Bacterially induced synthesis of low-Cd struvite coupled with Cd²⁺ biosorption under high cadmium stress

Chemical precipitation of struvite is promising for phosphorus recovery from wastewater. However, metal cations with high phosphate affinity, such as Cd²⁺, can reduce struvite phase purity and promote heavy-metal incorporation, limiting practical safety. In this study, seawater was used as the magnesium source and Shewanella oneidensis MR-1 as the mineralizing strain to establish a struvite biomineralization system under Cd²⁺ stress, aiming to remove aqueous Cd²⁺ using MR-1 while producing struvite with low Cd content. An artificial Cd²⁺ stress gradient of 1-30 mg L^-1 was applied, with 30 mg L^-1 representing an extreme high-Cd condition. The results showed that MR-1 induced the formation of struvite-dominated precipitates across the tested Cd²⁺ concentration range, with calculated struvite contents of 96.03-97.36%. MR-1 also removed 85.16-100% of Cd²⁺ from the aqueous phase through biosorption. At the same initial Cd²⁺ concentrations, bio-struvite contained 7.13-354.35 mg kg^-1 Cd, significantly lower than the 923.74-18339.83 mg kg^-1 measured in chemically precipitated struvite. Total phosphorus removal efficiencies were 88.27-91.46% and 58.12-60.23% in the biomineralization and chemical precipitation systems, respectively. TEM-EDS and FTIR analyses indicated that carboxyl, hydroxyl, amino, amide, and phosphorus-containing functional groups on the MR-1 surface were involved in Cd²⁺ immobilization. The good fit to the pseudo-second-order model further supported an important role of chemisorption in Cd²⁺ immobilization. This study provides a new strategy for the resource-oriented treatment of Cd-contaminated eutrophic wastewater.