Academic Journal of Science and Technology ISSN: 2771-3032 | Vol. 7, No. 3, 2023 1 High Efficient Degradation of Oilfield Waste Water Containing Polymers By Metal‐tartaric Complex Lanbing Wu1, Rong Su2, Pengfei Hu2, Xiaoliang Yang3 and Jie Zhang1, 4, * 1Shaanxi Province Key Laboratory of Environmental Pollution Control and Reservoir Protection Technology of Oilfields, Xi’an Shiyou University, Xi’an 710065, China 2Xi’an Changqing Chemical Group Company Ltd, Changqing Oilfield Branch Company, PetroChina, Xi’an 710060, China 3No. 1 Oil Production Plant of Changqing Oilfield, PetroChina, Xi’an 710060, China 4Engineering Research Center of Oil and Gas Field Chemistry, Universities of Shaanxi Provence, Xi’an Shiyou University, Xi’an, 710065, China * Corresponding author Abstract: This To provide an efficient catalyst for Fenton oxidation processing to the treatment in waste fracturing fluid containing hydroxypropyl guar gum under high pH value, a series of metal-tartaric acid complex was prepared for catalyzing the Fenton oxidation. The results show that Fe(III)L complex demonstrate high catalytic performance for degradation of polymer in a wide pH range from 8.0 to 12.0. The viscosity of hydroxypropyl guar gum can be reduced effectively to 2.3 after adding 10.0% H2O2 (mass ratio to hydroxypropyl guar gum) and using 10.0% Fe(III)L (mass ratio to H2O2) as catalysts. The COD value of the treated hydroxypropyl guar gum solution can be decreased to 107 mg/L from 8680 mg/L. The proposed mechanism of the activation of H2O2 by the complex was studied by UV spectrum. Keywords: Fenton oxidation; Metal-tartaric acid complex; Hydroxypropyl guar gum. 1. Introduction Guar gum is a galactomannan polysaccharide that contains small amount of arabinose, glucose and uronic acid, besides galactose and mannose [1], which is derived from the seed of a leguminous plant Cyamopsistetragonolobus. It is a naturally available water-soluble gum possessing a molecular weight reported to be 200,000-250,000 [2-4]. A modification product, hydroxypropyl guar gum (HPG), is widely used in many industrial sectors such as oil recovery, fracturing fluid and drilling fluid additive, food systems, paints, mineral industry and personal care, etc due to its better solubility and thermal stability in solution [5]. In the oilfield production, many processes generate wastewaters with a wide variety of contaminants, such as hydroxypropyl guar gum, polyacrylamide (PAM) and carboxymethyl cellulose (CMC) [6], which causes limitation for field application. A method meeting these requirements and deserving attention is the Fenton oxidation process [7]. 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 [8,9]. The typical Fenton oxidation with H2O2-Fe2+ only can be used below the pH 3.0, because of the reaction stops after the consumption of Fe2+ ions due to its hydration transformation to Fe(OH)2 under higher pH value. As a result, a lot of acid should be consumed to reach the discharge standard. Some reported Fenton-like oxidation of the industry wastewater in batch experiments using metal complexes as catalysts [10-13]. As reported, the degradation can be proceeded in relatively wide pH range from 5.0 to 9.0 by catalyzed with metal complex catalysts, where in the removal efficiencies of pyridine and 3-cyanopyridine were 84% and >99%, respectively. Based on this study, the oxidative degradation of hydroxypropyl guar gum by H2O2 in relative high pH range can be further explored by using complex metals [14]. To seek for efficient Fenton oxidation processing for the treatment in waste water containing hydroxypropyl guargum and other polymers under high pH value [15,16], in this work, a series of metal-tartaric acid complex was prepared for the Fenton oxidation catalysts. The catalyst was screened by the viscosity reduction of hydroxypropylguargum solution oxidized by H2O2. The experimental conditions of Fenton oxidation were optimized in detail and the structure of prepared catalysts was characterized. Our work can be described as Fig.1. 2. Materials and Methods 2.1. Materials All of reagent were of analytical grade and were purchased without further purification. Hydroxypropyl guar gum was obtained from Changqing Oilfield with an average molecular weight of 2 million. Regent-grade hydrogen peroxide solution is 30% volume ratio to water. 2.2. Catalyst preparation and glue preparation Metal chloride solution (FeCl2, FeCl3, CuCl2, CoCl2, NiCl2 and ZnCl2) with concentration of 0.20 mol/L was added into 500mL of 0.20mol/L tartaric acid solution under stirring at molar ratio of 1:1/1:2/1:3 respective at room temperature. Then the solution was diluted to a certain concentration for catalysis. One of the categories of metal complex was shown in Fig.2. 