During the years, adsorption has garnered considerable attention being one of the most cost-effective and efficient methods for separating contaminants out of liquid phase. A comprehensive understanding of adsorption mechanisms entails several crucial steps, including adsorbent characterization, batch and column adsorption tests, fitting of predefined kinetic and isotherm models, and meticulous thermodynamic analysis. These combined efforts serve to provide clarity and insights into the intricate workings of adsorption phenomena. However, the vast amount of literature published in the field each year is riddled with ill-considered model selections and incorrect parameter analyses. Therefore, the aim of this paper is to establish guidelines for the proper employment of these numerous kinetic, isotherm, and fixed-bed models in various applications. A thorough review has been undertaken, encompassing more than 45 kinetic models, 70 isotherm models, and 45 fixed bed models available hitherto, with their classification determined based on the adsorption mechanisms expounded within each of them. Moreover, five general approaches for modifying fixed-bed models were provided. The physical meanings, assumptions, and interconversion relationships of the models were discussed in detail, along with the information criterion used to evaluate their validity. In addition to commonly used activation energy and Gibbs energy analysis, the methods for calculating site energy distribution were also summarized.

Compound pollution of microplastics and estrogens is a growing ecotoxicological problem in aquatic environments. The adsorption isothermal properties of bisphenol A (BPA) and 17α-ethinyl estradiol (EE2) on polyamide (TPU) in monosolute and bisolute systems were studied. Under the same adsorption concentration (1-4 mg L-1), EE2 had a greater adsorption capacity than BPA in the monsolute system. Compared to the energy distribution features of the adsorption sites of EE2 and BPA, the BPA adsorption sites were located in the higher energy area and were more evenly distributed than those of EE2, while the quantity of BPA adsorption sites was less than that of EE2. In the bisolute system, the average site energy, site energy inhomogeneity, and adsorption site numbers of BPA increased by 1.674, -17.166, and 16.793%, respectively. In comparison, the average site energy, site energy inhomogeneity, and adsorption sites numbers of EE2 increased by 2.267, 4.416, and 8.585%, respectively. The results showed that BPA and EE2 had a cooperative effect on the competitive adsorption of TPU. XPS analysis showed that BPA and EE2 had electron transfer on TPU, although the chemisorption effects and hydrogen bonds between BPA and TPU were more significant. Comparing the changes in the relative functional group content of TPU in monosolute and bisolute systems, BPA and EE2 were synergistically absorbed on TPU. This study can provide a theoretical reference for the study of competitive adsorption between coexisting organic pollutants.

Competitive adsorption and complementary adsorption between emerging pollutants has been observed in multiple studies. Investigation of the preference of pollutants for different types of adsorption sites can provide a supplementary perspective for understanding complementary adsorption. In this study, the simultaneous adsorption of two typical emerging pollutants, sulfamethoxazole (SMX) and bisphenol A (BPA), on magnetic biochar (MBC-1) was investigated. The results showed that the modification with ferric chloride optimized the surface properties of biochar (aromaticity, hydrophobicity, and oxygen-containing functional groups, etc.), and helped to remove SMX and BPA through various interactions. The equilibrium adsorption capacity of the two adsorbents was inhibited by competitive adsorption in the mixed solute systems, which was due to the same adsorption mechanism. When pH = 7, the SMX and BPA adsorption mainly involved pore filling, hydrophobic effect, π-π EDA, and hydrogen bonding. In addition, electrostatic force, surface coordination, and ion exchange have also been proven to be related to the adsorption of SMX and BPA. In the co-adsorption system, BPA\'s competitive advantage might be due to its superior hydrophobicity, charge property, and molecular diameter. In the competitive adsorption experiment, the total adsorption capacity (Qi) of the competitive solute exceeded the adsorption inhibition (△Qi) of the main solute, indicating that the two solutes occupied their preferred adsorption sites, which confirmed the complementary adsorption phenomenon. Complementary adsorption can be explained by the preference of SMX and BPA for different types of adsorption sites. BPA preferentially occupied high-energy sites in the co-adsorption system, such as π-π EDA interaction, ion exchange, and surface coordination. At the same time, SMX tended to be removed by hydrophobic interaction and hydrogen bonding.

