In this study, we synthesized Ca/Mg biochar sorbents (CMZZs) by pyrolysis of biogas residue (ZZs) impregnated with calcium chloride and magnesium chloride and investigated their potential to adsorb P from water. The results showed that the content of calcium and magnesium in the modified biochar was 1.3 and 15.4 times, respectively, what they were before the modification. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) patterns showed that the type of chemical functional groups on the surface of biochar had not changed, but several new peaks appeared, indicating that Mg (OH)2 and MgO might be present on the surface of the CMZZs. The biochar equilibrium data were well described by the Freundlich model, and the adsorption reached equilibrium after 100 min. The adsorption kinetics followed a pseudo-second-order model, and the maximum adsorption capacity of the CMZZs was 76.92 mg·g-1 when the pH was 9 and the temperature 303 K. The results revealed that a soaking method can effectively load Ca2+ and Mg2+ onto the surface of ZZ, and CMZZs offer a promising adsorbent for P removal with a high adsorption capacity for P from wastewater.
Keywords: adsorption isotherms; adsorption kinetics; biochar; biogas residue; phosphorus.