In this work, a feasible one-pot approach to synthesize manganese oxide/graphene composites, the so-called plasma-enhanced electrochemical exfoliation process (PE3P), has been developed. Herein, a ...composite of graphene decorated with manganese oxide nanoparticles was prepared via PE3P from a KMnO4 solution and graphite electrode under a voltage of 70 V in an ambient environment. By controlling the initial KMnO4 concentration, we obtained distinct MnO2/graphene samples. The prepared samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and Raman spectroscopy. The electrochemical measurements of the MnO2/graphene composites revealed that the specific capacitance of the samples is approximately 320 F g-1 at a scan rate of 10 mV s-1, which is comparably very high for manganese oxide/carbon-based supercapacitor electrode materials. Considering the simple, low-cost, one-step and environmentally friendly preparation, our approach has the potential to be used for the fabrication of MnO2/graphene composites as the electrode materials of supercapacitors.
•Properties and mechanisms of Cr(VI) adsorption onto LDH-based materials reviewed.•Such materials: high anion exchange capacity and positively charged external surface.•Identified mechanism: anion ...exchange, adsorption-coupled reduction, electrostatic attraction.•Adsorption-coupled reduction: identified by some advanced techniques from 2016.•Thermodynamic parameters: ΔG° <0, ΔH° >0, and ΔS° >0 in the most observation cases.
An attempt has been made in this review to provide some insights into the possible adsorption mechanisms of hexavalent chromium onto layered double hydroxides-based adsorbents by critically examining the past and present literature. Layered double hydroxides (LDH) nanomaterials are typical dual-electronic adsorbents because they exhibit positively charged external surfaces and abundant interlayer anions. A high positive zeta potential value indicates that LDH has a high affinity to Cr(VI) anions in solution through electrostatic attraction. The host interlayer anions (i.e., Cl−, NO3−, SO42−, and CO32−) provide a high anion exchange capacity (53–520 meq/100 g) which is expected to have an excellent exchangeable capacity to Cr(VI) oxyanions in water. Regarding the adsorption-coupled reduction mechanism, when Cr(VI) anions make contact with the electron-donor groups in the LDH, they are partly reduced to Cr(III) cations. The reduced Cr(III) cations are then adsorbed by LDH via numerous interactions, such as isomorphic substitution and complexation. Nonetheless, the adsorption-coupled reduction mechanism is greatly dependent on: (1) the nature of divalent and trivalent salts utilized in LDH preparation, and the types of interlayer anions (i.e., guest intercalated organic anions), and (3) the adsorption experiment conditions. The low Brunauer–Emmett–Teller specific surface area of LDH (1.80–179 m2/g) suggests that pore filling played an insignificant role in Cr(VI) adsorption. The Langmuir maximum adsorption capacity of LDH (Qomax) toward Cr(VI) was significantly affected by the natures of used inorganic salts and synthetic methods of LDH. The Qomax values range from 16.3 mg/g to 726 mg/g. Almost all adsorption processes of Cr(VI) by LDH-based adsorbent occur spontaneously (ΔG° <0) and endothermically (ΔH° >0) and increase the randomness (ΔS° >0) in the system. Thus, LDH has much potential as a promising material that can effectively remove anion pollutants, especially Cr(VI) anions in industrial wastewater.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Calcium carbonate (CaCO3)-enriched biomaterial derived from freshwater mussel shells (FMS) was used as a non-porous biosorbent to explore the characteristics and mechanisms of cadmium adsorption in ...aqueous solution. The adsorption mechanism was proposed by comparing the FMS properties before and after adsorption alongside various adsorption studies. The FMS biosorbent was characterized using nitrogen adsorption/desorption isotherm, X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, Fourier-transform infrared spectroscopy, and point of zero charge. The results of batch experiments indicated that FMS possessed an excellent affinity to Cd(II) ions within solutions pH higher than 4.0. An increase in ionic strength resulted in a significant decrease in the amount of Cd(II) adsorbed onto FMS. Kinetic study demonstrated that the adsorption process quickly reached equilibrium at approximately 60 min. The FMS biosorbent exhibited the Langmuir maximum adsorption capacity as follows: 18.2 mg/g at 10 °C < 26.0 mg/g at 30 °C < 28.6 mg/g at 50 °C. The Cd(II) adsorption process was irreversible, spontaneous (−ΔG°), endothermic (+ΔH°), and more random (+ΔS°). Selective order (mmol/g) of metal cations followed as Pb2+ > Cd2+ > Cu2+ > Cr3+ > Zn2+. For column experiments, the highest Thomas adsorption capacity (7.86 mg/g) was achieved at a flow rate (9 mL/min), initial Cd(II) concentration (10 mg/L), and bed height (5 cm). The Cd(II) removal by FMS was regarded as non-activated chemisorption that occurred very rapidly (even at a low temperature) with a low magnitude of activation energy. Primary adsorption mechanism was surface precipitation. Cadmium precipitated in the primary (Cd,Ca)CO3 form with a calcite-type structure on the FMS surface. A crust of rhombohedral crystals on the substrate was observed by SEM. Freshwater mussel shells have the potential as a renewable adsorbent to remove cadmium from water.
