Inhibiting Substances as Tracers for the Reactivity Assessment of Fe(0)-PRBs

Passivation processes of Fe(0)-barriers are well known from lab-studies (Phillips et al., (2000), Schlicker et al., (2000)) and from field-sites. Normally the passivation processes are correlated with the groundwater composition but quantitative prediction and monitoring of the inhibition velocity under field conditions is a serious problem. Currently, only concentration profiles of contaminants, isotope studies or the measurement of reactivity loss with column-experiments of altered Fe(0)-material from the field sites are used for the characterization of Fe(0)-reactivity. All of theses approaches have serious disadvantages and limitations. Thus the sampling of unaltered Fe(0)-material out of the reactive barrier is difficult and the perturbed installation of the material in column experiments may lead to significant modification in the field behaviour of the Fe(0)-barrier. In addition, the concentration profile of the contaminant is not always a good tool for reactivity estimations due to uncertainties in hydrogeological boundary conditions. The same general restrictions apply also for isotope studies, in which the shift of the d13C signal is used as an indicator for degradation processes of the chlorinated aliphatics. Therefore here the use of Fe(0) inhibiting substances as reactive tracers is presented as a new approach for the characterization of the Fe(0)-reactivity. The methodology of reactive tracers to determine reactive surface areas of Fe(III) in porous was developed last year by Veehmayer et al. (2000) by interpretation of the breakthrough curves of species with known specific interactions with the solid phase. The concept is also applicable for the estimation of reactive sites in Fe(0)-columns, so that the breakthrough curves of oxidants like NO₃-, CrO₄²- or oxidizing organic substances may be interpreted as indicative of reactive reducing sites in the Fe(0)-column. Such correlation was already shown by Schlicker et al., (2000), who explained the movement of passivation fronts by the blocking of reactive sites at the Fe(0) surface. To investigate this approach different column experiments with passivated Fe(0) are being currently carried out. Initial results from the lab indicate that different inorganic as well as organic substances can be used for characterization of the passivation state of the Fe(0) surface. Application of reactive tracer combinations also give some clues about the surface properties of the inhibiting substances, which might be helpful with respect to reactivation approaches for passivated permeable Fe(0)-barriers. Despite the first encouraging but more phenomenological lab results some theoretical problems, like the alteration of the specific surface area during the lab experiments or competition processes between organic or inorganic compounds at the altered surface of the Fe particles have to be addressed more in detail.