Degradation of Nitrilotris(methylenephosphonic Acid) and Related (Amino)Phosphonate Chelating Agents in the Presence of Manganese and Molecular Oxygen

Abstract

Phosphonates are used in an increasing variety of industrial and household applications including cooling waters systems, oil production, textile industry, and detergents. Phosphonates are not biodegraded during wastewater treatment but instead are removed by adsorption onto sludge. This study investigates the degradation of phosphonates in the presence of MnII and molecular oxygen. The half-life for the reaction of nitrilotris(methylene)phosphonic acid (NTMP) in the presence of equimolar MnII and in equilibrium with 0.21 atm O2 is 10 min at pH 6.5, with the reaction occurring more slowly under more alkaline or acidic pH values. The presence of other cations such as CaII, ZnII, and CuII can considerably slow the reaction by competing with MnII for NTMP. Catalytic MnII is regenerated in cyclic fashion as the reaction takes place. Although generation of a MnIII-containing intermediate appears likely, electron transfer within the ternary complex O2−MnII−NTMP cannot be completely ruled out. Formate ion, orthophosphate ion, imino(dimethylene)phosphonic acid and N-formyl imino(dimethylene)phosphonic acid breakdown products have been identified. This work indicates that manganese-catalyzed thermal autoxidation of commercial (amino)phosphonate chelating agents is likely to be an important degradation mechanism in natural waters.