Slow Adsorption Research Articles

Effective elimination of hexavalent chromium and lead from solution by the modified biochar with MgMn2O4 nanoparticles: adsorption performance and mechanism.

Heavy metal pollution is one of the environmental problems that need to be solved urgently. The adsorption method is thought as the most effective and economical treatment technology. Nature biochar usually showed unsatisfactory adsorption capacity due to its relatively small adsorption capacity and slow adsorption rate. The metal of Mn has been widely applied in the modification of biochar, which could effectively improve the adsorption capacity of biochar. However, leaching of Mn2+ on the adsorbent materials would appear during the adsorption process. And it would increase the risk of secondary pollution. The multifunctional binary modified biochar could improve the adsorption capacity of environmental pollutant removal. In addition, it could also act as a metal support carrier, reducing the risk of secondary pollution. A novel effective biochar loaded by Mg-Mn binary oxide nanoparticles (MgMn2O4@Biochar) was prepared and applied for the Cr(VI) and Pb(II) removal in aqueous solution. The characteristic of MgMn2O4@Biochar was analyzed by SEM, TEM, FTIR, and XRD. The irregular and somewhat flaky shaped particles of different shape and sizes clustered on the surface of MgMn2O4@Biochar appeared. Abundant functional groups of O-H, -C-OH, C-O, and C-OOH could be observed on the surface of MgMn2O4@Biochar. The elements of Mg and Mn elements besides of C, O, and Si elements were presented on the surface of MgMn2O4@Biochar. The wt% of C, O, Mg, Mn, and Si were 42.82%, 48.99%, 2.83%, 4.44%, and 0.93%, respectively. The operational parameters had an important influence on adsorption capacity of Cr(VI) and Pb(II) removal. The results showed that the adsorption capacity of MgMn2O4@Biochar for Cr(VI) and Pb(II) would reach 33.5 mg/g and 536 mg/g, respectively, within 360 min. Additionally, the adsorption processes of Cr(VI) and Pb(II) in solution could be described with pseudo-second-order. For Cr(VI), the Langmuir model was suitable to the adsorption process. However, the adsorption process of Pb(II) in solution could be described with Freundlich model. Furthermore, it could be concluded that the possible mechanism of Cr(VI) and Pb(II) removal by MgMn2O4@Biochar was physical adsorption, surface complexation reaction, and electrostatic adsorption.

Performance and dynamic behavior of H2 layered-bed PSA processes using various activated carbons and zeolite LiX for steam methane reforming gas

Layered-bed pressure swing adsorption (PSA) processes using activated carbons (ACs) and zeolite LiX were investigated to elucidate the performance and dynamic behavior of each AC in H2 separation. Experimental adsorption and re-adsorption breakthrough characteristics of ACs (CAC: parent AC, KACa: KOH-activated CAC, and KACi: KOH-impregnated CAC) and zeolite LiX were compared using a steam methane reforming gas. A rigorous dynamic model was used to evaluate the performance of four and six layered-bed PSA processes in terms of purity, recovery, productivity, and impurities. The KACi bed demonstrated the longest breakthrough time of impurities, but its desorption performance was adversely affected by the strong affinity induced by intercalated potassium compounds. While KACa showed higher adsorption capacities than CAC, its slow adsorption rate resulted in the shortest breakthrough time. On the other hand, CAC presented a separation performance between KACi and KACa beds, but its weak CO adsorption affinity led to a relatively wide CO mass transfer zone. Zeolite LiX demonstrated the longest CO breakthrough time, indicating its potential to purify trace CO in the layered-bed, but its layer length must be carefully decided to prevent CO2 propagation. The four layered-bed PSA produced a fuel cell grade H2 (99.9999% purity, 81.54% recovery, 115 mol/kg/day productivity, and < 0.2 ppm CO), which was further improved by the six layered-bed PSA to 1.5% at the same H2 purity. The dynamic behaviors and sensitivity analysis (P/F ratio, adsorption time, AC/LiX ratio, AC type, and number of beds) provide valuable insights and guidelines for the design and optimization of H2 layered-bed PSA.

