Adsorption mechanism of Cr(VI) on woody-activated carbons

Valorizing Xanthoceras Sorbifolia Bunge seed coats: A novel lignin-based activated carbon for effective malachite green adsorption and wastewater treatment

In order to increase the added value of Xanthoceras Sorbifolia Bunge (XSB) and to obtain green biomass activated carbon with abundant pores for efficient MG removal, this study was the first to prepare XSB-based high-performance activated carbon using KOH activation. Activated at temperatures between 600 and 800 °C, XSBAC-800 exhibited the highest specific surface area (1580 m2/g) and pore volume (0.732 cm3/g), leading to superior MG adsorption. The maximum adsorption capacity (Qm) of XSBAC-800 for MG was 1241.25 mg/g at 313 K, pH=7, and 0.2 g/L concentration. The adsorption process was spontaneous, endothermic, and driven by physical mechanisms such as pore filling and electrostatic interactions. XSBAC-800 also demonstrated excellent reusability, maintaining a removal percentage above 89 % after five regeneration cycles. These findings highlight the effectiveness of XSB-based activated carbon as a sustainable adsorbent for the removal of MG dyes, providing a promising solution for water pollution control.

Exploring the thermal decomposition of cigarette butts and its role in chromium adsorption processes

Smoked cigarette butts (SCB) can serve as precursors for producing activated carbon (AC) due to their high carbon content, which originates from cellulose acetate and other components retained during tobacco combustion. This study examines the pyrolysis process and char formation from SCB residue. Kinetic analysis of char formation is conducted using thermogravimetry with heating rates of 5, 10, and 15 °C/min. Mathematical models, including Kissenger-Akahira-Sunose (KAS), Starink (STK), and Ozawa-Flynn-Wall (OFW) are applied to determine thermodynamic parameters, such as activation energy. The resulting char undergoes KOH activation to enhance surface area and pore volume, resulting in BET surface area and micropore volume values of 433 m2/g and 0.25 cm3/g, respectively. The activated carbon is then used for Cr(VI) removal, exhibiting a maximum adsorption capacity (Qmax) of 55.8 mg/g, as determined by the Langmuir model.

New insights into sustainable in-situ fixation of heavy metals in disturbed seafloor sediments

To address the issues of plume formation and heavy metal ion release during deep-sea mining operations, this study employed multi-sourced mineral composite roasting materials (MMCCM) of varying sizes. An in-situ capping technique was applied within a simulated system to immobilize heavy metals in contaminated sediments. The results demonstrated that capping with MMCCM of different sizes significantly suppressed the upward migration of Cu, Co, and Ni from sediments into the overlying seawater following disturbance. Ion diffusion was identified as a key mechanism driving heavy metal migration. By calculating the release rates of heavy metals during both the disturbed and undisturbed phases, it was found that the application of MMCCM induced a negative diffusion of heavy metals, indicating that the MMCCM-sediment layer functioned as a "sink" for heavy metals. FTIR and XPS analysis showed that the primary mechanisms for heavy metal removal by MMCCM were electrostatic attraction and complexation-precipitation. Additionally, capping with MMCCM facilitated the transition of heavy metals from labile to stable forms within the sediments. Through comprehensive evaluation, the long-term effectiveness of the fixed effects was demonstrated as follows: large MMCCM (L@MCM) > medium MMCCM (M@MCM) > small MMCCM (S@MCM) > powder MMCCM (P @ MCM). Finally, we proposed future research directions and introduced the DQSE framework for the sustainable application of MMCCM. Based on the above findings, this study provides new insights and research references for the in-situ immobilization of heavy metals and plume reduction during future deep-sea mining processes.

Adsorption of Cr(VI) Using Organoclay/Alginate Hydrogel Beads and Their Application to Tannery Effluent

The tanning industry is among the most environmentally harmful activities globally due to the pollution of lakes and rivers from its effluents. Hexavalent chromium, a metal in tannery effluents, has adverse effects on human health and ecosystems, requiring the development of removal techniques. This study assessed the efficacy of organobentonite/alginate hydrogel beads in removing Cr(VI) from a fixed-bed adsorption column system. The synthesized organobentonite (OBent) was encapsulated in alginate, utilizing calcium chloride as a crosslinking agent to generate hydrogel beads. The effects of the volumetric flow rate, bed height, and initial Cr(VI) concentration on a synthetic sample were analyzed in the experiments in fixed-bed columns. The fractal-like modified Thomas model showed a good fit to the experimental data for the asymmetric breakthrough curves, confirmed by the high R2 correlation coefficients and low χ2 values. The application of organoclay/alginate hydrogel beads was confirmed with a wastewater sample from an artisanal tannery industry in Belén (Nariño, Colombia), in which a Cr(VI) removal greater than 99.81% was achieved. Organobentonite/alginate hydrogels offer the additional advantage of being composed of a biodegradable polymer (sodium alginate) and a natural material (bentonite-type clay), resulting in promising adsorbents for the removal of Cr(VI) from aqueous solutions in both synthetic and real water samples.

