Miljøprojekt, 916 - Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Reaktive vægge og filtre med jernspåner - en sammenfatning

Reactive Barriers and Iron Filings - Overview of Projects

Summary and conclusions

Permeable reactive barriers and on-site canisters containing zero-valent iron are technologies for treatment of groundwater contaminated with especially chlorinated compounds and chromate. Intensive research is ongoing on a world-wide basis to optimise the technology, and to study some of the factors leading to reduced efficiency of the technology. The report gives an overview of processes in zero-valent iron, alternative technological construction methods, and an updated status of the research on zero-valent iron treatment of contaminated groundwater. The demonstration projects of zero-valent iron treatment carried out under the Danish EPA Technology Programme for Soil and Groundwater Contamination are presented. Finally, a detailed status of the zero-valent iron treatment technologies is given.

Iron filings have through the anaerobic corrosion process a potential for degradation of many types of organic pollutants, especially chlorinated compounds. Iron filings can also treat many inorganic compounds such as chromate, arsenic, and nitrate, through reduction, precipitation, and adsorption processes. Newly reported research adds more compounds to the list of compounds that offer a potential for being treated by zero-valent iron technologies. Technologies combining other metals with the iron filings have been developed in order to enhance the reactivity of the material. Also alternative installation technologies are invented and presented. A substantial fraction of the research is focusing on the longevity issues of the zero-valent iron technology: for how long will the barrier stay permeable, and be reactive towards the pollutants?

Several of the zero-valent iron projects under the Danish EPA Technology Programme for Soil and Groundwater Contamination were demonstration projects carried out in collaboration with the Danish counties in full scale in order to gather field experience under real Danish conditions. Five field projects were carried out: three permeable reactive barriers (PRBs), and two on-site groundwater treatment canisters. Two of the three projects were dealing with sites contaminated with chlorinated solvents (Vapokon Site and Copenhagen Freight Yard Site), one with a mixture of TCE and chromate (Haardkrom Site). The two on-site treatment projects were dealing with PCE contaminated groundwater (Lyndby Site) and chromate (Skt. Claravej Site).

Besides the field projects two projects were carried out in the laboratory. One project studied the chromate removal capacities of iron filings, and found a relatively limited capacity, which needs to be considered in cases of high chromate loads. The other project evaluated the effect of combining zero-valent iron reactors with reactors containing miscellaneous organic materials. Several of the reactors containing organic materials showed removal of chlorinated solvents. However, the combined organic matter reactor/zero-valent iron reactor did not show significantly better removal rates than the zero-valent iron reactor alone.

The technologies using zero-valent iron seem promising. The most important issues to consider are controlling the hydraulic conditions of permeable reactive barriers to ensure homogeneous retention times within the iron barrier, and the long-term reactivity of the iron material. Due to changes of the chemistry of the groundwater passing through the barrier, several compounds may be retained by precipitation and adsorption processes. The retained solids may affect the reactivity of the iron surfaces leading to reduced degradation rates and/or limited capacities for chromate removal. With the present knowledge it is difficult to predict the durability of a permeable reactive barrier from iron material properties and groundwater chemistry. There is a need for more focused research for developing predictive models of the long-term reactivity of barriers.