Characterization of phosphonate-based antiscalants used in drinking water treatment plants by anion-exchange chromatography coupled to electrospray ionization time-of-flight mass spectrometry and inductively coupled plasma mass spectrometry.

In drinking water production, phosphonates are frequently applied as antiscalants in order to prevent the precipitation of salts on reverse osmosis membranes. Whereas the nominal constituents are defined, phosphonate-based antiscalants were found to contain significant amounts of undeclared phosphorous contaminants, particularly noticeable technical diethylenetriamine penta(methylene phosphonic acid) (DTPMP). Following a literature review on impurities of phosphonic acids, separation by anion exchange chromatography coupled with ICP-MS- and ESI-TOF-detection allowed the comprehensive characterization of technical phosphonates. After the identification of synthesis by-products, intermediates and degradation products, the quantification of the phosphorus compounds was accomplished by complementary phosphorus-sensitive ICP-MS detection. The contents of the nominal constituents as well as the organophosphorous impurities of 24 technical antiscalants were determined and compared with the manufacturer's declaration. Of all compounds detected within the technical formulations, 86% were confirmed with reference compounds. Phosphorous impurities were found to contribute to the total phosphorus content with close to 20% (in ATMP- and PBTC-based products) and 38%-65% (in DTPMP-based products). Correspondingly, the nominal compounds were found to contribute with 78%-80% (based on 6 ATMP products) and with 81.5%-83% (based on 2 PBTC products), whereas in 10 out of 12 DTPMP products the nominal compound was found to contribute with 34%-44% to the total phosphorus. Only in two products, elevated DTPMP proportions of 55% and 62% with regard to total phosphorus were determined. The screening for aminomethylphosphonic acid (AMPA) revealed contents between 1.9 mg/L and 157 mg/L. This study represents the first in-depth characterization of phosphonate-based antiscalant products and stock solutions used for drinking water production.