Cr Adsorption Capacity Research Articles

Polydopamine-functionalized natural cellulosic Juncus effusus fiber for efficient and eco-friendly Cr (VI) removal from wastewater

Biomass adsorbents have received extensive attention for the removal of heavy metals, but most of them had very limited adsorption capacity or needed to be modified with environmentally unfriendly chemicals to achieve acceptable performance. In this study, a green biomass adsorbent was fabricated using an abundant natural cellulosic Juncus effusus (JE) fiber and natural melanin polydopamine (PDA) through a simple mussel-inspired polymerization strategy. JE's unique hollow three-dimensional network structure provided an ideal scaffold substrate with abundant sites for dopamine polymerization deposition. This enabled the modified PDA@JE fibers to be rich in amino, hydroxyl, and other active adsorption sites, thereby endowing the PDA@JE fiber adsorbent with a promising Cr (VI) adsorption capacity of up to 145.8 mg / g. This capacity was significantly higher than that of most previously reported biomass-based adsorbents. Furthermore, 10–20 times the concentration of common interfering ions such as Ca2+, Cd2+, Cu2+, NO3-, Cl-, SO42- and PO43- had little effect on the adsorption efficiency. This study provided a green and general mussel-inspired strategy for preparing effective biomass adsorbent, offering a promising approach for utilizing natural cellulosic fibers in eco-friendly applications.

Chromium adsorption capacity from tannery wastewater on thermally activated adsorbent derived from kitchen waste biomass

Environmental sustainability has gained greater acceptance as an approach to achieving the objective of a secure ecosystem with a reliable management system. The best strategy for maintaining a healthy ecosystem is waste management. In this present study, chromium (Cr) adsorption capacity from real tannery wastewater on thermally activated adsorbent, equipped from Cucurbita moschata (pumpkin) peel is described. The adsorbent features were investigated by implementing Fourier transform infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), and pHpzc (point of zero charges) analysis. The Cr adsorption was identified by EDX analysis. SEM images were analyzed to record surface morphological modifications. The adsorbent has a pHpzc of 8.7. The thermally activated adsorbent is used to treat the wastewater, in a batch experiment with varying parameters including contact time, adsorbent dose, and pH. Under optimal circumstances, 50 mL of wastewater was mixed with 2.5 g of adsorbent, agitated for 10 min, allowed to settle, and then the Cr concentration was evaluated. The Cr level in untreated effluent and in filtrate was 3178.6 mg/L, and 12.1 mg/L, respectively. The capability of Cr adsorption and Cr removal efficiency was 3164.46 mg/g and 99.32%, correspondingly, at a pH of 7.2. The biochemical oxygen demand (BOD), chloride (Cl-), and chemical oxygen demand (COD) showed a decreasing percentage of 96.58%, 55.62%, and 95.01%. The Cr adsorption fitting for pumpkin peel adsorbent follows the Pseudo second-order (PSO) kinetic and Freundlich isotherm model. Thus, this investigation established the efficiency of using pumpkin peel as an adsorbent for Cr tanning effluent treatment.

Synthesis of Chitosan/Halloysite Nanotubes Composite Aerogel as Adsorbents

A novel chitosan/halloysite nanotubes composite aerogel (CS/HNTs) was prepared by incorporation of halloysite nantubes into crosslinked chitosan network via vacuum freeze drying. Nitrogen adsorption—desorption isotherms analysis show it has a specific surface area of 51.24 m2g–1 with an average pore diameter of 8.96 nm, the resulting CS/HNTs was used as an efficient adsorbent material for removal of Cr(VI) from water. The adsorption performance of CS/HNTs for Cr(VI) under different experimental conditions were studied. The adsorption experiments show that the adsorption capacity of CS/HNTs composite aerogel for Cr(VI) increases slightly with the increase of temperature and the optimum pH value for Cr(VI) adsorption is found at pH = 2. The maximum adsorption capacity of Cr(VI) was estimated to be 49.85 mg g–1 with the optimum adsorbent dose of 0.10 g at 30 °C. The adsorption kinetics of the assay exhibit a strong correlation with the mathematical model known as the pseudo—second—order equation. The experimental results exhibit a high level of conformity with the Langmuir isotherm, providing evidence of a state of equilibrium. Moreover, detailed computations have been conducted to ascertain crucial thermodynamic parameters such as the change in Gibbs free energy (ΔG°), modification in enthalpy (ΔH°), and variation in entropy (ΔS°). These calculated parameters provide compelling evidence that the adsorption of Cr(VI) onto CS/HNTs is a spontaneous process driven by thermodynamic favorability. Furthermore, the process is characterized by the absorption of heat from the surroundings, indicating an endothermic nature.

