Particle Size- and Crystallinity-Controlled Phosphorus Release from Biochars

Controlled release sterile organic phosphorus fertilizers could be coproduced (as biochars) with biofuels using the existing thermochemical conversion platforms. However, the availability of nutrient elements in biochar changes in amended soils, as a result of sizing (fragmentation) and other biogeochemical processes. This study investigated particle size- (<38 vs <600 μm) and pyrolysis temperature-dependent (200-800 °C in 100 °C intervals) phosphorus dissolution kinetics (pH 8.5 NaHCO₃ over 18 d) of shrimp shell biochars. Two-dimensional infrared correlation spectroscopy revealed an increase in P crystallinity (P-O functional group at 1043 cm^-1) as a function of pyrolysis temperature and total P content of biochar. The broad and symmetrical P-O peak in 700 °C biochar (but not at lower pyrolysis temperatures) was attributable to semicrystalline phosphate phases. These thermodynamically stable P phases in 700 °C biochar released higher orthophosphate concentration when pulverized to smaller size fractions (<38 μm). Pyrolysis temperature-dependent phosphate release behaviors (400 °C biochar > feedstock > 700 °C biochar) indicated a complex interplay of solid-phase P speciation, dissolved organic carbon concentration, and size-dependent reactivity involving the colloidal fraction of phosphorus phases in biochars.