Site Energy Distribution Analysis Research Articles

Adsorption of anionic dyes from aqueous solutions by a novel CTAB/MXene/carbon nanotube composite: Characterization, experiments, and theoretical analysis

A novel three-dimensional composite was prepared by the self-assembly of MXene nanosheets and carbon nanotubes (CNTs) with the assistance of cetyltrimethylammonium bromide (CTAB), which was named as CMC composite. Adsorption of three anionic dyes, including Acid orange (AO7), Methyl orange (MO) and Congo red (CR), from aqueous solutions on the composite and the mechanisms were investigated in this study. Compared with MXene and CNTs, the CMC composite exhibited superior adsorption performance for AO7, MO and CR with maximum adsorption capacities of 367.9, 294.2, and 628.5 mg/g, respectively. The adsorption capacities of AO7 and MO on the CMC composite decreased, while that of CR increased with increasing temperatures. Quantum chemical calculations indicated that electrostatic interactions, π-π electron-donor-acceptor interactions and hydrogen bonding played important roles on the adsorption. The approximate site energy distribution analysis demonstrated that more adsorption active sites existed on the CMC composite than on MXene and CNTs, which endowed the CMC composite with higher adsorption capacities for these anionic dyes. The differences of adsorption behaviors for three anionic dyes could be ascribed to the number of available adsorption sites and the change of the site heterogeneity on the CMC surface.

Adsorption of atrazine and paraquat on montmorillonite loaded with layered double hydroxide and active site energy distribution analysis

Abstract Clay minerals are effective adsorbents for the remediation of pesticides in wastewater due to their large superficial areas and excellent cation-exchange capabilities. However, this adsorption effect can be reduced by the accumulation of adsorbents on clay minerals, amongst other problems. Therefore, in this study, montmorillonite (Mnt) modified by layered double hydroxide (LDH) with different loading amounts was successfully prepared using an in situ method. The results from X-ray diffraction, Fourier-transform infrared spectrometry, Brunauer–Emmet–Teller (BET) and scanning electron microscopy analyses revealed that LDH structures were successfully combined with the Mnt layer and formed a porous structure. However, excess LDH still caused the aggregation and accumulation of layers. The adsorption performance of LDH@Mnt for atrazine (ATZ) and paraquat (PQ) was investigated, and the removal efficiency of the LDH@Mnt composite was higher than those of Mnt and LDH alone. The kinetic study revealed that the adsorption process fitted the pseudo-second-order model and internal diffusion model, and 3-LDH@Mnt had the greatest absorbability efficiency for both ATZ and PQ, indicating the adsorption process was controlled by the number of active sites of the adsorbent. The generalized Langmuir model accurately characterized the adsorption process of ATZ and PQ elimination in the adsorption isotherm investigation, indicating that the adsorption energies of the active sites on the adsorbents were different. 3-LDH@Mnt had better absorbability performance for ATZ/PQ, and the sorption capacities were 7.03 and 91.9 mg g–1, respectively. According to site energy distribution theory, the amount of sorption sites of the composite adsorbent was large and the average adsorption energy was high, both of which being beneficial for the adsorption of ATZ and PQ. The effects of pH, coexisting anions and reuse experiments were also tested, indicating that the LDH@Mnt composite possessed high adsorption stability. This excellent removal performance represents a promising strategy for the remediation and elimination of pesticide contaminations from the environment.

Effect of iron nanoparticles on chromium adsorption in aqueous solution using magnetic biochar: A site energy distribution analysis