Cauliflower stem-derived biochar for effective adsorption and reduction of hexavalent chromium in synthetic wastewater: A sustainable approach

Contamination with hexavalent chromium (Cr(VI)) has drawn immense interest of national and global regulatory authorities due to its toxic and carcinogenic threat. Although various approaches have been applied for Cr(VI) removal from aqueous solution, adsorption has been proven to be the most promising method. Biochar, a cost-effective adsorbent, is one of the prominent adsorbents for environmental remediation that can be produced from different biowaste precursors. Cauliflower (Brassica oleracea) is one of the common vegetables around the world. The cauliflower stem is not commonly edible and therefore discarded as waste. In this work, the cauliflower waste-based biochar, with and without chemical modification, was prepared from a simple oxygen-limited pyrolysis. With the assistance of TGA, DSC, and SEM-EDS, the effect of pyrolysis temperature on the generation of biochar, reaction energy as well as reaction temperature, and morphology of the prepared biochar were analyzed. Pyrolytic temperature of 450°C was found to be the best for tailoring biochar. Chromium(VI) removal from the synthetic solution was performed by using biochar as an adsorbent. H3PO4-activated biochar (PBC) performs better than other biochar. The adsorption kinetics, isotherm, and thermodynamic parameters were studied in batch-wise experiments by varying the contact time, initial Cr(VI) concentration, and operating temperature. The Cr(VI) removal on PBC biochar follows a pseudo-second-order kinetics and fits with Langmuir isotherm resulting in Cr(VI) adsorption capacity of 64.10 mg/g. The thermodynamic parameters and XPS analysis of spent biochar adsorbent reveal that Cr(VI) reduction to Cr(III) happens simultaneously during the removal process. The spent biochar can be regenerated using 0.1 M NaCl and used for several cycles. Thus, the cauliflower stem-derived biochar can be effectively applied for Cr(VI) removal for multiple cycles through regeneration, which would contribute to the circular economy for environmental purposes.

Insight into a novel composite biosorbent by incorporating chitosan into electrolytic manganese residue for Cr(VI) removal from aqueous solutions

A novel adsorbent was obtained by using a typical solid waste electrolytic manganese residue (EMR) and a green material chitosan (CS) for the Cr(VI) removal from the aqueous environment. The tagged mineral phases and elements in the EMR were successfully detected in the synthesized composites according to the XRD and SEM-EDS results. Furthermore, the characteristic peak of metal and hydroxyl group binding appeared at 779 cm−1, and the Si-O bending vibration at 454 cm−1 in the FT-IR pattern of the synthesized composites indicating the functional Fe and Mn hydroxyl groups and the silicates in EMR were all successfully grafted onto the CS. This obtained composite showed a high Cr adsorption capacity with a maximum adsorption capacity of 133.64 mg·g−1, and the Cr(VI) removal rate could maintain a high level of 94.6 % after five adsorption–desorption cycles. Its adsorption isotherm matched with the Langmuir model, and the adsorption kinetics followed the pseudo-first-order model. According to the response surface CCD model, the optimal conditions for Cr(VI) removal were pH at 2, contact time at 53.0 min, and initial concentration at 100 mg·L-1. For the removal mechanism, Cr(VI) anions could be easily electrostatic adsorbed by –NH3+, or chelated with the Fe and Mn oxides. Meanwhile, partial Cr(VI) (∼35.7 %) was reduced into Cr(III) by the abundant hydroxyl group of chitosan on the surface of the as-obtained materials, and then the free Cr(III) could be easily chelated with three chitosan monomers. This work opens up a novel and green methodology to modify chitosan and reuse the solid waste EMR.

