Performance and mechanism of a biochar-based Ca-La composite for the adsorption of phosphate from water

In this study, a calcium-modified biochar with a high dephosphorization efficiency was obtained using sheep manure as a carbon source and oyster shells as a calcium(Ca) source. However, it was found that the calcium-modified biochar had a low-pH inadaptability. Based on this, lanthanum(La) was doped in place of Ca to prepare biochar with a strong phosphate adsorption ability in a wide range of pH. The results showed that with the increase of Ca content, the phosphorus adsorption capacity of the material with a mass ratio of biochar to oyster shells of 1:1 (BC-Ca5) increased gradually, and the maximum adsorption capacity that was fitted by the Langmuir equation reached 78.11 mg P/g. Also, the adsorption capacity was 48.70 mg P/g when the pH > 7, and lower than 6 mg P/g when pH ≤ 7. The maximum adsorption capacity fitted to the Langmuir equation of the material with a mass ratio of biochar, oyster shells, and La of 1:1(BC-La4) reached 88.34 mg P/g, and the adsorption capacity reached 50 mg P/g under all pH conditions except pH = 7. Ca was mostly found on the surface of the biochar, and La was mainly present in the internal pore structure. The FTIR, XRD, and XPS results showed that the main adsorption mechanism was complexation. This study provides a reference for the resource utilization of livestock and poultry manure and shell solid wastes, as well as metal sources for metal modified biochar, and improving the adsorption performance of calcium-modified biochar.