Various phosphorus-fertilizers
Source: BAM, division Contaminant Transport and Environmental Technologies and division Thermochemical Residues Treatment and Resource Recovery
Phosphorus (P) is essential for all living beings and is supplied by their respective nutrition. Eventually, P is removed from farmlands and must be replaced to allow for continuous farming. Mineral fertilizers made of phosphate rock are the major input stream for P. However, phosphate rock-based P-fertilizers are often contaminated with uranium (U) and thorium (Th), which might pose environmental and health hazards. Phosphorus recovery from sewage sludge ash (SSA) is an option to diminish these problems. SSA has a significant potential for P recovery and contains significantly less U and Th than phosphate rock. In addition to the total amount of U and Th in P-fertilizers, their mobility and bioavailability is important, which depends to a large extent on their chemical bonding, especially the oxidation state. Thus, we analyzed the chemical bonding of U and Th in various P-fertilizers by U and Th L3-edge X-ray absorption near-edge structure (XANES) spectroscopy. Phosphate rocks and sewage sludge-based P-fertilizers contain mainly U(IV) compounds which have only a low bioavailability. In contrast, acidic treatment of phosphate rock to produce super phosphates lead to an oxidation to U(VI) compounds (including formation of uranium phosphates) with a strongly increased bioavailability. On the contrary, all analyzed P-fertilizers contain Th in form of strongly insoluble phosphates and oxides with a low bioavailability.
Uranium and thorium species in phosphate rock and sewage sludge ash based phosphorus fertilizers
Christian Vogel, Marie Hoffmann, Mareike Taube, O. Krüger, R. Baran, Christian Adam
published in the Journal of Hazardous Materials, Vol. 382, pages 121100, 2020