Phosphorus release characterization of biochar loaded with inherent and exogenous phosphorus and impact on soil Pb immobilization

Phosphorus (P)-laden biochar has been widely used for environmental lead (Pb) remediation, but the P release risk and their adsorption and immobilization effect on soil potentially toxic elements (PTEs) between inherent P-laden biochar and exogenous P-laden biochar are rarely investigated. In this study, two biochars loaded with low and moderate inherent P (LPIB and MPIB) and two biochars loaded with moderate and high exogenous P (MPEB and HPEB) were produced by wheat straws with total P concentrations of 2.53, 4.71, 5.24, and 10.8 mg g−1, respectively. Water-extractable P was dominated in LPIB (49.5%), MPIB (35.9%), and MPEB (42.1%), while residual P was predominant in HPEB (43.5%). Meanwhile, labile P in MPEB (3002 mg kg−1) was 1.32 times higher than that in MPIB (2272 mg kg−1). Maximum P release capacity followed the order: LPIB (1.53 mg g−1) < MPIB (2.13 mg g−1) < MPEB (2.65 mg g−1) < HPEB (4.37 mg g−1), which was more suitable for the pseudo-second-order model. Lower pH, common anions (Cl−, NO3−, SO42−, HCO3−) and cations (Na+, K+) significantly promoted P release from biochar, but Ca2+ and Mg2+ inhibited P release. Langmuir sorption isotherms were better fitted the sorption of Pb2+ by the four biochars and the maximum adsorption capacities followed the order of HPEB (116 mg g−1) > LPIB (105 mg g−1) ≈ MPEB (104 mg g−1) ≈ MPIB (103 mg g−1). Four biochars applications significantly increased soil pH by 0.66–0.68 and decreased CaCl2-extractable Pb and exchangeable Pb by 72.1–92.7% and 27.3–33.0%, promoting Pb bounding to carbonate and organic matter by 57.2–60.9% and 367–467%, respectively. HPEB application increased labile P by 25.6% and 37.2% compared to MPIB and MPEB applications. This study implies that biochar loaded with exogenous P has higher P release risk than that loaded with inherent P when they have similar Pb adsorption and immobilization effects.