Commonly known as Industrial Vitamins in a layman's language, Rare Earth Elements (REE), are currently used in a wide range of technological devices, including magnets, superconductors, batteries, catalysts, e.tc. According to Farahmand (2016), the volume of mined REEs worldwide sharply increased from approximately 51 kt/year by 1990, to about 131 kt/year by 2010. Just to mention, the modern day hybrid car according to Wu, Sun and Wang (2013) contains up to about 14.5 kg of the REEs. The REEs typically include Yttrium of Z = 39 and 15-Lanthanide components of Z = 57 to 71. As Doulati et al (2008) assert, in the recent past, the process of separation and upgrading of Rare Earth Elements (REEs) has been on an increasing demand. This is due to the essential purposes of these elements as well as their compounds. Ogata, Narita and Tanaka (2015) thereby reveal some of the most crucial uses of these elements to include the production of radio-pharmaceuticals, control of nuclear reactors, construction of petrochemical catalysts, aluminium, colored glasses and steel products in their respective industries, the laser industry, crude oil refinement, manufacture of infrared wavelength absorbing glasses (Langmuir, 1918), the productions of super-magnets and super-conductors, computers hard-disks, chips, and colored-bulbs among other diverse uses.
The extraction or separation of REEs such as lanthanides from each other had been considered as a great challenge due to their similarity in the chemical and physical properties (Wu, Sun & Wang, 2013). It thereby became necessary to introduce an alternative method that could be used to make the separations of such elements easier. The solvent extractions and Ion exchange methods, over a long period of time, served as the effective and reliable approaches for the extraction, separation, and recovery of elements such as lanthanides (Das & Das, 2013). However, the use of the abovementioned methods proved to be less cost-effective owing to the high costs of organic solvents, coined to a range of technical limitations (Fan, Parker & smith, 2003). It was thereby necessary to introduce a new techniques, and adsorption was regarded as a new best method for this. In this context, adsorption is thereby briefly defined as the process of adhesion of molecules, ions, or atoms from the liquid, or molten/dissolved solid, or gas on to a surface, thereby creating a thin or thick layer of adsorbate on to the surface of the used adsorbent.
Currently, all rare earth elements in use today are extracted through the processes of mineral processing and mining of the REE ores. The most commonly recognized techniques for extraction or separation, such as the precipitation and solvent extraction methods are employed in the processes of metal recovery (Das & Das, 2013). These are particularly effective in large-scale operations of high metal-iron concentrations. On the contrary, adsorption is useful in the recovery of metal-ions from sources of low concentration through relatively simpler processes. The practical adsorbent requirements include relatively high-selectivity of RE metal-ions, easier desorption of the metal-ions, low pH adsorbability, faster desorption and adsorption rates, durability against repeated usage, high adsorption capacity, high mechanical strength, and low costs (Qiu et al, 2009).
In a related study conducted by Jung-Ah et al (2011) on the leaching of REEs and their related adsorptions using the blue-green algae, the study also incorporated and examined the adsorption method. During their study, a Leachate of (0.01 L), whose pH was put under a careful control, alongside a predetermined volume of phormidium were put into a vial of 0.02 L. The pH was then adjusted to regularly between pH 1 and pH 8. After which the vial was shaken under a temperature of 293 K at a rate of 150 rpm. On the course of the experiment sets, an adsorbent dosage was alternated between 0.5 to 200 grams per liter, and the contact time was regulated between 1 minute and 2 hours. Upon the completion of the adsorption experiment, the resultant solution was then filtered and subjected to the analysis for the presence or volume of REE ions through inductively-coupled-plasma optical emission. The ICP-OES (inductively-coupled-plasma optical emission), also referred to as the inductively-coupled-plasma atomic-emission-spectroscopy (ICP-AES), is a method for the analysis of detecting the presence of metal traces or metal ions (Jung-Ah, et al, 2011). Afterwards, the investigational results were finally described in terms of Nd-adsorbed concentrations, which is the Nd mass per unit mass of adsorbent). This is as well referred to as the adsorption-density. Subsequently, the adsorbed Nd amounts q were then calculated by the use of the equation provided bellow:
q=C0-C1VM (mg/g)Where, C1 and C0 represent the aqueous adsorbate concentration (mg/L) after and before the adsorption, correspondingly; V = the sample volume (in Liters); M = adsorbents mass (in grams). The results thereby indicated the presence of metal traces since the Nd was found positive in the experiment.
Another experiment was conducted by Smith, Travis, Misra and Dhiman (2016) on the topic of the Adsorption of Aqueous REEs using Carbon-black obtained from castoff tires. During the experiment, certain particles viz. Functional-Activated-Carbon (F-AC), Activated-Carbon (A-C, calcon); commercialized carbon-black (CC-B, acetylene derived, Alfa-Aesar, bulk density 80 120 g/L, 50% compressed); recycled-tires carbon-black (RT-CB, LLC, ORT Engineering Wheels) and functional commercial carbon-black (FCCB) were used in the examination of the efficiency of the adsorption of REEs in a solution (Chen et al, 2012). The CCB, RTCB, and AC carbon particles were employed as obtained minus any treatments, while the F-CCB and F-AC were operationalized by phosphine group. During this experiment, aqueous solutions with dissolved 100 mg per liter of REE were used for the studies of adsorption. As well, Nitrate compunds/salts of Ce, Y, Sm, Nd, and La were also incorporated. For the purposes of mimicking the geothermal surroundings, the rare earth elements were all mixed to form a single uniform solution, and certain carbon adsorbents (sorbent constituents) were then confirmed at same room-temperature (Smith et al, 2016). Out of every carbon sample, masses of between 0.6 and 10 grams were taken and weighed in 250 ml plastic-reactors, within which each carbon sample was contacted with 200 ml of 100 ppm mixed REEs solutions, then shaken for 24 hours at 25 *C at a shaker speed of 200 revolutions per minute. Finally, temperature effects were then scrutinized at a range of 25 to 80 *C, and adsorption percentages determined by the use of equation below:
%Adsorption=CREE,i-CREE,tCREE,ix 100Where CREE,I and CREE,t were the initial concentrations of REE and final concentrations after time t respectively.
