Turning waste coffee grounds into a high-performance magnetic adsorbent for Cu(II) removal via ball milling and pyrolysis

Coffee grounds are converted to magnetic biochar via one-step co-ball milling. • Cu(II) uptake increases by ∼66% with rapid magnetic separation. • Oxygen-containing groups and Fe 3 O 4 synergistically drive adsorption. • Over 71% capacity is retained after five cycles. The increasing prominence of Cu(II) pollution in water bodies has driven the development of efficient and easily recoverable adsorbents. In this study, M-CBC was prepared from waste coffee grounds via pyrolysis and ball milling. Ball milling significantly increased the specific surface area and introduced abundant oxygen-containing functional groups. The loaded Fe 3 O 4 provided favorable superparamagnetism (saturation magnetization: 8.87 emu/g), enabling rapid magnetic separation. Batch adsorption experiments showed that under optimal conditions (pH 5.0, dosage 0.8 g/L, time 180 min), the actual adsorption capacity for Cu(II) reached 23.54 ± 0.23 mg/g, representing an approximately 66% enhancement compared to unmodified biochar. The adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm model, with a theoretical maximum adsorption capacity of 49.06 ± 1.36 mg/g at 25 °C, suggesting predominantly monolayer adsorption behavior on relatively homogeneous active sites. Thermodynamic analysis revealed that the adsorption process exhibited spontaneous and temperature-dependent characteristics, indicating the possible coexistence of multiple adsorption interactions. After five consecutive adsorption–desorption cycles, M-CBC retained 71.4% of its initial capacity, demonstrating excellent regeneration stability. Mechanistic analysis revealed that surface complexation with oxygen-containing groups played a dominant role, while the redox activity of Fe 3 O 4 provided synergistic enhancement. The combined effects of ball milling and Fe 3 O 4 loading effectively improved the adsorption performance, magnetic recoverability, and structural stability of the biochar. This work offers a resource-efficient approach to valorize waste coffee grounds and highlights the promising potential of M-CBC for treating Cu(II)-containing industrial wastewater due to its high efficiency and easy recoverability. Download: Download high-res image (142KB) Download: Download full-size image