Abstract
Phosphorus (P) fertilizers from secondary resources became increasingly important in the last years. However, these novel P-fertilizers can also contain toxic pollutants such as chromium in its hexavalent state (Cr(VI)). This hazardous form of chromium is therefore regulated with low limit values for agricultural products even though the correct determination of Cr(VI) in these fertilizers may be hampered by redox processes, leading to false results. Thus, we applied the novel diffusive gradients in thin-films (DGT) technique for Cr(VI) in fertilizers and compared the results with the standard wet chemical extraction method (German norm DIN EN 15192) and Cr K-edge X-ray absorption near-edge structure (XANES) spectroscopy. We determined an overall good correlation between the wet chemical extraction and the DGT method. DGT was very sensitive and for most tested materials selective for the analysis of Cr(VI) in P-fertilizers. However, hardly soluble Cr(VI) compounds cannot be detected with the DGT method since only mobile Cr(VI) is analyzed. Furthermore, Cr K-edge XANES spectroscopy showed that the DGT binding layer also adsorbs small amounts of mobile Cr(III) so that Cr(VI) values are overestimated. Since certain types of the P-fertilizers contain mobile Cr(III) or partly immobile Cr(VI), it is necessary to optimize the DGT binding layers to avoid aforementioned over- or underestimation. Furthermore, our investigations showed that the Cr K-edge XANES spectroscopy technique is unsuitable to determine small amounts of Cr(VI) in fertilizers (below approx. 1% of Cr(VI) in relation to total Cr).
Highlights
Phosphorus (P) is essential for all living beings and is supplied by their respective nutrition
Since phosphate rock and P became a scarce resource in the European Union (EU 2017), several processes are under development to recover P from wastewater for fertilizer production (Schaum 2018; Kratz et al 2019)
Mg/kg bql bql bql bql 11.1 (11%) 19.3 (20%) 50.8 (58%) bql bql bql mg/kg bql bql bql 0.8 (0.8%) 6.8 (7%) 10.0 (11%) 36.7 (42%) bql bql bql bql below quantification limit, PR phosphate rock, triple super phosphate (TSP) triple superphosphate, CL converter lime of thermochemically treated SSA (SSA, SSA-Na1 – SSANa6), it can be clearly seen that the total Cr content decreases from 118.3 (SSA) to 86.9 mg/kg (SSA-Na6) due to the dilution with increasing amounts of added Na2CO3
Summary
Phosphorus (P) is essential for all living beings and is supplied by their respective nutrition It is removed from farmlands in the form of agricultural products and must be replaced by application of fertilizers to allow continuous farming. Since phosphate rock and P became a scarce resource in the European Union (EU 2017), several processes are under development to recover P from wastewater for fertilizer production (Schaum 2018; Kratz et al 2019). These novel P-fertilizers can contain pollutants that are toxic for plants, animals, and humans. In contrast to the trivalent form Cr(III), which is an essential trace element, Cr(VI) is a highly mobile, toxic, and carcinogenic compound
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