Process Intensification for Rare Earth Elements Adsorption by Resonant Vibratory Mixing (RVM)

Rare earth elements (REE) are critical to 21st-century technology, from electronics and defense applications to renewables and beyond. The processing of REE is primarily based on minerals (bastnasite, monazite, and xenotime), but alternative resources (coal ash, E-waste, and permanent magnets) are also gaining increasing interest. Adsorption remains one of the most efficient, environmentally friendly extraction methods despite its lengthy mixing time. In the present research article, a hemp biochar prepared by vacuum pyrolysis at 700 °C was examined for its applicability in the adsorption of selected REE (La3+, Nd3+, Dy3+) from synthetic solutions. An innovative technique, Resonant Vibratory Mixing (RVM), was applied to improve adsorption kinetics, with factors including time (5–30 min) and intensity (30–70%) at room temperature. Using the Thermo Scientific 4000 M shaker for mixing, the maximum adsorption capacities were 77.56 mg/g for Dy3+, 75.85 mg/g for La3+, and 72.65 mg/g for Nd3+ using 100 mg of hemp biochar and 10 mL solutions (1000 mg/L). The adsorption capacity of 100 mg hemp biochar was 79.79 mg/g for Dy3+, followed by 77.61 mg/g for La3+ and 75.75 mg/g for Nd3+, using RVM for only 40 min at 70% intensity. RVM increased the adsorption capacities of all REE in only 40 min. Surface and structural analyses were carried out using Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer-Emmett-Teller analysis (BET), Zeta Potential, and Carbon/Hydrogen/Nitrogen (CHN) methods. The adsorption recoveries of all REE in the single-element system were higher than 98.5%. However, in a multi-element system, the adsorption recoveries of La3+, Nd3+, and Dy3+ were 83.7%, 96.2%, and 99.2%, respectively, demonstrating that hemp biochar has low selectivity for Dy3+ and Nd3+. The adsorption process could be well described by the Langmuir isotherm and the pseudo-second-order kinetic model, indicating monolayer adsorption and chemical process involvement. Based on the characterization analysis of hemp biochar, electrostatic interaction was the dominant mechanism in this study. REE desorption using 0.5 M nitric acid was the most efficient, with >80% of REE recovered. The combination of hemp biochar as an adsorbent and RVM as a mixing technique demonstrated excellent performance in synthetic solutions; the reusability and application of hemp biochar to natural solutions require further study.