Insights into the Mechanism of Efficient Cr(VI) Removal from Aqueous Solution by Iron-Rich Wheat Straw Hydrochar: Coupling DFT Calculation with Experiments

Agricultural solid waste has become one of the raw materials for hydrothermal carbon production, promoting resource utilization. This study synthesized two types of ball-milling carbons (Fe-MHBC vs MHBC) with and without FeCl3 modification using wheat straw hydrochars. Cr(VI) adsorption on these two types of ball-milling carbons was investigated. According to Langmuir's maximum adsorption capacity analysis, Fe-MHBC had a capacity of 116.29 mg g-1. The thermodynamic analysis based on isothermal adsorption reveals the spontaneous process of the reaction between the two materials. The adsorption of Cr(VI) on Fe-MHBC exhibited excellent agreement with the pseudo-second-order kinetics model. Furthermore, X-ray photoelectron spectroscopy analysis showed that Fe(II) in the material reduced Cr(VI) when it participated in the reaction. The acidic conditions facilitate the elimination of Cr(VI). The Fe-MHBC has a higher zeta potential, which enhances the electrostatic attraction of Cr(VI) particles. Even with a starting pH of 10, the removal rate can be consistently maintained at over 64%. The adsorption of Cr(VI) was inhibited by various anions and higher ion concentrations. Density functional theory demonstrates that the presence of Fe enhances the adsorption capacity and electron transfer flux of Cr(VI). Fe-MHBC effectively eliminates Cr(VI) by the process of electrostatic adsorption, redox, and complexation reactions. This study demonstrated that hydrochar materials modified by FeCl3 through a ball-milling process show considerable potential as effective adsorbents in the treatment of Cr(VI) pollution, offering a viable and environmentally friendly solution for mitigating this prevalent environmental issue.

Fabrication of Iron-Containing Biochar by One-Step Ball Milling for Cr(VI) and Tetracycline Removal from Wastewater

Simple ball milling technology can simultaneously improve the adsorption performance of adsorbents for heavy metals and organic pollutants and has attracted increasing attention. Iron-modified biochar (Fe@MBC) was prepared by one-step ball milling, and the characterization results proved that FeCl3 was successfully loaded on biochar. The removal rates of Cr(VI) and tetracycline hydrochloride (TC) by Fe@MBC were increased by 88.27% and 82.64% compared with BC. The average pore size, oxygen-containing functional groups and graphitization degree of Fe@MBC are higher than those of BC, which is more conducive to promoting adsorption. The adsorption isotherms show that the adsorption of Cr(VI) and TC on the Fe@MBC surface conforms to the Langmuir type of single-layer adsorption and the Freundlich model of multilayer adsorption, respectively. The maximum adsorption capacities of Cr(VI) and TC are 25.46 and 66.91 mg·g-1, respectively. Kinetic experiments show that the adsorption process is more consistent with the pseudo-second-order model of chemical adsorption. The adsorption process of Cr(VI) and TC on the Fe@MBC surface is a spontaneous endothermic process that becomes more obvious as the temperature increases. The increase in solution pH has a significant impact on the removal rate of Fe@MBC. When the pH value increased from 3 to 11, the adsorption rates decreased by 53.74% and 17.16%, respectively. The presence of PO43-, CO32-, K+, and Cu2+ significantly affects the adsorption of TC by Fe@MBC, and PO43- and CO32- also affect the adsorption of Cr(VI). Mechanistic studies show that ion exchange, electrostatic interaction, pore filling, and hydrogen bonding contribute to the removal of Cr(VI) and TC by Fe@MBC. The removal mechanism of Cr(VI) also involves complexation and redox reactions, and the removal mechanism of TC involves π-π bonds and van der Waals forces. The results show that Fe@MBC is a green and efficient adsorbent.

