Polyacrylamide structural instability and degradation induced by nanobubbles: A molecular simulation study

Nanobubble cavitation presents an effective method for the degradation of polyacrylamide (PAM) wastewater. However, the efficiency of bubble energy utilization remains suboptimal, and the underlying degradation mechanisms require further elucidation. In this study, molecular dynamics simulations were employed to investigate the degradation mechanisms of PAM by modeling various nanobubble configurations and spatial arrangements. The results indicate that the collapse of nanobubbles generates high-velocity jets in the central region, causing deformation of PAM molecular chains that is proportional to both the impact velocity and bubble size. The collapse mechanism induces turbulent vortices due to strong shear forces, while discontinuities in local density, velocity, and pressure lead to the formation of secondary shock waves. Higher impact velocities and larger bubble sizes were found to enhance PAM degradation efficiency. Specifically, local shear effects following bubble collapse induce stretching of C-C single bonds and expansion of C-C-C bond angles in the PAM main chain, resulting in the fragmentation of long-chain structures into shorter segments. Notably, PAM undergoes mechanical degradation during this process, with its chemical structure largely maintained and breakage points primarily concentrated in the central region of the bubble. Additionally, increasing the number of bubbles does not necessarily improve degradation efficiency. Different multi-bubble arrangements significantly influence the distribution of kinetic energy post-collapse. Horizontal bubble arrangements, in particular, demonstrate enhanced efficiency through the superposition effects of secondary shock waves on PAM molecular chains, thereby maximizing nanobubble energy utilization.