Graphene oxide interactions with co-existing heavy metal cations: adsorption, colloidal properties and joint toxicity

To find the linkage among the surface adsorption, colloidal stability and combined toxicity of graphene oxide (GO) and divalent heavy metal cations (denoted as Me(II)), the adsorption affinity of GO and bacteria towards Me(II) (i.e., Cd2+, Co2+ and Zn2+), the destabilizing ability of Me(II) on GO, and the combined effects of GO and Me(II) on the survival of Gram-negative Escherichia coil (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) bacteria were investigated. The results show that the destabilizing ability of Me(II) is consistent with their adsorption affinity with GO, which is positively correlated with the ionic radius of Me(II) and negatively correlated with the hydration shell thickness of Me(II). Moreover, the adsorption affinity of bacteria towards Me(II) is negatively correlated with the toxicity of Me(II). Reasons causing the decreased joint toxicity of GO and Me(II) are (1) GO decreases the amount of free Me(II) ions by adsorption; (2) Me(II) change the size and edge sharpness of GO by complexation; (3) GO competes with Me(II) for the membrane receptor binding sites; (4) GO-Me(II) complexes cannot be assimilated by bacteria; (5) Me(II) change the surface properties of GO and bacteria, consequently affecting their interactions. These findings are important for the assessment of the ecological risk of GO after it was used as a heavy metal ion carrier.