56-Removal of Nitrate from Drinking Water by Chemical Reduction using Metallic Iron

Abstract

In India several parts of Rajasthan and Maharashtra drinking water is contaminated with elevated concentrations of nitrate from agriculture and untreated sewage. Nitrate if further reduced to nitrite, can cause methemoglobinemia among new-born infants and can form carcinogenic compounds in the stomach of adults. Current treatment strategies (ion exchange, reverse osmosis, electrodialysis, biological denitrification) are costly, difficult to maintain and potentially generate concentrated waste streams. To resolve the above issues, a chemical method of zero-valent iron reduction has been explored in the present work for removal of nitrate from drinking water in a lab-scale study. Studies were conducted in batch mode with metallic iron of 300-mesh size at nitrate-nitrogen concentration of 22 mg/L (Nitrate 100 mg/L). Almost 70% nitrate-nitrogen removal was achieved at reaction pH of 3 within 35-40 minutes time period. Nitrate reduction had followed first order kinetics and reaction rate constant was strongly dependent on reaction pH. Nitrate-nitrogen removal (%) was directly proportional to iron dose at a constant reaction pH. By products of the reaction were observed as ammonia and ferrous ions. Unaccounted nitrogen fraction was 20-78% depending on reaction pH. Nitrate reduction by zero-valent iron when followed by lime softening and aeration could be applied in conventional drinking water treatment system.