Effects of low pH on nitrate reduction by iron powder

Mechanism of enhanced nitrate reduction via micro-electrolysis at the powdered zero-valent iron/activated carbon interface

“…The efficiency by Fe 0 /AC decreased yet with all above 70%, whereas that by Fe 0 only dropped drastically to around 10%. This much poorer reduction efficiency by Fe 0 at initial pH higher than 2 has also been observed in other works, since the process is acid-promoted and the solution pH can severely affect iron corrosion, thus determining nitrate reduction efficiency [5].…”

“…6. In the case using Fe 0 only, nitrate is reduced to ammonium by the electron transferred directly from iron corrosion [5,29]: While mixing Fe 0 with AC, there will be a relative potential difference within this galvanic cell, which enhances electron transfer from iron to activated carbon. The adsorbed H + ion or H 2 O molecule on the AC surface thus accepts the electron and is converted to adsorbed atomic hydrogen (denoted as H ad ) under acidic or near-neutral condition, respectively.…”

“…Unfortunately, considerable studies [4][5][6] have revealed that the performance of Fe 0 in nitrate reduction is strongly dependent on the pH of aqueous solution, and the rapid process only occurs at pH < 4. The acidic condition must be maintained by adding acid solution in order to enhance iron corrosion and dissolve ferrous (hydr)oxides on the Fe 0 surface to keep nitrate being reduced at an appropriate rate [5,6].…”

“…Unfortunately, considerable studies [4][5][6] have revealed that the performance of Fe 0 in nitrate reduction is strongly dependent on the pH of aqueous solution, and the rapid process only occurs at pH < 4. The acidic condition must be maintained by adding acid solution in order to enhance iron corrosion and dissolve ferrous (hydr)oxides on the Fe 0 surface to keep nitrate being reduced at an appropriate rate [5,6]. Fortunately, nanoscale zero-valent iron (NZVI) has been demonstrated to be a promising alternative even under near-neutral and/or neutral conditions due to its large specific surface area and high reactivity [7][8][9].…”

“…Surprisingly, the reduction process was continued with 68% of nitrate reduced after 720 min at initial pH 6. The much more rapid reduction of nitrate under lower pH condition can be mainly due to a larger amount of protons that not only participate in the corrosion reaction but also dissolve iron (hydr)oxides coating on the Fe 0 surface to avoid their inhibition effect, with the similar mechanism to nitrate reduction by zero-valent iron [5]. Nonetheless, the nitrate reduction rate at higher pH was much slower, indicating an obviously different reaction mechanism.…”