Performance and Mechanism of a Green and Sustainable γ-nFe2O3-Based Magnetic Biochar for the Effective Adsorption of Cadmium

To improve the greenness and recyclability of adsorbent materials, a simple two-step method for the preparation of γ-nFe 2 O 3 -based magnetic biochar (FeMnBC) with high adsorption capacity and magnetic recovery properties from coffee waste was developed to achieve the cadmium (Cd) removal from aqueous solutions. Elemental analyzer, TEM, BET, XRD, FTIR and VSM (vibrating sample magnetometer) were used to characterize and evaluate the effects of each operation step or the introduction of materials during the synthesis of FeMnBC. The results showed that impregnation with KMnO 4 improved the porosity and Cd adsorption capacity of biochar. The FeMnBC had a maximum adsorption capacity of 111.36 mg/g according to the Langmuir model, and the Cd adsorption capacity was > 100.27 mg/g in the pH range of 4–9 for the application of raw coffee grounds with a certain organic load and acidity. In contrast, previous studies have found that the pH had a significant impact on the adsorption capacity of biochar, and the addition of magnetic matrix may result in a reduction in adsorption capacity (typically < 50 mg/g). The magnetization saturation of FeMnBC reached 16.61 emu/g, which was sufficient for the FeMnBC to be separated by a magnet (> 93.27%) due to the introduction of γ-nFe 2 O 3 . Moreover, the Cd removal capacity by secondary regeneration FeMnBC was still greater than 91.37 mg/g, which was of great significance to the greenness and sustainability of the adsorbent material. Furthermore, the mechanism of Cd adsorption by FeMnBC was the formation of complexes through oxygen-containing functional groups and Fe/Mn oxides, cation exchange reactions, π-bond complexation and precipitation reactions, as confirmed by XRD, FTIR, SEM–EDS and XPS analysis.