Efficient degradation of polyacrylamide using a 3-dimensional ultra-thin SnO2-Sb coated electrode

Characterization and degradation mechanism of a newly isolated hydrolyzed polyacrylamide-degrading bacterium Alcaligenes faecalis EPDB-5 from the oilfield sludge

Hydrolyzed polyacrylamide (HPAM) is posing serious threats to ecosystems. However, biodegradation is an effective method to remove HPAM owing to its low cost and environmental friendliness. In this study, Alcaligenes faecalis EPDB-5 was isolated as a highly efficient HPAM degrading strain from sludge contaminated with polymerized produced water from Daqing oilfield. Under the optimal conditions, the strain EPDB-5 demonstrated an impressive HPAM degradation rate of 86.05%, the total nitrogen (TN) removal of 71.96% and chemical oxygen demand (COD) removal of 67.98%. Meanwhile, it can maintain a stable degradation rate higher than 75% under different pH and temperature conditions. 27 genes that play a key role in HPAM degradation were annotated by metagenomics sequencing. The key genes were involved in multiple KEGG pathways, including biofilm formation, biosynthesis secondary metabolites, and metabolic pathways. SEM, GPC, and FTIR analyses revealed that the structure of HPAM after biodegradation showed pores, a significant decrease in molecular weight, -NH2 detachment, and carbon chain breakage. Particularly, we propose a possible mechanism of biofilm formation - HPAM degradation - biofilm disappearance and reorganization. Moreover, the degradation rate of strain EPDB-5 on real wastewater containing HPAM was 29.97% in only three days. This work expands our knowledge boundary about the HPAM degradation mechanism at the functional gene level, and supports the potential of strain EPDB-5 as a novel auxiliary microbial resource for the practical application of HPAM.

Application of cold plasma technology for the simultaneous degradation and viscosity removal of polyacrylamide and its copolymers from contaminated wastewater

Polyacrylamide and its copolymers are routinely employed for polymer flooding, generating an emulsifying and highly viscous wastewater that warrants enhanced treatment due to its recalcitrant nature. Herein, a falling-film dielectric barrier discharge (DBD) plasma reactor was used to examine the simultaneous viscosity removal and degradation of polyacrylamide (PAM), hydrolyzed, and cationic polyacrylamide (HPAM and CPAM) wastewater. The reactor conditions (pH: 6.3, airflow rate: 3 L/min, power: 200 W, recirculation flow rate: 150 mL/min, treatment time: 20 min) led to about 45% degradation and an approximately 48% viscosity removal ratio for the polymers. The recirculation and atomizing of the solution at the nozzle ensured a uniform mixture exploiting the falling-film effect of the reactor. The indispensable role of the radicals in the plasma treatment process was confirmed by scavenger tests, with hydroxyl and superoxide radicals influencing the polymer degradation process. The degradation of the polymers and viscosity removal increased 2 folds following the 1st order kinetics when the initial pH was adjusted to 3. The degradation and viscosity removal ratios for PAM, HPAM, and CPAM were 94.33%, 92.54%, and 90.66%, respectively, and 98.58%, 97.01%, and 97.01%, respectively, with an energy yield of 14.114, 13.813 and 13.513 g/kW∙h for PAM, HPAM, and CPAM, respectively. The results will be a requisite reference for implementing the DBD technology in wastewater treatment.

Heterogeneous electro-Fenton removal of polyacrylamide in aqueous solution over CoFe2O4 catalyst

Polyacrylamide (PAM) in environmental water has become a major problem in water pollution management due to its high molecular mass, high viscosity and non-absorption by soil. CoFe2O4 with strong magnetic properties was prepared by solvent-thermal synthesis method and used as the catalyst for the removal on PAM in heterogeneous Electro-Fenton (EF) system. It showed that the removal efficiency of PAM by the heterogeneous EF system using CoFe2O4 catalyst was 92.01% at pH 3 after 120 min. Further studies indicated that ·OH was the most significant active species for the removal of PAM, and the contribution of ·O2- and SO4·- for the removal of PAM was less than 15%. The reusability test and XRD, XPS, FTIR analyses proved that the catalyst had good stability. After a repeated use for five times, the catalyst still had a high PAM removal rate and stable structure. The valence distribution and functional groups of the phase components of the catalyst did not change significantly before and after the reaction. The possible mechanism of catalyst activation of H2O2 was deduced by mechanism investigation. The CoFe2O4 is an efficient and promising catalyst for the removal of PAM wastewater.

