A double network gel as low cost and easy recycle adsorbent: Highly efficient removal of Cd(II) and Pb(II) pollutants from wastewater

Self-separating core-shell spheres with a carboxymethyl chitosan/acrylic acid/Fe3O4 composite core for soil Cd removal

Cadmium (Cd) removal from soil is crucial as Cd enters the food chain and affect food safety, thus impose severe threaten to human health. We developed PPC@PC-Fe, a dual-functional core-shell sphere for efficient soil Cd reduction. The shell (PPC) was composed of encapsulated citric acid (CA) in a polylactic acid (PLA) and polyethylene glycol (PEG) network, which endows a function of activating Cd; and the core (PC-Fe) consisted of a polyacrylic acid/carboxymethyl chitosan (PAA/CMC) hydrogel with Fe3O4 nanoparticles to adsorb adjacent activated Cd. Upon water contact, the shell dissolved, releasing CA to activate soil Cd. Simultaneously, the swellable PC-Fe core absorbed water and expanded in size, promoting the disintegration of PLA in the shell, which triggered the automatic separation of core from shell, enabling the exposed PC-Fe core to rapidly adsorb Cd. Furthermore, the PC-Fe core can be magnetically removed after adsorption of Cd. Soil culture tests showed that 2 % PPC@PC-Fe reduced soil Cd from 6.009 mg/kg to 4.834 mg/kg in 10 days, with the acid-soluble Cd being the predominantly target to be activated and remove. This study demonstrates an effective stepwise activation and adsorption mechanism by a single carrier, with simple magnetic collection minimizing secondary pollution. It offers an innovative approach to the remediation of cadmium-contaminated sites in the field.

Stabilization of arsenic-cadmium co-contaminated soil with the iron-manganese sludge derived amendment: Effects and mechanisms

An iron-manganese sludge-derived amendment was proposed to remediate arsenic (As) and cadmium (Cd) co-contaminated soil, with a strong adsorptive capacity across pH 4 to 10. The Langmuir model defined maximum adsorption at 78.17 mg/g for As(III), 110.48 mg/kg for As(V), and 65.77 mg/g for Cd(II). The X-ray photoelectron spectroscopy (XPS) spectra provided insights into the chemical interactions: As was predominantly complexed or ligand exchanged with iron(hydr)oxides. In contrast, cadmium exhibited a tendency to bond with acylamino and carboxyl groups, in addition to the ferric hydroxyl groups. Notably, 42.15% of the adsorbed As(III) was oxidized into As(V) by Mn(IV) oxides present in the amendment. The soil-verification experiment demonstrated that an amendment dosage of 40 g/kg was efficacious in reducing the leaching concentration of As and Cd to maintained below the safety thresholds of 0.1 mg/L and 0.01 mg/L, respectively, for pH levels 4 to 11, meeting the Chinese Surface Water Quality Standard V (GB3838-2002). After the stabilization, the exchangeable fractions of As and the acid-soluble fractions of Cd were significantly reduced, with these elements being transformed into more stable forms. The amendment maintained the soil's neutral pH and adjusted the soil physicochemical properties. This article presents a holistic approach by examining the organic-inorganic composite of iron-manganese oxides with polyacrylamide, modified as a stabilizing amendment for As and Cd co-contaminated soil. This innovative amendment adeptly navigates the previously conflicting stabilization mechanisms for anionic and cationic metals. Offering dual advantages, the amendment not only remediates soil but also addresses the disposal of waste, presenting a win-win solution for environmental management.

Magnetic Fe3O4@SiO2 study on adsorption of methyl orange on nanoparticles

Magnetic core-shell Fe3O4@SiO2 nanoparticles were synthesized by sol-gel method. Based on the characterization and experimental results, the adsorbent was found to have an average particle size of approximately 120 nm, a pore size range of 2-5 nm and superparamagnetic properties. It exhibited electrostatic and hydrogen bonding interactions during adsorption of methyl orange (MO). The adsorption of MO on the magnetic Fe3O4@SiO2 nanoparticles exhibited pseudo-second-order kinetics, the adsorption process is a spontaneous endothermic adsorption process, which conforms to the Langmuir adsorption isotherm model. he maximum amount of MO was adsorbed at pH = 2, T = 45 °C and t = 30 min, and the highest adsorption capacity was 182.503 mg/g; The unit adsorption capacity of the Fe3O4@SiO2 nanoparticles still reached 83% of the original capacity after 5 cycles, so the material was reusable and met the requirements of environmental protection. This study reveals the great potential of magnetic mesoporous nanoparticles for removal of dyes from wastewater.

Qualitative and quantitative analysis for Cd2+ removal mechanisms by biochar composites from co-pyrolysis of corn straw and fly ash

Removal of heavy metals (e.g., Cd) from contaminated water using waste-converted adsorbents is promising, but the efficiency still needs to be improved. Here, we prepared a functional biochar composite as novel Cd adsorbents by co-pyrolysis of two typical solid wastes, i.e., agricultural corn straw and industrial fly ash. The adsorption behavior and mechanism were investigated using batch and column adsorption experiments and modern characterization techniques. Results showed that alkali-modified fly ash (AMFA) was loaded onto the surface of the corn straw biochar as some fine particle forms, with quartz (SiO₂) and silicate being the main mineral phases on the surface. The maximum sorption capacity fitted by Langmuir model for functionalized biochar composite (FBC700) was up to 137.1 mg g^-1, which was 7.7 times higher than that of the original corn straw biochar (BC700). Spectroscopic analysis revealed that adsorption mechanisms of Cd onto the FBC700 included mainly precipitation and ion exchange, with complexation and Cd-π interaction also contributing. The AMFA could effectively improve the mineral precipitation with Cd. The adsorption columns filled with FBC700 exhibited a longer breakthrough time than that filled with BC700. The adsorption capacity calculated by Thomas model for FBC700 was also approximately 6.0 times higher than that for BC700, showing that FBC700 was more suited to practical applications. This study provided a novel perspective for recycling solid wastes and treating Cd-contaminated water.

