Low-Carbon cadmium removal via cyanobacterial MICP: Metabolic mechanisms and species efficiency

Cadmium (Cd) is widely distributed in contaminated water. Globally, substantial resources and energy are consumed annually to treat Cd-contaminated water, thereby intensifying carbon emissions and contributing to global climate change. Microbially induced carbonate precipitation (MICP) by cyanobacteria has the potential to become a low-carbon method for treating Cd-contaminated wastewater. This study demonstrated that the MICP reaction of Synechococcus sp., Synechocystis sp., and Microcystis aeruginosa could effectively achieve the low-carbon removal of Cd²⁺. Irregular spherical CdCO₃ minerals with sizes ranging from 0.5 to 5.0 μm were produced during the MICP process. The removal rate and carbon sequestration amount of Synechococcus sp. were 97.13 ± 1.64 % and 0.101 ± 0.008 mg, respectively, which were significantly higher than those of Synechocystis sp. (47.52 ± 2.53 %, p < 0.01; 0.056 ± 0.001 mg, p < 0.01) and M. aeruginosa (45.73 ± 2.40 %, p < 0.005; 0.051 ± 0.002 mg, p < 0.01) when 10.00 mL of the supernatant was used to treat Cd²⁺ with an initial concentration of 1.00 mM. On the one hand, Synechococcus sp. exhibited a higher pH (10.667 ± 0.311) and CO₃^2- concentration (80.995 ± 2.500 mg·L^-1), which facilitated the mineralization of Cd²⁺ and inorganic carbon fixation. On the other hand, Synechococcus sp. had a higher cell density (OD₆₈₀: 1.451 ± 0.021) and chlorophyll-a concentration (5.262 ± 0.466 mg·L^-1), promoting organic carbon fixation through enhanced cyanobacterial photosynthesis. This phenomenon could be attributed to the elevated transcription levels of key functional genes (including CcmO, CcmK, PsbA, PsaD) involved in CO₂ concentration and photosynthesis processes. Cyanobacterial MICP reduces reliance on energy-intensive treatments, mitigating climate impacts. The integration of Cd remediation with carbon fixation offers a dual-benefit strategy for sustainable wastewater management, addressing hazardous metal pollution while contributing to global carbon neutrality goals.