At present "pump and treat" is the most commonly used method for groundwater and aquifer remediation . Due to the fact that the source of the organic contaminants very often is a separate non-aqueous phase and due to natural heterogeneities in the subsurface, pump and treat very often lacks from transport limitations. Most of these remediation projects have not been successful in spite of a long time of operation. As an alternative, contaminants can be immobilized or degraded in "Geochemical Barriers" under favourable Eh - pH- conditions.
If the spreading of the contaminants is significantly restricted by biogeochemical reactions in the aquifer without applying any technical means, "Natural Attenuation" is regarded to be sufficient. Otherwise the "Natural Geochemical Barriere" must be modified. Injection of liquids for the degradation and immobilisation of the contaminants encounters the problem of losses by dilution and mass transfer due to heterogeneities. These problems are mainly overcome by "Artificial Geochemical Barriers": Reactive solids are placed downstream the contaminant plume. Very often they are combined with impermeable walls ("funnel"), leading the contaminated plume to the permeable, reactive parts of the wall ("gate") (Starr and Cherry, 1994). Because of its high cost effectiveness, in situ reactors are being developed for various contaminants (BTEX, chlorinated solvents, heavy metals, NO3-). Reactive materials must meet the following requirements:
-bulk goods (low cost, homogeneity with respect to physical and chemical properties)
-high reactivity over long time periods with respect to the contaminants under various geochemical
-hydraulic stability (no clogging)
-low toxicity of the metabolites
The idea of using elemental iron in permeable reactive walls for the degradation of the chlorinated aliphatic
hydrocarbons (CAH) via abiotic reductive dechlorination originates from the research group around Prof. Gillham (Gillham and O'Hannesin, 1994).
Fe0 + R-Cl + H2O ¤ Fe2+ + R-H + Cl- + OH- (1)
Because iron is available in high quantities (prize × $500), research is focussing on iron and modified iron reactive walls. The reaction has proved to be pseudo first order with respect to the concentration of CAH and the reaction rate is proportional to the metal surface/solution volume in a wide range and decreasing at higher pH. Half lives of common CAH are in the range of hours (e.g. TCE 1 h*m2/ml), but some (c-DCE, DCA) are hardly degraded. Several large scale pilot plants in North America have been running rather successful.
Few research, however, has been done on the effect of high organic and inorganic groundwater constituents on the geochemical and hydraulic long-term stability of a Fe0-wall. High O2-concentrations and high alkalinity have been found to reduce the permeabiltiy of columns filled with zero-valent iron, and to decrease the TCE degradation rate by the formation of iron precipitates iron oxides and carbonates on the Fe0 -surface (Mackenzie et al., 1995).
Currently we test in batch experiments different metals and bimetals as to optimize the TCE degradation rates. We found that pH increases by the dechlorination reaction using Fe0 and low mineralized water up to 10. High pH, however, means reduction of the degradation rates by accumulation of the reaction product OH- and by formation of iron precipitates. Al0 showes higher degradation rates (by factor 3) buffering the solution around pH 6, where the formed Al(OH)3-precipitate strongly controls dissolved Al(III)-concentrations. Better degradation results than each single metal showed a 1:1 Fe0/Al0-mixture by formation of local cells buffering at pH 8,5-9. Pd0 is reported in the literature to have a catalytic effect and so Fe0 was palladized from aqueous Pd(II)- solution which increased the degradation rate of TCE by 100.
We will also present the results of batch experiments using high concentrations of organic and inorganic chemicals which may inhibit the TCE degradation itself (e.g. surfactants) or by formation of precipitates (e.g. phosphate).
Based on the results of the batch and subsequent column experiments, where changes in permeability due to precipitates or H2 -development will be investigated, an in situ application on a technical scale in the Research Facility for Subsurface Remediation (VEGAS) will be designed, where a modified Fe0-reactor will be tested.
Gillham, R.W. & O'Hannesin, S.F., Ground Water 32, 958-967 (1994).
Mackenzie, P.D. et al., ACS Symp. on Emerg. Tech. in Haz. Waste Man. VII, 59-62 (1995).
Starr, R.C. & Cherry, J.A., Ground Water 32, 465-476 (1994).