High-performance biochar-loaded MgAl-layered double oxide adsorbents derived from sewage sludge towards nanoplastics removal: Mechanism elucidation and QSAR modeling.

Utilization of sewage sludge for the fabrication of environmental functional materials is highly desirable to achieve pollution mitigation and resource recovery. In the present work, we introduced a novel MgAl-layered double oxide (LDO)@biochar composite adsorbent in-situ fabricated from Al-rich sewage sludge, and its excellent application in nanoplastics adsorption. Initially, fifteen model contaminants with varied conjugate structures, hydrogen bonding and ionic properties were selected for an investigation of adsorption behavior and adsorption selectivity on LDO@biochar. Structural variation of LDO@biochar suggested reconstruction of the layered double hydroxide (LDH) during the adsorption process due to the "memory effect". Under the synergy of LDH and biochar, the contaminants were adsorbed via multiple adsorbent-adsorbate interactions, including anion exchange, electrostatic interaction, hydrogen bonding and π-π conjugation. Then, a quantitative structure-activity relationship (QSAR) model was constructed by integrating the number of hydrogen bond acceptors, polarity surface area, number of aromatic rings, and Fukui index f(-)x together to reflect the affinity of each contaminant to the adsorbent. Guided by the QSAR model, the negatively charged polystyrene nanoplastics with continuously conjugated aromatic rings were predicted to be effectively adsorbed on LDO@biochar. Experimental tests confirmed a great capacity of LDO@biochar towards the polystyrene nanoplastics, given the equilibrium adsorption capacity as high as 360 mg g-1 at 30-50 °C. This work not only opened up a new avenue for sustainable utilization of sewage sludge towards high-performance environmental functional materials, but also demonstrated the potential of the QSAR analysis as a rapid and accurate approach for guiding the application of an adsorbent to new emerging containments.

Kinetics and mechanism study of dyes degradation in electric field-promoting catalytic wet air oxidation process.

Wet air oxidation (WAO) is a clean and eco-friendly technology for dyes removal, but the high operating temperature and pressure limit its practical application. In the present work, an electric field-promoting (EF-promoting) catalytic WAO process is developed to degrade dyes under room condition. The oxidation kinetics of four different types of dyes and their degradation pathways are studied. A kinetic model is constructed by including the exogenous electric field into the Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism framework, and quantitative structure-activity relationship (QSAR) analysis is conducted to correlate the kinetic parameters to the physicochemical properties of the dyes. A negative linear relationship is found between the adsorption equilibrium constants of the dyes and their first ionization energies, and their surface reaction rate constants are positively linearly associated to Esum (ELUMO + EHOMO). The degradation pathways of the different dyes are proposed according to the degradation intermediates and the activities of the atoms within the dye molecules. The heteroatoms N and S, and the atom C connecting the aromatic rings are identified as the susceptible sites upon the electrophilic attack of O2. Bond cleavage at these sites gives rise to aromatic fragments which are eventually mineralized via carboxyl acids. The results of this work is helpful for guiding the design and operation of the EF-promoting catalytic WAO process into the treatment of various dye wastewaters.

Bifunctional Ni@NiO catalyst supported on loofah sponge-derived carbon for electrocatalytic air oxidation of biorefractory pollutant in a coupling system.

Oxygen (O2) in the air is a green oxidant, and utilization of air for pollutant removal is highly desired. Herein, we report the preparation and utilization of a novel biomass-based three-dimensional (3D) Ni@NiO/carbon composite for the electro-activation of O2 under room condition. The carbon-coated Ni@NiO nanoparticles are fabricated on a hierarchical 3D porous loofah sponge-derived carbon (LSC) support as the bifunctional catalyst for the activation of O2 via both the electro-oxidation and electro-reduction reactions. An electrocatalytic air oxidation coupling system is constructed with the Ni@NiO/LSC shell-core electrodes for pollutant degradation. A variety of organic pollutants, including pharmaceutics and personal care products (PPCPs), dyes, phenolic compounds, and real waters are mineralized by more than 60% with significantly enhanced biodegradability. Notably, the coupling system obtains high mineralization efficiency of 70.2 ± 1.9% on landfill leachate with significant biodegradability enhancement. The specific energy consumptions of the coupling system are only 6.8 ± 0.7 to 60.2 ± 3.6 kWh kg-TOC-1 in mineralizing different pollutants. The hollow structure of the LSC fibers endows the loaded Ni@NiO with superior intrinsic catalytic activity, which is associated with low reaction resistance and facile electron transfer. The Ni@NiO on LSC presents an electrocatalytic wet air oxidation (ECWAO) catalytic activity higher by 35.8% and cathodic air oxidation (CAO) catalytic activity higher by 22.7% as compared to that loaded on commercial graphite felt.

