Degradation of hydroxypropyl guar gum at wide pH range by a heterogeneous Fenton-like process using bentonite-supported Cu(0)

Hydraulic fracturing of wells during oil and gas exploration consumes a lot of fresh water and generates large volumes of polluted wastewater (Hickenbottom et al. 2013). The development of unconventional oil and gas resources usually results in the production of wastewater containing chemical additives and components from the depths of the formation (Mumford et al. 2018), such as oil, polyacrylamide, methanol, emulsions, cuttings and clay particles. Under high temperature working conditions, the oil extraction wastewater is characterized as corrosive due to high chemical oxygen demand (COD), high salinity and strong acidity (Dai et al. 2019). If the wastewater is not disposed of in time, after a long period of storage, the release of a strong smell into the environment will become a threat to public health and cause environmental pollution. Hydroxypropyl guar gum (HPGG), polyacrylamide (PAM) and carboxymethyl cellulose (CMC) are widely used as thickeners during fracturing (Tang et al. 2019); they are the main cause of high COD and are hard to deal with effectively.

Currently, the policy agenda of many countries gives priority to wastewater treatment for environmental protection development (Kunjachan et al. 2017). Advanced oxidation processes, especially Fenton oxidation, have been shown to be effective and suitable processes for the abatement of persistent and recalcitrant water pollutants (Goi et al. 2008; Trapido et al. 2009; Dulova et al. 2011). However, traditional homogeneous Fenton catalysts work only in limited pH range to avoid the precipitation of iron, and the production of iron sludge needs a separation (Yang et al. 2015), which makes them uneconomical and limited in application. Heterogeneous Fenton-like processes have the advantage that they produce minimal sludge and coproducts (Zárate-Guzmán et al. 2020) and the working pH range is relatively wide (pH 3–7) (Lam et al. 2007); thus they are considered to be environmentally friendly. In the progress of the technology, the combinations of Fenton-like oxidation and zero-valent metals, such as iron, copper, nickel and zinc, have played a significant role in organic wastewater treatment (Chand et al. 2009). ElShafei et al. (2017) prepared the nano-zero valent metals of Fe, Cu, and Ni and examined them in degradation of nonylphenol (6 mg/L) by ultrasonic-assisted (20 kHz) Fenton-like process at the neutral pH under room temperature. Similarly, Babuponnusami & Muthukumar (2012) presented the removal of phenol using nano-zero valent iron in a heterogeneous photoelectro Fenton-like system, and they found that the removal efficiency was increased with an increase of nano-zero valent iron dosage, whereas it decreased with increase of initial phenol concentration and initial pH. However, it should be pointed out that zero-valent metals has some application limitations such as easy aggregation (Chen et al. 2011), difficult separation (Pliego et al. 2015) and low stability (Shi et al. 2011). Moreover, few research studies have explored the effective degradation of oilfield polymers containing HPGG through the heterogeneous Fenton-like processes using zero-valent metal catalyst at wide pH range. Therefore, it is meaningful to explore an appropriate heterogeneous catalyst to degrade wastewater of oil well drilling to an acceptable level.

Recently, it has been found that the dispersion of zero-valent metal particles can be enhanced by using porous materials as mechanical supports (Uezuem et al. 2009). Bentonite can be used as a good carrier due to its abundance, unique structural characteristics and good mechanical stability. In our research, five zero-valent metals are supported on the surface of bentonite as active phase to promote the heterogeneous Fenton-like reaction. The present study focuses on the following four objectives: (1) synthesis and screening of the bentonite-supported zero-valent metals; (2) characterization of the optical catalyst with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) technology; (3) evaluations of the relevant factors on HPGG degradation, such as H₂O₂ concentration, catalyst dosage, the temperature and pH; (4) free radical analysis and stability evaluation of the optimal catalyst.

All chemicals used were of analytical grade without further purification. Hydrogen peroxide solution was 30% volume ratio to water. Bentonite was obtained from Fengyun Chemical Co., Ltd, Xi'an, China. HPGG with molecular weight of about 2 million was obtained from Changqing Oilfield. CMC and PAM (purity > 95%) were obtained from Xinhe Environmental Protection Co., Zhengzhou, China.

The bentonite-supported zero-valent metals were prepared by using conventional liquid-phase reduction where bentonite acted as a support material. Theoretically, the mass ratio of bentonite to metal in the supported catalyst was 1:1. Firstly, 4.53 g FeCl₂, 4.41 g CoCl₂, 4.28 g NiCl₂, 4.23 g CuCl₂ and 4.17 g ZnCl₂ were dissolved in a 100 mL beaker with distilled water, respectively. Then, about 2 g of bentonite was added in turn and the solution was stirred for 24 h so as to complete full exchange of metal ions in the chloride with cations on bentonite. Subsequently, a freshly prepared 0.13 M NaBH₄ solution (100 mL) was added into the mixture with constant stirring for 30 min under nitrogen atmosphere as described by Huang et al. (2012). Finally, the formed sample was centrifuged and washed three times with absolute alcohol and dried overnight at 80 °C for further using.

