Microscopic mechanism about the selective adsorption of Cr(VI) from salt solution on nitrogen-doped carbon aerogel microsphere pyrolysis products

Gallic acid-functionalized chitosan composite for efficient removal of hexavalent chromium in aqueous

To address the pH limitations of traditional Cr(VI) adsorbents, converting Cr(VI) anions into the more stable Cr(III) cations through a reduction reaction is an effective strategy. Therefore, in this study, gallic acid (GA), with a high redox potential, was selected as the reducing agent, together with chitosan (CS) as the substrate, and sodium alginate (SA) as the gelling agent. A novel SCG composite material was successfully prepared using the sol-gel crosslinking method, which efficiently captured and reduced Cr(VI) while immobilizing Cr(III). At pH = 4, t = 150 min, and T = 30 °C, 1 g/L of SCG can remove 94.7 % of Cr(VI), with a reduction rate of 67.5 %, achieving efficient and stable Cr(VI) removal over a wide pH range. Furthermore, the SCG material demonstrated good pH adaptability (It can still remove 84 % of Cr(VI) at pH = 7) and practical applicability. Comparative experiments and DFT calculation results showed that the introduction of GA not only helped form a more compact network structure but also interacted synergistically with CS, significantly promoting the removal of Cr(VI). In conclusion, the SCG composite material provides a new approach for the treatment of Cr(VI)-contaminated wastewater.

Defect-Engineered MOF-801/Sodium Alginate Aerogel Beads for Boosting Adsorption of Pb(II)

Metal-organic frameworks (MOFs) are attractive adsorbents for heavy metal capture due to their superior stability, easy modification, and adjustable pore size. However, their inherent microporous structure poses challenges in achieving a higher adsorption capacity. Defect engineering is considered a simple method to create hierarchical MOFs with larger pores. Here, we employed l-aspartic acid as a mixed linker to bind Zr4+ clusters in competition with fumaric acid of MOF-801 to create defects, and the pore size was increased from 4.66 to 15.65 nm. Mercaptosuccinic acid was subsequently used as a postexchange ligand to graft the resultant MOF-801 by acid-ammonia condensation to further expand the pore size to 22.73 nm. Notably, the -NH2, -COOH, and -SH groups contributed by these two ligands increased the adsorption sites for Pb(II). The obtained defective MOF-801 with larger pores was thereafter loaded onto sodium alginate to form aerogel beads as adsorbents, and an adsorption capacity of 375.48 mg/g for Pb(II) was achieved, which is ∼51 times that of pristine MOF-801. The aerogel beads also exhibited outstanding reusability with a removal efficiency of ∼90.23% after 5 cycles of use. The adsorption mechanism of Pb(II) included ion-exchange interaction, as well as chelation interactions of Pb-O, Pb-NH2, and Pb-S. The versatile combination of defect engineering and composite beads provides novel inspirations for MOF modification for boosting heavy metal adsorption.

A novel iron oxide (Fe3O4)-laden titanium carbide (Ti3C2) MXene stacks for the efficient removal of tetracycline from aqueous solution

Fe3O4 has the advantages of unique magnetic stability and low biological toxicity, which can improve pollutants separation efficiency. MXenes are two-dimensional materials and easy surface functionalization that can provide suitable carriers for Fe3O4. In this work, we synthesized magnetic MXene composites by a one-pot method that relies on doping Fe3O4 particles onto Ti3C2 MXene nanosheets by heat treatment. The Fe3O4/Ti3C2 MXene was analyzed by SEM, XRD, FTIR and XPS techniques, which showed that the material has good tetracycline (TC) removal properties and magnetic separation ability. The results showed that the adsorption capacity of it was 46.42 mg g-1, and the removal efficiency of 0.06 g adsorbent for 50 mL of 30 mg L-1 TC could reach 92.1% in a wide pH range of 4-10, when the adsorption temperature was 25 °C, and the adsorption time was 3 h. The adsorption data were consistent with Langmuir and the proposed second-order kinetic model, and the thermodynamic experiments confirmed that the adsorption of TC was a monolayer physicochemical adsorption coexisting heat-trapping process (ΔH = 15.72 kJ mol-1). In addition, the adsorption of TC by Fe3O4/Ti3C2 MXene was attributed to the synergistic effect of electrostatic attraction, hydrogen bonding and π-π packing. In conclusion, the saturation magnetization of Fe3O4/Ti3C2 MXene is 27.3 emu/g and it can not only be separated from water using its strong magnetic properties to avoid secondary contamination, but also can be used as a promising material to effectively remove antibiotics from aqueous media.

