Geochemical stability of zero-valent iron modified raw wheat straw innovatively applicated to in situ permeable reactive barrier: N2 selectivity and long-term denitrification.

The zero-valent iron (ZVI) modified wheat straw materials are widely used for treating groundwater by permeable reactive barrier (PRB). We report the performance of a field-scale PRB filled with ZVI modified wheat straw materials for nitrate (NO3-)-contaminated groundwater. In lab-scale PRB filled with ZVI modified wheat straw material, NO3- concentration entering the PRB was varied (27.80-59.86 mg L-1) according to the in situ NO3- contamination. A stable NO3- removal rate of 90% was achieved at a controlled hydraulic retention time of 22 days, together with a proportion of denitrifying bacteria up to 34.37%. The field-scale PRB filled with ZVI modified wheat straw material was successful at removing NO3- from groundwater (removal percentages ≥60%) at a groundwater flow rate of 0.01 m3 d-1. Monitoring of groundwater within this PRB provided evidences that the nitrogen gas (N2) selectivity increased with lower ammonia (NH4+) generated from ZVI reduction of NO3-, and few emission of NO2- present due to denitrification capacity in this PRB. The results are finally compared with the few others reported existing PRBs for nitrate-contaminated groundwater worldwide, and demonstrated that the ZVI modified wheat straw material would be an effective fillings for field PRB to remediate groundwater.