Coupling donann dialysis and electro-reduction process for nitrate removal from simulated groundwater

Abstract

In this study, Donnan dialysis (DD) was applied to denitrify synthetic groundwater and electro-reduction was utilized as a post-treatment to remove the nitrate from the secondary waste generated during the DD process. Combined experiments were performed under continuous operating mode using a synthetic solution mimicking nitrate-contaminated groundwater found in agriculturally intensive regions of Italy. The influence of residence time on thie removal efficiency of nitrate in each reactor was studied. The effects of nitrate concentration, accompanying ions, and by-product formation were tested in the electrolyzer. In DD, nitrate, along with the accompanying ions bicarbonate and sulfate, was transported through an anion exchange membrane into a 100 mM NaCl receiving stream. Higher removal efficiencies were obtained with prolonged residence time, up to 65% at RT of 120 min. A maximum of 60% electro-reduction of nitrate, using a Cu-DSA electrode pair, was achieved at a residence time of ≥90 min. The specific energy consumption did not change significantly with the electrolysis RT ranging from 40 to 120 min. An increase in the nitrate concentration resulted in higher removal efficiency and reduced energy consumption. During the electrolysis of the DD waste, no ammonia was formed and 100% N2-selectivity was obtained.