"Chemical reduction of PCE by zero valent iron colloids – Batch and column experiments"In order to remove contaminants from the groundwater, a relatively new and promising method is the injection of nano-sized zero valent iron (nZVI) as suspensions directly into a source zone. In this way the reactant is brought in close contact to the contaminant phase and chemically reduces the dissolved chlorinated solvent before the solution is being transported with the flow of the groundwater. The reaction close to the pure phase strongly reduces the contaminant concentration in water and increases the rate of dissolution and hence the remediation time.
In order to determine degradation performance and longevity of different iron particles, batch experiments have been 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. But it is relatively simple to control the experimental conditions, and, therefore, it is possible to compare different nZVI suspensions to each other.
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, which is self-inhibiting due to the production of OH- (increasing pH) in batch experiments, is not suppressed in column experiments, where the flow of “fresh” water keeps the pH more stable. Therefore, in batch the anaerobic corrosion is different to flowing systems as in column experiments or in the field.
This anaerobic corrosion shortens the longevity and, moreover, hydrogen as a by-product of these processes will be produced. If this happens in larger amounts, this hydrogen becomes a separate (gas-) phase and clogs the porous medium.
Thus, column experiments have been performed as well and more elaborate column experiments are currently being set up. Preliminary experiments showed that the production of hydrogen gas could be reduced by adding burned lime (Ca(OH)2) to the nZVI suspension. Other experiments are currently performed to show, if the injection of Ca(OH)2 in the porous media as a pH increasing preconditioning of the aquifer is suitable. Also, the effect of Ca(OH)2 on the longevity and reactivity will be studied.
Several long term experiments have been planed for the column scale within the BMBF-funded 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.