Pretreatment of shale gas drilling flowback fluid (SGDF) by the microscale Fe0/persulfate/O3 process (mFe0/PS/O3)

Performances and mechanisms of the peroxymonosulfate/ferrate(VI) oxidation process in real shale gas flowback water treatment

Shale gas flowback water (SGFW), which is an inevitable waste product generated after hydraulic fracturing during development, poses a severe threat to the environment and human health. Managing high-salinity wastewater with complex physicochemical compositions is critical for ensuring environmental sustainability of shale gas development. Desalination processes have been recommended to treat SGFW to adhere to the discharge limits. However, organic fouling has become a significant concern in the steady operation of desalination processes, and the effective removal of organic compounds is challenging. This study aimed to develop an effective oxidation method to mitigate membrane fouling in real SGFW treatment process. It adopted the peroxymonosulfate (PMS)/ferrate (Fe(VI)) process, involving both free and non-free radical pathways that can alleviate the negative effects of high-salinity environments on oxidation. The operating parameters were optimized and removal effects were examined, while the synergistic oxidation mechanism and organic conversion of the PMS/Fe(VI) process were also analyzed. The results showed that the PMS/Fe(VI) process exhibited a synergistic effect compared with the PMS and Fe(VI) processes alone, with a total organic carbon (TOC) removal efficiency of 46.8% under optimal reaction conditions in real SGFW. In the Fe(VI)/PMS process, active species such as Fe(V)/Fe(IV), ·OH, and SO4-· were jointly involved in the oxidation of organic matter. Additionally, 99.5% of the total suspended solids and 95.2% of Ba2+ in the SGFW were removed owing to the formation of a coagulant (Fe3+) and SO42- during the reaction. Finally, an ultrafiltration membrane fouling experiment proved that oxidation processes can increase the membrane-specific flux and alleviate fouling resistance. This study can serve as a reference for the design of real SGFW treatment processes and is significant for the environmental management of shale gas development.

Fouling and chemically enhanced backwashing performance of low-pressure membranes during the treatment of shale gas produced water

The treatment of shale gas produced water (SGPW) for beneficial reuse is currently the most dominant and economical option. Membrane filtration is one preferred method to deal with SGPW, but membrane fouling is an unavoidable problem. In this study, two types of ultrafiltration (UF) membranes and one type of microfiltration (MF) membrane were investigated to treat SGPW from Sichuan basin. Results showed that increased total dissolved solid (31-40 g/L) and UV₂₅₄ (10-42.9 m^-1) were observed for the same shale gas plays, and the primary fluorescent organic substances were humic acid-like components. Compared to UF membranes with the flux decline by 2% to 60%, MF membranes with larger pore size were more likely to be fouled with the flux decline by 43% to 95%. Cake layer filtration was verified to be the primary membrane fouling mechanism. Statistical analysis showed that UV₂₅₄ played the most significant role in membrane fouling which had the highest correlation (0.76 to 0.93). Compared to permeate backwashing (13%), deionized water backwashing and chemically enhanced backwashing (CEB) using NaClO, H₂O₂ and citric acid improved the cleaning efficiencies (31%-95%). CEB using NaOH prepared by deionized water aggravated membrane fouling, while excellent cleaning efficiencies (39%-79%) were observed for CEB using NaOH prepared by permeate. The difference in cleaning behaviors for fouled membranes by SGPW was verified by morphology observation and element composition analysis.

Detection and treatment of organic matters in hydraulic fracturing wastewater from shale gas extraction: A critical review

Although shale gas has shown promising potential to alleviate energy crisis as a clean energy resource, more attention has been paid to the harmful environmental impacts during exploitation. It is a critical issue for the management of shale gas wastewater (SGW), especially the organic compounds. This review focuses on analytical methods and corresponding treatment technologies targeting organic matters in SGW. Firstly, detailed information about specific shale-derived organics and related organic compounds in SGW were overviewed. Secondly, the state-of-the art analytical methods for detecting organics in SGW were summarized. The gas chromatography paired with mass spectrometry was the most commonly used technique. Thirdly, relevant treatment technologies for SGW organic matters were systematically explored. Forward osmosis and membrane distillation ranked the top two most frequently used treatment processes. Moreover, quantitative analyses on the removal of general and single organic compounds by treatment technologies were conducted. Finally, challenges for the analytical methods and treatment technologies of organic matters in SGW were addressed. The lack of effective trace organic detection techniques and high cost of treatment technologies are the urgent problems to be solved. Advances in the extraction, detection, identification and disposal of trace organic matters are critical to address the issues.

