A sustainable environmentally friendly NO x removal process using Ag(II)/Ag(I)-mediated electrochemical oxidation

Abstract

This paper presents the results of investigations related to the development of a mediated electrochemical oxidation (MEO) process-based wet scrubbing method for NO and NO 2 abatement from a simulated NO-air flue gas mixture. The Ag(II) redox mediator in nitric acid medium generated in an electrochemical cell was circulated continuously through a packed-bed column by a closed loop, and the NO x gas was oxidatively removed by absorption/oxidation. The outlet gas from scrubber-I was subsequently admitted into scrubber-II for further continuous washings by 3 mol L −1 HNO 3. Numerous experimental runs were carried out in order to observe the performance of the wet scrubber-electrochemical cell integrated system as a function of the operating conditions: the initial NO concentration in the feed (100–400 mg L −1), gas flow rate (0.061–0.243 m s −1), liquid flow rate (0.012–0.049 m s −1), concentration of silver ion mediator (0.01–0.1 mol L −1), nitric acid concentration (3–6 mol L −1), and the temperature (15–45 °C). Low initial NO feed, low gas flow rates, high liquid flow rates, high HNO 3 acid concentration, Ag(I) concentration as low as 0.05 mol L −1 and low temperatures were most suitable for high removal performance. Total removal (100%) of NO and 80% removal of NO x were achieved in the oxidation with electrogenerated Ag(II) in a single stage gas scrubbing in scrubber-I. But, interestingly, a second stage gas scrubbing by a simple HNO 3 (3 mol L −1) wash in scrubber-II led to a better removal of NO x with overall efficiency reaching 90% as well. This is an alternative green treatment process for zero emission of waste gases. Some NO x removal studies were also performed using Ce(IV)/Ce(III) redox mediator system, and the effect of nitric acid concentration and temperature examined, for a comparison with Ag(II)/Ag(I).