Selenocarrageenan-inspired hybrid graphene hydrogel as recyclable adsorbent for efficient scavenging of dyes and Hg2+ in water environment

Polymeric hydrogels-based materials for wastewater treatment

Low-cost and reliable wastewater treatment is a relevant issue worldwide to reduce the concentration of environmental pollutants. Industrial effluents containing dyes, heavy metals, and other inorganic and organic compounds can pollute water resources; therefore, novel technologies are required to mitigate and control their release into the environment. Adsorption is one of the simplest methods for treating contaminated water in which a wide spectrum of adsorbents can be used to remove emerging compounds. Hydrogels are interesting materials with high adsorption capacities that can be synthesized via green routes. These adsorbents are promising for large-scale industrial wastewater treatment applications; however, gaps still exist in achieving sustainable commercial implementation. This review focuses on the discussion and analysis of preparation, characterization, and adsorption properties of hydrogels for water purification. The advantages of these polymeric materials for water treatment were analyzed, including their performance in the removal of different organic and inorganic contaminants. Recent advances in the functionalization of hydrogels and the synthesis of novel composites have also been described. The adsorption capacities of hydrogel-based adsorbents are higher than 500 mg/g for different organic and inorganic pollutants, and can reach values of up to >2000 mg/g for organic compounds, significantly outperforming other materials reported for water cleaning. The main interactions involved in the adsorption of water pollutants using hydrogel-based adsorbents were described and explained to allow the interpretation of their removal mechanisms. The current challenges in the implementation of hydrogels for water purification in real-life operations are also highlighted. This review provides an updated picture of hydrogels as interesting materials to address water depollution worldwide.

Effective assessment of biopolymer-based multifunctional sorbents for the remediation of environmentally hazardous contaminants from aqueous solutions

Persistent contaminants in wastewater effluent pose a significant threat to aquatic life and are one of the most significant environmental concerns of our time. Although there are a variety of traditional methods available in wastewater treatment, including adsorption, coagulation, flocculation, ion exchange, membrane filtration, co-precipitation and solvent extraction, none of these have been found to be significantly cost-effective in removing toxic pollutants from the water environment. The upfront costs of these treatment methods are extremely high, and they require the use of harmful synthetic chemicals. For this reason, the development of new technologies for the treatment and recycling of wastewater is an absolute necessity. Our way of life can be made more sustainable by the synthesis of adsorbents based on biomass, making the process less harmful to the environment. Biopolymers offer a sustainable alternative to synthetic polymers, which are manufactured by joining monomer units through covalent bonding. This review presents a detailed classification of biopolymers such as pectin, alginate, chitosan, lignin, cellulose, chitin, carrageen, certain proteins, and other microbial biomass compounds and composites, with a focus on their sources, methods of synthesis, and prospective applications in wastewater treatment. A concise summary of the extensive body of knowledge on the fate of biopolymers after adsorption is also provided. Finally, consideration is given to open questions about future developments leading to environmentally friendly and economically beneficial applications of biopolymers.

Green and Superior Adsorbents Derived from Natural Plant Gums for Removal of Contaminants: A Review

The ubiquitous presence of contaminants in water poses a major threat to the safety of ecosystems and human health, and so more materials or technologies are urgently needed to eliminate pollutants. Polymer materials have shown significant advantages over most other adsorption materials in the decontamination of wastewater by virtue of their relatively high adsorption capacity and fast adsorption rate. In recent years, "green development" has become the focus of global attention, and the environmental friendliness of materials themselves has been concerned. Therefore, natural polymers-derived materials are favored in the purification of wastewater due to their unique advantages of being renewable, low cost and environmentally friendly. Among them, natural plant gums show great potential in the synthesis of environmentally friendly polymer adsorption materials due to their rich sources, diverse structures and properties, as well as their renewable, non-toxic and biocompatible advantages. Natural plant gums can be easily modified by facile derivatization or a graft polymerization reaction to enhance the inherent properties or introduce new functions, thus obtaining new adsorption materials for the efficient purification of wastewater. This paper summarized the research progress on the fabrication of various gums-based adsorbents and their application in the decontamination of different types of pollutants. The general synthesis mechanism of gums-based adsorbents, and the adsorption mechanism of the adsorbent for different types of pollutants were also discussed. This paper was aimed at providing a reference for the design and development of more cost-effective and environmentally friendly water purification materials.

