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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.

Hexavalent chromium (Cr(VI)) in water systems is a major hazard for living organisms, including humans. The most popular technology currently used to remove Cr(VI) from polluted water is sorption for its effectiveness, ease of use, low cost and environmental friendliness. The electrostatic interactions between chromium species and the sorbent matrix are the main determinants of Cr(VI) sorption. The pH plays a central role in the process by affecting chromium speciation and the net charge on sorbent surface. In most cases, Cr(VI) sorption is an endothermic process whose kinetics is satisfactorily described by the pseudo second-order model. A critical survey of the recent literature, however, reveals that the thermodynamic and kinetic parameters reported for Cr(VI) sorption are often incorrect and/or erroneously interpreted.

In this study, new chitosan Schiff base functionalized malic acid (CS-SB/MA) and its Fe2O3 (CS-SB/MA/Fe) composite were prepared through a simple route and characterized by different techniques including FT-IR, XRD, TGA, DSC, FESEM, and BET. The characterization results indicated that the desired compounds, CS-SB/MA and CS-SB/MA/Fe, were successfully prepared. In addition, the adsorption of Eosin Y (EY) from aqueous solution using the as-prepared compound, CS-SB/MA/Fe, was studied. The influence of important parameters such as pH solution and contact time were also studied and reported. EY adsorption by the as-prepared CS-SB/MA/Fe2O3 composite was found to be optimal at a pH solution of 5, a contact time of 90 min, and an adsorbent dosage of 0.02 g. The maximum removal percentage and adsorption capacity were calculated to be 93.56% and 65.52 mg/g, respectively. The EY adsorption followed by the pseudo-second-order (PSO) kinetic model as a chemisorption process including different interactions between anionic groups of EY and the functional groups of CS-SB/MA/Fe. Therefore, the prepared chitosan-base adsorbent, CS-SB/MA/Fe, has great potential application for the removal of organic dyes from wastewater.

This study describes the adsorption of Cu (II), Co (II) and Ni (II) ions from wastewater on husk biomass. The ability of adsorbent to capture the metal ions has been found to be in the order of Ni (II)>Co (II) and Cu (II) depending upon the size and nature of metal ions to be adsorbed. It has been observed that percentage removal of Cu (II), Co (II) and Ni (II) ions increases with increase of adsorbent dosage, contact time and pH of the medium but up to a certain extent. Maximum adsorption capacity (q ) for Cu (II), Co (II) and Ni (II) ions has been found to be 11.05, 15.04 and 19.88 mg/g, respectively, under optimum conditions of adsorbent dosage, contact time and pH of the medium. Langmuir model best fits the adsorption process with R value approaches to unity for all metal ions as compared to other models because adsorption sites are seemed to be equivalent and only monolayer adsorption may occur as a result of binding of metal ion with a functional moiety of adsorbent. Pseudo second order kinetic model best interprets the adsorption process of Cu (II), Co (II) and Ni (II) ions. Thermodynamic parameters such as negative value of Gibbs energy (∆G°) gives information about feasibility and spontaneity of the process. Adsorption process was found to be endothermic for Cu (II) ions while exothermic for Co (II) and Ni (II) ions as signified by the value of enthalpy change (∆H°). Husk biomass was recycled three times for removal of Ni (II) from aqueous medium to investigate its recoverability and reusability. Moreover husk biomass has a potential to extract Cu (II) and Ni (II) from electroplating wastewater to overcome the industrial waste water pollution.