The effects of adding green-synthesized magnetic iron-containing nanoparticles (GSMFe) onto biochar in aqueous solution for the adsorptive removal of hexavalent chromium [Cr(VI)] were investigated in this study. Nanocomposites, denoted as green synthesis magnetic biochar (GSMB), were created using a green synthesis technique with white tea residue to introduce GSMFe into biochar. Six adsorbents, varying in GSMFe content, were tested for their effectiveness in eliminating Cr(VI), a globally significant hazardous heavy metal. The results demonstrated that incorporating GSMFe into biochar led to significant improvements in adsorption capacity and saturation magnetization. With an increasing amount of GSMFe, the maximum adsorption capacity increased from 2.47 mg/g (EWTWB) to 9.11 mg/g (GSMB4). The highest saturation magnetization was achieved at 13.4 Am2/kg at GSMB4. Similarly, surface areas rose up to 72.9 m2/g at GSMB3 but declined thereafter due to GSMFe aggregation and pore blockage. Sorption behavior for Cr(VI) was assessed using five isotherm models, with the Redlich-Peterson model showing the best fit. The analysis of approximate site energy distribution (SED) indicates that the incorporation of GSMFe enhances the frequency of the entire range of sorption energy sites, while the biochar matrix contributes to a slight increase in medium sorption energy sites within the GSMFe. Among the GSMBs, the difference were more pronounced at low-energy sites than at high-energy sites. At higher energy sites (27,500–40,000 J/mol), sorption site frequencies remained similar, regardless of GSMFe content and associated physicochemical properties. For sorption energy site values exceeding 17,500 J/mol (Cr(VI) concentration below 50 mg/L), GSMB2 is regarded as a more practical choice due to its relatively large area under the frequency distribution curve and commendable cost-effectiveness.

Cr(VI) adsorption on activated carbon, sludge derived biochar, and peanut shells derived biochar: Performance, mechanisms during the reuse process and site energy distribution analysis

The reusability of adsorbents for Cr(VI) removal and the related mechanism are important to practical application. In this study, commercial activated carbon (AC), sludge-derived biochar (SBC), and peanut shell-derived biochar (PBC) were reused to the tenth round on Cr(VI) removal and then characterized to analyze the ongoing adsorption mechanisms involved during the reuse process. The site energy distribution theory was used to analyze the changes in site energy during the reuse of synthesized adsorbents. Batch adsorption experiments revealed that Cr(VI) removal efficiency decreased from 99.97 % to 43.37 %, 47.22 %, and 40.13 % when reusing AC, SBC, and PBC, respectively, from the first to the tenth round at optimum conditions. The adsorption capacity decreased with each reuse round due to a decrease in -OH, CO, O=C-O, and CO on the surface of adsorbents. The site energy (E*) values decreased as adsorption capacity (qe) increased, indicating that HCrO4− and Cr2O72− first occupied high-energy adsorption sites and then the low-energy adsorption sites. Reduction of Cr(VI) to Cr(III) was identified as significant in the elimination of Cr(VI). These findings offer new insights into the reusability behavior and interaction mechanisms of adsorbents during Cr(VI) elimination from wastewater.

Biochar supported nanoscale zerovalent iron-calcium alginate composite for simultaneous removal of Mn(II) and Cr(VI) from wastewater: Sorption performance and mechanisms

Heavy metal pollution in water caused by industrial activities has become a global environmental issue. Among them, manganese mining and smelting activities have caused the combined pollution of Cr(VI) and Mn(II) in water, posing a serious ecotoxicological risk to ecological environments and human health. To efficiently remove Cr(VI) and Mn(II) from wastewater, a novel biochar supported nanoscale zerovalent iron-calcium alginate composite (CA/nZVI/RSBC) was synthesized by liquid-phase reduction and calcium alginate embedding methods. The adsorption performance and mechanisms of Cr(VI) and Mn(II) by CA/nZVI/RSBC were investigated. The maximum adsorption capacities of Cr(VI) and Mn(II) onto CA/nZVI/RSBC fitted by the Langmuir model were 5.38 and 39.78 mg/g, respectively, which were much higher than the pristine biochar. The iron release from CA/nZVI/RSBC was comparatively lower than that of nZVI/RSBC. Mn(II) presence enhanced the reduction of Cr(VI) by CA/nZVI/RSBC. The results of XRD, XPS, and site energy distribution analysis indicated that redox was the predominant mechanism of Cr(VI) adsorption, while electrostatic attraction dominated Mn(II) adsorption. This study provides a novel alternative way for the simultaneous removal of Cr(VI) and Mn(II) in wastewater.

