Transformation of graphene oxide by ferrous iron: Environmental implications

Abstract

Abiotic transformation of graphene oxide (GO) in aquatic environments can markedly affect the fate, transport, and effects of GO. The authors observed that ferrous iron (Fe[II])-an environmentally abundant, mild reductant-can significantly affect the physicochemical properties of GO (examined by treating aqueous GO suspensions with Fe(2+) at room temperature, with doses of 0.032 mM Fe(2+) per mg/L, 0.08 mM Fe(2+) per mg/L, and 0.32 mM Fe(2+) per mg/L GO). Microscopy data showed stacking of GO nanosheets on Fe(2+) treatment. Spectroscopy evidence (X-ray diffraction, Fourier transform infrared transmission, Raman and X-ray photoelectron spectroscopy) showed significant changes in GO surface O-functionalities, in terms of loss of epoxy and carbonyl groups but increase of carboxyl group. The reduction mechanisms were verified by treating model organic molecules (styrene oxide, p-benzoquinone, and benzoic acid) resembling O-containing fragments of GO macromolecules with Fe(2+). With sedimentation and adsorption experiments (using bisphenol A as a model contaminant), the authors demonstrated that Fe(2+) reduced GOs still maintained relatively high colloidal stability, whereas their adsorption affinities were significantly enhanced. Thus, reduction of GO by mild reductants might be of greater environmental concerns than by stronger reducing agents (e.g., N2H4 and S(2-)), because the latter can result in too significant losses of surface O-functionalities and colloidal stability of GO. This interesting aspect should be given consideration in the risk assessment of GO.

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

“…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.…”

Graphene-family nanomaterials in wastewater treatment plants

“…This fact, along with the ability of some bacteria [33] and naturally occurring organics [34], sulfides [35] and ferrous iron [36] to reduce GO as well, indicates that the majority of GO released to the environment could became RGO after certain time. In this regard, Chowdhury et al [37] used the quartz crystal microbalance with dissipation monitoring to analyse the deposition and release onto silica and NOM-coated silica surfaces of GO aerobically photoreduced for 1, 3, 11, 61 and 187 hours, and anaerobically photoreduced for 3, 11 and 61 hours.…”

“…So, by extrapolation, it can be inferred that, if there were low amounts of GO bound to the coarse solids and to the sand, there will be less RGO. However, due to the hydrophobicity of the reduced species, they could agglomerate through van der Waals forces more readily than GO, and a fraction of them could settle in the grit chamber, since, in some cases, sedimentation in deionised water after reduction took place in minutes [36]. Notwithstanding the foregoing, according to Chowdhury et al [37], NOM improves the stability of deionised water + electrolyte + RGO, and keeping in mind the relatively high amount of organic matter contained in the sewage, the reduced nanomaterial settled down together with the grit will be little if adsorption on other minerals than sand is not very strong (to the best of our knowledge, there are not studies similar to those of GO with kaolin/kaolinite, hematite, montmorillonite, goethite and layered double hydroxides for RGO).…”

Nanomaterial Transformation in the Soil–Plant System: Implications for Food Safety and Application in Agriculture

“…[53] For example, ferrous ions (Fe 2+ ) can reduce graphene oxide. [54] In an aerated soil with high oxygen content, oxidation of ENMs is more favorable. This may result in the formation of an oxide layer on the surface of some metallic ENMs which can act as a protective coating for the particle core.…”