Adsorption and Membrane Processes
Adsorption and membrane processes are an emerging development in science that has greatly influenced the way in which experts in biology and chemistry handle modern problems. At the core of adsorption and membrane processes, zeolites contribute the largest toward ensuring that these processes are regulated and completed efficiently. The development of the adsorption process has resulted in its application, in industrial processes such as heterogeneous catalysis, de-ionization of water and other processes. While the process may seem complex, the structure analysis section of the essay will simplify and clarify the phenomenon. The paper will also discuss the structure and functioning of isotherms and zeolites. Finally, the conclusion will address the main elements in the essay as well as emerging issues related to the application of adsorption processes in the world. Similarly, the membrane processes have contributed significantly toward the separation industry.
The processes of adsorption and absorption are normally interchanged and confused by scientists while this should not be the case. Adsorption refers to the adhesion of ions, molecules or atoms from a dissolved solid, liquid or gas to a surface. This process produces an adsorbate film on the exterior of the adsorbent (Clercq & Lant 28). This difference between this process and absorption is that, in absorption, the fluid (absorbate) is dissolved or infused by a solid or liquid (the absorbent). It is imperative to make a note that adsorption is a process that occurs on the surface while absorption entails the entire volume of the substance.
Adsorption is quite similar to surface tension because both occur due to a consequence of surface energy. In any mass, all the fusion requirements whether covalent, ionic or metallic, of the component atoms of the substance are filled by other particles in the material. The precise nature of the bonding relies on the particulars of the species in question, but the adsorption process is largely categorized as physisorption due to the weak features of van der Waals forces or chemisorption that is common of covalent bonding. Rarely, adsorption may also take place because of electrostatic attraction (Clercq & Lant 42).
The natural occurrence of adsorption in many different systems has increased its broad application in industrial circles for instance, activated charcoal, confining and utilizing heat to supply cold water for air conditioning (adsorption chilling), synthetic resins, maximizing the storage capability for carbide-derived carbons, and purification of water. Chromatography, adsorption and ion exchange, are sorption processes where certain adsorbates are discriminatingly transmitted from the fluid stage to the surface of impenetrable, rigid particles floating in a container or stacked in a column. More unfamiliar, are the chemical industry applications as a technique of prolonging neurological exposure to certain drugs.
An uncontaminated liquid lowers its free surface energy through surface tension, which is roughly equal to the amount of energy required to cover the expected reduction in free surface energy. A large volume of dissolvable materials can significantly modify the surface tension of a fluid. Abrasive detergents, for instance, reduce surface tension considerably. If a material that has activity on its surfaces is present in a liquid, a decline in the tension at the surface will happen when the solute moves through the surface. Movement of the material to the surface or periphery results in a decrease of the energy necessary to expand the surface area, the decrease being comparative to the amount of adsorbate on the surface. The energy equilibrium of the system thus supports adsorptive concentration of surface-active materials at the phase boundary. The propensity of an impurity to reduce the surface tension of water is called hydrophobicity.
Any typical adsorption process contains five main steps. The first step involves the introduction of the feed solution as well as the adsorbent particles. The combination of these two aspects heightens the energy levels between the particles and atoms. This also improves the rate of transfer of solutes from the liquid to the surface of the absorbent solid material. The third step involves the detachment of the bulk of the solution from the adsorbent and the adsorbate. The final step involves the exclusion of the absorbed solute from the solid’s surface in a process called elution. This constant and stable rate of adsorption creates equilibrium between the solid adsorbent and the solute concentration. All these processes are made possible through the activity of zeolites (Clercq & Lant 528).
(Source: Science Direct)
Structure of Zeolites
Zeolites are micro-permeable, aluminosilicate minerals frequently used as industrial adsorbents. The expression ‘zeolite’ was initially coined in 1756 by Axel Cronstedt, who discovered that, under high temperatures, the substance stilbite emitted massive volumes of steam from the water that was adsorbed by the substance (Clercq & Lant 68). Using this observation as his foundation, Axel identified the first of many unique zeolite frameworks. Later, other archeologists and scientists discovered over 35 natural zeolite frameworks. Zeolites are widely applied in industrial processes for purifying water, as catalysts and nuclear reprocessing. All naturally occurring zeolites have aluminum. This substance acts as a good adsorbent for polar substances especially water and substances that easily dissolve in water. By eliminating the aluminum component in zeolite, it transforms into a hydrophobic compound and can even absorb other substances such as volatile organic compounds (VOC).