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•Adsorption process: conducted in batch and laboratory-scale column experiments.•Maximum adsorption capacity: FMS (26 mg/g) and commercial activated carbon (32 mg/g).•Cadmium precipitates: identified as (Cd,Ca)CO3 and CdCO3.•Non-activated chemisorption: occurring very rapidly with low activation energy (10.2 kJ/mol).•Adsorption process: irreversible, spontaneous, endothermic, and more random.
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The use of nonlinear fitting of data of isotherm and kinetics is becoming more frequently used in the adsorption literature since the authors are becoming convinced that the use of linear isotherm ...and kinetics adsorption models could lead to extremally erroneous results. The utilization of nonlinear form of the van't Hoff equation for calculating the changes in enthalpy (∆H°) and changes in entropy (∆S°) of an adsorption process has not been reported in the literature until now. In this study, the values of equilibrium constants obtained from isotherms of adsorption (n = 57) at several temperatures (i.e., Langmuir, Liu, and Sips) were used to calculate the thermodynamic parameters of adsorption by the linear form and nonlinear form of the van't Hoff equation. Results demonstrated that the sign of thermodynamic parameters obtained from the linear and nonlinear forms was the same, whereas the magnitude of the nonlinear method is slightly higher than the linearized method. Two statistical methods—the parametric statistical (paired t-test) and non-parametric statistical (non-parametric Wilcoxon signed-rank paired test)—were applied for the comparison of difference. Although the difference on the magnitude of the thermodynamic parameters (obtained from the linear and nonlinear forms) was not apparently too enormously (n = 57), the statistical result at the 90%, 95% and 99% of probability level indicated that the difference was significant, being the values of the thermodynamic parameters obtained by the nonlinear method statistically higher than the values of these parameters obtained by the linearized method. Therefore, the nonlinear form of the van't Hoff equation should be used for accurately calculating the ∆H° and ∆S° parameters in the study of adsorption thermodynamics.
•Nonlinear and linear fitting of van't Hoff equation•Statistical comparison of nonlinear and linear fitting of van't Hoff equation•Thermodynamic parameters of nonlinear fitting are statistically higher than the linear method.
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This paper provided further discussion on some identified mistakes and inconsistencies. Such problems included (1) the determination and discussion on the pHPZC value of the biochar, (2) the ...excellent adsorption capacity of the biochar toward the dye contaminant, (3) the proposd adsorption mechanism (i.e., chemical adsorption) only based on the best fitting of the experimental data to the kinetic model (i.e., the pseudo-second-order or Elovich model), (4) the conception on the Freundlich model, and (5) the presentation of the adsorption mechanism involved in hydrogen bonding. Some other potential problems regarding the determination of adsorption capacity of an adsorbent towards an adsorbate (qe; mg/g) were additionally discussed herein. It gives readers a gentle reminder that the initial concentration of adsorbate (also known as the blank sample; Co) always plays a vital role in accurately calculating the qe value. The Co value from experiment (i.e., 254 mg/L or 245 mg/L) is often dissimilar to the Co value from theory (i.e., 250 mg/L). The difference becomes enormously significant when the Co value reaches very high concentration (i.e., 1000 mg/L) because an extremely high dilution factor is applied to determine the concentration of adsorbate in solution. The author hopes that the comments and contents in this paper will be particularly helpful for other researchers who are interested in the field of adsorption science and technology. Some highly-readable recent publications, which comprise the different types of paper as “comment”, “discussion”, “perspective”, and “critical review”, have also introduced in this paper. The expert reviewers and editors should give a great concern to such problems for further evaluations of submitted manuscripts in the field.
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•The fundamental aspects of wettability and surface tension phenomena are presented and discussed.•The latest etching developments and recent promising applications are discussed.•The ...advantages and drawbacks of superhydrophobic materials of preparation have been identified.•The rugosity of the surface is crucial to obtain a superhydrophobic surface.•Optically transparent and highly stable superhydrophobic coatings are needed to protect solar cells.