MOF-derived MnO@C with high activity for electric field-assisted catalytic oxidation of aqueous pollutants.

The activation of oxygen (O2) under room condition is important for the utilization of air to perform oxidation. Here, we report a porous carbon-encapsulated MnO (MnO@C) derived from Mn metal-organic framework (MOF)grown in-situ on a graphite felt (GF) support. The MnO@C exhibits superior catalytic activity in an electric field-assisted catalytic oxidation system for the degradation of organic pollutants under room condition. The catalytic oxidation reaction applies a surface reaction pathway in which the surface-bound chemisorbed oxygen species are electro-oxidized and then involved in the oxidation of co-adsorbed organic pollutants. The abundant oxygen vacancies and oxygenated functional groups in MnO@C provide active sites for the chemisorption of O2, and its conductive mesoporous structure allows facile electrons and mass transfer. As a result, the MnO@C/GF catalyst displays quite high turnover frequency (TOF) value as 0.038 mg-TOC mg-MnO-1 min-1, which is 6.66 times higher than that of the MnO/GF catalyst prepared by impregnation method as a comparison. With the aid of + 1.0 V of positive electric field, the catalytic oxidation system exhibits extensive effectiveness in mineralizing a variety of dyes, pharmaceuticals, personal care products, and phenolic compounds under room condition with significantly enhanced biodegradability.

Manganese-doped molybdenum oxide boosts catalytic performance of electrocatalytic wet air oxidation at ambient temperature.

Mn-doping strategy was adopted to modify the structure of MoO2 for enhancing its catalytic activity towards room-temperature electrocatalytic wet air oxidation (ECWAO) reaction. A series of Mn-doped MoO2 were prepared on carbon support, and their structures were investigated to elucidate the productive effect of Mn doping on the catalytic activity of MoO2. The incorporation of MnIII/MnII into the MoO2 lattice induced the transformation from MoIV to MoV and created more oxygen vacancies. Such structural modifications promoted the electron transfer of MoO2 through the redox couples between MoVI/MoV/MoIV and MnIII/MnII, and facilitated the transformation from O2 to adsorbed oxygen species on MoO2 surface. As a result, the ECWAO catalytic activities of Mn-doped MoO2/graphite felt (MoO2/GF) outperformed the activity of MoO2/GF. Among the synthesized series, Mn0.066:MoO2/GF exhibited the highest activity with the maximum turnover frequency (TOF) promoted by 59% than the undoped MoO2/GF. Under the catalysis of Mn0.066:MoO2/GF, the ECWAO process obtains mineralization efficiencies generally above 85% in degrading typical pharmaceutics and person care products (PPCPs). These findings are anticipated to open up a new venue in the design and fabrication of highly active catalysts for air oxidation reactions by using the strategy of selective dopant-induced structure modification.

Bimetal heterointerfaces towards enhanced electro-activation of O2 under room condition.