Zeolites are used to take out nitrogen from the air thus increasing the concentration of oxygen for both medical and industrial purposes. Their largest application is in the production of cleaning detergents. They are also utilized in agriculture and medicine. Zeolites have an absorbent structure that can contain a broad range of cations, such as Na+, Mg2+, Ca2+, K+ and others (Shoumkova 38). These positive ions are somewhat freely attached and can easily be replaced with others when they com into contact whilst in a solution. Zeolites belong to the aluminosilicate family of micro-permeable stones commonly referred to as molecular sieves. The expression ‘molecular sieve’ is in reference to a specific quality to arrange molecules based mainly on a size elimination process selectively.
Speaking in general terms, natural zeolites are hydrated aluminosilicates (Shoumkova 12). They are made up of an open, three-dimensional structure and an infinite network of open channels extending through the structure. Freely bound, positively charged cations, are connected at the joints of the negatively charged aluminosilicate web structure. The aluminosilicate structure offers extraordinary strength and steadiness to the network structure. The channels measure around 0.4 to 0.8 nanometers in thickness that is somewhat larger than a water ion, separate molecules depending on their size and transferable cations (Shoumkova 67).
Molecules having a large mass that cannot go through the entry channel are expelled which explains the expression “molecular sieve”. The molecular arrangement, cation exchange capacity (CEC), surface charge density, and surface area of each specific zeolite determines its loading, reduction, expansion and steadiness under a range of conditions (Shoumkova 36). Zeolites have a constant, three-dimensional crystalline arrangement that is comparable to the honeycomb that consists of a lattice of interconnected channels and cages. Zeolites have high specific surface areas, and their stable framework restricts reduction and swelling.
Possibly the most industrially valuable and dynamic quality of zeolite is its cation exchange capacity (CEC). Cation exchange takes place when positively charged elements or compounds switch positions on a negatively charged location. The most ordinary exchangeable cations found in zeolite molecules are potassium, calcium, magnesium, and sodium, many of which are sought-after in several industrial and biological processes. The capability to discharge beneficial elements while amassing and fusing with other, less popular, substances makes zeolite a perfect channel for the discriminatory adsorption of definite compounds and elements from water air, and soil.
An adsorption or a sorption isotherm refers to the balance of the sorption of a substance on a surface specifically its boundary at a steady temperature. Sorption isotherms represent the quantity of material attached at the surface that is the sorbate as a function of the substance present in the gas stage or within the mixture. Adsorption isotherms are frequently used as empirical models that do not declare statements on the essential methods and calculated variables. They are acquired through measuring data by the technique of regression analysis. The most regularly applied isotherms are the Freundlich isotherm, BET model, Langmuir isotherm, and the linear isotherm. Adsorption isotherms illustrate the equilibrium association of solute concentration between the solid adsorbent stage and the liquid solvent point at a specific pressure, total solute concentration, pH and temperature.
The Fundamental Adsorption Isotherm
Basing the argument on the to Le-Chatelier principle, the direction of stability would shift in that course where the stress can be alleviated (Danilczuk et al 45). When excessive pressure is applied to the equilibrium system, the balance shifts in the direction where the number of molecules diminishes. Since the number of molecules lowers towards the front, with pressure increases, forward direction of equilibrium will be preferable. There are different types of isotherms such as the Freundlich isotherm, BET model, Langmuir isotherm, and the linear isotherm.
