Studies on the mechanism of chelate degradation in iron-based, liquid redox H 2 S removal processes

Abstract

Available data on the oxidation of nitrilotriacetic acid (NTA), ethylenedinitrilotetraacetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA), and other aminopolycarboxylate chelating agents are reviewed and the intermediates and products of the oxidative degradation of each chelating agent are described and compared. The oxidation of these chelating agents occurs during the reoxidation of the ferrous chelate to the ferric chelate, during which a Fenton type side reaction occurs in which hydrogen peroxide is formed and which in turn generates the hydroxyl radical by reaction with ferrous ion. The site of oxidative attack by the hydroxyl radical on these ligands seems to be the -CH 2 - groups α to the carboxylate groups, as well as the -CH 2 - groups in the ethylene bridges between the nitrogens. The evidence for the implication of the hydroxyl radical as the active oxidant is discussed, and the use of phenyl-tert-butylnitrone (PBN) as the trapping agent for the hydroxyl radical is described. The use of chelating agents as Fe 3+ /Fe 2+ redox catalysts for the oxidation of H 2 S to sulfur that are less reactive toward the hydroxyl radical is recommended. Keywords: chelate degradation, H 2 S oxidation, nitrilotriacetic acid (NTA), ethylenedinitrilotetraacetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA).