Polyacrylamide (PAM) has critical functions in water-intensified industries including energy, agriculture, and water treatment. Although viewed as non-toxic due to their large molecular weights, PAM can undergo chain scission degradation under photo-chemical and mechanical triggers in environmental compartments. This degradation generates a complex chemical mixture that remains unexplored. Previous bulk infrared spectroscopy analysis revealed the presence of aldehyde containing products, which may cause health effects due to their electrophilicity. To decipher polymer degradation chemistry, we developed a new non-target workflow using liquid chromatography-high resolution mass spectrometry (Orbitrap, HRMS). Conventional non-target workflows cannot be directly applied due to the lack of HRMS database for polymers and degradation products. To combat this, we modeled degradation pathways and constructed databases of tentative molecular formulas of degradation products. We chose radical induced chain scission of PAM in deep shale under hydraulic fracturing conditions as the model system. Fracturing additive persulfate was the radical sources to form carbonyl terminals on polymer fragments (aldehydes or ketones that is dependent on the location of the carbon-centered radical on backbone). Based on these pathways, we constructed 100,892 unique molecular formulas: 14,534 for original polymer (including acrylic copolymers) and 86,359 for degradation products. From the 5-20% matches between detected MS1 features with our database, we observed a skewed aldehyde/ketone terminating fragments over degradation time, contradictory to previously proposed ratios of carbon radical position, likely due to the impact of radical steric hinderance of sulfate radicals compared to hydroxyl radicals. In addition to include three newly proposed degradation mechanisms to our database, tandem MS data will be utilized to elucidate structural information of matched formula. Finally, we will determine if the toxic monomer of PAM, acrylamide, forms in these conditions. This is a first attempt to resolve complex polymer degradation chemistry using of HRMS and database construction that can advance our understanding of the environmental impact of industrially important polymers.

Deciphering polymer degradation chemistry via integrating novel databases construction into non-target analysis

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ACS Fall 2022

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