With the recent progress in nanotechnology and material engineering, nano-based coatings have become multifunctional, smarter, efficient, versatile and durable. Superhydrophobic coatings are an important class of the smart coating family, which has gained recognition in coating science over the last few years. The uniqueness of superhydrophobic coatings arises from the various phenomenal innovations, and its development is expected to continue in the next decades. The bioinspired superhydrophobic surfaces are commonly obtained by designing a double-scale structure by using nanotechnology, followed by the addition of water repellent compounds. It lacks an overview article describing on the recent progress in superhydrophobic coatings and surfaces. In this perspective article, various fundamental aspects of wettability and related phenomena are discussed. We present and compare the existing methods for the preparation of superhydrophobic coatings. Properties of superhydrophobic coatings such as self-cleaning, anti-icing, anti-fouling, and anti-bacterial features were also introduced. The review also discusses various superhydrophobic technological breakthroughs and future trends in the preparation and application of these materials.
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•A natural laterite from Vietnam effectively removed arsenic from groundwater.•Thermodynamic constant ∆G° negative but ∆H° and ∆S° positive for As(III) and As(V).•A community water ...filter system was successfully designed using the laterite.•The filter reduced high arsenic levels in groundwater to below WHO drinking water level.
In the Red River Delta, Vietnam, arsenic (As) contamination of groundwater is a serious problem where more than seventeen million people are affected. Millions of people in this area are unable to access clean water from the existing centralized water treatment systems. They also cannot afford to buy expensive household water filters. Similar dangerous situations exist in many other countries and for this reason there is an urgent need to develop a cost-effective decentralized filtration system using new low-cost adsorbents for removing arsenic. In this study, seven locally available low-cost materials were tested for arsenic removal by conducting batch adsorption experiments. Of these materials, a natural laterite (48.7% Fe2O3 and 18.2% Al2O3) from Thach That (NLTT) was deemed the most suitable adsorbent based on arsenic removal performance, local availability, stability/low risk and cost (US$ 0.10/kg). Results demonstrated that the adsorption process was less dependent on the solution pH from 2.0 to 10. The coexisting anions competed with As(III) and As(V) in the order, phosphate > silicate > bicarbonate > sulphate > chloride. The adsorption process reached a fast equilibrium at approximately 120–360 min, depending on the initial arsenic concentrations. The Langmuir maximum adsorption capacities of NLTT at 30 °C were 512 μg/g for As(III) and 580 μg/g for As(V), respectively. Thermodynamic study conducted at 10 °C, 30 °C, and 50 °C suggested that the adsorption process of As(III) and As(V) was spontaneous and endothermic in nature. A water filtration system packed with NLTT was tested in a childcare centre in the most disadvantaged community in Ha Nam province, Vietnam, to determine arsenic removal performance in an operation lasting six months. Findings showed that the system reduced total arsenic concentration in groundwater from 122 to 237 μg/L to below the Vietnam drinking water standard of 10 μg/L.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This study developed a layered double hydroxides (Mn/Mg/Fe-LDH) material through a simple co-precipitation method. The Mn/Mg/Fe-LDH oxidized arsenite As(III) ions into arsenate As(V) anions. The ...As(III) and oxidized As(V) were then adsorbed onto Mn/Mg/Fe-LDH. The adsorption process of arseniate As(V) oxyanions by Mn/Mg/Fe-LDH was simultaneously conducted for comparison. Characterization results indicated that (i) the best Mg/Mn/Fe molar ratio was 1/1/1, (ii) Mn/Mg/Fe-LDH structure was similar to that of hydrotalcite, (iii) Mn/Mg/Fe-LDH possessed a positively charged surface (pHIEP of 10.15) and low Brunauer–Emmett–Teller surface area (SBET = 75.2 m2/g), and (iv) Fe2+/Fe3+ and Mn2+/Mn3+/Mn4+ coexisted in Mn/Mg/Fe-LDH. The As(III) adsorption process by Mn/Mg/Fe-LDH was similar to that of As(V) under different experimental conditions (initial solutions pH, coexisting foreign anions, contact times, initial As concentrations, temperatures, and desorbing agents). The Langmuir maximum adsorption capacity of Mn/Mg/Fe-LDH to As(III) (56.1 mg/g) was higher than that of As(V) (32.2 mg/g) at pH 7.0 and 25 °C. X-ray photoelectron spectroscopy was applied to identify the oxidation states of As in laden Mn/Mg/Fe-LDH. The key removal mechanism of As(III) by Mn/Mg/Fe-LDH was oxidation-coupled adsorption, and that of As(V) was reduction-coupled adsorption. The As(V) mechanism adsorption mainly involved: (1) the inner-sphere and outer-sphere complexation with OH groups of Mn/Mg/Fe-LDH and (2) anion exchange with host anions (NO3−) in its interlayer. The primary mechanism adsorption of As(III) was the inner-sphere complexation. The redox reactions made Mn/Mg/Fe-LDH lose its original layer structure after adsorbing As(V) or As(III). The adsorption process was highly irreversible. Mn/Mg/Fe-LDH can decontaminate As from real groundwater samples from 45–92 ppb to 0.35–7.9 ppb (using 1.0 g/L). Therefore, Mn/Mg/Fe-LDH has great potential as a material for removing As.
