Rare earth elements, also known as lanthanides, are important components of many latest technology applications and are widely used in the industrial sector, medicine and agriculture. Although lanthanides are ubiquitous in the environment, their increasing use in aforementioned sectors over the last few decades provided them with a title of "new pollutants".

Different methods are nowadays applied for removal of various pollutants from wastewaters, whereby the emphasis is placed on the adsorption due to its simplicity, high efficiency and low cost. The most common adsorbents for metal ions are active carbon, clay minerals, biomaterials and solid wastes in the form of geopolymers [1,2]. Previous studies reported an excellent adsorption capacity of fly ash based geopolymers for removal of heavy metals from aqueous solutions, e.g. Cu, Pb, Cr, Cd, Ni, Zn, Cs [1-3]. Given the fact that lanthanides are not significantly studied in this regard, our aim was to investigate the ability of the geopolymer matrices to remove lanthanides from the model aqueous solutions.

For this purpose, the geopolymers were prepared using the Raša coal ash (Istria, Croatia) as starting material. The Raša coal characteristics, summarized by Medunić et al. [4], are as follows: total moisture 5.80-19.1 wt%, ash content 10.3-23.9 wt%, carbon 58.3-67.5 wt%, hydrogen 4.10-5.00 wt%, sulfur (org.) 7.90-10.6 wt%, oxygen 5.90-12.9 wt%, nitrogen 1.00-1.80 wt%, and an average content of rare earth elements 81.1 mg kg^-1 [5].

The Raša coal ash, previously sieved through 2 mm sieve, was activated with a sodium silicate and sodium hydroxide solutions to prepare paste specimens. The fresh pastes were exposed to heat curing in a laboratory oven at 75°C for 24 h. After two weeks of ageing at room temperature, subsamples of prepared geopolymers were used for adsorption experiments.

Aqueous solutions containing rare earth elements were prepared from multielement reference standard (Analytika, Prague, Czech Republic) containing Ce, La, Nd and Pm (100 ± 0,2 mg L^-1) and Dy, Er, Eu, Gd, Ho, Lu, Sc, Sm, Tb, Tm, Y and Yb (20 ± 0,4 mg L^-1). The subsamples (0.5 g) of prepared geopolymers were added to 50 mL of the prepared multielement solution containing 1 mg L^-1 of rare earth elements. At certain time intervals of the adsorption experiment (5 min, 15 min, 30 min, 60 min and 120 min), an aliquot of the solution (~2-3 mL) was separated by filtration through 0.45 µm syringe filter. The obtained solutions were analyzed by High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS) using an Element 2 instrument (Thermo, Bremen, Germany).

The results indicate almost complete removal of lanthanides from the initial model solution within two hours. These preliminary data are an incentive for additional experiments that will provide a detailed insight into the mechanism of adsorption of lanthanides on geopolymer matrices, including the precursor and geopolymer characterization.

Keywords: rare earth elements, geopolymers, adsorption

References

[1] Al-Zboon, K., Al-Harahsheh, M.S., Hani, F.B., 2011. Fly ash-based geopolymer for Pb removal from aqueous solution. J. Hazard. Mater. 188, 414–421

[2] Muzek, M.N., Svilovic, S., Zelic, J., 2014. Fly ash-based geopolymeric adsorbent for copper

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[3] López, F.J., Sugita, S., Tagaya, M., Kobayashi, T., 2014. Metakaolin-based geopolymers

for targeted adsorbents to heavy metal ion separation. J. Mater. Sci. Chem. Eng. 2, 16–27

[4] Medunić, G., Rađenović, A., Bajramović M., Švec, M., Tomac, M., 2016a. Once grand, now forgotten: what do we know about the superhigh-organic-sulfur Raša coal? Min. Geol. Petrol. Eng. Bull. 27-45; doi: 10.17794/rgn.2016.3.3.

[5] Fiket, Ž., Medunić, G., Kniewald, G., 2016. Rare earth element distribution in soil nearby thermal power plant. Environ. Earth Sci. 75, 598, 1-9; doi: 10.1007/s12665-016-5410-2.