Recovery of rare earth elements by adsorption on biochar of dead Posidonia oceanica leaves

Abstract

In this study, a pristine biochar (BCP) from dead Posidonia oceanica leaves, a by-product of biofuel production, and its two chemically activated forms with KOH (BBCP) and with H 3 PO 4 (ABCP) were tested as new adsorbent materials for the recovery of three rare earth cations (REE), namely La 3+ , Dy 3+ and Nd 3+ from aqueous solutions. The biochars were characterized through elemental analysis, nitrogen adsorption-desorption analysis, attenuated total reflectance-Fourier transform infrared (ATR-FT-IR) spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX), and pH pzc measurements. From single batch adsorption experiments at different pH values, the pH = 5.0 was chosen as the best pH value for kinetic and isotherm adsorption studies. The effect of ionic medium on the adsorption ability of the best REE adsorbent ABCP was also evaluated by carrying out isotherm experiments in 0.1 mol/L NaNO 3 . Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to evaluate the REE concentration in the solutions. Kinetic and isotherm data of REE adsorption were tentatively subjected to regression analysis with various kinetic and isotherm equations. The parameter values of the best fit models and characterization results were analyzed to obtain information about the adsorption mechanism. The recyclability of ABCP adsorbent was also evaluated through recycle and reuse column experiments in which 0.1 mol/L HNO 3 and EDTA were used as extractant solutions. The chemical activation processes enhance the adsorption capacity of BCP by increasing the carbonization, the specific and microporous surface area, the pore volume and, in the case of activation with H 3 PO 4 , introducing phosphate groups in the biochar structure. The promising REE recovery results obtained with ABCP transform the biochar from a by-product to a high value-added material. This contributes to making biofuel production a more cost-effective and environmentally-friendly process. Synthetic representation in steps of the use of adsorbent material obtained from dead Posidonia oceanica leaves for the adsorption and subsequent recovery of rare earths ions. • Biochars of dead Posidonia oceanica leaves were used as adsorbents of rare earth ions; • Adsorbent materials have been extensively characterized; • Chemically activated biochars showed better recovery ability than pristine biochar; • kinetic and isotherm adsorption data were processed with different models; • the effects of pH and ionic medium on the REE adsorption were evaluated.

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