Biochar Composite with Enhanced Performance Prepared Through Microbial Modification for Water Pollutant Removal

This study developed mycelial biochar composites, BQH-AN and BQH-MV, with stable physicochemical properties and significantly improved adsorption capabilities through microbial modification. The results showed that the specific surface area and porosity of BQH-AN (3547.47 m2 g-1 and 2.37 cm3 g-1) and BQH-MV (3205.59 m2 g-1 and 2.46 cm3 g-1) were significantly higher than those of biochar BQH (2641.31 m2 g-1 and 1.81 cm3 g-1), which was produced without microbial treatment. In adsorption experiments using rhodamine B (RhB), tetracycline hydrochloride (TC), and Cr (VI), BQH-AN showed maximum adsorption capacities of 1450.79 mg g-1 for RhB, 1608.43 mg g-1 for TC, and 744.15 mg g-1 for Cr(VI). BQH-MV showed similarly strong performance, with 1329.85 mg g-1 for RhB, 1526.46 mg g-1 for TC, and 752.27 mg g-1 for Cr(VI). These values were not only higher than those of BQH but also outperformed most other biochar adsorbents. Additionally, after five reuse cycles, the pollutant removal efficiency of the mycelial biochar composites remained above 69%, demonstrating excellent regenerative ability. This study not only produced biochar with superior adsorption properties but also highlighted microbial modification as an effective way to enhance lignocellulosic biochar performance, paving the way for further biomass development.

Biosorption of Cr(VI) by Theobroma cacao pericarp

This paper reports a comprehensive study of Theobroma cacao pericarp (TCP) residues, which has been prepared, characterized, and tested as an inexpensive and efficient biosorbent of Cr(VI) from aqueous solutions. The maximum adsorption capacity of TCP obtained at optimal conditions (pH = 2, dose = 0.5 g L-1, C0 = 100 mg L-1) was qmax = 48.5 mg g-1, which is one of the highest values reported by the literature. Structural and morphological characterization has been performed by FTIR, SEM/EDX, and pHPZC measurements. FTIR analysis revealed the presence of O-H, -NH, -NH2, C = H, C = O, C = C, C-O, and C-C functional groups that would be involved in the Cr(VI) biosorption processes. The experimental equilibrium data of biosorption process were successfully fitted to non-linear Langmuir (R2 = 0.95, χ2 = 11.0), Freundlich (R2 = 0.93, χ2 = 14.8), and Temkin (R2 = 0.93, χ2 = 14.7) isotherm models. Kinetics experimental data were well adjustment to non-linear pseudo-2nd (R2 = 0.99, χ2 = 2.08)- and pseudo-1st-order kinetic models (R2 = 0.98, χ2 = 2.25) and also to intra-particle Weber-Morris (R2 = 0.98) and liquid film diffusion (R2 = 0.99) models. These results indicate that Cr(VI) biosorption on heterogeneous surfaces as well as on monolayers of TCP would be a complex process controlled by chemisorption and physisorption mechanisms. The thermodynamic results indicate that the Cr(VI) biosorption on TCP is a feasible, spontaneous, and endothermic process. TCP can be regenerated with NaOH and reused up to 3 times.

Superior comprehensive performance of modified activated carbon as a hexavalent chromium adsorbent

The presence of hexavalent chromium species (CrVI) in wastewater from manufacturing industries such as electroplating and leather production can pose serious health hazards. To address these concerns, this study developed a novel adsorbent based on activated carbon as the primary material to attract CrVI. Activated carbon has been modified with several other components to improve its comprehensive performance, including adsorption capacity, chemical stability, collectability, and reusability. Specifically, decoration with magnetite nanoparticles made it possible to collect the adsorbent magnetically and reuse it several times. On the one hand, the addition of chitosan not only increased the chemical stability of activated carbon, especially under acidic conditions, but also enhanced the Cr adsorption capacity at pH higher than 4, where adsorption of only activated carbon was significantly decreased, probably because the protonated amino groups attracted chromate anions. In addition, the co-existence of tannic acid did not increase the adsorption capacity significantly but appeared to promote the reductive adsorption of CrVI, where the reduction of CrVI means lowering the toxicity of Cr species. It was demonstrated that activated carbon modified with magnetite, chitosan, and tannic acid exhibited superior comprehensive performance that could be repeatedly used over a wide pH range as compared to the parent activated carbon.