A comprehensive evaluation of amine-impregnated silica materials for direct air capture of carbon dioxide

The potential of amine-impregnated silica materials for Direct Air Capture (DAC) of CO2 was investigated. A commercially available mesoporous silica was impregnated with tetraethylenepentamine (TEPA) and polyethylenimine (PEI) with molecular weights of 600, 1200, and 1800, each using amine loadings of 10–50 wt%. The materials were screened for equilibrium CO2 uptake, amine efficiency, and adsorption kinetics at temperatures of 25–35 °C. For all loadings, the CO2 uptakes followed the order of TEPA > PEI-600 > PEI-1200 ≈ PEI-1800. Also, for all amines, the materials synthesized with 50% loading achieved the highest equilibrium CO2 uptakes despite suffering from low amine efficiency and slow adsorption kinetics due to diffusional limitations. For these materials, increasing the adsorption temperature from 25 to 30 and 35 °C enhanced equilibrium CO2 uptake and amine efficiency, indicating kinetically controlled adsorption. In contrast, for all amines, the materials synthesized with 40% loading achieved a combination of high equilibrium CO2 uptake, good amine efficiency, and fast adsorption kinetics; therefore, they were selected to undergo 50 successive adsorption–desorption cycles. Despite an initially high CO2 uptake, TEPA suffered from low thermal stability due to its volatility, resulting in an amine loss of 55% and CO2 uptake loss of 29%. In contrast, bulkier amines such as PEI-600 outperformed TEPA, showing high thermal stability (1.4% amine loss) and low CO2 uptake loss (0.8%). Selected materials underwent column-breakthrough experiments in the presence of 400 ppmv CO2 in N2. Under dry conditions, TEPA achieved a longer breakthrough time than PEI-600 (13 versus 4 min). Under humid conditions, the breakthrough time for PEI-600 increased from 4 to 15 min, confirming previous reports that humidity boosts the CO2 uptake of amine-modified materials.

Dynamic polyiodide anions formation in cationic covalent organic framework for efficient iodine capture

Rapid and efficient capture of iodine is very important for the treatment of radioactive nuclear waste, but the traditional iodine adsorbents have the disadvantages, including the slow adsorption rate and the low adsorption capacity. To address these problems, herein, a new cationic vinyl-based covalent organic framework (COF) with abundant dissociative iodide anions was synthesized by Knoevenagel condensation and the post-ionization modification (PIM) reaction. The resulting ionic COF (iCOF-TMPT) has high specific surface area, excellent chemical stability, as well as the high-density pyridine cations and abundant dissociative iodide anions, which is beneficial for the highly efficient iodine adsorption. The iCOF-TMPT not only has the outstanding iodine adsorption capacity for iodine vapor and solution (6.61 g g−1 for iodine vapor and 1.55 g g−1 for iodine solution), but also possesses the remarkable iodine adsorption rate of 2.81 g h−1, which surpasses any other reported COF absorbents. The adsorption mechanism indicated that the dissociative iodide ions in iCOF-TMPT can induce neutral iodine molecules to generate polyiodide anions, and the pyridine cations in iCOF-TMPT can form strong Coulomb interaction with the formed polyiodide anions. This work not only provides a novel cationic vinyl-based two-dimensional COF with ultra-high chemical stability by Knoevenagel condensation and PIM reaction, the high-density pyridine cations and extra-framework iodide ions can achieve efficient I2 capture by inducing the formation of polyiodide.

MgH2Insights Used for Hydrogen Storage and Catalysis: Preparation, Characterization and Mechanism

Magnesium is a rich element on earth, with high reversible, cycle stability and antitoxicity, low cost, and high-volume hydrogen capacity, its theoretical hydrogen storage capacity up to 7.6 wt %. Magnesium-based hydrogen storage materials are considered as one of the most promising hydrogen storage media. However, due to its high thermodynamic stability and slow hydrogen adsorption kinetics, the high adsorption temperature and long adsorption time limit its practical application. In order to overcome the thermodynamic and kinetic obstacles of magnesium-based hydrogen storage materials in practical application and improve the hydrogen storage performance of MgH2, catalytic modification is a very effective method at present. This review summarizes the preparation methods, characterization methods and reaction basis of various catalysts modified by MgH2. Finally, we analyze the development trend of various catalysts in the future, as well as the application prospects.

