Editing Adsorption (section)

==Adsorbents==

===Characteristics and general requirements===
[[Image:Activated Carbon.jpg|thumb|Activated carbon is used as an adsorbent]]

Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths with a [[hydrodynamic radius]] between 0.25 and 5&nbsp;mm. They must have high [[Abrasion (mechanical)|abrasion]] resistance, high [[thermal stability]] and small pore diameters, which results in higher exposed surface area and hence high capacity for adsorption. The adsorbents must also have a distinct pore structure that enables fast transport of the gaseous vapors.<ref>{{cite journal |last1=Yi |first1=Honghong |title=Effect of the Adsorbent Pore Structure on the Separation of Carbon Dioxide and Methane Gas Mixtures |journal=Journal of Chemical & Engineering Data |date=April 2015 |volume=60 |issue=5 |pages=1388–1395 |doi=10.1021/je501109q |url=https://pubs.acs.org/doi/10.1021/je501109q |access-date=21 April 2023|url-access=subscription }}</ref>
Most industrial adsorbents fall into one of three classes:
* Oxygen-containing compounds – Are typically hydrophilic and polar, including materials such as [[silica gel]], [[limestone]] (calcium carbonate)<ref>{{Cite journal|title=Mixture of CaCO<sub>3</sub> Polymorphs Serves as Best Adsorbent of Heavy Metals in Quadruple System|doi=10.1061/(ASCE)HZ.2153-5515.0000651|year=2022|last1=Viswambari Devi|first1=R|last2=Nair|first2=Vijay V|last3=Sathyamoorthy|first3=P|last4=Doble|first4=Mukesh|journal=Journal of Hazardous, Toxic & Radioactive Waste|volume=26|issue=1|s2cid=240454883 }}</ref> and [[zeolite]]s.
* Carbon-based compounds – Are typically hydrophobic and non-polar, including materials such as [[activated carbon]] and [[graphite]].
* Polymer-based compounds – Are polar or non-polar, depending on the functional groups in the polymer matrix.

===Silica gel===
[[File:THC 2003.902.070 Silica Gel Adsorber for NO2.tif|thumb|right | Silica gel adsorber for NO<sub>2</sub>, Fixed Nitrogen Research Laboratory, ca.1930s]]

[[Silica gel]] is a chemically inert, non-toxic, polar and dimensionally stable (< {{convert|400|°C|F|disp=or|sigfig=2}}) amorphous form of SiO<sub>2</sub>. It is prepared by the reaction between sodium silicate and acetic acid, which is followed by a series of after-treatment processes such as aging, pickling, etc. These after-treatment methods results in various pore size distributions.

Silica is used for drying of process air (e.g. oxygen, natural gas) and adsorption of heavy (polar) hydrocarbons from natural gas.

===Zeolites===

[[Zeolite]]s are natural or synthetic crystalline [[aluminosilicate]]s, which have a repeating pore network and release water at high temperature. Zeolites are polar in nature.

They are manufactured by [[hydrothermal synthesis]] of sodium aluminosilicate or another silica source in an [[autoclave]] followed by ion exchange with certain cations (Na<sup>+</sup>, Li<sup>+</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, NH<sub>4</sub><sup>+</sup>). The channel diameter of zeolite cages usually ranges from 2 to 9 [[angstrom|Å]]. The ion exchange process is followed by drying of the crystals, which can be pelletized with a binder to form macroporous pellets.

Zeolites are applied in drying of process air, CO<sub>2</sub> removal from natural gas, CO removal from reforming gas, air separation, [[catalytic cracking]], and catalytic synthesis and reforming.

Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by dealumination of aluminum-containing zeolites. The dealumination process is done by treating the zeolite with steam at elevated temperatures, typically greater than {{convert|500|C|F|sigfig=2}}. This high temperature heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is expelled from the zeolite framework.

