Photolysis of hexamethylenediaminetetra(methylenephosphonic acid) (HDTMP) using manganese and hydrogen peroxide

Abstract

Abstract Aminophosphonates such as hexamethylenediaminetreta(methylene phosphonic acid) (HDTMP) are categorised as persistent substances. They are commonly used as scale inhibitors in cooling water systems and desalination processes. After utilisation, they are often discharged into aquatic environment without pre-treatment. Advanced oxidation processes (AOP) are promising pre-treatments for industrial wastewater treatments. We investigated the photodegradation of HDTMP with or without addition of manganese (Mn2+) and/or H2O2. Similar to results of our former photodegradation studies, we found that HDTMP also undergoes conversion with or without additives during the ultra violet (UV) irradiation. The reaction rate was most affected by the addition of H2O2, i.e. the HDTMP degradation was accelerated by a factor 3.85 compared with UV treatment without additives. The addition of Mn2+ accelerated the degradation of HDTMP only by a factor 1.53 compared with the UV treatment without additives. The combined addition of Mn2+ and H2O2 accelerated the HDTMP degradation by a factor 2.81. Interestingly, the initial cleavage is not initiated as expected at the C–N bond but at the C–P bond of the methyl carbon and the phosphorus of the methylenephosphonic acid group of HDTMP. This initial cleavage was independent whether the UV treatment was performed with or without additives. Therefore, we conclude that the degradation mechanism is similar independent of the four tested treatment conditions. We identified amino(methylenephosphonic acid) AMPA, dimethylamino(methylenephosphonic acid) DAMP and iminodi(methylenephosphonic acid) IDMP as the major breakdown products by performing LC/MS analyses. The major mineralisation products were ortho-phosphate, ammonium and carbon dioxide. The mass balances of unknown breakdown products allowed us to speculate about the molecular size of the unknown organic breakdown products.