Carpet Grass Polyphenol Reductive Degradation of Aqueous Nitrate─A Conceptual Field Application Study

Abstract

Rainwater flowing along the ground, and from hard surface such as pavement and roofs, becomes surface water runoff, which flows to surface waters, and infiltrates into the ground to become groundwater. Surface water runoff can contain elevated levels of nitrates (NO 3 − ) from various sources including animal wastes and fertilizers. Reducing elevated levels of NO 3 − in surface water runoff can minimize and/or prevent groundwater and surface water contamination. Natural polyphenols in carpet grass, due to phenolic hydroxyl groups, can degrade aqueous NO 3 − . This study evaluated the potential for carpet grass to purify water by denitrifying NO 3 − as surface water flows through grass-covered land. The research investigated nitrate removal efficiency and reaction kinetics under various flow rates and doses of carpet grass, validating the feasibility of using natural polyphenols for water purification. A grass dose of 100 g and a retention time of 24 h, which produced approximately 20−80 mg/L as gallic acid equivalent (GAE) of polyphenol in simulated surface water runoff, were demonstrated to effectively degrade NO 3 − in aqueous solution (110 mg/L) and result in the denitrification of NO 3 − to nitrite (NO 2 − ) and eventually N 2 . The first-order reaction kinetic rate constants for NO 3 − degradation, NO 2 − formation, and subsequent degradation of NO 2 − are k obs (NO3-degradation) = 6.89 × 10 −2 h −1 , k 1(NO2-formation) = 5.11 × 10 −2 h −1 , and k 2(NO2-degradation) = 4.63 × 10 −2 h −1 , respectively, with a conversion rate (α) of NO 3 − to NO 2 − to be 0.74. Implementing natural vegetation, such as carpet grass, in water management practices offers an environmentally sustainable approach to reducing nitrate contamination in surface water runoff.