Role of solution chemistry in the retention and release of graphene oxide nanomaterials in uncoated and iron oxide-coated sand

摘要

Understanding the fate and transport including remobilization of graphene oxide nanomaterials (GONMs) in the subsurface would enable us to expedite their benign use and evaluate their environmental impacts and health risks. In this study, the retention and release of GONMs were investigated in water-saturated columns packed with uncoated sand (Un-S) or iron oxide-coated sand (Fe-S) at environmentally relevant solution chemistries (1-100 mM KCl and 0.1-10 mM CaCl_2 at pH 7 and 11). Our results showed that increasing ionic strength (IS) inhibited GONMs' transport, and the impact of K~+ was less than Ca~(2+). The positively charged iron oxide coating on sand surfaces immobilized the negatively charged GONMs (pH 7) in the primary minimum, yielding hyperexponential retention profiles particularly in Ca~(2+). A stepwise decrease in pore-water IS caused detachment of previously retained GONMs. The mass of GONMs released during each detachment step correlated positively with the difference in secondary minimum depth (ΔΦ_(min2)) at each IS, indicating that the released GONMs were retained in the secondary minimum. While most retained GONMs were re-entrained upon lowering pore-water IS in Un-S, decreasing IS only released limited GONMs in Fe-S, which were captured in the primary minimum. Introducing 1 mM NaOH (pH 11) released most retained GONMs in Fe-S; and average hydrodynamic diameters of the detached GONMs upon injecting NaOH were significantly smaller than those of GONMs in the influent and retentate, suggesting that NaOH induced GONMs disaggregation. Our findings advance current knowledge to better predict NMs' fate and transport under various solution chemistries such as during rainfall events or in the mixing zones between sea water and fresh water where transient IS changes drastically.