Nitrogen enrichment of animal bone-derived biochars as an agricultural NP-fertilizer

Food processing creates many by-products, and not all of them are used efficiently. Especially animal-based side products are frequently considered as waste with costly disposal requirements. For recycling of the nutrients contained in these residues, also under consideration of the hygienic specifications, pyrolysis can be used to create animal bone-based biochars. A lab-scale pyrolysis reactor (Pyreka 3.0) was used to produce biochars from different bone fractions of cattle and pigs after these bones had originated as waste from abbatoir operations. This study had the objective to investigate the potential of the bone chars to serve as a phosphorus (P) supply for agricultural purposes and to study the ammonium sorption potential of these chars.

The total phosphorus content of bones reached up to 140 mg/g. The water-soluble phosphorus content was in the range of 0.16 – 0.93 mg/g, an increase in pyrolysis temperature from 350 °C to 500 °C or 650 °C increased the water-soluble content by 13.3 or 12.2 % respectively. The citric acid soluble phosphorus content was between 1.75 – 2.19 mg/g. After pyrolysis temperatures of 350 °C, slightly more phosphorus dissolved in the coal products than at 500 °C (+2.7 %) and at 650 °C (+5.5 %).

The ammonium sorption capacity of biochars produced by varying pyrolytic processes was investigated by a series of sorption experiments. The removal of ammonium by the biochars from an aqueous ammonium solution was measured by using colorimetric determination of the ammonium content. The maximum ammonium sorption results were achieved by biochars produced from bovine heads and feet respectively at a temperature of 900°C and activated with H₂O.

When exposed to a solution containing 50 mg/L of ammonium, these biochars adsorbed 1.23 and 1.14 mg ammonium/g biochar, respectively. The possibility to enrich abattoir waste biochars, which are depleted in nitrogen because of the pyrolysis process, with ammonium gained from a nitrogen-enriched biogas slurry produced from animal residues of the meat production process was tested using a substitute slurry made with ammonium sulfate. The highest absorbance rate using the substitute slurry containing 10 g/L ammonium was achieved by biochar made from bovine heads and resulted in 43.1 mg ammonium/g biochar.

This study shows that bone-based biochars enriched with nitrogen from e.g. biogas digestates have significant potential as an NP-fertilizer that supports the strategies of circular economy.