The Langmuir isotherm
Published in 1916 by Langmuir Irving, the Langmuir isotherm attempted to explain the adsorption of gases into solids (Danilczuk et al 81). The Langmuir isotherm was drawn from a proposed kinetic method and was based on four major assumptions. It was assumed that surface of the adsorbent substance was consistent and that all the adsorption sites were alike. The second assumption was that there was absolutely no interaction between the adsorbed molecules. The third assumption was that only one mechanism was used in the adsorption process. The last assumption was that even at the maximum rate of adsorption, only one layer was created and no pile up of molecules occurred. However, scientists proved that all these four assumptions seldom applied. Langmuir’s proposition was that adsorption used the following mechanism: considering A as the gas molecule and the adsorption site to be S, direct and inverse rate constants to be k and k_1. Basing the equation on these considerations, Langmuir calculated the equilibrium as follows:
Other representations included P as the partial pressure of the gas (Danilczuk et al 56).
Industrial Processes using Zeolites
The structural and chemical properties of zeolite makes them useful in a large number of industrial applications where useful, economical substances that absorb adsorb, bind, fill and sieve are demanded.
Advantage of Zeolites over other adsorbents
While most industrial organizations clearly prefer zeolites to other types of absorbers, it is imperative to note that the best results are achieved through a combination of different absorbers. A combination of zeolite and carbon eliminated mixed fumes of xylene and methanol more efficiently than an adsorbent made up of each independent compound. At the Spring 1997 Meeting of American Institute of Chemical Engineers, Gupta from Durr Industries reported this discovery (Shoumkova 87). They realized that the path through the combination was bigger than the path through either adsorbent when they were used independently. The organization accredited the improved adsorption to the greater path length. However, both methanol and xylene were adsorbed and this created new limits to the ability of carbon adsorption and its use as the initial adsorbent upstream of a zeolite finishing bed. The idea that zeolite is a natural element that has no side effects when consumed by human beings in whatever manner increases the appeal and benefits that it possess. Doctors in hospitals can use zeolite to treat radiation patients without using many resources and caution
The major disadvantage of using zeolites as water softeners in detergents is that they are insoluble in water and contribute toward the heavy sewage sludge. Experiment outcomes and calculations have revealed that zeolite type A normally contributed toward a 10% increase in the dry mass that made up the sludge. Nevertheless, studies have indicated that detergents having traces of zeolite do not directly increase the amount of sewage sludge, which makes them excellent in relation to sludge treatment and disposal (Shoumkova 36).
Wastewater and Sewage Treatment using Zeolite Technology
As of 2011, Procter & Gamble and Henkel were cited as the largest producers of synthetic zeolites (Danilczuk et al 18). These and other large companies typically use the ZELFLOCC wastewater treatment technology. ZELFLOCC is a procedure for the improvement of wastewater sanitization systems that utilizes the extraordinary qualities of the natural stone zeolite to augment the performance and capability of present treatment systems, or to lower capital spending on modern treatment systems. The ZELFLOCC process is normally applied to activated sludge structures that utilize uniquely prepared zeolitic substances to draw in dispersed particle matter and heavy metals. Apart from this function, the process also considerably enhances the ability of the sludge to settle, reduce sludge odor, and decrease both metal requirements for phosphorus precipitation as well as polymer necessities for dewatering. Dried sludge takes on an open structure that promotes composting and vermiculture.
In the ZELFLOCC procedure, specifically prepared zeolitic substance is introduced to wastewater during the treatment process. This ZELFLOCC concentrate draws scattered particulate material, creates a core on which organisms can attach themselves, and helps in the production of sludge with excellent settling and dewatering qualities. Odors are very
( Source: Science Direct)quickly eliminated by ZELFLOCC; heavy metals are extracted even when they exist at low concentrations in the water, as well as the reduction of metal salt requirements for phosphorus precipitation. When introduced during plant start-up, the foaming is negligible, and complete deduction of floating solids and oxygen demand takes place even before actual biological flocs have developed. Some advantages of ZELFLOCC are immediately realized after the dosing starts, demonstrating chemical mechanisms, but major improvements in the ability to settle normally take a couple weeks to become noticeable, as the process provokes biological transformations in the system.
