Extraction and Quantification of Synthetic Hydrophilic Polymers from Soil with Py-GC/MS

Current plastic research focuses on particulate water-insoluble polymers such as polyethylene and polystyrene. Synthetic hydrophilic polymers, a class of anthropogenic materials with an annual global production of over 35 Mt, have received little scientific attention. Synthetic hydrophilic polymers are used for the controlled release of agrochemicals and for seed coating. Despite being intentionally added to soil, little is known about their occurrence or fate in soil.

This study aimed to address this knowledge gap by developing and validating a pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) method for identifying and quantifying synthetic hydrophilic polymers in soil. We focused on the most common synthetic hydrophilic polymers: polyacrylic acid (PAA), polyethylene glycol (PEG), polyvinyl alcohol (PVOH) and polyvinyl pyrrolidone (PVP) in agricultural model soil.

The method was validated by measuring one solution containing all four polymers between 1 and 200 µg/mL using pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). Intra-day repeatability was determined by 10-fold measurement of a 150 µg/mL standard. To test polymer recovery from three agricultural model soils (5–47% clay, 1–3% organic carbon), 5 g of soil were spiked with 1 mL of hydrophilic polymer solution (2 mg/mL). Further, three extraction agents were tested: aqueous NH₃ solution (pH = 11), aqueous H₃PO₄ (pH = 3) solution and concentrated sodium pyrophosphate solution (TSPP, 0.1 M, pH = 9). 10 mL of extraction agent were added and agitated for up to 28 days. Samples were taken every seven days, to assess the optimal extraction time and the best possible recovery rate. Non-spiked reference soil was used as a blank. Samples and blanks were measured as duplicates.

Limits of detection (LODs) for PEG, PVOH and PVP were below 1 µg/mL and limits of quantification (LOQs) ranged from 68 to 87 µg/mL. LOD and LOQ for PAA were the highest with 25 µg/ml and 94 µg/mL, respectively. The pyrolysis of PAA and PVOH partly resulted in similar pyrolysis products, challenging the simultaneous and selective quantification of both polymers. The intra-day repeatability was 8–16%. The best recovery rates ranged from 20 to 133% and were achieved with TSPP. While the acidic solution led to recovery rates of 21–115% in soils with 5-16% clay and 1% organic carbon, polymer concentrations in a soil with 47% clay and 2.6% SOM were below LOD. The alkaline solution recovered 5–144% of the polymers. The optimal extraction time varied among soil types. On average, a 14-day extraction yielded the best recoveries with TSPP solution. Blank signals for PVP and PEG were below 10% of the sample spikes. For PVOH and PAA the blank signals were 26–71% and 15–63%, respectively. These results demonstrate the significant challenges of analyzing PVOH and PAA simultaneously, as both polymers produce similar pyrolysis products.

Py-GC/MS is a promising tool for identifying and quantifying synthetic hydrophilic polymers. However, further experiments using complementary analytical methods are required to improve analytical robustness.