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Bacterial reduction of iron in clay barriers: a new technology for the remediation of organic groundwater contaminants

Final Activity Report Summary - BIO FE-CLAY BARRIER (Bacterial reduction of iron in clay barriers: a new technology for the remediation of organic groundwater contaminants)

Groundwater pollution by domestic, agricultural and industrial chemicals is at an all-time high in many parts of the world. In Europe, the EC Water Framework Directive requires that surface, coastal and underground waters meet 'good status' within member states by 2015. With only few years to go, initiatives to develop cost-effective and sustainable remediation technologies are crucially needed.

Clay minerals are abundant in soils and sediments worldwide. New solutions to many groundwater contaminations could be developed by wisely selecting clay materials for the design of permeable reactive barriers (PRBs) and by manipulating their properties in situ. In particular, recent studies have shown that reduced Fe-bearing smectites can promote the reductive degradation of many redox-sensitive contaminants, such as chlorinated aliphatics, nitroaromatics and various pesticides. Yet, Fe-bearing clays have never been used in PRBs for the treatment of groundwater contaminated with any of these compounds.

The main goal of this project was to demonstrate the feasibility of a new remediation technology - the Bio Fe-Clay Barrier - which would use Fe-bearing clays and in situ bacterial Fe-reduction for the remediation of diverse groundwater contaminations. The project aimed to answer the following questions:
1. Which Fe-reducing bacteria would be the most suitable for reducing Fe in the Bio Fe-Clay Barrier and which growth medium and additives should be used?
2. Since clays are low permeability materials, how to design a Bio Fe-Clay Barrier with sufficient permeability?
3. What will happen in the field? Will Fe-reduction affect the permeability of the barrier?
4. Which organic compounds could be efficiently degraded by the Bio Fe-clay Barrier and how fast?

To answer these questions, series of small-scale laboratory experiments were completed using a multi-disciplinary approach, associating clay mineralogy, rock and soil mechanics, microbiology and organic geochemistry.

Networking and field activities were completed to identify and collect suitable Fe-bearing clays. After preparation (fractionation) and characterisation of the clay minerals, microcosm experiments were completed to identify the best commercially available Fe-reducing bacteria and the most suitable conditions. Shewanella algae BrY gave the best results, especially when anthraquinone disulfonic acid (AQDS) was added. One of the studied clay minerals (a glauconite) did not, however, respond well to Fe-reduction.

Degradation experiments were then completed to test the effect of bacterially reduced Fe-smectites on the redox degradation of a model organic (nitrobenzene). Ninety percent degradation to aniline was observed in less than 100 hours in contact with a 20 % reduced nontronite. The rate of degradation was limited by the proportion of active reductive sites and was approximately 20 times faster if AQDS was used.

Other experiments with p,p'-DDT and S. oneidensis MR-1 also showed that reduced nontronite can degrade up to 21 % of DDT to DDD in only 6 weeks, demonstrating that Fe-bearing clays can substantially contribute to the reductive degradation of recalcitrant compounds in natural and artificial systems.

Using results from this project, Dr Fialips applied for further funding and won a research grant from the United Kingdom EPSRC Research Councils to develop and test a new clay-based material for the Bio Fe-Clay Barrier. This new material uses a binder to attach Fe-bearing clays on sand allowing the required permeability to be reached while avoiding clogging of the permeable system through clay migration. Results of batch and column experiments are very promising and Dr Fialips intends to patent the new porous material.