Fruit waste-derived cellulose-polyaniline composite for adsorption-coupled reduction of chromium oxyanions

Hexavalent chromium oxyanions, known as potentially toxic micropollutants, exist in the effluents and discharges of metallurgical, electroplating, refractory, chemical, and tanning industries. The exposure of chromium-contaminated water causes severe health hazards. The present work outlines a facile approach to grow polyaniline (PANI) on fruit-waste-derived cellulose (CEL) via oxidative polymerization of aniline; followed by chemical processing with NH4OH to obtain CEL-PANI-EB composites for adsorptive separation-coupled reduction of highly toxic hexavalent chromium oxyanions. The spectroscopic analyses of the CEL-PANI-EB composite before and after adsorption of Cr(VI) oxyanions revealed hydrogen bonding, electrostatic, and complexation as major interactive pathways. The adsorbed hexavalent chromium oxyanions are reduced into Cr(III) species by oxidation of PANI-based benzenoid amine into quinoid imine in the CEL-PANI-EB composite. The adsorption of Cr(VI) oxyanions by the CEL-PANI-EB composite showed negligible effects of other anionic co-pollutants, like NO3- and SO42-. The CEL-PANI-EB composite adsorbed Cr(VI) oxyanions with a removal capacity of 469 mg g-1, based on the Langmuir adsorption isotherm model. The hydroxyl functionalities in cellulose and amine/imine functionalities in PANI facilitate the electrostatic attraction between the CEL-PANI-EB and Cr(VI) oxyanions in an acidic environment beside the hydrogen linkages. The adsorbed Cr(VI) oxyanions are reduced to Cr(III)-based species by the benzenoid amines of PANI, as revealed from the XPS analyses. The CEL-PANI-EB composite showed excellent recyclability and maintained 83.4% adsorption efficiency after seven runs of chromium adsorption-desorption. The current findings reveal the potential of CEL-PANI-EB composites for the adsorptive removal of Cr(VI) oxyanions and their conversion into a lesser toxic form, making them promising materials for wastewater treatment applications.

Facile Synthesis of Polymer-Reinforced Silica Aerogel Microspheres as Robust, Hydrophobic and Recyclable Sorbents for Oil Removal from Water

With the rapid development of industry and the acceleration of urbanization, oil pollution has caused serious damage to water, and its treatment has always been a research hotspot. Compared with traditional adsorption materials, aerogel has the advantages of light weight, large adsorption capacity and high selective adsorption, features that render it ideal as a high-performance sorbent for water treatment. The objective of this research was to develop novel hydrophobic polymer-reinforced silica aerogel microspheres (RSAMs) with water glass as the precursor, aminopropyltriethoxysilane as the modifier, and styrene as the crosslinker for oil removal from water. The effects of drying method and polymerization time on the structure and oil adsorption capacity were investigated. The drying method influenced the microstructure and pore structure in a noteworthy manner, and it also significantly depended on the polymerization time. More crosslinking time led to more volume shrinkage, thus resulting in a larger apparent density, lower pore volume, narrower pore size distribution and more compact network. Notably, the hydrophobicity increased with the increase in crosslinking time. After polymerization for 24 h, the RSAMs possessed the highest water contact angle of 126°. Owing to their excellent hydrophobicity, the RSAMs via supercritical CO2 drying exhibited significant oil and organic liquid adsorption capabilities ranging from 6.3 to 18.6 g/g, higher than their state-of-the-art counterparts. Moreover, their robust mechanical properties ensured excellent reusability and recyclability, allowing for multiple adsorption-desorption cycles without significant degradation in performance. The novel sorbent preparation method is facile and inspiring, and the resulting RSAMs are exceptional in capacity, efficiency, stability and regenerability.

Exploration on the role of different iron species in the remediation of As and Cd co-contamination by sewage sludge biochar

Numerous studies have explored the adsorption of cadmium (Cd) and arsenic (As) by iron (Fe)-modified biochar, but few studies have examined in-depth the similarities and differences in the adsorption behavior of different iron types on Cd and As. In this study, sewage sludge biochar (BC) was co-pyrolyzed with self-made Fe minerals (magnetite, hematite, ferrihydrite, goethite, and schwertmannite) to treat Cd and As co-contaminated water. The adsorption of Cd and As on the Fe-modified biochar was further analyzed by adsorption kinetics, adsorption isotherms, and adsorption thermodynamics combined with a series of characterization experiments. Both SEM-EDX and XRD results confirmed the successful loading of iron minerals onto BC. Both adsorption kinetics and adsorption isotherms experiments showed that the adsorption of Cd and As by BC and the other five Fe-modified biochar was mainly controlled by chemical interactions. The results also indicated that goethite biochar (GtBC) was the most effective for the adsorption of Cd among the five Fe-modified biochar. Ferrihydrite biochar (FhBC) formed more diverse complexes, coupled with the relatively stronger electrons accepting ability, thus making it more effective for As adsorption than the others. Additionally, GtBC and hematite biochar (HmBC) were found effective for the adsorption of both Cd and As, whereas MBC was not found effective for either metal. Furthermore, combined with XPS results, the adsorption of Cd by the materials was mainly governed by Cd²⁺-π interactions, complexation precipitation, and co-precipitation, while oxidation reactions also existed for As.