The results from this experiment revealed that the parent sorbent materials, including CCB and AC did not display substantial adsorption as compared to the examined domain weights, with the exemption of Nd and AC (~ 10 percent adsorption). Moreover, the functionality of CCB and AC with phosphine-group, such as F-CCB and F-AC exhibited an upsurge in the ability of adsorption (Smith et al, 2016). Noteworthy, the adsorption in the case of F-AC and F-CCB was found to be fairly independent of the sorbent materials percentage weight, but with the exemption of the Nd adsorptions. Nonetheless, the RTCB demonstrated a linear association between the sorbent materials weight and adsorption.
In a different but related study, Farahmand (2016) conducted an experiment on the Adsorptions of Cerium (iv) from Aqueous Solutions through the use of Activated-Carbon - AC developed from Rice Straws. During the experiment, the Ceriums adsorption from synthetic solutions that contain cerium-oxide was explored by the use of activated carbon that was developed or obtained from rice straws, which is activated by H3PO4. The techniques of SEM and FTIR were employed so as to examine the morphological and structural features of rice straws and activated-carbons. The optimal obtained settings with reported high adsorption levels included contact time of 500 minutes, a temperature of 35 *C, pH value of 4, adsorbent dosage of 0.02 gr, and cerium concentration of 300 ppm (Farahmand, 2016). The studys highest cerium adsorption was established by 4.12 mg/g, and the adsorption-kinetics for cerium alongside equilibrium behaviors were scrutinized. This was a close indication that the process of adsorption obeys the models of Langmuir-isotherm and Pseudo-first order kinetic (Temkin & Pyzhev, 1940). The study results thereby indicated that the activated-carbon obtained from the rice-straws that has been actuated by H3PO4 relatively predisposed the adsorbent of the ceriums adsorption from the used aqueous solutions, hence confirming the sentiments of Ho and McKay (1998) in their earlier studies.
From all the above studies and study cases, it can generally be concluded that the process and degree of adsorption are influenced by certain factors. These factors include the used solutions pH value, temperature, adsorbent dosage, the duration of contact time, and the initial concentration of metal (Ho, 2006). According to Foo and Hameed (2010), one of the most relevant tools for the thermodynamics and kinetics models of the process of adsorption is the use of diverse methods in the determination adsorption isotherms. Here, the adsorption-isotherm is expressed as the association between the concentrations of adsorbent on the solid surface and the concentrations of contaminants within the solution phase constant temperature and equilibrium (Lagergren, 1998). Here, the equilibrium cerium adsorption and time are dependent of the initial concentrations of the adsorbed materials. As asserted by Uner et al (1997), there has been an extensive examination and application of carbon-based materials in adsorption of heavy aqueous metals. The adsorptions of such heavy-metals on such materials is commonly attributed to the exchange of ions with phenolic hydroxyl and carboxylic functional groups (Jia, Wang, Li & Niu, 2004). Furthermore, rare earth elements also exhibit high affinities to dissolved organic matters, hence leading to the formations of complex organic compounds in ordinary or natural water (Shams, Darvishi & Jorfi, 2010). The presence of calcium and sodium minerals/salts also help improve the REE adsorption, whereas magnesium and zinc decreases adsorption (Freundlich, 1906). As such, adsorption mechanisms are thereby attributed to both ion and anion exchange mechanism.
Chen, Y. G., Zhu, B. H., Wu, D. B., Wang, Q. G., Yang, Y. H., Ye, W. M. & Guo, J. F. (2012). Eu(III) adsorption using di(2-ethylhexyl) phosphoric acid-immobilized magnetic GMZ bentonite. Chemical Engineering Journal, 181, 387-396.
Das, N. & Das, D. (2013). Recovery of rare earth metals through bio-sorption: An overview. Journal of Rare Earths Materials, 31, 933-943.
Doulati A, F., Badii, K., Yousefi L, N., Shafaei, S. & Mirhabibi, A. (2008). Adsorption of direct red 80 dye from aqueous solution onto almond shells: effect of ph, initial concentration and shell type. Journal of Hazardous Materials, 151, 730-737.
Fan, X., Parker, D.J. & smith, M. D. (2003). Adsorption kinetics of fluoride on low cost materials. Water Research, 37, 4929-4937.
Farahmand, E. (2016). Adsorption of cerium (IV) from aqueous solutions using activated carbon developed from rice straw. Open Journal of Geology, 6, 189-200.
Foo, K. Y. & Hameed, B. H. (2010). Insights into...
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