Towards improved characterization of the fate and impact of hydraulic fracturing chemicals to better secure regional water quality

Hydraulic fracturing (HF) of shale and other permeable rock formations to extract gas and oil is a water-intensive process that returns a significant amount of flowback and produced water (FPW). Due to the complex chemical composition of HF fluids and FPW, this process has led to public concern on the impacts of FPW disposal, spillage and spreading to regional freshwater resources, in particular to shallow groundwater aquifers. To address this, a better understanding of the chemical composition of HF fluid and FPW is needed, as well as the environmental fate properties of the chemical constituents, such as their persistence, mobility and toxicity (PMT) properties. Such research would support risk-based management strategies for the protection of regional water quality, including both the phase-out of problematic chemicals and better hydraulic safeguards against FPW contamination. This article presents recent strategies to advance the assessment and analysis of HF and FPW associated organic chemicals.

Cu-coupled Fe/Fe3C covered with thin carbon as stable win-win catalysts to boost electro-Fenton reaction for brewing leachate treatment

The electro-Fenton oxidation is one of the powerful approaches for achieving the complete mineralization of organic pollutants in water. The key dilemma for efficient industrial application of electro-Fenton oxidation is the complicated post-processing of iron sludge, and the cost and risk associated with H₂O₂ transportation and storage. Herein, Cu-coupled Fe/Fe₃C covered with carbon layer on carbon felt (Cu-Fe/Fe₃C@C), engineered by a hydrothermal reaction followed by the consequent thermal-treatment in N₂ atmosphere, as a self-supported integrated cathode were used for an onsite oxygen reduction reaction and a Fenton oxidation reaction. Experimental evidences demonstrate that, at the operating potential of -1.1 V, Fe₃C can selectively catalyze O₂ into H₂O₂ by 2e reduction pathways with assistance of metal Cu. Meanwhile, metal Fe and Cu incorporated into Cu-Fe/Fe₃C@C simultaneously motivate the onsite Fenton oxidation arose by H₂O₂. Such a win-win catalyst presented high activity in the electro-Fenton process. In acidic environment, the efficient mineralization rate of methylene blue, nitrobenzene, phenol, and bisphenol A can reach more than 70% in 60 min, as well as the excellent stability and durability due to the protection of graphited carbon layer. Compared with tradition electrochemical degrade system, the prepared Cu-Fe/Fe₃C@C electrode as cathode for practical refractory brewing leachate treatment reveal more efficient decolorization and mineralization, saving 14.3% of electricity.

Nano-Fe mediated treatment of real hydraulic fracturing flowback and its practical implication on membrane fouling in tandem anaerobic-oxic membrane bioreactor

The rising water-use intensity, and lack of cost-effective treatment strategy and reuse of hydraulic fracturing flowback (HFF) has become an increasing cause of concern. The present work evaluates the integration of parallel sets of tandem anaerobic-oxic membrane bioreactor (AMBR) with and without nano-Fe for treatment and reuse of real HFF obtained from Ordos Basin, China. Treatment efficiencies in terms of organic conversions, micro-pollutants degradation, resource recovery, and effects of nano-Fe release on membrane fouling were evaluated. Nano-Fe mediated AMBR (FAMBR) system effectively reduce target micro-pollutants (such as Acenaphthylene) at 94.4 % compared to the parallel AMBR system (17.1 % without nano-Fe). Moreover, recovery of potential economic chemicals like Al and P (1.0 and 0.6 mg/g spent nano-Fe) availed using FAMBR system. However, colonization of FAMBR membrane surface by Fe-protein/peptide hydroxocomplexes initiated by Fe-catalyzed microbial extrusions present a huge fouling challenge relative to the AMBR system. Additional evidences from microscopic/spectroscopic analysis of the FAMBR membrane system revealed that despite having a promising outlook, mediation of nano-Fe with AMBR system might result in a major fouling event during HFF treatment. Engineered design of nano-Fe to reduced leached nano-Fe ions in pre-treatment step prior to AMBR treatment system may be of potential research consideration.