Amino-functionalization of lignocellulosic biopolymer to be used as a green and sustainable adsorbent for anionic contaminant removal

In this work, natural biopolymer stemming from lignocellulosic peanut hull biomass was used as a green and low-cost adsorbent to eliminate anionic Congo red (CR) and Cr(VI) ions from aqueous sample. In order to enhance the removal performance, the lignocellulosic biopolymer was subjected to amino-modification by the graft copolymerization of (3-acrylamidopropyl) trimethylammonium chloride and N, N'-methylenebisacrylamide. The property of the prepared amino-functionalized biopolymer (AFB) was examined through FTIR, TG, SEM, particle size analysis, zeta potential determination and XPS. The adsorption efficacy of AFB for CR and Cr(VI) was tested at different pH, contact time and initial concentration. The kinetic, isotherm and thermodynamics investigations revealed that the uptakes of CR and Cr(VI) were the combination processes of chemical and physical interactions, and both endothermic in nature. The AFB exhibited good reusability without significant loss in adsorption capacity after five consecutive cycles. Mechanistic analysis indicated that the quaternary ammonium groups in AFB contributed a lot to the binding of anionic compounds through electrostatic attraction. In addition, n-π and hydrogen bonding while reduction and coordination were also responsible for the removal of CR and Cr(VI), respectively. The present study provides a favorable strategy for the removal of anionic contaminates in water by using green and sustainable lignocellulosic wastes.

Development of Regular Hydrophobic Silica Aerogel Microspheres for Efficient Oil Adsorption

The objective of this research was to develop new hydrophobic silica aerogel microspheres (HSAMs) with water glass and hexmethyldisilazane for oil adsorption. The effects of the hexmethyldisilazane concentration and drying method on the structure and organic liquid adsorption capacity were investigated. The hexmethyldisilazane concentration of the modification solution did not influence the microstructure and pore structure in a noteworthy manner, which depended more on the drying method. Vacuum drying led to more volume shrinkage of the silica gel microsphere (SGM) than supercritical CO₂ drying, thus resulting in a larger apparent density, lower pore volume, narrower pore size distribution, and more compact network. Owing to the large pore volume and pore size, the HSAMs synthesized via supercritical CO₂ drying had a larger organic liquid adsorption capacity. The adsorption capacities of the HSAMs with pore volumes of 4.04-6.44 cm³/g for colza oil, vacuum pump oil, and hexane are up to 18.3, 18.9, and 11.8 g/g, respectively, higher than for their state-of-the-art counterparts. The new sorbent preparation method is facile, cost-effective, safe, and ecofriendly, and the resulting HSAMs are exceptional in capacity, stability, and regenerability.

Removal of Chromium Species by Adsorption: Fundamental Principles, Newly Developed Adsorbents and Future Perspectives

Emerging chromium (Cr) species have attracted increasing concern. A majority of Cr species, especially hexavalent chromium (Cr(VI)), could lead to lethal effects on human beings, animals, and aquatic lives even at low concentrations. One of the conventional water-treatment methodologies, adsorption, could remove these toxic Cr species efficiently. Additionally, adsorption possesses many advantages, such as being cost-saving, easy to implement, highly efficient and facile to design. Previous research has shown that the application of different adsorbents, such as carbon nanotubes (carbon nanotubes (CNTs) and graphene oxide (GO) and its derivatives), activated carbons (ACs), biochars (BCs), metal-based composites, polymers and others, is being used for Cr species removal from contaminated water and wastewater. The research progress and application of adsorption for Cr removal in recent years are reviewed, the mechanisms of adsorption are also discussed and the development trend of Cr treatment by adsorption is proposed.

Auto-Continuous Synthesis of Robust and Hydrophobic Silica Aerogel Microspheres from Low-Cost Aqueous Sodium Silicate for Fast Dynamic Organics Removal

An efficient auto-continuous globing process was developed with a self-built apparatus to synthesize pure silica aerogel microspheres (PSAMs) using sodium silicate as a precursor and water as a solvent. A hydrophobic silica aerogel microsphere (HSAM) was obtained by methyl grafting. A reinforced silica aerogel microsphere (RSAM) was prepared by polymer cross-linking on the framework of the silica gel. The pH value of the reaction system and the temperature of the coagulating bath were critical to form perfect SAMs with a diameter of 3.0 ± 0.2 mm. The grafted methyl groups are thermally stable up to 400 °C. Polymer cross-linking increased the strength significantly, owing to the polymer coating on the framework of silica aerogel. The pore volumes of HSAM (6.44 cm³/g) and RSAM (3.17 cm³/g) were much higher than their state-of-the-art counterparts. Their specific surface areas were also at a high level. The HSAM and RSAM showed high organic sorption capacities, i.e., 17.9 g/g of pump oil, 11.8 g/g of hexane, and 22.2 mg/g of 10 mg/L methyl orange. The novel preparation method was facile, cost-effective, safe, and eco-friendly, and the resulting SAM sorbents were exceptional in capacity, dynamics, regenerability, and stability.