Synthesis of Cellulose-Poly(Acrylic Acid) Using Sugarcane Bagasse Extracted Cellulose Fibres for the Removal of Heavy Metal Ions

In this study, sugarcane bagasse (SCB) was treated with sodium hydroxide and bleached to separate the non-cellulose components to obtain cellulose (CE) fibres. Cross-linked cellulose-poly(sodium acrylic acid) hydrogel (CE-PAANa) was successfully synthesised via simple free-radical graft-polymerisation to remove heavy metal ions. The structure and morphology of the hydrogel display an open interconnected porous structure on the surface of the hydrogel. Various factors influencing batch adsorption capacity, including pH, contact time, and solution concentration, were investigated. The results showed that the adsorption kinetics were in good agreement with the pseudo-second-order kinetic model and that the adsorption isotherms followed the Langmuir model. The maximum adsorption capacities calculated by the Langmuir model are 106.3, 333.3, and 163.9 mg/g for Cu(II), Pb(II), and Cd(II), respectively. Furthermore, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectrometry (EDS) results demonstrated that cationic exchange and electrostatic interaction were the main heavy metal ions adsorption mechanisms. These results demonstrate that CE-PAANa graft copolymer sorbents from cellulose-rich SCB can potentially be used for the removal of heavy metal ions.

An all-biopolymer self-assembling hydrogel film consisting of chitosan and carboxymethyl guar gum: A novel bio-based composite adsorbent for Cu2+ adsorption from aqueous solution

A novel bio-based composite adsorbent, all biopolymer self-assembled hydrogel film has been prepared by eco-friendly amalgamating chitosan (CS) and carboxymethyl guar gum (CMGG) biopolymers in water without needing small molecules for cross-linking. Various analysis demonstrated the electrostatic interactions and hydrogen bondings within the network structure are responsible for gelling, crosslinking, and forming a 3D structure. Various experimental parameters were optimized to evaluate the CS/CMGG's potential for removing Cu²⁺ ions from aqueous solution, including pH, dosage, Cu(II) initial concentration, contact time, and temperature. The pseudo-second-order kinetic and Langmuir isotherm models are highly correlated with the kinetic and equilibrium isotherm data, respectively. Using the Langmuir isotherm model for an initial metal concentration of 50 mg/L at pH 6.0 and 25 °C, the maximum adsorption of Cu(II) was calculated to be 155.51 mg/g. A combination of adsorption-complexation and ion exchange must be involved in Cu(II) adsorption on the CS/CMGG. Five cycles of the loaded CS/CMGG hydrogel regeneration and reuse were successfully achieved without an appreciable difference in Cu(II) removal percentage. Thermodynamic analysis indicated that copper adsorption occurred spontaneously (ΔG°: -2.85 J/mol, 298 K) and exothermically (ΔH°: -27.58 J/mol). A reusable bio-adsorbent for removing heavy metal ions was developed that is eco-friendly, sustainable, and efficient.

Enhanced selective copper precipitation by mechanochemically activated benzene tricarboxylic acid

Toward the treatment of waste solution containing heavy metals, direct precipitation of the metal ions from an acidic solution without alkaline neutralisation is still the greatest challenge. Based on the ligand properties of benzene tricarboxylic acid (BTC) to copper ions, a simple ball milling with 90 min at 400 rpm was used to activate BTC to enhance its capacity for copper removal from the pH of the original solution around 3-4. A set of analytical methods were used to characterise the activated BTC sample and BTC-Cu precipitate before and after copper precipitation. Compared with the raw BTC, the activated BTC could efficiently remove copper ions over 90% from an initial copper concentration of 100 mg/L in a shorter time from an acidic media with lower pH of around 2.60 and the maximum adsorption capacity can be stable at about 111.70 mg/g, resulting from probably the enhanced deprotonation effect for copper incorporation. Furthermore, at controlled dosage, the activated BTC demonstrated much high selectivity on precipitating copper ions from other heavy metals of Ni, Mn, Zn and Cd and provided a new approach for easy copper recycling from waste solution as secondary sources. This process may serve the purpose of recycling both metal and acidic solutions after the purification.

