Advancing nanobubble technology for carbon-neutral water treatment and enhanced environmental sustainability.

Gaseous nanobubbles (NBs) with dimensions ranging from 1 to 1000 nm in the liquid phase have garnered significant interest due to their unique physicochemical characteristics, including specific surface area, low internal gas pressure, long-term stability, efficient mass transfer, interface potential, and free radical production. These remarkable properties have sparked considerable attention in the scientific community and industries alike. These hold immense promise for environmental applications, especially for carbon-neutral water remediation. Their long-lasting stability in aqueous systems and efficient mass transfer properties make them highly suitable for delivering gases in the vicinity of pollutants. This potential has prompted research into the use of NBs for targeted delivery of gases in contaminated water bodies, facilitating the degradation of harmful substances and advancing sustainable remediation practices. However, despite significant progress in understanding NBs physicochemical properties and potential applications, several challenges and knowledge gaps persist. This review thereby aims to summarize the current state of research on NBs environmental applications and potential for remediation. By discussing the generation processes, mechanisms, principles, and characterization techniques, it sheds light on the promising future of NBs in advancing environmental sustainability. It explores their role in improving oxygenation, aeration, and pollutant degradation in water systems. Finally, the review addresses future research perspectives, emphasizing the need to bridge knowledge gaps and overcome challenges to unlock the full potential of this frontier technology for enhanced environmental sustainability.

Synergistic roles of carbon dioxide nanobubbles and biochar for promoting direct CO2 assimilation by plants and optimizing nutrient uptake efficiency.

This study investigates the synergistic role of carbon dioxide nanobubbles (CNBs) and biochar (BC) on seed germination, plant growth, and soil quality, employing Solanum lycopersicum (tomato) and Phaseolus vulgaris (beans) as test plant species. CNBs, generated and dispersed in both distilled water (DW) and tap water (TW), exhibited distinct characteristics, with TW-CNBs being larger and more stable (peak values of around 18.17 nm and 299.5 nm, zeta potential (ZP) of -5.91 mV), while DW-CNBs have peak values of around 1.63 nm and 216.1 nm, ZP of -3.23 mV. The results suggest CNBs enhance seed germination by upto 20%. CNBs in BC amended soil further promoted plant height and leaf number. CNBs increased dissolved CO2 levels to 2-24 ppm within 40 min, while BC enriched soil organic carbon from 19.20 to 24.96 ppm in beans and 18.33 to 22.35 ppm in tomatoes. The pH levels decreased from 7.68 to 3.78 for TW-CNBs and from 7.41 to 2.13 for DW-CNBs. Additionally, the electrical conductivity (EC) decreased from 112.1 to 99.6 for TW-CNBs, while it increased from 4.15 to 32.1 for DW-CNBs. Together they significantly increased soil available phosphorus and potassium to 4.03-8.06 and 3.58-7.16 kg ha-1; and 5.67-55.74 and 17.57-43.79 kg ha-1 in bean and tomato, respectively. Variations in nutrient concentrations were observed, with substantial increase in Na (16.27% and 6.58%), Zn (3.39% and 0.46%), and Mg (5.05% and 1.44%) content for beans and tomatoes, respectively. Structural equation model and principal component analysis revealed differences between CNB and BC treated soils, highlighting positive impact on soil quality and plant growth compared to control. Integration of CNBs and BC presents a multifaceted approach to enhance soil quality and promote plant growth, offering promising solutions for sustainable agriculture and environmental management.

A green approach towards sorption of CO2 on waste derived biochar.

Carbon capture technologies have advanced in recent years to meet the ever-increasing quest to minimize excessive anthropogenic CO2 emissions. The most promising option for CO2 control has been identified as carbon capture and storage. Among the numerous sorbents, char generated from biomass thermal conversion has shown to be an efficient CO2 adsorbent. This study examines various characteristics that can be used to increase the yield of biochar suited for carbon sequestration. This review gives recent research progress in the area, stressing the variations and consequences of various preparation processes on textural features such as surface area, pore size and sorption performance with respect to CO2's sorption capacity. The adjoining gaps discovered in this field have also been highlighted herewith, which will serve as future work possibility. It aims to analyse and describe the possibilities and potential of employing pristine and modified biochar as a medium of CO2 capture. It also examines the parameters that influence biochar's CO2 adsorption ability and pertinent challenges regarding the production of biochar-based CO2 sorbent materials.

