Tuning Pb(II) Adsorption from Aqueous Solutions on Ultrathin Iron Oxychloride (FeOCl) Nanosheets

MXenes as emerging adsorbents for removal of environmental pollutants

MXenes are a recently emerging class of two-dimensional nanomaterials that have gained considerable interest in the field of environmental protection. Owing to their high surface area, abundant terminal groups, and unique two-dimensional layered structures, MXenes have demonstrated high efficacy as adsorbents for various pollutants. Here we focused on the latest developments in the field of MXene-based adsorbents, including the structure and properties of MXenes, their synthesis and modification methods, and their adsorption performance and mechanisms for various pollutants. Among the pollutants that have been reported to be adsorbed by MXenes are radionuclides (U(VI), Sr(II), Cs(I), Eu(III), Ba(II), Th(IV), and Tc(VII)/Re(VII)), heavy metals (Hg(II), Cu(II), Cr(VI), and Pb(II)), dyes, per- and polyfluoroalkyl substances (PFAS), antibiotics (tetracycline, ciprofloxacin, and sulfonamides), antibiotic resistance genes (ARGs), and other contaminates. Moreover, future directions in MXene research are also suggested in this review.

Aqueous Electroreduction of Nitric Oxide to Ammonia at Low Concentration via Vacancy Engineered FeOCl

Electroreduction of nitric oxide (NO) to NH3 (NORR) has gained extensive attention for the sake of low carbon emission and air pollutant treatment. Unfortunately, NORR is greatly hindered by its sluggish kinetics, especially under low concentrations of NO. Herein, we developed a chlorine (Cl) vacancy strategy to overcome this limitation over FeOCl nanosheets (FeOCl-VCl ). Density functional theory (DFT) calculations revealed that the Cl vacancy resulted in defective Fe with sharp d-states characteristics in FeOCl-VCl to enhance the absorption and activation of NO. In situ X-ray absorption near-edge structure (XANES) and attenuated total reflection-infrared spectroscopy (ATR-IR) verified the lower average oxidation state of defective Fe to enhance the electron transfer for NO adsorption/activation and facilitate the generation of key NHO and NHx intermediates. As a result, the FeOCl-VCl exhibited superior NORR activities with the NH3 Faradaic efficiency up to 91.1 % while maintaining a high NH3 yield rate of 455.4 μg cm-2  h-1 under 1.0 vol % NO concentration, competitive with those of previously reported literatures under higher NO concentration. Further, the assembled Zn-NO battery utilizing FeOCl-VCl as cathode delivered a record peak power density of 6.2 mW cm-2 , offering a new route for simultaneous NO removal, NH3 production, and energy supply.

Nanoscale Insights into the Interaction Mechanism Underlying the Adsorption and Retention of Heavy Metal Ions by Humic Acid

The mobility and distribution of heavy metal ions (HMs) in aquatic environments are significantly influenced by humic acid (HA), which is ubiquitous. A quantitative understanding of the interaction mechanism underlying the adsorption and retention of HMs by HA is of vital significance but remains elusive. Herein, the interaction mechanism between HA and different types of HMs (i.e., Cd(II), Pb(II), arsenate, and chromate) was quantitatively investigated at the nanoscale. Based on quartz crystal microbalance with dissipation tests, the adsorption capacities of Pb(II), Cd(II), As(V), and Cr(VI) ionic species on the HA surface were measured as ∼0.40, ∼0.25, ∼0.12, and ∼0.02 nmol cm-2, respectively. Atomic force microscopy force results showed that the presence of Pb(II)/Cd(II) cations suppressed the electrostatic double-layer repulsion during the approach of two HA surfaces and the adhesion energy during separation was considerably enhanced from ∼2.18 to ∼5.05/∼4.18 mJ m-2. Such strong adhesion stems from the synergistic metal-HA complexation and cation-π interaction, as evidenced by spectroscopic analysis and theoretical simulation. In contrast, As(V)/Cr(VI) oxo-anions could form only weak hydrogen bonds with HA, resulting in similar adhesion energies for HA-HA (∼2.18 mJ m-2) and HA-As(V)/Cr(VI)-HA systems (∼2.26/∼1.96 mJ m-2). This work provides nanoscale insights into quantitative HM-HA interactions, improving the understanding of HMs biogeochemical cycling.

Insights into the selectivity of metallic oxides for arsenic and phosphate from EXAFS and DFT calculations

Phosphate is the biggest competitor for arsenic removal. Nanoscale metal oxides (NMOs) are commonly used to treat arsenic-contaminated water, yet their selective adsorption mechanisms for arsenic and phosphate are poorly understood. We quantified the selectivity of iron oxide (Fe2O3), zinc oxide (ZnO), and titanium dioxide (TiO2) nanosheets for arsenic in systems containing arsenic and phosphate, and determined the interaction of phosphate and arsenate/arsenite on metal oxide surfaces through batch experiments, spectroscopic techniques, and DFT calculations. We found that Fe2O3, TiO2, and ZnO nanosheets exhibit selectivity for arsenate/arsenite in the presence of phosphate, with Fe2O3 the most selective, followed by TiO2 and ZnO. The bonding mechanism on these metallic oxide surfaces dominates the selectivity. The more stable inner-sphere complexes of arsenate on the surfaces of Fe2O3 (bidentate binuclear), TiO2 (bidentate binuclear), and ZnO nanosheets (tridentate trinuclear) contribute to their preference for arsenate over phosphate. This difference in arsenate selectivity can be reflected in the difference in adsorption energy, net electron transfer number, and M - O bond length of the most stable inner sphere complexes. Overall, our study elucidated the selective adsorption mechanisms of arsenate/arsenite on Fe2O3, TiO2, and ZnO surfaces and highlighted the need to consider the competition between arsenate and phosphate during their removal from contaminated water.

