DESIGNING A TRICHLOROETHENE SOURCE ZONE TREATMENT BASED UPON NANO SIZED ZERO VALENT IRON

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The widespread use of CHSs has provided countless opportunities for CHS entry into sub-surface soils and rocks through a combination of spillage, leaking storage tanks and deliberate disposal. CHSs have specific densities greater than 1 which means that a CHS release has the ability to sink below the water table. CHSs are immiscible with water and have low aqueous solubilities. Environmental engineers and scientists describe such liquids as DNAPLs (dense non-aqueous phase liquids) to emphasise their immiscibility in water and specific gravity, which is greater than 1. CHS DNAPLs slowly dissolve in flowing groundwater giving rise to a contaminant plume. Groundwater provides some 40% of the potable water needs of the UK and CHS contamination often results in public water abstractions being abandoned. This is unsustainable! Contaminated aquifer restoration is therefore a necessary aspect of sustainable water consumption. Restoration must entail the removal of trapped CHS droplets and pools which continuously feed the contaminant plume and some form of plume containment or degradation. Iron can be used to degrade dissolved CHS molecules to safer end-products through reductive dechlorination. However the sustainable technical solution to aquifer restoration requires the removal of CHS DNAPL. This is difficult! Pumping may remove some DNAPL but not all. This is due to the fact that the forces generated by pumping are unable to overcome the capillary forces that trap CHS DNAPL in pore spaces. In these cases much of the CHS mass can be removed via emulsion formation. Emulsions are usually stabilised by surfactants but they can be stabilised by colloidal particles. Such emulsions are called Pickering emulsions. The basic premise of this proposal is that iron nano-particles can be used as emulsion stabilisers. In a previously Royal Society supported project we were able to demonstrate that chemically modified clay particles could be used to remove trapped creosote from a liquid saturated porous system as a Pickering emulsion. We propose that chemically modified nZVI can be used to remove CHS DNAPLs in a similar fashion. However experimental reports of DNAPL removal via emulsion formation conclude that not all the DNAPL mass is removed. This proposal addresses the issue of degradation of the remaining mass. Field observation indicates that colloidal sized particles can be removed from the aqueous flow field through adhesive contact to surfaces. Normally the removal of emulsion stabiliser from a treatment injection is a problem. However we conjecture that the removal of nZVI by attachment to the CHS/water interface of trapped droplets (resilient to emulsion formation) will promote CHS degradation. Moreover nZVI particles trapped in pore spaces will also degrade dissolved CHS. We thus believe that we can develop a technology based upon the use of a commercially available inexpensive nZVI product which can be chemically modified by the use of relatively cheap amphiphilic molecules in a few straightforward steps that can be used to:(i) remove trapped CHS DNAPL through emulsion formation;(ii) degrade trapped CHS DNAPL (that is resilient to emulsion formation) through iron mediated reductive dechlorination;(iii) degrade dissolved CHS. Such a treatment will provide an effective technical solution to CHS DNAPL contaminated aquifer restoration. The purpose of the work plan is to investigate how nZVI can be chemically modified and subsequently deployed to provide this technical solution.