Efficient removal of hexavalent chromium by nano-cerium-based adsorbent: The critical role of valence state and oxygen vacancy

Cerium-based adsorbents have been gradually used for the adsorption removal of highly toxic Cr(VI) from wastewater due to their low toxicity and wide working pH. However, the intrinsic properties of adsorbents contribute significantly to their adsorption performance, and the relationship between them needs to be clarified. Herein, series of nano-cerium based adsorbents (Ce@Cs) with different surface defects and Ce(III) content were prepared to explore their effects on the Cr(VI) adsorption capacity. Results showed that the optimal Ce@C performed well over a wide pH range of 2.0–12.0, and the calculated Cr(VI) adsorption capacity reached 302.43 mg/g at 45 ℃. Ce(III) and surface defects in cerium-based adsorbents exhibited an important influence on the Cr(VI) adsorption performance of Ce@Cs, and their contents showed a good positive correlation with the Cr adsorption capacity (R2 =0.988 and 0.827). A series of evidences confirmed that the generated Ce(III) and oxygen vacancies could provide more sufficient coordination number to promote Cr(VI) complexation with Ce@Cs and lower the impedance of Ce@Cs to improve the reduction of Cr(VI) to low-toxic Cr(III). This work provides new insights into the Cr(VI) adsorption using cerium-based adsorbents, which helps to improve their potential in the purification of Cr(VI)-containing wastewater.

Synthesis and characterization of bio-nanocomposite based on chitosan and CaCO3 nanoparticles for heavy metals removal

Water is a vital component of life; therefore, it is critical to have access to pure water for various life-sustaining activities including agriculture and human consumption. An eco-friendly nanocomposite based on chitosan (Cs) and nanomaterials (CaCO3-NPs) were combined to amalgamate the advantages of biopolymers and nanomaterials to overcome the problems of instability, poor mechanical properties, and low removal percentage of biopolymers. The as-prepared samples were characterized and were used for the removal of heavy metal from wastewater. X-ray diffractometer, Fourier transform infrared spectroscopy, and transmission electron microscope were used to distinguish the prepared absorbents. The absorption of the heavy metals by as-prepared samples was examined at different conditions. The kinetic and isotherm models of the adsorption process were also studied. The data showed that the removal percentages of Cd, Cu, Pb, Zn, Cr and Ni by the composite were 98.0, 94.8, 99.0, 97.9, 97.4 and 98.3 %, respectively. The kinetic and isothermal studies showed that the absorption of these metal ions by the samples obeyed a pseudo-second-order mechanism and Langmuir isotherm model, respectively. In addition, the maximum adsorption capacities of Cd, Cu, Pb, Zn, Cr, and Ni ions by as-prepared nanocomposite were 83.33, 47.84, 98.03, 89.28, 62.11, and 63.69 mg/g, respectively.

Fabrication of a novel chitosan/polyaspartic acid composite: A pH-tunable sorbent for efficient adsorption of heavy metal ions and dyes from water

Heavy metal ions and dyes pollutants have caused significant environmental pollution, posing a threat to human life and ecosystems. Herein, a novel pH-tunable composite adsorbent CS-PA integrated chitosan (CS) with polyaspartic acid (PA) was fabricated. The morphology and structure of CS-PA were characterized. The results indicated that PA was successfully introduced into CS, and the particle size of CS-PA was irregular and porous with a pore diameter of 10–100 μm. Adsorption experiments demonstrated that CS-PA was efficient in removing various charge properties of heavy metal ions and dyes: cationic [Cu(II) and methylene blue (MB)], anionic [Cr(VI) and congo red (CR)] species. Also, the sorption process in the experimental conditions fitted pseudo-second-order kinetic model and Langmuir isotherm. The maximum adsorption capacity of Cu(II), MB, Cr(VI), and CR were 133.33, 172.41, 138.89, and 178.57 mg/g, respectively, based on the Langmuir isotherm study. CS-PA has strong selective sorption ability for MB over Cu(II) under the acidic condition and CR over Cr(VI) at a weakly alkaline solution in their binary systems. Multiple attractions occurred in adsorbing Cu(II), MB, Cr(VI), and CR because of their diverse charge properties and structural characteristics. Finally, regeneration studies further demonstrate that <5 % in sorption efficiency on CS-PA after 5 cycles. Thus, this composite adsorbent is tunable, versatile and potentially applicable to wastewater treatment.

