Aggregation of Antibiotic Resistance Plasmids under Different Hydrochemical Conditions

• • At pH 3–7, low pH induces inward condensation and protonation of ARP functional groups, reducing negative charge and increasing aggregate size; this pH sensitivity is critical for predicting ARP mobility in acidic soil and groundwater systems. • • In 0.001–0.1 mol·L−1 NaCl, ARP aggregation increases with ionic strength due to double-layer compression; however, Ca2+ (0.001–0.05 mol·L−1) triggers significantly stronger aggregation via cationic bridging, with a threshold effect at 0.01 mol·L−1 CaCl2 where hydrodynamic diameter (dp) and zeta potential shift markedly. • • In the presence of 20 mg·L−1 natural colloids, ARP heteroaggregation is enhanced; at 0.1 mol·L−1 NaCl versus 0.01 mol·L−1 CaCl2, both dp and zeta potential increase, indicating that higher ionic strength compresses the double layer, reduces surface charge, and promotes colloid-ARP aggregation. • • Increasing natural colloid concentration reduces steric hindrance and increases collision frequency, leading to enhanced adsorption or heteroaggregation; this effect is more pronounced with Ca2+ than Na+, underscoring the role of divalent cations in facilitating ARP transport in colloid-rich environments.