Bone char vs. S-enriched bone char: Multi-method characterization of bone chars and their transformation in soil

Highlights: • Rock phosphates for mineral P-fertilizers are limited; alternatives are required. • First complete characterization of bone char (BC) and S-enriched BC (BC^plus) • P was mainly hydroxyl apatite but in BC^plus some Ca(H₂PO₄)·2H₂O was detected by XANES. • S was mainly elemental S (62%) and sulfate (29%) in BC^plus. • P and S compounds of BC and BC^plus reacted differentially in soil. To decrease environmental impacts from usage of mineral P fertilizers based on rock phosphate, alternative P fertilizers are urgently necessary but have to be critically evaluated for their characteristics and behaviour or effects in soil. For this reason, bone char (BC) and S-enriched BC (BC^plus), original and after one vegetation period in soil, were analysed by wet chemical analyses and XANES spectroscopy. According to X-ray absorption near edge structure (XANES) spectroscopy, both chars were dominated by P bound in hydroxyapatite, which was well reflected by wet chemical P fractionation, where Ca-P was the dominant fraction. Sulfur fractionation of both chars confirmed low percentages of sulfate-S according to XANES analysis but failed to detect elemental S in BC^plus. Because S concentrations in BC^plus were comparable to that of activated carbon used for biogas desulfurization and sorbed S was dominantly elemental S, BC seems to be well suited for biogas desulfurization. After one year in soil the disappearance of more easily soluble Ca(H₂PO₄)·2H₂O and strongly reduced proportions of sulfates and sulfonates in soil-BC^plus compared to BC^plus pointed to considerable advantages of BC^plus over BC. Taking into consideration the acidic pH of BC^plus, the high Ca, P, and S concentrations and the expected microbial induced "in situ digestion" of BC by oxidation of elemental S, it can be concluded that a cascade usage of BC as biogas adsorber and following subsequent usage of BC^plus as S/P/Ca/Mg (multi-element) fertilizer could be an alternative to mineral fertilizers based on rock phosphate. The agronomic efficiency and detailed application guidelines must be derived from established and currently running longer-term plot and field experiments.