Macadamia nut shell biochar/Mg-Al LDH composite: A sustainable solution for highly effective phosphate ion removal from water

Phosphorus (P) is essential for life, but excessive phosphate (PO₄³⁻) causes eutrophication, leading to water quality deterioration. This study aims to investigate the potential of Macadamia nut shells as a sustainable material for removing PO₄^3– ions from water. A composite of biochar derived from Macadamia nut shells (MBC) and Mg-Al layered double hydroxides (Mg-Al LDH) was fabricated and utilized as an adsorbent for PO₄^3– ion removal. The materials were characterized using various techniques and the effect of various parameters on the adsorption performance of prepared adsorbents were evaluated. The highest adsorption capacity was recorded at pH 5.7, while an increment in temperatures 25 °C and 60 °C enhanced the adsorption capacity by 42 %. A variety of kinetic and isotherm models were employed to represent the experimental data accurately. The pseudo-second-order model provided the optimized fit for the PO₄^3– adsorption onto MBC/Mg-Al LDH, indicated by the highest R² value. The Langmuir isotherm model predicted the highest adsorption capacity of 43.05 mg/g for MBC/Mg-Al LDH, which is comparable to many reported adsorbents. The MBC/Mg-Al LDH composite exhibited excellent regeneration potential, with only a minor loss in adsorption capacity after eight cycles. Moreover, the MBC/Mg-Al LDH showcased remarkable selectivity for PO₄^3– ions, maintaining high adsorption efficiency even when other competing anions and humic acid were present in the solution. The impact of co-existing anions on PO₄³⁻ removal followed the sequence HCO₃⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻. The findings demonstrate that MBC/Mg-Al LDH is a promising and sustainable solution for pollutant remediation in water.