Absorption of Nitric Oxide into Aqueous Solutions of Ferrous Chelates Accompanied by Instantaneous Reaction

Abstract

The absorption of nitric oxide (NO) into aqueous solutions of ferrous chelates of nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), hydroxyethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA) was studied in a stirred cell reactor. Experimental conditions were as follows: 0.05 ≤ CFe(II)L ≤ 0.1 kmol/m3, 8 ≤ PNO ≤ 30 kPa, 3 ≤ pH ≤ 10 (NTA), or 7 ≤ pH ≤ 8 (EDTA, HEDTA, DTPA), CL ≤ CFe(II) ≤ 3CL (NTA) or CL = CFe(II) (EDTA, HEDTA, DTPA), and 293 ≤ T ≤ 333 K (NTA, EDTA, HEDTA) or T = 294 K (DTPA). The absorption leads to stable ferrous NO chelates. Due to the high reaction rate, in combination with the relatively high PNO applied, the absorption rate is strongly affected by mass transfer limitation only. By applying penetration theory, the ratio of the diffusion coefficients of ferrous chelates and NO was determined: at T = 294 K, (DFe(II)chelate/DNO)1/2 = 0.44 ± 0.01, 0.34 ± 0.01, 0.36 ± 0.01, and 0.31 ± 0.015, for the ferrous NTA, HEDTA, EDTA, and DTPA complexes, respectively. At elevated T, (DFe(II)chelate/DNO)1/2 decreases due to the unusual T-dependency of DNO. For ferrous NTA, the formation of the ferrous NO chelate is accompanied by pH effects that can be understood from iron chelate chemistry. In the case of ferrous NTA, pH < 5, or an excess of ligand, these effects lead to local pH gradients at the gas−liquid interface, that substantially affect the NO absorption rates. Kinetic data from the literature on the absorption of NO into ferrous chelates were evaluated using the mass transfer parameters determined. These kinetic data are often unreliable.