Different effects of copper (II), cadmium (II) and phosphate on the sorption of phenanthrene on the biomass of cyanobacteria

Abstract

Due to the large surface area and high organic carbon content of cyanobacteria, organic contaminants can be readily sorbed on cyanobacteria during algal blooms, and then be transferred to the food web. This process is likely to be affected by the coexisting metals and nutrients, however, the possible impacts remain unclear. Effects of Cu2+, Cd2+, and phosphate on the sorption of phenanthrene on cyanobacterial biomass collected from an algal bloom were therefore studied. Continuous decrease in phenanthrene sorption was observed in the presence of low concentrations of Cu2+, and Cd2+ (<0.04mmolL−1), because Cu2+ and Cd2+ were coadsorbed with phenanthrene on the surface of cyanobacteria as suggested by scanning electron microscopy-energy dispersive X-ray (SEM-EDX) and Fourier transform infrared (FTIR) analyses. Phenanthrene sorption began to increase with the further increase in Cu2+ concentration, but remained lower than that in the absence of Cu2+. This increase in sorption was ascribed to the cation–π interaction between Cu2+ and phenanthrene, as suggested by the enhanced ultraviolet absorbance at 251nm. In contrast, sorption rebounding of phenanthrene did not occur in the presence of higher concentrations of Cd2+. The different effects of Cu2+ and Cd2+ on phenanthrene sorption were attributed to that Cd2+ required much more energy than Cu2+ to form cation–π complexes with phenanthrene in the solutions. Phenanthrene sorption decreased continuously with the increase in phosphate concentration. Phosphate blocked the binding sites, modified the cell morphology, and increased the negative charge as well as the hydrophilicity of the cyanobacterial surface, thereby suppressing phenanthrene sorption. This study indicates that sorption of aromatic organic compounds by cyanobacteria could be significantly alerted by concentrations and properties of the coexisting transition metals and phosphates, which may subsequently affect their transfer to the food web in eutrophic waters.