Physicochemical and adsorptive properties of biochar derived from municipal sludge: sulfamethoxazole adsorption and underlying mechanism

Municipal sludge waste could be transformed into useful biochar through pyrolysis process. In this study, municipal sludge-derived biochar (SBC) was successfully synthesized via the one-pot pyrolysis method, and the yield of sludge biochar gradually decreased with the pyrolysis temperature increased from 300°C to 800°C. The sludge biochar exhibited an alkaline surface due to the gradual accumulation of ash and the formation of carbonate and organic anion during high-temperature pyrolysis process. Moreover, the prepared samples were analyzed by different characterization techniques including BET, SEM, and XPS. Adsorption experiments using the optimized biochar sample of SBC800 resulted in a 95% sulfamethoxazole (SMX) removal efficiency and the maximum adsorption capacity of 7033.4 mg/kg, which was 47.5 times higher than that of SBC300. The adsorption process of SBC800 for SMX was more in line with the Freundlich and D-A isotherm model, the whole process was an exothermic reaction. SBC800 could effectively remove SMX through pore filling effect, electrostatic attraction, hydrogen bonding, hydrophobic effect, and π-π EDA interaction. Site energy distribution analysis showed that SMX preferentially occupied the high-energy adsorption site of SBC800, and then gradually diffused to the low-energy adsorption site. This study proposed a sustainable method for recycling municipal sludge for organic pollutant removal.

The effect of micro- and nano-γ-Al2O3 on the synthetic hydrotalcite and Congo red adsorption: Site energy distribution

It is significant to reveal the structure and properties of micro-nano reactants and their relationships in preparing high-performance materials. In this study, the effect on the structures and Congo red (CR) removal properties of synthetic hydrotalcite (HT) by micro γ-Al2O3 (MA) and nano γ-Al2O3 (NA) (named HT-MA and HT-NA, respectively) were investigated. FTIR, XRD, TGA-DTG, SEM-EDS, and BET, amongst others, were used to analyze the morphological and structural differences between HT-MA and HT-NA. The effects of synthetic HT addition, initial pH, CR initial concentration, contact time, and other factors on CR adsorption were also investigated. The results showed that the CR adsorption efficiency of HT-NA was higher than that of HT-MA under the same conditions and was not affected by pH in the range of 4–12. Kinetic analysis indicated that the adsorption process fits well with pseudo-second-order and intraparticle diffusion models, and the isotherm curves fit better with the Freundlich and Dubinin-Ashtakhov models, suggesting a heterogeneous porous multilayer adsorption mechanism. The site energy distribution analysis results further elucidated that nano-γ-Al2O3 was more favorable to increasing the number of high-energy sites and a stronger heterogeneous surface interaction of synthetic HT, thus improving CR adsorption efficiency. HT-NA and HT-MA showed good regeneration performances after being recycled four times and are expected to be used in wastewater treatment.

Porous N-doped carbon converted from protein-rich shrub enables record-high removal of p-nitrophenol: superior performance and mechanism

The complete removal of harmful phenolic contaminants from water remains huge challenge because of the lack of cost-effective and powerful adsorbents. Herein, a sustainable nitrogen self-doped biochar (named KBC750N) with large specific surface area (1398.44 m2/g), high adsorption capacity (606.06 mg/g) and removal ratio (99.99%) for p-nitrophenol (PNP) in real waters (Seawater, Yangtze River water, and Yellow River water) was prepared from a naturally abundant protein-rich psammophyte Caragana korshinskii (CK) by a facile one-step pyrolysis process. The in-situ nitrogen-doped biochar shows much better adsorption removal efficiency of 99.99% toward PNP than expensive commercial adsorbents (only 77.52%). The biochar still keeps high adsorption efficiency after reused more than five times. Site energy distribution analysis revealed that the biochar captures PNP mainly through n–π and π–π electron-donor–acceptor interactions among the electron-rich functional groups (N-containing moieties, –COOH, and –OH), graphite carbon, and PNP. Density functional theory (DFT) calculations confirmed that the nitrogen site in biochar helps to strengthen its adsorption capability to PNP. This work provides a sustainable platform for the development of high-performance, cost-effective materials for efficient elimination of organic pollutants from water.

