Abstract
Agricultural wastes are considered as green adsorbents that can work as an alternative to recover critical and scarce metals from secondary sources. Critical elements as rare earth elements (REEs) can be obtained from electronic wastes or tailings and could be recovered using these green alternatives. In this study, walnut shell (WS) was tested to determine whether several REEs can be efficiently retained by this green adsorbent. Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Thermogravimetry and Differential scanning calorimetry (TG-DSC) were used to characterize WS before and after REE adsorption. Analytical performance of REE quantification was evaluated, besides of adsorption capacity and isotherms that were calculated in order to determine the model that fit well to REE adsorption. ICP-OES results indicated the lowest limit of detection and quantification (LOD and LOQ) with Eu (0.08 and 0.23ppb, respectively), nevertheless, quantification of other elements was also at the ppb level. In order to obtain the highest adsorption of metals, 75 and 2,000 µm particle sizes were studied, reaching >80% of adsorption with both sizes. Additionally, several pH values were tested in order to determine the optimum condition for maximal adsorption and adsorption capacity, noticing that pH 4 showed the best adsorption percentage (>85%, qe = 6.5 - 8mg/g). Langmuir isotherm model fitted well for Eu, La, Sm and Gd adsorption equilibrium. Characterization of WS was done using FTIR, TG-DSC and SEM. FTIR analysis showed several changes in the spectra after adsorption of REE tested but major changes were observed at the OH group, which shifted up to 31 cm-1 of wavelength. Additionally, TG-DSC showed that WS pyrolysis was divided in three stages: vaporization of moisture (about 10% of weight loss), thermal decomposition of hemicellulose, cellulose and lignin (higher than 60% of weight loss), and high-temperature calcination of residues (<25%). Finally, SEM characterization showed empty and filled pores of different sizes in WS after metal adsorption, and more rugged aspect was observed. This study reveals that WS is an efficient low-cost adsorbent for REEs and can be used for future recovery of these elements from secondary sources.
Highlights
The rare-earth elements (REEs) consist of lanthanum, scandium, yttrium, and fourteen other REEs
The high adsorption might be explained by the high carbon content of walnut shell compared to O, H, and N, which according to previous reports is composed by more than 50% of C (Wang et al, 2010; Zabihi et al, 2010; Ghasemi et al, 2015)
It was reported the use of commercial activated carbon (CAC) as an adsorbent that is effective in heavy metal removal was reported, showing an adsorption efficiency of ∼90% Cd and Cr removal (Babel and Kurniawan, 2004; Hydari et al, 2012)
Summary
The rare-earth elements (REEs) consist of lanthanum, scandium, yttrium, and fourteen other REEs. REEs have been shown to promote the growth of plants and the body weight gain of animals; agricultural and animal production industries are interested in these elements (Hu et al, 2006; Xu and Wang, 2007; Abdelnour et al, 2019; Agathokleous et al, 2019). Several materials have been used to recover heavy metals, such as peat, lignite and humic acids, fly ash, microbial biomass, and agricultural by-products such as soya bean hulls, walnut shells, and others (Laszlo and Dintzis, 1994; Alvarez Puebla et al, 2006; Pehlivan and Altun, 2008; Pehlivan et al, 2009; Módenes et al, 2015; Suresh Kumar et al, 2015; Darmayanti et al, 2017; Cheng et al, 2018; Ennoukh et al, 2019; Neris et al, 2019). The advantages of using a low-cost bioadsorbent are high efficiency at low cost, minimization of chemical and/or biological sludge regeneration of the biosorbent, and the possibility of metal recovery (Dai et al, 2018)
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