Development of Bi2WO6 and Bi2O3 - ZnO heterostructure for enhanced photocatalytic mineralization of Bisphenol A.

Present study proposed the synthesis of mixed p-type and n-type nanocomposite heterostructures by co-precipitation method. The as-synthesized heterostructures were characterized through different characterization techniques. The as-synthesized Bi2WO6 and Bi2O3-ZnO heterostructures were tested as photocatalysts during the photodegradation of Bisphenol A (BPA). The Bi2O3-ZnO heterostructure nanocomposite was found to be a more effective photocatalyst than Bi2WO6. The effect of operating parameters including catalytic dose (0.02-0.15 gL-1), initial BPA concentration (5-20 mgL-1), temperature change (5-20 °C) and solution pH changes (4, 5, 7, and 8) were evaluated with Bi2O3-ZnO under UV-light irradiation by selecting a 300 W Xe lamp. More than 90% BPA was degraded with 0.15 gL-1 Bi2O3-ZnO, keeping 1.0 mM H2O2 concentration fixed in 250 mL of reaction suspension. The HPLC and GC-MS were used to detect the reaction intermediates and final products. A plausible degradation pathway was proposed on the basis of the identification of reaction intermediates. Repeatability test analysis confirmed that the as-synthesized catalyst showed superb catalytic performance on its removal trend. The kinetics of degradation of BPA were well fitted by the power laws model. With the order of reaction being 0.6, 0.9, 1.2, and 1.3 for different operating parameters, i.e., catalyst dose, initial pH, temperature, and initial BPA concentration.

Synthesis and application of green mixed-metal oxide nano-composite materials from solid waste for dye degradation.

Present study demonstrates reutilization of electrochemical (EC) sludge as a potential low-cost green catalyst for dye degradation. Hexagonal Fe2O3 type phase with trevorite (NiFe2O4)-type cubic phase nanocomposite material (NCM) was synthesized from solid waste sludge generated during EC treatment of textile industry wastewater with stainless steel electrode. For NCM synthesis, sludge was heated at different temperatures under controlled condition. Various synthesized NCMs were characterized by powder X-ray diffraction (PXD), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis. The synthesized NCMs were found to contain iron, chromium, nickel and oxygen in the form of α-Fe2O3 (metal: oxygen = 40:60), (Fe,Cr,Ni)2O3 and trevorite NiFe2O4, (Ni,Fe,Cr) (Fe,Cr,Ni)2O4 (metal: oxygen = 43:57). Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), pore size distribution, and atomic force microscope (AFM) analysis showed distribution of grains of different shapes and sizes. Catalytic activity of NCM was studied by the methylene red dye degradation by using the catalytic wet peroxidation process. Zeta potential study was performed under different pH so as to determine the performance of the NCMs during dye degradation.

Photocatalytic hydrogen production of the CdS/TiO2-WO3 ternary hybrid under visible light irradiation.

An attractive and effective method for converting solar energy into clean and renewable hydrogen energy is photocatalytic water splitting over semiconductors. The study aimed at utilizing organic sacrificial agents in water, modeled by formic acid, in combination with visible light driven photocatalysts to produce hydrogen with high efficiencies. The photocatalytic hydrogen production of cadmium sulfide (CdS)/titanate nanotubes (TNTs) binary hybrid with specific CdS content was investigated. After visible light irradiation for 3 h, the hydrogen production rate of 25 wt% CdS/TNT achieved 179.35 µmol·h(-1). Thanks to the two-step process, CdS/TNTs-WO3 ternary hybrid can better promote the efficiency of water splitting compared with CdS/TNTs binary hybrid. The hydrogen production of 25 wt% CdS/TNTs-WO3 achieved 212.68 µmol·h(-1), under the same condition. Coating of platinum metal onto the WO3 could further promote the reaction. Results showed that 0.2 g 0.1 wt% Pt/WO3 + 0.2 g 25 wt% CdS/TNTs had the best hydrogen production rate of 428.43 µmol·h(-1). The resultant materials were well characterized by high-resolution transmission electron microscope, X-ray diffraction, scanning electron microscopy, and UV-Vis spectra.

Regeneration and reuse of leachate from a municipal solid waste landfill.

