The potential of sulfide to dissolve low solubility inorganic phosphates from anaerobically digested biosolids with a total P (TP) concentration of 0.97 ± 0.03 mmol P/dry g was investigated over a range of sulphide to total P molar ratios (S2-/TP) . Sodium sulfide was added to 10 % biosolids slurry at S2-/TP ratios from 0 – 4 for up to 96 hours. The maximum inorganic ortho-phosphate (o-Pi) concentration (48 ± 7 mM, or 42 ± 6 % of TP) was obtained for S2-/TP of 4 after 24 h at 25 °C. Iron concentration was reduced from 0.6 ± 0.1 mM to less than 0.01 mM (S2-/TP &gt; 1). Increasing S2-/TP decreased the relative biosolids P fraction and increased Na and S as measured with energy dispersive x-ray spectroscopy. Powder x-ray diffraction suggested that as-received biosolids contained vivianite and other substituted iron phosphate minerals: ferrorosemaryite, rockbridgeite, arrojadite, graftonite, and laueite. FTIR data suggest that sulphide treatment dissolved less crystalline iron phosphate solids preferentially to crystalline phases. The dissolved o-Pi can be precipitated for reuse upon mixing with calcined cementitious material from waste concrete.

Dissolution of iron phosphate in anaerobically digested biosolids with sodium sulphide for phosphorus recovery as calcium phosphate

ACS Spring 2022

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