===Activated carbon===

The term "adsorption" itself was coined by [[Heinrich Kayser]] in 1881 in the context of uptake of gases by carbons.<ref>{{cite web |url=https://application.wiley-vch.de/books/sample/3527324712_c01.pdf 
|title=Water and Wastewater Treatment: Historical Perspective of Activated Carbon Adsorption and its Integration with Biological Processes |website=application.wiley-vch.de
|first=Ferhan |last=((&Ccedil;e&ccedil;en)) |access-date=23 Aug 2024 |date=7 July 2011}}</ref>

[[Activated carbon]] is a highly porous, amorphous solid consisting of microcrystallites with a graphite lattice, usually prepared in small pellets or a powder. It is non-polar and cheap. One of its main drawbacks is that it reacts with oxygen at moderate temperatures (over 300&nbsp;°C).[[Image:Demac isoth.jpg|right|thumb|230px|Activated carbon nitrogen isotherm showing a marked microporous type I behavior]]

Activated carbon can be manufactured from carbonaceous material, including coal (bituminous, subbituminous, and lignite), peat, wood, or nutshells (e.g., coconut). The manufacturing process consists of two phases, [[carbonization]] and activation.<ref>Spessato, L. et al. KOH-super activated carbon from biomass waste: Insights into the paracetamol adsorption mechanism and thermal regeneration cycles. Journal of Hazardous Materials, Vol. 371, Pages 499-505, 2019.</ref><ref>Spessato, L. et al. Optimization of Sibipiruna activated carbon preparation by simplex-centroid mixture design for simultaneous adsorption of rhodamine B and metformin. Journal of Hazardous Materials, Vol. 411, Page 125166, 2021.</ref> The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons from the raw material, as well as to drive off any gases generated. The process is completed by heating the material over {{convert|400|°C|F|sigfig=2}} in an oxygen-free atmosphere that cannot support combustion. The carbonized particles are then "activated" by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. This agent burns off the pore blocking structures created during the carbonization phase and so, they develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time that they spend in this stage. Longer exposure times result in larger pore sizes. The most popular aqueous phase carbons are bituminous based because of their hardness, abrasion resistance, pore size distribution, and low cost, but their effectiveness needs to be tested in each application to determine the optimal product.

Activated carbon is used for adsorption of organic substances<ref>{{Cite journal|title=Tea waste derived activated carbon for the adsorption of sodium diclofenac from wastewater: adsorbent characteristics, adsorption isotherms, kinetics, and thermodynamics|doi=10.1007/s11356-018-3148-y|pmid=30221322|year=2018|last1=Malhotra|first1=Milan|last2=Suresh|first2=Sumathi|last3=Garg|first3=Anurag|journal=Environmental Science and Pollution Research|volume=25|issue=32|pages=32210–32220|bibcode=2018ESPR...2532210M |s2cid=52280860}}</ref> and non-polar adsorbates and it is also usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent since most of its chemical (e.g. surface groups) and physical properties (e.g. pore size distribution and surface area) can be tuned according to what is needed.<ref>{{Cite journal |last1=Blankenship |first1=L. Scott |last2=Mokaya |first2=Robert |date=2022-02-21 |title=Modulating the porosity of carbons for improved adsorption of hydrogen, carbon dioxide, and methane: a review |journal=Materials Advances |language=en |volume=3 |issue=4 |pages=1905–1930 |doi=10.1039/D1MA00911G |s2cid=245927099 |issn=2633-5409|doi-access=free }}</ref> Its usefulness also derives from its large micropore (and sometimes mesopore) volume and the resulting high surface area. Recent research works reported activated carbon as an effective agent to adsorb cationic species of toxic metals from multi-pollutant systems and also proposed possible adsorption mechanisms with supporting evidences.<ref>{{Cite journal|title=Comparative study of separation of heavy metals from leachate using activated carbon and fuel ash|doi=10.1061/(ASCE)HZ.2153-5515.0000520|pmid=04020031|year=2020|last1=Mohan|first1=S|last2=Nair|first2=Vijay V|journal=Journal of Hazardous, Toxic & Radioactive Waste|volume=24|issue=4|pages=473–491 |s2cid=219747988 }}</ref>