Assessing the potential of sewage sludge-derived biochar as a novel phosphorus fertilizer: Influence of extractant solutions and pyrolysis temperatures.

Sewage sludge-derived biochar (SSB) is a phosphorus (P) source with potential to replace soluble P fertilizers. However, SSB presents a diversity of P compounds, mainly in mineral forms with different degrees of chemical stability. This hinders the prediction of P bioavailability. In the present study we evaluated P solubility and bioavailability using different chemical extractants. Additionally, the relationships between extractable P and physicochemical properties were evaluated for SSB obtained over a wide range of temperatures (200 °C; 300 °C; 500 °C and 600 °C). Available phosphorus content was extracted using 2% citric acid (P-CA), neutral ammonium citrate + water (P-NAC) and Mehlich 1 solution (0.0125 mol L-1 H2SO4 + 0.050 mol L-1 HCl). Physicochemical properties and extractable P were strongly affected by pyrolysis temperature. Higher pyrolysis temperature resulted in increased pH, BET surface area, pore volume, ash, fixed carbon, Ca, Mg and Zn contents, as well as formation of stable Ca minerals (calcite and oxalate). The total P content increased with pyrolysis temperature (≥300 °C). Nevertheless, the solubility of biochar-P in the extractants presented different trends with temperature. The P-NAC content reached a maximum (79% of TP) at 300 °C and then declined at higher temperatures. Only at 600 °C P-CA and available P were affected by the temperature, where the P-CA increased and available P decreased. Therefore, it is recommended that the P solubility in different extractants should be considered when using SSB as an alternative to inorganic P fertilizers.