Comments on the calculation of the standard equilibrium constant using the Langmuir model in Journal of Hazardous Materials 422 (2022) 126863

Extraction and Purification of Aucubin from Eucommia ulmoides Seed Draff in Natural Deep Eutectic Solvents Using Macroporous Resins

Aucubin (AU) is an active ingredient that exerts strong antioxidant and anti-inflammatory effects in the treatment of several diseases. In order to improve the efficiency of resource utilization of traditional biomass waste, Eucommia ulmoides seed-draff (EUSD) waste biomass was used as the raw material, and a series of deep eutectic solvents were selected to evaluate the extraction efficiency of aucubin from EUSD. A response surface experiment was designed based on a single-factor experiment to optimize the extract conditions. The results showed that the best conditions for aucubin extraction were an HBD-HBA ratio of 2.18, a liquid-solid ratio of 46.92 mL/g, a water percentage of 37.95%, a temperature of 321.03 K, and an extraction time of 59.55 min. The maximum amount of aucubin was 156.4 mg/g, which was consistent with the theoretical value (156.8 mg/g). Then, the performance of 12 resins for adsorption and desorption was contrasted. The results revealed that HPD950 resin exhibited the best performance, with an adsorption capacity of 95.2% and a desorption capacity of 94.3%. Additionally, the pseudo-second-order model provided the best match to the kinetics data, the Langmuir model provided the best fit to the isotherm data, and adsorption was a beneficial, spontaneous, exothermic, and physical process. In the recyclability test, the HPD950 resin had great potential and excellent sustainability in aucubin recovery. In the antioxidant activity study, the aucubin extract exerted a strong antioxidant ability with scavenging capabilities for four free radicals. Furthermore, the antifungal activity study found that the aucubin extract exhibited a good antifungal effect against 5 tested pathogens. The research results can provide a theoretical basis for the extraction of high-value components from waste biomass by deep eutectic solvent and a certain application value for the development and utilization of natural aucubin products.

Outstanding adsorption capacity of iron oxide synthesized with extract of açaí berry residue: kinetic, isotherm, and thermodynamic study for dye removal

Contamination of water by toxic dyes is a serious environmental problem. Adsorbents prepared by an environmentally safe route have stood out for application in pollutant removal. Herein, iron oxide-based nanomaterial composed of Fe(III)-OOH and Fe(II/III) bound to proanthocyanidins, with particles in the order of 20 nm, was prepared by green synthesis assisted by extract of açaí (Euterpe oleracea Mart.) berry seeds from an agro-industrial residue. The nanomaterial was applied in the adsorption of cationic dyes. Screening tests were carried out for methylene blue (MB), resulting in an outstanding maximum adsorption capacity of 531.8 mg g-1 at 343 K, pH 10, 180 min. The kinetics followed a pseudo-second-order model and the isotherm of Fritz-Schülnder provided the best fit. Thermodynamic data show an endothermic process with entropy increase, typical of chemisorption. The proposed mechanism is based on the multilayer formation over a heterogeneous adsorbent surface, with chemical and electrostatic interactions of MB with the iron oxide nanoparticles and with the proanthocyanidins. The high adsorption efficiency was attributed to the network formed by the polymeric proanthocyanidins that entangled and protected the iron oxide nanoparticles, which allowed the reuse of the nanomaterial for seven cycles without loss of adsorption efficiency.

Synthesis and characterization of hydrogel-based magnetite nanocomposite adsorbents for the potential removal of Acid Orange 10 dye and Cr(VI) ions from aqueous solution

Magnetic responsive hydrogels (CMX-cl-P4VP/M-NPs) were successfully synthesized through in situ co-precipitation procedure and investigated using various techniques. The surface morphology analysis revealed that the M-NPs were uniformly distributed within the hydrogel matrix and had average sizes ranging from 4.98 to 15.02 nm. The graft copolymer containing nanoparticles exhibited a sensitive magnetic response, and their recovery could be facilitated by applying a magnetic field. The purpose of this research is to study the ability of the prepared magnetic hydrogel to remove AO-10 dye and hexavalent chromium ions (Cr(VI)) from the aqueous solution under various factors, namely contact time, pH, amount of adsorbent, coexisting ions and AO-10 and Cr(VI) concentrations. The outcomes of the batch adsorption demonstrated that the adsorbent hydrogel incorporated with a low percentage (10 %) of M-NPs had a strong affinity for the removal of AO-10 dye and Cr(VI) ions at an optimum pH = 3, and the removal percentage reached 99.3 and 97.4 % for 500 mg L^-1 and 300 mg L^-1 of AO-10 dye and Cr(VI) ions within 90, 50 min, respectively. The data were well-fitted by pseudo-first-order kinetics. The maximum adsorption capacities of AO-10 dye and Cr(VI) ions onto adsorbent were 2448 and 574.7 mg g^-1 at 298 K, calculated from the Langmuir model.