Identifying the Role of Surface Hydroxyl on FeOCl in Bridging Electron Transfer toward Efficient Persulfate Activation

FeOCl is a highly effective candidate material for advanced oxidation process (AOP) catalysts, but there remain enormous uncertainties about the essence of its outstanding activity. Herein, we clearly elucidate the mechanism involved in the FeOCl-catalyzed perdisulfate (PDS) activation, and the role of surface hydroxyls in bridging the electron transfer between Fe sites and PDS onto the FeOCl/H2O interface is highlighted. ATR-FTIR and Raman analyses reveal that phosphate could suppress the activity of FeOCl via substituting its surface hydroxyls, demonstrating the essential role of hydroxyl in PDS activation. By the use of X-ray absorption fine structure and density functional theory calculations, we found that the polar surface of FeOCl experienced prominent hydrolyzation, which enriched abundant electrons within the microarea around the Fe site, leading to a stronger attraction between FeOCl and PDS. As a result, PDS adsorption onto the FeOCl/H2O interface was obviously enhanced, the bond length of O-O in adsorbed PDS was lengthened, and the electron transfer from Fe atoms to O-O was also promoted. This work proposed a new strategy for PDS-based AOP development and a hint of building efficient heterogeneous AOP catalysts via regulating the hydroxylation of active sites.

Ferroelectric Polarization and Iron Substitution Synergistically Boost Electrocatalytic Oxygen Evolution Reaction in Bismuth Oxychloride Nanosheets

Ferroelectric materials have gained significant interest in various kinds of water splitting, but the study of ferroelectric materials for electrocatalytic water splitting is in its infancy. Ferroelectric materials have spontaneous polarization below their Curie temperature due to dipolar alignment, which results in surface charges. In 2D ferroelectric materials, spontaneous polarization depends on thickness. Herein, we report that thickness-dependent ferroelectric polarization in 2D nanosheets can also accelerate the oxygen evolution reaction (OER) along with the tailored active surface area of exposed crystalline facets, which improves the electrocatalytic activity relatively. Iron-substituted BiOCl nanosheets of varying thickness are fabricated by varying the pH using a facile coprecipitation method. The substituted iron enhances polarization and electrochemical active sites on the surface. The findings in this study show that the exposed (001) facet and higher thickness of the nanosheets have high ferroelectric polarization and, in turn, superior electrocatalytic activity and remarkable stability, requiring low overpotentials (348 mV and 270 mV at 100 mA/cm2 and 10 mA/cm2) in alkaline (1.0 M KOH) electrolyte. As the thickness of the nanosheets is decreased from 140 to 34 nm, the electrocatalytic performance of iron-substituted BiOCl nanosheets starts to reduce due to the lower Coulomb-Coulomb interaction and the increasing depolarization.

Arsenic removal and stabilization behavior of schwertmannite@BC (Sch@BC) in contaminated dual media (water/soil): Via sulfate exchange and chemical complexation

Arsenic (As) is extremely harmful to the ecological environment and human health owing to its high toxicity. The composite that biochar (BC) modified by Schwertmannite (Sch), marked as Sch@BC, were prepared to remediate As-contaminated water and soil with a high efficiency. The characterization results showed that the Sch particles were successfully loaded on the BC, providing more active sites for As(V) adsorption. Compared with the pristine BC, the adsorption capacity of Sch@BC-1 was significantly improved (50.00 mg/g), of which the adsorption capacity kept stable over a wide pH range (pH = 2-8). The adsorption process conformed to pseudo-second-order kinetics and Langmuir isotherm model, which indicated that chemical adsorption was the dominant mechanism and the adsorption rate was controlled by intraparticle diffusion. Sch@BC could adsorb As(V) through electrostatic interaction and ion exchange, forming a FeAsO₄ complex and removing As(V). The 5-week soil incubation experiment showed that 3% Sch@BC showed the optimal stabilization effect, while the proportion of stable crystalline Fe/Mn-bound fractionation (F4) increased. Moreover, the results of microbial community diversity showed that Sch@BC interacted with As-resistant dominant microorganisms such as Proteobacteria in soil, promoted their growth and reproduction, and improved the stability of As in soil. In summary, Sch@BC is an excellent agent with broad application prospects for remediating As-contaminated water and soil.

A novel scheme to improve the photo-Fenton performance of iron oxychloride by carbon: Three existent states and roles of carbon in the degradation of tetracycline in water

The photo-Fenton process is promising for sincerely treating contaminated water. In this work, carbon-decorated iron oxychloride (C-FeOCl) is synthesized as a photo-Fenton catalyst for removing tetracycline (TC) from water. Three actual states of carbon are identified and their different roles in enhancing photo-Fenton performance are revealed. All carbon on/in FeOCl, including graphite carbon, carbon dots and lattice carbon, enhance visible light adsorption. More importantly, a homogeneous graphite carbon on the outer surface of FeOCl accelerates the transportation-separation of photo-excited electrons along the horizontal direction of FeOCl. Meanwhile, the interlayered carbon dots offer a FeOC bridge in helping the transportation-separation of photo-excited electrons along the vertical direction of FeOCl. In this way, C-FeOCl acquires isotropy in conduction electrons to ensure an efficient Fe(II)/Fe(III) cycle. These interlayered carbon dots extend the layer spacing (d) of FeOCl to about 1.10 nm, exposing the internal iron centers. The lattice carbon significantly increases the amounts of coordinatively unsaturated iron sites (CUISs) in activating hydrogen peroxide (H₂O₂) to hydroxyl radical (OH). Density functional theory (DFT) calculations confirm this activation on inner and external CUISs with a significantly low activation energy of about 0.33 eV.

Unsaturated phospholipid modified FeOCl nanosheets for enhancing tumor ferroptosis

Iron-dependent accumulation of reactive oxygen species (ROS) and lipid peroxidation play key roles in ferroptosis, which has been an attractive strategy to kill tumor cells. However, the rapid annihilation of hydroxyl radicals (˙OH) produced from the Fenton reaction has become a major obstacle in inducing lipid peroxidation in cells. In this study, we develop a nano-delivery system of unsaturated phospholipid (Lip) and polyacrylic acid (PAA) functionalized FeOCl nanosheets (FeOCl@PAA-Lip). In this system, the ˙OH radicals produced from the Fenton reaction between FeOCl nanosheets and endogenous H₂O₂ of tumor cells attack Lip on the nanosheets in situ to initiate the lipid peroxidation chain reaction, which not only realizes free radical conversion but also leads to the amplification of ROS and lipid peroxides, thus enhancing tumor ferroptosis. The in vitro and in vivo results confirmed that FeOCl@PAA-Lip nanosheets exhibited specific tumor cell-killing effects, good biocompatibility, long circulation time, low side effects, high tumor targeting and an excellent tumor inhibition rate (73%). The Lip functionalization strategy offers a paradigm of enhancing ferroptosis treatment by conversion of ˙OH/phospholipid radicals/lipid peroxyl radicals and strengthening lipid peroxidation.

