"Laboratory Experiments to Characterize the Transport and Reactivity of Zero-Valent Iron Colloids in the Subsurface"1. Verification of Transport of Zero-Valent Iron Colloids in Porous Media In order to remove contaminants from the groundwater and soil, several methods are available. A relatively new and promising method is the injection of a zero valent iron colloid suspension in the subsurface using injection wells.
The goal is to deposit the iron in the pores space and hence form an injected reactive zone. However, little is known about the transport behavior of zero valent iron colloids during the injection. A better understanding of the transport behavior is necessary to design a field application such that the reactive zone will be continuous in space and sufficiently wide. So far, the spreading and distribution of the iron colloids in the subsurface during injection could not be proven due to the lack of appropriate measuring techniques.
In the ongoing work, measuring devices with a very high sensitivity based upon magnetic susceptibility have been developed to non-destructively determine the concentration of nano-scale iron during and after the injection in columns and in a large-scale container experiment. In a field application the injection in a well would result in a radial flow field around the well. The large-scale container experiment was constructed to create such a 2-D radial flow field. The experiment was equipped with the newly developed measuring technique providing a quantitative measurement of breakthrough curves.
Several column and container experiments were conducted to determine the parameters controlling the mobility of iron colloids in the subsurface during injection. Within the ongoing BMBF research project NAPASAN the measuring technique is being improved and optimized to make detection and verification also possible in the field. Another aim of NAPASAN is to develop manufacturing processes of nano-particles (iron and nonferrous metals) and to modify their characteristics on the basis of economic criteria. The particles must be modified in such a way that a transport in porous media (or rather groundwater zone) is enabled and a contact with the contaminants and their reduction is guaranteed.
2. Chemical reduction of PCE by zero valent Iron colloids – comparison of Batch and column experiments In order to remove contaminants from the groundwater, a relatively new and promising method is the injection of a zero valent iron colloid suspension in the subsurface using injection wells. The iron attaches to the sand grains and fills up favorite deposition sites in the porous medium to form a reactive zone.
One of the advantages of this technology is the possibility to inject the reactants directly into the source zone. The reactant is then very close to the contaminant phase and chemically reduces the dissolved chlorinated solvent before the solution is being diluted, as it would be the case further downstream. The reaction close to the pure phase strongly reduces the contaminant concentration in water. This in turn increases the rate of dissolution and hence the remediation time.
In order to determine degradation rates, and hence, longevity of the iron particles, batch experiments are conducted. However, the results are not directly transferable to the field scale since in batch experiments the contact between iron colloids and the chlorinated solvent is optimized by shaking or rotating the vial. In porous media flow, the distribution of the chlorinated solvent is diffusion controlled resulting in a decrease in the availability of contaminants for the chemical reduction. In addition, the corrosion is self inhibiting due to the rising pH which does not occur in column experiments, where the flow of fresh water keeps the pH neutral. Therefore, the corrosion is much stronger in flowing systems than in column experiments or in the field.
This shortens the longevity and, moreover, hydrogen as a by-product of the corrosion process will be produced. If this happens in larger amounts, this hydrogen becomes a separate (gas-) phase and can clog the porous medium.
Preliminary column experiments have been performed in previous research projects and more elaborate column experiments are being set up in the ongoing project. The preliminary experiments showed that the production of hydrogen gas could be reduced by adding burned chalk (Ca(OH)2) in granular form to the suspension during injection.
Several long term experiments have been planed for the column scale within the project NAPASAN. Among these are investigations on the longevity of zero valent iron particles in porous media, either with pure phase present or in contact with dissolved contaminants. Also, the effect Ca(OH)2 on the longevity and reactivity will be studied as a part of NAPASAN.