Enhancement of Cd(II) Adsorption on Microalgae–Montmorillonite Composite

As important parts of microbe–mineral associations, microalgae–montmorillonite composites are common in nature environment, which have been overlooked for a long time. In this study, the binding of Cd(II) in the montmorillonite, microalgae and their 1:5, 1:10 mass ratio composites interface were investigated. The results showed that maximum adsorption capacity of 1:5 and 1:10 mass ratio binary composites were 65.25 mg g−1 and 55.49 mg g−1, respectively, which both higher the theoretically calculated capacity from single montmorillonite and microalgae at pH 5.5. Fourier transform infrared spectrometer (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that phosphoryl and carboxyl functional groups on the algal cellular surface played major roles in both microalgae–montmorillonite composite formation and Cd(II) adsorption. Extended X-ray absorption fine structure (EXAFS) further probed that higher sorption on composites were attributed to Cd(II) bridging between microalgae and montmorillonite, in which monodentate phosphoryl-Cd and carboxyl-Cd complexes on microalgae side, leading Cd(II) to a more stabilized state. In addition, the montmorillonite promoted dispersion of microalgae, releasing more carboxyl and phosphoryl functional groups as Cd(II) binding sites on the microalgae surface. These findings highlight the function of microalgae in the microalgae–clay mineral associations for Cd(II) fixation and provide further insight into the sequestration and migration of toxic metals in natural environments.