Effect of bentonite and composite-modified carbonized fiber on Cu(II) and Cr(VI) adsorption by purple soil

Bentonite and different composite-modified carbonized fibers were used to amend purple soil (PS) to investigate the effects of material combination on Cu(Ⅱ) and Cr(Ⅵ) adsorption by PS. The thermodynamic and kinetic characteristics of Cu(Ⅱ) and Cr(Ⅵ) adsorption were investigated by batch treatment, and the adsorption changes of Cu(Ⅱ) and Cr(Ⅵ) on the amended PS under various pH, ionic strength, and temperature were compared. Results were as follows. (1) The adsorption isotherms of Cu(Ⅱ) and Cr(Ⅵ) on the amended PS conformed to the Freundlich model, and the maximal adsorption capacities for Cu(Ⅱ) and Cr(Ⅵ) ranged 343.33–741.89 and 151.11–311.39 mmol/kg, respectively. (2) The Cu(Ⅱ) and Cr(Ⅵ) adsorption by the amended PSs was mainly multilayer adsorption and primarily driven by chemisorption. (3) High and low pH levels were beneficial to Cu(Ⅱ) and Cr(Ⅵ) adsorption, respectively. The amount of Cu(Ⅱ) and Cr(Ⅵ) adsorption initially increased and then decreased with the increase in ionic strength, with the maximum adsorption observed at 0.1 mol/L ionic strength. (4) High temperature was conducive to the Cu(Ⅱ) and Cr(Ⅵ) adsorption, which was determined as a spontaneous, endothermic, and entropy-increasing process.

PANI/MCM-41 adsorption for removal of Cr(VI) ions and its application in enhancing electrokinetic remediation of Cr(VI)-contaminated soil.

In this study, a polyaniline/mesoporous silica (PANI/MCM-41) composite material that can be used as a filler for permeable reactive barrier (PRB) was prepared by in situ polymerization. Firstly, the adsorption capacity of PANI/MCM-41 on Cr (VI) in solution was investigated. The results show that the prepared PANI/MCM-41 exhibits a significant Cr (VI) adsorption capacity (~ 340mg/g), and the adsorption process is more accurately described by the Langmuir isotherm and pseudo-second-order kinetic model. The thermodynamic functions evidenced that the Cr(VI) adsorption was an endothermic spontaneous process. In addition, adsorption-desorption cycle experiments proved the excellent reusability of the material. Subsequently, the material was utilized as a filler in the PRB for the remediation of Cr(VI)-contaminated soil using electrokinetic-permeable reactive barrier (EK-PRB) technology. The results show that compared with traditional electrokinetic remediation, the use of PANI/MCM-41 as an active filler can enlarge the current during remediation and enhance the conductivity of soil, which increases the removal rates of total Cr and Cr(VI) in soil (17.4% and 10.2%).

Synthesis, characterization and chromium ions adsorption of the nanocomposite calix[6]arene-tetraester-silylated-clay

In this investigation, the nanocomposite (SC-C[6]) is prepared and characterized. The adsorption of chromium ions onto SC-C[6] as a function of pH solution, contact time, initial Cr(VI) concentration and effect of temperature is studied through batch experiments. The removal towards Cr(VI) increased with decreasing of pH solution, from 7.5 to 1.5, while it decreased with increasing of initial Cr(VI) concentration and temperature. The maximum Cr(VI) adsorption capacity for SC-C[6] reached at pH 1.5, 10.4mgL−1, 25 °C and 1 h, and the mechanisms may include electrostatic interaction and hydrogen bonding between the nanocomposite and Cr(VI) anions.

Enhanced synergistic removal of Cu(II) and Cr(VI) with multifunctional biomass hydrogel from strong-acid media

Simultaneous recovery of heavy metal ions (HMIs) such as Cu(II) and Cr(VI) from strong-acid media was a great challenge due to the inhibition of protons. Herein, a novel biomass hydrogel (CMC/PEI-PD) containing various groups (bis-picolylamine, amino, and hydroxyl groups) was newly prepared by a facile two-step process. The static experiments relating pH, kinetics and isothermal co-adsorption confirmed the synergistic effect towards Cu(II) and Cr(VI) consistently. Specifically, the adsorption capacities of Cu(II) and Cr(VI) at pH 2.0 increased by 23.73% and 40.18% in comparison with the single systems. Moreover, coexistence of inorganic anions and cations could further increase the adsorption of Cu(II) and Cr(VI) by 59.90% and 43.39%, respectively. At the same time, the adsorption and desorption ratios for both HMIs remained stable. The superior performance came from the two dominant mechanisms of co-removal. On the one hand, Cu(II) chelated by bis-picolylamine group attracted Cr(VI) in the form of cation bridge, thus promoting Cr(VI) adsorption. On the other hand, the protonated amine group attracted Cr(VI) by electrostatic interaction and weakened the inter-cationic repulsion by electrostatic shielding, thus promoting Cu(II) adsorption. In addition, the dynamic column experiment towards simulated acidic electroplating wastewater involving Cu(II)–Cr(VI)–Ni(II) certified the high efficiency and feasibility of the co-removal. Therefore, CMC/PEI-PD owned great potential in the separation of typical HMIs even directly from strong-acid media.

