Highly efficient and acid-resistant metal-organic frameworks of MIL-101(Cr)-NH2 for Pd(II) and Pt(IV) recovery from acidic solutions: Adsorption experiments, spectroscopic analyses, and theoretical computations

Charged functional groups modified porous spherical hollow carbon material as CDI electrode for salty water desalination

As a new electrochemical technology, capacitive deionization (CDI) has been increasingly applied in environmental water treatment and seawater desalination. In this study, functional groups modified porous hollow carbon (HC) were synthesized as CDI electrode material for removing Na+ and Cl- in salty water. Results showed that the average diameter of HC was approximately 180 nm, and the infrared spectrum showed that its surface was successfully modified with sulfonic and amino groups, respectively. The sulfonic acid functionalized HC (HC-S) showed better electrochemical and desalting performance than the amino-functionalized HC (HCN), with a maximum Faradic capacity of 287.4 F/g and an adsorptive capacity of 112.97 mg/g for NaCl. Additionally, 92.63% capacity retention after 100 adsorption/desorption cycles demonstrates the excellent stability of HC-S. The main findings prove that HC-S is viable as an electrode material for desalination by high-performance CDI applications.

The design of high-efficient MOFs for selective Ag(I) capture: DFT calculations and practical applications

Recovering silver from wastewater not only significantly reduces environmental harm but also meets the growing demand for silver in modern industry. Here, a novel metal-organic framework adsorbent (MOF-RD) using rhodanine derivatives as linkers is introduced for the efficient and selective capture of silver ions in real wastewater. The adsorption of MOF-RD followed pseudo-second-order and Sips models, and thermodynamic investigations revealed the process to be endothermic. MOF-RD demonstrated a remarkable adsorption capacity of 707.2 mg·g-1 for Ag(I) at pH 5 and 318 K. The interaction between silver ions and MOF-RD was mainly electrostatic attraction and coordination, with coordination primarily occurring at the CO and CS sites within the rhodanine motif. The practical applicability of MOF-RD for selective adsorption of Ag(I) was validated in actual wastewater with high-concentration competing metal ions. Furthermore, after 10 adsorption-desorption cycle experiments, MOF-RD still retained a strong regenerative capability. The results reveal the good potential of MOF-RD as an adsorbent for selectively recovering Ag(I) from industrial wastewater. Additionally, the strategies and methods adopted in this article also provide new perspectives and technical paths for the separation and recovery of other metal ions in wastewater.

Separation of Palladium by an Imine-Linked Cu(I)-Organic Framework

Selective capture of palladium (Pd) is one of the important works in science due to its high application and low content in the Earth's crust. To this end, we present herein a new Cu(I)-organic framework (ECUT-MOF-1) by introducing pyridine N active sites to chelate Pd(II). ECUT-MOF-1 demonstrated that the maximal adsorption capacity of Pd(II) was 350 mg/g in pH = 3 solution. In addition, kinetic analysis, cycle performance, selectivity, and adsorption mechanisms were also investigated. All of the results suggested its superior application in the recovery of Pd(II).

A post-synthetic modification strategy for enhancing Pt adsorption efficiency in MOF/polymer composites

Growing polymers inside porous metal-organic frameworks (MOFs) can allow incoming guests to access the backbone of otherwise non-porous polymers, boosting the number and/or strength of available adsorption sites inside the porous support. In the present work, we have devised a novel post-synthetic modification (PSM) strategy that allows one to graft metal-chelating functionality onto a polymer backbone while inside MOF pores, enhancing the material's ability to recover Pt(iv) from complex liquids. For this, polydopamine (PDA) was first grown inside of a MOF, known as Fe-BTC (or MIL-100 Fe). Next, a small thiol-containing molecule, 2,3-dimercapto-1-propanol (DIP), was grafted to the PDA via a Michael addition. After the modification of the PDA, the Pt adsorption capacity and selectivity were greatly enhanced, particularly in the low concentration regime, due to the high affinity of the thiols towards Pt. Moreover, the modified composite was found to be highly selective for precious metals (Pt, Pd, and Au) over common base metals found in electronic waste (i.e., Pb, Cu, Ni, and Zn). X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS) provided insight into the Pt adsorption/reduction process. Last, the PSM was extended to various thiols to demonstrate the versatility of the chemistry. It is hoped that this work will open pathways for the future design of novel adsorbents that are fine-tuned for the rapid, selective retrieval of high-value and/or critical metals from complex liquids.

