Electrosorption-promoted photodegradation of opaque wastewater on a novel TiO(2)/carbon aerogel electrode

A review of advanced oxidation process towards organic pollutants and its potential application in fracturing flowback fluid

Fracturing flowback fluid (FFF) including various kinds of organic pollutants that do harms to people and new treatments are urgently needed. Advanced oxidation processes (AOPs) are suitable methods in consideration with molecular weight, removal cost and efficiency. Here, we summarize the recent studies about AOP treatments towards organic pollutants and discuss the application prospects in treatment of FFF. Immobilization and loading methods of catalysts, evaluation method of degradation of FFF, and continuous treatment process flow are discussed in this review. In conclusion, further studies are urgently needed in aspects of catalyst loading methods, macromolecule organic evaluation methods, industrial process, and pathways of macromolecule organics' decomposition.

In Situ Synthetic ZIF-8/Carbon Aerogel Composites as Solid-Phase Microextraction Coating for the Detection of Phthalic Acid Esters in Water Samples

In this study, a hybrid composite featuring zeolitic imidazolate framework-8/carbon aerogel (ZIF-8/CA) was synthesized via in situ nucleation and growth of ZIF-8 nanoparticles inside carbon aerogels. The novel material was used as the solid-phase microextraction (SPME) coating for the five phthalic acid esters (PAEs) detection by coupling with a gas chromatography–flame ionization detector (GC-FID). Compared with bare carbon aerogel, the ZIF-8/CA presented the best performance, which is attributed to the unique advantages between the high surface area of CA and high hydrophobic properties, the thermal stability of ZIF-8, and their synergistic adsorption effects, such as molecular penetration, hydrogen bond, and π–π stacking interactions. Under the optimized conditions, the as-proposed ZIF-8/CA fiber provided a wide linearity range from 0.2 to 1000 μg L⁻¹ and a low detection limit of 0.17–0.48 μg L⁻¹ for PAEs analysis. The intra-day and inter-day of signal fiber and the fiber–fiber relative standard deviations were observed in the ranges of 3.50–8.16%, 5.02–10.57%, and 5.66–12.11%, respectively. The method was applied to the determination of five PAEs in plastic bottled and river water samples.

Electroreforming of Biomass for Value-Added Products

Humanity's overreliance on fossil fuels for chemical and energy production has resulted in uncontrollable carbon emissions that have warranted widespread concern regarding global warming. To address this issue, there is a growing body of research on renewable resources such as biomass, of which cellulose is the most abundant type. In particular, the electrochemical reforming of biomass is especially promising, as it allows greater control over valorization processes and requires milder conditions. Driven by renewable electricity, electroreforming of biomass can be green and sustainable. Moreover, green hydrogen generation can be coupled to anodic biomass electroforming, which has attracted ever-increasing attention. The following review is a summary of recent developments related to electroreforming cellulose and its derivatives (glucose, hydroxymethylfurfural, levulinic acid). The electroreforming of biomass can be achieved on the anode of an electrochemical cell through electrooxidation, as well as on the cathode through electroreduction. Recent advances in the anodic electroreforming of cellulose and cellulose-derived glucose and 5-hydrooxylmethoylfurural (5-HMF) are first summarized. Then, the key achievements in the cathodic electroreforming of cellulose and cellulose-derived 5-HMF and levulinic acid are discussed. Afterward, the emerging research focusing on coupling hydrogen evolution with anodic biomass reforming for the cogeneration of green hydrogen fuel and value-added chemicals is reviewed. The final chapter of this paper provides our perspective on the challenges and future research directions of biomass electroreforming.