High-efficiently and rapidly capturing cesium from water with Prussian blue@expanded graphite as adsorbent

The removal of radioactive 137Cs from nuclear wastewater is still needed to mitigate their high radiological toxicity to living creatures. However, the industrial application of adsorbents is still challenging owing to the complex and costly preparation and slow adsorption rate for Cs wastewater with low concentration. Herein, cost-effective and fast removal of Cs+ is achieved by vermiform expanded graphite (EG) immobilized with potassium copper hexacyanoferrate (KCuHCF) via the in-situ synthesis method. We revealed that the nano-scale KCuHCF granules are dispersed homogeneously on the EG interlayers. With this design, it achieves an excellent Cs removal capacity of 16.24 mg/g within 30 min in ultralow Cs concentration and shows high selectivity among various competing ions. We systematically studied the associated adsorption mechanism by XPS and ICP, which revealed that ion exchange between the lattice K and Cs played critical roles. Our work paves the way for the design of economical adsorbents for radioactive Cs remediation.

Effect of soluble surfactant on the interface dynamics of a rising droplet

In this study, we present numerical simulations examining the impact of soluble surfactant on the interface dynamics of a rising droplet. To achieve this, the droplet interface is tracked using an arbitrary Lagrangian–Eulerian approach, and the bulk and interfacial surfactant concentration evolution equations fully coupled with the incompressible Navier–Stokes equations are solved. We systematically evaluate the boundary of interfacial dynamics evolution by varying certain dimensionless parameters. Specifically, we study the effects of changes in parameters such as the Langmuir number, the Biot number, the Damkohler number, the bulk Peclet number, and the elastic number on interfacial tangential velocity, interfacial concentration and its gradient, interfacial viscous shear stress, and droplet rising velocity. Our findings confirm the validity of the stagnant-cap model for describing the interfacial fluidity of a surfactant-laden rising droplet. Increasing the Langmuir number and decreasing the Damkohler number can inhibit interface fluidity, but there is a threshold for the Damkohler number. Additionally, the overall increase in interface tension may mask the hindering effect of the locally increased concentration gradient on the interfacial fluidity. The Biot number has no impact on the steady state of the interface, but a slow adsorption rate may result in a bimodal retardation before the interface reaches a steady state. A clear threshold exists for the Peclet number to hinder the interface velocity, and a too high Peclet number leads to strong nonlinearity in the interface physical quantities. Variations in the elastic number significantly affect the evolution of the interface, causing the interface velocity to pass through several states, ranging from almost no retardation, uniform retardation, stagnant-cap retardation to complete retardation.

Synergy between hydrophilic silica nanoparticles and poly(vinylpyrrolidone) for long-term foam stabilization with little energy input

The role of solid particles to act as foam and emulsion stabilizers is well known, providing long-term stability to the two-phase system. However, the ability for particles to generate large volumes of foam with little energy input (low agitation speeds and short agitation times) is difficult to achieve. Poor foamability is due to the slow particle adsorption dynamics and the high adsorption barrier at the air–water interface. In contrast, common surfactants and some surface active polymers which adsorb more readily at the air–water interface exhibit good foamability but often poor foam stability due to the mutual action of multiple destabilization mechanisms. In the current study, we reported a simple bi-component system of hydrophilic silica nanoparticles and poly(vinylpyrrolidone) (PVP) polymer to produce foams with both high foamability and foam stability via simply hand shaking for 1 min. Foams are stabilized by PVP coated silica composite nanoparticles (CPs) with a well-defined core–shell structure. The strong adsorption of PVP onto silica was demonstrated using Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), confirming the formation of an irreversibly and robust adsorbed polymer layer. To better understand the mechanisms for enhanced foamability and foam stability, the interfacial dynamics and the 2-dimensional mechanical properties of the particle-polymer network partitioned at the air–water interface were assessed. The presence of the PVP polymer corona substantially enhanced the adsorption and retention of hydrophilic silica particles at the air/water interface, while the rigid interfacial network formed by the silica nanoparticles guaranteed long-term foam stability (excess of one week), with coarsening fund to be the dominating destabilization mechanism for foam. The enhanced performance is in stark contrast to the individual components, which could only provide good foamability other than foam stability.

