Effect of Iodide in Diethylenetriamine Oxidation in CO2 Capture: Role of iron ions and Enhancement by chelating agents

Oxidative degradation remains a key challenge in amine-based CO₂ capture, limiting solvent stability, increasing costs, and creating risk of environmental impact. Chain polyamines such as diethylenetriamine (DETA) exhibits high CO₂ absorption capacity and faster kinetics than the baseline solvent monoethanolamine (MEA). However, its severe oxidative degradation has hindered broader application. While iodide has been reported as an effective inhibitor in MEA oxidation, its performance in polyamines remains unclear. In this study, DETA was selected as a representative polyamine to systematically investigate its oxidative behavior and evaluate inhibition by iodide. Experiments were conducted using a high oxygen pressure reactor (HOPR) and a high gas flow reactor (HGF) to quantify amine retention, ammonia generation, and other degradation products. Results showed that in the presence of 1 mM Fe3+, 100 mmol/kg iodide showed significantly limited inhibition in DETA oxidation compared to MEA. This difference may be attributed to stronger iron-driven oxidation pathways in DETA, which compete with iodide in peroxide decomposition. Kinetic modeling and peroxide consumption rate measurements further support that iodide probably functions by terminating radical reactions, rather than physical effects such as reducing oxygen solubility. The effect of iodide should strongly depend on the relative rate of peroxide decomposition by Fe3+ and iodide. The iodide-to-Fe3+ ratio is also expected to be a critical parameter. To overcome this limitation, phosphonate-based chelating agents (HEDP or DTPMP) were used to reduce dissolved iron availability. The combined use of iodide and chelators achieved 100% inhibition of DETA oxidation, even in iron-rich environments. This work provides mechanistic insights and a practical strategy for improving the oxidative stability of polyamine solvents in CO₂ capture.