Polypyrrole-Bentonite composite as a highly efficient and low cost anionic adsorbent for removing hexavalent molybdenum from wastewater

Green crosslinking with oxidized sodium alginate for enhanced Mo(VI) adsorption in alginate-based membranes

The use of green and nontoxic crosslinking agents in material synthesis is both important and challenging. In this study, oxidized sodium alginate (OSA) was synthesized via sodium periodate oxidation and used for the preparation of sodium alginate/polyethyleneimine membranes. FTIR confirmed the presence of aldehyde groups after the oxidation of sodium alginate. In addition, the appearance of the Schiff base structure of the OSA membrane indicated that crosslinking was successful. SEM revealed the roughness and porosity of the surfaces of OSA membranes, which were favourable for the adsorption of Mo(VI). XPS indicated that adsorption was potentially related to the coordination reactions of amino groups on PEI and to electrostatic attraction. Permeation characterization revealed that the membrane had excellent mechanical strength and durability. The maximum adsorption amount calculated via the Sips equation was 452.112 mg g-1 at 50 °C. The adsorption process followed pseudo-second-order kinetics and was spontaneous, as confirmed by thermodynamic analysis. Competing ions, simulated of industrial wastewater and cyclic desorption experiments confirmed the good practical application of this material. Overall, the OSA membrane showed good performance in removing and recovering Mo(VI) from aqueous solutions, and OSA was determined to be a green crosslinker comparable to glutaraldehyde (GA).

Adsorption behavior of Re(VII) and Mo(VI) on mesoporous silica materials functionalized with nitrogen-containing groups

The deep removal of molybdenum (Mo) from rhenium (Re)-containing solutions is critical for mitigating molybdenum toxicity and enabling sustainable recycling of strategic rhenium resources. Selective adsorbents play a unique and critical role in this process, where the functional groups serve as decisive factors governing the Mo(VI)/Re(VII) separation efficiency. Herein, MCM-41 was grafted with primary amine, quaternary ammonium, and imidazole groups to systematically compare their Mo/Re adsorption and separation performance. The XRD patterns show a broadened (110) diffraction peak centered at approximately 2.35°. The grafting amounts of primary amine groups, quaternary ammonium groups, and imidazole groups measured by thermogravimetric analysis were 22.81 %, 19.08 %, and 16.66 %, respectively. MCM41 grafted by imidazole groups retained the highest specific surface area of 710.98 m2/g and mesoporous structure with 2.40 nm pore size, although its grafting amount was low, attributed to its aromatic stacking-driven pore preservation. In single and binary adsorption systems, imidazole-modified adsorbents demonstrated superior Mo(VI) selectivity with a separation factor of 1171.57, outperforming other amine counterparts. Langmuir isotherm modeling coupled with pseudo-second-order kinetic analysis demonstrated a chemisorption-dominated monolayer adsorption mechanism, achieving maximum Mo(VI) adsorption capacities of 169.60 mg/g for primary amine-functionalized materials, 183.20 mg/g for quaternary ammonium-modified systems, and 191.33 mg/g in imidazole-grafted adsorbents. Notably, while density functional theory calculations indicated a reduced adsorption energy for imidazole groups at -362.32 kJ/mol compared to -406.46 kJ/mol for quaternary ammonium counterparts, the hierarchical pore architecture of imidazole-modified composites provided abundant ion-accessible sites, underscoring the dual necessity of structural accommodation and chemical driving forces. This work establishes a mechanistic framework for designing selective adsorbents, offering a viable solution for deep Mo(VI) removal from Re-rich industrial effluents.

Enhanced Cr(vi) removal by Co and PPy co-modified Ca-Al-layered double hydroxides due to adsorption and reduction mechanisms