Pick-up of fluoroquinolones from the aqueous phase via magnetically propeled microrobots: kinetics, thermodynamics, and site energy distribution analysis

Removing fluoroquinolones (FQs) in marine culture tailwater is crucial for the coastal marine environment. The application of a bacteria-based microrobot for FQ removal was discussed. Norfloxacin (NOR) and levofloxacin (LEV) had static maximum adsorption capacities of 114.8 and 49.4 mg/g, respectively, by a magnetic microrobot. The experimental results of NOR adsorption by a magnetic microrobot were well supported by the Langmuir isotherm and Elovich kinetic models. Both the Langmuir isotherm model and the pseudo-second-order kinetic model may be able to accurately represent the LEV adsorption process. The mass transfer mechanism of the NOR and LEV adsorptions was divided into two steps and described better using the intraparticle diffusion (IPD) model. The exothermic and spontaneity of the sorption process were demonstrated through the study of thermodynamics. The magnetic microrobot’s heterogeneous surface was validated by the examination of site energy distribution. Additionally, this study demonstrated that the majority of the NOR and LEV sorption took place at sites with energies over 4.25 and 17.36 kJ/mol, respectively, supporting the notion that NOR and LEV adsorption constitute physical–chemical processes. Based on the above results, a magnetic microrobot, as a new-style green bio-adsorbent, can potentially be used to remove NOR and LEV from the mariculture in an inexpensive and effective manner.

Effects of Salinity, pH, and Cu(II) on the Adsorption Behaviors of Tetracycline onto Polyvinyl Chloride Microplastics: A Site Energy Distribution Analysis

Microplastics (MPs), as vectors for various pollutants in the environment, have been arousing public concern in recent years. The extensive use of antibiotics has led to their frequent detection in water environments, where they inevitably coexist with MPs and are actively adsorbed onto MPs’ surfaces. However, the information on the influence of the main environmental factors on the sorption behavior of MPs is not fully understood, and especially, information about the effect mechanism is limited. This study aims to comprehensively assess the main factors and mechanisms that affect antibiotic sorption onto microplastics. The results indicated that the adsorption of tetracycline (TC) onto PVC MPs fits the pseudo-second-order model well. The adsorption of TC onto PVC MPs decreased with increasing salinity and pH. With the help of SED analysis, the decrease at high salinity was attributed to the decreased quantities of the most distributed energy sites in high-salinity systems compared to freshwater. Additionally, the decreased adsorption capacity of TC onto MPs at high pH mainly resulted from the electrostatic repulsion between MPs and TC2−. The coexisting Cu2+ and TC could improve the affinity between PVC MPs and TC via “MPs-TC-Cu2+” and “MPs-Cu2+-TC” bridges, boosting the capacity of PVC MPs to adsorb TC. This study provides comprehensive insights into the influence and mechanism of the main factors on the environmental behaviors of the coexistence of MPs with antibiotics, which is of great importance for evaluating and controlling their risks.

Adsorption of antibiotics on montmorillonite and site energy distribution analysis

The migration and transformation of antibiotics requires further study in geochemical settings. In this study, the removal effect of montmorillonite (Mt) on antibiotics was investigated, using the broad-spectrum antibiotics ciprofloxacin (CIP) and erythromycin (ERY) as subjects. The results of adsorption kinetics and adsorption isotherm experiments showed that the generalized Langmuir model describe the adsorption process well, and the adsorption capacity of CIP was higher. The intercalation and adsorption behaviors of CIP and ERY on Mt surface were analyzed by FT-IR, SEM, TEM, XRD and Molecular simulation. In order to analyze the different adsorption capacity of these antibiotics on Mt, the theory of site energy distribution was employed. By fitting the experimental data with the Generalized-Langmuir model, the site energy distribution and average site energy were calculated, which revealed the competitive adsorption mechanism of CIP and ERY on Mt. Moreover, the adsorption of antibiotics on Mt under different pH conditions was studied, and the competitive adsorption results were also demonstrated by the potential energy distribution.

