Photodegradation of phosphonates in water

Phosphonates are widely used as chelating agents, e.g., in water cooling systems, in bleaching baths or as scale inhibitors in deflocculation agents. They are considered to be difficult to degrade and produce aminomethylphosphonic acid (AMPA) as a metabolite. As the fate of phosphonates in the environment is not very well known the present work aims at simulating the time dependent photodegradation of four selected phosphonates: nitrilotris-methylenephosphonic acid (NTMP), ethylenediamine-tetra-methylenephosphonic acid (EDTMP), diethylenetriaminepenta-methylenephosphonic acid (DTPMP) and hexaethylenediamine-tetra-methylenephosphonic acid (HDTMP), at concentrations of 1 mg/l (i.e. 3.2 μM NTMP, 2.3 μM EDTMP, 1.7 μM DTPMP and 2.0 μM HDTMP) irradiated by a middle pressure mercury lamp emitting between 190 and 600 nm. The influence of iron under different pH ranges (3, 5-6 and 10) are tested. The degradation of phosphonates is measured by the release of orthophosphates (PO4-P) and aminomethylphosphonic acid (AMPA). This study shows that phosphonates are substances that undergo UV light conversion, which is enhanced in the presence of iron. The half-life without iron is between 15 and 35 min at pH 3, between 10 and 35 min at pH 5-6 and between 50 and 75 min at pH 10. The half-life in the presence of 3.6 μM iron is between 5 and 10 min at pH 3, between 5 and 15 min at pH 5-6 and between 35 and 60 min at pH 10. The individual substances do not significantly influence the reaction rates whereas the presence of iron and the pH have significant effects. The total conversion of phosphonates after 90 min is 75-100% for pH values of 3 and 5-6 and 55-75% for a pH of 10 dependent on the presence of iron. In the environment longer degradation times are to be expected since natural light is weaker by a factor between 125 and 300 in the UVB, a factor between 3 and 8 in the UVA and of the same intensity in the visible range than the light in our study. Although orthophosphates are the major products, AMPA is also shown to be a by-product of the photodegradation of phosphonates that is later converted into orthophosphate.