High-capacity/high-rate hybrid column for high-performance ion exchange

Herein, a fixed-bed high-capacity/high-rate (HC/HR) hybrid column was developed using commercial ion-exchange beads (IEBs) and ion-exchange fibers (IEFs). The as-fabricated HC/HR hybrid column exhibited excellent breakthrough bed volume (BV) and utilization efficiency of capacity (UEC) at a high service flow rate (SFR) for the adsorption of Cd(II). The IEBs displayed a high adsorption capacity of 235.2 ± 9.8 mg g−1 and slow adsorption kinetics (k2 = 0.0001 g mg−1 min−1) for the sorption of Cd(II); meanwhile, the IEFs showed a maximum adsorption capacity of only 146.3 ± 7.5 mg g−1, which is lower than that of the IEBs, but fast kinetics (k2 = 0.0130 g mg−1 min−1). At an SFR of 104.23 BV h−1, the HC/HR hybrid column showed excellent performance for the sorption of Cd(II), having a high breakthrough BV of 1009.11 and a UEC of 92.86%; these values are much higher than those of the IEB-packed column. Furthermore, at an increased SFR (318.47 BV h−1), the HC/HR hybrid column maintained its high performance, demonstrating a breakthrough BV of 568.80 and UEC of 83.90%. The regeneration experiment indicates that 97% of the initial capacity was retained. Thus, the HC/HR hybrid column could easily be applied to existing column systems and shows promising performance in ion-exchange processes.

Adsorption of Tannic Acid onto Gold Surfaces

Thin film coatings are widely applicable in materials for consumer products, electronics, optical coatings, and even biomedical applications. Wet coating can be an effective method to obtain thin films of functional materials, and this technique has recently been studied in depth for the formation of bioinspired polyphenolic films. Naturally occurring polyphenols such as tannic acid (TA) have garnered interest due to their roles in biological processes and their applicability as antioxidants, antibacterial agents, and corrosion inhibitors. Understanding the adsorption of polyphenols to surfaces is a core aspect in the fabrication processes of thin films of these materials. In this work, the adsorption of TA to gold surfaces is measured using a quartz crystal microbalance with dissipation monitoring (QCMD) and surface plasmon resonance (SPR) for a wide range of TA concentrations. The adsorption kinetics, aggregation, and stability of TA solutions in physiological-like conditions are studied. Unexpectedly, it is found that the adsorption rates depend only weakly on concentration because of the presence of TA aggregates that do not adsorb. The mechanism of layer formation is also investigated, finding that TA monolayers readily adsorb onto gold with flat or edge-on molecular orientations dependent on the solution concentration. A mix of orientations in the intermediate case leads to slow multilayer adsorption.

Polymer-microsphere coated with MoS2 nanoflowers for the removal of Pb(II) from water: Polydopamine-mediated MoS2 assembly and efficient Pb(II) adsorption

