Received 20 June 2016; accepted 18 July 2016; published 21 July 2016
The last decade has shown an increase in health problems due to enhanced organic pollutants in environment (air and water)  -  . So, chemical, physical and biological processes have been adopted to remediate organic pollutants from environment  -  . The petroleum is a major pollutant of water which consists of alkanes, unsaturated hydrocarbons, cycloalkanes and aromatic hydrocarbons, and it is being remediated through bioremediation, absorption, membrane and filtration techniques, respectively    -  . However, petroleum remediation recognized by each individual technique has advantages and disadvantages, and it leaves the trace above the threshold values set by Environmental Protection Agency (EPA) and World Health Organization (WHO), except for bio-remediation which runs with its peculiar drawbacks  -  . The photocatalysts have been employed to remediate even trace amount of organic pollutants in water  -  . The catalyst, “titanium dioxide (TiO2)” has been extensively used for photocatalytic remediation of water   . The band gap of TiO2 is 3.2 eV, and has shorter wavelengths of light radiation (<415 nm, in UV light spectrum). TiO2 being chemically inert has widely used as industrial photocatalyst due to high efficiency, photostability, and lower cost    . Recently, a survey of photocatalytic materials has been performed for environmental decontamination based on TiO2 and TiO2 composite with fullerene, graphite and carbon  . TiO2 based photocatalyst faces two major drawbacks for decontamination of water; as it has faster electron hole recombination time  , and its operating band gap energy is too high (3.2 ev) falling under the UV light spectrum  . The research has been directed on modifying TiO2 by doping nitrogen, fluorine and metal ions (iron, silver, platinum etc.). The doping induces disorder in TiO2 through hydrogenation which reduces reduction in recombination time but increases the quantum efficiencies and modifies band structure for efficiently working under wide spectrum of solar radiation  -  . The control of operational parameters to remediate completely the azo dyes in textile waste water using TiO2 nanomaterials have been studied  . The structure modification of TiO2 in nanotubes form has been studied for dye degradation  .
Recently, optical absorption of TiO2 has been shifted by making composite with graphene  . Graphene (G) “a 2D material” is a monolayer of graphite (3D)  , has attracted attention due to its unique electrical  , photonics and optoelectronics  , energy storage  , photovoltaics  and photo-electrochemical  properties. Graphene increases photocatalytic properties of TiO2 due to its large surface area, absorbing, electronic and environmentally friendly characteristics   . The photocatalytic activities of TiO2 are greatly influenced due to particle size, dopant, surface area, structure and morphologies  . Based on our earlier studies of the use of silicon oxide-graphene composite as well as graphene-TiO2 composite, it has been agreed that surface area of composite nanoparticles can be enhanced than pristine TiO2 nanoparticles   . G-TiO2 can be synthesized by hydrothermal method, sol-gel, and colloidal blending methods  -  . The earlier study on G-TiO2 nanomaterials has shown the complete remediation of methyl orange in water under visible light  .
Under this manuscript, we optimized G-TiO2 nanoparticles synthesis process for obtaining the large surface area and toluene, naphthalene and diesel based organics in water were remediated in visible light. The G-TiO2 nanoparticles were synthesized using sol-gel technique, and studied. G-TiO2 nanoparticles were characterized by XRD, SEM, TEM, UV-vis and particle analyzer based techniques. The quantitative and qualitative remediation of toluene, naphthalene and diesel were met. In an experimental setup, initially, a given organic concentration in DI water (mL) in closed glass container with fixed amount of photocatalyst was used. Simulating the solar intensity of 800 - 1000 W/m2 using a soft light bulb illuminated in the visible light radiation, and samples were collected as a function of time (hours). The samples were centrifuged and gas chromatography (GC) was employed to measure the amount of organic pollutant in the water.
Hydrochloric acid (HCl), propanol and titanium (IV) isopropoxide, methyl orange, toluene, diesel, naphthalene, and other reagents were purchased from Sigma-Aldrich (USA), and used without purification unless and until reported. Toluene, naphthalene and diesel solutions in water were prepared as per requirement of the experiment. The graphene platelets of size < 20 - 50 nm were obtained from a commercial company “Angstron Materials’ (USA)”.
2.2. Synthesis of G-TiO2 Nanocomposite
The synthesis of G-TiO2 nanocomposite materials showed better yield through a sol-gel synthesis process  . The synthesis of nanocomposite G-TiO2 was initiated by using a mixture of titanium (IV) isopropoxide in propanol solution. Initially, 1.93 gram (g) of graphene was mixed with 200 ml of propanol with an addition of 40 mL of titanium (IV) isopropoxide, and left on stirring for 30 minutes. HCl was added drop wise and the solution was left stirring for 24 hours at room temperature. The precipitate formed after 24 h of reaction under stirring was washed in deionized water for removal of any unreacted organic residues by centrifugation process. The centrifuged G-TiO2 nanoparticles were dried at 100˚C for 24 h.