Release and migration of Hg during desulfurization wastewater evaporation process: Whole process evaluation by experimental and theoretical study

Hot flue gas evaporation technology is an effective strategy for zero liquid discharge of desulfurization wastewater. However, there is a potential risk that heavy metals such as Hg may be released from the wastewater during evaporation, disrupting the original balance of the power plant or even exceeding the Hg emission standard. Wastewater evaporation and Hg release behavior were obtained using a single droplet drying system. At an evaporation temperature of 300 °C, approximately 18.5% of Hg was released in the constant wet-bulb temperature period, and the remaining was released in the following evaporation periods. Furthermore, a fixed-bed experiment, in combination with density functional theory calculations, was used to investigate the possible migration mechanisms of released Hg. The results revealed that high HCl concentration, introduced fly ash, and precipitated evaporation products play a crucial role in the fate of Hg, and 85.3% of Hg finally turned into less harmful particulate-bound Hg. This study provides a new and effective strategy for evaluating the migration process of pollutants in wastewater treatment. Moreover, it will serve as an essential reference for advanced wastewater treatment and heavy metals control technologies in the future.

Steady release-activation of hydrogen peroxide and molecular oxygen towards the removal of ciprofloxacin in the FeOCl/CaO2 system

Difficult storage of hydrogen peroxide (H₂O₂), low production of reactive oxygen species (ROS), and inefficient Fe(II)/Fe(III) recycling limit the application of the Fenton-like process. Calcium peroxide (CaO₂) based iron oxychloride (FeOCl) system was developed for solving these deficiencies, and ciprofloxacin (CIP) was effectively degraded within 20 min treatment. 0.33 mmol/L H₂O₂ and 2.4 mg/L dissolved oxygen (DO) were produced via CaO₂. Quenching experiments and electron paramagnetic resonance results confirmed that hydroxyl radicals (·OH) and superoxide anion (·O₂^-) worked as the main ROS. Density functional theory (DFT) calculations and experimental results suggested that H atoms of H₂O₂ adsorbed on FeOCl favored the activation of H₂O₂ into ·OH and DO into ·O₂^-, and electrophilic Cl and O coordination in FeOCl contributed to the cycle of Fe(II)/Fe(III). ·OH and·O₂^- were responsible for CIP degradation, and toxicity assessments demonstrated that the developed system reduced the hazard of treated solution. Clarity of FeOCl/CaO₂ system triple roles, including H₂O₂ and O₂ production, activation into ROS, and Fe(II)/Fe(III) recycling, facilitates the efficient utilization of O₂ in Fenton-like system.

Capture Mechanism of Cadmium in Agricultural Soil Via Iron-Modified Graphene

Cadmium (Cd) contamination in agricultural soils has caused extensive concern to researchers. Biochar with iron-compound modifications could give rise to the synergistic effect for Cd restriction. However, the related capture mechanism based on physicochemical properties is unclear. In this study, first principles calculations are proposed to explore the adsorption ability and potential mechanism of the ferric hydroxide modified graphene (Fe@G) for capturing CdCl2. The simulation results show that the adsorption energy to CdCl2 could enhance to −1.60 eV when Fe(OH)3 is introduced on graphene. Subsequently, analyses of electronic properties demonstrated a significant electron transfer between Cd s-orbital and O p-orbital, thereby leading to strong adsorption energy. This theoretical study not only identifies a powerful adsorption material for Cd reduction in agricultural soils and reveals the capture mechanism of Fe@G for Cd but also provides a foundation and strategy for Cd reduction in agricultural soils.

Radix Astragali residue-derived porous amino-laced double-network hydrogel for efficient Pb(II) removal: Performance and modeling

Valorizing solid waste for heavy metal adsorption is highly desirable to avoid global natural resources depletion. In this study, we developed a new protocol to valorize Radix Astragali residue (one of the Chinese medicine residues) into a low-cost, chemically robust, and highly permeable (ca. 90%) amino-laced porous double-network hydrogel (NH₂-CNFs/PAA) for efficient Pb(II) adsorption. The NH₂-CNFs/PAA showed (i) excellent Pb(II) adsorption capacity (i.e., 994.5 mg g^-1, ~4.8 mmol g^-1), (ii) fast adsorption kinetics (k_f = 2.01 ×10^-5 m s^-1), (iii) broad working pH range (2.0-6.0), and (iv) excellent regeneration capability (~15 cycles). (v) excellent performance in various real water matrices on Pb(II) removal. Moreover, its high selectivity (distribution coefficient K_d ~2.4 ×10⁶ mL g^-1) toward Pb(II) was owing to the present of abundant amino groups (-NH₂). Furthermore, the fix-bed column test indicated the NH₂-CNFs/PAA can effectively remove 114.6 bed volumes (influent concentration ~5000 μg L^-1) with an enrichment factor 10.9. The full-scale system modeling (i.e., pore surface diffusion model (PSDM)) has been applied to predict the NH₂-CNFs/PAA performance on Pb(II) removal. Overall, we have provided an alternative "win-win" scenario that can resolve the Chinese medicine residues disposal issue by valorizing it into high performance gel-based adsorbents for efficient heavy metal removal.

