Guar gum is considered as a main component of oilfield wastewater. This work is intended to optimize the experimental conditions (H2O2 dosage, Fe2+ dosage, initial concentration of organics, initial pH and temperature) for the maximum oxidative degradation of guar gum by Fenton's reagent. The kinetics of guar gum removal were evaluated by means of the chemical oxygen demand (COD) and the absorbance measurements. The batch experiment results showed that the optimum conditions were: H2O2 dosage, 10,000 mg/L; Fe2+dosage, 2,000 mg/L; initial concentration of organics, 413 mg/L; pH, 3 and temperature, 35 °C, under which the COD removal could reach 61.07% and fairly good stability could be obtained. Under the optimum experimental conditions, using UV irradiation to treat the wastewater, the photo-Fenton systems can successfully eliminate COD from guar gum solution. The COD removal always obeyed a pseudo-first-order kinetics and the degradation rate (kapp) was increased by 25.7% in the photo-Fenton process compared to the Fenton process. The photo-Fenton system needed less time and consequently less quantity of H2O2 to obtain the same results as the Fenton process. The photo-Fenton process needs a dose of H2O2 20.46% lower than that used in the Fenton process to remove 79.54% of COD. The cost of the photo/Fenton process amounted to RMB9.43/m3, which was lower than that of the classic Fenton process alone (RMB10.58/m3) and the overall water quality of the final effluent could meet the class Ι national wastewater discharge standard for the petrochemical industry of China.

INTRODUCTION

Large amounts of wastewater are generated during hydraulic fracturing to recover oil from deep shale formations (Aguilera 1995; Neghaban et al. 1998; Hickenbottom et al. 2013). This oilfield wastewater contains various organics pollutants, solid impurities, heavy metals, polymers, chemical additives, proppants, oil, etc. (Olsson et al. 2013). The wastewater is characterized by high chemical oxygen demand (COD), high color and salinity, strong acidity and viscosity, and has become one of the main sources of water pollution in oilfields and can increase considerably environmental hazards if discharged without effective treatment (Chase et al. 1997). Nowadays, disposal of wastewater constitutes one of the most significant waste discharges associated with oil well drilling (Yang et al. 2014), and consequently it is vital to explore appropriate processes for the effective abatement of organics to an acceptable level.

Among the several processes used in the treatment of these wastes, the advanced oxidation processes (AOPs) appear to be a promising field of study due to the effective degradation of organic contaminants under mild conditions (Andreozzi et al. 2000; Bautista et al. 2007). AOPs are based on the use of a very strong oxidizing agent such as hydroxyl radical (HO•) with oxidation-reduction potential (HO•/H2O) = 2.8 V/NHE, which is generated in situ in the reaction medium (Hirvonen et al. 1996).

Fenton and UV-Fenton processes have proved to be effective and economical methods used for the detoxification and degradation of many organic compounds (Fusheng 1997; Yoon et al. 2001; Bagal & Gogate 2014). Oxidation with Fenton reagent is based on ferrous ion and hydrogen peroxide, and exploits the reactivity of the hydroxyl radical produced in acidic solution by the catalytic decomposition of H2O2 (Kang & Hwang 2000; Chamarro et al. 2001):
formula
1
Ferrous iron is slowly regenerated through the so-called Fenton-like reaction between ferric iron and H2O2 in acidic aqueous medium (Sedlak & Andren 1991; Pan et al. 2008):
formula
2
formula
3
Nevertheless, numerous competitive reactions can also occur, namely the following ones, which negatively affect the oxidation process (Bergendahl & Thies 2004; Ren 2014):
formula
4
formula
5
formula
6
formula
7
formula
8
formula
9
formula
10
In the presence of substrate, as a target contaminant, the generated hydroxyl radicals are able to attack most of the contaminant either by hydrogen abstraction, as shown in Equation (11), or by hydroxyl addition, as shown in Equation (12) (Flotron et al. 2005; Sun et al. 2007), where R represents a saturated hydrocarbon group.
formula
11
formula
12
In UV-Fenton process, in addition to the above reactions the formation of hydroxyl radical also occurs by the following reactions (Equations (13) and (14)).
formula
13
formula
14
At acidic pH (2.5‒5), the main compounds absorbing light in the UV-Fenton system are ferric ion complexes, e.g., Fe(OH)2+ and Fe(RCO2)2+, which produce additional Fe2+(Equations (15) and (16)) (Sagawe et al. 2001).
formula
15
formula
16