Unravelling co-catalyst integration methods in Ti-based metal-organic gels for photocatalytic H2 production

The synthesis, characterization and photocatalytic hydrogen evolution reaction (HER) performance of a series of metal-organic gels (MOGs) constructed from titanium(IV)-oxo clusters and dicarboxylato linkers (benzene-1,4-dicarboxylato and 2-aminobenzene-1,4-dicarboxylato) are described. All the MOGs exhibit a microstructure comprised of metal-organic nanoparticles intertwined into a highly meso-/macroporous structure, as demonstrated by cryogenic transmission electron microscopy and gas adsorption isotherms. Comprehensive chemical characterization enabled the estimation of the complex formula for these defective materials, which exhibit low crystallinity and linker vacancies. To gain deeper insights into the local structure, X-ray absorption fine structure (XAFS) spectroscopy experiments were performed and compared to that of the analogous crystalline metal-organic framework. Additionally, the ultraviolet-visible absorption properties and optical band gaps were determined from diffuse reflectance spectroscopy data. The MOGs were studied as light absorbers for the sacrificial photocatalytic HER under simulated solar light irradiation using a platinum co-catalyst by either (1) in situ photodeposition or (2) ex situ doping process, through a post-synthetic metalation of the MOG structure. The chemical analysis of the metalation, along with high-angle annular dark-field scanning transmission electron microscopy, revealed that although the in situ addition of the co-catalyst led to greater HER rates (227 vs. 110 μmolH2 gMOG-1 h-1 for in situ and ex situ, respectively), the ex situ modification provided a finer distribution of platinum nanoparticles along the porous microstructure and, as a result, it led to a more efficient utilization of the co-catalyst (45 vs. 110 mmolH2 gPt-1 h-1).

Modulating the pore and electronic structure for targeted recovery of platinum: Accelerated kinetic and reinforced coordination

Adsorption for recovery of low-concentration platinum (Pt) from the complex composition of acidic digestates was challenging because of slow kinetic and poor affinity. It was expected to be overcome by the improvement of pore size distribution and adsorption site activity. Herein, a series of Prussian blue etchings (PBE) with porosity-rich and activity-high cyano (CN) was synthesized to recover low-concentration Pt. The N2 isotherm results showed that the pore structure evolved from mesoporous to microporous. The Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations results revealed that the modulation of electronic structure converted FeII to FeIII in [FeII(CN)6]4-. The coexistence of micro- and meso-pore structures provided channels to accelerate adsorption and ensured PtII enrichment. The regulation of Fe valence state activated CN, which reinforced the strength of coordination interaction between Pt and Fe-CN- at N-atom. The adsorption rate and maximum capacity of PBE1 were 4.4 and 2.5 times higher than those of PB, respectively, due to the dual efficacy of accelerated kinetic and reinforced coordination. This study systematically analyzes the pivotal role of pore and electronic structure modulation in adsorption kinetic and affinity, which provides a novel strategy for PtII targeted recovery.

A molecular extraction process for vanadium based on tandem selective complexation and precipitation

Recycling vanadium from alternative sources is essential due to its expanding demand, depletion in natural sources, and environmental issues with terrestrial mining. Here, we present a complexation-precipitation method to selectively recover pentavalent vanadium ions, V(V), from complex metal ion mixtures, using an acid-stable metal binding agent, the cyclic imidedioxime, naphthalimidedioxime (H2CIDIII). H2CIDIII showed high extraction capacity and fast binding towards V(V) with crystal structures showing a 1:1 M:L dimer, [V2(O)3(C12H6N3O2)2]2-, 1, and 1:2 M:L non-oxido, [V(C12H6N3O2)2] ̶ complex, 2. Complexation selectivity studies showed only 1 and 2 were anionic, allowing facile separation of the V(V) complexes by pH-controlled precipitation, removing the need for solid support. The tandem complexation-precipitation technique achieved high recovery selectivity for V(V) with a selectivity coefficient above 3 × 105 from synthetic mixed metal solutions and real oil sand tailings. Zebrafish toxicity assay confirmed the non-toxicity of 1 and 2, highlighting H2CIDIII's potential for practical and large-scale V(V) recovery.