SEM was performed with a JSM-6390A with 20.0 kV of an accelerating voltage. XPS measurements were carried out on an ESCALAB250Xi electron spectrometer using 300 W Al K_α radiations.

HPGG, CMC and PAM, weighing 1.2 g, were dissolved in 200 mL distilled water respectively, stirred at room temperature for 30 min and then settled for 12 h to make them completely swollen. The degradation experiments were conducted in a 10 mL Ubbelohde viscometer. Firstly, 5 mL HPGG glue (0.6%, w/w), a certain concentration of H₂O₂, a certain amount of catalyst and distilled water were added in a small beaker and mixed uniformly by stirring. Then, the mixture was poured into the Ubbelohde viscometer at desired temperature and the absolute viscosity of glue solution was measured intermittently at different periods (0, 5, 10, 15, 20, 25, 30, 35 and 40 min) (Gu et al. 2013). In Fenton-like process, H₂O₂ concentration, pH value, catalyst amount and reaction temperature were studied thoroughly because of their significant effects on oxidation capacity. The initial pH was settled at 7 and then was adjusted to a designed value by adding sodium hydroxide solution (0.1 M). The COD values of polymers before and after degradation were determined by standard potassium dichromate method (Alves et al. 2010). Excessive H₂O₂ and appropriate amount of catalyst were required in order to remove most polymers under the optimal experimental conditions. The experimental data was obtained in triplicate to ensure the relative errors could be minimized, and the results presented here represent the average values of independent measurements. Recovered experiments were performed to evaluate the stability of the supported catalyst by centrifuging after each cycle.

Figure 1 shows the degradation of HPGG gum solution with the mass concentration of 0.6% by 6,000 mg/L H₂O₂ catalyzed by the bentonite-supported Fe(0), Co(0), Ni(0), Cu(0) and Zn(0) at 45 °C and pH 7.0. It can be found that the catalytic performance of all of the bentonite-supported catalysts kept improving during the initial 20 min and remained stable after 30 min if 6,000 mg/L H₂O₂ concentration and 6 g/L of catalyst were used. Among them, the viscosity reduction rate of HPGG catalyzed by the bentonite-supported zero-valence copper could be significantly enhanced. And the highest degradation performance reflected by the greatest absolute viscosity drop of HPGG from 18 mm²/s to 1.5 mm²/s was observed after 40 min, which might be due to the large specific surface area and large number of active sites (Elshafei et al. 2014). Therefore, the bentonite-supported zero-valent copper was used as catalyst for further experiments.

The structure and morphology of the bentonite and the bentonite-supported zero-valent copper catalyst were analyzed by SEM at 20,000 times magnification and shown in Figure 2. Figure 2(a) shows that the bentonite was composed of an aggregate of flake particles with smooth surface (Neetu et al. 2009), while Figure 2(b) clearly shows that the bentonite-supported zero-valent copper catalyst appeared as scattered clusters indicating the high dispersion of zero-valent copper. Consequently, it is suggested that the prepared supported zero-valent copper catalyst could effectively promote the catalytic effects of the fracturing wastewater containing HPGG.

XPS can be used to determine the chemical composition of samples as well as the oxidation state of the species (Su et al. 2011). Therefore, XPS technology was used to characterize the bentonite-supported zero-copper, shown in Figure 3. The photoelectron peak for C 1 s at 284.8 eV is used for energy calibration. The binding energy peak at 933.67 eV can be assigned to Cu(0), indicating the presence of zero-valent copper supported on the bentonite (Zhu et al. 2016). In addition, it is worth mentioning that the small peak at 935.77 eV indicates a Cu(II) oxidation state, mainly caused by atmospheric oxygen.

An increase in the catalyst dosage is beneficial to improve the removal efficiency of the contaminant in the wastewater. However, excessive adding of catalyst may have a negative effect in the heterogeneous Fenton-like processes. Therefore, optimization of the solid catalyst should be conducted in wastewater treatment. Figure 5 shows the changes in HPGG absolute viscosity oxidized by 6,000 mg/L H₂O₂ over various amount of catalyst ranging from 0.6 g/L to 9 g/L at 45 °C and pH 7.0. Evidently, the viscosity reduction rate of HPGG could be significantly enhanced with increasing catalyst dosage in the range from 0.6 g/L to 3.0 g/L. However, lower viscosity reduction was obtained when using 3.0 g/L catalyst, so further adding of catalyst is not necessary. In fact, the generated OH· would be consumed by the excess catalyst in the aqueous solution as suggested by Wang et al. (2016). In addition, considering the preparation cost of the solid catalyst, 6.0 g/L was selected as the optimum dosage.

Temperature has a significant effect on the degradation efficiency of polymers, since the thermal degradation reaction will exhibit an Arrhenius-type behavior with increasing the activation energy (Wang et al. 2017). Considering the application temperature of HPGG in an oilfield, Figure 6 shows the variation of absolute viscosity of HPGG solution in presence of 6 g/L catalyst and 6,000 mg/L H₂O₂. The blank without H₂O₂ and catalyst showed that HPGG had limited biodegradability. However, the viscosity reduction rate of HPGG observably improved in the experiment group with the increase of temperature. Almost 79.1% of viscosity reduction rate was achieved after 40 min at a higher temperature of 45 °C. The possible reason is that higher temperature can provide enough energy to overcome the reaction activation energy and then accelerate the reaction rate by increasing the reaction rate constant according to the Arrhenius equation (Iebuegu & Ezenwa 2011). Taking energy consumption into account, 45 °C was selected as the optimum temperature in this work.