New and highly efficient Ultra-thin g-C3N4/FeOCl nanocomposites as photo-Fenton catalysts for pollutants degradation and antibacterial effect under visible light

The photo-Fenton reaction was widely used in the removal of pollutants in waste water, which makes it exhibit great potential in the field of environmental remediation. Hence, it is crucial to explore a new efficient and stable photo-Fenton catalyst driven by visible light. In this work, a simple two-step calcination method was used to synthesize sheet-like stacked Ultra-thin g-C₃N₄/FeOCl (CNF) materials. The morphology, composition, photo-Fenton performance, and antibacterial properties were systematically analyzed. Research results exhibited that the synthesized CNF catalysts showed enhanced visible light absorption capacity and excellent photo-Fenton performance. Compared with FeOCl alone, CNF displayed stronger degradation ability for rhodamine B (RhB) and could achieve 97% degradation within 9 min, which was about 10 times that of pure FeOCl. At the same time, the composite catalysts exhibited excellent antibacterial effects under photo-Fenton conditions. The antibacterial rate of CNF composite catalyst under photo-Fenton conditions can reach almost 99%, which was 3 times that of photocatalysis alone and 2 times that of Fenton alone. The heterojunction formed between Ultra-thin g-C₃N₄ and FeOCl promoted the separation of e^- and h⁺. Simultaneously, the presence of e^- promoted the cycle of Fe³⁺ and Fe²⁺ in FeOCl, thereby promoting the generation of hydroxyl radicals (OH) from H₂O₂ and improving the photo-Fenton activity to achieve the effect of degrading pollutants and antibacterial. The photo-Fenton catalysis and degradation mechanism were analyzed in detail. This work provided a theoretical basis for the application of CNF material in the removal of wastewater.

Selective adsorption and separation of Cr(VI) by surface-imprinted microsphere based on thiosemicarbazide-functionalized sodium alginate

Cr(VI)-imprinting thiosemicarbazide-functionalized sodium alginate (IIPTSC@SA) microspheres were fabricated to achieve the good selective adsorption for Cr(VI) removal from aqueous solution containing multiple ions. The selective adsorption and reusability of IIPTSC@SA microsphere were intensively investigated. The results showed that the grafting conditions of thiosemicarbazide were optimized at 0.2 g of TSC at 60 °C for 5 h and the ion-imprinting conditions at 100 mL of 2 wt % glutaraldehyde at 60 °C for 6 h. IIPTSC@SA microspheres exhibited the maximum adsorption capacity of 252.5 mg/g and good selectivity to remove Cr(VI) from multi-ion coexisting water. The adsorption process complied with a monolayer adsorption by virtue of chemical interactions together with endothermic spontaneously. After eight adsorption-desorption, the adsorption capacity and selectivity of IIPTSC@SA microspheres remained relatively stable.

Mechanistic insight into selective adsorption and easy regeneration of carboxyl-functionalized MOFs towards heavy metals

The development of heavy metal adsorbents with high selectivity has become a research hotspot due to the interference of coexisting ions (e.g., Na⁺, Ca²⁺) in the actual wastewater, but the more difficult regeneration caused by high adsorption selectivity severely limits its practical applications. Herein, a carboxyl adsorbent, MIL-121, demonstrated high adsorption selectivity for heavy metals at 10,000 mg/L of Na⁺ (removal > 99% for Cu²⁺) as well as unexpected easy regeneration (desorption > 99%) at low H⁺ concentration (10^-3.5-10^-3.0 M), which is hundreds of times lower than that of ever reported selective adsorbents (> 10^-1 M H⁺). X-ray photoelectron spectrometry (XPS), extended X-ray absorption fine structure (EXAFS) coupled with Density functional theory (DFT) simulation unveil that the -COOH groups in MIL-121 for heavy metals adsorption is specific inner-sphere coordination with higher binding energy (1.31 eV for Cu), and less energy required for regeneration (0.26 eV for H). Similar high selectivity and easy regeneration were also satisfied with other heavy metals (e.g., Pb²⁺, Ni²⁺), and removal of heavy metals remained > 99% in 10 consecutive adsorption-desorption cycles. For actual copper electroplating wastewater treatment, MIL-121 could produce ~ 3600 mL clean water/g sample, outperforming 300 mL that of the benchmark commercial adsorbent D-113. This study shows the potential of MIL-121 for heavy metal wastewater treatment and provides mechanistic insight for developing adsorbents with high selective adsorption and easy regeneration.

Phytic acid-doped poly-N-phenylglycine potato peels for removal of anionic dyes: investigation of adsorption parameters

In this work, we report on enhanced adsorption of anionic dyes, Reactive Blue 49 (RB 49) and Direct Blue 199 (DB 199), using a phytic acid-doped poly-N-phenylglycine@potato peel (PA-PPG@PP) nanocomposite material.

Accelerating FeIII-Aqua Complex Reduction in an Efficient Solid-Liquid-Interfacial Fenton Reaction over the Mn-CNH Co-catalyst at Near-Neutral pH

The sluggish regeneration rate of Fe^II and low operating pH still restrict the wider application of classical Fenton process (Fe^II/H₂O₂) for practical water treatment. To overcome these challenges, we exploit the Mn-CNH co-catalyst to construct a solid-liquid interfacial Fenton reaction and accelerate the Fe^III/Fe^II redox cycle at the interface for sustainably generating ^•OH from H₂O₂ activation. The Mn-CNH co-catalyst exhibits an excellent regeneration rate of Fe^II (∼65%) and a high tetracycline removal rate (K_obs) of 0.0541 min^-1, which is 19.0 times higher than that of the Fe^II/H₂O₂ system (0.0027 min^-1) at a near-neutral pH (pH ≈ 5.8), and it also attains 100% degradation of sulfamethoxazole, rhodamine B, and methyl orange. The cyclic mechanism of Fe^III/Fe^II is further elucidated in an atomic scale by combining characterizations and density functional theory calculations, including Fe_aq^III specific adsorption and the electron-transfer process. Mn active sites can accumulate electrons from the matrix and adsorb Fe_aq^III to form Mn-Fe bonds at the solid-liquid interface, which accelerate electron transfer from Mn-CNH to Fe_aq^III and promote the regeneration of Fe^II at a wide pH range with a lower energy barrier. The regeneration rate of Fe^II in the Mn-CNH/Fe^II/H₂O₂ system outperforms the benchmark Fenton system and other typical metal nanomaterials, which has great potential to be widely applied in actual environment remediation.