Landfill leachate is deep brown in color with extremely complex composition and difficult to treat in order to meet the effluent standards. The leachate of Keelung City Tien Wai Tien landfill has an average flow of 350 CMD. In the present study following serially connected devices: Activated sludge/contact aeration (AS/CA) combined system, reverse osmosis (RO) and an ammonia stripping tower were used to treat the leachate. After treatment, the COD (removal rate of 91%), BOD (removal rate of 83%), SS (removal rate of 86%) and NH(4+)-N level (removal rate of 98%) significantly reduced in the leachate. The treated effluent was further recycled and used as RO back washing water and for sprinkling roads and watering plants in Keelung City. It is further required to evaluate whether the treated effluent can be reutilized for agriculture and extinguishing fire during shortage of water.

MXene/biochar composites for enhanced wastewater reclamation and bioenergy production: A kinetics and thermodynamics study.

The study explores the synthesis and utilization of biochar (BC) and multi-layer MXene to MXene/biochar (MB) composites for wastewater treatment. Simultaneously, it also investigates their energy generation potential through biomass and soil property assessments. The integrated column and batch treatments have shown significant results, elevating total dissolved solids from 63.7 to 125.5 mg L-1 with column treatment, while reducing them to 6.37 % and 1.35 % with BC and MB treatment, respectively. BC with high carbon content, demonstrated increased hydrophobicity, which was amplified by the integration of MXene, thereby enhancing its potential for advanced wastewater treatment. Treated wastewater exhibited elevated nutrient concentrations (Ca, Cu, Fe, K, Na, and NH4+), promoting the growth of Pennisetum purpureum. WW_B shows promising energy potential, with a higher heating value of 25.03 MJ kg-1 and a lower heating value of 23.57 MJ kg-1. They demonstrated high volatile matter exceeding 70.9 wt %, and a fixed carbon from 10.02 to 27.53 wt %, signifying their potential for efficient conversion and bio-oil yield during pyrolysis. The ultimate analysis emphasized significant carbon, with oxygen content ranging from 43.42 to 47.78 wt %., influencing combustion characteristics. MT_B exhibited its suitability for energy production through thermochemical conversion, underlined by its high flammability and low volatile ignition values. In the absence of BC, the Ea ranged from 24.77 to 77.88 kJ mol-1 in wastewater and from 21.67 to 69.6 kJ mol-1 in MB treated wastewater. Meanwhile, when soil contained BC and was irrigated with wastewater, the Ea varied from 24.66 to 80.91 kJ mol-1. In the case of MB treated wastewater, it ranged from 25.01 to 75.79 kJ mol-1. The research thereby affirms the potential of MB composites to advance water and energy sustainability setting us for broader nexus-based applications.

Calculating of river water quality sampling frequency by the analytic hierarchy process (AHP).

River water quality sampling frequency is an important aspect of the river water quality monitoring network. A suitable sampling frequency for each station as well as for the whole network will provide a measure of the real water quality status for the water quality managers as well as the decision makers. The analytic hierarchy process (AHP) is an effective method for decision analysis and calculation of weighting factors based on multiple criteria to solve complicated problems. This study introduces a new procedure to design river water quality sampling frequency by applying the AHP. We introduce and combine weighting factors of variables with the relative weights of stations to select the sampling frequency for each station, monthly and yearly. The new procedure was applied for Jingmei and Xindian rivers, Taipei, Taiwan. The results showed that sampling frequency should be increased at high weighted stations while decreased at low weighted stations. In addition, a detailed monitoring plan for each station and each month could be scheduled from the output results. Finally, the study showed that the AHP is a suitable method to design a system for sampling frequency as it could combine multiple weights and multiple levels for stations and variables to calculate a final weight for stations, variables, and months.

Carbon capture of biochar produced by microwave co-pyrolysis: adsorption capacity, kinetics, and benefits.