Experimental study on reducing the viscosity of sewage containing PAM catalyzed by low temperature plasma synergistic AC/Mn + TiO2

With the wide application of polymer flooding technology in oil fields, wastewater containing PAM (polyacrylamide) is produced. Its high viscosity makes it difficult to degrade. In this paper, the low-temperature plasma produced by DBD (Dielectric Barrier Discharge) was studied to reduce the viscosity of wastewater containing PAM under the synergistic action of AC (Activated carbon)/Mn + TiO2 catalyst. The effects of different amount of AC/Mn + TiO2 catalyst, discharge voltage and initial concentration of solution on viscosity reduction were studied. The change of functional groups in wastewater containing PAM was detected by FTIR (Fourier transform infrared absorption spectrometer), and the mechanism of catalytic viscosity reduction was analysed. The AC/Mn + TiO2 catalysts were analysed by XPS (X-ray photoelectron spectroscopy), XRD (X-Ray Diffraction) and ESEM (Field emission electron microscopy). With the increase of discharge voltage, the effect of catalytic viscosity reduction is enhanced. After 10 min of discharge, the effect of catalytic viscosity reduction is significantly enhanced. The catalytic viscosity reduction is best when discharge voltage is 18 KV and discharge time is 30 min. The viscosity reduction of polyacrylamide solution by low-temperature plasma AC/Mn + TiO2 is significant. When the amount of AC/Mn + TiO2 catalyst added is 544 mg, the viscosity of polymer containing solution can be reduced from 1758 mPa·s to 11.9 mPa·s, and the shear rate can be changed from 0 1/sec to 30 1/sec after the discharge for 30 min. The functional groups in solution did not change significantly and the element composition of AC/Mn + TiO2 catalyst did not change before and after catalytic discharge.

One-step synthesis of Pt-Nd co-doped Ti/SnO2-Sb nanosphere electrodes used to degrade nitrobenzene

Ti/SnO2-Sb electrodes possess high catalytic activity and efficiently degrade nitrobenzene (NB); however, their low service life limits their wide application. In this study, we used one-step hydrothermal synthesis to successfully prepare Pt-Nd co-doped Ti/SnO2-Sb nanosphere electrodes. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were applied to characterize the surface morphology, microstructure, and chemical composition of the electrodes, respectively. The electrochemical activity and stability of the electrodes were characterized via linear sweep and cyclic voltammetry, electrochemical impedance spectroscopy, and an accelerated service life test; their performance for NB degradation was also studied. An appropriate amount of Pt-Nd co-doping refined the average grain size of SnO2 and formed a uniform and compact coating on the electrode surface. The oxygen evolution potential, total voltammetric charge, and electron transfer resistance of the Ti/SnO2-Sb-Nd-Pt electrodes were 1.88 V, 3.77 mC/cm2, and 11.50 Ω, respectively. Hydroxy radical was the main active radical species during the electrolytic degradation of nitrobenzene with Ti/SnO2-Sb-Nd-Pt. After Pt-Nd co-doping, the accelerated service life of the electrodes was extended from 8.0 min to 78.2 h (500 mA/cm2); although the NB degradation rate decreased from 94.1 to 80.6%, the total amount of theoretical catalytic degradation of NB in the effective working time increased from 17.4 to 8754.1 mg/cm2. These findings reveal good application potential for the electrodes and provide a reference for developing efficient and stable electrode materials.

Study on synergistic catalytic degradation of wastewater containing polyacrylamide catalyzed by low-temperature plasma-H2O2

The degradation behavior of polyacrylamide (PAM) solution by low-temperature plasma was investigated, and the effect of some factors that might affect the degradation process was further examined. The PAM solution was treated with low-temperature plasma generated by dielectric barrier discharge (DBD) combined with H2O2 and a Mn + Cu/AC composite catalyst. The optimal conditions for the oxidation degradation of a PAM solution using low-temperature plasma-H2O2-Mn + Cu/AC were determined as follows: initial concentration of 1000 mg/L, discharge voltage of 18 kV, H2O2 addition of 2%, and catalyst addition of 810 mg. The results indicated that the degradation rate increased with the increase of the catalyst dosage at the same discharge time. The degradation rate of 180 min increases from 90 to 97.6% with an increase in voltage from 16 to 18 kV, and the molecular weight decreases from 2,720,204.23 to 1,370,815.54. The degradation effect caused by the change of H2O2 addition was considerable compared with other factors. When the discharge time was 180 min, the degradation rate increased 26.3% with the increase of 1.6% H2O2 addition. Under the optimal process conditions, the addition of the catalyst resulted in a more rapid initial decrease in the pH value of the solution compared to the system without the catalyst.