Uncoordinated amino groups of MIL-101 anchoring cobalt porphyrins for highly selective CO2 electroreduction

Electrocatalytic carbon dioxide reduction reaction (CO2RR) presents a sustainable route to address energy crisis and environmental issues, where the rational design of catalysts remains crucial. Metal-organic frameworks (MOFs) with high CO2 capture capacities have immense potential as CO2RR electrocatalysts but suffer from poor activity. Herein we report a redox-active cobalt protoporphyrin grafted MIL-101(Cr)-NH2 for CO2 electroreduction. Material characterizations reveal that porphyrin molecules are covalently attached to uncoordinated amino groups of the parent MOF without compromising its well-defined porous structure. Furthermore, in situ spectroscopic techniques suggest inherited CO2 concentrate ability and more abundant adsorbed carbonate species on the modified MOF. As a result, a maximum CO Faradaic efficiency (FECO) up to 97.1% and a turnover frequency of 0.63 s-1 are achieved, together with FECO above 90% within a wide potential window of 300 mV. This work sheds new light on the coupling of MOFs with molecular catalysts to enhance catalytic performances.

Efficient treatment of palladium from wastewater by acrolein cross-linked chitosan hydrogels: Adsorption, kinetics, and mechanisms

Herein we present a study on the preparation and properties of a hydrogel adsorbent for treatment of wasted palladium souring from actial petrochemical industrial wastewater. Chitosan was used as the raw material and acrolein as the cross-linking agent for the hydrogel (A/CS). The adsorption behaviors of the hydrogel for Pd(II) ions were characterized and analyzed. The effect of pH, temperature, adsorption kinetics, and thermodynamics were investigated. Langmuir models were employed to describe the adsorption isotherms, while the pseudo-second-order equation was applied to describe the adsorption kinetics. The experimental results demonstrated that the adsorption was a monolayer chemical adsorption, and the adsorption capacity was found to reach 505.05 mg/g under optimal conditions. In addition, FT-IR and XPS analyses, combined with MS calculations confirmed that chelation and electrostatic attraction were dominated in the adsorption process. Overall, the development of this hydrogel adsorbent will provide a practical approach to the treatment of industrial wastewater containing palladium and have great potential for practical applications.

Toward a mechanistic understanding of Rhenium(VII) adsorption behavior onto aminated polymeric adsorbents: Batch experiments, spectroscopic analyses, and theoretical computations

Rhenium, a rare and critical metal, existing in the industrial wastewater has been aroused extensive interests recently, due to its environmental and resource issues. Chitosan, an easily available, low-cost and eco-friendly biopolymer, was prepared and modified by grafting primary, secondary, tertiary and quaternary amino groups, respectively. Adsorption behaviors and interactions between ReO4- and these four types of aminated adsorbents were investigated through batch experiments, spectroscopic analysis, and theoretical computations. Chitosan modified with secondary amines showed an extremely high uptake of ReO4- with 742.0 mg g-1, which was higher than any reported adsorbents so far. Furthermore, a relatively high adsorption selectivity for Re(VII), as well as the stable and facile regeneration of these aminated adsorbents revealed a promising approach for Re(VII) recovery in full-scale applications. The electrostatic attraction was illustrated to be the main adsorption mechanism by Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy analyses. Significantly, the sub-steps of the adsorption process, encompassing the transformation of binding sites and the subsequent binding between these sites and the adsorbate, have been thoroughly investigated through the density functional theory (DFT) calculation method. This approach was firstly proposed to clearly demonstrate the differences in Re(VII) adsorption behavior onto four types of aminated adsorbents, resulting the importance of not only strong binding energy but also an appropriate binding spatial environmental for effective Re(VII) adsorption.

