Overview of Adsorbents
By chemical type, adsorbents can be classified as:
ŸOrganic: activated carbon, binder-formed activated carbon, activated carbon fiber, macroporous adsorption resins, etc.;
ŸInorganic: silica gel, molecular sieves (zeolites), alumina, activated bleaching earth, diatomaceous earth, etc.
In industry, activated carbon and macroporous resins are the most widely used; alumina, activated bleaching earth, and diatomaceous earth are also employed in specific decolorization/refining scenarios.
Activated Carbon Fundamentals
Activated carbon features abundant feedstocks, relatively low cost, high specific surface area (typically 500–1500 m²/g), and a well-developed pore structure (micropores <2 nm, mesopores 2–50 nm, macropores >50 nm). It is suitable for decolorization, deodorization, removal of trace organic impurities/color bodies, and adsorption of gaseous organics, among others. Different precursors (coconut shell, wood-based, coal-/resin-based) and preparation methods lead to differences in pore architecture and surface chemistry, resulting in notable differences in adsorption selectivity and capacity.
Common quality/selection metrics: BET specific surface area, iodine number (micropores/aqueous uptake of small molecules), methylene blue number (mesopores/larger organics), particle size and grading, hardness/attrition rate, ash/metal impurities, moisture content, pH, etc.
Forms and Classifications
ŸPowdered Activated Carbon (PAC): fine particle size, fast adsorption rate, high capacity per unit mass; however, high pressure drop/difficult filtration and strict dust control requirements. Typically used for batch decolorization with post-dosing agitation and filtration.
ŸGranular/Extruded Activated Carbon (GAC/EAC): larger particle size, stable packed-bed operation, controllable pressure drop; convenient for continuous fixed-bed/moving-bed operation.
ŸBinder-Formed Activated Carbon (e.g., using nylon/phenolic as binders): converts powdered carbon into regular granules/pellets/honeycombs; low dust, high mechanical strength, easy flow control, but slightly lower capacity per unit mass due to binder masking. Suitable for chromatographic or refining processes requiring controlled elution/regeneration. (Common binders: phenolic resins, pitch-based binders, PTFE/PVDF, PVA, etc.; some systems use engineering plastics or other polymers.)
ŸActivated Carbon Fiber (ACF): produced by activating carbon fibers derived from organic fibers; features high external surface area, narrow pore size distribution, and fast mass transfer. Can be made into felt, fabric, paper, honeycomb, and other forms; suitable for gas-phase purification, trace removal, and rapid cyclic adsorption–desorption.
Brief Comparison of Activated Carbon Forms
nPAC: high capacity/fast rate → difficult filtration, significant dust; suited to batch decolorization and short-process “rescue” treatments.
nGAC/EAC: stable operation, convenient for fixed beds and regeneration; capacity depends on the carbon’s pore structure/surface chemistry—form itself does not determine equilibrium capacity (PAC has faster kinetics) → suited to continuous fixed-bed operation and regenerable systems.
nBinder-formed activated carbon: slightly lower capacity, superior strength/operability → suited to controlled elution and low-dust requirements.
nACF: ultrafast mass transfer, thin-layer configurations → suited to trace purification and rapid cyclic adsorption–desorption.
General Rules of Adsorption and Desorption (primarily liquid/gas-phase physisorption)
lHydrophobicity/Aromaticity: aromatics > aliphatics (due to π–π interactions and polarizability); more hydrophobic species (high logKₒw) are more readily adsorbed.
lPolarity/Degree of Ionization: compounds with more polar groups and higher ionization in water show reduced adsorption tendency due to increased solubility and hydration.
lMolecular Size/Pore Matching: greater molecular weight/size with better matching to micro-/mesopores often yields higher capacity; excessively large molecules will be diffusion-limited.
lSolvent Effects: the more soluble the solute in the solvent, the weaker the driving force for adsorption. In most cases, adsorption is stronger in water or in aqueous phases with a low proportion of organics; increasing the fraction of alcohols/ketones markedly reduces adsorption. In aqueous-dominant systems, adding alcohols/ketones as cosolvents typically weakens hydrophobic interactions and lowers uptake; for nonaqueous systems, evaluate specific solvent–solute–carbon surface interactions.
lpH effects (ionization suppresses adsorption):
Ÿ Acidic compounds: adsorb more strongly under acidic conditions (predominantly unionized); basic conditions facilitate desorption.