2.3. Sample Preparation and Decontamination Setup
The organic contaminants (toluene, naphthalene and diesel) at different concentrations were used to decontaminate using G-TiO2 nanocomposite photocatalyst. A 100 W lamp was employed to simulate the solar light intensity of 800 - 1000 W/m2. The contaminants solution of G-TiO2 were stirred in closed glass container, and kept closed during the completion of the experiment. Samples were collected at regular intervals, and centrifuged to separate composite G-TiO2 particles from measuring solution. The centrifuged sample of 1 mL solution was passed through a gas chromatography. Diesel, toluene and naphthalene containing water samples have been kept in the identical conditions, and decontaminated water samples have been collected as a function of time using G-TiO2 photocatalyst. These petroleum molecules may get evaporated especially under stirring and light exposure conditions. It is useful to add a control experiment using the same equipment setup while changing the G-TiO2 to pure TiO2.
The retention time (in min) vs. area under curve was plotted to understand each organic contaminant in the water sample. The ratio of concentrations as Co (initial concentration) and Cn (concentration of solution at different timed samples with % of sample remained in the solution) were used to understand the change in percentage of concentration with the use of G-TiO2 nanocomposite photocatalyst  .
3.1. TEM Study
Figures 1(a)-(d) exhibits TEM picture of the G-TiO2 nanocomposite at different magnifications. The Figure 1(a) observed the particle size of 20 - 50 nm for G-TiO2 nanoparticle. Further, magnification in Figure 1(b) and Figure 1(c) reveals well-defined graphene coated TiO2 nanoparticles.
Figure 2 reveals the d-spacing with inter-planar structure of G-TiO2 nanocomposite. The Y-axis shows the d-spacing of different crystalline planes present in the G-TiO2 nanocomposite material. The x-axis in Figure 2
Figure 1. TEM pictures of G-TiO2 nanocomposite at different magnification (a)-(d).
is the characteristics ring used for calculation of inter-planar place of G-TiO2 based polycrystalline nanocomposite. The error bar is calculated using 10 different measurements per crystalline structure. It reveals polycrystalline structure in G-TiO2 nanocomposite.
3.2. SEM Study
The surface morphology of G-TiO2 nanocomposite have been studied using SEM as shown in Figures 3(a)-(d).
Figure 2. Cross sectional TEM image where Y-axis shows the d-spacing of the different crystalline planes and X-axis represents the ring used for calculation of interplanar place of the polycrystalline nanocomposite.
Figure 3. SEM images of G-TiO2 nanocomposite at different magnifications (a)-(d).
The sol-gel synthesis of G-TiO2 provides varying particle size from 20 - 50 nm as observed earlier in TEM measurement. The compact bundles of the nanomaterials have been observed in SEM studies. The compact particle distribution of G-TiO2 is observed in Figure 3(a) and Figure 3(b) which attributes the dispersion of TiO2 nanoparticles with graphene. The particle size varying from 20 - 50 nm for G-TiO2 nanoparticles in Figure 3(c) and Figure 3(d). We have not detected the size of nanoparticles in SEM studies due to aggregation of small particles in G-TiO2 composite structure. There is always aggregation of nanoparticles so care is taken to disperse well in the G-TiO2 composite material.
3.3. X-Ray Diffraction (XRD)
XRD analysis on G-TiO2 is as shown in Figure 4. The strong diffraction peak at 26.51 is indicative of presence of graphene in G-TiO2 structure. The presence of peaks at 25.27, 37.85, 47.83, 54.55, 63.59, 70.15, 83.1 degrees are due to TiO2 anatase phase present in G-TiO2 nanocomposite   . The structure indicates the forms of crystallinity in G-TiO2 nanomaterials.
3.4. Particle Analyzer
It is important to understand G-TiO2 particle distribution in water. To realize such behavior, G-TiO2 particles were dispersed in water. Figure 5 displays the agglomeration of nanocomposite in water solution, indicating the distribution of particles in contaminated water. The nanocomposite particles in water form aggregation, and it shows agglomeration up-to mm in size. The small aggregation of 100 nm is also observed in Figure 5. Figure 5 shows prominent 100 nm to 1 μm particle size distribution of G-TiO2 in water samples.
Figure 4. XRD image of G-TiO2 nanocomposite.
Figure 5. The particle distribution of G-TiO2 nanocomposite in water.
3.5. UV-Visible Spectroscopy
Figure 6 displays UV-visible absorption spectrum of G-TiO2 nanocomposite material. It has strong absorption band from 250 to 400 nm due to doping of graphene onto TiO2, the absorption spectra strongly continues till 620 nm, suggesting that G-TiO2 nanocomposite functions effectively in both UV and visible spectrum of light. The band gap of TiO2 nanopartciles are estimated to be 3.2 eV. It can be stated that there is a red shift of band edge and reduction of band gap to 2.7 eV is due to graphene doping.
4. Decontamination Study
Figure 7 shows step-by-step procedure for pollutant decontamination using G-TiO2 nanomaterials. It also shows sample collection, decontamination and sample analysis using gas chromatograph.
