Adsorptive removal of phosphate from water with biochar from acacia tree modified with iron and magnesium oxides

A novel biochar (BC) from Acacia tortilis trees pruning waste was synthesized and tested for the removal of phosphate from aqueous solutions. The BC was prepared by calcination at 600 °C and doped with Fe₃O₄ and MgO by hydrothermal process. The presence of iron and magnesium ions in the modified BC was confirmed by EDS analysis and X-ray diffraction (XRD) methods. Both unmodified and doped BCs were tested for phosphate removal from synthetic 1-500 ppm aqueous solutions. While the unmodified BC did not show any significant removal of phosphate from aqueous solutions, the modified BC almost completely removed phosphate from water. The enhancement in removal efficiency is due to an increase in the overall surface charge and surface area of BC as a result of doping with Fe₃O₄ and MgO salts. The average porosity and BET surface area corresponding to the plain BC increased by more than 20% from 322 to 394 m²/g after modification by impregnation with iron oxide and magnesium oxide. The modificaiton of BC with Fe₃O₄ and MgO nanoparticles was observed to increase the point of zero electric charge (PZC) from pH 3.4 (corresponding to plain BC) to pH 5.3 (corresponding to modified BC). The adsorption process was very fast and a phosphate removal value of 82.5% was reached only after 30 min of adsorption, while the removal efficiency after 4 h of adsorption was 97.5%. The rapid removal efficiency in short contact time is attributed to the high surface area of BC and strong bonding between the modified BC surface and PO₄^3- ions. The highest adsorption capacity was observed to correspond to 98.5 mg/g which was achieved at PO₄^3- concentration of 500 ppm and pH 8.5. Moreover, after fitting the adsorption data onto four of the most widely used adsorption isotherm models, the adsorption of PO₄^3- onto BC can be better described by the Langmuir isotherm model.