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 H3PO4 (ABCP) were tested as new adsorbent materials for the recovery of three rare earth cations (REE), namely La3+, Dy3+ and Nd3+ 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 pHpzc 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 NaNO3. 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 HNO3 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 H3PO4, 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.