A novel hydrangea-like magnetic composite Fe3O4@MnO2@ZIF-67 for efficient selective adsorption of Pd(II) from metallurgical wastewater

The selective adsorption of palladium from wastewater is a feasible solution to solving palladium pollution and resource scarcity. Because traditional solvent extraction methods often involve the use of considerable amounts of organic solvents, research is focused on investigating adsorption techniques that can selectively remove palladium from wastewater. In this paper, the magnetic composite Fe3O4@MnO2@ZIF-67 was synthesized and its performance for the adsorption of Pd(II) in acidic water was investigated. Fe3O4@MnO2@ZIF-67 was characterized by various analytical methods such as TEM, SEM, EDS, BET, XRD, FTIR, zeta potential analysis, VSM, and TGA. The effects of palladium ion concentration, contact time, pH, and temperature on adsorption were evaluated. The kinetics were shown to follow the pseudo-second-order kinetic model and Elovich model, and the rate-limiting step was chemisorption. Thermodynamic studies showed that increasing the temperature promoted the adsorption of Pd(II), and the maximum uptake capacity of Fe3O4@MnO2@ZIF-67 for Pd(II) was 531.91 mg g-1. Interestingly, Fe3O4@MnO2@ZIF-67 exhibited superior selectivity for Pd(II) in the presence of Ir(IV), Pt(IV), and Rh(III). The adsorbent can be used repeatedly for selective adsorption of palladium. Even at the fifth cycle, the uptake rate of Pd(II) remained as high as 83.1%, and it showed a favorable adsorption capacity and selectivity for Pd(II) in real metallurgical wastewater. The adsorption mechanism was analyzed by SEM, FTIR, XRD, XPS, and DFT calculations, which indicated that electrostatic interactions and coordination with nitrogen-containing groups were involved. Fe3O4@MnO2@ZIF-67 is a promising adsorbent for the efficient adsorption and selective separation of palladium ions.

Graphene oxide-based dendritic adsorbent for the excellent capturing of platinum group elements

Herein, we report amino functionalized thermally stable graphene oxide-based dendritic adsorbent (GODA) with the highest sorption capacity ever recorded for platinum group elements (PGEs), including platinum (Pt(IV), PtCl₆^2-) and palladium (Pd(II), PdCl₄^2-), from highly acidic aqueous solutions. The GODA was designed and synthesized to have fully ionized amine binding sites and was characterized in detail. The detail batch adsorption experiment along with kinetic, isotherm, and thermodynamic studies were carried out to investigate the adsorption efficacy of GODA. For both Pt(IV) and Pd(II), the experimental data are more accurately fitted with the pseudo-second-order and the intraparticle diffusion kinetic models and Langmuir isotherm model as compared to the pseudo-first-order kinetic model and Freundlich and Temkin isotherm models, respectively. The material showed the highest ever adsorption capacities of 827.8 ± 27.7 mg/g (4.24 ± 0.00 mmol/g) and 890.7 ± 29.1 mg/g (8.37 ± 0.00 mmol/g) for Pt(IV) and Pd(II), respectively, at pH 1. The adsorption equilibriums were achieved within 70 min and 65 min for Pt(IV) and Pd(II), respectively. The thermodynamic parameters indicate that the adsorptions of both metals are spontaneous. The binding mechanisms are considered to be electrostatic interactions, hydrogen bonding, cationic-π bonding, and surface complexation between the sorbent and the sorbates. Furthermore, the as-prepared GODA exhibited high thermal stability and significant acid-resistance at pH 1. The GODA demonstrated excellent regeneration and reusability for Pt(IV) and Pd(II) over five adsorption/desorption cycles, indicating its excellence in practical applications.

Spatial directional separation and synergetic treatment of Cr(VI) and Rhodamine B mixed pollutants on three-layered Pd@MIL-101/P25 photocatalyst

The development of efficacious photocatalysts for removal of heavy metal and dyes coexisting pollutants simultaneously remains a challenge. Herein, we designed a three-layered Pd@MIL-101/P25 composite photocatalyst, which had the characteristics of directional photogenerated carrier separation. Pd nanoparticles were encapsulated in the MIL-101 to enrich the e^-, while P25 was loaded on the outer surface of MIL-101 as the valence band of the heterojunction with MIL-101 to enrich the h⁺. The photocatalytic kinetic constants (K) of Pd@MIL-101/P25 for the removal of Cr (VI) and RhB were 3.4 and 4.2 times greater than that of MIL-101, respectively. The photocatalytic efficiency of the catalyst in the mixed pollutants of Cr(VI) and RhB was much higher than that when Cr(VI) and RhB were present separately. Due to the 1.2 and 1.6 nm windows of MIL-101, two target pollutants can be directionally separated. Cr (VI) was reduced by e^- on the inner surface, and RhB was blocked on the outer surface and oxidized by h⁺. These results suggested that the directional spatially separation of target pollutants are able to separate the reaction sites of oxidation and reduction, improving the utilization efficiency of photogenerated carriers. This work not only provided a new strategy for the design and construction of photocatalytic materials, but also provided a new idea for the treatment of mixed pollutants.