Highly compressible and macro-porous hydrogels via the synergy of cryogelation and double-network for efficient removal of Cr(VI)

Mechanically robust and macro-porous hydrogels are urgently required for the dynamic removal of heavy metals in wastewater purification field. Herein, a novel microfibrillated cellulose/polyethyleneimine hydrogel (MFC/PEI-CD) with high compressibility and macro-porous structures was fabricated via the synergy of cryogelation and double-network for Cr(VI) adsorption from wastewater. MFCs were pre-cross-linked by bis(vinyl sulfonyl)methane (BVSM) and then formed double-network hydrogels with PEIs and glutaraldehyde below freezing. The SEM showed that the MFC/PEI-CD possessed interconnected macropores with an average pore diameter of 52 μm. Mechanical tests indicated a high compressive stress of 116.4 kPa at 80 % strain, which was 4 times higher than the corresponding MFC/PEI with a single-network. The Cr(VI) adsorption performance of MFC/PEI-CDs was systematically investigated under different parameters. Kinetic studies indicated that the adsorption process was well described by the pseudo-second-order model. Isothermal adsorption behaviors accorded well with Langmuir model with the maximum adsorption capacity of 545.1 mg/g, which was superior to most adsorption materials. More importantly, the MFC/PEI-CD was applied to dynamically adsorb Cr(VI) with the treatment volume of 2070 mL/g. Therefore, this work demonstrates that the synergy of cryogelation and double-network is a novel method for preparing macro-porous and robust materials with promising heavy metal removal from wastewater.

Development of novel MOF-mixed matrix three-dimensional membrane capsules for eradicating potentially toxic metals from water and real electroplating wastewater

The stability and applicability of UiO-66-(NH₂)₂ metal-organic framework (MOF) nanoparticles (NPs) were successfully improved in this study by incorporating them into alginate biopolymer during the manifestation of crosslinking agents-calcium chloride and glutaraldehyde-via a simple, environment-friendly, and facile approach to eradicate potentially toxic metals (PTMs) such as Cr⁶⁺, Cr³⁺, Cu²⁺, and Cd²⁺ from water and real electroplating wastewater. Hydrophilic functional groups (i.e., -OH, -COOH, and -NH₂) are imperative in the smooth loading of UiO-66-(NH₂)₂ MOF- NPs into three-dimensional (3-D) membrane capsules (MCs). The X-ray photoelectron spectroscopy (XPS) results suggested that UiO-66-(NH₂)₂ MOF was effectively bonded in/on the capsule via electrostatic crosslinking between -H₃N⁺ and -COO^-. Scanning electron microscopy results revealed a porous honeycomb configuration of the 3-D SGMMCs (S: sodium alginate, G: glutaraldehyde, M: MOF NPs, and MCs: membrane capsules). The maximum monolayer absorption capacities for Cr⁶⁺, Cr³⁺, Cu²⁺, and Cd²⁺ were 495, 975, 1295, and 1350 mg/g, respectively. The results of Fourier transform infrared spectroscopy and XPS analyses showed that electrostatic attraction and ion exchange were the main processes for PTM removal used by the as-developed 3-D SGMMCs. The as-developed 3-D SGMMCs exhibited outstanding selectivity for removing the targeted PTMs under the specified pH/conditions and maintained >80% removal efficiency for up to six consecutive treatment cycles. Notably, > 60% removal efficiencies for Cr⁶⁺ and Cu²⁺ were observed when treating real electroplating wastewater. Therefore, the as-developed 3-D SGMMCs can be used as an exceptional multifunctional sorbent to remove and recover PTMs from real electroplating wastewater.

Magnetic responsive mesoporous alginate/β-cyclodextrin polymer beads enhance selectivity and adsorption of heavy metal ions

