title: Thermochemical treatment of sewage sludge ash with sodium salt additives for phosphorus fertilizer production--Analysis of underlying chemical reactions.
abstract: Stocks of high grade phosphate rock are becoming scarce, and there is growing concern about potentially harmful impurities in conventional phosphorus fertilizers. Sewage sludge ash is a promising secondary phosphorus source. However, to remove heavy metals and convert the phosphorus contained in sewage sludge ash into mineral phases available to plants, an after-treatment is required. Laboratory-scale calcination experiments of sewage sludge ash blended with sodium salts using dried sewage sludge as a reducing agent were carried out at 1000°C. Thus, the Ca3(PO4)2 or whitlockite component of raw sewage sludge ash, which is not readily plant available, was converted to CaNaPO4 (buchwaldite). Consequently, nearly complete phosphorus solubility in ammonium citrate (a well-established indicator for plant availability) was achieved. Moreover, it was shown that Na2CO3 may be replaced by moderately priced Na2SO4. However, molar ratios of Na/P>2 were required to achieve >80% phosphorus solubility. Such over-stoichiometric Na consumption is largely caused by side reactions with the SiO2 component of the sewage sludge ash - an explanation for which clear evidence is provided for the first time.
authors.affiliation: Department of Materials and Environment, BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany. Electronic address: jan.stemann@gmail.com.
authors.forename: Jan
authors.initials: J
authors.lastname: Stemann
authors.affiliation: Department of Analytical Chemistry, Reference Materials, BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany.
authors.forename: Burkhard
authors.initials: B
authors.lastname: Peplinski
authors.affiliation: Department of Materials and Environment, BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Adam
mesh_terms: D002118:Calcium / Q000737:chemistry; D060729:Coal Ash / Q000737:chemistry*; D005308:Fertilizers / Q000032:analysis*; D006358:Hot Temperature; D007477:Ions / Q000737:chemistry; D010758:Phosphorus / Q000032:analysis*; D012722:Sewage / Q000032:analysis*; D012964:Sodium / Q000737:chemistry*
chemical_list: D060729:Coal Ash; D005308:Fertilizers; D007477:Ions; D012722:Sewage; D010758:Phosphorus; D012964:Sodium; D002118:Calcium
keywords: Phosphorus recovery; Plant availability; Solubility in ammonium citrate; Whitlockite-to-buchwaldite transformation