Mechanistic study of Hg(II) interaction with three different α-aminophosphonate adsorbents: Insights from batch experiments and theoretical calculations

Herein, efficient and potential chelating α-aminophosphonate based sorbents (AP-) derived from three different amine origins (aniline/anthranilic acid/O-phenylenediamine) to form AP-H, carboxylated and aminated enhanced aminophosphonate as AP-H, AP-COOH, and AP-NH₂ were synthesized via a facile method. The structure of the synthesized sorbents was elucidated using different techniques; elemental analysis (CHNP/O), FT-IR, NMR (¹H-, ¹³C and ³¹P NMR), TGA and BET. The fabricated sorbents were exploited for Hg(II) removal from aqueous solution via sorption properties. Isotherm fitted by Langmuir equation: the maximum sorption capacities at optimum pH 5.5, and T:25 ± 1 °C, were found to be 1.33, 1.23, and 1.15 mmol Hg g^-1 for AP-COOH, AP-NH₂, AP-H, respectively, which is roughly correlated with the active sites density and the hard/soft characteristics of adsorbents' reactive groups. Metal-ligand binding affinities are qualitatively rationalized in terms of hard and soft acids and bases (HSAB) theory. The interaction of Hg(II) (soft) has a stronger affinity to AP-COOH can be considered a softer base compared with reference material (AP-H) over than AP-NH₂ (hard). This sequence result showed opposite trends consistent with their reciprocal properties according to the steric effect modulates and the specific surface area. Thermodynamics analysis for absolute values of ΔH°, ΔS° and ΔG° afford the selectivity towards Hg(II) sorption with the following order: AP-COOH > AP-NH₂ >AP-H. Elution and regeneration was carried out by HCl solution and recycled for a minimum of five cycles, the sorption and desorption efficiencies are greater than 91%. Such sorbents exhibit good durability, stability and promising potential for Hg(II) removal. Finally, a new modelling technique for quantitative non-linear description and comparison of equivalent geographical positions in 3D space of extended relationships. Exothermic and spontaneous behavior were observed using a proposed Floatotherm that included the Van't Hoff parameters model.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Remediation of heavy metal polluted waters using activated carbon from lignocellulosic biomass: An update of recent trends

The use of a cheap and effective adsorption approach based on biomass-activated carbon (AC) to remediate heavy metal contamination is clearly desirable for developing countries that are economically disadvantaged yet have abundant biomass. Therefore, this review provides an update of recent works utilizing biomass waste-AC to adsorb commonly-encountered adsorbates like Cr, Pb, Cu, Cd, Hg, and As. Various biomass wastes were employed in synthesizing AC via two-steps processing; oxygen-free carbonization followed by activation. In recent works related to the activation step, the microwave technique is growing in popularity compared to the more conventional physical/chemical activation method because the microwave technique can ensure a more uniform energy distribution in the solid adsorbent, resulting in enhanced surface area. Nonetheless, chemical activation is still generally preferred for its ease of operation, lower cost, and shorter preparation time. Several mechanisms related to heavy metal adsorption on biomass wastes-AC were also discussed in detail, such as (i) - physical adsorption/deposition of metals, (ii) - ion-exchange between protonated oxygen-containing functional groups (-OH, -COOH) and divalent metal cations (M²⁺), (iii) - electrostatic interaction between oppositely-charged ions, (iv) - surface complexation between functional groups (-OH, O^2-, -CO-NH-, and -COOH) and heavy metal ions/complexes, and (v) - precipitation/co-precipitation technique. Additionally, key parameters affecting the adsorption performance were scrutinized. In general, this review offers a comprehensive insight into the production of AC from lignocellulosic biomass and its application in treating heavy metals-polluted water, showing that biomass-originated AC could bring great benefits to the environment, economy, and sustainability.

Carrageenan-based nano-hybrid materials for the mitigation of hazardous environmental pollutants

Fast industrialization and population growth are associated with the increased release of hazardous contaminants in the environment. These hazardous substances, including pharmaceutical, biomedical, personal-care products, heavy metals, endocrine-disrupters, and colorants, pollute the ecosystem by disturbing nature's balance. Nanotechnology has paved new horizons in biochemical engineering by designing novel approaches of integrating nanoscale science with biotechnology to construct improved quality materials for target uptake of pollutants. Recently, nanostructured materials have emerged as research and development frontiers owing to their excellent properties. The tailored designing of nanohybrids constructs with physicochemical alteration enables the nano-bioadsorbent with high target specificity and efficiency. The development of eco-friendly, biodegradable, cost-efficient, and biopolymer-based nanohybrid constructs is gaining attention to remove hazardous environmental pollutants. κ-carrageenan biopolymer is frequently used with different nanomaterials to design nanohybrid bio-adsorbents to remove various contaminants. Herein, the potentialities of carrageenan-based nanohybrid constructs in environmental remediation have been summarized. Different nanostructures, e.g., silica, non-magnetic/magnetic, carbon nanotubes/nanorods, nanoclay/nanomembrane, metal organic frameworks, graphene oxide, and other nanomaterials have been described in combination with carrageenan biopolymers focusing on environmental remediation.