Recovery of Palladium and Gold from PGM Ore and Concentrates Using ZnAl-Layered Double Hydroxide@zeolitic Imidazolate Framework-8 Nanocomposite

Gold (Au) and palladium (Pd) are platinum group metals (PGMs) that are considered critical in society because they are required in several industrial applications. Their shortage has caused the urgent need for their recovery from secondary resources. Therefore, there is a need to develop functional materials with high adsorption capacity and selectivity for recovery of PGMs from various secondary sources. In this study, a Zn-Al-layered double hydroxide@zeolitic imidazolate framework-8 (Zn–Al–LDH@ZIF–8) nanocomposite was used as an adsorbent for the recovery of Au and Pd from ore concentrates. The Zn–Al–LDH@ZIF–8 nanocomposite was characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, zeta potential, and X-ray diffraction (XRD) spectroscopy. The recovery of Au(III) and Pd(II) was achieved using ultrasound-assisted dispersive µ-solid-phase extraction (UA-D-µ-SPE) and their quantification was attained using an inductively coupled plasma mass spectrometer (ICP-MS). The results showed that the surface of the adsorbent remained positively charged in a wide pH range, which endowed the nanocomposite with high adsorption affinity towards Au(III) and Pd(II). Under optimised conditions, the equilibrium studies revealed that the adsorption of Au(III) and Pd(II) ions followed the Langmuir isotherm model with maximum sorption capacities of 163 mg g−1 and 177 mg g−1 for Au(III) and Pd(II), respectively. The nanocomposite possessed relatively good regeneration, reusability, and stability characteristics, with its performance decreasing by only 10% after five adsorption–desorption cycles.

Highlighting the Importance of Characterization Techniques Employed in Adsorption Using Metal–Organic Frameworks for Water Treatment

The accumulation of toxic heavy metal ions continues to be a global concern due to their adverse effects on the health of human beings and animals. Adsorption technology has always been a preferred method for the removal of these pollutants from wastewater due to its cost-effectiveness and simplicity. Hence, the development of highly efficient adsorbents as a result of the advent of novel materials with interesting structural properties remains to be the ultimate objective to improve the adsorption efficiencies of this method. As such, advanced materials such as metal–organic frameworks (MOFs) that are highly porous crystalline materials have been explored as potential adsorbents for capturing metal ions. However, due to their diverse structures and tuneable surface functionalities, there is a need to find efficient characterization techniques to study their atomic arrangements for a better understanding of their adsorption capabilities on heavy metal ions. Moreover, the existence of various species of heavy metal ions and their ability to form complexes have triggered the need to qualitatively and quantitatively determine their concentrations in the environment. Hence, it is crucial to employ techniques that can provide insight into the structural arrangements in MOF composites as well as their possible interactions with heavy metal ions, to achieve high removal efficiency and adsorption capacities. Thus, this work provides an extensive review and discussion of various techniques such as X-ray diffraction, Brunauer–Emmett–Teller theory, scanning electron microscopy and transmission electron microscopy coupled with energy dispersive spectroscopy, and X-ray photoelectron spectroscopy employed for the characterization of MOF composites before and after their interaction with toxic metal ions. The review further looks into the analytical methods (i.e., inductively coupled plasma mass spectroscopy, ultraviolet-visible spectroscopy, and atomic absorption spectroscopy) used for the quantification of heavy metal ions present in wastewater treatment.

Highly efficient metal-organic frameworks adsorbent for Pd(II) and Au(III) recovery from solutions: Experiment and mechanism

With the boom of modern industry, the demand for precious metals palladium (Pd) and gold (Au) is increasing. However, the discharge of Pd(II) and Au(III) wastewater has caused environmental pollution and shortage of resources. Here, a new metal-organic frameworks adsorbent (MOF-AFH) was synthesized to efficiently separate Pd(II) and Au(III) from the water. The adsorption behavior of Pd(II) and Au(III) was explored at the same time. When gold and palladium are adsorbed separately, the adsorption capacity of gold and palladium is 389.02 mg/g and 191.27 mg/g, respectively. The equilibration time is 3 h. When gold and palladium coexist, the adsorption capacities of Au(III) and Pd(II) are 238.71 and 115.02 mg/g, respectively. The experimental results show that the adsorption of Pd(II) and Au(III) on MOF-AFH is a single-layer chemical adsorption, which is an endothermic process. MOF-AFH has excellent selectivity and after MOF-AFH is repeatedly used 4 times, the removal effect can still reach more than 90%. The adsorption mechanisms include reduction reaction and chelation with N and O-containing functional groups on the adsorbent. There is also electrostatic interaction for Au(III) adsorption. The adsorbent can be used to efficiently recover gold and palladium from wastewater.

