Molecular mechanistic insights towards aggregation of nano-biochar moderated by aromatic components in dissolved organic matter

Nano-biochar (NBC) is a promising tool for sustainable remediation of contaminants in aquatic environments. However, the presence of ubiquitous ions and dissolved organic matter (DOM) can impact NBC aggregation, resulting in reduced application efficacy and potential ecological risks. Understanding and regulating NBC aggregation offers valuable insights for its deployment. This study integrated batch aggregation experiments, theoretical models, Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS), and density functional theory (DFT) calculations to explore the behaviors and mechanisms of NBC aggregation with coexisting ions and model DOM. NBC aggregation kinetics followed the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in both NBC-ions and NBC-ions-fulvic acid (FA) solutions, indicating that the aggregation process is controlled by Van der Waals forces and electrostatic repulsion. Mono/di-valent electrolytes promoted NBC aggregation, whereas FA moderated it, with higher molecular weight FA fractions exhibiting superior performance. Three-dimensional excitation-emission (3D-EEM) fluorescence spectra and Parallel factor analysis (PARAFAC) analyses revealed that HA-like substances, followed by FA-like substances, can form a complex with ions, thereby moderating NBC aggregation. FTICR-MS scans identified lignin substances with aromatic structures as key components that effectively reduce the promoted NBC aggregation with coexisting mono/di-valent electrolytes. DFT calculations confirmed that the aromatic structures in FA spontaneously form complexes with electrolytes, thereby potentially regulating NBC aggregation. This research highlights potential strategies for regulating NBC applications and offers insights into the behavior of nanoparticles in aquatic environments.