Heteroaggregation-driven regulation of levofloxacin adsorption by nano-biochar-goethite complexes in aquatic...

The heteroaggregation of naturally occurring substances is crucial for the transportation, transformation, and fate of biochar nanoparticles (BNPs). This study systematically investigates the heteroaggregation behavior between BNPs and goethite (GT), with particular emphasis on their colloidal stability, interaction energetics, and the resulting effects on levofloxacin (LEV) adsorption. BNPs exhibited the highest colloidal stability at pH 7.5. In mixed systems, the coexistence of BNPs and GT enhanced the dispersion of GT particles while simultaneously diminishing the stability of BNPs. The maximum energy barriers (Φ_max) for homo- and hetero-aggregation followed the order GT-GT (-10.4 × 10^-18 J) < BNPs-GT (2.85 × 10^-18 J) < BNPs-BNPs (6.4 × 10^-17 J), indicating that GT tends to undergo homopolymerization prior to subsequent heteroaggregation with BNPs. Both Na⁺ and Ca²⁺ promoted BNP-GT heteroaggregation, with Ca²⁺ exhibiting a markedly stronger effect as evidenced by the lower critical coagulation concentration (CCC). When BNP concentration remained below the critical threshold (40 mg TOC per L), BNP effectively enhanced GT dispersion. However, above this threshold, extensive heteroaggregation occurred, forming large and compact heteroaggregates. Heteropolymer formation partially shielded the surface functional groups of BNPs, thereby restricting LEV access to available adsorption sites. Nonetheless, at elevated LEV concentrations, LEV molecules were able to penetrate the heteropolymer matrix or induce structural rearrangements, enabling access to internal sites and enhancing overall adsorption capacity. Interestingly, at low BNP concentrations, the BNP-GT mixture improved LEV removal by inhibiting GT agglomeration and exposing additional adsorption sites. In contrast, at high BNP concentrations, the formation of large heteroaggregates occluded active sites and diminished LEV adsorption efficiency. Overall, this study provides mechanistic insights into BNP-mineral interactions in natural waters and elucidates their implications for the environmental transport and fate of antibiotics. These findings advance the current understanding of nanoparticle-mineral heteroaggregation processes and offer important guidance for predicting contaminant mobility in aquatic environments.