Search Results (90)

This study explored the production and activation of biochar from rice straw residue for dye adsorption applications. Rice straw, a widely available but underutilized biomass, was processed to isolate lignin and generate biochar through pyrolysis at 450 °C and 550 °C. Activation using chemical agents (e.g., KOH and NaOH) was performed to enhance surface area and porosity. Among the tested conditions, KOH activation at a char-to-agent ratio of 1:3 produced activated carbon at 800 °C with the highest BET surface area (835.2 m²/g), and high fixed carbon (44.4%) after HCl washing. Thermogravimetric analysis was used to investigate pyrolysis kinetics, with activation energies determined using the Kissinger, Flynn–Wall–Ozawa, and Kissinger–Akahira–Sunose models. The brown solid showed a higher activation energy (264 kJ/mol) compared to isolated lignin (194 kJ/mol), indicating that more energy is required for decomposition. The AC was evaluated for the adsorption of methylene blue (MB) and methyl orange (MO) from aqueous solutions. Both dyes followed the Langmuir isotherm model, indicating that monolayer adsorption occurred. The maximum adsorption capacities reached 222 mg/g for MB and 244 mg/g for MO at 303 K, with higher values at elevated temperatures. Adsorption followed a pseudo-second-order kinetic model and was governed by a physisorption mechanism, as supported by thermodynamic analysis (ΔH < 20 kJ/mol and E_a < 40 kJ/mol). These findings demonstrate that KOH-activated biochar from rice straw residue is a high-performance, low-cost adsorbent for dye removal, contributing to sustainable biomass utilization and wastewater treatment. Full article

Tetracycline (TC) is widely used in medicine and livestock farming. TC is difficult to degrade and tends to persist and accumulate in aquatic environments, and it has gradually become an emerging pollutant. Biochar (BC) has strong potential for removing TC from water. This potential arises from its excellent surface properties, low-cost raw materials, and renewable nature. However, raw biomass materials are highly diverse, and their preparation conditions vary significantly. Modification methods differ in specificity and the application scenarios are complex. These factors collectively cause unstable TC removal efficiency by biochar. The chemical activation process using KOH/H₃PO₄ significantly enhanced porosity and surface functionality, transforming raw biochar into an activated carbon material with targeted adsorption capacity. Adjusting the application dosage and environmental factors (particularly pH) further enhanced the removal performance. Solution pH critically governs the adsorption efficiency: optimal conditions (pH 5–7) increased removal by 35–40% through strengthened electrostatic attraction, whereas acidic/alkaline extremes disrupted ionizable functional groups. The dominant adsorption mechanisms of biochar involved π–π interactions, pore filling, hydrophobic interactions, hydrogen bonding, electrostatic interactions, and surface complexation. In addition, the main challenges currently hindering the large-scale application of biochar for the removal of TC from water are highlighted: (i) secondary pollution risks of biochar application from heavy metals, persistent free radicals, and toxic organic leaching; (ii) economic–environmental conflicts due to high preparation/modification costs; and (iii) performance gaps between laboratory studies and real water applications. Full article

To address mass transport limitations in carbon nanofiber membrane electrodes for overall water splitting, a self-supporting nitrogen-doped hollow carbon nanofiber membrane embedded with Co/Co₂P heterojunctions (Co/Co₂P-NCNFs-H) was fabricated via continuous coaxial electrospinning. The architecture features uniform hollow channels (200–250 nm diameter, 30–50 nm wall thickness) and a high specific surface area (254 m² g⁻¹), as confirmed by SEM, TEM, and BET analysis. The Co/Co₂P heterojunction was uniformly dispersed on nitrogen-doped hollow carbon nanofibers through electrospinning, leverages interfacial electronic synergy to accelerate charge transfer and optimize the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Electrochemical tests demonstrated exceptional catalytic activity, achieving current densities of 100 mA cm⁻² at ultralow overpotentials of 405.6 mV (OER) and 247.9 mV (HER) in 1.0 M KOH—surpassing most reported transition metal catalysts for both half-reactions. Moreover, the electrode exhibited robust long-term stability, maintaining performance for nearly 20 h at 0.6 V (vs. Ag/AgCl) (OER) and over 250 h at −1.5 V (vs. Ag/AgCl) (HER), attributed to the mechanical integrity of the hollow architecture and strong metal–carbon interactions. This work demonstrates that integrating hollow nanostructures (enhanced mass transport) and heterojunction engineering (optimized electronic configurations) creates a scalable strategy for designing efficient bifunctional catalysts, offering significant promise for sustainable hydrogen production via water electrolysis. Full article

Efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) in alkaline media are essential for sustainable hydrogen production. In this study, Ni electrocatalysts were deposited on pencil graphite using a simple one-step pulsed current electrodeposition method, from both acidic Watts and alkaline citrate baths. The influence of bath type and electrodeposition parameters—current density and temperature—on catalyst morphology and performance for HER was systematically investigated by scanning electron microscopy and electrochemical methods. Linear sweep voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrocatalytic activity, stability, and HER mechanism. The best catalytic performance was achieved for the Ni electrocatalyst deposited from the citrate bath at 50 mA cm⁻² and 40 °C, showing an exchange current density of 0.93 mA cm⁻², a Tafel slope of −208 mV dec⁻¹, and overpotentials of −210 mV and −386 mV at 10 and 100 mA cm⁻², respectively, in 1 M KOH solution. Chronopotentiometry confirmed improved stability and an overpotential reduction of approximately 92 mV as compared to pure Ni, while EIS revealed the lowest charge transfer resistance. It was shown that the electrocatalysts deposited from the citrate bath outperform those from the Watts bath, and electrodeposition at 40 °C is optimal for achieving the highest electrocatalytic activity for HER. Full article

Activated carbon is widely recognized as an effective material for removing pollutants, especially pharmaceutical residues, from water. In this study, high-surface-area activated carbon derived from rice husks (RHAC) was synthesized via KOH activation and used for the adsorption of ciprofloxacin, a widely used fluoroquinolone antibiotic. Its adsorption behavior was systematically investigated through batch experiments varying the pH, adsorbent dosage, contact time, initial concentration, and temperature. The RHAC exhibited a high surface area of 1539.7 m²/g and achieved a maximum adsorption capacity of 398.4 mg·g⁻¹. The Freundlich isotherm best describes its adsorption equilibrium, suggesting multilayer adsorption on a heterogeneous surface. Kinetic modeling revealed that the adsorption process followed a pseudo second-order model (R² = 0.9981), indicating chemisorption as the rate-limiting mechanism. Thermodynamic parameters (ΔH° = 6.61 kJ/mol, ΔG° < 0) confirmed that the process was endothermic and spontaneous. These findings demonstrate that RHAC is a highly efficient, low-cost, and sustainable adsorbent for removing ciprofloxacin from aqueous environments. Full article

The indiscriminate discharge of common dyes, such as malachite green (MG), poses significant risks to water quality and human health. To address this issue, a biochar (MBC) was synthesized from waste Myriophyllum aquaticum biomass (MAB) and further activated with KOH to produce micro-mesoporous biochar (KMBC) with enhanced adsorption efficiency. Characterization results demonstrated that KMBC exhibits a higher specific surface area (1632.7 m²/g) and a larger pore volume (0.759 cm³/g) compared to MBC. Batch adsorption experiments revealed that the adsorption process follows pseudo-second-order kinetics and the Langmuir isotherm model, with the theoretical maximum adsorption capacities of MBC and KMBC reaching 1772.3 mg/g and 2570.7 mg/g, respectively and the adsorption is a spontaneous, endothermic, and entropy-driven process. Key mechanisms involved in the adsorption process include hydrogen bonding, hydrophobic interactions, and surface complexation. Due to electrostatic attraction, selective adsorption experiments confirmed that MBC can effectively separate cationic dyes such as MG from mixed anionic-cationic systems. Dynamic experiments showed that the breakthrough curve data fit well with the Thomas model. In summary, MAB-derived biochar demonstrates significant potential for practical applications in the treatment of MG-contaminated wastewater. Full article

