Transport of Sulfide-Reduced Graphene Oxide in Saturated Quartz Sand: Cation-Dependent Retention Mechanisms

Abstract

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 cation-dependent. 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 Ca(2+) (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 Ca(2+) 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.

Environmental transformation of natural and engineered carbon nanoparticles and implications for the fate of organic contaminants

“…2). 52,106,107 CNP attachment generally decreases with increasing negative surface charge. Biochar-derived pCNPs, which contain the most O-functional groups and negative surface charge could therefore form more mobile pCNPs than less oxidized pCNPs.…”

“…These observations indicate that the pCNPs discussed above may be more prone to transformation than eCNPs as they generally contain Environmental Science: Nano Critical review smaller aromatic sheets, as well as more structural irregularities and mineral phases. 28,50 Chemical transformation of pCNPs can be influenced by environmental factors such as light, 15,16,51 or the presence of oxidants (e.g., O 2 ), 15 reductants (e.g., Fe 2+ and S 2− ), 6,[52][53][54] and natural organic matter (NOM). 55,56 The factors driving chemical transformation differ markedly between (i) soot, which is mainly transported via the atmosphere and more susceptible to photochemical reactions involving reactive oxygen species (ROS), and (ii) other pCNPs, which are mainly transported by surface runoff and subsurface infiltration, where redox reactions with O 2 or S 2− are likely to be more important.…”

“…For example, GO can be transformed in aquatic environments because of its abundant surface O-functional groups, while Fe 2+ and S 2− can reduce GO thereby increasing its hydrophobicity. 6,52,54 However, Wang et al 53 found that GO reduction by Fe 2+ resulted in a reduced content of alkoxy and hydroxy groups and an increased content of carboxy groups because of the hydrolysis reaction of acid anhydrides on GO. Chemical reduction can therefore also result in the formation of polar surface groups and the effect of redox reactions needs to be evaluated on a case-by-case basis.…”

Effects of clay minerals on transport of graphene oxide in saturated porous media

“…To date, most of the studies on transport of graphene oxide have used purified quartz sands as the porous medium [2][3][4][5][6][7][8][9]13]. However, this oversimplified approach cannot fully address the transport properties of graphene oxide in natural subsurface environments-subsurface water-bearing units can be highly heterogeneous, and clay minerals, metal oxides, and soil organic matter all can interact with nanoparticles differently from quartz sand.…”

“…Thus, its transport is not very sensitive to changes of pH, increase of monovalent cation (e.g., Na þ and K þ ) concentrations, and the presence of dissolved organic matter [2,[5][6][7][8][9]. However, certain divalent cations (such as Ca 2 ) may significantly hinder the transport of graphene oxide by enhancing its retention via cation bridging between graphene oxide and porous media [3,4].…”