Sorption of chromium from aqueous solutions using Fucus vesiculosus algae biosorbent

The presence of heavy metals in wastewater is an environmental concern and the current treatment procedures are very expensive so it is necessary to find effective and inexpensive biosorbents. In this study, Fucus vesiculosus was used as a biosorbent for the biosorption of Cr(III) ions from the aqueous solutions. Biosorption parameters, such as pH, adsorbent dose, contact time, and initial concentrations of Cr(III) had the most impact on the sorption process. The required pH value for sorption was 5, the biosorbent dose was 4.0 g/L, the contact time was seen to occur after 90 min, and the Cr(III) removal decreased from 98.9 to 92%. The maximum biosorption capacity of chromium was 14.12 mg/g. FTIR analysis of Fucus vesiculosus biomass before the sorption process contains carboxyl, amino, hydroxyl, alkyne, and carbonyl groups, and according to the analysis after the sorption process, it was found that Cr(III) metal ions were incorporated within the sorbent during the interaction with (=C-H) active functional groups. The biosorption data were found to be perfectly suited by Langmuir equilibrium isotherm model. According to the results of this study, Fucus vesiculosus is an effective biosorbent for the removal of Cr(III) from aqueous solutions.

Comparison of surface adsorption efficacies of eco-sustainable agro/animal biomass-derived activated carbon for the removal of rhodamine B and hexavalent chromium

Effective management and remediation strategies are crucial to minimize the impacts of both organic and inorganic contaminants on environmental quality and human health. This study investigates a novel approach utilizing cotton shell activated carbon (CSAC), rice husk activated carbon (RHAC), and wasp hive activated carbon (WHAC), produced through alkali treatment and carbonization under N2 atmosphere at 600 °C. The adsorption capacities of biomass-derived mesoporous activated carbons (CSAC, RHAC, WHAC) alongside macroporous commercial activated carbons (CAC) were evaluated for removing rhodamine B (Rh B) and hexavalent chromium (Cr6+). The CSAC exhibits remarkable adsorption efficiency (255.4 mg.g-1) for Cr(VI) removal, while RHAC demonstrates superior efficacy (174.2 mg.g-1) for Rh B adsorption. Investigating various optimal parameters including initial pH (pH 3 for Cr and pH 7 for Rh B), catalyst dosage (200 mg.L-1), and initial concentration (20 mg.L-1), the Redlich-Peterson isotherm model is applied to reveal a hybrid adsorption mechanism encompassing monolayer (chemisorption) and multilayer (van der Waals adsorption) processes. Kinetic analysis highlights the pseudo-second-order and Elovich models as the most suitable, suggesting physiochemisorption mechanisms. Thermodynamic analysis indicates the endothermic nature of the adsorption process, with increased randomness at the solid-solution interface. Isosteric heat investigations using Clausius-Clapeyron, Arrhenius, and Eyring equations reveal a heterogeneous surface nature across all activated carbons. Further confirmation of Rh B and Cr(VI) adsorption onto activated carbons is provided through FTIR, FESEM, and EDAX analysis. This study highlights the innovation and promise of utilizing biomass-derived activated carbons for effective pollutant removal.

Equilibrium, Kinetic, and Diffusion Mechanism of lead(II) and cadmium(II) Adsorption onto Commercial Activated Carbons

The adsorption of Pb(II) and Cd(II) on three commercial microporous activated carbons was analysed. Adsorption kinetics and statistics were investigated, and the results were described with different models. The highest values of the correlation coefficient R2 were obtained for the pseudo-second-order kinetics model for all ions tested and all sorbents used. The adsorption process was found to be determined by both diffusion in the liquid layer and intraparticle diffusion. The adsorption equilibrium is very well described by Langmuir, Temkin, Thoth or Jovanovic isotherm models. Based on the values of n from the Freundlich isotherm and KL from the Langmuir isotherm, the adsorption of cadmium and lead ions was found to be favourable. The highest monolayer capacities were obtained during the adsorption of lead ions (162.19 mg/g) and for cadmium (126.34 mg/g) for activated carbon WG-12. This carbon is characterised by the highest amount of acid functional groups and the largest specific surface area. The adsorption efficiency of the tested ions from natural water is lower than that from a model solution made from deionised water. The lowest efficiencies are obtained when the process occurs from highly mineralised water.

