Biochar and nano-hydroxyapatite as green adsorbent: synergistic effect in remediating copper-contaminated water and...

Copper (Cu) contamination in water and soil seriously threatens environmental and human health. This study evaluates the synergistic effect of palm kernel shell biochar (PKB) and nano-hydroxyapatite (nHAp) as green adsorbents for Cu remediation. Materials were characterized using SEM, XRD, FTIR, and BET analysis. An adsorption study was evaluated using the batch method, and soil incubation experiments were conducted to determine the soil's physicochemical properties, Cu bioavailability, Cu fractionation in soil, and corn seed germination. PKB exhibited a higher surface area (158.44 m²/g) compared to nHAp (52.65 m²/g), while nHAp showed larger pore volume (0.26 cm³/g) and pore size (19.63 nm). Batch adsorption experiments demonstrated that the Langmuir isotherm model best fit the data (R² > 0.95), indicating monolayer adsorption. The combined adsorbent (CBnHAp at 2.2 g) achieved the highest maximum adsorption capacity (q_max = 3.49 mg/g). In a 6-week soil incubation experiment, CBnHAp (T5) significantly increased soil pH from 5.30 to 6.30 and enhanced soil organic matter (SOM) content to 7.75%. This treatment reduced the available Cu concentration from 166.38 mg/kg to as low as 29.93 mg/kg, achieving up to 74.0% reduction in Cu bioavailability. Sequential extraction revealed that CBnHAp shifted Cu from the more bioavailable fractions (F1 and F2) to less mobile fractions (F3 and F4). Furthermore, a corn seed germination bioassay demonstrated that CBnHAp (T5) improved germination rates by up to 73.00%, with significant increases in root and shoot growth compared to the control. This study demonstrates that CBnHAp is a highly effective, sustainable, and eco-friendly approach for remediating Cu-contaminated environments, offering a promising solution for water and soil decontamination.