Abstract
Cyanobacteria are promising adsorbents that are widely used for heavy metal removal in aqueous solutions. However, the underlying adsorption mechanism of Synechocystis sp. PCC6803 is currently unclear. In this study, the adsorption behavior and mechanism of cadmium (Cd2+) were investigated. Batch biosorption experiments showed that the optimal adsorption conditions were pH 7.0, 30 °C, 15 min, and an initial ion concentration of 4.0 mg L-1. The adsorption process fitted well with the pseudo-second order kinetic model, mainly based on chemisorption. Complexation of Cd2+ with carboxyl, hydroxyl, carbonyl, and amido groups was demonstrated by Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectrometry (EDX) analyses confirmed the presence of Cd2+ on the cyanobacterial cell surface and intracellularly. Cd2+ could lead to reactive oxygen species (ROS) accumulation and photosynthesis inhibition in cyanobacterial cells, and glutathione (GSH) played an important role in alleviating Cd2+ toxicity. Analyses of three-dimensional fluorescence spectroscopy (3D-EEM) and high performance anion exchange chromatography-pulsed amperometric detection (HPAEC-PAD) revealed the changes of the composition and content of EPS after Cd2+ adsorption, respectively. Real-time quantitative polymerase chain reaction (RT-qPCR) revealed the potential molecular regulatory mechanisms involved in Cd2+ biosorption. These results revealed the adsorption mechanism of Cd2+ by Synechocystis sp. PCC6803 and provided theoretical guidance for insight into the biosorption mechanisms of heavy metals by other strains.