The engineering of macroscopic architectures by loading MoS2 nanosheets onto larger framework materials is an efficient strategy to overcome the limitations preventing the application of nanosized MoS2-based adsorbents for Pb(II) removal. However, the large-scale utilization of these nanocomposites still faces several challenges as a result of their poor stability, slow adsorption kinetics, and low MoS2 utilization. Herein, a practical and efficient nanocomposite (MoS2/PDA/MPS) was prepared by coating MoS2 nanoflowers onto micron-sized polystyrene beads (MPS) via the construction of a strong adhesive polydopamine (PDA) interlayer. The PDA interface can adhere to the MPS surface and immobilize the MoS2 nanopartices, thus facilitating the application of MoS2/PDA/MPS in flow-through systems. Further, the flower-like structure of the MoS2 on the outer surface of the host exposes active sites and facilitates mass transfer compared to those of traditional nanocomposites. Meanwhile, the R-SO3- groups of MPS can enhance permeation and preconcentrate the Pb(II) ions via the Donnan membrane effect, resulting in improved adsorption kinetics and capacity. Batch adsorption experiments revealed that MoS2/PDA/MPS shows fast adsorption kinetics (<45 min), exceptional adsorption capacity (371.7 mg/g, calculated from the Langmuir model at 298 K), and high adsorption selectivity for Pb(II) uptake. The excellent stabilities and reusability of the nanocomposite were tested over 10 adsorption–desorption cycles. In addition, a considerable working capacity (approximately 2000 BV) in synthetic wastewater was observed in fixed-bed column tests, and the used adsorbent could be readily regenerated with a binary NaCl–EDTANa2 solution for reuse without significant capacity loss. Thus, MoS2/PDA/MPS has great applications for the remediation of lead-containing wastewater, and our findings provide new insights into the preparation of efficient MoS2-based adsorbents having practical applications.

Alkyl chain length of quaternized SBA-15 and solution conditions determine hydrophobic and electrostatic interactions for carbamazepine adsorption

Santa Barbara Amorphous-15 (SBA) is a stable and mesoporous silica material. Quaternized SBA-15 with alkyl chains (QSBA) exhibits electrostatic attraction for anionic molecules via the N+ moiety of the ammonium group, whereas its alkyl chain length determines its hydrophobic interactions. In this study, QSBA with different alkyl chain lengths were synthesized using the trimethyl, dimethyloctyl, and dimethyoctadecyl groups (C1QSBA, C8QSBA, and C18QSBA, respectively). Carbamazepine (CBZ) is a widely prescribed pharmaceutical compound, but is difficult to remove using conventional water treatments. The CBZ adsorption characteristics of QSBA were examined to determine its adsorption mechanism by changing the alkyl chain length and solution conditions (pH and ionic strength). A longer alkyl chain resulted in slower adsorption (up to 120 min), while the amount of CBZ adsorbed was higher for longer alkyl chains per unit mass of QSBA at equilibrium. The maximum adsorption capacities of C1QSBA, C8QSBA, and C18QSBA, were 3.14, 6.56, and 24.5 mg/g, respectively, as obtained using the Langmuir model. For the tested initial CBZ concentrations (2–100 mg/L), the adsorption capacity increased with increasing alkyl chain length. Because CBZ does not dissociate readily (pKa = 13.9), stable hydrophobic adsorption was observed despite the changes in pH (0.41–0.92, 1.70–2.24, and 7.56–9.10 mg/g for C1QSBA, C8QSBA, and C18QSBA, respectively); the exception was pH 2. Increasing the ionic strength from 0.1 to 100 mM enhanced the adsorption capacity of C18QSBA from 9.27 ± 0.42 to 14.94 ± 0.17 mg/g because the hydrophobic interactions were increased while the electrostatic attraction of the N+ was reduced. Thus, the ionic strength was a stronger control factor determining hydrophobic adsorption of CBZ than the solution pH. Based on the changes in hydrophobicity, which depends on the alkyl chain length, it was possible to enhance CBZ adsorption and investigate the adsorption mechanism in detail. Thus, this study aids the development of adsorbents suitable for pharmaceuticals with controlling molecular structure of QSBA and solution conditions.

“Partition Method”-Inspired Fabrication of Hierarchically Porous Polyetherimide via Supercritical CO2 Foaming: Achieving Efficient Adsorption of Carbon Dioxide

Although supercritical carbon dioxide foaming technology has achieved significant advances in the fabrication of porous polymers, the slow adsorption of carbon dioxide and the high melt strength of engineering plastics make it difficult to foam. Herein, the “partition method” is used to divide block polyetherimide (PEI) into many tiny parts through the interconnected pores to realize the rapid adsorption of CO2 molecules. Specifically, an adsorption capacity of 9.18 wt % for porous PEI was obtained at 35 °C for just 1 h which was considerably higher than that of 4.36 wt % for non-porous PEI. Then, PEI with a uniform and ordered hierarchically porous structure (micro- and nanoscale) was successfully prepared by external force-induced cell nucleation growth. Meanwhile, this treatment has little effect on the thermal properties of the polymer. This green strategy applies to other general plastics, for example, polystyrene, and paves the way for designing large-scale hierarchically porous components such as porous bearing cages, tissue engineering scaffolds, and thermal insulation foam.

