESEP), Environmental Science and Engineering Division, Colorado School of Mines, Golden, CO
2 Civil and Environmental Engineering Division, Carnegie Mellon University, Pittsburgh, PA 15213
3 Department of Earth Sciences, Uppsala University, Villavagen 16, Uppsala, Sweden
ABSTRACT
BACKGROUND
RESULTS (1)
Nano-scale zero-valent iron (NZVI) is a novel remediation technology for aquifers contaminated by chlorinated ethenes such as tetrachloroethene (PCE) and trichloroethene (TCE). Recent research has shown that the placement of NZVI in the subsurface is critical for efficient remediation, particularly because placement of NZVI directly in high saturation DNAPL sources may be inefficient. In this study we show that a significant reduction in down-gradient contaminant concentrations can efficiently be achieved by placing NZVI in the zone directly down-gradient of the DNAPL source, referred to as the zone of effective reactivity (ZER). The extent and location of this treatment zone depend on the entrapment morphology and mass-transfer characteristics of the DNAPL source, as well as the groundwater flow field controlled by the aquifer heterogeneity. The objective of an ongoing study was to first carefully study the dissolution dynamics of a PCE DNAPL source over a range of fluid saturations in well-controlled two-dimensional laboratory experiments. Here we determine the effect of the DNAPL source morphology on mass-transfer characteristics and the down-gradient dissolved contaminant plume. DNAPL saturations in the source zone are continuously measured in space and time using an automated x-ray-attenuation system, thereby recording changes in source morphology over time during the dissolution process. Dissolved PCE concentrations are simultaneously measured in a vertical profile down gradient of the source by analyzing water samples using gas chromatography (GC). The results constitute a detailed dataset on DNAPL source morphology and the corresponding down-gradient contaminant concentrations. It is shown that the dissolution of the source causes the PCE mass-flux pattern and down-gradient plume to change over time, influencing the requirements of the ZER. NZVI should be placed down-gradient of the source in such way that (1) the plume is captured before it spreads over an aquifer volume that is too large to be feasible for NZVI treatment, and (2) enough groundwater residence time is provided to produce the desired reduction in contaminant concentrations within the treatment zone. Optimal NZVI placement depends on the mass-transfer characteristics of the DNAPL source and therefore also on the source morphology and its development over time.
Ongoing work include large scale tank experiments and numerical modelling of the simultaneous rate-limited-dissolution and dechlorination reactions. This work involves a more generalized study of different reaction zone configurations and source settings, and provides more detailed information about the requirements of the ZER under a broader range of field conditions.
METHODS
ACKNOWLEDGEMENTS
SERDP funding through project ER-1485 is thankfully acknowledged. CSM undergraduate student Matt Bolt assisted in the assembly of large tank.
OBJECTIVES:
Compressed N2
De-aired water inflow reservoir
Peristaltic pump
2-D sand tank
Outflow reservoir (Teflon bag)
Water flow
Nano-scale zero-valent iron particles (NZVI) can be used to degrade chlorinated solvents such as tetrachloroethene (PCE). Modified zero-valent reactive nanoiron particles (MRNIP) have a large surface area to volume ratio which enhances reactions, and can be injected into contaminated aquifers for remediation. However, the placement of RNIP in subsurface is of critical importance for efficient remediation. We hypothesize that there exists an optimal placement zone directly down-gradient of the DNAPL source, referred to as the zone of effective reactivity (ZER).
The objectives of this study were:
to study DNAPL dissolution dynamics under well-controlled conditions in a small two-dimensional sand tank
(2) to study the effects of MRNIP treatment down-gradient of the DNAPL source zone and investigate if an optimal zone of effective reactivity (ZER) can be identified
(3) to develop numerical models of simultaneous DNAPL dissolution and dechlorination reactions to generalize the concepts of ZER (ongoing)
In the MRNIP treatment experiment, the effluent PCE concentration initially declined strongly in response to MRNIP injection down-gradient of the DNAPL source, but later rebounded as the MRNIP became inactive.
CONCLUDING REMARKS
The spatial distribution of DNAPL was continuously measured using an automated X-ray attenuation system. Thereby, changes in source morphology with time were recorded together with the spatially distributed mass transfer to the groundwater flowing through and past the DNAPL source.
