Removal of nitrate from groundwater by nano-scale zero-valent iron injection pulses in continuous-flow packed soil columns

Injection of zero-valent iron nanoparticles (nZVI) into aquifers has gained increasing attention of researchers for in-situ treatment of NO3 --contaminated groundwater. nZVI has proved efficient in chemically reducing NO3- and, according to recent research efforts, in supporting biological denitrification under favoured conditions. Given the scarce research on nZVI pulsed injection in continuous-flow systems, the objective of this study was to evaluate the effect of nZVI pulses on the removal of NO3- from groundwater in packed soil columns and, more particularly, to elucidate whether or not biotic NO3- removal processes were promoted by nZVI. Three identical columns were filled with aquifer soil samples and fed with the same nitrate polluted groundwater but operated under different conditions: (A) with application of nZVI pulses and biocide spiked in groundwater, (B) without application of nZVI pulses and (C) with application of nZVI pulses. Results showed that the application of nZVI (at 30 mg/L and 78 mg/L doses) resulted in an immediate and sharp removal of NO3 -(88-94%), accompanied by an increase in pH (from 7.0 to 9.0-10.0), a drop in redox potential (Eh) (from +420 mV to 100 mV) and a release of Fe(II) and Total Organic Carbon (TOC) in the effluent (to 200 mg/L and 150-200 mg/L, respectively). The released TOC came from the organic polymer used as stabilizer of the nZVI particles. Comparison against the sterilized control column revealed that, under the experimental conditions, no biological denitrification developed and that the removal of NO3- was due to chemical reduction by nZVI. The main by-product of the NO3- removal was NH4+, which at the prevailing pH was partially converted to NH3, which dissipated from the aqueous solution resulting in a net removal of total dissolved N. A mass balance of Fe permitted to quantify the percentage of injected nZVI trapped in the column ( 98%) and the NO3- retention capacity of the nZVI particles (13.2-85.5 mg NO3-/g nZVI).

Automated summary

Injection of zero-valent iron nanoparticles (nZVI) has shown significant removal efficiency of NO3- in groundwater through chemical reduction. The application of nZVI pulses resulted in pH increase, redox potential drop, and release of Fe(II) and TOC in the effluent.