Catalytic pyrolysis of chicken bone waste with potassium tartrate: availability of macro- and micronutrients

Biochar derived from bone waste (bone char) is a promising phosphorus fertiliser. However, limited nutrient availability restricts its agricultural application; therefore, different amendments have been investigated to modify nutrient release from biochar. This study evaluated the effects of pyrolysis temperature and the addition of potassium tartrate (KTar) as an in-situ catalyst on the water solubility of selected macro- and micronutrients in chicken bone waste-derived bone char. Chicken bone waste (CBW) was mixed with KTar at a 10:1 mass ratio and pyrolysed at 400-800 °C for 30 min. The KTar modification strongly affected nutrient retention and extractability. In particular, KTar enhanced sulphur retention at temperatures above 400 °C, resulting in total sulphur contents 55-97% higher than those in unmodified bone chars, and promoted almost complete water extractability of sulphur at 400 °C. Phosphorus availability was also increased by KTar. The extractable-to-total phosphorus ratios were 6 times higher at 400 °C and 1.7-2.5 times higher at higher temperatures compared with unmodified bone chars. In addition, KTar altered micronutrient behaviour as it increased the availability of iron and copper, likely through the formation of soluble salts, promoted zinc volatilisation at 800 °C and enhanced the extractability of the remaining zinc fraction. Overall, KTar modification improved the fertilising potential of bone chars by increasing the water-extractable fractions of essential nutrients. Specifically, KTar-modified bone char obtained at 500 °C represents the optimal treatment, with a balanced high yield, carbon stability, and water-extractable phosphorus, sulfur, iron, and copper. These findings suggest that KTar-assisted pyrolysis is a promising strategy for producing nutrient-enriched bone chars for agricultural applications.