The resistance mechanism of humic acids and proteins in soluble microbial products from activated sludge against silver nanoparticles

Abstract

In the process of activated sludge against silver nanoparticles (AgNPs) present in wastewater, soluble microbial products (SMP) produced by microbial secretion or macromolecular cell lysis formed a reactive barrier against AgNPs. Proteins (PN) and humic acids (HA), the primary constituents of SMP, were extracted from activated sludge in this study. The adsorption mechanism of HA, which had excellent adsorption properties, on AgNPs was first analyzed. Then, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were further used to explore the bonding mechanisms of AgNPs by HA and PN, respectively. The results showed that the surface structure of HA was cracked after resisting AgNPs, and the adsorption process on AgNPs was monomolecular layer adsorption, with the maximum adsorption capacity of 12.9 mg/g at 35 °C. Thus, HA was an adsorbent with medium adsorption capacity. The FTIR and XPS analyses demonstrated that HA chelated and chemisorbed with AgNPs through chemical bonds such as -OH, C-H, CC, C-N, CO, and C-O-C, thereby reducing the microbial cytotoxicity of AgNPs. The chemical reaction between PN and AgNPs occurred via -OH and C-H dependent hydrogen bonding, and N-H was oxidized to -NO3, which ultimately resulted in the destruction of the PN peptide chain structure and the change of PN conformation. The present study investigated the resistance mechanism of HA and PN, the main components of SMP in activated sludge system, under the stress of AgNPs, which can provide theoretical support for the in-situ bioremediation of AgNPs in wastewater and the potential application of SMP.