Carbon aerogel as a solid-phase microextraction fiber coating for the extraction and detection of trace tetracycline residues in food by coupling with high-performance liquid chromatography

A direct immersion solid-phase microextraction method for determining tetracyclines (TCs) was developed by coupling with high-performance liquid chromatography. A carbon aerogel (CA) was synthesized as a fiber coating with high extractive properties and a low density of 0.1855 g cm-3via ambient pressure drying and carbonization. The as-synthesized CA exhibited a high specific surface area and a cross-linked structure; it was characterized via scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and Brunauer-Emmett-Teller analysis, etc. The extraction performance for six TCs was investigated, and the main experimental parameters were optimized by the Box-Behnken design. Adsorption kinetics, Langmuir and Freundlich models were used to clarify the extraction mechanism. This method showed wide linear ranges of 1-500 μg L-1, low limits of detection of 0.52-1.05 μg L-1, good repeatability of 1.37-12.47%, and satisfactory inter-fiber reproducibility of 8.51-15.81% relative standard deviation for the detection of six TCs. Moreover, this study provided an interesting insight into the detection of TCs residues in food samples.

Environmental application of monolithic carbonaceous aerogels for the removal of emerging pollutants

In this study, the performance of three commercial available monolithic carbonaceous aerogels (NQ30A, NQ60A and NQ80A) for the removal of different emerging pollutants, detected in water sources, was evaluated. More specifically, the removal of two pharmaceuticals (antipyrine and sulfamethoxazole) and an anti-fungal agent (methyl paraben), widely used in cosmetics, was studied. The NQ60A demonstrated the best adsorption characteristics and effectively adsorbed over 50 mg/g of the antipyrine and around 30 mg/g sulfamethoxazole and methyl paraben. The kinetic study of the adsorption process revealed that pseudo-first order kinetic model described very well the kinetic behaviour of the selected pollutants onto the NQ60A aerogel. After that, the regeneration of the loaded aerogel, with antipyrine alone and in presence of the other two contaminants, was evaluated. The regeneration was accomplished in two ways: (1) by using directly the loaded aerogels as cathode during the electro-Fenton treatment and (2) by its regeneration immersed in the bulk volume of electro-Fenton cell (boron doped diamond as anode and carbon felt as cathode). Both approaches can provide an effective removal of the pollutants inside the aerogel. In addition, the regenerated aerogel proved to maintain its adsorptive properties and can be successfully reused in successive cycles of adsorption-regeneration. On the basis of these promising results, it can be concluded that the proposed strategy based on aerogels adsorption and electro-Fenton regeneration is a suitable alternative for emerging pollutants removal from water streams.

Fabrication of graphene oxide wrapped Ti/Co3O4 nanowire photoanode and its superior photoelectrocatalytic performance

Here, we successfully fabricated graphene oxide (GO) wrapped Ti/Co₃O₄ nanowires (NWs) by electrophoretic deposition based on the good dispersibility of GO in an aqueous solution. Interestingly, GO can adhere to the surface of Co₃O₄ NWs via an ultrathin gossamer-like sheet, and the coverage and wrapping of GO on the surface of Co₃O₄ NWs can be controlled by tuning the electrochemical deposition time and voltage. Our results also demonstrate that GO wrapped Co₃O₄ NWs had superior photoelectrochemical activity for the decolorization of dye (reactive brilliant blue KN-R) in wastewater, mainly because the introduction of GO can tune the oxygen evolution behavior, the transportation of reactant and induced carriers, electrochemical active areas, and the light-harvesting capability of Co₃O₄ NWs. Therefore, we anticipate that GO wrapped Ti/Co₃O₄ NWs could be considered as a promising photoanode for the treatment of organic pollutants in wastewater.

Carbon Gels-Modified TiO2: Promising Materials for Photocatalysis Applications

Carbon gels are a kind of porous organic polymer, which play pivotal roles in electrode, supercapacitor, hydrogen storage, and catalysis. Carbon gels are commonly prepared by the condensation of resorcinol and formaldehyde. The as-prepared polymers are further aged and sintered at a high temperature in an inert atmosphere to form cross-linked and intertwined porous structures. Owing to its large specific area and narrow pore size distribution, this kind of material is very appropriate for mass transfer, substrate absorption, and product desorption from the pores. In recent years, carbon gels have been discovered to function as effective hybrid materials with TiO₂ for photocatalytic applications. They could act as efficient deep-traps for photo-induced holes, which decreases the recombination probability of photo-induced carriers and lengthens their lifetime. In this mini-review, we will discuss the state-of-the-art paragon examples of carbon gels/TiO₂ composite materials applied in photo(electro)catalysis. The major challenges and gaps of its application in this field will also be emphasized.

