Dimethyl phthalate disrupts sulfur cycling and triggers toxic H2S pollution in sludge anaerobic system

Dimethyl phthalate (DMP), a crucial plastic additive widely employed, has been detected in sewers, sediments, and sewage sludge at elevated levels, particularly in regions where active sulfur cycling occurs. However, the effect of DMP on anaerobic sulfur conversion, especially toxic H2S production, has not been documented. This study aimed to explore the fate of DMP in anaerobic system and its influence on sulfur conversion through both batch and long-term tests using sludge as substrate. Experimental results showed that sludge can effectively adsorb DMP, which conformed to the Freundlich model and pseudo-second-order adsorption kinetic model, with the equilibrium adsorption capacity of DMP in sludge system being 1.024 mg/g. ∼ 85% of DMP was biodegraded in anaerobic system, with monomethyl phthalate (MMP) and phthalic acid (PAA) as its main degradations. Additionaly, DMP presence at 20-200 mg/kg TSS significantly increased H2S gas production by 35.1%-71.7%. Although PAA and MMP can slightly promote H2S production, DMP was the main contributor. Multidimensional analyses showed that DMP spontaneously binds to the EPS of sludge and increases the secretion of EPS, as well as altering its physical and chemical properties (i.e., increasing its direct electron transfer capacity), thereby promoting the bioprocesses of organic sulfur hydrolysis and sulfur containing amino acids degradation. Metagenomic profiling confirmed this metabolic shift, showing an increase in the abundance of hydrolytic bacteria such as Ferruginibacter, as well as an increase in the abundance of organic sulfur metabolic genes such as pepD and aspB, which accelerated sulfide production from organic sulfur hydrolysis. These findings uncover a novel toxicity mechanism where DMP actively trigger hazardous gas emissions, highlighting unrecognized ecological risks in sulfur rich environmental compartments.