Highly efficient As(III) removal in water using millimeter-sized porous granular MgO-biochar with high adsorption capacity

Biochar adsorbents for removing As(III) suffer from the problems of low adsorption capacity and ineffective removal. Herein, a granular MgO-embedded biochar (g-MgO-Bc) adsorbent is fabricated in the form of millimeter-sized particles through a simple gelation-calcination method using chitosan as biochar sources. High-density MgO nanoparticles are evenly dispersed throughout the biochar matrix and can be fully exposed to As(III) through the rich pores in g-MgO-Bc. These features endow the adsorbent with a high adsorption capacity of 249.1 mg/g for As(III). The g-MgO-Bc can efficiently remove As(III) over a wide pH of 3-10. The coexisting carbonate, nitrate, sulfate, silicate, and humic acid exert a negligible influence on As(III) removal. 300 μg/L of As(III) can be purified to far below 10 μg/L using only 0.3 g/L g-MgO-Bc. The spent g-MgO-Bc could be well regenerated by simple calcination. In fixed-bed column experiments, the effective treatment volume of As(III)-spiked groundwater achieves 1500 BV (30 L) (3 g of adsorbent, solution flow rate of 2.0 mL/min, C₀ = 50 μg/L). The Mg(OH)₂ generated in situ in g-MgO-Bc is responsible for the adsorption of As(III) through the inner-sphere complex mechanism. The work would extend the potential applicability of biochar adsorbent for As(III) removal to a great extent.

Catalytic wet peroxide oxidation degradation of magenta wastewater and preparation of FeOCl/montmorillonite

The iron oxychloride/pillared montmorillonite (FeOCl/MMT) catalyst was prepared by wet impregnation method and solid melting method. Various characterization techniques were used to analyze the microscopic morphology and structure of a series of catalysts. Moreover, the catalysts were used to treat magenta-simulated dye wastewater through catalytic wet peroxide oxidation (CWPO) degradation. The magenta removal rate and chemical oxygen demand (COD) removal rate of the magenta-simulated dye wastewater were used to evaluate the catalytic performance of the catalyst, and the optimal catalyst preparation conditions were selected. The results showed that the solid melting method was more favorable to the preparation of the catalyst, and the COD removal rate of wastewater can reach 70.8% when the FeOCl load was 3%. Moreover, 96.2% of the magenta in the solution was removed. The COD removal rate of the magenta wastewater decreased by only 12.4% after the catalyst was repeatedly used six times, indicating that the catalyst has good activity and stability. The Fermi equation can simulate the reaction process of the catalyst treating magenta wastewater at high temperature.

Novel recycling of incinerated sewage sludge ash (ISSA) and waste bentonite as ceramsite for Pb-containing wastewater treatment: Performance and mechanism

With rapid economic growth and urbanisation, the reuse and recycling of solid wastes has become a high priority for the sustainable development of modern cities. In this study, two typical solid wastes, incinerated sewage sludge ash (ISSA) and waste bentonite, were co-valorised to produce granular adsorbents through a simple and energy-saving pelletisation/sintering process. A mixture of ISSA and bentonite at a weight ratio of 3:1 was pelletised and sintered at 700 °C. The resultant ceramsite, with good mechanical strength, could effectively remove Pb(Ⅱ) from aqueous solutions. The adsorption kinetics can be described by the pseudo-first-order (PFO) model. The results indicated that the Pb(Ⅱ) adsorption process was dominated by electrostatic attraction, precipitation, and complexation. The isothermal data exhibited a good correlation with the Freundlich model, indicating that the adsorption process was non-ideal and spontaneous. The maximum adsorption capacity was approximately 21.6 ± 0.35 mg/g at 318 K. After 5 cycles of regeneration, the adsorbent maintained good adsorption performance. Moreover, the removal rate was not greatly affected by ionic strength. These findings demonstrate that the granular adsorbent prepared with ISSA and waste bentonite can be recognised as a promising adsorbent for Pb-containing wastewater treatment.

Critical Review of Advances in Engineering Nanomaterial Adsorbents for Metal Removal and Recovery from Water: Mechanism Identification and Engineering Design

Nanomaterial adsorbents (NAs) have shown promise to efficiently remove toxic metals from water, yet their practical use remains challenging. Limited understanding of adsorption mechanisms and scaling up evaluation are the two main obstacles. To fully realize the practical use of NAs for metal removal, we review the advanced tools and chemical principles to identify mechanisms, highlight the importance of adsorption capacity and kinetics on engineering design, and propose a systematic engineering scenario for full-scale NA implementation. Specifically, we provide in-depth insight for using density functional theory (DFT) and/or X-ray absorption fine structure (XAFS) to elucidate adsorption mechanisms in terms of active site verification and molecular interaction configuration. Furthermore, we discuss engineering issues for designing, scaling, and operating NA systems, including adsorption modeling, reactor selection, and NA regeneration, recovery, and disposal. This review also prioritizes research needs for (i) determining NA microstructure properties using DFT, XAFS, and machine learning and (ii) recovering NAs from treated water. Our critical review is expected to guide and advance the development of highly efficient NAs for engineering applications.

Experimental and DFT studies on the characteristics of PbO/PbCl2 adsorption by Si/Al- based sorbents in the simulated flue gas

Mineral oxides are effective in-furnace sorbents used to control lead (Pb) emissions in flue gas at high temperatures. In this paper, the PbO/PbCl₂ adsorption characteristics of sorbents were investigated via experimental and density functional theory (DFT) methods. The results show that Si/Al-based compounds can chemically adsorb Pb, and the adsorption is related to the Si-O and Al-O bonds in the sorbents. Exposed Si and O atoms on SiO₂ surfaces and exposed Al and O atoms on Al₂O₃ surfaces are the active sites for Pb adsorption, and PbO is easier to remove than PbCl₂. Pb adsorption is promoted in a mixture of SiO₂ and Al₂O₃. Doping Si atoms into Al₂O₃(100) promotes PbO adsorption, and doped three-coordinate Si atoms have a more obvious promotion effect than doped two-coordinate Si atoms. Doping Al atoms into SiO₂(001) has no obvious effect on PbO adsorption. The effect of temperature on Pb adsorption was studied by thermodynamic analysis. The Gibbs free energy difference for PbO adsorption on SiO₂(001) increases from -373.04-32.42 kJ/mol as the temperature increases from 300 to 1800 K. High-temperature calcination changes the bond length and bond angle of the system, affecting the stabilities of atomic configurations and decreasing the Pb adsorption capacity.