Photo-aged non-biodegradable and biodegradable mulching film microplastics alter the interfacial behaviors between agricultural soil and inorganic arsenic

The impacts of microplastics (MPs) prevalent in soil on the transport of pollutants were urged to be addressed, which has important implications for ecological risk assessment. Therefore, we investigated the influence of virgin/photo-aged biodegradable polylactic acid (PLA) and non-biodegradable black polyethylene (BPE) mulching films MPs on arsenic (As) transport behaviors in agricultural soil. Results showed that both virgin PLA (VPLA) and aged PLA (APLA) enhanced the adsorption of As(Ⅲ) (9.5%, 13.3%) and As(Ⅴ) (22.0%, 6.8%) due to the formation of abundant H-bonds. Conversely, virgin BPE (VBPE) reduced the adsorption of As(Ⅲ) (11.0%) and As(Ⅴ) (7.4%) in soil owing to the “dilution effect”, while aged BPE (ABPE) improved arsenic adsorption amount to the level of pure soil due to newly generated O-containing functional groups being feasible to form H-bonds with arsenic. Site energy distribution analysis indicated that the dominant adsorption mechanism of arsenic, chemisorption, was not impacted by MPs. The occurrence of biodegradable VPLA/APLA MPs rather than non-biodegradable VBPE/ABPE MPs resulted in an increased risk of soil accumulating As(Ⅲ) (moderate) and As(Ⅴ) (considerable). This work uncovers the role of biodegradable/non-biodegradable mulching film MPs in arsenic migration and potential risks in the soil ecosystem, depending on the types and aging of MPs.

Sorption Characteristics and Site Energy Distribution Theory of Typical Estrogens on Microplastics

Microplastics (MPs) and estrogens are high-profile emerging contaminants at present, and MPs might become the carrier of estrogens in the environment and induce combined pollution. To study the adsorption behavior of polyethylene (PE) microplastics to typical estrogens, the adsorption isothermal properties of the six estrogens[estrone (E1), 17α-estradiol (17α-E2), 17β-estradiol (17β-E2), estriol (E3), diethylstilbestrol (DES), and ethinylestradiol (17α-EE2)] in single-solute and mixed-solute systems were studied through batch equilibrium adsorption experiments, in which the PE microplastics before and after adsorption were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Then, the site energy distribution theory of the adsorption of six estrogens on PE microplastics was further analyzed based on the Freundlich model. The results showed that the adsorption process of selected estrogens with two concentrations (100 μg·L-1 and 1000 μg·L-1) on PE were more consistent with the pseudo-second order kinetic model. The increase in initial concentration reduced the equilibrium time of adsorption and increased the adsorbing capacity of estrogens on PE. In the single system (one estrogen) or mixed system (six estrogens) with different concentrations (10 μg·L-1-2000 μg·L-1), the Freundlich model showed the best fitting effect for the adsorption isotherm data (R2>0.94). The results of isothermal adsorption experiments and XPS and FTIR spectra showed that the adsorption of estrogens on PE in the two systems was heterogeneous adsorption, and hydrophobic distribution and van der Waals forces were the principal factors in the process of adsorption. The occurrence of C-O-C (in only the DES and 17α-EE2 systems) and O-C[FY=,1]O (in only the 17α-EE2 system) indicated that the adsorption of synthetic estrogens on PE was affected slightly by chemical bonding function, but no obvious effects were observed for natural estrogens. The results of site energy distribution analysis showed that, compared with the single system, the adsorption site energy of each estrogen shifted to the high-energy region in its entirety in the mixed system, and the site energy increased by 2.15%-40.98%. The energy change in DES was the most significant among all of the estrogens, indicating its competitive advantage in the mixed system. The above results of this study can provide some reference for the study of adsorption behavior, mechanism of action, and environmental risks under the coexisting condition of organic pollutants and MPs.

