Abstract
Graphene oxide (GO) is likely to encounter heavy metals due to its widespread use and inevitable release into the subsurface environment, where the ubiquitous presence of iron oxides (e.g., hematite) would affect their interaction and transport. The present study aimed to investigate the cotransport of GO (20 mg L−1) and copper (0.05 mM CuCl2) in the presence of varying degrees of geochemical heterogeneity represented by iron oxide-coated sand fractions (ω = 0‒0.45) in water-saturated columns under environmentally relevant physicochemical conditions (1 mM KCl at pH 5.0‒9.0). The Langmuir-fitted maximum adsorption capacity of Cu2+ by GO reached 137.6 mg g−1, and the presence of 0.05 mM Cu2+ decreased the colloidal stability and subsequent transport of GO in porous media. The iron oxide coating was found to significantly inhibit the transport of GO and Cu-loaded GO in sand-packed columns, which can be explained by the favorable deposition of the negatively charged GO onto patches of the positively charged iron oxide coatings at pH 5.0. Increasing the solution pH from 5.0 to 9.0 promoted the mobility of GO, with the exception of pH 7.5, in which the lowest breakthrough of GO was observed. This is possibly due to the fact that the surface charge of iron oxide approaches zero at pH 7.5, suggesting that new “favorable” sites are available for GO retention. This study deciphered the complicated interactions among engineered nanomaterials, heavy metals, and geochemical heterogeneity under environmentally relevant physicochemical conditions. Our results highlight the significant role of geochemical heterogeneity, such as iron oxide patches, in determining the fate and transport of GO and GO-heavy metal association in the subsurface environment.
Highlights
Graphene oxide (GO) is composed of hexagonal carbon sheets with abundant oxygen-containing functional groups on the basal and edge planes [1,2]
The main objective of this work was to examine the cotransport of Cu with GO in water-saturated sand columns packed with mixtures of iron oxide-coated and uncoated sand grains at a wide pH range of 5.0-9.0
The hydrodynamic diameters (Dh ) of GO decreased from 960 nm to
Summary
Graphene oxide (GO) is composed of hexagonal carbon sheets with abundant oxygen-containing functional groups (e.g., carboxyl, epoxy, and hydroxyl) on the basal and edge planes [1,2]. Water 2020, 12, 444 which is mainly attributable to electrostatic attraction and surface complexation of metal ions with the oxygen-containing functional groups of GO [5,8]. Extensive studies have reported high mobility, transport, and release of GO in porous media [9,10,11,12,13,14,15]. These findings collectively raised a newly emerging concern that GO could potentially facilitate the transport of heavy metals through the subsurface environment via colloid-facilitated transport of contaminants [16,17]
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