The rate of organic degradation could be increased by irradiation of Fenton with UV light. This leads not only to the formation of additional hydroxyl radicals but also to recycling of ferrous catalyst by reduction of Fe3+. In this way, the concentration of Fe2+ is increased and the overall reaction is accelerated and can be used as alternative to UV lamps to reduce the degradation process costs. Thus, UV-Fenton degradation of contaminants using UV lamps has been successfully used as an economically viable process (Primo et al. 2008; Monteagudo et al. 2009).

The aim of this work is the optimization of the Fenton process for the degradation of guar gum as a main ingredient in oilfield wastewater. In order to improve the reaction rate, solutions were subjected to UV radiation using a laboratory-scale reactor. The influence of different operational parameters such as H2O2 dose, initial concentrations of organics and Fe2+, temperature, and pH was investigated. Furthermore, the cost of the oilfield wastewater treatment in determining optimum Fenton reagent doses was also evaluated.

MATERIALS AND METHODS

Laboratory instruments and chemicals

An ultraviolet spectrophotometer (UV-8000, Metash Instrument Co., Ltd, China) and quartz cuvettes were used for measurements on absorbance analysis and COD using a dichromate solution as the oxidant in strong acid media. The pH values were determined by using an acidometer (PHS-3C, INESA Scientific Instrument Co., Ltd, China). Oxidation experiments were conducted with a six-joint heat-collection temperature magnetic stirrer (HJ-6, Changzhou ZOJE Experimental Instruction Manufacturing Co., Ltd, China). Ultraviolet radiation was generated through a UV lamp (300 W, Metash Instrument Co., Ltd, China). A vacuum filtration device (FS3310, Shanghai Collar Germany Instrument Co., Ltd, China) is also needed for treating the effluent.

The hydrogen peroxide (H2O2, 30%, w/w), sodium hydroxide (NaOH), hydrochloric acid (HCl), ferrous sulfate heptahydrate (FeSO4·7H2O), potassium dichromate (K2Cr2O7) were of analytical grade, and other chemicals used were analytical reagent grade. All solutions were prepared with distilled water.

Oilfield wastewater was obtained from the National Offshore Oil Corporation at Tianjin, China. This wastewater was skimmed before being used and determined. The wastewater was stored at 4 °C until required. The main characteristics of sample wastewater are listed in Table 1. As shown, the oilfield wastewater had high concentrations of COD, total dissolved solids and suspended solids (SS). Moreover, the oilfield wastewater had high color.

Table 1

Characteristics of the raw oilfield wastewater

ParameterUnitValue
pH 6.7 
COD mg/L 3,386 
BOD5 mg/L 631 
BOD5/COD 0.186 
Total dissolved solids mg/L 6,150 
SS mg/L 40.38 
Color 175 
Guar gum mg/L 3,297–3,516 
Alkalinity mg/L 5,126 
Oil and grease mg/L 37 
Total petroleum hydrocarbon mg/L 34 
ParameterUnitValue
pH 6.7 
COD mg/L 3,386 
BOD5 mg/L 631 
BOD5/COD 0.186 
Total dissolved solids mg/L 6,150 
SS mg/L 40.38 
Color 175 
Guar gum mg/L 3,297–3,516 
Alkalinity mg/L 5,126 
Oil and grease mg/L 37 
Total petroleum hydrocarbon mg/L 34 

