Pore-scale investigation of the use of reactive nanoparticles for in situ remediation of contaminated groundwater source

Tannaz Pak,Luiz Fernando De Lima Luz,Tiziana Tosco,Gabriel Schubert Ruiz Costa,Paola Rodrigues Rangel Rosa,Nathaly Lopes Archilha

doi:10.1073/pnas.1918683117

Abstract

Nanoscale zero-valent iron (nZVI) particles have excellent capacity for in situ remediation of groundwater resources contaminated by a range of organic and inorganic contaminants. Chlorinated solvents are by far the most treated compounds. Studies at column, pilot, and field scales have reported successful decrease in contaminant concentration upon injection of nZVI suspensions in the contaminated zones. However, the field application is far from optimized, particularly for treatments at-or close to-the source, in the presence of residual nonaqueous liquid (NAPL). The knowledge gaps surrounding the processes that occur within the pores of the sediments hosting those contaminants at microscale limit our ability to design nanoremediation processes that are optimized at larger scales. This contribution provides a pore-scale picture of the nanoremediation process. Our results reveal how the distribution of the trapped contaminant evolves as a result of contaminant degradation and generation of gaseous products. We have used state-of-the-art four-dimensional (4D) imaging (time-resolved three-dimensional [3D]) experiments to understand the details of this degradation reaction at the micrometer scale. This contribution shows that the gas released (from the reduction reaction) remobilizes the trapped contaminant by overcoming the capillary forces. Our results show that the secondary sources of NAPL contaminations can be effectively treated by nZVI, not only by in situ degradation, but also through pore-scale remobilization (induced by the evolved gas phase). The produced gas reduces the water relative permeability to less than 1% and, therefore, significantly limits the extent of plume migration in the short term.

Highlights

Nanoscale zero-valent iron particles have excellent capacity for in situ remediation of groundwater resources contaminated by a range of organic and inorganic contaminants
Our main hypothesis was that the emergence of a gas phase will cause fluid redistribution at the pore scale, changing the fluid arrangements and, impacting the DNAPL entrapment
While the nanoremediation concept is proved to be successful at laboratory, pilot, and field scales, the existing practice is far from optimized, when applied close to the secondary source of contamination, where residual DNAPL is still present as a segregated phase

Summary

Introduction

Nanoscale zero-valent iron (nZVI) particles have excellent capacity for in situ remediation of groundwater resources contaminated by a range of organic and inorganic contaminants. Our results show that the secondary sources of NAPL contaminations can be effectively treated by nZVI, by in situ degradation, and through pore-scale remobilization (induced by the evolved gas phase). The release of chlorinated solvents in groundwater resources is a widespread and a global problem [1] This family of contaminants is poorly soluble and denser than water (dense nonaqueous liquid [DNAPL] phase) and, can sink into deeper sediments located below a leakage source, resulting in a residual saturation of the DNAPL phase, which slowly releases the contaminants and may impact groundwater for decades. It is possible to remove most of the DNAPL phase from the host aquifer by combining injection of water (with chemical additives) and extraction of contaminated washing solution This process is known as aggressive soil flushing, which is Significance

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Conclusion