Microwave co-pyrolysis of sewage sludge and leucaena wood was conducted to produce biochar as an adsorbent for CO2 capture. Both microwave power level and blending ratio were crucial factors affecting the CO2 adsorption capacity of biochar. At a power level of 150 W, the biochar produced by microwave co-pyrolysis of 25% sewage sludge and 75% leucaena wood possessed the highest CO2 adsorption capacity. When the biochar was produced at 100 W, its CO2 adsorption capacity was higher than predicted. Based on the proximate and elemental compositions of biochar, two equations were obtained to predict CO2 adsorption capacity. The proximate composition of biochar can provide more precise prediction of CO2 adsorption capacity than elemental composition according to the higher R2 value provided. The blending ratio of 50% would be most appropriate to produce the biochar with acceptable reduction in CO2 adsorption capacity and loss of quantity. The pseudo-second-order model would be most suitable for simulating the kinetic of CO2 adsorption. The biochar produced from 1 metric tonne of sewage sludge and leucaena wood can offset carbon tax by 83 US dollars. Based on experimental results and findings, microwave co-pyrolysis should be a feasible technique to produce biochar possessing high CO2 adsorption capacity.

Adsorption and desorption characteristics of heavy metals onto conventional and biodegradable plastics.

Biodegradable plastics have been widely used to replace conventional plastics to minimize environmental impacts of plastic packaging. However, before biodegradable plastics decompose in the environment, they could pose a threat to terrestrial and aquatic creatures by acting as vectors of contaminants in the food chain. In this study, conventional plastic bags (CPBs) made of polyethylene and biodegradable plastic bags (BPBs) made of polylactic acid were examined for their heavy metal adsorption. Effects of solution pHs and temperatures on adsorption reactions were investigated. Because of a larger BET surface area, presence of oxygen-containing function groups, and smaller crystallinity, the heavy metal adsorption capacities of BPBs are significantly larger than those of CPBs. Among Cu (up to 791.48 mg⋅kg-1), Ni (up to 60.88 mg⋅kg-1), Pb (up to 1414.58 mg⋅kg-1), and Zn (up to 295.17 mg⋅kg-1), Pb and Ni show the largest and the lowest extents of adsorption onto the plastic bags, respectively. In the different waterbodies in nature, Pb adsorption on the CPBs and the BPBs were 318.09-379.91 and 528.41-764.22 mg⋅kg-1, respectively. Consequently, Pb was selected as the target contaminant in the desorption experiments. After Pb was adsorbed onto the CPBs and the BPBs, Pb could be completely desorbed and released into simulated digestive systems in 10 h. In conclusion, BPBs could be potential vectors of heavy metals, and their suitability as a substitute for CPBs must be thoroughly investigated and confirmed.

Peroxymonosulfate activation with an α-MnO2/Mn2O3/Mn3O4 hybrid system: parametric optimization and oxidative degradation of organic dye.

The present study proposed the synthesis of low-toxicity and eco-friendly spherically shaped manganese oxides (α-MnO2, Mn2O3, and Mn3O4) by using the chemical precipitation method. The unique variable oxidation states and different structural diversity of manganese-based materials have a strong effect on fast electron transfer reactions. XRD, SEM, and BET analyses were used to confirm the structure morphology, higher surface area, and excellent porosity. The catalytic activity of as-prepared manganese oxides (MnOx) was investigated for the rhodamine B (RhB) organic pollutant with peroxymonosulfate (PMS) activation under the condition of control pH. In acidic conditions (pH = 3), complete RhB degradation and 90% total organic carbon (TOC) reduction were attained in 60 min. The effects of operating parameters such as solution pH, PMS loading, catalyst dosage, and dye concentration on RhB removal reduction were also tested. The different oxidation states of MnOx promote the oxidative-reductive reaction under acidic conditions and enhance the SO4•-/•OH radical formation during the treatment, whereas the higher surface area offers sufficient absorption sites for interaction of the catalyst with pollutants. A scavenger experiment was used to investigate the generation of more reactive species that participate in dye degradation. The effect of inorganic anions on divalent metal ions that genuinely occur in water bodies was also studied. Additionally, separation and mass analysis were used to investigate the RhB dye degradation mechanism at optimum conditions based on the intermediate's identification. Repeatability tests confirmed that MnOx showed superb catalytic performance on its removal trend.

Evaluation of solar photovoltaic carport canopy with electric vehicle charging potential.