Sodium humate based double network hydrogel for Cu and Pb removal

Sodium humate (SH) is one of the derivatives humic substances, which can be utilized for heavy metal removal from water due to its containing plenty of functional groups. In this study, a double network hydrogel SH/polyacrylamide (SH/PAM) was synthesized by a simple free-radical polymerization and used for Cu²⁺ and Pb²⁺ removal from water. The adsorption process can be well described by Langmuir-Freundlich model, indicating that both physical and chemical adsorption were involved. X-ray photoelectron spectroscopy (XPS) characterization demonstrated that complexation was the main mechanism for the adsorption. Two-dimensional correlation analysis of FTIR (2D-FTIR-COS) results showed that the variation order of functional groups during Cu²⁺ and Pb²⁺ adsorption in the following order: COOH ≈ -CO > -OH > C-O and -COOH ≈ C-O > -CO > -OH, respectively. According to the density functional theory (DFT) calculation results, the O atom of SH in the COO^- was the main adsorption site. Meanwhile, the adsorption energy of Pb²⁺ was more negative than that of Cu²⁺ and the orbital hybridization between O atom of SH and Pb²⁺ was denser than that of Cu²⁺, which suggested that SH/PAM had a stronger combining capacity for Pb²⁺ than Cu²⁺. Therefore, the adsorption capacity for Pb²⁺ was larger than Cu²⁺. Moreover, the removal efficiencies are 30.2% for Al, 98.79% for Cu, 99.0% for Fe, 17.2% for Mn, 93.4% for Pb, and 62.4% for Zn in actual acid mine drainage using 6 g L^-1 adsorbent. Collectively, this study not only provided a new adsorbent for heavy metal removal but also explicated the mechanism of heavy metal removal by SH from molecule and electron perspective, which is helpful for the application of SH in the environmental field.

Porous boron nitride nanofibers enhanced sodium acrylate and acrylamide copolymer hydrogels for effective adsorption of Pb2

A new composite hydrogel adsorbent for adsorption of Pb²⁺ has been prepared by combining porous boron nitride nanofibers (BNNFs) and the acrylamide and sodium acrylate copolymer (P(AANa-co-AM)) via a chemical crosslinking method. Porous BNNFs with abundant hydroxyl functional groups can form hydrogen bond interactions with carboxyl and amino functional groups of the copolymer in the composite hydrogel and carry and dissipate forces for the composite hydrogels. So the mechanical performances of the copolymer hydrogels can be effectively improved, which is very valuable for the practical application of the composite hydrogel to remove Pb²⁺ from waste water. The thermal stability and swelling performance of the pure copolymer hydrogels were also greatly improved. This is not only because of the strong hydrogen bond interactions but also the good thermal stability and flexibility of BNNFs. The composite hydrogel adsorbent shows superior adsorption capacity for Pb²⁺ (490.2 mg g^-1) to most of the reported hydrogel adsorbents. The chemisorption dominates the whole adsorption process according to the pseudo-second-order kinetic and the Langmuir models. The composite hydrogel adsorbent also shows good reusability. Therefore, we believe that the prepared composite hydrogels will play an important role in removing Pb²⁺ from wastewater.

Natural-clay-reinforced hydrogel adsorbent: Rapid adsorption of heavy-metal ions and dyes from textile wastewater

In this study, two natural clay minerals were combined with hydrogels to study the influence of natural adsorbents on the adsorption performance of hydrogels. Here, we separately doped bentonite and vermiculite and discussed their mechanical properties and adsorption properties. It was found that the compressive performance of the hydrogel added with clay increased by 21.6% and the swelling performance decreased or increased to varying degrees. Regarding the adsorption performance of hydrogels, it can be seen from the adsorption Langmuir isotherm model that the adsorption capacity of clay-hydrogels is improved to varying degrees (6.6%-15.8%) compared with non-clay-hydrogels, and clay-hydrogels have different degrees of improvement (6.6%-15.8%). The hydrogel has a removal efficiency of more than 95% for low concentrations of heavy-metal ions and dyes. In addition, the clay-hydrogel has low cost and is easy to prepare, and can be recycled many times. Therefore, the material is of great significance for the treatment of pollutants. PRACTITIONER POINTS: The effect of natural clay on the adsorption performance of hydrogels was studied. Clay can enhance the compression and adsorption properties of hydrogels. The adsorption mechanism and adsorption capacity of clay hydrogels were evaluated.

Magnesium/aluminum layered double hydroxides intercalated with starch for effective adsorptive removal of anionic dyes

In this research, the adsorptive performance of a starch-magnesium/aluminum layered double hydroxide (S-Mg/Al LDH) composite was investigated for different organic dyes in single-component systems by conducting a series of batch mode experiments. S-Mg/Al LDH composite showed preferential adsorption of anionic dyes than cationic dyes. The marked impact of key process variables (e.g., contact time, adsorbent dosage, pH, and temperature) on its adsorption was investigated. Multiple isotherms, kinetics, and thermodynamic models were applied to describe adsorption behavior, diffusion, and uptake rates of the organic dyes over S-Mg/Al LDH composite. A better fitting of the non-linear Langmuir model reflects the predominance of monolayered adsorption of dye molecules on the composite surface. Partition coefficients (mg g^-1 μM^-1) for S-Mg/Al LDH were observed in the following descending order: Amaranth (665) > Tartrazine (186) > Sunset yellow (71) > Eosin yellow (65). Furthermore, comparative evaluation of the adsorption enthalpy, entropy, and Gibbs free energy values indicates that the adsorption process is spontaneous and exothermic. S-Mg/Al LDH composite maintained a stable adsorption/desorption recycling process over six consecutive cycles with the advantages of low cost, chemical/mechanical stability, and easy recovery. The results of this study are expected to expand the application of modified LDHs toward wastewater treatment.

