Limitations of the molybdenum blue method for phosphate quantification in the presence of organophosphonates

Organophosphonates (OPs) are widely used as chelating agents in domestic and industrial applications. While regarded as hardly biodegradable, OPs can undergo abiotic transformation with phosphate (PO₄^3-) as a main transformation product. As some OPs are suspected precursors of glyphosate in surface waters, their environmental fate is of current interest. Due to analytical challenges posed by quantification of individual OPs, monitoring PO₄^3- formation is a widely used proxy to monitor OP transformations. The molybdenum blue (MB) method, employing UV/Vis spectroscopy, is frequently used for PO₄^3- quantification due to its sensitivity and operational simplicity. However, while interference of certain inorganic ions is well-documented, the effects of OPs on the accuracy of the MB method remain unexplored. This study investigated the effects of six OPs, namely N-(phosphonomethyl)glycine (glyphosate), 1-hydroxyethylidene(1,1-diphosphonic acid) (HEDP), iminodi(methylene phosphonate) (IDMP), aminotris(methylene phosphonate) (ATMP), ethylenediaminetetra(methylene phosphonate) (EDTMP), and diethylenetriaminepenta(methylene phosphonate) (DTPMP). Spectral analysis of pure PO₄^3- standards using the MB method exhibits two characteristic absorption maxima (λ_max) at 710 and 880 nm. In the presence of OPs, a new λ_max appears around 760 nm. This is accompanied by an increase in absorbance values at both 710 and 880 nm, leading to significant over-quantification of PO₄^3- concentrations. Among the evaluated OPs, DTPMP exhibits the most substantial interference (PO₄^3- over-quantification by up to 240%), while glyphosate causes minimal interference (≤ 20%). The effects are most pronounced at OPs:PO₄^3- ratios ≥1. A case study simulating DTPMP transformation confirms PO₄^3- over-quantification of up to 350%, revealing limitations of the MB method. Therefore, careful data evaluation and complementary analytical techniques for accurate PO₄^3- measurements are indispensable in OP transformation research.