Occurrence and transformation of antimony in a full-scale municipal wastewater treatment plant

Antimony (Sb), a key controlled heavy metal sourcing from brake pads and tires as a trending friction stabilization, is increasingly detected in urban drainage systems. However, Sb’s occurrence and transformation in wastewater treatment facilities are missing. The Sb distribution, speciation, and transformation along the wastewater treatment processes were thus investigated in a full-scale municipal wastewater treatment plant (WWTP). Results showed dissolved Sb dominated by Sb(III) (81.7%) in the influent. Along the treatment train, a dynamic redox cycle was identified: near-complete oxidation of Sb(III) to Sb(V) occurred under aerobic conditions, while substantial Sb(V) reduction and Sb(III) re-accumulation prevailed in anaerobic/anoxic zones. This transformation was governed by an interplay of abiotic factors (e.g., Fe/Mn oxides) and microbial activity, evidenced by the abundance of key functional genes (e.g., arsC_gult , dsrA/B ). Consequently, Sb was efficiently removed (74.8%) via sequestration into sludge, where it was enriched to 20.2 mg/kg. The residual fraction dominated Sb speciation in sludge (85–96%), primarily attributed to sulfide precipitation as Sb 2 S 3 driven by microbial sulfate reduction. However, this effective removal led to significant sludge enrichment, with a geo-accumulation index (Igeo) of 3.36 indicating a moderate-to-heavy contamination risk. This study underscores that enhancing reductive conditions in terminal units is key to immobilizing Sb and highlights the need for integrated strategies to manage this emerging contaminant throughout the wastewater treatment cycle. • Sb from 7.3 -110 μg/L in influent and up to 20.2 mg/kgSS(dw) in sludge. • 74.8% of influent Sb is effectively removed. • The anaerobic stage sludge Sb accumulated via intracellular microbial reduction. • Sludge poses a moderate-to-heavy contamination risk (Igeo = 3.36). • The last aerobic stage potentially released more Sb into effluent.