In recent years, adsorption science and technology for water and wastewater treatment has attracted substantial attention from the scientific community. However, the number of publications containing ...inconsistent concepts is increasing. Many publications either reiterate previously discussed mistakes or create new mistakes. The inconsistencies are reflected by the increasing publication of certain types of article in this field, including “short communications”, “discussions”, “critical reviews”, “comments”, “letters to the editor”, and “correspondence (comment/rebuttal)”. This article aims to discuss (1) the inaccurate use of technical terms, (2) the problem associated with quantities for measuring adsorption performance, (3) the important roles of the adsorbate and adsorbent pKa, (4) mistakes related to the study of adsorption kinetics, isotherms, and thermodynamics, (5) several problems related to adsorption mechanisms, (6) inconsistent data points in experimental data and model fitting, (7) mistakes in measuring the specific surface area of an adsorbent, and (8) other mistakes found in the literature. Furthermore, correct expressions and original citations of the relevant models (i.e., adsorption kinetics and isotherms) are provided. The authors hope that this work will be helpful for readers, researchers, reviewers, and editors who are interested in the field of adsorption studies.
•This study summarizes common literature mistakes in the field of adsorption.•Reiteration and propagation of such mistakes in future publications should be avoided.•Correct expression and citation of the models used in adsorption studies are provided.•We also highlight some problems that need to be thoroughly discussed or further investigated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•The thermodynamic parameters were calculated from the Langmuir, Freundlich, Henry, and partition constants.•The thermodynamic parameters were strongly dependent on derivation of the ...applied constants.•The Langmuir and Freundlich constants (dimensionless) were appropriate for calculating the thermodynamic parameters.•The adsorption mechanism involved physical adsorption.•Electrostatic attraction played a primary role in the adsorption process (∼90%).
Thermodynamic adsorption investigation plays a key role in estimating adsorptive mechanisms (i.e., physical or chemical). Accuracy estimation of the thermodynamic parameters is directly dependent on the equilibrium constant between two phases (KC; dimensionless). In this study, the thermodynamic parameters were calculated from the KC constants derived from the adsorption isotherm constants (i.e., Langmuir, Freundlich, and Henry) and partition coefficient, with and without dimensionality consideration. Results showed that the optimal selection of KC is strongly dependent on: the adsorption characteristics (i.e., Henry, Freundlich, and Langmuir) where equilibrium data are actually located; and the correlation coefficient of the van’t Hoff equation. Both the Langmuir and Freundlich constants (dimensionless) are appropriate to calculate the thermodynamic parameters. The Langmuir constants from its four linear forms can be applied to calculate the thermodynamic parameters without significant difference. The Cd(II) biosorption process onto the orange peel (OP) occurred spontaneously (−ΔG°), in an exothermic nature (−ΔH°), and with increased randomness (+ΔS°). The biosorption process involved physical adsorption with minimal activation energy (Ea) and adsorption energy (E). The biosorption phenomenon reached fast equilibrium and reversibility. The negatively charged carboxylic groups (–COO−) on the OP’s surface play an important role (approximately 90%) in Cd2+ biosorption through electrostatic attractions (out-sphere complexes).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Activated carbon (AC) was synthesized from golden shower (GS) through a new chemical activation process. The three-stage process comprised (1) hydrothermal carbonization of GS to produce hydrochar, ...(2) pyrolysis of hydrochar to produce biochar, and (3) subsequent chemical activation of biochar with K2CO3 to obtain GSHBAC. The traditional synthesis processes (i.e., one-stage and two-stage) were also examined for comparison. In the one-stage process, GS that was impregnated with K2CO3 was directly pyrolyzed (GSAC), and the two-stage process consisted of (1) pyrolytic or hydrothermal carbonization to produce biochar or hydrochar and (2) subsequent chemical activation was defined as GSBAC and GSHAC, respectively. The synthesized ACs were characterized by scanning electron microscope, Brunauer–Emmett–Teller (BET) surface area analysis, Fourier transform infrared spectrometry, point zero charge, and Boehm titration. The adsorption