Experimental setup

The experiments which involved UV radiation were carried out in a batch cylindrical glass photoreactor of 500 mL with a medium-pressure 300 W Hg lamp immersed in a quartz sleeve placed in the middle of the reactor. It was surrounded by a water cooling jacket to maintain constant temperature. A broad radiation spectrum was emitted with wavelengths from 200 nm to 600 nm, including complete range of ultraviolet radiation. To determine the amount of radiation emitted by the UV lamp, actinometric experiments were performed. The photon flux entering the reactor was 8.8 × 10−5 Einstein/s estimated from hydrogen peroxide actinometry (Nicole et al. 1990). A magnetic stirrer was used to provide proper mixing (Primo et al. 2007).

Experimental procedure

Prior to the experiments, to minimize particulate effects in oxidation reactions and to determine the organic matter corresponding to the solid fraction, the wastewater samples were filtered through a 0.45 μm millipore filter. After filtration, the initial COD concentration scarcely decreased 2‒3% (Sedlak & Andren 1991). Batch experiments were carried out adjusting initial pH to 3‒3.5 with sulfuric acid to enhance the oxidation. Then, hydrogen peroxide and ferrous sulfate heptahydrate were added to 200 mL wastewater samples using a six-joint heat-collection temperature magnetic stirrer to provide proper mixing and various temperature according to the treatment conditions. For tests using only the Fenton reagent, the experimental device was kept away from UV irradiation by covering with a black plastic film and an aluminum foil. After the oxidation reaction, neutralization was carried out by increasing pH value up to 8‒9 with sodium hydroxide. After settling for 30 minutes, the samples was filtered by 0.45 μm membrane through the vacuum filtration device. Then, the absorbance and COD in the supernatant were measured.

All samples were tested in duplicate, and the test was reproduced three times for each sample, so that the relative errors could be minimized. All the figures show the average values.

RESULTS AND DISCUSSION

Oxidation of wastewater by the Fenton process

Kinetics of COD removal

Figure 1 shows the decrease of COD during the oxidation of guar gum (COD0 = 3,386 mg/L) by Fenton's reagent. It can be seen that COD values decreased almost exponentially and 59% of COD removal was obtained after approximately 110 min. If we suppose that the oxidation of guar gum and its by-products by the hydroxyl radicals is of a first order with respect to COD and that the hydroxyl radicals concentration is constant during the treatment, the oxidation rate (r) can be given by the following equation:
formula
17
where k is the reaction rate constant, α is the reaction order related to the hydroxyl radicals, and kapp = k[•OH]α the apparent rate constant. The integration of Equation (17) subject to the initial condition COD = COD0 at t = 0 leads to the following equation:
formula
18
Figure 1

Trend of COD during the oxidation of guar gum solution by Fenton process. The inset panel shows its kinetic analysis assuming that COD follows a pseudo-first-order reaction. Dosing of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

Figure 1

Trend of COD during the oxidation of guar gum solution by Fenton process. The inset panel shows its kinetic analysis assuming that COD follows a pseudo-first-order reaction. Dosing of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

The value of kapp could be calculated from the plot of Ln(COD0/CODt) versus t (inset of Figure 1). As can be seen, points lie satisfactory in a straight line with correlation coefficient greater than 0.96. The constant kapp was used to study the effect of different concentrations of H2O2, Fe2+ and guar gum, and different temperatures and pH.

