Acceleration of the Fe(III)EDTA− reduction rate in BioDeNOx reactors by dosing electron mediating compounds

BioDeNOx, a novel technique to remove NOx from industrial flue gases, is based on absorption of gaseous nitric oxide into an aqueous Fe(II)EDTA2− solution, followed by the biological reduction of Fe(II)EDTA2− complexed NO to N2. Besides NO reduction, high rate biological Fe(III)EDTA− reduction is a crucial factor for a succesful application of the BioDeNOx technology, as it determines the Fe(II)EDTA2− concentration in the scrubber liquor and thus the efficiency of NO removal from the gas phase. This paper investigates the mechanism and kinetics of biological Fe(III)EDTA− reduction by unadapted anaerobic methanogenic sludge and BioDeNOx reactor mixed liquor. The influence of different electron donors, electron mediating compounds and CaSO3 on the Fe(III)EDTA− reduction rate was determined in batch experiments (21 mM Fe(III)EDTA−, 55 °C, pH 7.2 ± 0.2). The Fe(III)EDTA− reduction rate depended on the type of electron donor, the highest rate (13.9 mM h−1) was observed with glucose, followed by ethanol, acetate and hydrogen. Fe(III)EDTA− reduction occurred at a relatively slow (4.1 mM h−1) rate with methanol as the electron donor. Small amounts (0.5 mM) of sulfide, cysteine or elemental sulfur accelerated the Fe(III)EDTA− reduction. The amount of iron reduced significantly exceeded the amount that can be formed by the chemical reaction of sulfide with Fe(III)EDTA−, suggesting that the Fe(III)EDTA− reduction was accelerated via an auto-catalytic process with an unidentified electron mediating compound, presumably polysulfides, formed out of the sulfur additives. Using ethanol as electron donor, the specific Fe(III)EDTA− reduction rate was linearly related to the amount of sulfide supplied. CaSO3 (0.5−100 mM) inhibited Fe(III)EDTA− reduction, probably because SO32- scavenged the electron mediating compound.