Fe-Mn Oxides Based Multifunctional Adsorptive/Electrosensing Nanoplatforms: Dynamic Site Rearrangement for Metal Ion Selectivity

Achieving structural requirements for the exclusive selectivity of adsorbent to a specific metal remains challenging, as certain metal ions show similar adsorptive behaviors and preference toward a given site. We reported the morphology and oxidation state-dependent selectivity manipulating of layered oxides by controlling the dynamic evolution of different adsorptive sites. The computational investigation predicted the site-specific partitioning trends of metal ions at two sites of manganese oxide (MnO₂) layers: the lateral edge sites (LESs) and octahedral vacancy sites (OVSs). In contrast to the predominant occupation of the OVSs for other metal ions, the binding of lead (Pb) ions was energetically favored at both the sites. We assembled ultrathin MnO₂ nanosheets on the magnetic iron oxides to first enhance the accessibility of the LESs. A sequential ligand-promoted partial reduction of the atomic MnO₂ layers induced the edge-to-interlayer migration of Mn atoms to block the nonspecific OVSs and activate the LESs, enabling a superior selectivity to Pb. In addition, the iron oxides helped construct a multifunctional adsorptive/electrosensing platform for Pb regarding their facile magnetic separation and electrochemical activity. Simultaneous selective adsorption and on-site monitoring of Pb(II) were achieved on this nanoplatform, owing to its satisfactory stability and sensitivity without an obvious matrix effect.

Adsorption of Pb(II) from Aqueous Solution by Mussel Shell-Based Adsorbent: Preparation, Characterization, and Adsorption Performance

As a natural biological adsorbent, shell powder is inexpensive, highly efficient, and does not leave any chemical residue; thus, it can be used to remove contaminants from water. In this study, we used mussel shells as a raw material to prepare an adsorbent. Scanning electron microscopy was used to observe the surface morphology of the mussel shell powder before and after calcination, and X-ray diffraction measurements, Fourier transform infrared spectroscopy, differential scanning calorimetry, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller measurements were performed to analyze the structure and composition of calcined mussel shell powder. Characterization of the shell powder before and after calcination revealed a change from calcium carbonate to calcium oxide, as well as the formation of a surface porous structure. Using Pb(II) as a representative contaminant, various factors affecting the adsorption were explored, and the adsorption mechanism was analyzed. It was found that the adsorption is consistent with the Freundlich adsorption isotherm and the pseudo second-order model. The calcined mussel shell powder exhibits excellent adsorption for Pb(II), with an adsorption capacity reaching 102.04 mg/g.

Enhanced bisphenol S anaerobic degradation using an NZVI-HA-modified anode in bioelectrochemical systems

As a substitute for bisphenol A (BPA), bisphenol S (BPS) has a longer half-life, higher chemical inertness and better skin permeability than BPA, and it also has a strong endocrine disruption effect. Relatively few studies have focused on the main processing technology for BPS biodegradation, and the findings indicate that the biodegradation efficiency of BPS was relatively low. Therefore, this paper used an NZVI-HA composite-modified bio-anode to enhance the anaerobic degradation of BPS in a Bioelectrochemical Systems (BES). The results showed that the degradation efficiency of BPS was improved from 31.1% to 92.2% with the NZVI-HA modification compared with the control group (CC-BES). FTIR and XPS analyzes demonstrated that HA can accelerate the reduction rate of Fe³⁺ and increase the ratio of Fe²⁺/Fe³⁺. In addition, HA can form Fe-O-HA complexes with NZVI to promote electron transfer. An analysis of the NZVI-HA-BES intermediate metabolites revealed that complex modification properties altered the BPS degradation pathway. An analysis of microbial diversity indicated that the bacteria related to the degradation of BPS may be Terrimonas, Lysobacter, and Acidovorax.

In situ quantitative determination of the intermolecular attraction between amines and a graphene surface using atomic force microscopy

The adsorption of pollutants on carbonaceous environmental media has been widely studied via batch sorption experiments and spectroscopic characterization. However, the molecular interactions between pollutants and interfacial sites on carbonaceous materials have only been indirectly investigated. To comprehend the adsorption mechanisms in situ, we applied atomic force microscopy force spectroscopy (AFM-FS) to quantitatively determine the molecular interactions between typical amines (methylamines and N-methylaniline) and the surface of highly oriented pyrolytic graphite (HOPG), which was supported by the single molecule interaction derived from density functional theory and batch adsorption experiments. This method achieved direct and in situ characterization of the molecular interactions in the adsorption process. The molecular interactions between the amines and the adsorption sites on the graphite surface were affected by pH and peaked at pH 7 due to strong cation-π interactions. When the pH was 11, the attractions were weak due to a lack of cation-π interaction, whereas, when the pH was 3, the competitive occupation of hydronium ions on the surface reduced the attraction between the amines and HOPG. Based on AFM-FS, the single molecule force of methylamine and N-methylaniline on the graphite surface was estimated to be 0.224 nN and 0.153 nN, respectively, which was consistent with density functional theory (DFT) calculations. This study broadens our comprehension of cation-π interactions between amines and electron-rich aromatic compounds at the micro/nanoscale.