Effect of H2O2 concentrations

The H2O2 dose is considered as one of the most important factors which should be considered in the Fenton process (Bagal & Gogate 2014). The effect of the dosing of hydrogen peroxide on the efficiency of the oxidation process was investigated under the operating conditions (COD0 = 3,386 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 25 °C (Figure 2). It was found that COD removal efficiency increases with increasing the concentrations of hydrogen peroxide from 5,000 to 13,000 mg/L. The highest percent removal of COD was attained at 110 min when using 10,000 mg/L H2O2 concentration, so further addition of H2O2 is not necessary. Excessive H2O2 reacts with HO• competing with organic pollutants and consequently reduces treatment efficiency (Pinar et al. 2013; Pliego et al. 2014).
Figure 2

Effect of H2O2 concentrations on the COD removal by the Fenton process. The inset panel shows kapp evolution at different dosing of H2O2. COD0 = 3,386 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

Figure 2

Effect of H2O2 concentrations on the COD removal by the Fenton process. The inset panel shows kapp evolution at different dosing of H2O2. COD0 = 3,386 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

The inset of Figure 2 shows the variation of the apparent rate constant (kapp) values, at different H2O2 concentrations, calculated from the straight lines considering a pseudo-first-order reaction. The value of kapp increased when the concentration of H2O2 increased from 5,000 to 10,000 mg/L, then began to decline from 10,000 to 13,000 mg/L. This is due to the effect of the additional HO• radicals produced. Given that the concentration of Fe2+ introduced initially in the solution is sufficient to react with H2O2, the competitive reactions (Equations (4), (6), and (8 )‒(10)) did not affect significantly the COD removal rate.

Furthermore, in order to follow the change in solution color during guar gum oxidation, absorbance measurements were carried out at a wavelength of 254 nm. In the first 10 minutes, the solution undergoes a fast color change from yellow to dark brown, reaching a peak level. Later, the solution begins to slowly clear to a light brown, and even turns a pale yellow residual color in some experimental conditions. The effect of concentration of hydrogen peroxide on color evolution was tested in a set of assays with constant catalyst concentration [Fe2+] = 2,000 mg/L at pH 3. Results are reported in Figure 3, which shows the temporal absorbance at 254 nm using different concentration of H2O2. In all cases, the COD removal efficiency is lower than color removal efficiency during guar gum oxidation. The rate of this decolorization stage increased with the increase of concentration of H2O2 and the solution being fully decolorized at shorter reaction times. With 10,000 mg/L H2O2 dose, the initial absorbance has a maximum (A254 = 0.517) after about 110 min of treatment and then the absorbance decreased and tended to 0.05, leading to the almost complete disappearance of color. The relationship between the dose of hydrogen peroxide and the final color of the solution is therefore established, and it is concluded that the color observed depends on the level of oxidation reached. Consequently, it can be said that current color is a good indicator of the degree of oxidation achieved during the reaction. Using at least 10,000 mg/L H2O2 dose is required in these conditions to remove almost completely the guar gum.
Figure 3

Temporal results of color evolution at 254 nm (A254). Effect of the concentration of H2O2 upon color formation and abatement. COD0 = 3,386 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

Figure 3

Temporal results of color evolution at 254 nm (A254). Effect of the concentration of H2O2 upon color formation and abatement. COD0 = 3,386 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

Effect of the initial concentration of ferrous iron

Ferrous ion acts as a catalyst in Fenton's reactions (Wang et al. 2014; Oh et al. 2016). To choose the optimal amount of Fe2+ added in the reaction solution, a set of tests was performed. Figure 4 illustrates the decrease of COD with time, during the oxidation of guar gum wastewater (COD0 = 3,386 mg/L) using different initial concentrations of Fe2+. As can be seen, Fe2+ dosage has a significant effect on the degradation of guar gum. The COD percent removal increased from 48% to 55.2% within 110 min reaction when the initial concentration of Fe2+ increased from 1,000 to 2,000 mg/L. As a catalyst, ferrous ion initiates the decomposition of hydrogen peroxide to generate the very reactive HO• in Fenton's reactions. Therefore, higher initial Fe2+ concentration led to higher generation of HO• and better degradation of guar gum solution and its by-products. However, for Fe2+ doses higher than 2,000 mg/L, the COD percent removal showed almost no change. This result is essentially due to competitive consumption of HO• and HO2• radicals (Equations (5) and (7)) (Buxton et al. 1988). It is worth noting that, in the Fenton process, the amounts of Fe2+ ions should be as low as possible for economic and environmental reasons; high amounts of Fe2+ ions might produce a larger quantity of Fe3+ sludge. The treatment of the sludge containing Fe3+ at the end of the wastewater treatment is expensive and needs a large amount of chemicals and manpower.) (Ramirez et al. 2007; Hansson et al. 2015).
Figure 4

Effect of the initial concentration of Fe2+ on the COD removal by Fenton process. The inset panel shows kapp evolution at different Fe2+ concentration. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, pH = 3, and T = 35 °C.