Single evaluation and selection of functional groups containing N or O atoms to heavy metal adsorption: Law of electric neutrality

Adsorption capacities and characteristics of heavy metal adsorbents have been investigated thoroughly, but the essential functional groups to bind heavy metal pollutants in composites have not been identified and confirmed separately. Previous researches reported that -OH, -NH₂, -CONH₂ or their protonation had binding effects to heavy metals. However, these descriptions were all based on the complex composites. The composites were consisted of different functional groups. Thus, this article aims to evaluate and discuss (1) the adsorption properties of different functional groups containing N or O atoms, (2) the protonation of groups containing N atoms, (3) the basis properties of adsorbents related to adsorption, (4) the physical adsorption by network structure. By a series of single evaluations, the results showed that the hydrophobic ester group (R₁-COO-R₂), non-ionic groups, including -OH, -NH₂, -CONH₂, ether (C-O-C), CO, tertiary amine (R3N), and protonation of -NH₂ and R3N, had no interaction with metals. Only negative groups, such as -COO^-, -SO₃^-, could adsorb cationic heavy metals. And positive -N(CH₃)₃⁺ group could bind with Cr₂O₇^2-. Furthermore, these conclusions have been verified by the law of electric neutrality in the heavy metal solutions and solid adsorbents. This study determined that the combining process between negative functional groups and cationic metals, or between positive groups and anionic metals, can be applied to decrease the heavy metal concentration.

Poly(vinyl alcohol) Hydrogels: The Old and New Functional Materials

Hydrogels have three-dimensional network structures, high water content, good flexibility, biocompatibility, and stimulation response, which have provided a unique role in many fields such as industry, agriculture, and medical treatment. Poly(vinyl alcohol) PVA hydrogel is one of the oldest composite hydrogels. It has been extensively explored due to its chemical stability, nontoxic, good biocompatibility, biological aging resistance, high water-absorbing capacity, and easy processing. PVA-based hydrogels have been widely investigated in drug carriers, articular cartilage, wound dressings, tissue engineering, and other intelligent materials, such as self-healing and shape-memory materials, supercapacitors, sensors, and other fields. In this paper, the discovery, development, preparation, modification methods, and applications of PVA functionalized hydrogels are reviewed, and their potential applications and future research trends are also prospected.

Low-cost hydrogel adsorbent enhanced by trihydroxy melamine and β-cyclodextrin for the removal of Pb(II) and Ni(II) in water

Hydrogels have extensively studied as adsorbents, raw materials for the preparation of adsorbent hydrogels have low strength, while high strength hydrogels have weak adsorption capacity. In this study, PVA hydrogel was crosslinked via trihydroxy melamine and epichlorohydrin, and β-cyclodextrin with strong adsorption capacity was added to remove the heavy metal ions. Results showed that the addition of trihydroxy melamine with 8%, the compressive strength of the hydrogel was increased by approximately 20%. The Langmuir isotherm model showed that the adsorption capacity of the hydrogel for Pb(II) and Ni(II) reached 505.9 mg/g and 286.7 mg/g, respectively, and the efficiency of removing the low-concentration heavy metal ions in water more than 99%. The hydrogel is low cost, and maintained highly removal efficiency under low pH. The removal efficiency of the hydrogel remained above 90% after five repeated adsorption-desorption experiments. The hydrogels have a potential to be used in wastewater treatment as adsorbents.

Removal of Cd(II) from Micro-Polluted Water by Magnetic Core-Shell Fe3O4@Prussian Blue

A novel core-shell magnetic Prussian blue-coated Fe3O4 composites (Fe3O4@PB) were designed and synthesized by in-situ replication and controlled etching of iron oxide (Fe3O4) to eliminate Cd (II) from micro-polluted water. The core-shell structure was confirmed by TEM, and the composites were characterized by XRD and FTIR. The pore diameter distribution from BET measurement revealed the micropore-dominated structure of Fe3O4@PB. The effects of adsorbents dosage, pH, and co-existing ions were investigated. Batch results revealed that the Cd (II) adsorption was very fast initially and reached equilibrium after 4 h. A pH of 6 was favorable for Cd (II) adsorption on Fe3O4@PB. The adsorption rate reached 98.78% at an initial Cd (II) concentration of 100 μg/L. The adsorption kinetics indicated that the pseudo-first-order and Elovich models could best describe the Cd (II) adsorption onto Fe3O4@PB, indicating that the sorption of Cd (II) ions on the binding sites of Fe3O4@PB was the main rate-limiting step of adsorption. The adsorption isotherm well fitted the Freundlich model with a maximum capacity of 9.25 mg·g-1 of Cd (II). The adsorption of Cd (II) on the Fe3O4@PB was affected by co-existing ions, including Cu (II), Ni (II), and Zn (II), due to the competitive effect of the co-adsorption of Cd (II) with other co-existing ions.

Recyclable and degradable nonwoven-based double-network composite hydrogel adsorbent for efficient removal of Pb(II) and Ni(II) from aqueous solution

This study reports a novel adsorbent structure and shows the satisfactory removal performance of Pb(II) and Ni(II). The fabric structure increases the strength of the hydrogel. The hydrogel plays a major role in the composite structure as a matrix, while the fabric bears the applied load and protects the structure from mechanical damage. The double-network composite hydrogel is reinforced by plasma grafted polylactic acid melt-blown non-woven fabric and polyethylene glycol dimaleate, and its compressive strength reaches 40.6 kPa at 60% strain. The interface substantially improves the compression strength by 42.9%. Through the adsorption isotherm model, the adsorption capacity of the hydrogel for Pb(II) and Ni(II) reaches 233.12 and 165.06 mg/g, respectively, and the removal rate of heavy metal ions in water at low concentrations exceeds 95%, showing the excellent removal rate of heavy metals. Even after the fifth cycle, the removal efficiency barely declines, indicating the feasibility of repeatedly use. Cost analysis reveals that the adsorbent is relatively low cost, solving the problems of difficult recovery, low strength, and easy damage of hydrogel adsorbents, and promoting the industrial application of hydrogels as adsorbents.

