Oxidation of ferrous nitrilotriacetic acid with oxygen: A model for oxygen mass transfer parallel to reaction kinetics

The kinetics of the reaction of ferrous chelate of nitrilotriacetic acid (NTA) and gaseous oxygen were studied in a stirred-cell reactor. The initial concentration of ferrous chelate was 0.100 kmol/m(3). Other reaction conditions include 293 < T < 333 K, 2 < pH < 10, and 20 < p(O2) < 90 kPa. The structure of ferrous chelates as a function of temperature and pH was studied by potentiometric titrations. Under the conditions applied, the reaction rate appeared to proceed according to R-O2 = k(1,2)C(O2)C(Fe(II))(2) with k(1,2)(infinity) = (1.4 +/- 0.35) x 10(7) m(6)/mol(2) s and E-a = 52.5 +/- 1.4 kJ/mol for no excess NTA. In the presence of excess NTA, a double-coordinated ferrous species is produced, whose reactivity appears to depend on pH and NTA concentration. Due to undesired side reactions with ligand NTA, more oxygen disappears than expected from the conversion of ferrous chelate. If no excess ligand is present, the gas absorption rate is pH-dependent for pH less than or equal to 4. Excess ligand results in a pH-dependence over the entire pH-range studied. The observed regeneration rate as a function of pH can be explained from a more comprehensive model, accounting for complex chemical reaction, in parallel with interfacial mass-transfer limitations.