The removal and recovery of Cr(VI) by Li/Al layered double hydroxide (LDH)

Ultrathin porous Pd metallene as highly efficient oxidase mimics for the colorimetric detection of chromium (VI)

Palladium (Pd)-based nanomaterials are the emerging class of catalysts with individual physiochemical properties. Unlike traditional catalysts, metallenes showed abundant active sites, large surface area, and high atomic utilization. Based on these benefits, we demonstrate a highly active 2D graphene-like Pd metallene with abundant accessible active sites serving as a highly efficient oxidase mimic. The structure and morphology of Pd metallenezymes were controlled to enhance the catalytic performance and to efficiently utilize all the Pd atoms. Pd metallenezymes with excellent oxidase-like activity were successfully applied for colorimetric-based chromium (VI) (Cr(VI)) detection in a real environment. 3,3',5,5'-Tetramethylbenzidine (TMB) was used as a typical chromogenic substrate catalyzed by Pd metallene to show that blue oxidized TMB (ox TMB) was significantly reduced to colorless TMB by the reducing agent 8-hydroxyquinoline (8-HQ). The reaction process was impressively reversed by the addition of Cr(VI), which interacted with 8-HQ to restore the blue color of TMB. Based on the above results, a facile and effective colorimetric sensing system for the detection of Cr(VI) with a low detection limit of 2.8 nM was developed and could be successfully applied to the detection of Cr(VI) in a real environment.

BSA-stabilized silver nanoclusters for efficient photoresponsive colorimetric detection of chromium(VI)

Hexavalent chromium is a highly toxic substance, which will pose a serious threat to human life and health and the entire ecosystem. Therefore, it is crucial to establish a simple and rapid detection method for hexavalent chromium. In this work, we fabricated bovine serum albumin-stabilized silver nanocluster (BSA-Ag₁₃ NC) which exhibited photoresponsive oxidase-like activity, catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to the blue oxidized state TMB (oxTMB) in a short time. Interestingly, 8-hydroxyquinoline (8-HQ) can significantly inhibit the color reaction of TMB oxidation while Cr(VI) can interact specifically with 8-HQ to restore this chromogenic reaction. Based on the above facts, a colorimetric sensing system for detecting Cr(VI) was developed. The sensing system shows a wide linear range, and good selectivity, with a low detection limit of 2.32 nM. Moreover, this sensing system could be successfully applied to the detection of Cr(VI) in lake water, tap water, and sewage with satisfactory results.

Adsorptive removal of heavy metal anions from water by layered double hydroxide: A review

High-valence heavy metals with high ecotoxicity are generally found in water in the form of anions, and this increases heavy metal pollution intensity and treatment difficulty. Recent studies have pointed to the potential efficiency of layered double hydroxides (LDHs) to meet this challenge. In this review, we retrospectively research the development of LDHs using a Java application called CiteSpace. We describe the unique layer structure, highly adjustable chemical properties, and diverse synthesis methods of LDHs, all of which decide the effective adsorption of heavy metal anions by LDHs. Subsequently, we focus on discussing the adsorption mechanism of LDHs on heavy metal anions, as well as the current state of research and future directions for microscopic interaction mechanisms. For practical applications, it is critical to improve the adsorption selectivity and stability. We then recommend solutions to improve the adsorption selectivity and stability after identifying the influencing mechanism. Finally, we provide our perspectives on the future development of LDHs adsorption of heavy metal anions.

Magnetic MnFe2O4/ZnFe-LDH for Enhanced Phosphate and Cr (VI) Removal from Water

Due to the layered structure and recycling characteristics, magnetic MnFe2O4/ZnFe-LDH was prepared by co-precipitation. In this study, we intensively explored MnFe2O4/ZnFe-LDH for water purification compared with the ZnFe-LDH. The morphological and structural characteristics of the obtained products were systematically characterized. MnFe2O4/ZnFe-LDH exhibits the maximum adsorption capacity of 94.52 mg/g for phosphate and 49.03 mg/g for Cr (VI), respectively, which is superior to that of the ZnFe-LDH, indicating that the magnetic MnFe2O4 effectively enhanced adsorption performance. Meanwhile, the mechanism of adsorption of phosphate and Cr (VI) was briefly studied, where the metal center ion in MnFe2O4/ZnFe-LDH and between layers of SO42- serves as capture sites for phosphate and Cr (VI) removal. Furthermore, the MnFe2O4/ZnFe-LDH can be used for magnetic recycled and still maintained excellent removal efficiency (70%) for phosphate and Cr (VI) after five adsorption-desorption cycles. This work could open a new vista of designing magnetic novel adsorbents in environmental remediation.