Deep Dehalogenation of Florfenicol Using Crystalline CoP Nanosheet Arrays on a Ti Plate via Direct Cathodic Reduction and Atomic H

Efficient elimination of antibacterial activity of halogenated antibiotics by dehalogenation pretreatment is desired for a biochemical treatment process. In this study, crystalline cobalt phosphide nanosheet arrays on a Ti plate (C-CoP/Ti) are fabricated by a simple electrodeposition and phosphorization process. The crystalline structure greatly promotes atomic hydrogen (H*) generation. Moreover, the nanosheet arrays can provide abundant active sites and accelerate electron transfer and mass transport. As a result, the dehalogenation rate of florfenicol (FLO, an emerging organic pollutant) on C-CoP/Ti is 11.1, 2.97, and 13.6 times higher than that on amorphous CoP/Ti, Pd/Ti, and bare Ti, respectively. The C-CoP/Ti electrode achieves 97.4% dehalogenation of FLO (20 mg L^-1) within 30 min at -1.2 V (vs Ag/AgCl). Nearly 100% of Cl and 20% of F are broken away within 120 min, showing the highest electrocatalytic defluorination efficiency reported so far. Both experimental results and theoretical calculations reveal that the dehalogenation of FLO on C-CoP/Ti is synergistically accomplished via direct reduction of electron transfer and indirect reduction of H*. This study develops a highly efficient non-noble metal electrode material for dehalogenation of halogenated organic compounds.

Photochemical Protection of Reactive Sites on Defective TiO2- x Surface for Electrochemical Water Treatment

The electrode is the key in electrochemical process for water and wastewater treatment. Many nonstoichiometric metal oxides are active electrode materials but have poor stability under strong anodic polarization due to their susceptible nature of the oxygen vacancies on surface and subsurface as defective reactive sites. In this work, a novel photochemical protecting strategy is proposed to stabilize the defective reactive sites on the TiO_{2- x} surface and subsurface for long-term anodic oxidation of pollutants. With this strategy, a novel photoassisted electrochemical system at low anodic bias is further constructed. Such a system exhibits a high protecting capacity at a low operation cost for electrochemical degradation of bisphenol A (BPA), a typical persistent organic pollutant. Its excellent photochemical protecting capacity is found to be mainly attributed to the mild non-band-gap excitation pathways on the defective TiO_{2- x} electrode under both visible-light irradiation and moderate anodic polarization. Under real sunlight irradiation, a 20 run cyclic test for BPA degradation demonstrates the excellent performance and stability of the constructed system at low bias without significant oxygen evolution. Our work provides a new opportunity to utilize the defective and reactive TiO_{2- x} for efficient, stable, and cost-effective electrochemical water treatment with the aid of its photo- and electrochemical bifunctional properties.

A Comparison of Electrochemical Performance of Carbon Aerogels with Adsorption Metal Ions for Super Capacitors

Environmental problems caused by metal ions have caused widespread concern in recent years. In this work, carbon aerogels (CAs) adsorbing different metal ions were prepared. The adsorption performance and kinetics of metal ions (Cu(II), Cr(VI), and Fe(III)) on carbon aerogels were systematically investigated. The results indicated that the maximum adsorption capacity of Cu(II) was 424 mg·g⁻¹ in 600 mg·L⁻¹ copper solution. Adsorption performances of Cu(II), Cr(VI), and Fe(III) on CAs well fitted with a pseudo-second-order kinetic model. The structures and morphologies of metal-containing samples were characterized by scanning electron micrographs (SEM), Energy Dispersive Spectrometer (EDS), transmission electron microscope (TEM), and X-ray diffraction (XRD). The results demonstrated that the texture and electrochemical performance of CAs adsorbing metal ions exhibited a clear change. The specific surface area of CAs for adsorbing copper ions was 450 m²·g⁻¹ and they showed a small average pore diameter (7.16 nm). Furthermore, CAs adsorbing metals could be used for the super capacitor. The specific capacitance of CAs adsorbing copper ions could reach 255 F·g⁻¹ at a current density of 1.0 A·g⁻¹. The CA-Cu electrode materials exhibited excellent reversibility with a cycling efficiency of 97% after 5000 cycles.