1 g of G-TiO2 nanocomposite have been used with toluene at 100 ppm (250 ml) under 100 W visible light bulb. Figure 8 shows ~90% of toluene decontamination in water for exposure of only an hour of visible light. Further, light exposure results in similar values indicating that toluene on surface of water mostly evaporated or there could be continual evaporation of toluene from water surface. We have earlier observed that without
Figure 6. UV-visible spectrum of G-TiO2 nanocomposite.
Figure 7. Schematic of sample collection and analysis shown in step in step process.
use of graphene in TiO2, remediation of organic in visible light excitation is poor.
The initial solution of naphthalene solution was 5000 μg/mL in methanol, analytical standard as obtained from Sigma Aldrich. 25 μg/mL naphthalene in DI water (250 mL) was prepared to recognize the effect of decontamination using G-TiO2 (1 g sample) composite material under visible light. Naphthalene is sparingly soluble in water so we used methanol naphthalene solution. 25 μg/mL of solution was prepared and decontaminated. Figure 9 shows decontamination of naphthalene under visible light in presence of G-TiO2 nano-compo- site particles.
There was only 50% reduction of naphthalene under visible light over a period of 48 h, measured using GC which is shown in Figure 9. The composition of diesel in water obtained from Sigma Aldrich contains acetone, methanol and mineral oil type. 25 μg/mL of diesel in DI water (250 mL) was used with 1 g of G-TiO2. The methanol is soluble in water whereas acetone and oil are springy soluble in water. It is clear that without organic molecules much in contact with G-TiO2 nanoparticle, it is difficult to decontaminate under visible light. Figure 10 shows the change area of diesel measured as a function of time (in hours) under 100 W of visible light lamp. So, organic molecules present in diesel are not in contact with G-TiO2 nanocomposite displaying ~ 40% reduction of diesel after 48 h of visible light irradiation.
90% of toluene, 50% of naphthalene and 40% of diesel have been remediated using G-TiO2 nanomaterial under visible light as shown in Figures 8-10. Figure 11 shows the pictorial representation for decontamination mechanism G-TiO2 with petroleum pollutants. The insolubility of petroleum pollutants in water brings contaminant to the surface of water thereby inhibiting photocatalytic effect with G-TiO2. The contaminants soluble in water remain in contact with G-TiO2 particles and are completely remediated. Surfactant (dodecyl sulphonate,
Figure 8. The change in the area in toluene measurement as function of hour for toluene decontaminated water in 100 W visible light.
Figure 9. The change in the area in naphthalene measurement as function of hour for naphthalene decontaminated water in 100 W visible light.
Figure 10. The change in the area in diesel measurement as function of hour for diesel decontaminated water in 100 W visible light.
Figure 11. Pictorial presentation of soluble and insoluble organic compound decontamination using G-TiO2 nanocomposite material.
polystyrene sulfonate) or biosurfactant (rhamnolipid) have been employed for increasing the solubility of petroleum contaminants, which helps contaminant to remain in contact with the photocatalyst. Table 1 shows the results produced from our group for naphthalene decontamination with surfactant with G-TiO2, suggesting importance of oil pollutant to come in contact with photocatalyst for complete remediation  .
The G-TiO2 nanocomposites have been synthesized using sol-gel synthesis process and characterized for mass production. The particle distribution has been studied in water, which shows the agglomeration from 100 nm to 1 mm size particle. G-TiO2 was able to decontaminate 90% of toluene whereas with naphthalene shows only 50% of reduction and diesel reveals only 40% of reduction from water solution. Naphthalene and diesel insolubility is reason behind the ineffective photocatalytic effect using G-TiO2 nanomaterial. The mechanism of petroleum based contaminant has been understood using G-TiO2 nanocomposite material. We have also compared toluene and naphthalene remediation using different types of TiO2 synthesized photocatalyst material with G-TiO2 nanocomposite material. However, we have obtained interesting results from toluene, naphthalene and diesel as
Table 1. The compared remediation of G-TiO2 with surfactant and without surfactant.
Table 2. Comparative study of petroleum production remediation.
shown in Table 2. The results shown in Table 2 reveal that it is easy to remediate toluene than naphthalene or diesel from water. The decontamination depends upon solubility of organics in water or the layer of organics to remain in contact with photocatalysts. Naphthalene as well as diesel are springily soluble in water and do not remain in contact with the G-TiO2 nanomaterials whereas toluene remains in contact with photocatalyst. Due to lower density than water both naphthalene and diesel molecules stay on the surface of water. The insolubility in water as well as no contact with G-TiO2 makes diesel and naphthalene to remediate partially up-to 50% and 40% than their initial values. Based on our understanding, we are using various surfactant (dodecyl sulphonate, polystyrene sulfonate) and biosurfactant (rhamnolipid) with G-TiO2 to effectively remediate various chemicals of petroleum including mineral oil A and B for future work as shown in Figure 11.
The authors are grateful to NSF for financial support. One of the Authors, Gunti is grateful to Dr. Yang Yang for help in GC measurement on various samples. The authors are grateful to NREC staffs for their support during SEM, TEM and X-ray diffraction measurements.
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