Synthesis of Nano-Fe@NdFeB/AC magnetic catalytic particle electrodes and application in the degradation of 2,4,6-trichlorophenol by electro-assisted peroxydisulfate process

The coupling of electrolysis and the peroxydisulfate (PDS) activation was selected in this study to degrade solution-phase 2,4,6-trichlorophenol (TCP). To enhance the PDS activation efficiency and catalytic recycling ratio, a novel magnetic activator, nano iron coated on neodymium iron boron/activated carbon nanocomposite (Nano-Fe@NdFeB/AC), was synthesized and utilized as catalytic particle electrodes. To increase the mass transfer ability, a novel magnetic internal circulation electrolytic reactor (MICE) was established. The results indicated that globular Fe, with sizes ranging from 25 nm to 300 nm, is present on the surface of the catalyst. This catalyst has sufficient magnetism to be separated by the magnetic separation method and its specific saturation magnetization and residual magnetization were 1.48 and 0.26 emu/g, respectively. At the optimal condition of [pH]₀= 9.0, [Na₂S₂O₈]₀= 2.0 mmol/L, [Nano-Fe@NdFeB/AC]₀= 5.0 g/L and I = 50 mA, the TOC percentage of removal could reach 84% after 30 min of reaction. The TCP mineralization follows pseudo-first-order kinetics. The intermediate products of 2,6-dichloro-2,5-cyclohexadiene-1,4-dione, Tetrachloro-hydroquinone, and 2,3,5,6-tetrachloro-p-benzoquinone were found during the reaction. TCP mineralization was confirmed to have a hybrid mechanism involving reductive dechlorination with Fe, •OH addition oxidation and electron capture by SO•₄ ^-. This study provides a new method for the treatment of degradation-resistant pollutants.

Alternated biodegradation of naphthalene (NAP), acenaphthylene (ACY) and acenaphthene (ACE) in an aerobic granular sludge reactor (GSBR)

The paper presents quantitative changes of selected 2- and 3-ring PAHs after process phases of GSBR reactor. The studies have been carried out for 264 cycles of GSBR reactor, during which concentration of naphthalene was increased in the range of 3.00-710.00 μg/L, acenaphthylene 1.00-160.00 μg/L, acenaphthene 3.00-440.00 μg/L. GSBR operating cycle consisted of filling (30 min), mixing (90 min), aeration (540 min), sedimentation (10 min), decanting (30 min) and downtime (20 min) phases. Activated sludge dry mass concentration was 4.00 kg/m³. Conducted studies showed that in GSBR reactor naphthalene was degraded with the highest intensity. Results of the statistical analysis confirmed that naphthalene concentrations were statistically significantly different (α = 0.05) after each individual GSBR process phase, while in case of acenaphthene and acenaphthylene, the differences were observed only between mixing and aeration phases. Additionally, equations estimating concentrations of PAHs in treated wastewater were developed. Selected activated sludge technological parameters (sludge volume index, sludge and hydraulic retention time) and concentration of PAHs were used for equations. The R² coefficients of equations were above 0.99, which indicates a good adjustment of estimation to observed values.

The efficient degradation of organic pollutants in an aqueous environment under visible light irradiation by persulfate catalytically activated with kaolin-Fe2O3

Characterizations and properties of catalysts.

Extended Fenton's process: toward improving biodegradability of drilling wastewater

In this study, an extended Fenton process was used to improve biodegradability of the waste drilling mud containing bio-refractory polymers. Variation of biodegradability and organics with different molecular weights with the oxidation time were investigated during the Fenton oxidation process. Although the residual total organic carbon (TOC) arrived at a stable level soon after oxidation reaction, organics with the lower molecular weight increased and its biodegradability was improved significantly in the extended oxidation process, which originated from decomposition of residual H₂O₂ catalyzed by transformation of the Fe³⁺/Fe²⁺ and organoradicals. Under the conditions that follow: pH 3.0, H₂O₂ 500 mg L^-1, Fe²⁺ 250 mg L^-1, oxidation time 120 min, further TOC removal of 35.9% and biochemical oxygen demand and total organic carbon (BOD/TOC) ratio of 0.83 was achieved. At the biological test, a substantial increase in TOC degradation by biological treatment with extension of Fenton oxidation time was observed. Finally, more than 90% biological removal of the TOC was achieved for the 120 min oxidation treatment. The experimental results highlight that an extended process can be adopted to improve the biodegradability of wastewater by utilization of the slow reaction of hydrogen peroxide with Fe³⁺ and organoradicals.

