Quantitative determination of vivianite in sewage sludge by a phosphate extraction protocol validated by PXRD, SEM-EDS, and 31P NMR spectroscopy towards efficient vivianite recovery

Qian Wang, Tae Hyun Kim, Kasper Reitzel, Nina Almind-Jørgensen, Ulla Gro Nielsen*

*Corresponding author for this work

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Vivianite (Fe3(PO4)2⋅8H2O) is a potential phosphorus (P) recovery product from wastewater treatment plants (WWTPs). However, routine methods for quantification of vivianite bound P (vivianite-P) are needed to establish the link between vivianite formation and operating conditions, as current approaches require specialized instrumentation (Mössbauer or synchrotron). This study modified a conventional sequential P extraction protocol by insertion of an extraction step (0.2% 2,2′-bipyridine + 0.1 M KCl) targeting vivianite-P (Gu et al., Water Research, 2016, 103, 352–361). This protocol was tested on digested and dewatered sludge from two WWTPs, in which vivianite (molar Fe:P ratios of 1.0–1.6) was unambiguously identified by optical microscopy, powder X-ray diffraction, and scanning electron microscopy with energy dispersive X-ray spectroscopy. The results showed that vivianite-P was separated from iron(III)-bound P (Fe(III)-P) in the sludge. Vivianite-P constituted about half of the total P (TP) in the sludge from a Fe dosing chemical P removal (CPR) WWTP, but only 16–26% of TP in the sludge from a WWTP using a combination of Fe dosing CPR and enhanced biological P removal (EBPR). The modified protocol revealed that Fe-bound P (Fe-P, i.e., vivianite-P + Fe(III)-P) was the dominant P fraction, in agreement with quantitative 31P nuclear magnetic resonance (NMR) experiments. Moreover, it was shown that the conventional P extraction protocol underestimated the Fe-P content by 6–35%. The established protocol represents a reliable in-house analytical method that can distinguish and quantify vivianite-P and Fe(III)-P in sludge, i.e. facilitate optimized vivianite production at WWTPs.

Original languageEnglish
Article number117411
JournalWater Research
Number of pages9
Publication statusPublished - 1. Sep 2021


  • Circular economy
  • Phosphate recovery
  • Sewage sludge
  • Solid-state P NMR
  • Vivianite


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