Removal of methylene blue and lead(ii) via PVA/SA double-cross-linked network gel beads loaded with Fe3O4@KHA nanoparticles

The adsorption of MB and Pb(ii) onto and regeneration of PVA/SA/Fe₃O₄@KHA magnetic gel beads.

Functional wastepaper-montmorillonite composite aerogel for Cd2+ adsorption

In this study, a composite aerogel (WP-MMT) composed of wastepaper (WP) and montmorillonite (MMT) was prepared by ambient pressure drying technology to adsorb Cd²⁺. The study of compression performance indicated that the composite aerogel had ideal mechanical strength when the mass ratio of WP to MMT was 1:1. The specific surface areas of the aerogels modified by hydrogen peroxide (WP-MMT-H₂O₂) and sodium hydroxide (WP-MMT-NaOH) were increased greatly. The sorption isotherms and kinetics of Cd²⁺ sorption on WP-MMT-H₂O₂ and WP-MMT-NaOH were investigated. The Cd²⁺ sorption data could be well described by a simple Langmuir model, and the pseudo-second-order kinetic model best fitted the kinetic data. The maximum sorption capacity obtained from the Langmuir model was 232.50 mg/g for WP-MMT-NaOH. The adsorption mechanism of WP-MMT was chemical adsorption of a single-molecule layer. In general, it was proved that the composite aerogel with high adsorption capacity of Cd²⁺ could be synthesized from modified WP and MMT by ambient pressure drying. The composite aerogel fabricated by wastepaper and montmorillonite showed bright application prospect in the aqueous heavy metal pollution control.

Advances in application of cotton-based adsorbents for heavy metals trapping, surface modifications and future perspectives

Cotton-based adsorbents (CBAs) are promising materials for combating the problem of heavy metal pollution of environmental waters. This is ascribed to the low cost, abundance, biodegradability and efficiency of CBAs. Herein we review the adsorption of heavy metals (HMs) onto CBAs. We found that several surface modifications were employed to improve the efficiency of the CBAs. These modifications were effected via thermal, physical and chemical means to obtain activated carbons, biochars, ionic liquids, aerogels, hydrogels, chitosans and nanoparticle-derived CBAs. The CBAs exhibited maximum HMs uptake as low as 0.002 mg/g to as high as 505.6 mg/g. Although, the cotton-derived activated carbons and biochars exhibited enhanced HM uptake from that of the unmodified CBAs, they were less efficient than CBAs modified by other methods. Recent chemical, ionic liquid, chitosan and nano-derived CBAs were the most efficient, with high uptake and fast kinetic removal. However, the nanoparticle-based adsorbents are preferred to the chemically modified forms, due to the possibility of secondary pollution and the noxious effect of the latter to the environment. Findings showed that chemical treatment produced CBAs most efficient for As(V), Pb(II) and Fe(III), while ionic liquid CBA was more efficient for Cu(II) and Ni(II). Nano-based treatment was suitable for the uptake of Co(II), Zn(II), Pb(II) and Cd(II), while the chitosan based adsorbent was viable for Hg(II). Isotherm and kinetic evaluation of CBAs mostly conformed to the Langmuir and pseudo-second order models, respectively. Spontaneous adsorption of HMs onto CBAs was deduced from thermodynamic analysis, with endothermic and exothermic characteristics. Over 88% desorption of HMs was obtained from the CBAs studied with good average reusability from 3 to 20 cycles. We also discussed the directions for future research.

Preparation of a Hydrogel-Based Adsorbent for Metal Ions through High Internal Phase Emulsion Polymerization

In this work, a porous hydrogel-based adsorbent for metal ions was prepared through the copolymerization of acrylic acid and 2-hydroxyethyl methacrylate using a high internal phase emulsion (HIPE) method. Stretched molecular chains in the hydrogel ensure the excellent accessibility of functional sites by the metal ions. A highly open cellular structure endows the P(AANa-co-HEMA) gel with high transport rates as a promising adsorbent. Adsorption properties were investigated by three isotherm models and two kinetic models. X-ray photoelectron spectroscopy analysis proved a chelating interaction between -COO^- and metal ions. The adsorption capacity reached 630 mg·g^-1 for Pb²⁺ under 303 K and a 400 μg·mL^-1 initial concentration. The results show that the as-prepared PolyHIPE-based P(AANa-co-HEMA) gel possesses an open cellular structure, high adsorption capacity, and high selectivity for Pb²⁺.

Preparation of Fe3O4@SiO2@ P(AANa-co-AM) Composites and Their Adsorption for Pb(II)

A series of magnetic composites of sodium polyacrylate and polyacrylamide copolymer [Fe₃O₄@SiO₂@P(AANa-co-AM)] were prepared. The investigation showed that the adsorption efficiency of Pb(II) was the best when the acrylamide/acrylic acid (AM/AA) mass ratio of composites was 5:5. Therefore, the composite of this ratio was selected as the adsorbent to systematically adsorb Pb(II) in aqueous solution. Static adsorption of Pb(II) to the magnetic composites in aqueous solutions was investigated by varying the solution pH and the concentration of Pb(II). The adsorption kinetics and isotherms model of Pb(II) on the Fe₃O₄@SiO₂@P(AANa-co-AM) composites followed a pseudo-second-order model and the Langmuir isotherm model, respectively. When the temperatures were 298.15, 308.15, and 318.15 K, the maximum adsorption capacities of Fe₃O₄@SiO₂@P(AANa-co-AM) composites were 237.53, 248.14, and 255.10 mg/g, respectively. The thermodynamic study of adsorption showed that the adsorption of Pb(II) on Fe₃O₄@SiO₂@P(AANa-co-AM) composites was a spontaneous endothermic process. The X-ray photoelectron spectroscopy (XPS) analysis showed that the adsorption of Pb(II) was due to the chelation between -COO^- and Pb(II). After four adsorption-desorption cycles, the adsorbent can still maintain a high adsorption capacity.