Novel Sb-SnO2 Electrode with Ti3+ Self-Doped Urchin-Like Rutile TiO2 Nanoclusters as the Interlayer for the Effective Degradation of Dye Pollutants

Stable and efficient SnO₂ electrodes are very promising for effectively degrading refractory organic pollutants in wastewater treatment. In this regard, we firstly prepared Ti³⁺ self-doped urchin-like rutile TiO₂ nanoclusters (TiO_2-x NCs) on a Ti mesh substrate by hydrothermal and electroreduction to serve as an interlayer for the deposition of Sb-SnO₂ . The TiO_2-x NCs/Sb-SnO₂ anode exhibited a high oxygen evolution potential (2.63 V vs. SCE) and strong ⋅OH generation ability for the enhanced amount of absorbed oxygen species. Thus, the degradation results demonstrated its good rhodamine B (RhB), methylene blue (MB), alizarin yellow R (AYR), and methyl orange (MO) removal performance, with the rate constant increased 5.0, 1.9, 1.9, and 4.7 times, respectively, compared to the control Sb-SnO₂ electrode. RhB and AYR degradation mechanisms are also proposed based on the results of high-performance liquid chromatography coupled with mass spectrometry and quenching experiments. More importantly, this unique rutile interlayer prolonged the anode lifetime sixfold, given its good lattice match with SnO₂ and the three-dimensional concave-convex structure. Consequently, this work paves a new way for designing the crystal form and structure of the interlayers to obtain efficient and stable SnO₂ electrodes for addressing dye wastewater problems.

Degradation of antibiotics by electrochemical advanced oxidation processes (EAOPs): Performance, mechanisms, and perspectives

Global consumption and discharge of antibiotics have led to the rapid development and spread of bacterial antibiotic resistance. Among treatment strategies, electrochemical advanced oxidation processes (EAOPs) are gaining popularity for treating water/wastewater containing antibiotics due to their high efficiency and easiness of operation. In this review, we summarize various forms of EAOPs that contribute to antibiotic degradation, including common electrochemical oxidation (EO), electrolyte enhanced EO, electro-Fenton (EF) processes, EF-like process, and EAOPs coupling with other processes. Then we assess the performance of various EAOPs in antibiotic degradation and discuss the influence of key factors, including electrode, initial concentration and type of antibiotic, operation conditions, electrolyte, and water quality. We also review mechanisms and degradation pathways of various antibiotics degradation by EAOPs, and address the species and toxicity of intermediates produced during antibiotics treatment. Finally, we highlight challenges and critical research needs to facilitate the application of EAOPs in antibiotic treatment.

Aerobic degradation of anionic polyacrylamide in oil sands tailings: Impact factor, degradation effect, and mechanism

An investigation was carried out to study the degradation of anionic polyacrylamide (A-PAM) under different temperature and microorganism conditions as well as to assess its effects on water chemistry and toxicity in oil sands tailings. The maximum removal efficiency of A-PAM was 41.0 % in tailings water with augmented microorganisms at 20 °C. No acrylamide (AMD) monomer was released during the A-PAM degradation, while residual AMD, from the manufacturing process to make A-PAM, was completely removed within 4 weeks. Both temperature and microorganisms showed significant effects (p < 0.05) on the degradation of A-PAM and residual AMD. Gel permeation chromatography (GPC) and Fourier transform infrared (FT-IR) analyses showed that biodegradation could be the active pathway for A-PAM degradation in oil sands tailings. These analyses also indicated that macromolecular A-PAM was degraded into lower molecular weight organic compounds. No remarkable changes of the total concentration of naphthenic acids (NAs) were observed in A-PAM treated tailings water. However, low concentrations of fatty acids (<2.5 mg/L), which fit the NAs formula, were detected in pure polymer solution, indicating that A-PAM degradation would not affect the total concentration of NAs in tailings water but affect their distribution. Our results also showed that total organic carbon (TOC) and chemical oxygen demand (COD) could be used as indicators of A-PAM degradation in tailings water due to their strong linear correlations (R² > 0.90). Only slight increases in zeta potential and pH were found during A-PAM degradation. Limited effect on acute toxicity and no genotoxicity were found in A-PAM treated tailings water. Furthermore, the results suggest that A-PAM undergoes hydrolysis of amide groups by amidase enzymes, releasing ammonia and smaller molecules like organic acids. This research provides valuable information regarding the stability and impacts of A-PAM and thus will be beneficial for the management of oil sands tailings in long period of time.