results demonstrated that the MG5 adsorption process was not remarkably affected by neither the solution pH (2.0–10) nor ionic strength (0–0.5 M NaCl). Kinetic studies showed that the adsorption equilibrium was quickly established, with a low activation energy required for adsorption (Ea; 3.30–27.8 kJ/mol), and the ACs removed 50–73% of the MG5 concentration from solution within 01 min. Desorption studies confirmed the adsorption was irreversible. Thermodynamic experiments suggested that the MG5 adsorption was spontaneous (−ΔG°) and endothermic (+ΔH°), and increased the randomness (+ΔS°) in the system. Although the specific surface areas of the ACs followed the order GSAC (1,413) > GSHAC (1,238) > GSHBAC (903) > GSBAC (812 m2/g), the maximum adsorption capacities determined from the Langmuir model (Qomax) at 30 °C exhibited the following order: GSHBAC (531) > GSAC (344) > GSHAC (332) > GSBAC (253 mg/g). Oxygenation of the ACs' surface through a hydrothermal process with acrylic acid resulted in a decrease in MG5 adsorption and identified the importance of π-π interactions to the adsorption process. The primary interactions in MG5 adsorption were π-π interactions and pore filling, while hydrogen bonding and n-π interactions were minor contributors. The three-stage process can be regarded as the effective preparation method of AC with a high adsorption capacity toward the cationic dye.
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•Activated carbon synthesized from golden shower through a new chemical activation method.•A removal rate of 50%–73% achieved within 1 min in kinetic study.•Maximum adsorption capacity at 30 °C (531 mg/g), 40 °C (614 mg/g), and 50 °C (678 mg/g).•Primary adsorption mechanisms: π-π interaction and non-micropore filling.•The π-π interaction identified by a new oxygenation method using acrylic acid.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Activated carbons (ACs) were synthesized from golden shower (GS) through chemical activation. Two synthesis processes were used: one-stage and two-stage processes. In the one-stage process, GS that ...was impregnated with K2CO3 was directly pyrolyzed (GSAC), and the two-stage process consisted of (1) pyrolytic or hydrolytic carbonization to produce biochar or hydrochar and (2) subsequent chemical activation was defined as GSBAC and GSHAC, respectively. The activated carbon’s characteristics—thermal stability and textural, physicochemical, structural, and crystal properties—were thoroughly investigated. Results demonstrated that the characteristics of activated carbons strongly depend on the method used for their synthesis. The Brunauer–Emmett–Teller surface area followed the order GSAC (1413 m2/g) > GSHAC (1238 m2/g) > GSBAC (812 m2/g). The existence of acidic groups was determined through Fourier transform infrared spectroscopy and Boehm titration. The excellent adsorptive capacities of the activated carbons were confirmed from the iodine number (1568–2695 mg/g) and methylene number (143–233 mg/g).
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Six lignocellulosic waste-derived biosorbents cantaloupe peel (CAN), pine cone (PC), litchi fruit peel (LP), annona squamosal (AS), bamboo shoot (BS), and sugarcane bagasse (SB) were selected as ...low-cost and renewable materials to prepare chemically modified biosorbent. The modified biosorbent was prepared through a newer carboxyl groups-grafting process onto the biosorbent’s surface using acrylic acid. The results showed that the cation exchange capacity (CEC) of biosorbents increased by approximately 66.3–104% after modified. The modified biosorbent exhibited significantly higher adsorption capacity of Pb
2+
, Cu
2+
, and Cd
2+
ions than the pristine biosorbent. The maximum Langmuir adsorption capacity (
Q
o
max
) of both pristine and modified biosorbents toward three metal ions (Pb
2+
, Cu
2+
, and Cd
2+
) followed the decreasing order: CAN > PC > LP > AS > BS > SB. The preference ranking of three metal ions on the pristine and modified biosorbents (mmol/kg) was generally in the order: Pb
2+
> Cu
2+
> Cd
2+
. Among these biosorbents, cantaloupe peel exhibited an excellent adsorption affinity to metal cations compared to the five others. The
Q
o
max
values of modified and pristine cantaloupe peels were ordered as follows: 143.2 and 81.1 mg/g for Pb
2+
adsorption, > 45.4 and 30.4 mg/g for Cd
2+
adsorption, > 33.1 and 23.5 mg/g for Cu
2+
adsorption. After five adsorption–desorption cycles, the removal efficiency of Pb
2+
by modified CAN was maintained at around 70%. The ion exchange played a determining role in adsorption mechanism. It can be concluded that modified cantaloupe peel can serve as a newer and promising biosorbent with a high adsorption capacity to various potentially toxic metals.
