Fenton process enhanced by metal sulfide for treating the actual evaporated mother liquid of gas field wastewater

Natural gas is considered low-carbon clean energy that ranks between traditional fossil fuels, such as oil and coal, and renewable energy. Natural gas emits less carbon than coal and oil and is cheaper than renewable energy, making it a suitable platform to foster China's energy transition toward achieving effective air pollution control and carbon neutrality (Ibrahim et al. 2022). China's natural gas production has increased recently, reaching 1,925.0 billion m³ at the end of 2020 (Resources 2021). However, during natural gas extraction, a huge volume of gas field wastewater is generated, including drilling wastewater, fracturing flow-back fluid, and high-salt wastewater (Feng et al. 2020; Jin et al. 2022). In addition, the triple-effect evaporation process has been identified as an effective technique for treating gas field wastewater due to the operability of evaporation equipment at the site of the gas field project. However, the evaporated mother liquor of gas field wastewater (EML-GFW), the concentrated liquid remaining after distillation, contains a range of organic compounds and inorganic salt ions at significantly higher concentrations than raw water (Wang et al. 2019; Feng et al. 2022).

There are only a handful of studies conducted on the actual EML-GFW due to the complexity of actual wastewater. EML-GFW removal using traditional adsorption methods, air flotation separation, and coagulation could only transform the phases of pollutants, thus necessitating additional treatment. In addition, a high-salt concentration leads to membrane fouling, thermal equipment scaling, and microbial death, thus hindering the application of these methods in EML-GFW treatment (Moussa et al. 2006; Osaka et al. 2008; Sheng et al. 2022).

The Fenton method is an established advanced oxidation technique (AOP) used to degrade refractory organic pollutants due to its strong oxidizing ability, fast reaction speed, and high treatment efficiency (Adil et al. 2020; Wang et al. 2021; Ziembowicz & Kida 2022). However, the decomposition efficiency of H₂O₂ in conventional Fenton is limited because of the weak cyclic reaction of Fe²⁺/Fe³⁺, which requires a catalyst or activator to accelerate Fe²⁺ regeneration (Liu et al. 2018; Ren et al. 2022). Metal sulfides are abundant in nature and have good electrical conductivity and strong stability, making them an ideal catalyst candidate to accelerate the speed and efficiency of pollutant removal by AOP (Fu & Lee 2019; Zhu et al. 2020; Li et al. 2021; Kumar et al. 2022). A diversity of metal sulfides have been reported to act as excellent catalysts to enhance •OH production, including CuS, MoS₂, WS₂, ZnS, Cr₂S₃, CoS₂, PbS, and so on. Xing et al. used the Fenton methods enhanced by various metal sulfides (MoS₂, WS₂, ZnS, Cr₂S₃, CoS₂, and PbS) to treat 20 mg/L rhodamine B (RhB). The results revealed that not only had the removal efficiency of RhB reached more than 90%, but also the reaction rate constant of RhB removal by the Fenton/MoS₂ method (3.7 × 10⁻²/s) was 18.5 times higher than that of the conventional Fenton method (0.2 × 10⁻²/s) (Xing et al. 2018). Dong et al. reported that in the WS₂ co-catalytic photoassisted Fe(II)/H₂O₂ Fenton system, accompanied by improved H₂O₂ decomposition efficiency, it was able to oxidize phenol (10 mg/L) and reduce Cr(VI) (40 mg/L) simultaneously, with removal efficiencies of 80.9 and 90.9%, respectively (Dong et al. 2018).

Four metal sulfides (CuS, ZnS, MoS₂, and WS₂) were used to enhance the Fenton process and to determine the process with the highest efficiency for actual EML-GFW. Based on the fundamental factors, namely the reaction time, initial pH of the solution, and the dosage of H₂O₂ and metal sulfides, optimization of the metal sulfide-enhanced process was conducted. Finally, simulated wastewater of characteristic pollutants, N,N-dimethylethanolamine (DMEA), was prepared to explore the TOC removal mechanism by Fenton/ZnS without the interference of complex organic matters in actual EML-GFW. This research offers theoretical insight into the implementation of the Fenton process, particularly for treating organic wastewater with complex constituents.

Sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂, 30%, w/w), ferrous sulfate heptahydrate (FeSO₄·7H₂O), tert-butanol ((CH₃)₃COH), copper sulfide (CuS), molybdenum disulfide (MoS₂), zinc sulfide (ZnS), tungsten disulfide (WS₂), dimethylamine ((CH₃)₂NH), N,N-dimethylethanolamine ((CH₃)₂NCH₂CH₂OH) used were all of the analytically pure (AR) grade. Distilled water was used in the experiments.

The pH of 100 mL of EML-GFW was adjusted to 4.0 with sulfuric acid and subsequently added with 22 g/L FeSO₄·7H₂O and 20 g/L metal sulfides (CuS, ZnS, MoS₂, or WS₂) with vigorous stirring. After a reaction with 0.8 mol/L H₂O₂ for 3 h, the Fenton reaction was terminated. The supernatant of the precipitated solution was passed through a filter with a pore of 0.45 μm in diameter to measure TOC, followed by the screening of metal sulfides with the best treatment efficiency. Then, the effects of reaction time (1, 2, 3, 4, 5, and 6 h), initial pH of the solution (2.0, 3.0, 4.0, 5.0, 6.0, and 7.0), the dosage of H₂O₂ (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 mol/L), FeSO₄·7H₂O (10, 15, 20, 25, 30, 35, and 40 g/L) and metal sulfide (0, 2, 4, 6, 8, 10, and 12 g/L) on EML-GFW were investigated. All experiments were performed three times to ensure the accuracy and reliability of the results.

Based on DMEA concentration in the actual wastewater (EML-GFW), the simulated wastewater (DSW) was prepared with 386 mg/L DMEA. Fenton/ZnS process dosages were adjusted in proportions equal to the TOC ratios of EML-GFW and DSW.

The pH of DSW was adjusted to 3.0, and 1 mol/L tert-butanol was added. After fully dissolving 4.6 g/L FeSO₄·7H₂O and 1.5 g/L metal sulfide in DSW, 0.185 mol/L H₂O₂ was slowly added. Following the termination of the Fenton reaction, the supernatant was passed through a filter membrane with a pore size of 0.45 μm and subjected to DMEA concentration determination.

The foil wrap was used to block the light and the next steps were performed according to Section 2.2.2. All experiments were conducted three times to ensure the accuracy and reliability of the results.

TOC measurement (vario TOC select, Germany) was conducted according to the combustion oxidation-non-dispersive infrared absorption method. pH was measured by a microelectrode system (unisense mmm 7221, Danish).

EML-GFW samples were subjected to solid-phase micro-extraction and analyzed by gas chromatography-mass spectrometry (GC-MS) to qualitatively determine the compositions of organic compounds. GC-MS analysis was carried out using the following protocols: DB-624UI (60 m × 0.25 mm × 1.40 μm) column; temperature procedures: 40 °C for 5 min, followed by a gradual increment to 150 °C at a rate of 5 °C/min, the final increment to 280 °C at a rate of 10 °C/min, and a holding time of 7 min; high-purity helium as a carrier gas; a diversion ratio of 3.0. The full scan mode was adopted at the scan range of 33–500 amu. Finally, the retention time of the compound was compared with the NIST database to determine the quality of the results. These tests were completed at the Analytical Testing Center of Sichuan University.

DMEA concentration was determined by gas chromatography using the following parameters: CAM (30 m × 0.32 mm × 0.25 μm) column; temperature procedure: 60 °C for 5 min, followed by the increment to 200 °C at a rate of 20 °C/min, and a holding time of 4 min. The injection volume was 1 μL and nitrogen was used as the carrier gas at a flow rate of 25 cm/s.