Photochemical Anti-Fouling Approach for Electrochemical Pollutant Degradation on Facet-Tailored TiO2 Single Crystals

Electrochemical degradation of refractory pollutants at low bias before oxygen evolution exhibits high current efficiency and low energy consumption, but its severe electrode fouling largely limits practical applications. In this work, a new antifouling strategy was developed and validated for electrochemical pollutant degradation by photochemical oxidation on facet-tailored {001}-exposed TiO₂ single crystals. Electrode fouling from anodic polymers at a low bias was greatly relieved by the free ·OH-mediated photocatalysis under UV irradiation, thus efficient and stable degradation of bisphenol A, a typical environmental endocrine disrupter, and treatment of landfill leachate were accomplished without remarkable oxygen evolution in synergistic photoassisted electrochemical system. Electrochemical and spectroscopic measurements indicated a clean electrode surface during cyclic pollutant degradation. Such a photochemical antifouling strategy for low-bias anodic pollutants degradation was mainly attributed to the improved electric conductivity and excellent electrochemical and photochemical activities of tailored TiO₂ anodic material, whose unique properties originated from the favorable surface atomic and electronic structures of high-energy {001} polar facet and single-crystalline structure. Our work opens up a brand new approach to develop catalytic systems for efficient degradation of refractory contaminants in water and wastewater.

Preparation of transition metal composite graphite felt cathode for efficient heterogeneous electro-Fenton process

A composite graphite felt (GF) modified with transition metal was fabricated and used as cathode in heterogeneous electro-Fenton (EF) for methyl orange (MO) degradation. Characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), the morphology and surface physicochemical properties of the cathodes after modification were observed considerably changed. After loading metals, the current response became higher, the accumulation of H₂O₂ and the degradation efficiency of MO were improved. Under the same conditions, GF-Co had the highest catalytic activity for electro-reduction of O₂ to H₂O₂ and MO degradation. At pH 3, 99 % of MO degradation efficiency was obtained using GF-Co after 120 min treatment and even at initial pH 9, 82 % of that was obtained. TOC removal efficiency reached 93.8 % using GF-Co at pH 3 after 120 min treatment while that was 12.3 % using GF. After ten-time runs, the mineralization ratio of the GF-Co was still 89.5 %, suggesting that GF-Co was very promising for wastewater treatment. The addition of isopropanol proved that ^·OH played an important role in degradation of MO.

Efficient Electrochemical Reduction of Nitrobenzene by Defect-Engineered TiO2-x Single Crystals

TiO2 is a typical semiconductor and has been extensively used as an effective photocatalyst for environmental pollution control. But it could not be used as an electrochemical reductive catalyst because of its low electric conductivity and electrocatalytic activity. In this work, however, we demonstrate that TiO2 can act as an excellent cathodic electrocatalyst when its crystal shape, exposed facet and oxygen-stoichiometry are finely tailored by the local geometric and electronic structures. The defect-engineered TiO2-x single crystals dominantly exposed by high-energy {001} facets exhibits a high cathodic activity and great stability for electrochemical reduction of nitrobenzene, a typical refractory pollutant with high toxicity in environment. The single crystalline structure, the high-energy {001} facet and the defective oxygen vacancy of the defect-engineered TiO2-x single crystals are found to be mainly responsible for their cathodic superiority. With the findings in this work, a more practical non-Pd cathodic electrocatalyst could be prepared and applied for electrocatalytic reduction of refractory pollutants in water and wastewater, and extend the promising applications of TiO2 in the fields of environmental science.

Enhancement of the photocatalytic activity of a TiO2/carbon aerogel based on a hydrophilic secondary pore structure

The surface/interface synergy effect plays a positive role on the spatial separation and utilization of electrons and holes in photocatalytic process, which suggests a potential strategy for designing high efficiency photocatalysts.