Transport-limited kinetics of phosphate retention on iron-coated sand and practical implications

Iron-coated sand (ICS) is a by-product from drinking water treatment made of sand coated with ferric iron (hydr)oxides. It is considered a suitable material for large-scale measures for phosphate removal from natural and agricultural waters to prevent eutrophication. Previous studies demonstrated that the residence time of water must be very long to reach equilibrium partitioning between phosphate and ICS but specifics for application are missing. First, SEM-EDX images were used to support the conceptual assumption that P adsorption inside the coating is a transport-limited process. Second, a conceptual model of phosphate adsorption was proposed considering two types of sites: one type with fast adsorption kinetics and reaching equilibrium with the percolating solution, and another type for which adsorption is also reversible but described by pseudo-first-order kinetics. The latter is conceived to account for transport-limited adsorption in the interior of the coating while the former fraction of sites is assumed to be easily accessible and located close to the grain surface. Third, the kinetics of phosphate adsorption on ICS were quantitatively determined to describe and predict phosphate retention in filters under various flow conditions. The model was calibrated and validated with long-term column experiments, which lasted for 3500 h to approach equilibrium on the slowly reacting sites. The model reproduced the outflowing phosphate concentrations: the pronounced increase after a few pore volumes and the slow increase over the remaining part of the experiment. The parameterized model was also able to predict the time evolution of phosphate concentrations in the outflow of column experiments with different flow velocities, flow interruption, and in desorption experiments. The equilibrium partition coefficient for the experimental conditions was identified as 28.1 L/g-Fe at pH 6.8 and a phosphate concentration of 1.7 mg-P / L. The optimized first-order mass transfer coefficient for the slow adsorption process was 1.56 10−4 h−1, implying that the slow adsorption process has a time scale of several months. However, based on the parameterized model, the slow adsorption process accounted for 95.5% of the equilibrium adsorption capacity, emphasizing the potential relevance of this process for practical applications. The implications for the design, operation, and lifespan of ICS filters are exemplarily illustrated for different scenarios.

Molecular Dynamics Simulation of Pore-Size Effects on Gas Adsorption Kinetics in Zeolites

AbstractStrong gas-mineral interactions or slow adsorption kinetics require a molecular-level understanding of both adsorption and diffusion for these interactions to be properly described in transport models. In this combined molecular simulation and experimental study, noble gas adsorption and mobility is investigated in two naturally abundant zeolites whose pores are similar in size (clinoptilolite) and greater than (mordenite) the gas diameters. Simulated adsorption isotherms obtained from grand canonical Monte Carlo simulations indicate that both zeolites can accommodate even the largest gas (Rn). However, gas mobility in clinoptilolite is significantly hindered at pore-limiting window sites, as seen from molecular dynamics simulations in both bulk and slab zeolite models. Experimental gas adsorption isotherms for clinoptilolite confirm the presence of a kinetic barrier to Xe uptake, resulting in the unusual property of reverse Kr/Xe selectivity. Finally, a kinetic model is used to fit the simulated gas loading profiles, allowing a comparison of trends in gas diffusivity in the zeolite pores.