The rapid and effective removal of pharmaceuticals and personal care products (PPCPs) from groundwater is challenging. In this paper, porous cyano group-rich g-C₃N₄ catalysts were prepared by urea (U) and a KOH-assisted thermal polymerization strategy. The thickness, active sites, and pores of g-C₃N₄ were successfully modulated by urea and KOH-assisted thermal polymerization. In addition, the charge separation efficiency of g-C₃N₄ was effectively improved by the above methods. We combine the g-C₃N₄ photocatalyst with peroxymonosulfate (PMS) to achieve the efficient degradation performance of ibuprofen. Meanwhile, we also explored the reaction mechanism of g-C₃N₄ in the photocatalytically coupled persulfate system, which illustrated the active roles of singlet oxygen and holes in the system in degrading pollutants. Our work demonstrates that the photocatalytically coupled persulfate system is an advanced technology necessary for the deep treatment of PPCPs in groundwater and suggests a feasible strategy for catalyst modulation. Full article

This study analyzes the potential of olive pomace fly ash (OPFA) as an alternative alkaline activator for electric arc furnace slag (EAFS) in the manufacture of sustainable cementitious materials. Cements were prepared by replacing 30–50 wt% of EAFS with OPFA and compared with control cements activated with potassium hydroxide (KOH) at concentrations of 4 and 8 M. Cements were characterized by bulk density, water absorption, total porosity, compressive and flexural strength, as well as analytical techniques such as XRD, FTIR and SEM-EDS. The results reveal that the incorporation of 40 wt% OPFA provides optimum properties, reaching maximum compressive and flexural strengths of 20.0 MPa and 5.7 MPa, respectively, after 28 days of curing. These improvements are attributed to the increased formation of C,K-A-S-H gel, which incorporates Fe, the main reaction product that densifies the matrix and reduces porosity. However, 30 wt% OPFA provides insufficient alkali content, which limits the reaction, while excess alkali at 50 wt% OPFA reduces mechanical performance due to unreacted residues and increased interconnected porosity. Compared to KOH-activated cements, which achieve maximum flexural and compressive strengths of 4.4 and 9.5 MPa (EAFS/KOH-8M binders), the results confirm the potential of OPFA as an alternative activator, with significant sustainability advantages. Full article

Anion Exchange Membranes (AEMs) are promising materials for electrochemical devices, such as fuel cells and electrolyzers. However, the main drawback of AEMs is their low durability in alkaline operating conditions. A possible solution is the use of composite ionomers containing inorganic fillers stable in a basic environment. In this work, composite anion exchange membranes are prepared from poly (2,6-dimethyl-1,4-phenylene oxide) with quaternary ammonium groups on long-side chains (PPO-LC) and exfoliated Mg/Al lamellar double hydroxide (LDH) as inorganic filler added in different percentages (2, 5, and 10%). The mechanical stiffness of the membranes increases significantly by the addition of exfoliated LDH up to 5%. The ionic conductivity is measured as a function of the temperature in fully humidified conditions and as a function of relative humidity (RH). The maximum conductivity is observed for 5% LDH. The average activation energy for conductivity amounts to 0.20 ± 0.01 eV in fully humidified conditions and >50% RH. Thermogravimetric analysis of membranes before and after alkaline degradation tests (2 M KOH @ 80 °C, 48 h) reveals that the sample with 5% LDH has improved stability (19% vs. 36% of degradation). The stability tests are also investigated, measuring the ionic conductivity and the water uptake. A protective effect of LDH on the alkaline degradation of quaternary ammonium groups is clearly evidenced and opens the way to the use of different compounds and exfoliation methods in the LDH family. Full article