Biomass-derived multifunctional TiO2/carbonaceous aerogel composite as a highly efficient photocatalyst

The fabrication of TiO₂/carbonaceous aerogel composite using wintermelon as source material exhibited outstanding photodegradation for organic pollutants.

Nitrogen-doped electrospun reduced graphene oxide–carbon nanofiber composite for capacitive deionization

A nitrogen-doped electrospun reduced graphene oxide–carbon nanofiber composite was synthesized though electrospinning and ammonia treatment for an electrode for capacitive deionization.

Hydrothermal synthesis of Mn-doped CdS hollow sphere nanocomposites as efficient visible-light driven photocatalysts

A series of Mn-doped CdS hollow sphere photocatalysts has been directly synthesized by a simple and facile hydrothermal route for the first time.

Highly efficient and recyclable nanocomplexed photocatalysts of AgBr/N-doped and amine-functionalized reduced graphene oxide

Although silver bromide has recently drawn considerable attention because of its high photocatalytic activity, it tends to form agglomerated metallic silver under the irradiation of visible light. Therefore, photocatalytic activity decreases with time and cannot be applied for repeated uses. To overcome this limitation, in the present work, we complexed AgBr with nitrogen doped (N-doped) and amine functionalized reduced graphene oxide (GN). N-doped and/or amine functionalized graphene shows intrinsically good catalytic activity. Besides, amine groups can undergo complexation with silver ions to suppress its reduction to metallic Ag. As a result, these complexed catalysts show excellent photocatalytic activity for the degradation of methylene blue (MB) dye under the irradiation of visible light. Photocatalytic degradation of MB shows that the catalytic activity is optimized at a condition of 0.5 wt % GN, under which ∼99% of MB was degraded only after 50 min of visible light irradiation. Notably, the complexed catalyst is quite stable and retained almost all of its catalytic activity even after greater than 10 repeated cycles. Moreover, the catalyst can also efficiently decompose 2-chlorophenol, a colorless organic contaminant, under visible light exposure. Detailed experimental investigation reveals that hydroxyl (·OH) radicals play an important role for dye degradation reactions. A relevant mechanism for dye degradation has also been proposed.

Electrochemical degradation of refractory pollutants using TiO2 single crystals exposed by high-energy {001} facets

Anodic material plays a vital role in electrochemical water treatment. Titanium dioxide (TiO2) has been widely recognized as an excellent semiconductor photocatalyst, rather than an efficient electrocatalyst, due to its relatively low electric conductivity and poor electrochemical activity. In this work, it is found that TiO2 can actually become a superior electrocatalyst when its crystal shape and exposed facet are finely tuned. The shape-engineered TiO2 single crystals with {001} facets exhibit an excellent electro-catalytic activity and stability for degrading typical organic pollutants such as rhodamine B and bisphenol A, and treating complex landfill leachate. Its electro-catalytic superiority is mainly attributed to the single-crystalline structure and exposed polar {001} facet. Our findings could provide new possibility of utilizing TiO2 for efficient electrochemical water treatment because of its high activity, great stability, low cost and no toxicity.

Electrosorption and photocatalytic one-stage combined process using a new type of nanosized TiO₂/activated charcoal plate electrode

In the present study, an activated charcoal (AC) plate was prepared by physical activation method. Its surface was coated with TiO₂ nanoparticles by electrophoretic deposition (EPD) method. The average crystallite size of TiO₂ nanoparticles was determined approximately 28 nm. The nature of prepared electrode was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area measurement before and after immobilization. The electrosorption and photocatalytic one-stage combined process was investigated in degradation of Lanasol Red 5B (LR5B), and the effect of dye concentration, electrolyte concentration, pH, voltage, and contact time was optimized and modeled using response surface methodology (RSM) approach. The dye concentration of 30 mg L(-1), Na₂SO₄ concentration of 4.38 g L(-1), pH of 4, voltage of 250 mV, and contact time of 120 min were determined as optimum conditions. Decolorization efficiency increased in combined process to 85.65% at optimum conditions compared to 66.03% in TiO₂/AC photocatalytic, 20.09% in TiO₂/AC electrosorption, and 1.91% in AC photocatalytic processes.