Efficient catalysts for oxygen (O2) activation under room condition are required for effective wet air oxidation (WAO) technology. Here, we report a novel manganese-cobalt-based composite (MnO-CoO@Co) fabricated on a graphite felt (GF) support for catalyzing the electro-activation of O2 under room condition. Abundant Co-MnO and CoO-MnO heterointerfaces are formed in the composite. In comparison to the single-metal counterparts, i.e. CoO@Co/GF (16.99 wt% Co) and MnO/GF (26.83 wt% Mn), the bimetal MnO-CoO@Co/GF (5.29 wt% Co and 8.79 wt% Mn) displays an improved oxygen storage capacity and provides more active sites to accommodate surface adsorbed oxygen species. Notably, the strong synergy derived from bimetal heterointerfaces enhances the electron transfer and oxygen mobilization during the electro-activation of O2, thereby significantly reducing the reaction barrier. MnO-CoO@Co/GF exhibits excellent efficiency and stability in electrocatalytic WAO (ECWAO) towards the removal of pharmaceuticals and personal care products (PPCPs) over a wide pH range from 4.0 to 10.0. A model pollutant sulfamethoxazole (SMX) acquires mineralization efficiency of 78.4 ± 2.1% and mineralization current efficiency of 157.89% at +1.0 V of electrode potential. The toxicity of PPCPs can be totally eliminated after the ECWAO treatment. This work highlights the synergy derived from bimetal heterointerfaces in O2 electrocatalysis, and provides a promising approach for advanced WAO catalysts in PPCPs pollution control.

Room-temperature air oxidation of organic pollutants via electrocatalysis by nanoscaled Co-CoO on graphite felt anode.

Oxygen (O2) in air is an eco-friendly and economical oxidant. However, its activation is an energy-intensive process requiring high operation temperature. Herein, we report the synthesis of nanoscaled Co-CoO on a graphite felt (GF) as an anode material for electrocatalytic wet air oxidation (ECWAO) of water contaminants at room temperature. Such an ECWAO process shows extensive effectiveness in mineralizing a variety of biorefractory organic pollutants. A probe pollutant, bisphenol A (BPA), is rapidly degraded in 180 min with mineralization efficiencies higher than 85% over a wide pH range from 3.0 to 11.0. The Co-CoO/GF electrode exhibits excellent stability in the ECWAO process, without loss of activity and leaching of metal. The ECWAO process is confirmed to be initiated by the electrochemical activation of O2 through a non-radical pathway. The CoO on the surface of Co nanoparticle is identified as the catalytically active site, at which O2 molecules are first converted to chemisorbed oxygen species and then electrochemically oxidized to their activated states. The ECWAO process with the Co-CoO/GF electrode presents the merits of high efficiency, low energy input and environmental friendliness, and has a great potential for practical wastewater treatment.

Electro-activation of O2 on MnO2/graphite felt for efficient oxidation of water contaminants under room condition.

The activation of oxygen (O2) under room condition is highly desirable for oxidative removal of organic pollutants in water. Herein, we report a graphite felt (GF)-supported α-MnO2 catalyst which is active for activating O2 with assistance of an anodic electric field. The electro-assisted catalytic wet air oxidation (ECWAO) process on MnO2/GF is able to rapidly degrade a variety of dyes, pharmaceutics and personal care products (PPCPs) under room condition. The congo red, basic fuchsin, neutral red and methylene blue are completely mineralized in 160 min, and the bisphenol A, triclosan and ciprofloxacin are mineralized by 89.9%, 81.5% and 65.4%, respectively, in 300 min. Mechanistic study indicates a surface-catalyzed non-free radical pathway for the oxidation of organic pollutants by O2 in the ECWAO process. The oxygen vacancies on MnO2 are identified as the catalytically active sites, at which oxygen atom is transferred from O2 to organic molecule through chemisorbed oxygen species. The anodic electric field assists such an oxygen transfer pathway by activating the complex of chemisorbed oxygen species and organic molecule through electro-oxidation reaction. The ECWAO process on MnO2/GF electrode exhibits a great potential for practical wastewater treatment under room condition.

Excellent performance of electro-assisted catalytic wet air oxidation of refractory organic pollutants.

Catalytic wet air oxidation (CWAO) is a clean process for the treatment of toxic and/or biorefractory contaminants while it is a challenge to perform the CWAO at room temperature. Herein, we report an electro-assisted CWAO (ECWAO) process using partially oxidized nickel (Ni@NiO) immobilized on a porous graphite felt (GF) as a catalytic anode. Such a process demonstrates extensive effectiveness and good stability in the deep oxidation of various organic pollutants including triclosan (TCS), bisphenol A, ciprofloxacin, sulfamethoxazole, congo red, crystal violent and rodamine B at room temperature. The typical pollutant TCS is rapidly degraded within 105 min with a mineralization efficiency of 86.0 ±â€¯1.4%, at specific energy consumption of 5.3 ±â€¯0.3 kW h kg-TOC-1. NiO is identified as the catalytically active site at which O2 is dissociated and electro-activated to oxidize the TCS at room temperature. The ECWAO process on the anodic Ni@NiO/GF presents a great potential as an environmental-friendly and energy-saving process for treating high concentrations of refractory organic pollutants under ambient condition.

