Chelating agents in high temperature aqueous chemistry. 2. The thermal decomposition of some transition metal complexes of nitrilotriacetate (NTA)

Abstract

The kinetics and mechanism of decomposition of NTA complexes of FeIII, FeII, CoII, NiII, and CuII under hydrothermal conditions (425–573 K) have been examined. The relative rates at 573 K are CoIINTA− < NTA3−[Formula: see text] < NiIINTA− < FeIIINTA0 < H3NTA0 < CuIINTA− < H4NTA+. Aqueous CoIINTA− and FeIINTA−, like NTA3−, decomposed at 573 K by decarboxylation, precipitating Co(OH)2 and Fe3O4 respectively; NiIINTA− precipitated Ni(OH)2 initially but subsequently Ni metal. At 530 K, FeIIINTA0 solutions precipitated FeII3(NTA)2•H2O, but at higher temperatures Fe3O4 formed, the NTA ligand being reduced to HCHO and iminodiacetate (IDA) rather than decarboxylated. Similarly, CuIINTA−gave IDA and HCHO at temperatures as low as 425 K, forming first CuI (which precipitated as CuCl in the presence of Cl−) and then metallic Cu. The applicability of NTA to corrosion control in boilers and to 60Co removal from water-cooled nuclear reactors is briefly considered. The half-life of FeIIINTA in the hydrosphere is estimated at 80 years (cf. 8 × 106 years for free NTA), in the absence of photolysis or biodégradation.

Homogeneous and Heterogeneous Oxidation of Nitrilotrismethylenephosphonic Acid (NTMP) in the Presence of Manganese(II, III) and Molecular Oxygen

“…Rates of EDTA hydrolytic breakdown in the presence of +II metal ions decrease in the order Mg II > Ca II > Zn II > Fe II > Ni II , reflecting increasing percentages of total EDTA complexed by the added metal ion . In a similar manner, rates of NTA (nitrilotriacetic acid) degradation at high temperatures decrease as the extent of complexation by +II metal ions is increased . Higher-valent metal ions, in contrast, may induce oxidation−reduction pathways for chelating agent breakdown.…”

Manganese-catalyzed degradation of phosphonic acids

“…1), in contrast, are not subject to oxidative degradation under the conditions just described. Under hydrothermal conditions (125 to 300 C), direct and metal ion-catalyzed C-N and C-P bond cleavage have been observed with NTA, EDTA, and NTMP (Booy and Swaddle 1977;Martell et al 1975;Motekaitis et al 1982;Kaslina et al 1985). Progressive loss of liganddonor groups eventually generates products that do not effectively coordinate metal ions, and hence do not further degrade via metal ion-catalyzed pathway.…”

From initial treatment design to final disposal of chelating agents: a review of corrosion and degradation mechanisms

“…Fe 3+ was observed to be reduced to Fe 2+ and Cu 2+ to copper metal (Cu 0 ). Booy and Swaddle 105 also decomposed NTA–Fe 3+ and NTA–Cu 2+ chelates and made similar observations with the former yielding IDA, sarcosine, and dimethylamine, with the latter producing IDA, formaldehyde, and CO 2 . Only 2.5% of NTA was degraded after 6 hours of exposure to 160 °C (320 °F).…”

“…The thermal stability of NTA with several divalent cations and Fe 3+ was tested at 300 °C (572 °F) by Booy and Swaddle. 105 They showed that NTA 3− was more stable than several NTA chelates such as Fe( ii )–NTA, Fe( iii )–NTA, and Cu( ii )–NTA. However, Co( ii )–NTA was demonstrated more thermal stability than NTA 3− .…”