Co and polypyrrole co-modified hierarchical CaAl-LDH microspheres (CCALP) were synthesized via hydrothermal and in situ polymerization methods. The synergistic effect of PPy and Co endowed CCALP with higher surface area and more reduction sites than CaAl-LDHs modified by Co or PPy alone, maintaining good recyclability for Cr(vi) removal efficiency over four cycles without any treatment. Compared to Co, PPy doping was the dominant reason for Cr(vi) reduction on CCALP. Under optimized conditions, the theoretical maximum adsorption capacity reached 845.25 mg g-1, and the removal efficiency of Cr(vi) achieved 98.83%. The Langmuir model fitted well with the Cr(vi) adsorption on CCALP, supporting the monolayer adsorption hypothesis. The adsorption process followed the Avrami fractional kinetics (AFO) model, suggesting complex and multiple kinetic stages. Thermodynamic experiments confirmed that the adsorption was a spontaneous exothermic process. The density functional theory (DFT) and electrostatic potential (ESP) calculations confirmed that the oxygen-containing parts of Cr2O7 2- and HCrO4 - were the affinity sites, and the co-doping of Co and PPy significantly improved the Cr(vi) adsorption energy on CCALP. Therefore, the Cr(vi) removal mechanism on CCALP was proposed with electrostatic interaction, ion exchange, complexation and reduction.

Polypyrrole-decorated bentonite magnetic nanocomposite: A green approach for adsorption of anionic methyl orange and cationic crystal violet dyes from contaminated water

In the presented study, a novel polypyrrole-decorated bentonite magnetic nanocomposite (MBnPPy) was synthesized for efficient removal of both anionic methyl orange (MO) and cationic crystal violet (CV) dyes from contaminated water. The synthesis of this novel adsorbent involved a two-step process: the magnetization of bentonite followed by its modification through in-situ chemical polymerization. The adsorbent was characterized by SEM/EDX, TEM/SAED, BET, TGA/DTA-DTG, FTIR, VSM, and XRD studies. The investigation of the adsorption properties of MBnPPy was focused on optimizing various parameters, such as dye concentration, medium pH, dosage, contact time, and temperature. The optimal conditions were established as follows: dye concentration of Co (CV/MO) at 100 mg/L, MBnPPy dosage at 2.0 g/L, equilibrium time set at 105 min for MO and 120 min for CV, medium pH adjusted to 5.0 for MO dye and 8.0 for CV dye, and a constant temperature of 303.15 K. The different kinetic and isotherm models were applied to fit the experimental results, and it was observed that the Pseudo-2nd-order kinetics and Langmuir adsorption isotherm were the best-fitted models. The maximal monolayer adsorption capacities of the adsorbent were found to be 78.74 mg/g and 98.04 mg/g (at 303.15 K) for CV and MO, respectively. The adsorption process for both dyes was exothermic and spontaneous. Furthermore, a reasonably good regeneration ability of MBnPPy (>83.45%/82.65% for CV/MO) was noted for up to 5 adsorption-desorption cycles with little degradation. The advantages of facile synthesis, cost-effectiveness, non-toxicity, strong adsorption capabilities for both anionic and cationic dyes, and easy separability with an external magnetic field make MBnPPy novel.

Microbial weathering of montmorillonite and its implication for Cd(II) immobilization

Interactions between silicate bacteria and silicates are very common in nature and hold great potential in altering their mutual physicochemical properties. But their interactions in regulating contaminants remediation involving performance and mechanisms are often overlooked. Here, we focused on the interactions between silicate bacteria (Paenibacillus polymyxa, PP; Bacillus circulans, BC) and a soil silicate montmorillonite (Mt), and their impact on Cd(II) immobilization. The obtained results showed that Mt greatly promoted the growth of the bacteria, resulting in a maximum 10.31 times increase in biomass production. In return, the bacteria strongly enhanced the Cd(II) adsorption on Mt, with adsorption capacities increased by 80.61%-104.45% in comparison to the raw Mt. Additionally, the bacteria-Mt interaction changed Cd(II) to a more stabilized state with a maximum reduction of 38.90%/g Mt in bioavailability. The enhancement of Cd(II) adsorption and immobilization on the bacterial modified Mt was caused by the following aspects: (1) the bacteria activities altered the aggregation state of Mt and made it better dispersed, thus more active sites were exposed; (2) the microbial activities brought about more rough and crumpled surface, as well as smaller Mt fragments; (3) a variety of microbial-derived functional groups were introduced onto the Mt surface, increasing its affinity for heavy metals; (4) the main Cd(II) immobilization mechanism was changed from ion exchange to the combination of ion exchange and functional groups induced adsorption. This work elucidates the potential ecological and evolutionary processes of silicate bacteria-soil clay mineral interactions, and bears direct implications for the clay-mediated bioremediation of heavy metals in natural environments.