4-chlorophenol removal from water using graphite and graphene oxides as photocatalysts
© Bustos-Ramírez et al.; licensee BioMed Central. 2015
Received: 26 November 2014
Accepted: 7 April 2015
Published: 19 April 2015
Graphite and graphene oxides have been studied amply in the last decade, due to their diverse properties and possible applications. Recently, their functionality as photocatalytic materials in water splitting was reported. Research in these materials is increasing due to their band gap values around 1.8-4 eV, and therefore, these are comparable with other photocatalysts currently used in heterogeneous photocatalytic processes. Thus, this research reports the photocatalytic effectiveness of graphite oxide (GO) and graphene oxide (GEO) in the degradation of 4-chlorophenol (4-CP) in water. Under the conditions defined for this research, 92 and 97% of 4-CP were degraded with GO and GEO respectively, also 97% of total organic carbon was removed. In addition, by-products of 4-CP that produce a yellow solution obtained only using photolysis are eliminated by photocatalyst process with GO and GEO. The degradation of 4-CP was monitored by UV-Vis spectroscopy, High Performance Liquid Chromatography (HPLC) and Chemical Oxygen Demand (COD). Thus, photocatalytic activity to remove 4-CP from water employing GO and GEO without doping is successfully showed, and therefore, a new gate in research for these materials is opened.
With increasing global air and water pollutions, photocatalysis has attracted considerable attention because this method provides a promising pathway to attenuate environmental pollution problems, mainly due to the capacity of photocatalyst to degrade organic contaminants .
Chlorophenols represent an important group of organic water pollutants due to their toxicity and low biodegradability. They are considered priority pollutants and are employed in numerous industrial processes; in particular, 4-CP is involved in the synthesis of many pesticides, pharmaceuticals and dyes [2-5].
Most polluting organic compounds, including 4-CP, are difficult to degrade, so, it is important to develop more effective methods to promote their degradation . Different biological, physical and chemical methods have been applied for chlorophenol degradation [7,8]. Other established alternative methods, such as Advanced Oxidation Processes (AOP) , have been reported to be effective for the degradation of soluble organic contaminants from water, providing almost total degradation. Several technologies are included under AOP such as photo-Fenton, ozonation, photocatalysis, etc. [10,11].
Thus, the remediation of wastewater with chlorophenols using photocatalysis has been widely studied, but not in the case of 4-CP; however, the complexity of degradation processes with heterogeneous catalysts and the high sensitivity of the reaction to experimental conditions represent two main disadvantages, that make it difficult to compare different related systems. Several studies have evaluated the effects of different substituent groups in the aromatic ring in phenolic compounds, as well as the influence of metal ions and/or oxidant compounds, on the adsorption equilibrium and kinetic parameters of degradation during UV irradiation of a suspension in a photocatalytic process with a semiconductor.
The photocatalytic activity is governed by the catalyst, which is a semiconductor (e.g., TiO2 (the most used), ZnO and Fe2O3) . These compounds have two primary characteristics: the first, their band edge energy; semiconductors with more negative band edge energy than the reduction potential of water (or protons), and that remain stable when are in contact with water can be considered as appropriate catalyst materials ; the second characteristic is the particle size, of primary importance in heterogeneous photocatalysis because it is directly related to the efficiency of the catalyst through the definition of its specific surface area .
An increasing amount of works has been addressed in searching alternative materials to TiO2. Anyway, the information gathered indicates that exist a valid alternative to the use of modified-TiO2 for photocatalytic process, although the intrinsic electronic and physico-chemical properties of some compounds reported in the literature suggest more investigation. In the degradation of 4-CP, the TiO2 has been applied in the following cases: 1) Photocatalytic degradation method using 1% silver loaded TiO2 (Ag-TiO2), the degradation/transformation of 4-CP is extremely high compared to neat TiO2, this latter requires longer exposure time to obtain the same degree of degradation than the compound Ag-TiO2 ; 2) Cobalt-modified TiO2 (Co-TiO2) is able to bring about the photocatalytic degradation of pollutants such as 4-chlorophenol, the best photoactivity was observed for an amount of cobalt between 0.2% and 0.5% Co/TiO2 w/w ; 3) the mesoporous anatase is able to degrade 100% 4-CP, while Degussa P-25 degraded 57% in time of 180 min, the enhanced photocatalytic activity of the mesoporous titania samples when compared to Degussa P-25 was related to smaller crystallite size, presence of pure anatase phase, higher average pore diameter, and surface area ; these are some published works, but in the case of research applying graphite oxide and graphene oxide without doping to degrade photocatalytically 4-CP, it is non-existent until this time.
In the development of new photocatalysts, the graphite oxide (GO), a polymer-like semiconductor made of only carbon, oxygen and hydrogen, has a large exposed area and can be extensively dispersed in water on the molecular scale. These structural features, both due to their chemical and physical aspects, suggest the favourable role of graphite oxide as a photocatalyst. Therefore, graphene oxide (GEO) derivatives of GO can also be good photocatalysts. The difference between them is the number of stacked layers. Both materials (GO and GEO) have photocatalytic activity, as it was reported recently and showed by water splitting and hydrogen production [12,17,18].
