Degradation of nitrilotris(methylenephosphonic acid) and related (amino)phosphonate chelating agents in the presence of manganese and molecular oxygen

Phosphonates are used in an increasing variety of industrial and household applications including cooling waters systems, oil production,textile indus try, and detergents. Phosphonates are not biodegraded during wastewater tre atment hut instead are removed by adsorption onto sludge. This study invest igates the degradation of phosphonates in the presence of Mn-II and molecul ar oxygen. The half-life for the reaction of nitrilotris(methylene)phosphon ic acid (NTMP) in the presence of equimolar Mn-II and in equilibrium with 0 .21 atm O-2 is 10 min at pH 6.5, with the reaction occurring more slowly un der more alkaline or acidic pH values. The presence of other cations such a s Ca-II, Zn-II, and Cu-II can considerably slow the reaction by competing w ith Mn-II for NTMP. Catalytic Mn-II is regenerated in cyclic fashion as the reaction takes place. Although generation of a Mn-III-containing intermedi ate appears likely, electron transfer within the ternary complex O-2-Mn-II- 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 manganes e catalyzed thermal autoxidation of commercial (amino)phosphonate chelating agents is likely to be an important degradation mechanism in natural water s.