Digestate from biogas plants is known to contain a large amount of phosphorus (P). Due to the importance of phosphorus in plant growth and the expected depletion of phosphorite, the raw material for phosphorous fertilizer, efficient P management strategy has become necessary. One such management plan is the pyrolysis process, which converts digestate into biochar, effectively recovering phosphorus. In addition to this effectiveness, however, it is necessary to study changes in chemical properties, such as biological availability and solubility, in accordance with pyrolysis conditions, to ensure phosphorus recycling efficiency in the process. In this study, the effect of heating temperature on the phosphorus (P) chemical species of digestates and the resulting releasing characteristics of phosphorus have been investigated.

Digestates from domestic anaerobic digester (Cheongyang, Hongcheon and Dongdaemun) were collected and slowly pyrolyzed at 300, 550 and 750°C. Hedley’s sequential extraction was performed to the raw digestate and biochars. The labile fraction (H₂O extractable) of P decreased and was converted into non-labile fraction as the pyrolysis temperature increased. At the low heating temperature condition, the amount of rapidly extractable phosphorus decreased and the amount of phosphorus available for plants in the short term increased.

XANES-LCF results confirmed that chemical species such as newberyite increased in biochars produced under the low temperature condition. In the case of biochars produced under the high temperature condition, most of the P was converted into the non-labile form. This was consistent with the XANES-LCF results, which showed an increase in the proportion of highly crystalized and stable materials like hydroxyapatite, magnesium pyrophosphate and farringtonite. Additionally, it was possible to confirm that the change in phosphorus chemical species was affected by the ratio of the main components (Ca, Mg, and K) of digestate. In the case of digestate with a high Ca/P ratio, phosphorus was highly stabilized due to the facilitated binding of phosphorus to calcium pyrophosphate and hydroxyapatite.

The concentration of major cations (K, Mg, and Ca) were measured along with the concentration of phosphorus during the Hedley’s sequential extraction. K and Mg were found to stabilize as the pyrolysis temperature increased, but Ca was not extracted at the labile and semi-labile fraction. Through this, it was indirectly confirmed that the change in the labile/semi-labile portion of phosphorus resulting from the heating temperature is attributed to the phosphorus chemical species combined with K and Mg. The XANES-LCF analysis showed that as the pyrolysis increased, the phosphorus that was originally combined with K was then combined with Mg and Ca. Such transformation is due to the decomposition of K-P species with low melting point and boiling point (400°C) that resulted in the increase of Ca-P and Mg-P species.  

To analyze the effect of pyrolysis temperature on the releasing characteristics of phosphorus, P releasing kinetic experiment and repeated extraction experiment of raw digestate and its biochar(300, 550, and 750) were conducted. In the phosphorus kinetic experiment, rate constant (k) of the raw digestate and biochars produced at 300-550°C was 1.75 and 0.13-0.02 respectively, indicating that k decreases as the pyrolysis temperature increases.

Since the kinetic experiment had limitations in identifying the releasing characteristics after equilibrium, the long-term elution characteristics of phosphorus were obtained by repeated extraction (10 times). In raw digestate, P was hardly released after the 4^th extraction, while in biochars produced at 300-550°C, phosphorus was continuously released up to the 10^th extraction. XANES-LCF analysis of the extracted raw digestate and biochars confirmed that the chemical species associated with slow P releasing characteristics are MgHPO₄ and CaHPO₄. The results of this study will contribute to the selection of conditions for improving P recycling efficiency in the digestate pyrolysis process.

Keywords: Biochar, P speciation, P releasing, Solid digestate, Transformation