This invention relates to iron oxide -graphene nano composite for the manufacture of electrodes therefrom for water purification

Contamination of water represents a major global problem for the availability of sufficient drinking water. Further, gradual depletion of fresh water sources for consumption requires alternative sources such as sea water to be made potable. A number of desalination technologies like thermal distillation, freezing, electro-dialysis and membrane separation have been introduced but could not be made commercially feasible due to their limitations with low production and high energy consumption. Techniques such as Co-precipitation, flotation, ion-exchange, ultrafiltration, reverse osmosis and adsorption used for arsenic removal have drawbacks. Adsorbents for Arsenic removal deal with low concentration and have limitations at high concentrations. Further, they cannot desalinate simultaneously. Thus there is a need for a low-cost and efficient approach that can remove arsenic and desalinate simultaneously.

In this invention, we demonstrate capacitive deionization using iron oxide-graphene nanocomposite for high desalination efficiency and metal (Arsenic, Mg) removal with low energy consumption. Graphene was synthesized by hydrogen- induced exfoliation of graphitic oxide and decorated with iron oxide (Fe3O4) nanocrystals to make iron oxide-graphene (Fe3O4-f-HEG) nanocomposite electrodes. The electrodes were used for removal of inorganic arsenic species [As (V) and As (III)] along with sodium from aqueous solutions and for desalination of sea water. Maximum adsorption capacities for arsenate, arsenite and sodium have been found to be nearly 172.1, 180.3 and 142.7 mg/g, respectively with our nanocomposite electrodes, which are higher than other reported values for carbon electrodes. This invention demonstrates high desalination efficiency (removal of sodium, magnesium, calcium and potassium) and further demonstrates simultaneous removal of arsenic while being low cost and energy efficient.

Iron oxide-graphene nanocomposite was prepared in two steps. Graphene sheets were prepared by hydrogen induced exfoliation of graphitic oxide. The sheets were then decorated with iron oxide nanocrystals using chemical technique. Graphitic oxide was prepared by oxidation of pure graphite using Hummers' method. This graphitic oxide was further thermally exfoliated at 200 °C under hydrogen atmosphere to synthesize graphene sheets. These hydrogen exfoliated graphene sheets (HEG) were further treated with concentrated HN03 acid, resulting in the hydrophilic functional groups (-COOH, -C=0, and -OH) at the surface of HEG. The functionalized graphene sheets (/-HEG) were further washed several times with water to achieve pH=7 followed by drying. These functional groups also act as anchoring sites for metal oxide nanoparticles, leading to the better dispersion of nanoparticles over the surface of graphene. In second step, functionalized graphene (f-HEG) was suspended in de-ionized water by ultrasonication method. FeCI3.6H2O and FeSO4.7H2O (Across Organics) were dissolved in de-ionized water in the stoichiometric ratio of 3:2 and the resulted solution was heated up to 90 °C. Ammonia solution (NH4OH-25%) and f-HEG dispersed solution, in the volumetric ratio of 1:5, were added in the above solution. This mixture solution was stirred at 90 °C for 30 min followed by cooling to room temperature. The black precipitate was obtained by filtration and neutralized with water. The dried black precipitate was iron oxide-graphene (Fe304-f-HEG) nanocomposite.

Gel solution of Fe3O4-f-HEG in ethanol and nafion (-20 μL), was coated on carbon fabric (supplied by SGL, Germany) using spray coating technique and hot pressed at 50 °C under 1 Ton force for 15 min to ensure the good mechanical strength of the electrodes. Nanocomposite coated carbon fabric electrodes were applied at both ends of a cylindrical Perspex with length 2 cm, width 0.5 cm and diameter 4 cm. Stainless steel plates were used as current collector and graphite plates were used to provide conducting support to the electrodes. Schematic of the design of water purification assembly used for the present study is reported elsewhere. DC regulated power supply was used to apply voltage of 1 V across the electrodes. Complete experiment was performed with 50 mg of Fe3O4-f-HEG nanocomposite at each electrode. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to measure the concentration of different metals in solution and seawater.

The morphology of the prepared nanocomposite was examined by electron microscopes. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of nanocomposite clearly suggest the almost uniform decoration of iron oxide nanoparticles, with the particle size in the range of 10-15 nm, over the surface of f-HEG (figure-1a,b). XRD pattern in figure-2 indicates the crystalline nature of Fe3O4-f-HEG nanocomposite. XRD pattern of Fe3-Of--HEG nanocomposite exhibits the peak of iron oxide at 29 values of 30.5°, 36°, 43.5°, 53.8°, 57.5°, 63.1° and 74.6°, suggesting the face centered cubic structure of iron oxide nanoparticles.

