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This article in JEQ

  1. Vol. 43 No. 3, p. 1050-1060
    Received: May 24, 2013
    Published: June 24, 2014

    * Corresponding author(s): simone.nanzer@usys.ethz.ch
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The Molecular Environment of Phosphorus in Sewage Sludge Ash: Implications for Bioavailability

  1. Simone Nanzer *a,
  2. Astrid Obersona,
  3. Thomas Huthwelkerb,
  4. Urs Eggenbergerc and
  5. Emmanuel Frossarda
  1. a Institute of Agricultural Sciences, ETH Zurich, Eschikon 33, 8315 Lindau, Switzerland
    b Swiss Light Source, Paul Scherrer Institute, WLGA 211, 5232 Villigen, Switzerland
    c Institute of Geological Sciences, Univ. of Bern, Baltzerstrasse 1 and 3, 3012 Bern, Switzerland


Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl2 addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state 31P direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil–sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P ≤ 2, SSAs contained combinations of polymorphs of AlPO4, β-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.

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