2 Figure 1. The waste fracturing fluid and the clean processing Figure 2. Synthesis of metal-tartaric acid complex 2.3. Fenton oxidation process The polymer was dispersed in water with a certain concentration and stirred continuously at a stirring rate of 100 rpm under room temperature for 12 h to form the glue solution. Then 20 ml glue solution containing a certain concentration of H2O2 and catalyst were added in a flask and stirred to form a homogeneous mixture. The mixture was poured into an Ubbelohde viscometer at a certain temperature to measure the viscosity intermittently [17]. The pH, temperature and catalyst amount were varied systematically. After the desired duration of oxidation, solution was allowed to settle for 1h to complete the oxidation. The relative molecular mass was measured using Ubbelohde viscometer at a certain temperature [18]. The chemical oxygen demand (COD) was determined by dichromate method according the GB11914 of China and ISO6060. Electronic spectra were recorded in the range 200– 400 nm with spectral on as standard on a UV-2600 spectrophotometer. 3. Results and Discussion 3.1. Catalytic performance of different metal complexes The degradation of 0.6% hydroxypropyl guar gum solution in presence of 10% H2O2 (mass ratio to hydroxypropyl guar gum) and 10% complex (mass ratio to H2O2), Fe(II)L, Fe(III)L, Cu(II)L, Co(II)L, Ni(II)L and Zn(II)L with metal to ligand ratio of 1:1 respectively, has been investigated under 45℃ at pH 9.0. The results were summarized in Fig.3. From the result, it can be found that all of complexes show their catalytic performance to degrade hydroxypropyl guar gum in presence of H2O2 under the condition, and the viscosity of hydroxypropyl guar gum solution shows a great decrease from 14 to 1.25 when Fe(III)L was used as catalyst, which is much more potent than the others, therefore it was selected for further investigation. The experimental results also indicate that the relative viscosity of hydroxypropyl guar gum solution decreases greatly in initial reaction time of 5 minutes, and further extension of reaction time plays no more roles to the degradation process. Figure 3. Effect of different metal complexes on the degradation 3 3.2. Effect of ratio of tartaric acid to Fe(III) Considering that the metal to ligand ratio is an important structure directing factor to the process of crystallization of obtained complex, the effect of tartaric acid to Fe(III) ratio on the degradation performance of hydroxypropyl guar gum was investigated by varying the ratio from 1:1 to 1:3 under the reaction conditions of 10% H2O2 (mass ratio to hydroxypropyl guar gum) and 10% complex (mass ratio to H2O2). It was observed from Fig. 4 that viscosity of hydroxypropyl guar gum solution keeps gradually low tendency when tartaric acid to Fe(III) ratio increasing continuously up to 1:3, and it is obvious that the optimal value with tartaric acid to Fe(III) ratio is 1:2. The high degradation performance of Fe(III)L complex with metal to ligand ratio of 1:2 should contribute to the moderate coordination of tartaric acid with Fe(III) to form water coordinated Fe(III) which is easier to coordinate with H2O2. Figure 4. Effect of metal-tartaric acid complex ratio on the degradation 3.3. Effect of concentration of H2O2 In Fenton process the H2O2 dose is considered as one of the most important factors which should be considered. To optimize the catalytic performance of Fe(III)L, the concentration of H2O2 in reaction was investigated under the operating conditions . Fig. 5 shown the results of the viscosity reduction of hydroxylpropyl guar gum oxidized by various concentration of H2O2 ranged from 1% to 15%. From the results, it was found that the relative viscosity of hydroxypropyl guar gum decreased significantly from 16 to 1.75 with increasing H2O2 concentration. The higher reduce of viscosity was attained at 20 min when using 10% H2O2 concentration [19]. Further addition of H2O2 did not affect significantly the degradation of hydroxypropyl guar gum. Similar result was obtained by Wang et al. by confirming that the excessive H2O2 is not necessary due to the competing with polymers. Figure 5. Effect of concentration of H2O2 3.4. Effect of time on the concentration of H2O2 It is important to study the residual concentration of H2O2 for the better describing the influence of H2O2 as related to the plausible mechanism. Therefore, the concentration of H2O2 with time was investigated at the optimized reaction condition (a certain amount of DMP solution, copper sulfate solution, phosphate buffer and sample constant volume absorbance measurement) using DMP method at 45℃ [18]. As presented in Fig. 6 we can see obvious decline of concentration of H2O2 with time, which confirms the high catalytic role of prepared Fe(III)L to increase of produced free hydroxyl radicals as suggested in Fig. 9. Figure 6. Effect of time on the