Aluminum Dialkyl Phosphinate Flame Retardants and Their Hydrolysates: Analytical Method and Occurrence in Soil and Sediment Samples from a Manufacturing Site

Abstract

Aluminum dialkyl phosphinates (ADPs) are emerging phosphorus flame retardants due to their superior characteristics, but their analytical method, and occurrence and fate in environments have never been reported. For the first time, we developed a method for the analysis of trace ADPs and their hydrolysates (dialkyl phosphinic acids, DPAs), and studied their occurrences and fates in soils and sediments. We found that ADPs are hardly dissolved in water and organic solvents, but are dissolved and hydrolyzed to DPAs in 30 mM NH3·H2O, thus both ADPs and DPAs can be determined by liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) in the form of DPAs. ADPs and DPAs in soil and sediment samples were determined by (i) extracting both ADPs and DPAs with 75 mM NH3 · H2O, and selectively extract DPAs only with formic acid-water-methanol (5:5:90, v/v/v); (ii) quantifying the total content of ADPs and DPAs, and DPAs by LC-MS/MS analysis of the DPA contents in the former and the latter extract, respectively; and (iii) calculating ADPs from the content difference between the former and the latter extracts. The limit of quantifications (LOQs) of the proposed method were 0.9-1.0 μg/kg, and the mean recoveries ranged from 69.0% to 112.4% with relative standard deviations ≤ 21% (n = 6). In soil and sediment samples around a manufacturing plant, ADPs and DPAs were detected in surface soils in the ranges of 3.9-1279.3 and 1.0-448.6 μg/kg, respectively. While ADPs were found in all the samples of the soil and sediment cores from the drain outlet and the waste residue treatment site at levels ranging from 30.8 to 4628.0 μg/kg, DPAs were found in more than 90% of these samples with concentrations in the range of 1.1-374.6 μg/kg. The occurrences of ADPs and DPAs are not in correlation with the total organic carbon, whereas the occurrences of DPAs are highly correlated with the sample pH. Our study also suggests that the DPAs in the samples sourced from the hydrolysis of ADPs. The high hydrolysis degrees of ADPs (up to 49.6%) suggest that once released into the environment, ADPs are likely to coexist with their hydrolysates. Thus, to evaluate the environmental safety of ADPs, the environmental behavior and toxicity of both ADPs and DPAs should be considered.