Effective adsorption of crystal violet onto magnetic nanoparticles decorated bacteria: Kinetic and site energy distribution analysis

The discharge of crystal violet wastewater will cause extremely serious harm to the natural environment and human health, due to its certain toxicity and carcinogenicity. The adsorption characteristics and mechanism of crystal violet on magnetic nanoparticles decorated bacteria (Fe3O4 @bacteria) were investigated especially the adsorption kinetic, equilibrium, mass transfer mechanism, and site energy distribution analysis. With the initial concentration of crystal violet 40 mg/L, pH= 4, the addition of 0.003 g Fe3O4 @bacteria, after adsorption for 2 h under 25 °C, the static maximum adsorption capacity of crystal violet was up to 114.8 mg/g, which was 2.07 times that of Fe3O4 under the same conditions. Fe3O4 @bacteria could be recycled more than 6 times, also exhibited a 76.2% reduction in COD for actual dyeing wastewater. The adsorption data fitted well with the Temkin isotherm model, Elovich kinetic, and pseudo-second-order kinetic models. As for the mass transfer mechanism of the adsorption process, the IPD model was more suitable to describe the mass transfer process of the crystal violet adsorption by Fe3O4 @bacteria, while the intraparticle diffusion is not the only mechanism controlling the adsorption process. The adsorption process is spontaneous and exothermic according to the thermodynamics study. Based on the analysis of site energy distribution, the biosorbent had a heterogeneous surface and most of the crystal violet sorption occurred on the sites with energy over 13.0 kJ/mol, which supported that crystal violet sorption was physical-chemical sorption. Therefore, the magnetic nanoparticles decorated bacteria can serve as promising adsorbents to eliminate the organic dye pollutants caused by textile industry.

Photoaging processes of polyvinyl chloride microplastics enhance the adsorption of tetracycline and facilitate the formation of antibiotic resistance

Microplastics (MPs), antibiotics and microorganism ubiquitously coexist in aquatic environments. MPs inevitably undergo photoaging processes in aquatic environments, affecting the interactions between MPs and antibiotics and the antibiotic resistance of microorganism. In this study, the impact of photoaging processes of MPs on their adsorption behavior of tetracycline (TC) and related formation of antibiotic resistance were investigated. It was found that the photoaging processes significantly increased the adsorption capacity of TC onto MPs, with the Qe increasing from 0.387 to 0.507 mg/g at 288 K and from 0.507 to 0.688 mg/g at 308 K. The site energy distribution (SED) analysis further confirmed that the enhanced adsorption capacity was attributed to more high-energy adsorption sites acquired from MPs photoaging processes. Moreover, the enhanced adsorption of TC further facilitated the formation of seven antibiotic resistance genes (i.e., tetA, tetB, tetC, tetD, tetE, tetG, tetK) when MPs adsorbed with TC was covered by biofilm. This study helps comprehensively understand the environmental behaviors of co-existing MPs, antibiotics and microorganisms, providing a theoretical basis for evaluating and mitigating their coexistence risks.

Molecular weight-dependent adsorption heterogeneities of humic acid on microplastics in aquatic environments: Further insights from fluorescence spectra combined with two-dimensional correlation spectroscopy and site energy distribution analysis

