Biochar-induced microregional pH elevation enhances synergistic immobilization of tetracycline and copper during transport in saline soil water-bearing media: Quantum and molecular insights

Tetracycline (TC) and copper (Cu) are among the most widely used additives in livestock and poultry production. When introduced into soils via organic fertilizers, they can co-occur and interact, creating complex contamination scenarios that may threaten environmental quality and human health. Biochar effectively inhibits the co-transport of TC and Cu in saline-alkali soils. However, the underlying quantum and molecular mechanisms remain unclear. In this study, column experiments, transport modeling, quantum chemical calculations, and molecular dynamics simulations demonstrated that biochar increases the microdomain pH (> 10), thereby facilitating the synergistic immobilization of TC and Cu within saturated porous media. Soil alkalinization promoted TC deprotonation, and di-deprotonation significantly enhanced the covalent complexation between TC and Cu. The binding energy for the di-deprotonated TC complexed with dual Cu reached –50.38 kcal·mol -1 . Cu binding at distinct TC sites formed molecular clusters with heterogeneous surface potentials, facilitating their aggregation and deposition, preferentially onto biochar and illite surfaces. This synergistic retention intensified with an increase in biochar dosage. Pig manure biochar (PMBC) exhibited a significantly higher retention capacity for TC-Cu complexes than that of cotton straw biochar, which was attributed to the substantially high hydroxyl and carboxyl functional groups and ash content of PMBC. The application of PMBC within a closed-loop cotton-pig farming system reduces environmental risks from combined antibiotic and heavy metal pollution while enhancing soil carbon sequestration. This provides a sustainable remediation strategy and its basis in environmental structural chemistry.