Amino-functionalized alginate/graphene double-network hydrogel beads for emerging contaminant removal from aqueous solution

Inorganic-organic composite hydrogels have attracted much attention in recent years. In this study, an amino-functionalized graphene/alginate double-network hydrogel (NH₂-DN) with excellent mechanical and adsorption properties was successfully prepared. Triethylenetetramine (TETA) was used as a crosslinker which promotes random few-layer graphene sheets stacking and resulted in a reduced graphene oxide (rGO) network, containing mesopore and macropore structures on the hydrogel surface. Compared to single network hydrogel, enhanced thermal stability and mechanical properties were achieved in NH₂-DN. The elasticity modulus was improved by approximately 3 times due to the formation of the double-network. More importantly, NH₂-DN exhibited excellent adsorption properties for typical emerging contaminants (Cu²⁺ and ciprofloxacin (CIP)). Compared with that of an ordinary graphene/alginate single-network hydrogel (SN), the adsorption capacity of the NH₂-DN for Cu²⁺ and CIP reached 153.91 mg g^-1 and 301.36 mg g^-1, respectively, which was increased by 130% and 182%, respectively. Adsorption isotherm and kinetic analyses reveal that the adsorption process of CIP onto the NH₂-DN was dominated by chemical affinity. Adsorption properties were comprehensively examined, including the effects of the solid-liquid ratios, pH, and ionic strength. NH₂-DN retained 94% of its adsorption capacity when the ionic strength was 0.5 mol L^-1 and maintained at least 87% of its adsorption capacity in weak acidic and alkaline solutions. This novel amino-functionalized organic-inorganic hydrogel has great potential in environmental applications owing to its outstanding physicochemical, mechanical, and adsorption properties for emerging contaminants in wastewater.

Key factors and microscopic mechanisms controlling adsorption of cadmium by surface oxidized and aminated biochars

Modified biochar has great potential for adsorbing cadmium (Cd) in the aquatic environment, but the micro-immobilization mechanisms, driven by surface modifications, remain unclear. There has been no attempt to determine the key adsorption factors by integrating the numerous physiochemical indicators. In this study, surface oxidized biochar (OPBC) and surface aminated biochar (APBC) were prepared from porous biochar (PBC), and the Cd adsorption mechanisms by the modified biochars at the molecular and electronic scales were investigated. The adsorption capacity of APBC and OPBC for Cd was 23.54 and 19.04 mg g^-1, respectively, which was about three times higher than that of PBC. Macroscopically, physicochemical adsorption and intraparticle diffusion dominated the Cd adsorption, and surface properties, such as functional groups, were identified as key factors controlling adsorption. Microscopically, the adsorption of Cd mainly occurred in regions rich in π electrons, lone pair electrons and electron donor groups. The interaction between carboxyl and Cd dominated the adsorption performance of OPBC, while the Cd²⁺-π interaction was weakened by increasing the π electron electrostatic potential of aromatic rings. The lone pair electrons of the amino groups dominated the complexation of APBC with Cd, and the π electron electrostatic potential was almost unaffected.

Removal of heavy metal ions from multi-component aqueous solutions by eco-friendly and low-cost composite sorbents with anisotropic pores

Copper, nickel, zinc, chromium, and iron ions are the prevailing contaminants in the aqueous effluents resulting from the photo-etching industry. In this context, we investigate here the metal ion sorption performance of an ion-imprinted cryogel (IIC), consisting of low-cost materials coming from renewable resources, towards multi-component metal ion solutions. The IIC sorbent, which is based on a chitosan matrix embedding a natural zeolite, was synthesized using a straightforward strategy by coupling copper-imprinting and unidirectional ice-templating methods. As consequence, the 1D-orientation and the interconnectivity of flow-channels sustain the fast metal ion diffusion within the IIC anisotropic structure. The removal efficiency of IIC sorbent reached 50% after 30 min, and the sorption equilibrium was attained within 150 min. For assessing the successful formation of imprinted cavities with well-defined sizes controlled by the radius of copper ions used as template, selectivity studies were performed on binary, ternary, and five-component synthetic mixtures. The efficiency of IIC as sorbent was further evaluated on real-life aqueous effluents discharged from photo-etching processes; thus, an IIC dosage of 6 g L^-1 was found to remove 98.89% of Cu²⁺, 94.56% of Fe³⁺, 91.67% of Ni²⁺, 92.24% of Zn²⁺, and 82.76% of Cr³⁺ ions from this type of industrial wastewaters.