One-Step Fabrication of Recyclable Konjac Glucomannan-Based Magnetic Nanoparticles for Highly Efficient Cr(VI) Adsorption.

Recently, the natural polymer polysaccharide konjac glucomannan (KGM) has received attention as a promising adsorbent in water treatment due to its low toxicity, cost-effectiveness and biocompatibility. However, the high-level water absorbency of KGM makes it difficult to recover in water treatment. In this study, by combining KGM with magnetic nanoparticles, KGM-based magnetic nanoparticles (KGM-Fe3O4 NPs) with excellent adsorption properties and recyclability for heavy metals were prepared using an one-step precipitation method. The as-prepared KGM-Fe3O4 NPs have a spherical morphology of superparamagnetism with a small particle size (ca. 7.0 nm) and a large specific surface area (160.1 m2·g-1). Taking Cr(VI) as the target heavy metal ion, the above nanoparticles have a high adsorption capacity and fast adsorption rate for Cr(VI). The pseudo-second order kinetic model is more suitable to describe the adsorption process of Cr(VI) by KGM-Fe3O4 NPs, and the maximum adsorption capacity of Cr(VI) onto KGM-Fe3O4 NPs was calculated to be 41.67 mg·g-1 using the Langmuir isotherm model. In addition, KGM-Fe3O4 NPs with adsorbed heavy metal ions can be quickly recovered from a solution, regenerated, and reused in the next cycle. KGM-based Fe3O4 nanoparticles are promising adsorbents that show significant reusability for the removal of metal ions in water and wastewater treatment.

Amino-Functionalized Silica@Resorcinol-Formaldehyde Nanocomposites for the Removal of Cr(VI) from Aqueous Solutions.

Amino-functionalized silica@resorcinol-formaldehyde nanocomposites (NH2-SiO2@RF) were synthesized for the removal of Cr(VI) from aqueous solutions using the sol-gel technique with two simple preparation steps, including the one-pot synthesis of SiO2@RF using the Stöber method and (3-aminopropyl)triethoxysilane (APTES) modification. The morphology, particle size, functional group, and thermal stability of the obtained nanocomposites were systematically characterized, with the results indicating a uniform sphericity with a particle size of 200 nm and high thermal stability. The adsorption results demonstrated that the preferred pH value was 2, and the data were well fitted with the Langmuir and Temkin isotherm models and quasi-second-order kinetic equation, indicating a high adsorption capacity. The maximum Cr(VI) adsorption capacity from the nonlinear form of the Langmuir model was 272.6 mg·g-1. The intra-particle diffusion model accurately described the adsorption of Cr(VI) onto NH2-SiO2@RF. The changes in Gibb's free energy, enthalpy, and entropy revealed that Cr(VI) adsorption onto NH2-SiO2@RF was a spontaneous and endothermic process. Furthermore, high selectivity was demonstrated in the material for the removal of Cr(VI) from commonly coexisting ions. The obtained nanocomposites had good regeneration properties and maintained a removal rate above 85% in the fifth adsorption-desorption experiments. Moreover, under the optimized adsorption conditions, the obtained nanocomposites were preliminarily applied to tannery wastewater, demonstrating an excellent removal effect, which indicates their potential application value.

