Novel Cross-Linking Behavior in Addition to Chain Scission during Free-Radical Degradation of Water-Soluble Polymers

Water-soluble polymers, such as polyacrylamide (PAM), are industrially important chemicals that end up in our wastewater streams. Their free-radical degradation impacts the fate and treatment during use and in waste streams, the understanding of which is limited to oxidative chain scission at relatively low polymer concentrations. Here, using persulfate as the radical source, we demonstrated that cross-linking could dominate over chain scission under oxic conditions at industrially relevant high polymer and radical concentrations. Due to cross-linking, hydrogel-like solids precipitated from the solutions, the extent of which increased with concentrations of PAM and persulfate and with radical-to-polymer ratio. X-ray photoelectron spectroscopy and sulfate analysis revealed the incorporation of inorganic ions into the solid. Some solids could be redissolved in water and were more cross-linked than the cross-linked polymer that remained dissolved. A higher initial molecular weight of PAM (40-150 kDa) further promoted an earlier solid precipitation during the reaction-cooling process at lower PAM and radical concentrations; the solids were water-insoluble due to the high degree of cross-linking. These findings challenge the conventional understanding of free-radical polymer degradation and highlight the role of cross-linking, which can drastically change the environmental mobility, biodegradability/bioavailability, and treatability of PAM in waste streams.