κ-Carrageenan/Sodium alginate double-network hydrogel with enhanced mechanical properties, anti-swelling, and adsorption capacity

The abuse of antibiotics is becoming increasingly serious, particularly in offshore aquaculture areas. Double-network polymer gel has good prospects for environmental application for the removal of antibiotics. In this work, κ-Carrageenan/Sodium Alginate (κ-car/SA) double-network hydrogels were designed and synthesized with enhanced mechanical properties, anti-swelling, and adsorption capacity. It was found that the intermolecular interaction and viscosity tend to increase with the increasing concentration of κ-carrageenan. The swelling degree of the composite hydrogel in NaCl solution presented a decreasing trend with the increase of carrageenan. SA can effectively improve the mechanical properties of κ-carrageenan composite gel and enhance its compressive resistance and elasticity. Ciprofloxacin (CIP) was used as the model pollutant for testing the adsorption performance. The results show that the Langmuir-Freundlich isotherm model is more suitable for fitting the adsorption isotherm data of CIP on gel beads, which indicates that κ-car/SA hydrogels have heterogeneous surface and different binding sites. κ-car/SA composite double-network hydrogels exhibit excellent adsorption properties for CIP (229 mg/g). The optimal adsorption capacity of κ-car/SA composite hydrogels was obtained at pH 4, and the adsorption capacity of the hydrogels increased with increasing ion concentration. FTIR spectroscopy and the Zeta potential test analyses showed that the adsorption mechanism may be explained by hydrogen bonding and the electrostatic interactions between κ-car/SA composite hydrogels and CIP. The formation of the new double-network hydrogel provided good properties and development potential for the adsorption of antibiotic in water.

Preparation of Carboxymethyl Cellulose-Based Macroporous Adsorbent by Eco-Friendly Pickering-MIPEs Template for Fast Removal of Pb2+ and Cd2

Recently, Pickering high internal phase emulsions (Pickering HIPEs) have been widely used to fabricate macroporous materials. However, the high usage of poisonous organic solvent in HIPEs not only greatly increases the cost but also is harmful to human health and environment, which leads to limited large-scale applications. In this study, we prepared a novel monolithic macroporous material of carboxymethyl cellulose-g-poly(acrylamide)/montmorillonite (CMC-g-PAM/MMT) by the free radical polymerization via oil-in-water Pickering medium internal phase emulsions (Pickering MIPEs), which used the non-toxic and eco-friendly flaxseed oil as continuous phase, MMT, and Tween-20 (T-20) as stabilizer. The pore structure of the resulting macroporous materials could be tuned easily by adjusting the content of MMT, co-surfactant T-20, and the oil phase volume fraction. The maximal adsorption capacities of the prepared macroporous material for Pb²⁺ and Cd²⁺ were 456.05 and 278.11 mg/g, respectively, and the adsorption equilibrium can be reached within 30 min. Otherwise, the macroporous monolith exhibited excellent reusability through five adsorption–desorption cycles. Thus, the eco-friendly Pickering-MIPEs is a potential alternative method to be used to fabricate multi-porous adsorption materials for environmental applications.

A facile hydrothermal method-fabricated robust and ultralight weight cellulose nanocrystal-based hydro/aerogels for metal ion removal

Heavy metal ion contamination, in particular that associated with Pb²⁺, Cd²⁺, and Cu²⁺, poses a considerable threat to aquatic environments and human health. To obtain a highly efficient adsorbent, in this work, a facile hydrothermal method was applied to prepare acrylic acid grafted onto cellulose nanocrystal (AA-g-CNC) hydro/aerogel as an adsorbent for Pb²⁺, Cd²⁺, and Cu²⁺ removal. The obtained AA-g-CNC hydrogels withstood up to 0.821 MPa of compression and showed good reciprocating performance when the deformation reached 40%. The as-formed AA-g-CNC aerogels had highly porous honeycomb structure, with many functional groups and a high zeta potential, all of which are essential features for an effective adsorbent. The maximum Pb²⁺, Cd²⁺, and Cu²⁺ removal capacities of AA-g-CNC aerogels reached 1026, 898.8, and 872.4 mg/g respectively. Their adsorption followed the Freundlich isotherm model and fitted well with pseudo-second-order kinetic models. The adsorption mechanism mainly attributed to electrostatic chelation between metal ions with sulfonate and carboxylate groups.

Ionic liquid-based click-ionogels

Gels that are freeze-resistant and heat-resistant and have high ultimate tensile strength are desirable in practical applications owing to their potential in designing flexible energy storage devices, actuators, and sensors. Here, a simple method for fabricating ionic liquid (IL)-based click-ionogels using thiol-ene click chemistry under mild condition is reported. These click-ionogels continue to exhibit excellent mechanical properties and resilience after 10,000 fatigue cycles. Moreover, due to several unique properties of ILs, these click-ionogels exhibit high ionic conductivity, transparency, and nonflammability performance over a wide temperature range (-75° to 340°C). Click-ionogel-based triboelectric nanogenerators exhibit excellent mechanical, freeze-thaw, and heat stability. These promising features of click-ionogels will promote innovative applications in flexible and safe device design.

Comparative studies on Pb(II) biosorption with three spongy microbe-based biosorbents: High performance, selectivity and application

Lead pollution in industrial-derived water has become an increasingly serious concern. The development of adsorbents with excellent efficiency, selectivity and separability using diverse microorganisms is ideal for treating lead pollution. In this study, gram-negative bacteria Pseudomonas putida I3, gram-positive bacteria Microbacterium sp. OLJ1 and mycelial fungus Talaromyces amestolkiae Pb served as raw materials to facilely synthesize sponge-like biosorbents via a one-step method at room temperature. SEM, EDS, FTIR, ¹³C NMR, XRD and XPS were used for investigating the morphology and surface properties of these three biosorbents. The obtained biosorbents possessed the same three-dimensional porous structure but different productivities and mechanical strengths due to the similar chemical compositions and different cell structures of their microorganisms. Pb(II) adsorption on X-PI3, X-OLJ1 and X-TPb was fast and pH dependent, with maximal adsorption capacities of 345.02, 237.02 and 199.02 mg/g, respectively. The biosorbents had a high selectivity for Pb(II), while Pb(II) remarkably suppressed the adsorption of co-existing heavy metal ions. The analyses indicated that Pb(II) removal was mainly achieved by ion exchange reactions, surface complexation with heteroatom-containing functional groups and microprecipitation. The treatment effects of synthetic and real wastewater revealed that the as-prepared biosorbents are promising for Pb(II) removal.