Carbon Microspheres with Cr(VI) Adsorption Performance were Prepared by In-situ Hydrothermal Carbonization Method

Biochar material is a renewable adsorbent widely used for treating contaminated wastewater. The hydrothermal carbon (HTC) were prepared from low polymeric sugars and low concentration glucose under hydrothermal carbonization reactions without using dispersants. The composition and structure of the biochar produced were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Raman spectroscopy (Raman), and N2 adsorption-desorption, indicating that amorphous graphitic carbon was obtained. Experimental results from the static adsorption of Cr(VI)-contaminated wastewater showed that HTCP-2 exhibited the highest adsorption capacity for Cr(VI), with a maximum adsorption capacity of 22.62 mg.g−1.The adsorption Cr(VI), MB, and RhB by the synthesized biochar all conformed to the pseudo-second-order kinetic model and Freundlich isotherm, suggesting a multilayer chemical adsorption process. Additionally, the synthesized HTC surface is enriched with a significant amount of oxygen-rich functional groups, which also has good adsorption performance for cationic dyes. Furthermore, the test results of fluorescence, photocurrent, and impedance indicate that HTCP-2 possesses the ability to generate and separate photoinduced charge carriers. This implied that HTCP-2 can be used for the preparation of adsorption photocatalysts, which effectively remove environmental pollutants through the synergistic effect of adsorption-photocatalysis. This study provides a research foundation for advancing water treatment technologies. Copyright © 2023 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Effective removal of Cr(VI) in water by bulk-size polyaniline/polyvinyl alcohol/amyloid fibril composite beads.

With the rapid expansion of industrial activities, chromium ions are discharged into the environment and cause water and soil pollution of various extents, which seriously endangers the natural ecological environment and human health. In this study, polyaniline/polyvinyl alcohol/amyloid fibril (PANI/PVA/AFL) composite gel beads (PPA) were prepared from polyaniline and amyloid fibrils with HCl as doping acid and PVA as a cross-linking agent. The results showed that PPA was an irregular composite bead with a diameter of 6 mm. The adsorption of Cr(VI) on the PPA gel beads followed the pseudo-second-order kinetics model, suggesting that chemical reactions were the controlling step in the Cr(VI) adsorption process. Though the Redlich-Peterson isotherm model had the best fit for the adsorption data, the isothermal adsorption process can be simplified using the Langmuir model. The maximum adsorption capacity for Cr(VI) in water was 51.5 mg g-1, comparable to or even higher than some PANI-based nanomaterials. Thermodynamic parameters showed that the adsorption process was a spontaneous, endothermic, and entropy-increasing process. Microscopic analysis revealed that the capture of Cr(VI) on PPA was mainly governed by electrostatic attraction, reduction, and complexation reactions. PPA can be used as a kind of effective remediation agent to remove Cr(VI) in water.

Simultaneous Cr(VI) reduction and Cr(III) sequestration in a wide pH range by using magnetic chitosan-based biopolymer

The simultaneous reduction of Cr(VI) and sequestration of the resulting Cr(III) in one process is highly desirable as a cost-effective and environmental-friendly approach for the decontamination of Cr(VI)-polluted wastewater. However, most of the existing adsorptive materials are only effective in low pH environments (pH = 1–3), severely restricting the adsorption efficiency and cost effectiveness. Herein, we proposed a chitosan-based magnetic porous microsphere (PPy@PMCS) for simultaneous Cr(VI) reduction and Cr(III) sequestration in a wide pH range. Benefiting from its abundant interaction sites, Cr(VI) was effectively adsorbed on the surface and then immediately reduced to Cr(III) with much lower toxicity. Most importantly, the resulting Cr(III) was in-situ sequestrated by the complexation of chitosan matrix. As a result, PPy@PMCS exhibited a maximum Cr(VI) adsorption capacity of 330.42 mg/g at pH 2.0 and an adsorption capacity of 167.82 mg/g even at near neutral pH (6.0), which is superior to most reported adsorbents. Furthermore, the exhausted PPy@PMCS can be rapidly separated from solutions under an external magnetic field and facilely regenerated. The proposed novel biopolymer-based material shows great application potentials in wastewater treatment.

Facet-Dependent Competitive Adsorption Mechanisms of Chromate and Oxalic Acid on γ-FeO(OH) Nanocrystals.

