Sorptive removal of methylene blue from water by magnetic multi-walled carbon nanotube composites

Experimental and DFT insights into the adsorption mechanism of methylene blue by alkali-modified corn straw biochar

As an efficient and cost-effective adsorbent, biochar has been widely used in the adsorption and removal of dyes. In this study, a simple NaOH-modified biochar with the pyrolysis temperature of 300 °C (NaCBC300) was synthesized, characterized, and investigated for the adsorption performances and mechanisms of methylene blue (MB). NaCBC300 exhibited excellent MB adsorption performance with maximum removal efficiency and adsorption capacity of 99.98% and 290.71 mg g-1, which were three and four times higher than biochar without modification, respectively. This might be attributed to the increased content of -OH and the formation of irregular flakes after NaOH modification. The Freundlich isotherm suggested multilayer adsorption between NaCBC300 and MB. Spectroscopic characterizations demonstrated that multiple mechanisms including π-π interaction, H-bonding, and pore-filling were involved in the adsorption. According to density functional theory (DFT) calculations, electrostatic interaction between NaCBC300 and MB was verified. The highest possibility of the attraction between NaCBC300 and MB was between -COOH in NaCBC300 and R-N(CH3)2 in MB. This work improved our understanding of the mechanism for MB adsorption by modified biochar and provided practical and theoretical guidance for adsorbent preparation with high adsorption ability for dyes.

Carbon nanotube-wastewater treatment nexus: Where are we heading to?

Water treatment is crucial in solving the rising people's appetite for water and global water shortages. Carbon nanotubes (CNTs) have considerable promise for water treatment because of their adjustable and distinctive arbitrary, physical, as well as chemical characteristics. This illustrates the benefits and risks of integrating CNT into the traditional water treatment resource. Due to their outstanding adsorbent ability and chemical and mechanical properties, CNTs have gained global consideration in environmental applications. The desalination and extraction capability of CNT were improved due to chemical or physical modifications in pure CNTs by various functional groups. The CNT-based composites have many benefits, such as antifouling performance, high selectivity, and increased water permeability. Nevertheless, their full-scale implementations are still constrained by their high costs. Functionalized CNTs and their promising nanocomposites to eliminate contaminants are advised for marketing and extensive water/wastewater treatment.

Magnetic layered double hydroxide-based composites as sustainable adsorbent materials for water treatment applications: Progress, challenges, and outlook

Layered double hydroxides (LDHs) have shown exciting applications in water treatment because of their unique physicochemical properties, which include high surface areas, tunable chemical composition, large interlayer spaces, exchangeable content in interlayer galleries, and ease of modification with other materials. Interestingly, their surface, as well as the intercalated materials within the layers, play a role in the adsorption of the contaminants. The surface area of LDH materials can be further enhanced by calcination. The calcined LDHs can reattain their structural features upon hydration through the "memory effect" and may uptake anionic species within their interlayer galleries. Besides, LDH layers are positively charged within the aqueous media and can interact with specific contaminants through electrostatic interactions. LDHs can be synthesized using various methods, allowing the incorporation of other materials within the layers or forming composites that can selectively capture target pollutants. They have been combined with magnetic nanoparticles to improve their separation after adsorption and enhance adsorptive features in many cases. LDHs are relatively greener materials because they are mostly composed of inorganic salts. Magnetic LDH-based composites have been widely employed for the purification of water contaminated with heavy metals, dyes, anions, organics, pharmaceuticals, and oil. Such materials have shown interesting applications for removing contaminants from real matrices. Moreover, they can be easily regenerated and used for several adsorption-desorption cycles. Magnetic LDHs can be regarded as greener and sustainable because of several green aspects in their synthesis and reusability. We have critically reviewed their synthesis, applications, factors affecting their adsorption performance, and related mechanisms in this review. In the end, some challenges and perspectives are also discussed.

