Phosphorus speciation in different sewage sludges and their biochars and its implications for movement of labile phosphate in two soils

• Published in: Journal of environmental management Vol. 370; p. 122565
• Main Authors: Kooij, Josephine, Yang, Puu-Tai, Bruun, Sander, Magid, Jakob, Gro Nielsen, Ulla, Theil Kuhn, Luise, Müller-Stöver, Dorette
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01-11-2024
• Subjects: 31P NMR; Biochar; Phosphorus mobility; Phosphorus speciation; Sewage sludge; XANES; Phosphorus mobility; Biochar; XANES; Sewage sludge; Phosphorus speciation; 31P NMR; P NMR
• ISSN: 0301-4797; 1095-8630; 1095-8630
• DOI: 10.1016/j.jenvman.2024.122565

It is essential to understand the P dynamics of recycled biomaterials, like biochar derived from sewage sludge, especially with potential application as fertilizers. The objective of this study was to understand how pyrolysis affects the speciation of P in sewage sludge and thereby the effect on labile P pools and mobility of P in soil. The P speciation and lability of two sewage sludges (one biologically treated and one iron-precipitated) and their biochars (pyrolyzed at 400 °C and 600 °C) were determined by liquid state 31P nuclear magnetic resonance spectroscopy, X-ray absorption near edge spectroscopy, and sequential chemical extraction. These biomaterials were applied in a concentrated band to two soils, and P lability was studied in the adjacent soil at varying distances.
Speciation techniques showed P was more closely associated with Ca and Fe for the iron-precipitated sludge and its biochars than the biologically treated sludge and its biochars. Instead, the P in the biologically treated biochars was found to be largely (40% or more) in polymeric forms (pyro- or poly-phosphates). The relationship between the speciation and the mobility of P in soil (as assessed by incubating biomaterials in a one-dimensional reaction system) was more evident when incubating the sewage sludges than the respective biochars. Particularly, the biologically treated sludge had a high proportion of labile P (56% water-extractable P), as determined by sequential extraction, and upon incubation, it was also the only material where water-extractable P remained significantly above the control soil level up to 3 mm from the biomaterial layer. After pyrolysis, this lability decreased significantly (up to a 25-fold decrease in water-extractable P), and this was reflected in the immobility of P in the biochars during incubation in the two soils. Differences in speciation between biochars were not reflected in the incubation experiment, as the differences in P release and mobility were not significant.
•Two sewage sludges (EBPR and Fe-precipitated) were pyrolyzed at 400 °C and 600 °C.•P in Fe-precipitated sludge and derived biochars was closely associated with Fe and Ca.•P in EBPR sludge was not highly metal-bound and was thus very mobile in soils.•Biochars from EBPR sludge contained high proportions of pyro- and poly-phopshates.•P in all biochars showed a similar low mobility in soil.