Sorption studies of the surface modified activated carbon with beta-cyclodextrin

Abstract

Activated Carbon (AC) is an amorphous carbon-based material characterized with a large surface area (~ 1,000 m²/g) and consists primarily of graphitic (sp² hybrid) layers. Its amphoteric chemical property results because of the chemical treatment of the surface of AC with oxidizing agents, reducing agents, and grafting agents. β-cyclodextrin (β-CD) is a very interesting carbohydrate oligomer that provides very strong binding ability for small organic guest molecules in its inner cavity (6.0 ~ 6.5 Å) by van der Waals interactions and hydrogen bond formation between the guest molecules and the host. Surface modification of AC with β-CD was synthesized by chemical methods: oxidation with HNO₃, reduction with LiAlH4, and grafting β-CD onto the surface of AC via organic linkers such as glutaraldehyde and 1,4-phenylene diisocyanate. This surface grafted AC with β-CD, then, was evaluated for its surface area and sorption performance by using a solution dye sorption method using dye adsorbates. Surface functional groups produced from oxidation (carboxylic acid, lactone, quinine, phenol, and nitro groups), reduction (alcohol and amine groups), and grafting (imine, hemiacetal, and urethane bonds) methods including microscopy of untreated, surface modified, and grafted ACs were characterized by various surface characterization methods: Diffuse Reflectance Infra-red Fourier Transform Spectroscopy (DRIFTS), Scanning Electron Microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Thermogravimetric analysis (TGA), Differential thermogravimetry (DTG), Matrix Assisted Laser Desorption Ionization Time of Flight mass spectrometry (MALDI TOF MS), and Electron spin resonance (ESR) spectroscopy. A chemical method, the Boehm method, was used for identifying surface bound acidic and basic functional groups. Nitrogen porosimetry was used to analyze the surface area and pore structure characteristics of AC, surface modified ACs, and grafted ACs. p-nitrophenol (PNP) and methylene blue (MB) were used as adsorbates for the dye sorption method. PNP and MB were used to measure the sorption performance of grafted ACs at equilibrium using UV-vis spectrophotometry in aqueous solution. Sorption capacity (Qe), surface area (m²/g), and binding affinity characteristics [KF (L/g), KL (g/mol), and KBET (L/g)] were determined at equilibrium conditions using fundamental sorption models such as Langmuir, Freundlich, and BET isotherms. The sorption performance of grafted ACs and granular AC were different according to the difference in surface area and pore structure characteristics of each material.