Fe-Modified Sewage Sludge Biochar for Efficient Removal of Nanoplastics from Water: Mechanistic Insights and Multi-Pathway Adsorption Analysis

Nanoplastics (NPs) have emerged as pervasive aquatic pollutants due to their small size, high surface activity, and potential ecological and health risks. Although sludge-derived biochar is a sustainable adsorbent for NP removal, the relative importance of coexisting adsorption mechanisms remains poorly quantified. Here, iron-modified sludge biochar (FeBC) was synthesized and evaluated for NP removal from water. Batch experiments showed that FeBC significantly outperformed pristine biochar, achieving a maximum removal efficiency of 96.09%. Adsorption was strongly pH-dependent, with enhanced removal under acidic conditions due to surface protonation and strengthened electrostatic attraction toward negatively charged NPs. SEM, BET, FTIR, and XPS analyses indicated that electrostatic interactions, hydrogen bonding, π-π interactions, and pore adsorption jointly contributed to NP capture. Importantly, structural equation modeling quantitatively disentangled these mechanisms, revealing electrostatic interactions as the dominant driver (52.6%), followed by hydrogen bonding (23%), pore adsorption (16.6%), and π-π interactions (7.9%), and further identified synergistic and antagonistic relationships among them. These results demonstrate that surface charge regulation governs NP adsorption efficiency, providing a quantitative mechanistic basis for the rational design of biochar-based adsorbents. This study advances a multi-mechanistic framework for understanding and optimizing NP removal while promoting sludge resource valorization.