Humic acid characteristics and effects on the reactivity of nano-scale zero-valent iron particles during nitrate reduction

Humic acid (HA) comprises a significant fraction of natural organic matter (NOM), greatly influencing the performance of water and wastewater treatment processes. HA is expected to significantly affect the reactivity of nano-scale zero-valent iron (NZVI), which is receiving increasing attention due to its high reactivity. The effects of HA on nitrate reduction by NZVI were investigated to evaluate the potential of NZVI in practical applications. HA was characterized to identify the mechanism whereby HA affects. Nitrate reduction was enhanced at low HA concentration, but inhibited as HA concentration increased. HA decreased to reach a plateau and considerable amount of Fe3+ was detected when HA was present in dissolved phase. The increase in the degree of condensation of HA was verified by UV–vis spectroscopy, fluorescence spectroscopy, and size exclusion chromatography. Fourier transform infrared spectra confirmed that the Fe3+–HA complexes were formed in both the dissolved and solid fractions, without coexisting metal ions. The HA aggregation on NZVI and discrete HA macroaggregates were observed by transmission electron microscopy. The results suggest that HA induces complex effects on NZVI reactivity by improving iron dissolution, inhibiting readsorption of released iron, enhancing electron transfer, and inhibiting nitrate mass transfer via forming HA accumulation. It indicates that the clogging by aggregated NOM should be considered when NZVI is used for groundwater treatment or in combination with porous materials.