Olive leaves were utilized to produce activated biomass for the removal of ciprofloxacin (CIP) from water. The raw biomass (ROLB) was activated with sodium hydroxide, phosphoric acid, and Dead Sea water to create co-precipitated adsorbent (COLB) with improved adsorption performance. The characteristics of the ROLB and COLB were examined using SEM images, BET surface area analyzer, and ATR-FTIR spectroscopy. COLB has a BET surface area of 7.763 m2/g, markedly higher than ROLB’s 2.8 m2/g, indicating a substantial increase in adsorption sites. Through investigations on operational parameters, the optimal adsorption efficiency was achieved by COLB is 77.9% within 60 min, obtained at pH 6, and CIP concentration of 2 mg/mL. Isotherm studies indicated that both Langmuir and Freundlich models fit the adsorption data well for CIP onto ROLB and COLB, with R 2 values exceeding 0.95, suggesting effective monolayer and heterogeneous surface adsorption. The Langmuir model revealed maximum adsorption capacities of 636 mg/g for ROLB and 1243 mg/g for COLB, highlighting COLB’s superior adsorption capability attributed to its enhanced surface characteristics post-modification. Kinetic data fitting the pseudo-second-order model with R 2 of 0.99 for ROLB and 1 for COLB, along with a higher calculated q e for COLB, suggest its modified surface provides more effective binding sites for CIP, enhancing adsorption capacity. Thermodynamic analysis revealed that the adsorption process is spontaneous (∆ G o  < 0), and exothermic (∆ H o  < 0), and exhibits a decrease in randomness (∆ S o  < 0) as the process progresses. The Δ H° value of 10.6 kJ/mol for ROLB signifies physisorption, whereas 35.97 kJ/mol for COLB implies that CIP adsorption on COLB occurs through a mixed physicochemical process.

The huge development of industrial production and human activities throughout the world has resulted in serious water pollution. Pesticides, dyes, phenol, and phenolic compounds may be life-threatening to humans and marine aquatic animals, even at low concentrations. These pollutants must be removed by using different techniques to provide high-quality water. So far, the adsorption process has been considered an effective method to remove pollutants from an aqueous solution. The adsorption method has several advantages, such as low cost, easy operation, and high performance. This article reviewed the removal of dye, pesticides, phenol, and phenolic compounds through zeolite, activated carbon, clay, ash fly, and carbon nanotubes by reviewing the existing literature from 2000 to 2022. The properties of the adsorbent, adsorption studies, the method of design, and optimization conditions were reported. The thermodynamic studies were conducted to determine Gibbs free energy change, standard entropy change, and standard enthalpy change. Adsorption isotherms (Langmuir, Freundlich, and Temkim models) revealed the relationship between the amount of adsorbates per unit adsorbent in the equilibrium process. The adsorption kinetic investigations (pseudo-first-order and pseudo-second-order kinetic model) were carried out to determine the rate of adsorption and correlate the experimental data. Experimental results indicated that a higher adsorption capacity can be observed when the adsorbents have a high surface area and high-porosity structure. The removal percentage of pollutants increased when the adsorbent dosage was increased due to the availability of a larger number of adsorption sites. In conclusion, cheaper adsorbents can be used in wastewater treatment to improve water quality and protect the environment.

This manuscript describes the synthesis of dimethylethanolamine (DMEA)-grafted anion exchange membrane (AEM) by incorporating dimethylethanolamine as ion-exchange content into the polymer matrix via the solution casting method. The synthesis of the DMEA-grafted AEM was demonstrated by Fourier transform infrared (FTIR) spectroscopy. The prepared DMEA-grafted AEM exhibited higher thermal stability, homogeneous morphology, water uptake (WR) of 115%, and an ion exchange capacity (IEC) of 2.70 meq/g. It was used for the adsorptive removal of methyl orange (MO) from an aqueous solution via batch processing. The effect of several operating factors, including contact time, membrane dosage, initial concentration of aqueous dye solution, and temperature on the percentage discharge of MO and adsorption capacity, was evaluated. Experimental data for adsorption of MO onto the DMEA-grafted AEM was analyzed with two parameter and three parameter nonlinear adsorption isotherm models but fitted best using a nonlinear Freundlich isotherm. Adsorption kinetics were studied by using several models, and attained results showed that experimental data fitted well to pseudo-second-order kinetics. A thermodynamic study showed that adsorption of MO onto the prepared DMEA-grafted AEM was an endothermic process. Moreover, it was a feasible and spontaneous process.

