1. Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), College of Environment and Ecology, Chongqing University, Chongqing 400044, China 2. Sichuan Education Press, Chengdu 610200, China 3. State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
• The NPs aggregation in the electrolyte solution is consistent with the DLVO theory.
• In NaNO3 and low Ca(NO3)2, EPS alleviates the NPs aggregation by steric repulsion.
• In high Ca(NO3)2, EPS accelerates the NPs aggregation by exopolysaccharide bridging.
• Ag2S NPs have stronger stability compared with Cit-Ag NPs in aqueous systems.
Extracellular polymeric substances (EPS) in activated sludge from wastewater treatment plants (WWTPs) could affect interactions between nanoparticles and alter their migration behavior. The influence mechanisms of silver nanoparticles (Ag NPs) and silver sulfide nanoparticles (Ag2S NPs) aggregated by active EPS sludge were studied in monovalent or divalent cation solutions. The aggregation behaviors of the NPs without EPS followed the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The counterions aggravated the aggregation of both NPs, and the divalent cation had a strong neutralizing effect due to the decrease in electrostatic repulsive force. Through extended DLVO (EDLVO) model analysis, in NaNO3 and low-concentration Ca(NO3)2 (<10 mmol/L) solutions, EPS could alleviate the aggregation behaviors of Cit-Ag NPs and Ag2S NPs due to the enhancement of steric repulsive forces. At high concentrations of Ca(NO3)2 (10‒100 mmol/L), exopolysaccharide macromolecules could promote the aggregation of Cit-Ag NPs and Ag2S NPs by interparticle bridging. As the final transformation form of Ag NPs in water environments, Ag2S NPs had better stability, possibly due to their small van der Waals forces and their strong steric repulsive forces. It is essential to elucidate the surface mechanisms between EPS and NPs to understand the different fates of metal-based and metal-sulfide NPs in WWTP systems.
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