Hydrophobic interaction between nanoplastics and surfactant antagonistically shielding the toxicity of surfactant to syntrophic methanogenesis

Surfactants and nanoplastics are emerging contaminants in organic wastewater. Although the influences of surfactants on the anaerobic digestion of organic wastewater were widely investigated and confirmed, the regulation of co-existed surfactants and nanoplastics on syntrophic methanogenesis was not clear. This study explored the inhibitory mechanism of sodium dodecyl sulfate (SDS) surfactant on syntrophic methanogenesis and deciphered how polypropylene (PP) nanoplastics regulating this inhibition. The results showed that individual 200 mg/L SDS dramatically decreased methane yield (P) by 31.7 % and prolonged the lag time (t) from 5.2 d to 12.1 d by damaging the cell membrane, disordering extracellular matrix structure, and decreasing cell viability. Meanwhile, the relative abundance of major hydrogenotrophic Methanoculleus decreased by 43.6 %. However, the addition of PP at concentrations between 0.5 and 200 mg/L effectively mitigated the release of intracellular lactate dehydrogenase and enhanced cell viability. Consequently, key parameters P and t recovered to 6.0-20.6 % and 5.0-8.4 d, respectively. This demonstrated an antagonistic effect of PP and SDS co-existence. The physical analysis further suggested that the hydrophobicity of PP acted as carriers for the aggregation of hydrophobic end structure of SDS, which formed surfactant-coated particles. Meanwhile, the exposed hydrophilic head structure of SDS with negative charges had less chance to attach to the cell membrane due to electrostatic repulsion effects, thereby shielded the toxicity of SDS to microbes. These outcomes provided new insights into understanding the role of surfactants and nanoplastics regulating syntrophic methanogenic process.