In this work, the adsorption capacities of CoFe 2 O 4 magnetic nanoparticles were enhanced successfully by modifying coated-CoFe 2 O 4 @SiO 2 magnetic nanoparticles with sodium dodecyl sulfate and dithiooxamide and became CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles. The CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles were characterized using FESEM, TEM, VSM, BET, TG-DTG, and FTIR techniques. The CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles were used as an adsorbent for the removal of cadmium ions in the aqueous solution. The CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles demonstrated high capability to remove cadmium ions in the aqueous solution under optimum conditions, such as pH = 6; dosage = 50 mg; time = 30 min; T  = 298 K. Furthermore, the Langmuir isotherm model was found to be more suitable for describing the adsorption of cadmium ions onto CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles compared to the Freundlich and Temkin isotherm models. The optimum adsorption capacity for CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles at 298 K was observed to be 350.08 mg/g. The thermodynamic studies indicated that the adsorption of cadmium ions onto CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles was a spontaneous process. Moreover, the kinetic studies were performed on the adsorption of cadmium ions onto CoFe 2 O 4 @SiO 2 –SDS–DTO magnetic nanoparticles, and the experimental data demonstrated a proper fit with the pseudo-second order model, which confirms the adsorption capacity of the pseudo-second-order model.

Hollow tubular structured kapok fibers (Ceiba pentandra) were coated with polyaniline (PANI) molecules using an in situ oxidative polymerization technique. The tubular morphology of the kapok fibers was retained after PANI coating. The Fourier transform infrared (FT-IR) spectrum of the PANI-coated kapok fibers illustrated the vibration modes associated with the presence of PANI molecules. The PANI-treated kapok fibers achieved complete wettability with water molecules (zero water contact angle) from initially being highly hydrophobic (contact angle = 120°). In the present work, the removal of contaminants such as methyl orange dye and Cu(II) from aqueous solution using polyaniline-coated kapok fibers was investigated. Isotherm studies show that the removal of methyl orange dye (R2 ≥ 0.959) and Cu(II) (R2 ≥ 0.972) using PANI-coated kapok fibers follow the Langmuir isotherm model with maximum sorption capacities determined to be 75.76 and 81.04 mg/g, respectively. Based from thermodynamic studies, the sorption of methyl orange dye and Cu(II) are endothermic, feasible and spontaneous. Furthermore, kinetic studies show that the both processes follow a pseudo-second-order model, implying that the rate-determining step is chemisorption.

This study aims to develop efficient and sustainable hydrogels for dye adsorption, addressing the critical need for improved wastewater treatment methods. Carboxymethyl cellulose (CMC)-based hydrogels grafted with AAc were synthesized using gamma radiation polymerization. Various AAc to CMC ratios (5:5, 5:7.5, 5:10, 5:15) were treated with 37% NaOH and exposed to 1–15 kGy radiation, with the optimal hydrogel obtained at 5 kGy. Swelling studies showed an increase in swelling with 5–7.5% AAc content, with the 5:7.5 hydrogel achieving the highest swelling at 18,774.60 (g/g). FTIR spectroscopy confirmed the interaction between AAc and CMC, indicating the successful formation of the hydrogel. DSC analysis revealed that higher AAc content led to increased glass transition and decomposition temperatures, thereby enhancing thermal stability. The swelling kinetics were linked to a reduction in pore size and improved AAc grafting. The 5:7.5 hydrogel demonstrated the highest adsorption capacity (681 mg/g) for methylene blue at 80 mg/L, achieving a desorption efficiency of 95% in 2M HCl. Kinetic analysis revealed non-uniform physisorption on a heterogeneous surface, which followed Schott’s pseudo-second-order model. This study advances the development of efficient hydrogels for water purification, providing a cost-effective and environmentally friendly solution for large-scale applications.