Also, nanostructured semiconductor materials are anticipated as new photocatalysts to open up new opportunities and employ the renewable energy sources, such as: Iodine-doped TiO2 nanoparticles, titanium oxynitride porous thin films, TiO2/SnO2 nanofibers, square Bi2WO6 nanoplates, and Si nanowires. All these materials have been explored and exhibited interesting photoactivities. These materials with optical gap of 2.7 eV have been recognized as a very promising metal free photocatalyst, and its photocatalytic activity is confirmed to be even higher than the commercial nitrogen-doped TiO2. But like the TiO2, the rapid recombination of electrons and holes is one of the main reasons for the low photocatalytic efficiency of these kinds of photocatalysts [19-21]. But in the case of GO and GEO, the anti-recombination is a given characteristic in current works of research related to the topic of photocatalysts. This effect can be attributed to their nanometric thickness, surface area and the presence of oxygenated groups. Also, both carbon structures can be doped with different materials or atoms to improve their performance as photocatalysts . Additionally, reduced graphene oxide (RGO) modified with different compounds has been used successfully as a photocatalytic material in the remediation treatment of heavy metals and organic compounds [23-26]. Also, a recent work has studied about the photocatalytic effectivity of graphene oxide in the removal of 4-CP by computational method . However, the photocatalytic activities of unmodified GO and GEO have not been studied experimentally, for the removal of organic compounds in water. Thus, it is important to verify the photocatalytic activity of these unmodified carbon materials for the removal of organic compounds in water. GEO has attracted great interest because of its easy availability in bulk quantities, readiness for functionalization by chemical reaction, good dispersion in water and high biocompatibility . Thus, in this research is showed for the first time the capacity of graphene and graphite oxides to remove 4-CP from aqueous solutions under UV-light. Both undoped oxides (GEO and GO) present sufficient activity to remove the organic pollutant from water.
Materials and methods
Graphite (Electron Microscopy Sciences, No. 70230), distilled water (J.T. Baker), 4-chlorophenol (≥99% Sigma Aldrich), Sulfuric acid (H2SO4, Baker, 98%), potassium permanganate (KMnO4, Merck), hydrogen peroxide (H2O2, Baker, 30%) and distilled water (H2O) were used as received. Graphite oxide (GO) and graphene oxide (GEO) were synthesized from graphite by a methodology reported in a previous work , and described briefly below.
Synthesis of graphite and graphene oxides
First, H2SO4 (46 mL) was added into the reaction flask maintained at 0°C (±2°C) (ice bath), then graphite (2 g) and KMnO4 (6 g) were added slowly. After an increase in temperature to 35°C (±2°C), the mixture was stirred by a magnetic stirring bar and mixed for 2 h. Later, the excess water was incorporated to the mixture and H2O2 (10 ml) was added until there was no gas production.
Then, filtration was performed with distilled water in a glass filter, and the obtained brown GO was dried in an oven (Barnstead Thermoline, Model 3478) at 65°C for 12 h. After that, a solution containing 100 mg of dried GO in 10 mL of H2O was prepared. This solution was sonicated for 3 h at room temperature with the assistance of an ultrasonic bath (Branson 1510R-MTH) at 55 degassing units, in order to obtain graphene oxide sheets (GEO).
Adsorption test consisted on placing an aqueous solution of 4-CP with a concentration of 30 ppm and neutral pH into a tubular glass reactor, adding graphite oxide or graphene oxide (0.8 g/L in both cases) with magnetic stirring (1000 rpm) and then, the reactor was covered with aluminium foil to prevent contact with external light. The reaction time was 100 minutes and aliquots were taken at 20, 40, 60, 80 and 100 minutes.
For these experiments, 30 mL of 4-CP solution (30 ppm) at neutral pH were placed in the same reactor used for the adsorption test and induced with UV irradiation from a lamp (Pencil UV lamp, 254 nm, 5.5 W). Light source was located in the centre of the vessel along with a magnetic stirrer (1000 rpm). The reactor temperature was maintained at 24°C +/- 2°C and the reactor was covered with aluminium foil to prevent contact with external light. The reaction time was 100 minutes and aliquots were taken after 20, 40, 60, 80 and 100 minutes.
The photocatalytic efficiencies of the GO and GEO were determined in the same tubular glass reactor used for the photolysis test with the same continuous stirring (1000 rpm). The total reaction volume was 30 ml. The tests were performed using 0.8 g/L of graphite and graphene oxides, with 30 ppm of 4-CP at neutral pH. The reactor temperature was maintained at 24°C +/- 2°C and aluminium foil was placed around the reactor. A UV lamp (Pencil UV lamp, 254 nm, 5.5 W) was placed in the centre of the reactor to provide UV light radiation. The reaction time was 100 minutes and aliquots were taken after 20, 40, 60, 80 and 100 minutes.
Chemical Oxygen Demand (COD)
Method 8000/reactor digestion method in the high range (0–1500 mg/L), programmed in the HACH DR5000 spectrophotometer was used to determine the COD in the initial and final solutions of 4-CP.