Adsorption thermodynamics is an integral part of the study of adsorption and plays a vital role in estimating adsorption mechanism (i.e., physisorption and chemisorption). For the liquid-phase ...adsorption, the equilibrium constant of some adsorption isotherm models (Langmuir, modified Langmuir, Langmuir–Freundlich, modified Langmuir–Freundlich, Liu, Khan, Sips, Hill, Toth, Redlich–Peterson, Koble–Corrigan, and Radke–Prausnitz models) and the constant of Henry and Freundlich models have been directly or indirectly applied for calculating the corresponding thermodynamic parameters (∆G°, ∆H°, and ∆S°). This study explored the effects of the selection of (1) the unit of adsorption isotherm (qevs. Ce), (2) the equilibrium constant of adsorption isotherm model, and (3) the linear and nonlinear forms of the van’t Hoff equation on the value (sign and magnitude) of the thermodynamic parameters. A commercial activated carbon was selected as a typical adsorbent to adsorb methylene green dye (target adsorbate) in solution. Results indicated that the constants of the Henry and Freundlich models could not be applied for calculating the thermodynamic parameters. The equilibrium constants of the modified Langmuir and modified Langmuir–Freundlich models were not suitable for calculating the thermodynamic parameters in this study compared to the other equilibrium constants of the Langmuir, Langmuir–Freundlich, Khan, Liu, Sips, Redlich–Peterson, Toth, Hill, Radke–Prausnitz, and Koble–Corrigan models. The thermodynamic parameters were calculated from thermodynamic equilibrium constant derived from the equilibrium constant of the Redlich–Peterson model or the Radke–Prausnitz model (the van’t Hoff equation: adj-R2 = 0.9999).
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•Equilibrium constants of 12 adsorption models used for calculating thermodynamic parameters.•Relationship between adsorption equilibrium constants and thermodynamic equilibrium constant reported.•∆S° and ∆H° obtained from nonlinear form of van’t Hoff equation insignificantly different to those from its linear form.•Equilibrium constant of Redlich–Peterson and Radke–Prausnitz models suggested for calculating thermodynamic parameters.•Adsorption mechanism: dominant physisorption (∆H° = 16.4 kJ/mol).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
This study aimed to develop a novel in-situ method to directly remove toxic hexavalent chromium anions from groundwater. The characteristics of Mg/Al-layered double hydroxides (LDH) involving in-situ ...synthesis and interlayer exchangeable anions can facilitate to remove Cr(VI) from solution. Two different methods of LDH preparation were employed to explore the adsorption efficiency of (di)chromates, such as traditional coprecipitation (CO3-LDH) and innovative in-situ synthesis (in-situ-LDH). The synthesized LDH samples were characterized using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and zeta potential. The results demonstrated that the adsorptive amount of Cr(VI) for the in-situ synthesis process dramatically increased with an increase in initial Cr(VI) concentrations from 100 mg/L to 900 mg/L. The kinetic study indicated that the constant rate (k2) of the pseudo-second-order equation significantly decreased when the initial concentration of Cr(VI) exceeded 500 mg/L. The removal efficiency of Cr(VI) was slightly dependent on solution pH (5.0–12) values. The in-situ-LDH absorbent (339 mg/g) exhibited the significantly higher Langmuir maximum adsorption capacity than CO3-LDH (246 mg/g). The primary adsorption mechanism was anion exchange; meanwhile, the adsorption-coupled reduction mechanism also played an integral role. The advanced in-situ synthetic method can be developed to efficiently remove toxic hexavalent chromium anions from groundwater.
