Insights into key factors controlling GO stability in natural surface waters

The effects of pH, cations (Na⁺, K⁺, Mg²⁺, Ca²⁺ and Al³⁺), and anions (Cl⁻, HCO₃⁻, HPO₄²⁻ and SO₄²⁻) on graphene oxide (GO) stability were investigated to address the current limitations in the knowledge regarding the stability of GO in natural surface water and its underlying mechanism. The threshold values of cations that destabilize GO were obtained and affected by both pH and anions. By employing elemental mapping and studying the effects of polyacrylic acid (PAA) on GO sedimentation and the re-dispersion of GO aggregates, we find that the GO aggregates induced by Na⁺ and K⁺ via electric double layer suppression and by Ca²⁺ and Al³⁺ via strong complxing are difficult to re-disperse completely. Specifically, more PAA is needed to re-disperse GO aggregates than to stabilize GO, which suggests that after GO binds with heavy metal ions. It is less likely to be transported over a long distance even in natural water that are rich in natural organic matter. Finally, we find that the key factor controlling GO sedimentation in natural surface waters is its binding with Mg²⁺ and Ca²⁺. This study is expected to provide critical knowledge to more accurately predict the fate of GO in natural surface aquatic environments.