Ÿ Basic compounds: adsorb more strongly under basic conditions (predominantly unprotonated/free base); acidic conditions facilitate desorption.
lTemperature: physisorption is typically exothermic; raising temperature usually lowers equilibrium capacity but can accelerate kinetics.
lCompetition/Coexistence: coexisting organics, natural organic matter (NOM), or surfactants compete for sites and/or block pores, reducing target removal.
Selection and Application Notes
1. Initial screening: prioritize granular/extruded activated carbon to balance operability and pressure drop; if the target is difficult to adsorb, try PAC or high-iodine/high–methylene blue grades; if adsorption is too strong with elution peak tailing, consider binder-formed/surface-modified activated carbon or lower carbon dosage.
2. Process variables: for liquids, ensure prefiltration/clarification (to prevent plugging), proper contact time (EBCT), control of organic solvent proportion, and optimized pH and temperature; for gases, pay attention to humidity (water vapor occupies micropores), bed temperature, and breakthrough curves.
3. Regeneration and replacement: steam/thermal regeneration is common in gas-phase service; liquid-phase systems can employ solvent desorption plus thermal regeneration. Capacity declines after multiple cycles—use breakthrough curves and iodine number/loading criteria for replacement decisions.
4. Safety and compliance: control dust to prevent explosion and dust hazards; manage adsorption exotherms and fire risk during regeneration/activation; treat off-gas. Monitor potential impacts of ash/metals on downstream processes (e.g., catalysis/pharmaceutical scenarios).
Spherical Activated Carbon (SAC) vs. “Spherical Carbonized Resin”
SAC is typically produced by carbonization followed by activation of spherical macroporous resins or polymer beads (e.g., phenolic or PS–DVB). It features highly regular sphericity, dense and uniform packing, low pressure drop, and high mechanical strength, making it suitable for fixed phases in gas- and liquid-chromatography, fine purification, and controlled fluidized-bed operations. Note the distinction: “spherical carbonized resin” (carbonized only) has limited porosity and specific surface area; high-capacity adsorption performance is achieved only after activation.
Adsorbents & Related Materials at a Glance
Category | English Name | Common CAS No. | Notes |
Carbon-based adsorbents | Activated carbon/Activated charcoal | 7440-44-0; also 64365-11-3 | 7440-44-0 (carbon/activated carbon, used by ECHA, etc.); some catalogs use 64365-11-3 specifically for “Activated carbon”. |
Carbon-based adsorbents | Spherical activated carbon (SAC) | Same as above (7440-44-0 commonly used) | Different form but the substance remains activated carbon; most suppliers do not assign a separate CAS. |
Carbon-based adsorbents | Activated carbon fiber (ACF) | Usually uses 7440-44-0 | Listed under “carbon/activated carbon”; no unified dedicated CAS—suppliers often label by product name. |
Inorganic adsorbents | Silica gel | 112926-00-8; also 7631-86-9 (silica/silica gel) | Both are common: 112926-00-8 (amorphous silica/colloidal silica), 7631-86-9 is also used by many suppliers for silica gel beads. |
Inorganic adsorbents | Zeolites (aluminosilicates) | Commercial frameworks (3A/4A/5A/13X, etc.) are generally grouped under this number. | |
Inorganic adsorbents | Activated alumina/Aluminum oxide, activated | Chromatography grades commonly labeled Brockmann I/II, etc. | |
Inorganic adsorbents | Diatomaceous earth/Kieselguhr | Widely used under this number as a filter aid/carrier. | |
Inorganic adsorbents | Activated bleaching earth / Acid-leached bentonite | 70131-50-9 | Also called Activated clay; Montmorillonite K10 (commercial grade). |
Polymeric organic adsorbents | Macroporous PS-DVB adsorption resin | 9003-70-7 (commonly for XAD-2); also 9052-95-3 (PS-DVB) | Belongs to “copolymer/commercial systems”; CAS varies with grade; refer to the supplier’s specification. |
Binder examples¹ | Nylon-6/Polycaprolactam | “Nylon–activated carbon” refers to binder-formed carbons; nylon is one common binder. | |
Binder examples¹ | Nylon-66/Poly(hexamethylene adipamide) | Another commonly used binder/engineering plastic resin. |
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