Enhanced Reduction of Nitrate in Groundwater by Zero-valent Iron with Activated Red Mud

Abstract

The objective of this study was to examine the feasibility of using red mud for enhancing the nitrate removal in groundwater by zero-valent iron (Fe(0)). Batch experiments were performed with both untreated and acid-treated red mud in conjunction with Fe(0) system to treat an actual groundwater sample containing 2.52 mM nitrate. Addition of untreated red mud into aqueous solution containing Fe(0) abruptly raised pH value to highly alkaline (11–12.5), leading to inhibition of nitrate reduction. On the other hand, the addition of acid-treated red mud significantly increased nitrate removal, resulting in 164% increase in the 1st-order rate constant (kobs) as compared to Fe(0) alone system when the red mud was dosed at 5 g/L to the solution containing 50 g/L Fe(0). Nitrate removal increased with increasing dose of acid-treated red mud, showing nearly 100% increase in removal efficiency when the dose was increased from 0 to 0.5 g in 20 mL solutions containing 0.2 g Fe(0). The enhancing effect of activated red mud was assumed to be the result from scavenging of the reaction precipitate, thereby minimizing passivation of Fe(0) surfaces, and formation of additional reactive sites (e.g. green rust) for nitrate reduction. The reactions of both untreated and acid-treated red mud in the absence of Fe(0) showed little removal of nitrate, indicating red mud alone lacks reduction or sorption capability for nitrate. Results from this study demonstrated that the potential utility of Fe(0)/activated red mud system may be developed into a viable technology for the removal of nitrate from groundwater.