While sustainable mobility and decarbonization of transportation sector are among the most comprehensive solutions to the problem of climate change, electric vehicles (EV) are becoming increasingly popular as the future mode of transport. In this study, the integration of a solar carport canopy to a potential EV charging station is analyzed using various operating conditions. A detailed analysis has been provided for the carport located in southern Taiwan, Kaohsiung city, where electricity generation, emission impacts, and financial analysis of the solar EV charging station are discussed. The results of a case study showed a potential of 140 MWh/year of solar energy yield, which could provide solar electricity of more than 3000 vehicles per month with 1-h parking time, generating 94% lower total carbon dioxide emission than the electricity produced from traditional grid methods. Taken into account the impact of carbon tax implementation on driver economics, the results demonstrated the viability of such photovoltaic (PV)-based charging stations, particularly for possible higher carbon tax scenarios in the future. The presented results can be implemented on a larger scale, offering guidelines and tools for constructing solar-powered EV charging station infrastructure.

Spatial allocation of LID practices with a water footprint approach.

Urban water problems due to stormwater have been aggravated by the higher frequency of high-intensity precipitation events and the increase of paved surfaces. However, with appropriate stormwater management practices, such as low-impact development (LID), stormwater can provide an additional urban water resources rather than cause damage. This study aims to apply a water footprint to location determination of LID practices in the urban area. The LID planning procedure was demonstrated with the highest population density region in Taipei, Taiwan. In order to improve the spatial resolution of LID allocation, the "first-level dissemination area" with 450 residents was used as a spatial unit. The performance of LID practices was then evaluated with the simulation using the Storm Water Management Model (SWMM). Three LID practices, rainwater harvesting systems, permeable pavements, and bioretention systems, were selected. After the water footprint accounting, ten sites were suggested for LID implementation. The runoff reduction rate reached up to 65 % by rainwater harvesting systems or at least 3 % by permeable pavements. This study provides a simpler and more effective approach to ways of integrating an urban water footprint into LID planning and stormwater management in urban areas.

Contribution of electrolyte in parametric optimization of perfluorooctanoic acid during electro-oxidation: Active chlorinated and sulfonated by-products formation and distribution.

The present study investigated the roles of peroxydisulfate (PDS) radicals and sulfate radicals (SO4•-) that formed from sulfate (SO42-) during electrochemical oxidation of perfluorooctanoic acid (PFOA). The effect of operating parameters such as different types of electrolytes (NaCl, NaClO4, and Na2SO4), initial pH, current density, dose of electrolyte, and initial concentration of PFOA using electrochemical oxidation for perfluorooctanoic acid (PFOA) decomposition study was investigated. A difference in the removal efficiency with different electrolytes (i.e., Cl-, ClO4-, and SO42-) illustrated an increasing effect of electrooxidation of PFOA in the order of ClO4- < Cl- < SO42-, which suggested that •OH induced oxidation and direct e- transfer reaction continued to play a crucial role in oxidation of PFOA. At the optimum treatment condition of j = 225.2 Am-2, Na2SO4 concentration = 1.5 gL-1, [PFOA]o = 50 mgL-1 and initial pH = 3.8 maximum PFOA removal of 92% and TOC removal of 80% was investigated at 240 min. The formation of three shorter-chain perfluorocarboxylates (i.e., perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA), and perfluoropentanoic acid (PFPeA) and formate (HCOO-) ions were detected as by-products of PFOA electro-oxidation, showing that the C-C bond first broken in C7F15 and then mineralized into CO2, and fluoride ion (F-). The fluorine recovery as F- ions and the organic fluorine as the shorter-chain by-products were also obtained. The degradation kinetic has also been studied using the nth-order kinetic model.

Cationic surfactants influencing the enhancement of energy efficiency for perfluorooctanoic acid (PFOA) removal in the electrocoagulation-flotation (ECF) system.

From an environmental perspective, approaching sustainability requires a fundamental conceptual shift from the wastewater treatment process toward integrated treatment systems that consider efficient and effective utilization. This study aims to investigate the effects of different surfactants on the removal of perfluorooctanoic acid (PFOA). We used cationic surfactants as both frothers and collectors in the electrocoagulation-flotation (ECF) method to improve the removal efficiency of PFOA. The results showed that, under a monopolar aluminum electrode and with an initial PFOA concentration of 0.25 mM, the ECF method with decyl-trimethyl-ammonium bromide (DTAB) was able to remove over 98% of PFOA within 10 min. Cationic surfactants with a similar linear alkyl chain shape to PFOA, but a longer chain length, are more effective at removing PFOA through the ECF process. The removal mechanism is thought to involve co-precipitation with aluminum hydroxides through Al-F bonding, co-flotation with cationic surfactants, and mixed micelle formation with cationic surfactants. The optimal conditions were tested in both synthetic and realistic wastewater matrices and produced similar results. It has the potential for real wastewater application. The energy yield (G50) of ECF with 5 mM DTAB is 497 g·kWh-1, superior to other treatments, and is an extremely energy-effective method for separating PFOA from wastewater.