POME is treated for removal of color from biologically treated POME in fixed bed column: applying wavelet neural network (WNN)

As Malaysia is one of the world's largest producer of palm oil, large amounts of palm oil mill effluent (POME) is generated. It was found that negatively charged components are accountable for POME color. An attempt was made to remove residual contaminants after conventional treatment using anion base resin. Adsorption experiments were carried out in fixed bed column. Various models such as the Thomas, the Yoon-Nelson, the Wolborska and BDST model were used to fit the experimental data. It was found that only the BDST model was fitted well at the initial breakthrough time. A wavelet neural network model (WNN) was developed to model the breakthrough curves in fixed bed column for multicomponent system. The results showed that the WNN model described breakthrough curves better than the commonly used models. The effects of pH, flow rate and bed depth on column performance were investigated. It was found that the highest uptake capacity was obtained at pH 3. The exhaustion time appeared to increase with increase in bed length and decrease in flow rate.

Three-dimensional homogeneous ferrite-carbon aerogel: one pot fabrication and enhanced electro-Fenton reactivity

This work focuses on constructing a high catalytic activity cathode of an electro-Fenton system, to overcome the defects of low activity, poor stability, and intricate fabrication of supported catalysts. A series of ferrite-carbon aerogel (FCA) monoliths with different iron/carbon ratios was synthesized directly from metal-resin precursors accompanied by phase transformation. Self-doped ferrite nanocrystals and carbon matrix were formed synchronously via moderate condensation and sol-gel processes, leading to homogeneous texture. An optimal 5% ferric content FCA was composed of coin-like carbon nano-plate with continuous porous structure, and the ferric particles with diameters of dozens of nanometers were uniformly embedded into the carbon framework. The FCA exhibited good conductivity, high catalytic efficiency, and distinguished stability. When it was used as an electro-Fenton cathode, metalaxyl degradation results demonstrated that 98% TOC elimination was realized after 4 h, which was 1.5 times higher than that of the iron oxide supported electrode. It was attributed to self-doped Fe@Fe(2)O(3) ensuring Fe(II) as the mediator, maintaining high activity via reversibe oxidation and reduction by electron transfer among iron species with different valences. Meanwhile, an abundance of independent reaction microspaces were provided for every ferric crystal to in situ decompose electrogenerated H(2)O(2). Moreover, the possible catalytic mechanism was also proposed. The FCA was a promising candidate as potential cathode materials for high-performance electro-Fenton oxidation.

Design of a novel Cu₂O/TiO₂/carbon aerogel electrode and its efficient electrosorption-assisted visible light photocatalytic degradation of 2,4,6-trichlorophenol

Cu(2)O/TiO(2) heterojunction photocatalyst is built on carbon aerogel (CA) substrate with good adsorption properties by sol impregnation and seed-mediated synthesis approach. The Cu(2)O/TiO(2)/CA electrode has excellent electrosorptive and high efficient photocatalytic properties. Its morphology and surface chemical composition are characterized with SEM, TEM, X-ray diffraction (XRD) and Raman spectra. The UV-vis diffuse reflectance spectra show that the optical absorption edge for Cu(2)O/TiO(2)/CA appears at 636 nm. Under visible-light (λ > 420 nm) irradiation, the photocurrent density increment on Cu(2)O/TiO(2)/CA is 60 times of that on Cu(2)O/TiO(2)/FTO. The electrochemical characteristic is investigated with electrochemical impedance spectrum (EIS). The Cu(2)O/TiO(2)/CA electrode is further applied in the electrosorptive photodegradation of the 2,4,6-trichlorophenol (2,4,6-TCP) wastewater. The result shows that the removal ratio of 2,4,6-TCP in 5.5 h on Cu(2)O/TiO(2)/CA is 96.3% and the TOC removal is 91.3%. The intermediates generated in the degradation process are analyzed by GC-MS and HPLC. The possible mechanism of visible light photocatalytic degradation of 2,4,6-TCP on Cu(2)O/TiO(2)/CA is also studied.