The quantitative description of the equilibrium data by the isotherm models is an indispensable link in adsorption studies. The previous review papers focus on the underlying assumptions, fitting methods, error functions and practical applications of the isotherm models, usually ignoring their curve characteristics, selection criteria and common controversies. The main contents of this review include: (i) effect of the model parameters on the isotherm curves; (ii) determination of the site energy distribution; (iii) selection criteria of the isotherm models; and (iv) elimination of some common controversies. It is of great significance to reveal the curve characteristics for selecting a proper isotherm model. The site energy distribution is conducive to understanding the physicochemical properties of the adsorbent surface. The complete isotherm is recommended to be correlated with the experimental data. The model parameter qmax should be cautiously adopted for comparison of the adsorbent performance. The residual plot can be used to diagnose the fitting quality of the isotherm models further. This review also addresses some common mistakes and controversies and thereby avoids their propagation in future publications.

As one of the large dosages of pharmaceutical and personal care products (PPCPs), acetaminophen is widely present in the water environment and presents potential environmental risks. Therefore, it is necessary to study the removal mechanism of acetaminophen from the environment. Based on the high-value conversion demand of agricultural straw resources in China, straw-derived biochar prepared by pyrolysis has a good application prospect for the sorption and purification of acetaminophen in water. However, the sorption process and mechanism of straw-derived biochar for acetaminophen remain unclear. Four types of straw (rice, wheat, maize, and bean straw) were chosen as raw materials, and straw-derived biochars were prepared through the pyrolysis method at 400℃ and 500℃. The batch sorption experiments were used to study the sorption of acetaminophen to different sources and different pyrolysis temperature biochars. The effect of humic acid and pH on the sorption process was also studied. The results showed that:based on the Freundlich and site energy distribution models, the sorption of acetaminophen on biochar at 500℃ biochar was significantly higher than that at 400℃ biochar (the sorption coefficient KF was 1.16-2.53 times higher), and 500℃ biochar had more high-energy sorption sites. For high-temperature pyrolysis biochar, the primary sorption mechanism was pore sorption and π-π effect; for low-temperature pyrolysis biochar, the primary sorption mechanism for removing acetaminophen was H-bonding. The presence of humic acid enhanced the sorption of acetaminophen, which was attributed to the strong interaction between the humic acid selected in the experiment and acetaminophen, thus promoting adsorption. The decrease in sorption capacity of biochar caused by the increasing pH was mainly attributed to the pore blockage resulting from the aggregation of acetaminophen molecules. The pore sorption and π-π interaction of acetaminophen on straw-derived biochar could be promoted by increasing pyrolysis temperature. These experiments on humic acid and pH show that straw-derived biochar is not affected by humic acid and has good sorption performance in a low pH environment.

This study explored biochar (BC) amendment effects on microcystin-LR (MCLR) concentration-dependent sorption and sequential desorption (SDE) by diverse soils to assess MCLR-trapping by BC-amended soils. Soil properties varied with rising BC dose and aging time. As aging proceeded, BC-amended soils shared a generally similar \'firstly increase and then decrease\' trend of MCLR sorption and \'firstly decrease and then increase\' trend of desorption at most cases. It appeared that MCLR sorption by BC-amended soils was most positively correlated with mesoporosity and surface basic functionality. BC-amendment increased MCLR-trapping for most soils, especially 4% BC at 3 month-aging maximized trapping ratio of GZ, SY and SX to 86.59%-95.43%, 80.01%-87.20% and 78.73%-90.85%, respectively, at 50-500 μg/L MCLR by largely increasing sorption and decreasing desorption. BC-amendment best matched GZ soil because MCLR-trapping of BC-amended GZ exceeded other amended soils at the same BC dose and aging time, but failed to obviously increase MCLR-trapping of HS soil at most cases, except only case with 2% BC at 3 month-aging. Site energy distribution verified that maximally enhanced MCLR-trapping of most soils was due to greatly enhanced sorption affinity during sorption and 1st desorption cycle, making closer MCLR-binding that more resistant to desorption. Contrarily, BC-amendment did not enhance sorption affinity of HS along sorption-SDE to compromise MCLR-trapping increase at most cases. This study validated 3 months as suitable BC-aging time to maximize MCLR-trapping in diverse soils, and elucidated influencing factors and mechanisms from view of site energy distribution, which shed novel insights on MCLR sorption-desorption by BC-amended soils, and guided to optimize BC-amendment strategy for efficient MCLR-immobilization and eco-risk elimination in diverse soils.