The present work analyzes the behavior of an activated carbon fabricated from almond shells for the removal of cationic dyes (methylene blue, MB, and malachite green, MG) by adsorption from aqueous solutions. The carbonized precursor was activated with KOH at a 1:2 (w/w) ratio with the objective of increasing both the surface area and the pore volume. Both non-activated and activated carbon were characterized in different aspects of interest in dye adsorption studies (surface structure, point of zero charge, specific surface area, and pore size distribution). The effect of the dye’s initial concentration and adsorbent dosage on dye removal efficiency and carbon adsorption capacity was studied. Adsorption kinetics were analyzed under different experimental conditions, and different models were assayed to determine the adsorption mechanism. Dye adsorption in the adsorbent surface could be considered the rate-limiting step. Different adsorption equilibrium models were evaluated to fit the experimental data. This adsorbent allowed us to reach high Langmuir adsorption capacity for both dyes (MB: 341 mg·g⁻¹, MG: 364 mg·g⁻¹ at 25 °C and 0.5 g·L⁻¹). Moreover, kinetic and equilibrium adsorption data have been used to simulate breakthrough curves in a packed-bed column using different conditions (bed length, liquid flowrate, and dye initial concentration). The simulation results showed that almond shell activated carbon is a suitable adsorbent for methylene blue and malachite green removal from wastewater. Full article

In this research, we produced two types of biochar (BC) using cotton stalks as raw material and KOH as an activator, and compared their performance and adsorption mechanisms in the removal of tetracycline (TC) and methylene blue (MB) from wastewater. The results showed that the biochar generated using both procedures formed pores that connected to the interior of the biochar and had extensive microporous and mesoporous structures. The molten salt approach produces biochar with a higher specific surface area, larger pore size, and higher pore volume than the impregnation method, with a maximum specific surface area of 3095 m²/g. KBCM-900 (the BC produced using the molten salt method at 900 °C) had a better adsorption effect on TC, with a clearance rate of more than 95% in 180 min and a maximum adsorption amount of 912.212 mg/g. The adsorption rates of the two BCs for MB did not differ significantly at low concentrations, but as the concentration increased, KBCI-900 (the BC generated by the impregnation method at 900 °C) exhibited better adsorption, with a maximum adsorption of 723.726 mg/g. The pseudo-second-order kinetic model and the Langmuir isotherm model may accurately describe the TC and MB adsorption processes of KBCI-900 and KBCM-900. The KBCI/KBCM-900 adsorption process combines physical and chemical adsorption, with the primary mechanisms being pore filling, π–π interactions, hydrogen bonding, and electrostatic interactions. As a result, biochar generated using the molten salt method is suitable for the removal of large-molecule pollutants such as TC, whereas biochar prepared using the impregnation method is suitable for the removal of small-molecule dyes such as MB. Full article

High-performance and cost-efficient electrocatalysts and electrodes are needed to improve the hydrogen evolution reaction (HER) for the hydrogen (H₂) generation in electrolysers, including microbial electrolysis cells (MECs). In this study, free-standing carbon nanofiber (CNF) films with supported cobalt phosphide nanoparticles have been prepared by means of an up-scalable electrospinning process followed by a thermal treatment under controlled conditions. The produced cobalt phosphide-supported CNF films show to be nanoporous (pore volume up to 0.33 cm³ g⁻¹) with a high surface area (up to 502 m² g⁻¹) and with a suitable catalyst mass loading (up to 0.49 mg cm⁻²). Values of overpotential less than 140 mV at 10 mA cm⁻² have been reached for the HER in alkaline media (1 M KOH), which demonstrates a high activity. The high electrical conductivity together with the mechanical stability of the free-standing CNF films allowed their direct use as cathodes in a MEC reactor, resulting in an exceptionally low voltage operation (0.75 V) with a current density demand of 5.4 A m⁻². This enabled the production of H₂ with an energy consumption below 30 kWh kg⁻¹ H₂, which is highly efficient. Full article