Figure 4

Effect of the initial concentration of Fe2+ on the COD removal by Fenton process. The inset panel shows kapp evolution at different Fe2+ concentration. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, pH = 3, and T = 35 °C.

On the other hand, the inset of Figure 4 shows that kapp values, calculated from the straight lines, considering a pseudo-first-order reaction, increased as a function of Fe2+ dosage and reached a maximum around 2,000 mg/L of Fe2+. Therefore, 2,000 mg/L Fe2+ was selected as the optimum Fe2+ dosage in this work.

Moreover, Figure 5 shows the change in absorbance of the solution versus time measured for four initial concentrations of Fe2+ ions. In all cases the curves showed the same shape. The absorbance decreased from an initial maximum whose intensity increased proportionally with the amount of Fe2+ ions. The final color can be deepened or modified by Fe3+ that appears in the reacting medium due to the oxidation of Fenton reagent. Colored chemical additive compounds are the main contributors to the color observed during the reaction, although the contribution of Fe3+ and its complexes is by no means negligible because the residual color of fully oxidized water increases with the initial concentration of iron. This suggestion is also supported by other authors (Federico et al. 2006).
Figure 5

Temporal results of color evolution at 254 nm (A254). Effect of the initial concentration of ferrous ions upon color formation and abatement. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, pH = 3, and T = 35 °C.

Figure 5

Temporal results of color evolution at 254 nm (A254). Effect of the initial concentration of ferrous ions upon color formation and abatement. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, pH = 3, and T = 35 °C.

Effect of the initial COD

It is important from an application point of view to study the dependence of removal efficiency on the initial concentration of the organics pollutant. Therefore, the effect of guar gum concentration on the degradation efficiency was investigated at different initial concentrations (COD0, 413, 792, 1,690, and 3,386 mg/L) and results are presented in Figure 6. It can be observed that the COD removal decreased with the increase of the initial concentration of the pollutant. Almost 80%, 75%, 68% and 59% of COD removal was achieved after about 110 min time of reaction for COD0 413, 792, 1,690, and 3,386 mg/L, respectively. However, at high concentration, the removal of COD needs more time and so more H2O2; this is because when the concentration of COD increases, the quantity of hydroxyl radicals produced continuously with time does not increase accordingly and hence the removal rate decreases. Also, from the inset of Figure 6, it can be seen that kapp decreased linearly with COD0. This behavior was similar to those reported by many researchers (Modirshahla et al. 2007; Francisco 2016).
Figure 6

Effect of the initial concentration of guar gum on the COD removal by the Fenton process. The inset panel shows kapp evolution at different initial COD. Concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

Figure 6

Effect of the initial concentration of guar gum on the COD removal by the Fenton process. The inset panel shows kapp evolution at different initial COD. Concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L, pH = 3, and T = 35 °C.

Effect of the initial pH

The effects of pH on the degradation of guar gum organics by the Fenton reaction has been illustrated and acidic conditions are required to produce the maximum amount of HO• by the decomposition of H2O2 catalyzed by ferrous ions (Xie et al. 2015). Several investigations have indicated that the optimum pH for the degradation of organics by the Fenton process is in the range 2.5‒3.5 and that the extent of degradation decreases with increasing pH for pH > 3.5 (Samet et al. 2009).