MIL series of metal organic frameworks (MOFs) as novel adsorbents for heavy metals in water: A review

With large specific surface area, abundant adsorption sites, flexible pore structure, and good water stability, Materials of Institute Lavoisier frameworks (MILs) have attracted increasing attention as effective environmental adsorbents. This review systematically analyzes and recapitulates recent progress in the synthesis and application of MIL-based adsorbents for the removal of aqueous heavy metal ions. Commonly used solvothermal, microwave, electrochemical, ultrasonic, and mechanochemical syntheses of MILs are first summarized and compared. Instead of focusing on adsorption process parameters, adsorption performances and governing mechanisms of virgin MILs, functional MILs, MIL-based composites, and carbonized MILs to representative metal(loid) ions (chromium, arsenic, lead, cadmium, and mercury) in water under various conditions are then systematically reviewed and discussed. In the end, this work also outlines prospects and future directions to promote the applications of MILs in treating heavy metal contaminated water.

Analysis of β-blockers and β2-agonists in environmental and biological samples by magnetic solid-phase extraction combined with high-performance liquid chromatography-tandem mass spectrometry

β-Blockers and β2-agonists are commonly prescribed for therapeutic treatments and are also administered to livestock, leading to their presence in both environmental and biological samples. Hence, the development of sensitive, rapid, and reliable analytical methods for the determination of β-blockers and β2-agonists in environmental and biological samples is important. In this study, MIL-101(Cr)-NH₂ &GO-coated SiO₂ /Fe₃ O₄ magnetic particles were prepared as sorbents for magnetic solid-phase extraction and then combined with high-performance liquid chromatography-tandem mass spectrometry for the analysis of 20 β-blockers and eight β2-agonists. The experimental parameters of magnetic solid-phase extraction were studied in detail, and the optimal conditions were established. Under optimal conditions, the limits of detection were in the range of 0.002-0.007 μg/L with enrichment factors of 20.2-24.9. The developed method was successfully applied for the determination of 20 β-blockers and eight β2-agonists in river water, human urine, and freeze-dried pork liver powder. Bisoprolol and salbutamol were detected at concentrations of 2.78 mg/L in human urine and 11.5 μg/kg in freeze-dried pork liver powder.

Fast and efficient adsorption of palladium from aqueous solution by magnetic metal-organic framework nanocomposite modified with poly(propylene imine) dendrimer

In this study, a magnetic metal-organic framework (MMOF) was synthesized and post-modified with poly(propyleneimine) dendrimer to fabricate a novel functional porous nanocomposite for adsorption and recovery of palladium (Pd(II)) from aqueous solution. The morphological and structural characteristics of the prepared material were identified by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmet-Teller (BET) isotherm, and vibrating sample magnetometer (VSM). The results confirmed the successful synthesis and post-modification of MMOF. Semispherical shape particles (20-50 nm) with appropriate magnetic properties and a high specific surface area of 120 m²/g were obtained. An experimental design approach was performed to show the effect of adsorption conditions on Pd(II) uptake efficiency of the dendrimer-modified magnetic adsorbent. The study showed that the Pd(II) uptake on dendrimer-modified MMOF was well described by the Langmuir isotherm model with the highest uptake capacity of 291 mg/g under optimal condition (adsorbent content of 12.5 mg, Pd ion concentration of 80 ppm, pH = 4, and contact time of 40 min). The adsorption kinetics of Pd(II) ions was suggested to be a pseudo-first-order model. The results revealed a faster adsorption rate and higher adsorption capacity (about 43%) for dendrimer-modified MMOF. Finally, the reusability of the provided adsorbent was evaluated. This work provides a valuable strategy for designing and developing efficient magnetic adsorbents based on MOFs for the adsorption and recovery of precious metals.

