Acid leaching of vivianite separated from sewage sludge for recovering phosphorus and iron

• Published in: Water research (Oxford) 2024-11, Vol.266, p.122361, Article 122361
• Main Authors: Zhao, Yudong, Korving, Leon, Grönfors, Outi, Prot, Thomas, Suopajärvi, Terhi, Luukkonen, Tero, Liimatainen, Henrikki
• Format: Article
• Language: English
• Subjects: Ferric Compounds - chemistry; Ferrous oxalate; Iron - chemistry; Iron and phosphate separation; Oxalic Acid - chemistry; Phosphorus - chemistry; Phosphorus recovery; Sewage - chemistry; Sewage sludge ash; Sulfuric Acids - chemistry; Vivianite; Waste Disposal, Fluid - methods
• ISSN: 0043-1354; 1879-2448; 1879-2448
• DOI: 10.1016/j.watres.2024.122361

•Vivianite is completely dissolved in HCl and H2SO4 at a H+/P molar ratio of 2.5.•Oxalate precipitates Fe(II), Ca, and Mg ions to minimize impurities in leachate.•Phosphoric acid leaching + oxalate precipitation yields pure ferrous oxalate.•Integrating Fe recovery and oxalic acid reuse with P recovery. This paper examines the acid leaching efficiencies of Fe and P from vivianite slurry (VS, Fe3(PO4)2·8H2O), which is magnetically separated from anaerobic digested sludge, and elaborates on Fe and P reuse routes. The characteristics and dissolution behavior of raw VS in hydrochloric, sulfuric, phosphoric, oxalic, and citric acids are investigated. Results reveal that the primary impurities in VS are organic matter, other phosphate compounds, and Mg present in the vivianite crystal structure. Hydrochloric and sulfuric acids could effectively extract P (90%) from VS at an optimal hydrogen-to-phosphorus (H⁺/P) ratio of 2.5, compared with sewage sludge ash (SSA) that normally needs an H⁺/P ratio greater than 3. Hence, VS can be employed as an alternative P resource following a similar recovery route used with SSA. However, in comparison to SSA, VS use can decrease acid consumption in P extraction and the requirement for the extensive purification of cationic impurities. Furthermore, oxalic acid effectively facilitates the separation of P and Fe in VS by precipitating Fe as insoluble ferrous oxalate in acidic conditions, leading to a high Fe recovery rate of 95%. The recovery and reuse of Fe through the oxalic acid route further improves the feasibility of VS as an alternate resource. [Display omitted]