Facet-dependent toxic metal adsorption of iron oxides widely occurred in natural environments. It is known that organic acids can alter the adsorption behaviors of trace elements by cooperative or competitive effects. However, the coadsorption mechanisms of the specific facets are still not fully understood. In the current investigation, Cr(VI) adsorption onto the lepidocrocite (γ-FeO(OH))-exposed facets in the presence of oxalic acid (OA) was studied using macroexperiments, in situ attenuated total reflectance Fourier transform infrared spectroscopy, X-ray adsorption fine structure, and density functional theory calculations. Rod-like lepidocrocite (R-LEP) with a high ratio of {001}/{010} facet showed excellent Cr(VI) adsorption capacity than that of plate-like lepidocrocite (P-LEP, the dominant facet is {010}) in the absence/presence of OA. Interestingly, OA reacted with R-LEP would be easier to diminish Cr(VI) adsorption than with P-LEP. The competitive adsorption occurred on the {001} facet due to the formation of inner-sphere OA configurations (monodentate mononuclear and bidentate mononuclear structures) and a bidentate binuclear Cr(VI) complex. However, OA coordinated with {010} facets via the outer-sphere complexes, while Cr(VI) could form a protonated monodentate binuclear configuration. These observations suggest that the competitive adsorption processes between OA and Cr(VI) exhibit facet dependence. Furthermore, lepidocrocite-exposed facets determine the interfacial interactions and geochemical behaviors of Cr(VI) in polluted environments.

Research on the Effect of Coordinated Remediation of Heavy Metal Pollution in Roadbed Slope Soil by Combining Biochar with Reed

This study investigated the effect of modified activated carbon combined with plants on the coordinated restoration of heavy metal pollution in roadbed slope soil. This study used wheat husks as raw materials, potassium hydroxide and permanganate as modifiers, and aquatic plant reeds to remediate heavy metals in roadbed slope soil. The adsorption amount and removal rate of heavy metals Cu, Pb, and Cr were investigated under adsorbent dosage, adsorbent physical structure parameters, different pH environments, and ternary composite heavy metal body experimental conditions. According to the experimental results, after activation, the total pore volume and total specific surface area of biochar increased significantly, 21.4 and 111.7 times those before activation, respectively. The order of removal rates of heavy metal ions by different adsorbents was Pb &gt; Cr &gt; Cu. When the adsorbent was biochar–reed joint remediation, the maximum adsorption capacity of Cu, Pb, and Cr was 31.25, 136, and 52.3 mg/g. The adsorption capacities of the two adsorbents (biochar and biochar–reed) for Pb were the highest, and the adsorption sensitivity of the adsorbents for Pb was higher than that for Cr and Cu. Under single-component conditions, the adsorption capacity of biochar to heavy metals Cu, Pb, and Cr was ranked as Pb &gt; Cu &gt; Cr. In a ternary solution system, the adsorption capacity of biochar to heavy metals Cu, Pb, and Cr was ranked as Cu &gt; Cr &gt; Pb. Considering the adsorption capacity and removal rate of nutrient-rich substances, as well as the cost and effect of the use of the adsorbent, the recommended amount of the adsorbent was 4–6 g.

Selective removal of Cr(VI) using polyvinylpyrrolidone and polyacrylamide co-modified MoS2 composites by adsorption combined with reduction

Cr(VI), one of the most hazardous metal pollutants, poses significant threats to the environment and human health. Herein, a novel MoS2 composite (MoS2/PVP/PAM) modified by polyvinylpyrrolidone (PVP) and polyacrylamide (PAM) was synthesized to enhance the removal of Cr(VI). Characterization analysis including SEM, XRD, FTIR, and XPS indicated that PVP and PAM could increase the interlayer spacing and the dispersibility of MoS2, and introduce pyrrolic N and amino functional groups. The batch experiments showed that MoS2/PVP/PAM represented excellent Cr(VI) removal performance over a wide pH range, and exhibited a significantly higher maximum Cr(VI) adsorption capacity (274.73 mg/g, at pH 3.0, and 298 K) than pure MoS2. The adsorption of Cr(VI) followed Langmuir and pseudo-second-order kinetic model, which was a homogeneous monolayer chemisorption process. MoS2/PVP/PAM showed stable removal of Cr(VI) in the presence of humic acid (HA), interfering cations and anions at different concentrations. Moreover, it had excellent selectivity for Cr(VI) (Kd value of 1.69 × 107 mL/g) when coexisting with a variety of competing ions. Multiple characterization revealed that Cr(VI) was reduced to low toxicity Cr(III) by Mo4+ and S2−, and then chelated on the surface of the adsorbent by pyrrolic N. This research expanded the design concept for MoS2 composites by demonstrating the potential of MoS2/PVP/PAM as a promising material for selective elimination of Cr(VI) in water.