Harvest and utilization of chemical energy in wastes by microbial fuel cells.

Organic wastes are now increasingly viewed as a resource of energy that can be harvested by suitable biotechnologies. One promising technology is microbial fuel cells (MFC), which can generate electricity from the degradation of organic pollutants. While the environmental benefits of MFC in waste treatment have been recognized, their potential as an energy producer is not fully understood. Although progresses in material and engineering have greatly improved the power output from MFC, how to efficiently utilize the MFC's energy in real-world scenario remains a challenge. In this review, fundamental understandings on the energy-generating capacity of MFC from real waste treatment are provided and the challenges and opportunities are discussed. The limiting factors restricting the energy output and impairing the long-term reliability of MFC are also analyzed. Several energy storage and in situ utilization strategies for the management of MFC's energy are proposed, and future research needs for real-world application of this approach are explored.

Bioelectricity-assisted partial degradation of linear polyacrylamide in a bioelectrochemical system.

The wide application of water-soluble linear polyacrylamides (PAMs) can cause serious environmental pollution. Biological treatment of PAMs receives very limited efficiency due to their recalcitrance to the microbial degradation. Here, we show the bioelectrochemical system (BES) can be used as an effective strategy to improve the biodegradation efficiency of PAMs. A linear PAM with viscosity-average molecular weight of 5 × 10(6) was treated in the anodic chamber of BES reactor, and the change of PAM structure during the degradation process was investigated. The anodic bacteria in the BES demonstrated abilities to utilize the PAM as the sole carbon and nitrogen source to generate electricity. Both the anode-attached and planktonic bacteria contributed to the electricity generation, while the anode-attached community exhibited stronger electron transfer ability than the planktonic one. The closed-circuit and open-circuit operations of the BES reactor obtained chemical oxygen demand (COD) removal efficiencies of 32.5 and 7.4 %, respectively, implying the generation of bioelectricity could enhance the biodegradation of PAM. Structure analysis suggested the carbon chain of PAM was partially degraded in the BES, producing polymeric products with lower molecular weight. The microbial cleavage of the carbon chain was proposed to start from the "head-to-head" linkages and end with the formation of ether bonds.

Effective sulfur and energy recovery from hydrogen sulfide through incorporating an air-cathode fuel cell into chelated-iron process.

The chelated-iron process is among the most promising techniques for the hydrogen sulfide (H2S) removal due to its double advantage of waste minimization and resource recovery. However, this technology has encountered the problem of chelate degradation which made it difficult to ensure reliable and economical operation. This work aims to develop a novel fuel-cell-assisted chelated-iron process which employs an air-cathode fuel cell for the catalyst regeneration. By using such a process, sulfur and electricity were effectively recovered from H2S and the problem of chelate degradation was well controlled. Experiment on a synthetic sulfide solution showed the fuel-cell-assisted chelated-iron process could maintain high sulfur recovery efficiencies generally above 90.0%. The EDTA was preferable to NTA as the chelating agent for electricity generation, given the Coulombic efficiencies (CEs) of 17.8 ± 0.5% to 75.1 ± 0.5% for the EDTA-chelated process versus 9.6 ± 0.8% to 51.1 ± 2.7% for the NTA-chelated process in the pH range of 4.0-10.0. The Fe (III)/S(2-) ratio exhibited notable influence on the electricity generation, with the CEs improved by more than 25% as the Fe (III)/S(2-) molar ratio increased from 2.5:1 to 3.5:1. Application of this novel process in treating a H2S-containing biogas stream achieved 99% of H2S removal efficiency, 78% of sulfur recovery efficiency, and 78.6% of energy recovery efficiency, suggesting the fuel-cell-assisted chelated-iron process was effective to remove the H2S from gas streams with favorable sulfur and energy recovery efficiencies.

