Fabrication and Characterization of Effective Biochar Biosorbent Derived from Agricultural Waste to Remove Cationic Dyes from Wastewater

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.

Agrowaste-generated biochar for the sustainable remediation of refractory pollutants

The rapid growth of various industries has led to a significant, alarming increase in recalcitrant pollutants in the environment. Hazardous dyes, heavy metals, pesticides, pharmaceutical products, and other associated polycyclic aromatic hydrocarbons (such as acenaphthene, fluorene, fluoranthene, phenanthrene, and pyrene) have posed a significant threat to the surroundings due to their refractory nature. Although activated carbon has been reported to be an adsorbent for removing contaminants from wastewater, it has its limitations. Hence, this review provides an elaborate account of converting agricultural waste into biochar with nanotextured surfaces that can serve as low-cost adsorbents with promising pollutant-removing properties. A detailed mechanism rationalized that this strategy involves the conversion of agrowaste to promising adsorbents that can be reduced, reused, and recycled. The potential of biowaste-derived biochar can be exploited for developing biofuel for renewable energy and also for improving soil fertility. This strategy can provide a solution to control greenhouse gas emissions by preventing the open burning of agricultural residues in fields. Furthermore, this serves a dual purpose for environmental remediation as well as effective management of agricultural waste rich in both organic and inorganic components that are generated during various agricultural operations. In this manner, this review provides recent advances in the use of agrowaste-generated biochar for cleaning the environment.

Polyvinyl Alcohol-Citric Acid: A New Material for Green and Efficient Removal of Cationic Dye Wastewater

The citric acid (CA) cross-linked polyvinyl alcohol (PVA) adsorbent, PVA-CA, was efficiently synthesized and its application to the removal of dyes in water, particularly the cationic dye, methylene blue (MB), was thoroughly investigated. The morphologies and physiochemical characteristics of PVA-CA were fully characterized by SEM, FT-IR, XRD, TGA, BET, and XPS. The effects of contact time, adsorbent dosage, MB concentration, solution pH, and temperature on the adsorption performance were compared using controllable methods. The maximum adsorption capacity of PVA-CA was 709.86 mg g-1 and the removal rate remained high through several adsorption-desorption cycles, demonstrating that such a composite absorbent has a good adsorption performance and recoverability. Further analysis by the density functional theory (DFT) showed that van der Waals interactions, electrostatic interactions and hydrogen bonding interactions between PVA-CA and MB played significant roles in the adsorption mechanism.

Characterization and Co-Adsorption Mechanism of Magnetic Clay-Biochar Composite for De-Risking Cd(II) and Methyl Orange Contaminated Water

The application of the adsorption method in sewage treatment has recently become a hot spot. A novel magnetic clay-biochar composite (BNT-MBC) was fabricated by co-pyrolysis of bentonite and biomass after being impregnated with Fe (NO3)3·9H2O. Its adsorption capacity for Cd(II) and methyl orange was approximately doubled, reaching a maximum of 26.22 and 63.34 mg/g, and could be easily separated from the solution by using external magnets with its saturation magnetization of 9.71 emu/g. A series of characterizations including surface morphology and pore structure, elemental analysis, functional group analysis and graphitization were carried out, showing that the specific surface area was increased 50 times by loading 20 wt.% bentonite, while its graphitization and oxygen-containing functional groups were also enhanced. The isotherm fitting indicated that Cd(II) was adsorbed in multiple layers, while methyl orange was in both monolayer and multilayer adsorptions. The kinetic fitting indicated that chemisorption was the rate-limiting step of both, and it was also a complex process controlled by two steps with the fitting of intra-particle diffusion. In the binary system of Cd(II) and methyl orange, the co-existing pollutants facilitated the adsorption of the original one, and there was no competition between adsorption sites of Cd(II) and methyl orange. BNT-MBC also exhibited good reusability and can be magnetically recovered for recycling. Thus, the magnetic clay-biochar composite BNT-MBC is a cost-effective and promising adsorbent for simultaneous removing Cd(II) and methyl orange from wastewater.