Figure 7 shows the change of COD removal with time and the kapp curve in the oxidation of guar gum solution (COD0 = 3,386 mg/L) as a function of the initial pH. Compared with 59.6% at pH 2 and 57.6% at pH 4, the optimum pH point of COD removal is 3 with 62.8%. Obviously, the COD removal efficiency was influenced by the initial pH and the optimum experiment pH was 3. The values of kapp increase when pH increases from 2 to 3, then decrease when pH is raised from 3 to 6. The contributing factors for the low kapp in lower pH range (<3) include the formation of oxonium ion (i.e., H3O2+) due to the strong proton solvating ability of H2O2, complex species [Fe(H2O)6]2+ and [Fe(H2O)6]3+ and enhanced HO• scavenging by H+. The poor degradation of guar gum at a high pH value (>3) was caused by the formation of ferrous and ferric hydroxide complexes with much lower catalytic capability than Fe2+ (Kang & Wang 2000; Zhao & Hu 2008).
Figure 7

Effect of the initial pH on the COD removal by the Fenton process. The inset panel shows kapp evolution at different pH. Concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and T = 35 °C.

Figure 7

Effect of the initial pH on the COD removal by the Fenton process. The inset panel shows kapp evolution at different pH. Concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and T = 35 °C.

In addition, the influence of the initial pH on the evolution with time of the absorbance at 254 nm wavelength is shown in Figure 8. As can be seen, initial pH has no significant effect on the final color removal efficiency at pH values between 2 and 5 and can lead to the almost disappearance of color at the end of the treatment.
Figure 8

Temporal results of color evolution at 254 nm (A254). Effect of the initial pH upon color formation and abatement. Concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and T = 35 °C.

Figure 8

Temporal results of color evolution at 254 nm (A254). Effect of the initial pH upon color formation and abatement. Concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and T = 35 °C.

Effect of the temperature

The temperature plays an important role in chemical oxidation, because it represents a determinant parameter in the kinetics of homogeneous reactions. The influence of this parameter on the kinetic rate constant, kapp, for the guar gum degradation was investigated in the range between 25 °C and 60 °C with test conditions at COD0 3,386 mg/L, H2O2 10,000 mg/L, Fe2+ 2,000 mg/L and pH 3. The obtained results shown in Figure 9 indicate that kapp was significantly influenced by the temperature with an optimal value of 35 °C. The values of kapp quickly increased when the temperature increased from 25 °C to 35 °C, suddenly decreased when the temperature was raised from 35 °C to 50 °C, and then gradually decreased with the increase of temperature in the range of 50–60 °C. The decrease of kapp at temperature higher than 35 °C is due to the accelerated decomposition of H2O2 into oxygen and water. Similar results were reported by researchers (Wang 2008). The data for temperatures between 25 °C and 35 °C exhibit an Arrhenius-type behavior with apparent activation energy of 17,543 J/mol (Equation (19)) calculated from the usual Lnkapp versus 1/T (Figure 10).
formula
19
Figure 9

Plot of Ln(COD0/COD) versus time at different temperatures. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and pH = 3.

Figure 9

Plot of Ln(COD0/COD) versus time at different temperatures. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and pH = 3.

Figure 10

Plot of Ln kapp versus (1/T) for the degradation of guar gum by Fenton oxidation process. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and pH = 3.

Figure 10

Plot of Ln kapp versus (1/T) for the degradation of guar gum by Fenton oxidation process. COD0 = 3,386 mg/L, concentration of H2O2 10,000 mg/L, [Fe2+] = 2,000 mg/L and pH = 3.

where R is the ideal gas constant (8.314 J/mol K) and T is the reaction absolute temperature (K).