Correction to the thermodynamic calculation using the Langmuir isotherm model by Saeed et al. (2022)

The thermodynamic parameters are usually used to analyze the spontaneity, thermal and random change of the adsorption process, therefore, it is important to obtain these parameters accurately. Recently, Saeed et al. (2022) published a high academic paper to reveal the adsorption properties and mechanism of dyes onto the chitosan composite of the iron metal-organic framework (CS/MOF-235). However, the thermodynamic parameters of ΔG and ΔS need to be corrected because their results affected the analyzed conclusions. In the present study, the thermodynamic parameters were recalculated by using the correct method through the Langmuir isotherm model. The recalculated thermodynamic parameters indicated that the change in free energy (ΔG) for the removal of methylene blue (MB) and methyl orange (MO) onto CS/MOF-235 is negative but not positive, meaning that the adsorption of dyes (MB and MO) is spontaneous but not non-spontaneous. The change in entropy (ΔS) is positive but not negative, implying that the randomness increases but not decreases during the adsorption process. The enthalpy (ΔH) maintained negative value and the entropy (ΔS) obtained positive value indicate that both of them are the driving forces of the adsorption.

Improper Estimation of Thermodynamic Parameters in Adsorption Studies with Distribution Coefficient KD (qe/Ce) or Freundlich Constant (KF): Considerations from the Derivation of Dimensionless Thermodynamic Equilibrium Constant and Suggestions

Adsorption processes often include three important components: kinetics, isotherm, and thermodynamics. In the study of solid–liquid adsorption, “standard” thermodynamic equilibrium constant $\left(K_{\text{Eq}}^{\text{o}}\right.$ ; dimensionless) plays an essential role in accurately calculating three thermodynamic parameters: the standard Gibbs energy change (∆G°; kJ/mol), the standard change in enthalpy (∆H°; kJ/mol), and the standard change in entropy [∆S°; J/(mol × K)] of an adsorption process. Misconception of the derivation of the $K_{\text{Eq}}^{\text{o}}$ constant that can cause calculative errors in values (magnitude and sign) of the thermodynamic parameters has been intensively reflected through certain kinds of papers (i.e., letters to editor, discussions, short communications, and correspondence like comment/rebuttal). The distribution coefficient (KD) and Freundlich constant (KF) have been intensively applied for calculating the thermodynamic parameters. However, a critical question is whether KD or KF is equal to $K_{\text{Eq}}^{\text{o}}$ . This paper gives (1) thorough discussion on the derivation of thermodynamic equilibrium constant of solid–liquid adsorption process, (2) reasonable explanation on the inconsistency of (direct and indirect) application of KD or KF for calculating the thermodynamic parameters based on the derivation of $K_{\text{Eq}}^{\text{o}}$ , and (3) helpful suggestions for improving the quality of papers published in this field.

Improvement in adsorption of Hg2+ from aqueous media using sodium-type fine zeolite grains

To increase the adsorption capability of Hg²⁺ from aqueous media, we prepared sodium-type fine zeolite grains with various particle sizes (denoted as ZE1, ZE2 and ZE3). The particle sizes of ZE1, ZE2 and ZE3 were 16.363 ± 0.365, 1.454 ± 0.357 and 0.607 ± 0.377 μm, respectively. Moreover, the CEC, specific surface area and pore volume were in the order ZE1 (42 mmol/g and 23.5 m²/g) < ZE2 (72 mmol/g and 67.1 m²/g) < ZE3 (135 mmol/g and 176.6 m²/g). Subsequently, the Hg²⁺ adsorption capability was investigated. The performance of tested agents on Hg²⁺ adsorbed was in the order ZE1 (5.0 mg/g) < ZE2 (9.4 mg/g) < ZE3 (20.2 mg/g). It was concluded that fine crystalline zeolite was important in enhancing the adsorption capability of Hg²⁺. In addition, the mechanism of adsorption of Hg²⁺ on the ZE samples was evaluated. Our results suggested that Hg²⁺ was exchanged with sodium ions in the interlayers of ZE samples with correlation coefficients of 0.966-0.979. Our findings revealed that these ZE samples constitute potential agents for the adsorption of Hg²⁺ from aqueous media.