In comparison with other separation technologies, adsorption is one of the most used due to its operational characteristics and the possibility of its uses in dilute solutions and even nonclarified ones [1
]. Thus, from a long time ago, adsorbents of various natures are used as a medium to remove both valuable and nonvaluable, including hazardous, metals or solutes from aqueous solutions [2
]. Including these adsorbents, carbon nanotubes in various configurations are considered of a particular interest in this task [4
Among these applications is the case of the recovery of rare earths from various aqueous solutions [8
]. Nowadays, with a global mine production of 2.1 × 105
ton in 2019 [10
], the recovery of these rare earths is of particular interest due to their use in smart technologies and scarce availability of their resources (i.e., here in the European Union); thus, the recycling of these metals became of interest. However, in the European Union, the rate of their recyclability is around 6% and 7% for the heavy and light rare earths, respectively [11
Cerium and cerium compounds are of particular importance due to their application in strategical sectors, including metal alloys, glass, adsorbent, catalysts, biomedical applications [13
], and also in a growing sector such as rechargeable batteries [21
]; thus, cerium is considered a critical raw material by the EU [24
Recent investigations on the adsorption of cerium(III) showed that a variety of adsorbents is useful for this task, i.e., K10-montmorillonite removes Ce(III) from solutions via a cation exchange mechanism [25
], whereas simulated radioactive Ce(III) can be removed from solutions using magnetic trioctylamine–polystyrene composite microspheres [26
]. Nitrolite is used as an adsorbent for a series of light rare earths and Cr(III) [27
]; in all the cases, the adsorption increases with the increase in the pH (up to pH 9); however, metal precipitation phenomena occur together with a true adsorption process. Other adsorbents recently used for the removal of cerium(III) from aqueous solutions are various soils [28
], montmorillonite nanoclay [29
], HKUST-1 framework [30
], and chert rocks [31
] in this work, and being one of the rare exception in adsorption experiments, the stirring speed applied to the system is considered. Relying on not a true adsorption process but on an ion exchange process, several ion exchangers resins (Amberlite 200C Na, Amberlite 200C, Dowex M4195 and Diphonix) are used to investigate the recovery of Ce(III) (among other metals) from acidic solutions [32
From several years ago, we have used the two particular adsorbents mentioned below in the removal of several metals from aqueous solutions [4
]. The reason is that both materials are easily accessible in the market and, based in our experience, represented good alternatives with respect to other adsorbents, at least in terms of availability and price. Continuing in our research efforts, the present investigation shows an experimental work on the adsorption of cerium(III) from aqueous solutions using two carbon nanomaterials: nonoxidized multiwalled carbon nanotubes (MWCNTs) and oxidized multiwalled carbon nanotubes (ox-MWCNTs); in this case, the material has been functionalized by carboxylic acid groups. Several experimental variables affecting cerium(III) adsorption were investigated, and the data were fitted to several models to explain the kinetics, rate law, and adsorption isotherms associated with this adsorption. Metal loaded onto both nanomaterials can be desorbed by the use of acidic solutions.
Conceptualization, F.J.A. and F.A.L.; methodology, F.J.A. and F.A.L.; formal analysis, I.G.-D. and F.J.A.; investigation, F.J.A., F.A.L., I.G.-D., E.E.B. and O.R.L.; resources, F.A.L.; writing—original draft preparation, F.J.A.; writing—review and editing, F.J.A., F.A.L., I.G.-D., E.E.B. and O.R.L. All authors have read and agreed to the published version of the manuscript.
This research has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 776851 (CarEService).
Conflicts of Interest
The authors declare no conflict of interest.
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Influence of the stirring speed on the adsorption of cerium(III). Aqueous phase: 0.01 g/L Ce(III) at pH 6. ox-MWCNTs dosage: 1 g/L. Temperature: 25 °C.
Plot of Ce(III) adsorption (%) vs. pH.
Influence of stirring speed on cerium adsorption.
|Stirring Speed, min−1||Ce(III) Adsorption, %|
Influence of the aqueous pH.
|pH||Ce(III) Adsorption, %|
Variation in the metal adsorption with the initial cerium concentration.
|[Ce]aq,0, g/L||Ce(III) Adsorption, %|
Cerium adsorption vs. multiwalled carbon nanotube (MWCNT) dosage.
|Nanotubes Dosage, g/L||Ce(III) Adsorption, %|
Influence of initial cerium concentration in metal adsorption.
|[Ce]aq,0, g/L||Ce(III) Adsorption, %|
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