Mesoporous (~7-8 nm) biopolymer hydrogel beads (HNTs-FeNPs@Alg/β-CD) were synthesised via ionic polymerisation route to separate heavy metal ions. The adsorption capacity of HNTs-FeNPs@Alg/β-CD was higher than that of raw halloysite nano tubes (HNTs), iron nanoparticles (FeNPs), and bare alginate beads. FeNPs induce the magnetic properties of adsorbent and metal-based functional groups in and around the hydrogel beads. The mesoporous surface of the adsorbent permits access of heavy metal ions onto the polymer beads to interact with internal active sites and the mesoporous polymer network. Maximum adsorption capacities of lead (Pb), copper (Cu), cadmium (Cd), and nickel (Ni) were 21.09 mg/g, 15.54 mg/g, 2.47 mg/g, and 2.68 mg/g, respectively. HNTs-FeNPs@Alg/β-CD was able to adsorb heavy metals efficiently (75-99%) under environment-relevant concentrations (200 μg/L) from mixed metal contaminants. The adsorption and selectivity trends of heavy metals were Pb > Cu > Cd > Ni, despite electrostatic binding strength of Cd > Cu > Pb > Ni and covalent binding strength of Pb > Ni > Cu > Cd. It demonstrated that not only chemosorption but also physisorption acts as the sorption mechanism. The reduction in surface area, porosity, and pore volume of the expended adsorbent, along with sorption study results, confirmed that pore filling and intra-particle diffusion played a considerable role in removing heavy metals.

Fabrication of MXene/PEI functionalized sodium alginate aerogel and its excellent adsorption behavior for Cr(VI) and Congo Red from aqueous solution

MXene/PEI modified sodium alginate aerogel (MPA) was facilely prepared by introducing polyethylenimine (PEI) and amino functionalized Ti₃C₂T_x into sodium alginate (SA) aerogel matrix through cross-linking reactions. Abundant active groups of PEI coupled with in situ reduction ability of MXene dramatically promoted the removal of Cr(VI), realizing the adsorption capacity of 538.97 mg/g. MPA also possessed an ultrahigh adsorption capacity (3568 mg/g) towards Congo Red (CR), ascribing to the strong electrostatic attraction and the synergetic effect of surface adsorption and intercalation adsorption. The Cr(VI) and CR adsorption mechanisms by MPA were further validated using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Batch adsorption experiments were conducted to investigate pH impacts, kinetics, isotherms and thermodynamics. The results demonstrated that adsorption processes of both Cr(VI) and CR fitted felicitously with the Langmuir isotherm and the pseudo-second-order kinetic model. More importantly, having a double-network structure constructed by polymeric SA and PEI, the mechanical strength of the aerogel was significantly reinforced, which was easily recycled without secondary pollution and the capacity decreased few after five cycles. Furthermore, provided with outstanding antibacterial properties against S. aureus and E. coli, MPA can be extensively applied for the water treatment as a both highly efficient adsorbent and antimicrobial.

Preparation of a porous carbon material by hydrothermal activation of itaconic acid fermentation waste liquid and its efficient adsorption of Cr(vi)

Recycling organic waste liquid to treat chromium-containing wastewater to achieve the purpose of treating waste with waste.

Unexpected effect of magnetic nanoparticles on the performance of aqueous removal of toxic Cr(VI) using modified biopolymer chitosan

Modified biopolymer chitosan namely 2-hydroxy-1-naphthaldehyde chitosan (CTS-Nap) has been synthesized for the removal of toxic chromium from aqueous solutions. In an attempt to enhance the adsorption capacity of toxic chromium on the prepared modified biopolymer, magnetic Fe₃O₄ nanoparticles have been loaded on the modified adsorbent to form the magnetite adsorbent (Fe₃O₄@CTS-Nap). The adsorption mechanism of both adsorbents is explored by batch experiments, FT-IR, SEM, TEM, XRD, VSM, and EDS. The optimum adsorption is achieved at pH 1.5 for CTS-Nap and 1.0 for Fe₃O₄@CTS-Nap. Pseudo second order illustrated the best description for the adsorption process with correlation coefficient R² = 0.999 and the film diffusion or chemisorption is the rate-limiting step. The equilibrium data is analyzed using five isotherm models, the experimental data agreed well with the Freundlich model with a maximum adsorption capacity of 78.12 mg g-1 and 57.14 mg g-1 for CTS-Nap and Fe₃O₄@CTS-Nap, respectively. However, this unexpected result revealed that the presence of magnetic nanoparticles does not always enhance the adsorption process and many other factors could control the adsorption process. Generally, these outcomes revealed that the unmagnetite modified adsorbent CTS-Nap have practical greater influence on wastewater treatment management rather than the magnetic modified chitosan Fe₃O₄@CTS-Nap.