Facile asymmetric modification of graphene nanosheets using κ-carrageenan as a green template

The synthesis of Janus nanosheets using κ-carrageenan (κ-Ca) as a green template endows a greener and more straightforward method compared to traditional approaches of using wax template. We hypothesize that the hydrogen bonding interaction between κ-Ca and graphene oxide (GO) allows partial masking of GO's single facet, paving the way for the asymmetric modification of the exposed surface. GO is first encapsulated within the porous hydrogel matrix formed by κ-Ca to isolate one of the facets. The exposed surface was then selectively hydrophobized to produce an amphiphilic asymmetrically modified graphene oxide (AMGO). The properties of AMGO synthesized under different κ-Ca/GO ratios were studied. The κ-Ca/GO interactions and the properties of GO and AMGO were investigated and characterized. AMGO was successfully produced with a yield of 90.37 % under optimized synthesis conditions. The separation of κ-Ca and AMGO was conducted without organic solvents, and the κ-Ca could be subsequently recovered. Furthermore, the porous hydrogel matrix formed by κ-Ca and GO exhibited excellent shape-retaining properties with high thermal tolerance of up to 50 °C. Given these benefits, this newly developed method endows sustainability and open the possibility of formulating more flexible material synthesis protocols.

Efficient removal of Hg2+ from aqueous solution by a novel composite of nano humboldtine decorated almandine (NHDA): Ion exchange, reducing-oxidation and adsorption

Efficient removal of Hg²⁺ from aqueous solution is key for environmental protection and human health. Herein, a novel composite of nano humboldtine decorated almandine was synthesized from almandine for the removal of Hg²⁺. Results showed that the Hg²⁺ removal process followed pseudo-second-order kinetic model and Langmuir equation, and the maximum adsorption capacity was 575.17 mg/g. Furthermore, Hg²⁺ removal by the composite was pH-dependent and low pH value facilitated the removal of Hg²⁺. SEM and HADDF-STEM results suggested a new rod morphology was generated and the adsorbed mercury was mainly enriched into this structure after reaction with Hg²⁺ solution. The removal mechanisms of Hg²⁺ by the composite was pH dependent, and included ion exchange, surface complexation, reduction and oxidation. Our results demonstrated that the composite was an ideal material for Hg²⁺ removal and the transformation ways of mercury related species could be a significant but currently underestimated pathway in natural and engineered systems.

Processing and modification of hydrogel and its application in emerging contaminant adsorption and in catalyst immobilization: a review

Due to the wonderful property of hydrogels, they can provide a platform for a wide range of applications. Recently, there is a growing research interest in the development of potential hydrogel adsorbents in wastewater treatment due to their adsorption ability toward aqueous pollutants. It is important to prepare such a hydrogel that possesses appropriate robustness, adsorption capacity, and adsorption efficiency to meet the need of water treatment. In order to improve the property of hydrogels, much effort has been made by researchers to modify hydrogels, among which incorporating inorganic components into the polymeric networks is the most common method, which can reduce the product cost and simplify the preparation procedure. Not only can hydrogel be applied as adsorbent, but it also can be used as matrix for catalyst immobilization. In this review, the key advancement on the preparation and modification of hydrogels is discussed, with special emphasis on the introduction of inorganic materials into polymeric networks and consequential changes in the properties of mechanical strength, swelling, and adsorption. Besides, hydrogels used as adsorbents for removal of dyes and inorganic pollutants have been widely explored, but their use for adsorbing emerging contaminants from aqueous solution has not received much attention. Thus, this review is mainly focused on hydrogels' application in removing emerging contaminants by adsorption. Furthermore, hydrogels can be also applied in immobilizing catalysts, such as enzyme and photocatalyst, to remove pollutants completely and avoid secondary pollution, so their progress as catalyst matrix is overviewed.

