Temperature-Driven Shift in the Role of Dissolved Black Carbon in Ferrihydrite–Phosphorus Interactions: From Inhibition to Promotion

Abstract

Dissolved black carbon (DBC) is widespread in natural environments and interacts strongly with Fe(III) (oxyhydr)oxides, yet how these interactions regulate phosphorus (P) retention by ferrihydrite (Fh) remains unclear. Here, we prepared DBC from biochars pyrolyzed at 300 °C (DBC300) and 550 °C (DBC550) to investigate their impacts using experimental studies and DFT simulations. DBC300 reduced Fh’s maximum P adsorption capacity by 10% via competitive ligand exchange, while DBC550 increased it by 42%. With low molecular weight and high aromaticity, DBC550 promoted Fe(III) dissolution to enhance phosphate retention, as evidenced by the Fe(III)-P correlation. This Fe(III) dissolution-mediated phosphate immobilization process dominated over competitive inhibition across a pH range of 4.0–8.5 and C/P ratios of 0.25–2.25. DFT confirms that DBC550 lengthens Fe–O bonds to destabilize Fh. Our findings show that rising pyrolysis temperature shifts DBC from a P mobilizer to a fixator; overlooking this pathway may overestimate DBC-induced eutrophication risk.