Nitrate removal in zero-valent iron packed columns

Nitrate removal by laboratory and field continuous-flow zero-valent iron (Fe0) packed bed columns was evaluated for different influent water qualities (pH, dissolved oxygen (DO), nitrate concentration) and several months of operation (600–1500 bed volumes (BVs)). In contrast to previous batch experiments with Fe0 where nitrate was stoichiometrically converted to ammonium, only 70% of the applied nitrogen was recovered as nitrate, ammonium, or nitrite (<0.1 mg/L) during shorter-term column tests (2–20 BVs) and less than 25% of the applied nitrogen was recovered during longer-term field testing (500–1000 BVs) at elevated nitrate levels (~25 mg N/L). Nitrate removal was accompanied by a pH increase, DO decrease, and soluble iron increase. During longer-term operation (500–1500 BVs) iron and calcium precipitates were observed, by SEM and EDX analyses, to form in the packed columns. Precipitation led to cementation and reduction in permeability for the Fe0/sand media in the packed column. Different abiotic and microbial-mediated mechanisms may be involved during shorter- and longer-term operation of Fe0 systems and the role of iron precipitates should be further evaluated.