"Injection of Nano-Scale Iron for the In-Situ Remediation of Chlorinated Hydrocarbons in Soil and Groundwater"In order to remove contaminant plumes from the groundwater, permeable reactive barriers can be used. State of the art is to excavate a trench and fill it with granular iron or to build a funnel and gate system where the funnel forces the plume towards the gate consisting of granular iron. The disadvantages of these systems include the cost of implementation which increases with increasing depth and the requirement of open access to machinery.
A relatively new and promising method to overcome these obstacles is the injection of nano-scale iron suspended in water into the subsurface using injection wells. The idea is that this iron will be transported with the injected water, deposit in the pores and hence form a reactive wall.
A further advantage is that nano-sized iron is expected to have a much higher reactivity due to the larger specific surface, reducing the necessary thickness of the barrier. However, this increased reactivity may have to be paid for with a reduced longevity.
Little is known about the transport behavior of nano-sized iron during injection in the subsurface. Transport distances from few centimeters to several meters (or even tens of meters) have been reported. Thus, without additional experiments, it is not possible to design a remediation measure that depends on the minimum distance between injection wells to ensure continuity of the injected barrier. Currently, there are no measuring techniques available to non-destructive determine the nano-iron concentration in soil directly and hence the spreading of the nano-sized iron in the subsurface. For several pilot tests the spreading has been determined based on secondary parameters such as pH value or redox potential.
In the presented research several experiments were developed to get a better understanding of the effectiveness of the use of nano-sized iron for subsurface in-situ contaminant treatment.
Batch and column experiments were conducted to find the parameters influencing the reactivity, longevity and transportability.
In the batch experiments the aging-effect of aggregation / disaggregation was investigated, which was found to be of great influence on the sedimentation rate and thus the stability of the suspension. Furthermore the effectiveness of the reduction of PCE was investigated. Experiments with low and saturated concentrations as well as with PCE as a separate phase present were conducted. For the experiments with PCE phase present the nano-iron concentration was varied.
The column experiments to test the nano-iron longevity were performed in horizontal 2 meter long glass columns with an inner diameter of 3.5 cm. A flow with a constant PCE concentration (low or max.) in degassed water was passed through the columns. The iron content in the columns was regularly measured to determine the amount of iron utilized for the reduction, parallel the iron concentration in the effluent was photometrical determined to verify the non-destructive measurement.
The transport experiments were conducted in horizontal Plexiglas® columns (also 2 m x 3.5 cm) filled with sand. The following parameters were varied: disaggregation of suspension, injection rate and injection concentration. Mechanical disaggregation resulted in a great improvement of the transport properties. The iron concentration of the injected fluid greatly influenced the transport distance (higher iron concentration resulted in a greater transport distance). Furthermore, a correlation between rate of injection and transport distance could be observed.
Methods to systematically transfer the batch and column results to a 3-D system were to be found and tested. For this, a radial flow field experiment was constructed. It consisted of a segment of one-sixth of a confined aquifer around an injection well. The radius of the segment was 2 m and the height was 60 cm, which is a near-field scale setup. The main advantages of this setup were the well controlled boundary conditions (const. flux at the injection well and const. head at the outflow perimeter), the easy installation of measuring probes and the visual observation made possible through a glass plate on one side of the segment.
Since the difference of geogenic to injected iron concentration can be several orders of magnitudes, measuring devices with a very high resolution based upon the magnetic susceptibility (as used in mine detectors) had to be developed. The measuring technique makes it possible to non-destructively determine the concentration of nano-iron during and after the injection in a column as well as in a 3-D large-scale experiment.
The results of the column and batch experiments will be presented and further explained in the poster at the Gordon conference. Furthermore the first 3-D experiments are expected to have been performed before the summer and will then be presented in poster as well.