Oxidation kinetics of Iron(II) complexes of trans-1,2-diaminocyclohexanetetraacetate (cdta) with dissolved oxygen: Reaction mechanism, parameters of activation and kinetic salt effects

Abstract

The present investigation takes concern about a spiny environmental problem afflicting the pulp mill industry exploiting the Kraft sulfate-pulp process where dilute total reduced sulfur contaminants are co-mixed with oxygen in large-volume gas effluents. A potential Redox process for removing the total reduced sulfurs consists in oxidizing them by means of iron(III) organometallic complexes while the co-mixed oxygen mediates the oxidative regeneration of iron(II) into iron(III) complexes. In this work, the oxidation kinetics of iron(II) trans-1,2,-diaminocyclohexanetetraacetate (cdta) complexes with molecular oxygen (O 2) as the source oxidant was investigated for a wide pH range ( 1.75 < pH < 12 ) in a 3.2 dm 3 single-phase stirred cell reactor within the [281–323 K] temperature range. Simultaneous measurements of iron(II)–cdta (50– 400 μ mol dm - 3 ) and O 2 (0.5– 6 mg dm - 3 ) were used to clarify the reaction mechanism which has been interpreted differently in previous works. The observed kinetic data in alkaline solutions could be accounted for in terms of three forward [ Fe 2 + cdta 4 - + O 2 (rate-limiting, k 1 , app ), Fe 2 + cdta 4 - + O 2 · - ( k 2 , app ) , 2 Fe 2 + cdta 4 - + H 2 O 2 ] and one reverse [ Fe 3 + ( OH - ) n cdta 4 - + O 2 · - ( k - 1 , app , n = 0 or 1)] elementary steps. Assessment of the rate-limiting apparent rate constant led to the following results ( k 1 , app = 38.0 ± 2.4 dm 3 mol - 1 s - 1 at T = 297.2 K and I c = 0.05 mol NaCl dm - 3 , Δ ‡ H 1 0 = 92.6 ± 0.6 kJ mol - 1 , Δ ‡ S 1 0 = 38.6 J mol - 1 K - 1 ). Fe 3 + OH - cdta 4 - , being the dominating iron(III) product at pH > 10 , was found to be less reactive than Fe 3 + cdta 4 - with the superoxide intermediate ( O 2 · - ) , thus reducing the effect of the reverse step at higher pH. A study on the effect of electrolytes on the reaction rate led to the conclusion that salts increase the rate constant k 1 , app . Finally, kinetic results in acidic conditions leading to the formation of other iron(II)–cdta complexes (i.e., Fe 2 + cdta 4 - H + ) and another superoxide intermediates ( HO 2 · ) are reported and discussed.