Oxidative removal of recalcitrant organics in shale gas flowback fluid by the microwave-activated persulfate process

Shale gas flowback fluid (SGF) is generated during shale gas extraction and typically contains a variety of toxic and refractory organic compounds. In this work, a microwave-activated persulfate process (MW-PS process) was developed to pretreat SGF. The major factors influencing the treatment efficiency of the MW-PS process (PS dose, initial pH, MW power, and reaction time) were optimized, and the synergetic effect (SE), degradation of recalcitrant matter, and energy consumption were systematically investigated. Results showed that the SE of the process reached a high index (i.e., 9.85), suggesting a significant synergetic effect of MW and PS. In addition, under the optimal MW-PS condition (PS dose of 2.5 g/L, MW power of 900 W, and initial pH of 2), chemical oxygen demand removal reached 66.40% in a short reaction time of 10 min. Other analyses demonstrated that benzene series compounds, organic acids, lipid substances, alkanes, antioxidants, and fluorescent dissolved organic matter in SGF were decomposed to smaller-molecule organic matter, suggesting that refractory and toxic organic matter was removed by the MW-PS treatment process. Overall, the results of this study showed that MW-PS technology is an effective and promising method to treat SGF once the operation parameters are optimized.

Natural mackinawite catalytic ozonation for N, N-dimethylacetamide (DMAC) degradation in aqueous solution: Kinetic, performance, biotoxicity and mechanism

To enhance the degradation of N, N-dimethylacetamide (DMAC) in aqueous solution, the natural mackinawite (NM) is introduced for catalytic ozonation in this study as it is an environmentally friendly catalyst with low cost and easy availability. The properties of the NM were initially characterized via scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Then, impact factors including NM dosage, ozone gas concentration and initial pH were investigated and the optimal conditions (i.e., NM dosage = 3.5 g/L, ozone gas concentration = 300 L/min, initial pH = 6.8) were obtained in NM/O3 process. Besides, the superiority of the NM/O₃ process was confirmed by the experiments that the degradation efficiency of DMAC in the NM/O₃ process (i.e., 95.4%) was much higher than that in the zero-valent iron (ZVI)/O₃ process (i.e., 46.1%) and the synthetic FeS/O₃ process (i.e., 68.6%). Furthermore, the intermediate and possible degradation pathway of DMAC were proposed, and the biological toxicity of the intermediate was subsequently evaluated by the activated sludge. Finally, the mechanism of the NM/O3 process was proposed in this study based on control experiment and radical scavenging experiment. The extraordinary efficiency for DMAC degradation was found to be mainly caused by HO^• of the reactive oxygen species (ROS) (i.e., HO^•, O₂^•- and H₂O₂) generated in the NM/O₃ process. Therefore, this study confirmed that NM was a high efficient catalyst for degradation the toxic and refractory pollutants in catalytic ozonation system.

Pretreatment of water-based seed coating wastewater by combined coagulation and sponge-iron-catalyzed ozonation technology

Coagulation-sedimentation combined with sponge iron/ozone (CS-SFe/O₃) technology was applied to pretreat water-based seed coating wastewater (WSCW) from pesticide manufacturing. Coagulation with polyferric sulfate at a dosage of 1.5 g L^-1 and a pH of 8.0 was effective, with color and chemical oxygen demand (COD) removal rates of 96.8 and 83.4%, respectively. SFe/O₃ treatment further reduced the organic content in the effluents, especially concerning the degradation of aromatic pollutants, as demonstrated via ultraviolet-visible spectrophotometry (UV-vis), excitation-emission matrix (EEM) fluorescence spectrometry, and gas chromatography-mass spectrometry (GC/MS) analyses. The residual color and COD values of the effluent were 581.0 times and 640.0 mg L^-1, respectively, under optimal conditions (ozone concentration of 0.48 mg L^-1, SFe dosage of 20.0 g L^-1, initial pH of 9.0, and reaction time of 30 min). Organic pollutants were also degraded by the high amounts of HO, which may have been generated via the transformation of ozone into HO on the SFe's surface and in the solution. Meanwhile, the biochemical oxygen demand (BOD₅)/COD ratio of the WSCW increased, which indicates that the biodegradability improved significantly. The amount of iron leached from SFe particles was 4.5 mg L^-1, which shows that the SFe catalyst has good stability. The operating cost of the combined CS-SFe/O₃ technology was estimated at approximately 2.79 USD t^-1. The results of this study suggest that the application of the combined CS-SFe/O₃ technology in WSCW pretreatment can be beneficial for removing suspended solids, degrading recalcitrant pollutants, and enhancing biodegradability for the subsequent bioprocessing treatment.