Information on the interactions of microplastics (MPs) with dissolved organic matter (DOM) is essential for understanding their environmental impacts. This study selected fulvic acid (FA) as a typical DOM to investigate the influence of contact time, temperature, dosage, solution pH, salinity, and coexisting metal ions on the adsorption of FA onto polyamide 6 (PA6) MPs. The adsorption kinetic and isotherm can be successfully described by mixed-order (MO) and Freundlich models. The adsorption site energy distribution based on the Freundlich equation was applied to analyze the interaction between FA and PA6-MPs and the adsorption site heterogeneity. Thermodynamic analysis demonstrated that the values of parameters (ΔGads°, ΔSads°, ΔHads°) were significantly affected by initial solution concentrations and the adsorption process was spontaneous, endothermic, and randomness-increased. Fourier transform-infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed the importance of amide functional groups of PA6-MPs in controlling FA adsorption. Hydrogen bonds, hydrophobic, electrostatic, and n-π electron donor-acceptor (n-π EDA) interactions played different roles on adsorption of FA under different conditions of solution chemistry. These findings are beneficial to provide new insights involving the adsorption behavior and interaction mechanisms of FA onto PA6-MPs for the environmental risk assessment of MPs.

The primitive biochar (BC) and NiFe2O4/biochar composites (NFBC), biological adsorbents prepared from vinasse wastes, possess the environmental application in levofloxacin (LEV) removal. In this study, the efficient adsorption of LEV onto biochar synthesized by pyrolysis of vinasse wastes from aqueous environment was investigated. The influencing factors (i.e., pH, reaction time, and temperature) of adsorption process were also well studied. The results indicated that the maximum adsorption capacities of both BC and NFBC were occurred in mildly acidic condition (pH 6). In addition, the biochar adsorption capacities were obviously increased in higher temperature (25-45 °C). The chemistry adsorption and monolayer homogeneous dominated adsorption process of LEV onto BC and NFBC. The adsorption process was spontaneous and endothermic by thermodynamic analysis. The SEDA (site energy distribution analysis) explained that the adsorption effectivity increased by increasing site energy of biochar surface. The SEDA revealed the more energy heterogeneity in NFBC, fitting the characterization result of Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The electron-donor-acceptor (EDA) interactions and hydrogen bonds is suggested as the major adsorption mechanism. And as for the adsorption of the various biowaste recycled synthetic, this study can be referred in discussion of performance analysis and optimal condition.

This study accurately assessed microcystin-LR (MCLR)-trapping capabilities of diverse biochars based on sorption and sequential desorption (SDE), and elucidated MCLR sorption-desorption mechanisms from novel views of sorption domains and site energy distribution along sorption-SDE process. Results showed that maize straw biochar (MSB) and chicken manure biochar (CMB) excelled in trapping MCLR (91.0%-97.4% and 85.7%-96.4%, respectively, at 60-600 μg/L of initial MCLR amount), followed by their respective HCl-treated ones (HCMB, HSMB), while HCl-treated bamboo biochar and pine sawdust biochar poorly trapped MCLR (48.9%-77.8% for HBB, 22.6%-67.2% for HPSB). Non-partition sorption domains (NPSD) contributed more than partition sorption domain (PSD) to MCLR sorption by each biochar. Higher NPSD contribution to MCLR sorption in CMBs and MSBs than other biochars resulted from their higher pHPZC and mesoporosity, which provided stronger electrostatic and pore-filling interaction for MCLR. Desorption hysteresis was weaken with rising aqueous MCLR amount for most biochars. Along SDE process, remaining MCLR in PSD of MSBs, HPSB and HBB could transfer to NPSD, thus desorption ratio continuously decreased with increasing desorption cycle. Differently, remaining MCLR in NPSD of CMBs converted into PSD during 1st-3rd desorption, causing fluctuated desorption ratio without obvious decrease as desorption cycle increased. These implied that MCLR in PSD was more easily desorbed than NPSD for each biochar. Site energy distribution dynamics supported the results of PSD and NPSD contribution changes along SDE. This study was greatly implicated in cost-efficient emergent MCLR-pollution remediation and deeply understanding MCLR sorption-desorption mechanisms of diverse biochars.