Sorption of benzoic acid from aqueous solution by cetyltrimethylammonium bromide modified birnessite.

Layered manganese oxide (birnessite) has been studied for its use as catalytic materials. The research presented in this study investigates the sorption of benzoic acid from water on synthesized cetyltrimethylammonium bromide modified birnessite (CTAB-birnessite). The synthesized CTAB-birnessite was characterized by X-ray powder diffraction (XRD). The experimental results of sorption kinetic were well fitted to the pseudo-second-order equation. The sorption isotherms were linear at different pH values, and it indicates a partition mechanism. Up to about 53% of the dissolved benzoic acid was sorbed by CTAB-birnessite; in contrast, only 16% of the dissolved benzoic acid was sorbed by birnessite. These results indicate that CTAB-birnessite can be a potential sorbent for benzoic acid removal.

Satellite imagery: a way to monitor water quality for the future?

Monitoring water at high spatial and temporal resolutions is important for maintaining water quality because the cost of pollution remediation is often higher than the cost of early prevention or intervention. In recent decades, the availability and affordability of satellite images have regularly increased, thus supporting higher-frequency and lower-cost alternative methods for monitoring water quality. The core step in satellite remote sensing detection is inverse modeling, which is used to calibrate model parameters and enhance the similarity between the model and the real system being simulated. The reflectance values measured at water quality stations are extracted from atmosphere-corrected satellite imagery for analysis. However, various external environmental, hydrological, and meteorological factors affect the evaluation results, and the results obtained with different parameters can vary. This literature review shows that nonpoint-source pollution caused by stormwater runoff can also be monitored using satellite imagery. To improve the accuracy of satellite-based water quality prediction, the temporal resolution of field measurements can be increased, thus better considering the influence of seasonality. Then, the atmospheric correction module can be improved by using available atmospheric water content products. Moreover, because water surface ripples affect reflectance, wind speed and direction should be considered when comparing water quality scenes.

Application of synthetic data to establish the working framework for multivariate statistical analysis of river pollution traceability - the heavy metals in Nankan River, Taiwan.

This study applied multivariate statistical analysis (MSA) to synthetic data simulated by a river water quality model to verify whether the MSA can correctly infer the pollution scenario assigned in the river water quality model. The results showed that when assessing the number and possible locations of pollution sources based on the results of cluster analysis (CA), two instead of three pollution point source were identified when considering the hydraulic variations of surface water. When discussing the principal component analysis (PCA) result, the second principal component (PC2) and the Pearson correlation coefficients among the pollutants should also be considered, which can infer that Cu, Pb, Cr, and Ni are contributed by the same pollutant point source, and Cu is also influenced by another pollutant point source. This result also implies that the solid and liquid partition coefficients (Kd) of pollutants can affect the interpretation of the PCA results, so the Kd values should be determined before tracing the pollution sources to facilitate the evaluation of the source characteristics and potential targets. This study established a working framework for surface water pollution traceability to enhance the effectiveness of pollution traceability.

Effects of zinc salt addition on perfluorooctanoic acid (PFOA) removal by electrocoagulation with aluminum electrodes.

In this study, the electrocoagulation (EC) of perfluorooctanoic acid (PFOA) by an aluminum electrode with the addition of zinc salt was investigated. Adding ZnCl2 successfully prevented a rise in pH during EC and increased the efficiency from 73.7% to over 99%. In addition, the longer the carbon chain of a PFA was, the better the removal of that PFA by electrocoagulation. The main functions of ZnCl2 were to prevent the rise in pH and improve flotation because the flocs with added ZnCl2 were easy to gather together and had a faster floating speed. The XPS results demonstrated the occurrence of bonding between aluminum and fluoride. This finding indicates that complexation between aluminum and fluoride may be the main mechanism for removal when aluminum electrodes are used to remove perfluoroalkyl (PFA) compounds.