Comparing the influence of humic/fulvic acid and tannic acid on Cr(VI) adsorption onto polystyrene microplastics: Evidence for the formation of Cr(OH)3 colloids

Microplastics (MPs) can act as vectors for various contaminants in the aquatic environment. Although some research has investigated the adsorption characteristics and influencing factors of metals/organic molecules on MPs, the effects of dissolved organic matter (DOM) (which are ubiquitous active species in ecosystems) on metal oxyanions such as Cr(VI) capture by MPs are largely unknown. This study explored the adsorption behaviors and mechanisms of Cr(VI) oxyanions onto polystyrene (PS) MPs using batch adsorption experiments and multiple spectroscopic methods. The effects of representative DOM components (i.e., humic acid (HA), fulvic acid (FA) and tannic acid (TA)) on Cr(VI) capture by PS were particularly studied. Results revealed a significantly enhanced adsorption of Cr(VI) on PS in the presence of TA. The Cr(VI) adsorption capacity was increased from 2876 μg g^-1 to 4259 μg g^-1 and 5135 μg g^-1 when the TA concentrations raised from 0 to 10 and 20 mg L^-1, respectively. Combined microscopic and spectroscopic investigations revealed that Cr(VI) was reduced to Cr(III) by TA and formed stable Cr(OH)₃ colloids on PS surfaces. Contrarily, HA and FA inhibited Cr(VI) adsorption onto PS, especially at pH > 2.0 and higher DOM concentrations, due to site competition and electrostatic repulsion. Increase in pH was found to reduce zeta potentials of MPs, resulting in inhibited Cr(VI) adsorption. The adsorbed Cr(VI) declined with increasing ionic strength, implying that outer-sphere surface complexation affected the adsorption process in the presence of DOM. These new findings improved our fundamental understanding of the fate of Cr(VI) and MPs in DOM-rich environmental matrices.

Removal of car battery heavy metals from wastewater by activated carbons: a brief review

Spent automobile batteries are one of the most significant secondary sources of harmful heavy metals for the environment. After being incorporated into the aquatic ecosystems, these metals disseminate to various plants, microorganisms, and the human body and cause multiple adverse effects. Activated carbons (ACs) have long been used as an effective adsorbent for different heavy metals in wastewater treatment processes. Although numerous research works have been published to date on this topic, they are scattered in the literature. In this review, we have assembled these works and provided an extensive overview of the application of ACs for treating spent car battery heavy metals (CBHMs) from aquatic systems. The preparation of ACs from different precursor materials, their application in the adsorption of CBHMs, the adsorption mechanism, kinetics, adsorption isotherms and various parameters that may affect the adsorption processes have been discussed in detail. A brief comparative analysis of the adsorption performances of ACs prepared from different precursor materials is also provided. Finally, recommendations for future research works are also offered.

Effectiveness and mechanisms of the adsorption of carbendazim from wastewater onto commercial activated carbon

The wide application of fungicides is becoming one of the main causes of water pollution. Activated carbon (AC) is a frequently-used adsorbent in water treatment. In this work, aiming to obtain a better understanding of fungicides on AC, carbendazim was selected as a model fungicide in water. The effects of AC dosage, adsorption temperature, adsorption time and pH value of solution on carbendazim adsorption by AC were investigated. When the initial concentration of carbendazim was 500 mg L^-1 and the volume of wastewater was 25 mL, the optimum dosage of AC and reaction time was determined to be 0.3 g and 150 min, respectively. The pH ranging from 3.0 to 10.0 exhibited little effect on the adsorption capability of AC. The higher the adsorption temperature was, the better adsorption capacity was. Adsorption capacity could reach 32.31 mg g^-1 under the optimal adsorption conditions. The kinetics study reveals that the adsorption of carbendazim occurred on the surface of adsorbent during initial stage. The adsorption data was well fitted by Langmuir adsorption isotherm, indicating that the adsorption process was monolayer adsorption. The thermodynamic experiments confirmed that the adsorption of carbendazim was an endothermic process with the coexistence of physical and chemical adsorption. Because the main components of AC used in this research work is amorphous carbon with low impurity and its surface has not been modified with additional functional groups, the conclusion of the study was easy to be replicated by repeated experiments. Therefore, the findings of this study could guide the adsorption of carbendazim onto the other kinds of AC with high specific surface area, and provide useful information for application of commercial AC in treatment of fungicides wastewater.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.