Pd(II)-Imprinted Chitosan Adsorbent for Selective Adsorption of Pd(II): Optimizing the Imprinting Process through Box-Behnken Experimental Design

The ion/molecular imprinting technique is an efficient method for developing materials with high adsorption selectivity. However, it is still difficult to obtain an imprinted adsorbent with desirably high selectivity when the preparation processes are not well designed and optimized. In this present work, a chitosan-based ion-imprinted adsorbent was optimally prepared through Box-Behnken experimental design to achieve desirably high selectivity for Pd anions (PdCl₄ ^2-) from aqueous solutions with high acidity. The dosage of epichlorohydrin (ECH) used in the first and second steps of cross-linking as well as the pH of the imprinting reaction medium is likely one of the key factors affecting the selectivity of the synthesized ion-imprinted chitosan adsorbent, which were selected as factors in a three-level factorial Box-Behnken design. As a result, the effects of these three factors on Pd(II) selectivity were able to be described by using a second-order polynomial model with a high regression coefficient (R ²; 0.996). The obtained optimal conditions via the response surface methodology were 0.10% (v/v) of first cross-linking ECH, an imprinting pH of 1.0, and 1.00% of second cross-linking ECH. Competitive adsorption was performed to investigate the selectivities of the ion-imprinted chitosan adsorbents prepared under the optimal conditions. The selectivity coefficient of Pd(II) versus Pt(IV) (β_Pd/Pt) of the Pd(II)-imprinted adsorbent was 115.83, much greater than that of the chitosan adsorbent without imprinting and various reported selective adsorbents. Therefore, the Box-Behnken design can be a useful method for optimizing the synthesis of ion-imprinted adsorbents with desirably high adsorptive selectivity for precious metals.

Highly selective removal of Ni(II) from plating rinsing wastewaters containing [Ni-xNH3-yP2O7]n complexes using N-chelating resins

Inorganic complexants, such as ammonia (AA) and pyrophosphate (PP), are often present alongside heavy metal ions in alkaline plating rinsing wastewater. We investigated the removal capacity of Ni(II) from waters containing [Ni-xNH₃-yP₂O₇]ⁿ complexes by chelating and ion-exchange resins in sole and dual-ligand systems. D463 (containing iminodiacetic groups) and PAMD (possessing polyamine groups) exerted superior performance under all conditions. Ni(II) adsorption on D463 decreased with AA and PP by 10.3% and 64.4%, respectively. Conversely, the adsorption on PAMD increased by 57.3% and 75.8%, respectively. PAMD exhibited high selectivity toward anionic [Ni-PP] species over free PP. More Ni(II) was captured by PAMD in the dual-ligand systems than sole systems, while the case for D463 was opposite. As confirmed by species tracking and DFT/XPS analyses, complexes breaking-Ni²⁺ capture was the dominant mechanism for D463, while the dual-site (non-charged and protonated amines) interactions with NiP₂O₇^2- on PAMD promoted its adsorption. The tandem combination D463-PAMD was the optimal mode to remove the most Ni(II). The actual wastewater test demonstrated that >210 BV effluent met the limit of 0.1 mg Ni(II)/L and the eluent contained 15 g Ni(II)/L. This study guides the application of chelating adsorption processes in the advanced treatment of plating rinsing wastewaters.

Adsorption isotherm models: Classification, physical meaning, application and solving method

Adsorption is widely applied separation process, especially in environmental remediation, due to its low cost and high efficiency. Adsorption isotherm models can provide mechanism information of the adsorption process, which is important for the design of adsorption system. However, the classification, physical meaning, application and solving method of the isotherms have not been systematical analyzed and summarized. In this paper, the adsorption isotherms were classified into adsorption empirical isotherms, isotherms based on Polanyi's theory, chemical adsorption isotherms, physical adsorption isotherms, and the ion exchange model. The derivation and physical meaning of the isotherm models were discussed in detail. In addition, the application of the isotherm models were analyzed and summarized based on over 200 adsorption equilibrium data in literature. The statistical parameters for evaluating the fitness of the models were also discussed. Finally, a user interface (UI) was developed based on Excel software for solving the isotherm models, which was provided in supplemental material and can be easily used to model the adsorption equilibrium data. This paper will provide theoretical basis and guiding methodology for the selection and use of the adsorption isotherms.