Behaviors and Mechanisms of Adsorption of MB and Cr(VI) by Geopolymer Microspheres under Single and Binary Systems

Geopolymers show great potential in complex wastewater treatment to improve water quality. In this work, general geopolymers, porous geopolymers and geopolymer microspheres were prepared by the suspension curing method using three solid waste products, coal gangue, fly ash and blast furnace slag. The microstructure, morphology and surface functional groups of the geopolymers were studied by SEM, XRD, XRF, MIP, FTIR and XPS. It was found that the geopolymers possess good adsorption capacities for both organic and inorganic pollutants. With methylene blue and potassium dichromate as the representative pollutants, in order to obtain the best removal rate, the effects of the adsorbent type, dosage of adsorbent, concentration of methylene blue and potassium dichromate and pH on the adsorption process were studied in detail. The results showed that the adsorption efficiency of the geopolymers for methylene blue and potassium dichromate was in the order of general geopolymers < porous geopolymers < geopolymer microspheres, and the removal rates were up to 94.56% and 79.46%, respectively. Additionally, the competitive adsorption of methylene blue and potassium dichromate in a binary system was also studied. The mechanism study showed that the adsorption of methylene blue was mainly through pore diffusion, hydrogen bond formation and electrostatic adsorption, and the adsorption of potassium dichromate was mainly through pore diffusion and redox reaction. These findings demonstrate the potential of geopolymer microspheres in adsorbing organic and inorganic pollutants, and, through five cycles of experiments, it is demonstrated that MGP exhibits excellent recyclability.

Hexavalent chromium ion removal from wastewater using novel nanocomposite based on the impregnation of zero-valent iron nanoparticles into polyurethane foam

In this study, we developed a novel nanocomposite, polyurethane foam impregnated with zero-valent iron nanoparticles (PU@nZVI), for the effective removal of chromium(VI) from various water sources. The characterization of nanocomposite (PU@nZVI) was performed by XRD, SEM-EDS, TEM and FT-IR techniques. Using the response surface methodology, we optimized the removal conditions, achieving an optimal pH of 2 and a dose of 0.5 g/L. The PU@nZVI demonstrated an excellent maximum adsorption capacity of 600.0 mg/g for Cr6+. The adsorption kinetics and isotherms were best described by the pseudo-second-order model and the Freundlich isotherm, respectively. Significantly, the nanocomposite removed 99.98% of Cr6+ from tap water, 96.81% from industrial effluent, and 94.57% from treated sewage wastewater. Furthermore, the PU@nZVI maintained its efficiency over five adsorption-desorption cycles, highlighting its reusability. These results suggest that the PU@nZVI nanocomposite is a highly efficient and sustainable option for chromium(VI) removal in water treatment applications.

Entrapment behaviours of trivalent and hexavalent chromium from aqueous medium using edible alkali-derived activated carbon of Eichhornia crassipes (water hyacinth)

The study examines the adsorption capabilities of an environmentally friendly activated carbon derived from a novel activating agent, i.e., an edible alkali prepared from black gram plant ash, for the removal of Cr(III) and Cr(VI) ions from an aqueous environment. The results of the systematic research show impressive removal efficiencies of 95.12% for Cr(III) ions and 99.6% for Cr(VI) ions. The kinetics and equilibrium data of the adsorption process confirm to the pseudo-second-order kinetics and Freundlich isotherm model. The thermodynamic analysis reveals the adsorption process as feasible and spontaneous across the temperature range of 298-313 K. The mechanism entails electrostatic attraction and adsorption of Cr(III) and Cr(VI) ions on oppositely charged surfaces and the participation of oxygen-containing functional groups on WHAC-BGA surface in the reduction of Cr(VI) to Cr(III). This study provides valuable insights for optimizing strategies to combat chromium contamination in water sources, offering a sustainable solution with the potential for real-world application.

