MECHANISMS OF NITRATE REMOVAL WITH METTALIC IRON POWER

Batch experiments using 10 ml serum bottles as reactors were conducted to investigate the mechanisms of nitrate reduction in an iron/water/nitrate system under anoxic conditions. At neutral pH without initial pH adjustment, the zerovalent iron (ZVI) was passivated, and in 48 h less than 10% of nitrate was removed. With the initial pH adjusted to 2.3, nitrate reduction occurred in three sequential stages. In stage 1 with a low pH, nitrate was reduced rapidly while the pH rapidly increased from 2.3 to 4.8 due to the instant acid corrosion reaction between Fe0 and H+. Stage 2 was characterized by a much more moderate nitrate reduction rate when pH further rose from 4.8 to 6.2, and a black coating was formed on the Fe0 surface. Stage 3 was featured by a rapid nitrate reduction concurrent with a rapid aqueous Fe2+ depletion and black precipitate formation. The oxide coating and precipitate formed in stage 3 were confirmed as magnetite by X-ray diffraction spectroscopy. Dissolved Fe2+ was identified as the key factor via screening tests on the effects of pH, the magnetite coating, the magnetite precipitate and dissolved Fe2+. Based on the stoichiometric test, the total reaction was summarized asNO−3 + 2.82 Fe0 + 0.75 Fe2+ + 2.25 H2O → NH+4 + 1.19 Fe3O4 + 0.50 OH−The equation indicates that the major electron donor should be Fe0, not the dissolved Fe2+. A hypothetical two-layer semiconductor model is proposed as the mechanism for further investigation. A kinetic model with a double-Langmuir-adsorption formulation was developed to represent the site saturation effects of aqueous Fe2+ and NO3− on nitrate reduction in a ZVI system at neutral pH. The proposed kinetic model can fit the experimental data very well and, therefore, partly supports the mechanism. The study indicates that the iron oxide can facilitate nitrate reduction provided that aqueous Fe2+ is present in the system.