Groundwater Remediation using Nanotechnology

Chlorinated solvent contaminants are among the most recalcitrant aquifer contaminants which can cause serious health problems (e.g. kidney and liver damage) and some are considered as carcinogenic. They are classified as DNAPLs, i.e. dense non-aqueous phase liquids. The scale of the problem posed by these contaminants is globally significant due to their wide industrial use since the beginning of 20^th century e.g. in metal processing plants. Removal of chlorinated contaminants from the host aquifers can be done by water injection (soil flushing). In an aquifer water is the wetting phase and the chlorinated solvent acts as the non-wetting phase. It is known that such a displacement is always less than 100% efficient due to capillary action, therefore, a portion of these contaminants will remain trapped in the host aquifer¹. Previous studies have shown that a non-wetting oil phase becomes disconnected into small oil droplets trapped in pores of the host rock as water is injected into the rock. Aquifer remediation technologies are designed to target this trapped contaminant to either (i) pump it out for ex-situ treatment, or (ii) degrade it, in-situ, into less harmful substances.  The latter has received more attention recently as it offers a non-invasive means for contamination elimination. 

Nanoremediation is an emerging technology with great potential for in-situ degradation of chlorinated contaminants and many other metal contaminants (e.g. Cr, As, etc.). The technology injects Fe⁰ nanoparticle in form of aqueous suspensions into contaminant bearing sediments². These nanoparticles are highly reactive and excellent electron donors (Fe⁰  Fe²⁺+ 2e¯). Chlorinated solvents can readily accept those electrons and release their chlorine atoms in form of ions. Example reaction: (C₂H₂Cl₂+ Fe⁰ + 2H⁺C₂H₄ + 2Cl¯+ Fe²⁺). Specifically, nanoremediation has benefited from recent developments in industrial scale manufacturing of engineered nanoparticles at low cost. While nanoremediation concept is proven to be successful at laboratory, pilot, and field scales, the existing practice is far from optimised. This study contributes to the design of an optimum nanoremediation process, targeting remediation of the contamination source that contains the chlorinated solvent in form of a disconnected residual phase. We have studied the mechanisms that occur within the pore space of the contaminant-bearing sediment as the chemical reactions explained above occur. 

We present the outcomes of our 4D (time-resolved, 3D) experiments comprising flow injections and simultaneous 3D imaging using X-ray computed micro-tomography (micro-CT) technique.  The study has been conducted at the Brazilian synchrotron. The specific chlorinated contaminant under study is Trichloroethane (known as TCE). For the first time, we have captured the evolution of TCE phase structure/distribution, in 3D, during the nanoremediation process. Our data show that the gas phase released during the chemical reaction remobilises the trapped TCE phase, facilitating its complete removal in subsequent soil flushing processes. Our findings provide new insights into the pore-scale physics of the nanoremediation process and contribute to optimisation of this process.