Dynamic interplay of protein-humic complexes dictates perfluoroalkyl acids phase partitioning during sequential sludge treatment: multiscale mechanistic insights from composition-structure-energy interdependencies.

Sewage sludge acts as a substantial reservoir and potential secondary source of perfluoroalkyl acids (PFAAs) in the environment. The phase partitioning and interaction mechanisms of PFAAs during sludge treatment and disposal are not fully understood. Herein, a holistic approach that incorporated the analysis of sludge composition, structural evolution, and surface energy dynamics was employed to systematically elucidate PFAA occurrence, migration, and fate throughout the sludge treatment process. Various treatment processes minimally affected the overall PFAA concentration. PFAAs partitioning between solid and liquid phases of sludge was predominantly modulated by interactions with organic matter. 3D-EEM fluorescence spectroscopy revealed that specific protein fractions (tryptophan and tyrosine) and humic substances from sludge solid played a crucial role in PFAA adsorption. FTIR spectroscopy revealed that hydroxyl, amide, and aromatic functional groups primarily mediated PFAA binding through hydrophobic and electrostatic interactions. Thermodynamic analysis revealed that PFAA interactions in the sludge were primarily governed by the Lewis acid-base interaction free energy rather than van der Waals forces or simple electrostatic attraction. Hierarchical clustering analysis confirmed that organic matter was a principal determinant of PFAA distribution and mobility. A multiple linear regression model revealed that a 1% increase in the contents of proteins and humic substances increased PFAA partitioning coefficients (Log Kd) by 0.063 and 0.056, respectively. This study provides a comprehensive understanding of PFAA behavior in sludge, which can contribute to the development of a refined PFAA risk assessment framework and informed mitigation strategies for sludge treatment and disposal.