Characterizing Bacillus globigii as a Bacillus anthracis surrogate for wastewater treatment studies and bioaerosol emissions

This study characterized Bacillus globigii (BG) as a Bacillus anthracis Sterne (BAS) surrogate for wastewater treatment-related studies of UV inactivation, adsorption onto powdered activated carbon (PAC), and bioaerosol emission. The inactivation of BG was faster than that of BAS in DI water (pseudo first-order rate constants of 0.065 and 0.016 min-1 respectively) and in PBS solution (0.030 and 0.005 min-1 respectively). BG was also removed more quickly than BAS by PAC adsorption in DI (0.07 and 0.05 min-1 respectively) and in PBS (0.09 and 0.04 min-1 respectively). In DI, BG aggregated more (P < 0.05) than BAS when the pH was 7 or greater but there were no statistically significant differences in NaCl solution. Spore aggregation was also studied with extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) models. Less than 1% of all spores were released as bioaerosols, and there was no significant difference (P > 0.05) in emission between BG and BAS. To the author's knowledge, this study is the first to demonstrate that BG is a suitable surrogate for BAS for bioaerosol emissions, but a poor surrogate for both UV inactivation and PAC adsorption. These results can be used to understand the ability of BAS to act as a surrogate for BA Ames because of its genetic and morphological similarities with BAS.

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.

Efficient Photocatalytic Reduction of Hexavalent Chromium by BiVO4-Decorated MXene Photocatalysts and Their Charge Carrier Dynamics

The synergistically MXene (Ti3C2Tx) co-catalyst-decorated BiVO4-based heterostructured photocatalysts have been synthesized by a hydrothermal approach with varied loading concentrations of MXene (Ti3C2Tx) to drive the hexavalent chromium reduction efficiently. The formation of the heterostructured photocatalyst was confirmed by the appearance of X-ray diffraction (XRD) peaks corresponding to the monoclinic BiVO4 phase and MXene (Ti3C2Tx) and also the antisymmetric (834 cm-1) and symmetric stretching (715 cm-1) of tetrahedral VO4 and D (1330 cm-1) and G (1570 cm-1) bands corresponding to MXene (Ti3C2Tx) in the Raman spectrum. The worm-like structures of BiVO4 nanocrystals grew onto the lamellar sheets of MXene (Ti3C2Tx), as shown by field emission scanning electron microscopy (FESEM), and has an increased surface area of 15.62 m2g-1 in the case of BVO-20-TC. X-ray photoelectron spectroscopy (XPS) analysis confirms the presence of V5+ and Ti3+states, and the uniform distribution of BiVO4 nanocrystals over lamellar sheets of MXene (Ti3C2Tx) is evident from energy-dispersive X-ray (EDX) analysis. The ultraviolet-diffuse reflectance spectroscopy (UV-DRS) spectra suggest a decrease in the band gap energy of BVO-20-TC to 2.335 eV, promoting a higher degree of visible light harvesting. Upon optimization, by varying the pH, the amount of the photocatalyst, and the concentration of Cr(IV), BVO-20-TC exhibits the highest photocatalytic efficiency (96.39%) while using a Cr(VI) concentration of 10 ppm at pH 2 and 15 mg of the photocatalyst, and the photoreduction of Cr(VI) to Cr(III) follows the pseudo-first-order reaction. The decrease in the PL intensity in BVO-20-TC reveals a faster transfer of electrons from MXene (Ti3C2Tx) to BiVO4. Further, the higher degree of band bending at the BiVO4/MXene (Ti3C2Tx) heterojunction, revealed from the Mott-Schottky analysis, facilitates efficient charge transfer and eventually faster and efficient photoreduction of Cr(VI) to Cr(III). The reusability and stability test undertaken for BVO-20-TC reveals that even after five cycles, the Cr (VI) photoreduction efficacy is retained. This work provides insights into photoreduction of Cr (VI) by using such heterostructures.

Mesoporous Materials for Metal-Laden Wastewater Treatment

Rapid technological, industrial and agricultural development has resulted in the release of large volumes of pollutants, including metal ions, into the environment. Heavy metals have become of great concern due to their toxicity, persistence, and adverse effects caused to the environment and population. In this regard, municipal and industrial effluents should be thoroughly treated before being discharged into natural water or used for irrigation. The physical, chemical, and biological techniques applied for wastewater treatment adsorption have a special place in enabling effective pollutant removal. Currently, plenty of adsorbents of different origins are applied for the treatment of metal-containing aqueous solution and wastewater. The present review is focused on mesoporous materials. In particular, the recent achievements in mesoporous materials' synthesis and application in wastewater treatment are discussed. The mechanisms of metal adsorption onto mesoporous materials are highlighted and examples of their multiple uses for metal removal are presented. The information contained in the review can be used by researchers and environmental engineers involved in the development of new adsorbents and the improvement of wastewater treatment technologies.