Recent Advances in Applications of Hybrid Graphene Materials for Metals Removal from Wastewater

The presence of traces of heavy metals in wastewater causes adverse health effects on humans and the ecosystem. Adsorption is a low cost and eco-friendly method for the removal of low concentrations of heavy metals from wastewater streams. Over the past several years, graphene-based materials have been researched as exceptional adsorbents. In this review, the applications of graphene oxide (GO), reduce graphene oxide (rGO), and graphene-based nanocomposites (GNCs) for the removal of various metals are analyzed. Firstly, the common synthesis routes for GO, rGO, and GNCs are discussed. Secondly, the available literature on the adsorption of heavy metals including arsenic, lead, cadmium, nickel, mercury, chromium and copper using graphene-based materials are reviewed and analyzed. The adsorption isotherms, kinetics, capacity, and removal efficiency for each metal on different graphene materials, as well as the effects of the synthesis method and the adsorption process conditions on the recyclability of the graphene materials, are discussed. Finally, future perspectives and trends in the field are also highlighted.

Nanocellulose-based composites for the removal of contaminants from wastewater

Polymers derived from plant and animal sources are of great interest in wastewater remediation due to their cost-effectiveness and renewable adsorption capabilities, one such polymer is nanocellulose (NC). NC has gained a lot of attention in various research fields due to its abundance in nature, nano-dimension, high surface area, stability and bio-compatibility. As a result, NC has emerged as a great potential adsorbent for the removal of contaminants such as heavy metals, organic dyes, oils, pharmaceutical and etc. in the environmental remediation. This review focuses on the description of the building blocks, structure, properties, isolation and also discusses the potential of nanocellulose based composites materials with reinforcements such as activated carbon, carbon nanotube, graphene oxides, metals, non-metals and ceramics that were effectively used as an adsorbents for diverse organic and inorganic contaminants in water.

Comparison of the adsorption behaviors for methylene blue on two renewable gels with different physical state

High removal efficiency and excellent recyclability are the fundamental qualities that an outstanding adsorbent used for organic dye removal should possess. In this study, two recyclable gels (sodium alginate/Ca/fiber: SCFA hydrogels; cellulose nanofiber/chitosan: CNFCS aerogels) were successfully fabricated using the facile method. Additionally, the as-prepared adsorbents were investigated using a series of characterizations. The adsorption behavior and anti-interference performance of the synthesized gels were compared by choosing methylene blue (MB) as the model pollutant. The kinetic behavior of the gels towards MB was consistent with the pseudo first-order model, and the SCFA hydrogels reached adsorption equilibrium faster than the CNFCS aerogels. The maximum adsorption capacity of MB on the SCFA hydrogels and CNFCS aerogels was 1335.0 and 164.5 mg g^-1 (pH = 7.0, dosage: 0.5 g/L; initial concentration from 15 to 180 mg L^-1), respectively. More specifically, we found that the co-existing anions had different effects on MB adsorption over the gels used for MB removal. Furthermore, for the SCFA hydrogels, co-existing natural organic matter (NOM) at low concentrations enhanced MB adsorption, and then stabilized as the concentration of NOM increased. However, this increasing trend was not observed for MB adsorption on CNFCS aerogels; these gels exhibited a slight decrease at first, and then showed no change. Nevertheless, both the gels exhibited superior regeneration and recycling abilities.

Thermal Flow Self-Assembled Anisotropic Chemically Derived Graphene Aerogels and Their Thermal Conductivity Enhancement

In this study, we investigated the directional heating of graphene oxide (GO) dispersion to generate a temperature gradient and form a simulated “ocean current” inside the dispersion so that GO sheets could be aligned in a directional manner and then reduced and self-assembled into anisotropic reduced graphene oxide (rGO) gel. After freeze-drying and varying degrees of vacuum microwave treatment, anisotropic chemically derived graphene aerogels (AGAs) were obtained. Through performance detection and the analysis of the results, it was verified that the AGAs with certain characteristics of “ocean current” were prepared in this experiment, and its axial direction has obvious directional arrangement. After being treated by vacuum microwave for a short time (1 min.), the axial thermal conductivity of the composite materials (AGA-adsorbed paraffin) was observed to be 1.074 W/mK, and the thermal conductivity enhancement efficiency was 995%; as compared with similar thermal conductivity enhancement composites that were found in previous studies, the proposed method in this paper has the advantages of simple processing, high efficiency, and energy conservation.