Graphene in the Aquatic Environment: Adsorption, Dispersion, Toxicity and Transformation

Abstract

Graphene-family nanomaterials (GFNs) including pristine graphene, reduced graphene oxide (rGO) and graphene oxide (GO) offer great application potential, leading to the possibility of their release into aquatic environments. Upon exposure, graphene/rGO and GO exhibit different adsorption properties toward environmental adsorbates, thus the molecular interactions at the GFN-water interface are discussed. After solute adsorption, the dispersion/aggregation behaviors of GFNs can be altered by solution chemistry, as well as by the presence of colloidal particles and biocolloids. GO has different dispersion performance from pristine graphene and rGO, which is further demonstrated from surface properties. Upon exposure in aquatic environments, GFNs have adverse impacts on aquatic organisms (e.g., bacteria, algae, plants, invertebrates, and fish). The mechanisms of GFNs toxicity at the cellular level are reviewed and the remaining unclear points on toxic mechanisms such as membrane damage are presented. Moreover, we highlight the transformation routes of GO to rGO. The degradation of GFNs upon exposure to UV irradiation and/or biota is also reviewed. In view of the unanswered questions, future research should include comprehensive characterization of GFNs, new approaches for explaining GFNs aggregation, environmental behaviors of metastable GO, and the relationship between dispersion of GFNs and the related adsorption properties.

Heteroaggregation of nanoparticles with biocolloids and geocolloids

“…Biocolloids include viruses, bacteria, proteins, DNA, spores, algae, protozoa and other microorganisms [45][46][47][48][49]. Biocolloids are of significant interest in the fate and transport of contaminants in aqueous phase, not only in groundwater, but also in surface water [45,50].…”

“…Biocolloids are of significant interest in the fate and transport of contaminants in aqueous phase, not only in groundwater, but also in surface water [45,50]. Similar with inorganic colloids (e.g., clays, aluminoscilicates and iron oxyhydroxides), biocolloids can alter the aggregation and dispersion behavior of nanomaterials [46][47][48].…”

“…Hydrogen bonding is common in water and between hydrogen and several electronegative elements; and has been described in a number of NPs-based heteroaggregates. Lewis acid-base interactions are a type of chemical bonding that lead to heteroaggregation of NPs (and colloids in general) in aqueous and non-aqueous systems [47]. A heteroaggregation system can also arise from interplay of multiple mechanisms -both physical and chemical interactions [99,128,129].…”

“…In fact, heteroaggregation via hydrogen bonding has been employed for separation/purification of NPs using functionalized or decorated magnetic NPs [149,150]. In aqueous media, formation of hydrogen bonds between water molecules and oxygencontaining groups on the surface of NPs may weaken heteroaggregation arising from hydrogen bonding [47]. Hydrogen bonds (5-10 kT) are generally stronger than common van der Waals bonds (~1 kT) but weaker than typical covalent bonds (~100 kT) [132].…”

“…Heteroaggregation between different types of NPs or NPs and other colloids/surfaces arising from chemical interactions have been mostly ascribed to Lewis acid-base interactions, a type of coordinate covalent bond [47,154]. Heteroaggregation (with interaction energy in orders of tens of kT units) between two SiO 2 particles in a non-polar medium was ascribed to Lewis acid-base interactions between the acidic and basic monomers of the particles' surface coating polymers, which contained carboxylic and tertiary amine groups, respectively [154].…”