Phosphate adsorption using metal-modified biochar has awakened much attention and triggered extensive research. In this study, the effect of lanthanum (La)-modified sludge using impregnation-co-precipitation was used for phosphate adsorption. Consequently, La-coated biochar at a pyrolysis temperature of 600 °C had the highest phosphate adsorption and the lowest heavy metal leaching potential. The treatment of virgin biochar with alkali before La loading was found to be beneficial for the increase of phosphate adsorption capacity. The adsorption kinetics was well depicted by the pseudo-second-order model, and indicating that intraparticle diffusion played a crucial role in the adsorption process. The good fitness between adsorption data and the Langmuir isotherm model showed a maximal adsorption capacity of 93.91 mg/g, where phosphate absorption was highly correlated to its concentration in the solution. The La-coated biochar showed high adsorption capacity when the solution pH varied from 3.0 to 6.0, and was insensitive to the coexisting chloride, nitrate, sulfate, bicarbonate and citrate. Moreover, the adsorption mechanism was further explored by using Zeta potential analysis, FTIR and XPS, indicating that the phosphate is adsorbed through electrostatic attraction in the form of the inner-sphere complexation. All the results suggested that the sludge-based biochar, as a support material for La, could serve as a promising adsorbent for phosphate in real applications.