A method of preparing a Calcium-Aluminium-layered double oxide (CaAl-LDO)-doped biochar composite is provided. The method comprising providing a solution containing calcium nitrate and aluminium nitrate in a molar ratio of calcium nitrate to aluminium nitrate is 3:1, providing a pomace, providing an alkaline solution, mixing the pomace, the alkaline solution, and the solution containing calcium nitrate and aluminium nitrate, thereby to form a precipitate Calcium-Aluminium-layered double hydroxide (CaAL-LDH) on the pomace, and pyrolyzing or calcinating the mixture to prepare CaAL LDO-doped biochar composite. The pomace may be a natural pomace or pyrolyzed pomace. The (CaAl-LDO)-doped biochar composite may be used for removing phosphate-based impurities from water. The (CaAl-LDO)-doped biochar composite may adsorb up to 98% of phosphates from water.