Radioactive Cleanup using Zeolite Technology
Catastrophic radiation events that have taken place globally within nuclear plants have resulted in disseminating radiation into the atmosphere and within human bodies. Victims of exposure to radiation immediately become critical with diarrhea, nausea and vomiting. Even worse, radiation can trigger long-term complications such as leukemia (cancer), genetic corruption and bodily distortion. The solution to the exposure to radiation lies in the application of natural zeolites that has been medically certified for use in eliminating heavy metals and radiation with a high rate of success that can tackle uranium 238 levels. The reason for this effectiveness lays in the exceptional, negatively charged structure enabling the zeolite to confine the hazardous elements from the human body into a molecular enclosure and eliminate it through normal excretion.
Proposed as a safe and cheap medical option for radiation-sick patients, the use of natural zeolites eliminates the need for painful and unsanitary needles, regular consumption of pills and other side effects. The Richland Hanford Nuclear Facility situated in Washington was among the first institutions to apply zeolite technology in treating radiation patients. People exposed to radioactive Strontium (Sr 90) and Cesium have been treated by subjecting them through equipment fitted with a type of natural zeolite called clinoptilolite (Genuis 125). The advantage of using clinoptilolite is that it is safe for consumption orally.
In fact, during the Chernobyl and Three Mile disasters, the society was forced to consume cookies and bread baked with clinoptilolite that directly removed the radiation chemicals from their bodies. Zeolites respond rapidly to glass, cement structures, and in the process, allow the radioactive toxins to be captured and enclosed securely. Zeolites are physically strong and unaffected by nuclear degradation. They are also cheaper than other solutions to the radiation problem. Cancers are normally known to grow after contact with toxic substances or procedures such as radiation or asbestosis. The use of zeolite progressively eliminates toxic impurities, metals, toxins and viral particles from the body thus dealing with malignancies.
Odor elimination using Zeolite Technology
Zeolite can facilitate proper ventilation and fresh air by lowering or eliminating the substance odors, carpet outgassing, volatile organic compounds (VOC), neurotoxic aggravations and formaldehyde that can cause various respiratory ailments (Genuis 45). Zeolite has the capability to desorb and adsorb water and gas atoms without going through chemical or physical alterations in the rock itself. Zeolite substance can handle and overpower odors and moisture many times their own weight. Furthermore, the desorbing process for zeolite odor systems only requires exposure to sunlight making it very cheap for companies that may be seeking cost-effective odor measures.
The hygroscopic nature of zeolite makes it the most efficient and safe desiccant that can be installed in offices and other premises that have large populations or odor problems (Genuis 28). However, the unique quality of zeolite is not its ability as a hygroscopic mineral. Its strength lies in the ability to adsorb moisture to the point of saturation. When the humidity levels in the air stream drops past the saturation point, the reverse happens. The moisture is released back into the air stream. Therefore, it is simply does not removes moisture, but instead holds or release it depending on the relative humidity of the air stream.
Zeolites are frequently installed in dehumidification equipment known normally as desiccant wheels. These contrivances reduce humidity levels and offer many advantages when implemented in the long term. Nonetheless, it is very imperative to become conscious of the fact that these equipments regularly restore the zeolite and other similar desiccants by subjecting the saturated desiccant through a hot stream of air and emitting the moisture-laden waste air from the air stream and conditioned premises.
(Source: Science Direct)
Based on the different reviewed publications, it can be concluded that, owing to their exclusive qualities, zeolites have an immense potential as useful sorbent substances for a great amount of water treatment applications. These applications such as water softening for human consumption, ammonia elimination from public sewage, factory wastewaters, swimming pools, fishponds, and animal farms, extraction of heavy metals from acid mine drainages, phosphates removal, and many other functions. Within the agricultural sector, zeolite has played a major role in boosting plant growth, reducing loss of nutrients and improving water retention and infiltration. This advancement in science therefore has inadvertently contributed toward global food security through promoting sustainable agriculture.
Chromatography is another field that has been majorly influenced by the development of the adsorption and membrane technology. Artificial zeolites developed by recycling of manufacturing, public or farming waste materials, mainly RHA and CFA, have huge possibilities as an inexpensive, environmental-friendly resolutions that could enhance the efficiency of water sanitization (Genuis 17). The cheap costs and additional environmental profit of reducing solid wastes disposal within the production of zeolites from waste materials have significantly enhanced their recognition and inspired rising interest among zeolite investigators and producers globally.
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