Microplastics as emerging environmental contaminants have attracted global attention. Ubiquitous dissolved organic matter (DOM), a heterogeneous mixture with various organic components and continuous molecular weight (MW) distribution, can significantly influence the fate, bioavailability and toxicity of microplastics in aquatic environments. However, to date, understanding of the MW-dependent adsorption heterogeneities of DOM on microplastics in aquatic environments has remained unknown. In this study, the soil humic acid (HA), a representative DOM, was fractionated into > 100 kDa HA, 30–100 kDa HA, 10–30 kDa HA, 3–10 kDa HA and < 3 kDa HA, whose adsorption behaviors on polystyrene microplastics (PSMPs) under different electrolytes at pH6.0 investigated by multispectral techniques, as well as two-dimensional correlation spectroscopy and site energy distribution analysis. High molecular weight MW-fractionated HAs dominated the soil HA. Compared to the lower molecular weight MW-fractionated HAs, the higher molecular weight counterparts exhibited higher aromaticity, showing higher adsorption affinities onto PSMPs in aquatic environments. Adsorption isotherms and kinetics of the MW-fractionated HAs onto PSMPS can be reasonably explained by Langmuir models and the linear driving force (LDF) model, respectively. Enhanced aromaticity in residual solutions was observed, suggesting that the preferable adsorption of the higher aromatic moieties in the pristine and MW-fractionated HAs on PSMPs could be subsequently replaced by the lower aromatic moieties for the limited binding sites. Combined, the humic-like materials in MW-fractionated HAs could be significantly impacted the complexation of PSMPs in aquatic environments. The heterogeneous/complicated distributions of active adsorption sites in the humic-like materials of the MW-fractionated HAs, and the subsequent subtle changes of these sites to PSMPs were characterized by synchronous fluorescence spectroscopy coupled with two-dimensional correlation spectroscopy. High molecular weight MW-fractionated HAs provided more active adsorption sites than those in the low molecular weight counterparts, which also possessed stronger adsorption affinities and higher complexation capacities to PSMPs. The site energy distributions for the adsorption of the MW-fractionated HAs on PSMPs were calculated based on Langmuir model. The adsorption heterogeneities of PSMPs at the experimental conditions were close. This study provides deep insights into the MW-dependent adsorption heterogeneities of HA on PSMPs, and the results are of significance to understand the biogeochemical behaviors of DOM and MPs in aquatic environments.

Microfiltration Membranes for the Removal of Bisphenol A from Aqueous Solution: Adsorption Behavior and Mechanism

This study mainly investigated the adsorption behavior and mechanism of microfiltration membranes (MFMs) with different physiochemical properties (polyamide (PA), polyvinylidene fluoride (PVDF), nitrocellulose (NC), and polytetrafluoroethylene (PTFE)) for bisphenol A (BPA). According to the adsorption isotherm and kinetic, the maximum adsorption capacity of these MFMs was PA (161.29 mg/g) &gt; PVDF (80.00 mg/g) &gt; NC (18.02 mg/g) &gt; PTFE (1.56 mg/g), and the adsorption rate was PVDF (K1 = 2.373 h−1) &gt; PA (K1 = 1.739 h−1) &gt; NC (K1 = 1.086 h−1). The site energy distribution analysis showed that PA MFMs had the greatest adsorption sites, followed by PVDF and NC MFMs. The study of the adsorption mechanism suggested that the hydrophilic microdomain and hydrophobic microdomain had a micro-separation for PA and PVDF, which resulted in a higher adsorption capacity of PA and PVDF MFMs. The hydrophilic microdomain providing hydrogen bonding sites and the hydrophobic microdomain providing hydrophobic interaction, play a synergetic role in improving the BPA adsorption. Due to the hydrogen bonding force being greater than the hydrophobic force, more hydrogen bonding sites on the hydrophobic surface resulted in a higher adsorption capacity, but the hydrophobic interaction contributed to improving the adsorption rate. Therefore, the distribution of the hydrophilic microdomain and hydrophobic microdomain on MFMs can influence the adsorption capacity and the adsorption rate for BPA or its analogues. These consequences provide a novel insight for better understanding the adsorption behavior and mechanism on MFMs.

