Since phosphate rock and phosphorus (P) became a scarce resource in the European Union several processes are under development to recover P from wastewater for fertilizer production. However, these novel P-fertilizers can also contain toxic pollutants such as chromium in its hexavalent state (Cr(VI)). In contrast to the trivalent form Cr(III), which is an essential trace element, Cr(VI) is a highly mobile, toxic and carcinogenic compound. This hazardous form of chromium is therefore regulated with low limit value of 2 mg/kg for agricultural products. However, the Cr(VI) determination with the standard wet chemical extraction method (German norm DIN EN 15192) may be hampered using elevated temperatures and high pH-values especially in organic and or Fe(II) rich matrices. Thus, we applied the novel passive sampler Diffusive Gradients in Thin-films (DGT) technique for Cr(VI) in fertilizers and compared the results with the standard wet chemical extraction method and Chromium 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. The DGT method was very sensitive and for most tested P-fertilizers made from recycled materials selective for the analysis of Cr(VI). However, hardly soluble Cr(VI) compounds cannot be detected with the DGT method since only mobile Cr(VI) is analyzed. This, however, is also an advantage of this method because only the mobile and bioavailable Cr(VI), which is an important parameter for plant growth, will be determined. 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.
Chromium (VI) in phosphorus fertilizers determined with the diffusive gradients in thin-films (DGT) technique
Christian Vogel, Marie Hoffmann, O. Krüger, V. Murzin, W. Caliebe, Christian Adam
published in Environmental Science and Pollution Research, Vol. 27, pages 24320-24328, 2020
BAM, division Contaminant Transport and Environmental Technologies and division Thermochemical Residues Treatment and Resource