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•Novel in-situ method using Mg/Al-LDH to remove (di)chromate from groundwater was developed.•Concentration effects of Mg and Al on performance of in-situ synthesis were evaluated.•Effects of pH, time, and Cr(VI) concentration on removal efficiency were investigated.•XRD indicated interlayer CO32− anions exchanged with aqueous CrO42− anions.•XPS demonstrated the reduction phenomenon of Cr(VI) into Cr(III).
A novel in-situ method using layered double hydroxides (LDH) for directly removing toxic (di)chromate anions from groundwater was developed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The mechanism and capacity of methylene green (MG5) adsorption onto commercial activated-charcoal (CAC, Norit RB4C) were investigated in batch experiments. The microporous CAC material was found to ...exhibit a large specific surface area (1026m2/g) and high total pore (0.502cm3/g) and micropore (0.347cm3/g) volumes. The point of zero charge (9.81±0.07) of CAC was determined by the “drift method” and found to be insignificantly dependent on the varying operation conditions. The dye adsorption process was low relative to the solution pH (2.0–10) and ionic strength (0–0.5M). Kinetic studies indicated that the adsorption equilibrium was quickly reached based on low activation energy required for adsorption (Ea; 4.12kJ/mol). CAC can remove 53–64% of the MG5 concentration from solution within 1min. The maximum adsorption capacities determined from Langmuir model at 10°C, 30°C, 40°C, and 50°C were 361mg/g, 489mg/g, 543mg/g, and 581mg/g, respectively. Desorption studies demonstrated that the MG5 adsorption was irreversible. The MG5 adsorption process was found to be spontaneous (−ΔG°), endothermic (+ΔH°), and increased the randomness (+ΔS°) in the system. Oxygenation of the CAC surface through a hydrothermal process with acrylic acid resulted in a decrease in MG5 adsorption and identified the importance of π–π interactions to the adsorption process. The analysis of Fourier transform infrared spectroscopy revealed that the aromatic CC bonds decreased in intensity and upshifted after MG5 adsorption, which additionally confirms the significant contribution of π–π interactions. The combined results of our studies highly indicated that the primary mechanisms in MG5 adsorption were π–π interactions and pore filling, while hydrogen bonding and n–π interactions were minor contributors.
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IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Hydrochars derived from golden shower pod (GSH), coconut shell (CCH), and orange peel (OPH) were synthesized and applied to remove methylene green (MG5). The results indicated that the hydrochars ...possessed low specific surface areas (6.65-14.7m
2
/g), but abundant oxygen functionalities (1.69-2.12mmol/g). The hydrochars exhibited cellular and spherical morphologies. Adsorption was strongly dependent on the solution pH (2-10) and ionic strength (0-0.5M NaCl). Equilibrium can be quickly established in the kinetic study (60-120 min). The maximum Langmuir adsorption capacities at 30 °C followed the order GSH (59.6mg/g)>CCH (32.7mg/g)>OPH (15.6mg/g)> commercial glucose-prepared hydrochar (12.6mg/g). The dye adsorption efficiency was determined by the concentrations of oxygen-containing functionalities on the hydrochar surface. The adsorption process occurred spontaneously (− ΔG
o
) and exothermically (−ΔH
o
). Desorption studies confirmed the reversible adsorption process. Oxygenation of the hydrochar surface through a hydrothermal process with acrylic acid contributed to increasing MG5 adsorption and identifying the negligible role of
π-π
interaction to the adsorption process. The analysis of Fourier transform infrared spectrometry demonstrated that the aromatic C=C peak did not significantly decrease in intensity or shift toward higher/lower wavenumbers after adsorption, which further confirms the insignificant contribution of
π-π
interaction. Electrostatic attraction played a major role in adsorption mechanisms, while minor contributions were accounted for hydrogen bonding and n-
π
interactions. The primary adsorption mechanisms of MG5 onto hydrochar were similar to biosorbent, but dissimilar to biochar and activated carbon (i.e.,
π-π
interaction and pore filling).
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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