In previous studies, the researchers generally reached the conclusion that pH 3 was the best reaction condition (Yuan et al. 2013; Yang et al. 2014); however, Feng et al. (2012) revealed that neutral (pH 6) and slightly alkaline (pH 9) conditions could treat the organic waste water effectively. Since the heterogeneous reaction has revealed some satisfactory results in wastewater treatment under neutral/alkaline conditions, great effort has been made to broaden the pH range in view of the alkaline nature of the oilfield sewage. Figure 7 shows the change of absolute viscosity of HPGG solution with pH values at the same reaction condition of 6,000 mg/L H₂O₂ and 6 g/L catalyst at 45 °C. Obviously, the viscosity reduction rate increased when pH increased from 8 to 9 compared with the blank (pH = 7.0), and then decreased gradually when pH was raised from 10 to 11. The results showed that the bentonite-supported zero-valent copper catalyst has good alkali resistance, which could be attributed to the activities of zero-valent copper rather than the solubility of metal ions on the catalyst surface (Wang et al. 2016). Therefore, the most suitable pH value is selected as 9 in following experiments.

It has been widely accepted that HPGG, PAM and CMC can be used as thickeners during the oil fracturing process (Tang et al. 2019), so the three polymers were chosen to evaluate the COD removal of the bentonite-supported zero-valent copper/H₂O₂ oxidation process. As can be seen from Figure 8, the COD removal rate of the three polymers increased as the reaction time increased. Almost 76, 70 and 68% of COD removal was achieved after about 45 min of reaction for COD₀ 8,747, 7,147, and 6,800 mg/L, respectively. The maximum COD removal rate of HPGG, PAM and CMC increased to 93, 91 and 92% at the reaction time of 240 min, respectively. In fact, as shown in Figure 9, guar gum is a kind of natural linear galactomannan gum consisting of a linear backbone of β-1,4 linked mannose units and randomly attached α-1,6 linked galactose units as side chains (Moreira 2008) indicating that it is more easily degraded than the other two polymers because of the easy decomposition characteristics of side chain galactose.

In order to further understand the degradation mechanism of HPGG, radical quenching experiments were carried out. It is well known that 2-propanol can act as an effective scavenging agent for OH· (Ji et al. 2013). In this study, methanol was used to explain the role of hydroxyl radical. As depicted in Figure 10, the viscosity reduction rate of HPGG decreased with relatively low concentration of methanol (50 mM) compared with the blank group at the same time, and the degradation effect was obviously inhibited while methanol was at higher concentration (100 mM), indicating that OH· was the dominant reactive oxygen species in presence of the bentonite-supported zero-valent copper.

In heterogeneous Fenton-like processes, the stability of recycled catalyst is of great importance from synthesis and economical points of view (Saeedi et al. 2013). Hence, the recycling of the bentonite-supported zero-valent copper catalyst was investigated. The absolute viscosity of the reused catalyst, recovered by centrifuging after each cycle, was measured at 45 °C and pH 9. As shown in Figure 11, the performance of viscosity reduction was almost unaffected after being reused twice at 45 min, changing from 85.2% to 82.1%, which showed the good stability of the supported catalyst in the application process. After five consecutive cycles, the viscosity reduction ratio decreased probably caused by the reduction of the specific surface area of the bentonite-supported zero-valent copper.

In this paper, five bentonite-supported zero-valent metal catalysts were prepared for catalytic degradation of HPGG solution so as to achieve the purpose of polymer wastewater treatment at wide pH range. The results showed that the bentonite-supported zero-valent copper exhibited high catalytic performance reflected by a large absolute viscosity drop of HPGG from 18 mm²/s to 1.5 mm²/s within 40 minutes. The high efficient degradation performance could be attibuted to high dispersion morphology of the supported Cu(0) catalyst, and the XPS analysis demonstrated the presence of zero-valent copper on bentonite. Over 76% COD removal rate of the HPGG can be achieve when 9,000 mg/L H₂O₂ and 6 g/L bentonite-supported Cu(0) were used after 45 min at 45 °C and pH 9.0. The results will provide an attractive method for fracturing wastewater treatment containing HPGG in heterogeneous Fenton-like systems.

This work was financially supported by National Science Foundation of China (No. 21763030), Scientific Research Plan Projects of Shaanxi Science and Technology Department (2019GY-136), Xi'an Science and Technology Project (201805038YD16CG22(3)) and Postgraduate Innovation and Practical Ability Training Project of Xi'an Shiyou University (YCS18211017). And we thanks the work of Modern Analysis and Testing Center of Xi'an Shiyou University.

All the data in this study are available on request to the corresponding author.