Adsorption of Water Vapor on Activated Carbon: A Brief Overview

I. Introduction 230II. States of Water Adsorbed in Micropores 231III. Porous Structure of Activated Carbon 232IV. Chemical Nature of the Activated Carbon Surface 233V. Mechanism of Water Adsorption 235 A. BET Model 236 B. Dubinin-Astakhov and Dubinin-Serpinsky Models 236 C. Talu and Meunier Association Model 237 D. Other Models 237 E. Discussion 237VI. Isotherm Equations for Water Adsorption 239 A. D'Arcy-Watt Equation 239 B. Dubinin-Astakhov Equation 240 C. Dubinin-Serpinsky Equation 240 D. Talu-Meunier Equation 241VII. Enthalpy of ImmersionVIII. Characteristics of Isotherm and Hysteresis A. Shape of Adsorption Isotherm B. Hysteresis LoopIX. Carbon Pretreatment A. Effects of Heat Treatment B. Effects of C02 Activation C. Effects of Oxidizing Acid Concentration D. Effects of Aging E. Effects of Mechanoactivation F. Effects of PreadsorptionX. Form of Adsorbed Water MoleculesXL ConclusionReferences246 246 248251 252 253 254 256 256 257Activated carbon (AC) has been used to remove and recover volatile organic compounds (VOCs) from contaminated air. AC is particularly attractive as an adsorbent due to its high surface area and good uptake capacity for VOCs. Also, it has a variety of morphology, such as fibers, films, and monoliths, in addition to traditional granules. Thereby, activated carbon has a high potential for further development in environmental engineering. Water vapor usually coexists with VOCs in the air. Due to the relatively hydrophobic nature of the AC, the adsorption equilibrium is not so much hindered by the coexistence of water vapor if the relative humidity is low. Unfortunately, the relative humidity of the coexisting water vapor is usually high, and this causes a reduction in the adsorption capacity of VOCs [33,107]. This points to the importance of the problem of water adsorption in chemical and environmental engineering. The adsorption of water vapor by microporous carbon is highly specific, being dependent on surface chemistry as well as on pore size and shape. Due to the low energy of dispersion between water molecules and aromatic surfaces, the carbonaceous surfaces are essentially hydrophobic. Here the hydrophobic term does not mean that the carbonaceous surfaces repel water molecules as there is always some dispersion attraction, but water molecules simply prefer to remain in the vapor phase rather than to be in the confined spaces of the carbon micropores (intermolecular interaction is greater than molecular-surface interaction). However, the water molecules can be adsorbed specifically on oxygen-containing sites, which typically cover only a small fraction of the total surface area of carbonaceous materials. So not only