Nanotechnology water remediation with bulky graphene materials

(Nanowerk Spotlight) The unique properties of nanomaterials are beneficial in applications to remove pollutants from the environment. The extremely small size of nanomaterial particles creates a large surface area in relation to their volume, which makes them highly reactive, compared to non-nano forms of the same materials.

The potential impact areas for nanotechnology in water applications are divided into three categories: treatment and remediation; sensing and detection: and pollution prevention (read more: "Nanotechnology and water treatment").

Silver, iron, gold, titanium oxides and iron oxides are some of the commonly used nanoscale metals and metal oxides cited by the researchers that can be used in environmental remediation (read more: "Overview of nanomaterials for cleaning up the environment").

A more recent entrant into this nanomaterial arsenal is graphene. Individual graphene sheets and their functionalized derivatives have been used to remove metal ions and organic pollutants from water. These graphene-based nanomaterials show quite high adsorption performance as adsorbents. However they also cause additional cost because the removal of these adsorbent materials after usage is difficult and there is the risk of secondary environmental pollution unless the nanomaterials are collected completely after usage.

One solution to this problem would be the assembly of individual sheets into three-dimensional (3D) macroscopic structures which would preserve the unique properties of individual graphene sheets, and offer easy collecting and recycling after water remediation.

(a) Optical and (b) SEM images of graphene oxide (GO) architecture prepared by a simple centrifugal vacuum evaporation method. (c) Digital images of the original methylene blue dye solution (left), the pale color solution with precipitated methylene blue adsorbed GO architecture (middle), and the colorless water after filtering the methylene blue adsorbed GO architecture (right). (©American Chemical Society)

Although great progress has been achieved in both preparation of bulky graphene porous architectures and their application in water remediation, much work remains to be done. A recent review article in the March 12, 2014, online edition of Small ("Porous Graphene Materials for Water Remediation") summarizes the recent developments in this area.

In particularly, the article focuses on the rational design and application of bulky graphene materials in the cleanup of oil, removal of heavy metal ions, and elimination of water-soluble organic pollutants. The authors also suggest future prospects of bulky graphene materials for environmental remediation.

The application of graphene-based bulky porous architectures in water remediation greatly depends on their surface properties and microstructure, such as the spacing size among graphene sheets and their orientation. While a variety of graphene porous architectures have been successfully obtained by different methods, most of their microstructure is disordered and random.

Ordered and controllable microstructures and surface design would effectively improve the performance of graphene porous architectures in water remediation.

Clean-up of oil

Bulky porous materials based on graphene and its derivatives exhibit highly selective adsorption ability of oil from aqueous solution due to their high specific surface area and superhydrophobic-oleophilic surface. Furthermore, they can be easily utilized during the oil cleanup process and collected after usage. In addition to easy manipulation, they show excellent recycling ability.



One challenge in fabricating bulky graphene materials for oil cleanup is to achieve a porous architecture while at the same time keeping a large accessible surface area of 2D graphene sheets. Leavening strategy, a process to produce porous structures by in situ gas formation, was developed to form reduced graphene oxide (rGO) foams with open porous and continuous crosslink structures by autoclaved leavening and steaming of graphene oxide layered films (read more: "Making graphene 'bread' - leavening technique results in freestanding graphene oxide films" ).



In addition to pure graphene materials, graphene composites and carbon aerogels can serve as an adsorbent to clean up oil from water.



Removal of heavy metal ions

Various materials, such as clay minerals, oxides, zeolites, and carbon materials, have been used as adsorbents to remove heavy metal ions from water. Unfortunately, their relatively low adsorption capacity, poor chemical stability as well as unsatisfactory recycling ability limit their practical application.

Different from the demand of oil cleanup, the adsorption capability of adsorbent materials for heavy metal ions depends on their specific surface area and the interactions between them and heavy metal ions. Therefore, favorable porous structure and active graphene surface are required in the design of bulky graphene materials for selective adsorption of heavy metal ions from water. These rationally designed bulky graphene materials exhibited excellent adsorption capacity as well as recycling ability.

Elimination of water-soluble organic pollutants

A number of adsorbent materials, such as mesoporous silica, mesoporous hybrid aerogel and activated carbon, have been used as adsorbents for elimination of water soluble organic pollutants. However, these adsorbents often suffer from either low/limited adsorption capacity or inefficient desorption; moreover, almost all of them lack the ability of recycling and reuse.

Alternatively, recent work has illustrated that bulky porous graphene materials shows high adsorption capacity and excellent recycling ability. Like the requirement for removal of heavy metal ions, specific surface area and the interactions between graphene and water soluble organic pollutants are the two main factors that determine the adsorption and desorption capacity of porous graphene architectures for water soluble organic pollutants.

Apart from the adsorption capacity, the recycling ability of graphene-based adsorbent materials is another key parameter for their practical application in water remediation. An ideal adsorbent should not only possess high adsorption capability, but also show good desorption property. In order to recycle porous graphene materials, desorption of pollutants from them should be considered. One major advantage of the porous graphene materials is their structural and chemical stability, thus, they can be regenerated after each use by desorbing pollutants from them.

The authors conclude that graphene-based composites comprising multiple materials will greatly extend the scope of graphene?s functionality far beyond what pure graphene materials have already been achieved in water remediation application. Therefore, new approaches that control the assembly of graphene and other functional materials into macroscopic hybrid architectures are desired.