Phosphate fertilizer production from renewable resources like sewage sludge and livestock waste helps to ensure future phosphate supply, while also solving waste management issues. After combustion, the resulting ash contains heavy metals at a restrictively high level, preventing its direct use as fertilizer. In this study, several organic acids and sulfuric acid are used to dissolve phosphates from ash. Acetic, maleic and citric acids perform as expected, but oxalic acid outperforms all, including sulfuric acid. All phosphate is dissolved at pH 4 when using oxalic acid, while pH 2 is needed in the case of sulfuric acid. Furthermore, less of the heavy metals end up in the resulting solution when using oxalic acid. Nearly all calcium is retrieved in the solid residue when oxalic acid is used, pointing towards formation of calcium oxalate, not chelating complexes as often assumed, as the cause of oxalic acid outperforming the other acids in this study.
- Heavy metal
- Oxalic acid