Spectroscopic analysis of the interaction mechanism between manganese(ii) and microbial extracellular polymeric substances in landfill leachate treatment

Abstract

Microbial extracellular polymeric substances (EPS) play a vital role in microbial metabolism and interact with manganese(II), which is commonly present in landfill leachate, during anaerobic biological treatment. This study investigates the molecular mechanisms underlying their interaction. The results indicate that as the manganese(II) concentration gradually increases from 0 to 200 mg L⁻¹, the protein content in the tightly bound EPS (TB-EPS) rises progressively, but then begins to decrease at higher concentrations. Meanwhile, the humic acid content in TB-EPS shows a gradual decline with increasing manganese(II) levels. In contrast, both the protein and humic acid contents in the loosely bound EPS (LB-EPS) continuously increase while polysaccharide content remained largely unaffected by manganese(II). Three-dimensional excitation–emission matrix spectroscopy revealed that the fluorescence intensity of tryptophan-like organics in LB-EPS enhanced with increasing manganese(II), whereas in TB-EPS, it increased initially and then declined. Humic acid-like fluorescence intensified in LB-EPS but weakened gradually in TB-EPS. UV-visible spectroscopy further demonstrated stronger interactions between manganese(II) and aromatic structures or unsaturated aliphatic chains in TB-EPS compared to LB-EPS. Analysis of the protein secondary structure indicated a decreased α-helix/(β-turn + random coil) ratio in TB-EPS at 200 mg L⁻¹ manganese(II), which contributed to enhanced sludge adsorption and flocculation; this trend was reversed at higher concentrations. This multi-spectral approach clarifies the molecular-level effects of manganese(II) on EPS components and provides insights for optimizing the biological treatment of landfill leachate.