Fourier transform infrared spectroscopy (FT-IR) of GO and GEO samples was performed using a Bruker-Vector 33 with a scanning range of 4000-500 cm−1 with resolution of 1 cm−1.
Raman spectroscopy of the carbon samples was carried on a Micro-Raman (Dilor, Lab Ram), with measurements at 488 nm incident laser light with a spectral resolution of 1 cm−1.
UV-vis spectroscopy was carried out on a HACH DR5000 spectrophotometer at wavelengths of 200–1100 nm to determine the absorption bands characteristic of the 4-CP solution and monitor the progress of 4-CP removal from the solution by adsorption, photolysis and the photocatalytic processes.
HPLC (High Performance Liquid Chromatography) was performed with a mobile phase of H2O2 with 5 mmol H2SO4 and methanol (80:20), a flow rate of 1 mL/min and an Ascentis Express C18 3 cm × 4.6 mm (2.7 μm) SUPELCO column for the determination of intermediate compounds.
Mineralization of 4-CP was followed by measuring the chemical oxygen demand (COD) in a typical reaction. An aliquot was taken at the end of the reaction and the COD was measured by a colorimetric method on the HACH DR5000 spectrophotometer.
Results and discussion
Photo dissociation of the C-Cl bond in 4-CP has been reported using UV laser excitation . The photochemical reaction is the first stage in the C-Cl bond cleavage of 4-CP, with subsequent decomposition of the intermediate ion or radical . It is important to consider the band at 280 nm, which defines the generation of intermediate compounds, in the progression of 4-chlorophenol degradation.
Materials used as photocatalyst to removal 4-CP at different reaction times
Removal of 4-CP (%)
Reaction time (min)
TP2.5 (P-modified TiO2)
Graphite oxide (GO)
Graphene oxide (GEO)
Intermediate degradation of 4-chlorophenol analysed by HPLC
Concentration in ppm of aromatic intermediate compounds generated during the degradation of a 4-CP solution in photolysis and photocatalysis
Aromatics compounds (ppm)
Concentration in ppm of intermediate compounds (carboxylic acids) generated during the degradation of a 4-CP solution by photolysis and photocatalysis
Carboxylic acids (ppm)
Also, in Table 3, the carboxylic acids generated in the 4-CP degradation process are showed. The highest concentration obtained is oxalic acid, at 5.1 ppm after photolysis; however, in the photocatalytic reactions with GO and GEO the concentration of oxalic acid is 3.8 and 3.3 ppm, respectively. Moreover, in the production of succinic acid the lowest concentration is present in the solution from the photocatalytic process with GEO. The by-products generated in our experimental processes are very similar with those found in other reports, although the amounts or concentrations of these compounds are not reported previously [29,38].
Mineralization of 4-chlorophenol evaluated by COD
According with the results showed in this research, it can be concluded that GO and GEO are materials with sufficient capacity to degrade 4-CP present in water as contaminant. This fact promises a new trend in the application of carbon nanomaterials without doping as photocatalysts. UV determinations showed that the degradations of 4-CP in 92% and 97% using GO and GEO at 100 min respectively, are superior to photolysis (50%) during the same time, it was found that the reaction mechanism is favourable for 4-CP degradation with carbon oxides. The main by-products generated during the photolysis, and photocatalytic method with GO and GEO, correspond to aromatic compounds and carboxylic acids. In the photocatalytic process the by-products generated were lower in comparison with photolysis process, and better results were obtained with photocatalysis performed with GEO, in this case hydroquinone was not observed at the end of the reaction. Also, mineralization measured by COD indicates that up to 97% of organic matter is removed through photocatalysis using GEO. In addition, research findings show that nanometric size plays an important role in photocatalytic processes, reflected in the fact that GEO show better results in eliminating 4-CP than GO. Materials such as GO and GEO that have recently been used in photocatalytic applications such as water splitting [13,17] are showed in this research as an efficient alternative to eliminate 4-CP in water, using a small quantity of photocatalyst and a short reaction time. Thus, both materials are suggested as effective for their use in advanced oxidation processes. Thus, it can be concluded that, photocatalysis with GO and GEO is simple and effective, because, avoids the use of any oxidizing agent. Furthermore, their lower rates of recombination could be an important factor in this photocatalytic process. Therefore, the highly efficient degradation of 4-CP using GO and GEO as photocatalytic materials could be extended to other organic materials with aromatic rings, and thereby a new line of research in photocatalysis with carbon nanomaterials focused in the treatment of contaminated water, depending on the structural features of these materials, their electronic properties, dimensions and functional groups in the materials’ surface.
The authors are grateful to Mr. Eduardo Martín del Campo for his assistance with the HPLC analyses. Ph.D. student Bustos-Ramírez Karina is thankful to CONACYT for grant no. 218707 and to Centro en Conjunto de Investigación en Quimica Sustentable UAEM-UNAM, especially to Dr. Gabriela Roa for the use of facilities in her laboratory. Also, Velasco-Santos Carlos appreciates the support of TecNM. Article in Memoriam Dr. Adolfo M. Espindola-Gonzalez.
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