Microscopic understanding about adsorption and transport of different Cr(VI) species at mineral interfaces

Cr(VI) ranks as one of the most toxic heavy metals and herein, microscopic mechanisms for adsorption and transport of different Cr(VI) oxyanions (Cr₂O₇^2- and CrO₄^2-) at kaolinite interfaces are addressed by dispersion-corrected periodic density functional theory calculations. Cr(VI) oxyanions adsorb favorably at both tetrahedral and octahedral surfaces, and K⁺ ions serve as bridge for Cr(VI) oxyanions and tetrahedral surfaces while Cr(VI) oxyanions serve as bridge for K⁺ ions and octahedral surfaces. Adsorption structures are altered significantly by pH variation, and stability trends at different pH ranges are deciphered by the dominant interaction force with clay surfaces: Electrostatic interaction with K⁺ ions at tetrahedral surfaces whereas combined action of electrostatic and H-bonding interactions with Cr(VI) oxyanions at octahedral surfaces. Electron transfers are strongly pH-dependent, and clay surfaces serve as electron reservoirs. CrO₄^2- rather than Cr₂O₇^2- is dominant at clay interfaces, and HCrO₄^- can co-exist under acidic conditions. Cr₂O₇^2- transformation to CrO₄^2- is kinetically blocked at pH ≈ PZC while preferred at pH < PZC. Cr(VI) removal and reclamation should proceed at pH > 7.0 and pH < PZC, respectively. Results greatly promote the understanding about Cr(VI) bioavailability and fate in surficial environments and are also useful for Cr(VI) removal and reclamation.

Kinetic, thermodynamic and isotherm investigations of Cu2+ and Zn2+ adsorption on LiAl hydrotalcite-like compound

LiAl hydrotalcite-like compound (LiAl HTlc) was synthesized via a hydrothermal method and used to adsorb Cu²⁺ and Zn²⁺ for investigating the adsorption characteristics of heavy metal cations. The X-Raydiffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and transmission electron microscopy (TEM) characterizations revealed the interconnecting flower-like layered structure of LiAl HTlc. The adsorption kinetics and isotherms of Cu²⁺ and Zn²⁺ on LiAl HTlc agreed with the pseudo-second-order model and the Langmuir model at a given sorbent concentration (C_s), respectively. The C_s-effect on the adsorption kinetics and isotherms was observed, and the Langmuir-surface component activity (SCA) equation could be utilized to characterize the effect of C_s in the adsorption isotherms. The adsorption process was spontaneous and endothermic. The adsorption mechanism denoted that the adsorption process was controlled using two main mechanisms, i.e., surface complexation and isomorphic substitution. This is the first report, to the best of our knowledge, on the usage of LiAl HTlc for the removal of heavy metal cations Cu²⁺ and Zn²⁺ from a solution. LiAl HTlc is a promising sorbent for treating water containing heavy metal cations.

Simultaneous removal of cationic and anionic heavy metal contaminants from electroplating effluent by hydrotalcite adsorbent with disulfide (S2-) intercalation

Hydrotalcite materials are generally utilized for anionic pollutants due to its interlayered anion exchange ability. Their potentiality for cationic contaminants is rarely explored. In this study, disulfide (S^2-) intercalated LDH material demonstrated capability to remove both heavy metal cations and oxyanions simultaneously from water. The S^2- intercalation of LDH significantly improved its adsorption capability towards both heavy metal cations (Co²⁺ and Ni²⁺) and oxyanion (CrO₄^2-). The adsorption amount of S-LDH towards Co²⁺ and Ni²⁺ reached 88.6mg/g and 76.2mg/g, which are 405% and 281% higher than that of pristine LDH. For CrO₄^2- removal, the adsorption amount reached 34.7mg/g, 402% higher than that of pristine LDH. The cations capture mechanism mainly depends on the novel layer sheet cation substitution mechanism based on irreversible precipitation and the generation of metal sulfide precipitates. Meanwhile, the interlayered S^2- can be easily replaced by CrO₄^2- to realize the simultaneous removal of both heavy metal cations and oxyanions. In the fixed-bed column experiments, 448 bed volume (BV) (672 mL) of simulating electroplating wastewater can be efficiently treated by yielding only 1 BV(15 mL) of chemical sludge, which is practically acceptable. This work provided a highly practical adsorption technology based on the S^2- modification hydrotalcite material for the purification of heavy metal ions contaminated wastewater.

