PCE dissolution and simultaneous dechlorination by nanoscale zero-valent iron particles in a DNAPL source zone

Area of Science:

• Environmental Engineering
• Geochemistry
• Remediation Technologies

Background:

• Nanoscale zero-valent iron (NZVI) is effective for dechlorinating organic compounds in aqueous solutions.
• The application of NZVI for dense non-aqueous phase liquid (DNAPL) removal from source zones under field-like conditions remains under-investigated.
• Understanding simultaneous DNAPL dissolution and NZVI-mediated dechlorination is crucial for effective in-situ remediation.

Purpose of the Study:

• To investigate the efficacy of direct NZVI emplacement in DNAPL source zones for contaminant removal.
• To evaluate the interplay between DNAPL dissolution and NZVI-driven dechlorination reactions.
• To develop and utilize a numerical model simulating these coupled processes.

Main Methods:

• Experimental setup involving a custom flow cell with a DNAPL tetrachloroethene (PCE) source zone and emplaced NZVI.
• Monitoring of effluent PCE and dechlorination byproducts over time.
• Development of a 3D numerical model incorporating rate-limited DNAPL dissolution, NZVI dechlorination, and byproduct partitioning.

Main Results:

• Direct subsurface emplacement of NZVI (10 g/L) did not significantly accelerate DNAPL dissolution or mass depletion rates.
• The rate of DNAPL dissolution into groundwater controls the overall longevity of the DNAPL source.
• While faster NZVI reactions reduce aqueous contaminant levels, there's a limit to mass removal rate enhancement.

Conclusions:

• Direct NZVI application within DNAPL source zones is not an optimal strategy for accelerating mass removal.
• DNAPL dissolution rate is the limiting factor for NZVI-mediated remediation effectiveness.
• Strategic NZVI placement in down-gradient capture zones is recommended for efficient reduction of aqueous contaminant concentrations.