In Situ Synthesis of FeOCl in Hollow Dendritic Mesoporous Organosilicon for Ascorbic Acid-Enhanced and MR Imaging-Guided Chemodynamic Therapy in Neutral pH Conditions

Chemodynamic therapy (CDT) based on the Fenton reaction is a promising strategy for nonlight cancer treatment. However, the traditional Fenton reaction is only efficient in strongly acidic conditions (pH = 2-4), resulting in the limited curative effect in a weakly acidic tumor microenvironment (TME). Herein, we first developed a simple in situ growth method to confine FeOCl nanosheets into hollow dendritic mesoporous organosilicon (H-DMOS) nanoparticles to obtain FeOCl@H-DMOS nanospheres. Ascorbic acid (AA) was then absorbed on the nanosystem as a H₂O₂ prodrug and, meanwhile, was used for the regeneration of Fentons reagent for Fe²⁺. Finally, poly(ethylene glycol) (PEG) was coated on FeOCl@H-DMOS-AA to enhance the permeability and retention (EPR) effect in tumor tissue. The as-fabricated FeOCl@H-DMOS-AA/PEG can generate a large amount of highly toxic hydroxyl radicals (^•OH) by catalyzing H₂O₂ even in neutral pH conditions with the help of AA. As a result, the effect of CDT has been markedly enhanced by the increased amount of H₂O₂ and the efficient Fenton reaction in mild acidic TME, which can remove almost all of the tumors in mice. In addition, FeOCl also endows the nanosystem with T₂-weighted MR imaging capability (r₂ = 34.08 mM^-1 s^-1), thus realizing the imaging-guided cancer therapy. All in all, our study may contribute a new direction and may have a bright future for enhanced CDT with a neutral pH range.

Highly Efficient and Selective Hg(II) Removal from Water Using Multilayered Ti3C2Ox MXene via Adsorption Coupled with Catalytic Reduction Mechanism

Mercury (Hg) removal is crucial to the safety of water resources, yet it lacks an effective removal technology, especially for emergency on-site remediation. Herein, multilayered oxygen-functionalized Ti₃C₂ (Ti₃C₂O_x) (abbreviated as M-Ti₃C₂) nanosheets were prepared to remove Hg(II) from water. The M-Ti₃C₂ has demonstrated ultrafast adsorption kinetics (the concentration decreased from 10 400 to 33 μg L^-1 in 10 s), impressively high capacity (4806 mg g^-1), high selectivity, and broad working pH range (3-12). The density functional theory (DFT) calculations and experimental characterizations unveil that this exceptional Hg(II) removal is owing to the distinct interaction (e.g., adsorption coupled with catalytic reduction). Specifically, Ti atoms on the {001} facets of M-Ti₃C₂ prefer to adsorb Hg(II) in the form of HgClOH, which subsequently undergoes homolytic cleavage to form radical species (e.g., ^•OH and ^•HgCl). Immediately, the ^•HgCl radicals dimerize and form crystalline Hg₂Cl₂ on the edges of M-Ti₃C₂. Up to ∼95% of dimeric Hg₂Cl₂ can be efficiently recovered via facile thermal treatment. Notably, owing to the adsorbed ^•OH and energy released during the distinct interaction, M-Ti₃C₂ has been oxidized to TiO₂/C nanocomposites. And the TiO₂/C nanocomposites have shown to have better performance on the photocatalytic degradation of organic pollutants than Degussa P25. These exceptional features coupled with mercuric recyclable nature make M-Ti₃C₂ an outstanding candidate for rapid/urgent Hg(II) removal and recovery.

Reactive oxygen species generation in FeOCl nanosheets activated peroxymonosulfate system: Radicals and non-radical pathways

Iron oxychloride (FeOCl) is utilized as a activator of peroxymonosulfate (PMS) for the degradation of paracetamol (APAP) and phenacetin (PNCT) in response to the water pollution by persistent pharmaceuticals. The degradation process was well fitted with a pseudo-first order kinetic pattern, and the excellent catalytic performance towards APAP (100 % removal) and PNCT (86.5 % removal) was obtained in the presence of 0.2 g/L FeOCl and 2.0 mM PMS at pH 7.0 in 30 min. In-situ electron spin resonance (ESR) and scavenging tests revealed the generation of a series of ROS (·OH, SO₄^-, O₂^-, ¹O₂), which was highly dependent on pH. Besides, the non-radical pathways process involved ¹O₂ was dominant in APAP oxidation, while both ·OH and ¹O₂ are significant in PNCT removal. Furthermore, the formation of disinfection by-products (DBPs) during post-chlorination showed neglectable increment at neutral and alkaline condition with FeOCl/PMS pre-oxidation, and the calculated cytotoxicity would experience a continuous deterioration with pH increase. These results displayed high efficiency of FeOCl/PMS system in micropollutants degradation and a relatively comprehensive activation process of PMS, which may promote practical application in environmental remediation.

Integrating the (311) facet of MnO2 and the fuctional groups of poly(m-phenylenediamine) in core-shell MnO2@poly(m-phenylenediamine) adsorbent to remove Pb ions from water

Exposed active facets and functional groups are critical for adsorbents obtaining excellent adsorption properties. In the present study, MnO₂@PmPD with exposed active facets was successfully prepared. MnO₂,which came from KMnO₄ by the sacrificial reductant of PmPD, deposited on the surface of PmPD. Meanwhile, we combined experimental study and theoretical calculations to elucidate the distinct adsorption nature of MnO₂@PmPD towards Pb. The surface adsorption of MnO₂@PmPD toward Pb was achieved by the interaction between Pb and O atoms on the surface of MnO₂. The DFT calculations revealed the facet-dependent adsorption of MnO₂ toward Pb. The adsorption affinity of facets toward Pb was in the order of (311) > (111) > (400) > (440), and (311) facet was predominantly adsorption site for Pb. The analysis of partial density of state revealed the strong hybridization between the Pb-p state and O-p states of MnO₂. Additionally, the pores of MnO₂ provide the interstitial channels for the transportation of Pb into PmPD. The Pb entered the internal of MnO₂@PmPD was bonded by the amine and newly formed carboxy groups on PmPD. This study not only develops an efficient adsorbent for heavy metals removing, but also throws light on exemplifying the interaction of Pb with MnO₂ based materials.