Arsenic mobilization in nZVI residue by Alkaliphilus sp. IMB: Comparison between static and flowing incubation

Arsenate reducing bacteria (AsRB) enhance arsenic (As) release via reducing As(V) to As(III), and As mobility is usually controlled by As(III) re-uptake on in-situ formed secondary iron minerals. The re-uptake of As(III) under groundwater flow conditions significantly impacts the fate and transport of As. Herein, a novel As(V)-reducing bacterium Alkaliphilus IMB was isolated in an As-contaminated soil. Scanning transmission X-ray microscopy showed that dissolved As(V) was mainly bound to the cell walls whereas dissolved As(III) was homogeneously distributed around IMB, indicating that As(V) reduction occurs outside the cell membrane. To explore the effect of IMB on As mobility, IMB was incubated with As-loaded nanoscale zero-valent iron (nZVI) residues under static and flowing conditions. IMB reduced 100% dissolved As(V) to As(III) even in a short contact time (∼1 h) during flowing incubation. The formation of As(III) did not influence As mobility under static condition as evidenced by the comparable concentrations of released As in the presence of IMB (8.5% to total As) and the abiotic control (10% to total As). Biogenic As(III) was re-adsorbed on the solids as shown by the higher ratio of solid-bound As(III) to total As in the presence of IMB (54%) than that in the abiotic control (12%). By contrast, the degree of As(III) re-adsorption was inhibited in the flowing environment, as suggested by the lower As(III) ratio in the solid (31%). This inhibition can be ascribed to the relatively slow adsorption of As(III) compared with the quick reduction of As(V) (∼1 h). Thus, IMB significantly enhanced As release during flowing incubation as shown that 9.8% As was released in the presence of IMB while 2.1% As in the abiotic control. This study found the contrary effect of AsRB on As mobility in static and flowing environments, highlighting the importance of re-adsorption rate of As(III).

Adsorption-Reduction Behavior of Cr(VI) by Ferrihydrite-Fulvic Acid Complexes with Different C/Fe Mole Ratios

Ferrihydrite and fulvic acid are prevalent components of soils and can significantly influence the environmental behavior of Cr(VI) in these matrices. Prior research has investigated the sorption behavior of Cr(VI) by ferrihydrite and fulvic acid independently; however, the sorption-reduction capacity of Cr(VI) by the ferrihydrite-fulvic acid complex, which is ubiquitously present in soils, has been less explored. In this study, ferrihydrite-fulvic acid complexes (Fh-FA) with varying C/Fe mole ratios were synthesized, and the adsorption-reduction behaviors of Cr(VI) on Fh-FA were examined using batch experiments, FTIR, BET, XRD, SEM-EDS, and XPS. The results demonstrate that the pseudo-second-order kinetic model can accurately describe the adsorption process of Fh-FA on Cr(VI), which can be delineated into three stages: rapid adsorption (0-30 min), slow adsorption (30-120 min), and reaction equilibrium (&gt;120 min). The adsorption of Cr(VI) on Fh-FA primarily occurs through chemisorption. FTIR and XPS analyses revealed that Cr(VI) is initially adsorbed by Fe-OH on the Fh-FA surface. However, as the C/Fe mole ratio of Fh-FA increases, more Fe-OH is complexed by -COOH from FA, resulting in a decrease in the adsorption capacity of Cr(VI) by Fh-FA. The number of phenolic hydroxyl groups from FA also increases, providing additional electrons to reduce Cr(VI) to Cr(III) with increasing C/Fe mole ratio. This study not only emphasizes the adsorption-reduction behavior of Cr(VI) by ferrihydrite-fulvic acid complexes but also uncovers the interplay between C/Fe mole ratios and Cr(VI) adsorption-reduction, contributing to a more comprehensive understanding of the relative roles of Fe hydroxides and natural organic matter in soil environments. These findings have significant implications for Cr(VI) management in soils, enhancing our capability to protect and sustain the environmental quality.

Transport of double-stranded ribonucleic acids (dsRNA) and deoxyribonucleic acids (DNA) in sand and iron oxide-coated sand columns under varying solution chemistries.