Recyclable polyvinyl alcohol sponge containing flower-like layered double hydroxide microspheres for efficient removal of As(V) anions and anionic dyes from water

Layered double hydroxides (LDHs) are very promising adsorbents for the removal of anionic pollutants from water. However, the low adsorption efficiency and recycling difficulty of conventional LDH powders are obstacles to practical applications. Herein, a novel Zn/Fe-LDH composite sponge was successfully fabricated using a simple in-situ hydrothermal method. Characterization studies revealed that the composite sponge contained flower-like Zn/Fe LDH microspheres uniformly dispersed throughout a poly vinyl alcohol (PVA) sponge matrix. The specific surface area of the Zn/Fe-LDH composite sponge was 42.5 m² g^-1, approximately 5 times higher than the pristine PVA sponge (8.9 m² g^-1). Adsorption experiments revealed that Zn/Fe-LDH composite sponge exhibited a much higher adsorption ability for As(V) anions and methyl orange (MO) compared with a Zn/Fe-LDH powder or the pristine PVA sponge. The maximum adsorption capacity for As(V) was found to be 85.7 mg g^-1. Furthermore, the Zn/Fe-LDH composite sponge showed high thermal stability, good mechanical stability and easy recoverability, thereby allowing reuse. Results guide the development of improved, low cost water treatment materials.

[SnS4]4- clusters modified MgAl-LDH composites for mercury ions removal from acid wastewater

The high acidity of mercury ions (Hg²⁺) contained wastewater can complicate its safe disposal. MgAl-LDHs supported [SnS₄]^4- clusters were synthesized for Hg²⁺ removal from acid wastewater. The active sites of [SnS₄]^4- clusters were inserted into the interlayers of MgAl-LDHs using an ion-exchange method. The experimental results indicated that [SnS₄]^4-/MgAl-LDHs composite can obtain higher than 99% Hg²⁺ removal efficiency under low pH values. The maximum mercury adsorption capacity is 360.6 mg g^-1. It indicated that [SnS₄]^4- clusters were the primary active sites for mercury uptake, existing as stable Hg₂(SnS₄) on the surface of the composite. Under low pH values, such a composite seems like a "net" for HgSO₄ molecules, exhibiting great potential for mercury removal from acid solutions. Moreover, the co-exist metal ions such as Zn²⁺, Na⁺, Cd²⁺, Cr³⁺, Pb²⁺, Co²⁺, and Ni²⁺ have no significant influences on Hg²⁺ removal. The adsorption isotherms and kinetics were also studied, indicating that the adsorption mechanism follows a monolayer chemical adsorption model. The [SnS₄]^4-/MgAl-LDHs composite exhibits a great potential for Hg²⁺ removal from acid wastewater.

Adsorption mechanism of Cr(VI) onto GO/PAMAMs composites

Graphene oxide/polyamidoamine dendrimers (GO/PAMAMs) composites were used to remove Cr(VI) from simulated effluents, the adsorption kinetics and thermodynamics of Cr(VI) onto GO/PAMAMs were systematically investigated. The results showed that the optimum pH value was 2.5, the removal percentage reached 90.7% for 30 mg/L of Cr(VI) within 120 min. The adsorption process was well described by pseudo-second-order kinetic model. The maximum adsorption capacities of Cr(VI) onto GO/PAMAMs were found to be 131.58, 183.82 and 211.42 mg/g at 293.15, 303.15 and 313.15 K, respectively, which were calculated from the Langmuir model equation. The adsorption thermodynamic parameters indicate that the adsorption of Cr(VI) onto GO/PAMAMs is a spontaneous endothermic process. The XPS analysis reveals the adsorption and removal mechanism of Cr(VI) on GO/PAMAMs that first the Cr(VI) binds to the protonated amine of GO/PAMAMs, then Cr(VI) be reduced to Cr(III) with the assistance of π-electrons on the carbocyclic six-membered ring of GO in GO/PAMAMs, and then Cr(III) was released into solution under the electrostatic repulsion between the Cr(III) and the protonated amine groups.

Removal of Pb(ii) and Cd(ii) from wastewater using arginine cross-linked chitosan-carboxymethyl cellulose beads as green adsorbent

A one pot approach has been explored to synthesize crosslinked beads from chitosan (CS) and carboxymethyl cellulose (CM) using arginine (ag) as a crosslinker. The synthesized beads were characterized by FTIR, SEM, EDX, XRD, TGA and XPS analysis. The results showed that CS and CM were crosslinked successfully and the obtained material (beads) was analyzed for adsorption of Cd(ii) and Pb(ii) by using batch adsorption experiments; parameters such as temperature, contact time, pH and initial ion concentration were studied. Different kinetic and thermodynamic models were used to check the best fit of the adsorption data. The results revealed that the kinetics data of the adsorption of Pb(ii) and Cd(ii) ions shows the best fit with the pseudo second order model whereas the thermodynamics data shows the best fit with the Langmuir isotherm with maximum adsorption capacities of 182.5 mg g-1 and 168.5 mg g-1 for Pb(ii) ions Cd(ii) ions, respectively. For the recovery and the regeneration after the one use of the beads, several adsorption-desorption cycles were carried out to check the reusability and recovery of both the metal ion and the adsorbent without the loss of maximum adsorption efficiency.