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•Mn/Mg/Fe-LDH: successfully synthesized through a simple co-precipitation method.•As(III) and As(V) adsorption capacity of Mn/Mg/Fe-LDH: 56.1 mg/g and 32.2 mg/g.•Key removal mechanism of As(III): oxidation-coupled adsorption.•As(V) removal mechanism: inner-sphere complexation outer-sphere complexation, anion exchange.•Adsorption process of arsenic ions onto Mn/Mg/Fe-LDH: highly irreversible.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This paper primarily aimed to provide some concerns and continue discussion about the previous published paper in this journal. First, when the mechanism of Cr(VI) removal from solution involved in ...adsorption-coupled reduction was proposed, the X-ray photoelectron spectroscopy (XPS) of Cr 2p spectrum of laden adsorbent (i.e., DDTC-LDH after adsorption) needs to demonstrate the co-existence of Cr(VI) and Cr(III). The detection of reduced Cr(III) in solution after the completed adsorption of Cr(VI) only provides information on the mechanism regarding reduction, not adsorption-coupled reduction. Second, adsorption mechanism (chemisorption or physisorption) cannot be drawn only based on the best statistical fit between the time-dependent data of adsorption experiment and the kinetic model (i.e., the pseudo-second-order, Elovich, or Avrami model). Third, the constant KRP (liters per grams of adsorbent not adsorbate; L/g) of the Redlich–Peterson isotherm model is not equal to or used as the thermodynamic equilibrium constant KEqo. The application of the constant KRP for calculating the thermodynamic parameters of adsorption process (∆G°, ∆H°, and ∆G°) using the van't Hoff equation leads to a certain error in the values (sign and magnitude) of those parameters. Fourth, the pHPZC of adsorbent is significant different to its pHIEP on both meanings and analysis methods. The use of those terminologies in the fields of material and sorption (adsorption and absorption) must be correct. Finally, some important information needs to provide in the studies of adsorption isotherm and mechanism (i.e., solution pH) and characteristics of diethyldithiocarbamate intercalated-LDH (i.e., arrangement and orientation of diethyldithiocarbamate in the interlayer region of DDTC-LDH).
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Initial periods of adsorption kinetics play an important role in estimating the initial adsorption rate and rate constant of an adsorption process. Several adsorption processes rapidly occur, and the ...experimental data of adsorption kinetics under the initial periods can contain potential errors. The pseudo-second-order (PSO) kinetic model has been popularly applied in the field of adsorption. The use of the nonlinear optimization method to obtain the parameters of the PSO model can minimize error functions during modelling compared to the linear method. However, the nonlinear method has limitations in that it cannot directly recognize potential errors in the experimental points of time-dependent adsorption, especially under the initial periods. In this study, for the first time, the different linear types (Types 1–6) of the PSO model are applied to discover the error points under the initial periods. Results indicated that the fitting method using its linear equations (Types 2–5) is really helpful for identifying the error (doubtful) experimental points from the initial periods of adsorption kinetics. The imprecise points lead to low adjusted R2 (adj-R2), high reduced χ2 (red-χ2), and high Bayesian information criterion (BIC) values. After removing these points, the experimental data were adequately fitted with the PSO model. Statistical analyses demonstrated that the nonlinear method must be used for modelling the PSO model because its red-χ2 and BIC were lower than the linear method. Type 1 has been extensively applied in the literature because of its very high adj-R2 value (0.9999) and its excellent fitting to experimental points. However, its application should be limited because the potential errors from experimental points are not identified by this type. For comparison, the other kinetic models (i.e., pseudo-first-order, pseudo-nth-order, Avrami, and Elovich) are applied. The modelling result using the nonlinear forms of these models indicated that the fault experimental points from the initial periods were not detected in this study.
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