Nanoscale zero-valent iron intercalated 2D titanium carbides for removal of Cr(VI) in aqueous solution and the mechanistic aspect

MXenes, as new members of the 2D materials group, are regarded as good candidates for heavy metal removal particularly for radioactive metal element because of their high irradiation stability. However, the small interlayer distance and lack of active sites on the surface limit their further application. In this report, nanoscale zero-valent iron has been introduced into the inter-layer structure of alkaline intercalated Ti₃C₂ (Alk-Ti₃C₂) and investigated to Cr(VI) removal. The XPS spectra, SEM images, TEM images, and N₂ adsorption-desorption isotherms characterizations revealed that the OH groups on the Alk-Ti₃C₂ surface assisted the introducing of nZVI into the inter-layer of Alk-Ti₃C₂ and subsequently stabilized the nZVI. The increased active sites of nZVI and extended interlayer space of Alk-Ti₃C₂ could improve the uptake capacity of Cr(VI) (194.87 mg/g at pH = 2). The highly efficient removal of Cr(VI) was maintained even in the presence of coexisting cations, which showed great potential for real environment remediation. Mechanistic study indicated that the synergistic effects of nZVI and Alk-Ti₃C₂ nanosheet in nZVI-Alk-Ti₃C₂ composites are keys for Cr(VI) removal.

Mechanistic insights into heavy metals affinity in magnetic MnO2@Fe3O4/poly(m-phenylenediamine) core-shell adsorbent

Adsorption represents an attractive mean to remediate polluted water. Unfortunately, the surface positive charges, low surface area and complicated separation procedures inhibit the usability of poly (m-phenylenediamine) (PmPD) as an adsorbent for heavy metal removing. To overcome these drawbacks, a magnetic MnO₂@Fe₃O₄/PmPD core-shell adsorbent was designed to remove heavy metals from water. The MnO₂ shell, came from the redox reaction between KMnO₄ and PmPD, increased the surface area and changed the surface electronegativity. MnO₂@Fe₃O₄/PmPD could be easily separated from water. It showed a significant increase in heavy metals removal efficiency, with maximum capacities of 438.6 mg/g for Pb(II) and 121.5 mg/g for Cd(II), respectively. The affinity between heavy metals and MnO₂@Fe₃O₄/PmPD were mainly due to electrostatic attraction, ion exchanges and coordinated interaction. Density functional theory (DFT) calculations further confirmed that Pb and Cd were bonded with O atoms. The calculated adsorption energy indicated that the (111) MnO₂ facet presented stronger adsorption affinity toward Pb(II) than Cd(II). Additionally, FM150 (150 mg) could regenerate 22 L Pb(II) wastewater upon single passage through the filterable column with a flux of 20 mL/min. Thus, the present work demonstrates the promising potential of using MnO₂@Fe₃O₄/PmPD for efficiently removing heavy metals from wastewater.

Ultra-rapid detoxification of Sb(III) using a flow-through electro-fenton system

Environmental pollution caused by antimony (Sb) has attracted worldwide attention recently. Here, we employed a flow-through electro-Fenton system for the rapid and efficient detoxification of highly toxic Sb(III). A FeOCl modified carbon nanotube (CNT) filter served as functional cathode, where FeOCl as nanocatalyst promoted the generation of HO by facilitating effective Fe³⁺/Fe²⁺ cycling. Upon application of a proper potential, an ultra-rapid conversion of Sb(III) to less toxic Sb(V) can be achieved in situ just by a single-pass filtration (>99% within 2 s). Compared with the conventional batch reactor, the proposed system demonstrated ultra-rapid Sb(III) detoxification kinetics due to the convection-enhanced mass transport. The proposed flow-through E-Fenton system works effectively across a wide pH range (e.g., 3-9). EPR technique and radical quenching experiments indicate that HO and HO₂ were the dominant radical species responsible for Sb(III) detoxification. At -0.4 V vs. Ag/AgCl, a >96.4% Sb(III) conversion efficiency still can be achieved when challenged with 500 μg L^-1 Sb(III)-spiked tap water. The as-produced Sb(V) can be removed effectively by another Sb(V)-specific CNT filter functionalized with nanoscale iron oxides. The outcome of this research provides a promising strategy by integrating state-of-the-art electro-Fenton, membrane separation, carboncatalysis and nanotechnology for detoxification of Sb(III) and other similar heavy metal ions in polluted water.

Efficient Selective Removal of Pb(II) by Using 6-Aminothiouracil-Modified Zr-Based Organic Frameworks: From Experiments to Mechanisms

We report an efficient, reusable, and selective 6-aminothiouracil (ATA)-modified Zr(IV)-based adsorbent (defined as UiO-66-ATA(Zr)) for lead ion removal in water. The adsorption equilibrium time and the maximum sorption capacity of UiO-66-ATA(Zr) for Pb(II) are, respectively, 120 min and 386.98 mg/g at pH 4 and 298 K. The Pb(II) removal rate reaches 96% at 60 min and exceeds 99% at the equilibrium state in the pH range of 2.0-5.8. Hill and pseudo-second-order models can well describe the sorption process. Pb(II) adsorbing onto UiO-66-ATA(Zr) is an irreversible, favorable chemisorption process with multimolecule participation and film diffusion control. The calculations of density functional theory, the experimental results, and the characterization analyses suggest that the binding mechanisms are the chelation and ion-exchange/electrostatic interactions between hydroxyl/amino/sulfhydryl groups of UiO-66-ATA(Zr) and Pb(II). Besides, UiO-66-ATA(Zr) has a better affinity to Pb(II) than the coexisting ions in water and an excellent repeatability at eight cycles of adsorption. Moreover, the thermodynamic study shows that UiO-66-ATA(Zr) adsorbing Pb(II) is an endothermic reaction. Thus, UiO-66-ATA(Zr) is a prospective sorbent for Pb(II) removal under the initiative of environmental protection and water purification, and this work may also provide an idea for industrial catalysis.

Change in Magnetic Properties upon Chemical Exfoliation of FeOCl

The development of novel, intrinsic two-dimensional (2D) antiferromagnets presents the opportunity to vastly improve the efficiency of spintronic devices and sensors. The strong intrinsic antiferromagnetism and van der Waals layered structure exhibited by the bulk transition-metal oxychlorides provide a convenient system for the synthesis of such materials. In this work, we report the exfoliation of bulk FeOCl into and subsequent characterization of intrinsically antiferromagnetic thin-layer FeOCl nanosheets. The magnetic properties of bulk FeOCl, its lithium intercalate, and its nanosheet pellet are measured to determine the evolution of magnetic properties from the three-dimensional to the quasi-two-dimensional system. This work establishes FeOCl and isostructural compounds as a source for the development of two-dimensional intrinsic antiferromagnets.