Novel M (Mg/Ni/Cu)-Al-CO3 layered double hydroxides synthesized by aqueous miscible organic solvent treatment (AMOST) method for CO2 capture

Layered double hydroxides (LDHs) have been intensively studied in recent years owing to their great potential in CO₂ capture. However, the severe aggregation between platelets and low surface area restricted it from exhibiting very high CO₂ adsorption capacity and CO₂/N₂ selectivity. In this research, we for the first time synthesized Ni-Al-CO₃ and Cu-Al-CO₃ LDHs using aqueous miscible organic solvent treatment (AMOST) method. The as-synthesized materials were evaluated for CO₂ adsorption at three different temperatures (50, 80, 120 °C) applicable to post-combustion CO₂ capture. Characterized with XRD, N₂ adsorption-desorption, TEM, EDX, and TGA, we found the newly synthesized Ni-Al-CO₃ LDH showed a nano-flower-like morphology comprising randomly oriented 2D nanoplatelets with both high surface area (249.45 m²/g) and pore volume (0.59 cc/g). Experimental results demonstrated that un-calcined Ni-Al-CO₃ LDH is superior in terms of CO₂ capture among the three LDHs, with a maximum CO₂ adsorption capacity of 0.87 mmol/g and the ideal CO₂/N₂ selectivity of 166 at 50 °C under 1200 mbar for typical flue gas CO₂/N₂ composition (CO₂:N₂ = 15:85, v/v). This is the first report of a delaminated Ni-Al-CO₃ LDH showing better CO₂ capture performance than the well-reported optimal Mg layered double hydroxide.

Adsorption mechanisms of chromate and phosphate on hydrotalcite: A combination of macroscopic and spectroscopic studies

Hydrotalcite (HT) is a layered double hydroxide (LDH), which is considered as a potential adsorbent to remove anion contaminants. In this study, adsorption of chromate (CrO₄) and phosphate (PO₄) on HT was conducted at various pH and temperatures. Related adsorption mechanisms were determined via the isotherm, kinetic, and competitive adsorption studies as well as the Cr K-edge X-ray absorption fine-structure (XAFS) spectroscopy. The maximum adsorption capacities for CrO₄ and PO₄ on HT were 0.16 and 0.23 mmol g^-1. Regarding adsorption kinetics, CrO₄ and PO₄ adsorption on HT could be well described by the second order model, and the rate coefficient of CrO₄ and PO₄ on HT decreased significantly with the increasing pH from 5 to 9. The adsorption kinetics for CrO₄ and PO₄ were divided into fast and slow stages with the boundary at 15 min. This biphasic adsorption behavior might be partially attributed to multiple reactive pathways including anion exchange and surface complexation. Fitting results of Cr K-edge EXAFS analysis showed a direct bonding between CrO₄ and Al on HT surfaces. Such a surface complexation appeared to be the rate-limiting step for CrO₄ adsorption on HT. By contrast, the diffusion through the hydrated interlayer space of HT was the major rate-limiting step for PO₄. This study determined the adsorption behaviors of CrO₄ and PO₄ on HT, including the initial transfer process and the subsequent adsorption mechanisms. Such information could improve the strategy to use HT as the potential adsorbent for the remediation of anionic pollutants.

Utilization of calcium-based and aluminum-based materials for the treatment of stabilized landfill leachate: a comparative study

This study explored the efficiencies and mechanisms of refractory organic matters removal in the stabilized landfill leachate by adding different reagents. Calcium-based and aluminum-based materials were added into the leachate as comparing experiments. XRD, FTIR, and EEM were adopted to analyze the solid products and leachate. As a result, the in situ synthesized CaAl-LDHs were more beneficial for refractory organic matters removal, especially for benzodiazepines. When CaAl-LDHs were formed, the removal efficiencies of COD, UV254, and TOC were best and achieved 58.48, 81.22, and 71.30%, respectively. For fluorescent substances, humic acid-like and fulvic acid-like compounds were efficiently removed by CaAl-LDHs. In particular, CaAl-LDHs had selective removal effects on fulvic acid-like compounds, which were characteristic of small molecular weight and major carboxyl groups.

Competitive adsorption behavior toward metal ions on nano-Fe/Mg/Ni ternary layered double hydroxide proved by XPS: Evidence of selective and sensitive detection of Pb(II)

Hypersensitive and highly selective nanomaterials for the measurement of heavy metal ions (HMIs) hold a key to electro-analysis. Various works improved the results of analysis but without scientific understanding. Herein, Fe/Mg/Ni-layered double hydroxide (LDH) has been successfully prepared and its electrochemical behavior for Pb(II) detection is also studied using square wave anodic stripping voltammetry (SWASV). The well performance of electrochemistry suggest that the modification with Fe/Mg/Ni-LDH significantly promotes the selectivity and sensitivity toward Pb(II). The sensitivity on Fe/Mg/Ni-LDH modified glassy carbon electrode (GCE) is 68.1μAμM^-1 over the range from 0.03 to 1.0μM under the optimized conditions. Otherwise, the selectivity, anti-interference, stability measurements and practical implications of Fe/Mg/Ni-LDH modified GCE are also performed. What^,s more, a reasonable mechanism of detection for Pb(II) including selectivity and sensitivity is proposed based on adsorption and characterized using XPS and XRD. These findings provide a potentially excellent material to improve the sensitivity and selectivity for toxic metal ions as well as a deep understanding of detection.

