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

Nitrate reduction by zero-valent iron (Fe-0) powder always works well only at controlled pH lower than 4 due to the formation of iron (hydr)oxides on its surface. Fe-0 powder combined with activated carbon (AC), i.e., Fe-0/AC micro-electrolysis system, was first introduced to enhance nitrate reduction in aqueous solution. Comparative study was carried out to investigate nitrate reduction by Fe-0/AC system and Fe-0 under near-neutral conditions, showing that the Fe-0/AC system successfully reduced nitrate even at initial pH 6 with the reduction efficiency of up to 73%, whereas for Fe-0 only similar to 10%. The effect of Fe-0 to AC mass ratio on nitrate reduction efficiency was examined. Easier nitrate reduction was achieved with more contact between Fe-0 and AC as the result of decreasing Fe-0 to AC mass ratio. Ferrous ion and oxidation-reduction potential were measured to understand the mechanism of enhanced nitrate reduction by Fe-0/AC microelectrolysis. The results suggest that a relative potential difference drives much more electrons from Fe-0 to AC, thus generating adsorbed atomic hydrogen which makes it possible for nitrate to be reduced at near-neural pH. Fe-0/AC micro-electrolysis thus presents a great potential for practical application in nitrate wastewater treatment without excessive pH adjustment. (C) 2014 Elsevier Inc. All rights reserved.