Phosphorus Release from Calcium-Based Modified Sludge-Derived Biochar under Low-Molecular-Weight Organic Acids: Dual Role of Inorganic Anions

Calcium-based substances carrying different inorganic anions will lead to the generation of different phosphorus (P) compounds in sludge pyrolysis products (biochar), which will inevitably affect P release efficiency. The effects of CaO and CaCl2 (2.5%–20%) on P conversion during pyrolysis at 700 °C were studied. Simultaneously, the influence of three representative low-molecular-weight organic acids (LMWOAs: acetic acid (AA), oxalic acid (OA), and citric acid (CA)) on P release behavior was investigated. The results showed that when the addition amount of CaO and CaCl2 was 10%, the AP concentration reached the maximum values of 47.05 and 46.98 mg/g, respectively. The main Ca–P of CaO-modified biochar was Ca5(PO4)3OH, while CaCl2-modified biochar was mainly Ca–P (Ca5(PO4)3Cl and Ca2PO4Cl) containing Cl. The P release rate of CaCl2-modified biochar in LMWOA was faster than that of CaO-modified biochar. Additionally, the order of P release efficiency in biochar is: CA > OA > AA at low concentrations (0.5 and 1.0 mM), OA > CA > AA at high concentrations (2.0 and 5.0 mM). The environmental factors affecting P release were ranked by significance: pH > organic matter concentration > temperature. Heavy metals showed minimal mobility during P release, confirming biochar’s stabilization capacity. Moreover, CaO-modified biochar has good stability and a slow P rate, while CaCl2-modified biochar could quickly address P deficiency and may be suitable for a severe P deficiency environment.