Assessing ecological risks associated with the use of genetically modified RNA interference crops demands an understanding of the fate of crop-released insecticidal double-stranded RNA (dsRNA) molecules in soils. We studied the adsorption of one dsRNA and two double-stranded DNA as model nucleic acids (NAs) during transport through sand- and iron oxide-coated sand (IOCS)-filled columns over a range of solution pH and ionic compositions. Consistent with NA-sand electrostatic repulsion, we observed only slight retention of NAs in sand columns. Conversely, pronounced NA retention in IOCS columns is consistent with strong and irreversible NA adsorption involving electrostatic attraction to and inner-sphere complex formation of NAs with iron oxide coatings. Adsorption of NAs to iron oxides revealed a fast and a slow kinetic adsorption regime, possibly caused by the excluded-area effect. Adsorption of NAs to sand and IOCS increased in the presence of dissolved Mg2+ and with increasing ionic strength, reflecting cation-bridging and screening of repulsive electrostatics, respectively. The co-solute phosphate and a pre-adsorbed dissolved organic matter isolate competitively suppressed dsRNA adsorption to IOCS. Similar adsorption characteristics of dsRNA and similarly sized DNA suggest that existing information on DNA adsorption to soil particles helps in predicting adsorption and fate of dsRNA molecules in soils.

Lysine-modulated synthesis of enzyme-embedded hydrogen-bonded organic frameworks for efficient carbon dioxide fixation

Carbonic anhydrase (CA) is an important carbon fixation enzyme. Immobilization of CA can expand its application in the realm of adsorption, catalysis, and so on. As a typical metal-free framework, hydrogen-bonded organic frameworks (HOFs) featuring mild synthesis process, exquisite framework structure and good enzyme compatibility have been used for enzyme embedding. However, the catalytic performance of CA-embedded HOFs (CA@HOFs) is limited by the micropore size of HOFs and the slow adsorption of CO2. Herein, CA@Lys-HOF-1 was synthesized by introducing lysine (Lys), a basic amino acid, during the coprecipitation of CA and HOFs for CO2 fixation. The addition of Lys enlarged the average pore size of HOF-1 from 1.8 to 3.2 nm, whereas the introduced -NH2 groups increased the initial adsorption of CO2 from 0.55 to 1.21 cm3 g-1. Compared to CA@HOF-1, the activity of CA@Lys-HOF-1 was enhanced by 71.25%, and the corresponding production of CaCO3 was enhanced by 12.7%. After eight reaction cycles, CA@Lys-HOF-1 still maintained an output of 9.97 mg of CaCO3 every 5 min, 83.7% of the initial production. It is hoped that the CA@Lys-HOF-1 reported offers a platform for efficient and continuous fixation of CO2.

An overview of atmospheric water harvesting methods, the inevitable path of the future in water supply.

Although science has made great strides in recent years, access to fresh water remains a major challenge for humanity due to water shortage for two-thirds of the world's population. Limited access to fresh water becomes more difficult due to the lack of natural resources of water. Many of these resources are also contaminated by human activities. Many attempts have been made to harvest water from the atmosphere, and condensation systems have received much attention. One of the challenges in generation systems is the high consumption energy of the cooling feed, despite the generation of large amounts of water from the atmosphere. As other airborne contaminants condense with water vapor, the water after harvesting needs to be treated, which adds to construction and maintenance costs. Also, the need for high relative humidity in condensation systems has led scientists to find ways of atmospheric water harvesting at low relative humidity and use renewable energy sources. Sorption systems can absorb atmospheric water without the need for an energy supply and spontaneously. Desiccants such as silica gel and zeolite, due to their high affinity for water, can absorb water vapor in the air through physical or physicochemical bonding, but all of these have slow adsorption kinetics. Therefore, it takes a long time for the water harvesting cycle or they are not able to absorb water at low relative humidity, and others need a lot of energy for the water desorption phase. Metal-Organic Frameworks (MOF) are porous materials that, due to their special structure, are considered the most promising material for atmospheric water harvesting at low relative humidity. MOF-303 has been identified as the most efficient material to date and can harvest 0.7 liters of water per kilogram of MOF-303 at 10% RH and 27 °C. MOFs can harvest atmospheric water even in desert areas using only solar energy, and the water produced is drinkable and does not need to be treated. In this review, systems and methods of atmospheric water harvesting will be studied and compared and then the mechanism of adsorption and desorption in sorption systems will be discussed in detail.