This study aims at investigating the abiotic degradation pathway of
ethylenediaminetetra(methylenephosphonic acid) (EDTMP) simulated applying UV
irradiation. The degradation of EDTMP and formation of degradation products was
determined using LC-MS and ³¹P-NMR. In the laboratory scale experiments, EDTMP was
degraded within 30 min and the degradation products, iminodi(methylenephosphonic
acid) (IDMP), ethylaminobis(methylenephosphonic acid) (EABMP), and amino-
(methylenephosphonic acid) (AMPA), were simultaneously released. IDMP was the
main degradation product of EDTMP. Therefore, we conclude that the initial cleavage of
EDTMP is a heterolytically driven process, which starts the degradation process at the
intramolecular C-N bond. In contrast, the main product of a possible homolytic C-C
cleavage of methylaminobis(methylenephosphonic acid) could not be confirmed with
either LC-MS or ³¹P-NMR. Additionally, there was no evidence for a primary attack on
the C-P bond. All identified degradation products of EDTMP have been mineralized to
carbon dioxide (CO₂). Three additional degradation products (M1, M2, and M3) have
been found using the ³¹P-NMR analysis but have not yet been quantified using LC-MS.
We assume that the unidentified degradation product M1 is related to m/z 312, M2
to m/z 341, and M3 to m/z 409. Thus we concluded that EDTMP undergoes photochemical
conversion to IDMP, the main degradation product. EABMP and AMPA also accumulate,
but in smaller amounts. All intermediates are further mineralized to CO₂.
DTPMP is predominantly utilized as scale inhibitor. We investigated the reaction rates and degradation mechanism of DTPMP with and without addition of Fe²⁺, Mg²⁺ and Ca²⁺ by performing LC/MS and ³¹P-NMR analyses. DTPMP undergoes conversion with and without addition of bivalent metal ions. The initial cleavage of DTPMP is initiated at the C-N bond leading to release of IDMP as its major breakdown product. The release of smaller quantities of EABMP and AMPA confirmed the nucleophilic attack on the DTPMP amines. Oxidation of Fe²⁺ to Fe³⁺ during the initial 30 min indicated an intramolecular electron transfer changing the electron density distribution at the nitrogen centre, which increased the radical attack during UV irradiation. Independent of the fact that Fe acted as catalyst and Mg²⁺ and Ca²⁺ acted as reactants, we found no significant differences in their degradation mechanisms. However, the reaction rates were strongly affected by the addition of the bivalent metal ions as Fe²⁺ accelerated most DTPMP degradation followed by Mg²⁺ and Ca²⁺. The UV treatment without metal ion addition was four times slower compared with Fe²⁺ addition. We conclude that in environments rich in ferrous iron and/or at reduced redox potential, photolysis of DTPMP will be catalysed by iron and will lead to accumulation of IDMP, EABMP and AMPA and several other none-quantifiable breakdown products.
Photodegradation of aminophosphonates such as ethylenediaminetetra(methylenephosphonic acid) (EDTMP) is recently assumed being the major degradation pathway in aquatic environments. Several photolysis studies were reported about EDTMP and possible breakdown products occurring in natural ecosystems. Reliable prediction of environmental photolysis of parent compounds and possible release of breakdown products requires different
set-up conditions and varying the parameters influencing the photodegradation. We studied the influence of three different system configurations during UV degradation of EDTMP. These three configurations differed either in geometry and/or treated sample volumes. System 1 was equipped with a direct cooling jacket at the UV lamp. System 2 had the geometry of system 1 but there was no usage of a direct cooling jacket. System 3 was a gas-tight system with a larger sample volume. Using the chemical actinometer potassium ferrioxalate, we determined the highest photon flux for system 3 followed by system 2 and 1. In addition, we performed scavenger experiments with methanol and ascorbic acid in order to prove the dominating radical species. In system 1, the addition of methanol showed almost no effect while the ascorbic acid resulted in a reduction of 57.1% orthophosphate released. Therefore we conclude that in system 1 the radical-drive degradation of EDTMP is mainly
based on superoxide radicals. In system 2 and 3 both radical species, i.e., hydroxyl radicals and superoxide radicals, contribute to the photodegradation of EDTMP. We determined different half-lives for EDTMP for the three different systems configurations. For system 1, the estimated half-life achieved was 14.09 ± 0.15 min. For
system 2 and 3, the half-lives were almost similar and averaged 4.75 ± 0.05 min and 5.02 ± 0.20 min, respectively. Contrary to our assumption to also find the highest degradation rate for system 3, we found the highest degradation rate for system configuration 2 as a result of the differences in the construction and geometry
of the three systems. Our findings lead us to recommend the three system configuration for different research purposes. Thus, we recommend system 1 for detailed studies on the degradation pathway of the parent compound and their breakdown products. System 2 is recommended as a suitable configuration for kinetic studies of the parent compound. And finally, we recommend the system configuration 3 for complete mass balances. The gas-tight system allows determining all soluble and gaseous compounds.