Adsorption isotherm studies were performed to examine the quantity of the metallic impurities that could be removed using Fe3O4-f-HEG nanocomposite electrodes. Here, Langmuir and Freundlich isotherm models were applied for simultaneous removal of sodium and arsenic. Sodium arsenate and sodium arsenite containing aqueous solutions were used for this study with the initial arsenic concentrations in the range 50-300 mg/L. In case of Calculated isotherm constants are given in table-1. Experimental results show that both models fit for arsenic impurities. Maximum adsorption capacities (obtained from Langmuir isotherm) of 172.1 and 180.3 mg/g were found for arsenate and arsenite, respectively. Values of 'n' were found to be greater than one in each case (table-1), known to be a favorable condition for adsorption. Data in the table-2 clearly demonstrates the superiority of this invention approach in comparison to other available reports for arsenic removal.

Table 1 Isotherm constants for sodium and arsenic removal

Table 2 Comparison of different existing reports

The removal efficiency in percentage was calculated by using the followina formula -
Where, 'C0' and 'Cf' are the initial and final concentrations of metals, respectively.

A number of cycles were performed with high concentration (300 mg/L of arsenic) of arsenic solution and seawater using the same electrode in each cycle. Since industrial wastewater contains high concentration of metallic impurities, performance of Fe3O4-f-HEG nanocomposite electrodes was checked with high concentration of arsenic. Arsenic solutions and seawater were treated for 50 min in each cycle.

Figure 4 shows the removal efficiency of nanocomposite electrodes for sodium, both inorganic arsenic species and for desalination of seawater. Nearly 54% of Arsenic (As) and 55% of sodium (Na) removal efficiency was achieved in case of arsenate solution with 20 numbers of repeated cycles. Removal efficiency was found to be 61% and 59% for 'As' and 'Na', respectively, in arsenite solution with the same electrodes and same number of cycles as above. In each case the initial concentration of arsenic was 300 mg/L. Linear variation of removal efficiency with number of cycles performed suggests the good cyclic repeatability of electrodes for simultaneous removal of arsenic and sodium. Simultaneous and nearly equal removal efficiency for arsenic and sodium also suggests the utilization of Fe3O4-f-HEG nanocomposite electrodes for the removal of multiple metal impurities from water (like sea water). Sodium (Na), magnesium (Mg), calcium (Ca) and potassium (K) are found to be most copious metal impurities in sea water. Hence, removal of the above metals was tested using Fe3O4-f-HEG nanocomposite electrodes for desalination of seawater. Initial concentrations of Na, Mg, Ca and K in seawater were observed to be 10000, 1920, 680 and 570 mg/L, respectively. Removal efficiencies of 51, 55, 50 and 50% were achieved for Na, Mg, Ca and K, respectively, with 20 numbers of repeated cycles. Nearly linear variation of removal efficiency with respect to different numbers of cycles was observed, suggesting the good cyclic repeatability of electrodes for removal of multiple metals in seawater.

It is noteworthy:

• Water purification and treatment and sea water desalination for human consumption are the major application areas of this invention.

• This invention involves a significant reduction in the cost of raw materials involved in its preparation.

• This invention is a simple and scalable process adaptable to mass manufacturing and immediate adoption as retrofit to existing systems.

• This invention has low power requirements and is highly energy efficient compared to existing technologies.

We Claim:

1. A method of manufacture of iron oxide-graphene nano composite for the manufacture of electrodes therefrom for water purification comprising the steps of preparing graphene sheets by hydrogen induced exfoliation of graphitic oxide; decorating the iron oxide nanocrystals[ preparing graphitic oxide oxidation of pure graphite; further thermally exfoliating the graphitic oxide at 200 °C under hydrogen atmosphere to synthesize graphene sheets; treating the resulting hydrogen exfoliated graphene sheets (HEG) with concentrated HN03acid, resulting in the hydrophilic functional groups (-COOH, -C=O, and -OH) at the surface of HEG; washing the functionalized graphene sheets (f-HEG) several times with water to achieve pH=7 followed by drying; suspending the functionalized graphene (f-HEG) in de-ionized water by ultrasonication; dissolving FeCI3.6H2O and FeS04.7H2O in de-ionized water in the stoichiometric ratio of 3:2 and heating the resulting solution up to 90 °C; adding Ammonia solution (NH4OH-25%) and f-HEG dispersed solution, in the volumetric ratio of 1:5, to the above solution; stirring solution at 90 °C for 30 min followed by cooling to room temperature to obtain iron oxide-graphene (Fe3O4-f-HEG) nanocomposite as a black precipitate; filtering and washing the precipitate to neutral with water.

2. A method of manufacture of iron oxide-graphene nano composite for the manufacture of electrodes therefrom for water purification substantially as herein described and illustrated.

3. Manufacture of iron oxide-graphene nano composite for the manufacture of electrodes therefrom for water purification when manufactured by a method as claimed in any one of the preceding Claims.