Water. Desalination + reuse
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RESEARCH H+, Fe Exfoliation First-stage reduction 3- -SO - -SO 3- -SO3- 3-SO 3--S-SO O 3-S O3 -SO 3- -SO 3- Second-stage reduction -SO 3--SO -SO 3- 3- -SO 3- -SO3-SO 3- -SO 3- 3-SO 3- -SO Hydrazine -SO3- 3- ImpRovIng ElECtRodES It is reasonable to assume that current graphene-based CDI electrodes could further be improved by modifying the reduction process of the GO, to prepare well-dispersed graphene with one or only a few layers that could feature larger surface areas and better electrolyte wetting. A novel multiple-step reducing process has been developed to transform GO into well-dispersed, more isolated graphene nanosheets with one or a few more layers. The resultant graphene nanosheets can be used as a high-performance electrode material for CDI process. In the multi-step reducing process, iron powder—a mild and environmentally friendly reducing agent—was used to replace hydrazine, in the partial reduction Oxidization -SO the efficiency of the desalting process. The ideal electrode materials for CDI should both be highly conductive and have high surface area with suitable pore size and pore structures. Currently available carbon electrodes limit the desalination efficiency of CDI due to their low conductivity and non-ideal pore properties. Many kinds of carbon materials have been investigated as CDI electrodes such as carbon aerogel, carbon cloth, carbon nanotubes and mesoporous carbons. Having a one-atom-thick planar structure, graphene possesses many fascinating structural and electrical properties that are ideal for the CDI process, such as an exceptionally high theoretical specific area, superior room temperature electrical conductivity and excellent mechanical and thermal strength. Among the available techniques for producing graphenes, chemical reduction of graphite oxide (GO), or chemically converted graphenes, provides not only an established, low-cost and scalable approach, but also a highly flexible method for the chemical functionalisation of graphene materials. At present, the chemical reduction of GO is typically achieved using harsh reducing agents such as hydrazine monohydrate, in spite of their high toxicity. However, the reduced graphene sheets usually form heavy aggregates and precipitate easily from the reaction medium because the recovered graphite domain increases the sheet's hydrophobic property and corresponding stacking interactions. These kinds of aggregates interfere with the ideal two-dimensional structure of the graphene, lowering its performance as a CDI electrode. Sulphonation Figure 2. Overcome heavy aggregation of graphene nanosheets by controlled sulphonation. of the GO. Then, sulphonic functional groups were introduced onto the surface of the GO and finally further reduction was achieved by hydrazine (see Figure 2). The resultant graphene nanosheets exhibited good dispersion in water, while an enhanced specific surface area of 464 m2/g with a mean pore size of 3.3 nm and specific capacitance of 149.8 F/g were achieved. Electrodes based on this kind of graphene nanosheet demonstrated a high NaCl removal efficiency of 83.4% and specific electrosorptive capacity of 8.6 mg/g. The electrodes also showed an outstanding regenerative capability (Fig 3). In addition, the wetting behaviour of graphene nanosheets and their application in capacitive deionisation were also investigated. By altering the hydrophilicity of graphene nanosheets (GNS) through controlled introduction of sulphonic groups, the water contact angle of GNS is greatly reduced, indicating an improved wettability. The findings of this research prove that graphene nanosheets synthesised from this multi-step reducing process can be applied as high-performance electrodes in the CDI process. It is anticipated that the novel graphene-based electrode materials will be transferred into CDI production through further development of the electrode and the technology. CdI In bRACkISH wAtER dESAlInAtIon Apart from developing graphene electrodes with better desalting performance, the research team at the University of South Australia has also conducted field trials to investigate the prototype CDI unit provided by its industry partners LT Green Energy (with AQUA EWP, USA), and Power & Water. In this work, a portable CDI unit was tested first time as an alternative desalination technique to the conventional RO process used at Wilora, a remote Aboriginal community in the Northern Territory, Australia (Figure 4). The field test was then carried out in October 2011. The feasibility of using CDI to desalt in such locations was studied, with overall salinity removal efficiency, energy efficiency and ion selectivity assessed. The highest removal efficiency for total dissolved solids (TDS) during one purification cycle of each flow rate is shown in Figure 5 (b). Another observation is that the onset time of highest TDS removal rate, ie, the lowest TDS points at each flow rate marked by arrows in Figure 5 (a), appears later during the 90 s purification cycle as the flow rate decreases. The increased flow rate tended to decrease the overall TDS removal efficiency. However, in terms of energy Figure 3. a) Electrosorption of salt ions onto the graphene electrodes and with inset of the changes of water contact angles. b) The electrode regeneration cycles. | 44 | Desalination & Water Reuse | August-September 2012