Facile hydrothermal synthesis of MXene@antimony nanoneedle composites for toxic pollutants removal

A new 2D transition metal carbides family noted that MXene with antimony (Sb) nano-needles composites have demonstrated potential applications for photocatalytic dye degradations applications. Single-step synthesis of novel structures two/one-dimensional MXene@antimony nanoneedle (MX@Sb-H) nanocomposite-based photocatalysts is produced employing hydrothermal technique. The preparations and characterizations were compared with hand mixture preparations of pure TiO₂@Sb and MXene (MX@Sb-M). The crystallographic structure was identified employing X-ray diffraction (XRD) studies and main sharp XRD peaks were observed with diffraction angle with orientations planes for all three samples TiO₂@Sb, MX@Sb-M and MX@Sb-H. The micro-Raman spectroscopy explored key vibration modes centered at 151.72 and 637.52 cm^-1 corresponding to Ti and Sb hybrid composites respectively. Fourier transform infrared spectroscopy (FTIR) spectrum of functional group peaks at 609.16 and 868.80 cm^-1 revealed Ti-OH/Sb-O-C stretching. The morphological investigations of horizontal growth for "Sb" nanoneedle on MXene nanosheets were explored by scanning electron microscopy (SEM). The degradation efficiency was calculated. The efficiency calculated were 27%, 38%, 68% and 82% for MB solution, TiO₂@Sb added MB, MX-Sb-M added MB and MX-Sb-H added MB solution and the efficiency were 32%, 38%, 50% and 65% for pure RhB solution, TiO₂@Sb added RhB, MX-Sb-M added RhB and MX-Sb-H added RhB solution. The photocatalytic activity of TiO₂@Sb, MX@Sb-M and MX@Sb-H was examined. Among these MX@Sb-H nanocomposite was demonstrated the high photocatalytic action in expressions of rate stability of photocatalytic dye degradations.

A facile route to synthesize n-SnO2/p-CuFe2O4 to rapidly degrade toxic methylene blue dye under natural sunlight

In the present study, the n-SnO₂/p-CuFe₂O₄ (p-CFO) complex was prepared by a two-step process. p-CFO synthesized by the molten salt method was coated with SnO₂ synthesized by a facile in situ chemical precipitation method. The formation of n-SnO₂/p-CFO was confirmed by powder X-ray diffraction (PXRD). Scanning electron microscopy (SEM) images showed that the sharp edges of uncoated pyramid-like p-CFO particles were covered by a thick layer of n-SnO₂ on coated p-CFO particles. The complete absence of Cu and only 3 wt% Fe on the surface of the n–p complex observed in the elemental analysis using energy-dispersive X-ray spectroscopy (EDX) on the n–p complex confirmed the presence of a thick layer of SnO₂ on the p-CFO surface. Diffuse reflectance spectroscopy (DRS) was employed to elucidate the bandgap engineering. The n-SnO₂/p-CFO complex and p-CFO showed 87% and 58.7% methylene blue (MB) degradation in 120 min under sunlight, respectively. The efficiency of the n–p complex recovered after 5 cycles (73.5%) and was found to be higher than that of the uncoated p-CFO (58.7%). The magnetically separable property of the n–p complex was evaluated by using vibration sample magnetometry (VSM) measurements and it was confirmed that the prepared photocatalyst can be easily recovered using an external magnet. The study reveals that the prepared complex could be a potential candidate for efficient photodegradation of organic dyes under sunlight due to its efficient recovery and reusability owing to its magnetic properties.

The synthesis of n-SnO₂/p-CuFe₂O₄ to degrade toxic methylene blue dye under natural sunlight and its mechanism.

When MXene (Ti3C2Tx) meet Ti/PbO2: An improved electrocatalytic activity and stability

Stable electrode materials with high catalytic activity are urgently required for electrochemical degradation of refractory organic pollutants in wastewater treatment. Herein, high conductive MXene (Ti₃C₂T_x) was firstly fabricated by electrophoretic deposition (EPD) as an interlayer for preparing a novel PbO₂ electrode. The well-conducted Ti₃C₂T_x interlayer significantly improved the electrochemical performance of the EPD-2.0/PbO₂ (EPD time was 2.0 min) electrode with the charge transfer resistance decreased by 9.51 times, the inner active sites increased by 5.21 times and the ∙OH radicals generation ability enhanced by 4.07 times than the control EPD-0/PbO₂ anode. Consequently, the EPD-2.0/PbO₂ electrode achieved nearly 100% basic fuchsin (BF) and 86.78% COD removal efficiency after 3.0 h electrolysis. Therefore, this new PbO₂ electrode presented a promising potential for electrochemical degradation of BF and the new Ti₃C₂T_x middle layer could also be used to fabricate other efficient and stable anodes, such as SnO₂, MnO₂, TiO₂, etc.