Adsorption of Tannic Acid and Macromolecular Humic/Fulvic Acid onto Polystyrene Microplastics: A Comparison Study

Dissolved organic matter (DOM) has been widely reported to influence the environmental behavior of microplastics (MPs), but little is known about the properties and mechanisms of interaction between specific DOM components and MPs. Here, we studied the adsorption of three representative DOM components (humic acid, HA; fulvic acid, FA; and tannic acid, TA) on polystyrene (PS) MPs in batch adsorption experiments. Results revealed that HA/FA adsorption was greater under acidic conditions, while higher TA adsorption on PS was found at pH 4 and 6. The divalent cation (Ca2+) exerted a more prominent role in enhancing HA, FA, and TA adsorption on PS than did monovalent ones (K+ and Na+). The adsorption process fitted well with the Freundlich isotherm model and the pseudo-second-order kinetics model. The adsorption site heterogeneity was evaluated using the site energy distribution analysis based on the Freundlich model. The greater binding ability of HA on the PS surface caused a more negatively charged surface than FA/TA, as reflected by Zeta potential values. The findings of this study not only provide valuable information about the adsorption behavior and interaction processes of various DOM components on PS MPs, but also aid our efforts to evaluate the environmental behaviors of MPs.

Effects of humic acid on Pb2+ adsorption onto polystyrene microplastics from spectroscopic analysis and site energy distribution analysis

Microplastics (MPs), act as vectors of heavy metal pollutants in the environment, is of practical significance to study the adsorption process and mechanism on heavy metals. In this study, polystyrene microplastics (PSMPs) were used as model MPs to study the adsorption of Pb2+ on PSMPs and the effects of humic acid (HA) on the adsorption process. The results showed that HA promoted the adsorption of Pb2+ on PSMPs, and the higher the concentration of HA, the greater the adsorption of Pb2+. With the increase of pH value and decrease of ionic strength, the adsorption capacity of PSMPs for Pb2+ increased. The scanning electron microscope equipped with the energy dispersive spectroscope (SEM–EDS), fourier transform-infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis showed that Pb2+ could be adsorbed directly onto PSMPs and also indirectly by HA. The higher KSV values in the PSMPs-HA-Pb2+ system than PSMPs-HA system by fluorescence analysis of HA suggested that HA acted as a bridging role in the adsorption of Pb2+ on PSMPs. The site energy distribution analysis further revealed that HA increased the average site energy μ(E*) and its standard deviation σe* of PSMPs by introducing more adsorption sites, thus enhanced the adsorption affinity of PSMPs. This study provided more thoughts and insights into the adsorption behavior and mechanism of MPs for Pb2+ in aquatic environments.

Adsorption of tetracycline hydrochloride on layered double hydroxide loaded carbon nanotubes and site energy distribution analysis

In this study, a novel composite material of growing layered double hydroxides on carbon nanotubes (LDH@CNT) was prepared by in-situ method and possessed a favorable adsorbent structure by the mutual support of LDHs and CNTs. For the removal of antibiotic tetracycline, the adsorption kinetics of LDHs, CNTs, and LDH@CNT composites were well fitted by intraparticle diffusion model and pseudo-second-order model, revealing that the adsorption site was the main factor for the adsorption process. In the isotherm study, Generalized Langmuir model described the adsorption process better than the Langmuir model, indicating that there are multiple interactions between tetracycline and composite material. In addition, site energy distribution theory combined with molecular simulation to explain the adsorption mechanisms, and the removal of tetracycline by LDHs, CNTs, and LDH@CNT composites was calculated. The results showed that the average site energies of LDH@CNT composites were lower than that of LDHs (19.09 kJ /mol) and CNTs (16.41 kJ /mol), but exposed adsorption active sites in LDH@CNT composites were much more than that of LDHs and CNTs, which was beneficial to removal of tetracycline. The innovative growth of LDHs on CNTs overcame the disadvantage of easy aggregation, which exposed more adsorption sites and improved the removal efficiency of TC. In addition, the effects of pH, ion strength, antibiotic competition and reuse tests on LDH@CNT composite samples were also analyzed and showed a good stability of composite. Therefore, the excellent removal performance of LDH@CNT composite suggested a promising strategy for the removal and remediation of antibiotics contaminated environment.