Sulfonated magnetic multi-walled carbon nanotubes with enhanced bonding stability, high adsorption performance, and reusability for water remediation

In view of the simple and rapid conveniency of magnetic separation, magnetic nanocomposites had notably gained attention from researchers for environmental field applications. In this work, carboxylated magnetic multi-walled carbon nanotubes (c-MMWCNTs) and novel sulfonated MMWCNTs (s-MMWCNTs) were synthesized by a facile solvent-free direct doping method. Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, energy dispersive X-ray, vibrating sample magnetometer, and point of zero charge analyses confirmed the successful doping of the Fe₃O₄ nanoparticles into the functionalized MWCNTs to form MMWCNTs. Besides, the bonding stabilities of both c-MMWCNTs and s-MMWCNTs were compared, and results showed that s-MMWCNTs possessed more substantial bonding stability than that of c-MMWCNTs with significantly less leaching amount of Fe₃O₄. The adsorption capacity of s-MMWCNTs was higher than that of c-MMWCNTs owing to the stronger electronegativity sulfonic group in s-MMWCNTs. Moreover, the reusability experiments proved that the adsorbent remained consistently excellent MB removal efficiency (R > 94%) even reused for twelve cycles of batch adsorption. The finding of the present work highlights the simple fabrication of novel s-MMWCNTs and its potential to be served as a promising and sustainable adsorbent for water remediation owing to its enhanced bonding stability, high adsorption performance, magnetic separability, and supreme recyclability.

Efficient degradation of organic pollutants using peroxydisulfate activated by magnetic carbon nanotube

The magnetic composite of Fe₃O₄ and carbon nanotube (MCNT) was fabricated in a facile one-pot solvothermal method and employed to activate peroxydisulfate (PDS) for degradation of Rhodamine B (RhB) and other pollutants. The effects of operational factors including MCNT dosage and PDS dosage were studied, and high removal efficiencies of 84.2-99.5% were achieved for these pollutants with 0.3 g/L MCNT and 4 mM PDS. The effects of environmental factors including initial pH, inorganic cations, inorganic anions, humic acid and water matrix were also studied. Reusability test showed that the removal efficiency declined in four consecutive runs, which was attributed to the adsorbed oxidation products on the catalyst surface. Based on quenching experiments, solvent exchange (H₂O to D₂O), inductively coupled plasma and open circuit potential tests, it was concluded that radicals of ^·OH/SO₄^·- and the non-radical electron-transfer pathway were involved in the MCNT/PDS system, and the contributions of O₂^·-, ¹O₂, high-valent iron-oxo species and homogenous activation were insignificant. Moreover, the orbital-weighted Fukui functions of RhB were calculated by density functional theory, and its plausible degradation pathway was proposed based on the calculation results. Finally, toxicity evaluation of the degradation products was performed in the quantitative structure-activity relationship approach.

Magnetic graphene oxide for methylene blue removal: adsorption performance and comparison of regeneration methods

A series of Fe₃O₄-graphene oxide (GO) composite materials (MGOs) with abundant surface area, rich oxygen-containing functional groups, and magnetic properties were prepared in a facile coprecipitation method and then employed for the adsorptive removal of methylene blue (MB) from water. The kinetic data were better fitted in the pseudo-second-order model than in the pseudo-first-order model, and the intraparticle diffusion model revealed the two-step diffusion process including diffusion in the boundary layer and in the porous structures. The maximum adsorption amounts of MB were calculated to be 37.5-108 mg/g at 25 °C and pH 9 using the Langmuir isotherm model. Thermodynamic study showed that the adsorption process was spontaneous, with ΔH° of 23.0-49.6 kJ/mol and ΔS° of 131-249 J∙mol^-1∙K^-1. The adsorption amount of MB increased with pH in the range of 4-10. Inorganic ions including Na⁺ and Ca²⁺ suppressed the adsorption of MB, and the more pronounced impact of Ca²⁺ was ascribed to its higher valence state. The cetyltrimethylammonium bromide (CTAB) surfactant showed a stronger inhibitory effect than Ca²⁺. The adsorption mechanism was proposed to be a combination of electrostatic interactions, hydrophobic adsorption, and electron donor-acceptor interactions. Two methods were used for the regeneration of spent MGO, and the results showed that the peroxomonosulfate (PMS) oxidation method was more favorable than the acid washing method, considering the better regeneration ability and lower amount of washing water used. Finally, the reaction mechanism of PMS oxidation was analyzed based on quenching tests and in situ open circuit potential measurements, which proved that OH and ¹O₂ played dominant roles and that the fine adsorption ability of MGO promoted the reaction between them and MB.