Resistance mechanism of soluble microbial products to silver nanoparticles in activated sludge: adsorption, bonding and influencing factors

The extensive application of silver nanoparticles (AgNPs) has resulted in them ending up in wastewater treatment plant (WWTP) facilities at concentrations varying between 0.13 and 20.02 mg L⁻¹. Soluble microbial products (SMPs) in activated sludge systems can form the first barrier against AgNPs before attached metabolites and microorganisms react with these biotoxicants. Based on this, the present study investigated the adsorption and bonding resistance mechanisms of AgNPs on SMPs in activated sludge systems and analysed the typical influencing factors. Analysis using quasi-second-order kinetics and the Freundlich isotherm model revealed that the adsorption of AgNPs onto SMP is chemisorption-controlled, with multi-layer adsorption being the main mechanism, and the adsorption capacity reached 263.9 mg g⁻¹ at 35 °C. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses indicated that aldehydes, carbonyl groups, and amide groups in SMPs were able to interact with AgNPs through electrostatic forces, hydrogen bonding and chelate-forming, metal-bound organometallic complexes (M–CNOHCNOH–M), resulting in conformational changes in SMP proteins. An increase in pH and ionic strength favoured the resistance of SMPs to AgNPs, and as the valence of the metal cation increased, the enhancement of this resistance became more significant. This study confirmed that SMP functions as an effective natural adsorbent for immobilizing AgNPs in activated sludge systems and provides a mechanistic foundation for developing targeted in situ bioremediation strategies.