Carbonate-mediated Fe(II) oxidation in the air-cathode fuel cell: a kinetic model in terms of Fe(II) speciation.

Due to the high redox activity of Fe(II) and its abundance in natural waters, the electro-oxidation of Fe(II) can be found in many air-cathode fuel cell systems, such as acid mine drainage fuel cells and sediment microbial fuel cells. To deeply understand these iron-related systems, it is essential to elucidate the kinetics and mechanisms involved in the electro-oxidation of Fe(II). This work aims to develop a kinetic model that adequately describes the electro-oxidation process of Fe(II) in air-cathode fuel cells. The speciation of Fe(II) is incorporated into the model, and contributions of individual Fe(II) species to the overall Fe(II) oxidation rate are quantitatively evaluated. The results show that the kinetic model can accurately predict the electro-oxidation rate of Fe(II) in air-cathode fuel cells. FeCO3, Fe(OH)2, and Fe(CO3)2(2-) are the most important species determining the electro-oxidation kinetics of Fe(II). The Fe(II) oxidation rate is primarily controlled by the oxidation of FeCO3 species at low pH, whereas at high pH Fe(OH)2 and Fe(CO3)2(2-) are the dominant species. Solution pH, carbonate concentration, and solution salinity are able to influence the electro-oxidation kinetics of Fe(II) through changing both distribution and kinetic activity of Fe(II) species.

A fuel-cell-assisted iron redox process for simultaneous sulfur recovery and electricity production from synthetic sulfide wastewater.

Sulfide present in wastewaters and waste gases should be removed due to its toxicity, corrosivity, and malodorous property. Development of effective, stable, and feasible methods for sulfur recovery from sulfide attains a double objective of waste minimization and resource recovery. Here we report a novel fuel-cell-assisted iron redox (FC-IR) process for simultaneously recovering sulfur and electricity from synthetic sulfide wastewater. The FC-IR system consists of an oxidizing reactor where sulfide is oxidized to elemental sulfur by Fe(III), and a fuel cell where Fe(III) is regenerated from Fe(II) concomitantly with electricity producing. The oxidation of sulfide by Fe(III) is significantly dependent on solution pH. Increasing the pH from 0.88 to 1.96 accelerates the oxidation of sulfide, however, lowers the purity of the produced elemental sulfur. The performance of fuel cell is also a strong function of solution pH. Fe(II) is completely oxidized to Fe(III) when the fuel cell is operated at a pH above 6.0, whereas only partially oxidized below pH 6.0. At pH 6.0, the highest columbic efficiency of 75.7% is achieved and electricity production maintains for the longest time of 106 h. Coupling operation of the FC-IR system obtains sulfide removal efficiency of 99.90%, sulfur recovery efficiency of 78.6 ± 8.3%, and columbic efficiency of 58.6 ± 1.6%, respectively. These results suggest that the FC-IR process is a promising tool to recover sulfur and energy from sulfide.

An integrated approach to optimize the conditioning chemicals for enhanced sludge conditioning in a pilot-scale sludge dewatering process.

An integrated approach incorporating response surface methodology (RSM), grey relational analysis, and fuzzy logic analysis was developed to quantitatively evaluate the conditioning chemicals in sludge dewatering process. The polyacrylamide (PAM), ferric chloride (FeCl(3)) and calcium-based mineral powders were combined to be used as the sludge conditioners in a pilot-scale sludge dewatering process. The performance of conditioners at varied dosages was comprehensively evaluated by taking into consideration the sludge dewatering efficiency and chemical cost of conditioner. In the evaluation procedure, RSM was employed to design the experiment and to optimize the dosage of each conditioner. The grey-fuzzy logic was established to quantify the conditioning performance on the basis of grey relational coefficient generation, membership function construction, and fuzzy rule description. Based on the evaluation results, the optimal chemical composition for conditioning was determined as PAM at 4.62 g/kg DS, FeCl(3) at 55.4 g/kg DS, and mineral powders at 30.0 g/kg DS.