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

## TL;DR

This study shows how iron-modified biochar can efficiently remove nanoplastics from water by understanding the key mechanisms involved.

## Contribution

The study quantifies the relative roles of multiple adsorption mechanisms using structural equation modeling for NP removal by biochar.

## Key findings

- Fe-modified biochar achieved 96.09% nanoplastic removal efficiency.
- Electrostatic interactions were the dominant adsorption mechanism (52.6%).
- Surface protonation and pH strongly influence NP adsorption efficiency.

## Abstract

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.