Hybrid n-Alkylamine Intercalated Layered Titanates for Solid Lubrication

The intercalation of different primary n-alkylamines in the structure of a layered titanate of the lepidocrocite type (H_1.07Ti_1.73O₄) for application in high-temperature solid lubrication is reported. The intercalation process of the amines was explored by means of in situ small-angle X-ray scattering (SAXS), with variations in alkyl chain length (3-12 carbon atoms) and the amine/titanate ratio. The intercalation process was found to be completed within 5 min after mixing of the precursors in water at 80 °C. The topotactic transformation of the layered titanate is driven by an acid-base reaction. The thermal degradation of the modified titanates was investigated by thermogravimetric analysis (TGA), and the chemical changes were investigated by temperature-dependent infrared spectroscopy (DRIFTS). The coefficient of friction of the lubricants was assessed by means of high-temperature pin-on-disc experiments up to 580 °C. The intercalation of amine rendered a deformable layered ceramic upon heating. It was found that the hydrocarbon chain length exerts an influence on the mechanical properties of the titanates, resulting in lower friction forces for lubricants with longer intercalated amine molecules. Films of solid lubricants with longer amine chain lengths showed coefficients of friction as low as 0.01, lower than that of the state-of-the-art material graphite.

Adsorption of heavy metals from aqueous solutions by Mg-Al-Zn mingled oxides adsorbent

In our study, Mg-Al-Zn mingled oxides were prepared by the co-precipitation method. The structure, composition, morphology and thermal stability of the synthesized Mg-Al-Zn mingled oxides were analyzed by powder X-ray diffraction, Fourier transform infrared spectrometry, N₂ physisorption, scanning electron microscopy, differential scanning calorimetry and thermogravimetry. Batch experiments were performed to study the adsorption behavior of cobalt(II) and nickel(II) as a function of pH, contact time, initial metal ion concentration, and adsorbent dose. The maximum adsorption capacity of Mg-Al-Zn mingled oxides for cobalt and nickel metal ions was 116.7 mg g^-1, and 70.4 mg g^-1, respectively. The experimental data were analyzed using pseudo-first- and pseudo-second-order kinetic models in linear and nonlinear regression analysis. The kinetic studies showed that the adsorption process could be described by the pseudo-second-order kinetic model. Experimental equilibrium data were well represented by Langmuir and Freundlich isotherm models. Also, the maximum monolayer capacity, q_max, obtained was 113.8 mg g^-1, and 79.4 mg g^-1 for Co(II), and Ni(II), respectively. Our results showed that Mg-Al-Zn mingled oxides can be used as an efficient adsorbent material for removal of heavy metals from industrial wastewater samples.

Uptake of Nd3+ and Sr2+ by Li–Al layered double hydroxide intercalated with triethylenetetramine-hexaacetic acid: kinetic and equilibrium studies

Li–Al layered double hydroxide intercalated with triethylenetetramine-hexaacetic acid (TTHA) was found to take up Nd³⁺ ions preferentially over Sr²⁺ ions from solutions, due to the difference in the stabilities of the Nd–TTHA and Sr–TTHA complexes.

Synergistic effect of humic and fulvic acids on Ni removal by the calcined Mg/Al layered double hydroxide

The calcined Mg/Al LDH exhibits notable capacity to remove Ni(ii), especially in the presence of organic matters.

Kinetic and thermodynamic studies of the biosorption of Cr(VI) by Pinus sylvestris Linn

Biosorption equilibrium, kinetics and thermodynamics of chromium(VI) ions onto cone biomass were studied in a batch system with respect to temperature and initial metal ion concentration. The biosorption efficiency of chromium ions to the cone biomass decreased as the initial concentration of metal ions was increased. But cone biomass of Pinus sylvestris Linn. exhibited the highest Cr(VI) uptake capacity at 45 degrees C. The biosorption efficiency increased from 67% to 84% with an increase in temperature from 25 to 45 degrees C at an initial Cr(VI) concentration of 300 mg/L. The Langmuir isotherm model was applied to experimental equilibrium data of Cr(VI) biosorption depending on temperature. According to Langmuir isotherm, the monolayer saturation capacity (Q(max)) is 238.10 mg/g. The pseudo-first-order and pseudo-second-order kinetic models were applied to test the experimental data for initial Cr(VI). The pseudo-second-order kinetic model provided the best correlation of the used experimental data compared to the pseudo-first-order kinetic model. The activation energy of biosorption (E(a)) was determined as 41.74 kJ/mol using the Arrhenius equation. Using the thermodynamic equilibrium coefficients obtained at different temperatures, the thermodynamic constants of biosorption (DeltaG(0), DeltaH(0) and DeltaS(0)) were also evaluated.