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

Abstract

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).

Keywords: Column experiment; Groundwater remediation; Injection pulse; Nano-scale zero-valent iron; Nitrate reduction.