We describe how the reduction of graphene
oxide (GO) via environmentally relevant pathways affects its
transport behavior in porous media. A pair of sulfide-reduced
GOs (RGOs), prepared by reducing 10 mg/L GO with 0.1 mM
Na2S for 3 and 5 days, respectively, exhibited lower mobility
than did parent GO in saturated quartz sand. Interestingly,
decreased mobility cannot simply be attributed to the increased
hydrophobicity and aggregation upon GO reduction because
the retention mechanisms of RGOs were highly cationdependent.
In the presence of Na+ (a representative
monovalent cation), the main retention mechanism was
deposition in the secondary energy minimum. However, in
the presence of Ca2+ (a model divalent cation), cation bridging
between RGO and sand grains became the most predominant retention mechanism; this was because sulfide reduction markedly
increased the amount of hydroxyl groups (a strong metal-complexing moiety) on GO. When Na+ was the background cation,
increasing pH (which increased the accumulation of large hydrated Na+ ions on grain surface) and the presence of Suwannee
River humic acid (SRHA) significantly enhanced the transport of RGO, mainly due to steric hindrance. However, pH and SRHA
had little effect when Ca2+ was the background cation because neither affected the extent of cation bridging that controlled
particle retention. These findings highlight the significance of abiotic transformations on the fate and transport of GO in aqueous
systems.
We describe how the reduction of grapheneoxide (GO) via environmentally relevant pathways affects itstransport behavior in porous media. A pair of sulfide-reducedGOs (RGOs), prepared by reducing 10 mg/L GO with 0.1 mMNa2S for 3 and 5 days, respectively, exhibited lower mobilitythan did parent GO in saturated quartz sand. Interestingly,decreased mobility cannot simply be attributed to the increasedhydrophobicity and aggregation upon GO reduction becausethe retention mechanisms of RGOs were highly cationdependent.In the presence of Na+ (a representativemonovalent cation), the main retention mechanism wasdeposition in the secondary energy minimum. However, inthe presence of Ca2+ (a model divalent cation), cation bridgingbetween RGO and sand grains became the most predominant retention mechanism; this was because sulfide reduction markedlyincreased the amount of hydroxyl groups (a strong metal-complexing moiety) on GO. When Na+ was the background cation,increasing pH (which increased the accumulation of large hydrated Na+ ions on grain surface) and the presence of SuwanneeRiver humic acid (SRHA) significantly enhanced the transport of RGO, mainly due to steric hindrance. However, pH and SRHAhad little effect when Ca2+ was the background cation because neither affected the extent of cation bridging that controlledparticle retention. These findings highlight the significance of abiotic transformations on the fate and transport of GO in aqueoussystems.
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