Understanding the mechanism of KOH activation in the preparation of activated carbon (AC) enables more efficient utilization of biomass. In this study, brewer’s spent grains (BSGs) were carbonized at 500 °C to produce biochar (BC), followed by KOH activation under different activation conditions. The gas and solid products generated during the activation process were analyzed by gas chromatography (GC), X-ray diffraction (XRD), Raman analysis, a surface area and pore size analyzer, and X-ray photoelectron spectroscopy (XPS). The results show that increasing the KOH/BC ratio or the activation temperature could both promote gas production. XPS results indicated that the activator reacted first with -COOH and then with -OH of ACs, with AC_5-700 having the highest C-OH content (50.04%). As the KOH/BC ratio increased, more aromatic structures were destroyed, and the porosity of ACs was significantly enhanced, with AC_7-700 having the highest Brunauer–Emmett–Teller (BET) specific surface area (S_BET) (2997.69 m²/g). At low temperatures, KOH reacted with the active groups of BC and carbon at the edge of the aromatic structure. At high temperatures, the activator (KOH, K₂O, and K₂CO₃) reacted with carbon in the aromatic structure to generate a large number of pores on ACs and expand them. ACs exhibited more pores with higher KOH addition, and a higher activation temperature did not generate more new pores, but expanded the pores more significantly than high KOH addition. Full article

The excellent mechanical properties of alkaline-activated tailings are essential for their increased use in building materials. While numerous studies have been conducted on activated tailings, the strength of alkaline-activated tungsten slag has not been extensively explored due to the low reactivity of silicon and aluminum in these tailings. This research delves into the early unconfined compressive strength of tungsten tailings activated by two alkali solutions (NaOH and KOH) at three different alkali concentrations (mass ratio of alkali to tungsten tailings), cured at 80 °C over periods of one day, three days, and seven days. The study finds significant improvements in the stability of tungsten tailings when forming (C, N)-A-S-H or (C, K)-A-S-H gels with both alkalis. Scanning Electron Microscope (SEM) results show that the morphology of the (C, N)-A-S-H gels transitions from membranous to flocculated and then to a three-dimensional network as the NaOH content and curing time increase. Conversely, the (C, K)-A-S-H gels primarily exhibit thin-film morphology with some three-dimensional network structures. The presence of flocculation and three-dimensional mesh in the gels fosters the formation of a robust skeletal structure, enhancing the strength of the samples. Furthermore, specimens treated with NaOH solution exhibit a higher gel content compared to those treated with KOH solution. These factors contribute to the superior efficacy of sodium hydroxide in enhancing the strength of tungsten tailings compared to potassium hydroxide. X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) results identify the formation of new phases such as pirssonite, buetschliite, potassium bicarbonate, and potassium carbonate. The first new phase results from the carbonization of excess NaOH solution, while the latter phases arise from the carbonization of excess KOH solution. These carbonization processes negatively impact the strength of the materials. Full article

This work synthesizes a xerogel from a sol–gel synthesis strategy and supports it on N-doped carbon support from spent coffee biomass (Mn(II)O/N-CC, hereafter MnO) as an efficient oxygen reduction reaction (ORR) catalyst in alkaline electrolytes. The effects of N-CC carbon content on MnO nanoparticle size, dispersion, distribution, morphology, and electrochemistry on ORR are discussed. The SEM and TEM measurements show that increasing the N-CC content during the MnO gelation reaction improved MnO dispersion and particle size during thermal treatment, increasing the ORR’s electrochemical active surface area. Several physiochemical and electrochemical characterizations show a clear relationship between N-CC catalysts and ORR activities. The best catalyst, MnO/N-CC-5, had an even distribution of 27 nm MnO nanoparticles on the N-CC support. The MnO/N-CC-5 catalyst had almost identical ORR kinetics and stability to those of the state-of-the-art Pt/C catalyst in 0.1 M KOH electrolytes, losing only 10 mV in half-wave potential after 5000 potential cycles and retaining 96% of current for over 10 h of continuous chronoamperometric stability. By measuring the electrochemical active surface areas of various catalysts by cyclic voltammetry at different scan rates and measuring the double layer capacitance (C_dl) and ECSA, MnO/N-CC-5 catalysts were shown to have enhanced ORR activity. The XPS analysis explains the ORR activity in terms of the Mn³⁺/Mn⁴⁺ ratio, and a mechanism was proposed. These findings suggest that the MnO/N-CC-5 catalyst could be a cathode catalyst in fuel cells, biofuel cells, metal–air batteries, and other energy conversion devices. Full article