Oxidation of wastewater by the UV-Fenton process

In order to improve the reaction rate and COD abatement efficiency, solutions were subjected to UV radiation using a laboratory-scale reactor. Figure 11 shows the trend of the COD0/CODt ratio during the treatment of wastewater by the two processes under the optimum experimental conditions already found when using the Fenton process (COD0 413 mg/L, concentration of H2O2 10,000 mg/L, [Fe2+] 2,000 mg/L, pH = 3 and T 35 °C). It can be seen that the UV-Fenton system needed less time to reach the same COD removal efficiency. After reaction, the concentration of residual H2O2 in solution is 2,046 mg/L, and consequently a smaller quantity of H2O2 is required to reach the maximum COD removal. In fact, under the optimum experimental conditions, the UV-Fenton process need a dose of H2O2 20.46% lower than that used in the Fenton process to remove 79.54% of COD. Furthermore, the COD removal rate is higher with the UV-Fenton process as shown by the kapp values in the inset of Figure 11.
Figure 11

Effect of UV radiation on the kinetic abatement of COD during the treatment of wastewater (COD0 = 413 mg/L). The inset panel shows the fitting of the experimental data to a first-order reaction kinetic model concentration of H2O2 10,000 mg/L, [Fe2+] 2,000 mg/L, pH = 3 and T 35 °C.

Figure 11

Effect of UV radiation on the kinetic abatement of COD during the treatment of wastewater (COD0 = 413 mg/L). The inset panel shows the fitting of the experimental data to a first-order reaction kinetic model concentration of H2O2 10,000 mg/L, [Fe2+] 2,000 mg/L, pH = 3 and T 35 °C.

Economic analysis

An efficient and cost-effective treatment process must strike a balance between treatment efficiency and operating costs. The consumption of the chemical reagents in the treatment technology of photo/Fenton and classic Fenton alone are shown in Table 2. The cost of the treatment of the classic Fenton process alone was about RMB10.58/m3, while in the case of the photo/Fenton process, the cost was only RMB9.43/m3 when met with the same COD discharge requirement. From the perspective of economy, the photocatalytic process reduced the cost of H2O2 in the subsequent Fenton process, thereby resulting in lower treatment cost. This indicated that the photo/Fenton process was more economical for the degradation of oilfield wastewater. Thus the photo/Fenton process can be potentially used in industrial applications.

Table 2

Consumption of the reagents and their costs

 Dosage (kg/m3)
 
Treatment technologyHClNaOHFeSO4·7H2OH2O2UV (W)Operating cost (RMB/m3)
Fenton process 0.015 0.05 10 10.58 
Photo/Fenton process 0.015 0.05 1.793 7.986 300 9.43 
 Dosage (kg/m3)
 
Treatment technologyHClNaOHFeSO4·7H2OH2O2UV (W)Operating cost (RMB/m3)
Fenton process 0.015 0.05 10 10.58 
Photo/Fenton process 0.015 0.05 1.793 7.986 300 9.43 

Note: Unit price of reagent (RMB/t): HCl, 100; NaOH, 2,000; FeSO4·7H2O (99.0 wt.% purity), 240; H2O2 (30%, w/w), 1,000; UV lamp (300 W), 0.5 RMB (KW/h).

CONCLUSION

On the basis of the experimental data obtained in this study, the degradation of guar gum has been studied by applying homogeneous Fenton and photo-Fenton processes. The results showed that the photo-Fenton process was more efficient than the Fenton process for COD removal. In the photo-Fenton process, the degradation rate (kapp) was increased by 25.7%, which reduced the operating cost greatly. Furthermore, the COD and color removal increased with the increase of the concentration of hydrogen peroxide, the ferrous ion as catalyst accelerated the COD removal, and Fe2+ concentration of 2,000 mg/L could be used as an optimum dosage for the Fenton process. Furthermore, the optimum pH for both COD and color removal was 3. In addition, the degradation rate was significantly influenced by the temperature with an optimum value of 35 °C.

The costs of the photo/Fenton process amounted to RMB9.43/m3. Therefore, the photo-Fenton process can be considered as an effective, alternative and economic method for the treatment of oilfield wastewater.

ACKNOWLEDGEMENTS

The authors thank the kind help of China National Offshore Oil Corporation in Tianjin for providing water samples.

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