Polypropylene nanoplastic-induced changes in nitrogen removal and extracellular polymeric substances in activated sludge

Abstract

Polypropylene nanoplastics (PP NPs) inhibit biological nitrogen removal, yet the mechanisms remain unclear. Extracellular polymeric substances (EPS) play a vital role in microorganisms. Therefore, this study aimed to investigate whether PP NPs alter EPS properties and consequently affect biological denitrification. Three-dimensional fluorescence spectroscopy (3D-EEM) and Fourier-transform infrared spectroscopy (FTIR) were employed to analyze the effects of PP NPs on EPS during biological denitrification. The results showed that as the PP NP concentration increased from 0 to 10 mg L⁻¹, biological nitrogen removal was inhibited, with effluent ammonia nitrogen rising from 0.11 ± 0.06 to 0.44 ± 0.05 mg L⁻¹ and nitrate nitrogen from 1.65 ± 0.49 to 6.85 ± 0.49 mg L⁻¹. The EPS content indicated an initial increase followed by a decrease with increasing PP NP concentration. The 3D-EEM analysis showed the maximum fluorescence intensities of tryptophan-like and tyrosine-like substances in loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), peaking at 746.7, 792.7, 1001, and 1157 a.u., respectively. The FTIR analysis showed that PP NPs disrupted the EPS structure by interacting with N–H, amide I, and CO bonds. Analysis of the protein secondary structure revealed that a reduction in α-helices and an increase in β-sheets, turns, and random coils destabilized protein structure stability, thereby impairing nitrogen removal efficiency.