concentration of H2O2 3.5. Effect of degradation temperature The temperature plays an important role in chemical oxidation. The effect of reaction temperature was evaluated on degradation of hydroxypropyl guar gum solution using Fe(III)L as catalyst. Fig. 7 demonstrates the variation of viscosity of hydroxypropyl guar gum solution at various reaction temperatures in presence of 10% H2O2. As shown, the values of viscosity decreased when the temperature increased from 15℃ to 55℃.The values quickly decreased in the beginning 10 minutes, and then gradually drop off with the increase of time. This is consistent with the results that the oxidation degradation is an endothermal reaction, and higher reaction temperature is good to the degradation process. 4 Figure 7. Effect of reaction temperature on the degradation 3.6. Effect of 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 metal ions [19]. Several investigations have manifest that the optimum pH for the degradation of organics is in the range 8-12 [22]. The poor degradation performance at a high pH value was caused by the formation of ferrous and ferric hydroxide complexes with much lower catalytic capability than Fe2+ [21]. In our research work, the degradation performance of hydroxypropyl guar gum was examined in pH range 8-12 with the hydroxypropyl guar gum concentration of 0.6 %, and the results were summarized in Fig. 8. From the result, it can be found that the reaction was conducted under high pH conditions, and the degradation efficiency decreases with increasing the solution pH up to 14. The generation of reactive oxygen species by Fe-based catalyst is pH-dependent since it influences the dissolution and existential state of Fe(III) from composites and the evolution of the superoxide radical. It was observed that in our research the maximum hydroxypropyl guar gum degradation was obtained at pH of 9.0 [22]. Figure 8. Effect of pH value on the degradation 3.7. Proposed mechanism UV spectra have been widely used for characterization of complex substances and can provide valuable information on the structural and functional properties of complex. From the UV spectra results shown in Fig. 9, it was found that the tartaric acid and the corresponding Fe(III)L complex show similar UV spectra with obvious absorbance peaks at 220 and 275 nm are known to be π-π* transformations in the aromatic ring and chromophore, which became less intense over Fe(III)L when tartaric acid is coordination with Fe(III) as shown in Fig.2. Furthermore, the absorbance decreases obviously, and the absorption intensity of the peak at 196 nm decrease obviously when H2O2 was added in Fe(III)L solution, which indicates the coordination of H2O2 with Fe(III)L, as a result, Fe(III)L can active H2O2 to generate ∙OH radical, as shown in Fig. 10 [25].    Figure 9. UV spectra of ligand corresponding Fe(Ⅲ) complex Figure 10. The activation of H2O2 by Fe(III)-tartaric acid complex 3.8. COD removal Hydroxypropyl guar gum, polyacrylamide (PAM) and carboxymethyl cellulose (CMC) are commonly used in fracturing process, so the three polymers were choose to evaluate the COD removal of this catalytic oxidation system. The three polymers with the mass concentration of 0.6% were oxidized by 100% H2O2 (the demand of H2O2 to oxidize the polymer to CO2 and H2O) at 45℃ and pH 9 respectively, and the results were presented in Fig. 11. As can be seen, the COD values of all polymers decreased rapidly within 45 min with high decreasing ratio of 75.41%, 54.54% and 65.34% respectively, and it was decreased by 98.76%, 95.34% and 96.28% respectively within 240 min. It also can be found that hydroxypropyl guar gum is easier to be oxidized than that of others [26]. The results showed that Fe(III)L has great catalytic degradation performance to various polymers. 5 Figure 11. COD removal of Fe(Ⅲ)L catalyzed oxidation for different polymers 4. Conclusions A series of metal-tartaric acid complex was prepared for catalyzing the Fenton oxidation of fracturing waste water containing much polymer under high pH value. The catalyst was screened through the viscosity reduction of hydroxypropyl guar gum solution oxidized by H2O2, among which Fe(III)L exhibits high catalytic performance for degradation of hydroxypropyl guar gum in a wide pH range 8.0-12.0, and the viscosity of hydroxypropyl guar gum can be reduced effectively with the 10.0% H2O2 (mass ratio to hydroxypropyl guar gum) in presence of 5.0% Fe(III)L (mass ratio to H2O2). Furthermore, the COD value of various polymers can be greatly decreased from 8680 mg/L to 107 mg/L within 4h and the degradation can be proceed efficiently for various polymers at high pH value up to 9. The results offer an attractive alternative in disposing of the recalcitrant fracturing waste water. 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