Recyclable chitosan–biochar biocomposite films for sustainable removal of congo red dye pollutants from water

Abstract

Synthetic dyes, such as congo red (CR), are among the most hazardous pollutants released into aquatic environments due to their toxicity, persistence, and potential health risks. This study prepared and extensively characterized novel chitosan–biochar biocomposite films using SEM-EDS, N₂ adsorption–desorption, FTIR, XPS, TGA, tensile testing, water contact angle measurement, and water absorption and solubility testing to efficiently remove CR dye from aqueous solutions. These characterization techniques revealed that the films exhibited excellent mechanical strength, surfaces enriched with –OH, –NH₂, and –COOH functional groups and low porosity. Furthermore, the prepared films could be easily separated from the solution after adsorption. The CR removal percentages of the composites were ∼74%, 73%, and 85% for pristine biochar (CH₂BC), acid-modified (CH₂ABC), and base-modified (CH₂BBC), respectively, after 420 minutes with a solution pH of 6.5 at 25 °C. The adsorption data suggested that the kinetic models were those of pseudo-first-order and Elovich models, while the Freundlich model best described the adsorption isotherm. Moreover, CH₂BBC retained up to 79% of its adsorption capacity after four adsorption–desorption cycles, confirming its reusability. The CR adsorption mechanism involves electrostatic attraction, π–π stacking, H-bonding, n–π interaction, imine and sulfone bridging. The environmentally friendly preparation, low cost, and easy regeneration of the prepared chitosan–biochar biocomposite films offer a promising sustainable solution for remediating dye-contaminated water.