Both the MRNIP treatment and the dissolution experiment were performed in a custom made, small 2-dimensional sand tank. A constant flow of de-aired (anaerobic) water was maintained through the tank at a pore water velocity of 0.5 m/day and the outflow was collected in gas-tight Teflon bags, preserving dissolved gases and allowing integrated sampling of out-flowing water.
Water samples were collected in a vertical profile down-gradient of source zone and analyzed on a GC/ECD for PCE concentration after extraction to hexane. Additional integrated samples from the Teflon bags were also collected.
Injection of nano-scale zero-valent iron particles (NZVI) is an emerging remediation technique for aquifers contaminated with chlorinated hydrocarbons. Recent research has shown that the placement of NZVI in the subsurface is critical for efficient remediation. In this work we investigate the hypothesis that optimal contaminant reductions are obtained by placing NZVI in the zone directly down-gradient of the DNAPL source, referred to as the zone of effective reactivity (ZER).
The detailed measurements of DNAPL saturations in the source together with down-gradient measurements of PCE concentrations show:
how the DNAPL source changes over time
the characteristics of the mass-transfer to the groundwater
the development of the contaminant load on the reaction zone over time
Our preliminary results indicate that an optimal zone of effective reactivity (ZER) exists and that NZVI should be placed down-gradient of the source in such way that:
the plume is captured before it spreads over an aquifer volume that is too large to be feasible for NZVI treatment, and
enough groundwater residence time is provided to produce the desired reduction in contaminant concentrations within the treatment zone.
Optimal NZVI placement depends on the spatial and temporal evolution of the DNAPL source morphology and its mass-transfer characteristics.
The data collected is currently used to test a numerical model of simultaneous DNAPL dissolution and dechlorination reactions, which will further aid the identification of the ZER.
X-Ray Attenuation System
The PCE concentrations measured in the vertical profile are strongly correlated to the mass loss by dissolution in the DNAPL source, measured using x-ray attenuation.
The source first dissolves in areas of low saturation. The concentration at port 2, which is down-gradient from the uppermost part of the source therefore peaks early.
In the first 2 weeks the mass flux by dissolution is highest in the mid-upper part of the source which is correlated to port 3. At later stages mass is depleted here and port 4 gets the highest concentrations.
Port 6 shows the average concentration over the vertical profile.
Sampling ports
Well 1
Well 2
Well 3
Source zone
Inflow
Outflow A B C D
RESULTS (2)
In the MRNIP treatment experiment, 25 mL of 50 g/L aqueous MRNIP solution was injected down-gradient of the DNAPL pool through the ports in the back of the tank. After the MRNIP injection, water samples were collected upstream and downstream of the MRNIP barrier using the ports in the back to measure reduction in PCE concentration.
MRNIP barrier downstream of DNAPL pool
Mass in and out of MRNIP barrier
Large tank experiments
In the 8 by 4 feet tank dechlorination processes will be studied at intermediate scales, at the same time as the mass flux from the DNAPL source can be carefully monitored using x-ray attenuation measurements
outflow (port 6)
2D sand tank
inflow
27.5 cm
width across tank: 2.8 cm
medium/fine sand
NAPL
coarse sand
17 cm
very fine sand
sampling port 1 2 3 4 5
The DNAPL PCE source was created by first injecting to create an average 80% DNAPL saturation in the coarse lens, and then slowly withdrawing to 40%
Initial DNAPL source before dissolution
Before and during the dissolution experiment the DNAPL saturation was carefully recorded using x-ray attenuation measurements with a spatial resolution of 2 x 2 mm
Source after 13.5 days of dissolution
Comparing the source after 13.5 days to the initial source gives the spatial distribution of mass loss by dissolution to the groundwater
A DNAPL PCE source was created in a small lens of coarse sand.
Ports 1-5 are located down-gradient of the DNAPL source in a vertical profile. Port 6 is refers to the samples from the effluent line and Bag refers to integrated samples from the outflow bag.
Different configurations of the reactivity zone will be explored in the 8’x4’ tank, in order to identify the optimal extent and placement of the reactivity zone.
Reactivity zones where PCE is de- chlorinated are created by injecting MRNIP through specifically designed injection wells.
In an integrated approach, numerical models of simultaneous DNAPL dissolution from the source and dechlorination reactions will aid the identification of the zone of effective reactivity (ZER)
Preliminary model of PCE dissolution from the DNAPL source