The optimization of the Fenton/ZnS process was conducted based on several factors: reaction time, initial solution pH, as well as the dosages of H₂O₂, FeSO₄·7H₂O, and ZnS.

ZnS addition was beneficial to improve the treatment effect of the Fenton/ZnS process on EML-GFW and the influence of its dosage on wastewater treatment is demonstrated in Figure 3(e). With the increase of ZnS dosage, TOC removal efficiency exhibited a trend of increasing, followed by stabilizing. The maximum TOC removal efficiency was 74.5% when ZnS dosage was increased from 0 to 12 g/L. The photocatalytic activity of ZnS helped improve the oxidation activity of Fenton because ZnS contributed to Fe³⁺ reduction to Fe²⁺ when illuminated, and the regenerated Fe²⁺ continued to react with H₂O₂ to generate more •OH, thus promoting the continuous progress of the Fenton reaction and the mineralization of organic pollutants (Xing et al. 2018). However, when ZnS was added in excess, TOC removal effect was not improved due to the limited oxidant H₂O₂ content in the solution. Meanwhile, the excess ZnS led to turbidity, which hindered photogenerated radiation and subsequently decreased catalyst efficiency. In addition, the formation of particle agglomerates impaired the excitation process and the formation of electron/hole pairs due to ZnS overdosing (Suave et al. 2018; Garg et al. 2019; Manny Porto Barros et al. 2023). Compared with the single Fenton method, the Fenton/ZnS process with 10 g/L ZnS addition could improve the TOC removal efficiency of EML-GFW by 32.9%.

Under optimized conditions, i.e., initial pH of 3.0, 30 g/L of FeSO₄ 7H₂O, 10 g/L ZnS, and 1.2 mol/L H₂O₂, despite the presence of high salt and multiple organics in EML-GFW, TOC removal efficiency by the Fenton/ZnS process could reach up to 74.5% after 3 h of reaction.

DMEA was undetected in EML-GFW and DSW subjected to the optimized Fenton/ZnS process and TOC removal efficiencies were recorded as 74.5 and 46.1%, respectively, indicating the easy removal yet difficult mineralization of DMEA. The main organic compounds identified in DSW subjected to the Fenton/ZnS process include dimethylamine, with a relative percentage of 90.8%, suggesting that part of DMEA might have completely mineralized to produce CO₂ and H₂O, and the rest was mainly oxidized to form dimethylamine, which remained in the solution.

The effects of the Fenton process enhanced by metal sulfides (CuS, ZnS, MoS₂, and WS₂) on actual EML-GFW were investigated, revealing that the optimum condition was achieved by applying the Fenton/ZnS process. The TOC removal efficiency of EML-GFW reached 74.5% when the process was carried out with an initial pH of 3.0, and the doses of FeSO₄·7H₂O, ZnS, and H₂O₂ used were 30, 10 g/L, and 1.2 mol/L, respectively. Furthermore, GC-MS analysis identified DMEA as one of the main pollutants of EML-GFW at different time points. Finally, the mechanism of DSW treatment via the Fenton/ZnS process was proposed. The photocatalytic activity of ZnS under illumination could accelerate the Fenton reaction by promoting the production of the active component, •OH, increasing the removal efficiency of TOC. In summary, the Fenton/ZnS process may allow for a better treatment efficiency of the actual wastewater EML-GFW composed of complex organic contaminants in the laboratory, offering valuable insights for application in wastewater treatment at the gas field project site.

B.Y. conceptualized the system, investigated the study, did a formal analysis, and wrote the original draft. Y.C. and M.L. supervised the study, wrote the review, edited the file, managed resources, and acquired funds. M.W. investigated the study, wrote the review and edited the file.

This study was supported by the National Major Science and Technology Project of the 13th Five-Year Plan ‘High-efficiency development of ultra-deep bio-herm gas reservoirs with bottom water’ of China (2016ZX05017-005).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.