Superior removal of Hg (II) ions from wastewater using hierarchically porous, functionalized carbon

The removal of heavy metal ions from industrial wastewater by adsorption has been central to the environment for decades, where common adsorbent materials are often limited by poor efficiency, complex fabrication and long processing time. Porous carbon derived from biospecies holds promise to address the limitations. In this study we converted bagasse into a carbon composite having hierarchically porous structure; the composite's dispersion phases - iron oxide and manganese oxide - were synthesized by a simple one-step liquid-phase reaction method. Featuring large specific surface area of 350.8 m² g^-1, the composite demonstrated exceptional Hg (II) removal efficiency of 96.8%, adsorption rate of up to 96.8% within 150 min and adsorption capacity of 9.8 mg g^-1. In comparison with other removal materials, our work is outstanding in terms of both removal efficiency and synthesis simplicity. The high efficiency is attributed to the synergy between physical adsorption referring to hierarchically porous structure and chemical adsorption relating to functional complexation processes. It provides a new avenue for the development of high-performance adsorbent materials for heavy metal removal from aqueous media.

Removal of Hg(ii) in aqueous solutions through physical and chemical adsorption principles

Adsorption has been the focus of research on the treatment of heavy metal mercury pollution since it is among the most toxic heavy metals in existence.

Enhanced removal of hexavalent chromium from aqueous media using a highly stable and magnetically separable rosin-biochar-coated TiO2@C nanocomposite

Recently, nanosized metal-oxides have been extensively investigated for their ability to remove metal ions from aqueous media. However, the activity and capacity of these nanosized metal-oxides for removing metal ions decrease owing to their agglomeration in aqueous media. Herein, we synthesized a highly stable and magnetically separable rosin-biochar-coated (RBC) TiO₂@C nanocomposite through a facile and environment-friendly wet chemical coating process, followed by a one-step heating route (pyrolysis) for efficient removal of Cr(vi) from aqueous solution. An array of techniques, namely, TEM, HRTEM, TEM-EDS, XRD, FTIR, VSM, BET and TGA, were used to characterize the prepared nanocomposite. The pyrolysis of rosin into biochar and the fabrication of Fe onto the RBC-TiO₂@C nanocomposite were confirmed by FTIR and XRD examination, respectively. Moreover, TEM and HRTEM images and elemental mapping using TEM-EDS showed good dispersion of iron and carbon on the surface of the RBC-TiO₂@C nanocomposite. Sorption of Cr(vi) ions on the surface of the RBC-TiO₂@C nanocomposite was very fast and efficient, having a removal efficiency of ∼95% within the 1^st minute of reaction. Furthermore, thermodynamic analysis showed negative values of Gibb's free energy at all five temperatures, indicating that the adsorption of Cr(vi) ions on the RBC-TiO₂@C nanocomposite was favorable and spontaneous. Conclusively, our results indicate that the RBC-TiO₂@C nanocomposite can be used for efficient removal of Cr(vi) from aqueous media due to its novel synthesis and extraordinary adsorption efficacy during a short time period.

A biochar-coated RBC-TiO₂@C nanocomposite was synthesized using a wet chemical coating followed by a one-step heating route (pyrolysis) for the efficient removal of Cr(vi).

Preparation of PEI/CS aerogel beads with a high density of reactive sites for efficient Cr(vi) sorption: batch and column studies

PEI/CS aerogel beads with a high density of reactive amino groups, with the aim of efficient Cr(vi) removal from aqueous solutions under column conditions, were easily prepared via combinational yet controllable sol–gel and freeze drying processes.

Synthesis, characterization, and highly acid-resistant properties of crosslinking β-chitosan with polyamines for heavy metal ion adsorption

A novel crosslinking modification of β-chitosan and successfully analyzed its fast adsorption characteristics for different heavy metal ions in highly acidic environments.